Method of shaping the edges of slices of semiconductor material and corresponding apparatus

Abstract: The invention concerns a method in which the edge of the slices (L) is made to slide in contact with the surfaces of a pad (B) soaked in acid substances and in which, during the process, the areas of the edge of the slices that have been in contact with the pad are periodically washed with a special liquid, contained in the tank (M).

Description: This invention concerns a method for shaping the edges of slices of semiconductor material, especially epitaxial material, and the corresponding equipment. In the technology of slices of semiconductor material, a recurring problem consists of shaping the edge of the slices in order to remove the circular protuberances ("epitaxial crowns") which form on the edge of the slices as a result of the growth on them of epitaxial layers and which, unless they are removed, would give rise to greater fragility of the slices and difficulty in the subsequent diffusion and photomasking operations. In order to shape the edge and remove the epitaxial crown, currently known equipment carry out mechanical abrasion of the edge by means of diamond wheels or discs of abrasive paper. This type of equipment, however, has the drawback of producing dust when in operation, and dust notoriously represents a source of contamination for semiconductor layers. The present invention consits of a method and an apparatus as defined in claims 1 and 3. respectively, which overcome the drawbacks above. This invention provides a method for shaping the edges of substrates of semiconductor material which does not present the aforesaid problem. For this purpose, in the method according to the invention the edge of the slice is made to slide in contact with the surfaces of a pad soaked in acid substances and, during the process, the areas that have been in contact with the pad are periodically washed with a special liquid. These and other features of the invention will be more clearly evident from the following description and the accompanying drawings of a non-restrictive embodiment, in which: Figs. 1a, 1b and 1c : show a schematic cross-sectional view of the edge of a slice, respectively, in the virgin state, after epitaxial growth, and after the shaping process according to the invention; Fig.2a : shows a partial cross-sectional plan view of a pad used in the process according to the invention; Fig.2b : shows a side view of the pad of Fig. 2a; Fig. 3 : shows a plan view of an apparatus for carrying out the process according to the invention; Fig. 4 : shows a cross-sectional view along the line A-A of Fig. 1; Fig. 5 : shows an enlarged view of a detail of Fig. 3. Figs. 1a, 1b and 1c show a schematical cross-sectional view of the edge of the same slice, respectively, before the growth of an epitaxial layer (Fig. 1a), after growth (Fig. 1b) and, lastly, (Fig. 1c) at the end of the edge shaping process according to the invention. In particular, Fig. 1b shows the epitaxial crown K which forms along the edge during the growth of the epitaxial layer I, said crown being considerably higher (in the region of 40%) then the layer itself. Figs. 2a and 2b show a pad used in the process according to the invention. Said pad consists of a cylinder-shaped body B made of flexible material resistant to the acids used in the process and capable of soaking up the acids themselves. The pad is provided on one side with a series of notches C, the cross-sections of which, according to a plane passing through the axis of the pad, are V-shaped. On the bottom of each notch is the mouth of a channel D which passes radially through the pad until it comes into contact with the internal cavity J of the cylinder F, which serves the dual purpose of supporting the pad and of conveying the acid substances. The purpose of said channels is to enable the mixture of acids present in the cavity of the cylinder F (consisting, for example, of 10% hydrofluoric acid, 30% nitric acid and 60% acetic acid, in volume) to overflow into the channels themselves and then soak uniformly through the entire body of the pad. In the apparatus of Figs. 3 and 4 , the various parts represent: B: a pad of the type shown in Figs. 2a and 2b; G and H: pairs of rollers; L: a slice of semiconductor material resting on the rollers G and H are slidingly engaged by one part of its edge in a notch in the pad B; M: a tank containing on the bottom continuously interchanged deionized water; N: a motor for driving the rollers G and H by means of a belt and pulley transmission system O; Q and R: bars for supporting a slice-holding basket P (shown in fig. 5). Fig. 5 shows an enlarged view of the tank of Fig. 4, in which reference P indicates a basket-shaped container designed to receive the slices and to hold them in a vertical position and equally spaced apart from one another by means of slots on the right hand side and on the bottom of the basket. Said container is also provided with apertures in correspondence with the rollers G and H and with the pad B, which enable the edge of each slice to engage with the surface of the aforesaid cylinders and with a respective notch in the pad. During the process, the slices in the container are made to rotate around their own axes by means of the rollers G and H. As a result of the rotatory movement the edge of each slice brushes against the pad for one or more turns, thereby undergoing chemical attack by the acids in which the pad is soaked, with consequent removal of the surface in contact. Said removal is lesser or greater in relation to the composition of the acid and to the number of turns made by the slices. During the course of the rotatory movement, the area of the edge of the slice that has been in contact with the pad runs through the water on the bottom of the tank (which is continuously interchanged by means of inlet and outlet apertures, not visible in the figures), thus quenching any acid which may still remain on the surface of the slice. It is obvious that numerous modifications, adjustments or replacement of elements with other functionally equivalent elements may be made to the previously described embodiments, given by way of illustration. In particular, it is obvious that the shape of the notches in the pad shown in Fig. 2 (which is symmetrical with respect to the planes perpendicular to the axis of the pad and crossing trough the axis of each channel) is not binding. In fact, said notches may be differently shaped and, in particular, asymmetrical whenever it is desired to obtain an equally asymmetrical shape on the edge of the slices.

Claims: Method of shaping, in a reaction tank, the edge of a number of slices of substantially circular-shaped semiconductor material, whereby the edges of the slices are made to slide, by rotation of each slice on its axis perpendicular to the plane of the slice, in contact with the surfaces of a corresponding number of notches present in a pad soaked in acid substances, and that the areas of the edges of the slices that have been in contact with the pad are, during each turn of the slices, washed with a liquid present in the reaction tank. Method as claimed in Claim 1, characterized by the fact that said liquid is deionized water. Apparatus for shaping the edge of slices of substantially circular-shaped semiconductor material in accordance with the method of claim 1 or 2, comprising: a reaction tank; a number of slices of substantially circular-shaped semiconductor material, each slice having an axis perpendicular to its plane: supporting means inside the reaction tank for supporting said number of slices in order to keep lined up said axes of the slices: a pad, inside the reaction tank, soaked in acid substances, presenting a number of notches corresponding to the above number of slices, designed in such a way as to enable the edge of each slice to enter a respective notch of the pad; driving means designed to make each slice rotate on its own aforesaid axis; washing means inside the reaction tank, designed to wash, with a liquid and during each turn of the slices, the areas of the edge of each slice that has been in contact with the surface of the aforesaid notches. Apparatus as claimed in Claim 3, characterized by the fact that the aforesaid supporting means consist of a basket provided with slots on one or both sides and/or on the bottom, designed to receive the edge of a respective slice and to keep the slice in a substantially vertical position, and a lateral aperture designed to enable the entry of at least the part of said pad provided with said notches. Apparatus as claimed in Claim 4, characterized by the fact that the aforesaid driving means consist of two rollers, which are made to rotate, with their axes parallel with the aforesaid axes of each slice and in contact with the edge of each slice, on opposite sides with respect to the vertical plane passing through said axis of the slice. Apparatus as claimed in Claim 5, characterized by the fact that, in order to realize the aforesaid washing means, said reaction tank is provided on the bottom with suitable inlet and outlet apertures which enable said reaction tank itself to contain a sufficient amount of continuously interchanged deionized water to wash the lower edge of each slice during its rotatory movement.

Inventors: BERETTA GIORGIO, INSERRA ANTONINO, BERETTA, GIORGIO, INSERRA, ANTONINO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1988
Main classification: H01L21
IPC codes: H01L21