WO2015069452A1

WO2015069452A1 - Methods and apparatus for thin-film substrate formation

Info

Publication number: WO2015069452A1
Application number: PCT/US2014/061617
Authority: WO
Inventors: James Gee
Original assignee: Applied Materials, Inc.
Priority date: 2013-11-05
Filing date: 2014-10-21
Publication date: 2015-05-14

Abstract

Embodiments of the present invention generally provide methods and substrate supports for the fabrication of thin silicon film substrates. In some embodiments, a method of forming thin film substrates includes supporting a bottom surface of an ingot on a chucking surface of a substrate support, applying a first chucking force to the bottom surface of the ingot sufficient to prevent the bottom surface from moving relative to the chucking surface, wherein the chucking surface is formed from a second material having a different coefficient of thermal expansion (CTE) than the first material of the ingot, and adjusting the temperature along the bottom surface of the the ingot to separate a thin film layer of the ingot along a fracture plane.

Description

METHODS AND APPARATUS FOR THIN-FILM SUBSTRATE FORMATION FIELD

[0001] Embodiments of the invention relate to the fabrication of thin silicon film substrates.

BACKGROUND

[0002] Some silicon substrates for semiconductor devices are manufactured by growing a silicon ingot and slicing the ingot into substrates or "wafers". The slicing process is very wasteful due to material lost during the cutting operation, sometimes referred to as kerf or kerf loss. Current silicon substrate production for silicon solar cells loses around 50% of the silicon during the slicing step due to the kerf loss. A technique for producing silicon films directly from the ingot without any kerf loss would eliminate this loss and make much more efficient use of the expensive high-purity silicon ingot.

[0003] Methods that eliminate the use of the slicing step, and thus the associated kerf loss, to produce thin silicon films are known as "kerfless" fabrication. One kerfless separation technique applies a layer in tension (stressor layer) on the silicon substrate to initiate horizontal fracture and lift a film from the substrate off the ingot. Thus, this technique requires application of a stressor material on the surface of the silicon substrate, and removal of the stressor layer after exfoliation of the film from the substrate. For example, aluminum (Al) and silver (Ag) films fired at a high temperature may be used as the stressor layer. However, application of a metal stressor layer, exposure to elevated temperatures, and removal of the stressor layer is expensive. The high temperature exposure induces very significant stresses so that the fracture is difficult to control. In addition, the lifted films with the stressor layer immediately form tight rolls/curls that are difficult to handle and could cause damage to the semiconductor film. In some kerflless separation processes, electroplated nickel (Ni) may be deposited near room temperature with a large internal stress. However, the application of thin-film metal adhesion layer and electroplated plated Ni layer has high processing cost, and the Ni stressor layer needs to be removed. The net result is that thin silicon film substrate fabrication is very costly.

[0004] Therefore, the inventor has provided methods for improved thin silicon film substrate formation.

SUMMARY

[0005] Embodiments of the present invention generally provide methods and substrate supports for the fabrication of thin silicon film substrates. In some embodiments, a method of forming thin film substrates includes supporting a bottom surface of an ingot on a chucking surface of a substrate support, applying a first chucking force to the bottom surface of the ingot sufficient to prevent the bottom surface from moving relative to the chucking surface, wherein the chucking surface is formed from a second material having a different coefficient of thermal expansion (CTE) than the first material of the ingot, and adjusting the temperature along the bottom surface of the the ingot to separate a thin film layer of the ingot along a fracture plane.

[0006] In some embodiments, a method of forming thin film substrates may include applying a first chucking force to retain a bottom surface of an ingot on a chucking surface of a substrate support, adjusting the temperature of the chucking surface and the ingot to a first temperature, applying a second chucking force to a top surface of the ingot and in same direction of the first chucking force, and selectively adjusting the temperature from the first temperature along the bottom surface of the ingot to separate a thin film layer of the ingot along a fracture plane.

[0007] In some embodiments, a substrate support for use in forming thin film substrates may include a body having a chucking surface to retain a bottom surface of an ingot when disposed thereon via chucking forces, a heater disposed in the body to heat the chucking surface and the ingot, a cooling system disposed in the body to selectively cool the bottom surface of the ingot from a first side of the ingot to a second side of the ingot.

