Method of Making a Semiconductor Device Including via Holes

The present invention relates to a "via-holed" semiconductor device which includes "via-holes" (also called through-holes) which are formed in a semiconductor substrate to extend therethrough and through which components formed on one surface of the semiconductor substrate are electrically connected to another surface of the substrate. The present invention relates also to a method of making such a semiconductor device.

One example of a "via-holed" semiconductor device and method of making it is shown in FIGS. 1(a) through 1(d).

A plurality of semiconductor elements having source electrodes 2, drain electrodes 3 and gate electrodes 4, respectively, are formed on a front surface of a semiconductor substrate 1. Then, the semiconductor substrate 1 is thinned to a desired thickness by etching or grinding (or any other suitable technique) the substrate 1 from the opposing rear surface. Next, as shown in FIG. 1(a), a first resist coating 21 is used as a mask to etch a large recess 22 in a rear surface region corresponding to the semiconductor elements, particularly the source electrodes 2 in the illustrated example, of the respective semiconductor elements. After the formation of the recess 22, the first resist coating 21 is removed. Then, as shown in FIG. 1(b) and 1(c), a second resist coating 23 is formed, which coating 23 acts as a mask for etching the substrate 1 in the bottom of the recess 22 to form a through-hole 24 extending to each of the source electrodes 2. After that, the second resist coating 23 is removed. Finally, as shown in FIG. 1(d), the rear surface of the substrate 1, the inner surface of the recess 22 and the inner surface of each of the through-holes 24 are coated with a metal layer 25 so that the respective source electrodes 2 and the rear surface of the semiconductor substrate 1 are electrically interconnected.

Another example of conventional "via-holed" semiconductor devices is disclosed in Japanese Published Patent application 63-155673 published on June 28, 1988. The semiconductor device of this publication has a structure as shown in FIG. 2, which is fabricated by thinning a substrate 1 to a thickness on the order of 30,.mu.m, forming individual stepped through-holes 30 one for each source electrode 2, and thereafter coating the inner surfaces of the through-holes 30 and the rear surface of the substrate 1 with a metal layer 31.

According to the conventional method of making a via-holed semiconductor device shown in FIG. 1, because the large recess 22 must be first formed by the first etching process before etching the through-holes 24, a large step D is formed between the rear surface of the substrate 1 and the bottom surface of the recess 22. That bottom substrate is coated with the second resist coating 23 acting as a mask for forming the through-holes 24. Because of this step D, it is difficult to form the second coating 23 with a uniform thickness. This step, furthermore, prevents accurate exposure of the second resist coating 23 to a pattern of radiation for patterning the coating 23.

As for the semiconductor device shown in FIG. 2, because the substrate 1 must be thinned to a small thickness on the order of 30 .mu.m, there is a decrease in strength of the substrate and, when the device is a monolithic microwave integrated circuit device, an increase in transmission loss between microstrips. Further, since in the conventional semiconductor device of FIG. 2 the through-holes 30 are individually formed for each source electrode 2, the thermal resistance of the device is undesirably large.

Therefore, the object of the present invention is to eliminate the above-stated problems.

According to the present invention, a semiconductor substrate has a plurality of semiconductor elements having electrodes formed on a first surface thereof, and a plurality of small recesses formed in a second surface of the substrate at locations corresponding to those electrodes of the semiconductor elements for which through-holes are to be formed. The material of the substrate in and around the small recesses is etched away to form a single large recess which encompasses the small recesses and, at the same time, to cause the small recesses to extend through the substrate to thereby form a plurality of through-holes which extend from the bottom of the large recess to the said respective electrodes.

FIGS. 1(a) through 1(d) are cross-sectional views of a manufacturing process for a via-holed semiconductor device;

FIG. 2 is a cross-sectional view illustrating the structure of a conventional via-holed semiconductor device;

FIGS. 3(a) through 3(d) are cross-sectional views of a semiconductor device according to one embodiment of the present invention, for explaining various manufacturing steps; and

FIG. 4 is a cross-sectional view of a semiconductor device according to another embodiment of the present invention.

Referring to FIG. 3, a plurality of semiconductor elements with source electrodes 2, drain electrodes 3 and gate electrodes 4 are formed in an upper surface of a semiconductor substrate 1. Next, the semiconductor substrate 1 is thinned to a predetermined thickness (e.g. a thickness of more than 70 .mu.m) by etching or grinding the substrate from an opposing bottom surface. (Any other suitable technique may be used for thinning the substrate.) Next, as shown in FIG. 3(a), a first resist coating 5 is formed on the bottom surface of the substrate 1, which acts as a mask for etching the substrate 1 from the bottom surface of the substrate 1 toward the upper surface. The etching forms small recesses 6 in the rear surface at locations opposite the respective source electrodes 2. After the formation of the small recesses 6, the first resist coating 5 is removed. Next, as shown in FIGS. 3(b) and 3(c), a second resist coating 7 is deposited and used as a mask to etch away the material of the substrate 1 in and around the small recesses 6. This etching results in the formation of a single large recess 8 in a region which encompasses the respective small recesses 6. This etching also causes the small recesses in the bottom surface to extend farther through the substrate so that a plurality of through-holes 9 are formed, which extend from the large recess 8 to the respective source electrodes 2. After that, the second resist coating 7 is removed.

Better results can be obtained if a dry-etching technique is used for forming the small recesses 6, the large recess 8 and the through-holes 9, because etching in the lateral direction is smaller in dry-etching technique than in wet etching.

Finally, as shown in FIG. 3(d), a metallic layer 10 is formed by vapor-deposition, covering the bottom surface of the substrate 1, the inner surface in the large recess 8, and the inner surfaces of the respective through-holes 9 whereby the respective source electrodes 2 and the bottom surface of the semiconductor substrate 1 are electrically connected together.

In place of the metallic layer 10 of FIG. 3, a metallic layer 11 shown in FIG. 4 may be formed by plating as a PHS (plated heat sink). When the structure shown in FIG. 4 is employed, reductions in the source inductances of the respective semiconductor elements and a reduction in the thermal resistance of the semiconductor device are achieved.

The present invention has been described by means of embodiments in which source electrodes of semiconductor elements are connected to a bottom surface of a semiconductor substrate by a metallic layer, but the electrodes are not limited to source electrodes.