New SOI process

12/01/1998

New SOI process

Campbell, CA, start-up Silicon Genesis Corporation (SiGen) has developed a new silicon-on-insulator (SOI) technology that purports to offer a less expensive way to get higher quality wafers. SiGen has exclusive license to SOI technology from UC-Berkeley, with the school`s professor Nathan Cheung as a company co-founder and consultant.

SOI processing can be divided into two approaches: oxygen implant/anneal (i.e., SIMOX), and layer transfer techniques (i.e., SOITEC`s Smart-Cut, and Canon`s ELTRAN). SiGen`s layer transfer Genesis Process involves three basic steps:

 implant hydrogen to a uniform depth to form a cleave plane,

 bond the wafer to the surface of a thermally oxidized second wafer, and

 cleave the hydrogen layer to leave a thin silicon layer on top of the oxide.

The cleaved wafer can then be re-used to save material costs. The SiGen process differs substantially from other layer transfer technologies in how these steps are accomplished.

Hydrogen implant. SiGen uses plasma immersion ion implantation (PIII) to form the hydrogen cleave layer. One of the company`s co-founders, Dr. Chung Chan of Northeastern University, founded privately held Waban Technology Inc. (WTI) in 1993 to commercialize PIII processing. WTI is now the East-Coast R&D arm of SiGen, where the first 300-mm wafer compatible alpha-tool was designed and built.

Wafer bonding. The formation of a strong, uniform bond between wafers is absolutely critical for any layer transfer SOI process. SiGen`s process first requires that a thermal oxide layer is grown on both wafers, and then a plasma process step "activates" the surfaces for bonding. The company states that this step is required, though it is not exactly clear what changes to surface states occur.

Cleaving. The cleaving step is the most unique piece of intellectual property developed by SiGen. SOITEC uses a thermal anneal step to cleave at the hydrogen plane; bonded wafers are heated until hydrogen gas at the implant level builds up sufficient pressure to force the wafers apart. The SiGen process relies on a pressurized air burst to initiate a horizontal crack between the wafers that rapidly moves through the weakened hydrogen implant plane - the "room-temperature controlled cleave process (rT-CCP)."

A bonded wafer-pair for cleaving is loaded into a simple jig that constrains movement within the plane while allowing only enough vertical space for separation. An air burst first initiates separation at the bond plane (where a pressure concentrating "V" shape is formed by the two wafer edge bevels), but the cleave almost immediately jumps to the weaker implant plane. Maintaining the air pressure forces the crack to propagate in 2D through the implant plane. After complete separation (which typically takes under a second), the bond interface is annealed and the surface touch-polished.

SiGen president and co-founder Francois Henley, formerly president and co-founder of Photon Dynamics, said, "After identifying the process-window for nonthermal cleavage, we examined numerous ways to transfer sufficient mechanical energy. The air burst was the last one we tried, and it`s somewhat surprising how well it works." Though first developing the air burst technique, the company has also applied for patent protection on the other mechanical alternatives.

The air-burst cleave allegedly produces a smoother surface than thermal cleaving, so less polishing is required. A shorter overall processing time combined with hypothetically less-expensive equipment should result in a lower overall manufacturing cost.

SiGen`s facility in Campbell is currently running SOI samples. Though SiGen wants to initially sell both wafers and equipment to seed the market, the company ultimately plans to license the process IP to wafer manufacturers while licensing equipment manufacturing, sales, and service to an OEM. The company`s IP may also be applicable to other microelectronic substrate applications, such as silicon-on-glass (for FPDs) and epi. - E.K.