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Encapsulation of GaAs and GaAsPd in furnace annealing
Abstract
Arsenic has a tendency to evaporate during high temperature annealing of GaAs. This process can have deleterious consequences for electrical properties of the contact to GaAs and must be controlled during annealing by encapsulants. It would also be advantageous if this cap could be integrated in the contact metallization scheme for GaAs. In this work we compare the behavior of conducting thin-film encapsulants of mere GaAs and of GaAs covered with a Pd film upon furnace annealing. Thin films of Pd are used on GaAs to form a contacting layer by reaction with GaAs. This reaction complicates the encapsulation task because it causes stresses in an already strained thin film encapsulant. We report on experimental study of conducting encapsulants mainly; W, Hf, HfN, and TaSiN, but others have been tried, too. They were sputter-deposited on top of GaAs and GaAsPd samples. During annealing in argon we monitored the evaporants with a thin Cr film deposited on silicon nitride and resting face to face on the encapsulated sample. The captor layer was then analysed by backscattering spectrometry. We found out that Pd enhances As evaporation and GaAsPd could not be encapsulated. The reasons for this will be discussed.
Shallow Si n+p junctions equipped with a reactively sputtered Ta36Si14N50 amorphous metallic diffusion barrier of about 100 nm thickness have survived, without a detectable degradation of their reverse current, a 30 min long vacuum annealing test of 700 deg C with Al (Al melts at 660 deg C), 750 deg C with Au, and 950 deg C with Cu. For all three metallizations, these temperatures are records that much surpass any previously known results. This same Ta-Si-N film also acts as an excellent conducting annealing cap for GaAs. Implications of these findings are briefly discussed. A list of collaborators and of papers acknowledging the support of this contract are given.
The novel field of ternary amorphous metallic thin films made of an early transition metal and a combination of B, C, N, Si, and P is briefly reviewed. Ternary alloys composed of a transition metal, silicon, and nitrogen applied as thin-film diffusion barriers for Cu contacts to Si devices are emphasized. The synthesis of these films by reactive sputtering is described, their structural and electrical properties are discussed, and the reason for their unique performance as diffusion barriers for copper are explained. Shallow junction diodes provided with a 10 nm thick barrier layer of this type and a copper overlayer can withstand 650°C for 30 min in vaccum without significant degradation of the reverse current. A table lists all diode tests published so far in the open literature.
We have directly measured the amounts of arsenic evaporated through very thin encapsulating films during rapid thermal processing (RTP). The evaporated molecules are condensed onto a copper film on a sapphire substrate and subsequently counted using 5 MeV Rutherford backscattering analysis. For 20‐nm‐thick Si, SiO 2 , and Si 3 N 4 caps, we have determined the temperature ranges for which a 10‐s RTP cycle produces less than 1×10<sup>1</sup><sup>5</sup>/cm<sup>2</sup> of evaporated As. Preliminary measurements at higher temperatures of the thickness and time dependence of evaporation through SiO 2 caps of 20 to 60 nm thicknesses are also presented and discussed in terms of the mechanism of As loss.
A technique has been developed for direct, quantitative measurement of the amount of Ga and As evaporated from uncapped GaAs surfaces during rapid thermal annealing (RTA). The method involves collection of the evaporated molecules on a nearby copper film, followed by compositional analysis of the copper film using 5 MeV Rutherford backscattering. We have measured the rates of evaporation from uncapped GaAs surfaces during RTA in the temperature range 600–725 °C and found them to be in reasonable agreement with rates predicted from available measurements of the equilibrium vapor pressures of Ga and As.
We have deposited a thin-film of W60N40 as encapsulant to reduce the loss of arsenic from GaAs when this films of palladium (27 nm) and GaAs (55 nm) deposited on an inert substrate react with each other on thermal annealing in an argon ambient. Results from Rutherford backscattering spectrometry, scanning electron microscopy, and four-point probe sheet resistance measurements show that W60N40 is an effective cap for the Pd-GaAs system at least up to 400°C for 24 h. Without the W60N40 cap, a drastic degradation of the Pd-GaAs system starts at about 400°C after 90 min.
