Method and apparatus for connecting vertically stacked integrated circuit chips

[0001] 1. Field of the Invention

[0002] The invention relates to electronic modules comprised of stacked integrated circuit carrying chips and more particularly to the method and apparatus for providing a connection among and with memory chips as an assembly of stacked memory chips.

[0003] 2. Description of the Prior Art

[0004] High-density electronic packages, which are capable of incorporating more electronic capacity in a given space, or reducing the space required for a given amount of electronic capacity. Such packages are particularly useful as computer memories, control logic, arithmetic units, and the like wherein a plurality of similar chips can be advantageous combined into a single assembly or module.

[0005] The electronic density is obtained by means of a structure in which integrated circuit (IC) chips are stacked to form a three-dimensional structure. The stacked chip structure: (a) has at least one interconnect plane which is adapted to be electrically connected to external circuitry; and (b) contains within its volume a very extensive electronic system. The term interconnect plane signifies that electrical leads extend to that planar surface of the stacked chip structure.

[0006] In various prior applications and patents assigned to the assignee of this application, stacks of silicon IC chips have been proposed. One of those applications is U.S. Pat. No. 4,706,166. That patent discloses a three-dimensional module of stacked layers, or chips, each of which layers carries IC circuits whose leads extend to a common interconnect plane of the module. Electrically conductive bumps deposited on the access plane of the module are aligned with, and bonded to, electrically conductive bumps on a supporting substrate, thereby connecting the circuitry in the stacked layers to external circuitry.

[0007] Various limitations and deficiencies in the prior developments have led to the present invention. One such limitation is the fact that IC chips, such as memory devices, which are preferably obtained as standard (off-the-shelf) items from suppliers, must be modified to provide external leads only at one edge, instead of two edges, of each chip.

[0008] Perhaps the most critical problems encountered have been due to the electrically conductive properties of the material of the stacked chips, except for such materials as gallium arsenide and sapphire. Because the electrical leads at the interconnect plane must be insulated from the semiconductor material, it has been necessary to apply passivation material on the interconnect plane, and then to form T-shaped electrical connections by applying thin-film metallization to the interconnect plane.

[0009] These T-connects are fragile and therefore not very reliable. In the case of a silicon stack, the reliability of the T-connects depends to a large extent on the quality of the passivation layer. Another problem centers around the epoxy glue between layers, which is troublesome in several ways. Glue thickness variations, for example, can cause problems during certain processing steps, and the glue limits the stack's operating temperature to about 10 C. It also limits the choice of material for the bonding bumps (to avoid degrading the glue and passivation due to high temperature). In addition to the T-connect problem and the glue problem, there is also a problem with flip-chip bonding (bump bonding) of the stacked chip module to a substrate. Flip-chip bonding has been less reliable as a method for making electrical interconnections than other methods, such as TAB bonding and wire bonding. In particular, it is not very practical in a mass production environment.

[0010] Another issue addressed by the present invention concerns heat transfer, particularly where the IC chips have high power requirements. Although silicon has reasonable heat-conducting properties, there is still the possibility of overheating problems in silicon stacks. Furthermore, the heat dissipation problem appears almost insurmountable (in stacked chip modules), if non-heat-conducting chips made of poor thermally-conducting material, such as gallium arsenide (GaAs), are used.

[0011] Such chips have certain advantages over silicon, including their ability to provide much higher speed electronic signals. However, the use of GaAs devices at higher speeds and temperatures, in the future can be expected to create packaging problems. As operating frequency increases into the gigahertz range, chip temperature increases and electrical/material properties begin to vary significantly. As a result, many other electrical properties are also affected; they include signal propagation delay, signal rise time, and characteristic impedances. Requirements for innovative denser packaging to help alleviate these problems have become critical. It is therefore obvious that special temperature considerations must be given to the packaging of GaAs devices to avoid degradation of their high speed performance.

[0012] To satisfy these needs the prior art has provided a frame carrier in which the chip is embedded as shown in U.S. Pat. No. 4,764,846. While effective to meet the foregoing problems, this approach entails processing and manufacturing steps which add significantly to the cost of the assembled stack of chips.

