THERMO-COMPRESSION BONDING A SEMICONDUCTOR TO STRAIN BUFFER

Abstract: A diffusion bonding press (10) and method are provided for thermocompression diffusion bonding structured copper strain buffers (55, 65) directly to one or each of the two major opposed surfaces (70a, 70b) of a substrateless semiconductor device wafer (70) having a first metal coating (94, 96) of titanium, chromium or nickel and a layer (98, 100) of copper, silver or gold. The expensive tungsten or molybdenum support plate conventionally employed to provide structural integrity to the relatively fragile semiconductor device wafer is thus eliminated. The wafer may have a passivated beveled outer edge surface (70c) prior to bonding. A selected portion of each structured copper strain buffer is diffusion bonded to the semiconductor device wafer while subjecting the wafer only to substantially compressive force, thus avoiding wafer fracture during the bonding process. The structured copper strain buffers may further have lateral extents no greater than the lateral extent of the respective semiconductor device wafer surface in contact therewith, allowing the beveled surface to be cleaned and passivated after attachment of the strain buffers to the wafer via diffusion bonding. When only one buffer (55 or 65) is bonded to the wafer, a rigid member, such as quartz, may be interposed between the plate of the press and the noncoated surface of the wafer.

Description: THERMO-COMPRESSION BONDING A SEMICONDUCTOR TO STRAIN BUFFER This invention relates to methods for diffusion bonding surfaces together and, more particularly, to a method for thermo-compres sion diffusion bonding a structured copper strain buffer directly to one or each of the two major oppos ed surfaces of a substrateles s s e icon- ductor devic e wafer . When attaching heat sinks to the respective opposed surfac es of a s emiconductor devic e wafer, it is often desirable that thes e attachments occur as clos e to the semiconductor device wafer as pos sible in order to optimize heat removal from the wafer. Such is the cas e when heat sinks are attached via respective structured copper strain buffers to a s emiconductor device -wafer . Briefly, a structured copper strain buffer comprises a bundle of substantially parallel, closely packed strands of copper of substantially equal length, one common end thereof being thermo-compression diffusion bonded to a metallic sheet. One type of prior high power s emiconductor device includes a metallic support plate or substrate, typically compris ed of tungsten or molybdenum., attached to one of the oppos ed surfac es of a s emiconductor devic e wafer to provide the inher ently fragile wafer with structural integrity. In thos e devic es , heat sinks have been attached via structur ed copper strain buffers to one or both sides of the wafer support plate structures . It is desirable, however, to diffusion bond respective heat sinks via structured copper strain buffers "directly" to each of the opposed surfaces of a semiconductor device wafer, thereby eliminating the rather expensive metallic support plate and simultaneously providing heat sinks attached to the strain buffers with a more intimate thermal connection to the semiconductor device wafer . More efficient heat conduction away from the wafer results . The term "directly" as used herein signifies that actually such bonds are made to the metallized layers situated on the respective major oppos ed surface of the wafer, and not to the wafer itself. Brief Summary of the Invention The pres ent invention is directed to increasing the conduction of heat away from a semiconductor devic e wafer having respective structured copper strain buffers diffusion bonded to one or. each of the two major surfaces thereof. The support plate attached to the s emiconductor device wafer as needed in many conventional semicon¬ ductor device structures is not required and respective structured copper strain buffers are thermo-compression diffusion bonded "directly" to one or each of the two major surfaces of the wafer. In accordance with one preferred embodiment of the invention, a improved press for diffusion bonding metal surfaces of an ass embly together includes first and second metallic plate means, each having a predetermined thermal coefficient of expansion. The second plate means are oriented parallel to the first plate means and spaced apart therefrom so as to allow acceptance of the as sembly between the first and second plate means . A metallic support connects the first plate means to the second plate means . The support is selected to exhibit thermal expansion smaller than the total thermal expansion in thickness of the first and s econd plate means when the press is heated to an elevated temperature. The first and second metallic plate means each include first and second raised portions extending therefrom, respectively, and facing each other . Each rais ed portion includes a flat surface having an identical geometric pattern, respectively, the patterns being in axial and rotational alignment with each other such that equal and opposite pres sures are extended- on the to-be-bonded metallic surfaces of the assembly situated between the first and second rais ed portions thereby squeezing the metallic surfaces together at high pressure when the press is heated to an elevated temperature. In accordanc e with another preferred embodiment of the invention, a method for fcermo-compres sion diffusion bonding a structured copper strain buffer to a substrateles s s emiconductor device wafer having two major oppos ed surfaces is provided. A substrateless semiconductor device wafer is defined to be one which has no support plate attached thereto. Each of the major oppos ed surfaces is smoothed. One of the major oppos ed surfac es is coated with a first metallic layer and a second metallic layer may be coated atop the first metallic layer . The strain buffer to be bonded to the wafer includes a bundle of substantially parallel, close-packed strands of copper of substantially equal length. One common end of thes e strands is thermo-compression diffusion bonded to a metallic sheet, while the common end of the copper strands opposite the metallic sheet is to be thermo-compression diffusion bonded to the metal-coated major surface of the semiconductor device wafer. The uncoated major surface of the semiconductor devic e is positioned in abutment with a flat referenc e surface of a rigid member while the common end of the copper strands opposite the metallic sheet of the strain buffer is positioned in abutment with the metal- coated surface of the s emiconductor device wafer. The strain buffer - wafer-rigid member as s embly thus formed is surrounded with an inert atmospher e and is squeezed together at high pres sure. The assembly is heated at a temperature within the range of 300 C. to o 400 C. while the same is squeezed together. Because the rigid member essentially does not yield under pressure, a thermo- compres sion diffusion bond between the structur ed copper strain buffer and the s emiconductor device wafer is produced without wafer fracture. In accordance with another preferred embodiment of the inventio a method is provided for thermo-compression diffusion bonding first and second structured copper strain buffers to a