No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
In Situ Monitoring of Silicon Plasma Etching Using a Quantum Cascade Laser Arrangement
Abstract
In etch plasmas used for semiconductor processing, concentrations of the precursor gas NF3 and of the etch product SiF4 are measured online and in situ using a new diagnostic arrangement, the Q-MACS Etch system, which is based on quantum cascade laser absorption spectroscopy (QCLAS). In addition, the etch rates of SiO2 layers and of the silicon wafer are monitored including plasma-etching endpoint detection. For this purpose the Q-MACS Etch system is working as an interferometer arrangement. The experiments are performed in an industrial, dual-frequency, capacitively coupled, magnetically enhanced, reactive ion etcher (MERIE), which is a plasma reactor developed for dynamic random access memory (DRAM) technologies. In the spectral range 1028 ± 0.3 cm–1, the absorption cross-sections of SiF4 and NF3 are determined to be σ = (7.7 ± 0.7) × 10–18 cm2 molecule–1 and σ = (8.7 ± 0.8) × 10–20 cm2 molecule–1, respectively.
... Sensors for etching processes must fulfill specific criteria, such as the ability to perform real-time measurements, nonintrusiveness, and compliance with cleanroom operation requirements. Laser-based sensors show all the desired characteristics and have been employed for the in situ process monitoring of the industryscale production of dynamic random access memory [557,558]. Stancu et al. used a QCL near 9727 nm to measure absorption cross sections of etching products, SiF 4 and NF 3 , to quantify spectral interference from NF 3 [557] (Fig. 30). The first demonstration of online and in situ concentration measurements of SiF 4 and NF 3 by Stancu et al [557]. ...
... Laser-based sensors show all the desired characteristics and have been employed for the in situ process monitoring of the industryscale production of dynamic random access memory [557,558]. Stancu et al. used a QCL near 9727 nm to measure absorption cross sections of etching products, SiF 4 and NF 3 , to quantify spectral interference from NF 3 [557] (Fig. 30). The first demonstration of online and in situ concentration measurements of SiF 4 and NF 3 by Stancu et al [557]. ...
... Stancu et al. used a QCL near 9727 nm to measure absorption cross sections of etching products, SiF 4 and NF 3 , to quantify spectral interference from NF 3 [557] (Fig. 30). The first demonstration of online and in situ concentration measurements of SiF 4 and NF 3 by Stancu et al [557]. was followed by the work of Lang et al [558]., in which the sensor was synchronized with the etching process to facilitate automated measurements. ...
Sensors are perhaps the most important and integral components of our modern society. With global warming and environmental pollution garnering ever-increasing attention, as well as solutions for sustainabile and smart cities, the optimized performance of current and future energy systems and process industries is paramount. The accurate sensing and quantification of key parameters of such systems are essential for monitoring, controlling, and optimization efforts. In situ laser-based optical sensors are most suitable for achieving the desired characteristics of accuracy, sensitivity, selectivity, portability, speed, safety, and intelligence. In recent decades, significant progress has been made in the development and deployment of laser-based sensing solutions, although new challenges and opportunities continue to emerge rapidly. In this review paper, we focus on laser absorption spectroscopy (LAS)-based sensors owing to their simple architecture, easy implementation, and market penetration. We detail recent advancements made in LAS variants using new laser sources and techniques. A brief discussion on other laser-based sensing techniques, namely, photoacoustic spectroscopy, laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy, and laser-induced breakdown spectroscopy, is provided to compare these strategies with LAS. The applications of laser-based sensors in various energy systemsincluding engines, turbines, power plants, furnaces, and boilers—as well as process industriessuch as petrochemical, semiconductor, natural gas leak detection, and corrosion detectionare presented, illustrating their many benefits and possible uses. A distinguishing aspect of this review paper is that we present the comparison of previous studies in tabular formats, making it easy to appreciate the recent progress in laser-based sensing solutions. Finally, suggestions on future directions and emerging technologies to pursue for the further enhancement, development, and deployment of laser-based sensors are proposed.
... In contrast QCL absorption spectroscopy (QCLAS) has only recently been recognized as an effective plasma diagnostic tool [30,31]. Further development of sophisticated plasma process monitoring [32] and control [33] devices in industrial environments should be forthcoming due to the room temperature operating capabilities of such spectrometers. ...
... For the majority of relevant polyatomic molecules neither high resolution absorption cross sections nor spectral data for additional calculations are reported in the literature. The absorption spectra are commonly complex in nature and as a result, they lack any appearance of the rapid passage phenomena (see Figure 5 or [32,57,58]). It has been demonstrated that defining effective absorption cross sections σ eff within a spectral micro-window <ν> in both the inter or intra pulse method may also lead to an adequate calibration (case ii) [32,58]. ...
... The absorption spectra are commonly complex in nature and as a result, they lack any appearance of the rapid passage phenomena (see Figure 5 or [32,57,58]). It has been demonstrated that defining effective absorption cross sections σ eff within a spectral micro-window <ν> in both the inter or intra pulse method may also lead to an adequate calibration (case ii) [32,58]. Absolute values for σ eff are obtained from a measurement of the incident and transmitted intensity, I 0 and I, of a standardized gas sample of known number density n and absorption length L: Figure 5b shows a CF 4 overview spectrum merged from several chirped QCL pulses. ...
The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.
... Additionally, comprehensive calibration is required for achieving quantitative data on QMS monitoring. 28,29) Laser Absorption Spectroscopy (LAS) delivers highly sensitive measurements with multiple-reflection gas cell by taking advantage of laser directivity. 30,31) Since longer optical path length derived from multiple-reflection increases the sensitivity, LAS is suitable to Page 2 of 21 AUTHOR SUBMITTED MANUSCRIPT -JJAP-S1103264.R3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t 3 monitor the behavior and etch endpoint even in processes with the small amount of generated by-product such as contact etching of HAR. ...
... In addition, the high wavelength resolution of laser light enables high selectivity toward target gases. 27,29,[32][33][34] In this study, we report on the use of LAS for chamber in-situ etching process monitoring. ...
The behavior of the partial pressure of SiF4, a byproduct in fluorine-based plasma etching, has been measured in real-time using a method based on Laser Absorption Spectroscopy (LAS). The partial pressure of SiF4 is highly correlated with the etch rate of SiO2 (R2 = 0.999). Etch endpoints were clearly observed from the signal transitions, whose period indicate the etch rate uniformity. In addition, integrating the partial pressure of SiF4 with respect to time is correlated with the number of Si atoms etched regardless of the composition of the etched materials. Specifically, Si, SiO2 and Si3N4 were examined in this work. Based on the strong relationship between the measured SiF4 partial pressure and the etching profiles, real-time monitoring by LAS is useful for the prediction of etch profiles.
... quantum cascade laser, diode laser-are spectrally narrow (typically 10 -3 cm -1 , [213]), tunable sources that allow scanning ro-vibrational lines with high spectral resolution [213], [214]. This spectral region is thus particularly attractive for trace gas molecular spectroscopy [215] because it allows detection of species with high selectivity and sensitivity [213], [216]- [218]. ...
