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The purpose of this paper is to define standard methods for effective and efficient image-based dimensional metrology for microlithography applications in the manufacture of integrated circuits. This paper represents a consensual view of the co-authors, not necessarily in total agreement across all subjects, but in complete agreement on the fundamentals of dimensional metrology in this application. Fundamental expectations in the conventional comparison-based metrology of width are reviewed, with its reliance on calibration and standards, and how it is different from metrology of pitch and image placement. We discuss the wealth of a priori information in an image of a feature on a mask or a wafer. We define the estimates of deviations from these expectations and their applications to effective detection and identification of the measurement errors attributable to the measurement procedure or the metrology tool, as well as to the sample and the process o fits manufacture. Although many individuals and organizations already use such efficient methods, industry-wide standard methods do not exist today. This group of professionals expects that, by placing de facto standard meth-odologies into public domain, we can help reduce waste and risks inherent in a "spontaneous" technology build-out, thereby enabling a seamless proliferation of these methods by equipment vendors and users of dimensional metrology. Progress in this key technology, with the new dimensional metrology capabilities enabled, leads to improved perform-ance and yield of IC products, as well as increased automation and manufacturing efficiency, ensuring the long-term health of our industry.

Traditionally, overlay metrology has ben primarily used for quality control of developed wafers. The secondary roles of overlay metrology, stepper modeling and feedback control, have become more prominent in recent years. In particular, the design of overlay sampling plans is usually strongly influenced by the requirements imposed by stepper modeling. Previous research has shown that intrafield sampling plans must be symmetric and repeating in order to support stable feedback control. These constraints impact the tradeoff between overlay tool throughput and quality control by imposing a higher than necessary cost on measuring additional exposure fields for irregular errors. In this paper we examine the concept of using super sparse overlays among plans along with innovative analysis methods and algorithms for stepper modeling. Super sparse sampling plans are defined as where the field by field intrafield sampling is irregular and asymmetric. When compared to typical sampling plans, super sparse plans allow the overlay tool to examine a 2 to 4 times the number of fields on a wafer with the same number of total measurements. In order to support super sparse sampling, it is necessary to employ innovative algorithms, which are capable of performing stepper modeling without losing result integrity. In this paper we report on the relative performance of regular versus super spares sampling plans for quality control, stepper modeling and overlay tool throughput.

The introduction of DUV step-and-scan exposure tools into a mix-and-match manufacturing environment with traditional i- line step-and-repeat systems has presented many unique challenges to lithographic process engineers. One of these challenges has been the development and selection of reliable methods for controlling intrafield patten overlay registration. We examined a spectrum of overlay control methods and compared the benefits and costs of each. We analyzed six different intrafield overly control approaches including: (1) traditional static tuning to a fixed archive wafer; (2) dedicated stepper routing; (3) static LEMSYS matching; (4) static cluster sorting; (5) global feedforward control, and; (6) combined cluster sorting with feed-forward control. In traditional static tuning, each stepper is calibrated to an arbitrary fixed reference and wafers are allowed to flow freely within the entire stepper population. Dedicated stepper routing imposes a restriction that wafers must return to the same stepper during critical layer processing. With static LEMSYS matching each stepper is calibrated to an ideal reference that is generated to minimize higher order intrafield errors. Static cluster sorting uses LEMSYS data to divide steppers into clusters then critical layer exposures for a given wafer are kept within a local cluster. Feedforward control attempts to aggressively adapt magnification offsets based upon the known lens signatures of steppers used to print previous layers. Our comparisons were based upon data from 12 actual exposure systems. The result showed that significant gains in overlay control can be achieved with incremental costs in dollars and complexity. Cost-benefits analysis showed that the more aggressive control techniques, involving feed- forward control, were best suited to large fabs operating near the physical limits of their steppers.

