Microcontamination control is a key parameter for enhancing yield in terms of density of voids within the interface of a bonded wafer. One important cause of void density is time-dependent haze (TDH) or surface-particulate defect density. TDH on silicon surfaces is generated by the interaction of organic volatiles with moisture on the silicon surfaces. Sub-micrometer particle defect density, or haze level, is mainly affected by contamination arising from five factors: people, process, equipment, environment, and processing chemicals and gases.
Direct wafer bonding (DWB) is a three-step sequence consisting of ultra-clean surface preparation, contacting, and thermal annealing. Contacting is an operation of ultrafine alignment and face-to-face joining of two substrates. Prior to joining, each contacting substrate is chemically cleaned in order to minimize surface haze and particulate contamination. Both sub-micrometer particle density and surface haze can be greatly reduced by point-of-use (POU) microfiltration and purification of deionized (DI) water as well as application of ultra-dilute concentration of ultra-pure chemicals containing extremely low levels of liquid particle counts. Ultra-pure DI water is filtered with a high-efficiency membrane filter that removes particles down to 0.02 micrometer (μm), thus reducing metal ions to sub-parts-per-billion (ppb) levels. As a result, surface haze and sub-micrometer light-point defects density on a contacting surface are minimized to reduce the density of voided or disbonded regions. A 1-μm particle, for instance, can cause a void as large as 1 centimeter (cm) in diameter during the bonding operation. Data is presented in terms of fractional areas of the voided regions for direct silicon wafer-bonded substrates.
In addition to semiconductor wafer substrates, surface contamination is discussed for optical substrates such as fused silica. Minimizing microcontamination and TDH on the surface of optical substrates is important in maximizing ultraviolet (UV) transmission rates at wavelengths between 190 and 250 nanometers (nm). Methods and techniques are recommended for minimizing trace levels of surface contamination to improve UV transmission rates. No data for bonding of optical components is included in this study.