Editorial
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Overlay in the New Reality Years ago, when I first began working in lithography, there were two kinds of litho engineers: CD engineers and overlay engineers. The CD engineers cared about critical dimension control. They worried about the resist process and stepper dose and focus, and tried to make sure the resulting wafer CDs stayed in spec. They spent a lot of time on a CD SEM, complained about reticle quality, and tried to live with smaller and smaller process windows. The overlay engineers just worried about overlay. They kept the stepper’s wafer and reticle stages working well to make sure that every printed pattern properly overlaid with the wafer’s previous patterns. Using optical overlay metrology tools the overlay engineers regularly made fine adjustments to the stepper to keep each wafer lot in spec. They compared the distortion performance of one stepper with another and figured out how to mix and match different stepper types or models for best performance. The CD engineers and the overlay engineers rarely talked to each other: they didn’t need to. All of the many fine adjustments that the overlay engineers made to the stepper had no discernable impact on critical dimensions. Adjustments to the resist process or stepper dose and focus did not impact overlay enough to worry about. For all practical purposes, the work of these two engineers was orthogonal. And they remained blissfully ignorant of each other’s worlds. Certainly the CD specifications and the overlay specifications were combined to create the design rules that governed how transistors could be packed together to make a chip, but the combination required only simple geometry, and CD and overlay remained independent.
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Summer 2003
Yield Management Solutions
Things began to change in the last five or eight years. To help live with smaller depth of focus, chemical mechanical polishing flattened the wafer surface and resulted in significantly better CD control. This same CMP process, however, made overlay targets more difficult to measure, and the resulting overlay corrections sometimes less reliable. Design of the overlay targets became dependent on processes designed to enhance CD control. People began experimenting with alternatives to the standard box-in-box overlay measurement targets and carefully looked at how CMP process flows affected stepper alignment and overlay measurements. Today we are pushing the requirements for overlay performance to levels that were hard to even imagine a few years ago. In the regime of 65 nm to 90 nm node processes, total overlay requirements drop below 25 nm. With numbers as small as these, every nanometer of overlay error becomes significant. And a new trend is emerging. Some of the same sources of CD errors are also causing overlay errors. Aberrations in the stepper or scanner cause pattern placement errors that vary across the field and are different for different pattern types. The impact of processes such as CMP on measurement targets is dependent on the exact shapes and sizes of the targets. Even focus and exposure errors can affect overlay results and overlay measurements. And the use of non-perfect phase shift masks combine with all of the above errors in very complex ways to impact pattern placement.
Yield Management S O L U T I O N S
E D I T O R- I N - CH I E F Uma Subramaniam MANAGING E DITOR Siiri Hage S E N I O R ED I T O R Aparjot Dehal
Critical dimension performance and overlay performance are no longer separate, independent quantities. The days when the CD engineer did not need to talk to the overlay engineer are gone. Today it is a new reality, an unavoidable complexity, where all aspects of lithographic imaging are related and interdependent. New tools, processes, and approaches are required to tackle the challenges now facing us. And metrology is no longer a passive add-on to the process flow of making a wafer, but an integral part of manufacturing. In this issue of Yield Management Solutions magazine you’ll see some of the steps in this direction being taken by our customers.
C ONTRIBUTING EDITORS David Moreno Tom Salinas ART DIRECTOR AND P RODUCTION MANAGER Carlos Hueso D E S I G N C O N S U LTA N T Terry Rieckhoff Mike Garnica C I R C U L AT I O N E D I T O R David Viera
KLA-Tencor Worldwide C O R P O R AT E H E A D Q U A RT E R S
KLA-Tencor Corporation 160 Rio Robles San Jose, California 95134 408.875.3000 I N T E R N AT I O N A L O F F I C E S
KLA-Tencor France SARL Evry Cedex, France 33 16 936 6969
Chris A. Mack Vice President, Lithography Technology
KLA-Tencor GmbH Munich, Germany 49 89 8902 170 KLA-Tencor (Israel) Corporation Migdal Ha’Emek, Israel 972 6 6449449 KLA-Tencor Japan Ltd. Yokohama, Japan 81 45 335 8200 KLA-Tencor Korea Inc. Seoul, Korea 822 41 50552 KLA-Tencor (Malaysia) Sdn. Bhd. Johor Bahru, Malaysia 607 557 1946 KLA-Tencor (Singapore) Pte. Ltd. Singapore 65 782 6788 KLA-Tencor Taiwan Branch Hsinchu, Taiwan 886 35 335163 KLA-Tencor Limited Wokingham, United Kingdom 44 118 936 5700
Summer 2003
Yield Management Solutions
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