Process for the production of non-deformable p-SiC wafers

Abstract

The present invention comprises the following steps: Heat treatment of the polycrystalline silicon carbide slab (1); Thinning of a polycrystalline silicon carbide slab, said thinning including correction of deformations caused by heat treatment by removal of material from the polycrystalline silicon carbide slab; The present invention relates to a process for the manufacture of polycrystalline silicon carbide wafers, comprising:

Description

[Technical field]

[0001] The field of the invention is that of the manufacture of polycrystalline silicon carbide wafers, intended in particular to act as a support for thin layers of monocrystalline silicon carbide. [Background technology]

[0002] (prior art) Silicon carbide (SiC) is being increasingly used in power electronics applications, particularly to meet the needs of an ever-increasing range of electronic devices, such as electric vehicles. Power devices and integrated power systems based on single crystal SiC can indeed manage much higher power densities than their traditional silicon counterparts, and can do so with smaller sized active areas.

[0003] Nevertheless, substrates made of monocrystalline SiC, where SiC is intended for the microelectronics industry, remain expensive and difficult to supply in large sizes. It is therefore advantageous to resort to layer transfer solutions to generate composite structures comprising thin layers made of monocrystalline SiC, generally on lower-cost carrier substrates. One well-known thin layer transfer solution is the Smart Cut™ process, which is based on the implantation of light ions and assembling by direct bonding. Such a process makes it possible to produce composite structures comprising thin layers made of monocrystalline SiC, taken from a donor substrate made of monocrystalline SiC, and in direct contact with a carrier substrate made of polycrystalline SiC, for example.

[0004] Nevertheless, these composite structures tend to exhibit high bow ("bow" means a parabolic curve with rotational symmetry, especially with respect to the center of the slab) and warp ("warp" means a deformation with a radius of curvature that is positive in one axis and negative in the other axis) values. These high values ​​are especially the result of the relaxation of stresses caused by the manufacture of the carrier substrate made of polycrystalline SiC, which is likely to occur when the carrier substrate is subjected to a heat treatment during the Smart Cut process (for example, during the separation annealing, which is generally carried out at temperatures of the order of 1300-1700° C. to manipulate the transfer of the thin layer from the donor substrate to the carrier substrate) or, after the Smart Cut process, during the formation of electronic components on the transferred thin layers (generally at temperatures of the order of 1800-2000° C.).

[0005] These high bow and deformation values ​​are problematic in that they can result in fracture of the composite structure or cause alignment problems during the lithography steps required to form the power components. Summary of the Invention

[0006] DISCLOSURE OF THE PRESENT ART The object of the present invention is to provide a technique for the production of polycrystalline SiC wafers which makes it possible to limit, and indeed even to eliminate, the risk of deformation of the wafers during subsequent heat treatments.

[0007] For this purpose, the present invention comprises the following steps: Heat treating the polycrystalline silicon carbide slab; and thinning a polycrystalline silicon carbide slab, said thinning including correction of deformations caused by heat treatment by removal of material from the polycrystalline silicon carbide slab.

[0008] Some preferred, but non-limiting aspects of this process are as follows: The removal of material is carried out by grinding a polycrystalline silicon carbide slab; Removal of material from the polycrystalline silicon carbide slab is carried out by electrical discharge machining; Material removal is performed on both the front and back sides of the polycrystalline silicon carbide slab; The removal of material is performed so that the wafer presents a front and back surface that are flat and parallel to each other; the thinning step includes removing a thickness of material of 50 micrometers or more on at least one of the faces of the slab; This process comprises a stage of production of a slab by deposition of material on a growth substrate, a stage of heat treatment preceded by a stage of separation of the slab and the growth substrate; The heat treatment is carried out at a temperature between 1650° C. and 2000° C. for a period of more than 10 minutes. The heat treatment includes a steady state period of 1850°C. The heat treatment includes a stationary phase and a temperature decrease from the stationary phase to a target temperature. The process involves deposition of polycrystalline silicon carbide onto a growth substrate to form a polycrystalline silicon carbide slab, followed by removal of the growth substrate, prior to a heat treatment. The heat treatment is carried out at a temperature higher than the temperature of deposition of the polycrystalline silicon carbide on the growth substrate.

