Method for thinning and reducing warpage of composite structure carried by polycrystalline SiC carrier substrate
By performing combined grinding and polishing on the back of the polycrystalline silicon carbide carrier substrate of the composite structure, the problem of excessive warping is solved, and effective thinning and warping of the composite structure is achieved, ensuring the manufacturing quality of electronic components.
Patent Information
- Application Number
- CN202380063045.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2022-09-07
- Filing Date
- 2023-09-06
- Publication Date
- 2025-04-15
AI Technical Summary
During the grinding and thinning process of the back of the composite structure, the warping amplitude is too large, exceeding the acceptable level of the back-end processing equipment, affecting the manufacturing quality of electronic components.
By performing a combination of coarse and fine grinding on the back of the polycrystalline silicon carbide carrier substrate, materials with thicknesses greater than 100 μm are removed and work hardened layers are removed by polishing, limiting warping to an acceptable range.
It effectively reduces the thickness of the composite structure while limiting warping to an acceptable level of rear-end processing, ensuring the manufacturing quality of electronic components and the stability of equipment.
Smart Images
Figure CN119836676A_ABST
Abstract
Description
Technical Field
[0001] The field of the invention is that of machining composite structures carrying elements of electronic components produced on a monocrystalline silicon carbide layer on a polycrystalline silicon carbide carrier substrate. Background Art
[0002] Silicon carbide (SiC) is increasingly being used in power electronics applications, especially to meet the needs of new application areas of electronics, such as electric vehicles. Specifically, single-crystal SiC-based power devices and integrated power supply systems are able to handle higher power densities than conventional silicon power devices and integrated power supply systems, and do so with smaller active area sizes.
[0003] However, substrates made of single-crystal SiC intended for use in the microelectronics industry remain expensive and difficult to supply on a large scale. It is therefore advantageous to adopt solutions that can produce composite structures, which typically include a thin layer made of single-crystal SiC on a low-cost carrier substrate. One such solution is Smart Cut TM A process which, for example, enables the production of a composite structure comprising a thin layer made of single-crystalline SiC (sampled from a donor substrate made of single-crystalline SiC) in direct contact with a carrier substrate made of polycrystalline SiC. The composite structure has a front side on the side of the single-crystalline SiC thin layer and a back side on the side opposite to the front side.
[0004] During the front-end processing, the elements of the electronic components (e.g., elements that allow the formation of vertical transistors) are formed on the front side of the composite structure by a combination of semiconductor thin film deposition or epitaxial growth, lithography, etching, doping, metal deposition and passivation steps. The back side of the composite structure is then thinned by grinding, for example, for a carrier substrate with a diameter of 150 mm, its thickness is reduced from 350 μm to 180 μm, or even to 100 μm. The target thickness reduced by this thinning depends on the subsequent back-end processing to be carried out (in which the chips are cut and packaged (packed)). The thickness is a compromise between the physical properties of the material (the chip needs to be mechanically supported) and its electrical properties (thinning reduces the contribution of the polycrystalline SiC carrier substrate to electrical losses).
[0005] Grinding causes warping of the composite structure. After grinding the back side of a monocrystalline SiC carrier substrate, the magnitude of this warping is still limited, e.g., about 400 μm to 600 μm for a carrier substrate with a diameter of 150 mm. In contrast, after the same type of grinding of the back side of a polycrystalline SiC carrier substrate, the magnitude of this warping is larger, e.g., close to 1000 μm for a carrier substrate with a diameter of 150 mm. This large magnitude may exceed the acceptable warping level (e.g., about 600 μm) for processing the composite structure after thinning by the equipment used during the back-end processing operation. Summary of the invention
[0006] The object of the present invention is to provide a solution that enables thinning of the back side of a composite structure comprising a thin monocrystalline SiC layer on which the elements of the electronic device are produced and which is carried by a polycrystalline SiC carrier substrate, while limiting the warpage to a level acceptable for the subsequent back-end processing.
