With the bulk of lithography equipment users searching for effective methods to push the resolution limits of their steppers, a test reticle that implements patented phase-shift technology is making it possible to rapidly and accurately analyze focus-related anomalies, characterize system and lens performance, and extend system capabilities to enhance overall tool performance. Use of this test reticle is so widespread today that it is used with all major brands of overlay measurement tools; one major supplier of such equipment has incorporated the reticle into its capabilities, and a variety of software applications call for its exclusive use.
Focus is one of the most critical parameters in submicron optical lithography. As minimum feature sizes shrink, corresponding focus effects become more prevalent, directly resulting in reduced stepper performance. When resolution falls below 0.35µm or exceeds the specified limits of the lithography system, the slightest miscalculation of optimum focus can manifest itself more readily since the allowable range of variation and error is tight.
Effective characterization and control of focus effects in a stepper results in increased production and higher yield. As a result, selection of the method used for evaluation of focus-related effects is important.
Several methods are used for collection and evaluation of focus effects on a stepper. Most require a focus-exposure matrix that calls for multiple exposure and focus settings in order to calculate best focus. An ideal method would provide focus data that is insensitive to exposure, thus eliminating the need for this matrix for focus determination and, resultantly, exposure of multiple die.
Evaluation of the data gathered using most focus collection strategies is also subject to operator interpretation since wafers must be inspected either optically or using SEMs. Such results are not highly repeatable. An optimum method would allow for automated collection and evaluation of data using overlay measurement tools to increase reliability and repeatability of the results and decrease the amount of time required to characterize the data. Finally, some techniques rely on the stepper itself to measure focus offset, meaning that an independent technique would still be periodically required to calibrate the system.
Phase-shift focus monitor reticle
Figure 1. Data gathered using the PSFM reticle showed a) a significant autoleveling problems that was b) fixed with a system software correction and sent to customers.
Another focus evaluation method uses the Phase Shift Focus Monitor reticle (PSFM reticle). This method provides several advantages over other techniques:
- It provides data over several points within a single field without the need for multiple exposure settings (and therefore a focus-exposure matrix). A real-time snapshot of focus is gathered, and the time required collecting such data is reduced since several die are not required.
- Data acquired using the PSFM reticle allow automated collection of focus data and are compatible with all presently available overlay measurement tools. Operator interpretation is eliminated, thus increasing repeatability. The time required for evaluation and characterization of the data is substantially reduced with the use of these tools.
- PSFM reticles are available for all existing stepper-lithography tools; this allows data to be readily evaluated across a variety of models, manufacturers, reduction ratios, and lens types.
- It allows rapid characterization of several types of focus errors to evaluate the effects of the stepper lens, autofocus and autoleveling subsystems (Fig. 1), and wafer-chuck flatness.
The PSFM reticle is primarily used to determine and regularly monitor best focus (see "The Phase Shift Focus Monitor reticle" sidebar). In this application, substantial time is saved since automated metrology tools can collect and evaluate the data. For example, regular use of the PSFM reticle at Advanced Micro Devices (AMD) Fab 25 for comprehensive collection of focus data has decreased the time required from 6 hrs/wafer (using SEM evaluation) to less than 30 min.
The PSFM reticle also gathers several other types of focus-related data, making it an effective tool for evaluation of new or existing equipment. Regular use of the PSFM reticle in this application has proven effective in extending the capabilities of lithography-tool performance.
Both lithography manufacturers and users utilize the PSFM reticle. Most major US chip makers, as well as several others throughout the world, use the PSFM reticle in a variety of applications. It is compatible with all brands of stepper-lithography equipment; data have been effectively collected on ASML, SVGL, Nikon, Canon, Ultratech Stepper, and GCA-ISI systems (the latter acquired by Ultratech Stepper).
An extensive library of data has been gathered regarding performance of the PSFM reticle. Results have been presented at a variety of symposia, including those held by IEEE, BACUS and SPIE, as well as several lithography-manufacturer user groups. To date, over 25 papers or articles have been published detailing its use and applications.
Characterization of autofocus, autoleveling errors
The PSFM reticle has been successfully applied to rapidly characterize errors in autoleveling and autofocus deviations. Since the data provide a real-time snapshot of autofocus-autoleveler performance using a single exposure, offsets can be quickly determined to establish the lithography tool`s repeatability. By analyzing the data from several exposures, the performance of these subsystems can be characterized over time.
In 1997, the PSFM reticle was used by AMD to evaluate problems on some of the company`s lithography tools at Fab 25 in Austin, Texas; a substantial yield loss was exhibited on the outer 5-6mm periphery of the wafers exposed . Data acquired using the PSFM reticle revealed a significantly large focus deviation - as much as 1.4 µm at the wafer edge. As a result, the lithography-tool manufacturer revised its autofocus software. The same tests used to gather the first round of data were then repeated (see Fig. 1b), and a considerable decrease in focus variation was observed. As a result, the lithography-tool company released the software to all of its users.
Further studies conducted by Benchmark Technologies in collaboration with users of other types of lithography equipment yielded similar results, confirming that the PSFM reticle can be successfully applied to rapidly characterize errors in autoleveling and autofocus subsystems.