[0009] The invention further extends to the manufacture of composite structures by transfer of a thin layer made of monocrystalline silicon carbide from a monocrystalline silicon carbide substrate to a polycrystalline silicon carbide wafer manufactured according to the invention, which may additionally include the formation of electronic components in the transferred thin layer at a temperature lower than the temperature of the heat treatment applied during the manufacture of the wafer. [Brief description of the drawings]

[0010] Other aspects, objects, advantages and features of the present invention will become more clearly apparent from a reading of the following detailed description of preferred embodiments of the invention, given as non-limiting examples and made with reference to the accompanying drawings, in which: [Figure 1] FIG. 2 shows a polycrystalline silicon carbide slab after a surface smoothing step. [Diagram 2]FIG. 2 illustrates the deformation of a polycrystalline silicon carbide slab caused by a heat treatment step. [Diagram 3] FIG. 13 illustrates the correction of deformations caused by heat treatment by removal of material. [Figure 4] FIG. 2 illustrates a multicrystalline silicon carbide wafer obtained according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Detailed Disclosure of Specific Embodiments The present invention relates to a process for the production of polycrystalline silicon carbide (p-SiC) wafers from p-SiC slabs, which wafers, of course, exhibit a reduced thickness relative to the thickness of the slab.

[0012] Deposition of p-SiC onto a growth substrate (e.g., a graphite substrate), typically by chemical vapor deposition at temperatures between 1200° C. and 1400° C., allows for the formation of relatively thick (e.g., 2-3 mm thick) p-SiC slabs. Different crystalline forms (also called polytypes) of silicon carbide exist. The most common are the 4H, 6H, and 3C forms. Preferably, the polytype of the p-SiC slab thus formed is the 3C polytype, although all polytypes can be envisaged for the implementation of the present invention.

[0013] After removal of the growth substrate, the p-SiC slab undergoes a process of formation of one or more wafers (wafering process), which includes various cleaning, etching, grinding and polishing steps, making it possible to obtain one or more p-SiC wafers with the desired morphology (in particular with chamfered edges) and the desired thickness. Sawing may also be performed during this process, in particular if several wafers have to be produced from one and the same slab.

[0014] The removal of the growth substrate, if it is made of graphite, is carried out, for example, by combustion of the graphite. For this purpose, a heating step in the presence of oxygen, for example at a combustion temperature of 800° C. or higher, is often used. The combustion temperature is often below 1000° C.

[0015] According to the invention, a thermal treatment is inserted into this wafering process in order to prepare a wafer that will not be deformed during a subsequent thermal treatment, for example during the implementation of the Smart Cut process or during the preparation of electronic components, which wafering process is further adapted so that the wafer thus prepared is flat and does not exhibit bow or warp.

[0016] Thus, starting from a p-SiC slab separated from its growth substrate, the process according to the invention for the production of p-SiC wafers comprises heat treatment of the slab and thinning of the slab.

[0017] In the possible embodiment illustrated by figure 1, the step of heat treatment may be preceded by a step of smoothing of the p-SiC slab 1. This surface smoothing may be carried out by grinding or by mechanical or chemical-mechanical polishing. This surface smoothing may furthermore make it possible to remove the growth seeds of the p-SiC crystal used during the formation of the slab 1.

[0018] In another possible embodiment, which may or may not be combined with the previous embodiment, the heat treatment step is preceded by a step of cleaning the p-SiC slab 1.

[0019] The heat treatment is performed at a temperature higher than the temperature of deposition of p-SiC on the growth substrate during formation of the slab, and also higher than the maximum temperature of any subsequent heat treatment, e.g., higher than the temperature of any subsequent fabrication heat treatment of an electronic component.

[0020] The heat treatment is preferably carried out at a temperature between 1650° C. and 2000° C. for a period of more than 10 minutes. This heat treatment can in particular be carried out at a temperature of at least 1700° C., for example at 1850° C., at 1900° C. or even at 2000° C. The heat treatment can include a temperature increase / decrease ramp of between 10° C. / min and 100° C. / min.

