In wafer bonding, two flat substrates are permanently joined by applying carefully calculated pressure, temperature, and voltage. Of the three parameters, pressure is often the most difficult to precisely ascertain and control. Traditional methods assume perfectly flat pressure plates. However, in nearly all real environments pressure plates are not ideal due to manufacturing or installation variability and degradation over time from wear. The result is difficult-to-detect pressure variations across the surface that can lead to unbonded wafer areas, cracked wafers, and premature wear of the pressure plates.
Micralyne Inc., an independent foundry located in Canada, uses Pressurex® film from Sensor Products Inc. to adjust the compression exerted by their bonding tools. Pressurex® is a quick, accurate, and economical way to detect and correct pressure variations, which can result in flaws in the wafer bonding process. When placed between contacting surfaces, it instantaneously and permanently changes color in direct proportion to the actual pressure applied. Variations in pressure that lead to imperfections or irregularities in the bonding process can be accurately detected and corrected -- improving yield, decreasing scrap, and boosting productivity.
Early testing by Micralyne of bonding results analyzed with the film reveals a donut-shaped high pressure ring with relatively little pressure applied to the center (right). After a series of adjustments the final result is a more uniform distribution of pressure (left).
Precise pressure magnitude is easily determined by comparing color variation results to a color correlation chart, conceptually similar to interpreting Litmus paper. Additional pressure analysis of the film is available, which assimilates the pressure distribution into 2D and 3D pseudo color images, histograms, and line scans, among other features.
Wafer-to-wafer bonding has become an enabling semiconductor technology in industries such as 3D packaging, MEMS, MOEMS, and SOI. Pressurex® is being used across a wide range of bonding technologies, including metal eutectic, anodic, fusion, metal diffusion, glass frit, and polymer adhesive bonding. The “snapshot” imaging results can be used as a control to compare processes and tools, making it ideal for six-sigma and other closely monitored high performance manufacturing operations. Pressurex® measures pressures from 2 to 43,200 psi; free samples are available.