GB2175234A - Method of and apparatus for pressure bonding two bodies together - Google Patents

Abstract

Method of an apparatus for pressure bonding two bodies 2, 4 together, in which method a layer of a malleable metal is evaporated onto each of the bonding surfaces which are substantially flat. When the total thickness of bonding metal between the bodies must be small, for example, less than 1 micrometre when bonding a block of an acoustic medium to a piezoelectric element in order that the acoustic losses in the bond material should be acceptable, the surfaces of the bond material must be quite clean when the bond is established. The layers of the malleable metal are produced from an evaporation source 20, 21, 22 which is situated below both the bonding surfaces in order to mitigate deposition of spits on the bonding surfaces. The pressure bond between the two bodies is established inside the evaporation chamber 19 without exposing the bonding surfaces of the malleable metal to the ambient atmosphere. This is achieved by employing a jig formed in two parts 1, 3 linked by articulated links 5. Both parts holding the respective bodies, are initially suspended over the evaporation source (Figure 1), then, before pressing, one part 3 is pushed off the carrier rod 12 by a lever 14 and falls into position (Figure 4) below the first part 1. <IMAGE>

Description

SPECIFICATION Method of and apparatus for pressure bonding two bodies together The invention relates to a method of pressure bonding two bodies together by providing each of the bonding surfaces of the bodies with a malleable metal layer, which bonding surfaces are substantially flat, and establishing a mechanical bond between the bonding surfaces by pressing the bonding surfaces together by applying a steady pressure normal to the bonding surfaces, in which the malleable metal layers are produced on the bonding surfaces by evaporation under reduced pressure from an evaporation source situated below both the bonding surfaces, and the mechanical bond between the bonding surfaces is established while the bodies are still present in the reduced pressure atmosphere and to apparatus for carrying out such a method.The invention also relates to the manufacture of an acousto-optic transducer comprising a piezoelectric crystal bonded to an acoustic medium element by means of such a method and apparatus, and to an acousto-optic laser modulator comprising such an acousto-optic transducer.

Such a method and apparatus is disclosed in European Patent EP 0 001 220 Al, and is suitable for use in the manufacture of acousto-optic laser modulator elements where it is necessary to bond a piezoelectric crystal element to a block of an acoustic medium in such a way as to obtain efficient mechanical coupling between the two blocks.

United Kingdom Patent Specification 1,426,873 relates to a method of securing a ceramic member to another member which is of metal or ceramic material by pressure bonding, wherein between substantially flat facing surfaces of the two members there is applied a malleable metal body and a mechanical bond between the members via the intermediate malleable body is obtained by pressing the members together in a press at a steady pressure normal to said flat surfaces of between 1 and 5 tons per square inch at a temperature above ambient but below the melting point of the malleable metal body at said pressure and below the lowest temperature at which a liquid phase would form at said pressure by interaction of the elemental components of the malleable metal body and members of the facing surfaces, said temperature and pressure being together applied to achieve within a period not exceeding 30 second a bond between the members having a bond tensile strength minimally of 20% of the Ultimate Tensile Strength of the malleable metal body. G.B. Specification 1,426,873 also refers to a similar method of securing a semiconductor body, for example, of germanium, to a support by a similar pressure bonding method.

When making an acousto-optic transducer by pressure bonding two bodies together using a soft metal to make the bond while obtaining a transducer which has acceptable acoustic losses, it is necessary to use significantly thinner bonding layers than can be used when the bonding metal is introduced between the bonding surfaces in the form of a metal foil. The bonding metal in such cases is provided on the bonding surfaces by evaporation, and preferably the total thickness of metal between the two bodies is less than 1 micrometre. This is described in a letter by E.K.Sittig et al in Proc IEEE 56, (1968) pages 1375-1376, in which an initial layer of chromium about 10nm thick is first applied by evaporation to the cleaned surface of a body followed by a gold layer of about 0.2#m thick on top of which a layer of the bonding material, e.g. indium, is evaporated to a thickness of about 0.2cm.

