# Patent

## Title
Microfeature systems including adhered microfeature workpieces and support members

## Abstract

Methods and systems for adhering microfeature workpieces to support members are disclosed. A method in accordance with one embodiment of the invention includes disposing a first adhesive on a surface of a microfeature workpiece, and disposing a second adhesive on a surface of a support member. The method can further include adhesively attaching the microfeature workpiece to the support member by contacting the first adhesive with the second adhesive while the second adhesive is only partially cured. In further particular embodiments, the first and second adhesives can have different compositions, and the second adhesive can be fully cured after the microfeature workpiece and support member are adhesively attached.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. patent application Ser. No. 11/053,984, filed Feb. 8, 2005, which is incorporated herein by reference in its entirety.


TECHNICAL FIELD
The present invention relates generally to methods and systems for adhering microfeature workpieces to support members.
BACKGROUND
Packaged microelectronics assemblies, such as memory chips and microprocessor chips, typically include a microelectronic die mounted to a substrate and encased in a plastic protective covering. The die includes functional features, such as memory cells, processor circuits and interconnecting circuitry. The die also typically includes bond pads electrically coupled to the functional features. The bond pads are electrically connected to pins or other types of terminals that extend outside the protective covering for connecting the die to buses, circuits, and/or other microelectronic assemblies.
In one conventional arrangement, the die is mounted to a supporting substrate (e.g., a printed circuit board), and the die bond pads are electrically coupled to corresponding bond pads of the substrate with wire bonds. After encapsulation, the substrate can be electrically connected to external devices. Accordingly, the substrate supports the die and provides an electrical link between the die and the external devices.
Prior to encapsulation, the microelectronic die is typically attached to the substrate with an adhesive. For example, one conventional assembly includes a wafer backside laminate (WBL) that adheres the microelectronic die to the substrate. Other assemblies include an epoxy that adheres the microelectronic die to the substrate. The foregoing adhesion techniques have both advantages and drawbacks. For example, a WBL generally increases the strength of the die, but does not fill in surface features on the substrate. Accordingly, the bond formed between the die and the substrate may fail due to an inadequate adhesive bond. Epoxies tend to flow more than WBLs do and can accordingly fill such features, but epoxies can fail in shear at the junction between the epoxy and the die, and/or at the junction between the epoxy and the substrate. Accordingly, it is desirable to adhere dies to substrates in a more robust manner.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a microfeature workpiece having a first adhesive in accordance with an embodiment of the invention.
FIGS. 2A-2F illustrate a process for manufacturing a microfeature workpiece assembly using first and second adhesives in accordance with an embodiment of the invention.
FIG. 3 is a partially schematic illustration of a microfeature workpiece attached to a support member in accordance with another embodiment of the invention.
FIG. 4 is a partially schematic illustration of a support member carrying two microfeature workpieces in accordance with another embodiment of the invention.
FIG. 5 is a partially schematic illustration of a support member carrying two microfeature workpieces in accordance with still another embodiment of the invention.



