Surface mount IC using silicon vias in an area array format or same size as die array

[0002] This is a division of application Ser. No. 08/601,302, filed Feb. 16, 1996 now U.S. Pat. No. 5,973,396.

[0003] 1. Field of the Invention

[0004] The present invention relates to semiconductor devices and associated integrated circuit configurations and, more particularly, to bare die configurations and stacked multi-chip (bare die) assemblies with chip-integral vertical connection circuitry and a method of fabricating such die and assemblies.

[0005] 2. State of the Art

[0006] As computers and other microprocessor-dependent electronics become physically more compact and operate at ever-faster speeds, the amount of “real estate” available on circuit boards and other component-supporting substrates becomes ever-smaller. Various die packaging schemes have evolved to promote greater component density, one of the most recently prevalent being surface mount technology (SMT). In an SMT package, “footprint” size is reduced by use of small leads placed at a fine pitch, sometimes in combination with staggered rows of terminal bond pads about substantially the entire exterior of a circuit package. The SMT pads or lead ends generally extend immediately below the package and not first laterally, as in most prior art dual-in-line (DIP) and small-outline J-lead packages (SOJ). Another attempt to increase component density by vertical component orientation has resulted in the so-called zig-zag inline package (ZIP).

[0007] Multi-chip modules (MCM's), wherein several chips or dice for the same or different functions are mounted in a common package, generally mount all dice in the module on a supporting substrate in the same horizontal plane, such as is the case in a single in-line memory module (SIMM) including a plurality of dynamic random access memory (DRAM) dice.

[0008] It has, however, been recognized that it may be desirable in certain applications to enhance component density by laterally stacking vertically-oriented die or die support substrates on a transversely-oriented carrier substrate to which electrical access is secured via edge connect structures on the die or die support substrates. U.S. Pat. Nos. 5,266,833; 4,983,533; and 4,764,846 are exemplary of state of the art lateral die stacking approaches. Such structures, while increasing component density, obviously require conductors extending to at least one side of the stacked die or die support substrate, which may effectively limit the number of connections to the carrier substrate. If terminals are placed on more than one edge of a die (see the '833 patent), the die support substrate or other bus structure becomes somewhat complex. Further, edge-accessing die or die support substrates dictates that all inter-die or inter-die support electrical connections must be effected through the carrier substrate without the potential for direct die-to-die connection.

[0009] It has also, however, been recognized that it may be desirable in certain applications to vertically stack horizontally-disposed dice in two or more layers. U.S. Pat. Nos. 5,481,134; 5,481,133; 5,468,997; 5,455,445; 5,434,745; and 5,128,831 are exemplary of state of the art vertical die stacking approaches. Also illustrative of the state of the art in that regard is “Laminated Memory: A new 3-Dimensional Packaging Technology for MCM's”, Proceedings of1994 IEEE Multi-Chip Module Conference,pp. 58-63. Further examples of such structures are the commercially available Micro SMT Packages from Micro SMT, Inc., depicted and described in a Micro SMT, Inc. brochure bearing a 1993 copyright. Existing vertical stack MCM's, while increasing component density, usually laterally offset active and passive devices in a given plane or layer of a stack from the vertical, interlayer conductors, necessitating the use of horizontal circuit traces extending from the devices to the edge of each die or at the very least to a peripheral area of the die. Such offsets increase the lateral size or footprint of the stack and are somewhat limiting with respect to the number of devices per layer, even if the vertical conductors are formed as contact holes or vias in the silicon die material itself, which is not common practice. It has also been suggested in the art to employ diffused metal pillars connected to horizontally offset active devices of the die by laterally extending circuit traces.

[0010] Therefore, it would be extremely advantageous to form passive and active devices and vertical conductors, hereinafter generally referred to herein as vias, respectively on and within dice, and in vertically superimposed relationship to the extent possible. Such an approach would permit electrical connection of a die to other dice or a supporting substrate such as a printed circuit board (PCB) through the back side of the die, using that heretofore unused or underemployed die side in lieu of the so-called “active” or top side. It would also be highly beneficial to minimize the number of vias when stacking dice by accessing a single, commonly-required via within the die perimeter for (by way of example for a DRAM die) CAS, RAS, I/O, power, and ground from each level or plane in the die stack, thus requiring dedicated vias to each level only for the chip-select function. Finally, it would simplify and expedite fabrication of such die structures to employ via formation techniques which do not involve forming and filling holes extending vertically through the die.

