111111111111111111111111111111111111111111111111111111111111111111111111111 US006387829Bl (12) (54) (75) (73) United States Patent (10) Usenko et al. (45) SEPARATION PROCESS FOR SILICON-ONINSUIATOR WAFER FABRICATION 5,250,460 5,374,564 5,710,057 5,994,207 6,013,567 6,221,774 Inventors: Alexander Yuri Usenko, Murray Hill; William Ned Carr, Montclair, both of NJ (US) Assignee: Silicon Wafer Technologies, Inc., Newark, NJ (US) ( * ) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U.s.c. 154(b) by 0 days. (21) Appl. No.: 09/543,998 (22) Filed: (60) Related U.S. Application Data Provisional application No. 60/139,851, filed on Jun. 18, (51) (52) (58) Int. CI? U.S. CI Field of Search Apr. 6, 2000 1999. (56) HOiL 21/425; HOlL 21/265 438/977; 438/120 438/15, 977, 120 References Cited U.S. PATENT DOCUMENTS 4,846,931 A 7/1989 Gmitter Patent No.: US 6,387,829 Bl Date of Patent: May 14,2002 A A A A A B1 * * 10/1993 12/1994 1/1998 11/1999 1/2000 4/2001 Yamagata Brnel Kenney Henley et al. Henley Malik 438/515 438/690 * cited by examiner Primary Examiner-Amir Zarabian Assistant Examiner-Beth E. Owens (74) Attorney, Agent, or Firm-DeMont & Breyer, LLC (57) ABSTRACT A process for manufacturing a silicon-an-insulator wafer from a silicon wafer assembly. The assembly is made of two wafers. One of the wafers contains a fragile layer. The fragile layer is a layer containing a high amount of hydrogen. An amount of energy from an energy source is applied to the assembly to separate the assembly along the fragile layer thus forming a silicon-on-insulator wafer and a leftover wafer. The energy source is selected from the group consisting of: ultrasound, infrared, hydrostatic pressure, hydrodynamic pressure, or mechanical energy. The amount of energy is chosen to be sufficient to transform the fragile layer into a quasi-continuous gaseous layer. Under separation the hydrogen-enriched layer transforms into layer consisting of hydrogen platelets and hydrogen microbubbles. 13 Claims, 9 Drawing Sheets 1::\0 120 110 100 u.s. Patent US 6,387,829 Bl Sheet 1 of 9 May 14,2002 c Fig. L L ｾ ｾ｟ ｾ ｾ ｾ ｾ Ｍｃｾ L ｾ ｾ｟ ｾ 1. Prior Art __;r Fig. __;r Fig. 3. Prior Art :::l- Fig. 4. Prior Art __;r 2. Prior Art u.s. Patent ｾＱＰＶ ＶＰＱｾ｜ May 14,2002 Sheet 2 of 9 US 6,387,829 Bl 601 rIO L-----------------J ----------------- 60.1 Fig.5. Prior art /,00 10 EXPANDING CLEAVE FRONT Fig.6. Prior art u.s. Patent [3 May 14,2002 Sheet 3 of 9 ＮｆＭ Ｍ Ｍ Ｍ Ｍ ｾ Fig. 7. Prior Art Fig. 8. Prior Art US 6,387,829 Bl u.s. Patent May 14,2002 Sheet 4 of 9 US 6,387,829 Bl Fig.9A 1:10 120 110 100 Fig.9B ｾＱＰＰ Fig.9C u.s. Patent May 14,2002 Sheet 5 of 9 .........•.•...••.•..........•.••............... Fig. to US 6,387,829 Bl u.s. Patent Sheet 6 of 9 May 14,2002 Fig. 11 US 6,387,829 Bl u.s. Patent Sheet 7 of 9 May 14,2002 121 120 Fig.l2A Fig.12B US 6,387,829 Bl u.s. Patent May 14,2002 Sheet 8 of 9 US 6,387,829 Bl 35 Fig.13A Fig.13B u.s. Patent May 14,2002 Sheet 9 of 9 Fig.l4A 14.2 l.p 140 Fig.14B US 6,387,829 Bl US 6,387,829 Bl 1 2 patents on the ELTRAN process of fabrication of siliconon-insulator wafers. In this technique the film separation is obtained by etching off a sacrificial layer comprised of porous silicon. This application claims priority from the provisional patent application entitled "Separation Process For Silicon- 5 Still another variation of the epitaxial lift-off process for On-Insulator Wafer" and, filed Jun. 18, 1999 and assigned film separation is described by Kenney in U.S. Pat. No. application No. 60/139,851, the disclosure of which is 5,710,057 [5]. In this technique an etchant distribution is hereby incorporated incus entirety for all purposes. facilitated by capillary action in trenches preformed in the seed substrate. BACKGROUND OF THE INVENTION 10 A disadvantage of the Bruel [lJ technique is that the The present invention relates to the manufacture of roughness of the as-cut surface requires polishing (e.g., silicon-on-insulator substrates. More particularly, the invenCMP) to smooth the surface. This polishing affects the tion provides a technique for separating (cleaving) a subthickness uniformity of the device layer across the wafer. strate as part of the fabrication process of silicon-onThus, the polishing process, while improving local insulator wafers for semiconductor integrated circuits and 15 roughness, simultaneously increases thickness variations. microelectromechanical systems. Another disadvantage of the Bruel process [1 J is that the assembly sometimes cleaves along an undesired plane. The Delamination of thin films (micron range thickness) from solids such as a single crystal is a processing step that is desired plane