Methods of Forming Channels and Vias in Insulating Layers

The present invention is directed to trenching techniques for conductive electronic interconnects, and more particularly to methods for fabricating channels and vias in high density multilayers such as substrates and integrated circuits in few and simple steps.

High density copper/polyimide interconnects can be fabricated by providing conductors on a base, building interlayer pillars on the conductors, and coating the conductor-pillar structure with polyimide. This has several drawbacks: the metal conductor-pillar structure is vulnerable to mechanical damage before polyimide encapsulation; resist scumming can develop on underlying patterned layers after wet chemical etching; particles on the surface prior to plating can plate up quickly causing interlayer short circuits; and cooling after the 400.degree. C polyimide cure can create stress between the copper and polyimide since copper has a greater thermal coefficient of expansion.

These drawbacks associated with conductor-pillar structures can be overcome by forming trenches for vias and channels in a polyimide layer and then filling the trenches with a conductor. One such method for fabricating high density electronic circuits having narrow conductors is disclosed by Becker in U.S. Pat. No. 4,417,393. There are several drawbacks to Becker's method, including the need for two insulating layers with different depths to form vias and conductors in an insulating layer, the need for laser machining, and the inability to incorporate electrodeposition.

Furthermore, current interconnect fabrication methods with relatively few steps form conductive channels and vias simultaneously but fail to provide via stacking. Alternative methods which provide via stacking require a relatively large number of process steps.

The present invention provides trenching techniques for fabrication of electrical interconnects by forming the conductor channels and vias using well known low cost processes such as die stamping, etching, plating and polishing in relatively few steps while retaining design flexibility. Multilevel electrical interconnects suitable for the present invention include substrates, printed circuit boards, printed wiring boards, thin film interconnects, planar integrated circuit interconnects, multilevel integrated circuit wiring and the like.

An object of the present invention is to form vias and channels in an insulating layer with relatively few and simple process steps, as well as filling the vias and channels with a conductor with relatively few and simple process steps.

Still another object of the present invention is to form conductive channels and vias simultaneously as well as to provide via stacking.

Another object of the present invention is to minimize yield loss mechanisms that often accompany multilayer interconnect assembly, such as pinholes and mechanical weakness.

In accomplishing the foregoing objects, there is provided in accordance with one aspect of the present invention a method for fabricating an electrically conductive interconnect, comprising the sequential steps of providing an insulating layer on a base, forming a channel in the top surface and partially through the thickness of the insulating layer, forming a via in the top surface and completely through the thickness of the insulating layer adjacent the channel, depositing an electrical conductor into the channel and via, and planarizing the interconnect top surface so that the electrically conductive layer remains only in the channel and via and is otherwise removed from the top surface of the insulating layer, and the interconnect top surface is substantially smooth, thereby forming an electrically conducting channel interconnected to an electrically conducting via.

Further objects, features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.

FIGS. 1a-1f are cross-sectional views detailing a method of forming the vias and channels with a hard mask and soft mask in accordance with a first embodiment of the invention;

FIGS. 2a-2f are cross-sectional views detailing a method of forming the vias and channels with two stacked soft masks in accordance with a second embodiment of the invention;

FIGS. 3a-3f are cross-sectional views detailing another method of forming the vias and channels with two stacked soft masks in accordance with a third embodiment of the invention;

FIGS. 4a-4g are cross-sectional views detailing a method of forming the vias and channels with two soft masks applied and etched sequentially in accordance with a fourth embodiment of the invention;

FIGS. 5a-5g are cross-sectional views detailing another method of forming the vias and channels with two soft masks applied and etched sequentially in accordance with a fifth embodiment of the invention;

FIGS. 6a-6e are cross-sectional views detailing a method of forming the vias and channels with a metal die stamping plate in accordance with a sixth embodiment of the invention;

FIGS. 7a-7f are cross-sectional views detailing a method of forming the vias and channels with two layers of photoimagible polyimide in accordance with a seventh embodiment of the invention;

FIGS. 8a-8f are cross-sectional views detailing a method of forming the vias and channels with a mask followed by laser drilling in accordance with an eighth embodiment of the invention;

