Abstract:
Bonding machines have transparent elements on their bonding axes. Magnified optical means are positioned to view bonding tips and integrated-circuit chips simultaneously through the transparent elements thereby facilitating alignment of the chips to the tips. In one embodiment a bonding tip is transparent and a transparent compliant bonding member is utilized in conjunction with the tip to produce compliant bonds. Another embodiment utilizes a transparent tip in a &#34;hard tip&#34; type of bonding operation. A third embodiment utilizes a transparent tray closely positionable to the bonding tip. After alignment of the tip to one of the chips is achieved the tray is removed and bonding proceeds on an accurately positioned substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods of and apparatus for bonding workpieces together wherein rapid and accurate alignment of workpieces to a bonding tip is achieved. In particular, the invention relates to systems wherein viewing of the workpieces and the tip to which alignment is to be achieved is performed through transparent elements on the bonding axes of machines used for making the bonds. 
     2. Description of the Prior Art 
     In apparatus used for bonding electronic devices such as integrated-circuit chips to substrates, one of the principal problems is aligning the integrated-circuit chips to bonding tips or apertures in compliant bonding members, then engaging the chips to the tips or apertures, and aligning the chips with a pattern on a substrate. A broad spectrum of optical systems have been utilized to facilitate such alignment. Most of the systems have some inherent limitations and are thus not entirely satisfactory. 
     One of the most common systems employs a half silvered mirror suspended between a bonding tip and a plane on which either chips or a substrate lie. An optical axis of a microscope is obliquely directed to the half silvered mirror and an image of both the tip and a chip or substrate on the plane over the mirror appear simultaneously in the microscope eyepiece. However, the oblique viewing causes perspective distortions and accurate alignment is difficult to achieve because of such distortions. Additionally, because the mirror is suspended between the tip and the workpiece with which the tip must be engaged, the mirror must be mounted on a swing-away mechanism which adds complexity to the bonding apparatus. 
     The oblique viewing problems associated with half silvered mirrors are eliminated to a certain extent through the use of beam-splitter prism combinations which are suspended similarly to the mirror but which direct an image of both a tip and a workpiece perpendicularly of a bonding axis on which it is suspended. These combinations however, require very accurate alignment to the bonding axis and thus the supporting mechanism, in addition to being capable of swinging away from the bonding axis, must have adjustments provided thereon to achieve precise planar alignment of the prism. 
     Systems employing reference reticles have also been used to accomplish the desired alingments. Such systems are described in U.S. Pat. No. 3,477,630 issued to F. J. Schneider on Nov. 11, 1969. These systems, while performing admirably when properly adjusted, do suffer from the limitation that accurate alignment of a mechanism which positions a movable head requires very precise adjustment with respect to the reference reticle in order to achieve the necessary degree of accuracy. 
     Even more complex systems have been utilized employing rapidly oscillated mirrors to project images of workpieces and bonding tips simultaneously to an operator while an operator tries to achieve alignment therebetween. Obviously, systems that use oscillating mirrors suffer from problems associated with inherent complexity. 
     Except for the reference reticle system, the above described systems have one common shortcoming. Some optical element of the apparatus must be at least temporarily suspended between the bonding tip and the workpiece. In order to provide space for these elements, the bonding tip must travel extensive distances in accomplishing either engagement with the workpiece for pickup purposes or for bonding purposes. Extensive travel of the bonding tip inherently carries with it difficulty in providing precise positioning of the bonding tip throughout its range of operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide systems for facilitating rapid and accurate alignment of workpieces and force-producing elements in bonding apparatus. 
     It is another object of the invention to provide systems wherein force-producing elements in bonding apparatus are required to move only very short distances. 
     It is a further object of the invention to provide bonding apparatus in which workpieces and force-producing elements can be viewed in a direct line of sight along a bonding axis. 
     It is a more particular object of the invention to provide systems wherein bonding of workpieces can be performed under continuous visual surveillance. 
     It is an even further object of the invention to provide bonding apparatus wherein the workpieces can be observed during bonding therebetween so that distortions of the beam leads caused by bonding can be controlled in response to the viewing thereof. 
     The foregoing and other objects are accomplished in accordance with the invention by utilizing transparent elements positioned on bonding axes of bonding apparatus. Workpieces are aligned to force-producing members in response to images of the workpieces and the members which pass through the transparent elements. 