[0008] Other and further embodiments of the present invention are described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the present invention, briefly summarized above and discussed in greater detail below, can be understood by reference to the illustrative embodiments of the invention depicted in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0010] Figures 1A-1 F illustrate schematic cross-sectional views of an ingot and a substrate support during different stages in fabrication sequence for forming thin film substrates according to some embodiment of the invention.

[0011] Figure 2 depicts a flowchart of a method for forming thin-film substrates in accordance with some embodiments of the present invention.

[0012] Figure 3 depicts a schematic cross-sectional view of a vacuum chuck according to some embodiment of the invention.

[0013] Figure 4 depicts a schematic cross-sectional view of an electrostatic chuck according to some embodiment of the invention.

[0014] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The figures are not drawn to scale and may be simplified for clarity. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0015] Embodiments of the present invention generally provide methods and apparatus to efficiently exfoliate/peel thin layers of a material from an ingot of the material without the use of sawing or cutting of the material (i.e., kerfless film formation). The exfoliated thin layers of material may be used as substrates in the manufacture of semiconductor devices. For example, thin silicon films may be processed into solar cells, and the like. In some embodiments, materials other than, or in addition to, silicon may be used, such as, for example, germanium, gallium arsenide, sapphire, and the like. Embodiments of the kerfless thin-film substrate formation described herein advantageously do not require a stressor layer material for removing the thin-film layer from the ingot, described above, thereby reducing process steps and cost, as well enhancing control over the removed thin-film layer.

[0016] Figures 1A-1 F illustrate schematic cross-sectional views of an ingot 102 and a thin-film formation apparatus during different stages in fabrication sequence for forming thin film substrates according to some embodiments of the invention. Figure 2 depicts a flowchart of a method 200 for forming thin-film substrates in accordance with some embodiments of the present invention. Figures 1A-1 F will be described in conjunction with the method 200 of Figure 2 for forming thin-film substrates below.

[0017] As shown in figure 1A, the ingot 102 includes a side surface 104 and a bottom surface 106. The ingot 102 may be a block of material that the thin-film substrate is to be formed from. In some embodiments, the ingot 102 may be a rectangular block of material or a circular disk of material from which the thin-film substrate is to be formed. In the process described below, layers of the ingot 102 will be removed. The layers that are removed will be used, for example, as the thin-film substrates in the manufacture of semiconductor devices. The materials that ingot 102 may be formed from may include, but are not limited to, monocrystalline silicon, multicrystalline silicon, germanium, GaAs, SiC, sapphire, and the like. The thickness of the ingot 102 may be about 0.5 mm to about 200 mm. The ingot 102 may be reused, and additional thin-film substrates formed from the ingot, until the thickness of the ingot is no longer suitable to remove additional thin-film substrate layers off of the ingot 102.

[0018] As shown in Figure 1 B, the thin-film formation apparatus 100 includes a substrate support 1 10 having a chucking support surface 1 12 to support the ingot 102. The substrate support 1 10 includes a retention mechanism, such as a vacuum chuck (as shown and described in Figure 3) or an electrostatic chuck (as shown and described in Figure 4) to secure the ingot 102 to the thin-film formation apparatus 100. The substrate support 1 10 generally includes a body 1 14 having a heater 1 16 and cooling device 122 disposed therein. In some embodiments, a shaft 120 may be provided for supporting the body 1 14. The body 1 14 may be made out of any materials suitable to withstand processing conditions, such as copper, aluminum nitride, aluminum oxide, stainless steel, aluminum, pyrolytic boron nitride, or the like. In some embodiments, the material of the body 1 14 and chucking support surface 1 12 are selected such that the chucking support surface 1 12 and the ingot 102 have different coefficients of thermal expansion (CTE). In some embodiments, the body 1 14 has a substantially planar chucking support surface 1 12 for supporting a substrate thereupon. However, the inventors have observed that by allowing the chucking support surface 1 12 to have a slight radius, such that the center of the chucking support surface 1 12 is raised relative to the ends of the chucking support surface 1 12, fracture initiation of the ingot 102 and/or dechucking/lift-off of the thin-film substrate from the chucking support surface 1 12 may be improved. For example, in some embodiments, the chucking support surface 1 12 may have a radius of about 3 meters to about 30 meters.

