Sealing and Stress Relief Layers and Use Thereof

This invention relates to hermetic substrates for microelectronic applications. More particularly, it relates to the use of sealing layers to seal substrates and to provide a low stress base for the mounting of devices on the substrates.

In the microelectronic environment, there is a need for high density, high strength packaging to provide interconnection between semiconductor devices and connection from the devices to the electrical power supply. The electrical properties which are desirable include a highly conductive medium in a highly insulative carrier medium having a low dielectric constant. Thermally, the package must withstand not only the operating environment but also the thermal excursions encountered during the processing and fabrication of the part. Mechanically, it is preferable to have a substrate package which can withstand chip and pin joining stresses and stresses related to interconnecting with the next level of packaging. Furthermore, the package should be hermetic to prevent degradation of any of the desired properties due to leakage. Alumina has offered many of the mechanical properties needed, but does not have the optimum electrical and thermal characteristics. From an electrical standpoint, alumina has a relatively high dielectric constant. The thermal coefficient of expansion for alumina is not compatible with that of the silicon material used in chips. The thermal expansion incompatibility results in fatigue failures in the alumina-to-silicon interconnections. An additional drawback to the use of alumina is the fact that alumina has a very high sintering temperature (in the range of 1100.degree.-1600.degree. C.) which limits the choices for a co-sinterable metallurgy to high melting point refractory metals such as molybdenum or tungsten. The electrical properties of the high temperature metallurgies are less desirable; most notably, they have relatively high resistivity values.

Glass-ceramic composites have been explored as the dielectric materials to replace alumina. The compositions, having a low dielectric value and a favorable coefficient of thermal expansion, appear to be good alternatives to alumina. An added benefit to the use of composites is the lower temperature at which the materials sinter. Sintering temperatures in the range of 850.degree.-1000.degree. C. permit the use of cosinterable metallurgies such as gold, silver and copper, all of which have superior conductivity over the previously employed refractory metals. The disadvantage of using glass-ceramic compositions is that they generally have low strength and toughness as compared with the prior art dielectric materials. Low fracture strength results in cracking due to thermal expansion mismatch between the dielectric materials and the associated metallurgy. Should cracking occur during processing, the package may become permeable to processing solutions which could compromise the integrity of the package. A method has been taught in an accompanying pending application Ser. No. 167,606 filed concurrently with the subject application on Mar. 11, 1988, the teachings of which are hereby incorporated by reference, wherein a uniform gap is created between the ceramic and the associated internal metallurgy in order to provide an expansion zone between the thermally mismatched materials. Necessarily, the creation of such a gap results in exposure of the substrate to processing solvents which may directly affect the materials and less directly affect the performance of the part. The invention taught and claimed herein provides a solution to the hermeticity problem while simultaneously providing stress dissipation to prevent ceramic fracture.

Prior solutions to the stated problems include the use of dielectric materials to fill voids or cracks such as is taught in IBM Technical Disclosure Bulletin Volume 16, Number 2, page 624 (July 1973) and in U.S. Pat. No. 4,237,606. Filling voids and cracks with a glass layer, such as is taught in each of the references, can eliminate the porosity, provided the size of the pores and the surface tension of the material are not incompatible so as to prevent a complete fill. The problem of incomplete fill, mentioned above, is not an insignificant consideration. Filling the micropores encountered in ceramics and glass-ceramics processing requires materials which can fill spaces of submicron dimensions, which many of the non-reactive, inorganic dielectric materials cannot achieve. A further concern with the use of certain inorganic layers/fillers is that the fill material will not provide any relief from the stresses associated with the package and its use. Organic layers have been proposed as protective layers over completed modules wherein the devices have been mounted on the substrate and all interconnection is complete. In one example, IBM Technical Disclosure Bulletin Vol. 15, No. 6, page 1974 (November 1972), an article describes the use of an organic material layer to seal the assembly from the subsequently introduced cooling fluids. A similar use of an organic material deposited over a substrate assembly can be seen in IBM Technical Disclosure Bulletin Vol. 29, No. 3, page 1073 (August 1986) wherein a polymeric layer is used as a barrier to emission of alpha particles from the substrate. In each instance, the use of the organics is restricted to final processing, by which time many harmful fluids may already have been trapped in the assembly. Furthermore, planarity is not a concern in a finished assembly, as it is prior to device joining

It is therefore an objective of the subject invention to provide a method and structure for obtaining an hermetic substrate and for providing a planar low stress base for making interconnection to said substrate.

