Influence of immersion lithography on wafer edge defectivity
02/01/2008
EXECUTIVE OVERVIEW
In semiconductor manufacturing, the control of defects at the edge of the wafer is a key factor to keep the number of yielding die on a wafer as high as possible. Using dry lithography, this control is typically accomplished by a well-understood edge bead removal (EBR) process. Immersion lithography, however, changes the situation significantly. During immersion exposure, the wafer edge is in contact with water from the immersion hood, and particles can be transported to the printed area or scanner wafer stage from the wafer edge area. The dynamic force of the immersion hood movement can potentially damage material in the EBR region. This paper investigates the impact of immersion lithography on wafer edge defectivity.
In the past, research on wafer edge defectivity has been limited to the inspection of the flat top part of the wafer edge, due to the inspection challenges at the curved wafer edge and lack of a comprehensive defect inspection solution. This study uses a new automated edge inspection system that provides full wafer edge imaging (top near edge, top bevel, apex, bottom bevel, and bottom near edge) using laser-based optics and multisensor detection.
Process control at the wafer edge
In semiconductor manufacturing, control of the process at the wafer edge is a key parameter in determining the total number of yielding die on a wafer. The removal of photoresist from the wafer backside and edges is especially important to avoid contact between the resist and the scanner stage or wafer handling hardware. Typically, a solvent EBR step is the last step in the coating recipe: the combination of a solvent stream from a static nozzle toward the wafer back side and a dynamic nozzle toward the wafer front side dissolves the resist up to a few millimeters from the wafer’ outer edge. The desired position of the EBR material edge at top side (the so-called EBR-width) can depend on the coated material (e.g., antireflective topcoat vs. photoresist material) and/or on the layer within the device (e.g., a contact hole lithography process might use a slightly different EBR width than the gate process). To increase die yield, it’s desirable to have EBR widths that are as small as possible.
Immersion lithography [1-4] changed the defectivity issues at the wafer edge significantly. During the immersion exposure sequence, the wafer edge is in contact with the water from the immersion hood (IH), introducing additional concerns beyond direct contact of resist with the scanner. First, when the IH is scanning in the EBR region, its movement can damage material edges (Fig. 1a). IMEC’s program on immersion lithography found that, for example, photoresist material can partially peel off during the IH pass (Fig. 1b).
A second concern involves the cleanliness of the wafer edge outside the EBR edges. The IH pass wets not only the near-edge top surface, but also the curved wafer edge and even part of the bottom surface. Defects can be released from this area and redeposited either on the wafer or on the wafer stage. In the first case, there will be a direct impact on the wafer defectivity. In the latter case, defects present on the wafer stage can still be transported onto wafers in subsequent wafer processing. IMEC’s program on immersion lithography found that resist residues left on the curved wafer part by an incomplete EBR step can be damaged by the IH pass, releasing fragments into the system (Fig. 1c).
Traditional defect inspection techniques have serious limitations when monitoring these new issues. Conventional dark field or bright field inspection tools cannot access the wafer edge since these systems typically have an edge exclusion of ~3mm. While microscopy tools can inspect the edge area, they can only give qualitative information, and give typically limited sampling information for the wafer edge.
This study used KLA-Tencor’s VisEdge CV300 automated edge inspection system that provides full wafer edge imaging using laser-based optics and multisensor detection. The system then uses automated defect classification (ADC) software to classify the defects of interest (DOI).
Technology for wafer edge defect inspection
The new technology for wafer edge defect inspection (Fig. 2) is based on a laser source directed to the wafer edge surface. Four detectors simultaneously collect the scattered light, the specular or reflected light, the phase shift in different polarizations, and topography information. As the laser scans the wafer edge surface, each signal can be converted into an image. Each type of defect produces a specific combination of signals, making ADC possible.
 Figure 2. Schematic representation of the VisEdge measurement principle. |
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Imaging of the wafer edge. Imaging covers the entire edge region including the following areas: ~5mm bottom near edge, bottom bevel, apex, top bevel, and ~5mm top near edge. Scanning generates a continuous high-resolution image for the entire wafer edge, which can be interpreted as a Mercator projection or an unfolding of the wafer edge surface into a flat plane.
Since the full wafer edge is scanned during the measurement, it is possible to represent the whole circumferential edge as an image. Excursions in eccentricity and/or in EBR width, which might result in a layer’s edge ending on the wrong underlying substrate, can be easily monitored and corrected using this kind of inspection.
For immersion-related work on wafer edge cleanliness, a high-resolution view of the wafer edge is typically more useful. Here, the images view only a few millimeter of the edge. Figure 3 uses this representation to show resist flakes observed along the apex-bevel regions in the specular channel.