Characterization of lens effects
The PSFM reticle has proven extremely effective for providing detailed characterization of lens effects, including astigmatism, aberrations, and heating effects.
Figure 2. Lens heating evaluation; 300nm focus change over 3 wafers
Lens heating is rapidly evaluated using the PSFM reticle. Figure 2 shows data gathered during exposure of three wafer lots on a 0.48 i-line stepper using a 16 mm x 11 mm field size at nominal focus and dose.
Measurements were taken at the center of each field. The plot of focus error versus exposure indicates that lens heating moved focus by about 350 nm, while astigmatism remained constant. Notice that the first and last few exposures of each wafer (see Fig. 2) have significant focus deviations, apparently due to being on or near the edge of the wafer.
Figure 3. A measure of lens field curvature. (Source: KLA-Tencor)
Curvature is characterized by measuring numerous points throughout each field and plotting a 3D map that details either concave or convex results. In Fig. 3, KLA-Tencor`s PSF (phase shift focus) software is used to collect the PSFM data to characterize system optics; this is done by calculating the lens field curvature after tilt data are removed (Fig. 3).
Astigmatism is calculated using the PSFM reticle by measuring numerous points across the field and plotting the difference between vertical and horizontal focus (dZx - dZy).
Characterization of wafer and chuck flatness
Nonflatness of the wafer can manifest itself as a variety of errors, such as tilt (see autoleveling, above) and chuck flatness. Using the PSFM reticle, data provide useful information about wafer and chuck flatness by measuring the error from several points throughout the field.
Compatible software and hardware
The PSFM reticle allows data to be automatically collected and analyzed by several overlay measurement tools, including those from KLA-Tencor, Biorad, IVS (acquired by Schlumberger), and OSI (acquired by Nanometrics). Standard software available on these systems can be used for rapid evaluation of focus data collected. In addition, several software applications are currently available that work exclusively with the PSFM reticle and the types of data it collects. These include KLA-Tencor`s (San Jose, CA) PSF software, TEA Systems` (Alburtis, PA) Focal Plane Analysis (FPAex) software, and New Vision Systems` (Cambridge, MA) Phase Shift Focus Viewer software.
KLA-Tencor`s PSF is an optional application available for its KLASS software, that enables calculation and evaluation of data collected exclusively using the PSFM reticle. Results are mathematically modeled and provided on focus, lens characteristics (Fig. 4), and wafer flatness. Data are presented in vector plots, 3D graphs, and x-y plots, and can be archived over time to evaluate long-term performance of lithography tools. PSF is also available for use on tools other than those manufactured by KLA-Tencor.
FPAex software available from TEA Systems and Phase Shift Focus Viewer from NVS provide similar results on focus, field curvature, field tilt, lens astigmatism, lens heating, lens coma, spherical aberration, and a variety of other focus-related data collected using the PSFM reticle. Report generation, spreadsheet output, vector plots, 3D graphs, and x-y plots can also be generated. Both applications are available for use on most metrology tools.
Use of the PSFM reticle helps enhance lithography-tool performance, evaluate new steppers, and extend the capability of new or existing equipment. Substantial time is saved with its use, since data from a single exposure can be collected using automated overlay measurement tools for rapid and accurate evaluation. It is compatible with presently available metrology tools, and several software applications have adopted exclusive use of the PSFM reticle to graphically characterize and display the data collected.
The PSFM reticle has been successfully used with all stepper-lithography systems, enabling evaluation of system and lens performance across a variety of tools.
Phase Shift Focus Monitor and PSFM are trademarks of Benchmark Technologies Inc. The phase-shifted technology used in the PSFM reticle is patented by IBM and exclusively licensed to Benchmark Technologies.
The authors thank the following for their contributions: Patrick Lord of KLA-Tencor for providing the PSF figures presented in this article; Ric Edwards and Paul Ackmann at AMD for providing the corresponding autofocus and autoleveling data and figures; Terry Zavecz at TEA Systems for data and assistance; and Susan Riley of Tech Text for editorial work.
1. R. Edwards, et. al., "Characterization of Autofocus Uniformity and Precision on ASML Steppers using the Phase Shift Focus Monitor Reticle," presented at SPIE Microlithography, March 1997.
2. D. Wheeler, et. al., "Phase Shift Focus Monitor Applications to Lithography Tool Control," presented at SPIE, March 1997.
Craig Sager has a BS in photographic science and instrumentation from Rochester Institute of Technology. His industry experience spans various positions, including manager of technical marketing and manager of wafer stepper applications at GCA. In 1986, he formed Benchmark Technologies, which designs and supplies test reticles for photolithographic applications. Benchmark Technologies Inc., 7E Kimball Lane, Lynnfield, MA 01940; ph 781/246-3303, fax 781/246-0308, www.benchmarktech.com.
Patrick Reynolds has held a variety of positions in photolithography engineering and applications at both GCA and Analog Devices. Reynolds cofounded Benchmark Technologies and, in 1990, joined the company as vice president. He has authored numerous papers in the areas of photolithography, its applications, and the use of Benchmark`s PSFM reticle.