the cost advantages associated with implementing a mix-and- match photolithography process have led to a dramatic increase in the interest and development of these manufacturing environments. This is especially true for older fabs with high production lithography tools already in place but technology that has increased beyond the capability of the tools. For he process engineer the challenge is to define a method of optimizing the exposure field registration between each of the different imaging systems. In this paper a procedure used to evaluate intrafield and interfield overlay errors between six ASML 5X steppers and sixteen Ultratech 1X steppers is described. With this technique reticle data, stage registration and a commercially available software analysis package are used to model pattern displacement of each stepper within this population. Wafers from each steppes are first patterned with nine fields, each consisting f a 9 by 9 array of ASML alignment marks. The X and Y stage coordinate of each alignment mark is then measured using a standard ASML 5500/60 intrafield analysis routine. Spreadsheeting the resulting stage registration data, subtracting the expected or 'ideal' stage position and correcting for any reticle pattern shifts, grid and intrafield data are obtained. Using this process a data sheet for each stepper was developed and, once formatted properly, loaded onto the software analysis package for registration modeling. Use of multiple exposure fields per wafer enabled the software to characterize both intrafield and interfield registration by first modeling the grid errors, subtracting these values, and then performing intrafield analysis of the remaining data. Further, by collapsing the intrafield data into a single field a 'lens fingerprint' of each stepper lens was derived. Using vector subtraction a direct comparison was made between the lenses of each stepper and an indexed table of exposure field translation errors created. The stepper lenses were also sorted from best to worst matches. This approach generated the required 231 paired data sets needed to match each stepper to all others while exposing and measuring only 44 wafers and required no artifact wafers. Measured evaluation results will be reviewed and expansion of this procedure to mapping 1X wide field lenses and matching of non-concentric exposure fields discussed.

In this study, a large field 1x stepper was matched to an advanced 5x reduction stepper using a 2:1 field matching scheme. The 1x field is a 44 X 22 mm rectangle that is symmetrically aligned to two 22 X 22 mm 5x reduction fields. Overlay measurements were collected at 33 sites per reduction field (or 66 sites per Ultratech field) and the resulting data was analyzed using a modified grid registration model that fully supports the 2:1 matching geometry. Two complementary optimization techniques were developed, the first of which assumes corrective action only on the 1x stepper. The more sophisticated approach supports corrective action on both the 1x and 5x reduction stepper. Next, both techniques were applied to the measured mix-and-match data with the results suggesting a specific set of corrective action that could be applied to the 1x and 5x reduction steppers. Based on these results, it was found that there is a substantial registration benefit to exerting simultaneous corrections on both stepper types as opposed to controlling each stepper individually.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The behavior of the focus-exposure window for a half micron process was analyzed across a range of resist thicknesses. The range examined was selected to encompass a minima through maxima region of the I-line swing curve. This analysis is shown for an I-line resist; the undyed version contrasted to the dyed. The process tube characterization method calculates the common corridor exposure latitude for the two resist data sets. The common corridor for a given resist system defines that area in the focus-exposure plane that meets a single process specification for all the resist thicknesses in the selected swing curve range. In this study the undyed resist demonstrates a superior process window at a given resist thickness but is found to be inferior to the dyed resist as determined by the process tube characterization method.

Our goal for the CD metrology process is to understand the influence of defocus and illumination latitude upon measured CDs and to optimize the focus and illumination settings for smallest measured CD range. For consistent submicron optical metrology process, the measured CD range should be better than +/- 10 nm (6S). The defocus and illumination latitudes are generated when the features are measured through focus at different illuminations. The process step is the factor that sets the conditions for different illumination modes. Traditional illumination modes in mask metrology are transmitted and reflected. The confocal reflected illumination mode adds a new dimension to the standard reflected light because of the increase in both transversal and longitudinal resolutions. The combined illumination between transmitted and reflected confocal modes proposed here, provide increased feature edge gradient and decreased intensity background noise. To emphasize the effects of the illumination modes, an algorithm is developed for CD metrology optimization using traditional and combined illumination techniques.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Lens performance parameters have been traditionally described using terms such as resolution, astigmatism, field curvature, proximity effects, and distortion. However, with decreasing geometries, decreasing exposure wavelengths and tighter bandwidths (such as 248 nm excimer laser lithography), lens characterization in terms of depth of focus (DOF) and exposure latitudes for the entire lens becomes more critical. The challenge is to define a focus and exposure setting that allows one to operate in the `common corridor.' `Common corridor' is defined as the resulting focus-exposure process window for the entire field of the lens encompassing all geometries of a specific line size. A modification to the MONO-LITH software package allows this calculation to be done quickly and easily.