substrateless semi¬ conductor device wafer having first and second oppos ed surfac es of unequal lateral extents and a beveled outer edge surface. The beveled surface is cleaned to remove contaminants ther efrom and then coated with a pas sivation material. The semiconductor devic e wafer with the above-described metallic layers and metallizations dispos ed thereon is s andwiched between the first and s econd structur ed copper strain buffers . Each of these strain buffers is of lateral extent equal to or les s or greater than the lateral extent of the respective s emicon¬ ductor devic e wafer surfac e in contact therewith such that the beveled edge surfac e either remains uncovered or covered by either of the structured copper strain buffers . When the lateral extent of the strain buffer is greater than the lateral extent of the wafer, the strain buffer is positioned so as to overhand the entire outer edge surfac e of the wafer. Each structured copper strain buffer is positioned such that the common end of the copper strands opposite the metallic sheet thereof faces the semiconductor device wafer . The so-positioned semiconductor device wafer structur e and structur ed copper strain buffers are surrounded by an inert atmosphere. A loading force is applied to identical, axially aligned portions of the first and second strain buffers , respectively, to squeeze them together at high pressure and, at the same time, the so-positioned s emiconductor device wafer and structured copper strain buffers are heated at a temperature within the range of 300 C. to 400 C . Thermo- compres sion diffusion bonds are thus formed between the two major, OM?i opposed, metal-coated surfaces of a substrateless , beveled semicon ¬ ductor device wafer and respective first and second structured copper strain buffers, without wafer fracture. In an alternative embodiment of the invention, first and s econd structured copper strain buffers are thermo-compres sion diffusion bonded to each of the two metallized major surfaces of a semicon¬ ductor device wafer. The outer edge surface of the semiconductor device wafer is beveled preferably prior to s andwiching the semicon¬ ductor device wafer between the first and second structured copper strain buffers. The separate metallic layers and metallizations are applied to each of the oppos ed surfaces of the s emiconductor devic e wafer as previously s et forth, but with the metallic layers and metallizations having a lateral extent sufficiently small to avoid over¬ lapping the beveled surface. The metallic layer and metallization on the surface of the wafer which is beveled are axially aligned with each other and with the metallic layer and metallization on the opposite surface of the wafer, which likewis e are axially aligned with each other . All metallic layers and metallizations are formed with .an equal lateral extent. The semiconductor device wafer is then sandwiched between the first and second strain buffers and the s emiconductor devic e wafer and first and second structured copper strain buffers are squeezed together while being heated in an inert atmosphere, as described above. The metallic sheet of the first structured copper strain buffer is thereafter cut, preferably with a las er beam to remove the porti on os the first strain buffer not bonded to the semiconductor device wafer. The beveled edge surfac e of the wafer is cleaned, typically by sputter etching, and is then pas sivated. Thermo-compres sion diffusion bonds are thus formed between the two major oppos ed surfaces of the beveled semiconductor device wafer and respective first and s econd structured copper strain buffers without wafer fracture. The invention may best be understood by r eferenc e to the following description taken in conjunction with the accompanying drawings . Λ . Figur e 1 is a cros s -s ectional side view of a thermo-compres sion diffusion bonding press showing materials situated in the pres s to be bonded together in practicing the invention. Figure 2 illustrates the diffusion bonding press of Figure 1 with a layer of compactible material situated between a metallic pres sing block and a structured copper strain buffer to avoid the undesirable bonding of the pressing block to the strain buffer in accordance with the invention. Figure 3 is a cros s- s ectional view of a beveled semiconductor device with structured copper strain buffers having a lateral extent greater than the lateral extent of the wafer thermo-compres sion diffusi bonded to each of the oppos ed surfac es of the wafer . Figure 4 illustrates laser cutting of a portion of a structured copper strain buffer in preparation for pas sivating the s emiconductor device wafer diffusion bonded thereto. Figure 5 is a top view of the bonded s emiconductor device wafer- strain buffer structure shown in Figur e 4 after removal of the portions of the strain buffer cut away by the las er. Figure 6 is a cros s -s ectional side view of the wafer- strain buffer structure shown in Figure 5, after pas sivation. Figure 7 is a cross- s ectional view of a beveled semiconductor device wafer with structur ed copper strain buffers having a lateral extent greater than the lateral extent of the wafer thermo-compression diffusion bonded to each of the metal-coated, oppos ed surfaces of the wafer . Figure 8 is a cros s -s ectional side view of an improved thermo- compres sion diffusion bonding pr es s showing materials to be bonded together situated therein. Figure 9 is a cros s - s ectional side view of the thermo-compr es si diffusion bonding pres s of Figure 8 showing materials of alternative configuration situated therein to be bonded together . Figure 1 shows a diffusion bonding press 10 which may be used in thermo-compression diffusion bonding a structured copper strain buffer 12 directly to a substrateless semiconductor device wafer 16. Press 10 is comprised of an upper metallic plate 22 oriented parallel to a lower metallic plate 24 with a space provided therebetween. A metallic pressing block 26 is positioned at the center of the side of upper plate 22 facing lower plate 24. Metallic bolts 28 and 30 pass through respective holes in upper plate 22 and lower plate 24 and are threaded into lower plate 24 to connect the two plates together as illustrated in Figure 1. Metallic bolts 28 and 30 are comprised of a steel other than stainless steel, while upper plate 22, lower plate 24 and metallic pressing block 26 are comprised of stainless steel. Metallic pressing block 26 may alternatively be comprised of Dural, an aluminum alloy, or other metals having a coefficient of thermal expansion greater than that of steel. To achieve a thermo-compression diffusion bond between structured copper strain buffer 12 and semiconductor device wafer 16, a disk of rigid material 20 having a flat reference surface 20a is situated within press 10 on lower plate 24. Member 20 is comprised of a rigid material such as quartz, for example, which will withstand high pressures of 20, 000 through 50, 000 psi without measurably yielding. Each of the opposed major surfaces of semiconductor device wafer 16 is smoothened to removed surface damage therefrom. Such surface damage would otherwise cause nonuniform distribution of pressures within wafer 16 and, thus, wafer breakage when the wafer is subjected to the high