Immersion defect process characterization and optimization. As indicated in the introduction, immersion-related defects at the wafer edge can be due to coated material edge damage in the EBR area, when the IH is passing over this region. On the other hand, defects can be caused by transport of particles present on the bevel. These might be released by forces of the immersion hood, transported by the water in the hood, and redeposited on the wafer and/or stage. This work focuses on the latter, and in particular on the flake defects observed in past work [5].
Edge region flake defects
Flake defects are related to material residues that are present on the wafer edge after coating. Typically, these residues are only present on the apex part of the bevel, and therefore are difficult to detect by conventional top-down inspection methods. The residues result from a nonoptimized EBR process: since the coated material on the wafer edge can be significantly thicker than on the flat top region, an insufficient solvent supply can leave edge residues while the top surface is clean. This phenomenon is more commonly observed with photoresist materials, rather than with BARC and topcoat materials.
The morphology of edge residues can depend on the resist. For some resists, the residue can be quite uniform along the apex. For other resists, large areas of thick residues are combined with areas with thin residues.
Once detected, the problem can be solved fairly easily by adjusting the EBR recipe. Because making the EBR recipe longer limits the throughput of the immersion cluster, however, fabs try to avoid this adjustment if possible. Since conventional inspection tools like tilted SEM can only measure a limited area of the wafer edge, there is a clear risk of handing wafers with resist residues over to the immersion scanner.
When wafers with resist residues are exposed on an immersion scanner, it is difficult to predict if the IH pass over the edge of the wafer will damage the resist residue, and if (part of) the residues will redeposit on the wafer top-side or on the scanner wafer stage. Tilted SEM inspections suggested qualitatively that such damage can happen with certain resists.
Experimental conditions of edge flake characterization. We experimented with three resists with different chemistries (Fig. 4). Sensitivity to edge damage was expected to vary across the three resist types. A dedicated exposure job spatially separated the areas where flakes are expected and where they are not expected. One section, consisting of two rows of 11 fields, was exposed close to the wafer edge at the opposite side of the notch. A similar area of two rows of 11 fields was exposed in the region of the notch. During the exposure of these 2 × 11 sections at both locations near the wafer edge (Region II), the IH makes continuous up- and down-scans over the wafer edge area, increasing the probability of defect generation. The exposure job was also designed so that on another part of the wafer (Region I, on the right hand side), the immersion hood did not pass over the wafer edge. In Region I, no flake-like defects should be detected.
 Figure 4. IH exposure sequence for edge flake characterization. |
Qualification. The specular image of regions with resist residues clearly showed a difference in reflected intensity: dark areas in the resist residues refer to thick layers, while light areas indicate much thinner layers. The results obtained from Resist Type A are detailed below.
We compared the SideScan images of areas where the IH did and did not pass. Figure 5a is a typical SideScan specular image for region I (i.e., where the IH did not pass). Differences in thick and thin resist residues are visible, but no fragments of the resist residues are released. In contrast, in Fig. 5b, taken from Region II, parts from the thick residue at the bottom of the apex are released. The close-up in the image indicates that some, but not all, of these edge flakes are redeposited on the apex closer to the top area of the apex.
 Figure 5. Specular and scatter images from Region I (a and c) and Region II (b and d) of a test wafer. IH damage is more likely in Region II. |
To determine whether any of these edge flakes end up on the top region (where possibly edge dies can be damaged), we analyzed the TopScan image of the corresponding areas of Fig. 5a and 5b using the scatter signal, shown in Figs. 5c and 5d. In Region II, a lot of particles were detected, while in Region I, no particles were observed in the images. This observation was encouraging for further ADC work.
In specifying a classification algorithm to detect the edge region flakes, redeposited edge flakes on the apex side were detected by setting the threshold levels for the reflectivity in the specular channel of the SideScan image. For redeposited defects in the top near-edge region, a combination of signals in the specular and scatter channels gave more accurate detection. Once all the measurement parameters for both areas are fixed, all can be combined in a single measurement recipe. A single measurement sequence on a wafer provides defect classification and mapping for all the wafer edge areas of interest. (Fig. 6).
Immersion process characterization and optimization. Having qualified the inspection to classify and map edge flake defects, we used our results in a design of experiment to improve our understanding of this kind of defect source and its key impact parameters.
As indicated above, Resist A tends to generate flakes when the IH is passing over its edge. The non-optimized coating process left residues for two other resists, resist B and C, however the residue morphology was different.
 Figure 7. Edge flake defects as a function of resist chemistry and EBR recipe. |
When the same immersion exposure job was used, significantly fewer edge flakes were detected in the near top region for Resist B and C than for Resist A (Fig. 7). Moreover, the residual defects were less confined to the exposure zone, so some of these defects might not be related to flaking, but caused by coating and wafer handling. In the TopScan images, no clear sign of damage was seen. Clearly the choice of resist chemistry can be important to prevent these kinds of defects.