Les méthodes de stérilisation à basse température prennent une importance croissante pour la décontamination de matériaux thermosensibles utilisés dans les appareils exposés aux risques microbiologiques tels que les endoscopes dans les hôpitaux, les emballages dans l'industrie agroalimentaire, ou les équipements soumis aux agents microbiologiques de guerre dans les zones de conflits. Les méthodes de stérilisation standards non-thermiques souffrent de limitations liées à leur toxicité, leurs coûts élevés, leurs faibles compatibilités avec les matériaux, et/ou leurs longs cycles de stérilisation (quelques heures). Une approche alternative consiste à utiliser des plasmas hors équilibre à pression atmosphérique produits par décharges électriques. Les plasmas permettent des cycles de stérilisation pluscourts car les surfaces traitées sont exposées à de nombreux agents biocides, notamment à du rayonnement, à des espèces réactives oxygénées et azotées (RONS), et à des espèces chargées. Cependant, à pression atmosphérique, le volume du plasma est généralement faible. Les traitements en post-décharge permettent d'augmenter la surface de traitement, tout en réduisant la dégradation du matériau par les espèces chargées. Dans la post-décharge, les principaux agents biocides sont les RONS. L'objectif de cette thèse est d'étudier la production et le transport des RONS générés par des décharges pulsées non-thermiques dans l'air et l'azote à pression atmosphérique au moyen de diagnositcs de spectroscopie d'absorption UV et mid-IR (QCLAS), de fluorescence
... This is followed by results in section 4 and conclusions in section 5. Figure 1 shows a schematic of the QCLAS setup, which includes a Neoplas Control Q-MACS laser driver with a two-path detection system. The apparatus is similar to that used in reference [34]. Here a thermoelectrically cooled QCL (from Alpes Laser) operating at 5.2 µm is installed in the Q-MACS. ...
Radial distributions of absolute nitric oxide (NO) density and gas temperature are measured in atmospheric confined plasmas using Mid-IR quantum cascade laser absorption spectroscopy (QCLAS). Two ro-vibrational transitions of the fundamental band are probed in the electronic ground state NO(X) at 1900.076 cm⁻¹ and 1900.517 cm⁻¹, respectively. Plasmas are generated using nanosecond repetitively pulsed (NRP) discharges in air at atmospheric pressure. The spatial measurements are radially performed halfway between the discharge electrodes with a resolution down to 0.3 mm by the Abel inversion technique. The gas temperature is determined using two methods: (i) based on the ratio of the two ro-vibrational absorption lines and (ii) based on the collisional broadening line shape. The local NO density is obtained from local absorption coefficients and temperature dependent line strengths. The results were found in good agreement. The time averaged gas temperature and density at the discharge center are found at 800(±100) K and 2(±0.2) × 10¹⁵ cm⁻³, respectively. The FWHM of the NO density radial profile is found at 6-7 mm. This is large compared to the discharge channel width, i.e. typically below 0.5 mm, which is explained mainly by hot reactive jets induced in the post-discharge.
... In the short space of 18 years since their first demonstration, QCLs have been widely applied in various disciplines: physics, chemistry, biomedicine, engineering, and so on. In addition to the suitable wavelength range, QCLs usually feature a relatively narrow line width and good wavelength tenability, making them suitable for a wide variety of spectroscopic applications covering atmospheric and environmental trace gas sensing (25,26), molecular spectroscopy (27,28), isotope ratio measurements (29,30), breath gas analysis (31,32) as well as industrial process control (33), and high-temperature combustion and plasma diagnostics (34)(35)(36), to name a few. From a technical point of view, many laboratories around the world have explored the use of a variety of analytical techniques for carrying out mid-IR quantum cascade laser spectroscopy (QCLS), which includes direct absorption spectroscopy (DAS) (37)(38)(39)(40), photoacoustic spectroscopy (PAS) (41)(42)(43)(44)(45)(46)(47)(48)(49), quartz-enhanced photoacoustic spectroscopy (50)(51)(52), cavity ring-down spectroscopy (CRDS) (53,54), cavity-enhanced absorption spectroscopy (CEAS) (55,56), integrated cavity output spectroscopy (ICOS) (57)(58)(59)(60)(61), and so on. ...
Infrared laser absorption spectroscopy (LAS) is a promising modern technique for sensing trace gases with high sensitivity, selectivity, and high time resolution. Mid-infrared quantum cascade lasers, operating in a pulsed or continuous wave mode, have potential as spectroscopic sources
because of their narrow linewidths, single mode operation, tunability, high output power, reliability, low power consumption, and compactness. This paper reviews some important developments in modern laser absorption spectroscopy based on the use of quantum cascade laser (QCL) sources. Among
the various laser spectroscopic methods, this review is focused on selected absorption spectroscopy applications of QCLs, with particular emphasis on molecular spectroscopy, industrial process control, combustion diagnostics, and medical breath analysis.
... In the short space of 18 years since their first demonstration, QCLs have been widely applied in various disciplines: physics, chemistry, biomedicine, engineering, and so on. In addition to the suitable wavelength range, QCLs usually feature a relatively narrow line width and good wavelength tenability, making them suitable for a wide variety of spectroscopic applications covering atmospheric and environmental trace gas sensing (25,26), molecular spectroscopy (27,28), isotope ratio measurements (29,30), breath gas analysis (31,32) as well as industrial process control (33), and high-temperature combustion and plasma diagnostics (34)(35)(36), to name a few. From a technical point of view, many laboratories around the world have explored the use of a variety of analytical techniques for carrying out mid-IR quantum cascade laser spectroscopy (QCLS), which includes direct absorption spectroscopy (DAS) (37)(38)(39)(40), photoacoustic spectroscopy (PAS) (41)(42)(43)(44)(45)(46)(47)(48)(49), quartz-enhanced photoacoustic spectroscopy (50)(51)(52), cavity ring-down spectroscopy (CRDS) (53,54), cavity-enhanced absorption spectroscopy (CEAS) (55,56), integrated cavity output spectroscopy (ICOS) (57)(58)(59)(60)(61), and so on. ...
Abstract Since the first demonstration in 1994, progress in the development of quantum cascade lasers (QCLs) has been breathtakingly rapid. Various techniques based upon novel QCLs have attracted much interest from researchers working in science and engineering disciplines (atmospheric environmental monitoring, chemical analysis, industrial process control, medical diagnostics, applications of life science, etc.) over the course of approximately the last two decades. Some background and recent advances in the development of QCLs are discussed together with a brief outline of a few representative atmospheric chemical species and their spectral features, as well as a short summary of terahertz-QCL. Among the various laser spectroscopic methods, the focus in this review is directed toward selected applications of QCL absorption spectroscopic techniques, which are commonly used to measure atmospheric trace gases, with particular emphasis on ground-based eddy covariance measurements, isotope measurements, and airborne-platform atmospheric measurements.