The subject invention not only deals with the sealing and stress difficulties but does so in situ at the substrate level thereby providing a superior, planar base for mounting devices.

A first embodiment of the present invention teaches a structure including a capture pad deposited on the top and bottom surfaces of the fired substrate in alignment with the internal metal features of the substrate. A stress relief layer is deposited over the capture pads and over the entire surface of the substrate. Interconnection is made from the capture pad on the substrate surface to the outer surface of the stress relief layer whereupon is deposited a bonding pad on which to connect pins, devices, wires, etc.

A second embodiment similarly deposits a stress relief/sealing layer on the sintered substrate surface. The second embodiment, however, does not require the use of a capture pad and interconnection structure associated with the internal metal feature. Rather, the stress relief/sealing layer is selectively treated to exhibit conductivity in the regions in alignment with and abutting the internal metal features while the remaining regions of the layer act as a conformal dielectric sealing layer. In each embodiment, connection is then made at or about the surface of the sealing/stress relief layer.

Still another embodiment provides a co-sintered metal stud for interconnection at the top surface of the sealing/stress layer and the method of implementing same.

The invention will be described in greater detail with specific reference to the drawings wherein:

FIG. 1 is a cross-sectional view of the first embodiment of the inventive structure as applied to the bottom surface of a substrate.

FIG. 2 is a cross-sectional view of the first embodiment of the invention applied to the top surface of the substrate.

FIGS. 3A and 3B illustrate a cross-sectional view of the method and structure of the second embodiment of the invention wherein conductivity is induced in selected regions of a non-conductive sealing layer.

FIGS. 4A, 4B and 4C illustrate, in cross-sectional view, the method and structure of the second embodiment of the invention wherein the regions of a conductive sealing layer which are not in alignment with the internal features of the substrate are treated to become insulative/non-conductive.

FIG. 5 illustrates a layered polymer structure exhibiting both good adhesion and superior stress absorption.

FIG. 6 illustrates the inventive structure wherein the interconnection between the substrate and the top surface of the sealing/stress layer is made through a co-sintered connection.

FIG. 7 illustrates, in cross-sectional view, the use of a selected conformal sealing film wherein the film is conductive is certain geometric arrangements and non-conductive in others.

As discussed above, the selection of dielectric materials for packaging in semiconductor applications involves mechanical and thermal prerequisites along with the preferred electrical properties. The ideal electrical properties of glass-ceramic composites, of the type taught and claimed in U.S. Pat. No. 4,301,324 of Kumar, et al (the teachings of which are herein incorporated by reference), along with their thermal compatibility for use with silicon-based devices, and co-sinterability with low melting point, superior conductivity metallurgies, make the glass-ceramic composites most desirable. Again, the mechanical strength of the materials is somewhat lower than is required to withstand the standard multilayer ceramic processing.

Standard multilayer ceramic processing has been taught in several patent documents. An example is the teaching found in U.S. Pat. No. 3,518,756 wherein the basic steps are: mixing a slurry of ceramic particles and binders; casting the slurry into sheets; drying the sheets; forming a pattern of holes in the sheets by punching or other methods; metallizing the sheets by screening into the holes or by another deposition technique; stacking the metallized sheets; laminating the stack; and, firing the stack to burn off the binders and to sinter the ceramic and metallurgy. Processing after the formation of the basic package includes top surface metallization and device joining along with bottom surface pin joining. The steps in these processes which impart the greatest stresses to the glass-ceramic materials include firing at high temperatures and post-sintering thermal processes.

