System Uses Diffuse Reflectance Spectroscopy to Gauge Wafer Temperature

Kathleen G. Tatterson

SEATTLE -- Researchers from the University of Washington and the University of British Columbia have developed a noninvasive instrument for precisely measuring the temperature of a semiconductor substrate in real-time.

Using a technique called diffuse reflectance spectroscopy (DRS), the DRS 1000 system avoids the current practice of placing thermocouples near the wafer, ensuring that no part of the probe attaches to the wafer. Thermocouples disrupt temperature gauging by extracting heat from the point being measured. The diffuse reflectance technique determines the temperature of the semiconductor by simply looking at the wafer.

"The DRS 1000 allows measurement in hostile environments where you cannot attach mechanical temperature probes," said Jim Booth, a research scientist at Thermionics Northwest Inc. of Port Townsend, Wash., which is marketing the system. With this capability, scientists can more uniformly measure the temperature across an entire wafer and perform point-by-point measurements as well.

The device measures a wafer's temperature by gauging its diffuse reflectance when a broadband light source shines on it. For instance, when the sun shines on the roof of a red car, the color red is the diffuse reflection; the image of the sun on the roof is the specular reflection. The diffuse reflectance contains information about the object being measured, whereas the spectral reflectance primarily contains information about the light source.

Mirrors and lenses used

By using optical mirrors and lenses, the instrument separates the diffuse reflectance from the spectral reflectance to detect an object's color. From the color reading, it measures the temperature of the object to within 1 °C in 2 s. Thomas P. Pearsall, professor of electrical engineering at the University of Washington, likened the process to how enamel on some bakeware changes color when heated.

Using the new technology, researchers discovered that the temperature of the wafer not only fluctuates throughout the process, but can also change from batch to batch, even if the temperature of the heating element remains constant. The more accurately the wafer temperature is monitored and controlled, the less semiconductor structures need to compensate for temperature fluctuation. "If we can control temperature uniformity, we can improve chip performance by using more aggressive designs," Pearsall said.

The project was funded locally by the Washington Technology Center, while federal support came from a Small Business Innovation Research (SBIR) grant from the National Science Foundation. As part of the SBIR Phase II, academic and commercial laboratories around the country are testing the equipment. Based on their feedback, Pearsall said the team expects to make improvements and have the instrument market-ready within the next two years.