[0011] The present invention comprises a method for forming a bare die suitable for back side electrical connection to a carrier, as well as for multi-layer, or vertically stacked, multi-chip (bare die) assemblies employing metalzed silicon vias for vertical interconnection of the dice to each other and to a carrier substrate to which the die stack is electrically connected. The present invention provides the ability to fabricate a protected package of stacked dice with a surface-mount terminal array on the bottom or back side of the lowermost die for mounting to a conductor-bearing carrier substrate.

[0012] The present invention includes at least one, and preferably a plurality, of silicon or other suitable die (such as gallium arsenide) on which integrated electronic devices are formed and through which vertically-extending electrical conductors (vias) extend in a pattern to maximize component and circuit density on the die.

[0013] In the method of the invention, vias are formed as conductive pillars through the die material from the top or active surface thereof to the bottom. The pillars may be formed by doping of a conductive material onto the top die surface and gravity diffusion of the conductor through the silicon. Vertical diffusion may be enhanced, and lateral diffusion curtailed, by electrochemically anodizing the die using a mask to create the desired via pattern, or by appropriate crystallographic orientation of the silicon die material to facilitate channeling of the conductive material through the silicon matrix. Alternatively, ion implantation of the conductor through a suitably patterned mask may be employed to create the vias.

[0014] Subsequent to via formation, active or passive integrated devices or components and other structures may be formed on the active surface of the die. If an active component, the structure is formed by one of several epitaxial film deposition and doping processes as known in the art. If a passive component or bond pad, the structure may likewise be formed by any additive or subtractive material deposition process, or combination thereof, as known in the art. The components and surface conductors, such as bond pads, are preferably formed in superimposition to the vias rather than laterally offset therefrom.

[0015] If the die in question is employed singly on a supporting substrate, or is the uppermost die of a stack, an insulating, protective layer such as silicon dioxide, doped silicon dioxide (PSG, BPSG), silicon nitride or a polyimide may be applied over the entire active surface since all electrical connections are made from the back of the die. If a die stack is formed, a protective layer as required may additionally be formed about the sides of the stack or a shielding adhesive applied between the stacked dice and the lowermost die and the carrier substrate.

[0016] Each die of the preferred plurality is provided with vias to interconnect with dice above or below it or a carrier substrate, as the case may be. Vertically-aligned vias extending from the carrier substrate through each die to and through the uppermost die comprise a commonly-accessed conductive vertical pathway from the substrate to each die in the stack for such commonly-required functions as power, ground, I/O, CAS, RAS, etc. Discrete or individual vertical pathways for chip-selects extend to each of the various dice, so that each die accesses the carrier substrate for commonly-required functions commonly and discrete functions individually. It is contemplated that dice to be employed in a stack may have the same or different components but a common via layout for easy superimposition. The overall circuit defined by the die stack may then be customized by the number and type of die employed and the use of laser-blown or electrically-blown fuse elements, as known in the art, incorporated in the die circuitry of each die. Thus, a via stack may be electrically connected to components at one die level, but not at those above or below, serving only at the other (unconnected) die levels as a bypass conductor.

[0017] The back of each die may include terminal structures such as bond pads for interconnection to the next lower die, and the silicon may be etched using a suitable mask to cause the lower portions of the pillar-vias to protrude for connection to bond pads or components on the next adjacent lower layer. If the die in question is designated as the “base” or lowermost die of the stack, appropriate metalization in the form of circuit traces may be applied to the die back side over a suitable insulating layer, if required, (to, among other things, prevent inadvertent “doping” of the back side of the die by the metal traces) to achieve a desired connection pattern for flip-chip style connection (as, for example, by controlled collapse chip connection, or C4, bonding) to the carrier substrate.

[0018] FIGS. 1A, 1B and 1C schematically depict the basic fabrication sequence of a die according to the present invention;

[0019] FIG. 2 schematically depicts a die according to the present invention configured for singulated affixation to a carrier substrate and including a protective layer over the active surface, as well as back side contact structures for electrical connection to the carrier;

[0020] FIG. 3 schematically depicts a stack of three dice according to the present invention mounted to a carrier substrate;

[0021] FIG. 4 depicts a perspective schematic of a stack of eight dice according to the present invention, illustrating the use of vertically-superimposed vias for connections commonly required by each die in the stack; and

[0022] FIG. 5 is a top elevation of an active surface of a die depicting an exemplary fuse element usable in the present invention.

[0023] Referring now to FIGS. 1A, 1B and 1C, an exemplary silicon die 10 is depicted. It should be understood that fabrication of the present invention will normally be effected at least partially at the wafer level, and that reference to a single die is for descriptive purposes only, and not limiting of the applicability of the invention to such a structure.