FIGS. 9a-9b are cross-sectional views detailing a method of depositing an electrical conductor in the vias and channels;

FIGS. 10a-10c are cross-sectional views detailing another method of depositing an electrical conductor in the vias and channels;

FIGS. 11a-11c are cross-sectional views detailing planarizing the substrate top surface by polishing;

FIG. 12 is a cross-sectional view showing via stacking in a multilayer electrical interconnect;

FIG. 13 is a cross-sectional view similar to FIG. 12 showing the conductors in the top layer slightly etched back to provide registration for bumps on an integrated circuit;

FIGS. 14a-14b are photomicrographs of vias and channels formed according to the first embodiment of the present invention; and

FIGS. 15a-15b are photomicrographs of vias and channels formed according to the fifth embodiment of the present invention.

Eight embodiments for forming conductor channels and vias in multilayer electrical interconnects are now described. For illustration purposes, a multilayer electrical interconnect will be depicted by a copper/polyimide substrate 10 in the examples that follow. In embodiments 1, 2, 3 and 6 the vias and channels are formed in a single step, whereas in embodiments 4, 5, 7 and 8 the vias and channels are formed in separate steps. In embodiments 7 and 8 an irradiation source is required. In embodiment 7 two polyimide layers are required.

Referring now to the drawings, a high density electronic interconnect substrate designated as 10 is fabricated on a base 12. Base 12 is shown as a ceramic such as alumina ceramic or glass ceramic, however, it is understood that base 12 can be an organic or inorganic insulator, a conductor, an integrated circuit, or a preceding layer if the interconnect is a multilevel structure. Base 12 may require appropriate surface preparation, such as polishing, cleaning, etching, or roughening to assure acceptable contamination removal and/or surface finish. For example it ma be advantageous to roughen a copper/polyimide base by plasma etching, or to smoothen a ceramic, plastic, or metal base by polishing. An insulating layer shown as polyimide layer 14 having 15 microns thickness is deposited over and cured on base 12.

FIGS. 1a-1f show a first embodiment for forming the conductor channels and vias. In FIG. 1a a thin blanket layer of metal such as 2500 angstroms copper over 700 angstroms chromium is sputtered over polyimide layer 14 to form hard mask 16, which after conventional patterning has openings to expose via regions 20 and channel regions 22. In FIG. 1b soft mask 18 of photoresist, which can also be silicon-dioxide or silicon-nitride, is placed over hard mask 16 and after conventional patterning has openings to expose via regions 20 but covers channel regions 22. Thus via regions 20 are exposed but channel regions 22 are not exposed. In FIG. 1c a suitable etch is applied wherein hard mask 16 is non-erodible but soft mask 18 erodes. For illustration purposes a dry etcher can generate plasma etch 24. That is, as plasma etch 24 is applied hard mask 16 etchs slowly or not at all while soft mask 18 etchs rapidly and merely causes a time delay until plasma contacts the regions covered by soft mask 18. Soft mask 18 is made thick enough to provide the correct etching time delay for underlying polyimide layer 14. In this example via regions 20 are intended to be 10 microns deeper than channel regions 22, so soft mask 18 can be 10 microns thick photoresist provided it etches at the same rate as underlying polyimide layer 14. Thus the thickness of soft mask 18 can be adjusted for differential etch rates between soft mask 18 and underlying polyimide 14. Plasma etch 24 can comprise 90% O.sub.2 and 10% SF6 at 600 watts RF power and 150 millitorrs pressure, although the parameters for plasma etch 24 such as power, pressure, chemistry, and electrode spacing are highly variable depending on the desired etch rates, profiles, and etcher machines. As via regions 20 begin to etch channel regions 22 remain covered and undisturbed, and soft mask 18 begins to erode. Plasma etch 24 will tend to etch vertically and attack the exposed surfaces at the same rate. FIG. 1d shows soft mask 18 completely eroded, and via regions 20 partially etched through polyimide layer 14. If desired, via regions 20 could be fully etched at this stage. As plasma etch 24 continues, FIG. 1e shows via regions 20 etched 15 microns through the entire thickness of polyimide layer 14, and channel regions 22 etched 5 microns through a partial thickness of polyimide layer 14. The depth of 5 micron channels in a 15 micron insulating layer was selected as a compromise between the Scylla and Charybdis of high line resistance from too thin a channel and low characteristic impedance (i.e. capacitive loading) from too thick a channel. Via sidewalls 26 preferably are tilted at sidewall angle 28 of 20.degree. and converge toward the bottom of polyimide layer 14 to improve fabrication of additional materials thereon. The selection of sidewall angle 28 is a trade-off between denser vias from more vertical sidewall slopes and improved step coverage vias from less vertical sidewall slopes. Sloped via sidewalls may be difficult to accomplish with a hard mask and a dry etch since the plasma may etch anisotropically. In FIG. 1f plasma etch 24 is discontinued and hard mask 16 is removed from polyimide layer 14. Via regions 20 and channel regions 22 are now formed. Substrate 10 may now require surface cleaning if contamination develops from etch products not carried away by the etch process. The formation of vias and channels in a polyimide layer in accordance with the first embodiment was demonstrated as shown in FIGS. 14a and 14b.