[0013] Therefore, what is needed is an inexpensive and easily fabricated method and apparatus by which similar prefabricated, packaged chips may be stacked together and connected both to each other and to the external world.

[0014] The invention is an apparatus comprising a plurality of prepackaged integrated circuits, each with a first plurality of exterior leads connected to a second plurality of interior wires, having at least one exterior lead which is not connected to any interior wire. The plurality of prepackaged integrated circuits are vertically stacked on each other so that like leads are aligned with each. The prepackaged integrated circuits are modified so that the first plurality of exterior leads are removed exposing ends of the second plurality of interior wires on a collective lateral surface of the vertically stacked prepackaged integrated circuits. A plurality of metallizations are disposed on the collective lateral surface connecting selected ones of the second plurality of interior wires together and separately connected to selected other ones of second plurality of interior wires.

[0015] The apparatus further comprises a contact layer having contact pads defined therein. The contact layer is disposed on the vertically stacked prepackaged integrated circuits. The plurality of metallizations are coupled to the contact pads. The plurality of metallizations are coupled to the contact pads via leads disposed in the contact layer and extending to the collective lateral surface. The contact layer has an exposed top surface and the contact pads are defined on the exposed top surface.

[0016] 5. The apparatus of claim 2 wherein the contact layer has an exposed top surface and wherein the contact pads are defined on the exposed top surface.

[0017] The prepackaged integrated circuits have a plurality of types of circuit terminals. A first group of metallizations, which connect selected ones of the second plurality of interior or bonding wires together, are disposed vertically on the collective lateral surface and are connected to a selected one of the types of circuit terminals of each of the plurality of prepackaged integrated circuits to form a common connection among the selected type of circuit terminal. A second group of metallizations, which are separately connected to a selected one of second plurality of interior or bonding wires, correspond to a selected type of circuit terminal for each of the plurality of prepackaged integrated circuits. This second group of metallizations are disposed in part vertically on the collective lateral surface and are laterally offset one from the other to allow separate access to each one of the plurality of prepackaged integrated circuits through the selected type of circuit terminal.

[0018] This second group of offset metallizations are disposed in an NC or no connection region of the collective lateral surface. The collective lateral surface created when the first plurality of exterior leads are removed, i.e. NC or no connection region, includes an insulative surface devoid of any exposed ends of the interior wires. The plurality of metallizations which are separately connected to selected other ones of second plurality of interior wires are disposed at least in part thereon.

[0019] In the illustrated embodiment the plurality of prepackaged integrated circuits comprise a plurality of memory circuits. The selected ones of the second plurality of interior wires which are coupled together to the metallization include power terminals. The second plurality of interior wires separately connected to the metallization include chip control or data input/output terminals.

[0020] The invention is also characterized as a method for making the above described apparatus.

[0021] While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of means or steps limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.

[0022] FIG. 1 is a side cross-sectional view of a prepackaged chip devised according to the prior art.

[0023] FIG. 2 is a side cross-sectional view of a plurality of prepackaged chips of FIG. 1 vertically stacked according to the invention.

[0024] FIG. 3 is a side cross-sectional view of a plurality of prepackaged chips of FIG. 2 which have been modified to remove the lead frames according to the invention.

[0025] FIG. 4 is a side cross-sectional view of a prepackaged chip devised according to the prior art in which there are two back-to-back chips within the same package.

[0026] FIG. 5 is a side cross-sectional view of a plurality of prepackaged chips of FIG. 4 vertically stacked according to the invention.

[0027] FIG. 6 is a side cross-sectional view of a plurality of prepackaged chips of FIG. 5 which have been modified to remove the lead frames according to the invention.

[0028] FIG. 7 is a plan view of the side metallizations disposed on the collective lateral surface of the stack in either the embodiments of FIG. 3 or 6.

[0029] The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below.