The thermal stresses associated with firing arise from the mismatch in coefficients of thermal expansion of the materials, specifically the mismatch between the glass-ceramic expansion and that of the metallurgy. A metallurgy based upon copper or gold will expand, under high heating conditions, such as the 700.degree.-1000.degree. C. discussed in Kumar, et al, at a rate which is roughly quadruple the expansion of the ceramic material. For the relatively brief period that the materials are subjected to extremely high temperatures, the ceramic material is placed under great stresses which can result in cracking of the ceramic. As discussed above, the cracking of the ceramic material will permit processing solvents to permeate the substrate. Trapped solvents in the vicinity of the metallurgy will lead to corrosion; whereas fluids trapped in the bulk of the dielectric material can lead not only to material break-down but also to further stresses and bloating of the substrate during the subsequent temperature excursions. Ultimately, the performance of the entire package is compromised by the presence of any porosity in the substrate. Rather than alter the materials and, unavoidably, change those electrical properties for which one has specifically chosen the glass-ceramic composition and the metallurgy, it is preferable to minimize the effects and thereby remedy the hermeticity problem. Thermal expansion mismatch stresses can be minimized by adjusting the TCE (thermal co-efficient of expansion) of the associated materials; however, this approach will alter the electrical properties of the final package. The present invention seeks not necessarily to eliminate the cracking but to eliminate the permeability associated with cracks. With regard to the post-sintering stresses, the thermal processing steps will occasion the same TCE problems. In addition, there are mechanical stresses imparted by the bonding arrangements which can cause the underlying ceramic to fracture. As with the thermal properties, the mechanical strength of the substrate material cannot readily be changed without impacting the dielectric constant of the insulative material. However, the mechanical stresses on the substrate can be minimized by the subject invention.

The first embodiment of the invention is a structure comprising a glass-ceramic substrate comprising a crystallizable glass such as a cordierite-based system processed through sintering in accordance with known techniques such as taught in U.S. Pat. No. 3,518,756 and having a blanket stress relief and sealing layer deposited over the entire horizontal surfaces of the package. The sealing layer is preferably deposited over the sintered substrate and its co-sintered surface metallurgy. The sealing layer is most advantageously a polymeric material. A temperature stable polyimide, such as a BTDA-APB, is ideal with Thermid (a Trademarked product of the National Starch Company) being the best mode known to date. Temperature stability of the polyimide or other sealing material is critical not only for operation, but also to provide a thermally stable underlayer for deposition of bonding pads and joining of pins or devices, all of which will involve temperature excursions in excess of operating temperatures. The choice for the overlying layer is preferably a polymeric material given the superior mechanical properties of a polymer. When bonding pads or other features are deposited onto the polymeric layer, any mechanical stresses are readily dissipated in the flexible polymer medium; thereby, alleviating the stress on the low-fracture strength glass-ceramic. The polymeric layer must be chosen to be impervious to whatever processing solvents are needed for completion of the package. Furthermore, polymeric coatings are favorable in that they are readily deposited in a planar fashion. Deposition of the polymeric layer can be achieved by spin coating or any number of well-known deposition techniques. Furthermore, it is not critical to the subject invention whether the polymer is deposited "neat" (i.e. without solvents), or deposited in a solvent solution with the subsequent driving-off of solvents. Neither is it envisioned that the method of curing or polymerizing the polymer would be significant to the invention, provided that the underlying substrate will not be exposed to any processing solvents or byproducts which could permeate into the body of the metallized substrate.

A multilayer polymeric overcoat is illustrated in FIG. 5 wherein layers, 57 and 58, of materials having superior adhesive qualities surround a layer, 59, of material having superior mechanical ability with regard to stress dissipation. In a sample structure, a thin (1-4 microns) layer, 58, of a PMDA-ODA derived polyimide, was first deposited as a polymeric sealing layer exhibiting high adhesion to glass-ceramic and the metallurgy. A thicker (6-15 micron), intermediate layer, 59, of a polymer having superior "cushioning" effects such as the BTDA-APB derived polyimide is deposited over the thinner layer, 58. Finally, another thin (1-4 micron) layer, 57, of, for example, the PMDA-ODA derived polyimide is deposited over the intermediate layer to provide a good adhesive base for deposition of a bonding pad. With respect to the superior adhesive qualities, adhesion to both the ceramic and the metal features is a factor for the first-deposited layer, 58. Good adhesion to metal is the criterion for the uppermost layer, 57.