If however the metal layers are deposited on the bonding surfaces by evaporation from sources which are disposed between the bonding surfaces, one bonding surface being disposed above the evaporation source and the other bonding surface being below the source, inhomogeneities will be found in the smoothness of the bonding surfaces formed by evaporation, notwithstanding the care which may have been taken to polish the surfaces onto which metal was evaporated. These inhomogenities in the smoothness of the bonding surfaces are caused by "spits", that is to say globules of metal (1 to 5 micrometres in diameter), projected from the source onto the deposition surfaces, more especially the surface which is disposed below the source.This is unsatisfactory because modulator elements formed by pressure bonding pairs of components having such inhomogeneities on the bonding surfaces, tend to exhibit a high incidence of cracking of the piezoelectric crystal elements.

This problem is mitigated by the arrangement disclosed in EP 1220 in which both the surfaces to be coated are located above the evaporation source, however the process of evaporation and subsequent pressure bonding have to take place within an evacuated space, and the apparatus employed hitherto to carry this out involved a complex arrangement of body support arms attached to shafts having drive units associated therewith forming a bulky assembly which required a large and costly vacuum chamber to accommodate such an assembly.

An object of the invention is to provide an improved method and apparatus by means of which a pressure bond can be made in a reduced pressure atmosphere between two bodies inside an evaporation chamber in which material forming the bond has been deposited, such that relative movement between the bodies from a position in which bond material is evaporated onto the bonding surfaces to a disposition in which the pressure bond is established, can be performed in a simple and compact manner.

The invention provides a method of pressure bonding two bodies together by providing each of the bonding surfaces of the bodies with a malleable metal layer, which bonding surfaces are substantially flat, and establishing a mechanical bond between the bonding surfaces by pressing the bonding surfaces together by applying a steadya pressure normal to the bonding surfaces, in which the malleable metal layers are produced on the bonding surfaces by evaporation under reduced pressure from an evaporation source situated below both the bonding surfaces, and the mechanical bond between the bonding surfaces is established while the bodies are still present in the reduced pressure atmosphere, wherein the two bodies are held in respective portions of a jig, which portions are interconnected by means of articulated links which permit both translational and rotary movement of one jig portion relative to the other jig portion, wherein during the evaporation of metal onto the bonding surfaces of the bodies the jig portions are disposed in a first position in which the bonding surfaces are substantially coplanar, and then prior to pressing the bonding surfaces together one of the jig portions is moved into a second position so that the bonding surfaces of the two bodies are opposed to each other and are mutually parallel during the establishment of the mechanical bond.

The jig portions are preferably elongate and each body can then be held at one end of the corresponding jig portion, the jig portions each being suspended from a support adjacent the other end thereof in the first position during the evaporation of metal onto the bonding surfaces, and then, prior to pressing the bonding surfaces together, one of the jig portions is released from the support so that it falls under the control of the articulated links into the second position.

The invention further provides apparatus for pressure bonding two bodies together by providing each of the bonding surfaces of the bodies with a malleable metal layer, which bonding surfaces are substantially flat, and establishing a mechanical bond between the bonding surfaces by pressing the bonding surfaces together by applying a steady pressure normal to the bonding surfaces, the apparatus comprising an evacuable chamber having therein evaporation sources and displaceable support means for supporting respective bodies to be bonded so that the prospective bonding surfaces can be located above a respective evaporation source while evaporating material from the source, the displaceable support means then being capable of relocating the respective bodies so that the bonding surfaces are facing one another and are mutually parallel, and of applying pressure thereto so as to establish a mechanical bond between said bonding surface while the bodies are maintained in an atmosphere of reduced pressure, wherein the displaceable support means comprises a jig including respective jig portions for holding a corresponding one of said bodies, articulated links interconnecting said jig portions and arranged to permit both translational and rotary movement of one jig portion relative to the other jig portion, a carrier from which both said jig portions are initially suspended so that the bonding surfaces of bodies held by the jig portions are disposed in a first position in which the bonding surfaces are substantially coplanar and downwardly directed towards the evaporation source during the evaporation of metal onto the bonding surfaces, and means for releasing one of said jig portions from said carrier prior to pressing the bonding surfaces together so that the released jig portion moves under gravity and under the control of the articulated links into a second position so that the bonding surfaces of the two bodies are disposed facing one another and are thereby maintained mutually parallel during the establishment of the mechanical bond.