DETAILED DESCRIPTION
The present invention relates generally to microfeature dies and methods for adhering such dies to support members, for example, printed circuit boards. A method for attaching a microfeature workpiece to a support member in accordance with one aspect of the invention includes disposing a first adhesive on a surface of the microfeature workpiece and disposing a second adhesive on a surface of the support member. The method can further include adhesively attaching the microfeature workpiece to the support member by contacting the first adhesive with the second adhesive while the second adhesive is only partially cured. For example, the method can include attaching a film adhesive to the microfeature workpiece, and stenciling a liquid epoxy on the support member. The method can further include partially curing the epoxy (for example, to B-stage) before attaching the microfeature workpiece to the support member by contacting the two adhesives with each other. The method can include further curing the second adhesive to complete the bond between the microfeature workpiece and the support member.
In particular embodiments, the microfeature workpiece can have a first major surface, a second major surface facing opposite from the first major surface, and an edge surface between the first and second major surfaces. The method can further comprise electrically coupling the support member to the microfeature workpiece at the second major surface of the workpiece. In still a further aspect of the invention, the microfeature workpiece can be attached to the support member without either the first adhesive or the second adhesive contacting the edge surface of the microfeature workpiece.
A method in accordance with another aspect of the invention includes stiffening a microfeature workpiece by disposing a first adhesive layer on a surface of the microfeature workpiece, with the first adhesive layer having a first composition. The method can further include filling voids in a surface of a support member by disposing a second adhesive layer on a surface of the support member, with the second adhesive layer having a second composition different than the first composition. The method can still further include adhesively attaching the microfeature workpiece to the support member by contacting the first adhesive layer with the second adhesive layer.
Other aspects of the invention are directed toward microfeature workpiece systems or assemblies. A system in accordance with one aspect of the invention includes a microfeature workpiece having a first major surface, a second major surface facing away from the first major surface, and an edge surface between the first and second major surfaces. The system can further include a support member positioned adjacent to the microfeature workpiece, and an adhesive assembly attached between the microfeature workpiece and the support member. The adhesive assembly can include a first layer having a first composition and a first generally uniform thickness, and a second layer having a second composition different than the first with a second generally uniform thickness. In further specific aspects, the adhesive assembly can be generally out of contact with the edge surface of the microfeature workpiece, and the microfeature workpiece can be electrically coupled to the support member with an electrical coupling connected between the support member and the second major surface of the microfeature workpiece.
FIG. 1 is a partially schematic, cross-sectional side view of a microfeature workpiece 110 that can be attached to a support member to form an assembly (e.g., a package) in accordance with an embodiment of the invention. The microfeature workpiece 110 can have a first major surface 111, a second major surface 112 facing generally opposite from the first major surface 111, and an edge surface 113 between the first and second major surfaces 111, 112. The microfeature workpiece 110 can include a microelectronic die that has been separated from a larger substrate, e.g., a semiconductor wafer. The microfeature workpiece 110 can accordingly include internal microelectronic features and structures (not visible in FIG. 1) that are electrically coupled to workpiece bond pads 114. The bond pads 114 can provide for electrical communication between features and structures within the microfeature workpiece 110, and components and structures external to the microfeature workpiece 110.