The structural effects of heating 1500 Å Au/GaAs (001) encapsulated with 2000 Å of SiO2 were examined by scanning electron microscopy and X-ray diffraction. It was observed that SiO2/Au/GaAs (capped) in vacuum up to 500°C remained shiny and gold in color, whereas similar heating of Au/GaAs (uncapped) caused a change of color from shiny gold to dull silver. Furthermore, mass spectroscopy showed that the amount of arsenic vapor evolved was much less for the capped sample. However, X-ray diffraction showed that Au7Ga2 formed abundantly in both types of samples after heating at 500°C, though the epitaxial relationship was mainly Au7Ga2 (001) {norm of matrix} GaAs (001) for capped and Au7Ga2 (100) {norm of matrix} GaAs (001) for uncapped. SEM revealed gold-rich aligned rectangular protrusions on the surfaces of SiO2/Au/GaAs as well as Au/GaAs after heating at 500°C, though the average length of these rectangles was 1.5 μm for the capped sample and 6.7 μm for the uncapped sample. Moreover, new morphological features absent in Au/GaAs were observed in SiO2/Au/GaAs. These features are a gold-rich maze with a line width of μm and gold-rich protruded lines with a line width of 9 μm. The gold-rich protruded lines were formed by the growth and joining together of some gold-rich aligned rectangular protrusions. The gold-rich maze was observed in SiO2/Au/GaAs after heating in vacuum, but was not observed in SiO2/Au/GaAs after heating in nitrogen.
This review provides numerical and graphical information about many (but by no means all) of the physical and electronic properties of GaAs that are useful to those engaged in experimental research and development on this material. The emphasis is on properties of GaAs itself, and the host of effects associated with the presence of specific impurities and defects is excluded from coverage. The geometry of the sphalerite lattice and of the first Brillouin zone of reciprocal space are used to pave the way for material concerning elastic moduli, speeds of sound, and phonon dispersion curves. A section on thermal properties includes material on the phase diagram and liquidus curve, thermal expansion coefficient as a function of temperature, specific heat and equivalent Debye temperature behavior, and thermal conduction. The discussion of optical properties focusses on dispersion of the dielectric constant from low frequencies [κ 0 (300)=12.85] through the reststrahlen range to the intrinsic edge, and on the associated absorption and reflectance behavior. Experimental information concerning the valence and conduction band systems, and on the direct and indirect intrinsic gaps, is used to develop workable approximations for the statitistical weights N v (T) and N c (T), and for the intrinsic density. Experimental data concerning mobilities of holes and electrons are briefly reviewed, as is also the v n (E) characteristic for the conduction band system.
This paper evaluates reactively sputtered WSiN films as an annealing cap for GaAs substrate. WSiN film successfully suppresses As and Ga outdiffusion. The WSiN/SiO 2 /SiN/substrate multilayer annealing cap reconfirms this. As and Ga accumulate in the WSiN/SiO 2 interface after annealing at 800 °C for 20 min. The accumulated As peak concentration is estimated to be 10<sup>19</sup> atoms/cm<sup>3</sup> . X‐ray diffraction shows WSiN film remains in an amorphous phase, exhibiting no recrystallized grain boundary. Film stress only changes by one‐half before and after annealing. These characteristics are very different from other refractory metals such as WN and WSi which change from amorphous to crystalline phase after annealing. It is therefore concluded that reactively sputtered WSiN films prevent As and Ga outdiffusion and form a good annealing cap for GaAs.
AIN films were deposited onto GaAs and vitreous carbon substrates held at room temperature, by reactive evaporation of aluminium in the presence of nitrogen and/or NH3 gas mixture. These films and their combination with very thin layers of Si3N4 were successfully used as encapsulants for GaAs and were found to withstand annealing temperatures of up to 1100°C. Films grown by this novel method were analysed by Rutherford backscattering spectrometry and reflection high energy electron diffraction. Oxygen, nitrogen and aluminium were the only elements detected in the encapsulants. However, the best encapsulants were found to have the lowest oxygen content. The deposition conditions were found to be very important in preventing the reaction of the films with the surface of GaAs during heat treatment.
Evidence is given that Au and Au-based contacts do not react with GaAs substrate when alloyed with a CVD SiO 2 layer deposited on top of the metal. Reduced reactivity results in a significantly better morphology of the contact as compared with those alloyed conventionally. Lower electrical resistivities of Au(Zn)/p-GaAs and Au(GeNi)/n-GaAs ohmic contacts are obtained in a wider range of processing temperatures.
A novel cap-annealing technique for n<sup>+</sup> implanted layers
in GaAs MESFET's was investigated from the viewpoint of thermal stress
relaxation. In this technique, the same metal as the gate's-tungsten
nitride (WN<sub>x</sub>) was used for an underlayer of a double-layer
annealing cap. FET parameters V <sub>th</sub>, K -value,
and N <sub>g</sub> showed behavior in the short-channel region
just as if the short-channel effects were suppressed