# Patent

## Title
Semiconductor die mount by conformal die coating

## Abstract

A conformal coating on a semiconductor die provides adhesion between the die and a support. No additional adhesive is necessary to affix the die on the support. The conformal coating protects the die during assembly, and serves to electrically insulate the die from electrically conductive parts that the die may contact. The conformal coating may be an organic polymer, such as a parylene, for example. Also, a method for adhering a die onto a support, which may optionally be another die, includes providing a coating of a conformal between the die and the support, and heating the coating between the die and the support. The conformal coating may be provided on a die attach area of a surface of the die, or on a die mount region of a surface of the support, or on both a die attach area of a surface of the die and on a die mount region of a surface of the support; and the conformal coating may be provided following placement of the die on the support.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a divisional of U.S. patent application Ser. No. 12/199,080, filed Aug. 27, 2008, which claims the benefit of the filing date of U.S. Provisional Application No. 60/971,203, titled “Semiconductor die mount by conformal die coating”, filed on Sep. 10, 2007, each of said applications being incorporated by reference herein.
This application is related to S. J. S McElrea of al. U.S. application Ser. No. 12/124,077, titled “Electrically interconnected stacked die assemblies”, which was filed on May 20, 2008, and which is incorporated by reference herein.


BACKGROUND
This invention relates to mounting semiconductor die on a support.
A semiconductor integrated circuit die may be mounted directly onto a printed circuit board; or on a leadframe or a substrate; or on another semiconductor die in a stacked die assembly.
A semiconductor integrated circuit die is conventionally mounted on the support using an adhesive. Die mount adhesives may take the form of a die attach epoxy, as illustrated for example in FIGS. 1 and 2; or an adhesive film, as illustrated for example in FIGS. 3 and 4.
FIG. 1 shows an assembly including a die 22 mounted on a substrate 10 using a die attach epoxy. The die 22 has a front side (“active” side) 23 adjacent which the integrated circuitry is formed, and a back side 21, and sidewalls meeting the front and back sides at front and back edges. Die pads at the front side of the die, arranged in this example as peripheral pads 24, provide for electrical connection of the integrated circuitry of the die with underlying circuitry.
The substrate 10 in this example includes a patterned metal layer 12 on a dielectric layer. The patterned metal layer 12 is covered by an electrically insulative layer 13, which has openings exposing bond fingers 11. The substrate may include one or more additional patterned metal layers, variously connected at vias through intervening dielectric layers, including a patterned metal layer at the surface opposite the die mount surface, at which lands are exposed to provide for connection to circuitry in a device in which the assembly is deployed.
The die 22 is in this example affixed to a die mount region of the substrate by a die attach epoxy 32. The die mount procedure includes dispensing a specified quantity of epoxy onto the insulative layer 13, placing the die onto the epoxy, and curing the epoxy. Typically a “fillet” 33 is formed outside the die footprint at the periphery of the die mount region. The epoxy die attach process is carefully controlled to provide for a uniform die attach epoxy thickness and a limited spread of epoxy onto the substrate outside the die footprint. Following die attach, the die is electrically connected to the circuitry on the substrate by wire bonds 14 connecting pads 24 with bond fingers 11. A molding or encapsulation (not shown in the FIG.) may be formed over the assembly to protect the active side of the die and the interconnects.
FIG. 2 shows an assembly including a first die 22 mounted on a substrate 10 generally as described with reference to FIG. 1, and including an additional die 26 mounted on the first die 22 using a die attach epoxy. The additional die 26 has a front side (“active” side) 27 adjacent which the integrated circuitry is formed, and a back side 25, and sidewalls meeting the front and back sides at front and back edges. Die pads at the front side of the die, arranged in this example as peripheral pads 28, provide for electrical connection of the integrated circuitry of the die with underlying circuitry.
