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Defect Analysis and Metrology

Extending inspection limits using a DUV-laser-based bright-field inspection tool

Nurit Raccah, Daniel Rogers, Ehud Tzuri, and Arik Shavit, Applied Materials

As IC feature sizes continue to shrink well below the wavelength of visible light, there is an increasing need for high-resolution, high-throughput inspection tools to accelerate process development and optimize production. At the 90-nm technology node and beyond, characterizing shrinking yield-critical defects requires the detection of the new defect types that result from the integration of new materials and processes. These requirements lead to a natural trade-off between sensitivity and throughput. The increasing cost of inspection suggests that new inspection approaches are needed.

A deep-ultraviolet (DUV) laser-based bright-field inspection technology developed by Applied Materials (Santa Clara, CA) was used to characterize two advanced DRAM process modules. The inspection method enabled engineers to address specific processing issues rapidly. This article focuses on inspection strategies for three DRAM layers. In addition, the article compares current optical inspection tools and correlates tool data results with end-of-line probe results.

DUV Bright-Field Inspection

Automated bright-field wafer inspection tools, which were introduced more than 20 years ago, have traditionally used lamps as illumination sources. While these tools have evolved to become industry standards, they face limitations as chipmakers begin to address the needs  of 65-nm processes and below. The achievable illumination levels of lamp-based bright-field inspection tools are inherently limited. Time-delay-integration cameras can compensate in part for the low signal levels of traditional bright-field tools, but they too are limited in the low-light-flux regime. As a result, the instruments' sensitivity for materials that are susceptible to light damage or lack reflectivity can degrade substantially.

The DUV-laser-based inspection tool discussed in this article offers simultaneous bright-field and 3-D-channel imaging to perform in-line inspection of advanced processes. With a photomultiplier tube (PMT) detector, the tool operates at high resolution with a throughput of several wafers per hour, making it suitable for production applications. To assess the instrument's capabilities, tests were conducted on three layers from advanced DRAM technology nodes. Containing challenging defect types, the layers selected were shallow-trench isolation (STI), contact etch, and metal line after photoresist developing. For each of the layers, the challenges facing the inspection technique and the methodologies that were developed to characterize the defects are discussed.

Methodology

To conduct the evaluation, many wafers with the targeted layers were inspected. From previous probe-test data and failure analyses, specific difficult-to-detect defect types were selected. Inspection recipes were created for optimum sensitivity to the defects of interest. The recipes were subsequently assessed for throughput and false rate to determine whether the inspection recipe needed to be modified. For all three layers, the recipes met the throughput requirement specifications, so sensitivity did not have to be compromised.

The tool's sensitivity and throughput performance can be attributed to its design, which uses DUV-laser illumination, simultaneous bright-field and 3-D-channel imaging, and a photomultiplier (PMT) module for light collection. The combination of laser illumination and PMT detection enables the system to maintain high detection sensitivity, even in low-light situations. The bright-field channel is sensitive to shallow-pattern defects, while the 3-D channel offers high sensitivity to defects with some topography, such as particles and voids.

The system's ability to collect signals from the bright-field and the 3-D channels at the same time enables it to detect a broad range of defects in a single scan. In addition, it can efficiently perform simultaneous cell-to-cell and die-to-die inspection in one wafer pass, offering excellent sensitivity across different regions of the die without requiring multiple passes for different inspection conditions. Advanced filters and algorithms were employed to suppress wafer noise, achieving a high signal-to-noise ratio. Real-time nuisance filtering was also used to amplify defects of interest, enabling efficient scanning electron microscope (SEM) defect review and accelerating defect root-cause analysis.

Case Study

Inspection 1: STI Layer. The chemical-mechanical polishing (CMP) process used in the formation of STI features can introduce particle contaminants and surface scratches. In addition, void defects caused by inherent oxide problems often emerge after CMP. STI void defects are known to cause functional yield failures and reliability issues by inducing direct shorts between consecutive active areas, as illustrated in Figure 1, or two poly lines, as shown in Figure 2. If such void defects are not detected in-line at an early stage of the transistor-formation sequence, their presence may not be discovered until final probe test. During that period of days or weeks, all the material in the production pipeline can be placed at risk.

Figure 1: Schematic diagram of an oxide void shorting two active areas.
Figure 2: Schematic diagram of an oxide void shorting two poly lines.

