Abstract:
A method for forming an oblique recess of minimum dimension smaller than 10 μm in a wafer arranged in a plasma etch reactor in which the plasma extends along the wafer surface, including forming discontinuities in the contour of the plasma and of its sheath in the immediate vicinity of the wafer in areas where recesses delimited by openings in a protection mask are desired to be formed at the wafer surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the forming of recesses, bores, or trenches in a material, and especially applies to the field of semiconductor materials, intended for the forming of discrete or integrated components.  
         [0003]     The present invention more specifically relates to the forming of recesses, bores, or trenches which are oblique with respect to a main surface of the material to be etched and have a small transverse dimension.  
         [0004]     2. Discussion of the Related Art  
         [0005]     When recesses are desired to be etched according to a pattern having at least one very small dimension (smaller than 10 μm and possibly smaller than 1 μm) into the surface of a material, one of the currently-used methods, especially in the field of microelectronics, is a plasma etch method.  
         [0006]      FIG. 1  very schematically shows a plasma etch reactor. The reactor comprises an enclosure  1  provided with a bottom  2  on which is placed a support or susceptor  3 , that may be isolated from bottom  2 . A wafer  4  in which etch operations are desired to be performed is placed on support  3 . A plasma  5  is created by any means in the enclosure, for example, by a radio frequency field, and may be confined by electromagnetic fields. Of course, the enclosure further comprises means, not shown, of gas introduction, circulation, and extraction, and possibly means for imposing a temperature, for example, for cooling down, to support  3 , as well as means for biasing this support. As schematically illustrated in  FIG. 1 , the plasma generally extends into the entire enclosure, and by all means in the vicinity of wafer  4  to be etched, and is separated from all the internal enclosure surfaces by a distance s, in which a neutral area, called a sheath, is present. In the case where wafer  4  is a silicon wafer, plasmas are often obtained from fluorinated or chlorinated gases.  
         [0007]     Generally, anisotropic plasma etch methods involve two mechanisms: an etch mechanism and a passivation mechanism that may be alternated or not. The etch mechanism is considered as involving two phenomena, on the one hand a bombarding by the ions present in the plasma, on the other hand a chemical etch by reactive substances present in the plasma. The two phenomena are indissociable. The ion bombarding is used to eliminate the reaction products arranged at the bottom of a recess being formed.  
         [0008]     These explanations have been given to expose the context of the present invention, given that plasma etch methods are well known per se and that there exist many variations thereof that can be found in specialized works.  
         [0009]      FIG. 2  is a cross-section view of a wafer  4 , coated with a masking layer  7  comprising openings  8 , arranged in the plasma etch reactor of  FIG. 1 , that is, it shows a plasma region  5  separated from the wafer by a sheath thickness s.  
         [0010]     The ions bombard the silicon wafer perpendicularly to the surface thereof and are active, with chemical species located in the plasma, at the level of openings  8  in mask  7  to etch in wafer  4  recesses  10  orthogonal to the wafer surface.  
         [0011]     Thus, anisotropic plasma etch techniques do normally not enable forming recesses which are not orthogonal to a main surface of a wafer to be processed.  
         [0012]     Japanese patent application 2002-134470 suggests to deviate ions due to a lateral field created at the surface of a wafer. This field results from a charge accumulation under insulating layers having different thicknesses, the thickness difference being of about 100 nm. This method is not adapted to provide deep trenches nor substantial inclination angles.  
         [0013]     So, if oblique recesses or trenches are desired to be formed, techniques other than plasma etching are used. An oblique trench will for example be formed by sawing with diamond saws (JP-A-56054066), by sand blasting (JP-A-03225961), or with a laser (EP-A-0070692), or also by slanted ionic etching (JP-A-08130220 and JP-A-62173737). The methods do not provide thin trenches or enable forming patterns of complex shapes.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a modification of usual plasma etch methods to enable obtaining recesses, trenches, or vias, extending obliquely with respect to the surface of a wafer and not only perpendicularly thereto, these recesses being likely to have small lateral dimensions, such as conventionally obtained by plasma etch.  
         [0015]     More specifically, the present invention provides a method for forming an oblique recess of minimum dimension smaller than 10 μm in a wafer arranged in a plasma etch reactor in which the plasma extends along the surface of the wafer, comprising forming discontinuities in the contour of the plasma and of its sheath in the immediate vicinity of the wafer in areas where recesses delimited by openings in a protection mask are desired to be formed at the wafer surface. The discontinuities result, for example, from protrusions formed on the wafer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  very schematically shows a plasma etch reactor;  
         [0017]      FIG. 2  is a cross-section view of a wafer conventionally etched by plasma;  
         [0018]      FIG. 3  is a cross-section view of a wafer etched by plasma according to a first embodiment of the present invention;  
         [0019]      FIG. 4  is a cross-section view of a plasma-etched wafer according to a second embodiment of the present invention;  
         [0020]      FIG. 5  shows a conventionally-formed trench-isolation diode; and  
         [0021]      FIG. 6  shows a trench-isolation diode formed according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, as usual in the representation of semiconductor components, the various drawings are not drawn to scale.  
         [0023]     The present invention is based on the observation according to which, in a conventional plasma etch, the etching is of course orthogonal to the surface of the wafer to be etched, but above all occurs perpendicularly to the plasma surface at the sheath limit.  
         [0024]     Thus, the present invention provides deforming the plasma contour so that ions, projected orthogonally to this contour, will be projected in a direction which is not parallel to the wafer surface.  
         [0025]     According to an embodiment of the present invention, this plasma contour deformation is performed by forming protrusions on the wafer surface. However, other means may be envisaged to deform this plasma contour.  