[0021] The heat treatment can be carried out at low pressure (generally below 100 mbar, for example below 50 mbar, in particular between 10 and 30 mbar) or even at pressures higher than 100 mbar, indeed even at atmospheric pressure.

[0022] The heat treatment is generally carried out under a neutral atmosphere, for example under an argon or nitrogen atmosphere.

[0023] The heat treatment may include a stationary phase. The heat treatment may further be carried out by adjusting the temperature drop from the stationary phase to the target temperature. In an embodiment, the heat treatment includes a stationary phase of 1850°C. This stationary phase may have a duration of 30 minutes. The temperature increase may be carried out with a gradient of 10°C / min. The temperature drop may be adjusted, for example, to drop to 1000°C with a gradient of 10°C / min. The temperature drop from the target temperature to ambient temperature is then carried out by following the thermal inertia of the furnace used to carry out this heat treatment.

[0024] As shown in FIG. 2, the heat treatment tends to cause deformations (bow, warp) of the slab 1. The thinning of the slab is then adapted to include (and, where appropriate, consist of) the correction of the deformations caused by the heat treatment by removal of material from the polycrystalline silicon carbide slab. The removal of material is generally adapted locally so that the slab is not thinned the same at all points. This correction of the deformations results in a reduction in the bow or warp values ​​observed after the heat treatment.

[0025] Figure 3 shows a possible embodiment of this thinning, which in this case is carried out until the parallel dotted lines are reached. On the other hand, Figure 4 shows a wafer 2 obtained according to the invention from the slab 1 of Figure 1.

[0026] According to a possible embodiment, the removal of material aimed at correcting the deformations caused by the heat treatment is performed by grinding the p-SiC slab. In another embodiment, this material removal is performed by electro-discharge machining. Compared to grinding, the removal of material by electro-discharge machining presents the advantage that it can be performed without contact with the slab and without artificially creating deformations by elastic bending. In yet another embodiment, the removal of material combines electro-discharge machining and grinding. In this latter embodiment, electro-discharge machining can perform a coarse thinning, while grinding performs a finer thinning.

[0027] In an embodiment, the thinning comprises successively a very coarse thinning (by electro-discharge machining or grinding) which will result in the removal of a thickness of, for example, 150 μm or more, a coarse grinding which will result in the removal of a thickness of, for example, 20 μm, and a fine grinding which will result in the removal of a thickness of, for example, 3 μm. The different grinding steps differ in the size of the particles of the grinding wheels used, which particles become progressively smaller in the sequence of the grinding steps.

[0028] Optionally, a mechanical or chemical-mechanical polishing step is performed after the final grinding step.

[0029] As shown by Figure 3, thinning can be performed on both the front and back sides of the p-SiC slab. And as shown by Figures 3 and 4, this thinning is selectively performed so that the front and back sides of the wafer obtained at the end of the process are flat and parallel to each other. These front and back sides of the wafer 2 are not necessarily parallel to the front and back sides of the starting slab 1.

[0030] By way of example, thinning amounts to removing a thickness at least equal to the value of the deformation after heat treatment minus 25 μm.

[0031] Typically the thickness removed from each of the faces of the slab during post heat treatment thinning will be, for example, 50 μm or more, in particular 100 μm or more, indeed even 150 μm or more.

[0032] The thinning is in particular such that it results in a free-standing wafer, i.e. the thickness of the wafer is such that it does not break or plastically deform under the effect of its own weight, for example 200 μm or more, in particular 300 μm or more.

[0033] In particular, a thickness between 175 μm and 200 μm can be removed from each of the faces of the slab for a total thinning of 350-400 μm. Thus, wafers with a thickness between 325 μm and 375 μm can be obtained from a slab that has been subjected to a first thinning before a heat treatment resulting in a thickness of 725 μm.

[0034] The thinning of the slab may then be followed by a step of wafer finishing, particularly aimed at making the wafer smoother.