The second die 26 is in this example affixed to a die mount region on the front side 23 of the first die 22 by a die attach epoxy 34. The second die mount procedure, as with the first die mount procedure, includes dispensing a specified quantity of epoxy onto a die attach region of the front side of die 22, placing the second die onto the epoxy, and curing the epoxy. Typically a “fillet” 35 is formed outside the die footprint at the periphery of the die mount region. As in the first die attach process, this second epoxy die attach process is carefully controlled to provide for a uniform die attach epoxy thickness and a limited spread of epoxy onto the first die surface 23 outside the die footprint. Following die attach, the die is electrically connected to the circuitry on the substrate by wire bonds 16 connecting pads 28 with bond fingers 11. A molding or encapsulation (not shown in the FIG.) may be formed over the assembly to protect the active side of the die and the interconnects.
FIG. 3 shows an assembly including a die 22 mounted on a substrate 10 using a film adhesive 42. The die and the substrate, and their electrical connections to the substrate, are as in the example shown in FIG. 1.
The die 22 is in this example affixed to a die mount region of the substrate by a film adhesive 42. The die mount procedure includes applying the film to either the backside of the die 22 or to the die mount region of the substrate 10, thereafter placing the die onto the insulative layer 13 on the substrate, and thereafter (where the adhesive is curable) curing the adhesive. Where the adhesive film is applied to the substrate, the film must first be cut to size, and then positioned on the substrate surface; where the film is applied to the die, this step may be performed at the wafer level, so that the film is cut to size during die singulation. Following die attach, the die is electrically connected to the circuitry on the substrate by wire bonds 14 connecting pads 24 with bond fingers 11. A molding or encapsulation (not shown in the FIG.) may be formed over the assembly to protect the active side of the die and the interconnects.
FIG. 4 shows an assembly including a first die 22 mounted on a substrate 10 generally as described with reference to FIG. 3, and including an additional die 26 mounted on the first die 22. The die and the substrate, and their electrical connections to the substrate, are as in the example shown in FIG. 3. The second die mount procedure, as with the first die mount procedure, includes applying the film to either the backside of the die 26 or to the die mount region of the front side of the first die 22, thereafter placing the second die onto the first die, and thereafter (where the adhesive is curable) curing the adhesive. Where the adhesive film is applied to the substrate, the film must first be cut to size, and then positioned on the surface of the first die; where the film is applied to the second die, this step may be performed at the wafer level, so that the film is cut to size during second die singulation. Following die attach, the second die is electrically connected to the circuitry on the substrate by wire bonds 16 connecting pads 28 with bond fingers 11. A molding or encapsulation (not shown in the FIG.) may be formed over the assembly to protect the active side of the die and the interconnects.
SUMMARY
In a general aspect of the disclosure, a conformal coating on a semiconductor die provides adhesion between the die and a support. No additional adhesive is necessary to affix the die on the support. The conformal coating protects the die during assembly, and serves to electrically insulate the die from electrically conductive parts that the die may contact.
In various embodiments the support may include a printed circuit board, or a substrate, or a leadframe, or another die, or an interposer, for example.
In some embodiments the material of the conformal coating includes an organic polymer, for example a polymer of p-xylene or a derivative thereof, such as a polyxylylene polymer, e.g., a parylene C or a parylene N, or a parylene A, or a parylene SR. In some embodiments the conformal coating is formed by deposition, for example by vapor deposition, or liquid phase deposition, or by solid phase deposition.
The conformal coating is formed on at least a die mount area of at least one side of the die, and the die is mounted on a die mount region of a support surface; adhesion of the die to the support is provided by the conformal coating on the die.
In some embodiments the conformal coating covers the back side of the die, and may additionally cover one or more sidewalls, and may additionally cover the front side of the die; where the conformal coating covers the front side of the die, openings are provided in the coating to expose die pads for electrical connection of the die.
In some embodiments the support also has a conformal coating, on at least the die mount region. The material of the conformal coating on the support may be the same as, or different from, the conformal coating on the die; in some embodiments the material of the conformal coating includes an organic polymer, for example a polymer of p-xylene or a derivative thereof, such as a polyxylylene polymer, e.g., a parylene C or a parylene N, or a parylene A, or a parylene SR. In some embodiments the conformal coating is formed by deposition, for example by vapor deposition, or liquid phase deposition, or by solid phase deposition.