The performance of the STI module was evaluated on products being fabricated at both the 95- and 80-nm technology nodes. While other optical inspection systems were being used to detect void defects, the researchers found that the laser-based bright-field tool could detect voids as small as 30 nm. Since STI voids are typically associated with surface topography, their signal-to-noise level was improved dramatically by employing the 3-D channel. A proprietary spatial filtering method, optimized for the repetitive small-pitch patterns that are typical in DRAM devices, was used to eliminate pattern noise and further enhance the defect signal.

The inspection technology was highly sensitive to void defects down to 30 nm for both the 95- and 80-nm products. Figure 3a presents a laser bright-field wafer map of void defects at the STI inspection step, while Figure 3b shows a SEM image of a void. The tool's high detection capacity led to a successful reduction of overall defect levels. The tool's detection accuracy was confirmed by SEM review to be 97%. In addition, tool repeatability was measured at 98%.

Figure 3: Laser-based bright-field wafer map (a) showing void defects from the STI inspection step. Typical defects of interest (b) include particles, scratches, and voids.

Inspection 2: Contact Etch after Clean Layer. The second critical layer that was inspected during these tests was contact etch immediately after a cleaning step was performed. At that level, typical defects of interest include overetch contacts, blocked contacts, and deformed contacts. This study focused on detecting overetch contacts on a contact etch layer in 80-nm devices. Overetched contacts create a direct short between the contact and the word line.

The test parameters used to inspect wafers for overetched contacts were similar to those established in the STI layer study. Using a combination of the high-resolution 3-D channel, the bright-field channel, and a unique spatial filtering mask to block the diffraction lobes, the inspection tool detected defects with a distinct wafer-level signature, as illustrated in Figure 4.

Figure 4: (a) Laser-based bright-field contact etch wafer map showing a clear defect signature on the wafer edge, and (b) SEM image of an overetched contact.

The effect of overetch on end-of-line yield was then determined by correlating laser bright-field inspection maps to an end-of-line probe map. Laser bright-field and probe maps from two wafers used in the study are presented in Figures 5a and 5b, respectively. Occurring primarily at the wafer edge, the die that failed the probe test appear black in Figure 5b. Overetched contacts were also clustered at the wafer edge. Similar to STI layer inspection, contact etch after clean inspection detected defects of interest with a <3% false-alarm rate and >95% repeatability.

Figure 5: Edge-shading signature resulting from overetched contacts: (a) laser-based bright-field map, and (b) end-of-line probe map. The bright-field signature correlated strongly to the probe results.

Inspection 3: Metal Line after Photoresist Developing. The third process targeted in this evaluation was the metal line after photoresist developing but before metal etch. The ability of the inspection tool to isolate and detect photoresist microbridges in an after-develop inspection (ADI) step enables engineers to make process adjustments that can prevent the migration of defects to later process steps. More importantly, the wafer can be reworked as long as it has not undergone etch.

The metal line layer after photoresist developing is known for having a variety of defect types, including full and partial bridging defects, pattern defects, peeling (delamination), residues, bumps, particles, and embedded particles. Some of these defects may have originated at previous layers, making it difficult to find and focus on defects of interest quickly. In this study, the researchers identified microbridges as their highest priority. Their goal was to detect isolated micro-bridges and review them using a SEM tool. For efficient SEM review, the inspection results should ideally include defects of interest while ignoring nuisance defects.

Detecting defects on ADI layers is challenging for optical inspection tools that use visible light, because the percentage of light transmission increases with wavelength. Transmitted visible light scatters from the underlying pattern and generates noise from the previous layer, especially in the case of rough and grainy metal films. Two fundamental features of the laser bright-field technology help avoid this problem.

The first feature is the tool's DUV wavelength. As shown in Figure 6, the DUV antireflective coating (DARC) layer associated with the photoresist strongly absorbs DUV light at 266 nm, effectively eliminating unwanted signals that originate from underlying layers. The second feature is simultaneous signal detection in the bright-field and 3-D channels. With that arrangement, it is possible to achieve high sensitivity for detecting a wide range of defect types in a single scan.

Figure 6: Photoresist DARC light absorption versus wavelength. A much higher percentage (80%) of DUV (266-nm) light is absorbed than UV and visible light.

