AS researchers focus on ever-smaller dimensions to engineer advanced materials, they increasingly demand new tools to scrutinize these materials. The need is particularly acute for semiconductor chip makers as they continue to shrink the size of chips and their internal features. Lawrence Livermore researchers also want a better way to image and characterize the all-important surfaces of critical materials. Now a team from Livermore's Physics and Space Technology, Chemistry and Materials Science, and Engineering directorates has developed a diagnostic instrument called a time-of-flight secondary ion mass spectrometry (SIMS) emission microscope. For the first time, the instrument simultaneously provides extremely sensitive surface analysis, high-resolution imaging, and chemical determination of surface constituents. Recent tests on a variety of materials show that the new microscope may well prove valuable in solving vexing surface analysis problems in fields as diverse as precision optics and amino acid sequencing. SIMS is a widespread technique in which a stream of energetic, primary ions bombards the surface of a material under investigation. Upon impact, these ions generate positively and negatively charged secondary ions, which are gathered by electrically charged lenses, imaged, and identified. (Neutral atoms and molecules are also given off but are not detected.) NASA scientists used the first SIMS instrument in the 1960s to analyze moon rocks. Today, SIMS is widely used for analyzing trace elements and contaminants in solid materials, especially semiconductors and thin films. Traditional SIMS instruments employ a stream of single-charged primary ions (for example, xenon +1) to bombard a sample. With this technique, about a thousand bombardments are needed to produce one secondary ion, a slow process during which a spectrum of surface constituents is gradually built up. Greater "Pop" The new Livermore instrument uses not single-charged, but multiple-charged ions (for example, gold +69), which produce a thousandfold increase in secondary ions. "Highly charged ions make our instrument unique," says materials scientist Alex Hamza. "The higher the charge, the greater the 'pop,' the more ions that come off." More ions mean more--and faster--information about the composition of the surface layer, including any contaminants. Hamza says studies at Livermore show that during the first few femtoseconds (quadrillionths of a second) of impact, the highly charged ions deposit a huge amount of potential energy into a surface area several nanometers (billionths of a meter) square. In contrast, single-charged ions deposit large amounts of kinetic, not potential, energy. This kinetic energy transfer is not localized at the surface but is distributed more deeply into the sample. Although the exact mechanism of highly charged ion energy transfer isn't fully elucidated, Hamza says it is probable that electrons from nearby surface atoms are attracted to the strongly positive primary ion. The resulting electron transfer removes the "glue" that once held the nearby atoms in place, allowing them to fly off. As they leave the surface, they are attracted to the electrostatic lens of the microscope and accelerated to a detector located about a half meter from the sample. Finally, an image of the surface magnified at from 40 to 400 times is created (Figure 1).

The chemical determination of the secondary ions is performed through time-of-flight techniques in which the time a secondary ion takes to arrive at the detector is directly related to the mass. Histograms of the arrival times are built up to form mass spectra of the secondary ions emitted from the sample (Figure 2). With a collision rate of about a thousand per second, the Livermore instrument takes roughly 15 minutes (corresponding to about a million events) to build up a useful image such as that in Figure 1. Because of the number of secondary ions produced per collision and the small area being investigated, the microscope is particularly useful in determining the location of secondary ions through coincidence counting. By seeing what molecules come off together from the impact of primary ions, the instrument can reveal impurities in the location of interest. This feature is particularly important as chips shrink and can be contaminated by fewer impure atoms or molecules, which, nevertheless, must be detected.

Focusing on Sensitivity, Resolution The new Livermore instrument can detect 10 parts per million, a sensitivity equal to that found in a typical SIMS instrument. Resolution of feature sizes has been demonstrated at 6 micrometers. The development team is confident it can achieve resolutions down to 10 nanometers within a year through better lens design and improved detector resolution. The instrument uses beams of highly charged ions generated by the electron-beam ion trap (EBIT), developed by a Lawrence Livermore research team a few years ago. With this device, the charge, energy, and mass of the primary beam can be varied independently. Electrostatic lenses and apertures control the intensity and width of the primary ion beam. Conceived, designed, and fabricated by Livermore physicist Alan Barnes with the assistance of mechanical engineer Ed Magee, the SIMS emission microscope features a novel "acorn-shaped" objective lens used to image the secondary ions while a sensitive detector determines the up­down position and time of arrival of the secondary ions at the microscope image plane. Contrast in the image can be based on the intensity of the electrons detected or the presence of particular secondary ions. Because of this technology's potential importance to the semiconductor industry, the team has used it to analyze the deposition of tungsten on patterned silicon wafers, a common step in computer chip manufacturing. Figure 1 is an image collected from a wire-mesh-covered sample of a silicon wafer patterned with silicon dioxide and tungsten. The colors in the image indicate the type of secondary ion observed, as measured by time of flight: red indicates a tungsten-related region and blue a silicon dioxide­dominated region. By selecting events from the blue and red regions, researchers can provide a spatially resolved analysis of the surface composition. Figure 2a shows the time-of-flight spectrum of the blue region, while Figure 2b is the spectrum from the red region. Both spectra reveal the outstanding sensitivity of the instrument. According to Hamza, an impressive array of instruments is available to image materials with very high resolution--examples include transmission electron microscopy, scanning electron microscopy, and scanning tunneling microscopy. An equally impressive array of instruments and techniques is available to determine material composition (Auger electron spectroscopy, photoelectron spectroscopy, and SIMS). However, no other technique combines high-resolution images, high sensitivity to trace elements, and the chemical structure of the secondary ions, all in one package. An Eye on the Future The first Livermore instrument was built to demonstrate the concepts necessary to construct more powerful versions. Plans for the next two years include improved resolution, data collection, and primary beam focusing. To image smaller areas, the team will experiment with using ion streams chilled to low temperatures. Hamza reports significant interest from Semitech, a national semiconductor industry forum. Semitech officials have suggested that semiconductor companies could send samples to a central location housing several of the Livermore microscopes. As for other applications, the research team sees significant potential wherever chemical structure must be determined at high resolution. A natural fit is stockpile surveillance activities (for example, investigating corrosion in metals such as uranium) and inspecting high explosives to determine their reliability. In fact, the research team has already used the new microscope to examine the distribution of high-explosive molecules in their binding material--a factor affecting reliability. Another important application is the investigation of possible links between glass failure and polishing residue in optical components used in powerful lasers such as Lawrence Livermore's forthcoming National Ignition Facility. One intriguing application is analyzing biological materials. By using a highly charged ion stream to break molecular bonds, the microscope could be used to determine the sequence of amino acids forming proteins and thereby become a powerful tool used in molecular biology as well as forensics. If planned refinements succeed, the instrument could well become a mainstay in research laboratories everywhere. --Arnie Heller

Key Words: highly charged ions, SIMS (secondary ion mass spectrometry), time-of-flight secondary ion mass spectrometry (SIMS) emission microscope.

For further information contact Alex Hamza (925) 423-9198 (hamza1@llnl.gov).