METHOD OF ATTACHING INTEGRATED CIRCUIT DIES BY ROLLING ADHESIVES ONTO SEMICONDUCTOR WAFERS

Abstract: A method of attaching an integrated circuit die (182) to a support surface (188) is disclosed. An adhesive (164) is rolled onto a surface of a semiconductor wafer (160) using a roller to form an adhesive coating (164) on the semiconductor wafer surface. The adhesive coating (164) is dried, and the semiconductor wafer (160) is diced to form several integrated circuit dies (182) each having an integrated circuit and an adhesive layer (184) that constitutes a portion of the adhesive coating. One of the dies (182) is attached to a die attach pad (188) by placing the die's adhesive layer (184) in contact with the pad (188) which is then cured. A packaged integrated circuit (180) containing a die (182) with an adhesive layer (184) is also described.

Description: METHOD OF ATTACHING INTEGRATED CIRCUIT DIES BY ROLLING ADHESIVES ONTO SEMICONDUCTOR WAFERS BACKGROUND OF THE INVENΗON The present invention relates generally to attaching integrated circuit dies to support surfaces. More particularly, it relates to rolling adhesives onto semiconductor wafers to form adhesive layers on semiconductor dies. Semiconductor integrated circuits are currently mass produced for a broad range of purposes. Therefore, continual cost and quality improvements in manufacturing are valuable. Significant savings in the packaging of one integrated circuit can generate large overall cost savings under mass production. Generally, once an integrated circuit die has been fabricated, it is attached to a support structure such as a die attach pad (DAP), a heat spreader, a connection substrate, or the like during packaging. Gluing and taping are conventional techniques for attaching the die to a support surface. The most wide spread gluing techniques contemplate applying a glue or an adhesive directly to the support surface 10, or in some cases directly to the die just before it is placed in contact with the support surface. Figs, la-c are top views of a support surface 10 illustrating conventional techniques for applying glues or adhesives to the support surface 10. Fig. la illustrates a pattern of glue drops 12 that are placed directly on the support surface 10. Such a pattern 12 can be formed using an adhesive applicator with several apertures, similar to a "shower head," or a needle programmed to serially apply the glue drops in the pattern 12. It is noted that when the die is attached, the adhesive will tend to spread, but there is no guarantee that the adhesive will cover the entire interface between the die and the support surface 10 and thus, the strength of the resulting bond is not optimum. The shower head and needle apertures may also partially clog during an application or between successive applications further diminishing the application area at the interface and degrading bond strength. When the needle approach is used, the needles must be programmed to form the glue drop pattern 12, and the pattern formation itself is time-consuming. When the "shower head" approach is used, a distinct head must often be designed for a particular size chip. In custom chip applications, this can be relatively expensive. Fig lb illustrates the formation of a continuous pattern 14 on the support surface 10 using a needle programmed to deposit adhesive while tracing the continuous pattern 14. This method suffers the same drawbacks as described above with respect to Fig. la. Another adhesive based approach contemplates "stamping" a single (large) glue drop 16 onto the support surface 10 as illustrated in Fig. Ic. While the single glue drop 16 may cover most or all of the interface, it tends to interfere with connections to the attached die by jutting up around the die's edges. When taping is contemplated the tape is typically applied either to the back surface of a semiconductor wafer prior to dicing or to the die support surface prior to placing the die thereon. However, tape has the drawback of being relatively expensive. In view of the forgoing, an improved cost effective method for applying integrated circuit dies to support surfaces such that the adhesive is spread evenly and does not jut around the edges of the die would be advantageous. SUMMARY OF THE INVENΗON To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a method of attaching an integrated circuit die to a support surface is disclosed. An adhesive is rolled onto a surface of a semiconductor wafer using a roller to form an adhesive coating on the semiconductor wafer surface. The adhesive coating is dried, and the semiconductor wafer is diced to form several integrated circuit dies each having an integrated circuit and an adhesive layer that constitutes a portion of the adhesive coating. A selected die is then attached to an associated support surface by placing the adhesive layer of the selected die in contact with the associated support surface. The adhesive layer connecting the selected die and the associated support surface is then cured. In a preferred embodiment, the adhesive is a B-stageable adhesive. In one embodiment, the die support surface is preheated prior to the die attaching step. In another embodiment, the die support surface is flash heated substantially simultaneously with the attaching step. In yet another embodiment, the roller used to apply the adhesive to the wafer is sized such that when the adhesive coating is applied, substantially no portion of the rolling surface contacts the wafer twice. By way of example, in some embodiments, the adhesive may be a B-stageable