sion defects. These systematic defects may indicate a need to review the reticle manufacturing process. High frequency chrome edge roughness will translate into a loss of edge sharpness in the stepper aerial image, thus resulting in a loss of linewidth control on the wafer. With the resolution limits advanced DUV steppers being challenged by today’s rapidly shrinking design rules, poorly resolved reticle features and OPC geometries reduce the wafer process window. Improperly formed OPC geometries have caused critical features to bridge on the wafer and result in device failure. Transmission errors, another important class of defects, will absorb DUV light as it exposes the reticle pattern onto the wafer and distort the IC design. The results of all these types of defects will be low device yield and possible device failure. Considering the large investment in wafer fabs, it is critical to catch these problems before they impact wafer production. One missed defect could result in the loss of millions of dollars — in less than a week. Reticles have emerged as a critical factor in enabling IC manufacturers to accelerate their technology develop-
ment and to extend the life of optical lithography beyond the 0.25 µm generation. Optical technology is approaching production limitations and requires creative photomask design to go beyond 0.18 µm devices. For instance, OPC and phase shift techniques are used to compensate for proximity effects and line-end shortening that occur during wafer exposure. The true challenge will be to keep reticles from becoming a limiting factor in semiconductor manufacturing. Even more aggressive reticle technology will be needed for 0.15 and 0.13 µm devices. If there is not a strong, supportive infrastructure involving manufacturing, inspection and lithography, however, advanced reticles will not be able to keep pace with the accelerated demands of semiconductor technology. circle RS#017
Redefining Reticle Quality Management — RQM Any advanced photolithographer has bad dreams and nightmares. A bad dream would be having a reticle defect print on a wafer and cause low speed binning. A nightmare would be having that same defect cause a yield bust. In some ways, the bad dream could be worse — it can go undetected for weeks or months or result in device failures in the field. There are two ways to help ensure these nocturnal anxieties do not become reality. First, the lithographer could perform wafer image qualification. In this process, the reticle is patterned onto a wafer and reviewed by wafer inspection and metrology tools. However, this process is time consuming, expensive and does not optimize the wafer manufacturing process. Moreover, after the initial test, the reticle is seldom requalified until a yield excursion occurs, to determine fault. The other alternative is reticle quality management (RQM). This comprehensive, integrated system inspects, measures and analyzes the entire reticle cycle — including materials, manufacturing, cleaning and lithography applications. This is a more effective strategy because it detects reticle changes as they occur — not after they print on the wafer. Reticle Inspection checks the reticle for both pattern and
contamination anomalies. The pattern on the reticle is inspected for data gaps, chrome extensions and other pat-
tern mistakes. Any deviation from the circuit design will most likely lead to immediate device failure. Once qualified, the reticle continues to be monitored for contaminants or pattern degradation. Over time or use, particles can move, crystals can form, pellicles can be damaged, or chrome disfiguration can occur — especially from electrostatic discharges. Reticle CD SEM (scanning electron
microscopy) provides precise measurements of critical lines and geometries on the reticle. Accurate linewidths optimize speed performance and maximize economic yield. CD SEM provides additional value by imaging and sizing reticle defects — especially edge roughness, line shortening and optical proximity correction (OPC)-specific defects. Reticle Analysis is the final component of RQM. Analysis provides the vital link between inspection, measurement and results. The information gathered by RQM will help manage the impact of defects, repairs and other reticle quality parameters.
Used together, the integrated RQM strategy can help optimize the lithography process and let the lithographers sleep a little easier at night. circle RS#017 Summer 1998
Yield Management Solutions
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