[0019] The heater 1 16 generally comprises one or more resistive elements embedded in the body 1 14. The resistive elements may be independently controllable to create a plurality of heater zones. A temperature indicator 1 15 may be provided to monitor the processing temperature. As one example, the temperature indicator can be a thermocouple, positioned such that it provides data correlating to the temperature at the chucking support surface 1 12 (or at the surface of a substrate disposed thereon). The heater 1 16 and temperature indicator 1 15 may be coupled to a controller 1 18 via conduit 1 17 to control the operation of the heater 1 16 during processing. A thermocouple or other temperature measurement metrology may also be used to monitor the temperature of the ingot 104.

[0020] The cooling device 122 may include various ways to cool the bottom surface of the ingot, such as for example, one or more cooling channels or cooling elements that may be independently controllable to create cooling zones. In some embodiments, the cooling device 122 may include thermoelectric coolers. The cooling device 122 may be controlled such that cooling may be selectively initiated on one end of the bottom surface of the ingot 102, and progress to the opposite end of the bottom surface of the ingot 102. In embodiments where the cooling device 122 is one or more coolant channels, the coolant device 122 may be coupled to a coolant source 124 via a conduit 123 to circulate coolant from the coolant source 124 through the coolant channels. In some embodiments, the coolant device 122 may include multiple cooling zones, wherein the cooling zones may be independently controlled by a controller (e.g., controller 1 18 or a separate controller). Although depicted as being disposed above the heater 1 16, the cooling device 122 may alternatively be disposed beneath the heater 1 16 or interspersed with the heater 1 16. Alternatively, the ingot 104 could be heated directly with an external heater and the substrate support 1 10 would only include the cooling device 122.

[0021] In embodiments where the substrate support 1 10 is a vacuum chuck as shown in Figure 3, the shaft 120 may have one or more openings 302 (or other mechanism, such as tubes, hoses, or the like) that fluidly couple chucking holes 304 disposed on the chucking support surface 1 12 to a vacuum system 308 via conduits 310. In some embodiments, vacuum channels 306 may be fluidly coupled to chucking holes 304. Accordingly, in operation, a substrate may be disposed on the chucking support surface 1 12 of the substrate support 1 10 and retained thereon by application and maintenance of vacuum pressure within the vacuum channels 306 via the chucking holes 304. Accordingly, in operation, the ingot 102 may be disposed on the chucking support surface 1 12 of the substrate support 1 10 and retained thereon by application and maintenance of vacuum pressure within the chucking holes. The shaft 120 further comprises a central passageway to facilitate routing of facilities or connectors to the body 1 14 of substrate support 1 10.

[0022] In embodiments where the substrate support 1 10 is an electrostatic chuck as shown in Figure 4, an electrode 402 may be provided within the body 1 14 and coupled to a power source 404 (such as a DC power supply) to produce electrostatic forces to retain the ingot 102 to the chucking support surface 1 12.

[0023] The method 200 begins at 202 and proceeds to 204 where an ingot (e.g., ingot 102 shown in Figure 1A) of a selected material is formed. After the ingot 102 is formed, a fracture initiation crack 108 may optionally be formed along the outer surface of the ingot 102 at 206 of method 200. The fracture initiation crack 108 may be formed on one edge of the ingot, such as for example, surface 104. In other embodiments, the fracture initiation crack 108 may be formed about a plurality of sides, including the entire perimeter of ingot 102. In embodiments where the ingot is round, the fracture initiation crack may be formed around the cylinder surface. In some embodiments, the fracture initiation crack may be formed with laser ablation, electrical discharge machining (EDM), or by dicing a groove. The fracture initiation crack 108 may be formed at a distance from the bottom surface 106 of the ingot 102 selected to produce a thin-film substrate of a desired thickness. For example, in some embodiments, the fracture initiation crack 108 may be formed about 30 microns to about 1000 microns above the bottom surface 106 of the ingot 102. The fracture initiation crack 108 may extend into the ingot 102 (i.e., depth of the crack) by about 10 microns to about 200 microns. The inventors have observed that by using a fracture initiation crack 108, the thickness of the thin-film substrate that is removed from the ingot 102 can be better controlled. [0024] At 208, the ingot 102 is placed on a chucking surface 1 12 of the substrate support 1 10. In some embodiments, the ingot 102 is at room temperature when it is placed on the chucking surface 1 12. In other embodiments, the ingot 102 may be heated or cooled prior to placing it on the chucking surface 1 12. In some embodiments, the ingot is kept at an elevated temperature and only the chuck is actively heated and cooled.