epoxy. In others, it may be a thermo plastic. A packaged integrated circuit is also disclosed having a die support surface, a die, an adhesive layer, a plurality of leads, and a package. The die has a plurality of I/O pads formed on a first surface of the die and a mounting surface located opposite the first surface. The adhesive layer covers the entire mounting surface and attaches the die to the support surface. The adhesive layer material substantially does not extend beyond the mounting area boundary. The leads are electrically connected to associated I/O pads and the package encapsulates the integrated circuit die but leaves portions of the leads exposed to permit electrical connections between the integrated circuit and components external to the packaged integrated circuit. BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: Figs, la-c illustrate conventional methods of applying adhesive paste to support surfaces. Fig. 2 is a flowchart illustrating a method of attaching an integrated circuit die to a support surface in accordance with one embodiment of the present invention. Figs. 3 is a diagrammatic perspective view illustrating rolling a roller on an adhesive reservoir pad to deposit an adhesive onto the roller. Figs. 4a is a diagrammatic perspective view illustrating step 104 of Fig. 2 wherein an adhesive is rolled onto a surface of a semiconductor wafer using a roller to form an adhesive coating on the semiconductor wafer surface. Figs. 4b is a diagrammatic top view of the wafer shown in Fig. 4a. Figs. 4c is a diagrammatic cross sectional side view of the wafer shown in Fig. 4a. Fig. 5a is a cross sectional side view of the semiconductor wafer mounted on a film supported by a dicing frame resulting from step 108 of Fig. 2. Fig. 5b is a top view of the semiconductor wafer mounted on a dicing frame shown in Fig. 5a. Fig. 5c is a top view illustrating the result of a dicing step 1 10 of Fig. 2 forrning a multiplicity of integrated circuit dies. Fig. 6a is a diagrammatic cross sectional side view of a packaged integrated circuit with a die attached to a support surface by an adhesive layer in accordance with the present invention. Fig. 6b is a top view of a packaged integrated circuit made in accordance with the present invention and shows the die attached to a die attach pad and connected to a plurality of leads. DETAILED DESCRIPTION OF THE INVENΗON Referring initially to Fig. 2, a method 100 for attaching an integrated circuit die to a support surface in accordance with one embodiment of the present invention will be described. The process begins in step 102 with the fabrication of a semiconductor wafer having a multiplicity of integrated circuits formed thereon. After fabrication of the semiconductor wafer, an adhesive is then rolled onto a mounting surface of the wafer to form an adhesive coating thereon in a rolling step 104. Although a variety of adhesives can be used, B-stageable adhesives have been found to work well and the invention will be described referring to embodiments which use B-stageable adhesives. As used herein, a B-stageable adhesive is an adhesive that can be applied, dried without inducing full polymerization, reflowed (typically by heating the adhesive), and then cured to form more permanent bonds. A wide variety of B- stageable adhesive may be used including B-stageable epoxies such as Ablebond 961- 2™ available from Ablestik Labs and thermo plastics. During rolling step 104, the B- stageable adhesive will be in its initial fluid-based form of a paste, gel, or liquid. Many B-stageable adhesives form a relatively hard and relatively non-sticky surface during the drying step which make them particularly easy to handle. The fluid-based adhesive is initially deposited onto the roller by any suitable method. Transferring a technique from ink printing to the art of semiconductor packaging, the roller may be rolled on an adhesive reservoir pad to deposit the adhesive onto the roller prior to rolling the B-stageable adhesive onto the wafer mounting surface. The reservoir pad may be rotated relative to the roller to facilitate distribution of the adhesive over a reservoir pad surface. Such a process is diagrammatically illustrated in Figs. 3. As seen therein, a roller 150 is rolled across an adhesive reservoir pad 152 to coat the roller 150 with an adhesive. To form an even distribution of adhesive on the roller, the adhesive reservoir pad 152 may be rotated during the rolling step. In some cases, it may be desirable to rotate the pad 152 between occurrences of rolling the roller 150 on the pad 152. The arrow directions in Fig. 3 illustrate the roller 150 rolling to the right or to the left and the pad 152 rotating clockwise or counter-clockwise. The rolling and rotating directions may be varied to suit the particular embodiment of the invention. To facilitate saturation of the roller surface with adhesive, ends 154 of the roller 150 preferably do not extend beyond the edges of the reservoir pad 152. Once the adhesive has been coated on the roller 150, it is rolled onto the semiconductor wafer 160 in step 104. Figs. 4a-c diagrammatically illustrate the step of rolling the fluid-based adhesive onto a semiconductor wafer 160. In the embodiment shown, the roller ends 154 are arranged to extend beyond a wafer surface 162 during the application of the adhesive. This insures that an adhesive coating 164 is formed over the entire wafer surface 162. Further, the circumference of the roller is preferably at least as large the length L of the wafer and the roller surface area is at