[0024] FIG. 1A depicts die 10 in an essentially pristine state, being modified only by the addition of conductive exemplary vias 12 and 14, extending from what will become the front side or active side 16 of die 10 to the back side 18 thereof. Vias 12 and 14 may be formed by any process known in the art. However, it is preferred and significant to the invention that the vias be formed in situ in the silicon matrix of the die without prior physical removal of the die material.

[0025] By way of example only, vias may be formed by gravitationally-induced diffusion of metal applied (as by doping) through apertures in a mask or other protective layer on active side 16. Exemplary diffusible metals possessing suitable conductivity and without undue resistivity include, by way of example, Au and Al. Other suitable metals are known in the art. One limitation to the utility of diffusion-forming vias is significant lateral migration of the diffusing metal, which may be somewhat limiting as far as via density due to spacing requirements. Vertical diffusion speed may be enhanced and lateral migration reduced through selective electrochemical anodization of the die using a masking technique as referenced in the aforementioned U.S. Pat. No. 5,455,455 and described in detail in U.S. Pat. No. 5,386,142, the disclosure of both of these patents being incorporated herein by this reference. Another approach to enhancing beneficial vertical diffusion while curtailing lateral spread of the diffused conductor is selective crystallographic orientation of the silicon die material to facilitate channeling of the conductive material through the silicon matrix. An alternative to diffusion is ion-implantation of the conductor material, which is relatively rapidly effected in comparison to diffusion techniques, and similarly limits lateral spread of the conductive material of the via.

[0026] Referring to FIG. 1B, after vias 12 and 14 are formed, epitaxial layer 20 can be formed on active side 16 if active devices are to be fabricated over vias 12 on die 10. Active devices such as 22, superimposed on or immediately adjacent via 12, are then formed by means well known in the art. Active devices may include, by way of example and not limitation, any such devices known in the art such as diodes, transistors, and memory (ROM, PROM, EPROM, EEPROM, bubble, DRAM, SRAM) including subcomponents such as transistors, capacitors and fuses. Active devices may also be fabricated in the original silicon of the die, as known in the art. Passive devices 23 (FIG. 3) (resistors, capacitors, fuses, etc.) may similarly be formed on or into die 10, again by means well known in the art. Devices offset from vias 12 and 14 may be connected to such by lateral or horizontal circuit traces. Finally, if a via is to serve as a through or relay via, a bond pad 24 as depicted capping via 14 may be formed as well known in the art to provide an electrical path to a higher die in a stack of which die 10 is a part.

[0027] Referring to FIG. 1C, if die 10 is to be employed as a singulated bare die and secured to a carrier substrate such as substrate 30 bearing conductive traces 32, or is employed as the top die in a stack as depicted in FIG. 2, a protective layer 50, such as silicon dioxide, doped silicon dioxide (PSG, BPSG), silicon nitride or a polyimide, may be applied over the entire active surface and all components thereon since all electrical connections are made from the back of the die.

[0028] Again referring to FIG. 1C, die 10 includes back side terminal structures 26 and 28 for connection to bond pads 24 on the top of a lower die of a stack or for connection to traces, 32 of carrier substrate 30. Terminal structures may comprise applied metalization in the form of bond pads 34. Alternatively, or in addition to, the formation of bond pads 34, a selected depth 19 of the back side 18 of die 10 may be selectively etched after being masked, as known in the art, to cause vias 12 and 14 to project downwardly from back side 18, as shown, and provide enhanced clearance between die 10 and the next lower die in the stack, or between die 10 and carrier substrate 30. If the latter application is intended for die 10, C4 or similar flip-chip style conductive bumps or protrusions 36 may be formed directly under a via. Alternatively, back side 18 may be provided with metalization in the form of circuit traces 38, which re-pattern some or all of the external electrical connections of the die 10 or of the entire die stack as required by the substrate input/output or I/O pattern, circuit traces 38 thus terminating in conductive bumps or protrusions 36. As known in the art, a non-conductive underfill may then be introduced between back side 18 and carrier substrate 30.

[0029] Referring to FIG. 2 of the drawings, exemplary singulated die 10′, intended for non-stacked usage, is depicted affixed to a carrier substrate 30, die 10′ being employed alone or in combination with other such dice or dice of other configurations such as in a SIMM or other MCM. Elements as previously identified with regard to FIGS. 1A through 1C are referenced in FIG. 2 by the same numerals. Non-conductive protective underfill 40 is depicted in FIG. 2.