Where the support is another die, the conformal coating may cover the front side of the die, and may additionally cover one or more sidewalls, and may additionally cover the back side of the die; where the conformal coating covers the front side of the die, openings are provided in the coating to expose die pads for electrical connection of the die.
In another general aspect of the disclosure, a method for affixing a die onto a support includes providing a die having a conformal coating, contacting a die attach area of the conformal coating with a die mount region of the support, and heating the die attach area of the conformal coating. In some embodiments the heating procedure follows the contacting procedure; in some embodiments the contacting procedure follows the heating procedure. That is, the coated die may be heated prior to contacting the die and the support to make an assembly; or, the die and the support may be contacted to form an assembly, and the assembly may then be heated to effect adhesion.
In another general aspect of the disclosure, a method for affixing a die onto a support includes providing a die having a conformal coating, providing a support having a conformal coating, contacting a die attach area of the conformal coating on the die with a die mount region of the conformal coating on the support, and heating the die attach area of the conformal coating on the die or the die mount region of the conformal coating on the support.
In some embodiments the heating procedure follows the contacting procedure; in some embodiments the contacting procedure precedes the heating procedure. That is, the coated die may be heated prior to contacting the die and the support to make an assembly; or, the die and the support may be contacted to form an assembly, and the assembly may then be heated. In some embodiments the heating procedure follows the contacting procedure; in some embodiments the contacting procedure precedes the heating procedure.
In another general aspect of the disclosure, a method for affixing a die onto a support includes placing the die onto the support, providing a standoff between a die attach surface of the die and a die mount surface of the support, and carrying out a conformal coating process to coat at least the die attach surface and the die mount surface. The conformal coating material may be an organic polymer, such as a parylene, for example, and the conformal coating process may be a vapor deposition process; the polymer that forms in the space between the die surface and the support surface provides adhesion between the die and the support.
The assemblies according to the disclosure can be used for building computers, telecommunications equipment, and consumer and industrial electronics devices.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic sketch in a sectional view showing a die mounted onto a substrate using a conventional die mount epoxy.
FIG. 2 is a diagrammatic sketch in a sectional view showing a stack of two die mounted onto one another and mounted onto a substrate using a conventional die mount epoxy.
FIG. 3 is a diagrammatic sketch in a sectional view showing a die mounted onto a substrate using a conventional film adhesive.
FIG. 4 is a diagrammatic sketch in a sectional view showing a stack of two die mounted onto one another and mounted onto a substrate using a conventional film adhesive.
FIG. 5 is a diagrammatic sketch in a sectional view showing a die having a conformal coating over the front side, the back side, and sidewalls, the coating having openings exposing interconnect pads at the front side.
FIG. 6 is a diagrammatic sketch in a sectional view showing a die mounted onto a substrate according to an embodiment.
FIG. 7 is a diagrammatic sketch in a sectional view showing die mounted one upon the other in a die stack mounted onto a substrate according to an embodiment.
FIG. 8 is a diagrammatic sketch in a sectional view showing a die having a conformal coating over the front side, the back side, and sidewalls, the coating having openings exposing interconnect pads at the sidewalls.
FIG. 9 is a diagrammatic sketch in a sectional view showing a die having a conformal coating over the front side, the back side, and sidewalls, the coating having openings exposing interconnect pads at the sidewalls and at the front side adjacent front side edges.
FIGS. 10A, 11A are diagrammatic sketches in sectional view each showing a stack of four die mounted one onto another according to an embodiment.
FIGS. 10B, 11B are diagrammatic sketches in sectional view each showing a four die stack as in FIGS. 10A, 11A mounted onto and electrically interconnected with a substrate according to an embodiment.
FIGS. 12A-12C are diagrammatic sketches in sectional view showing stages in construction of a conformally coated four die stack assembly according to an embodiment.