The microfeature workpiece 110 can include a first adhesive 115 adhesively attached to the second surface 112, and configured to bond the microfeature workpiece 110 to a support member. In one embodiment, the first adhesive 115 can include a wafer backside laminate (WBL) that is applied to the microfeature workpiece 110 at the wafer level (e.g., before the microfeature workpiece 110 is separated or diced from a larger wafer or other substrate). For example, a sheet of the first adhesive 115 can initially be carried by a dicing frame and attached to a wafer. The wafer attachment process can occur at an elevated temperature (e.g., 40° C.-80° C.) and/or pressure (e.g., 20-50 psi). After dicing the wafer, a small portion of the first adhesive 115 remains attached to each microfeature workpiece 110, and a standard pick and place process can be used to move the microfeature workpiece 110. The first adhesive 115 can include a film substrate having adhesive material attached to both opposing surfaces of the film (e.g., a double-backed adhesive film). In another embodiment, the first adhesive 115 can be a monolayer, e.g., a single layer of adhesive applied to the second surface 112 of the microfeature workpiece 110. Suitable first adhesives are available from Ablestick Laboratories of Rancho Dominguez, Calif. under part number RP787-3DS, and from the Hitachi Chemical Company Ltd. of Tokyo, Japan under part number FH-800.
The first adhesive 115 can act not only to adhere the microfeature workpiece 110 to a support member, but also to stiffen the microfeature workpiece 110. As a result, the microfeature workpiece 110 can be less likely to break during normal handling operations. This can be particularly important for very thin microfeature workpieces 110, which may be more susceptible to such breakage. Such microfeature workpieces 110 can have a thickness of 150 microns or less, e.g., 100 microns or less.
FIGS. 2A-2F illustrate a support member 220 and a method for attaching the support member 220 to the microfeature workpiece 110 described above with reference to FIG. 1. Beginning with FIG. 2A, the support member 220 can include a printed circuit board or other suitable structure having provisions for electrical connections to the microfeature workpiece 110 (FIG. 1). For example, the support member 220 can include first bond pads 221 at a first surface 227, second bond pads 222 at an opposite, second surface 228, and internal conductive structures (not visible in FIG. 2A) coupling the first bond pads 221 to the second bond pads 222. In preparation for applying an adhesive to the support member 220, a stencil 223 can be positioned adjacent to the first surface 227. The stencil 223 can include one or more openings 224 sized in accordance with the desired attachment surface area between the support member 220 and the microfeature workpiece 110.
Referring next to FIG. 2B, a second adhesive 225 can be applied to the support member 220 in a stenciling process. Accordingly, the second adhesive 225 can be distributed over the stencil 223 using techniques well-known to those of ordinary skill in the relevant art to fill the opening 224. In one aspect of this embodiment, the second adhesive 225 can include an epoxy that is applied to the support member 220 when in a liquid or otherwise generally flowable state. Accordingly, the second adhesive 225 can readily fill the opening 224 in the stencil 223. The second adhesive 225 can also readily fill gaps, recesses, projections, roughness elements, and/or other surface features 229 in the first surface 227 of the support member 220. As a result, the second adhesive 225 can be in intimate contact with the surface features 229 to provide a robust bond with the support member 220.
In FIG. 2C, the stencil 223 is removed and the second adhesive 225 can be partially cured. The process of partially curing the second adhesive 225 can include applying heat 226 and, optionally, pressure, to the second adhesive 225. For example, the temperature of the second adhesive can be elevated to about 100° C. for about one hour, at atmospheric pressure, in an inert (e.g., nitrogen) environment. When the second adhesive 225 includes an epoxy, the partial curing process shown in FIG. 2C can include forming a B-stage epoxy. As used herein, the term “B-stage” includes, but is not necessarily limited to, an intermediate stage in the reaction of a thermosetting resin in which the material softens when heated and swells when in contact with certain liquids, but does not entirely fuse or dissolve. In this stage, the second adhesive 225 is generally non-tacky (e.g., tack-free). Suitable epoxies include Sumitomo X2225 available from Sumitomo Electric Industries, Ltd. of Osaka, Japan, and Ablestik RP809-1A available from Ablestik Laboratories of Rancho Dominguez, Calif.
