Whole Wafer Thermal Measurements by Means of Laser Ultrasound

The production of semi-conductor wafers is a multibillion dollar industry. The 1996 worldwide semiconductor market is estimated at $162 billion with a growth potential to $309 billion by the year 2000. Thus, even a small improvement in control during the manufacturing process could have a large impact on the economic viability of many companies and products. A critical factor in all semiconductor production processes is improved control of production processing parameters. Current methods of temperature measurement such as thermocouples and optical pyrometry are not adequate.We propose a novel technique to measure in situ the wafer temperature during the semiconductor processing. The measurement technique is based on the principle that the velocity of ultrasonic Lamb waves propagating in most materials is a function of the temperature. We propose to generate and detect these waves in a non-contact manner with an emerging technology called Laser Ultrasonics. This technique does not depend on optical or electrical properties of the wafers and is insensitive to the processing environment. Current advancements in technology have made the components of laser ultrasonic instrumentation quite affordable. Temperature measurements of plus or minus one degree Celsius are quite achievable at reasonable cost and without adversely affecting the process environment using the proposed technology.