While dry etching with a plasma etch is the preferred method of etching, other etching methods are suitable for selectively removing material from the insulating layer. A wet chemical etch can be used, and may be more isotropic then a plasma etch thereby facilitating a slope on the via sidewalls, however a wet etch may lead to a via sidewall slope which is too large and/or a wet etch may not have sufficient accuracy for fine pitch geometries. Another suitable etching method is photoablation, or light based etching. The use of an excimer laser to remove selected regions of materials by photoablation in order to pattern the underlying layer is well known in the art. This is termed "ablative photodecomposition" (APD) and requires high powered pulsed sources. U.S. Pat. No. 4,414,059 describes an APD process in which ultraviolet radiation of wavelengths less than 220 nm cause fragmentation of resist polymer chains and the immediate escape of the fragmented portions from the resist layer. In effect the incident energy is transferred to kinetic energy in rapid and large amounts of bond breaking whereby the polymer chains are fragmented and explode-off as volatile by-products. This is a "positive" process since after the substrate is flooded with light the exposed regions of the polyimide as well as a soft mask will etch. The use of ultraviolet radiation particularly for APD and etching of polyimide is known in the art. U.S. Pat. No. 4,508,749 by Brannon et al describes the use a of U.V. source between 240-400 nm to etch through a polyimide layer. Brannon et al is primarily directed to producing tapered openings in the polyimide structure between metallization layers so that metallization can then be deposited in the openings to connect metallic layers adjacent to the top and bottom surfaces of the polyimide. Plasma etching is preferred over photoablation since plasma etching is fairly easy to apply over a wide field whereas currently photoablation is not.

FIGS. 2a-2f show a second embodiment for forming the conductor channels and vias in which any description for the first embodiment in FIGS. 1a-1f is incorporated herein insofar as the same is applicable, and the same description is not repeated. FIG. 2a shows lower soft mask 30 having openings 32 exposing via regions 20 but covering channel regions 22 placed over polyimide layer 14. FIG. 2b shows upper soft mask 34 having openings 36 exposing via regions 20 and channel regions 22 placed over lower soft mask 30. Via openings 32 and 36 are aligned to assure proper placement of via regions 20. FIG. 2c shows plasma etch 24 applied to upper soft mask 34. As plasma etch 24 is applied via regions 20 begin to etch and upper soft mask 34 begins to erode. FIG. 2d shows upper soft mask 34 completely eroded, lower soft mask 30 eroded over channel regions 22, and via regions 20 partially etched through polyimide layer 14. Plasma etch 24 continues, and in FIG. 2e via regions 20 are etched entirely through polyimide layer 14, channel regions 22 are etched partially through polyimide layer 14, and lower soft mask 30 has completely eroded. In FIG. 2f plasma pattern transfer has occurred and etch 24 is discontinued. It is understood that soft masks 30 and 34 are shown as completely eroded for illustration purposes only, since pattern transfer can also be achieved if one or both of soft masks 30, 34 are only partially eroded, in which case the non-eroded portions remaining after via regions 20 and channel regions 22 are formed and plasma etch 24 is discontinued would be removed from substrate 10 such as by stripping. The use of two soft masks instead of the soft mask and hard mask combination described in the first embodiment is a trade-off since two soft masks require fewer process steps but may require greater process control.

