Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W90/00—Package configurations
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W70/00—Package substrates; Interposers; Redistribution layers [RDL]
  • H10W70/01—Manufacture or treatment
  • H10W70/05—Manufacture or treatment of insulating or insulated package substrates, or of interposers, or of redistribution layers
  • H10W70/093—Connecting or disconnecting other interconnections thereto or therefrom, e.g. connecting bond wires or bumps
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W70/00—Package substrates; Interposers; Redistribution layers [RDL]
  • H10W70/60—Insulating or insulated package substrates; Interposers; Redistribution layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W70/00—Package substrates; Interposers; Redistribution layers [RDL]
  • H10W70/60—Insulating or insulated package substrates; Interposers; Redistribution layers
  • H10W70/611—Insulating or insulated package substrates; Interposers; Redistribution layers for connecting multiple chips together
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W70/00—Package substrates; Interposers; Redistribution layers [RDL]
  • H10W70/60—Insulating or insulated package substrates; Interposers; Redistribution layers
  • H10W70/611—Insulating or insulated package substrates; Interposers; Redistribution layers for connecting multiple chips together
  • H10W70/614—Insulating or insulated package substrates; Interposers; Redistribution layers for connecting multiple chips together the multiple chips being integrally enclosed
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/071—Connecting or disconnecting
  • H10W72/0711—Apparatus therefor
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/851—Dispositions of multiple connectors or interconnections
  • H10W72/874—On different surfaces
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/90—Bond pads, in general
  • H10W72/941—Dispositions of bond pads
  • H10W72/9413—Dispositions of bond pads on encapsulations
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W90/00—Package configurations
  • H10W90/10—Configurations of laterally-adjacent chips
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W90/00—Package configurations
  • H10W90/701—Package configurations characterised by the relative positions of pads or connectors relative to package parts
  • H10W90/731—Package configurations characterised by the relative positions of pads or connectors relative to package parts of die-attach connectors
  • H10W90/734—Package configurations characterised by the relative positions of pads or connectors relative to package parts of die-attach connectors between a chip and a stacked insulating package substrate, interposer or RDL
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W90/00—Package configurations
  • H10W90/701—Package configurations characterised by the relative positions of pads or connectors relative to package parts
  • H10W90/731—Package configurations characterised by the relative positions of pads or connectors relative to package parts of die-attach connectors
  • H10W90/736—Package configurations characterised by the relative positions of pads or connectors relative to package parts of die-attach connectors between a chip and a stacked lead frame, conducting package substrate or heat sink

Definitions

• the present invention is generally directed to an improved multichip integrated circuit module. More particularly, the present invention relates to a packaging method for electronic integrated circuit chips, particularly very large scale integrated circuit (VLSI) devices, on a substrate also having a polymer encapsulant overlying the chips on the substrate and providing a means for supporting inter chip and intra chip connection conductors. Even more particularly, the present invention relates to a repairable multichip module structure and corresponding repair method; a multichip module structure having high I/O capacity with optimal heat removal through one side and high performance I/O through an opposite side; multichip module structures optimized for speed; multichip module structures having the ability to incorporate an assortment of components of varying thickness and function therein; and multichip modules having an integrated hermetic structure with high I/O count.
• VLSI very large scale integrated circuit
• Multichip modules are divided into two basic structures. In the most common structure, a miniature circuit board is provided upon which integrated circuits are mounted and electrically connected.
• the second multichip module structure involves mounting chips on a substrate, and subsequently providing interconnect to the chips by essentially building an interconnecting circuit board over the top of the chips. These two approaches are referred to herein as “chip on board” for the first approach, and “circuit board above chips” for the second approach.
• the circuit board In the "chip on board” approach, the circuit board is typically fabricated using alumina or silicon substrate, with copper or aluminum interconnection metallization. The most frequently used dielectric is polyimide. Silicon dioxide can be used as a dielectric on silicon substrates with certain thermal advantages. There are three primary methods for making connection from the pads of the chips to the miniature circuit board. These are wire bonding, tape automated bonding or tab bonding, and flip chip or solder bump bonding. Each of these approaches, including their advantages and disadvantages, are discussed below.
• This overlay bridges the gaps between the chips. Again interconnection is provided by forming via holes in the polymer dielectric, metallizing the entire surface of the overlay and pattering the metal to form the interconnect.
• a discussion of the HDI overlay approach is provided by Eichelberger et al. in U.S. Patent No. 4,783,695, entitled “Multichip Integrated Circuit Packaging Configuration and Method,” and U.S. Patent No. 4,918,811, entitled “Multichip Integrated Circuit Packaging Method.”
• the subject invention falls into the category of "circuit board above chips” and most closely resembles the STD approach.
• a multichip integrated circuit package comprises a substrate and a plurality of integrated circuit chips disposed on the substrate.
• the chips include interconnection pads for connecting to other integrated circuit components or for connecting to other pads of the same chip.
• a polymer encapsulant completely surrounds the integrated circuit chips disposed on the substrate.
• the polymer encapsulant has an upper surface, located above the tops of the integrated circuit chips, which has a plurality of via openings therein so as to expose at least some of the interconnection pads on the chips.
• a pattern of electrical conductors is provided on the polymer encapsulant such that the conductors extend between selected via openings so as to electrically connect selected interconnection pads.
• the substrate has a flat upper surface, i.e., no milling is required to provide for the integrated circuit chips.
• the integrated circuit package may further include a dielectric layer overlying the polymer encapsulant with its interconnection conductors disposed thereon.
• the dielectric layer also includes a plurality of via openings therein which are aligned with at least some of the interconnection conductors disposed on the polymer encapsulant.
• a second plurality of interconnection conductors is disposed on the dielectric layer to extend between at least some of the openings in the dielectric layer so as to provide electrical connection with interconnection pattern conductors disposed on the polymer encapsulant.
• the module may be rendered repairable by selecting a solvent-sensitive material for the dielectric layer.
• Additional package enhancements can include the disposition of one or more preprocessed chips on the substrate's flat upper surface.
• a preprocessed chip may include a flex tab, a chip having a series of conductive lands on a top surface thereof for wire bonding thereto, a tiered power and ground busing structure, and/or a termination resistor.
• the present invention is also believed to encompass a particularly novel structure wherein an array of electrical contact pads are provided on the upper surface of the package to provide electrical interface to circuitry external to the package, while the substrate's lower surface provides a thermal interface (for example, to a heat sink) for dissipation of heat generated by the integrated circuit chips. Each interface is, in effect, coupled in a direct line path to the integrated circuit chips contained within the module.
• a plurality of circuit chips is disposed on a flat upper surface of a substrate.
• Each chip includes at least one interconnection pad.
• a low viscosity polymer material is employed to surround the chips and the upper surface of the substrate so that all space between the chips is filled thereby.
• This polymer material is then cured to a hardened, high viscosity polymer encapsulant.
• a plurality of via openings is provided in the polymer encapsulant, each via opening being disposed over an interconnection pad. Then, a pattern of electrical conductors is provided on the encapsulant such that the conductors extend between the via openings so as to electrically connect selected integrated circuit interconnection pads.
• the polymer encapsulant may be lapped to form a substantially flat upper surface which is parallel to the substantially flat upper surface of the substrate; and/or the integrated circuit chips may be lapped, prior to the encapsulation process, to reduce the thickness thereof.
• Specific techniques for repair of a module and for disposition of integrated circuit chips on the substrate are described and claimed herein, as well as additional method features hereof.
• an object of the present invention is to provide a direct interconnection between integrated circuit chips, said interconnection being highly reliable and requiring a least number of interconnections.
• Another object of the present invention is to provide encapsulating layers which can be removed and reapplied to the module so that repair of the assembly is achieved without degrading remaining chip parts which have been tested and found not to be defective.
• Yet another object of the present invention is to provide a method of directly interconnecting circuit chips and other electronic components.
• a further object of the present invention is to provide an interconnect method with very high speed capability due to the minimum capacitance of the interconnect, minimum length of the interconnect and the use of a polymer dielectric.
• a still further object of the present invention is to provide an interconnect method which allows simple attachment of the integrated circuit chip to the substrate for the purpose of heat removal and electrical connection, while accommodating chips of varying thickness.
• Yet a further object of the present invention is to provide an interconnect which reduces the overall system size such that the area of the total electronic system is not substantially greater than the area of the individually incorporated electronic circuit components.
• a still further object of the present invention is to provide an interconnect system with built in flexibility of the interconnection mechanism so as to accommodate thermal expansion and thermal mismatch between system components.
• Still another object of the present invention is to provide a multichip module in a highly planar structure having enhanced resolution, reduced electrical interference to the next level, reduced thermal interference to a heat sink and the capability of stacking modules.
• Yet another object of the present invention is to provide a multichip module having a high input output interface capability on one side with effective heat removal capability on the other side.
• a related object is to provide a high input output interconnect interface capability in a hermetic module wherein heat is removed on one side and I/O electrical connection is provided on the other side.
• an object of the present invention is to provide an interconnection method wherein: the process produces little or no stress on the electronic components with a low potential for damage during normal processing; allows chips to be placed with sufficient accuracy that unmodified art work can be use to pattern direct connection to the chips; allows a variety of materials to be used including thermoplastics and thermal sets while still maintaining a high degree of planarity in the final module; allows the use of completely flat substrates, without the requirement for wells or substrate frames; and allows high volume production.