pressure employed in the thermo-compression diffusion bonding process of the invention. This step of smoothing may be accomplished, for example, by polishing or etching each of the opposed major surfaces. One of the two major opposed surfaces of wafer 16 is provided with a first metallic layer 21, comprised of one titanium , chromium and nickel, coated ther eon. A s econd metallic layer 23 compris ed of one of such metals as copper, silver and gold is applied over the metallic layer 21. Metallic layers 21 and 23 may be applied to wafer 16 by sputtering or evaporation, for example. Structures to be diffusion bonded together are situated upon surface 20a of member 20. Specifically, wafer 16 is situated on surfa 20a with the uncoated surface of wafer 16 positioned facing surface 20 Structured copper strain buffer 12 is positioned in abutment with the exposed surface of metallic layer 23 on wafer 16. Strain buffer 12 comprises a bundle of substantially parallel, close-packed strands of copper 40 of substantially equal length, one common end thereof being thermo-compression diffusion bonded into" a metallic sheet 14, typically compris ed of copper . The remaining common end of strands 40 is situated in abutment with metallic layer 23 on wafer 16. A conventional press (not shown) is us ed to squeeze upper plate 22 and lower plate 24 together and while such pressure is applied to thes e plates, bolts 28 and 30 are tightened. The thermo-compres sion diffusion bond between structured copper strain buffer 12 and semiconductor device wafer 16 is actually formed when press 10 containing rigid member 20, semiconductor device wafer 16 with metallic layers 21 and 23 disposed thereon and structured copper strain buffer 12, positioned as described above and as shown in Figure 1, are placed in an inert atmosphere and heated at a temperature within the range of 300 C. to 400 C , typically 325 C , for approximately 15 minutes to 5 hours . When press 10 is heated in this manner, upper plate 22, lower plate 24, and metallic pressing block 26 expand to a greater total extent than do metallic bolts 28 and 30. Therefore, a force is exerted between pres sing block 26 and lower plate 24 resulting in the squeezing of structured copper strain buffer 12 and semiconductor device wafer 16 together and the thermo- compres sion diffusion bonding of each to the other . Although frequent referenc e is made herein to the thermo-compression diffusion bonding OM? of structured copper strain buffer 12 to semiconductor device wafer 16 for simplicity of description, thos e skilled in the art will appreciate that the thermo-compres sion diffusion bond is actually formed between the commond end of copper strands 40 opposite metallic sheet 14 and metallic layer 23 on wafer 16. During thermo-compression diffusion bonding, s emiconductor device wafer 16 is subjected to high pressure, specifically, 20, 000 psi to 50, 000 psi. If this force is not purely compressive, that is , if s emiconductor devic e wafer 16 is subjected to bowing or tensile forc es, wafer 16 will likely fracture, resulting in a damaged s emiconductor device. It is thus extremely important that uniform high pres sure be applied over the entire wafer 16. The surfaces of the members to be bonded together must be flat and par allel to each other and to the oppos ed, facing surfaces of lower plate 24 and metallic pres sing block 26. Situating rigid member 20 on lower plate 24 provides an ess entially non-yielding, flat reference surface for s emiconductor device wafer 16 placed thereon. Employing rigid member 20 in this manner as sures that semiconductor device wafer 16 is subj ected to substantially pure compres sive forces during thermo-compres sion diffusion bonding, with negligible tensile forc es being exerted on the wafer, enabling the diffusion bond to be achieved without fracture of the substrateless wafer . The resulting bonded semiconductor device wafer- strain buffer-heat sink assembly includes no substrate or support plate. Heat is conducted more efficiently away from the wafer to the heat sink than if the ass embly were to include such substrate. Figure 2 is identical to Figure 1 with like numerals signifying like components , except forthe plac ement of a layer of nonreactive compactible material 32 between pressing block 26 and strain buffer 12. In accordance with the preferred embodiment of the invention, layer 32 is so positioned prior to application of the loading force discus sed in the method s et forth above. Such layer of compactible material tends to compress under application of the loading force, filling in any irregularities in the facing surface of metallic sheet 14 and resulting in a more uniform distribution of the loading force on strain buffer 12 The thermo-compression diffusion bond between strain buffer 12 and wafer 16 is thus made substantially void free. Layer 32 additionally prevents strain buffer 12 fro undesirably adhering to metallic pressing block 26 during thermo-compres sion diffusion bonding. Layer 32 may be compris ed of glas s wood, or Glas s Fiber Filter paper available from Fisher Scientific Company, Clifton, New Jerse or other similarly compactible materials . Figure 3 shows a diffusion bonding press 10 suitable for thermo compression diffusion bonding a first structured copper strain buffer 12 and a second structur ed copper strain buffer 74, respectively, to the oppos ed major surfaces 16a and 16b of substrateless semiconducto device wafer 16. The outer edge surface 16c of s emiconductor devic wafer 16 is preferably beveled, as shown in Figure 3, although the invention encompasses wafers both with and without a beveled outer edge surface. To achieve the respective thermo-compression diffusion bonds between strain buffers 12 and 74 and wafer 16, surfaces 16a and 16b ar smoothened to remove surfac e damage therefrom. Such surface damage would otherwise cause nonuniform distribution of pres sures within wafer 16 and thus wafer breakage when wafer 16 is subjected to the high pressures employed in the thermo-compression diffusion bonding process of the invention. This step of smoothing may be accomplished, for example, by polishing or etching surfac es 16a and 16b. Metallic layers 31 and 37 are applied to wafer surfaces 16a and 16b, respectively. Each of metallic layers 31 and 37 is comprised of one of such metals as titanium, chromium and nickel. Metallizations 34 and 36 are respectively applied over metallic layers 31 and 37. Each of metallizations 34 and 36 are respectively comprised of one of such metals as copper, gold and silver . Thes e metallic layers and metallizations may be applied to wafer 16 by sputtering or evaporatio for example. The wafer -metallic layer-metallization structure thus formed is sandwiched between structured copper strain buffers 12 and 74. Structured copper strain buffer 12 is compris ed of a bundle of substantially parallel, closely packed strands of copper 40 of substantially equal length with one common end thereof thermo- compression diffusion bonded to a metallic sheet 42, typically comprised of copper. The opposite common end of copper strands 40 is positioned in abutment with metallization 34. Structured copper strain buffer 14 is es s entially identical to structured copper strain buffer 12 and is comprised of copper strands 