[0007] To this end, the present invention provides a method for processing a composite structure, the composite structure comprising a single-crystal silicon carbide thin layer on a polycrystalline silicon carbide carrier substrate. The composite structure has a front side on the side of the single-crystal silicon carbide thin layer and a back side opposite to the front side. After forming an element of an electronic component on the front side of the composite structure, the method comprises thinning the composite structure from the back side by grinding the polycrystalline silicon carbide carrier substrate and removing the work hardening layer present on the surface of the back side after grinding.
[0008] Some preferred but non-limiting aspects of the method are as follows:
[0009] the polycrystalline silicon carbide carrier substrate has a thickness greater than 300 μm before said thinning and a thickness less than 200 μm after said thinning, preferably a thickness between 100 μm and 200 μm;
[0010] - The back side grinding includes coarse grinding and fine grinding in sequence;
[0011] - Rough grinding is performed using a grinding wheel with an abrasive size characterized by a mesh size of less than 5000;
[0012] - Fine grinding is performed using a grinding wheel with an abrasive size characterized by a mesh size greater than 5000;
[0013] - fine grinding is performed so as to remove material with a thickness between 1 μm and 3 μm;
[0014] - Rough grinding is performed so as to remove material with a thickness greater than 100 μm;
[0015] -Remove the work-hardened layer by polishing the back side;
[0016] - polishing is mechanical polishing or chemical-mechanical polishing;
[0017] - polishing is carried out for a time between 5 minutes and 30 minutes;
[0018] - polishing is performed to remove a material thickness between 0.2 μm and 2 μm;
[0019] - it further comprises metallizing the back side after removing the work hardened layer;
[0020] - After metallization, the composite structure is subjected to final manufacturing operations including chip dicing. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Other aspects, objects, advantages and features of the present invention will become more apparent on reading the following detailed description of preferred embodiments given by way of non-limiting examples with reference to the accompanying drawings, in which:
[0022] - Figure 1 is a schematic cross-sectional view of a single crystal SiC donor substrate;
[0023] - Figure 2 is achieved by implanting ions into Figure 1 Schematic cross-sectional view of a weakened plane formed in a donor substrate of the invention, the purpose of which is to define a single-crystalline SiC thin layer to be transferred;
[0024] - Figure 3 yes Figure 2 Schematic cross-sectional view of the bonding of a donor substrate with a polycrystalline SiC carrier substrate;
[0025] - Figure 4 is a schematic cross-sectional view of the separation of a donor substrate along a weakening plane, the purpose of which is to transfer a single-crystalline SiC thin layer to a multicrystalline SiC carrier substrate and thus form a composite structure;
[0026] - Figure 5 is Figure 4 A schematic cross-sectional view of an element of an electronic component formed on the front side of the composite structure;
[0027] - Figure 6 Yes Figure 5 A schematic cross-sectional view of thinning the back side of a composite structure;
[0028] - Figure 7 is a schematic cross-sectional view of removing a work-hardened layer existing on the surface of the back side after thinning;
[0029] - Figure 8Graphs showing warpage measurements at various steps of a post-manufacturing processing method for an electronic component. DETAILED DESCRIPTION
[0030] The invention relates to a method for processing a composite structure, wherein the composite structure comprises a single-crystal SiC thin layer located on a polycrystalline SiC carrier substrate and an element of an electronic device formed on the single-crystal SiC thin layer.
[0031] Before this processing, it can be based on Smart Cut TM The process transfers a single-crystal SiC thin layer from a donor substrate, at least part of the surface of which is made of single-crystal SiC, to a carrier substrate.
[0032] The donor substrate may be a bulk substrate of single crystal SiC. In other embodiments, the donor substrate may be a composite substrate comprising a surface layer of single crystal SiC and at least one other layer of another material. In this case, the thickness of the single crystal SiC layer is preferably greater than or equal to 0.3 μm.
[0033] The polycrystalline SiC carrier substrate is usually produced by chemical vapor deposition of polycrystalline SiC on a growth substrate (e.g. a graphite substrate). Therefore, the carrier substrate has columnar grains oriented in the growth direction of the deposition. The initial thickness of the polycrystalline SiC carrier substrate is preferably greater than 300 μm. For example, the polycrystalline SiC carrier substrate can take the form of a wafer with a diameter of 150 mm and an initial thickness of 350 + / - 25 μm. Alternatively, the polycrystalline SiC carrier substrate can take the form of a wafer with a diameter of 200 mm and an initial thickness of about 500 μm.
