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04/17/95

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SLAC, HP designing better detector for wafer surface impurities

STANFORD -- The Stanford Linear Accelerator Center has signed a cooperative agreement with Hewlett-Packard Co. to develop an ultra- sensitive X-ray detector for measuring extremely low levels of metallic impurities on the surfaces of silicon wafers used for computer chips and other microelectronic circuits.

The sensitivity of commercially available systems designed to detect surface metallic impurities is limited by the intensity and wavelength definition of the X-rays produced by conventional X-ray tubes. If a suitable detector is built, high-power synchrotron X-ray sources like those at the Stanford Synchrotron Radiation Laboratory at SLAC should allow an immediate factor of 10 improvement in detection sensitivity.

The goal of the Cooperative Research and Development Agreement between SLAC and Hewlett-Packard, signed on Jan. 6, is to develop the design parameters for such an ultra-sensitive X-ray detector. The 12- month, $150,000 effort is receiving funds from the Department of Energy and Hewlett-Packard.

As integrated circuits move to the next level of miniaturization, called ultra large scale integration (ULSI), metallic contaminants are expected to become a serious problem. ULSI chips contain millions of tiny, interconnected transistors only 0.5 microns in size, all manufactured out of the same piece of silicon and all of which must work correctly to make the chip functional. During processing, even widely spaced atoms of certain metals lying on the wafer's surface can gather to form defects that create leakage or other electrical problems.

Consequently, the semiconductor industry needs the capability to detect as few as 100 million metal atoms per square centimeter in order to develop efficient ULSI manufacturing methods. This level of contamination -- only one contaminant atom for every 10 million silicon atoms on the surface -- is invisible to commercially available detection methods.

The new detector design will be used in conjunction with the best available technique for measuring metallic contaminants on a silicon wafer surface, a method called Total External Reflection X-Ray Fluorescence. In TRXRF an X-ray beam grazes the silicon's surface at a low enough angle so the beam is reflected after penetrating only a few atoms deep into the wafer. As a result, it only illuminates impurities at or near the surface. When illuminated, the metal impurities are excited and emit X-rays in every direction and at wavelengths that are different from those in the illuminating beam. Because these secondary X-rays are characteristic of the particular atom that emitted them, a properly positioned detector can measure a range of X-ray wavelengths simultaneously and use this to calculate the relative concentrations of different metallic impurity atoms on the surface.

Recent experiments at SSRL on the detection of impurities on silicon wafer surfaces have detected nickel at a concentration of 300 million atoms per square centimeter. The chief limitation on sensitivity for some other important elements such as iron, copper and zinc was found to be materials in the detector itself that caused background noise. As part of the joint project, Hewlett-Packard and SLAC intend to reduce these background signals and make other improvements to the apparatus, which should lead to much better sensitivity. They expect to achieve detection levels of 300 million atoms per square centimeter or better for titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc.

Hewlett-Packard Co. of Palo Alto, Calif. designs, manufactures and services electronic products and systems for measurement, computation and communications. Its products and services are used in industry, business, engineering, science, medicine and education worldwide.

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