... The development of compact and robust optical sensors for molecular detection is of interest for an increasing number of applications, such as environmental monitoring and atmospheric chemistry 60-62 , plasma diagnostics and industrial process control 23,63 , combustion studies, explosive detection and medical diagnostics [64][65][66][67][68] . Absorption spectroscopy in the midinfrared spectral region using lasers as radiation sources is an effective method for monitoring molecular species. ...
Mid infrared (MIR) absorption spectroscopy between 3 and 20 µm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas and for trace gas analysis. The increasing interest in molecular processing plasmas has lead to further applications of IRLAS. IRLAS provides a means of determining the absolute concentrations and temperatures of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition and semiconductor processing this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes and for trace gas analysis. The aim of the present contribution is threefold: (i) to report on selected studies of the spectroscopic properties and kinetic behaviour of the methyl radical, (ii) to review recent achievements in our understanding of molecular phenomena in plasmas and the influence of surfaces, and (iii) to describe the current status of advanced instrumentation for quantum cascade laser absorption spectroscopy (QCLAS).
... Room temperature QC lasers are an attractive solution for trace gas sensing applications that require fast, portable, high-sensitivity measurements such as environmental monitoring and medical diagnostics [1,2]. However, the emission of a conventional Fabry-Perot (FP) QC laser is typically multi-mode [3] and is unsuitable for many desired applications requiring a single frequency and narrow-linewidth source, such as highresolution spectroscopy or metrology [4]. Distributed Feedback (DFB) QC lasers achieve single-mode emission but require complex fabrication techniques with specialized equipment often resulting in a low yield [5]. ...
An external-cavity (EC) quantum cascade (QC) laser using optical feedback from a partial-reflector is reported. With this configuration, the otherwise multi-mode emission of a Fabry-Perot QC laser was made single-mode with optical output powers exceeding 40 mW. A mode-hop free tuning range of 2.46 cm(-1) was achieved by synchronously tuning the EC length and QC laser current. The linewidth of the partial-reflector EC-QC laser was measured for integration times from 100 μs to 4 seconds, and compared to a distributed feedback QC laser. Linewidths as small as 480 kHz were recorded for the EC-QC laser.
The invention of quantum cascade laser (QCL) technology rejuvenates the spectroscopic monitoring of trace gas species providing an unprecedented access to the ‘molecular fingerprint region’ covering mid-infrared (mid-IR) spectral range of 3–25 μm. The recent technological advancements of QCL fulfil all the critical parameters as an ideal optical source of a portable and field-deployable mid-IR spectrometer for trace gas analysis in real time. Hence, QCL-based spectroscopic techniques have now widely been employed in various fields covering molecular spectroscopy, atmospheric and environmental trace gas sensing, medical diagnostics, chemical and explosive analysis, industrial process control, and many more. In this chapter, we first discuss various critical features of an optical source for trace gas sensing applications with subsequent brief mention of different mid-IR sources prior to the QCL. Thereafter, the importance of trace gas analysis in environmental and biomedical sciences has been presented in short, followed by a historic overview of QCL. Finally, we focus on the applications of QCL-based spectroscopic techniques in the field of atmospheric and biomedical sciences.
Plasma patterning processes play a key role in modern Ultra Large Scale Integration (ULSI) device fabrication. With further scaling of device dimensions, plasma process characteristics become more and more the limiting factor in pattern minimization, not least due to parasitic effects caused by different plasma properties. Such effects are geometrical deviations in the etch profiles, the loss of critical dimensions, the undercut as well as the most critical sidewall damage while etching trenches and vias to fabricate the circuit contact and metallization system. Especially for modern dielectric materials on the base of carbon-doped silica these drawbacks are much pronounced and worsen performance and power consumption of microelectronic devices. Finally, the charge damage of thin gate oxides during plasma processing results in reduced reliability and functionality of microelectronic devices. The complex interactions between conditions in the plasma bulk and the impacts on the wafer surface are not yet understood completely. With respect to the increasing complexity of modern semiconductor plasma etch equipment, the optimization of patterning processes using the empirical method becomes more and more impossible. This paper overviews typical challenges during plasma patterning processes in 28 nm and 22 nm technology nodes by means of today's Back-End of Line integration schemes. Different plasma diagnostic methods provide an insight into the physical and chemical conditions of different etch chambers and their influence on process results and damaging behaviour.
In situ measurements are reported giving insight into the plasma chemical conversion of the precursor BCl3 in industrial applications of boriding plasmas. For the online monitoring of its ground state concentration, quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range was applied in a plasma assisted chemical vapor deposition (PACVD) reactor. A compact quantum cascade laser measurement and control system (Q-MACS) was developed to allow a flexible and completely dust-sealed optical coupling to the reactor chamber of an industrial plasma surface modification system. The process under the study was a pulsed DC plasma with periodically injected BCl3 at 200 Pa. A synchronization of the Q-MACS with the process control unit enabled an insight into individual process cycles with a sensitivity of 10⁻⁶ cm⁻¹Hz-1/2. Different fragmentation rates of the precursor were found during an individual process cycle. The detected BCl3 concentrations were in the order of 10¹⁴ moleculescm⁻³. The reported results of in situ monitoring with QCLAS demonstrate the potential for effective optimization procedures in industrial PACVD processes.
The considerably higher power and wider frequency coverage available from quantum cascade lasers (QCLs) in comparison to lead salt diode lasers has led to substantial advances when QCLs are used in pure and applied infrared spectroscopy. Furthermore, they can be used in both pulsed and continuous wave (cw) operation, opening up new possibilities in quantitative time resolved applications in plasmas both in the laboratory and in industry as shown in this article. However, in order to determine absolute concentrations accurately using pulsed QCLs, careful attention has to be paid to features like power saturation phenomena. Hence, we begin with a discussion of the non-linear effects which must be considered when using short or long pulse mode operation. More recently, cw QCLs have been introduced which have the advantage of higher power, better spectral resolution and lower fluctuations in light intensity compared to pulsed devices. They have proved particularly useful in sensing applications in plasmas when very low concentrations have to be monitored. Finally, the use of cw external cavity QCLs (EC-QCLs) for multi species detection is described, using a diagnostics study of a methane/nitrogen plasma as an example. The wide frequency coverage of this type of QCL laser, which is significantly broader than from a distributed feedback QCL (DFB-QCL), is a substantial advantage for multi species detection. Therefore, cw EC-QCLs are state of the art devices and have enormous potential for future plasma diagnostic studies.
Over the past few years mid infrared absorption spectroscopy (MIR-AS) over the region from 3 to 20 ?m has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of MIR-AS because most of these compounds and their decomposition products are infrared active. MIR-AS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a micro second, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from MIR-AS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of MIR-AS techniques to industrial requirements including the development of new diagnostic equipment. The aim of the present chapter is fourfold: (i) to briefly summarize the basic principles of infrared absorption spectroscopy and related instrumentation, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas using different types of MIR-AS techniques, (iii) to describe examples of industrial process monitoring in the mid infrared and (iv) to discuss the potential of advanced instrumentation based on quantum cascade lasers (QCLs) for plasma diagnostics.