DETAILED DESCRIPTION
The invention will now be described in further detail by reference to the drawings, which illustrate alternative embodiments. The drawings are diagrammatic, showing features of the invention and their relation to other features and structures, and are not made to scale. For improved clarity of presentation, in the FIGS. illustrating various embodiments, elements corresponding to elements shown in other drawings are not all particularly renumbered, although they are all readily identifiable in all the FIGS. Also for clarity of presentation certain features are not shown in the FIGS., where not necessary for an understanding of the disclosure.
U.S. Pat. No. 7,215,018 describes a stacked die assembly including die each having a conformal coating on all six sides. The conformal coating is said to provide electrical insulation and to reduce physical damage to the die during handling and processing. A polymer, for example parylene, is suggested as the die coating insulating material. The coated die in the stack are laminated on top of each other with an electrically insulating polymer or epoxy preformed sheet, which may include solid (for example, glass) spheres to maintain a fixed spacing and separation between the semiconductor die after lamination.
Turning now to FIG. 5, there is shown in a diagrammatic sectional view a die 52 having a conformal coating 56 over the front side 53, the back side 51, and sidewalls 55. Openings indicated by arrows 57 expose peripheral die pads 54 for electrical connection of the die. The material of the conformal coating may be an organic polymer, for example a polymer of p-xylene or a derivative thereof, such as a polyxylylene polymer, e.g., a parylene C or a parylene N, or a parylene A, or a parylene SR.
A die may be coated with a parylene as shown in FIG. 5 by a vapor phase polymerization process. Processes for forming parylene are known. In one process, a para-xylylene dimer is vaporized at a pressure below atmosphere and at a temperature sufficient to vaporize the dimer at that pressure; then the vaporized dimer is pyrolized by heating to form the reactive para-xylylene monomer; then the reactive monomer and the item to be coated are brought together in a chamber with ambient temperature and reduced pressure, whereupon the reactive monomer condenses on all exposed surfaces within the chamber, including all exposed surface of the item, and polymerizes in situ. As is well known various process parameters can be controlled to determine the thickness of the resulting coating. Where the parylene coating on the die surface provides electrical insulation between adjacent parts, the thickness should be at least sufficient to provide insulation; and where the parylene coating on a die surface provides a barrier, for example to moisture, the thickness should be at least sufficient to provide such a barrier. In some examples the parylene coating on a die surface to be mated to another die surface has a thickness in a range about 0.1 micron to about 100 microns, more usually in a range in the order of one to a few microns to a few tens of microns.
Various of these procedures may be carried out sequentially or at least partly concurrently, and they may be carried out in a staged processing apparatus. The die may be positioned in the chamber such that one or more of the six sides of the die are entirely or partially obscured. The coating may be carried out, for example, at the wafer level, and as a result only the exposed surface of the wafer will be coated; subsequent die singulation results in die coated only on the corresponding surface of the die. Or, the coating may be carried out at a die array level, in which the wafer is partly or fully sawn; the resulting coating will cover part or the entirety of the sidewalls and either the die back sides (if the back side of the wafer is exposed) or the front sides (if the front side of the wafer is exposed) of the resulting die will be covered. Or, coating of all sides of the die may be obtained by carrying out two coating procedures, and turning the die (or wafer) over between the two procedures. Examples of various procedures for coating die at the wafer level and the die array level are described in, for example, S. J. S. McElrea et al. U.S. application Ser. No. 12/142,589, titled “Wafer level surface passivation of stackable integrated circuit chips”, which was filed on Jun. 19, 2008, and which is incorporated by reference herein.
Where as a result of the vapor phase polymerization process the front side of the die is covered, openings in the coating may be formed, for example, by laser ablation.
FIG. 6 shows an assembly including a conformally coated die mounted onto a substrate according to an embodiment of the invention. The substrate 10 is configured in this example substantially as in FIGS. 1-4, and the die 52 is configured substantially as in FIG. 5. The coated die in this example rests directly on the substrate (that is, directly on the dielectric layer 13 overlying the patterned metal layer 12), and adhesion of the coated die to the die attach region of the substrate is provided by the coating 56 at the die attach area on the back side of the die.
Adhesion of the coated die to the substrate is accomplished by heating the coating at the die attach area on the back side of the die to a temperature sufficient to affect adhesion of the die attach area with the substrate. This may be accomplished by, for example, heating the coated die to a temperature at which the material of the coating has some degree of surface tack, and then placing the die onto the die attach region of the substrate with the tacky surface contacting the substrate surface (that is, contacting the dielectric layer 13 overlying the patterned metal layer 12). Or, the coated die may be placed upon the substrate to make a die-on-substrate assembly, and then the assembly may be heated to heat the coating. As may be appreciated, the placing and the heating may be carried out sequentially or to some extent concurrently. The coating may be heated by convection or by conduction, for example, employing radiation (such as microwave radiation) or employing a heated platform, for example; and in some embodiments the substrate may be heated so that the coating is heated upon contact with the heated substrate. For example, where a parylene is used, adhesion may be accomplished by heating to a temperature in a range about 120° C. to about 230° C., usually not greater than about 175° C. to about 200° C. and more usually in a range about 125° C. to about 150° C. (excessive heating may damage the parylene coating). Adhesion may optionally be facilitated by application of pressure between the die and the substrate in addition to heat, and in some examples, where a parylene is used, applying a pressure about 5 N per square cm, or weighting the die on the substrate with a mass equivalent to about 10 kg per square cm, may be useful. Uniform adhesion may be facilitated by heating (and optionally applying pressure) for a time in a range about 1 second to about 30 minutes, for example about 10 minutes.
