HYBRID TYPE INTEGRATED CIRCUIT DEVICE

Abstract: In an integrated circuit device comprising a heat sink (3), an insulating layer (4) formed on the heat sink conduc­ tor films (5a,5b) disposed on the insulating layer (4) in a desired pattern, and circuit elements such as semi­ conductors connected between the conductor films to con­ stitute a desired circuit, the heat sink has fibers of carbon or the like embedded in a metal matrix of copper or the like. The fibers have a thermal expansion coefficient sub­ stantially equal to or lower than that of the insulating layer. With the above construction, heat generated in the circuit elements can sufficiently be transferred to the heat sink by way of the thin insulating layer and distortion caused by the heat generation in the interface between the heat sink and the insulating layer can be minimized to thereby pre­ vent damage of the insulating layer.

Description: IiCEGRAT".D CIRCUIT DEVICE This invention relates generally to integrated circuit devices and more particularly to an integrated circuit device with circuit elements which generate a large amount of heat. A recent development of semiconductor technology has been to develop not only a power module in which large-capacity thyristors, transistors and the like, along with their driver circuit elements, are arranged on a single substrate but also a hybrid IC in which a plurality of circuit elements are arranged on a single substrate. The integrated circuit device of the type mentioned above usually comprises a metal heat sink, an insulating layer disposed on the metal heat sink, a number of conductor films for forming a desired circuit network on the insulating layer, and semiconductor elements and the like circuit elements connected between the conductor films. The heat sink and the conductor film are made of electrically conductive materials. In general, copper is used for the heat sink and aluminum, copper or silver is widely used for the conductor film. the It is/practice that the insulating layer is made of when alumina and, / incorporating the insulating layer into the device, an alumina plate with its one surface metalized is placed on the heat sink and bonded thereto by soldering. In the integrated circuit device, heat generated in the circuit elements during its operation is transferred to the insulating layer and then dissipated from the heat sink. To obtain better heat dissipation characteristics, in some applications, a heat sink is used having its surface coated with nickel and an insulation layer is used with its surface facln the heat sink coated with copper. As a prior art concerning the above expedient, one may refer to USP 3,469,148, for example. The construction of this prior device art/was, however, insufficient to dissipate heat generated in circuit elements. Therefore, this prior device art/is somewhat troublesome because of bad dissipation of heat generated in the circuit elements. This tendency is aggravated especially when semiconductor elements which generate a large amount of heat are used as the circuit elements. Of troubles encountered in the prior art, there is mentioned herein a disadvantage that the semiconductor elements used as the circuit elements are overheated, resulting in increase in leakage current of the semiconductor elements and decrease in blocking voltage of the same. Therefore, it is of great significance to effectively dissipate heat generated in the circuit elements for the sake of suppressing degradation in performance of the integrated circuit device. One reason for the insufficient heat dissipation characteristics of the prior art integrated circuit device is such that the thickness of the insulating layer typically amounting up to 500 to 800 um is too thick. If the insulating layer might be reduced to the order of thickness of 200 to 300 vm, the heat dissipation characteristics would be improved considerably. is If the thickness of the insulating layer/reduced to meet the heat dissipation characteristics, arise in however, additional problems/that the insulating layer will yield owing to thermal distortion due to heat generated in the circuit elements during current conduction and,in extremity, it will be broken down. This is the because/thermal expansion coefficients of the insulating layer and the heat sink are very different. For example, while alumina generally used for the insulating layer has a coefficient of linear expansion of about 6.3 x 10 / C, copper generally used for the heat sink has a coefficient of linear expansion of about 17 x 10 6/or, exhibiting a great difference. Use of a material other than alumina for the insulating layer and of a material other than copper for the heat sink cannot be an essential solution to this problem. One may also refer to USP 3,969,754 which approaches a similar problem in connection with a supporting electrode for a semiconductor substrate and gives a solution thereto. This patent is however silent on the specific problem on the interface between the heat sink and the insul 2 ting layer and of any solutions thereto. An object of this invention is to provide an inte grated circuit device being capable of effectively dissipating heat generated in circuit elements. Another object of this invention is to provide an integrated circuit device immune to damage due to heat generated in circuit elements. Still another object of this invention is to provide an integrated circuit device with a heat sink having a thermal expansion coefficient which is comparable to that of an insulating layer. According to this invention, the above objects can be accomplished by providing an integrated circuit device comprising an insulating layer of sufficiently small thermal resistance, a heat sink being contiguous to one surface of the insulating layer and in the form in which of --.