The focus of this paper is the improvement of etch processes for SiCOH dielectrics in CF4 plasmas with respect to the sidewall damage. Three different additives, Ar, O2 and C4F8, were admixed to the parent gas in different concentrations to provoke a change of the etch process. Argon was added to enhance the sputter effect on the SiO 4/2 matrix of the SiCOH. Using the additive O2, a serious damage of the SiCOH in combination with a CH3 extraction, an enhanced hydrophilic behavior and a surface densification was aspired. Finally the C4F8 was used to enhance the polymerization efficiency of the etch process. The real occurrence of the theoretical predicted mechanism was analyzed using different bulk and surface sensitive measurement methods, e.g. contact angle measurements, spectral ellipsometry, Hg-probe investigation and FTIR analysis. A slight reduction of the sidewall damage with C 4F8 addition and a small increase of this value using argon were measured using SEM cross sections. The plasma dissociation of the process gas and the formation of SiFx and COx compounds as reaction products were clear influenced by the additives, measured with quadrupole mass spectrometry (QMS). Measurements with the quantum cascade laser absorption spectroscopy (QCLAS) show a surprising saturation effect for SiF 4 in the reaction chamber. The maximum SiF4 concentration was 1.5·1013 molecules/cm3. It seems that higher SiF4 concentrations are not possible, when SiCOH will be etched. Finally a new evaluation method on the basis of the correlation coefficient was applied to find dependencies between changes in the surface energy and dielectric constant and the results of the optical emission spectroscopy (OES). Therewith it was possible to identify species in the plasma, which promote and hamper the process induced rise of the dielectric constant and the surface energy. A promoting behavior was found for fluorine and different oxidizing species. Compounds of carbon, nitrogen and silicon as well as argon hamper the process induced material degradation.
Plasmas in interaction with surfaces are of key importance in a wide range of applications, which include materials technology, microelectronics, environmental issues, and biomedicine. The intense use of plasma technological processes demands proper plasma diagnostic techniques for monitoring, controlling and optimization purposes in industrial environments. In particular, for the improvement of the efficiency of a production process, in situ diagnostic techniques with online capabilities are favorable. From the middle of the last decade a variety of phenomena in molecular non-equilibrium plasmas in which many short-lived and stable species are produced have been successfully studied with quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range. It has been possible to determine absolute concentrations of species, temperatures, degrees of dissociation, dynamics of reaction processes and phenomena involving plasma-surface interactions using spectroscopy, thereby providing a link with chemical and kinetic modelling of the plasma. Since quantum cascade lasers (QCLs) emit near room temperature, i.e., without the need of cryogenic cooling, very compact and robust spectroscopic instruments can be designed. This has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. Recent applications of infrared absorption spectroscopy using QCLs for in situ monitoring of plasma processes in industrial environments are reviewed. Examples which emphasize the capabilities of QCLAS as plasma diagnostic technique in industrial plasma processes will be given. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Dielectric
etching
plasma processes for modern interlevel dielectrics become more and more complex by the introduction of new ultra low-k dielectrics. One challenge is the minimization of sidewall damage, while etching ultra low-k porous SiCOH by fluorocarbon plasmas. The optimization of this process requires a deeper understanding of the concentration of the CF2 radical, which acts as precursor in the polymerization of the etch sample surfaces. In an industrial dielectric
etching
plasma reactor, the CF2 radical was measured
in situ using a continuous wave quantum cascade laser (cw-QCL) around 1106.2 cm−1. We measured Doppler-resolved ro-vibrational absorption lines and determined absolute densities using transitions in the ν
3 fundamental band of CF2 with the aid of an improved simulation of the line strengths. We found that the CF2 radical concentration during the etching
plasma process directly correlates to the layer structure of the etched wafer. Hence, this correlation can serve as a diagnostic tool of dielectric
etching
plasma processes. Applying QCL based absorption spectroscopy opens up the way for advanced process monitoring and etching
controlling in semiconductor manufacturing.
Low-pressure rf plasmas have been applied for etching of ultra-low-k SiCOH wafers using an Oxford Plasmalab System 100. In pure CF4 plasmas, SiCOH layers have been etched for different power values. Using quantum cascade laser absorption spectroscopy in the mid-infrared spectral range, the correlation of online and in situ measured concentrations of two etching products, CO and SiF4, with the ex situ determined etching rates has been studied. The concentration of SiF4 was found to range between 0.6 and 1.4 × 1013 molecules cm−3. In contrast the concentrations of CO were measured to be only about 50 % of the SiF4 density with 7 × 1012 molecules cm−3 in maximum. The production rate of SiF4, determined from the time behavior of its concentration after plasma ignition, was found to be between 1 and 5 × 1012 cm−3 s−1. The etching rates varied between 2 and 7 nm s−1. Both parameters increase nearly linearly with the applied rf power. It was found that for power values of up to 1.1 kW, the etching rate depends nearly linearly on the in situ monitored concentrations of both etching products. Therefore, the concentration of the etching products can be directly used as a measure of the etching rate.
Spektroskopischen Verfahren zur Gasösung, sie waren unhandlich, nicht robust genug oder schlicht zu teuer. Durch Quantenkaskadenlaser als Strahlungsquelle kann nun die Anwendung der MIR-Absorptionsspektroskopie in der Industrie revolutioniert werden. Zwar steht die industrielle Nutzung in der Plasmaprozesstechnik noch am Anfang, jedoch ist ihr enormes Potenzial bereits jetzt erkennbar, was durch Messungen an Plasmaätzanlagen der Halbleiterindustrie eindrucksvoll belegt wird.
Infrared laser absorption spectroscopy with quantum cascade lasers in industrial application
Spectroscopic methods for gas analysis and plasma diagnostics in the field of the mid infrared spectral range (MIR) were lacking in sufficient time resolution up to now, they were cumbersome and not robust enough or simply too expensive. Through quantum cascade lasers as radiation source the application of the MIR absorption spectroscopy in the industry can be revolutionized. Although the industrial use in the plasma process is still in its infancy, their enormous potential is already evident, as demonstrated by measurements on plasma etching systems in the semiconductor industry, which is impressively documented.
Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics.
A rapidly swept quantum cascade laser (QCL) has been used to probe a small region of the ν
6 band of formaldehyde around 8 μm by direct absorption spectroscopy. Two of the strongest transitions accessible within the operating region of the QCL are assigned as (1,1,1)←(2,0,2) and (10,1,9)←(9,2,8). In this paper, we report pressure-induced broadening and shift coefficients for these transitions with a variety of gases (He, Ne, Kr, Ar, N2, O2, and CO2). The pressure broadening coefficient measurements are shown (in some cases) to be chirp rate dependent, even under conditions where rapid passage effects are negligible, highlighting the marked difference that can occur between studies with rapidly swept fields and those employing conventional diode laser devices.