50 and metallic sheet 52. The common end of copper strands 50 opposite metallic sheet 52 is positioned in abutment with metallization 36. A layer 54 of nonreactive compactible material is situated in abutment with metallic sheet 42 of structured copper strain buffer 12. Layer 54 may be comprised of glas s wood or Glas s Fiber Filter paper available from Fisher Scientific Company, Clifton, New Jersey, or other similarly compactible material. A layer of compactible material 56 preferably comprised of the same material as layer 54 is positioned in abutment with metallic sheet 52 of structured copper strain buffer 14. The combined structure formed by s emiconductor device wafer 16, structured copper strain buffers 12 and 74, and metallic layers 31 and 37 and metallizations 34 and 36 dispos ed therebetween and compactible layers 54 and 56 is positioned in pres s 10 between pr essing block 26 and lower plate 24. A conventional press (not shown) is us ed to squeeze upper plate 22 and lower plate 24 together and while such pressure is applied to thes e plates, bolts 28 and 30 are tightened. The thermo-compression diffusion bonds between structured copper strain buffer 12 and wafer 16, and between structured copper strain buffer 74 and wafer 16 are actually formed when pres s 10 containing the above-described combined structure, illustrated in Figure 1, is surrounded by an inert atmosphere and heated at a temperature within the range of 300 C. to 400 C , typically 325 C , for approximately 15 minutes to 5 hours . When press 10 is heated in this manner, upper plate 22, lower plate 24 and metallic pressing bl 36 expand to a greater total extent than do metallic bolts 28 and 30. Therefore, a forc e is exerted between pr essing block 26 and lower plate 24, resulting in the squeezing of structured copper strain buffe 12 and 14 and semiconductor devic e wafer 16 together and the thermo- compression diffusion bonding of buffers 12 and 14 to wafer 16. The now-formed strain buffer- afer ass embly 60 is removed from pres s 10 by loos ening bolts 28 and 30. Although reference is made herein t the thermo-compres sion diffusion bonding of strain buffer 12 to wafer 16 and strain buffer 74 to wafer 16 for simplicity of description, thos skilled in the art will appreciate that the actual thermo-compression diffusion bonds are formed at the interface between the common end of copper strands 40 and metallization 34, and at the interface betwe the common end of copper strands 50 and metallization 36. During thermo-compression diffusion bonding, substrateles s semiconductor device wafer 16 is subjected to high pressures, specifically, 20, 000 psi to 50, 000 psi. If this force is not purely compressive, that is, if semiconductor device wafer 16 is subj ected to bowing or tensile forces, wafer 16 will likely fracture, resulting i a damaged s emiconductor device. It is thus extremely important tha uniform high pres sur e be applied over the entir e wafer 16 . Prior methods of thermo-compression diffusion bonding us ed a support plate attached to the s emiconductor device wafer to enable th wafer to withstand some degr ee of bowing forces and nonuniform pressure without fracture. When attempting thermo-compres sion diffusion bonding of members to a "substrateless " semiconductor device wafer, it is important that the surfac es of the members to be bonded together be flat and parallel to each other and to the opposed facing surfaces of lower plate 24 and metallic pressing block 26. To solve the problem of bowing forces being generated near the edge surfac e of wafer 16 during thermo-compression diffusion bonding as would occur if strain buffers 12 and 74 did not extent over the entire lateral extent of wafer 16, the lateral extent of structured copper strain buffers 12 and 74 is made greater than the lateral extent of wafer 16 such that buffers 12 and 74 overhang wafer 16 around the entirety of its edge surface. Layers of compactible material 54 and 56 are positioned as described above to assure that during thermo-compression diffusion bonding, structured copper strain buffer 12 does not adhere undesirably to pressing block 26 and to as sure that structured copper strain buffer 74 similarly does not bond to lower plate 24. Use of such layers of compactible material helps as sure the creation of uniform, substantially void-free diffusion bonds . Voids in diffusion bonds may result when a thermo-compres sion diffusion bond between a compliant metallic member (such as a structured copper strain buffer) and another member having some degree of surface irregularity is attempted. When compres s ed, the layers of compactible material fill in the irregularities in the surfac e of the respective structured copper strain buffers allowing the diffusion bonding press 10 to apply a more evenly distributed pr essure to the members which are to be bonded together. Use of layers of compactible material 54 and 56 is preferable but not es sential to practice of the method of the invention. If semiconductor device wafer 16 is of the nonbeveled variety, no further processing in accordance with the invention is required. However, if wafer 16 includes a beveled outer edge surfac e 16c, as illustrated, it is desirable that surface 16c be cleaned and pas sivated to protect it from external contamination. As shown in Figure 4, beveled edge surface 16c lies rec es s ed under structur ed copper strain buffer 12 and is " thus inaccessible for cleaning and pas sivation purpos es . It would be undesirable to pas sivate beveled surface 16c prior to thermo-compression diffusion bonding sinc e the additional thickness of the pas sivant, which would likely become affixed to the outer edge r s of wafer surfaces 16a and 16b would cause uneven pres sure on the wafer during such diffusion bonding and likely result in fracturing the wafer. Therefore, pas sivation of beveled surface 16c is achieved in the following manner. Prior to sandwiching wafer 16 between structured copper strain buffers 12 and 74, metallic layers 31 and 37, applied to major surfaces 16a and 16b, respectively, of wafer 16, are formed with a lateral extent sufficiently small so as to avoid over¬ lapping beveled surface 16c . That is , the lateral extent of layers 31 and 37 may be equal to or less than the lateral extent of surface 16a. Metallizations 34 and 36 are thereafter applied over metallic layers 31 and 37, r espectively, with lateral extents equal to thos e of metalli layers 31 and 37, all of the above-mentioned lateral extents being equal and axially aligned with each other, as shown in Figure 4. When the thermo-compres sion diffusion bonding process is carried out upon the metallized wafer structure 16, the common end of copper strands 40 opposite metallic sheet 42 of strain buffer 12 becomes thermo-compression diffusion bonded only to the metallized portions of surface 16a. Similarly, the common end of copper strand 50 opposite metallic sheet 52 of strain buffer 74 diffusion bonds only to metallized portions of surface 16b. A laser device 62 such as a puls ed las er, typically having a peak pulsed power of 16 KW although not limited thereto, generates a beam of coherent light which is directed along a selected path on metallic sheet 42 of strain buffer wafer assembly 60, fabricated as previously described, so