Method for the production of semiconductor devices with an integral heatsink and of related semiconductor equipment

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The present invention concerns a procedure for the production of semiconductor devices with an integral heatsink or means for dissipating heat through attachments, and also for the production of related semiconductor devices. The invention deals especially with a method for the production of semiconductor devices of the type where the separation of the individual devices occurs automatically in the same incision device which creates the very structure thereof. As is known, heat dissipation is a very delicate problem for all microwave devices, because their operating conditions call for high power densities, usually in excess of 10.sup.8 watts per cm.sup.3. Therefore, the operating temperature of the device becomes rather high in the active zone of the diode, usually much higher than 100.degree. C. The most common solution for this heat problem, at present, is to establish a direct contact of the active zone of the device with a heat-diffusion member, for example, a block of copper which is gold-plated, and this can be produced by a soldering process under use of pressure, as described in the article by A. M. Cowley, "Design and Application of Silicon Impatt Diodes", Hewlett-Packard Journal, May, 1970, Page 4. Another solution is to use an integral heatsink having a continuous electrolytic coating in the heat dissipator on the surface of the wafer on the side of the P.sup.+ / N junction of the diode, whereafter one forms the Mesa (center) structure directly upon the heat-dissipating part, and then separates the individual diodes by a mechanical cutting from the dissipator; this is shown in R. A. Zettler and A. W. Cowley, "Batch Fabrication of Integral Heatsink IMPATT Diodes", Electronics Letters, Vol. 5, No. 26 (December 27, 1969); also B. Hoefflinger, "Recent Developments on Avalanche Diode Oscillators", Microwave Journal, Vol. 12, No. 3 (March, 1969). It should be pointed out that the technique of thermocompression as offered in the first solution is beset with serious problems, because it is difficult to consistently reproduce the same soldering step with the same quality thermal and mechanical characteristics, of the soldered spot.

With respect to the second prior art solution outlined above, it must be kept in mind that the electrolytic deposition of a heat dissipator of appreciable thickness, which is a continuous metal layer of a thickness of 100 or more microns upon a large surface which often is greater than 10 cm.sup.2, as this is the case for the constructions dealt with here, is bound to bring about the creation of an internal stress which could give rise to possible deformations in the equipment, with the consequence of deterioration of its mechanical characteristics. Furthermore, the separation of the individual semiconductor devices produced by a mechanical cutting of the heatsink, because the thickness of the metal of this sink is so great, easily introduces additional strains in the metal, which is frequently the cause of a damage to the semiconductor device, so that the electrical characteristics thereof deteriorate. Further, such mechanical technique always brings about a surface contamination of the semiconductor, with a possibility of losses due to surface currents.

In the solution proposed by the invention described hereafter, the electrolytic deposition occurs in what one may term "an insular manner", i.e., individual, small depositions form the heatsink.

It is now the fundamental concept of the present invention to bring about a process for the production of semiconductor devices with an integral heatsink which eliminates or at least reduces essentially the disadvantages of prior methods. The invention has in its scope furthermore to disclose a process which is fully compatible with the operatory techniques presently used in the production of semiconductor devices.

The technique proposed in the present invention, when it is used to prepare equipment in the manner of a batch process, offers the advantage of bringing about very high yields, which are superior to the yields experienced under prior art procedures. The new technique of the present invention allows an automatic separation of all the devices from each other, which were created as one batch, whereas the procedures of the prior art require a mechanical cutting. Another advantage of the present invention is the great flexibility of the planning with respect to dimensions and structure of the integral heatsink. The present invention avoids the prior art limitations with respect to the shape of the dissipator. Where the prior art process of mechanical cutting is involved, it is impossible to create shaped dissipators, as for example, circular.

Another advantage of the present invention is that it permits obtaining of very small dimensions for parts to be used in microcircuits.

The invention thus has as special scope a procedure for the production of semiconductor devices with integral heatsinks starting out from semiconductor materials which have a P.sup.+ / N or an N.sup.+ / P junction, and it involves the steps of covering uniformly a wafer of semiconductor material with a layer of dielectric; applying photographically incisions in the dielectric layer of the surface of the wafer covering the junction, which is limited to a finite number of zones which shall constitute the locations of the sinks; covering such zones with a layer of a flexible metal, the thickness of which is built up electrolytically; attaching said wafer with an adhesive to a support upon the surface which covers the junction; thinning or finishing or smoothing the opposite surface of the wafer; depositing a flexible metal layer upon said second surface in zones which will form the electrical resistance contact of the device, which zones correspond to the zones of metal deposition upon the first surface and which are exactly aligned therewith; and removing from the body of the wafer, in the spaces which form the interstices between and among those zones, the semiconductor material so that one obtains the separation of the semiconductor units with Mesa profile, equipped with sinks.

It should be pointed out that the process of the invention is applied to slices or wafers of semiconductor material where the P.sup.+ / N or the N.sup.+ / P junction was created either by thermal diffusion or by ionic implant by formation of a rectifier barrier metal-semiconductor, or also by epitaxial growing.

The dielectric layer can be obtained by a heating in an oxidizing atmosphere, by a cathodic deposition at low pressure of the dielectric material, by precipitation by evaporation in vacuo of the dielectric material, by a reactive deposition of silicon oxide (or another dielectric) at a low temperature, by anodic oxidation, or by deposition of oxides in the shape of emulsions.

With regard to the deposition of the flexible metal layer, this can be accomplished by use of one of the following techniques: Evaporation in vacuo, cathodic deposition at low pressure, electrolytic deposition, or chemical deposition without the passage of any electrical current.