Electrical connection from the surface of the preferably polymeric overlayer to the underlying metal features at the substrate surface can be achieved in a variety of manners. The first embodiment, above and illustrated in FIGS. 1 and 2 for the bottom and top surfaces respectively, provides for the overlayer to be deposited over the substrate surface inclusive of co-sintered metallurgical features, 14 in FIG. 1 and 24 in FIG. 2. The metal features illustrated therein are deposited as "capture pads" to provide not only a bonding pad for subsequent joining but also a seal about the via itself. Surface capture pads can have a diameter which is substantially greater than the diameter of the underlying internal feature; thereby providing a large target for subsequent interconnection. As illustrated in FIGS. 1 and 2, and disclosed in the concurrently filed patent application referenced above, the package can be fabricated with a uniform gap about the via after sintering. That gap may be filled as taught in the referenced application, or can be sealed off with the sealing layer or with a metal capture/bonding pad as illustrated herein. The capture pad may be deposited in a known metallization step either prior to the sintering of the part, or after the firing has been completed. The use of such a bonding pad would necessarily create stresses on both the via and the surrounding ceramic if the bonding of a pin or device were to be done directly to the pad at the substrate's surface. Therefore, the sealing layer is deposited over the substrate surface, inclusive of the metallurgical features. The sealing layer then provides the stress relief necessary to allow joining of pins or devices without undue stresses on the package.

Connection may be made through the overlayer by etching a via in the overlayer. Laser etching is a preferred method of directly etching a polymeric layer. Masking is not imperative if ablation by laser writing is used. Another method of providing a via in the overlayer is to use a photosensitive polymer for the overlayer, mask the regions of the overlayer not in alignment with the underlying metal, selectively expose the portion of the overlayer which is in alignment with the underlying capture pad and, with appropriate solvents, dissolve away that exposed portion of the overlayer. Those skilled in the art of photosensitive polymers will recognize that polymers can be chosen to operate as discussed above as negative resists, or other polymers can be chosen to operate as positive resists wherein the irradiated portion of the overlayer will remain intact and the masked regions in alignment with the vias will be dissolved away. Other methods of creating the opening for electrical connection to the underlying metal will be readily ascertainable by those having skill in the art. The capture pad provides sealing as well as a polymer-ablation target which minimizes alignment concerns; however, the polymer layer itself will seal off any via gaps in the absence of the capture pad. Ideally, direct alignment of interconnection metallurgy to via metallurgy can be achieved using the masks utilized for screening the top surface via metallurgy. The electrical connector, i.e. the interconnection metal as illustrated as 15 in FIG. 1 and 25 in FIG. 2, between the underlying metal and the surface of the overlayer may be a metal or metal-based paste which is screened into the via channel, a metal deposited through a mask and into the channel, or metal flowing into the channel during deposition of the bonding pad, 16 of FIG. 1 and 26 of FIG. 2. The only parameters for the interconnection metallurgy are that it provide electrical connection and that deposition of the metal does not alter the overlayer, the latter being an attribute of the overlayer material rather than of the metallurgy itself. Multiple layers of polymer and interconnect metallurgy may be fabricated in this manner to provide the desired package.

Interconnection to another related structure is illustrated in FIG. 6, further discussed below, wherein the co-sintered metal or metal-based via, 63, protrudes beyond the surface of the substrate after sintering. Therein, the overlayer will be conformally coated and any excess removed from the surface of the metal feature. As the illustration shows, if there is a gap about the via/metal feature, the overlayer material will seal the gap off in the absence of a capture pad. This form of sealing may be particularly advantageous as it allows the via material an impermeable air gap for expansion without the via material contacting the ceramic. Furthermore, the superior dielectric properties of air make it the perfect insulator adjacent the conductors for improved speed performance. In the instance of the raised metal feature, there is no alignment concern when interconnecting to the overlayer surface. If the structure was sintered with the metal feature abutting the ceramic, as opposed to with an air gap, cracking of the ceramic would be seen in all probability. The overlayer would, of course, eliminate any permeability created thereby by filling the cracks. To complete the structure, a metal or metal-based bonding pad will be deposited onto the surface of the overlayer. The devices, pins or wires bonded thereto will create stresses in the polymer, which can readily dissipate those stresses given its superior mechanical properties. The glass-ceramic body is, therefore, not exposed to any post-sintering stresses sufficient to cause cracking or fracture.

