Separating one silicon layer from another silicon layer by fracturing a thin and mechanically weak/fragile intermediate silicon layer has been widely known in making silicon-on-insulator (SOI) wafers for producing semiconductor devices. However, prior methods have several drawbacks. For example, most prior art methods require a planar intermediate layer separating the device layer and the substrate layer. U.S. application Ser. No. 11/868,489 having common inventor, Mehrdad Moslehi, of the present disclosure discloses a 3-D thin-film semiconductor device where prior art manufacturing methods may not be suitable.
Instead of having a flat porous silicon layer, the honeycomb 3-D TFSS and the template wafer comprise three-dimensional microstructures with high-aspect-ratio deep trenches made into the silicon template. As a result, the effective interface area between porous silicon layer and nonporous silicon layers is at least five times larger than that of a flat substrate. The large interface area per unit volume increases the magnitude of external energy/force that is required for fracturing the porous silicon layer. Prior art methods may not be suited to fracture the porous silicon, while mitigating damage to both the template and 3-D TFSS.
In addition, most release methods in the prior arts require a mechanical supporting plate bonded or attached by adhesive on top of the thin epitaxial silicon layer to be released. In addition to serving as a mechanical support, the bonded top plate may also absorb the external energy and generate a stress on the layer to be released. Without the top supporting plate, many of the prior art release methods are either less effective or cause mechanical damage to the released thin-film. U.S. application Ser. No. 11/868,489 discloses a 3-D TFSS which is not conducive to the use of a top supporting plate for release and post-release processes because: (i) it is not convenient to bond a supporting plate on top of the square 3-D TFSS to be released while preventing the supporting plate from attaching to the wafer surface outside of the 3-D TFSS square; (ii) it is difficult to de-bond the supporting plate from the released 3-D TFSS. In the case that the bonding adhesive has to be wet removed, extensive cleaning may need to be performed to prevent adhesive contaminations to the honeycomb surfaces.
Further, most of the release methods in the prior arts initiate a single separation front in the porous silicon layer at the beginning of release that propagates through the entire wafer to complete the release. In most cases, the separation front starts from the wafer perimeter and the released portion of the epitaxial silicon layer curves upward as the separation progresses towards to the wafer center. Such a release mechanism works well for a planar release, however it does not work for the 3-D TFSS release for the following reasons: (i) because of its three dimensional structural design, the early released portion of honey-comb structure can not be tilted. A slight out-of-plane curving by an external force or an intrinsic stress will have the 3-D TFSS locked into the template and prevent a full release; (ii) larger external energy/force applied unevenly to the partially released and locked-in 3-D TFSS could cause mechanical damages. Therefore, the release energy/force should be uniform and applied in a well controlled manner for the 3-D TFSS release.
It is known that the mechanical strength of porous silicon depends on the porosity of the layer, and that porous silicon mechanical strength is sufficiently lower than that of non-porous silicon. As an example, a porous silicon layer having a porosity of 50% may have a mechanical strength about one-half of that of a corresponding bulk silicon layer. When a porous silicon layer is subjected to compressive, tensile, or shearing forces, it can be fractured, collapsed, or mechanically destroyed. A porous silicon layer, which has higher porosity, can be fractured with less applied stress.
One method for collapsing the mechanically weak porous silicon layer employs injecting the porous layer with a fluid. This method not only succumbs to the difficulties of the prior art method mentioned above, but is also complex and requires precise alignment of the fluid injection nozzle with the porous silicon layer so as not to damage the thin-film layer.
In another prior art method, a process of manufacturing a SOI wafer includes separating a wafer assembly into two wafers at a fragile silicon layer containing a high amount of hydrogen. The separation energy source can be selected from a group consisting of: ultrasound, infrared, hydrostatic pressure, hydrodynamic pressure, or mechanical energy. Also, yet another prior art method applies a force to a laminating material separating a nonporous silicon and porous silicon layer to separate the two layers.
Besides succumbing to the disadvantages mentioned previously, these methods may often damage the template layer which is undesirable for releasing TFSS substrate of U.S. application Ser. No. 11/868,489. Other advantages of the present disclosure may be apparent to those skilled in the art.