least as large as the area of the semiconductor wafer surface 162. With such an arrangement, the roller 150 can coat the semiconductor wafer surface 162 in a single pass without having any portion of the rolling surface contacting the wafer surface twice. This tends to improve the likelihood that a uniform adhesive coating will be formed of the wafer surface 162. Referring back to Fig. 2, once the adhesive has been applied to the wafer to form the adhesive coating in step 104, the adhesive coating is dried in a drying step 106. By way of example, when a B-stageable epoxy such as Ablebond 961-2, is used as the adhesive material, drying at a temperature of between about 80°C and 120°C has been found to work well. When the adhesive is good B-stageable epoxy, step 106 turns the adhesive coating into a hard surface, but the epoxy does not undergo a full polymerization. Generally, the drying step 106 is performed by placing the semiconductor wafer in an oven. The semiconductor wafer is then mounted onto a film supported by a conventional dicing frame in a mounting step 108. As illustrated in Figs. 5a and 5b, the dicing frame 170 is usually a roughly circular hoop 171 spanned by a film 172. In conventional dicing operations, one side of the film will be "sticky" or "tacky" both to attach the wafer to the film. The wafer is positioned such that the sticky film surface will engage the hardened adhesive coating 164 to attach the film to the semiconductor wafer. After the wafer has been placed on the dicing frame, the film 172 is smoothed to removes air bubbles which may be trapped between the sticky portion of the film 172 and the adhesive coating of the semiconductor wafer 164. NITTO™ tapes are common films used in the industry in conventional processes. After the wafer has been mounted, the wafer is cut into a multiplicity of dies in dicing step 110. Each die includes an integrated circuit and an adhesive layer that constitutes a portion of the dried adhesive coating. It is noted that in the described process, the adhesive is applied to the semiconductor wafer as opposed to an individual support surface or an individual die as is common in the conventional processes illustrated in Figs. la-c. The result of dicing the semiconductor wafer 160 in step 110 to form several integrated circuit dies 174 is shown in Fig. 5c. The dicing step 110 is generally performed with a wafer saw although other suitable instruments may be used. After the dicing has been completed, the dicing frame is conveyed to a die attach machine in step 112 to facilitate attaching individual dies to associated support surfaces. In the die attach machine, a plunger pushes on the film opposite the side with the sticky surface to raise a selected die somewhat above the other dies on the sticky film. This "plunge-up" technique is followed by a step 1 14 of lifting the selected die off the sticky portion of the film using a conventional technique such as a vacuum head. If the selected integrated circuit die is to be applied to a metallic support surface such as a die attach pad (DAP), it is often desirable to pre-heat the support surface to facilitate curing. Step 116. When the support surface is a die attach pad that constitutes part of a lead frame, the support surface may be heated by heating a lead frame containing the die attach pad in step 116. Alternatively, the die attach pad may be heated directly. By way of example, when the aforementioned B-stageable epoxy is applied in step 104, the pre-heating the die attach pad surface to a temperature between about 50°C and 60°C has been found to work well. After the pre-heating step 116, the die attach pad may further be flash heated in 117 just before or during the attachment of the selected die to the die attach pad in step B-stageable epoxy, flash heating the die attach pad surface to a temperature between about 100°C and 120 * C has been found to work well. It should be appreciated that either the pre-heating or the flash heating steps could be used individually rather than in sequence depending upon the needs of a particular adhesive. When the above referenced B-stageable epoxy is used, it is possible to pre-heat and maintain the DAP at temperatures between about 100°C and 120°C. However, this tends to waste energy and in some cases may cause unnecessary stresses on the die. Alternatively, one may choose to heat the DAP to about 100"C and 120°C only during the attaching step, but this approach is slower than the combination of pre¬ heating step 116 and flash heating step 117. In any case, by heating the die attach pad surface, the B-stageable adhesive dried in step 106 will reflow to form a stronger bond between the integrated circuit die and a die attach pad during the attachment step 118. It should be appreciated that since the adhesive layer is rolled on, it is exactly the same size as the mounting surface of the die and is quite even. Thus, when the die is attached to its supporting surface the adhesive does not extend beyond a mounting surface of the die. An optional step of pressing the IC die onto the DAP may occur substantially simultaneously with the attachment step 118 to form a stronger bond between the die and the DAP. Any of the DAP heating schemes just described may be used in conjunction with the pressing step. Subsequently, the adhesive layer attaching the integrated circuit die to the die attach pad is cured in a step 120. Depending upon adhesive used and the length of time permitted for curing, the curing temperature may vary. To cure the above referenced B-stageable epoxy in one