Yield Enhancement of Bonded Silicon Wafers by Point-Of-Use Micro-Filtration and Purification Of DI Water During Chemical Cleaning

摘要

Direct wafer bonding (DWB) is a 3-step sequence consisting of surface preparation, contacting and thermal annealing. The contacting is an operation of ultra-fine alignment and face-to-face joining of two silicon wafers. Prior to joining, each silicon wafer is chemically cleaned in order to minimize surface contamination. The control of surface contamination is a key parameter in enhancing the product yield. The sub-micron particle density can be greatly reduced by (a) Point-of-use (POU) micro-filtration and purification of deionized (DI) water and (b) Application of ultra-dilute concentration of ultra-pure chemicals containing extremely low levels of liquid particle counts. Prior to joining operation, a laser beam Surfscan is employed to directly measure haze and light point defects (LPD's) on the surfaces of silicon wafers. The sub-micron LPD's counts must be minimal in order to reduce density of voided or "disbonded" regions. A 1-micron size particle, for instance, can cause a void as large as 1-centimeter in diameter during the bonding operation. A scanning acoustic microscope is used to quantatively detect the presence of micro-voids, delamination and other defects with an interface layer of a bonded wafer. Detailed product throughput and yield data in terms of voided density are presented in this technical paper. The role of ultra-pure DI water as well as ultra-dilute concentration of ultra-pure chemicals is discussed in order to produce clean silicon wafer surfaces. The final wafer rinse is performed using ultra-pure DI water, which is first filtered with the high efficiency membrane filter that removes particles down to 0.02-μm size and is then purified to remove metal ions to sub-ppb levels. Detailed voided data are compared with and without POU micro-filtration of DI water. This paper will describe the role of pure water and cleaning chemicals, wet cleaning processes, and contamination control using advanced filtration/purification methods in producing clean wafer surfaces. Test results showing the effectiveness of POU purification will be presented along with the wafer level data that correlate high yields to cleaner wafer environment.