Whereas when pressure bonding is performed using metal foils, the surface of the foils may be significantly contaminated with oxide, it is necessary to make the pressure bond at an elevated temperature which may be not far below the melting point of the foil material, and the foil material spreads significantly thus diluting the effects of the presence of contaminants on the surfaces of the foil. The surfaces of the evaporated metal layers are very clean, and it is essential that the pressure bond should be made while these surfaces are still clean and before they have been grossly contaminated as a result of exposure to the ambient atmosphere. Therefore the pressure bond is made while the bodies to be joined are still under the reduced pressure atmosphere in which evaporation was performed.It is then unnecessary to heat the bodies being bonded together, as would be the case were the bond to be made in air. The process of evaporating malleable metal onto the bonding surfaces of the bodies will have warmed the bodies to a temperature of, for example, 60 degress centigrade, and it is not necessary to heat the bodies further before establishing the mechanical bond.

However in carrying out this process hitherto, the respective bodies to be bonded were held by corresponding mounts carried by arms attached to shafts and having associated drive units. Such an assembly was bulky and required a large and costly vacuum chamber to accommodate it.

The invention is based on the realisation that by holding each body at one end of a preferably elongate corresponding individual jig portion which is suspended adjacent the other end from a carrier, the force of gravity can be used initially to direct both bonding surfaces downwardly towards an associated evaporation source while a corresponding metal layer is deposited thereon, and by connecting the two jig portions together by means of suitably disposed articulated links and releasing one jig portion from the supporting carrier, it can be made to fall to a position below the other jig portion with the respective coated surfaces parallel and facing one another so that the jig assembly can then be located between the plattens of a press enabling the coated surfaces to be brought together and bonded.

It was then realised that such a jig assembly could be made small and compact and that it would even be possible in the case of relatively small bodies such as the transducer and the associated block of optical medium required to form an acousto optic modulator, to accommodate several such jig assemblies, for example six, at a given evaporation and bonding station located within an evacuation chamber of normal size and modest di mensions.

A method or apparatus according to the invention may be used, for example, to mechanically bond a piezoelectric crystal element, such as a lithium niobate element, to a block of an acoustic delay medium, such as germanium. The malleable metal may be, for example, aluminium, gold, indium or tin.

In order to improve the adhesion of the malleable metal layers of the respective bodies, it is preferred to evaporate a flash layer of chromium directly onto the bonding surfaces of the bodies.

When the malleable metal is indium, it was found that a gold layer deposited between the chromium layer and the indium layer improved the thermal bond between the bodies.

An embodiment of the invention will now be described with reference to the accompanying schematic drawings, in which: Figure 1 is a side-view of a jig supported on a carrier and used to hold a lithium niobate crystal and a germanium block, and is shown in a first disposition in which metal layers are evaporated on bonding surfaces of the crystal and of the block, Figure 2 is an end-view of the jig shown in Figure 1, Figure 3 shows the jig of Figure 1 in a second disposition in which the bonding surfaces of the crystal and of the block are opposed to each other and are mutually parallel, and Figure 4 is a side view of an apparatus in which bonding surfaces of a lithium niobate crystal and of a germanium block are provided with metal layers by evaporation and are then mechanically bonded together by pressure bonding.

