Method for thinning a composite structure carried by a polycrystalline sic carrier substrate, with reduced warpage
Patent Information
- Application Number
- EP2023777328
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-09-07
- Filing Date
- 2023-09-06
- Publication Date
- 2025-07-16
Smart Images
Figure 1.1
Abstract
Description
[0001] DESCRIPTION
[0002] TITLE: Method for thinning a composite structure carried by a polycrystalline SiC support substrate, with reduced warping
[0003] TECHNICAL FIELD
[0004] The field of the invention is that of the packaging of a composite structure carrying electronic component elements produced on a layer of monocrystalline silicon carbide arranged on a support substrate of polycrystalline silicon carbide.
[0005] PRIOR ART
[0006] Silicon carbide (SiC) is increasingly widely used in power electronics applications, particularly to meet the needs of emerging electronics fields such as electric vehicles. Power devices and integrated power systems based on single-crystal SiC can effectively handle much higher power density than their traditional silicon counterparts, even with smaller active area dimensions.
[0007] Monocrystalline SiC substrates for the microelectronics industry remain expensive and difficult to source in large quantities. It is therefore advantageous to use solutions for producing composite structures typically comprising a thin monocrystalline SiC layer on a lower-cost support substrate. One such solution is the Smart Cut™ process, which, for example, makes it possible to manufacture a composite structure comprising a thin monocrystalline SiC layer, taken from a monocrystalline SiC donor substrate, in direct contact with a polycrystalline SiC support substrate. The composite structure has a front face on the side of the thin monocrystalline SiC layer and a back face on the side opposite the front face.
[0008] During an initial manufacturing process called "Front-End", electronic component elements (for example elements for forming vertical transistors) are formed on the front face of the composite structure by a combination of semiconductor film deposition or epitaxy, lithography, etching, doping, metal deposition and passivation steps. A thinning on the back face of the composite structure is then carried out by grinding which, for example for a 150 mm diameter support substrate, reduces its thickness from 350 pm to 180 pm, or even down to 100 pm. The reduced thickness targeted by this thinning depends on the final manufacturing process called "Back-End" which is subsequently implemented and in which we find the cutting of the chips and their encapsulation (packaging).This thickness results from a compromise between the physical properties of the material (maintaining mechanical support for the chips) and its electrical performance (thinning allows the contribution of the polycrystalline SiC support substrate to electrical losses to be reduced).
[0009] Grinding introduces warping of the composite structure. This warping remains of limited amplitude after grinding the back face of a monocrystalline SiC support substrate, for example of the order of 400 to 600 pm for a support substrate of 150 mm in diameter. On the other hand, this warping has a significant amplitude after grinding of the same type of the back face of a polycrystalline SiC support substrate, for example close to 1000 pm for a support substrate of 150 mm in diameter. This significant amplitude may exceed an acceptable level of warping, for example of the order of 600 pm, so that after thinning the composite structure can be handled by the equipment used during the final manufacturing operations of the "Back-End" process.
[0010] STATEMENT OF THE INVENTION
[0011] The invention aims to propose a solution for thinning the rear face of a composite structure comprising a thin layer of monocrystalline SiC on which electronic component elements have been produced and which is carried by a polycrystalline SiC support substrate, while limiting warping to an acceptable level for the rest of the "Back-End" process.
[0012] To this end, the invention proposes a method for conditioning a composite structure comprising a thin layer of monocrystalline silicon carbide arranged on a support substrate of polycrystalline silicon carbide. The composite structure has a front face on the side of the thin layer of monocrystalline silicon carbide and a rear face opposite the front face. The method comprises, after forming electronic component elements on the front face of the composite structure, thinning the composite structure from its rear face after grinding the support substrate of polycrystalline silicon carbide and removing a work-hardened layer present on the surface of the rear face after grinding.
