Silicon Genesis Corporation has developed a next-generation layer transfer (LT) technology called PolyMax™ a "clean" process technology. This technology is an outgrowth of SiGen's bonded layer transfer technology for the semiconductor, opto-electronic and display markets.
Since the founding of the company, SiGen has developed a number of advanced engineered substrates based on its layer transfer technology. These include Silicon-on-Insulator (SOI), Silicon-on-Glass (SOG), and Silicon-on-Quartz (SOQ). SiGen has also developed equipment to support the process technology for plasma activation (SPA - Standalone Plasma Activation Tool), and cleaving (DB&C - De-bond and Cleave Tool). These processes and equipment are commercially available and used by some of the largest semi, opto-electronic and display manufacturers in the world.
Now, SiGen has leveraged that layer transfer technology and applied it to the production of PV wafers for high efficiency, low silicon content solar cells.
The principal difference between the two cleaving technologies is that the NanoCleave® process requires a bond before cleaving, while the PolyMax™ process is cleaving without the need of a bond. Since the cleaved wafers are free-standing in the PV process, they are appropriate substrates for the construction of solar PV cells.
PolyMax™: two-step process approach to wafering
The process consists of two main steps. The first step is implantation, in which the surface of a brick of cz-Si is exposed to an energetic beam of light ions such as hydrogen. The ions come to rest in a thin layer at a well-defined depth below the surface of the silicon. This cleave layer establishes a plane along which a fracture, subsequently is introduced into the silicon crystal, which will preferentially propagate.
The second key step of the process is the actual cleaving of the silicon. The cleaving step consists of a proprietary thermally driven treatment of the brick whereby a fracture is initiated, and caused to propagate along the cleave layer, separating the thin upper layer of silicon from the remainder of the brick. For films more than a few tens of microns thick, the resulting film will be a monolithic, self-supporting wafer. The wafer is removed and the two-step PolyMax™ process is repeated on the newly exposed surface of the brick, each cycle producing a single wafer.
Since PolyMax™ wafers are formed by a cleaving process rather than sawing, they do not have any substantial kerf-loss. In addition, since the thickness of a PolyMax™ wafer is determined by the highly repeatable physical process of mono-energetic ions stopping at the same depth, the variations in thickness are much lower than for wire-sawn wafers. SiGen's PolyMax System, including Implant and Cleave Units which is a high energy, proton beam implanter capable of cleaving silicon into various thicknesses from 20um to 150um without any silicon waste or kerf losses.
SiGen's Polymax™ System, including implant and Cleave Unites which is a high energy, proton beam implanter capable of cleaving silicon into various thicknesses from 20um to 150um without any silicon waste or kerf losses.
The major benefits of the SiGen approach for wafer formation are as follows:
- At least 2X silicon savings due to "kerf-free" cleaving
- A clean, dry process that eliminates the need to clean and separate
wafers after cleaving resulting in higher yields
- Technology extendibility from 150um to 20um
- Higher Mechanical strength
- TTV <5%
- Less breakage for downstream failures
- Low surface roughness