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Inspection of Advanced OPC Reticles During the July 1998 YMS2, DuPont Photomask (DPI) presented its evaluation of OPC techniques and the inspection capabilities of KLA-Tencor’s new 353UV reticle inspection system. In the results, DPI emphasized the importance of OPC technology and the 353UV’s superior ability to inspect these features
Simply defined, optical proximity correction (OPC) techniques consist of incorporating features in the production of photomasks that neutralize optical distortion when a semiconductor design pattern is transferred to silicon. The value and use of OPC technology is growing because of the ongoing progression to increasingly smaller semiconductor design features. Semiconductor manufacturers are recognizing that OPC technology can extend the life of lithography steppers by extracting the maximum resolution performance possible from their equipment.
OPC addresses many issues OPC features range in complexity from simple edge jogs and line size adjustments to more complex, isolated mixed-tone features that are embedded in both the clear and chrome areas of the photomask. Serifs and hammerheads — squares and rectangles — are the most common OPC features on the photomask and are added to the end of a design line to prevent corner rounding and line-end shortening. Another technique involves adding assist features that change the way design features behave during the lithogra-
Figure 1. An oversized OPC feature can result in bridging on the wafer pattern.
phy process. For example, some OPC software packages automatically add scatter bars to isolated design features, causing those structures to print in the same manner as more dense design areas to produce a uniform result during the semiconductor manufacturing process. The features are too small to print during the lithography process, but preserve design integrity by increasing the resolution achieved by the stepper.
Monitoring the effects of OPC OPC features generated on a photomask are usually between 20 and 50 percent of the size of design features, requiring advanced production equipment to complete the manufacturing process successfully. Just as
Autumn 1998
properly implemented OPC features improve the pattern accuracy, improper OPCs can create pattern complications. For instance, a missing or under-sized serif can cause significant drawback, while oversized serifs can result in bridging on the wafer (figure 1). Inspection technology is being pushed to new levels of functionality by the use of OPC. Although these features are sub-resolution, they still must be inspected and verified. As part of a strategic alliance, DPI and KLA-Tencor are working closely together to test advanced photomask inspection technology and new software algorithms that enable pattern recognition of advanced OPC fea-
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353UV, AOP algorithm, 0.18 µm
351, APA algorithm, 0.25 µm
Figure 2. The 353UV significantly improves defect sensitivity over previous inspection systems.
tures, as well as identify OPC defects. Recently, DPI evaluated and characterized KLA-Tencor’s new Ultraviolet UV inspection system, the 353UV. During the evaluation, DPI engineers used both standard test plates and specially designed OPC reticles to characterize the new 0.18 nm pixel inspection, AOP algorithm, and defect highlighting tools.
System evaluation The 353UV demonstrated improved defect sensitivity on OPC features as a result of: 1) shorter wavelength, 2)
smaller pixel size, and 3) improved detection algorithms. Defects among serifs, hammerheads and assist lines were detected in isolated and dense geometries. Additionally, the 353UV system also showed improved sensitivity on UV-opaque defects and better resolution on DUV EA phase shift masks (figure 2). DPI engineers also realized that improved review tools are required to accurately classify the smaller defects. New review features such as edge sharpening and pixel displacement helped to classify 50 nm localized gate CD errors and process defects.
Inspection issues are closely linked to a photomask producer’s ability to generate features with sufficient clarity. The better a feature image is resolved on a photomask, the easier it becomes to do pattern inspections, saving valuable cycle time. The interdependent nature of production and inspection technology ensures that the development process will be ongoing, with advancements in production capabilities necessitating new developments in inspection technology.
KLA-Tencor Fall-Winter ’98 Trade Show Calendar
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October 19-21
ITC – International Test Conference, Washington, DC, USA
November 4-6
SEMICON Taiwan, Taipei, Taiwan
November 16-17
VDE/VDI Conference, Munich, Germany
December 1-3
Fall MRS, Boston, MA, USA
December 2-4
SEMICON Japan, Makuhari, Japan
Autumn 1998
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