The formation of the zones where the sinks are located and of the zones of ohmic contact may be brought about by a photographic etching or by deposition across a screen.

To accomplish the thinning or smoothening of surfaces, one uses the techniques of deburring, sanding and the electrochemical process.

The technique for the formation of the Mesa structure involves chemical attack or sanding.

The invention shall be described with reference to the accompanying drawings, which describe a preferred embodiment.

In the drawings:

FIG. 1 shows a view, as a section, of a sheet or wafer of the semiconductor material which has junctions and is covered with oxide;

FIG. 2 is a view as a section of the sheet of FIG. 1 where the layer of oxide, which covers the junction, is partially cut away due to a photoengraving;

FIG. 3 is a view as a section of the sheet of FIG. 2 after the deposition of the flexible metal layer upon this layer or coating which covers the junction;

FIG. 4 is a view as a section of a sheet according to FIG. 3 where the metal layer has become subjected to a photoengraving;

FIG. 5 is a view as a section of a sheet according to FIG. 4 in which the zones which become the supports of the sink have been caused to be built up on the base of an electrolytic deposition;

FIG. 6 is a view, as a section, of a sheet according to FIG. 5, adhered to a support and rendered thin (smoothed);

FIG. 7 is a view as a section of a sheet according to FIG. 6 where the free surface has become coated with a flexible metal layer;

FIG. 8 finally shows as a section, two semiconductor portions or units with Mesa structure according to the invention.

Referring to FIG. 1, there is shown a sheet or wafer 2 of a semiconductor material provided with junction 1 and coated upon both of its surfaces with a layer 3 of oxide. The first step of the process of the present invention may be recognized from FIG. 2 where there are shown as 4, two zones in which the layer of oxide which covers the junction has become removed. In FIG. 3 is shown the deposition of a flexible metal layer 5 over all of the surface of the sheet which has undergone photoengraving. Thereafter, there occurs a removal of the flexible metal layer across the full surface of the sheet with the exception of those areas where the oxide had become removed, these areas remain coated with metal as indicated at 6 in FIG. 4.

In FIG. 5 there is shown the deposit 7 upon these areas 6. Thereafter, the sheet 2 becomes attached under use of the adhesive 9 upon a support 8. This is followed by a smoothing or deburring or thinning of the sheet, as is shown in FIG. 6. Next, as is shown in FIG. 7, a metal layer 10 is deposited upon all of the free surface of the sheet 2. The final step in the process of this invention is the creation of the Mesa structure, thereby the semiconductor units 11 become separated, they are equipped with integral thermal sinks 12, as one may recognize from FIG. 8.

The following is a description of one example of the manufacture of semiconductor units in accordance with the teaching of this invention.

The process described relates to creation of microwave diodes for operation in the IMPATT manner for the generation of and the amplification in the X-band. The process involves the following manufacturing steps:

a. Creation of the P.sup.+ / N junction by thermal diffusion of boron, starting with a solid source of boron nitride in a two-phase process in which, first, a dope is deposited and, secondly, it is sunk or implanted into silicon. The correct time and temperature for the diffusion are selected so that a depth of the junction of about 2 microns is obtained.

b. Performing the sinking step, which occurs in an oxidizing atmosphere in a manner that a layer of silicon dioxide is formed upon the surface of the sheet or wafer.

c. Outlining by use of an ordinary photoetching process on the side of the p.sup.+ layer, geometrically regular squares of 500 .times. 500 micron dimension. This removes the silicon dioxide by a chemical attack. The back of the sheet is protected by a photoresist when the oxide is being etched.

d. Deposition from the vapor phase in vacuo of a flexible chromium layer of 500 A and of a flexible gold layer of 3000 A.

e. Outlining by photoetching, using a screen which complements the one used under c), geometrically regular squares, whereafter is formed by a chemical attack upon the chromium and upon the gold, a number of ascending islands which thus are separated from each other by the thermal oxide.

f. Creation of an electrolytic deposit of gold which allows forming of sinks in a thickness between 100 and 150 microns; the electric connection of the individual sinks in the rising phase is obtained by making use of the high conductivity of the P.sup.+ layer.

g. Bonding of the sheet with Sylgard 182 (a registered trade mark of Dow Corning Corporation, Midland, Mich., for silicone resin encapsulants) onto a support, which is followed by a deburring or grinding-down or a thinning, from the N- side, so as to obtain a thickness of the order of 25 to 50 microns. This thinning occurs by a chemical attack through use of hydroiodic, nitric and hydrofluoric acids.

h. Precipitating from the gas phase upon the surface of the N- type of a flexible layer of chromium of 500 A and of another layer of gold of 3000 A. By photoengraving or photoetching next become outlined geometrically circular regions of 150 microns diameter, whereby one uses devices which allow an electrolytic deposition of gold in the thickness of 2 to 3 microns, and a subsequent chemical attack upon the gold and upon the chromium. These circular regions are made to occur so that they will correspond to the center of the sinks which become deposited upon the opposite surface, whereby one obtains an orientation if one refers to the lines which occur at the edge of the sheet.

i. A chemical attack is next under use of hydrofluoric and nitric acids, to simultaneously bring about the formation of the Mesa structure upon the sinks, and their automatic separation. Typical dimensions are: Upper diameter 130 microns, lower diameter 160 microns, height 25 microns, and such Mesas are located upon a sink of 600 microns .times. 600 microns which is 100 microns thick.

The invention has been described as its preferred embodiment. It may be modified and variants may be applied by parties versed in the art, and all such changes still will fall within the purview of the appended claims.

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