is along the peak density of the implanted useful for variety of technologies, including semiconductor hydrogen. The undesirable plane is at a prebonded interface processing technology. Prior art includes separation methods 20 between wafers of the assembly. Defective silicon-oninsulator wafers are the result. as (1) ion cut with thermal initiation (Bruel), (2) ion cut with jet initiation (Henley), and (3) variations of sacrificial layer A disadvantage ofthe epitaxial lift-off technique [3J is that etching (Gmitter, Yonehara, Kenney). the area of delaminated film is limited to about 1 square inch, The separation technique through a hydrogen-rich layer 25 that is much less that typical silicon wafer size (4-12 inches was described by Bruel, U.S. Pat. No. 5,374,564 [1]. This in diameter). So the process is not applicable to mainstream technique is a part of Bruel's process [lJ used to fabricate semiconductor processing. silicon-on-insulator wafers. The technique uses thermal A disadvantage of etch-stop based separation techniques treating of a wafer assembly that includes a hydrogen[3,4,5J is the difficulty in obtaining a uniform layer thickness implanted wafer. The annealing temperature used is above 30 for large areas. Since the etchant etches silicon in addition to that at which ion implantation takes place. Typical annealing the sacrificial layer, there is a tendency to reduce the temperatures are in range from 400 to 500 0 C. Under thickness at the thin film silicon near the outer perimeter of annealing the implanted hydrogen begins to diffuse inside of the wafer. The result is a separated thin film of decreasing the wafer. Hydrogen coagulates into precipitates that serve thickness along radii toward the wafer perimeter. as nuclei for subsequent structure transfonnations. Then flat 35 Adisadvantage of the side jet technique [2J is that plasma platelets consisting of hydrogen are formed from the nuclei. immersion ion implantation dose needed in the process is The platelets are arranged along <100> silicon crystallo10" 8 cm- 2 . Such a high dose severely deteriorates the quality graphic planes. The platelets have top and bottom silicon of the delaminated layer. <100> surfaces with dangling bonds terminated by hydrogen. Next, bigger platelets continue to grow in expense of 40 SUMMARY OF THE INVENTION smaller platelets according to the Ostvald ripening mechaA technique is detailed for forming a silicon film from a nism. Finally, the continuous hydrogen layer is formed along donor silicon substrate with <100> or <111> surface orienthe plane of the maximum implanted hydrogen. Following tations. annealing, the former single wafer is separated into two 45 A The technique utilizes a step of forming a hydrogen-rich thinner wafers. layer in a donor substrate at a selected depth underneath the Another technique to delaminate a top layer from a silicon surface where the hydrogen atoms have a relatively high wafer using the buried hydrogen-rich layer is described by concentration to define a donor substrate material above the Henley in U.S. Pat. No. 6,013,567 [2]. The wafer is cleaved selected depth. The hydrogen-rich layer may be obtained by along the hydrogen-rich plane using a pressurized fluid jet applied initially to the edge of the wafer. The cleavage 50 implanting hydrogen ions through a surface into the donor wafer. initiates at the edge due to the jet action and the cleavage To initiate separation of thus prepared wafer, an energy wave then propagates through the substrate to release a thin source is applied to the substrate. The source is selected from film of material from the substrate. the group consisting of ultrasound, hydrostatic pressure, Still another technique to selectively peel a film from a single crystal is described by Gmitter in U.S. Pat. No. 55 hydrodynamic pressure, infrared light, mechanical, or combination thereof. Said energy source is applied in such a way 4,846,931 [3]. The technique is usually referred as epitaxial that energy is deposited preferentially in the hydrogen-rich lift-off. In this technique an epitaxial film is released from a layer. single crystal substrate upon which it is grown. The technique comprises (a) providing a thin release layer (1000 A) The application of said energy source coagulates hydrobetween the film to be grown and the substrate; (b) growing 60 gen into nuclei having platelet shapes oriented along crysthe epitaxial film(s); (c) applying a polymeric support layer tallographic cleavage planes, that is usually the <100> plane. which is under tension over the film; and (d) selectively The platelets then form a continuous layer thus releasing