     Particular examples of apparatus useful in practicing the invention include bonding apparatus with transparent bonding heads, bonding apparatus which employs transparent film as a compliant bonding medium and bonding apparatus in which workpieces are supported on a transparent workpiece-supporting tray which swings away from a bonding axis after a workpiece is aligned and engaged with a bonding tip in response to an image passing through the tray. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will be more readily understood from the following detailed description of specific embodiments thereof when read in conjunction with the appended drawings in which: 
     FIG. 1 is a perspective view of one embodiment of the inventive apparatus; 
     FIG. 2 is a partially sectioned elevation view of a bonding head portion of the machine of FIG. 1; 
     FIG. 3 is a view of a monitor portion of the machine of FIG. 1 showing a workpiece misaligned with respect to a portion of the machine; 
     FIG. 4 is a view of the monitor of FIG. 3 showing the workpiece aligned; 
     FIG. 5 is a view of the monitor of FIG. 3 showing a misalignment of a second workpiece; 
     FIG. 6 is a view of the monitor of FIG. 3 showing an alignment between the two workpieces; 
     FIG. 7 is a partially sectioned elevational view of a bonding head which is an alternate embodiment of the invention apparatus; 
     FIG. 8 is a view of the monitor of the machine of FIG. 1 showing portions of the bonding head of FIG. 7; 
     FIG. 9 is a view of the monitor of FIG. 8 showing alignment between a workpiece and portions of the head of FIG. 7. 
     FIG. 10 is a view of the monitor of FIG. 9 showing an additional workpiece; 
     FIG. 11 is a view of the monitor of FIG. 10 showing the workpieces bonded together; 
     FIG. 12 is an elevational view of a bonding machine which is an alternate embodiment of the inventive apparatus; 
     FIG. 13 is a view taken along the lines 13--13 of FIG. 12 showing a monitor which is portraying a workpiece and a bonding tip; 
     FIG. 14 is an elevational view of the machine of FIG. 12 in a workpiece pickup position; 
     FIG. 15 is a view taken along the lines 15--15 of FIG. 14 showing a transparent tray supporting a number of workpieces; 
     FIG. 16 is a view of the monitor of FIG. 13 illustrating a workpiece aligned with a bonding tip; 
     FIG. 17 is a view of the machine of FIG. 12 showing the machine in a bonding configuration and on which a portion of a support tray has been removed for purposes of clarity; 
     FIG. 18 is a view taken along the line 18--18 showing a substrate positioned for bonding on a bonding axis of the machine of FIG. 17. 
    
    
     DETAILED DESCRIPTION 
     Illustratively, the invention is described in connection with bonding beam-lead integrated-circuit chips to conductive elements which are pattern generated on ceramic substrates. However, it is to be understood that the invention can be useful in the bonding of many types of small articles where accurate alignment between parts is important. 
     One embodiment of the inventive apparatus a compliant bondig machine, designated generally by the numeral 20, is illustrated in FIG. 1. The bonding machine includes a movable head assembly, designated generally by the numeral 22, a base 24, a conventional positioner assembly designated generally by the numeral 26, and an electronic optical system including a television camera, designated generally by the numeral 27 and a monitor designated generally by the numeral 28. Even though the machine 20 is illustrated with an electronic optical system however, a conventional microscope might also be used with equal effectiveness. 
     Operation of the machine 20 can best be understood by a discussion and description of one complete bonding cycle. As illustrated in FIG. 1, a beam-lead integrated-circuit, designated generally by the numeral 30, is illustrated on a screen 32 of the monitor 28. The image on the screen illustrates leads 36 of the chip 30 being well aligned with conductive elements 38 which are patterns generated on a ceramic substrate, designated generally by the numeral 40. 
     At the point in time shown in FIG. 1, the monitor 28 is showing an image of the chip 30 engaged with a bonding aperture 42 of a compliant member, designated generally by the numeral 44. The compliant member 44 is transparent and for this reason, the substrate 40 below the compliant member is visible through the optical system 28. 
     One example of a material suitable for use as the compliant member 44 is polyimide film. One example of such a film is marketed by E. D. DuPont DeNemours and Co. of Wilmington, Delaware under the tradename &#34;Kapton.&#34; By way of example, a film thickness of 0.005 inch has been found to be suitable for chips which are approximately 0.002 inch thick. 
     For increased visability, the compliant member 44 may be colored. Sensing the color is done readily with a microscope but an electronic optical system must have color sensing capability to take advantage of the additional visibility which color imparts to the compliant member 44. 
     The chip 30 is held in engagement with the compliant member 44 by vacuum developed within a bonding head, designated generally by the numeral 46. The inner structure of the head 46 is more clearly illustrated in FIG. 2. A vacuum chamber 48 is formed in the ram 46 and is connected to a vacuum source 49 through a port 50 and tubing 52 (see FIG. 1). 
