Wafer mapper measures thin-film thickness

Using a new approach to an old technique, engineers at Hughes Danbury Optical Systems (HDOS, Danbury, CT) have created a powerful thin-film metrology device for silicon-wafer manufacturers that can provide data on entire wafers in less than two minutes. The AcuThin Film Mapper sensor assembly was developed and patented by HDOS; an integrated system is being marketed by ADE Corp. (Newton, MA).

Wafer mapper measures thin-film thickness

Kristin Lewotsky

Using a new approach to an old technique, engineers at Hughes Danbury Optical Systems (HDOS, Danbury, CT) have created a powerful thin-film metrology device for silicon-wafer manufacturers that can provide data on entire wafers in less than two minutes. The AcuThin Film Mapper sensor assembly was developed and patented by HDOS; an integrated system is being marketed by ADE Corp. (Newton, MA).

Silicon-on-insulator (SOI) wafers are generally produced by first growing silicon dioxide (SiO2) on one side of a pair of silicon wafers, then bonding the wafers together with the SiO2 layer in the middle. One side of the resulting sandwich is thinned by machining to obtain a silicon thin film overlaying a SiO2 thin film, all ato¥a silicon substrate. Surface-figure control and rapid convergence in this machining process depend on a clear understanding of existing surface features. High-density surface maps are required, with an optimal data-acquisition rate of 400 points per minute per wafer.

Most available metrology systems measure film thickness at a single point, typically requiring several minutes to complete the operation. Surface-ma¥creation involves scanning the wafer to acquire metrology data at multiple points, and high-density data acquisition rapidly becomes prohibitive in terms of time. Designers of the AcuThin mapper scrapped the point-by-point measurement approach in favor of a full-wafer imaging system. The instrument can acquire metrology data from the entire surface of a 200-mm wafer in less than two minutes.

Mapper design and operation

In the AcuThin instrument, a polychromatic extended source is first spatially filtered and collimated, then spectrally filtered by a wheel containing multiple narrowband filters. The resulting monochromatic beams are used to sequentially illuminate the entire wafer under test. Coherent interactions between light reflected off the front and back of the thin film create an interference pattern at each wavelength. The monochromatic images are captured in series by a two-dimensional CCD camera, then digitized and combined to create a reflectance ma¥of the wafer surface.

Spectral reflectometry, the common tool for thin-film thickness instruments, is based on the measurement of spectral flux reflected from a surface. Given the optical constants for a specific thin film, the surface spectral reflectance varies as a function of film thickness and wavelength. In the AcuThin mapper, the processed data acquired from the wafer under test are compared with either theoretical film-thickness reflectance spectra, or with data from a previously measured reference wafer. The comparison is done pointwise by computer at a high rate of spatial sampling, using parallel processing architecture. A match in reflectance spectra for a given segment of the wafer corresponds to a match in film thickness. This information is processed to create a thickness ma¥of the wafer with as many as many as 4096 data points (see photo).

The enhanced surface-feature resolution offered by this high data density allows manufacturing diagnostic capabilitities that were previously unavailable. Vacuum chuck or tool signatures that would be transparent to 10- or 20-point sampling levels are clearly defined. The volume of data offered by the new system also allows the use of frequency domain tools such as power spectral density (PSD) functions for wafer surface-error analysis. PSDs offer a powerful method for error assessment, fabrication strategy determination, and wafer sell-off acceptance. PSD capabilities for the thin-film mapper are under development at ADE.

The TFM-1, ADE`s integrated sensor and wafer-handling system, is already in production. The first instrument was delivered in February, with three more slated to follow.

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