[0034] Reference Figure 1 , according to Smart Cut TM The transfer process starts with providing a donor substrate 10, at least part of the surface of which is made of single-crystal SiC. In the drawings, a bulk substrate 10 of single-crystal SiC is shown.
[0035] Reference Figure 2 The transfer further includes implanting ionic species into the donor substrate 10 to form a weakened plane 12 defining the single crystal SiC thin layer 11 to be transferred. The implanted species typically include hydrogen and / or helium. Those skilled in the art will be able to determine the required implantation dose and energy.
[0036] When the donor substrate is a composite substrate, implantation is performed to form a weakened plane in the surface layer of single crystal SiC of the donor substrate.
[0037] Preferably, the thickness of the single crystal SiC thin layer 11 is less than 1 μm. Specifically, this thickness can be reduced by Smart Cut TMThe process is implemented on an industrial scale. In particular, the injection devices available on industrial production lines allow such injection depths to be obtained.
[0038] Reference Figure 3 After the implantation, the transfer includes bonding the donor substrate to the carrier substrate 20, with the single crystal SiC thin layer 11 located at the interface. The bonding may be atomic diffusion bonding (ADB).
[0039] Reference Figure 4 , the transfer then comprises separating the donor substrate 10 along the weakening plane 12, thereby transferring the single-crystalline SiC thin layer 11 to the carrier substrate 20. In a known manner, this separation can be achieved by heat treatment, mechanical action or a combination of these means. The remaining part 10 'of the donor substrate can optionally be recycled for another use.
[0040] Then, one or more finishing operations may be applied to the transferred single-crystal SiC layer 11. For example, smoothing, cleaning or even polishing (e.g. chemical-mechanical polishing (CMP)) may be performed to remove defects related to the implantation of ionic species and to reduce the roughness of the transferred single-crystal SiC layer 11. A high-temperature heat treatment may also be performed, which has the effect of stabilizing the structure and thus securing its geometry in the following steps, provided that surface elements of the electrical transistor elements are not deposited on the surface.
[0041] like Figure 5 As shown, then, usually, the elements 30 of the electronic components are formed on the single crystal SiC thin layer 11 by a combination of the steps of semiconductor thin film deposition or epitaxial growth, photolithography, etching, doping, metal deposition and passivation. For example, these elements 30 of the electronic components include elements of vertical transistors. These elements 30 are located on the front side FF of the composite structure, that is, on the side of the single crystal SiC thin layer 11. In addition, the composite structure also has a back side BF opposite to the front side FF.
[0042] After forming these elements 30 of the electronic components on the front side FF, the composite structure is processed. This processing includes thinning the composite structure from the back side BF to reduce its thickness to a target thickness that meets the requirements of the back end processing. For example, the target thickness is 180 μm, or even less.
[0043] like Figure 6As shown, the machining consists of grinding the back side BF of the composite structure, i.e. grinding the free side of the polycrystalline SiC carrier substrate 20. The front side FF itself is usually covered with a protective tape. After grinding the back side of the composite structure, the thickness of the thinned composite structure is preferably between 100 μm and 200 μm. Alternatively, if precautions are taken to ensure the mechanical stability of the wafer after thinning, the thickness of the thinned composite structure can be less than 100 μm. As mentioned above, the warpage caused by such grinding may be more than twice the warpage observed when the same grinding is applied to the back side of a single-crystalline SiC carrier substrate carrying an element similar to an electronic component on the front side.
[0044] According to the inventors, this significant warping may be due to:
[0045] -Adopt Smart Cut TM processes that bond a thin layer of single-crystal SiC to a polycrystalline SiC carrier substrate (i.e., materials with slightly different coefficients of thermal expansion), and whose bonded interface may therefore exhibit residual stresses capable of causing warpage sensitivity;
[0046] - the contribution of the bulk carrier substrate, since its manufacturing process (which involves chemical vapor deposition followed by annealing) may retain residual stresses;
[0047] - Contribution of the surface of the carrier substrate, which, under the effect of grinding, ultimately leads to a certain surface roughness or damaged surface areas, thus causing surface stresses.