     In order that the optical system 28 can receive images of the chip 30, the substrate 40 and the compliant member 44, the vacuum chamber 48 is constructed so that the walls through which an optical axis 54 passes are transparent. It will be noted that the optical axis 54 is also the bonding axis of the machine 20. The uppermost wall is a clear glass or quartz disc 56. Directly below the disc 56 is a clear glass or quartz engagement portion or bonding tip, designated generally by the numeral 58. The tip 58 is provided with an aperture or vacuum port 60 through which the force of vacuum within the chamber 48 can be transmitted to the chip 30. 
     The chip 30 is shown in &#34;engagement&#34; with the tip 58 under the influence of the forces of the vacuum source 49. Although the chip 30 is being held compressively against the tip 58, there is no actual contact therewith. The thickness of the compliant member 44 is greater than the thickness of a body portion 61 of the chip 30 and thus, no actual contact between the chip and the tip occurs in this embodiment during &#34;engagement.&#34; 
     The tip 58 is advantageously provided with a projection 62. The projection provides a way of localizing bonding forces to a small region of one of the substrates 40. This localization is often desirable when a number of the chips 30 are bonded on one of the substrates 40. The projection 62 will fit into a small space on the substrate 40 during bonding and the likelihood is reduced of disturbing another one of the chips which has previously been bonded nearby on the substrate. 
     Conventional cartridge heaters 63 are also provided in the head 46 in order to provide for heating of the chip 30 prior to and during bonding. 
     It can be seen in FIG. 1 then, that the chip 30 is in a position wherein it can be properly bonded to the metallic elements 38. Bonding is accomplished by lowering the movable head assembly 22 along conventional slide assemblies 64. After the assembly 22 is lowered, a conventional compliant bond is produced between the leads 36 and the conductive elements 38. A detailed description of the characteristics and techniques for producing compliant bonds with polyimide can be had by referring to an application for U.S. Pat. Ser. No. 864,856 filed in the names of J. A. Burns and A. Coucoulas on Oct. 8, 1969, and assigned to the assignee of record of this application. 
     A substantial amount of heat is required to produce an effective thermocompression bond and it is found desirable to supplement the heating produced by the heater 63 in the head 46 by heating the substrate 40. This is best accomplished by putting conventional heaters 65 in a substrate support designated generally by the numeral 66. 
     After the bond is complete, the head assembly 22 is raised again to the position shown in FIG. 1. The positioner assembly is moved to the right along conventional slide members 68 until a chip tray 70 is positioned on the optical axis 54. The compliant member 44 is also indexed so that a subsequent one of the bonding apertures 42 is aligned with the axis 54. 
     It can be seen from FIG. 1 that the compliant member 44 is a continuous strip which is wound from a supply reel 72 to a takeup reel 74. The takeup reel 74 has torque continuously applied to it by a conventional torque motor 76. Control of the movement of the takeup reel 74 is provided through an indexing sprocket 78. 
     The indexing sprocket 78 is moved in accurately controlled 45° increments. Movement of the member 44 is initiated when a cylinder-actuated pin 80 is withdrawn from one of eight holes 82 in a plate 84. The pin 80 is controlled by a spring-biased cylinder 86 and the cylinder acts only momentarily to pull the pin out of the hole 82 with which it is engaged. As soon as the pin 80 is free from the hole 82, the plate 84, which is on the same shaft as the sprocket 78, begins to rotate because of the force exerted on the sprocket by the member 44. After 45° of rotation the pin 80 snaps into the next one of the holes 82 and motion of the compliant member 44 stops. Spacing between bonding aperture 42 is such that 45° rotation of the sprocket 78 results in accurate location of the subsequent one of the apertures 42 on the optical axis 54. 
     After a new one of the bonding apertures is aligned with the optical axis 54, an image, such as that shown in FIG. 3, appears on the screen 32 of the monitor 28. An operator can see the aperture 42 and the projection 62 of the tip 58 as well as any one of the chips 30 which is within the field of the camera 27. The chips 30 are not necessarily in an ordered array on the chip tray 70 and because of this, it is quite likely that the chip 30 may not be aligned with the aperture 42. 
     To achieve alignment, an operator moves a handle 88 of the positioner assembly 26 and rotates the chip tray 70 while viewing the screen 32 until the image shown on FIG. 4 is achieved. FIG. 4, of course, illustrates the chip 30 being properly aligned with the aperture 42. 
     After the image of FIG. 4 is achieved, the head assembly is lowered and the force of vacuum within the chamber 48 brings the chip 30 into engagement with the head 46 by being drawn against the compliant member 44. After this engagement, the head assembly 22 is raised to the position shown in FIG. 1. 