Electrical signals are used for end point detection in plasma etching, but the origin of the electrical changes observed at end point is not well understood. As an etch breaks through one layer and exposes an underlayer, the fluxes and densities of etch products and reactants in the gas phase will change. The resulting perturbation in gas composition may alter the plasma electron density, which in turn may affect the electrical signals. Alternatively, changes in substrate electrical properties or surface properties, such as work function or emitted electron yield, may be involved. To investigate these effects, experiments were performed in a radio-frequency (rf)-biased, inductively coupled reactor, during CF{sub 4}/Ar plasma etching of silicon dioxide films on silicon substrates. A complete set of electrical parameters, for the bias as well as the inductive source, was measured and compared. The most useful end point signal was found to be the fundamental rf bias impedance, which decreases when the oxide is removed. A simultaneous increase in plasma electron density was measured by a wave cutoff probe. Analytical sheath models indicate that the measured change in electron density accounts for nearly all of the impedance decrease. The change in electron density can in turn be explained by the effects of etch products or reactants on gas composition. In contrast, electrons emitted from the wafer surface play at most a minor role in the changes in electron density and impedance observed at end point.
Quantum Cascade Lasers offer attractive options for applications of MIR absorption spectroscopy for basic research and industrial process control. The contribution reviews applications for plasma diagnostics and trace gas monitoring in research and industry.
In situ measurements are reported giving insight into the plasma chemical conversion of the precursor BCl3 in industrial applications of boriding plasmas. For the online monitoring of its ground state concentration, quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range was applied in a plasma assisted chemical vapor deposition (PACVD) reactor. A compact quantum cascade laser measurement and control system (Q-MACS) was developed to allow a flexible and completely dust-sealed optical coupling to the reactor chamber of an industrial plasma surface modification system. The process under the study was a pulsed DC plasma with periodically injected BCl3 at 200 Pa. A synchronization of the Q-MACS with the process control unit enabled an insight into individual process cycles with a sensitivity of 10-6 cm-1\cdotHz-1/2. Different fragmentation rates of the precursor were found during an individual process cycle. The detected BCl3 concentrations were in the order of 1014 molecules\cdotcm-3. The reported results of in situ monitoring with QCLAS demonstrate the potential for effective optimization procedures in industrial PACVD processes.
The application of quantum cascade lasers in the intrapulse operation mode for low-pressure plasma spectroscopy is hampered by the observation of rapid passage effects, leading to lower quantitative accuracy. We demonstrate that accurate densities and rotational temperatures of CH4 within a CH4 plasma can be obtained by characterizing the rapid passage effects in gas phase conditions prior to carrying out the plasma measurements. Furthermore, we show that the ratios of the integrated absorption of two transitions are not affected by the rapid passage effect and, thus, rotational temperatures of species can be obtained.
Volatile organic compounds (VOC) are known to be dangerous for human health. If they are part of a precursor gas mixture undesired effects in e.g. industrial processes may occur. Therefore, the destruction of unwanted VOCs in a gas stream is an important aim. For that purpose a five stage planar dielectric barrier discharge reactor working at atmospheric pressure has been developed and tested. Each stage consists of two parallel stainless steel meshes serving as electrodes. The space between them is filled with dielectric beads. The ac voltage applied to the electrodes is in the kV and kHz range and several gas flow rates are possible. The gases of interest have been measured using a Fourier Transform Infrared Spectrometer (FTIR) combined with a 32.5m long path cell. First result showed, that the VOC ethylene (C2H4) can be removed.
In a pulsed dc discharge of an Ar–N2 mixture containing 0.91% of NO the kinetics of the destruction of NO has been studied under static and flowing conditions, i.e. in a closed and open discharge tube (p = 266 Pa). For this purpose quantum cascade laser absorption spectroscopy (QCLAS) in the infrared spectral range has been applied as a new approach for fast in situ plasma diagnostics which is capable of achieving a time resolution below 100 ns. The time decay of the NO concentration was measured in single discharge pulses of 1 ms duration. Additionally, the temporal behaviour of the electric field and the applied power was followed during the pulse. The comparison of the time evolution of the NO concentration under static and flowing conditions and simplified model calculations enabled an analysis of the dynamics of the plasma heating to be made. The temperature increase during the pulse is below 40 K, but has a strong influence on the line strength of the NO absorption line. The apparent decrease in the NO concentration in a single pulse of about 20% is due to the heating of the gas which in turn makes the line strength vary while the concentration remains constant for several successive pulses. Therefore the QCLAS measurements combined with model calculations are a powerful non-invasive temperature probe with a remarkable time resolution approaching the sub-microsecond time scale.
Concentrations of the etch product SiF4 were measured online and in situ in technological etch plasmas with an especially designed quantum cascade laser arrangement for application in semiconductor industrial environment, the Q-MACS Etch. The combination of quantum cascade lasers and infra red absorption spectroscopy (QCLAS) opens up new attractive possibilities for plasma process monitoring and control. With the realization of a specific interface the Q-MACS Etch system is synchronized to the etch process and allows therefore automated measurements, which is important in a high volume production environment.
Boron trichloride (BCl3) is used as a source gas in various industrial plasma applications. The online monitoring of its ground state concentration in the plasma process reactor is vital for improved insight into the plasma chemistry and to increase productivity, reliability and reproducibility of the process. Infrared absorption spectroscopy was applied for in situ measurements of absolute densities of BCl3 and HCl in planar microwave plasma sources as well as in an industrial pulsed dc discharge reactor. In this paper procedures for the determination of HITRAN format line strength data files for BCl3 are described together with results of concentration measurements. The results show the influence of the discharge parameters on the dissociation of BCl3 and its conversion to HCl. Observations using tuneable diode lasers are compared with measurements using quantum cascade lasers and the applicability of this diagnostic technique for industrial plasma process monitoring is shown.
Quantum cascade lasers (QCL) offer attractive options for the application of mid-infrared absorption spectroscopy for industrial process monitoring and control. In particular the in-situ measurement of reactive plasma compounds can give new insight into the reaction kinetics of industrial plasma processes. For these purposes the compact quantum cascade laser measurement and control system (Q-MACS) has been combined with an infrared-fibre to allow a flexible and completely dust-sealed incoupling of the infrared radiation into the reactor chamber of an industrial plasma surface modification system. The system has been completed with a fast and compact detector and a data acquisition system to monitor concentrations of species of interest in the plasma process online. First tests proofed the potential of this approach.
The recent advent of quantum cascade lasers (QCLs) enables room-temperature mid-infrared spectrometer operation which is particularly favourable for industrial process monitoring and control, i.e. the detection of transient and stable molecular species. Conversely, fluorocarbon containing radio-frequency discharges are of special interest for plasma etching and deposition as well as for fundamental studies on gas phase and plasma surface reactions. The application of QCL absorption spectroscopy to such low pressure plasmas is typically hampered by non-linear effects connected with the pulsed mode of the lasers. Nevertheless, adequate calibration can eliminate such effects, especially in the case of complex spectra where single line parameters are not available. In order to facilitate measurements in fluorocarbon plasmas, studies on complex spectra of CF4 and C3F8 at 7.86 μm (1269 – 1275 cm-1) under low pressure conditions have been performed. The intra-pulse mode, i.e. pulses of up to 300 ns, was applied yielding highly resolved spectral scans of ~ 1 cm-1 coverage. Effective absorption cross sections were determined and their temperature dependence was studied in the relevant range up to 400 K and found to be non-negligible.
