Imaging Integrated Circuits with X-ray Microscopy Michael Bajuraa, Greg Bovermana, John Tana, Gene Wagenbretha, Craig Milo Rogersa, Michael Feserb, Juana Rudatib, Andrei Tkachukb, Stephen Aylwardc, Patrick Reynoldsc aUniversity of Southern California, Information Sciences Institute, 4676 Admirality Way #1001, Marina Del Rey, CA 90292 USA bXradia, Inc., 4385 Hopyard Rd. #100, Pleasanton, CA 94588 USA cKitware, Inc. 101 East Weaver St., Carrboro, NC 27510 USA
Abstract: We introduce x-ray microscopy as a method for scanning depending on the circuit’s radiation tolerance imaging integrated circuits. Compared to physical de- properties. layering combined with scanning electron microscopy, this Compared to SEM approaches, the current resolution of x- method is non-destructive, requiring only sample thinning ray microscopy is lower, currently around 30 nm, to approximately 100 um but otherwise leaving the sample compared to sub-nanometer resolution possible with SEM. intact. Using multiple views and 3D tomography, However the imaging resolution of x-ray microscopy has individual integrated circuit layers can be imaged. Current been on a “Moore’s Law” like path related to the ability to imaging resolution is approximately 30 nm, and is fabricate the zone plate nano-structures required to focus x- constantly improving with advances in nano-fabrication for rays. This trend is depicted in Figure 1 below, for both hard x-ray optics. hard (greater than 1 keV photon energy) and soft (less than 1 keV photon energy) x-rays. Over time, this trend is Keywords: Trust; x-ray microscopy; reverse engineering expected to continue to the single-digit nanometer resolution range2. Introduction Because of the need for trusted integrated circuits (ICs) in X-ray Microscope Description electronic systems and the difficulty of finding trusted A high-level schematic of an x-ray microscope is presented sources for leading-edge ICs, it has been proposed that ICs at the top of Figure 2. A picture of a microscope could be procured from un-trusted sources and inspected to constructed atop an optical table is shown in Figure 3. The ensure a supply of trusted ICs for critical applications. x-ray microscope consists of two focusing elements similar Since it is impractical to discover potentially harmful to a light microscope. An elliptical condenser refocuses circuits in an IC by electrical testing alone, physical light from an x-ray source onto the sample. After passing inspection is required to completely determine circuit through the sample, x-rays are focused onto a scintillator structure. This work describes an approach for IC imaging with a diffractive zone plate, and then imaged with a cooled using hard x-ray microscopy, which we have used to non- CCD camera. The x-ray optics are key to the microscope’s destructively image 90 nm technology test chips which operation: Without them, it would be very difficult to have been thinned to 100 um. Our challenge was to produce sub-micron resolution images, because of the develop an automated imaging method capable of scanning miniscule dimensions of the point x-ray source which a large IC fast enough to make it a practical inspection would be required. method. By varying the x-ray imaging energy with a tunable source, the technique shows some sensitivity to To scan a sample, it is mounted onto a motorized stage and circuit structures such as gate and active areas in addition to calibrated. A control computer moves the stage to the back end wiring interconnect layers. required positions, sets the exposure times and saves images from the CCD camera to disk. Comparison of Imaging Modalities The requirement of the zone plate for monochromatic (or Table 1 below summarizes the capabilities of different near-monochromatic) light for crisp focusing constrains the imaging modalities. Optical methods lack the resolution to imaging throughput. A high-brightness source suited to the effectively image deep submicron ICs, while electron- specific optical input requirements of the microscope is based methods (SEM, TEM) have the needed resolution needed to produce images in a timely matter. Tunability of but require physically destructive sample preparation. the x-ray source is desirable to perform elemental scans Destructive sample preparation can be a concern if there across specific x-ray absorption edges. The current source are only a limited number of devices available for testing, of choice is a synchrotron x-ray source, however compact since there are no do-overs after a physical delayering high-flux x-ray sources are a growing and active research process. X-ray microscopy addresses these concerns, and area3. has the potential to leave a circuit operational after