• FIG 1 is a cross-sectional side elevational view illustrating an advanced multichip integrated circuit module (AMCM) in accordance with the present invention
• Figure 2a is a plan view of a fixture plate with chips symmetrically attached thereto pursuant to the present invention
• Figure 2b is a cross-sectional side elevational view of the assembly of Figure 2a taken along line b- b and overlaid with a protective sealant;
• Figure 3a is a cross-sectional elevational view of one embodiment of a chip recovery process pursuant to the present invention
• Figure 3b is a cross-sectional side elevational view of a second embodiment of a chip recovery process pursuant to the present invention
• Figure 4a is a simplified plan view of one embodiment of a die attach apparatus pursuant to the present invention
• Figure 4b is a cross-sectional elevational view of the die attach apparatus of Figure 4a taken along line b-b;
• Figure 5a is a plan view of a chip/substrate structure positioned within a containment frame used in one embodiment of an encapsulation process pursuant to the present invention
• Figure 5b is a cross-sectional elevational view of the structure depicted in Figure 5a taken along line b-b;
• Figure 6a is a cross-sectional elevational view of an embodiment of a controlled space molding apparatus pursuant to the present invention.
• Figure 6b is a bottom plan view of the space molding apparatus depicted in Figure 6a;
• Figure 7a is a plan view of a second embodiment of a controlled space molding apparatus pursuant to the present invention.
• Figure 7b is a cross-sectional elevational view of the space molding apparatus of Figure 7a taken along line b-b;
• Figure 8a is a cross-sectional elevational view of one embodiment of a multichip integrated circuit module prior to lapping pursuant to one processing embodiment of the present invention
• Figure 8b is a cross-sectional elevational view of the module depicted in Figure 8a after lapping in the depicted apparatus;
• Figure 9 is a cross-sectional elevational view of a multichip integrated circuit module having a preprocessed flexible tab incorporated therein;
• Figure 10a is a cross-sectional elevational view of the module of Figure 9 during an intermediate step in the fabrication thereof;
• Figure 10b is a cross-sectional elevational view of the module of Figure 10a after selective excimer ablation of the polymer;
• Figure 11a is a plan view of a substrate with integrated circuit chips and preprocessed chips having wire bond lands, pursuant to one embodiment of the present invention
• Figure lib is a cross-sectional elevational view of the structure of Figure 11a taken along lines b-b and after encapsulation and metalization thereof;
• Figure 12 is a cross-sectional elevational view of an AMCM structure pursuant to one embodiment of the present invention incorporating a two layer power and ground preprocessed chip
• Figure 13 is a perspective view of one embodiment of the present invention wherein an AMCM with an area pad array is shown for electrical interface to an external circuit (not shown) ;
• Figure 14 is a cross-sectional elevational view showing the structure of Figure 13 oriented upside down and positioned in electrical interface with a conventional printed circuit board, using button contacts, and thermal interface to a heat sink;
• Figure 15 is a cross-sectional elevational view of one embodiment of a speed optimized AMCM circuit pursuant to the present invention.
• Figure 16 is a cross-sectional elevational view of one embodiment of an AMCM structure having certain thick components and circuits in wells, pursuant to the present invention
• Figure 17a is a plan view of one embodiment of a preprocessed chip having a plurality of resistor arrays thereon;
• Figure 17b is a plan view of one embodiment of a resistor array of Figure 17a;
• Figure 17c is an end elevational view of the resistor array depicted in Figure 17b;
• Figure 18a is a plan view of another embodiment of a resistor array assembly pursuant to the present invention
• Figure 18b is a cross-sectional elevational view of a circuit assembly incorporating the resistor array of Figure 18a;
• Figure 19 is a cross-sectional elevational view of an AMCM having a solvent-sensitive layer for circuit repair pursuant to the present invention
• Figures 20a-20d are cross-sectional elevational views of an AMCM at different stages during the module repair process pursuant to the present invention.
• Figure 21a is a cross-sectional elevational view of a chip removal apparatus pursuant to the present invention, shown in a first operative position;
• Figure 21b is a cross-sectional elevational view of the chip removal apparatus of Figure 21a, shown in a second operative position;
• Figure 22 is a cross-sectional elevational view of one embodiment of a hermetically sealed AMCM structure pursuant to the present invention
• Figure 23a is a cross-sectional elevational view of another embodiment of a hermetically sealed AMCM structure pursuant to the present invention.
• Figure 23b is a cross-sectional elevational view of a modified hermetically sealed AMCM structure similar to the structure depicted in Figure 23a.
• ACM Advanced Multichip Module
• FIG. 1 shows a cross-section diagram of the basic structure, generally denoted 10, of the present invention.
• Structure 10 includes a base plate or substrate 12.
• Substrate 12 can be formed from a large variety of materials including glass, metal, ceramic, plastic, silicon, alumina, aluminum nitride, copper clad molybdenum, Kovar ® (a Westinghouse product) and many other materials.
• the base plate does not require machining of grooves or wells of any kind for placement of the integrated circuits, which is a distinct departure from all know prior art approaches.
• the only ' requirement is that the substrate's upper surface 13 be sufficiently flat that the desired degree of planarity can be maintained.
• Integrated circuit chips 14 are attached to the base plate using a thin die attach material 16, which holds the chips accurately in place during processing and which presents a low thermal impedance for heat removal from the chips through substrate 12.
• the exact positioning of chips 14 is governed by features on the chips themselves and not by the accuracy of the saw cut edges of the chips.
• all the chips are thinned to exactly the same thickness so that the top surfaces of the chips are in a plane parallel to upper surface 13 of substrate 12. Specifically, the chips are thinned to a thickness of between 3 and 10 mils, and in a presently preferred embodiment, to a thickness of 6 mils.
• the chips are encapsulated in a polymer encapsulant 18, which, in another novel aspect, is applied in a low viscosity or liquid state and subsequently caused to harden in place. This is distinguished from the STD process described above wherein a thermoplastic is pressed over the tops of the chips and forced at high temperature and pressure into the gaps between the chips to an eventual level above the tops of the chips.
• the polymer material is preferably a formulation based on a UV curable cycloaliphatic epoxy type ZTI1004 obtained from Zeon Technologies of Nashua, New Hampshire. This material allows the polymer to be cured virtually instantly under intense ultraviolet light.
• the polymer top surface 19 lies above the tops of ICs 14, for example, by a thickness of 1 to 2 mils, and is planar everywhere with upper surface 13 of the base plate and the top surfaces of the IC chips 14.
• via holes are formed directly in polymer encapsulant 18 and metal 20 is deposited and patterned such that contact is made thru the via holes to the pads 22 of IC chips 14.
• the metal is patterned to form an interconnect adhered to the polymer surface which interconnects the IC chips. Additional interconnect layers are formed by coating a layer of a dielectric 24, forming via holes in that dielectric to circuitry on the first layer and metalizing and patterning conductors 26 on the second layer to form interconnects between conductors in the first layer. As many layers as required by the circuit can be added in this fashion. Note also that in structure
• the IC chips can be placed right to the edge of the base plate. Further, since the chips are completely surrounded by polymer encapsulant 18 they are protected during processing from coming in contact with the process chemicals.
• the starting substrate or base plate requires very little or no processing. This distinguishes the present invention over other techniques such as the above- described STD approach wherein the substrate must be provided with indentations to align the chips accurately. Also, in the overlay approach, substrates must be machined to various depths to accommodate different thicknesses of chips.
• the only processing step necessary for a given substrate material is to prepare the substrate (12) for good adhesion to the die attach material (16) and the encapsulant material (18) (see Figure 1) . This step varies depending on the type of substrate. By way of example, three different types of substrate are discussed here. These are ceramic, metal and silicon.
• Example 1 In a presently preferred embodiment, ceramic, and specifically alumina, is the substrate of choice. This is because ceramic substrates are readily available, strong and provide good thermal expansion coefficient match to the integrated circuits, which are typically silicon or GaAs. As-fired alumina substrates have been used in this process, but the preferred substrate is a substrate lapped to a specific flatness and thickness specification. Substrates so processed can be obtained from Acumet Corporation of Hudson, Massachusetts. A specification of 25 mils plus or minus 0.2 mil with a 20 micro inch finish gives an ideal starting substrate. Good adhesion is obtained between the die attach material and the polymer encapsulant through the use of a simple acid cleaning step. The step is performed as follows.
• a fresh solution of sulfuric peroxide is prepared by mixing concentrated sulfuric acid and 30% hydrogen peroxide in a 50/50 volumetric ratio. Substrates are dipped in the solution for a period of ten minutes, subsequently rinsed in DI water and spun dry in a spin rinser.
• Example 2 In the case of metal substrates, the metal is cleaned according to various acid cleaning steps well known in the art. For example, molybdenum may be cleaned in an acid pickling solution consisting of 10% nitric acid in DI water. Copper clad molybdenum may be cleaned by brush or pumice cleaning followed by a dip in a solution of Nutra-Clean ® (available from Shipley Chemical Company of Newton, MA) .
• a dip of one minute is usually sufficient, followed by rinsing in DI water and spin drying.
• a 200 to 1,000 angstrom coating of chrome or titanium is sufficient for this purpose.
• the chrome may be applied by electroplating techniques and the titanium by sputtering.
• Silicon substrates are usually coated with a coating of adhesion promoter such as hexamethyldisilane. Methods for coating by dip or vapor phase are well known in the semiconductor art.
• adhesion promoter provides a bridge between the glass characteristics of the silicon dioxide on the silicon surface and the organic molecule of the polymer die attach or encapsulant material.
• the typical range of thickness for substrates is 25 to 50 mils. This gives good thermal conductivity and" adequate strength for most applications.
• Metal substrates as thin as 1-5 mils may be used where the application calls for a structure of very high volumetric efficiency. Die Thinning
• the subject invention requires that IC die be thinned such that they are all the same thickness and that the thickness be in the range of 3 to 10 mils for optimum reliability.
• IC die are typically available commercially already sawed, and often placed in waffle packs. Die from different vendors are typically of different thickness, and virtually no commercially available die are available in thicknesses as low as 3 to 10 mils.