the etching the release layer, the tension in the support layer adjacent silicon film. causing the edges of the film to curve upward as the release For said ultrasound and infrared energy cases, choosing layer is etched away. 65 preferable parameters means the wavelength is chosen smaller than said donor substrate material thickness A variation of the epitaxial lift-of process is described by Yamagata in U.S. Pat. No. 5,250,460 [4J and subsequent (typically less than 10 micrometers). SEPARATION PROCESS FOR SILICON-ONINSUIATOR WAFER FABRICATION US 6,387,829 Bl 3 4 separation of a thin film from a single crystal silicon wafer Advantage of the present invention is the technique adds previously implanted with proton particles or with hydrogen flexibility for creating customized silicon-on-insulator based ions. The implanted wafer can be covered with thermally microstructures and integrated circuits. Also the technique grown silicon dioxide layer 105 (FIG. 9) before the implanrises the yield of silicon layer transfer as the hydrogen platelet coagulation along the interface of prebonded silicon 5 tation stage. FIG. 9A is a simplified cross-sectional view of the initial oxidized and wafer 100 implanted with protons wafers is suppressed. Also the technique improves a surface 200 that form a hydrogen-rich layer 110 and a thin film to be roughness of the silicon-on-insulator wafers obtained. separated 120. FIG. 9B is a simplified cross-sectional view diagram of the wafer 100 with a stiffener wafer 130 attached. BRIEF DESCRIPTION OF THE DRAWINGS This is an ioitial structure to be separated. FIG. 9C shows the FIGS. 1 through 4 show prior art that is related to 10 final step of the separation process when the top assembly consisting of stiffener wafer 130 and separated thin film 120 heat-caused separation [1]. is completely detached from initial implanted substrate 100. FIG. 1 shows the concentration profile of the hydrogen The process is used for the manufacture of silicon-onions as a function of the penetration depth. insulator wafers from regular silicon wafers that are preFIG. 2 shows the monocrystalline semiconductor wafer 15 implanted with ions. The wafer undergoes an energy load exposed to a bombardment of H+ ions and within which has that transforms the pre-implanted layer into of a layer 110 of appeared a gas microbubble layer produced by the implanted gaseous microbubbles in the wafer. The wafer can be than particles. easy separated into two wafers along the layer of FIG. 3 shows the semiconductor wafer of FIG. 2 and microbubbles. In the FIG. 9A the implantation 200 occurs 20 through an upper surface of the wafer 100 that is substancovered with a stiffener. tially parallel to a main crystallographic plane of the FIG. 4 shows the cross-section view of assembly of the substrate, for example the <100> plane. The implanted semiconductor wafer and the stiffener wafer shown in FIG. specie is preferably protons. 3 at the end ofthe separation phase, when cleaving has taken The implantation allows for the formation of a layer of place between the film and the substrate mass. gaseous microblisters 110 within the volume of the wafer at FIGS. 5 through 8 shows previous art that is related to 25 a depth approximately equal to the average ion penetration jet-caused separation [2]. depth. This layer of microblisters delimits a surface layer FIG. 5 is a simplified cross-sectional view of an implanted 120 within wafer 100, which will form the top part 120 of substrate 10 using selective positioning of cleave energy silicon-on-insulator wafer 140 in FIG. 9C. In particular, it 601. 30 should be noted that during ion implantation the wafer is FIG. 6 illustrates the controlled-propagating cleave with preferably kept at a temperature below the temperature at which the atoms of the implanted gas can move away by successive impulses 701, 705, 709 to initiate and then propagate a cleariog process 700. thermal diffusion from the crystaL For all preferred embodiments described below, an FIG. 7 shows a cross-sectional view of a pressurized fluid jet from a fluid nozzle to perform a controlled cleaving 35 amount of energy from an energy source is applied to the wafer assembly FIG. 9B. After applying the amount of process. energy a continuous hydrogen layer is formed at the place of FIG. 8 shows a cross-sectional view of a pressurized fluid the hydrogen-rich layer 110 thereby releasing silicon-onsource 607. insulator wafer 150 (FIG. 9C) from byproduct wafer 100 FIG. 9 shows a cross-sectional view that illustrates the 40 (FIG. 9C.) separation process due to present invention. Preferred Embodiment 1: Separating with