FIGS. 3a-3f show a third embodiment for forming the conductor channels and vias in a single step which is identical to the second embodiment in FIGS. 2a-2f, except the lower and upper soft masks have been switched. Lower soft mask 30a has openings 32a exposing via regions 20 and channel regions 22, while upper soft mask 34a has openings 36a exposing via regions 20a but covers channel regions 22a. The same via and channel etching results.

The first, second, and third embodiments for forming the conductor channels and vias in a single step are further advantageous in that the adverse effect of mask pinholes is minimized. Pinholes commonly form in masks to create unwanted openings. However, the location of pinholes tends to be random and highly scattered and the chances of pinholes lining up in an upper and lower mask are slim.

FIGS. 4a-4g show a fourth embodiment for forming the conductor channels and vias in which any description for the first embodiment in FIGS. 1a-1f is incorporated herein insofar as the same is applicable, and the same description is not repeated. FIG. 4a shows first soft mask 37, having openings exposing via regions 20, placed over polyimide layer 14 and covering channel regions 22. FIG. 4b shows first plasma etch 24a applied to first soft mask 37. As first plasma etch 24a is applied via regions 20 begin to etch and first soft mask 37 begins to erode. In FIG. 4c first soft mask 37 is either partially or completely eroded, via regions 20 are partially etched through polyimide layer 14, and channel regions 22 remain unetched. Alternatively, if desired, at this point via regions 20 can be completely etched. In FIG. 4d first plasma etch 24a is discontinued and second soft mask 38, having openings to expose via regions 20 and channel regions 22, is placed over polyimide layer 14. FIG. 4e shows second plasma etch 24b applied to second soft mask 38. Second plasma etch 24b continues. In FIG. 4f via regions 20 are etched completely through polyimide layer 14, channel regions 22 are etched partially through polyimide layer 14, and second soft mask 38 is either partially or completely eroded. In FIG. 4g second plasma etch 24b is discontinued and via regions 20 and channel regions 22 are formed. If photoresist from mask 37 or 38 remains then it is stripped.

FIGS. 5a-5g show a fifth embodiment for forming the conductor channels and vias which is identical to the fourth embodiment in FIGS. 4a-4g except the first and second soft masks have been switched. That is, first soft mask 37a has openings exposing via regions 20 and channel regions 22 whereas second soft mask 34a has openings exposing via regions 20 but covers channel regions 22. Consequently, via regions 20 can not be fully etched by the first etch 24a, otherwise the via and channel etching is the same as the fourth embodiment. The formation of vias and channels in a polyimide layer in accordance with the fifth embodiment was demonstrated as shown in FIGS. 15a and 15b.

FIGS. 6a-6e show a sixth embodiment for forming the conductor channels and vias in which any description for the first embodiment in FIGS. 1a-1f is incorporated herein insofar as the same is applicable, and the same description is not repeated. In FIG. 6a die stamp plate 40 has via abutments 42 and channel abutments 44 extending from plate surface 46. Via abutments 42 and channel abutments 44 are configured to the dimensions and placement of the desired via regions 20 and channel regions 22, respectively. Unlike the first embodiment, polyimide layer 14 is not fully cured (e.g. still in liquid form or soft baked). Preferably plate 40 is heated between 150.degree. C.-200.degree. C. if polyimide layer 14 is in liquid form. In FIG. 6b plate 40 is stamped against polyimide layer 14 until plate surface 46 contacts top polyimide surface 48. Abutments 42 and 44 selectively displace material in polyimide layer 14. In FIG. 6c plate 40 is retracted leaving via indentations 50 and channel indentations 52 in polyimide layer 14. Since excess polyimide 54 is likely to remain in via regions 20 beneath via indentations 50 it will be necessary to remove excess polyimide 54 to assure reliable interconnection between the conductive metal to be placed in the via and the underlying surface. In FIG. 6d a brief descumming plasma etch 56 similar to plasma etch 24 is applied to polyimide layer 14 in order to remove excess polyimide 54 and properly form via regions 20. In FIG. 6e brief descumming plasma etch 56 is discontinued. Post-etch surface 58 is slightly and uniformly lower than pre-etch surface 59 (shown as the broken line) by the thickness of excess polyimide 54. The relaxed requirements of brief descumming etch 56 allow a wide variety of preferred etches, such as plasma, wet chemistry, and photoablation. An alternative polymeric insulator such as epoxy may be advantageous due to the relatively low cost, low cure temperature and short cure time as compared to polyimide. Forming vias and channels by stamping the insulator with a plate may be advantageous for vias and channels with large geometries.