• the following description discloses a method for die thinning and die recovery which produces die of very uniform thickness regardless of starting die thickness or size.
• the disclosed inventive approach completely protects the active die surface during the thinning and subsequent recovery operations.
• the disclosed invention is amenable to high volume batch fabrication techniques and has experimentally been operated with batch yields of 100%.
• the chip thinning process starts with a fixture plate 30 (see Figures 2a & 2b) which is used to hold the chips 14 throughout the processing.
• this fixture plate 30 is a 0.090" thick glass plate.
• the major requirement of this plate is that it be flat to the desired tolerance to assure consistent processing of the chips.
• Commercially available window glass can meet this requirement.
• the glass should be lapped flat to a thickness tolerance of within 0.1 mil.
• Adhesive 32 is now coated on one surface of the plate, which can be achieved by spin coating or spray coating techniques. An adhesive material suitable for spin coating is disclosed below.
• an epoxy resin of high molecular weight is used, such as ECN1229, which has a melt point of approximately 100°C.
• This resin is mixed with an equal portion by weight of cellosolve acetate solvent.
• FC430 a fluoro-carbon wetting agent from 3M Corporation of St. Paul, MN.
• FC430 a fluoro-carbon wetting agent from 3M Corporation of St. Paul, MN.
• the resulting mixture is filtered through a one micron filter to remove all particulate above the one micron level.
• This mix is then spun at a spin speed of 1,500 rp for twenty seconds.
• the plate is baked on a hot plate for three minutes at 150°C followed by five minutes at 220°C. This removes essentially all of the solvent and leaves the surface dry to the touch at room temperature.
• the resulting thickness of the adhesive is approximately 10 microns.
• the plate 30 with adhesive 32 is then heated to a temperature of 100"C and chips 14 are placed face down in a symmetrical pattern on the plate.
• Chips 14 can be placed by picking them from a waffle pack using a vacuum pencil or preferably by using the pick and place machine described in the die attach section of this disclosure with reference to Figures 4a and 4b. Note that once the chips have been placed the assembly can be cooled and the chips are held rigidly by the adhesive. Note also that the adhesive material is extremely uniform and very little pressure is necessary to completely wet the surface of the chip with the adhesive such that the chip surface is completely protected by the plate 30 and sealed by the attachment adhesive 32. Because the attach adhesive is low viscosity during the attachment operation, it cannot place any force on the chip surface.
• Chips 14 are preferably placed symmetrically on the fixture plate 30 because this aids in balancing the actual thinning operation which will be described.
• the actual thinning operation is done on a commercial lapping machine such as a Spitfire SP-ML-15. More than one fixture plate can be accommodated at one time in the lap machines.
• stops 34 of a very hard material such as alumina are mounted on the fixture plate 30, preferably in the corners of the plate. These stops are of a thickness equal to the final thickness desired for the chips being thinned. The chips continue to be lapped until the stops are encountered at which time the lapping slows dramatically because the very hard alumina is lapped slowly if at all relative to the silicon or GaAs chips.
• adjustable lapping stops with diamond tips are used. These are attached to the lap pressure plate (not shown) and then adjusted so that the sum of the desired chip thickness and the thickness of the fixture plate is equal to the extension of the diamond stop.
• Such pressure plate fixtures provided with diamond stops are available from Lap Master Incorporated of Chicago, Illinois.
• the entire assembly is coated with a sealing layer 31, which prevents any material from being lodged under chips 14.
• the sealing layer on top of chips 14, which is lapped away during the processing, also provides buffering on the chip edges.
• a lapping media which has been experimentally used with success consists of 300 milliliters of 5 micron SMA powder available from Spitfire mixed in one gallon of vehicle which consists of 1/3 SAC-5 (available from Spitfire) with the residual water.
• SAC-5 available from Spitfire
• For a 15" lapping wheel a speed of 48 revolutions per minute was used.
• a pressure of three pounds per square inch was used for silicon chips. Chips could be consistently thinned from approximately 20 mils to 6 mils plus or minus 0.1 mil within a time period of 12 minutes.
• FIGs 3a & 3b show two embodiments of the recovery apparatus.
• Apparatus 40 includes a container 41 and a recovery vessel 42. Note that the fixture plate 30 is positioned so that the chips 14 fall away from the plate to the bottom of the recovery vessel 42. Once the solvent 44 has dissolved the attach adhesive in this way the chips never come in contact with a hard material during any portion of the cycle.
• a side protection material (31) is applied, preferably by spinning techniques. This material further seals the chips to prevent any possible lapping material from contacting the active surface of the chip.
• the same material as used for the adhesive can be used, along with a spin speed of 800 rpm and drying temperature of 150°C.
• the advantage of this approach is that the sealant can be easily dissolved in a solvent such as acetone. Dissolving the sealant also removes any of the lapping media which may have deposited on the fixture plate surface. This keeps both the chip active surface clean as well as the fixture plate.
• a clean fixture plate is advantageous in a presently preferred chip recovery embodiment ( Figure 3b) .
• the fixture place is heated above the melting point of the adhesive (e.g., 120°C).
• the fixture plate 30 is placed on a waffle pack 46 such that the chips 14 are in the wells 48.
• the plate is drawn slowly across the waffle pack 46. Chips are prevented from moving by the walls 50 of the waffle pack as the carrier plate 30 moves away. As a result the chips quickly loose adherence to the fixture plate and fall into the waffle pack.
• the flow dynamics are such that the adhesive adheres to the chips and protects them. Since the fixture plate is clean there is no danger of damage to the chips due to foreign substances. If the waffle pack is further provided with holes in the bottom and a cover with holes, chips can be soak cleaned in acetone (not shown) .
• the subject invention depends upon placing die with sufficient accuracy that the pads of the chips line up with fixed positions for via holes and interconnect pads. By doing this the need for adaptive lithograph is eliminated and standard mask type processing can be used. As described initially herein, it is necessary both to place the die accurately according to features on the die and to provide a means by which the die remain in place without swimming or moving by capillary attraction during the full curing process.
• Figures 4a & 4b show plan and cross-sectional elevational views, respectively, of a die attach apparatus 52 pursuant to the present invention.
• Apparatus 52 includes a high accuracy XY table 53 whose position is ultimately controlled and monitored by an AT type personal computer (not shown) .
• XY table 53 Mounted on XY table 53 are two rotational adjustment stages 54a, 54b.
• On one stage 54a is provided a chip positioning fixture which consists of a flat plate 56a with a hole in the center connected to a controllable vacuum source 57. Chips (e.g., 55) are placed on this alignment stage 54a and held in position by the vacuum 57.
• the second stage 54b holds a plate 56b which is machined to accept the desired substrate 58.
• Shims are provided so that the height of the active portion of the chip on the alignment stage 54a is the same as the height of the active portion of the chip 55 when placed on a substrate 58 which is mounted on the substrate alignment stage 54b. This is done to reduce the number of times that the focus of an alignment microscope must be changed during operation.
• a bridging structure 60 is provided over the top of the XY stage which is used to hold an alignment microscope 62 and a vacuum die pickup 64.
• the throw of the XY table 53 and the position of the alignment microscope 62 and die pickup tool 64 are chosen so that all points on the die 55 and all point on the substrate 58 can be placed under both.
• the alignment microscope 62 is mounted such that it can be focused and the focusing direction is directly perpendicular to the plane of the chip 55 on the alignment stage 54a and the substrate 58.
• the die pickup tool 64 is mounted on a two stage motion device 68.
• the first stage 70 is mounted rigidly to the bridge 60 such that it may be moved vertically in a direction perpendicular to the plane of the chip 55 on the alignment stage 54a and the substrate 58.
• a second stage 72 which holds the actual alignment tool 64, is mounted on the first stage 70. While the first stage 70 is raised and lowered under control of a micrometer, the second stage can move freely up and down in the same direction as the first stage but there is no hard positioning control 74, such as on the first stage 70. As such the second stage 72 is held against the lower stop 74 until the pick up tool is lowered by the first stage 70 and comes in contact with the top of the chip 55. At this point, the second stage 72 begins to rise and the weight of the second stage 72 is placed on the top of the chip 55 through the pickup tool 64.
• a controlled pressure equal to the weight of the second stage 72 (e.g., 400 grams for a quarter inch chip) is placed on the chip 55 regardless of the position of the first stage 70 until the second stage engages the stop on the first stage and lifts it vertically.
• the amount of weight in the second stage can be adjusted by placing weights on the second stage 72.
• the actual operation of the die attach apparatus proceeds as follows: A substrate 58 which has been coated with the die attach material is placed on the substrate alignment stage. (A full description of the die attach material invention is given in a subsequent portion of this section.)
• the necessary characteristic of the die attach material is that it be uniform without large particulate and that it be sticky.
• the die 55 to be placed is placed on the die alignment stage 54a and the vacuum hold- down 57 to that stage is energized.
• the first step in the process is to rotate the substrate 58 until it is square with the chosen fiducial marks 76 on the substrate. All subsequent chip placement will be in relation to the chosen fiducial marks 76.
• the next step is to note the exact position of the reference fiducial mark. This is done by placing the reference fiducial mark under the cross hair of the alignment microscope 62 and interrogating the XY table 53 control to determine the absolute position of the reference mark. The substrate reference mark position is saved.
• the chip 55 to be placed is made square with the table motion by aligning two pads on the chip whose position is known. Once the chip is square, a note is made of the exact position of the reference pad. A file containing data from a measurement of this chip is used to determine the relative distance from this reference pad to the center of the chip. A measurement of the distance from the center of the cross hair on the alignment microscope to the center of the pickup tool 64 is known. The stage 54a then moves from the position with the reference pad under the microscope cross hair to the position where the center of the chip is directly under the center of the pickup head.