[0030] A plurality of identical prepackaged chips, such a memory chips, are vertically stacked and bonded together with their terminals aligned. The exterior lead frames are removed by grinding down the collective lateral surface of the stack, including that portion of the exterior lead frames extending into the packaging of the chip. The bonding wires which were connected to the lead frames are now exposed on the collective lateral surface of the stack. In those areas collective lateral surface of the stack where no bonding wire was connected to the lead frame, a bare insulative surface is left. A contact layer is disposed on top of the stack and vertical metallizations defined on the stack to connect the ends of the wires to the contact layer and hence to contact pads on the top surface of the contact layer. The vertical metallizations are arranged and configured on the collective lateral surface of the stack to connect all commonly shared terminals of the chips, such a power and ground terminals to a single vertical metallization, while the control and data input/output signals of each chip are separately connected to offset vertical metallizations defined on the collective lateral surface, which metallizations are disposed in part on the bare insulative surface portions of the collective lateral surface.

[0031] FIG. 1 is a side cross-sectional view of a conventional integrated circuit chip, such as a flash memory chip 10. The packaged memory chip 10 is comprised of a semiconductor integrated chip, symbolically denoted by reference numeral 12 included within a passivating package 14, such as molded plastic. Chip 12 is connected to a conductive lead assembly 16 which electrically communicates chip 10 to the exterior world and extends into the side surface 18 of packaging 14. wire bonded leads 20 connect lead frame 16 to connection points 22 on chip 12. In this manner chip 10 is encapsulated or passivated from the exterior environment while providing the required number of electrical connections typically through a dual-in-line package.

[0032] Thus, FIG. 1 represents an integrated circuit memory chip 10 as it is typically provided by the manufacturer. A plurality of such chips 10 can be vertically stacked together according to the invention as depicted in FIG. 2. In the illustrated embodiment of FIG. 2 four such chips 10a, 10b, 10c and 10d are shown as vertically stacked one on top of each other. Any number of such chips may be vertically stacked and only four chips are shown only for the purposes of illustration. Chips 10a-10d are bonded together by adhesive or any other equivalent means now known or later devised.

[0033] As depicted in FIG. 2, corresponding lead frame's 16a-16d extent from side surface 18a-18d which side surface is collectively defines the side surface of the stack denoted by reference numeral 24 in FIG. 2. While the illustrated embodiment shows a dual-in-line lead frame 16, it is to be expressly understood that any kind of connection device now known or later devised for providing electrical access into the chip may be equivalently substituted.

[0034] A top connection player, generally denoted by reference numeral 26, is disposed on the top surface of the uppermost chip 10a. Contact layer 26 as a plurality of side leads 28 defined therein which extend to side surface 30 in a manner which will be described in greater detail below. Leads 28 then connect to vertical vias 32 defined within layer 26, which in turn are connected to contact pads 34 defined on the upper surface 36 of layer 26. The manufacture of contact layer 26 is conventional and may include known or later discovered semiconductor lithographic and/or printed circuit board manufacturing techniques. For example, contact layer 26 may be comprised of an insulating printed circuit board substrate in which metallic or conductive leads 28, vias 32 and contact pads 34 have been conventional defined or formed. Contact layer 26 is then conventionally bonded to the top of the stack of chips 10.

[0035] FIG. 3 is a side cross-sectional view of assembly 24 as shown in FIG. 2 after the lateral side portions of assembly 24 have been removed by grinding or any other equivalent means now known or later devised. This operation removes the extending portions of lead frames 16a-16d and portions of packaging 14 of each of the chips 10a-10d to the interior end 38 of lead frames 16a-16d. This leaves only a cross-sectional end portion of bonding wire 20 exposed on side 30 of assembly 24. To the extent then that each of the bonding wires 20 are connected to common contact points 22 within chips 10a-10d, wires 20 can be coupled together by means of defined metallizations 40 which are lithographically disposed on side surfaces 30 of assembly 24. Metallizations 40 are directed vertically upwardly and downwardly along sides 30 to contact leads 28 exposed on the side surface is 30 of layer 26. In this manner contact points 22 within chips 10a-10d are appropriately connected to contact pads 34 defined on the upper surface of layer 26.

[0036] Similarly, chips 10 which include two back-to-back integrated circuits 12a and 12b within a single packaging 14 as shown in the side cross-sectional view of FIG. 4 may be similarly stacked and interconnected. As shown in the side cross-sectional view in FIG. 4 chips 12a and 12b are separated by an interposer layer 42, but are otherwise identical in construction to the chip shown in FIG. 1. Chips 12a and 12b are connected to by their corresponding bonding wires 20a and 20b to a shared to lead frame 16. In this manner the amount of memory, for example, included within chip 10 may be doubled without doubling the number of exterior leads connected to package 14. While the interior wires 20a and 20b are shown in the illustrated embodiment as conventional bonding wires, any electrical connection used or capable of being used to connect an exterior terminal to a semiconductor chip now know or later devised may be equivalently substituted.