[0013] Some preferred but non-limiting aspects of this method are as follows: the polycrystalline silicon carbide support substrate has, before said thinning, a thickness greater than 300 μm and after said thinning a thickness less than 200 μm, preferably a thickness of between 100 μm and 200 μm; the grinding of the rear face comprises, in succession, coarse grinding and fine grinding; the coarse grinding is carried out with a grinding wheel whose abrasive grain size is characterized by a mesh of less than 5000; the fine grinding is carried out with a grinding wheel whose abrasive grain size is characterized by a mesh of greater than 5000; the fine grinding is carried out so as to remove material with a thickness of between 1 μm and 3 μm; the coarse grinding is carried out so as to remove material with a thickness greater than 100 μm; the removal of the work-hardened layer is carried out by polishing the rear face;the polishing is a mechanical polishing or a chemical-mechanical polishing; the polishing is carried out for a period of between 5 and 30 minutes; the polishing is carried out so as to remove a material thickness of between 0.2 pm and 2 pm; it further comprises a metallization of the rear face following the removal of the work-hardened layer; following the metallization, the composite structure is subjected to final manufacturing operations which include chip cutting. BRIEF DESCRIPTION OF THE DRAWINGS;
[0014] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:
[0015] - Figure 1 is a schematic sectional view of a monocrystalline SiC donor substrate;
[0016] - Figure 2 is a schematic sectional view of the formation, by implantation of ionic species, of a weakening plane in the donor substrate of Figure 1 to delimit a thin layer of monocrystalline SiC to be transferred;
[0017] - Figure 3 is a schematic sectional view of the assembly of the donor substrate of Figure 2 and a polycrystalline SiC support substrate;
[0018] - Figure 4 is a schematic sectional view of the detachment of the donor substrate along the embrittlement plane to transfer the thin layer of monocrystalline SiC onto the support substrate of polycrystalline SiC and thus form a composite structure;
[0019] - Figure 5 is a schematic sectional view of the formation of electronic component elements on the front face of the composite structure of Figure 4;
[0020] - Figure 6 is a schematic sectional view of the thinning on the rear face of the composite structure of Figure 5;
[0021] - figure 7 is a schematic sectional view of the removal of the work-hardened layer present on the surface of the rear face at the end of thinning;
[0022] - Figure 8 is a diagram illustrating warping measurements at different stages of a post-manufacturing packaging process for electronic component elements.
[0023] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0024] A method of packaging a composite structure comprising a thin film of monocrystalline SiC disposed on a support substrate of polycrystalline SiC and electronic component elements formed on the thin film of monocrystalline SiC is disclosed. This packaging may have been preceded by transferring, in accordance with the Smart Cut™ process, the thin film of monocrystalline SiC carbide to the support substrate from a donor substrate of which at least a surface portion is made of monocrystalline SiC.
[0025] The donor substrate may be a bulk substrate of monocrystalline SiC. In other embodiments, the donor substrate may be a composite substrate, comprising a surface layer of monocrystalline SiC and at least one other layer of another material. In this case, the layer of monocrystalline SiC preferably has a thickness greater than or equal to 0.3 μm.
[0026] The polycrystalline SiC support substrate is typically produced by chemical vapor deposition of polycrystalline SiC on a growth substrate, for example a graphite substrate. The support substrate thus has columnar grains oriented in the growth direction of the deposit. The polycrystalline SiC support substrate preferably has an initial thickness greater than 300 μm. For example, the polycrystalline SiC support substrate may be in the form of a 150 mm diameter plate having an initial thickness of 350 + / - 25 μm. Alternatively, the polycrystalline SiC support substrate may be in the form of a 200 mm diameter plate having an initial thickness of approximately 500 μm.
[0027] Referring to Figure 1, the transfer according to the Smart Cut™ process begins by providing a donor substrate 10 of which at least a surface portion is made of monocrystalline SiC. In the figures, a solid substrate 10 of monocrystalline SiC is shown.
[0028] With reference to Figure 2, this transfer further comprises an implantation of ionic species in the donor substrate 10 so as to form a weakening plane 12 delimiting a thin layer of monocrystalline SiC to be transferred 11. The implanted species typically comprise hydrogen and / or helium. A person skilled in the art is able to define the required implantation energy and dose.