Fluorocarbon containing capacitively coupled radio frequency (cc-rf) plasmas are widely used in technical applications and as model systems for fundamental investigations of complex plasmas. Absorption spectroscopy based on pulsed quantum cascade lasers (QCL) was applied in the mid-IR spectral range of 1269-1275 cm-1. Absolute densities of the precursor molecule CF4 and of the stable product C3F8 were measured with a time resolution of up to 1 ms in pulsed CF4/H2 asymmetrical cc-rf (13.56 MHz) discharges. For this purpose both the non-negligible temperature dependence of the absorption coefficients and the interference of the absorption features of CF4 and C3F8 had to be taken into account in the target spectral range. Therefore, at two different spectral positions composite absorption spectra were acquired under the same plasma conditions in order to discriminate between CF4 and C3F8 contributions. A total consumption of~ 12 % was observed for CF4 during a 1 s plasma pulse, whereas C3F8 appeared to be produced mainly from amorphous fluorocarbon layers deposited at the reactor walls. A gas temperature increase by ~ 100 K in the plasma pulse was estimated from the measurements. Additionally, not yet identified unresolved absorption (potentially from the excited CF4 molecule) was found during the àon-phase'.
The recent development of quantum cascade lasers (QCLs) offers an attractive new option for the monitoring and control of industrial plasma processes and for trace-gas analysis as well as for highly time-resolved studies on the kinetics of plasma processes. The contribution reviews selected examples of the application of QCLs for infrared absorption studies in basic research and for plasma monitoring and control in industry.
The kinetics of stable species has been studied in situ in pulsed CF4/H2 radio frequency discharges by means of time resolved quantum cascade laser absorption spectroscopy. The absorption spectra were usually recorded with a time resolution of 5 ms and required a multi-species analysis, because of interfering complex absorption features of CF4 and C3F8. For this reason, measurements were carried out at two different spectral positions. High resolution spectroscopic data were established by calibrating effective absorption cross sections and their relative temperature dependences for the relevant low pressure conditions (10 Pa). During the discharge a decrease in the CF4 density by ~12% was observed. The off-phase was characterized mainly by the gas exchange. The C3F8 density in the off-phase was found to be of the order of the detection limit (3 × 1013 cm−3). Spectra acquired during the plasma-on phase showed a rapid temperature-induced increase in the absorption signal and, additionally, suggested the influence of a short-lived broadband absorbing species. The reasonable assumption of the presence of CF4 hotbands has not yet enabled a further quantification.
Reactive ion etch processes for modern interlevel dielectrics become more and more complex, especially for further scaling of interconnect dimensions. The materials will be damaged within such processes with the result of an increase in their dielectric constants. The capability of selected additives to minimize the low-k sidewall damage during reactive ion etching (RIE) of SiCOH materials in fluorocarbon plasmas was shown in different works in the past. Most of the investigated additive gases alter the fluorine to carbon ratio as well as the dissociation of the parent gas inside the etch plasma. The result is a changed etch rate, a modified polymerization behavior and other characteristics of the process induced SiCOH damage. Heavy inert ions like argon will be accelerated to the sample surface in the cathode dark space and enhance therewith the sputter yield on the SiCOH network [1]. In this paper the additives Ar, O2, C4F8, H2, N2 and CO were added to a conventional CF4 etch plasma. We try to provoke different changes in the plasma conditions and therewith in the process results. Contact angle measurements, spectroscopic ellipsometry, Hg-probe analysis, FTIR measurements and SEM cross-sections were used to overview the additive induced modifications. To understand the influences of the additives gases more exactly, changes in the physical and chemical plasma behavior must be analyzed. Therefore quadrupole mass spectrometry (QMS) and quantum cascade laser absorption spectroscopy (QCLAS) were used.
Within the last decade mid-infrared absorption spectroscopy between 3 and 20 μm, known as infrared laser absorption spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has lead to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes.
The aim of the present article is twofold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas using TDLs and (ii) to report on selected new applications of QCLs in the mid-infrared.
Within the last decade, mid-infrared absorption spectroscopy between 3 and 20μm – known as infrared laser absorption spectroscopy (IRLAS) and based on tunable semiconductor lasers, namely lead salt diode lasers, often called tunable diode lasers (TDLs), and quantum cascade lasers (QCLs) – has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, and organosilicon compounds has led to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Information about gas temperature and population densities can also be derived from IRLAS measurements. A variety of free radicals and molecular ions have been detected, especially using TDLs. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes.
Quantum cascade lasers (QCLs) have attracted considerable interest as an alternative tuneable narrow bandwidth light source
in the mid-infrared spectral range for chemical sensing. Pulsed QCL spectrometers are often used with short laser pulses and
a bias current ramp similar to diode laser spectroscopy. Artefacts in the recorded spectra such as disturbed line shapes or
underestimated absorption coefficients have been reported. A detailed time-resolved high-bandwidth analysis of individual
pulses during a laser sweep has been performed. Quantitative results for CH4 absorption features around 1347cm−1 (7.42μm) fell short of the expected values for reasonable operating conditions of the QCL. The origin of the artefacts using
short pulses was identified to be partly of the same nature as in the case of long laser pulses. A complex combination with
the tuning principle was found, leading to an apparently increased instrumental broadening (effective line width) and underestimated
concentrations at low-pressure conditions.
Achieving the high sensitivity necessary for trace gas detection in the midinfrared molecular fingerprint region generally requires long absorption path lengths. In addition, for wider application, especially for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelectrically (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ∼0.5 m long cavity having a small sample volume of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 μ m yielded path lengths of 1080 m and a noise equivalent absorption of 2×10<sup>-7</sup> cm <sup>-1</sup> Hz <sup>-1/2</sup> . The molecular concentration detection limit with a 20 s integration time was found to be 6×10<sup>8</sup> molecules / cm <sup>3</sup> for N <sub>2</sub> O and 2×10<sup>9</sup> molecules / cm <sup>3</sup> for CH <sub>4</sub> , which is good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar. The main limiting factor for achieving even higher sensitivity, such as that found for larger volume multi pass cell spectrometers, is the residual mode noise of the cavity. On the other hand the application of TE cooled pulsed QCLs for integrated cavity output spectroscopy and cavity ring-down spectroscopy (CRDS) was found to be limited by the intrinsic frequency chirp-
of the laser. Consequently the accuracy and advantage of an absolute internal absorption calibration, in theory inherent for CRDS experiments, are not achievable.
In this paper, first measurements with a particularly designed quantum-cascade-laser (QCL) arrangement for application in semiconductor industrial environments for in situ wafer-to-wafer etch monitoring are reported. The combination of QCLs and infrared absorption spectroscopy (QCLAS) opens up new possibilities for plasma process monitoring and control. In silicon etch plasmas, concentrations of the etch product SiF<sub>4</sub> were measured real time in an industrial-production environment. The comparison of the results with inline data of the processed wafers shows a correlation between the amount of produced SiF<sub>4</sub> and the measures of the trench depth and the bottom void. Furthermore, it is shown that the characteristics of the refractive index of Si and SiO<sub>2</sub> in the mid-infrared can be used to determine etch rates of SiO<sub>2</sub> and Si wafers during the processing.