FIGS. 7a-7f show a seventh embodiment for forming the conductor channels and vias by photodefinition in which any description for the first embodiment in FIGS. 1a-1f is incorporated herein insofar as the same is applicable, and the same description is not repeated. In FIG. 7a over base 12 is provided a first photoimagible layer such as photoimagible polymeric having a thickness that defines the distance from the bottom of channel region 22 to the top of base 12, shown as an uncured 10 micron thick first photoimagible polyimide layer 14a. The use of photoimagible polyimide is conventional, see for instance K. K. Chakravorty et al, "Photosensitive Polyimide as a Dielectric in High Density Thin Film Copper-Polyimide Interconnect Structures", The Electrochemical Society Extended Abstracts, Vol. 88-1, Abstract No. 54, pp. 77-78 (May, 1988). In FIG. 7b first photoimagible polyimide layer 14a is selectively exposed to first mercury light 60 through first mask 62 which defines the vias in order to selectively alter the solubility of the exposed polyimide 14a. Preferably, as shown a negative process is used wherein the exposed polyimide 14a is outside via regions 20 and is rendered insoluable from irradiation, whereas the unexposed non-irradiated polyimide 14a in via regions 20 remains soluble. In FIG. 7c first mercury light 60 is discontinued and the soluble polyimide 14a is removed to form openings 64 in lower 10 microns of via regions 20. A soft bake can be applied to polyimide layer 14a to render it harder for further processing. In FIG. 7d a 5 micron thick second photoimagible layer shown as uncured photoimagible polyimide layer 14b is provided over layer 14a, wherein the thickness of layer 14b defines the depth of channel regions 22. In FIG. 7e second photoimagible polyimide layer 14b is selectively exposed to second mercury light 66 through second mask 68 to render insoluable the the exposed irradiated polyimide 14b outside via regions 20 and channel regions 22. In FIG. 7f second mercury light 66 is discontinued and the soluble polyimide 14b is then removed to form openings in channels 22 and upper portions 69 of via regions 20. Thereafter photoimagible polyimide layers 14a and 14b are fully cured. If desired a brief descumming etch 56 (not shown) may be applied. In the seventh embodiment two separate photoimagible insulating layers are necessary to form the vias and channels in a plane, however, the two layers can advantageously be coatings of the same material unlike the use of a separate etch-stop material in the prior art. The seventh embodiment has the advantages that the photodefinition of the vias and channels takes place in the final substrate insulating layer rather than a mask, and the need for a separate etch step is eliminated.

FIGS. 8a-8f show a eighth embodiment for forming the conductor channels and vias in which any description for the first embodiment in FIGS. 1a-1f is incorporated herein insofar as the same is applicable, and the same description is not repeated. In FIG. 8a a mask 70 is placed over polyimide layer 14, and then patternwise developed to have openings exposing via regions 20 and channel regions 22. For illustration purposes mask 70 will be a soft mask. While a hard mask is also suitable, soft mask 70 is preferred since after it erodes the plasma roughens the exposed surface of polyimide layer 14 which increases adhesion to subsequently applied materials. In FIG. 8b plasma etch 24 is applied. In FIG. 8c channel regions 22 and upper portions 72 of via regions 20 are formed, and soft mask 70 is completely eroded. In FIG. 8d plasma etch 24 is discontinued and laser beam 74 is applied from a CO.sub.2 laser source to thermally ablate and remove insulating material from the lower portions 76 of via regions 20. The power of laser beam 74 can vary widely since unlike the channels there is not critical depth control for the vias and the underlying polyimide is essentially drilled-out. In FIG. 8e via regions 20 are formed. In FIG. 8f laser beam 74 is discontinued. Thereafter a brief descumming etch 56 (not shown) may be applied if desired.