Ultrasound FIG. 10 illustrates the first preferred embodiment of the Energy Load separation process due to the present invention where an FIG. 10 shows the preferred embodiment utilizing ultraultrasound load is used. sound as the activation energy for separation. The ultrasound FIG. 11 illustrates the second preferred embodiment of 45 energy load is applied to the wafer assembly FIG. 9C the separation process due to the present invention where a through either the top or bottom surface. hydrostatic pressure load is used. For the separation, the implanted wafer is subjected to a sensitizing load for a given time, chosen such that the FIG. 12 illustrates the third preferred embodiment of the hydrogen in the substrate which was introduced by the separation process due to the present invention where infra50 hydrogen implantation is partly released from its attachred laser scanning is used. ments to the defects (which were generated by the hydrogen FIG. 13 illustrates the forth preferred embodiment of the trap-inducing implantation) and as well by the hydrogen separation process due to the present invention where a implaimplantation itself. This treatment causes the formawater jet is used. tion and growth of hydrogen filled microcracks at a depth FIG. 14 illustrates the fifth preferred embodiment of the 55 close to the maximum in the concentration depth profile of separation process due to the present invention where implanted hydrogen. This step must not cause hydrogen mechanical structures are used. induced surface blisters, which would prevent subsequent bonding of the first substrate to a second substrate. DETAILED DESCRIPTION OF THE At the end of energy activation the implanted hydrogen in PREFERRED EMBODIMENTS 60 the substrate is fully released from chemical bonding to the The present invention provides a technique for removing defects. These defects were generated by the hydrogen a thin film of material from a substrate while maintainiog the trap-inducing implantation as well as by the hydrogen structural integrity of both separated parts. Said thin film of implantation itself, to cause growth, overlapping and coamaterial is prebonded to a stiffener, for example, an oxidized lescence of hydrogen-filled microcracks, which split the silicon wafer thus forming silicon-on-insulator wafer. 65 monocrystalline thin layer from the rest of the first substrate The embodiments which will now be described in conthereby allowing the transferrance of the thin monocrystalline layer to the second substrate. junction with the above drawings FIGS. 9 to 14, relate to the US 6,387,829 Bl 5 6 promoting hydrogen movement with formation of platelets Ultrasound waves propagated through the bulk of a solid 94 (FIG. 10) affect the properties of point and extended the similar way as it happens under heat treatment. The defects. This processing is referred to as the process of difference with the thermal treatment caused separation ultrasound treatment. Ultrasound vibrations applied to a described by Bruel is that the separation front is single and semiconductor with a power density W, exceeding some 5 it propagates through the entire wafer. Finally it leaves smoother surfaces after the separation as compared to a threshold value, W rh are able either to generate Frenkel pairs or to force the dissociation of complex centers composed of multiple separation front case. Steps on the surface after two or more point defects. The value ofW th was found to be separation appear mostly at places where the two separation of the order of 10 W/cm 3 . In the opposite case, i.e. when fronts meet. Typical laser processing characteristics are: W<W rh , a different effect related to the interaction of point 10 beam cross-section diameter 1 millimeter, laser pulse repetidefects and extended lattice defects has been found in single tive rate 100 Hz, pulse duration 70 nanoseconds, horizontal crystals. This effect of ultrasound is an enhancement of scanning speed 50 mm/sec, energy per pulse 0.01 Joule, gettering by sinks (dislocations, grain boundaries, vertical scanning step 0.5 millimeters. In the case of using precipitates) of both intrinsic and extrinsic point defects. The the neodymium laser (wavelength 1.06 micrometer), the ultrasound vibrations can reduce the energy barrier for the 15 silicon absorption coefficient is 50 cm- 1 for single crystal silicon, and more than 103 cm- 1 for amorphous silicon. The diffusion of defects as well as the barrier for their capture by hydrogen-rich region has about the same absorption coeffisinks. The present invention uses the effect of the enhancecient as the amorphous silicon, so silicon