[0025] At 210, a weak chucking force 130 is optionally applied to hold the ingot 102 on the chucking surface 1 12 as shown in Figure 1 B. As used herein, a weak chucking force is a force sufficient to hold the ingot on the substrate support while allowing mating surfaces of the ingot and the substrate support (i.e., the bottom 106 and the chucking surface 1 12) to slip or move relative to each other. In embodiments where substrate support 1 10 is a vacuum chuck, the chucking forces 130 applied are vacuum chucking forces. In embodiments where substrate support 1 10 is an electrostatic chuck, the chucking forces 130 applied are electrostatic chucking forces. In some embodiments, the weak chucking force 130 may not be applied if the ingot can be secured by other methods, or if the ingot does to move during heating.

[0026] At 212, the chucking surface 1 12 and the ingot 102 are heated to a first temperature by heater 1 16 (e.g., by heat 132) as shown in Figure 1 C. In some embodiments, the temperature that the chucking surface 1 12 and the ingot 102 are heated to may depend on the material, and associated material properties, that the body 1 14 and the ingot 102 are made of. In some embodiments, the temperature would need to be raised such that the fracture strength of the ingot material is reached. For example, if the body 1 14 and chucking surface 1 12 are made from copper having a coefficient of thermal expansion (CTE) equal to about 16 ppm/K, and the ingot 102 is formed on silicon, the temperature would need to be raised by about 100°C (ΔΤ) to reach the fracture strength of silicon. Thus, if the silicon ingot 102 was placed on the chucking surface 1 12 at room temperature of about 30°C, the total temperature the ingot 102 and chucking surface 1 12 would have to be heated to about 130°C. A large tensile stress is applied to the surface of the silicon ingot due to the CTE mismatch after securing the silicon to the chuck and cooling the ingot and chuck back to room temperature. Exfoliation of a thin layer from the ingot occurs when the internal stress exceeds the fracture strength of the ingot material. Thus, in the example above, when the internal stress of a silicon ingot gets above the stress fracture threshold for silicon, exfoliation will occur. In some embodiments, if the chucking surface 1 12 includes a slight radius as described above, a smaller ΔΤ may be used since the radius of the chucking surface 1 12 would help create tensile stress within the ingot 102. The change in temperature ΔΤ described above depends upon the film thickness desired and on the material properties of the substrate support 1 10 relative to the ingot. Typical ΔΤ values may be about 25 degrees C to about 200 degrees C for ingot materials such as silicon, germanium, gallium arsenide, sapphire, and the like.

[0027] In some embodiments, the ingot 102 may be separately heated via external heating devices and placed on the chucking support surface 1 12. That is, the ingot might be directly heated (inductively or convectively) to an elevated temperature and only the temperature of the chucking support surface 1 12 is cycled. Specifically, the ingot might have a high mass and therefore require a lot of time to heat and cool. Therefore, it may be more efficient in some embodiments to externally heat the ingot prior to being placed on the chucking support surface 1 12.

[0028] At 214, after the ingot 102 and chucking support surface 1 12 are heated at 212 to the desired temperature, strong chucking forces 134 are applied, as shown in Figure 1 D. As used herein, a strong chucking force is a force sufficient to hold the ingot on the substrate support and not allowing mating surfaces of the ingot and the substrate support (i.e., the bottom 106 and the chucking support surface 1 12) to slip or move relative to each other. Thus, the strong chucking forces 134 bond the bottom surface of the ingot 102 and the chucking support surface 1 12 with substantially zero CTE stress at the first heated temperature. [0029] In some embodiments, an additional chucking force may be applied to a top surface of the ignot 102 in a direction to augment that of the strong chucking forces 134 to assist in exfoliation of the thin-film layer and ingot 102. Such a force could be applied, for example, by a mechanical press.

[0030] At 216, the temperature of the bottom surface 106 of the ingot 102 is selectively adjusted from the first temperature to another temperature to separate the thin-film layer from the ingot 102 along fracture plane 140, as shown in Figure 1 E. The temperature is selectively adjusted while the strong chucking forces 134 are still applied to prevent slippage between the bottom 106 of the ingot 102 and the chucking support surface 1 12. In some embodiments, selectively adjusting the temperature, indicated by cooling lines 141 , from the first temperature along the bottom surface includes cooling of the bottom surface of ingot starting from the first side of the ingot with the facture initiation crack. In some embodiments, selective or active cooling of the ingot along the bottom surface of the ingot controls fracture plane initiation and direction. Selectively adjusting the temperature from the first temperature creates internal tensile stresses within the ingot to initiate separation of the thin film layer along the fracture plane 140. In some embodiments, the thickness of the thin film layer may be controlled by the first temperature of ingot and chucking surface. In other embodiments where a fracture initiation crack 108 is formed in a surface of ingot 102, selectively cooling the bottom surface 106 of the ingot 102 will initiate fracture along fracture initiation crack 108.