[0029] When the donor substrate is a composite substrate, the implantation is carried out so as to form the embrittlement plane in the surface layer of monocrystalline SiC of said donor substrate. Preferably, the thin layer 11 of monocrystalline SiC has a thickness of less than 1 μm. Such a thickness is in fact accessible on an industrial scale with the Smart Cut™ process. In particular, the implantation devices available in industrial manufacturing lines make it possible to achieve such an implantation depth.
[0030] With reference to Figure 3, the transfer comprises, after said implantation, the bonding of the donor substrate and the support substrate 20, the thin layer 11 of monocrystalline SiC being at the interface. The bonding may be an atomic diffusion bonding known as ADB for “Atomic Diffusion Bonding”.
[0031] With reference to Figure 4, the transfer then comprises the detachment of the donor substrate 10 along the embrittlement plane 12 so as to transfer the thin layer of monocrystalline SiC 11 onto the support substrate 20. In a known manner, this detachment can be caused by a heat treatment, a mechanical action or a combination of these means. The remainder 10' of the donor substrate can possibly be recycled for another use.
[0032] One or more finishing operations can then be applied to the transferred monocrystalline SiC layer 11. It is for example possible to carry out smoothing, cleaning or even polishing, for example by chemical mechanical polishing (CMP), to remove the defects linked to the implantation of ionic species and reduce the roughness of the transferred monocrystalline SiC layer 11. A high temperature heat treatment can also be carried out which has the effect of stabilizing the structure and thus guaranteeing its geometry for the following steps, as long as transistor electrical component elements on the surface have not been deposited on the surface.
[0033] As shown in FIG. 5, electronic component elements 30 are then formed on the thin layer of monocrystalline SiC 11, typically by a combination of steps of deposition or epitaxy of semiconductor film, lithography, etching, doping, metal deposition and passivation. These electronic component elements 30 comprise for example vertical transistor elements. These elements 30 are arranged on a front face FF of the composite structure located on the side of the thin layer of monocrystalline SiC 11. The composite structure also has a rear face BF opposite the front face FF.
[0034] Once these electronic component elements 30 have been formed on the front face FF side, the composite structure is conditioned. This conditioning includes thinning the composite structure from its rear face BF to bring it to a target thickness that meets the requirements of the “Back-End” process. This target thickness is, for example, 180 μm, or even less.
[0035] As shown in Figure 6, this conditioning comprises grinding of the rear face BF of the composite structure, i.e. grinding of the free face of the polycrystalline SiC support substrate 20. The front face FF is typically covered with a protective tape. After grinding on the rear face of the composite structure, the thinned composite structure has a thickness preferably between 100 μm and 200 μm. Alternatively, the thinned composite structure may have a thickness of less than 100 μm if precautions are taken to ensure the mechanical stability of the plate after thinning. As indicated previously, this grinding causes warping which may be more than twice as significant as that observed when identical grinding is applied to the rear face of a monocrystalline SiC support substrate carrying similar electronic component elements on the front face.
[0036] This significant warping could, according to the inventors, be explained by:
[0037] - through the implementation of the Smart Cut™ process which combines a thin layer of monocrystalline SiC and a support substrate of polycrystalline SiC, i.e. materials which have slightly different thermal expansion coefficients and whose bonding interface can therefore present a residual stress capable of causing sensitivity to warping;
[0038] - by a contribution from the solid support substrate which, due to its manufacturing process by chemical vapor deposition followed by annealing, can retain a residual stress; - by a contribution from the surface of the support substrate which, under the effect of grinding, results in a specific surface roughness or a damaged surface area inducing a surface stress.
[0039] According to the invention, the grinding on the back face of the composite structure may comprise rough grinding followed by fine grinding.