as to form an incision 64 in sheet 42 and thus allow the removal of most of the portion of strain buffer 12 not bonded to wafer metallization 34. The portion of structured copper strain buffer 12 outside incision 64 is removed to form wafer-buffer structure 170 shown in Figure 5. The remaining portion of strain buffer 12 is designated strain buffer 12' . Beveled edge surface 16c is thus made accessible for cleaning and pas sivation. Chemically etching the surfac e of a beveled s emiconductor device wafer is a method for wafer cleaning well known in the art. ϊτ> r O FI However, such an etching step, if applied to wafer -buffer , structure 170 would result in the undesirable chemical attack of the structured copper strain buffers 12 and 74 by the etchant. Rather, beveled surface 16 is subj ected to sputter etching to remove contaminants therefrom prior to passivation. Subsequently, beveled surface 16c is coated with a pas sivation layer 82 comprised of one of the many pas sivation materials known in the art, polyimide siloxane, for example, as illustrated in Figure 6. To complete the device thus formed, heat sinks (not shown) are respectively attached to metallic layers 42 and 52 of strain buffers 12' and 74. This is preferably accomplished by thermo-compres sion diffusion bonding during the cours e of attachment of strain buffers 12 and 74 to wafer 16 or at a later time. Although heat sink attach¬ ment may be accomplished by other means of joining metals together, soldering, for example, thermo-compres sion diffusion bonding is preferred because it inherently achieves superior thermal conductivity between the joined metallic members . Thermo-compres sion diffusion bonding of the above method results in the intermediate structure shown in Figure 7 wherein a s emiconductor device wafer 16 includes oppos ed major surfaces 16a and 16b and beveled edge surface 16c . First and s econd metallic layers 31 and 37 are coated respectively on surfaces 10a and 10b. Metallizations 34 and 36 are coated, r espectively, on metallic layers 31 and 37. Structured copper strain buffers 12 and 74 are thermo- compres sion diffusion bonded respectively to metallizations 34 and 36. Structured copper strain buffer 12 includes a bundle of substantially parallel closely packed strands of copper 40, one common end thereof thermo-compression diffusion bonded into a metallic sheet 42. Structur ed copper strain buffer 74 is substantially similar to strain buffer 12 and includes copper strands 50 and metallic sheet 52. As pointed out above, structured copper strain buffers 12 and 74 are formed in such a manner as to have a lateral extent greater than that of silicon wafer 16, so as to prevent the silicon wafer 16 from being subjected to bowing forces which cause wafer fracture. Bowing forces are thus substantially eliminated; however, this bonding method ess entially prohibits cleaning and pas passivating beveled surface 16c prior to bonding because of the high probability that passivant applied to surface 16c would flow onto portions of surfac es 16a and 16b, causing wafer 16 to experienc e bowing forc es and nonuniform pressure when thermo-compression diffusion bonding is attempted. To allow pas sivation of the beveled edge 16c therefore, a laser beam is used to cut away the portion of structured copper strain buffer 12 overhanging beveled surface 16c. Beveled surface 16c is thereby expos ed for cleaning, preferably via sputter etching, and pas sivating. Conventional economical chemical etching is not a preferable method for cleaning beveled surface 10c becaus e the etchant would chemically attack the structur ed copper strain buffers 12 and 74. This embodiment constitutes an alternative solution to the problem of preventing bo-wing forces when thermo-compres sion diffusion bonding structured copper strain buffer s to be a substrate- les s semiconductor device wafer. One embodiment of the invention desirably allows cleaning of the beveled surface via conventional chemical etching and pas sivation prior to diffusion bonding. As illustrated in Figure 8, a press apparatus 10 is provided to bond first and second structured copper strain buffers 55 and 65, respectively, to the opposed major surfaces 70a and 70b of substrateless s emiconductor device wafer 70. The outer edge 70c of semiconductor devic e wafer 70 is beveled as shown in Figure 8. Bonding pres s 10 includes an upper metallic plate 22 oriented parallel to a lower metallic plate 24 with a space provided there- between. Metallic bolts 28 and 30 pass through respective holes in upper plate 22 and lower plate 24 and are thr eaded into lower plate 24 to connect the two plates together as illustrated in Figur e 8. A metallic pressing block (or expansion block) 88 is p JoOsS itioned at the \ O P center of the lower surface of upper plate 22 and attached thereto . A similar metallic pr essing block (or expansion block) 90 is positioned at the center of the upper surfac e of lower plate 24 and attached thereto. Pressing blocks 88 and 90 each include rais ed portions 88a and 90a respectively extending therefrom and facing each other . Raised portions 88a has a flat surface of a s elected geometric pattern, circular, square, rectangular or various other polygons , ' for example, which is axially and rotationally aligned with a like flat surface of identical geometric pattern on rais ed portion 90a. The flat surfaces of rais ed portions 88a and 90a are parallel to each other. Rais ed portions 88a and 90a are held in alignment by a guide member 92 positioned in abutment with pres sing blocks 88 and 90. Guide member 92 is conveniently of cylindrical shape having an inner diameter sufficiently large to accommodate the maximum lateral extent of pressing blocks 88 and 90 therein. Metallic pressing blocks 88 and 90 are compris ed of a metal having a thermal coefficient of expansion greater than that of metallic bolts 28 and 30. For example, pres sing blocks 88 and 90 may be compris ed of such metals as stainless steel, Dural and other aluminum alloys . Guide member 92 is comprised of a material having a thermal coefficient of expansion equal to or greater than that of pres sing blocks 88 and 90. To achieve the respective thermo-compression diffusion bonds between strain buffer s 55 and 65 and wafer 70, surfac es 70a and 70b are smoothened to remove surface damage therefrom. Such surface damage would otherwise caus e nonunifor distribution of pres sures within wafer 70 and thus wafer fracture when wafer 70 is subj ected to the high pressures employed in the thermo-compression diffusion bonding proces s of the invention. Metallic layers 94 and 96 are then applied to surfaces 70a and 70b, respectively. Each of metallic layers 94 and 96 are comprised of one of such metals as titanium, chromium and nickel. Metallizations 98 and 100 are respectively applied over .