Following die attach, the die 52 is electrically connected to circuitry in the substrate 10, in this example by wire bonds connecting exposed die pads 54 to bond fingers 11. Openings are made in the coating at selected sites, for example by laser ablation, to expose the die pads prior to wire bond formation; opening may be carried out at any stage after the front side is coated and before the wire bond is attached, as may be convenient for the particular process.
FIG. 7 shows an assembly including a first coated die 52 mounted on a substrate 10 generally as described with reference to FIG. 6, and including an additional coated die 56 mounted on the first die 52. The additional die 56 has a front side (“active” side) adjacent which the integrated circuitry is formed, and a back side, and sidewalls meeting the front and back sides at front and back edges. Die pads at the front side of the die, arranged in this example as peripheral pads, provide for electrical connection of the integrated circuitry of the additional die with underlying circuitry. The additional die is affixed to a die mount region on the front side of the first die 52 by adhesion between the coatings on the opposed surfaces of the respective die, that is, on the front side of the first die and on the back side of the additional die.
The second die mount procedure, as with the first die mount procedure, may be carried out substantially as for the first die mount. That is, the coated second die may be heated and then placed upon the first die (and, optionally, pressure may be applied between the second die and the first die), or the second die may be placed on the first die and then the assembly may be heated (and, optionally, pressure may be applied). Or, alternatively, all the die may be placed one over another on the substrate to form a stacked die-on-substrate assembly, and then the assembly may be heated (and, optionally, pressure may be applied) to effect adhesion. Following die attach, the additional die is electrically connected to the circuitry on the substrate by wire bonds 16 connecting die pads on the additional die with bond fingers 11. Openings are made in the coating at selected sites, to expose die pads prior to wire bond formation. A molding or encapsulation (not shown in this FIG.) may be formed over the assembly to protect the active side of the die and the interconnects.
A die stack, not affixed to a non-die support, may optionally be made by providing a first coated die and a second coated die, and heating a contact area of the coating on at least one of the die, and then placing one die onto the die attach region of the other die with a contact area of the surface of one coating contacting the die attach region of the surface of the other coating (and, optionally, applying pressure). This can be completed for any number of additional die, to form a multiple die stack of adhered die. Or, the die may be positioned one upon another to form a stack, and then the stack can be heated (and, optionally, pressure applied) to effect adhesion of all the die at once to form a multiple die stack of adhered die. Optionally one or more stacks of die may be held in a fixture, adapted to maintain the relative positions of the die in the stack, and optionally adapted to provide a compressive force on the stack. Examples of suitable fixtures are described, for example, in McGrath et al. International Application No. PCT/US08/6651, titled “Method for optimized integrated circuit chip interconnection”, and in McGrath et al. International Application No. PCT/US08/66568, tiled “Semiconductor die coating and interconnection fixture and method”, each of which was filed Jun. 11, 2008. The completed stack of adhered die can then be mounted on a non-die support either by adhesion of the coating on the die that contacts the non-die support (such as, for example, a printed circuit board, or a package substrate, or a leadframe), or by a conventional adhesive.
The examples shown in FIGS. 6 and 7 employ electrical connection by wire bonds. Other interconnects may be employed, and may be preferred. For example, a trace of electrically conductive polymer, for example electrically conductive epoxy or electrically conductive ink, may be laid down to connect a die pad with a bond site in electrical circuitry on a support. Where the conformal coating covers at least those areas of the die that are directly overlain by the epoxy trace, electrical shorting of the epoxy interconnect is avoided. Referring to FIG. 7, for example, a conductive epoxy trace may begin with electrical contact at an exposed die pad on the second die 56, may be drawn across the coated (thereby electrically insulated) margin and front edge of the second die 56, down over the sidewall to the coated (and thereby electrically insulated) front side of the first die 52, may optionally make electrical contact with an exposed die pad on the first die 52, may be drawn across the coated (thereby electrically insulated) margin and front edge of the first die 52, down over the sidewall to the surface of the substrate 10, which is insulated by the dielectric layer 13, and may end with electrical contact with an exposed bond finger 11.
FIGS. 8 and 9 show, in diagrammatic sectional view, embodiments of conformally coated die having interconnect terminals exposed at the sidewalls of the die. In FIG. 8, a die 82 has a front side 83 and a back side 81 and sidewalls 85, all coated with a conformal coating 86, except that interconnect terminals 84 are situated at the sidewalls and exposed as indicted by arrows 87. In FIG. 9, a die 92 has a front side 93 and a back side 91 and sidewalls 95, all coated with a conformal coating 96, except that interconnect terminals 94 wrap around a front edge of the die (which may optionally be chamfered) and are exposed at the sidewall and at the margin of the front side, and at the die edge as indicated by arrows 97.