to two hours, curing in a temperature range between about 150°C and 180°C has been found to work well. After the die has been securely attached, it may be electrically coupled to associated leads, traces, contacts or the like and then packaged as desired in step 122 using conventional processes. A packaged integrated circuit 180 having a die 182 attached to its support surface 188 (in this case a die attach pad of a lead frame) as described above is illustrated in Fig. 6a and 6b. The adhesive layer 184 covers an entire mounting surface 190 of the selected integrated circuit die 182 and does not extend beyond a boundary of the mounting surface 190. Several I O pads 198 may be formed on a connecting surface 192 opposite the mounting surface 190. The I/O pads are connected to associated leads, traces or contacts as required by the particular devices that are packaged. In the embodiments shown in Figs. 6a and 6b, the I/O pads are wire bonded to associated lead frame leads 194. The die and other components may then be packaged using any conventional technique including plastic packaging, using metal or ceramic packaging etc. Alternatively, the die may be attached directly to a circuit board or other substrate without being encapsulated. Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, any suitable adhesive may be rolled onto the semiconductor wafer surface. Fluid-based adhesives are understood to include pastes, gels, cements, putty substances, and liquids. Suitable B-stageable adhesives are not restricted to the described B-stageable epoxies and thermo plastics. Temperature ranges for curing the B-stageable adhesive may vary greatly in accordance with the adhesive used and the needs of the system. Similarly, the drying, heating, pre-heating, and flash heating temperature ranges may vary upon the adhesive used and the specific embodiment. A wide range of methods for depositing glues and adhesives onto the roller, such as pouring, may be used instead of the adhesive reservoir pad. If desired, the roller ends may extend beyond the periphery of the adhesive reservoir pad during deposition of adhesive onto the roller. Likewise, the roller ends may remain inside the periphery of the semiconductor wafer during the rolling step for applying the adhesive coating if desired. The roller surfaces and circumferences may be smaller than the areas and lengths of the semiconductor wafer surface. Only portions of the semiconductor wafer surface and the selected integrated circuit die may be coated with adhesive instead of the entire wafer surface and the entire mounting surface of the die. For example, this might be the case when the roller ends extend beyond the periphery of the adhesive reservoir pad or when the roller ends do not reach the edge of the wafer surface The semiconductor wafer may be diced into a multiple dies without using the dicing frame. Lasers and other cutting instruments may dice the wafer instead of the wafer saw. In some embodiments, the drying, heating, pre-heating, and flash heating steps may be optional. Other support surfaces such as jumper chips and connection substrates may be substituted for the die attach pad. In addition, more than one integrated circuit die may be packaged in a single encapsulating package to form multicbip packages (MCPs) or multichip modules (MCMs). Also, a selected die may be attached to an associated support surface without drying the fluid-based adhesive applied to the semiconductor wafer surface. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims: IN THE CLAIMS 1 . A method of attaching an integrated circuit die to a support surface, the method comprising the steps of: rolling an adhesive onto a surface of a semiconductor wafer using a roller to form an adhesive coating on the semiconductor wafer surface; drying the adhesive coating on the semiconductor wafer surface; dicing the semiconductor wafer to define a multiplicity of integrated circuit dies each having an integrated circuit thereon and an adhesive layer that constitutes a portion of the adhesive coating; and attaching a selected die to an associated support surface by placing the adhesive layer of the selected die in contact with the associated support surface; and curing the adhesive layer, after the attaching step. 2. A method of attaching an integrated circuit die according to claim 1 wherein the adhesive coating covers the semiconductor wafer surface in its entirety and wherein the roller has a rolling surface having a circumference at least as large as a length of the semiconductor wafer surface such that when the adhesive coating is applied, substantially no portion of the rolling surface contacts the wafer twice. 3. A method of attaching an integrated circuit die according to claim 1 further comprising the step of rolling the roller on an adhesive reservoir pad to deposit the adhesive onto the roller. 4. A method of attaching an integrated circuit die according to claim 3 further comprising the step of rotating the adhesive reservoir pad while the roller is rolled across the adhesive reservoir pad. 5. A method of attaching an integrated circuit die according to claim 1 wherein the associated support surface is a die attach pad surface, the method further comprising the step of preheating the die attach pad surface prior to the attaching step. 6. A method of attaching an integrated circuit die according to claim 5 further comprising the step of flash heating the die attach pad surface substantially simultaneously with the attaching step. 7. A method of attaching an integrated circuit die according to claim 1 wherein the drying step turns the adhesive coating into a hard surface, the method further comprising the step of mounting the semiconductor wafer onto a film supported by a dicing frame, the film having sticky surface and the wafer being mounted such that the sticky surface engages the hardened adhesive coating. 