As indicated earlier, resist residues can be optimized by changing the EBR recipe on the coat track. Resist A showed several hundred defect flakes with the regular (short) EBR sequence. After optimization, this resist achieved defect values similar to the background values obtained with the non-flaking resists B and C.
Further wafer edge challenges
More kinds of defects besides the edge region flakes can be important in immersion litho. This section discusses other possible defect sources.
Wafer handling marks and resist rework process. A variety of artifacts were seen even in fresh Si wafers, primarily on the bevel and apex region. These wafers had very limited processing and handling. Even on a fresh Si wafer with minimal wafer handling, damage was visible in the form of particles in the apex/bevel region. This introduces an additional concern with transport-related artifacts, and illustrates the need for an assessment of wafer edge quality and handling before introduction to the immersion process.
Resist rework processes. At IMEC, resist work is typically done by a combination of a dry ashing step, followed by a wet clean. In some cases, rework is used to redo a lithography step, for instance after an out-of-spec condition. In other cases, such as on monitoring wafers for (daily) focus/dose/CD or overlay, rework can be done more frequently. Limited rework typically results in an increased presence of scratches (typically at the lower bottom bevel), and an overall increase in reflectivity variation. Where wafers are reworked more often (estimated to be ~10 times or more), the bevel/apex area is much more affected. These defects could pose a risk when the immersion hood is passing over the wafer.
Conclusion
In this paper, we investigated the impact of immersion lithography on wafer edge defectivity. In the past, such work has been limited to inspection of the flat top part of the wafer edge due to the inspection challenges at the curved wafer edge and lack of a comprehensive defect inspection solution. This made it very difficult to detect and control defects on the non-flat part of the wafer edge. Our study used a new automated edge inspection system that provides full wafer edge imaging (top, side, bottom) using laser-based optics and multisensor detection, and then classifies the defects of interest with ADC software.
This technology demonstrated the impact from the immersion hood on wafer edge defectivity. Moreover, the work revealed several key challenges to keep wafer edge-related defectivity under control, including choice of resist, optimization of EBR recipes, wafer handling, and so forth.
Acknowledgments
The authors thank Diziana Vangoidsenhoven, Christie Delvaux, Bart Baudemprez, and Tom Vandeweyer for help in processing and wafer selection; Thomas Hoffmann for help in immersion soak time simulations; and Philippe Foubert, Dieter Van Den Heuvel, Shinichi Hatakeyama (TEL), Kathleen Nafus (TEL), Sean O’Brien (TI), Mireille Maenhoudt, and Richard Bruls (ASML) for helpful discussions on immersion tools and related defectivity. VisEdge is a trademark of KLA-Tencor Corp.
References
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- M. Kocsis et al., “Immersion-specific Defect Mechanisms: Findings and Recommendations for Their Control,” Proc. SPIE, 6154, 6154-180, 2006.
- M. Maenhoudt et al., Jour. of Photopolymer Science and Technology, 19, 585, 2006.
- M. Ercken et al., Jour. of Photopolymer Science and Technology, 19, 539, 2006.
- I. Pollentier et al., Proc. SPIE, 5754, 129, 2005.
I. Pollentier received his PhD in physics engineering from the University of Ghent in Belgium. In 1993, he joined IMEC, where he was initially responsible for the support of lithography in IMEC’s pilot line. In 1996, he moved to the lithography process development department, where he was involved in integrating i-line, 248nm, and 193nm lithography in IMEC’s mainstream semiconductor technologies. Currently, he manages the lithography integration and metrology group within the Lithography Department, where he is responsible for lithography processes and the metrology used at IMEC, Kapeldreef 75, 3001-Leuven, Belgium.
A. Somanchi received his masters in mechanical engineering from Louisiana State U. He is currently an applications development engineer who has been employed with KLA-Tencor for more than three years. He previously worked on Candela top surface inspection wafer tools targeting markets such as LED and CMP. He is currently with the Growth & Emerging Markets Division, where he focuses on the wafer edge inspection product, VisEdge. He can be reached at KLA-Tencor Corp., One Technology Drive, Milpitas, CA 94538 USA.
F. Burkeen received his BS in chemical engineering from Cal Poly Pomona and his MBA/MS in engineering management from Cal Poly San Luis Obispo. He is senior director of marketing for KLA-Tencor’s Growth and Emerging Markets (GEM) Division, where he is responsible for the direction of the company’s growing compound semiconductor wafer business.
S. Vedula received his PhD in chemical engineering from the U. of Tennessee and is senior product marketing manager in KLA-Tencor’s GEM Division with a focus on wafer edge inspection products. He has been employed with KLA-Tencor for more than nine years and has worked on metrology- and inspection-related applications.