Referring to Figures 1 and 2, the jig comprises a first portion 1 in which a lithium niobate crystal 2 (12 x 3 x 1 mm) was held and a second portion 3 in which a germanium block 4 (20 x 15 x 5 mm) was held. The jig portions 1 and 3 were loosely linked by two articulated links in the form of slotted links each comprising an elongate plate 5 provided with a longitudinal slot 6, stop members 7 and 8 on the respective portions 1 and 3 of the jig limiting the relative separation between the two jig portions 1 and 3. Each of the jig portions 1 and 3 were provided with two symmetrically positioned bores 9 and 10 through their thickness (only shown for jig portion 3 in Figure 2) and the jig was loaded onto two supporting rods 11 and 12 which extended from a carrier 13, so that the supporting rods 11 and 12 passed through the respective bores 9 and 10 of both jig portions 1 and 3.The diameters (3.0 mm) of the supporting rods 11 and 12 were slightly less than the diameters (3.3 mm) of the bores 9 and 10, both in order to allow the jig portion 3 to be easily slipped off the ends of the supporting rods 11 and 12 and to allow the jig portion to be displaced upwards relative to the supporting rods 11 and 12 by a small distance. In the first disposition of the jig portions 1 and 3 which is shown in Figure 1, the left-hand end of the slot 6 is spaced from the stop member 8.

When metal layers required for mechanically bonding the lithium niobate crystal 2 to the germanium block 4 have been evaporated onto the bonding surfaces of the crystal and of the block 4, the second jig portion 3 is pushed off the ends of the supporting rods 11 and 12 by means of a horizontally disposed finger 14 and falls into the disposition relative to the first jig portion 1 shown in Figure 3. The finger 14 was moved by rotating a vertically disposed rod 15 integrally formed with the finger 14 and which extended through a vacuum-tight seal in the base of an evaporation chamber in which the metal layers had been evaporated.

In this disposition the respective bonding surfaces 16 and 17 of the crystal 2 and of the block 4 are opposed and mutually parallel.

Referring to Figure 4, an apparatus in which the lithium niobate crystal 2 was mechanically bonded to the germanium block 4 comprises a base-plate 18 on which a bell jar 19 rested to provide a gastight enclosure. For the sake of clarity, the scale on which the jig portions 1, 3, the crystal 2, the block 4 and the supporting rod 12 are shown in Figure 4 is considerably exaggerated with respect to the scale on which the other components in Figure 4 are shown. This apparatus contained the abovementioned carrier 13, and respective evaporation sources 20, 21 and 22 for chromium, gold and indium.A hydraulic arm 23 extends into the apparatus though a gas-tight seal (not shown) in the base-plate 18 and has a head 24 which bears against the lower end of the second jig portion 3 during a pressure bonding step whereby a mechanical bond is formed between the lithium niobate crystal 2 and the germanium block 4. The upper end of the first jig portion 1 is forced against a steel platen 25 which is mounted on the baseplate 18.

The articulated links each formed by the plate 5 provided with a slot 6, may alternatively each take the form of two or more component plates, each component plate being attached to the next adjacent component plate by a pivotal joint, and the assembly can take the form of a chain-like structure if desired. The slot in the plate 5, or the plurality of pivoted links function so that when totally extended by gravity as in Figure 3, the lower jig portion 3 and the block 4 reliably clears the crystal 2 both at rest and while falling under gravity and under control of the articulated links after it is released from the support rods 11 and 12.The presence of the slot 5 or the use of a succession of two or more mutually pivoted links or of a chain-like arrangement for each articulated link, is not only to permit the two jig portions to be disposed side by side as in Figure 1 and then for one of the jig portions 3 to be pushed sideways off the ends of the rods 11 and 12 without displacing the other jig portion 1, but also to enable the lower platen 24 to raise the lower jig portion 3 so that the bonding surface 17 is brought into contact with the other bonding surface 16. Further raising of the platen 24 then upwardly displaces the upper jig portion 1 so that it is lifted off the supporting rods 11 and 12 into contact with the upper platen 25, and the oversize diameters of the bores 9 and 10 are arranged to enable this to occur freely.