         [0026]      FIG. 3  illustrates an embodiment of the present invention. On a silicon wafer  11  to be processed is formed a mask  12  provided with openings  13 . In the vicinity of at least one of these openings, a protrusion  15  is formed on the wafer. This protrusion may be formed by deposition and etch of a thick layer and its surface will possibly be coated with a coating  16  to make it insensitive to the plasma etch.  
         [0027]     The plasma contour deforms in the vicinity of the protrusion, as indicated by curve  18  in dotted lines, and especially comprises a rounded area  19  in the vicinity of the protrusion base. In the curvature area, the ion bombarding paths  20  are orthogonal to this curvature, that is, are oblique with respect to the surface of wafer  11 . Oblique recesses  21  can thus be formed in the wafer. The inclination of recesses  21  depends on the proximity of openings  13  in mask  12  with respect to the base of protrusion  15 . The closer openings  13  in mask  12  are to the base of protrusion  15 , the more oblique the recesses will be.  
         [0028]     As shown in  FIG. 4 , in which the same elements have been designated with the same reference numerals as in  FIG. 3 , several protrusions  15 . 1 ,  15 . 2 ,  15 . 3  may be created at the wafer surface and various oblique recesses may be created at the bases of these protrusions, on one side and/or on the other of each protrusion, which enables orienting the recesses in different directions.  
         [0029]     The distance between protrusions  15 . 1  and  15 . 2  is sufficient for the sheath to be deformed and for the plasma to penetrate more or less deeply between the protrusions, that is, the distance between two protrusions must be greater than sheath thickness s.  
         [0030]     In practice, the sheath thickness currently has, according to the plasma conditions, a value of from 0.1 to 0.6 mm. These values are given as an example only and will depend on the plasma conditions in the reactor as well as on the reactor type. Especially, sheath thickness s is increased by increasing the pressure in the enclosure. So, to obtain a substantial deformation of the sheath shape, the protrusions should have a height of at least 100 μm, preferentially of the same thickness range as the thickness s of the sheath.  
         [0031]     All alternative plasma etches may be used, especially various substrate biasing systems and various possible substrate cooling systems as well as various systems for circulating the gases in the enclosure.  
         [0032]     Many means may be used to form the structure comprising the protrusions. In the case where the substrate to be etched is a silicon wafer, this wafer may first be coated with a mask, after which a second silicon wafer provided with bores around protrusions  15  of  FIGS. 3 and 4  may be laid on the first wafer. The apparent surfaces of the second wafer will preferably be oxidized to avoid an etching of the silicon by the plasma. Several second wafers comprising selected patterns may, if several trenches close to one another and/or of different orientations are desired to be formed in the wafer to be etched, be successively used.  
         [0033]     Generally, the present invention has the advantage of enabling forming in a wafer oblique recesses, vias, or trenches of a width much smaller than 10 μm, which is much smaller than the minimum dimension which can currently be achieved by using saws. Especially, in the field of plasma etch, recesses having minimum dimensions much smaller than one micrometer, but that can reach a few micrometers, can currently be etched, and the depth of the formed recesses can extend down to depths on the order of a few micrometers, but reaching a few hundreds of micrometers.  
         [0034]     It should thus be understood that the present invention provides many applications in various fields and especially in the field of microelectronics.  
         [0035]     Protrusions  15  may take the shape of rectilinear ribs if rectilinear-trench shaped recesses  21  are desired to be formed. Protrusions  15  may also take the shape of pins, circular or having any other selected shape, centered on corresponding openings  13  formed in mask  12 ; recesses  21  will then have the shape of inverted tapers.  
         [0036]      FIGS. 5 and 6  illustrate an example of application of the present invention to the forming of a vertical high-voltage diode,  FIG. 5  illustrating a conventional structure and  FIG. 6  illustrating a structure according to the present invention. The diode is formed from a lightly-doped N-type substrate  32  having its upper surface comprising a P-type doped region  33  and having its lower surface comprising a heavily-doped N-type region  34 . An active area is delimited by a trench  31  having its walls coated with an insulator, for example, silicon oxide. Trench  31  crosses P layer  33  and, at least partially, substrate  32 . A cathode metallization, not shown, is formed on the lower surface and an anode metallization, not shown, is formed on the upper surface on the active area.  
         [0037]     One of the important features of high-voltage diodes is that they must stand a significant reverse voltage. This voltage is essentially supported by lightly-doped substrate  32 . The equipotential surfaces, shown in dotted lines, extend substantially horizontally and distribute mainly in layer  32 . However, in the conventional case illustrated in  FIG. 5 , a curvature of equipotential surfaces  35  at the level of their periphery can be observed. Thus, the equipotential surfaces are closer to one another at the level of the component periphery, in the vicinity of vertical trench  31 , than in the central portion. The thickness of N-type layer  32  must thus be greater than its theoretical thickness to enable standing the voltage with no breakdown at the level of the component periphery.  
         [0038]     If, as illustrated in  FIG. 6 , instead of a vertical trench such as trench  31 , an oblique trench  41  according to the present invention is used, equipotential surfaces  36  become horizontal even in the peripheral region. The diode will have a higher breakdown voltage and the thickness selected for the lightly-doped N-type region will be able to be closer to its theoretical value. As indicated previously, the peripheral oblique trench may be formed in one or several steps. It may also be formed of a succession of close bores (as pinholes) joining after an oxidization step.  
         [0039]     More generally, a trench may be formed by providing a series of recesses close to one another, the intervals between recesses being specifically processed, for example, by forming from these recesses specifically diffused and/or oxidized regions joining from one recess to the adjacent recess.  
         [0040]     This is an example only of application of the present invention. Many other examples will occur to those skilled in the art. For example, strongly oblique trenches joining inside of the substrate may be used to delimit isolated areas in the substrate. Oblique vias may also be formed to solve topological connection problems.  
         [0041]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.