     The positioner assembly 26 is then moved to the left so that one of the substrates 40 is roughly aligned with the optical axis 54. At this time, the operator sees an image similar to that shown in FIG. 5 on the screen 32. 
     The operator then moves the positioner handle 88 to manipulate the substrate 40 while viewing the screen 32 to achieve alignment of the conductive elements 38 with the leads 36. After an image like that shown in FIG. 6 is achieved, bonding of the leads 36 to the elements 38 can take place and a full cycle of operation is thus completed. 
     It is important to note that the vertical movement of the head assembly 22 can be quite small. This is because there is no need to place complex optical system components between the tip 58 and the workpieces. It is desirable to take advantage of the fact that the movement can be small so that great precision can be developed in the machine 20. 
     Keeping the movement of the assembly 22 small also assures that the image of the aperture 42 and the image of the chip 30 supported on the tray 70 as well as the image of the conductive elements 38 supported on the substrate holder 66 are simultaneously within the depth of field of the optical system 28. It is also important to construct the machine 20 so that the chips 30 and the conductive elements 38 are supported at substantially the same elevation so that each of the respective images will be in focus when they are placed on the optical axis 54 by movement of the positioner assembly 26. 
     Another embodiment of the inventive apparatus illustrated in FIG. 7 includes a non-compliant bonding head, designated generally by the numeral 146. The bonding head includes a vacuum chamber 148, a vacuum port 150, a transparent disc 156 and a transparent bonding tip 158. The head 146 can be used on a bonding machine similar to that shown in FIG. 1 with the exception that it is not necessary to use a compliant member. The head 146 is designed to be used in a so-called &#34;hard tip&#34; mode of operation. Provision must be made for permitting &#34;compensation&#34; to equalize bonding forces to all of the leads 36. One machine in which the head 146 may be advantageously employed is described in U.S. Pat. No. 3,475,814 issued to J. A. Santangini on Nov. 4, 1969. The head 46 may also be incorporated into a &#34;compensating&#34; arrangement by pivotally mounting the head. 
     The tip 158 includes a projection 162 and an aperture 160. The aperture 160 is large enough so that the body portion 61 of one of the chips 30 will fit into the aperture with some clearance around all sides of the body portion. The relative size of the aperture 160 to the body portion 61 is shown quite clearly in FIG. 9. The projection 162 serves substantially the same purpose as the projection 62 (FIG. 2). 
     Some heat is transmitted through the tip 158 from cartridge heaters 163. Of course, it is still desirable to provide heating from sources other than the ram 146. 
     In using the ram 146, an operator usually first encounters a situation in which the aperture 160 is misaligned with respect to one of the chips 30 such as that shown in FIG. 8. This is somewhat analogous to the situation shown in FIG. 3. The chip 30 is moved into proper alignment with the aperture 160 while being viewed along an optical-bonding axis 164 through an optical system that is operative because of the transparency of the disc 156 and the tip 158. FIG. 9 represents an image that an operator sees after proper alignment of the chip 30 and the aperture 160 is achieved. 
     FIG. 10 illustrates the conductive elements 38 of one of the substrates 40 as they appear to an operator prior to final aligment of the substrate and the chip 30. The substrate 40 is carefully moved until the conductive elements 38 are properly aligned with the leads 36. 
     FIG. 11 illustrates an image that an operator sees during bonding of the leads 36 to the elements 38. One of the very significant advantages of this inventive apparatus is that it permits an operator to visually determine the extent of &#34;squash out&#34; of the leads 36 while bonding is proceeding. Thus, an operator has full visual capabilities for viewing a bonding process as it occurs. The operator can respond to excessive or insufficient &#34;squash-out&#34; and the likelihood of getting good quality bonds is greatly enhanced with this system. 
     Another embodiment of the inventive apparatus is illustrated in FIGS. 12 through 18. A bonding machine, designated generally by the numeral 180, includes a ram assembly, designated generally by the numeral 182; a chip handling assembly, designated generally by the numeral 184; a substrate positioner, designated generally by the numeral 186; and an optical system, designated generally by the numeral 188. 
     Here again an understanding of the structure and operation of the machine 180 can best be understood by discussion of one full cycle of the operation. FIG. 12 illustrates the ram assembly 182 in a raised position prior to alignment of one of the chips 30 supported on the chip handling assembly 184. The ram assembly 182 is raised by the action of an eccentric cam 189 rotating on a shaft 190 causing displacement of a roller 192 connecting to a lever arm, designated generally by the numeral 194. The lever arm pivots around a fulcrum 196. An engagement portion of the arm 194 presses against an adjustably positionable collar 200 when the cam 189 rotates to the position shown in FIG. 12, thus urging the ram assembly 182 upwardly against the force of a compression spring 202. 
