1. Field of the Invention
This invention involves the removal of defects and disorder from crystalline layers and the epitaxial regrowth of such layers.
2. Description of the Prior Art
In the course of semiconductor device fabrication, starting semiconductor materials are usually "doped" with additional elements to yield the ultimate electrical properties of the desired device. Such addition of appropriate atoms or molecules is effected primarily by one of two broad processes. The first process, referred to as ion implantation, involves bombarding the semiconductor with appropriate, highly energetic dopant ions. The energetic ions enter the material and penetrate to depths as high as one micrometer before their kinetic energy is totally dissipated. In the course of this implantation, the single crystal starting material suffers severe damage to its ordered crystalline structure as a result of the large amount of kinetic energy that is transferred to the host lattice. This damage must be subsequently removed or "annealed" before the desired electrical properties can be realized. This annealing usually proceeds by heating the damaged crystal to temperatures generally in the neighborhood of 1000 degrees C. for material such as silicon. Diffusion occurs more readily in heated material and consequently, both dopant and displaced host atoms can incorporate themselves relatively rapidly (e.g., an hour) into the lattice structure of the single crystal. In addition, dopant atoms may diffuse into the substrate beyond the implanted region. While the annealing process generally eliminates the heavy damage that occurs in the region of initial implantation, some damage may remain in the implanted as opposed to the diffused region. This implanted surface region may be either etched away or the device may be designed such that this residual, post-annealing damage is not detrimental to its operation. Despite the annealing requirement and the occurrence of residual damage, the ion implantation technique is sufficiently powerful to warrant its application in a number of prevalent manufacturing sequences.
The second of the more prevalent doping processes involves the thermal diffusion of dopant atoms from the surface of the semiconductor to its interior. The appropriate dopant constituents may exist as a gas environment in which the semiconductor is placed, or may be deposited directly on the substrate, either prior to or during diffusion. Heating the entire semiconductor crystal results in diffusion of the dopant constituent into the interior region of the material under the influence of the chemical potential gradient which is maintained during the diffusion process. The diffusion may occur in more than one step and, in a prevalent practice, occurs first in a high temperature, predeposition phase and then over a much longer time period in a lower temperature diffusion step. It should be noted that in contradistinction to the ion implantation process, the diffusion process generally results in considerably less damage to the semiconductor substrate. This is due to the low energy dynamics involved in thermal annealing which is inherently less capable of producing damage, and to the fact that any damage which may occur may be essentially simultaneously annealed under the influence of the thermal conditions required for diffusion. However, dislocations due to misfit stresses that arise during cooling may appear during this process.
As mentioned above, the heavy damage which occurs during the ion implantation process must be annealed, generally by exposing the substrate to elevated temperatures. This annealing process is sufficiently powerful so that a surface layer that has been rendered amorphous by ion implantation may be annealed or "regrown" to a single crystal, as a result of the imposition of appropriate thermal conditions and under the influence of an appropriate underlying single crystal substrate. (See, e.g., Csepregi, L., Mayer, J. W., and Sigmon, T. W., Physics Letters, 54A, 157 (1975). Similarly, amorphous surface layers produced by vapor deposition of Si atoms onto specially cleaned silicon single crystals may be "regrown" by thermal annealing (Roth, J. A., and Anderson, C. L., Applied Physics Letters 31, 689 (1977)). In both cases, however, the regrown material may contain residual defects.
Recently, workers in the field have demonstrated the efficacy of laser radiation as a source of intense thermal energy in the annealing of damaged semiconductor materials. Essentially, the laser is used as a convenient means of depositing energy into the damaged semiconductor material, thereby raising its temperature, and increasing the appropriate diffusion rates so as to permit efficient annealing. (e.g., I. B. Khabullin, et al, Soviet Physics--Semiconductor Vol. 11, page 190 (1977)). Both pulsed and cw lasers have been used to obtain limited annealed regions, and a cw laser has been used to scan the damaged semiconductor thereby obtaining a patterned annealed region. (e.g., Kachurin, et al., Soviet Physics--Semiconductor 10 1128 (1977)). However, the exact nature of the process involved in laser annealing has not been understood.
Prior applications of the laser have included etching processes using a stationary laser spot, a scanning laser spot, or a series of overlapping laser spots to properly define the etched pattern. (See, e.g., Ready, J. F. (1971) "Effects of High Power Laser Radiation" Academic Press, New York).