bulk remains ment of gettering by sinks, i.e. when W<Wrh" In the preferred embodiment, ultrasound vibrations were almost unheated, while the hydrogen-rich region melts at generated in the wafer using a circular 100 mm diameter and 20 laser pulse energies higher than about 1 Joule/cm 2 . Pulsed 3 millimeter thickness piezoelectric transducer PZT-5A. character of the laser processing is also important in this case. For the nanosecond-range pulse duration the heat Transducers were driven by a function generator and power amplifier adjusted to the resonance frequency of the transproduced by the pulse dissipates during the pulse on disducer radial or thickness vibrations. For good acoustic tances less than 1 micrometer, thus providing an adiabatic contact, the wafer was pressed against a transducer front 25 mode of the processing. The pulsed laser treatment heats predominantly the hydrogen-rich layer. In the heated layer surface with a spring. The ultrasound transducer was operthe implanted interstitial hydrogen forms hydrogen clusters. ated at resonance of its radial vibrations of 25 kHz. The wafer assembly can be placed with either the stiffener or the The clusters are nuclei of hydrogen platelets. Further heating causes the nuclei of platelets to grow into bigger platelets. cleavable surface to the transducer. The ultrasound load time was 5 minutes or more (up to 120 minutes). The amplitude 30 Further, the growing platelets begin to overlap. The part of of sample vibrations in acoustic contact with the transducer the wafer assembly with the dense layer of platelets is a was monitored by a calibrated contact acoustic probe. The separated part. A border between separated and nonmaximum acoustic strain amplitude on the film surface was separated parts is a separating front. The front moves of the order of 10- 5 . The temperature of the sample under following the scanning laser beam. FIG. 12B shows the ultrasound load was stabilized at 50° c., and monitored by 35 different geometry of separation with pulsed infrared proa thermocouple attached to the wafer surface. cessing where the entire wafer is processed in a single pulse. Preferred Embodiment 2: Separating with Hydrostatic PresThis preferred embodiment is requires the high power laser of a flash lamp source. The threshold energy for the sepasure Energy Load ration is about 0.2 Joule/cm 2 , which translates into a 15 FIG. 11 shows the preferred embodiment using a hydrostatic pressure load to cause separation. The wafer assembly 40 Joule/pulse light source requirement for 100 mm diameter 31 is placed into hydrostatic pressure cell 111 and loaded wafers. with a pressure higher than 0.2 GPa. In silicon (and other Preferred Embodiment 4: Separating with Hydrodynamic Energy Source diamond or zinc-blend structured materials), the diffusion FIG. 13 shows the preferred embodiment utilizing a water constant increases with pressure. This is in contrast with the situation in close-packed materials in which diffusion dimin- 45 jet source to separate the donor substrate material. The water jet nozzle 33 is initially aligned to the edge of the wafer ishes with compression. Pressure cells, which use comassembly 31 to be separated. The wafer assembly consists of pressed gas, can accommodate wafer size samples. The implanted wafer 35 with hydrogen layer 36 and stiffener compressed gas cells create a pressure up to 1.5 Gpa. This wafer 37 attached to implantation side of the wafer 35. The pressure is enough to cause separation of the silicon wafers. Argon 112 is introduced into the cell 111 to approach 50 water jet nozzle moves with a speed of 0.01 to 0.1 cm/sec hydrostatic conditions. towards the center of the wafer assembly 31. The wafer assembly is placed onto a supporting turntable that has a Preferred Embodiment 3: Separating with Infrared Energy spherical shaped surface with radius of curvature between 1 Load and 2 meters. The turntable is made of material that has FIG. 12 shows the preferred embodiment using an infrared energy source to cause separation. FIG. 12A illustrates 55 adequate rigidity, for example, stainless steel. The turntable using of neodymium glass laser 120 in scanning mode. Said is rotating with speed 0.1 to 10 rotations per second. The laser produces a light beam 121 that is directed normally to water jet beam trajectory on the wafer assembly 31 forms a spiral 38 of continuously decreasing radius. The wafer the surface of the wafer assembly 31 to be separated. Scanning begins from the edge of said assembly 31 thus assembly cleavage begins at the edge of the wafers. The creating a separation front. The separation front propagates 60 