Referring next to FIG. 2D, the microfeature workpiece 110 can be positioned proximate to the support member 220, with the first adhesive 115 of the microfeature workpiece 110 facing toward the second adhesive 225 of the support member 220. The microfeature workpiece 110 and the support member 220 can be moved toward each other so that the first adhesive 115 contacts the second adhesive 225. The local temperature can be elevated (e.g., up to 100° C.-150° C.) for 1-3 seconds to “re-tack” the first and second adhesives 115, 125. The local pressure can also be elevated (e.g., by from about 1 g/mm2 to about 8 g/mm2) in a standard air environment. FIG. 2E illustrates the microfeature workpiece 110 with its first adhesive 115 contacting the second adhesive 225 of the support member 220. At this point, the second adhesive 225 can be fully cured by applying heat 226 (and optionally, pressure) to the second adhesive 225 to form an adhesive assembly 230 that includes both the first adhesive 115 and the second adhesive 225. In one embodiment, the adhesive assembly can be cured at 100° C.-150° C. for 1-2 hours in an inert environment. The resulting combination of the microfeature workpiece 110, the support member 220, and the adhesive assembly 230 forms an overall assembly or system 200 (e.g., a microfeature package).
Referring next to FIG. 2F, the assembly 200 can include electrical couplings between the microfeature workpiece 110 and the support member 220. For example, the assembly 200 can include wirebonds 201 extending between the workpiece bond pads 114 and the first bond pads 221 of the support member 220. The assembly 200 can optionally include an encapsulant 202 that at least partially surrounds the microfeature workpiece 110 and the wirebonds 201 or other electrical couplings between the microfeature workpiece 110 and the support member 220. The completed assembly 200 can be electrically coupled to other components via the second bond pads 222 located at the second surface 228 of the support member 220.
One feature of embodiments of the assembly 200 described above with reference to FIGS. 1-2F is that they can include multiple adhesives. A further particular feature of at least some of these embodiments is that an adhesive having a first composition can be positioned adjacent to the microfeature workpiece 110, and an adhesive having a second, different composition can be positioned adjacent to the support member 220. This combination of adhesives can provide one or more of several advantages. For example, the first adhesive 115 can stiffen and strengthen the microfeature workpiece 110 and can accordingly reduce or eliminate the tendency for the microfeature workpiece 110 to warp, chip, and/or deform in other manners. Preventing the microfeature workpiece 110 from warping can be particularly important for workpieces 110 having high aspect ratios, e.g., about 4:1, 5:1 or up to 10:1. The second adhesive 225, on the other hand, can more readily fill in the surface features 229 of the support member 220. Accordingly, the second adhesive can be less likely to leave air voids or other imperfections that may reduce the bond strength between the second adhesive 225 and the support member 220.
Another feature of at least some of the foregoing embodiments is that each of the adhesive layers can have a generally uniform thickness. For example, the first adhesive 115 can have the form of a generally uniform film. The second adhesive 225 can be applied with a generally uniform thickness by using the stencil 223. Because each adhesive layer has a generally uniform thickness, the microfeature workpiece 110 will be less likely to be tilted relative to the support member 220. An advantage of this arrangement is that it can reduce the likelihood for the microfeature workpiece 110 (and/or the wirebonds 201) to be exposed through the encapsulant 202 as a result of the microfeature workpiece 110 being tilted or otherwise misaligned relative to the support member 220.
Another feature associated with embodiments of the generally uniform layer thicknesses described above is that the adhesive assembly 230 can be attached directly between the support member 240 and the second surface 112 of the microfeature workpiece 110, without extending along the edge surfaces 113 of the microfeature workpiece 110. An advantage of this “fillet-less” arrangement is that the adhesive assembly 230 may be less likely to subject the edge surfaces 113 of the microfeature workpiece 110 to stresses that can crack or otherwise damage the microfeature workpiece 110.
Still another advantage of at least some of the foregoing embodiments is that the adhesive assembly 230, formed from adhesives having different chemical constituencies, can form a tight bond with both the microfeature workpiece 110 and the support member 220. When subjected to excessive stresses, the failure mode of the adhesive assembly 230 can be cohesive rather than adhesive. For example, the adhesive assembly 230 can fail internally rather than at the interfaces between the adhesive assembly 230 and either the microfeature workpiece 110 or the support member 220. Because higher stresses are typically required for failure in the cohesive mode than in the adhesive mode, the overall strength of the bond between the support member 220 and the microfeature workpiece 110 can be greater than it is for existing adhesive arrangements.