[0048] According to the present invention, grinding the back side of the composite structure may include coarse grinding followed by fine grinding.
[0049] Rough grinding allows the removal of polycrystalline SiC material of significant thickness (greater than 100 μm) at a rate compatible with industrial processes. For example, for a substrate having a diameter of 150 mm, rough grinding can remove a thickness of polycrystalline SiC between 150 μm and 250 μm, and for a substrate having a diameter of 200 mm, a thickness of polycrystalline SiC between 300 μm and 400 μm can be removed. Rough grinding can remove material at a rate greater than 0.2 μm / min, for example at a rate of 0.3 μm / min. Rough grinding can be performed by a grinding wheel having an abrasive size characterized by a mesh of less than 5000 (e.g., a mesh of 2000).
[0050] Rough grinding may generate significant surface stress on polycrystalline SiC, resulting in significant warpage. In particular, rough grinding may generate crystal defects with a depth of several microns in the columnar microstructure of polycrystalline SiC and cause a work-hardened layer to form on the surface of the back side of the carrier substrate.
[0051] Fine grinding can reduce pre-generated stresses by removing material, for example, between 1 μm and 3 μm. Fine grinding can be performed by a grinding wheel whose abrasive size is characterized by a mesh greater than 5000 (for example, a mesh of 8000). Fine grinding is performed at a rate less than coarse grinding, preferably at a rate less than 0.2 μm / min. At the end of fine grinding, the warpage is significantly reduced, but still greater than the acceptable warpage level for performing back-end processing. Therefore, fine grinding reduces the thickness of the work-hardened layer without completely eliminating the work-hardened layer.
[0052] According to a specific embodiment of rough grinding, the rough grinding itself can include very coarse grinding and not too coarse grinding in turn. Very coarse grinding can be performed by a grinding wheel with a feature of abrasive size that the mesh is less than 1000 (e.g., the mesh is 300), and not too coarse grinding can be performed by a grinding wheel with a feature of abrasive size that the mesh is less than 5000 (e.g., the mesh is 2000). The rate of very coarse grinding is preferably greater than the rate of not too coarse grinding. In this embodiment of rough grinding, very coarse grinding removes most of the total thickness of the polycrystalline SiC removed by rough grinding, and not too coarse grinding removes the last few microns. For example, not too coarse grinding removes 20μm of the total thickness of the polycrystalline SiC removed by rough grinding. Specifically, very coarse grinding can shorten the total length of the grinding process, but will produce a very thick work-hardened layer on the surface of the back side of the carrier substrate, the thickness of which is about 20μm. Not too coarse grinding allows the removal of the very thick work-hardened layer. As described above, not too coarse grinding also produces a work-hardened layer, but the thickness of the work-hardened layer produced by not too coarse grinding is about several micrometers.
[0053] In the embodiment where the coarse grinding comprises successively very coarse grinding and less coarse grinding, the coarse grinding will be followed by fine grinding as described above. The less coarse grinding makes it possible to obtain a work-hardened layer with a thickness less than that of a very thick work-hardened layer produced by very coarse grinding, which can be thinned by fine grinding and then finally removed in a time compatible with industrial processes. Figure 6 As shown, the composite structure thus has, after thinning from the rear side by grinding, a work-hardened layer 22 on the surface of the thinned polycrystalline SiC substrate 21 , which is not completely removed by fine grinding.
[0054] According to the present invention, referring to Figure 7, then remove the work-hardened layer 22 present on the surface of the back side of the composite structure after thinning. This removal is achieved, for example, by polishing the thinned back side. This polishing can be performed to remove a thickness of less than 3 μm, for example less than 2 μm or even between 0.2 μm and 1 μm or preferably between 0.2 μm and 0.5 μm. The advantage of this polishing is that, in addition to reducing the surface roughness, it can also reduce the warpage to a level acceptable for carrying out back-end processing.
[0055] Polishing can be carried out for a time between 5 minutes and 30 minutes, for example 10 minutes. Polishing can be carried out at a pressure between 5 decaN and 100 decaN, preferably between 7 decaN and 30 decaN. Polishing can be mechanical polishing (simple mechanical action, no chemical action) or chemical-mechanical polishing (CMP).