• the pickup head 64 is then lowered until it makes contact with the top of the chip 55. Lowering continues until the full weight of the second stage 72 is transferred through the pickup head to the top of the chip. At this point the vacuum for the pickup head is turned on and the vacuum 57 for the chip alignment stage 54a is turned off. The pickup head is then raised until the chip is picked up from the alignment stage and is sufficiently high to clear the substrate and any chips mounted thereon.
• the exact position of the substrate reference fiducial has been noted. As an option, this value can be input to the XY table controller and the substrate reference fiducial placed directly under the cross hair of the alignment microscope.
• a section of memory in the control computer is preferably programmed with a table of the desired position of the chip reference pad relative to the reference fiducial mark on the substrate.
• the chip is now being held by the pickup tool with the center of the pickup tool preferably aligned to the center of the chip.
• the substrate is moved to a position which represents the substrate fiducial directly under the chip reference pad plus an offset inserted from the table of relative positions for the given chip reference pad relative to the substrate fiducial.
• the chip With the substrate properly positioned under the chip, the chip is lowered until it contacts the surface of the substrate which is covered with the die attach adhesive. Lowering of the mechanism continues until the full weight of the pickup tool second stage is applied to the top of the chip. This position is held for a period of time (preferably five seconds) to give good wet out to the bottom of the chip by the die attach adhesive. This exact process is repeated for each chip to be placed.
• a material system which has several advantageous properties. Specifically, it can be applied by spin or spray coating techniques to achieve a very thin uniform coating. It can be dried free of solvent but remains extremely tacky or sticky in order to hold chips in place. It an be cured either by UV light or high temperatures; and its viscosity reduces and wet out improves with increases in temperature.
• a presently preferred embodiment of the method of die attach is as follows.
• a clean flat substrate is used as the starting point.
• the die attach material is spun at 1,500 rp for a period of 20 seconds.
• the substrate with material is dried on a hot plate at 100°C for a period of 7 minutes.
• the die attach material is approximately 7 microns thick and very sticky although free of solvent.
• the die are placed as described in the preceding paragraphs.
• the material is sufficiently thin in coating and highly viscous such that there is no interference between adjacent die. That is, little material is squeezed from under the die and forced up between the adjacent die. Although some is, it is not sufficient to put enough shear force on the die to cause them to move.
• the substrate is exposed to UV light. A total energy of 5 joules per square centimeter is used. This cures the die attach material solidly around each die and cures the material slightly under each die due to light scattering effects.
• the substrate is placed on a hot plate at a temperature of 150"C for a period of 5 minutes.
• the die attach material can be baked at a temperature of 220°C for a period of 20 minutes. This effectively totally cures the die attach material.
• This last post bake step is unnecessary if subsequent processing will eventually lead to a post bake step of 220°C for a period of 20 minutes or more.
• the material system is free from solvent. In this way baking at high temperature can occur without evolution of solvent and forming of blisters under the die.
• the thermal curing mechanism is chosen so that the temperature of the die attach material can be raised after coating to a sufficient temperature to allow evolution of all solvent without curing the die attach material. Note also that because the coating is very thin and exposed to the atmosphere during the drying process that effective and thorough solvent removal can occur.
• This system is mixed and baked in an oven at 100°C for a period of one hour twenty minutes. In order to effect mixing and not allow the material to cure in concentrated locations, the mix should be shaken every fifteen minutes during cure. After the above mixture has cooled to room temperature, 1.6 grams of Cyracure UVI6974 (available from Union Carbide Corporation) is added. This is the ultraviolet curing agent. The mixture is then filtered through a filter of 3 microns. This removes any particulate above 3 microns and prevents particulate from causing chip damage and offset or improper wetting of the bottom of the chips.
• chips have been experimentally placed relative to a fiducial on the substrate using only a flat substrate without pockets or alignment marks machined in it.
• the chips were placed by alignment to features on the chips themselves.
• the maximum degree of misplacement for any chip was less than 10 microns. This clearly allows the use of present IC chips with bond pad sizes of 75 microns square and allows expansion to future generations of chips with even smaller bond pad dimensions and spacing.
• This material is actually a solid at room temperature but it approaches water like viscosities at temperatures, e.g., 100°C. At these temperatures the material takes approximately one hour to reach the gel state. At temperatures of 150"C to 180°C the material cures in approximately ten minutes.
• a substrate 80 with chips 82 attached is provided with a frame 84 which is at least the same thickness as the chips
• This frame 84 acts as a dam for containing the encapsulant material (not shown) .
• the frame is permanently attached (e.g., via an adhesive 83) to the substrate at the same time the die are placed.
• the frame can be made of either alumina or silicon.
• the frame is temporary and consists of high temperature tape adhesively bonded to the substrate.
• a suitable high temperature tape is M797 available from CHR Industries of New Haven, CN.
• the substrate 80 with frame 84 attached is placed on a hot plate (not shown) at 100 ⁇ C.
• the gap fill material is introduced to any free portion of the substrate within the containment frame (and not directly on top of the chips) .
• the first encapsulant is to only fill in the spaces between the chips, afterwhich a second layer is placed over the top.
• the low viscosity encapsulant material flows to all points within the containment frame. It is necessary to keep the substrate level and to provide the encapsulant material in such quantity and at such a rate that the material does not exceed the height of the IC chips. If the material is applied at too high a rate, then a buildup in a portion of the substrate will occur and the tops of the chips will be covered by the gap filling material.
• a convenient way to assure that the correct amount of gap filling material has been applied is to use a high accuracy scale which weighs the difference between the substrate plus hot plate and the added gap filling material.
• the substrate is transferred to a
• any desired dielectric material can be sprayed or spun over the tops of the chips to complete the encapsulation.
• SPI129 a silicone polyimide available from MICRO SI of Phoenix, Arizona can be spun at a speed of 2,000 rp for a period of twenty seconds and dried ten minutes at 100°C, ten minutes at 150°C and twenty minutes at 220'C.
• a UV curable encapsulant material such as ZTI1004 available from Zeon Technologies of Nashua, New Hampshire can be used. In this approach material is actually filled to above the chip line.
• This material is liquid at room temperature and need not be raised in temperature to achieve a sufficiently low initial viscosity.
• the back surface of the substrate is radiated with UV light.
• the alumina substrate allows a substantial portion of the UV light to pass through to the polymer.
• the silicon chips however absorb the UV energy and do not allow any UV to pass in the area where the chips are. This results in selective curing in all the area around the chips, that is, in the gaps between chips and not in the area above the chips.
• Acetone or other suitable solvent is then used to wash away the encapsulant material above the chips.
• an overcoat layer is applied which coats over the tops of the chips and over the gap filling material. This eliminates the need for any high degree of care in filling the gaps while trying to avoid covering the chips with encapsulant material.
• a substrate with chips attached thereto is provided with a containment frame which is slightly higher than the tops of the chips (e.g., see Figures 5a and 5b).
• Encapsulate material is applied to the substrate by doctor blading techniques.
• a bead of material is dispensed at one end of the substrate.
• a doctor blade or straight edge is drawn across the substrate. Because the frame sits higher than the tops of the highest chip.
• the material is drawn across the substrate to a height just slightly higher than the tops of the chips.
• Curing is then effected by heat or UV light depending on the material used. Either ZTI1004, which is UV curable, or ZOL3A, which is heat curable, can be used. These materials are both available from Zeon Technologies of Nashua, New Hampshire.
• the frame for material containment can be a temporary frame, as described in the previous section, or it can be a permanent frame which is attached to the substrate at the same time that the die are attached.
• the substrate 90 with the chips 92 attached thereto is spaced a precise distance away from a flat plate 94 by spacing elements 91.
• the distance 'd' is set so that the tops of the chips are between 1 and 2 mils away from the flat plate 94.
• a sealing material 93 wraps around the three sides of the structure.
• Encapsulant (not shown) is then introduced at one end 96 of the flat plate 94.
• Figures 6a and 6b show a plan view and cross-sectional elevational view of the controlled space mold.
• Z0L3A can be used as the encapsulant. If this material is used, then the mold must be heated to a temperature exceeding 150°C for a period of ten to fifteen minutes to solidify the encapsulant material.
• the apparatus is cooled and the substrate removed.
• conventional mold release agents such as silicone or fluorocarbon can be used.
• the flat plate can be a glass plate, which has the attributes of a high degree of flatness, thermal stability and ready availability.
• a novel variation, depicted in Figures 7a and 7b, of the controlled space molding technique involves the use of a unique molding apparatus 100 coupled with a UV curable encapsulant (not shown) .
• the substrate 102 is held by a vacuum holddown chuck 104. UV curing material is introduced to the assembly at one end of the substrate.
• a glass plate 106 attached to a hinge apparatus 108 is hinged down over the top of the substrate.
• Precision stops 110 between the plate 106 and the substrate 102 ensure that the glass plate is held between 1 and 2 mils above the tops of the chips 112. As the plate is hinged down the encapsulant material is forced across the entire substrate. By dispensing the proper amount of material, excess material squeezed out around the edges of the substrate can be kept to a minimum. Because of surface tension effects, the encapsulant material stays in contact with the glass plate. The encapsulant material is now exposed using UV light which is irradiated through the glass. A mask is used to prevent UV light from curing material beyond the edge of the substrate.
• the encapsulant When the curing process is completed, the encapsulant is cured in all areas above the substrate but will not be cured in those areas where encapsulant material was squeezed out beyond the substrate.
• the uncured encapsulant material can easily be washed away in a solvent such as acetone and the encapsulated substrate removed from the glass plate.
• silicon or fluorocarbon release agents can be applied to the glass plate.
• An acceptable encapsulant compound, which is liquid at room temperature and UV curable, is ZTI1004 available from Zeon Technologies of Nashua, New Hampshire. The end result is an encapsulated substrate wherein the encapsulant comes to the edges of the substrate without the use of containment frames.