Yet another feature of the foregoing embodiments is that the adhesive assembly 230 can produce a bond that more readily withstands typical moisture sensitivity testing environments. Such environments typically include transitions in temperature up to about 260° C., and an elevated humidity. Improved moisture sensitivity can also result in lower package stresses in challenging environmental conditions. As a result of any of the foregoing features, the package 200 can be more robust than existing packages, and can accordingly be more reliable in a wider variety of applications.
Methods and adhesive assemblies at least generally similar to those described above with reference to FIGS. 1-2F can be used to bond microfeature workpieces to support members in other configurations as well. Referring now to FIG. 3, a microfeature workpiece assembly 300 can include a microfeature workpiece 310 having centrally located workpiece bond pads 314 in accordance with another embodiment of the invention. A corresponding support member 320 can include an aperture 331 aligned with the workpiece bond pads 314. The first adhesive 115 can be positioned adjacent to the microfeature workpiece 310 (outwardly from the workpiece bond pads 314), and the second adhesive 225 can be positioned adjacent to the support member 320 (outwardly from the aperture 331). After the second adhesive 225 has been partially cured, the microfeature workpiece 310 can be bonded to the support member 320 using a second curing process. The workpiece bond pads 314 can then be electrically coupled (e.g., with wirebonds 301) to first bond pads 321 of the support member 320. The resulting connection can be at least partially surrounded with an encapsulant 302. The first bond pads 321 can be electrically coupled to second bond pads 322, which can in turn provide communication with other devices, for example, via solder balls 303.
FIG. 4 illustrates a microfeature workpiece package or assembly 400 that includes two microfeature workpieces 410, shown as a first microfeature workpiece 410a and a second microfeature workpiece 410b. The first microfeature workpiece 410a can be adhesively secured to a support member 420 with a first adhesive assembly 430a. The first adhesive assembly 430a can include a first adhesive 115 and a second adhesive 225, configured in a manner generally similar to that described above. The second microfeature workpiece 410b can be adhered directly to the first microfeature workpiece 410a with a second adhesive assembly 430b that also includes a portion of the first adhesive 115 and the second adhesive 225. First wirebonds 401a can electrically couple the first microfeature workpiece 410a to the support member 420, and second wirebonds 401b can electrically couple the second microfeature workpiece 410b to the support member 420. The entire package 400 can be surrounded by an encapsulant 402.
In one aspect of an embodiment shown in FIG. 4, the first microfeature workpiece 410a can have a configuration generally similar to that of the second microfeature workpiece 410b, and the second microfeature workpiece 410b can overhang the first microfeature workpiece 410a to allow for electrical coupling to both microfeature workpieces. In other embodiments, multiple microfeature workpieces can be stacked in other configurations. For example, referring now to FIG. 5, a package 500 can include a first microfeature workpiece 510a and a second, smaller microfeature workpiece 510b, stacked upon the first microfeature workpiece 510a. This arrangement allows bond pads 514 on the outer edges of each microfeature workpiece 510 to remain exposed despite the adhesive connection between the two microfeature workpieces. The first microfeature workpiece 510a can be secured to a support member 520 with a first adhesive assembly 530a, and the second microfeature workpiece 510b can be secured to the first microfeature workpiece 510a with a second adhesive assembly 530b. Each adhesive assembly 530a, 530b can include the first adhesive 115 and the second adhesive 225. In other embodiments, arrangements generally similar to those described above can be used to attach one or more microfeature workpieces to corresponding support members and/or other microfeature workpieces.
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. For example, the microfeature workpiece can be electrically coupled to a corresponding support member with an electrical link other than a wirebond. Aspects of the invention described in the context of particular embodiments may be combined or eliminated in other embodiments. Although advantages associated with certain embodiments of the invention have been described in the context of those embodiments, other embodiments may also exhibit such advantages. Additionally, none of the foregoing embodiments need necessarily exhibit such advantages to fall within the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