-»•>« metallic layers 94 and 96 and are comprised of such metals as copper, silver and gold. B eveled surface 70c is then cleaned to remove contaminants therefrom. A chemical etch may be used to perform this cleaning since at this juncture the structured copper strain buffers are not yet bonded to wafer 70 and thus there is no opportunity for the etch to chemically attack the structured copper strain buffers. Alternately, sputter etching may be used to clean beveled surface 70c. After being cleaned, beveled surface 70c is coated with a passivation material 102 such as polyimide siloxane, for example. Pas sivation material 102 may partially extend on to maj or surfaces 70a and 70b without causing the undesirable generatio of bowing forces in wafer 16 during subsequent thermo-compression diffusion bonding. The wafer -metallic -layer -metallization-pas sivation ass embly thus formed is sandwiched between structured copper strain buffers 55 and 65. Structured copper strain buff er 55 is comprised of a bundle of substantially parallel, clos ely packed strands of copper 62 of substantially equal length with one common end thereof being thermo-compression diffusion bonded to a metallic sheet 57. The opposite common end of copper strands 62 is positioned in abutment with metallization 98. Structured copper strain buffer 65 is es s ential identical to structured copper buffer 55 and is comprised of copper strands 58 and metallic sheet 59. The common end of copper strands 58 opposite metallic sheet 59 is positioned in abutment with metallization 100. Strain buffers 55 and 65 each have a laterial extent equal to or les s than the lateral extent of the r espective wafer 70 surface in contact therewith such that beveled surface 70c remains uncovered by either of the strain buffers . Additionally, the lateral extent of strain buffers 55 and 65 is sufficiently small such that buffer s 55 and 65 do not overlap any passivation material 102 which may be pres ent on wafer surfaces 70a and 70b, respectively. If such overlap were to occur, undesirable bowing forces would likely result. » • A layer 104 of nonreactive, compactible material is situated in abutment with metallic sheet 57 of structured copper strain buffer 55. Layer 104 may be comprised of glass wool or Glass Fiber Filter paper available from Fisher Scientific Company, Clifton, New Jersey, or other similarly compactible materials . A layer of compactible material 106 compris ed of the same material as layer 104 is positioned in abutment with metallic sheet 59 of structured copper strain buffer 65. The as sembly formed by s emiconductor devic e wafer 70, structured copper strain buffers 55 and 65, and metallic layers 94 and 96 and metallizations 98 and 100 disposed therebetween, together with compactible layers 104 and 106, is positioned in pres s 55 between raised portions 88a and 90a as shown in Figure 8, A conventional pres s (not shown) is us ed to squeeze upper plate 22 and lower plate 24 together and while such pressure is applied to thos e plates , bolts 28 and 30 are tightened. The thermo-compres sion diffusion bonds between structured copper strain buff er 55 and wafer 70, and between structured copper strain buffer 60 and wafer 70 are actually formed when pres s 10 containing the above-described as sembly, shown in Figur e 8, is surrounded by an inert atmospher e and heated at a temperatur e within the range of 300 C. to 400°C. , typically 325°C , for approximately 15 minutes to 5 hours . When press 10 is heated in this manner, upper plate 22, lower plate 24 and metallic pres sing blocks 88 and 90 expand to a greater total extent than do metallic bolts 84 and 86. Therefore, a forc e is exerted between raised portions 88a and 90a, resulting in the squeezing of structur ed copper strain buffers 55 and 65 and semiconductor device safer 70 together and the thermo-compression diffusion bonding of buffers 55 and 65 to wafer 70. The respective thermo-compression diffusion bonds are formed between the portions of wafer 70 and buffers 55 and 65, which experience compres sive stres s . Guide members 92 expands with 6) temperature to an equal or greater total extent than do metallic pressing blocks 88 and 90 such that guide 92 does not become thermo- compression diffusion bonded thereto. Although reference is made herein to the thermo-compression diffusion bonding of strain buffers 55 and 65 to wafer 70 for simplicity of description, those skilled in the art will appreciate that the actual thermo-compression diffusion bonds are formed at the interface between the common end of copper strands 62 and metallization 98, and at the interface between the common end of copper strands 58 and metallization 100. During thermo-compression diffusion bonding, substrateless semiconductor device wafer 70 is subjected to high pressures, specifically, 20, 000 psi to 50, 000 psi. If this force were not purely compressive, that is , if the s emiconductor device wafer 70 is subject to bowing forces, especially in the region of beveled surface 70c, wafer 70 would likely fracture, resulting in; a damaged, unusable semiconductor device. It is thus extremely important that uniform, high pres sure be applied over wafer 70. Prior bonding methods utilized a support plate attached to the semiconductor device wafer to enable the wafer to withstand some degree of bowing forces and nonuniform pressure without fracturing. No such support plate structure is necessary in the present invention, however, since essentially equal pressures are applied to essentially identical selected regions of wafer surfaces 70a and 70b, respectively which are precisely aligned both axially and rotationally. Such pressures are applied via rais ed portions 88a and 90a through the intermediate layers of compactible material 104 and 106, strain buffers 55 and 65, metallizations 98 and 100, and metallic layers 94 and 96. Because wafer 70 thus experiences purely compressive pressures, it is not subjected to bowing forces and fracturing that may result therefrom. Layers of compactible material 104 and 106 are positioned as described above to assure that during thermo-compression diffusion bonding, structured copper strain buffers 55 and 65 do not undesirably diffusion bond to rais ed surfac es 88a and 90a, respectively. Use of such layers of compactible material facilitates the creation of uniform, substantially void-fr ee diffusion bonds which can otherwise occur when the bonding together of irregular surfac es is attempted. When compres s ed under high pressure, the layers of compactible material fill in the irregularities in the surfaces allowing rais ed surfaces 88a and 90a to apply a more evenly distributed pressur e to strain buffers 50 and 60 and hence to each of the individual members which are to be bonded together . Us e of layers of compactible material 104 and 106 is preferable but not ess ential to practice of the invention. Upon completion of the bonding operation, bolts 28 and 30 are loosened to permit removal of the bonded strain buffer -wafer ass embly fro press 10. In accordanc e with another embodiment of the invention, as illustrated in Figure 9, an "oversized" st ructured copper strain buffer 160, that is, a strain buffer having a lateral extent equal to or greater than the larger major surface of substrateles s wafer 70 having a beveled edge surface 70c , is thermo-compression diffusion bonded to surface 70b of wafer 70, and a structured copper strain buffer 55 having a lateral extent sufficiently small so as to overlap beveled surfac e 70c of wafer 70 is thermo-compres sion diffusion bonded to surface 70a of substrateless wafer 70. The diffusion bonds are formed without subj ecting semiconductor device wafer 70 to bowing forc es which would likely cause it to fracture. Figure 