We review the use of both pulsed and continuous wave quantum cascade
lasers in high-resolution spectroscopic studies of gas phase species. In
particular, the application of pulsed systems for probing kinetic
processes and the inherent rapid passage structure that accompanies
observations of low-pressure samples using these rapidly chirped devices
are highlighted. Broadband absorber spectroscopy and time-resolved
concentration measurements of short-lived species, respectively
exploiting the wide intrapulse tuning range and the pulse temporal
resolution, are also mentioned. For comparison, we also present recent
sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using
a continuous wave external cavity quantum cascade laser system. Using
this methodology the stability and resolution of this source is
quantified. We find that the laser linewidth as measured via the
Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates,
but decreases to 1.3 MHz as the laser scan rate is increased such that
the transition is observed at 30 kHz. Using this source, wavelength
modulation spectroscopy of NO is presented.
The combination of quantum cascade lasers and infra red absorption
spectroscopy (QCLAS) opens up new possibilities for plasma process
monitoring and control. First measurements are reported with an
especially designed quantum cascade laser arrangement for application in
semiconductor industrial environments to track the approach for in situ
process control in silicon etch plasmas. In gas mixtures with N2 and in
microwave (MW) plasmas at pressures below 30 Pa concentrations of C4F6
and of SiF4 were measured simultaneously online for the first time. It
could be demonstrated, that using quantum cascade lasers (QCL) it is
possible to control ex situ mass flow controllers (MFC) based on in situ
measured species concentrations in the gas phase and in the MW plasma
bulk.
A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out
of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering.
Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum
wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A
strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2
micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely
determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure
material.
Within the last decade mid-infrared absorption spectroscopy over a region from 3 to 17µm and based on tuneable lead salt diode lasers, often called tuneable diode laser absorption spectroscopy or TDLAS, has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry in molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has led to further applications of TDLAS because most of these compounds and their decomposition products are infrared active. TDLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetic phenomena. Information about gas temperature and population densities can also be derived from TDLAS measurements. A variety of free radicals and molecular ions have been detected by TDLAS. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of quantum cascade lasers (QCLs) offers an attractive new option for the monitoring and control of industrial plasma processes. The aim of the present paper is threefold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas, (ii) to report on selected studies of the spectroscopic properties and kinetic behaviour of radicals and (iii) to describe the current status of advanced instrumentation for TDLAS in the mid-infrared.
The translational temperature and absolute density of Si atoms in
capacitively coupled SiF4 plasmas driving with the very high
frequency (VHF) of 27 MHz or 60 MHz were measured by ultraviolet
absorption spectroscopy employing a ring dye laser and a hollow cathode
lamp. High temperatures of Si atoms above 1000 K at 27 MHz were obtained
at an electron density of 5.0× 1010 cm-3,
which is considerably larger than that at 60 MHz at the same electron
density. The translational temperature and absolute density of
SiF4 molecules were also evaluated using infrared diode laser
absorption spectroscopy. From the results measured systematically, it is
proposed that the hot Si atom is created due to the difference in the
energy of Si atoms produced from the electron impact dissociation of
SiFx (x = 1-3) radicals and SiF4 molecules. This
result is very important from the viewpoint of diagnostics of atoms and
plasma chemistries.
Questions concerning the absorption and emission of light are
investigated, taking into account cavity modes, thermal radiation and
Planck's law, basic photometric quantities, discrete and continuous
spectra, absorption and dispersion, transition probabilities, linear and
nonlinear absorption, a semiclassical description, and aspects of
coherence. Widths and profiles of spectral lines are considered along
with spectroscopic instrumentation, the fundamental principles of
lasers, lasers as spectroscopic light sources, tunable coherent light
sources, Doppler-limited absorption and fluorescence spectroscopy with
lasers, laser Raman spectroscopy, and high-resolution sub-Doppler laser
spectroscopy. Attention is given to time-resolved laser spectroscopy,
the laser spectroscopy of collision processes, and the ultimate
resolution limit. Applications of laser spectroscopy are related to
laser photochemistry, laser isotope separation, laser monitoring of the
atmosphere, laser spectroscopy in biology, and medical research and
hospital practice.
Improved values of molecular constants of group IVA tetrahalides are reported. Accurate frequencies, including those of different isotopically substituted species, have been determined using matrix isolation spectroscopy in conjunction with isotope techniques (for SiF4, GeF4, SiCl4, Si35Cl4, SiBr4, GeBr4, 74GeBr4, 70/76GeBr4, SnBr4, 116SnBr4 and 124SnBr4) and from vapour -phase IR spectra (for SiF4, GeF4, 74GeF4, SiCl4, Si35Cl4 SiBr4, GeBr4, 74GeBr4, SnBr4, 116SnBr4 and 124SnBr4). Band contour analyses, often rendered difficult by isotopic effects and hot-band progressions, were simplified by measuring the spectra of isotopically pure compounds (e.g. 74GeF4, 116Sn35Cl4) and by measurements at lower temperatures (SiF)4. The isotopic shifts, Coriolis coupling and force constant values determined in this study are more accurate than those previously reported for some of the molecules. In addition to the presentation of new data the known spectral information concerning the other tetrahalides of silicon, germanium and tin are reviewed.
The measurement method for determining absolute densities of SiF4 molecules in a reactive plasma has been established using infrared diode laser absorption spectroscopy (IRLAS). The spatial distribution of line averaged absolute densities of SiF4 molecules has been investigated in the electron cyclotron resonance (ECR) plasma reactor employing SiF4 gas using infrared diode laser absorption spectroscopy without a multiple reflection cell, namely, single-path IRLAS@. Furthermore, the spatial distributions of SiF2 radical densities are measured using laser-induced fluorescence (LIF) technique. It was found that the line averaged absolute density of SiF4 molecules indicated the hollow-type distribution in the reactor, while SiF2 radical densities had the maximum value near the plasma boundary. On the basis of these measured results, the formation mechanism of spatial distribution of SiF4 and SiF2 densities in the plasma reactor are clarified.
A laser interferometer system to monitor plasma etch rate, and to control etched depth of isolation areas in silicon for oxide
isolated bipolar devices, is reported. The etching process is stopped as soon as the desired etched depth is achieved. The
accuracy of the system is 4–6%, depending on the masking material. It is found that this method of laser interferometry can
be particularly useful in process development for observations in real time of changes in etch rate.