## Claims

1. A microfeature workpiece system, comprising:
a microfeature workpiece that includes a first microelectronic die having a first major surface, a second major surface facing away from the first major surface, and an edge surface between the first and second major surfaces;
a support member positioned adjacent to the microfeature workpiece;
a first adhesive assembly attached between the microfeature workpiece and the support member, the first adhesive assembly including
a first layer forming a first adhesive bond with the microfeature workpiece at a first interface between the first adhesive assembly and the microfeature workpiece, the first layer having a first composition and a first generally uniform thickness,
a second layer forming a second adhesive bond with the support member at a second interface between the first adhesive assembly and the support member, the second layer having a second composition different than the first composition and having a second generally uniform thickness,
wherein the first adhesive bond and the second adhesive bond are stronger than an adhesive bond between the first layer and the second layer of the first adhesive assembly such that the first adhesive assembly fails at a location between the first interface and the second interface when the first adhesive assembly is subjected to a stress; and

a second microelectronic die attached to the first microelectronic die with a second adhesive assembly having a composition generally the same as a composition of the first adhesive assembly.

2. The system of claim 1 wherein the first adhesive assembly is at least generally out of contact with the edge surface of the microfeature workpiece.

3. The system of claim 1, further comprising an electrical link connecting the support member to the microfeature workpiece at the second major surface of the microfeature workpiece.

4. The system of claim 1 wherein the microfeature workpiece has a thickness of 150 microns or less..

5. The system of claim 1 wherein the microfeature workpiece has a thickness of 100 microns or less.

6. The system of claim 1 wherein the microfeature workpiece has an aspect ratio of between about one and about ten.

7. A microfeature workpiece system, comprising:
a microfeature workpiece having a first major surface, a second major surface facing away from the first major surface, and an edge surface between the first and second major surfaces;
a support member positioned adjacent to the microfeature workpiece;
an adhesive assembly disposed between the microfeature workpiece and the support member to couple the microfeature workpiece and the support member, the adhesive assembly including
a first layer forming a first adhesive bond with the microfeature workpiece at a first interface between the adhesive assembly and the microfeature workpiece, the first layer having a first composition and a first generally uniform thickness,
a second layer forming a second adhesive bond with the support member at a second interface between the adhesive assembly and the support member, the second layer having a second composition different than the first composition and having a second generally uniform thickness,
wherein the adhesive assembly is at least generally out of contact with the edge surface of the microfeature workpiece, and wherein the first adhesive bond and the second adhesive bond are stronger than an adhesive bond between the first layer and the second layer of the adhesive assembly such that the adhesive assembly fails at a location between the first interface and the second interface when the assembly is subjected to a stress; and

an electrical coupling connected between the support member and the second major surface of the microfeature workpiece.

8. The system of claim 7 wherein the second layer includes an epoxy.

9. A microfeature workpiece assembly, comprising:
a microfeature workpiece having a first major surface, a second major surface facing away from the first major surface, and an edge surface between the first and second major surfaces;
a support member positioned adjacent to the microfeature workpiece; and
an adhesive adhering the microfeature workpiece to the support member, the adhesive having
a first layer forming a first adhesive bond with the microfeature workpiece at a first interface between the adhesive and the microfeature workpiece, the first layer having a first composition,
a second layer forming a second adhesive bond with the support member at a second interface between the adhesive and the support member, the second layer having a second composition different than the first composition,

wherein the first adhesive bond and the second adhesive bond are stronger than an adhesive bond between the first layer and the second layer of the adhesive such that the adhesive has a failure mode that is within the adhesive, between the first interface and the second interface.

10. The assembly of claim 9 wherein the adhesive includes an adhesive assembly having a first layer having a first composition and a second layer adjacent to the first layer and having a second composition different than the first composition.

11. A microfeature workpiece system, comprising:
a microelectronic die having a first major surface, a second major surface facing away from the first major surface, and an edge surface between the first and second major surfaces;
a support member positioned adjacent to the microfeature die; and
a first adhesive assembly attached between the microfeature die and the support member, the first adhesive assembly including
a first layer forming a first adhesive bond with the microfeature die at a first interface between the first adhesive assembly and the microfeature die, the first layer having a first composition,
a second layer in direct contact with the first layer and forming a second adhesive bond with the support member at a second interface between the first adhesive assembly and the support member, the second layer having a second composition different than the first composition,
wherein the first adhesive bond and the second adhesive bond are stronger than an adhesive bond between the first layer and the second layer of the first adhesive assembly.

12. The system of claim 11 wherein the first adhesive assembly is at least generally out of contact with the edge surface of the microfeature die.

13. The system of claim 11, further comprising an electrical link connecting the support member to the microfeature die at the second major surface of the microfeature workpiece.