Multilayer Structure and Method for Fabricating the Same

1. Field of the Invention

The present invention is related to a multilayer structure employed in hybrid IC (integrated circuit) or semiconductor devices, etc. More particularly, the invention is related to a reliable method and means for electrically interconnecting conductive layers having an insulative resin layer disposed therebetween.

2. Description of the Related Arts

Many kinds of multilayer structures and fabrication methods have been employed for fabricating hybrid ICs (integrated circuits) and semiconductor devices. A typical structure and manufacturing method are described in Japanese Unexamined Patent Publication Sho-63-244796 and shown here in FIG. 1. In accordance with this method, a wiring pattern 31 is formed on a first electrically conductive copper layer 38 that is disposed on an insulative glass substrate layer 37. The wiring pattern 31 is formed using a first resist pattern 32. A second resist pattern 33 having a hole 34 located above wiring pattern 31 is formed on resist pattern 32 (FIG. 1a) and a pillar 35 is formed in hole 34 by an electroplating. The first and second resist patterns 32 and 33 are removed and then a polyimide resin layer 36 (FIG. 1b) is formed over pillar 35, wiring pattern 31 and conductive layer 38. The top of pillar 35 is exposed by polishing or etching the surface of the polyimide resin layer 36 and a second electrically conductive layer 38' (FIG. 1c) and a second wiring pattern 31' (FIG. 1d) are formed over polyimide resin layer 36 and the top of pillar 35.

The spirit of this prior art structure is to flatten the top of pillar 35 as can be comprehended from the foregoing description and from FIGS. 1(b) and 1(c). However, when the top of the pillar is flat the electrical contact between the pillar top and electrically conductive layer 8 is not strong enough to withstand the mechanical stresses resulting from heating cycles, etc. This presents a possibility that the connection will break resulting in a deterioration in the reliability of the electrical connection between the multiple layers. Moreover, if the pillar top is flattened by mechanically polishing the surface of insulative layer 36, the polishing operation is very difficult because the substrate might have a tendency to bend due to the presence of the polyimide resin coating.

It is a general object of the invention to provide a multilayer structure and method for fabricating such structure wherein two patterns isolated by an insulative layer therebetween are reliably electrically interconnected

It is another object of the invention to provide a multilayer structure and a method for fabricating such structure wherein the improved flatness of each wiring pattern allows for a greater number of layers in the overall structure.

A multilayer structure is fabricated by first forming a first electrically conductive layer on an insulative substrate. A first electrically conductive pattern is then formed on the substrate. A resist pattern is formed over the first layer and the first pattern and the resist layer is provided with a via hole therethrough which communicates with the first pattern. A via lead is formed in the via hole by depositing a metal material in the via hole by electroplating utilizing the first layer as an electroplating lead. A predetermined portion of the first layer and all of the resist pattern are removed and an insulative resin layer is formed over the substrate, the first pattern and the via lead. The surface portion of the resin layer is removed chemically until the top segment of the via lead protrudes to a predetermined height above the surface of the resin layer. A second electrically conductive layer is formed over the resin layer and over the top of the via lead and a second pattern is formed on the second layer. The above steps are repeated for fabricating the required number of layers for the multilayer structure.

The above-mentioned features and advantages of the present invention, together with other objects and advantages which will become apparent, will be more fully described hereinbelow, with reference being made to the accompanying drawings which form a part of the description. In the drawings, like numerals refer to like parts throughout.

FIGS. 1(a) through 1(d) schematically illustrate the fabrication steps for a prior art multilayer structure; and

FIGS. 2(a) through 2(q) schematically illustrate the fabrication steps of a preferred embodiment of the present invention, and wherein FIG. 2(q) schematically illustrates a completed three-layer structure.