Coated die as in FIGS. 8 and 9 may optionally be stacked with sidewalls aligned, as shown in FIGS. 10A and 11A, respectively, and affixed by adhesion of the coatings as described above. Referring to these FIGS., the interconnect terminals 87, 87′, 87′′, 87′′′ (in FIG. 10A) and the interconnect terminals 97, 97′, 97′′, 97′′′ (in FIG. 11A), are now aligned, “vertically” with respect to an arbitrarily assigned “horizontal” plane of the front or back sides of the die and, accordingly, they may be vertically interconnected, for example using a trace of electrically conductive epoxy. FIGS. 10B and 11B respectively show stacked die as in FIGS. 10A and 11A with the die in each stack electrically interconnected and each stack connected to a substrate. Referring to FIG. 10B, a substrate 100 is shown having a patterned metal layer 102 on a dielectric layer, and covered by an insulative layer 103 except where, as indicated at 109, vertical interconnection is to be made. Here a stack of die as in FIG. 10A is mounted onto the substrate 100, affixed by adhesion of the conformal coating on the backside of the die 82 to a die mount region of the substrate surface. Vertical interconnection is made by conductive epoxy traces 107, which make electrical contact with interconnect terminals 87 at the sidewalls of the die and with the exposed interconnect sites 109.
Referring to FIG. 11B, a substrate 110 is shown having a patterned metal layer 112 on a dielectric layer, and covered by an insulative layer 113 except where, as indicated at 119, vertical interconnection is to be made. Here a stack of die as in FIG. 11A is mounted onto the substrate 110, affixed by adhesion of the conformal coating on the backside of the die 92 to a die mount region of the substrate surface. Vertical interconnection is made by conductive epoxy traces 117, which make electrical contact with interconnect terminals 97 at the sidewalls of the die and with the exposed interconnect sites 119.
In some embodiments, as described above, individual die are provided prior to stacking and adhesion with a conformal coating over at least a contact area; and the coated die are stacked and treated to effect adhesion. In the illustrated examples, the die are coated on the front side, the back side, and the sidewalls; but it is not necessary that all the surfaces of the die be coated. Particularly, the conformal coating may be applied to only one of the mutually opposed die attach surfaces, to effect adhesion of the die with a non-die support or with another die. Further surfaces (or areas of surfaces) of the die may be coated, for example to provide electrical insulation, so that shorting is avoided between the die surface and overarching or overlying electrical interconnects (bond wires or conductive epoxy traces, for example).
In other embodiments, uncoated die may be stacked and then a coating process may be applied to the stack. Where a parylene is formed using a dry vapor deposition, the active monomer condenses on surfaces within very small (narrow) spaces and, accordingly, the polymer can form in a very thin space between adjacent die. Such a very thin layer of polymer can suffice to effect adhesion between adjacent die. Stages in an example of such a process are shown by way of example in FIGS. 12A, 12B, 12C.
It may be preferable, when stacking uncoated die and thereafter applying the coating, to provide for uniform space between the adjacent die. This may be accomplished by providing a small piece of electrically nonconductive film or providing small electrically nonconductive spacers over the surface of each support (die or substrate, for example) and then placing the next die in the stack upon the film or the spacers. Where a small piece of film is employed, the film thickness is selected to provide the desired gap between the surfaces; the film can be roughly shaped and dimensioned, and need not cover any more of the surface than necessary to provide support and the desired spacing. Where small spacers are employed, the height of the spacers as provided on the surface is selected to provide the desired gap; the number of small spacers and their arrangement on the surface can be selected to provide support and the desired spacing. In some embodiments the film or support is substantially incompressible under the processing conditions, or compressible to a determinable extent. The film or spacers may optionally be adhesive, and may optionally be a curable adhesive, so that the film or spacers can provide temporary adhesion between the surfaces, to maintain the relative positions of the parts. This temporary adhesion (where employed) is typically insufficient to securely affix the die to the support, and permanent adhesion is provided by a parylene filling the gap, as described below. The nonconducting film may be an epoxy preform, for example; and the spacers may be small spots or puddles of an epoxy, for example, dispensed or printed in a suitable pattern. Or, the small supports may be (or include) spacer elements of an electrically nonconductive material (such as a glass or an organic polymer) that are dimensioned to provide the desired spacing, and, optionally, they may be provided with a thin surface adhesive to provide temporary adhesion. The spacer elements may have any of a variety of shapes; if spherical, for example, their diameter can provide the appropriate spacing and if spheroidal, for example, their smaller diameter can provide the appropriate spacing.
In some instances, the die as provided by the manufacturer has a protective passivation layer covering at least the active surface. This protective layer may be an organic polymer such as a polyimide, for example, and typically may have a thickness in a range about 5 μm to about 6 μm. Where a gap between surfaces in a range about the same as the passivation layer is desired, a significant portion of the passivation layer may be selectively removed (for example, in a patterned selective etch process) leaving small spots or areas of the passivation layer (for example, at the corners of the die; or at the corners and abut the center of the die) to serve as spacers.