[0040] Rough grinding allows the removal of a significant thickness of material, greater than 100 pm of polycrystalline SiC, at a speed compatible with an industrial process. For example, rough grinding can remove a thickness of polycrystalline SiC between 150 pm and 250 pm for a substrate with a diameter of 150 mm and a thickness of polycrystalline SiC between 300 pm and 400 pm for a substrate with a diameter of 200 mm. Rough grinding can achieve material removal at a speed greater than 0.2 pm / min, for example at a speed of 0.3 pm / min. It can be carried out using a grinding wheel whose abrasive grain size is characterized by a mesh size of less than 5000, for example a mesh size of 2000.
[0041] Rough grinding is likely to generate significant surface stress on polycrystalline SiC, leading to significant warping. In particular, rough grinding generates crystal defects a few micrometers deep in the columnar microstructure of polycrystalline SiC and leads to the formation of a work-hardened layer on the surface of the backside of the support substrate.
[0042] Fine grinding reduces the stress previously created by removing, for example, between 1 and 3 µm of material. It can be carried out using a grinding wheel with an abrasive grain size characterized by a mesh greater than 5000, for example a mesh of 8000. Fine grinding is carried out at a slower speed than coarse grinding, preferably at a speed lower than 0.2 µm / min. At the end of fine grinding, the warping is considerably reduced but remains higher than the acceptable warping level for the implementation of the "Back-End" process. Fine grinding thus reduces the thickness of the work-hardened layer, without completely removing it.
[0043] According to a particular embodiment of the coarse grinding, said coarse grinding may itself successively comprise very coarse grinding and less coarse grinding. The very coarse grinding may be carried out using a grinding wheel whose abrasive grain size is characterized by a mesh of less than 1000, for example a mesh of 300, and the less coarse grinding may be carried out using a grinding wheel whose grain size is characterized by a mesh of less than 5000, for example a mesh of 2000. The very coarse grinding is preferably carried out at a speed higher than the speed of the less coarse grinding. According to this embodiment of the coarse grinding, the very coarse grinding removes the majority of the total thickness of the polycrystalline SiC removed by coarse grinding and the less coarse grinding removes the last micrometers thereof.For example, less coarse grinding removes 20 μm of the total thickness of the polycrystalline SiC removed by rough grinding. Indeed, very coarse grinding allows to shorten the total duration of the grinding process but generates a very thick work-hardened layer on the surface of the back face of the support substrate of about 20 μm thickness. Less coarse grinding allows to remove said very thick work-hardened layer. Less coarse grinding also generates a work-hardened layer, but the thickness of said work-hardened layer generated by less coarse grinding is of the order of a few micrometers as previously mentioned.
[0044] In the embodiment according to which the coarse grinding successively comprises very coarse grinding and less coarse grinding, said coarse grinding is also followed by fine grinding as previously described. The less coarse grinding makes it possible to obtain a work-hardened layer that is less thick than the very thick work-hardened layer generated by the very coarse grinding, which can be thinned by fine grinding and then finally removed in a time compatible with an industrial process. As shown in FIG. 6, the composite structure thus has, following its thinning on the rear face by grinding, a work-hardened layer 22 on the surface of the thinned polycrystalline SiC substrate 21, this work-hardened layer 22 not having been completely removed by the fine grinding.
[0045] According to the invention, and with reference to Figure 7, the work-hardened layer 22 present on the surface of the rear face of the composite structure is then removed after thinning. This removal is for example carried out by polishing the thinned rear face. This polishing can be carried out so as to remove a thickness of less than 3 μm, for example a thickness of less than 2 μm or a thickness of between 0.2 μm and 1 μm or preferably between 0.2 μm and 0.5 μm. This polishing has the advantage, in addition to reducing the surface roughness, of reducing the warping to an acceptable level for implementing the “Back-End” process.
[0046] Polishing can be carried out for a period of between 5 and 30 minutes, for example a period of 10 minutes. It can be carried out under a pressure of between 5 and 100 decaN, preferably between 7 decaN and 30 decaN. Polishing can be mechanical polishing (simple mechanical action, without chemistry) or chemical-mechanical polishing of the CMP type.
