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Non-Destructive, Non-Contacting Test of Si Wafers by Thermore-Flectance

IP.com Disclosure Number: IPCOM000038694D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 8 page(s) / 78K

Publishing Venue

IBM

Related People

DiStefano, TH: AUTHOR [+4]

Abstract

This is a method of testing of silicon wafers by means of thermoreflectance measurements wherein an energy beam, preferably in the form of a laser beam, is used as a pump to energize specific points on a sample under test (e.g., a semiconductor wafer), causing low levels of heating of the sample at the point where the energy beam is directed, which is at such a low level of energy that it is nondestructive. Then an additional wave comprising a probe beam is directed at the same point on the sample and that beam is monitored to determine the energy level of the sample under test. The reflectivity of silicon, and most other solids, is a known function of its temperature. This effect is known as thermoreflectance.

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Non-Destructive, Non-Contacting Test of Si Wafers by Thermore-Flectance

This is a method of testing of silicon wafers by means of thermoreflectance measurements wherein an energy beam, preferably in the form of a laser beam, is used as a pump to energize specific points on a sample under test (e.g., a semiconductor wafer), causing low levels of heating of the sample at the point where the energy beam is directed, which is at such a low level of energy that it is nondestructive. Then an additional wave comprising a probe beam is directed at the same point on the sample and that beam is monitored to determine the energy level of the sample under test. The reflectivity of silicon, and most other solids, is a known function of its temperature. This effect is known as thermoreflectance.

Thus, by changing the temperature, the

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percentage of the probe beam, which is reflected from the surface of the silicon, is a function of the temperature beneath the pump beam. A parallel array of pump and probe beams can be employed. In addition, circuits can be provided to make the measurements desired as to the response to variations in reflectivity. In VLSI wafer processing it is highly desirable to be able to detect the introduction of yield-degrading defects, such as contamination, slip, scratching, or precipitation, as quickly and as closely to the source of the problem as possible, so corrective action can minimize the loss. Of course, concomitant testing must be non-destructive and as non-invasive as possible. This implies no mechanical contact

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with the wafers. Testing must be rapid (on the order of 1 wafer per minute) and have spatial resolution of about 100 microns or better. Here, thermoreflectance, one of the modes of modulation spectroscopy that has been used for almost 20 years to establish the band structure of semiconductors, has been modified and adapted to the present purpose. In contrast to conventional thermoreflectance, where Joule heating is induced by electrical contacts made into broad areas of a perfect sample, we induce localized and periodic heating by optical excitation. In further contrast to conventional thermoreflectance, where one varies the probe wavelength to obtain a spectrum from which band structure information is obtained, we, having established the band structure of Si and other materials of interest, keep the probe wavelength fixed and measure the variation of the amplitude of the reflectance transient, which is proportional to the local variation of surface temperature, as a function of the location on the wafer. The degree to which each region of the sample heats is affected by processes which are strong functions of the degree of perfection of the wafer in the region excited. These include: i) the rate of nonradiative recombination of the photoexcited carriers; ii) the rate of diffusion

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of these carriers; iii) the optical absorption constant at the excitation, or "...