A preferred embodiment of a multilayer structure of the present invention is hereinafter described with reference to the fabrication steps shown in FIGS. 2(a) through 2(q). With reference to FIG. 2(a), a first electrically conductive layer 2 is formed upon a surface of an insulative substrate 1 which is formed, for example, of a ceramic material, by depositing electrically conductive materials. Layer 2 is formed such as a chromium film having a thickness of 1500 angstroms, a titanium film having a thickness of 500 angstroms and a copper film having a thickness of 10000 angstroms on substrate 1. Next, an electrically conductive pattern 3 is formed on by depositing thereon, for example, a 5 .mu.m thick copper layer, a 2 .mu.m thick nickel layer and a 1500 angstrom chromium layer using known electroplating methodology. In such electroplating process, the first electrically conductive layer 2 can be utilized as an electrical lead for the electroplating process. In this preferred embodiment, ceramic substrate 1 has been provided in advance with a via lead 6 which extends through substrate 1 at a predetermined location, whereby pattern 3 is provided with an electrical connection which is led out to the rear of substrate 1. As can be seen in FIG. 2(b), first electrically conductive layer 2 and first pattern 3 are coated with a photosensitive resist layer 4 having a predetermined thickness, for example, of approximately 20 .mu.m.

With reference to FIG. 2(c), it can be seen that a via hole 4A having a diameter of approximately 80 .mu.m is provided at a predetermined location in photosensitive resist layer 4, above first pattern, using a known method such as light exposure, developing and etching. As can be seen, hole 4A communicates with the upper surface of pattern 3.

With reference to FIG. 2(d), copper is electroplated to a thickness of approximately 25 .mu.m in via hole 4A so as to form a via lead 5 therein. This electroplating process is carried out under carefully controlled plating conditions such that a brightener is added in the plating solution and the current density is typically 5 to 10 A/cm.sup.2, so that an excellent leveling effect is achieved and lead 5 is provided with high tensile strength as well as low electrical resistance.

FIG. 2(e) illustrates the next step where photo resist 4 is removed completely and first electrically conductive layer 2 is removed except for the portion thereof which is beneath first pattern 4.

As illustrated in FIG. 2(f), a coating 7-1 of a non-photosensitive resin, such as polyamic acid resin, is coated over substrate 1, first pattern 3 and via lead 5. Thereafter the resin coating is heating at 350 to 450 degrees C. for 30 to 60 minutes, so that the resin hardens and coating 7-1 becomes a first insulative polyimide resin layer 7-1. The coating conditions, such as the viscosity of the polyamic acid resin, are adjusted so that the coating thickness is approximately 22 .mu.m at flat areas of the structures, and 0.5 to 0.8 .mu.m over the top of via lead 5 which has a height of 25 .mu.m

Thereafter, with reference to FIG. 2(g), the surface of first insulative layer 7-1 is removed using the known RIE (reactive ion etching) procedure or an ion milling technique employing oxygen (O.sub.2) and tetrafluoride carbon (CF.sub.4) as etchants, until the top segment 5A of via lead 5 is exposed to a height of 3 to 5 .mu.m above the surface of first insulative layer 7-1. During this etching process, the selected etchants do not react with the metal of via lead 5.

As can be seen in FIG. 2(h), a second electrically conductive layer 8 and a second pattern 9 are formed over first insulative layer 7-1 as well as over the exposed via top segment 5A in the same way that the first electrically conductive layer 2 and first pattern 3 were initially formed over substrate 1. The plating conditions can be chosen from among a number of widely known techniques so as to provide excellent adhesion between the layers as well as a leveling effect so that the thickness of layer 8 and pattern 9 over the via top segment 5A are less than at other flatter portions of the structure. As a result, the upper surface of the thus formed second pattern 9 can be relatively flat even over the 3 to 5 .mu.m protrusion of via top segment 5A. Furthermore, the 3 to 5 .mu.m protrusion of via top segment 5A provides a non-flat shape, which distributes the stresses generated at the contact borders and provides a wider area of contact with the pattern 9 plated thereover than was provided by the flat top of the pillar 35 of the prior art. As a result of the favorable shape of via top segment 5A and favorable plating conditions, first pattern 3 and second pattern 9 are more securely connected by via lead 5.

As described above in connection with FIG. 2(f) the polyamic acid resin first adheres to the via lead 5, and then the polyamic acid resin is hardened by heat so as to become a polyimide resin. Thus, the adhesion between the via lead 5 and the resin 7-1 is excellent. On the other hand, in the prior art structure where the via and the upper pattern are concurrently plated in a previously provided via hole in the insulative layer, there can be no adhesion between the via lead and the insulative layer.