A sample of the first embodiment of the invention was fabricated in the following manner;

A slurry of binders, solvents, plasticizers and crystallizable glass having its principal crystalline phase of cordierite was cast into greensheets as taught in the referenced art. The dried greensheets were patterned and the pattern metallized with a copper-based metallurgy. The metallized greensheets were then stacked, at which time an additional Cu-based metallurgy was applied through masks to the top and bottom surfaces in contact with the metallization pattern on the top and bottom greensheets. Lamination and sintering were conducted in accordance with the known procedures as set out in Herron, et al U.S. Pat. No. 4,234,367. Subsequent to firing, the top and bottom substrate surfaces were spray-coated with a thin layer, 5 .mu.m layer of a PMDA-ODA derived polyimide. The polyimide was thermally cured in accordance with the specifications. A mask having dimensions substantially smaller than the metallization mask was utilized for projection etching of the polyimide to form holes in the polyimide and expose the surface metallization below. Using the same mask as was utilized in the earlier surface metallization step, bonding pads were applied to the surface of the polyimide layer. The holes in the polyimide were filled by the screening of the first metal applied for the bonding pads, so that an additional deposition step was not required. On the bottom, or pin-joining surface, multilayer metal was applied for the bonding pads as is known in the art. Different metallurgy systems would be used for the top and bottom surfaces given the bonding requirements which each surface has. For the pin-joining surface, the appropriate metallurgical system used was a multilayer stack of 180 .ANG. of Zr (having superior adhesion), 5 .mu.m of Cu (for electromigration resistance), 1 .mu.m of Ti (to promote adhesion between to Cu and the braze metal) and 5000 .ANG. of Au for brazing. Pins were brazed to the bonding pads and several pin pull tests were conducted. The pins withstood the maximum pull strength of 10.7.+-.0.3 lbs. with no ceramic fails/fractures. For the device-joining surface, bonding pads having the appropriate metallurgy for the desired bonds is deposited in accordance with well-known techniques. For example, for a wire bonding site, deposition of a multilayer stack including a thick gold bonding layer was conducted using known sputtering techniques. For flip-chip bonding sites, multilayer pads suitable for solder bonding were deposited in accordance with well-known practices. In each instance, the bonds remained intact through several thermal excursions, indicating that both thermal and mechanical stresses on the substrate had been relieved.

A second embodiment, as illustrated by FIGS. 3A-B and 4A-C and 7, teaches the use of a conformal overlayer, but does not involve subtractive etching of any portion of the layer for achieving electrical connection in the regions of the overlayer aligned to the underlying metallurgical features. This embodiment provides for continual coverage by the conformable overlayer with no possibility of exposure of the substrate surface after deposition of the overlayer. Specifically, one example of this "second" embodiment, seen in FIGS. 3A and B, is the use of an electrically insulative or semiconductive overlayer, herein polysilicon, 39, blanket deposited onto the surface of the substrate, 32, by chemical vapor deposition or other appropriate technique. Subsequent to the deposition of the insulative overlayer, conductivity is selectively induced into the overlayer in those regions, 35, in alignment with and abutting the underlying metal features, 33. Inducing conductivity in a polysilicon layer can be accomplished by ion implantation through the same mask which could be used for subsequent deposition of bonding pads. Although the aligned regions are conductive, they would in most instances be poor sites for direct attach of pins, devices, wires, etc. Therefore, subsequent deposition of bonding pads would be preferable. Another example of a material which can be deposited as a insulative layer and in which conductivity can be selectively induced is chalcogenide glass which can be rendered conductive by thermal fusion with the use of radiant heat. A preferred mode of the subject conformal layer approach is to deposit a polymeric layer, which has the desired stress-dissipating mechanical properties, and to selectively induce conductivity in those regions in alignment with the underlying metal features, by chemical doping for example. With the deposition of such a polymeric material, capture pads may not be necessary for sealing purposes as the polymer could fill at least the surfaces of any gap that may be present, see area 31 in FIG. 3B. The self-alignment feature of this embodiment obviates the need for a large diameter target for subtractive etching and interconnection. The reduction to practice of this embodiment of the invention has not been achieved; however, it is speculated that one skilled in the art could successfully attain such a structure.