[0056] After removing the work-hardening layer, the method comprises the step of metallizing (e.g., local metallization) the back side of the composite structure. The purpose of this metallization is to form contacts or electrodes (e.g., drains of vertical transistors) on the back side for elements of the electronic components formed on the front side. In this case, the work-hardening layer is preferably removed in advance by mechanical polishing. Specifically, unlike chemical-mechanical polishing or pure chemical polishing, the advantage of mechanical polishing is that it avoids additional roughness due to the decoration of grain boundaries and thus obtains a surface finish that is more conducive to metallization attachment.
[0057] After such metallization, the composite structure processed according to the invention, which has a significantly reduced warpage, can be subjected to the final manufacturing operations of back-end processing, in particular chip dicing.
[0058] Figure 8 The figures show measurements of the warpage G (in μm) of five composite structures W1-W5, each made of a thin layer of single-crystalline SiC on a carrier substrate made of polycrystalline SiC and carrying an element of an electronic component on its front side, and of two bulk substrates W6-W7 of single-crystalline SiC carrying the same element of an electronic component on their front side. More precisely, Figure 8 In the figure, the square represents the warpage before thinning from the back side, the upward triangle represents the warpage after thinning from the back side including rough grinding and fine grinding in sequence, the downward triangle represents the warpage after the work hardening layer is removed by mechanical polishing, and the diamond represents the warpage after the work hardening layer is removed by chemical-mechanical polishing.
[0059] It can be seen that the warpage of bulk single crystal SiC substrates W6-W7 after thinning is already acceptable, while the warpage of composite structures W1-W5 after thinning is significant, greater than the maximum acceptable value of 600 μm. However, it can be seen that after removing the work hardening layer, the warpage of composite structures W1-W5 is significantly reduced and is less than the acceptable limit of 600 μm.
Claims
1. A method for machining a composite structure comprising a thin monocrystalline silicon carbide layer (11) on a polycrystalline silicon carbide carrier substrate (20), the composite structure having a front side (FF) on the side of the thin monocrystalline silicon carbide layer and a back side (BF) opposite the front side, After forming the elements (30) of the electronic device on the front side of the composite structure, the method includes thinning the composite structure from the back side by grinding the polycrystalline silicon carbide carrier substrate (20) and removing the work hardening layer (22) present on the surface of the back side after grinding.
2. The method according to claim 1, wherein: The polycrystalline silicon carbide carrier substrate has a thickness of more than 300 μm before the thinning and a thickness of less than 200 μm after the thinning, preferably a thickness between 100 μm and 200 μm.
3. The method according to any one of claims 1 and 2, wherein: The back side grinding includes coarse grinding and fine grinding in sequence.
4. The method according to claim 3, wherein: Rough grinding is performed using a grinding wheel having an abrasive size characterized by a mesh size of less than 5000.
5. The method according to any one of claims 3 and 4, wherein: Fine grinding is performed using a grinding wheel having an abrasive size characterized by a mesh size greater than 5000.
6. The method according to any one of claims 3 to 5, wherein: Fine grinding is performed so that a material thickness of between 1 μm and 3 μm is removed.
7. The method according to any one of claims 3 to 6, wherein: The coarse grinding is performed so that a material thickness of more than 100 μm is removed.
8. The method according to any one of claims 1 to 7, wherein: The work-hardened layer is removed by polishing the back side.
9. The method according to claim 8, wherein: Polishing is mechanical polishing.
10. The method according to claim 8, wherein: The polishing is chemical-mechanical polishing.
11. The method according to any one of claims 8 to 10, wherein: Polishing is performed for a time between 5 minutes and 30 minutes.
12. The method according to any one of claims 8 to 11, wherein: Polishing is performed to remove less than 3 μm of material thickness.
13. The method according to any one of claims 1 to 12, wherein: The method further includes metallizing the back side after removing the work hardened layer.
14. The method according to claim 13, wherein: After metallization, the composite structure undergoes final manufacturing operations including chip dicing.