System Maps Wafers Using Space Technology
Rapid measurements to characterize 300 mm process tools and thin films can be done with whole-wafer scan.
Alexander E. Braun, Associate Editor -- Semiconductor International, 7/1/1998
Alexander E. Braun Associate Editor
Precision measurement of thin films to a few thousand angstroms is becoming routine in semiconductor processing, particularly with the coming of 300 mm. Although a constant awareness of film thickness on wafers is essential for process control, this kind of metrology can be time-consuming, particularly if these data are needed to characterize new processes and tools for 300 mm.
Some time ago, IPEC Precision (Bethel, Conn.) developed its ÅcuMap tool. Originally intended for measuring the thickness of layers used in SOI wafers by imaging the entire wafer, eliminating the need for a stage to measure from one spot to another, it is now finding new applications in 300 mm.
Digital imaging camera technology originally developed by Kodak for resolving the Earth's surface from orbit is at the system's heart. It uses a CCD with a 2048 x 2048 pixel array as well as light from a halogen lamp passed through a 24-filter wheel, each representing a discrete wavelength. The light from the filters illuminates the entire wafer and is captured digitally as it reflects from a precisely adjusted mirror. The reflective spectrum of the wafer surface is measured simultaneously at up to 4 million sample points (pixels), and these spectra are matched to stored values to determine film thickness. It takes 90 sec to capture and process up to 33,000 points.
Today's applications go beyond the measurement of SOI bonded wafers. They include thickness measurements of dielectric thin films -- oxide, photoresist or any other semitransparent dielectric.
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| The ÅcuMap system provides several types of displays from a single measurement. The examples shown here are a 2-D color thickness plot and a 3-D wire grid map. (Source: IPEC Precision) |
CMP for 300 mm requires different pad configurations and process technology. Being able to see across the wafer's entire surface (see Figure), getting 3-D, color displays of film topography all the way out to the edge, becomes a definite advantage, particularly considering that the SIA Roadmap shows the edge exclusion specification decreasing from 3 mm to 2 mm by next year and to 1 mm in the future.
The system provides a selection of different displays and statistics, depending on the kind of information being sought, across the whole wafer, instead of just discrete data points. This capability is valuable when running a resist track, because it makes it possible to fine-tune dispensers; dispense positions, angles and spin speeds; and immediately see the results on the resist film. Resist tolerances are much tighter, and defects are more subtle, for 300 mm. With resist coat specs now in the ±10-15 Å range, a system with the capability of a 0.5 Å resolution programmable in 0.5 Å increments assumes new importance.
Although an optimum resource for process and tool characterization, ÅcuMap is a complementary tool: It is not designed to measure specific features or sites, for it lacks a pattern recognition and cannot be trained to go to a specific point. Its purpose is to provide a visual "big picture" view across the entire wafer, which is useful for characterizing a wide range of processes. In the case of CMP process characterization, for example, if there is a minor crease in the backing pad or any other problem with the process, this is readily evident in the visual color display, as opposed to just looking at number tables. Any process problem -- such as one at the notch, at the edge itself or someplace else on the wafer, such as the wafer's exact center -- can be quickly identified.
Because the system maps the entire wafer -- rather than a relatively small number of data points -- it provides a statistically significant confidence level in the gathered data, ensuring that when a process decision is made, local defects and problems have not been missed or overstated. The system is not designed for patterned wafers and has a relatively large spot size, so it is not a total solution. It is a complementary niche tool that can help characterize new processes and tools much more rapidly than attempting to take thousands of points with more conventional point-by-point metrology tools.
