Known techniques for preparing a wafer that includes a thin semiconductor layer for forming circuitry (e.g., electronic, optoelectronic, or optical circuits or components) include Smart-Cut® type processes. In general, such processes involve implantation of a gaseous species at a controlled depth in a bulk donor wafer in order to create a weakness at a desired depth in the donor wafer, and the application of stresses to cause a separation at the desired depth due to the weakness. Molecular adhesion or wafer bonding may have been used before separation to bond a receiving wafer with a layer of the bulk donor wafer to be separated. Molecular adhesion or wafer bonding may be typical techniques by which the separated layer from the donor wafer and the receiving wafer are assembled. Once separated, further processing for producing circuits or components for the circuits in the separated layer may take place.
In some circumstances (e.g., when re-use is desired), it may be desired to subject the remaining structure of the donor wafer to further processing. The remaining structure may be the subject of mechanical, chemical-mechanical, or other polishing steps to ready remaining portions of the donating material of the donor wafer for further use. Other processing activities may involve chemical cleaning steps, relatively high temperature operations (e.g., 300 to 900° C., such as for oxide deposition), or substantially high temperature operations (e.g., 1150° C., such as for thermal oxidation in cases such as a silicon carbide wafer).
In some circumstances, it may be desired to recycle the bulk donor wafer through reuse. In such circumstances, the remaining structure may be required to be subject to additional implantation of one or more gaseous species, bonding with a receiving wafer, or further separation steps (e.g., through thermal or mechanical stresses).
Such reuse may progressively decrease the thickness of the donor wafer through consecutive removal of thin layers from the donor wafer. Progressively decreasing the thickness of the donor wafer may lead to an excessively thin donor wafer, which may not be reusable for further recycling.
There are other difficulties or deficiencies that are faced in recycling a donor wafer. There may be a high risk of fracture during predominantly mechanical operations such as when stress is applied to separate a thin layer from the donor wafer or such as when bonding is performed through CMP planarization of a surface oxide, etc. A high risk of fracture also arises for example during high-temperature heat treatment. The risk may be due to non-uniform temperatures in a donor wafer.
There may also be a high risk of fracture when an operator or processing machinery is required to handle a donor wafer. Another deficiency may involve large strains that are induced in certain operating steps when a donor wafer has been thinned through reuse. Operations such as implanting gaseous species or certain deposition steps may induce strains in thinner donor wafers that may cause the wafers to sag significantly (e.g., causing a wafer to take on a convex profile). Sagging may seriously compromise operations that require suitably flat contacting surfaces. Thus, a donor wafer may not be usable for further recycling once a minimum donor wafer thickness has been reached (e.g., a thickness at which deficiencies or drawbacks mentioned above may exist).
Discontinuing recycling at a minimum donor wafer thickness may be economically detrimental and/or inefficient in material consumption because the remaining material is typically discarded as waste material. Deficiency in this process is heightened in cases where the semiconductor material of a donor wafer is relatively expensive (e.g., is a high quality semiconductor material) or relatively fragile. For example, in the case of a standard silicon carbide wafer (e.g., a silicon carbide wafer having a standard diameter of 2 inches), a wafer thinned to about 200 μm may become unusable either because of frequent fractures during the process or because of a sag caused by implantation of gases prevents the wafer from suitably bonding to a receiving wafer.
In other applications, thickness may be relatively thin from a starting donor wafer (e.g., because wafers for a particular semiconductor material are typically offered on the market at that thickness). Gallium Nitride donor wafer may be one such example. Known techniques for producing such wafers involve using a thick epitaxy technique called HPVE (Hybrid Vapor Phase Epitaxy) on an epitaxially grown substrate (seed layer) that is removed after epitaxy. This technique, however, has two major drawbacks. Firstly, it only makes it possible to obtain self-supporting wafers having a thickness of at most around 200 to 300 μm. If a greater thickness is sought, imperfect lattice matching with the seed layer may generate excessive strains. Secondly, the rate of growth using the thick epitaxy technique is extremely slow (typically, 10 to 100 μm per hour). Such drawbacks may seriously handicap manufacturing costs.
Drawbacks may also be associated with conventional techniques in which ingots of some single crystal semiconductor material such as SiC are used for producing bulk donor wafers. In conventional techniques in which ingots of semiconductor material such as SiC are used for producing bulk donor wafers, the following operating steps are typically implemented: the ingot may be cut (e.g., using a saw) into slices having a thickness of around 1 mm, each of the faces of the slice may be coarsely polished to remove crystal damaged by sawing and to obtain good planarity, and the future front face (the removal face) may be successively polished to obtain appropriate surface roughness. Such techniques, which may start from relatively thick slices, may often involve substantial losses of material during the successive polishing steps. This obviously affects the manufacturing cost.
Thus, there is a need for providing such processes and structures in a more economically advantageous and efficient way. Within this context, there may also be a need to continue recycling even when extremely small thickness is reached.