The jig to which the lithium niobate crystal 2 and the germanium block 4 were secured was placed on the carrier 13 in the apparatus shown in Figure 4 with the jig portions 1 and 3 arranged as shown in Figure 1. The bell jar 19 was sealed to the baseplate 18, the apparatus was purged with nitrogen and then pressure inside the bell jar was reduced to 10-3Pa.

The respective bonding surfaces 16 and 17 of the crystal 2 and of the block 4 were then cleaned with a nitrogen glow discharge. A 10 nm thick chromium layer was deposited by evaporation from the chromium source 20 on each of the bonding surfaces 16 and 17. A 100 nm thick gold layer was deposited on each chromium layer and a 300 nm thick indium layer was deposited on each gold layer, by evaporation from the respective gold and indium sources 21 and 22. The bonding surfaces 16 and 17 were substantially coplanar, the difference between their heights (which was 20 cms) above the evaporation source being about 1 mm.

After the indium layers had been deposited on the bonding surfaces, the second jig portion 3 was slipped off the supporting rods 11 and 12 so that the jig portions 1 and 3 were disposed in the manner shown in Figure 3. The lithium niobate crystal 2 was then mechanically bonded to the germanium block 4 by applying a pressure of 1 ton/sq.in. (1.58 kN/cm2) to the lower end of the second jig portion 3 by means of the hydraulic ram 23, urging the indium layers together and forming a pressure bond.

Guide means (not shown) which co-operate with the hydraulic ram 23 engaged with the first and second jig portions 1 and 3 so as to maintain the bonding surfaces 16 and 17 mutually parallel. Pressure was maintained for 10 minutes so as to permit the stresses produced in the crystal 2 and in the block 4 to be relieved.

Although no heat was supplied directly to the jig, the temperature of the crystal 2 and the block 4 during the formation of the pressure bond was about 600C as a result of the heat dissipated in the apparatus during the previous steps of the process.

The yield of acceptable bonded assemblies produced by this process was 80%.

The bonded assemblies were then further processed to form acousto-optic transducers. The thickness of the lithium niobate crystal was reduced by grinding and polishing to a desired value, and then a gold electrode structure was evaporated onto the polished surface. The germanium block was provided with absorbing means, and the completed acousto-optic transducer was built into an acoustooptic laser modulator.

Claims (1)

1. A method of pressure bonding two bodies together by providing each of the bonding surfaces of the bodies with a malleable metal layer, which bonding surfaces are substantially flat, and establishing a mechanical bond between the bonding surfaces by pressing the bonding surfaces together by applying a steady pressure normal to the bonding surfaces, in which the malleable metal layers are produced on the bonding surfaces by evaporation under reduced pressure from an evaporation source situated below both the bonding surfaces, and the mechanical bond between the bonding surfaces is established while the bodies are still present in the reduced pressure atmosphere, wherein the two bodies are held in respective portions of a jig, which portions are interconnected by means of articulated links which permit both translational and rotary movement of one jig portion relative to the other jig portion, wherein during the evaporation of metal onto the bonding surfaces of the bodies the jig portions are disposed in a first position in which the bonding surfaces are substantially coplanar, and then prior to pressing the bonding surfaces together one of the jig portions is moved into a second position so that the bonding surfaces of the two bodies are opposed to each other and are mutually parallel during the establishment of the mechanical bond.

2. A method as claimed in Claim 1, wherein said jig portions are elongate and each body is held at one end of the corresponding jig portion, the jig portions each being suspended from a support adjacent the other end thereof in said first position during the evaporation of metal onto the bonding surfaces, and then, prior to pressing the bonding surfaces together, one of the jig portions is released from the support so that it falls under the control of the articulated links into the second position.