- a composite structure / fibers having a thermal expansion coefficient substantially equal to or lower than that of the insulating layer are embedded in a metal matrix, predetermined circuit elements disposed on the insulating layer, and metal conductors interconnecting the circuit elements. Since, according to this invention, the thickness of the insulating layer can be reduced to exhibit a small thermal resistance, it is possible to effectively dissipate heat generated in the circuit elements. Therefore, the troubles which would otherwise be caused by bad heat dissipation characteristics can be obviated. In addition, the circuit elements being freed from overheating permit the conduction of a larger amount of current in the device than in the prior art device. In accordance with this invention, the heat sink takes the form of the composite structure wherein fibers of low thermal expansion are embedded in the thermally conductive metal matrix because the heat sink canhave, because of the composite structure, compatibility with both properties of good heat conduction and low thermal expansion. The thermally conductive metal for the matrix is preferably selected from copper, silver, or an alloy mainly containing at least one of copper and silver. Graphite, carbon, tungsten or silicon carbonate is used for the fiber. Most preferably, graphite or carbon is used which is easy to make into a composite with copper or silver and which can prevent the creation of reactive substances in the composite structure which impair mechanical strength and thermal and electrical conductivity. Embodiments of / this invention will be now described in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic sectional view of a hybrid type integrated circuit device in accordance with a first embodiment of the invention; Figs. 2A to 2D and Figs. 3 to 5 are plan views showing examples of fiber pattern in the heat sink in accordance with the invention; Figs. 6 and 7 are schematic sectional views of the other embodiments in accordance with the invention; and Fig. 8 is a graph showing the relation between the thickness of insulating layer and the blocking voltage. Referring to Fig. 1, there is shown a hybrid type integrated circuit device in accordance with a first embodiment of the invention which comprises a heat sink 3, an insulating layer 4 of alumina disposed on the heat sink 3, a conductor film 5 formed on the insulating layer 4 and including copper films 5a and silver films 5b, and circuit elements interconnected to form a desired circuit network above the conductor film. Each of the circuit elements includes a semiconductor element 7 disposed on a layer 6 of a chromium-nickel-silver alloy and a lead wire 8. Thus, the semiconductor element 7 is disposed above the silver conductor film 5b with interposition of the chrominum-nickel-silver alloy layer 6 therebetween and is connected with the copper conductor film 5a by means of the lead wire 8 of copper. The insulating layer 4 is 100 ijm thick. The device of this embodiment was fabricated via the following steps. For formation of the heat sink, carbon fibers of a diameter of several microns plated with copper were netted into a sheet and a plurality of the sheets were stacked and hot-pressed at about 80000 in nitrogen atmosphere. Tnis composite structure contained an average amount of carbon fibers of 30 by volume and exhibited an average thermal expansion coefficient of 6 9 x 10 /OC within the range from room temperature to 3000C. It will be appreciated that when compared with the thermal expansion coefficient of alumina (about 6.3 X 10-6/ C) and the thermal expansion coefficient of -6 copper (about 17 x 10 6/oC), the average thermal expansion coefficient is far from the thermal expansion coefficient of copper but close to that of alumina. Next, onto a major surface of the thus prepared copper-carbon composite structure which was 500 vm thick and of a square configuration of about 25 mm side, alumina was flame-sprayed by plasma process. The flame -ing spray/of alumina was conducted by subjecting powder of alumina of 5 um average grain size to plasma jet in argon (5 to 15 by volume)-helium atmosphere under the conditions of 800 A current and 2 kg/hr rate. The insulating layer thus prepared exhibited a blocking voltage of 2500 V. Copper and silver were vapor deposited on the thus prepared insulating layer to form copper and silver conductor films of 10 Um thickness in a desired pattern, films of chromium-nickel-silver alloy of 1 Um thickness were formed on the silver conductor films by vapor depostion, semiconductor elements were bonded to the chromium- nickel-silver alloy films by soldering with lead-tin-silver alloy, and the semiconductor elements were connected with the copper conductor films by means of lead wires. The semiconductor element used herein includes various types of thyristor, transistor and diode. Between electrodes of the semiconductor element or between electrode of the se.i- conductor element and the conductor film are connected resistors, capacitors and other circuit elements for driv- ing the semiconductor element. In this embodiment, the fibers in the composite structure are concentrated in the side thereof contiguous to the insulating layer for the following reasons. It is ideal that the heat sink