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Processing Raw Images into IC Layers algorithms described above. For reference the associated Raw x-ray images of a 90 nm IC with viewing angles of CAD file images for these chip areas are shown. While the normal and 45-degrees are presented in Figure 4 and contact layer complicates these images, the image response Figure 5. With a field-of-view of just over 30 um, and a to the target layers is visible. depth-of-focus of approximately 40 um, it is possible to view a significant amount of circuitry in a single image. At Conclusions and Future Work normal view, much of the circuitry appears self-evident We have demonstrated an early, new technology for with an approximately 3% darkening, or absorption, for imaging integrated circuits with x-rays instead of electrons, every metal wire structure imaged. However a single on a scale of automation and accuracy which did not exist normal view cannot discern depth. Attempting to view the prior to our effort. As higher resolution zone plates are IC at an angle as in Figure 5 is confusing because of the developed with new nano-lithographic methods, and new metal fill structures added during the IC’s manufacturing compact tunable x-ray sources are developed, we expect process. Computed tomography is needed to combine this to be a viable technology for critical needs in the views of an IC from different angles to determine the future. structure on each layer. While the mathematics of computed tomography with Acknowledgements filtered backprojection are well-known, the challenge of This work was sponsored by Defense Advanced Research performing an accurate 3D reconstruction is aligning all the Projects Agency Microsystems Technology Office (MTO). images of the chip before processing. The stated Program “Enhancing trust with X-ray Phase-Optimized mechanical uncertainty of the sample positioning stage is a Scanning Equipment (EXPOSE) ARPA Order No. few microns, while the wiring structures present can be 100 X040/07 Program Code: 7720 Issued by DARPA/CMO nm or less in width. This means that image registration under Contract No. HR0011-07-C-0102. “The views and methods must be used to align the input images for final conclusions contained in this document are those of the positioning instead of the mechanical stage. This process is authors and should not be interpreted as representing the depicted in Figure 2 where a mosaic scan for each imaging official policies, either expressly or implied, of the Defense angle of the chip is assembled, registered, and converted Advanced Research Projects Agency or the U.S. into a 3D volume dataset with computed tomography Government.” mathematics. Special thanks also to the support of Stanford / SSRL including Piero Pianetta, Joy Hayter, and Sean Brennan Once the 3D volume is formed, the next challenge is who spent many hours assisting with our experimental orienting and identifying images of individual IC layers setup. corresponding to layers in a Computer-Aided-Design (CAD) file. Orientation is a challenge because of References uncertainties in the sample stage and sample mounting on 1. Tkachuk, A., Feser, M., Cui, H., Duewer, F., Chang, the order of milliradians which can cause the slicing H., Yun, W., “High-Resolution X-ray Tomography algorithm to skew- Using Laboratory Sources,” Developments in X-Ray sample across multiple IC layers. Similarly layer Tomography V, edited by Ulrich Bonse, Proc. of SPIE identification is a challenge due to variance in the stated Vol. 6318, 63181D, (2006) manufacturing thicknesses and horizontal alignment of layers relative to each other. 2. Yun, W., Feser, M., Lyon, A., Duewer, F., Wang, Y., “Pathways to sub-10 nm X–ray Imaging Using Zone Layer Imaging Detail Plate Lens,”, Design and Microfabrication of Novel X- Figure 6 depicts the quality of images formed by slicing Ray Optics II, edited by Anatoly A. Snigirev, Derrick the 3D data volume at the right layers. The contact layer is C. Mancini, Proceedings of SPIE Vol. 5539 (SPIE, resolved into substrate and poly-contacts, and the metal Bellingham, WA, 2004) layers bear a striking resemblance to their CAD file 3. “Advanced X-Ray integrated Sources counterparts. (AXiS),”DARPA Broad Agency Announcement , Figure 7 and Figure 8 show exploratory images of the poly DARPA-BAA-11-11, November 22, 2010. and active layers formed by tuning the x-ray microscope to specific x-ray energies and repeating the 3D reconstruction
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Table 1 Comparison of Imaging Modalities Figure 1 Historical Trend for Imaging Resolution
Figure 2 X-ray Inspection Flowchart Figure 3 X-ray Microscope at Stanford / SSRL
Figure 4 Normal View of 90 nm Integrated Circuit Figure 5 45-Degree View of 90 nm Integrated Circuit
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Figure 7 Gate Layer Imaging and Comparison with Figure 6 Imaging on Individual Wiring Layers CAD File
Figure 8 Active Layer Imaging and Comparison with CAD File
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