[0047] Following the removal of the work-hardened layer, the process includes a metallization step, for example localized, of the rear face of the composite structure. This metallization aims to form contacts or electrodes (for example vertical transistor drains) on the rear face for the electronic component elements formed on the front face. In this context, it is preferable to have previously carried out the removal of the work-hardened layer by mechanical polishing. Unlike mechanical-chemical or purely chemical polishing, mechanical polishing has the advantage of avoiding the creation of additional roughness by decorating the grain boundaries and hence leading to a surface condition more favorable to the adhesion of the metallization.
[0048] Once this metallization has been carried out, the composite structure packaged according to the invention with greatly reduced warping can be subjected to final manufacturing operations of a “Back-End” process, in particular chip cutting.
[0049] Figure 8 shows warping measurements G (in pm) of five composite structures W1-W5, each consisting of a thin layer of monocrystalline SiC on a polycrystalline SiC support substrate and carrying electronic component elements on the front face, and two solid substrates W6-W7 of monocrystalline SiC carrying identical electronic component elements on the front face. More precisely, in this figure 8, the squares represent the warping before thinning on the back face, the upward pointing triangles represent the warping after thinning on the back face comprising in succession a rough grinding and a fine grinding, the downward pointing triangles represent the warping after removal of the work-hardened layer by mechanical polishing and the diamonds represent the warping after removal of the work-hardened layer by chemical-mechanical polishing.
[0050] It is observed that the warping of the solid monocrystalline SiC substrates W6-W7 is already acceptable after thinning, while the warping of the composite structures W1-W5 is significant after thinning, exceeding the maximum acceptable value of 600 pm. However, it appears that after removal of the work-hardened layer, the warping of the composite structures W1-W5 is significantly reduced and is now below the acceptable limit value of 600 pm.
Claims
CLAIMS 1. Method for conditioning a composite structure comprising a thin layer of monocrystalline silicon carbide (11) arranged on a support substrate of polycrystalline silicon carbide (20), the composite structure having a front face (FF) on the side of the thin layer of monocrystalline silicon carbide and a rear face (BF) opposite the front face, the method comprising, after forming electronic component elements (30) on the front face of the composite structure, thinning the composite structure from its rear face by grinding the support substrate of polycrystalline silicon carbide (20) and removing a work-hardened layer (22) present on the surface of the rear face at the end of the grinding.
2. Method according to claim 1, in which the polycrystalline silicon carbide support substrate has, before said thinning, a thickness greater than 300 pm and after said thinning a thickness less than 200 pm, preferably a thickness of between 100 pm and 200 pm.
3. Method according to one of claims 1 or 2, in which the grinding of the rear face comprises in succession a coarse grinding and a fine grinding.
4. Method according to claim 3, in which the coarse grinding is carried out with a grinding wheel whose abrasive grain size is characterized by a mesh of less than 5000.
5. Method according to one of claims 3 or 4, in which the fine grinding is carried out with a grinding wheel whose abrasive grain size is characterized by a mesh greater than 5000.
6. Method according to one of claims 3 to 5, in which the fine grinding is carried out so as to remove material with a thickness of between 1 μm and 3 μm.
7. Method according to one of claims 3 to 6, in which the rough grinding is carried out so as to remove material with a thickness greater than 100 μm.
8. Method according to one of claims 1 to 7, in which the removal of the work-hardened layer is carried out by polishing the rear face.
9. The method of claim 8, wherein the polishing is mechanical polishing.
10. The method of claim 8, wherein the polishing is chemical-mechanical polishing.
11. Method according to one of claims 8 to 10, in which the polishing is carried out for a period of between 5 and 30 minutes.
12. Method according to one of claims 8 to 11, in which the polishing is carried out so as to remove a thickness of material less than 3 μm.
13. Method according to one of claims 1 to 12, further comprising metallization of the rear face following removal of the work-hardened layer.
14. The method of claim 13, wherein, following metallization, the composite structure is subjected to final manufacturing operations which include chip dicing.