has a thermal expansion coefficient substantially equal to that of the insulating layer. In order to approximate the thermal expansion coefficient of the composite structure to that of the insulating layer, incorporation of a fairly great deal of fibers is required which would be attained at the cost of degradation in thermal conductivity because the thermal conductivity of the fiber is lower than that of the metal matrix. To minimize the degradation in thermal conductivity and at the same time to prevent breakdown of the insulating layer, the amount of fibers is desirably localized in the side of the composite structure contiguous to the insulating layer, as in this embodiment. Specifically, the fibers were present 5 vol. A in the side of the heat sink contiguous to the insulating layer and 15 vol. i in the opposite side. In addition to the above consideration for the purpose of prevention that a local thermal distortion is caused at the interface between the heat sink and the insulating layer to thereby break down or deform the insulating layer, it is preferable to pattern the fibers in the surface of the composite structure contiguous to the insulating layer in such a manner that the thermal expansion of fibers proceeds with twodimensional homogeneity. To this end, the fibers are netted and stacked in the thermally conductive metal matrix as in the foregoing embodiment, or sheets of netted fibers are coated with thermally conductive metal and stacked and assembled together. In this case, a pattern as shown in FF. 2A or 2B is obtained. Another way is to cut fibers at 2 suitable length and to embed the cut fibers 2 in the thermally conductive metal matrix 1 in random directions as shown in Fig. 2C. Still another way is to wind a fiber coated with thermally conductive then metal in a spiral form and/a plurality of such fibers are press-bonded and assembed together, thereby obtaining a pattern as shown in Fig. 2D or Fig. 3. Many other patterns . of the fibers are possible such as a radial arrangement of the fibers as shown in Fig. 4 and an annular arrangement of the fibers as shown in Fig. 5. Of these patterns, constructions of Figs. 3 through 5 are suitably adapted to a circular or circlelike heat sink, and constructions of Figs. 2A, 2B and 2D are suitably adapted to a rectangular or rectangle-like heat sink. The amount of the fibers in the heat sink is preferably different in the side of the heat sink contiguous to the insulating layer and in the opposite side and it depends on what material is used for the insulating layer. Experimental experience shows, however, that an average amount of 15 to 40% by volume satisfactory. Alumina A#2O3) is preferably used as the material for the insulating layer as being relatively inexpensive and not poisonous. Silicon dioxide (SiO2), boron nitride (BN) or beryllium oxide (BeO) may also be used. Silicon dioxide is preferable since it can be tninned to several microns while exhibiting a high blockIng voltage of about 1000 V/um. Beryllium oxide has a large thermal conductivity of about 0.07 W/0C and is used preferably. the The insulating layer is formed by/flame spray process or alternatively, a previously treated thin plate may be used as the insulating layer. In fomation by flame spray-ing, the material for the insulating layer can advantageously be bonded directly to the heat sink and this manner of formation is suited for mass produclion. Also, a thin insulating layer can readily be obld. Disadvantageously, however, voids tend to occur n the insulating layer formed by flame spray process, thereby increasing thermal resistance or decreasing blocking voltage. An insulating layer in the form of the previously treated thin plate is expected to be of high quality but is not so suited for mass production as the insulating layer formed by flame spray process. Accordingly, either an insulating layer formed by flame spray process or an insulating layer in the form of a . previously treated thin plate may be selectively used n accordance with the intended purposes. The insulating layer formed by flame spray process has preferably a thickness of not less than 50 Vm. Ss insulating layer of a thickness of less than 50 um tends to be reduced in blocking voltage. The upper limit of the thickness (irrespective of the fact that the insulating layer is formed by flame spray process or not) is preferably less than 300 pm to ensure suitably that the thermal resistance can be/lower A second embodiment of the invention will now be described with reference to Fig. 6 in which the same members as those in Fig. 1 are designated by the same characters. A heat sink 3 was the same as that of the first embodiment. As insulating layer 4, a sintered boron nitride plate of 250 um thickness was used. On one surface of the insulating layer 4 were successively formed by sputtering a chromium (Cr) layer 301 of 0.6 ijm thickness, a chromium-copper alloy layer 302 of 0.6 pm thickness, and a copper (Cu) layer 303 of 1.5 jjm thickness. Thereafter, the copper layer 303 and the heat sink 3 with a surface metalizing coating of silver (A) were bonded together by a solder layer 304 of 50 um thickness. Desired conductor films and circuit elements such as semiconductor elements were arranged llke the Irt embodiment, above the insulating layer 4. A third embodiment of the invention will be described with reference to Fig. 7 in which the same members as Whose in Fig. 1 are designated by the same characters. A heat sink 3 was the same as that of the first embodimnt. As insulating layer 4, an alumina sintered