The behavior of the densities of Si, SiF, and SiF2 radicals and SiF4 molecule were investigated for variations in electron density at pressure of 40 mTorr in a very high frequency (VHF) 60 MHz capacitively coupled plasma employing SiF4 gas. The SiF4 molecule was measured by infrared diode laser absorption spectroscopy, the SiF2 and SiF radicals were measured by laser-induced fluorescence, the Si atom was measured by ultraviolet absorption spectroscopy, and the F atom was measured by actinometric optical emission spectroscopy. The SiF4 density decreased with an increase in electron density, and the dissociation ratio of SiF4 was saturated to about 63% at electron densities of above 8.7×1010 cm−3. The SiF2 density decreased due to electron impact dissociation, whereas the F, Si, and SiF densities increased with an increase in electron density above 1.2×1011 cm−3. The total Si density was estimated to be of the order of 1010–1011 cm−3. Furthermore, the spatial distributions of SiF, SiF2, and SiF4 densities were investigated. They were almost flat inside the plasma region, but the SiF and SiF2 radicals decreased gradually outside the plasma region, and the SiF4 molecule increased. The behavior of the Si, SiF, and SiF2 radicals was in good agreement with the emission intensity of rare gases injected as trace gases. The kinetics of the species in VHF 60 MHz SiF4 plasma was clarified on the basis of the densities of the species and electrons measured. These experimental results are useful in predicting the radical densities of VHF SiF4 plasma using simulation. © 2003 American Institute of Physics.
Standard laser interferometry is used in dry etch fabrication of semiconductor and MEMS devices to measure etch depth, rate and to detect the process end point. However, many wafer materials, such as silicon are absorbing at probing wavelengths in the visible, severely limiting the amount of information that can be obtained using this technique. At infrared (IR) wavelengths around 1500 nm and above, silicon is highly transparent. In this paper we describe an instrument that can be used to monitor etch depth throughout a thru-wafer etch. The provision of this information could eliminate the requirement of an `etch stop' layer and improve the performance of fabricated devices.
We have added a further new capability by using tuneable lasers to scan through wavelengths in the near IR to generate an interference pattern. Fitting a theoretical curve to this interference pattern gives in situ measurement of film thickness. Whereas conventional interferometry would only allow etch depth to be monitored in real time, we can use a pre-etch thickness measurement to terminate the etch on a remaining thickness of film material.
This paper discusses the capabilities of, and the opportunities offered by, this new technique and gives examples of applications in MEMS and waveguides.
We report the use of a tunable diode laser locked to a molecular vibrational absorption line as a sensitive plasma etching endpoint detector. Measurements were made on multilayer silicon wafers etched in a SF 6 plasma discharge. We show that polycrystalline silicon to silicon dioxide endpoint transitions on wafers with exposed area as small as 33 mm<sup>2</sup> should be observable by detecting the etch end product SiF 4 . The method shows considerable potential as an endpoint detection technique for applications such as contact hole etching wherein very small areas are being etched.
Line positions, intensities, transition-moment squared, and lower state energies are calculated for the three most abundant isotopomers of the oxygen molecule in the terrestrial atmosphere, 16O2, 18O16O, and 17O16O. All lines passing a wavenumber dependent cutoff procedure (3.7 × 10−30 cm−1/(moleculu cm−1) at 2000 cm−1) are retained for the 1996 HITRAN database. Halfwidths as a function of the transition quantum numbers are determined from the available experimental measurements. Explicit expressions are obtained relating line intensities to the transition-moment squared, the vibrational band intensity, and the electronic-vibrational Einstein-A coefficient. The statistical degeneracy factors are presented and misuse of these factors in previous works is explained. Finally, band-by-band comparisons between the new calculations and the data from the previous HITRAN database are made.
The bonded species in hydrogenated fluorinated amorphous silicon ({ital a}-Si:H:F) have been identified in the infrared spectra as SiH{sub 2} (2100, 890, and 630 cm{sup {minus}1}), SiH (2000 and 630 cm{sup {minus}1}), SiF{sub 4} (1015 cm{sup {minus}1}), SiF{sub 2} (930 cm{sup {minus}1}), and SiF (830 cm{sup {minus}1}), or alternatively as SiH (2000 and 630 cm{sup {minus}1}), SiF{sub 4} (1015 cm{sup {minus}1}), SiF{sub 2} (930 cm{sup {minus}1}), and SiHF (2100, 890, 830, and 630 cm{sup {minus}1}). To distinguish between these configurations, we have deposited deuterated fluorinated amorphous silicon ({ital a}-Si:D:F) by dc glow discharge and measured the infrared spectrum. Based on the isotopic shifts of the vibrational modes, we conclude that SiHF is not present, and thus support the former assignments.
The fundamental mechanism behind laser action leads in general only to narrowband, single-wavelength emission. Several approaches for achieving spectrally broadband laser action have been put forward, such as enhancing the optical feedback in the wings of the gain spectrum, multi-peaked gain spectra, and the most favoured technique at present, ultrashort pulse excitation. Each of these approaches has drawbacks, such as a complex external laser cavity configuration, a non-flat optical gain envelope function, or an inability to operate in continuous mode, respectively. Here we present a monolithic, mid-infrared 'supercontinuum' semiconductor laser that has none of these drawbacks. We adopt a quantum cascade configuration, where a number of dissimilar intersubband optical transitions are made to cooperate in order to provide broadband optical gain from 5 to 8 microm wavelength. Laser action with a Fabry-Pérot spectrum covering all wavelengths from 6 to 8 microm simultaneously is demonstrated with this approach. Lasers that emit light over such an extremely wide wavelength range are of interest for applications as varied as terabit optical data communications or ultra-precision metrology and spectroscopy.
The line strengths of nine Q-branch lines in the nu(2) fundamental band of the methyl radical in its ground electronic state have been measured by diode laser absorption spectroscopy. The vibration-rotation spectrum of methyl was recorded in a microwave discharge in ditertiary butyl peroxide heavily diluted in argon. The absolute concentration of the radical was determined by measuring its kinetic decay when the discharge was extinguished. The translational, rotational, and vibrational temperatures, also required to relate the line strengths to the transition dipole moment, were determined from relative integrated line intensities and from the Doppler widths of the lines after allowing for instrumental factors. The line strengths of the nine Q-branch lines were used to derive a more accurate value of the transition dipole moment of this band, mu(2)=0.215(25) D. Improved accuracy over earlier measurements of mu(2) (derived from line strengths of single lines) was obtained by integrating over the complete line profile instead of measuring the peak absorption and assuming a Doppler linewidth to deduce the concentration. In addition, a more precise value for the rate constant for methyl radical recombination than available earlier was employed. The new value of mu(2) is in very good agreement with high-quality ab initio calculations. Furthermore, the ratio of the transition dipole moments of the nu(2) and nu(3) fundamental bands in the gas phase is now in highly satisfactory agreement with the ratio determined for the condensed phase.
The Quantum cascade (QC) laser is an entirely new type of semiconductor device in which the laser wavelength depends on the band-gap engineering. It can be made to operate over a much larger range than lead salt lasers, covering significant parts of both the infrared and submillimetre regions, and with higher output power. In this tutorial review we survey some of the applications of these new lasers, which range from trace gas detection for atmospheric or medical purposes to sub-Doppler and time dependent non-linear spectroscopy.
Laser Spectroscopy 3rd Edition Springer-Verlag Berlin2003
- W Demtröder