9 shows press 10 with materials situated ther ein for bonding. Pres s 10 of Figure 9 is similar to that of Figure 8, with like numerals indicating like elements . The method for creating such thermo-compres sion diffusion bonds between wafer 70 and each of strain buffer 55 and "oversized" strain buffer 160 is identical to that described in conjunction with Figure 8, except for the following differ enc es . "Oversized" strain buffer 160 is employed instead of strain buffer 65 shown in Figure 8. Additionally, beveled surface 70c is cleaned and pas sivated only after the desired thermo-compres sion diffusion bonds have been formed, preventing passivation material intended for beveled surface 70c from coating portions of major surfaces 70a and 70b near beveled surfac e 70c before bonding and thereby creating an irregularity in these major surfac es which migh result in generation of some unacceptable bowing force during therm compres sion diffusion bonding and consequential wafer fracture. After the bonding operation, beveled surface 70c is cleaned by such techniques as sputter etching, for example, which is preferable to conventional hot chemical etch cleaning because such an etch likely would chemically attack the structured copper strain buffers bonded to semiconductor devic e wafer 70. Subs equent to cleaning, beveled surfac e 70c is coated with a passivation material 202 such a polyimide siloxane, for example. In actual practice of the invention, heat sinks (not shown) are respectively attached to metallic sheets 57 and 59 of strain buffers -55 and 65. This is accomplished by thermo-compres sion diffusion bonding during the course of the bonding of strain buffers 55 and 65 to wafer 70, or at a later time as is convenient. Alternatively, heat sink attachment may be accomplished by other suitable means of joining metals together, such as soldering, for example. Thermo- compression diffusion bonding, however, is preferred because it inherently results in superior thermal conductivity between the metallic members which it joins together, allowing more efficient removal of heat from the s emiconductor device wafer . Practice of the invention is not limited to circular geometries for the various wafers, strain buffers and other layers being bonded into s emiconductor device assemblies . Rather, other geometries , such as squares, rectangles, polygons and the like may be used in a manner consistent with the practice of the invention. The foregoing describes a press apparatus and methods dbr thermo-compression diffusion bonding structured copper strain buffers to one or each of the opposed major surfaces of a substrateless semiconductor device wafer. Such bonding is achieved without subjecting the wafer to forces, such as bowing forces, which would tend to cause wafer fracture. The method accommodates both beveled and non-beveled semiconductor device wafers. O PI

Claims: What Is Claimed Is : 1. A method for thermo-compres sion diffusion bonding a structured copper strain buffer to a substrateles s s emiconductor device wafer having two major oppos ed surfaces, said strain buffer including a bundle of substantially parallel, clos ely packed strands of copper of substantially equal length, one common end of said strands being thermo-compression diffusion bonded to a metallic sheet, said method characterized by the steps of: applying a first metallic layer to one of said major surfaces; applying a s econd metallic layer to said first metallic layer; positioning the common end of the strands of said strain buffer opposite said metallic sheet in abutment with said second metallic layer; surrounding the s emiconductor devic e wafer and strain buffer w an inert atmosphere; applying a loading force to squeeze together at high pr es sure, said s emiconductor device wafer and said strain buffer; and heating said s emiconductor device wafer and said strain buffer at a temperature within the range of 300 C . to 400 C. while said loading force is applied. 2. A method according to Claim 1 characterized by the steps of applying first and s econd metallic layers, respectively, over th first and second major opposed surfaces of said semiconductor devic wafer; applying first and second metallizations, respectively, over sai first and second metallic layers; sandwiching said s emiconductor device wafer between said first and second structured copper strain buffers, the surface of each structured copper strain buffer opposite the respective metallic sheet thereof being positioned facing said wafer, said structured copper str buffers each having a lateral extent greater than the lateral extent of said s emiconductor device wafer and being s o positioned as to overha the entire outer edge surfac e of said semiconductor device waf er^ 3. A method according to Claims 1 or 2 characterized by positioning in abutment with a flat reference surface of a rigid member the surface of the semiconductor devic e wafer opposite the wafer surface having the first and s econd metallic layers disposed thereon. 4. The method of Claims 1 or 3 characterized in that the rigid member is comprised of quartz. 5. A method according to Claim 1 characterized in that the first and second major oppos ed surfaces of unequal lateral extents and a beveled outer edge surface; coating said beveled surfac e with a pas sivation material; sandwiching said semiconductor devic e wafer between and in axial alignment with said first and s econd structured copper strain buffers , each of said structured copper strain buffers having a lateral extent equal to or less than the lateral extent of the respective major surfac e of s aid semiconductor devic e wafer in contact therewith such that said beveled edge remains uncovered by either of said structured copper strain buffers, the common end of the copper strands of each of said first and second structured copper strain buffers opposite the respective metallic sheet thereof being positioned facing said s emiconductor device wafers; applying a loading force to selected portions of said first and s econd structured copper strain buffers to squeeze said first and second structured copper strain buffers and said semiconductor device wafer together at high pressure, the selected portion of s aid first structured copper strain buffer being substantially of identical geometrical size and shape as, and substantially rotationally and axially aligned with, the selected portion of said s econd structured copper strain buffer. 6 . The method of Claim 5 characterized by the step of situating first and s econd layers of nonreactive compactible material in contact with the metallic sheets of said first and s econd structured copper strain buffers , respectively, prior to surrounding said s emiconductor device wafer and structured copper strain buffers with ?rn t inert atmosphere. *p f Qj.iPI 7. A method according to Claim 4 characterized in that the first major surface being smaller than said s econd major surface, said first structured copper strain buffer being situated on said first surface and having a lateral extent no greater than the lateral extent of said first major surface, said second structured copper strain buffer having a lateral extent at least equal to the lateral extent of said s econd surfac e. 8. The method of Claims 1 or 3 or 4 or 5 characterized in that the first metallic layer is comprised of one of the group consisting o titanium, chromium and nickel and said s econd metallic layer is comprised of one of the group consisting of copper, silver and gold. 9. The method of Claims 2 or 5 characterized in that the first and second metallic layers ar e compris ed of one of the group consist of titanium, chromium and nickel, and said first and s econd metalliz tions are compris ed of one of the group consisting of copper, silver and gold. 10. The method of Claim 4 characterized in that the rigid member, said semiconductor device wafer and said strain buffer are heated at approximately 325 C. for a time within the range of approximately 15 minutes to 3 hours while said rigid member, said s emiconductor device wafer and said strain buffer are being squeeze together. 