[0030] Referring to FIG. 3 of the drawings, a three-die stack including die 110a, 110band 110cis depicted assembled and mounted to a carrier substrate 130. Elements in FIG. 3, which are the same as those in FIGS. 1 and 2, are identified by the same reference numerals. Die stack 110 is encapsulated, top and sides, by a protective layer 150 such as polyimide or low temperature chemical vapor deposition (CVD) nitride. Dies 110aand 110bare provided with protective dams 120 projecting downwardly from back sides 18 about the entire die periphery to about the active sides 16 of the next lower die. An adhesive or other protective non-conductive filer or sealant 122, as desired, may be applied between the lower end of each dam 120 and the active side 16 to effect a die-to-die seal and eliminate the need to side-coat the die stack 110 with a protective layer 150, so that only a protective layer 150 on the top die 110ais required. As shown in broken lines, vias 12 and 14 may be placed in any configuration in dice 110a, 110b, and 110c.

[0031] Referring to FIG. 4 of the drawings, an eight-die stack including dice 210a, 210b, 210c, 210d, 210e, 210f, 210g, and 210hare depicted mounted on and electrically connected to a carrier substrate 230. As can be seen schematically, a series of common-access via stacks 212a, 212b, 212c, 212dand 212eextend vertically through all eight dice and are accessed by each. A second series of discrete access via stacks 214a, 214b, 214c, 214d, 214e, 214f, 214g, and 214hprovide an individual chipselect function to each die in the stack. Thus, the total number of required via stacks equals only the number required for common access to the carrier substrate, plus the number required for discrete access (generally equal to the number of dice in the stack).

[0032] As the number of die in the stack increases, the benefits of the present invention become more apparent in terms of reducing the relative die real estate required for conductors to access the carrier substrate and permitting shrinkage of the die stack. Several exemplary programmable blown or open fuse elements 260a, 260band 260care also depicted in FIG. 4, such open fuse elements isolating a die above a certain level from a particular via stack. Thus, for example, a chip-select via for the base dice 210hmay be isolated from the remaining seven (7) die above it. Other vias may similarly be isolated. Programmable antifuses, as known in the art, may similarly be employed to selectively connect, rather than disconnect, conductors on and between dice. Several programmable activated or closed fuse elements are depicted at 264a, 264b, and 264c.Fuses and antifuses may, of course, be employed in combination on the same die and on adjacent dice.

[0033] While the activated fuses and antifuses have been shown as part of the vias themselves, it should be understood that the concept of isolating or selectively connecting die levels is not so limited, as horizontally extending fuse and antifuse elements may also be employed, as may antifuses.

[0034] If a vertical fuse-isolation approach is used, polysilicon or oxide 262, located in a via segment (see FIG. 4), is a suitable fuse element which may be blown with a high current.

[0035] If a horizontal fuse-isolation approach is used (see FIG. 5), a thin film 70 of nichrome or titanium-tungsten, optionally with a necked-down portion 72, may be employed on the active side 16 or back side 18 of a die 10 between a horizontal conductive path 74 and a vertically-extending via 12 or 14.

[0036] Suitable antifuse technology is disclosed, by way of example, in U.S. Pat. Nos. 5,110,754 and 5,324,681, the disclosures of which are hereby incorporated by this reference. See also U.S. Pat. Nos. 5,485,031 and 5,485,032.

[0037] Of course, depending upon the fuse and antifuse locations and the overall circuitry pattern, fuses may be blown and antifuses activated or closed on the individual die prior to stacking or after the stack is assembled. It is preferable to qualify known good die (KGD) after the fuses are blown and antifuses closed at die or wafer-level, and then fabricate a KGD stack for assured greater yield of the ultimate, resulting MCM.

[0038] It is contemplated that a die on top of a stack may be configured with antifuse connections to all chip selects of the lower die in the stack. With such an arrangement, if a die of the stack fails during burn-in or test, one can then contact the upper die with probes or through electrical programming and activate or close the antifuses to “add” the top die to the circuit to replace the defective lower die. Standard fuse methodology is used to “remove” the bad die from the circuit. A known good die is preferably employed as the top die in the stack.

[0039] While the present invention has been described in terms of certain illustrated embodiments, it is not so limited. Those of ordinary skill in the art will understand and appreciate that many additions, deletions and modifications to the embodiments as specifically disclosed herein may be made without departing from the scope of the invention as hereinafter claimed. For example, a lowermost die may be used to laterally reroute the via pattern from one designed to best accommodate the sizes, configurations and various placements of components on the die above to a standardized or other selective pattern on a carrier substrate. Rerouting conductors may be formed on the upper side of the rerouting die to contact terminals on the back side of the next-uppermost die. The rerouting conductors would then extend to vias in the rerouting die for connection to traces on the carrier. It is also contemplated that dice of different size and shape may be stacked, as long as the vias vertically electrically connecting them are aligned. It is further contemplated that the invention has utility in paralleling processors by stacking the processors. In such a case, there would be additional pin types connected, such as clock inputs/outputs, etc., as well known in the art.