8. A method of attaching an integrated circuit die according to claim 7 further comprising the step of lifting the selected die off of the sticky portion of the film after the dicing step. 9. A method of packaging an integrated circuit as recited in claim 7 further comprising the step of smoothing the film after the mounting step to remove air bubbles which may be trapped between the sticky portion of the film and the adhesive coating of the semiconductor wafer. 10. A method of attaching an integrated circuit die according to claim 1 wherein the adhesive is a thermo plastic. 1 1. A method of attaching an integrated circuit die according to claim 1 wherein the adhesive is a B-stageable epoxy. 12. A method of attaching an integrated circuit die according to claim 1 1 wherein the drying step is performed at a temperature between about 80 degrees centigrade and 120 degrees centigrade and the support surface is a die attach pad surface, the method further comprising the step of: preheating the die attach pad surface at a temperature between about 50 degrees centigrade and 60 degrees centigrade prior to the attaching step . 13. A method of attaching an integrated circuit die according to claim 11 further comprising the step of flash heating the die attach pad surface at a temperature between about 150 degrees centigrade and 180 degrees centigrade substantially simultaneously with the attaching step. 14. A method of attaching an integrated circuit die according to claim 11 wherein the curing occurs in a temperature range between about 150 degrees centigrade and 180 degrees centigrade. 15. A packaged integrated circuit having an integrated circuit die attached to a support surface in accordance with the method of claim 1. 16. A method of attaching an integrated circuit die to a support surface, the method comprising the steps of: rolling a fluid-based adhesive onto a surface of a semiconductor wafer by using a roller thereby forming an adhesive coating on the semiconductor wafer surface; dicing the semiconductor wafer to define a multiplicity of integrated circuit dies each having an integrated circuit thereon and an adhesive layer that constitutes a portion of the adhesive coating; and attaching a selected die to an associated support surface by placing the adhesive layer of the selected die in contact with the associated support surface. 17. A method of attaching an integrated circuit die as recited in claim 16 wherein the adhesive coating covers the semiconductor wafer surface in its entirety and wherein the roller has a roller surface having an area at least as large as an area of the semiconductor wafer surface such that when the adhesive coating is applied, substantially no portion of the rolling surface contacts the wafer twice. 18. A method of attaching an integrated circuit die as recited in claim 16 wherein the attaching step includes a substep of pressing the selected integrated circuit die onto the support surface. 19. A method of attaching an integrated circuit die as recited in claim 16 further comprising the step of drying the adhesive coating of the semiconductor wafer surface to permit easier handling of the semiconductor wafer whereby the adhesive layer on the selected integrated circuit die becomes a dry adhesive layer. 20. A method of attaching an integrated circuit die as recited in claim 19 wherein the support surface is a die attach pad surface, the method further comprising the step of heating the die attach pad surface to heat the dry adhesive layer of the integrated circuit die to permit greater adhesion between the integrated circuit die and the die attach pad surface. 21. A method of attaching an integrated circuit die as recited in claim 20 further comprising the step of curing the adhesive layer of the selected integrated circuit die after the attaching and heating steps. 22. A packaged integrated circuit comprising: a die support surface; an die having an integrated circuit thereon, a plurality of I/O pads electrically connected to the integrated circuit, the I/O pads being formed on a first surface of the die, and a mounting surface located opposite the first surface, the mounting surface having a mounting area boundary; an adhesive layer that covers the entire mounting surface for attaching the die to the support surface, wherein the adhesive layer is formed from an adhesive material and the adhesive material substantially does not extend beyond the mounting area boundary; a plurality of leads electrically connected to the plurality of I/O pads of the integrated circuit die; and a package encapsulating the integrated circuit die while leaving portions of the plurahty of leads exposed to permit electrical connections between the integrated circuit and components external to the packaged integrated circuit. 23. A packaged integrated die as recited in claim 22 wherein the support surface is a die attach pad surface. 24. A packaged integrated die as recited in claim 22 wherein the adhesive layer is a B-stageable adhesive.

Inventors: MEKDHANASARN BOONMI, MOSTAFAZADEH SHAHRAM, TAKIAR HEM P, MEKDHANASARN, BOONMI, MOSTAFAZADEH, SHAHRAM, TAKIAR, HEM, P.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1995
Main classification: H01L23
IPC codes: H01L21, H01L23