[0031] At 218, the remaining portion of the ingot 102' is removed completely, as shown in Figure 1 F. The remaining portion of the ingot 102' may be reused in the process described above to remove additional thin-film substrate layers to be used in semiconductor manufacturing process.

[0032] At 220, the chucking forces are removed (i.e., dechucking) and the thin-film layer 142 is removed for use as desired, as shown in Figure 1 G. The release of the chucking forces at end of process removes all stress on the thin-film layer 142. Since the temperature of the chucking support surface 1 12 and the thin-film layer 142 are typically at the same temperature after processing, the removed film is planar, or substantially planar, at all times.

[0033] In some embodiments, the thin film layer 142 formed has a thickness from about 20 microns to about 200 microns. The process ends at 222.

[0034] While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.

Claims

What is claimed is:

1 . A method of forming thin film substrates from an ingot formed from a first material, comprising:

supporting a bottom surface of an ingot on a chucking surface of a substrate support;

applying a first chucking force to the bottom surface of the ingot sufficient to prevent the bottom surface from moving relative to the chucking surface, wherein the chucking surface is formed from a second material having a different coefficient of thermal expansion (CTE) than the first material of the ingot; and

adjusting a temperature along the bottom surface of the the ingot to separate a thin film layer of the ingot along a fracture plane.

2. The method of claim 1 , further comprsing:

forming a fracture initiation crack along a surface of a first side of the ingot, wherein the fracture plane is along the fracture initiation crack.

3. The method of claim 2, wherein a fracture initiation crack is one of laser scribed, electrical discharge machined, or mechanically sawed.

4. The method of claim 2, wherein adjusting the temperature along the bottom surface of the ingot includes selectively cooling the bottom surface of the ingot starting from the first side of the ingot with the fracture initiation crack to an opposite side of the ingot.

5. The method of any of claims 1 to 4, wherein supporting a bottom surface of the ingot on the chucking surface includes applying a second chucking force that is sufficient to hold the ingot on the substrate support while allowing the bottom surface of the ingot to move relative to the chucking surface, and wherein the second chucking force is less than the first chucking force.

6. The method of any of claims 1 to 4, wherein prior to applying the first chucking force, the method further includes heating the chucking surface and the ingot to a first temperature.

7. The method of claim 6, wherein a thickness of the thin film layer is controlled by a magnitude of the first temperature and the difference in CTE between the first material and the second material.

8. The method of any of claims 1 to 4, wherein the first material that the ingot is formed from is one of silicon, germanium, SiC, germanium, gallium arsenide, or sapphire.

9. The method of any of claims 1 to 4, wherein the first chucking force is sufficient to hold the ingot on the chucking surface while not allowing mating surfaces of the ingot and the chucking surface to move relative to each other.

10. The method of any of claims 1 to 4, wherein adjusting the temperature along the bottom surface of the ingot controls fracture plane initiation and direction.

1 1 . The method of any of claims 1 to 4, wherein adjusting the temperature along the bottom surface of the ingot includes cooling the chucking surface and the bottom surface of the ingot.

12. The method of any of claims 1 to 4, wherein adjusting the temperature creates internal tensile stresses within the ingot to initiate separation of the thin film layer along the fracture plane.

13. The method of any of claims 1 to 4, wherein the thin film layer has a thickness from about 20 microns to about 100 microns.

14. A substrate support for use in forming thin film substrates, comprising:

a body having a chucking surface to retain a bottom surface of an ingot formed from a first material when disposed thereon via chucking forces;

a heater disposed in the body to heat the chucking surface and the ingot; and a cooling system disposed in the body to selectively cool the bottom surface of the ingot from a first side of the ingot to a second side of the ingot.

15. The substrate support for claim 14, wherein the substrate support is one of a vacuum chuck capable of provide vacuum chucking forces to retain the ingot, or an electrostatic chuck capable of provide electrostatic chucking forces to retain the ingot.