3. A method as claimed in Claim 1 or Claim 2, wherein the articulated links comprise slotted links.

4. A method as claimed in any one of the preceding claims, wherein a piezoelectric crystal element is mechanically bonded to a block of an acoustic medium.

5. A method as claimed in Claim 4, wherein the piezoelectric crystal element consists of lithium niobate and the acoustic medium is germanium.

6. A method as claimed in any one of the preceding claims, wherein the malleable metal is aluminium, gold, indium or tin.

7. A method as claimed in any one of the preceding claims, wherein a chromium layer is evaporated directly on the bonding surfaces of the two bodies.

8. A method as claimed in Claim 7, wherein the malleable metal is indium and a gold layer is evaporated between the chromium layer and the indium layer.

9. A method as claimed in any one of the preceding claims, wherein the total thickness of metal between the two bodies is less than 1 micrometre.

11. A method of pressure bonding two bodies together, substantially as herein described with reference to the drawings.

12. Apparatus for pressure bonding two bodies together by providing each of the bonding surfaces of the bodies with a malleable metal layer, which bonding surfaces are substantially flat, and establishing a mechanical bond between the bonding surfaces by pressing the bonding surfaces together by applying a steady pressure normal to the bonding surfaces, the apparatus comprising an evacua ble chamber having therein evaporation sources and displaceable support means for supporting respective bodies to be bonded so that the prospective bonding surfaces can be located above a respective evaporation source while evaporating material from the source, the displaceable support means then being capable of relocating the respective bodies so that the bonding surfaces are facing one another and are mutually parallel, and of applying pressure thereto so as to establish a mechanical bond between said bonding surface while the bodies are maintained in an atmosphere of reduced pressure, wherein the displaceable support means comprises a jig including respective jig portions for holding a corresponding one of said bodies, articulated links interconnecting said jig portions and arranged to permit both translational and rotary movement of one jig portion relative to the other jig portion, a carrier from which both said jig portions are initially suspended so that the bonding surfaces of bodies held by the jig portions are disposed in a first position in which the bonding surfaces are substantially coplanar and downwardly directed towards the evaporation source during the evaporation of metal onto the bonding surfaces, and means for releasing one of said jig portions from said carrier prior to pressing the bonding surfaces together so that the released jig portion moves under gravity and under the control of the articulated links into a second position so that the bonding surfaces of the two bodies are disposed facing one another and are thereby maintained mutually parallel during the establishment of the mechanical bond.

13. Apparatus as claimed in Claim 12, wherein said jig portions are elongate and the corresponding body is held at one end of a respective jig portion, each said jig portion being initially suspended from the carrier adjacent the other end thereof.

14. Apparatus as claimed in Claim 12 or Claim 13, wherein the articulated links comprise slotted links.

15. Apparatus as claimed in any one of Claims 12 to 14, wherein one said body is a piezoelectric crystal element and the other said body is a block of acoustic medium.

16. Apparatus as claimed in Claim 15, wherein the piezoelectric crystal element consists of lithium niobate and the acoustic medium is germanium.

17. Apparatus as claimed in any one of Claims 12 to 16, wherein the malleable metal is aluminium, gold, indium or tin.

18. Apparatus as claimed in any one of Claims 12 to 17, wherein one of said sources contains chromium and is operated initially to evaporate a chromium layer on the bonding surface of the two bodies.

19. Apparatus as claimed in Claim 18, wherein the malleable metal is indium and another of said sources contains gold which is operated to form a gold layer on top of the chromium layer prior to evaporating an indium layer.

20. Apparatus for pressure bonding two bodies together, substantially as herein described with reference to the drawings.

21. An acousto-optic transducer comprising a piezoelectric crystal element bonded to an acoustic medium element by a method as claimed in any one of Claims 4 to 11, or by apparatus as claimed in any one of Claims 12 to 20.

22. An acousto-optic laser modulator comprising an acousto-optic transducer as claimed in Claim 21.