• the top of the encapsulant mimics the surface of the glass which is extremely flat and free of defects.
• the process can be conducted at a high rate of speed with little wastage of material.
• the energy required to cure the substrate to the point that it can be removed from the glass plate is one joule per square centimeter, at a wave length below 330 nanometers.
• quartz instead of a sodalime glass plate it is preferred to use quartz due to its high transmission at the UV wave length used.
• the apply, cure and lap method is especially desirable because it allows the use of essentially any material, it gives a very precise control over flatness and parallelism of the encapsulant surface and it allows the application process and the precision thickness control to the separated.
• a solvent born encapsulant material such as Silicone Polyimide type SPI135. Since this material is in a solvent the removal of the solvent in any of the other known encapsulation techniques would result in significant shrinkage in areas where the encapsulant was thick and less shrinkage in area, such as over the tops of the chips, where the encapsulant was thin.
• the desired high degree of planarity of the encapsulant could probably not be achieved, i.e., without the apply cure and lap process of present invention.
• the material can be applied by spin coating at a very low speed and subsequently baking to remove solvent. Although the resulting encapsulant surface after baking would not be flat, the lapping operation would achieve the required degree of flatness as long as the drying operation resulted in encapsulant material sufficiently above the chips on all parts of the substrate. It can be seen that the effects of shrinkage in the encapsulant material due to the drying and curing operations can be totally eliminated by the use of this technique. Another advantage of this technique is the simplification of the process of applying the encapsulant material.
• the ZTI1004 is applied to a substrate with chips attached and spun at a speed of 400 rpm for 15 seconds.
• the substrate with encapsulant applied is then placed under UV radiation and radiated with five joules per square centimeter of UV energy.
• the entire process can be accomplished in under a minute. Because the material is substantially above the tops of the chip and because it is a low viscosity liquid, trapped bubbles which can occur in other processing steps or particles due to mold surface or doctor blade contamination are eliminated.
• the process is extremely simple to perform with very wide margins of processing error.
• the lapping abrasive is SMA5 mixed 600 millimeters of SMA5 with one gallon of vehicle consisting of 1/3 SAC5 and the rest water.
• a 15" Spitfire lapping machine with a wheel speed of 60 rpm, a consistently flat and planar encapsulant surface can be achieved with a thickness of l mil above the tops of the chip by lapping for a total time of twelve minutes.
• a so called hard coat layer can be applied. This is done by spinning the desired dielectric material usually at a relatively high speed or spraying a relatively thin coat of the hard coat material to fill in the inherent scratches caused by the lapping process.
• ZTI1004 is also used as the hard coat. This is spun at 6,000 rpm, UV exposed with 0.5 joule per square centimeter of UV energy and then baked for five minutes at 150"C and twenty minutes at 220°C. The lapped surface provides for excellent wetting and flow out.
• Three different methods can be used to form via holes in polymer dielectrics. These are reactive ion etching, photo patterning and laser ablation.
• the preferred method of metallization is sputtering since it gives the ability to clean oxides from the metal pads as well as giving excellent adhesion of metal to polymers.
• materials prepared as described in the encapsulation and via formation sections were placed in a Balzers Model 450 sputtering system. The following conditions were used: The unit was pumped to a starting pressure of 1E-6 torr. Argon was admitted at a pressure of 1 mtorr and a flow rate of 10 cc per minute. The substrates were first RF back sputtered at a power level 1,000 watts for a period of three minutes. This was done to remove oxide from the surface of the metal pads in the vias.
• a titanium target was cleaned at a power level of 2.2 kilowatts using a Magnatron sputtering unit. A cleaning time of one minute was used. Subsequently, titanium was sputtered on the part for a period of eight minutes. This gave a coating of approximately 1,000 angstroms thick.
• copper was sputtered at 2.2 kilowatts using a Magnatron sputtering head. The copper target was first cleaned for a period of one minute and then copper was sputtered on the substrate for a period of forty minutes. This gave a copper thickness of 2 microns. This was followed by again sputtering titanium to a thickness of 1000 angstroms giving a titanium-copper-titanium sandwich. Two microns of copper is sufficient for a large majority of applications.
• An alternative technique which allows for thicker metallization, is to back sputter for three minutes and sputter titanium for eight minutes as described but then sputter copper for eight minutes also. This gives a copper thickness of approximately two to three thousand angstroms.
• the substrate is removed from the sputtering chamber and the copper is built up by electroplating.
• the cooper is plated to the substrate at a plating current of 35 amps per square foot. Electroplating at the prescribed current density for ten minutes gives a 6 micron thick copper coating. Plating for twenty minutes gives a 12 micron thick coating. Twelve microns is desirable for power supply and certain I/O pad configurations.
• a top layer of adhesion metal is applied either by electroplating (e.g., chrome) or by sputtering (e.g., chrome or titanium). Titanium sputtering proceeds as described before with a pump down followed by a cleanup for three minutes followed by cleaning the target for one minute followed by eight minutes of sputtering of titanium as previously described.
• electroplating e.g., chrome
• sputtering e.g., chrome or titanium
• Patterning is conducted by spin coating a resist, patterning the resist and then etching in suitable etchants.
• suitable etchants As an example, type AZP4620 resist can be used. This resist is spin coated at 2,000 rpm for twenty seconds, and then dried at 100°C for ten minutes. This is a positive acting resist which can be exposed through a mask with 200 mjoules per square centimeter of energy. The resist is then developed in a 0.1N solution of sodium silicate. Assuming a metallization of titanium- copper- titanium, the etch process takes place as follows. First, a dip in TFT etch available from Transene Company of Rowley, Massachusetts diluted twelve to one with water. This etch takes approximately twenty-two seconds.
• Additional interconnect layers are fabricated by spin or spray coating a dielectric material onto the module, forming via holes, applying metallization, and patterning that metallization.
• the only step which has not been described above is the step of applying a new dielectric layer.
• epoxy this is done as follows. The epoxy material is dipped in a concentrated sulfuric acid solution for a period of ten seconds followed by a thorough rinsing for one minute in DI water and spin drying or hot propanol drying. At this point, ZTI1004 is spin coated at 3,000 rpm and UV cured using an energy of one joule per square centimeter. Following the UV curing, via holes are formed by excimer laser and metallization is added and patterned. A post bake of the dielectric material can be done either before or after metallization and patterning. Clearly, the process is speeded up if for each dielectric layer there is no post bake, i.e., until the end of the process.
• An alternative interlayer dielectric is to use the above-described VAQS material from DuPont specially modified to eliminate glass filler and pigment. This material is spin coated at a speed of 2,000 rpm, dried for ten minutes at 100°C and then photo exposed with an energy of 100 mjoule per square centimeter. Material is then developed in a solution of 1% sodium carbonate for a period of two minutes. Post curing is achieved by exposure to two joules per square centimeter of UV energy, followed by a twenty minute bake at 220°C.
• this section various variations to the basic structure will be disclosed which allow for optimization or improvement in a particular area.
• this section will cover structures and methods for input output, connection to the next level, optimization for high speed, and repairable and hermetic structures.
• integrated circuits are not the only type of electronic component that can be interconnected by this technology.
• specially fabricated structures are disclosed which can be incorporated on the substrate and interconnected along with the rest of the ICs on the substrate, to add to the overall functional capability of the multichip module.
• the advantage of this approach is that special processing can be conducted separately on the preprocessed circuits, and the advantage of that special processing can be enjoyed by the multichip module.
• four preprocessed circuits and their incorporation in the basic AMCM will be discussed. These are flexible tab interconnect, wire bond lands, leadframe assembly and power distribution system.
• Figure 9 shows a cross-section elevational view of a flexible tab 130 incorporated in the basic advanced multichip module 132.
• the basic flex circuit is of a type available, for example, from Sheldal Incorporated. Many such flex circuits can be fabricated at one time and subsequently cut into appropriately sized strips. These strips when incorporated in the multichip module can be used as a flexible high I/O count interconnect for connecting the multichip module to a printed circuit board, for example.
• the preprocessed flex interconnect 130 is incorporated on the multichip module at the same time the chips 134 are placed on the substrate 136.
• the tab interconnect is adhesively bonded 138 to the substrate base 136.
• the top surface of the tab interconnect is essentially planar with the top surface of the integrated circuit chips (see Figures 10a and 10b) .
• circuit layers 140 that interconnect the integrated circuit chips can simultaneously provide interconnect to the input output tab.
• Care must be taken to provide a means for keeping the outer portion 131 of the tab 130 free of encapsulant and dielectric material, and protected throughout the processing. This can be done by depositing a layer of metal 142 to a thickness of approximately 1 micron and patterning that metal over the area where protection is desired.
• the processing then continues as described above with the result that the entire surface of the tab is covered with the encapsulant polymer 144. These can subsequently be removed by excimer laser ablation.
• Figure 10b shows the part after excimer ablation of the polymer.
• Figure 9 depicts the finished product after selective etching of the aluminum metallization.
• the addition of the patterned aluminum protection layer can be achieved as follows. An array of processed tab circuits is placed in the Balzers 450 sputtering unit. After a thirty minute pump down to 1E-6 torr. , argon is admitted to the chamber to a pressure of 1 mtorr. The aluminum target is cleaned by sputtering at 2.2 kilowatts for a period of one minute. Then aluminum is sputtered on the array of tab interconnects for a period of thirty minutes, which gives a coating thickness of 1 micron.
• the aluminum is then patterned by spin coating AZP4620 resist at 2,000 rpm for a period of twenty seconds and then baking the structure in an oven for a period of twenty minutes at 95°C. Exposure conditions are 120 mjoule. Development is in 1% sodium silicate for a period of thirty seconds. The aluminum is etched in a 2% sodium hydroxide solution and the resist removed by a dip in acetone for one minute followed by a dip in hot methanol for one minute. The tab circuit is then allowed to dry. At this point, individual tab circuits are cut from the array using conventional shearing techniques.