FIG. 12A shows four die 112, 112′, 112′′, 112′′′, selected 120 for assembly into a four-die stack. Each die except the topmost one in the stack is provided with (in this example) a number of generally spheroidal spacer elements 114 all having approximately the same nominal height. The material of the spacer elements is electrically nonconductive. When the die are placed one over another to form a stack 122, as illustrated in FIG. 12B, the spacer elements provide a standoff, or gap or space 113, between adjacent die, and the height of the spacer elements determines the thickness of the standoff. The spacer elements may have a nominal height in a range about 5 μm to about 40 μm, for example; in some embodiments the gap between adjacent die may be about 10 μm, or about 20 μm. As will be appreciated, for a given number of die in the stack thinner gaps between adjacent die results, desirably in some applications, in a thinner overall stack and in some embodiments the gap may be made less than 5 μm.
The assembly of spaced-apart die is then subjected to a conformal coating process, resulting in a coated stacked die assembly 124, as shown in FIG. 12C. The coating 116 forms on all exposed surfaces of the assembly, including a die active side at the “top” of the stack, a die backside at the “bottom” of the stack, and die sidewalls; and including the spaced-apart front sides and back sides of adjacent die in the stack, as indicated at 117.
As noted above, the parylene coating is formed by a dry vapor deposition. The active monomer condenses on surfaces within very small (narrow) spaces and, accordingly, the polymer can form in a very thin space between adjacent die. The thinness of a gap between surfaces that can be successfully filled by a parylene depends upon the distance from the edge to the center of the surfaces in the gap, and upon the mean free path of the parylene under the processing conditions employed. The capacity of the parylene monomer to invade the gap can limit the extent of thinness of the gap. The various parylenes have different characteristics, including mean free path characteristics, and for gap filling the choice of parylene and selection of processing conditions can be readily determined accordingly. The gap surfaces to be coated can, for example, be differentially cooled, to promote coating formation from within the gap toward the outside to fill the gap.
In other embodiments the individual die are provided prior to stacking and adhesion with a conformal coating over at least a contact area; the coated die are stacked to form an assembly of coated die; and the assembly of coated die is subjected to a further conformal coating process to encapsulate the stack and, optionally, to effect adhesion of the die in the stack and/or the stack to an additional stack or to a non-die support.
A greater or lesser number of spacer elements may be required to provide adequate support, depending for example upon the flexibility of the die (thinner die may require more support) and upon the height of the stack and the number of die in the stack.
The spacer elements may be, for example, glass or nonconductive polymer spheres; and they may be scattered over the surface of each die. Or, for example, the spacers may be small polymer dots, which may be adhesive to some extent (they may be a polymer adhesive, for example an epoxy) to affix the adjacent die temporarily, so that the die in the stack do not shift during processing steps leading up to the completion of the conformal coating over the stack; and they may be printed (for example screen printed or mask printed) onto the die surface, or dispensed onto the die surface using a needle or nozzle, for example.
As may be appreciated, to obtain adhesion of a die to a support, whether the support be another die, a substrate, a motherboard, or other surface, the coating need be situated only within an area where contact of the surface of the coated die and the surface of the substrate is expected. For example, where the backside of the coated die contacts a surface of the substrate, for adhesion purposes the coating may be only within the die attach area of the die.
The support may include a patterned metal layer at the die mount side, as shown by way of example in FIGS. 6, 7, 10B and 11B. Where no uncoated die surface contacts the surface of the substrate, the dielectric layer over the patterned metal layer (e.g. 13, 103, 113) is not required, as the dielectric properties of the conformal coating prevents shorting of the die and the circuitry on the substrate.
In the illustrated examples of assemblies having two or more die stacked die interconnected by electrically conductive polymer traces, the die are shown as having the same width (in the sectional view shown), and the die are arranged in the stack so that the die sidewalls in overlying die are generally vertically aligned. That is, the resulting die stack assemblies present a generally planar stack face for interconnection. In other embodiments, not all the die in the stack need have the same dimensions, and the die sidewalls adjacent the interconnects may be arranged so that they are not vertically aligned. For example, the die in the stack may be offset, or may be staggered. Examples of various die stacking configurations are shown in S. J. S. McElrea et al. U.S. application Ser. No. 12/124,077, referenced above.
Examples of various electrically conductive polymer interconnect materials, and techniques for applying interconnect traces onto a variety of stacked die assembly configurations, are described for example in T. Caskey et al. U.S. application Ser. No. 12/124,097, titled “Electrical interconnect formed by pulsed dispense”, which was filed May 20, 2008, and which is hereby incorporated herein by reference.
Other embodiments are contemplated by the invention. For example, the die may be interconnected by tab bonds, for example, or by ribbon bonds configured as off-die interconnects. Where “vertical” interconnection is employed, for example using an electrically conductive epoxy trace, adjacent die in the stack may be offset. An offset arrangement may be particularly useful where the die pads or interconnects are situated in the active surface of the die and are accordingly would not be presented at the face of the die stack for interconnection. A stack of several offset die may have a stairstep appearance, for example.
All patents and patent applications referred to above are hereby incorporated by reference herein.

