S.R. Brown's research while affiliated with University of Central Florida and other places

A site specific technique for cross-section transmission electron microscopy specimen preparation of difficult materials is presented. Focused ion beams are used to slice an electron transparent sliver of the specimen from a specific area of interest. Micromanipulation lift-out procedures are then used to transport the electron transparent specimen to a carbon coated copper grid for subsequent TEM analysis. The experimental procedures are described in detail and an example of the lift-out technique is presented.

The focused ion beam (FIB) tool has been successfully used as both a stand alone analytical instrument and a means to prepare specimens for subsequent analysis by SEM, TEM, SIMS, XPS, and AUGER. In this work, special emphasis is given to TEM specimen preparation by the FIB lift-out technique. The fundamental ion/solid interactions that govern the FIB milling process are examined and discussed with respect to the preparation of electron transparent membranes. TRIM, a Monte Carlo simulation code, is used to physically model variables that influence FIB sputtering behavior. The results of such computer generated models are compared with empirical observations in a number of materials processed with an FEI 611 FIB workstation. The roles of incident ion attack angle, beam current, trench geometry, raster pattern, and target-material-dependent removal rates are considered. These interrelationships are used to explain observed phenomena and predict expected milling behaviors, thus increasing the potential for the FIB to be used more efficiently with reproducible results.

Advantages of the FIB lift-out technique over traditional H-bar TEM specimen preparation have been recognized. The ability to rapidly (< 1 hour) prepare a site specific TEM specimen without destroying the entire bulk specimen has led to a wide spread reliance on this method. The main disadvantage of this technique is an inability to accomplish additional membrane thinning if required. Traditional H-bar preparation allows additional thinning. However, mechanical polishing is time consuming and the bulk sample is destroyed. A method has been developed which combines the efficient, site specific advantages of the lift-out method with the H-bar's ability to accomplish additional thinning. in this procedure a lift-out specimen is removed from the bulk sample and mounted onto a half-grid in a configuration similar to that employed by the H-bar technique. A 1.0-micron thick lift-out specimen was prepared using a FEI Strata DB-235 FIB dual-beam workstation by sputtering away bulk material leaving a thin membrane containing a desired feature (FIG 1).

Commercially available focused ion beam (FIB) workstations with spatial resolution of 5–7 nm can prepare specimens with excellent lateral resolution. This capability has been utilized extensively by the semiconductor industry to obtain materials characterization from continually smaller areas. The FIB has been adopted generally as a preparation tool for scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ability to prepare site-specific specimens that can be removed from the bulk of a sample provides enhanced SEM and TEM analyses and new approaches for other analytical tools. Dedicated scanning transmission electron microscopy (STEM) can provide images through samples several micrometers thick. Auger electron spectroscopy (AES) can analyze with improved ability to identify a small particle. Secondary ion mass spectrometry (SIMS) can provide trace analysis at high mass resolution. Automatic operation of FIB workstations permits the creation of multiple lift-out samples without the presence of an operator. Copyright © 2001 John Wiley & Sons, Ltd.

The focused ion beam system (FIB) has become a valuable tool for the preparation of transmission electron microscope (TEM) samples. Several FIB preparation techniques exist but of particular interest is the lift-out technique, which allows for the extraction of a thin membrane from a bulk material. This technique has seen great success in the preparation of cross sectional samples. We explore the use of this technique for planar sample preparation to examine grain deformation due to nanoindentation in a reference copper material.

The continued decrease in microelectronic feature dimensions has led to a reliance on the focused ion beam (FIB) for site-specific transmission electron microscopy (TEM) specimen preparation. To maximize the capabilities of the FIB, methods must be developed to consistently produce specimens thin enough to generate TEM lattice images. The limiting factor in producing quality TEM specimens by either the traditional or lift-out method is the final thickness of the specimen. The FIB is used to prepare TEM specimens by removing the bulk material that surrounds a desired feature by sputtering with a focused gallium ion beam. Successively lower beam currents are used to sputter away material until an electron transparent membrane (-0.2 μm) containing the desired feature remains. For a 300 keV TEM, lattice imaging of silicon requires additional membrane thinning to less than 0.05 μm. The loss of rigidity during the thinning process makes the membrane highly prone to deformation due to residual stresses, linear expansion, and ion beam interaction.

Reduced feature dimensions in microelectronic devices has led to a growing reliance on the focused ion beam (FIB) for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) specimen preparation. The introduction of copper for use in electrical interconnects will increase this reliance. Copper, much more so than aluminum, tends to smear when conventional polishing techniques are employed, rendering mechanical polishing unsuitable for quality specimen preparation. The sputtering characteristics of copper differ greatly from the aluminum based materials currently in use. Copper exhibits a grain orientation dependent sputtering rate that tends to produce uneven FIB polished surfaces. The sputtering rate for copper is also much higher than the silicon and dielectric materials that must be simultaneously removed. These differences in sputtering rates lead to the formation of curtains and ledges that also affect specimen quality. Another FIB induced artifact was observed when preparing SEM specimens (Figure 3).

Defects present in microelectronic devices are often present in test structures as well. This makes test structures useful in identifying defect mechanisms. Chain patterns consists of thousands of contacts and plugs in series. The presence of an open contact in a chain can be detected by a loss of electrical continuity. The specific site of an open contact is difficult to locate for further analysis. The application of the focused ion beam (FIB) for passive voltage contrast (PVC) provides an effective method for contact defect location. Once the defect is located the FIB facilitates efficient site-specific specimen preparation for scanning electron microscopy (SEM), transmission electron microscopy (TEM) or Auger analysis. The FIB is capable of PVC analysis by distinguishing electrically isolated conductors from grounded conductors. Isolated conductors charge as a result of ion beam interaction. Once charged, the quantity of secondary electrons available for detection is greatly reduced.

The semiconductor industry demands elemental information from ever smaller regions. The sensitivity of secondary ion mass spectrometry, coupled with the lateral resolution of a focused ion beam, can provide nanoscale elemental data that are competitive with that from other analytical techniques. Ion images of the sidewalls in repetitive semiconductor features tilted to present a large surface area have shown boron contamination after an etch process. The boron is removed by a specific cleaning step. Spot defect analysis was enhanced by the use of mass spectra that provide information on a range of elements before the defect is removed by sputtering. Ion implanted samples were analyzed in cross section and the implant shape detected. Summation of the secondary ion counts in the implant cross section over a few micrometers resulted in detection limits below 0.1 at. %. Implantation profiles have been detected for Al, Cr, Na, Li, and K without the aid of secondary ion enhancing species, such as oxygen or cesium. © 1999 American Vacuum Society.