Optics: measuring and testing – Lens or reflective image former testing
Reexamination Certificate
2001-04-13
2003-06-03
Mai, Huy (Department: 2873)
Optics: measuring and testing
Lens or reflective image former testing
C356S401000, C430S022000
Reexamination Certificate
active
06573986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical metrology and more particularly to characterizing and monitoring the intra-field distortions of projection imaging systems used in semiconductor manufacturing.
2. Description of the Related Art
Today's lithographic processing requires ever tighter layer-to-layer overlay tolerances to meet device performance requirements. Overlay registration is defined as the translational error that exists between features exposed layer to layer in the vertical fabrication process of semiconductor devices on silicon wafers. Other names for overlay registration include, registration error and pattern placement error, and overlay error. Overlay registration on critical layers can directly impact device performance, yield and repeatability. Increasing device densities, decreasing device feature sizes and greater overall device size conspire to make pattern overlay one of the most important performance issues during the semiconductor manufacturing process. The ability to accurately determine correctable and uncorrectable pattern placement error depends on the fundamental techniques and algorithms used to calculate lens distortion, stage error, and reticle error.
A typical microelectronic device or circuit may consist of 20-30 levels or pattern layers. The placement of pattern features on a given level must match the placement of corresponding features on other levels, i.e., overlap, within an accuracy which is some fraction of the minimum feature size or critical dimension (CD). Overlay error is typically, although not exclusively, measured with a metrology tool appropriately called an overlay tool using several techniques. See for example, Semiconductor Pattern Overlay, N. Sullivan, SPIE Critical Reviews Vol. CR52, 160:188. The term overlay metrology tool or overlay tool means any tool capable of determining the relative position of two pattern features or alignment attributes, that are separated within 500 um (microns) of each other. The importance of overlay error and its impact to yield can be found elsewhere. See Measuring Fab Overlay Programs, R. Martin, X. Chen, I. Goldberger, SPIE Conference on Metrology, Inspection, and Process Control for Microlithography XIII, 64:71, March, 1999; New Approach to Correlating Overlay and Yield, M. Preil, J. McCormack, SPIE Conference on Metrology, Inspection, and Process Control for Microlithography XIII, 208:216, March, 1999.
Lithographers have created statistical computer algorithms (for example, Klass II and Monolith) that attempt to quantify and divide overlay error into repeatable or systematic and non-repeatable or random effects. See Matching of Multiple Wafer Steppers for 0.35 micron Lithography using advanced optimization schemes, M. van den Brink, et. Al., SPIE VOL. 1926, 188:207, 1993; A Computer Aided Engineering Workstation for registration control, E. McFadden, C. Ausschnitt, SPIE Vol. 1087, 255:266, 1989; Semiconductor Pattern Overlay, supra; Machine Models and Registration, T. Zavecz, SPIE Critical Reviews Vol. CR52, 134:159. An overall theoretical review of overlay modeling can be found in Semiconductor Pattern Overlay, supra.
Overlay error is typically divided into the following two major categories. The first category, inter-field or grid overlay error, is concerned with the actual position of the overall device pattern imaged into the photoresist on a silicon wafer using an exposure tool, i.e., stepper or scanner as referenced from the nominal center of the wafer, see FIG.
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Obviously, the alignment of the device pattern on the silicon wafer depends on the accuracy of the stepper or scanner wafer handling stage or wafer stage. Overlay modeling algorithms typically divide inter-field or grid error into five sub-categories or components, each named for a particular effect: translation, rotation, magnification or scale (in both x and y directions), non-orthogonality, and residuals. See A Computer Aided Engineering Workstation for registration control, supra.
The second category, intra-field overlay error, is the positional offset of an individual point inside a field referenced to the nominal center of an individual exposure field, as illustrated in FIG.
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. The term “nominal center” means the exact location of the center of a “perfectly” aligned exposure field; this is the same as the requested field center coordinates given to the lithography tool when it is programmed for the job. Intra-field overlay errors are generally related to lens aberrations, scanning irregularities, and reticle alignment. Four sub-categories or components of intra-field overlay error include: translation, rotation, magnification and lens distortion. It is common practice to make certain assumptions concerning the magnitude and interaction of stage error and lens distortion error in modern overlay algorithms that calculate lens distortion. The common rule is: “trust the accuracy of the stage during the creation of the overlay targets by making the simple assumption that only a small amount of stage error is introduced and can be accounted for statistically”. See A “golden standard” wafer design for optical stepper characterization, K. Kenp, C. King, W. W, C. Stager, SPIE Vol. 1464, 260:266, 1991; Matching Performance for multiple wafer steppers using an advanced metrology procedure, M. Van den Brink, et. Al., SPIE Vol. 921, 180:197, 1988.
It is important for this discussion to realize that most overlay measurements are made on silicon product wafers after each photolithographic process, prior to final etch. Product wafers cannot be etched until the resist target patterns are properly aligned to the underlying target patterns. See Super Sparse overlay sampling plans: An evaluation of Methods and Algorithms for Optimizing overlay quality control and Metrology tool Throughput, J. Pellegrini, SPIE Vol. 3677, 72:82, 36220. Manufacturing facilities rely heavily on exposure tool alignment and calibration procedures. See Stepper Matching for Optimum line performance, T. Dooly, Y. Yang, SPIE Vol. 3051, 426:432, 1997; Mix-And-Match: A necessary Choice, R. DeJule, Semiconductor International, 66:76, Feb, 2000; Matching Performance for multiple wafer steppers using an advanced metrology procedure, supra, to help insure that the stepper or scanner tools are aligning properly; inaccurate overlay modeling algorithms can corrupt the exposure tool calibration procedures and degrade the alignment accuracy of the exposure tool system. See Super Sparse overlay sampling plans: An evaluation of Methods and Algorithms for Optimizing overlay quality control and Metrology tool Throughput, supra.
Over the past 30 years the microelectronics industry has experienced dramatic rapid decreases in critical dimension by moving constantly improving photolithographic imaging systems. Today, these photolithographic systems are pushed to performance limits. As the critical dimensions of semiconductor devices approach 50 nm the overlay error requirements will soon approach atomic dimensions. See Life Beyond Mix-and-Match: Controlling Sub-0.18 micron Overlay Errors, T. Zavecz, Semiconductor International, July, 2000. To meet the needs of next generation device specifications new overlay methodologies will need to be developed. In particular, overlay methodologies that can accurately separate out systematic and random effects and break them into assignable causes will greatly improve device process yields. See A New Approach to Correlating Overlay and Yield, supra.
In particular, those new overlay methodologies that can be implemented into advanced process control or automated control loops will be most important. See Comparisons of Six Different Intra-field Control Paradigms in an advanced mix and match environment, J. Pellegrini, SPIE Vol. 3050, 398:406, 1997; Characterizing overlay registration of concentric 5× and 1× stepper Exposure Fields using Inter-field Data, F. Goodwin, J. Pellegrini, SPIE Vol. 3050, 407:417, 1997. Finally, another area where quantifying lens dist
Hunter, Jr. Robert
McArthur Bruce
Smith Adlai
Litel Instruments
Mai Huy
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