After via regions 20 and channel regions 22 are formed there are many ways of depositing an electrically conducting layer into the via and channel including electrolytic deposition, electroless deposition, evaporation, sputtering, and squeegeeing. By way of example, unless otherwise noted electrolytic depostion is described for FIGS. 9a-9b and FIGS. 10a-10c.

Referring now to FIG. 9a, after via regions 20 and channel regions 22 have been removed from polyimide layer 14, seed layer 80 of 2500 angstroms copper over 700 angstroms chrome is sputter deposited over top surface 82 of polyimide layer 14, walls 84 of channel regions 22, sidewalls 86 of vias 20, and base regions 87 exposed beneath vias regions 20. That is, seed layer 80 covers the entire surface of substrate 10 to provide adhesion for subsequent electrolytic deposition of an electrical conductor. While seed layer 80 will provide a more uniform coverage of via sidewalls 86 when they are sloped, should via sidewalls 86 be vertical then sputtered seed layer 80 would be thinner but still of sufficient thickness.

In FIG. 9b electrically conductive layer 90 such as copper is electrolytically deposited over the entire seed layer 80 so that the electrically conductive layer 90 fills via regions 20 and channel regions 22. Electrically conductive layer 90 is preferably the same thickness as the deepest via region 20 to assure via regions 20 are entirely filled to the top of polyimide layer 14 since valleys 91 are likely to form in electrically conductive layer 90 above via regions 20. In this example polyimide layer 14 (and thus initially via regions 20) are 15 microns thick and seed layer 80 is very thin, therefore after seed layer 80 is deposited the deepest via region 20 and the preferred thickness of electrically conductive layer 90 is about 15 microns. Should electrically conductive layer 90 be thicker than 15 microns then additional planarization will be required as described in FIGS. 11a-11c. Should electrically conductive layer 90 be thinner than 15 microns then via regions 20 may not be entirely filled; however, if electrically conductive layer 90 substantially fills via regions 20, such as by covering sidewalls 88 with pit 89 therebetween, then proper electrical interconnection can still be made between conductors 90 in via region 20 and an overlaying conductor. During electrodeposition, high current densities at the top edges of the trenches may cause ridge build-up of the conductive material thereon which blocks the conductive material from filling the trenches. In this case reverse pulse plating can be used during electrodeposition to continually shave back ridge build-ups.

In FIGS. 10a-10c another method is shown for depositing conductors in the channels and vias. A first electrical conductor 92 is plated, preferably by electroless deposition, into the vias in lower regions 94 which extend to bottom surface 96 of channel regions 22, wherein an electrically conductive base 12a can serve as a first seed layer 98. In FIG. 8b second seed layer 100 is sputtered over intermediate surface 102. In FIG. 8c second electrically conductive layer 104 is electrolytically deposited over second seed layer 100. It is noted that since the lower via regions 94 are already filled with first electrical conductor 92 it is only necessary to deposit 5 microns of second electrical conductor 104 in order to assure that via regions 20 and channel regions 22 are filled. This is advantageous since 10 microns less of electrical conductor will need to be removed above polyimide layer 14 then if the vias and channels are filled with a single 15 micron conductive layer as shown in FIG. 9b.

Alternatively, the electrical conductors 90, 92 and/or 104 can be deposited by other processes such as evaporation or sputtering which may obviate the need for seed layers 80, 98 and/or 100, respectively. While evaporation is faster than sputtering, sputtering may be preferred for thin metallization, for instance if the vias and channels have been formed in an integrated circuit.