• a second example of a preprocessed circuit is a series of lands for wire bonding. These circuits are fabricated in a batch process and sawed or laser scribed in the same way as integrated circuit chips. They are placed during the die attach process and subjected to the normal processing steps. Typically, an alumina substrate is used with aluminum deposited to a thickness of 1 to 2 microns, and patterned to form landing areas for wire bonding.
• Figures 11a and lib show plan and cross-section elevational views of the module 150 with the wire bond lands 152 and landing areas 153. Note that electrical connection 156 is made by the exact same processing that makes electrical connection to the pads of the integrated circuit chips 154.
• FIG. 12 shows a third example of a preprocessed circuit. This is a two tiered power and ground distribution system. Again the power and ground strips 160 and 162 are fabricated separately from the fabrication of the advanced multichip module 164. Relatively complex power and ground busing structures can be fabricated to allow for a substantial number of layers of power and ground busses 160 and 162 made with thick conductor material.
• the die 166 are placed and can be used to provide stiff power and ground distribution of a number of power levels without increasing the number of signal layers which are required.
• this invention allows signal layers to provide interconnection pathways above the power and ground distribution circuits.
• the next interconnect level can, for example, be a simple conventional printed circuit board.
• a structure is provided in which the entire top surface of the multichip module can be covered with an array of input output pads which make contact to circuitry in the multichip module. Connection from this array of pads can be made to a conventional circuit board by using, for example, button contacts available from Cinch Incorporated.
• Button contacts are intended for interconnection from one circuit board to another circuit board or for interconnection from a package containing an integrated circuit to a circuit board.
• the particular advantages of the disclosed structure are that the input output pads can cover the entire top surface of the multichip module.
• Figure 13 shows an area pad array structure 172 incorporated in the multichip module 170.
• Figure 14 shows this structure in a cross-sectional elevational view making connection to a conventional printed circuit board 174 using button contacts 176.
• An aspect of the invention which is thought to be novel is the ability to provide the array of input output pads 172 over the entire top surface 171 of the module 170 without requiring any special separate areas to accommodate the interconnected components of the module.
• areas are set aside for interconnect of the components with wiring to peripheral areas of the circuit board where the button contacts are provided.
• pads are placed directly over the interconnected components 173. If, for example, the overlay type approach were used for this structure, pads placed between components could not accommodate the forces of the button contacts because the bridging between adjacent electrical components does not render the structure capable of supporting the button contact forces.
• Another aspect of the disclosed invention which is believed novel is the ability to provide contact pads having a very short interconnect length to the electronic circuitry.
• the distance from a pad to the associated interconnected electronics is on the order of several mils.
• the wiring lengths required in systems where the pads and the components are separated must necessarily be large fractions of an inch (boards with pads on periphery of components) .
• Another novel aspect of the structure is that the I/O pad array which makes interconnection to the circuit board is on one side of the module while the surface of heat removal is on the other side. This can easily be seen in Figure 14. Since the chips are mounted directly on a flat substrate heat can be easily removed through the opposite side 180 of the substrate 182. This is detailed in an earlier part of this disclosure.
• the novel aspect of the structure of Figure 14 is that heat removal takes place in a direct path from the chips 173 through the substrate 182 to a heat sink 184 while input output takes place in a direct path but in the opposite direction from the chips 173, to interconnect 181, to input output pads 173.
• Circuit board approaches preclude direct thermal connection on the same side as the input output.
• the input output distance must be sacrificed or a minimum thermal path length to the heat sink must be sacrificed.
• Another novel aspect of the presently disclosed structure is the ability to provide and make contact to internal test points. The ability to contact internal test points has been used extensively in testing conventional circuit boards. So called bed of nails testers make contact to pads connected to circuit runs on all points of a printed circuit board.
• the structure is capable of direct interconnection to a conventional printed circuit board through the use of button contacts.
• the area array of pads can be provided over the entire surface of the multichip module including the area both above and adjacent to the electronic components. Additionally, the input output interface and the thermal interface are both optimized and positioned on opposite sides of the multichip module.
• the processing steps of providing a substrate, covering the substrate with a die attach material, accurately attaching die which are thinned, encapsulating those die, providing holes through the encapsulation to the pads of the integrated circuit chips, metallizing, and patterning the metallization to provide interconnection between the integrated circuit chips has been disclosed above.
• the following additional steps are necessary in a given layer of the interconnect in order to provide input output pads in a array form which can cover the entire surface of the multichip module.
• a layer of dielectric is applied to the module by spin or spray techniques.
• ZII1004 can be used at a spin speed of 2,000 rpm for a period of twenty seconds. This material is then cured under UV light with an energy of 2 joules per square centimeter. The material is postbaked at 150"C for five minutes and 220°C for a period of twenty minutes. Following postbake, via holes are formed to the interconnect layer beneath using an excimer laser as described previously.
• Metallization is applied (as describe earlier) in which titanium is sputtered to a thickness of 1,000 angstroms and copper is sputtered to a thickness of 3,000 angstroms.
• the module is removed from the sputtering chamber and coated with a thick coating of photopatternable resist.
• a negative acting resist type F360 can be used. This material is available from Chem Line Incorporated. The resist is spun at a speed of 1,500 rpm for a period of twenty seconds. The resist is then baked for twelve minutes at 100°C, and subsequently patterned using an exposure energy of 100 mjoule per square centimeter and a development time of 100 seconds in a 1% sodium carbonate solution. This process leaves the area where pads are desired exposed. Electrical connection is then made to the metal of the substrate and electroplating of copper proceeds. An electroplating current of 35 amperes per square centimeter is used for a time of forty minutes to achieve a total thickness of electroplated copper of greater than 12 microns.
• the thickness of copper is important to this invention.
• the copper thickness must exceed 12 microns in order that the contact forces associated with the buttons be dispersed over the entire surface area of the contact pad so that the polymer underneath does not obtain a permanent set and eventually reduce the forces between the pad and the button contact.
• the permanent set can also punch through dielectrics and short underlying layers.
• the assembly is plated in a nickel bath to build up the thickness of nickel to approximately 100 micro inches and to provide a barrier between the nickel and the gold which will subsequently be plated.
• the substrate is rinsed and placed directly in an acid hard gold palting bath. Gold is plated to a thickness of at least 50 micro inches.
• the resist is removed by dipping the substrate in a 5% ammonium hydroxide solution for a period of one minute.
• the substrate is rinsed and placed in a copper etch consisting of one part ferric chloride and ten parts DI water. This etch takes place for a period of twenty seconds to remove the background copper and leave only titanium exposed.
• the titanium is etched in a one to twelve solution of TFT etch available from Transene Corporation. At this point, an environmental coating is added which coats the entire top surface of the module except for openings provided above each input output pad.
• the metallization described here is no different than the other metallizations in the multichip module except that it is typically thicker and covered with nickel and gold to give the highest reliability contact.
• This metallization layer can be used to provide additional interconnect capability. Specifically by leaving a small gap around each pad and then providing metal everywhere else, this layer can be used both for input output pads and for distribution of power and/or ground. Alternatively, the layer can be used to provide additional interconnection capability not provided in the layers beneath.
• the environmental coating is used to protect this layer and also to allow the existence of power and ground on this surface without the danger of shorts to other structures. Environmental coating can be supplied by using VAQS especially prepared as disclosed above. Spin coating at a speed of 2,000 rpm for a period of twenty seconds and baking and patterning is also accomplished as described earlier.
• an opaquing coating may be used.
• This coating consists of a pigment filled material; for example, black pigment in SPI 129. This mix is spin coated at 1,000 rpm and baked at 100°C for 10 min., 150°C 10 min., and 220°C for 30 min. Openings to the gold I/O pads are formed by excimer laser using an aperture slightly smaller than the pad size and pulse rates and energies as previously disclosed.
• Optimization For High Speed Figure 15 is a cross-sectional elevational view of a speed optimized advanced multichip module. The following discussion centers on those aspects of the advanced multichip module which together define a structure capable of very high speed operation. In some cases, the inherent structure of the basic advanced multichip module allows operation at very high speed. In other cases, particular novel structure variations provide enhanced or improved speed capabilities.
• An aspect of the basic structure of the invention is the ability to provide impedance controlled interconnect in combination with very efficient heat removal.
• the chips 190 are mounted directly on the substrate 192.
• the die attach glue line (not shown) is very thin and can be made thermally conducting by filling it with diamond powder or silver powder, depending whether thermal or both thermal and electrical conductivity are desired.
• the chips are thinned so that the thermal drop in the integrated circuit material is also reduced by the ratio of thinning. Typically, 21 mil chips are thinned to 7 mils which gives a 3 to 1 reduction in thermal resistance of the chip material.
• the actual substrate base plate 192 for the multichip module can be chosen to be highly thermally conductive.
• aluminum nitride has high strength, and good thermal expansion match between silicon and GaAs.
• Copper clad molybdenum offers custom tailored thermal coefficient match as well as improved heat spreading due to the copper and a conductive substrate to provide ground reference for high speed circuitry.
• the structure provides ideal thermal interface for chips, it is inherently capable of providing an impedance controlled strip line and microstrip line connections directly to the pads of the chips with no discontinuity in the signal path.
• Systems which build a miniature circuit board are capable of providing the controlled impedance strip line and microstrip line, but no matter how chips are mounted on this circuit board the thermal interface is not optimum. If chips are mounted directly on the circuit board, heat must be removed through the dielectric layers on the miniature circuit board. This presents a substantial thermal resistance. If the chips are mounted in the flip chip fashion, a special complex system must be provided for contacting the backside of the chips and removing heat.
