Film thickness mapping using interferometric spectral imaging

Details Film thickness mapping using interferometric spectral imaging Film thickness mapping using interferometric spectral imaging

1997-01-21

1999-01-05

Kim, Robert

Optics: measuring and testing

By particle light scattering

With photocell detection

356359, 356357, G01B 902

Patent

active

058568719

DESCRIPTION:

BRIEF SUMMARY
FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for spectral analysis of images to determine the thickness of a thin film, and particularly for spatially resolving film thicknesses of a coating deposited over the surface of a silicon wafer or other similar materials (for example, a flat panel display).
Measuring film thickness by reflectance spectroscopy is well known: see for example P. S. Hauge, "Polycrystalline silicon film thickness measurement from analysis of visible reflectance spectra", J. Opt. Soc. Am., No. 8, August 1979, and the book by Milton Ohring: "The material science of thin films", Academic Press Ltd., 1992. In today's microelectronic device manufacturing processes, the uniformity of the deposited films over a wafer is gaining importance as time goes on, because a good uniformity insures identity among the finished product chips. The size of the chips is also decreasing, so that the uniformity tolerance is becoming stricter. In addition, to insure high yield (low rejects) and efficient (low cost) manufacturing, the wafer inspection requires higher automation, shorter time, higher accuracy, and wider thickness range.
As a result, the film thickness map of the wafer, as one of the many inspections done during the manufacturing, must be done accurately, fast, on a large number of points (test sites), and at a wide thickness range.
Today, film thickness mapping instruments are based on ellipsometry or on reflectance spectroscopy. Only the latter is addressed herein. As examples of prior art in this field we mention the SpectraMap SM-300 and the FT-500 of Prometrix. The spectra are measured point by point by moving the wafer on a translation stage, in order to complete one thickness map. This takes time, it requires high movement accuracy, because of the high spatial resolution required, and increases the wafer handling, which is practical only when the wafer is outside a deposition chamber (therefore it cannot be done in-situ). In fact, the fastest thickness mapping mentioned by present manufacturers of film thickness equipment is hundreds of points in a few seconds.
There is thus a recognized need for, and it would be highly advantageous to have a method and apparatus for determining the spatial distribution of the thickness of a film overlying a substrate, more quickly, with higher spatial resolution (more test sites), without the need to move the wafer with respect to the measuring instrument when going from a test site to another (higher accuracy, and less wafer handling with the potential for in-situ monitoring), and easily measure the widest thickness range possible.
The present invention relates to a method and apparatus for mapping film thickness on Silicon wafers or similar substrates, which does not require moving the wafer (making the results faster and spatially more accurate, and potentially capable of being done in-situ), reaching tens of thousands of pixels in a few seconds (not hundreds as stated in the present commercial literature), and which has the potential to measure, in the same time as other potentially competing methods (mentioned below), a wider thickness range.
A spectrometer is an apparatus designed to accept light, to separate (disperse) it into its component wavelengths, and detect the spectrum. An imaging spectrometer is one which collects incident light from a scene and analyzes it to determine the spectral intensity of each pixel thereof.
The former measures the spectrum only at one point, therefore, with such an instrument, the wafer must be moved point by point relative to the instrument, and will have the above mentioned drawbacks of long measurement time and position accuracy.
The latter, i.e., an imaging spectrometer or spectral imager, can be of different types: a technology similar to the one used for resource mapping of the earth surface from airplanes and satellites could be used for film thickness mapping (see, for example, J. B. Wellman, Imaging Spectrometers for Terrestrial and Planetary Remote Sensing

REFERENCES:
patent: 5341205 (1994-08-01), McLandrich et al.

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