The metal line layer after photoresist developing was evaluated at the 95-nm technology node. The tool's inspection range was optimized to cover the full wafer, including both array and periphery regions. The array region, in which microbridges were abundant, was scanned using the system's cell-to-cell mode with the 3-D channel. The wafer's periphery area, which suffered from peeling defects, was scanned using the tool's die-to-die mode with the bright-field channel. The schematic diagram in Figure 7 highlights the inspection strategy. The system's ability to scan the array and periphery areas of the wafer simultaneously without compromising sensitivity in either area represents a throughput advantage over other bright-field tools. More importantly, the tool was highly sensitive to photoresist microbridge defects, as shown in the wafer maps in Figure 8.

Figure 7: Schematic diagram (a) depicting how simultaneous bright-field and 3-D-channel detection is used to detect microbridges (b) and peeling defects (c) in a single scan.
Figure 8: Metal line ADI wafer maps from two types of lamp-based bright-field tools (a and b) and a laser-based bright-field tool (c). The laser-based wafer image (d) shows a typical photoresist array bridge.

Subsequent SEM review confirmed that the laser-based system is more sensitive to microbridges than other optical inspection tools and returns fewer reports of nuisance defects. The system's ability to filter out nuisance defects and selectively report defects of interest provides a more reliable process indicator and enables faster feedback than other tools.

Conclusion

At the 95- and 80-nm technology nodes evaluated in this study, a laser-based bright-field technology was able to detect and resolve yield-limiting defects. Contact etch inspection results also correlated nicely to final wafer yield data. The system achieved high detection sensitivity without lowering throughput or missing defects of interest. The system's detection capability can be attributed to several key technologies: First, a 266-nm laser source and an efficient PMT detector render the tool sensitive to small and challenging defects; second, simultaneous bright-field and 3-D-channel imaging enables users to detect all critical defect types in a single scan; and third, effective filtering strategies reduce the rate of nuisance defects.

Looking forward, given its sensitivity and throughput, laser-based bright-field technology promises to be extendable to the 65-nm technology node and beyond, since it detects increasingly small defects as well as critical defects in complex structures.

Acknowledgments

The authors would like to thank Justin Arrington and Pete Frank of Micron Technology; and Noam Shachar, Ido Hammer, Silviu Reinhorn, and Ofir Montal of Applied Materials' PDC group.

Bibliography

Kim, YH. "Effect of UV Inspection on Defect Management for Advanced Devices." Paper presented at the Semicon Europa Yield Management Solution Seminar. April 24–26, 2001. Munich.

Lin, L, M Cheng, and T Han. "Detecting and Classifying DRAM Contact Defects in Real Time." MICRO 21, no. 7 (2003): 71–75.

Reinhorn, S, et al. "UVision for Advanced DRAM Process Monitoring." Paper presented at the Applied Materials Engineering Technology Conference. May 24–26, 2004. Santa Clara, CA.


Nurit Raccah is a product marketing manager in the wafer inspection division at Applied Materials Process Diagnostic and Control business group. In the past five years, she has served in various positions in the PDC division, including as a technologist and an application development engineer. Before joining Applied Materials, Raccah worked at Intel Electronics, Israel, as an etch process engineer. She holds a BS in chemical engineering and an MS in technology management from the Hebrew University in Jerusalem. (Raccah can be reached at nurit_raccah@amat.com.)

Daniel Rogers is a wafer inspection process engineer at Applied Materials. He received a BS in physics from Boise State University in Idaho. (Rogers can be reached at daniel_rogers@amat.com.)

Ehud Tzuri is the general product manager of the inspection division at Applied Materials. He began his career as a wafer inspection application engineer and then held positions in product and marketing management. For the last four years, Tzuri has been responsible for the management of all marketing activities in the inspection division. Previously, he held key positions at Orbot-Instruments. He received a BS in electronics and computer engineering from the Ben-Gurion University in Beer-Sheva, Israel. (Tzuri can be reached at ehud_tzuri@amat.com.)

Arik Shavit is an application development engineer in the wafer inspection division at Applied Materials Process Diagnostic and Control group. He has served in this position for the last two years. He received a BS in electrical and electronics engineering and a BS in physics and astronomy, both from Tel Aviv University in Israel. (Shavit can be reached at arik_shavit@amat.com.)


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