Another method for depositing the conductors in the channels and vias is electroless deposition (not shown). The insulating layer must first be treated to be receptive to an electroless plating bath. U.S. Pat. No. 4,701,351 by Jackson discloses a seeding process for electroless metal deposition in which a substrate is coated with a thin (1 micron) layer of a polymer which complexes with a noble metal compound, and the layer of polymer is contacted with a noble metal compound to form a complexed polymer. Thereafter the substrate is placed in an electroless plating bath. A mask can be used to expose selected regions of the polymer to the noble metal compound and/or electroless plating bath. After electroless deposition the substrate surface is planarized such as by polishing. An advantage of electroless deposition is that the pre-deposition treatment layer need not be a conductor. A disadvantage of the predeposition polymer treatment in Jackson is that the polymer seed layer does not typically adhere to the substrate as well as a sputtered conductor seed layer. As compared to electrolytic deposition, electroless deposition tends to provide more uniform coverage but the process takes more time.

Another way (not shown) of filling the vias and channels with a conductor is by squeegeeing a liquid metal or metal paste across the surface. After squeegeeing the liquid metal may stay clear of the polyimide surface. In the likely event excess liquid metal remains on the substrate surface it must be removed, as by lightly laping the substrate surface to assure a planarized and non-short-circuited substrate top surface. A disadvantage of squeegeeing is that liquid metals are not as conductive as, say, copper, and thereby may seriously degrade the performance of high density substrates with fine line pitches.

Referring now to FIGS. 11a-11c, the final step of the present invention is planarizing the substrate top surface until electrical conductor 90 remains only in the channels and the vias and the substrate top surface is substantially smooth. For illustration purposes a substrate with seed layer 80 and electrically conductive layer 90 deposited in accordance with FIGS. 9a-9b will be shown. In FIG. 11a top surface 106 of substrate 10 is contacted by polishing means 108 to begin planarization. The polishing should not locally conform to the unevenness in conductor 90 but rather shave off the highest parts of conductor 90 with little or no removal of material elsewhere. Polishing can be performed by mechanical polishing, mechanical/chemical polishing, electropolishing and the like. Mechanical polishing and mechanical/chemical polishing of polyimide are well known in the art and will not be detailed herein. Electropolishing is also known in the art, for instance R. J. Contolini et al in "Embedded Conductors by Electrochemical Planarization", Abstract No. 184, Abstracts From the Spring Electrochemical Society Meeting, Los Angeles, Calif., May 7, 1989 discuss a process which can produce planarized multilayer interconnects incorporating vertical-walled conductors that is applicable to any metal which can be electroplated and electropolished, such as copper and gold. See also C. W. Koburger, "Trench Planarization Technique", IBM Technical Disclosure Bulletin, Vol. 27, No. 6, pp. 3242-3243 (Nov., 1984) in which a nonselective etch is used to etch a trench structure just past the original surface, resulting in a surface that is neither roughened nor otherwise degraded by the etch. In FIG. 11b polishing means 108 is actuated toward substrate 10 and planarly removes the top of substrate 10 until contact is made with top surface 110 of polyimide layer 14. Seed layer 80 and electrically conductive layer 90 are thus removed by polishing means 108 from the non-via/non-channel regions 112 on top surface 110. That is, seed layer 80 and electrically conductive layer 90 are shaved back until they substantially fill via regions 20 and channel regions 22 but do not extend above polyimide layer 14. In FIG. 11c polishing means 108 is retracted. Polishing can be discontinued a fixed time 109 after polyimide layer 14 is exposed, and fixed time 109 can be selected so that only a negligible amount of polyimide will be removed. Optionally, a brief plasma etch-back (not shown) of the polished polyimide surface may be applied to provide roughening and thereby improve adhesion to subsequently deposited materials. The top surface 110 of substrate 10 is substantially smooth and electrically conductive via 113 is adjacent and interconnected to electrically conductive channel 114.

Referring now to FIG. 12, substrate 10 can be fabricated into a multilayer electrical interconnect 116 with stacked vias by repeating the above steps.

In FIG. 13 conductor 90 is etched-back a slight distance 118 to provide registration for electrical component 120 containing conductive bumps 122.

The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the invention have been described for the purpose of disclosure, numerous changes in the details of construction, and steps of the process, will be readily apparent to those skilled in the art and which are encompassed within the spirit of the present invention and the scope of the appended claims.