## Claims

1. A method for affixing a die onto a support, comprising providing a first die having a conformal coating on a surface thereof, providing a support having a conformal coating on a surface thereof, contacting a die attach area of the conformal coating on the first die directly with a die mount region of the conformal coating on the support, wherein the conformal coatings on the surfaces of the die and the support are of polyxylylene polymer material, and accomplishing adhesion of the die attach area with the die mount region of the support by heating at least one of the die attach area of the conformal coating on the first die or the die mount region of the conformal coating on the support to a temperature sufficient to affect adhesion.

2. The method of claim 1 wherein the heating procedure follows the contacting procedure.

3. The method of claim 2 wherein the contacting procedure follows the heating procedure.

4. The method of claim 1, wherein the polyxylylene polymer is a parylene selected from the group consisting of a parylene C, a parylene N, a parylene A, and a parylene SR.

5. The method of claim 4, wherein heating the die attach region includes heating the conformal coating to a temperature of about 120 to 230 degrees Celsius.

6. The method of claim 4, wherein heating the die attach region includes heating the conformal coating to a temperature of about 175 to 200 degrees Celsius.

7. The method of claim 4, wherein heating the die attach region includes heating the conformal coating to a temperature of about 125 to 150 degrees Celsius.

8. The method of claim 4, further comprising applying pressure between the first die and support.

9. The method of claim 8, wherein the pressure is about 5N per square cm.

10. The method of claim 1, further comprising providing a second die having a conformal coating on a surface thereof, contacting a die attach area of the conformal coating of the second die with a die mount region of the first die, and heating the die attach area of the conformal coating of the second die.

11. The method of claim 10, further comprising electrically connecting the first and second die to the support via one of an electrically conductive epoxy or electrically conductive ink.

12. The method of claim 1, wherein the support is a second die.

13. The method of claim 12, further comprising contacting a die attach area of the conformal coating of the second die with a die mount region on a non-die support, and heating the die attach area of the conformal coating of the second die.

14. The method of claim 13, further comprising electrically connecting the first and second die to the support via one of an electrically conductive epoxy or electrically conductive ink.

15. The method of claim 1, wherein the heating comprises heating both the die attach area of the conformal coating on the first die and the die mount region of the conformal coating on the support.