Past methods for producing silicon-on-insulator (SOI) wafers have involved epitaxial growth of silicon on an insulating substrate, or implantation of oxygen directly into silicon to form buried silicon dioxide layers (SIMOX™). In recent years, other methods have involved the transfer of a thin layer of semiconductor. One example of such a transfer method can be found in U.S. Pat. No. 4,846,931 to Gmitter et al. entitled “Method for Lifting-off Epitaxial Films”. According to the '931 patent, an epitaxial film is grown on a single crystal substrate. Afterward, a thin release layer positioned in between the epitaxial film and the substrate is selectively etched away. As the release layer is removed, the edges of the epitaxial film curl upward and away from the substrate and the epitaxial layer is peeled away. This approach is presently unsuitable for the preparation of SOI wafers because it is limited for lift-off of a film having a small area (about 1 cm2), while films having an area of 100 to 1000 cm2 are presently required for the fabrication of SOI wafers.
A method for transferring monocrystalline layers over to thermally oxidized silicon handle wafers by bonding and single etch back of porous silicon is described by Yonehara et al. in “Epitaxial Layer Transfer by Bond and Etch Back of Porous Si”, Applied Physics Letters, vol. 64, (1994) pp. 2108–2110. According to this paper, a thick substrate is made thinner by etching away the substrate until an etch stop (a porous silicon layer) is reached. The method has the disadvantage of the high cost to etch an entire semiconductor wafer.
Another method for transferring a semiconductor layer is described in U.S. Pat. No. 5,374,564 to Bruel, entitled “Process for the Production of Thin Semiconductor Material Films”. According to the '564 patent, hydrogen ions are implanted into a semiconductor substrate, and then are transformed into a quasi-continuous hydrogen layer. This method has disadvantages of the requirement of a high fluence of hydrogen (above 5×1016 cm−2), the difficulty in transferring an ultra thin (<0.1 micron) layer, and the low crystalline quality of the transferred layer due to surface damage induced by the hydrogen ion implantation.
Attempts were made to improve the Bruel method. In U.S. Pat. No. 5,877,070 to Goesele et al. entitled “Method for the Transfer of Thin Layers of Monocrystalline Material to a Desirable Substrate,” for example, a hydrogen-trap-inducing element such as boron or phosphorus is implanted into a substrate to create a disordered layer that divides the substrate into a lower portion (most of the substrate) and an upper portion that is transferred to a different substrate. After the creation of the disordered layer, hydrogen is implanted near the disordered layer and the substrate is then subjected to heat treatment. The upper portion of the substrate is then bonded to another substrate and the disordered layer is split, thereby transferring the upper portion (i.e. the thin layer) from the first substrate to the second substrate. While this method allows a somewhat reduced dosage requirement for the hydrogen implantation, it is still affected by the same problems as described above for the Bruel method.
U.S. Pat. No. 6,352,909 to Usenko entitled “Process for Lift-Off of a Layer From a Substrate” is concerned with another attempt at improving the Bruel method. The '909 patent describes forming a buried layer of defects by implantation. The buried defect layer is used to trap hydrogen. A disadvantage of this method is that the surface of the layer to be transferred is heavily damaged during the implantation and the damage is difficult to fix, even by annealing at a relatively high-temperature.
In U.S. Pat. No. 6,806,171 to Ulyashin et al. entitled “Method of Producing a Thin Layer of Crystalline Material,” a porous silicon layer is created on a silicon substrate, and a nonporous epitaxial layer is grown on the porous layer. The porosity of the now-buried porous layer is increased by hydrogenation techniques, and then the epitaxial layer is cleaved from the sandwich structure at the porous layer. After cleavage, the transferred layer needs to be smoothened. Similar to all of the prior art methods mentioned above, this method does not provide any improvement on the smoothness of the transferred layer.
U.S. Patent Application 20050070071 and U.S. Pat. No. 6,790,747 to Henley et al., both entitled “Method and Device for Controlled Cleaving Process” disclose a controlled cleaving process that involves introducing H atoms directly into a stressed region by hydrogen ion implantation. Damage cascades created within the stressed region degrade the crystalline quality and increase the roughness of the transferred layer.
Two approaches that are described by Cheng et al. in U.S. Pat. No. 6,573,126 and in U.S. Pat. No. 6,713,326, both entitled “Process for Producing Semiconductor Article Using Graded Epitaxial Growth”, involve using hydrogen ion implantation for lift-off of a semiconductor layer from a heterostructure that includes both a graded SiGe layer and a strain-relaxed SiGe layer. After thermal annealing, a zigzag network of microcracks results in a rough surface of the transferred layer. These approaches have the same limitations as those described by Bruel et al. in U.S. Pat. No. 5,374,564. In particular, a fluctuation in thickness as high as several tens of percent occurs when forming a layer of submicron thickness, and the formation of a uniform layer becomes a large problem for transferring a layer of material having a thickness of less than about 100 nanometers (1 nm=10−9 m). The difficulty of forming a thin film with high crystalline quality becomes more severe with an increase in wafer diameter.
There remains a heed for a better method for transferring ultrathin layers of crystalline semiconductor material.