[0037] Again a plurality of chips 10a-10d can be stacked as shown in FIG. 5 using the double chip embodiment of FIG. 4. In the same manner as described in connection with FIGS. 2 and 3, the stacked assembly 24 of FIG. 5 can be processed so that lead frames 16 are removed and contacts are then made on the lateral side surface 30 of assembly 24 with bonding wires 20a and 20b of chips 10a-10d. Connection is again made through vertical metallizations 40 to leads 28 and hence contact pads 34 on upper layer 26.

[0038] FIG. 7 is a diagrammatic plan view of lateral surface 30 of assembly 24 which illustrates the general principle by which side metallizations 40 may be used to connect the plurality of chips 10. In a typical integrated circuit chip 10 one or more leads within lead frame 16 will be unconnected or dummy leads, that is, leads which are designated as no connection leads, NC. Only certain ones of the leads will be connected to active circuit or contact points 22 within chips 12. For example in a memory, these active leads will include data input and outputs I/O as well as command signals such as: address latch enable, ALE; chip enable, CE; read enable, RE; write enable, WE; power, VCC; ground, VSS and similar control signals or shared circuit terminals. Since chips 10a-10d are vertically stacked and aligned with each other, similar leads will be vertically positioned above corresponding leads in adjacent chips. For example as shown in FIG. 7 in the vertical line connected to metallizations 40a will be all of the power connect leads VCC for each of the chips with an assembly 24. Similarly connected to vertical metallization 40b shown in FIG. 7, will be all of the addressed latch enable ALE contacts within chips 10 in assembly 24. However, since each of the chips 10a-10d must be separately accessed with respect to various ones of the other input and output data terminals, the separate metallizations 40c are defined on side surface 30 to connect, for example, to a ready/busy R/B terminal 44 in chip 10c. The corresponding ready/busy R/B terminal 44 the next lower chip 10d will then be provided with the separate metallizations 40d. Metallizations 40c and 40d as shown in FIG. 7 include an all lateral portions 46 which extend horizontally across side surface 30 until a region is reached on side surface 30 which has space available for the vertical run of the corresponding metallizations 40c or 40d. Space is available on side surface 30 for such offset vertical runs of lines 40c and 40d and other metallizations similar thereto by reason of the plurality of adjacent no connection terminals, NC, provided in a typical integrated memory chip 10. The removal step between FIGS. 2 and 3 or FIGS. 5 and 6 remove all the metal contact of lead frame 16 which is normally positioned in the NC region of lead frames 16. This leaves then bare passivation of packaging 14 as the exposed side surface 30. No end contacts of wires 20 are provided to adjacent an NC lead and hence the insulated surface 30 is available for deposition of metallizations 40 without interference.

[0039] FIG. 7 is a diagrammatic depiction of an actual assembly 24 of flash memory chips which illustrates that the number of NC connections are sufficient and spaced appropriately to allow ample surface on the side surface 30 for both the commonly shared metallizations 40a and nonshared or chip specific metallizations such as 40d and 40c. It is to be expressly understood that the geometric pattern of NC regions and active wire ends shown in FIG. 7 is illustrative only of one specific memory chip and that the pattern will vary with each prepackaged chip design. However, the principles of the invention are generally applicable to any geometric configuration that might be presented. For example, while NC regions are beneficially utilized, it is also to be understood that the area of the collective lateral side surface of the stack between normally spaced active terminals can be considered as equivalent to an NC region and used for metallizations of a more narrow width down to the limits of reliable manufacturing line width rules.

[0040] It is also to be contemplated as being within the scope of the invention that the metallizations disposed on the collective lateral side surface of the stack may be multilayered with intervening insulating layers between the multiple conductive lines with connecting vias to the terminals, if desired or necessary.

[0041] Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations.

[0042] The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.

[0043] The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.

[0044] Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.

[0045] The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.