plate of 250 um thickness was used. The overall surface of the heat sink 3 was coated with a thin film of copper oxide (CuO or Cuss) and the insulating layer 4 was superimposed on the heat sink 3. The insulating layer and heat sink were then bonded together directly by applying a high voltage of about 2000 V across them. This method advantageously simplifies the fabricution process. Preparation of the heat sink and insulating layer and manner of bonding together these members have been described by way of examples in the foregoing but they may be achieved in other manner than the above. For example, the heat sink and the insulating layer may be bonded together by diffusion bondig. The following table shows the characteristics of samples fabricated in accordance with teachings of the invention in comparison with a sample of the prior art. The samples of the invention were fabricated in accordance with the first embodiment and the prior art sample was an integrated circuit device having a heat sink of copper and an alumina sintered plate of 500 urn thickness bonded to one surface of the heat sink in a similar manner to the second embodiment. Talble EMI13.1 --------------------------------------------------------------------------------------------------- <SEP> Thermal <SEP> expansion <SEP> Blocking <SEP> voltage <SEP> Total <SEP> thermal <tb> Samples <SEP> coefficient <SEP> of <SEP> heat <SEP> sink <SEP> of <SEP> insulating <SEP> layer <SEP> resistance <tb> --------------------------------------------------------------------------------------------------- <SEP> Averag <SEP> amount <tb> <SEP> of <SEP> fibers <SEP> 30 <SEP> % <SEP> 9 <SEP> x <SEP> 10-6 <SEP> / C <SEP> 2,500 <SEP> V <SEP> 0,8 C/W <tb> <SEP> ------------------------------------------------------------------------------------- <SEP> This <SEP> Average <SEP> amount <tb> <SEP> invention <SEP> of <SEP> fibers <SEP> 40 <SEP> % <SEP> 7 <SEP> x <SEP> 10-6/ C <SEP> 2,500 <SEP> V <SEP> 0.9 C/W <tb> ---------------------------------------------------------------------------------------------------- <SEP> Prior <SEP> art <SEP> 17 <SEP> x <SEP> 10-6/ C <SEP> 4,000 <SEP> V <SEP> 1.7 C/W <tb> ---------------------------------------------------------------------------------------------------- As will be seen from the above table, the samples of the invention were reduced in thermal resistance by about 2 as compared with the prior art sample and/improved heat dissipation characteristics. Therefore, the permissible current conduction in the semiconductor elements was increased. Values of blockiizg voltage were sufficient for practical use. Breakdown of the insulating layer was not caused. Fng. 8 shows blocking voltages of an insulating layer formed by flame spray process in accordance with the first embodiment when the thickness of the insulating layer is varied. As shown, for the thickness less than 50 ssm, the blocking voltage decreases rapidly and is therefore unsatisfactory. It is also noted that the blocking voltage varies greatly when the thickness varies within the range of less than 50 pm. As described above, the invention can provide the integrated circuit device which fully fulfils the objects of the invention.

Claims: CLAIMS: 1. ¯ knA±7i integrated circuit device comprising a heat sink, an insulating layer bonded to one major surface of said heat sink, metal conductor films disposed on said insulating layer in a desired pattern, and circuit elements connected between said metal conductor films to constitute a desired circuit, characterized in that. said heat sink comprises a composite structure of a thermally conductive metal and fibers embedded in the thermally conductive metal and having a thermal expansion coefficient the substantially equal to or lower than that of / material of said insulating layer. 2. An integrated circuit device according to Claim 1, erin said thermally conductive metal consists essentially of one metallic material selected from the group consisting of copper, silver, and an alloy mainly containing at least one of copper and silver. 3. An integrated circuit device according to or Claim 2 Claim 2, wherein said fiber consists essentially of at least one material selected from the group consisting of graphite, carbon, tungsten, and silicon carbonate. any one of 4. An integrate circuit device according to/Claiccs to 3 said wherein said insulating layer consists essentially of at least one material selected from the group consisting of alumina (A1203), silicon dioxide (SiO2), boron nitrde (BN), and beryllium oxide (BeO). 5. An integrated circuit device according to Claim 4, wherein said insulating layer comprises a flame spray coating formed on said heat sink. 6. An integrated circuit device according to Claim 4, wherein said insulating layer comprises a thin sintered plate bonded to said heat sink. any one of 7An integrated circuit device according to/Claims 1 to, 6wherein said fibers in the side of said heat sink contiguous to said insulating layer have homogeneous thermal expansion characteristics. any one of 8. one integrated circuit device according to/Clairns to 7 wherein said circuit elements comprise thyristors and/ or transistors. any one of 9. An integrated circuit device according to/Claims to 8 are 1/ wherein the fibers /-t concentrated in a portion of said heat sink close said insulating layer.

Inventors: ARAKAWA, HIDEO, ASAHI, NAOTATSU, KUNIYA, KEIICHI, NAMEKAWA, TAKASHI, OGAWA, TAKUZO, OHASHI, MASABUMI
Applicants: HITACHI, LTD.
Application year: 1979
Main classification: H01L23
IPC codes: C22C49, H01L23, H01L21