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Finding a New Generation of Random and Process Defects on Advanced Reticles

As integrated circuit design rules move below 180 nm, the introduction of UV inspection brings to the surface new concerns with the reticle manufacturing process.

Recently, KLA-Tencor introduced ultraviolet inspection for reticles, and there are signs that the defects detected by this capability will revolutionize the wafer lithography and photomask industry in a manner similar to the introduction of STARlight inspection in 1995. STARlight represented a major advancement in reticle contamination inspection. Unlike the previous darkfield laser scattering technology, brightfield Simultaneous Transmitted And Reflected Light (STARlight) is used to detect submicron contamination defects — such as stains, dust and transmission errors. These new capabilities revealed a quality gap in the reticle cleaning and monitoring process.

Photomask manufacturers were able to detect significantly greater numbers of contaminants on the chrome surface. Wafer fabs saw evidence that deteriorating reticles were linked to poor wafer performance and yield excursions in their lithography process.

Within two years the added inspection capabilities motivated photomask manufacturers to implement improved manufacturing, cleaning and handling processes. A new integrated approach, reticle quality management (RQM) strategies, was implemented to continuously monitor reticles [see adjacent article “Redefining Reticle Quality Management — RQM”].

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Summer 1998

Yield Management Solutions

With advanced reticles being used for design rules of 180 nm and below, lithographers will need a reticle inspection tool with greater sensitivity than has been previously available. UV inspection, designed with a 364 nm laser, provides 150 nm sensitivity, an improvement of 50 nm over existing technology. This heightened capability will allow lithographers to see new levels of random and process defects. A new inspection technology may once again impact the reticle manufacturing process.

UV inspection is a much-anticipated technology for advanced photomask manufacturing. Narrower design rules can only be inspected with smaller pixels. New pattern enhancement techniques, such as optical proximity correction (OPC) and phase shifting, are further complicating photomasks. DUV stepper imaging is far less tolerant of reticle errors, which will increasingly image onto the wafer.

During preliminary evaluations, UV inspection found both random pattern defects and systematic defects in the reticle and lithography process. UV inspection showed significant improvement in detecting programmed defects on Verimask and SEMI-standard test reticles designed to calibrate inspection performance on traditional photomask errors such as chrome spots, pinholes, edge defects and CD errors.

UV inspection also captured process defects, such as edge roughness, poorly resolved corners and transmis-

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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