• the encapsulant material is extremely flat and forms a parallel plane with the surface of the active chip area.
• the following typical spacings are used to provide a 50 ohm matched impedance interconnect.
• a shield layer i.e.. Ground (0) .
• This layer provides two functions. First, it prevents any capacitive coupling between the Signal (1) line and the interconnecting lines on the chips themselves. Secondly, it presents a completely electrically uniform surface to the signal line so that the Signal (1) line does not see discontinuities between the ground field associated with chips 190 which appear to be at ground potential and the dielectric 191 between chips 190 which appears to be an electrically high impedance.
• the next layer in the structure is the dielectric between the shield layer. Ground (0) and the Signal (1) layer.
• this layer is configured to be between Signal (1) and the shield layer and thereby allow Signal (1) to approximate a micro strip configuration wherein the second dielectric is thinner than the first dielectric. This allows Signal (1) to be wider at 50 ohm impedance, thereby reducing copper losses associated with a less wide run.
• Typical width of Signal (1) lines for optimum performance will be approximately 25 microns.
• a typical thickness of dielectric 193 is approximately 20 microns.
• the spacing between Signal (1) lines is approximately 75 microns. This provides low levels of cross talk between adjacent lines as well as an approximately 50 ohm characteristic impedance associated with the strip line.
• the line resistance of this structure is 5 ohms per inch. This allows several inches of line length before line losses become significant. These typical values are given assuming a dielectric material with a dielectric constant of approximately three, such as ZTI1004.
• a very low impedance power and ground can be provided in the structure as shown.
• Power (1) and ground (1) conductors are essentially in uninterrupted planes with exceptions for vias from the layers above.
• Very low inductance is achieved in the power and ground planes by decreasing the dielectric thickness between power (1) and ground (1) . It is important to note that because the structure is extremely flat and planar it is possible to apply a very thin coating of dielectric material which is pinhole free. In the HDI overlay approach, nonplanarity at the edges of chips causes thinning of dielectric between power and ground.
• the power and ground plane are built up by electroplating copper to thicknesses of 12 to 20 microns. This gives ground plane resistance related voltage drops on the order of less than 50 millivolts for 100 amps current.
• Additional transient stabilization of the power and ground plane can be achieved by increasing the dielectric constant of the dielectric that separates power (1) and ground (1) . This is done by filling the dielectric material with a high K dielectric powder such as bariumtitanate or titanium dioxide. Using bariumtitanate mixed 50/50 by weight with ZTI1004 should give a dielectric constant of 115 in 5 micron thick dielectric coating. This results in a capacitance of 0.1 microfarads per square centimeter. It is extremely important to note that this level of capacitance is more than sufficient to provide capacitive decoupling of power supplies at very high frequencies. As a result, there is no need for distributed capacitance in this structure.
• the disclosed power ground structure provides the most effective means of high frequency bypassing of circuitry on the module. However, it is still necessary to provide energy storage for stabilizing the inductance of lead wires which supply power to the module. Ideally, these storage capacitors would be of as high a value as possible. It is also desirable, however, to have a relatively thin module typically with a substrate thickness of 25 to 50 mils and a chip thickness of 6 mils. If chip capacitors were treated as ordinary IC components they would have to be only 6 mils thick. This is substantially thinner than commercially available chip capacitors.
• the following structure discloses an enhancement of the basic advanced multichip module structure which accommodates thick capacitors 200 or other conventional chip components such as crystals 202 and inductors 204 while still maintaining complete planarity of the system.
• holes are cut completely through the substrate base 206. These holes are slightly larger (by the tolerance of the size of the component) than the capacitor 200 or other component involved.
• Laser Services of Westford, Massachusetts will laser cut holes in substrates for a nominal fee.
• high power C0 2 laser machining systems can be used.
• the process starts in a normal fashion with die attach material (not shown) coated on the substrate 206 and die 208 (and resistors 209) placed and cured. At this point, the substrate is turned upside down on a flat soft surface (not shown) . Thick components to be accommodated are placed in each of the holes provided in the substrate.
• a dot of UV curable material 201 such as ZTI1004, is dispensed either by hyperdermic needle or commercial dispensing equipment into each of the holes. The material is subsequently cured using 2 joules per square centimeter of UV energy. This holds the thick components in place so that the top of the thick components are even with the tops of the IC chips.
• Platers tape or other method of sealing the back of the hole is then used to close off the back of the hole temporarily, and the encapsulation process described in the section on encapsulation is accomplished with the net effect that the entire thick component is encapsulated with encapsulation material filling the hole.
• the platers tape can then be removed and the process continued in exactly the same way as described in the other processing steps.
• Via holes can be formed down to the thick component, metal 210 is deposited and patterned to make connection between the component and other interconnects to the ICs and I/O of the system. Such an assembly is shown in Figure 16.
• the thickness of the component 200 can be as great as the thickness of the thinned IC chips 208 plus the total thickness of the substrate base 206.
• Commercially available capacitor and resistor components are available in thicknesses from 20 to 50 mils and therefore these components can easily be accommodated in this invention without impact on the planarity of the system.
• crystal 202 and coil 204 are accommodated within wells 203 and 205, respectively, in the backside of substrate 206.
• Laser drilled holes are provided in substrate 206 and filled with a conductive material 212 to the components 202 and 204 to the upper surface of substrate 206 and thereby, the patterned metallization 210.
• resistors are preprocessed on an insulating substrate which is the same thickness as the thinned chips.
• the substrate is cut into sections which can be placed when the chips are placed and typically arrays of termination resistors are placed in the space between adjacent chips.
• These resistor arrays can be provided with power bus interconnections such that the ground side of any array of termination resistors can be preconnected and requires only one power connection for the array. This simplifies the wiring associated with the circuitry above the chips. It also separates the termination resistor processing steps from the multichip module processing steps, and allows each process to be optimized.
• FIGS 17a-17c show a resistor array 220 with one end of each resistor 221 bussed 222.
• Resistor 221 includes a substrate 223 upon which is positioned the resistive material 224, bus 222 and discrete pads 226.
• Figure 15 shows incorporation of termination resistors in the speed optimized advanced multichip module.
• FIG. 18a and 18b An alternative method (see Figures 18a and 18b) is to sputter a resistive material 230 on the starting substrate 232, pattern resistors as appropriate with the conductor leads 234 placed so that they will terminate in the spaces between chips. A thin insulating material 236 is then coated over the resistors. This is followed by the die attach material and the subsequent placement and curing of the die 238 in place. After the encapsulant 240 has been applied and planarized, via holes and metallization are formed to the pads of the chips and between chips to the termination resistor contact pads on the surface of the substrate base plate 232. This configuration is shown in Figures 18a and 18b. Repairable Structures And Methods
• FIG 19 shows the basic AMCM with repair capability. Processing of the basic AMCM continues as normal through chip placement, encapsulation, planarization, via formation and metal deposition and patterning for the first interconnect layer. At this point, a solvent sensitive dielectric layer 250 is applied either by spin coating or spray techniques. This structure is shown in Figure 19. Solvent sensitive layers which can be used include SPI129 which can be spin coated at a speed of 3,000 rpm for a period of twenty seconds and baked ten minutes at 100°C, ten minutes at 150 ⁇ C and twenty minutes at 200°C. Another material that melts at a specific melting point is Ultem ® resin available from GE Company. This resin can be applied by spin coating using the mix shown in Table 2.
• Probimide 200 available from Ciba Giegy. This material has a low dielectric constant and essentially will not melt at any normal operating temperature.
• Probimide 200 is deposited from a 15 percent mix of the basic polymer and Gama Butyro Lactone. Once the given solvent sensitive layer 250 has been applied, a second dielectric layer 257 can be applied as described previously. Via holes are formed, also as previously described, and metallization is applied and patterned.
• the second layer is removed.
• This can be achieved in one of three different ways.
• the substrate is heated above the melting point of the solvent sensitive layer. At this point the circuit layers above can be peeled off leaving behind remnants of the solvent sensitive layer and the first layer interconnect.
• the solvent sensitive layer can be soaked at room temperature in the solvent. This lifts off all layers above the first interconnect layer.
• the third method which is presently preferred, involves lapping the substrate in exactly the same way as described in the encapsulation section. This removes both the polymer and all interconnects including the first level interconnect. In all cases, the residual solvent sensitive layer is removed by a cleanup process which involves dipping the substrate in the appropriate solvent ( Figure 20a) .
• etching the metallization in an etch which attacks the interconnect metallization but does not attack the metallization on the chips ( Figure 20b) .
• the titanium can be removed using buffered pad etch which attacks the titanium but does not attack the aluminum of the chip pads.
• the copper can be removed by nitric acid, which attacks the copper but does not attack the aluminum of the chip pads, and the bottom titanium layer can be removed by buffered pad etch available from Ashland Chemical Company of Columbus, Ohio. This leaves behind chips which are encapsulated with via holes without any metallization going to the pads of the chips.
• the procedure from this point is the same. If the interconnect itself was defective, then a new interconnect is started and processing proceeds in exactly the fashion described above. If a defective chip must be replaced, then the encapsulant is first removed from around the chip, the substrate heated to the softening point of the die attach material and the chip pulled out ( Figure 20c) . Once the chip has been removed, all surfaces are cleaned up by mechanical abrasion in the die attach area under the defective chip, i.e., if necessary. This is followed by a relatively extended plasma etch step which plasma etches the surface of all polymer areas, including the surface above the encapsulant and the edges of the encapsulant where the defective chip was removed.
• die attach material is deposited in the area where the chip was removed and a new chip is placed and cured in place.
• New encapsulant material is now deposited over the surface of the entire substrate as described in the encapsulation section hereof ( Figure 20d) .