11. The method of Claims 1 or 3 or 4 or 5 or 8 characterized b the step of positioning a layer of nonreactive compactible material in abutment with the metallic sheet of said structured copper strain buffer prior to application of said loading forc e. 12. The method of Claim 11 characterized in that the layer of compactible material, s aid rigid member, said semiconductor devic o wafer and said strain buffer are heated at approximately 325 C . for time within the range of approximately 15 minutes to 3 hours while said layer of compactible material, said rigid member, said s emiconductor devic e wafer and said strain buffer are being squeeze together. 13. The method of Claims 1 or 2 or 9 characterized by the step of positioning first and s econd layers of nonreactive compactible material in abutment, respectively, with the metallic sheets of said first and second structur ed copper strain buffers, prior to application of said loading force. 14. The method of Claim 2 characterized by the step of beveling the outer edge of said s emiconductor devic e wafer to form a beveled edge surface prior to sandwiching said semiconductor device wafer between said first and s econd structured copper strain buffers , s aid first structured copper strain buffer being positioned facing said beveled surfac e. 15. The method of Claim 14 characterized by the step of applying said first and s econd metallic layers compris es depositing first and second metallic layers having a lateral extent less than the lateral extent of s aid wafer and sufficiently small so as to avoid overlapping said beveled surface, said first and s econd metallic layers being axially aligned with each other. 16. The method of Claim 15 characterized by the step of applying said first and s econd metallizations comprises depositing said first and s econd metallizations in such manner that said metallizations are axially aligned with said metallic layers and have a lateral extent equal to the lateral extent of said metallic layers . 17. The method of Claim 14 characterized by the step of cutting with a las er beam the metallic sheet of said first structured copper strain buffer in a manner allowing removal of most of the portion of said first structured copper strain buffer not bonded to said semicon¬ ductor devic e wafer after heating said wafer . 18. The method of Claim 17 characterized by the step of sputter etching s aid beveled edge surface of said semiconductor devic e wafer after cutting said metallic sheet. 19. The method of Claim 18 characterized by the step of pas siva¬ ting said beveled edge after sputter etching s aid beveled edge. 20. The method of any one of the preceding claims characteriz in that the high pressure is within the range of approximately 20, 000 to 50, 000 psi. 21. The method of any one of the preceding claims characteriz by the step of smoothing said first and second major opposed surfaces prior to applying said first and second metallic layers thereon, respectively. 22. The method of any one of the preceding claims characteriz by the step of smoothing is accomplished by polishing said first and s econd major opposed surfaces . 23. The method of any one of the preceding claims characteriz by the step of smoothing is accomplished by etching said first and s econd major opposed surfaces . 24. An improved press for diffusion bonding metallic surfaces of an assembly together, characterized by a first metallic plate mean having a predetermined thermal coefficient of expansion; second meta plate means having a predetermined thermal coefficient of expansion, said s econd plate means oriented parallel to said first plate means and spaced apart therefrom so as to accept said as sembly between sai first and second plate means; first and second rais ed portions include in and extending from said first and second metallic plate means, respectively, toward each other, each raised portion including a flat surface having a s elected geometric pattern, the geometric patterns of said first and second raised portions being substantially identical and axially and rotationally aligned with each other, ' each said flat surface being substantially parallel to the other; and metallic support means connecting said first plate means to said second plate means, said support means having a thermal coefficient of expansion s elected to restrict the thermal expansion thereof to a value less than the total thermal expansion in thickness of said first and second plate means when said press is heated to an elevated temperature; whereby equal pressures may be exerted on the metallic surfac es of said as s embly t squeeze said surfaces together at high pressure when said -pTess i heated to an elevated temperatur e. 25. The press of Claim 24 characterized in that the first and s econd plate means include first and second plates, respectively, and first and second expansion blocks facing each other and attached, respectively, to said first and second plates, said first and second expansion blocks respectively including said first and s econd rais ed portions . 26. The press of Claim 25 wherein said first and s econd expansion blocks are fabricated of material having thermal coefficients of expansion equal to the thermal coefficient of expansion of said first and second plate means . 27. The press of Claims 25 or 26 characterized in that the first and second expansion blocks are fabricated of material having thermal coefficients of expansion greater than the thermal coefficient of expansion of said first and s econd plate means . 28. The press of Claims 25 or 26 or 27 characterized by guide means extending from said first expansion block to said second expansion block for axially aligning said first and s econd rais ed portions . 29. The press of Claim 28 characterized in that the guide means comprises a metallic cylinder surrounding, and in abutment with, said first and second expansion blocks . 30. The press of Claim 29 characterized in that the guide means is fabricated of a metal having a thermal coefficient of expansion equal to the thermal coefficient of expansion of said first and second blocks . 31. The press of Claim 29 characterized in that the guide means is fabricated of a metal having a thermal coefficient of expansion greater than the thermal coefficient of expansion of said first and s econd blocks .

Inventors: HOUSTON D
Applicants: GEN ELECTRIC, GENERAL ELECTRIC CO
Application year: 1980
Main classification: H01L23
IPC codes: H01L21, H01L23