Sensitive strain measurements of bonded SOI films using Moiré

The authors have developed a simple technique to quantify strain in bonded Si films and used it to compare the strain induced by two distinct wafer bonding methods. This method consists of patterning sets of moire gratings on silicon-on-insulator (SOI) substrates prior to bonding using a G-line stepper. After planarization, bonding, and etch-back, the same lithography step is performed on the flipped patterns. The resultant interference between upper and lower gratings produces moire fringes which is a measure of the strain. In the experiments, the sensitivity of the measurement is approximately 20 nm. This approach has been used to compare two methods of wafer bonding. The first method, a manual bonding technique, yielded strain of up to 100 nm/mm. The second method employed a commercial-grade bonder and resulted in film strains below 40 nm/mm. In the bonding schemes the authors have studied, they believe strain results mainly from induced wafer bow during bonding and stress contributions of deposited films. This scheme was developed to address wafer strain that arises from a direct-alignment double-gate MOSFET fabrication scheme (Meinhold, 1994).