cleaved part propagates inward with the jet and turntable movements. The wafer assembly separates completely when following the scanning laser beam. To complete separation, an entire area of the wafer assembly 31 shoud be scanned. the jet reaches the center of the wafer assembly 31. The Said light wavelength should be chosen outside of the typical water jet characteristics are: nozzle diameter of 1.6 silicon band edge absorption. If the energy of the photons is millimeter, nozzle exit velocity 27 cm/sec (that gives a lower than the forbidden gap, the photons are absorbed 65 corresponding Reynolds number Re=430). The nozzle diameter can vary from 0.1 to 2 millimeters, and the nozzle exit mostly by defects. The hydrogen-rich layer is the defect-rich and the light energy is absorbed mostly in this layer thus velocity can vary from 10 to 100 cm/sec for typical wafer US 6,387,829 Bl 7 8 energy source selected from the group consisting of configurations. Under the water jet action the wafer assembly 31 bends, the strain concentrates at the hydrogen-rich ultrasound, infrared, hydrostatic pressure, hydrodylayer, thus encouraging the hydrogen atom movements namiic pressure, mechanical, and a combination inside the silicon. The displaced hydrogen forms platelets thereof, wherein said amount of energy is sufficient to the same way as under diffusion activation at elevated 5 transform said hydrogen-rich layer into a continuous temperatures. The platelets form a continuous hydrogen hydrogen layer thereby releasing said single-crystalline layer that separates the wafer assembly 31 into two wafers silicon layer. with cleaving along the <100> preferred plane. 2. The process of claim 1, wherein said donor substrate is Preferred Embodiment 5: Separating with Mechanical Linka silicon wafer. 10 ages and Structure 3. The process of claim 1, wherein said donor substrate FIG. 14 shows the preferred embodiment applying comprises single-crystalline silicon. mechanical linkages and structure. FIG. 14A illustrates the 4. The process of claim 1, wherein said insulator layer initial position when the wafer assembly 31 is placed onto comprises silicon dioxide. anvil 140 and piston 141 is in its top position. Next, a 5. The process of claim 1, wherein said acceptor substrate mechanical force 142 is applied to the piston and the piston 15 is single-crystalline silicon. moves all the way down reaching its bottom position as 6. The process of claim 1, wherein said energy source is shown on FIG. 14B. With this preferred embodiment the ultrasound. wafer assembly 31 bends and gets a shape predetermined by 7. The process of claim 6, wherein said ultrasound is a said anvil 140 and piston 141. The bend gives the mechanical stress to the wafer and the stress promotes hydrogen 20 self-resonance mode ultrasound with an acoustic strain amplitude of at least 10- 5 . movement inside the wafer assembly 31 that has the 8. The process of claim 1, wherein said energy source is hydrogen-rich layer. If the radii of curvature are chosen properly, the wafer assembly gets stress that is not enough an infrared energy source. to break it in an unintentional place, but enough to separate 9. The process of claim 1, wherein said infrared energy it along the weakened hydrogen-rich plane. The radii of 25 source provides infrared light that has a wavelength outside curvature for the assembly is typically from 0.6 to 3 meters. the silicon band edge absorption. What is claimed is: 10. The process of claim 1, wherein said hydrostatic 1. A process for manufacturing a silicon-on-insulator pressure energy source provides a hydrostatic pressure havwafer from a silicon wafer assembly, said process comprising an amplitude of at least 0.2 GPa. ing the steps of: 30 11. The process of claim 1, wherein said hydrodynamic providing a wafer assembly comprising a donor substrate pressure energy source comprises a water jet. having an insulator layer on a surface and a hydrogen12. The process of claim 1, wherein said mechanical rich layer at a selected depth below said surface definsource comprises a structure to which a force may be applied ing a single-crystalline silicon layer above said selected in order to provide mechanical energy. depth, and an acceptor substrate bonded to said surface 35 13. The process of claim 12, wherein said mechanical for accepting said insulator layer and said singleenergy is applied to said structure which has a radius of crystalline silicon layer above said selected depth, and; curvature from about 0.6 to 3 meters. applying to said wafer assembly an amount of energy directed to a flat surface of said wafer assembly from an * * * * *