Still another approach to the implementation of the second embodiment of the invention, as seen in FIGS. 4A-C, is the blanket deposition of a conducting overlayer, 49, and the subsequent passivation of those portions of the overlayer, 49', which are not in alignment with the underlying conductors, 43. The regions of the overlayer, 45, which are in alignment with the underlying features can be masked from the processing which passivates the remaining portions of the overlayer, for example an oxidation procedure. The material which remains conductive (e.g. non-oxidized) can serve as the bonding pad, or, if deemed necessary, another deposition can be made to provide a pad. In the alternative, as seen in FIG. 4C, the bonding pad, 48, can be deposited prior to the passivating step with the pad, itself, serving as the mask to protect the underlying material, 45, from the passivation. Necessarily, the metal chosen for the bonding pad should be impervious to the passivation technique employed to render the non-masked portions of the overlayer non-conducting. Many variations and implementations of the proposed conformal layer can be envisioned and it is intended that the claimed invention not be limited solely to the examples recited herein.

An additional means for implementing the second embodiment would be to choose a material for which the resistive value of the material would render it conductive in certain geometric arrangements with respect to the other conductive media in the package and non-conductive in other geometries (specifically, not capable of conduction between features which could cause shorts, but conductive from the overlying bonding pad to the underlying metal feature). This embodiment is illustrated in FIG. 7 wherein the conformal film, 79, is chosen having a resistivity such that the resistance, R.sub.v, between the overlying feature, 76, and the via, 73, of t/L.sup.2 is at a functional level so that conduction can take place between the respective metal features whereas the resistance R.sub.ns, referred to as the non-shorting resistance between one metal feature, 78, and another, 76, of L/Lt, is sufficient to prohibit conduction between the respective features, where L is the distance between features on the same plane (i.e. the surface of the conformal film) and t is the thickness of the conformal film.

As noted above, the following inventive method for obtaining a co-sintered stud on the internal metallurgy extending above the surface of the substrate provides ease of interconnection to the top surface of a subsequently deposited overlying sealing/stress relief layer. The processing of the substrate is conducted in accordance with typical procedures up through the stacking of the metallized greensheets. An additional metallized greensheet is then added to the top and/or bottom surfaces of the package, and the known processing steps resumed. This additional greensheet is metallized with the same conductive metallurgy as is used throughout the substrate and the metallurgy will, therefore, co-sinter. However, the additional greensheet is chosen to be a material which will not co-sinter with the bulk ceramic but will be readily removable from the surface of the sintered body. In the context of a glass-ceramic which sinters in the range of 700.degree.-1000.degree. C., it is appropriate to use a metallized alumina greensheet as the top and/or bottom additional greensheet. The alumina, which has a sintering temperature well in excess of 1200.degree. C., will not sinter at the relatively low temperatures used for the sintering of the glass-ceramic. Additionally, the alumina will remain sufficiently porous to allow for binder removal from the underlying glass-ceramic. Furthermore, with the removal of the binders from the alumina sheet, all that remains at the substrate surface after the sintering of the glass-ceramic is a powdery alumina which is readily removed from the surface of the substrate without the necessity of abrading the substrate surface and possibly shearing the sintered metallurgical studs. After removal of the alumina powder, a polyimide or other overlayer material can be conformally deposited on the glass-ceramic surface and over the studs and planarized back by any standard process to re-expose the studs' surfaces. The use of such a process as the co-sintered stud formation provides strong, self-aligned, co-sintered connections eliminating several potential sources for difficulties in the alignment and planarization processes. It is envisioned that the disclosed co-sintered stud technology could be utilized for many structures apart from the disclosed planar, metallized sealing layer. Ease of testing, for example, for any metallized part will be enhanced by the presence of protruding metal. The inventors do not wish the teachings to be interpreted as limited to the specific embodiments discussed herein.

The foregoing teachings represent the best known modes of achieving the inventive hermetic substrate. It should be understood to those having skill in the art that the invention is not limited to the specific examples and materials discussed. Changes in form and detail may be made without departing from the spirit and scope of the invention as hereinafter claimed.