[0035] The invention further extends to a process for the manufacture of composite structures, comprising the manufacture of the p-SiC wafers described above and the transfer of a thin layer made of monocrystalline silicon carbide from a monocrystalline silicon carbide substrate to a polycrystalline silicon carbide wafer. This transfer can be carried out according to the Smart Cut technology and therefore comprises the implantation of ionic elements into the monocrystalline silicon carbide substrate to form a weakened surface that defines the boundary of the thin layer to be transferred, the bonding of the monocrystalline silicon carbide substrate to the polycrystalline silicon carbide wafer (if appropriate via one or more bonding layers), and then the detachment of the monocrystalline silicon carbide substrate along the weakened surface to transfer the thin active layer to the polycrystalline silicon carbide wafer (caused by heat treatment, mechanical action or a combination of these means). The process for the manufacture of composite structures can additionally comprise the formation of electronic components, in particular power or radio frequency components, in the transferred thin layer.

[0036] The fact that the heat treatment is carried out upstream of the wafering process makes it possible to avoid excessive deformation of the wafers after they have been thinned and flattened under the influence of subsequent exposure to high temperatures.

[0037] If the thermal treatment is performed after separation of the slab and the growth substrate, the treatment does not cause further deformation of the slab that would be associated with the stresses exerted on the slab by the growth substrate during heating of the slab / substrate assembly, which further contributes to improving the flatness and stability of the final wafer.

[0038] According to an alternative form that can be envisaged, the step of thermal treatment is shared with a step of removal by combustion of the carrier substrate made of graphite. For example, during this step, it is the assembly formed by the wafer 1 and the carrier substrate that is brought to a temperature of 1650° C. or more as described above in the presence of oxygen so as to cause the combustion of the graphite while thermally treating the slab 1. According to another form that can be envisaged, the step of removal by combustion and the step of thermal treatment are carried out successively in the same furnace, preferably in this order. In this case, the furnace is first heated to 800° C. or more in the presence of oxygen and then, after the injected oxygen has been purged from the furnace, heated to above 1650° C., for example in a neutral atmosphere.

Claims

1. A process for the manufacture of polycrystalline silicon carbide wafers (2) comprising the following steps: Heat treatment of the polycrystalline silicon carbide slab (1); thinning the polycrystalline silicon carbide slab; the thinning comprises correcting deformation caused by the heat treatment by removing material from the polycrystalline silicon carbide slab; process.

2. The process of claim 1 , wherein the material removal is performed by grinding the polycrystalline silicon carbide slab.

3. The process of claim 1 , wherein the removal of material from the polycrystalline silicon carbide slab is performed by electrical discharge machining.

4. The process of any one of claims 1 to 3, wherein the removal of material is carried out on both the front and back sides of the polycrystalline silicon carbide slab.

5. The process of claim 4 , wherein the removal of material is performed so that the wafer presents front and back surfaces that are flat and parallel to each other.

6. The process of claim 4 , wherein the thinning step comprises the removal of a thickness of material on at least one of the faces of the slab of 50 micrometers or more.

7. 4. The process according to claim 1, comprising a step of producing the slab by deposition of material on a growth substrate, the step of heat treatment being preceded by a step of separation of the slab and the growth substrate.

8. A process according to any one of claims 1 to 3, wherein the heat treatment is carried out at a temperature between 1650°C and 2000°C for a period of more than 10 minutes.

9. 9. The process of claim 8, wherein the heat treatment comprises a steady state period of 1850°C.

10. 9. The process of claim 8, wherein the heat treatment comprises a steady-state phase and a controlled decrease in temperature from the steady-state phase to a target temperature.

11. 4. The process of claim 1, further comprising depositing polycrystalline silicon carbide on a growth substrate to form the polycrystalline silicon carbide slab prior to the heat treatment, followed by removal of the growth substrate.

12. The process of claim 11 , wherein the heat treatment is carried out at a temperature greater than the temperature of the deposition of the polycrystalline silicon carbide on the growth substrate.

13. 4. A process for the manufacture of a composite structure, comprising the manufacture of a polycrystalline silicon carbide wafer by the process of any one of claims 1 to 3 and the transfer of a thin layer made of monocrystalline silicon carbide from a monocrystalline silicon carbide substrate to the polycrystalline silicon carbide wafer.

14. 14. The process of claim 13, additionally comprising forming electronic components on the transferred thin layer at a temperature lower than the temperature of the heat treatment of the process of any one of claims 1 to 3.

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