This invention relates to a multilayer wiring substrate and a process for producing the same. More particularly, the present invention relates to a multilayer wiring substrate realizing thin film multilayering containing a metallic layer structure excellent in chemical resistance and prevention of generation of hillocks and whiskers.
Multilayer wiring substrates heretofore used in electronic computers and the like are formed by alternately laminating wiring layers and insulating layers on a substrate and connecting upper and lower wiring layers via through-holes formed through insulating layers. The material for insulating layers used in such multilayer wiring substrates is required to have properties such as a high glass transition point, excellent heat resistance, a small thermal expansion coefficient, security for processing precision. As a material meeting such requirements, there have generally been used polyimide resins.
As processes for producing multilayer substrates using a polyimide as an insulating layer, there are known a so-called wet etching method wherein a polyimide is subjected to pattern formation by wet etching (disclosed in, for example, IEEE, 33rd, Electronic Components Conferrence, pp. 610 to 615, 1983), and a so-called photosensitive polyimide method wherein a photosensitive polyimide is used for pattern formation (disclosed in, for example, IEEE, 34th Electronic Components Conference, pp. 82 to 87, 1984).
According to the wet etching method, the polyimide is coated on a substrate and cured with heating to form a film of predetermined thickness, followed by formation of a resist pattern thereon and etching of through-hole portions. Thus, processable film thickness depends on resistance to etching solution of the resist. When a metal resist such as Mo or the like is used, it is possible to process a film of upto about 50 .mu.m in thickness. Thus, this method is advantageous for forming a thick film polyimide pattern necessary for multilayer wiring substrates compared with the photosensitive polyimide method. On the other hand, according to the photosensitive polyimide method, since a polyimide precursor is exposed to light and developed to directly form a pattern, followed by curing with heating, it is difficult to form a pattern of polyimide film of 10 .mu.m or more in thickness considering light transmittance of the resulting photosensitive film.
In the case of applying the wet etching method or the photosensitive polyimide method, when Al is used as a wiring metal, there take place hillocks or whiskers and surface oxidation due to the thermal hysteresis of coated polyimide film during the curing step. When Cu is used as a wiring metal, surface oxidation easily takes place. In either case, such phenomena bring about problems of causing short circuit between wirings on the multilayer wiring substrate, and increasing a resistance value at connecting portions.
Further, according to the wet etching method, since a strong basic etching solution such as a hydrazine hydrate solution, an aqueous solution of potassium hydroxide, or the like is used for etching, many problems are brought about by generating corrosion of undercoating wiring metal, generating an undercut (a kind of erosion) at a tapered surface of lower portion of through-hole in polyimide film due to the generation of corrosion, generating falling of an upper wiring at the undercut portion, and damaging electric reliability of the multilayer substrate.
The wet etching method is explained referring to FIGS. 3A to 3D. In FIGS. 3A to 3D, numeral 1 denotes a mullite substrate, numeral 2 denotes an Al metal wiring layer (Al wiring layer) formed by sputtering on the mullite substrate 1, numeral 3 denotes a polyimide insulating layer formed on the Al wiring layer, and numeral 4 denotes an etching resist necessary for patterning the polyimide insulating layer 3. When the polyimide insulating layer 3 in the state as shown in FIG. 3A is etched by using a hydrazine hydrate etching solution (e.g. hydrazine hydrate/ethylenediamine =7/3 by volume), the undercoating Al wiring layer 2 is exposed as shown in FIG. 3B. By this exposure, the exposed portion of the Al wiring layer 2 is corroded (hereinafter referred to as "Al corrosion") to generate the undercut 5 encircled by dotted line at the bottom portion of the tapered surface of the through-hole 7 in the polyimide insulating layer 3. The Al corrosion and the undercut 5 gradually grow until the state shown by FIG. 3C. Then, the etching resist 4 is peeled off. After peeling, an upper wiring layer 6 is formed on the polyimide insulating film 3 by sputtering or the like. But a portion of the upper wiring layer 6 formed is fallen as shown in FIG. 3D to cause failure in burying laminating metal wiring in a later step and generation of pin holes in the polyimide insulating layer 3, resulting in not only damaging electric reliability of multilayer wiring substrate but also lowering productivity of elements.