     While the ram assembly 182 is raised, a tip 203 is held with its lower most surface approximately 0.015 inch from the surface of a transparent chip tray 204 of the chip handling assembly 184. This spacing can be varied by adjusting the collar 200 to assure that the chip 30 and the tip 203 are within the depth of field of the optical system 188. 
     When the tip 203 is held just above the tray 212, the optical system 188 can be used to produce an image of both the tip 203 and one of the chips 30. The optical system 188 is illustrated in FIG. 12 as being a conventional TV camera with a conventional right angle lens system that can be set to view along an optical axis 206. A microscope might also be used with equal success. 
     The optical axis is aligned with the bonding axis of the machine 180 and a support member 207 is provided with an aperture 208 through which the optical system 188 can receive the image of the chip 30 and the tip 203. 
     The chip handling assembly 184 is manipulated in the X and Y direction by a conventional manipulator 210. Angular manipulation is accomplished by rotating the tray 204. During manipulation an operator views the tip 203 and the chip 30 through a screen 212 of a monitor designated generally by the numeral 214 (FIG. 13), associated with the optical system 188. When the chip 30 is aligned properly with the tip 203, the tip is lowered to the position shown in FIG. 15, vacuum is applied within a bonding head 216 the chip 30 is held against the tip 203. The bonding head is advantageously of the type shown in U.S. Pat. No. 3,452,917 issued to F. J. Schneider on July 1, 1969. An image of one of the chips 30 properly aligned with the tip 203 is shown in FIG. 16. 
     When the ram assembly 182 is in the pickup position of FIG. 15 a stop member 218 is engaged with a latch 220 which is operated by a cylinder 222. The cylinder 222 is shown in its extended position in FIG. 15. The stop member 218 is adjustable in length so that the force of the spring 202 is not transmitted to the tray 204 by the tip 203. 
     After the chip 30 and tip 203 are engaged, the shaft 190 is rotated and the ram assembly 182 is raised. The latch 220 is retracted by the cylinder 222 and the tray 204 is swung away from the optical and bonding axis 206 under the action of a gear drive positioning mechanism designated generally by the numeral 223 of the chip handling assembly 184. FIG. 18 is an illustration of the extent to which the chip tray 204 is displaced from the axis 206. 
     With the ram assembly 182 in its raised position, a carefully machined substrate support block, designated generally by the numeral 224, is placed into engagement with the substrate positioner 186 as shown in FIGS. 17 and 18. The substrate positioner 186 includes two accurately positionable stop members 226. the members 226 are positioned by adjustment of micrometer adjusting screws 228. 
     The support block 224 is provided with two positioning members 230 placed at right angles to each other and a spring clamp 232. A substrate, designated generally by the numeral 234, is held against the members 230 by the clamp 232. Conductive elements 236 on the substrate are very accurately positioned with respect to the outer surface of the substrate so that the conductive elements become very accurately located with respect to the sides of the block 224. The micrometer screws 228 are adjusted so that a desired portion of the substrate 234 lies on the axis 206 after the block 224 is placed against the stop members 226. Thus, the chip 30 which is engaged with the tip 203 can be bonded at a desired location on the substrate without actual visual control of alignment of the substrate 234. 
     Bonding occurs when the ram assembly 182 is lowered with the latch 220 withdrawn. Rotation of the cam 188 permits the ram assembly 182 to move downwardly under the action of the spring 202 until the tip 203 presses the leads 36 against the substrate 234, at which time bonding between the leads 36 and the conductive elements 236 takes place with the spring 202 providing the bonding force. 
     The ram assembly 182 is again raising by rotation of the cam 188. The latch 220 is moved outwardly, the chip tray 204 is swung back into the axis 206, and a new cycle of operation can begin. 
     It can be recognized that in a situation where a number of the chips are to be bonded onto one substrate, that a number of the bonding machine 180 can be positioned along an assembly line and each of the substrate positioners 186 can be adjusted to accommodate a particular chip location on the substrate. Thus, one of the substrates 234 mounted on one of the blocks 224 can be passed from one of the machines 180 to successive ones of the machine where chips can be bonded at various locations on the substrates thereby creating a highly efficient manufacturing operation. It should also be recognized that such a manufacturing scheme requires that the conductive elements 236 are very accurately located with respect to the outer surfaces of the substrates 234. The system is quite useful where the conductive elements 236 are produced by accurate photo-projection techniques. 
     It is to be understood that the above-described embodiments are merely illustrative of the principles of the invention, and that various modifications may be made from the specific details described without departing from the spirit and scope of the invention.