Moreover, in the above-described structure and fabrication method, first insulative layer 7-1 does not need to be photosensitive. Accordingly, a polyimide resin which has a thermal expansion coefficient that is substantially equal to that of via 5 can be employed since many kinds of non-photosensitive polyimide resins having a variety of thermal expansion coefficients are available. Therefore, the problem which is encountered when a photosensitive polyimide resin is employed as an insulative layer, in that the stresses generated in the insulative layer cause separation of the pattern from the via lead, does not exist in the device and method of the present invention. These factors together with the abovedescribed secure connection between the via lead and the pattern prevent breaking of the connection between the via and the pattern and contribute to the enhancement of reliability of the connections.

If a third pattern is to be fabricated over second pattern 9, the processes described above are repeated. That is to say, as seen in FIG. 2(i) a photoresist 4' is coated over the second electrically conductive layer 8 and the second pattern 9.

Next, as seen in FIG. 2(j), a second via hole 4A' is etched out at a predetermined place in photoresist 4'.

With reference to FIG. 2(k), copper is electroplated in second via hole 4A' 50 as to form a second via lead 5' on first pattern 9 by utilizing second electrically conductive layer 8 as a lead for the electroplating procedure.

Photoresist 4' is removed completely and second electrically conductive layer 8 is removed in all areas except for the areas beneath second pattern 4', as can be seen in FIG. 2(l).

As illustrated in FIG. 2(m) polyamic acid resin is coated over first insulative layer 7-1 and second pattern 9, and is then heated to become polyimide resin and present a second insulative layer 7-2.

FIG. 2(n) shows the next step where the surface of second insulative layer 7-2 is removed so as to expose 3 to 5 .mu.m of the top segment 5A' of second via lead 5'.

As shown in FIG. 2(p), a third electrically conductive layer 10 and a third pattern 11 are formed over second insulative layer 7-2 and over the exposed via top segment 5A'.

Finally, as illustrated in FIG. 2(q), third electrically conductive layer 10 is removed except for the portion thereof beneath third pattern 11.

Thus, a multilayer structure having a second pattern which is connected to a first pattern and a third pattern which is, and third pattern connected to the second pattern, by via leads 5 and 5' respectively, is shown in FIG. 2(q).

If a fourth pattern is required, layer 10 is not removed and instead the above-described steps as illustrated in FIGS. 2(i) through 2(q), are repeated. By repetition of the above-described steps, as many layers as required can be stacked, causing no problems in reliability or deterioration due to non flat surfaces of the layers, because the patterns are adequately flat even when plated over the via top segments 5A and 5A'. The thicknesses of the various layers in FIGS. 2(a) through 2(q) are shown enlarged; therefore, the figures do not show the real flatness.

In the above description of the preferred embodiment, the first, second and third patterns 3, 9 and 11 are described as being formed as copper, nickel and chromium laminations however, it is apparent that other metals, such as gold, chromium/gold laminations, or chromium/platinum laminations are applicable for forming the layers of the invention. Chromium surfaces provide excellent adhesion with polyimide resin.

In the above description of the preferred embodiment, the first, second and third electrically conductive layers 2, 8 and 10 are described as being formed as chromium, titanium and copper laminations. However, it is apparent that the layers may also be formed of other metals, such as titanium/copper laminations, titanium, tungsten and copper laminations or tantalum, nickel and copper laminations, in the present invention.

In the above description of the preferred embodiment, first pattern 3 is formed over first electrically conductive layer 2. However, it is apparent that first electrically conductive layer 2 might as well be formed over first pattern 3 which has been formed directly on substrate 1. In this case the via lead 5 of the invention will be formed on the first electrically conductive layer 2.

In the above description of the preferred embodiment, the first and second insulative layers 7-1 and 7-2 are formed of polyimide resin. However, it is apparent that the insulative layers might also be formed of other insulative materials, such as a fluororesin.

In the above description of the preferred embodiment, substrate 1 has a via lead 6 therein which has been provided in advance. However, it is apparent that the invention can be embodied in a structure having a substrate that has no via lead therein. On the other hand, the first pattern 3 and the via lead 5 may be formed by electroplated using the via lead 6 as a lead for the plating operation in cases where first electrically conductive layer 2 is deleted.

The many features and advantages of the invention are apparent from the foregoing detailed specification and it is intended by the appended claims to cover all such features and advantages which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, the description is not intended to limit the invention and accordingly, all suitable modifications or equivalents falling within the scope of the invention or intended to be covered.