• the extensive plasma etching will have reduced the thickness of the original encapsulant so that a substantial thickness of new encapsulant material is provided over all chips.
• planarization of the encapsulant as described in the encapsulation section the process proceeds in exactly the same way as if creating a new module. Because the encapsulant material is thinned and then built up to the same original thickness, the process of repair can be repeated a large number of times.
• UV curable material 261 is now exposed through the glass plate in a selective manner by scanning a small aperture 262 over just the section of the glass plate that overlies the chip to be removed. This cures the UV curable material 261 both to the glass plate 260 and to the top surface of the encapsulant 264 connected to the chip 265.
• the substrate is heated to the softening point of the die attach material.
• This technique is especially effective for removing tightly spaced chips and especially for removing very small chips which are difficult to selectively remove.
• the technique is also of value because it can be completely automated wherein selection of chips to be removed and selective application of hardening can all be done under computer control. When the chips have been removed the uncured UV curable resin can be washed away with acetone solvent. Table 3 shows a formulation for the UV curable chip removal adhesive.
• the encapsulant material does not adhere sufficiently well to the tops of the chips, it can be removed from the tops of the chips by scanning an aperture of the excimer laser over the tops of the chips to be removed, thus ablating the encapsulant material, and then performing the process described in this paragraph.
• the structures provided are hermetically sealed.
• Prior art methods of providing hermetically sealed multichip modules have always involved fabricating a module, placing that module in a second package and then subsequently bonding from the pins of the package to the pads of the multichip module.
• a cover is placed on the hermetic package and sealed in place.
• the package is necessarily complex and expensive. Additionally, the package increases substantially the total size of the assembly, i.e., over the size of the multichip module itself.
• This invention is a step forward in that it achieves the hermetic enclosure within essentially the same footprint as the multichip module. It also does this with a very simple structure which has high thermal and electrical performance.
• a first embodiment of the invention is shown in Figure 22. This structure consists of two major sections. These are multichip module with area array pads 270 and hermetic sealing assembly 272 with hermetically sealed input/output conductors. Processing of the multichip module with area array pads proceeds exactly as described above in the area array input output section. The only additional step is the use of an excimer laser to ablate polymer material along the periphery of the substrate base so that proper hermetic sealing can occur. The second part of the structure is the hermetic sealing assembly 272 with hermetic input output conductors 274.
• This assembly consists of a ceramic I/O lid 276.
• This lid 276 has input output feed through that are hermetically sealed.
• the lid has a hermetic sealing ring 278 attached to its periphery.
• a ceramic lid as described can be obtained from Ceramic Process Systems of Massachusetts. This corporation specializes in forming holes in ceramic, and filling those holes with hermetic plugs which are electrically conductive. They also will provide a hermetic sealing ring in a variety of materials attached to the basic flat ceramic structure with conductive plugs.
• the ceramic input output lid is processed to provide gold pads on both sides of the substrate. This improves the reliability of the final assembled structure.
• the gold pads are provided as follows.
• metal is sputtered on both sides of the lid. This is done by sputtring 1,000 angstroms of titanium followed by 3,000 angstroms of copper.
• F360 photo resist is spun on the hermetic seal ring side of the ceramic lid. A spin speed of 1,500 rpm and spin period of twenty seconds can be used. The resist is then dried on a hot plate for twelve minutes at 95°C. At this point, resist is spun on the other side of the substrate using the same conditions. Although not critical to the operation, initially spinning on the sealing ring side allows the seal ring to prevent contamination of the resist when baking the other side.
• the resist can be exposed with an energy of 100 mjoules per square centimeter. After exposure of both sides, the resist is developed in 1% sodium carbonate for a period of one minute with continuous agitation. Copper is then electroplated in a two sided apparatus such that a final thickness of the copper plate is approximately 1 mil. This is followed by nickel electroplating. After thorough rinsing the nickel is overplated with 50 micro inches of hard gold using the a gold plate bath available from Transene. After gold plating, the resist is removed by one minute dip in 5% ammonium hydroxide.
• the background copper is etched in a ten to one solution of ferric chloride for a period of twenty seconds and the titanium removed in a twelve to one solution of TFT etch available from Transene Corporation.
• the ceramic lid now consists of a flat piece of ceramic with through conductors that have gold plated copper pads on both sides for good electrical contact plus a hermetic seal ring hermetically attached to the periphery of the lid.
• the assembly is now completed by placing an array of button contacts 280 in the ceramic lid assembly and then placing the advanced multichip module 270 with array pads over the top of the button contacts 280.
• the final operation involves sealing the seal ring to the base of the advanced multichip module. This can be accomplished in three different ways depending on the substrate base material.
• the substrate base could be fabricated of Kovar plated with nickel which is available as the lid material for Kovar cans from either Isotronics or
• the substrate base could be ceramic previously provided with a solder preform as is well known in the packaging art. Such ceramic plates are used as the tops of ceramic packages. They are plated with materials finally ending in gold plate and can then be soldered by reflow solder techniques to a seal ring.
• the base material could be copper clad molybdenum in which case solder sealing or weld sealing can be used. It is important to note that the structure which results is only slightly wider than the multichip module, by the width of the seal ring.
• the resulting structure has good electrical contact through an electrical interface which is less than one tenth inch between the outside pads and the internal integrated circuits.
• the thermal interface is directly from the backs of the chips through the base plate of the advanced multichip module itself.
• the final capability which is extremely important in military applications is that this assembled unit can be leak tested by ordinary leak testing means. This is especially important because it means that the quality of the hermetic seal can be checked before and after stress testing and therefore assure a highly reliable final product.
• the seal ring 278 is sized so that the assembly provides adequate pressure on the internal button contacts.
• a second novel hermetic structure is ⁇ disclosed which provides very high output capability through the surface nearest the interconnect and optimized thermal interface to the surface attached directly to the back of the chips.
• Figures 23a and 23b show the structure. Again the starting point of the structure is the basic advanced multichip module 290 with area array pads. Processing of the basic module proceeds exactly as described in the area array section of this disclosure.
• the processing departs at the point that copper is electroplated to form thick input output pads 291.
• nickel followed by gold is used.
• a layer of chrome is electroplated instead of nickel gold. This is done to provide adhesion to a subsequent polymer layer which will be applied.
• the gold is not required because it does not give particularly good adhesion to polymer, and the pad will not be exposed to the elements because it will be hermetically sealed.
• the resist is removed and the background metals are etched as described in the area array section of this disclosure.
• the hermetic sealing assembly consists of a ceramic plate 292 in which holes 293 have been drilled and to which a seal ring 294 has been bonded. Methods well known in the art can be used to bond a Kovar sealing ring 294 to ceramic material 292.
• the ceramic material can be obtained with laser drilled holes from Laser Services Incorporated.
• the hermetic sealing assembly with holes is bonded to the top of the multichip module with array pads by using pressure of five pounds per square inch supplied by placing a weight on top of the sealing assembly and placing the assembly on a hot plate at 150°C. This cures the adhesive material but not before the adhesive material flows wetting the entire sealing assembly and partially filling the holes. Once the adhesive bonding has been completed, the assembly is removed from the hot plate.
• the hermetic seal is completed by welding or soldering the seal ring to the base plate.
• the polymer in the holes is removed by using an excimer laser which now provides a clean hole down to the pads in the area array of pads in the multichip module.
• the assembly is placed in a sputtering chamber and 1,000 angstroms of titanium followed by two microns of copper, are sputtered. The extra copper is sputtered in order to give good coverage of copper inside the hole.
• the assembly is removed from the sputtering chamber. Additional copper is built up by electroplating until a thickness of copper exceeding 1.5 mils is obtained. This effectively seals all of the holes in the system. Nickel is then built up to a thickness of 100 mi ⁇ roinches.
• resist is applied using F360 resist spun at a spin speed of 1,500 rpm. After drying the resist it is exposed with 100 mjoules per square centimeter. Exposure opens the holes and provides large pads directly adjacent to the holes for electrical connections. Gold is then plated to a thickness of greater than 50 microinches. At this point the resist is removed in 5% ammonium hydroxide. The nickel and copper can be etched in ferric chloride using the gold as an etch resist. Once the nickel and copper have been removed the titanium is removed in TFT etch. In an alternative embodiment, no seal ring is used. Instead when the hermetic sealing assembly without seal ring is pressed on to the adhesive polymer area the thickness of the chips plus the interconnect is exposed around the periphery of the substrate.
• both the hermetic sealing assembly and the base plate can be relatively thin because they do not need to support forces normally associated with large hermetic cans. That is, the inside of the hermetic enclosure is filled completely with polymer so that any force will be distributed by the polymer. This allows pressurization or forces used for contacting the module to be amortized over the entire area.
• the hermetic sealing assembly for input output and a very thin base plate can be provided to give extremely low thermal drop from the chips to the base plate.
• the structure is hermetic it is extremely thin and, especially in the case of the sealing ring formed at the same time as the holes, the assembly is no larger than the original multichip module.

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• Structures Or Materials For Encapsulating Or Coating Semiconductor Devices Or Solid State Devices (AREA)
• Variable-Direction Aerials And Aerial Arrays (AREA)
• Non-Metallic Protective Coatings For Printed Circuits (AREA)
• Structures For Mounting Electric Components On Printed Circuit Boards (AREA)

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• 1992
  • 1992-03-26 JP JP4510234A patent/JPH06502744A/ja active Pending
  • 1992-03-26 EP EP19920917353 patent/EP0577779A4/en not_active Withdrawn
  • 1992-03-26 AU AU18743/92A patent/AU648417B2/en not_active Ceased
  • 1992-03-26 WO PCT/US1992/002623 patent/WO1992017901A1/en not_active Ceased
  • 1992-03-26 CA CA002106872A patent/CA2106872A1/en not_active Abandoned

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