Semiconductor Manufacturing Processes and Laser Displacement Sensors

One of the driving forces in the semiconductor manufacturing industry today is to reduce manufacturing cost and increase efficiency. In the past, laser displacement sensors have been used infrequently due to lack of speed, accuracy and flexibility. Recently, laser displacement sensors have become more accurate (0.02 μm repeatability), faster (392 kHz sampling rate) and more flexible. Because of their greater speed, accuracy and flexibility, these sensors can be used in a wider variety of applications ranging from wafer production and handling all the way to wire bonding and final packaging. Some common applications of laser displacement sensor, specifically when producing semiconductor wafers are noncontact thickness, flatness, perpendicularity, and warpage.

Speed can be explained as a measure of how often a sensor takes measurements on a target (sampling rate). The advantage of high speed is the more measurement samples the sensor makes on a target, the more averaging of these measurements can occur, giving a more stable reading.

Accuracy can be thought of as if you take a measurement anywhere in the measuring range of the sensor, how much error there can be. This number is the total error, including the error caused by the sensor (linearity), caused by temperature fluctuations, sensor mounting errors, etc. By understanding and limiting these sources of error, more accurate measurements can be made. When gauging the accuracy of a measurement, the accuracy of the sensor itself is only one component. Depending on the required tolerances, this may or may not be significant. In many cases, full scale linearity may not be as important as resolution and repeatability. Resolution is the smallest amount of change the sensor will register. Repeatability of the sensor is a measurement of differences seen when a target is measured over and over again in the same position under the same conditions. If the part is placed in the same spot in the sensors measuring range over and over again, this is a measure of the sensor repeatability and what ever method of placing the parts’ repeatability. To make a long story short, by increasing the resolution and repeatability, the sensors accuracy can be improved.