SEMICONDUCTOR DEVICE COMPRISING A SUBSTRATE

Abstract: A semiconductor device which comprises a copper base plate (4) bonded to a C.D.B. substrate (2) is allowed to have a thickness at least over 35 times the extent to which the copper base plate is extended when thermally bonded to said C.D.B. substrate.

Description: Semiconductor device This invention relates to a semiconductor device whose construction is adapted for the manufacture of a large power module and fitted with a heat sink plate electrically insulated from a semiconductor chip. The conventional semiconductor device provided with an electrically insulated heat sink plate comprises a Cu direct bond (hereafter referred to as "a C.D.B. substrate"), which is constructed by bonding a sheet of copper or copper alloy directly to both planes of a nonmetallic refractory plate prepared from, for example, ceramic material. With the above-mentioned conventional semiconductor device, a semiconductor chip is bonded to one plane of the C.D.B. substrate, and a copper base plate concurrently acting as a heat sink plate is, for example, soldered to the opposite plane of said C.D.B. substrate. In this case, the semiconductor chip and copper base plate are electrically insulated from each other. The copper base plate is hermetically sealed by coating a silicone resin adhesive on the surroundings of the copper base plate and further tightly attaching a case to the surface of said copper base plate by means of said adhesive. Silicone gel is poured into the case to protect the semiconductor chip and an aluminium wire connecting said semiconductor chip to an electrode terminal. After the curing of said silicon gel, epoxy resin is poured and cured to finish the subject semiconductor device. The copper sheet bonded to both planes of said C.D.B. substrate indicates a substantially equal thermal expansion coefficient to that of the ceramic plate during the manufacture of said C.D.B. substrate, thereby ensuring extremely strong bonding between th#e ceramic plate and copper sheet. Consequently, the C.D.t3. substrate and copper sheet bonded thereto may be regarded as an integral body. Now, let it be assumed that the C.D.B. substrate and copper base plate acting as a heat sink plate are, for example, soldered together. The copper has a thermal expansion coefficient (a = 17 x 10-6/OC), and the ceramic material has a thermal expansion coefficient (a = 7.7 x 10-6/OC). Even if, therefore, both materials can be bonded together in the flat form at the melting point (about 1850C) of solder, the difference between the above-mentioned thermal expansion coefficients causes the bonded mass, when cooled to the normal temperature of 250C, to indicate at the center such a warped shape as protrudes toward the ceramic side due to the bimetal effect. Now it let be assumed that the copper base plate measures 85 mm x 30 mm x 3 mm and the C.D.B. substrate measures 65 mm x 26 mm and is constructed by bonding a copper sheet having a thickness of 0.3 mm to both planes of a ceramic plate having a thickness of 0.6 mm. In this case, the above-mentioned warping reaches as large a level as 200 to 250 microns. When, therefore, the bonded mass warped to the above-mentioned extent is set in place by screwing both and portions of the copper base plate, a gap measuring more than 200 microns appears between said copper base plate and the member used on the subject semiconductor device-fixing side. As a result, a heat releasing effect is noticeably obstructed. Further, if it is attempted to eliminate such warping by forcefully applying a stress, then difficulties will arise such as the breakage of the semiconductor chip or the cracking of the ceramic component of the C.D.B. substrate, thereby rendering the subject semiconductor device unadapted for use. Further when the copper base plate is warped, a gap appears in the opening of the case when it is adhered to the copper base plate, thus making it necessary to apply a large amount of silicon adhesive to fill up said gap, and consequently presenting considerable difficulties in assembling a semiconductor device. Moreover, such a semiconductor device is accompanied with the drawbacks that adhesion between the case and copper base plate is weak, the hermetical sealing of the interstice between the case and copper base plate is ready to break, and a semiconductor device held in said interstice is reduced in its moisture proof property. It is accordingly the object of this invention to provide a semiconductor device free from warping by applying a copper base plate having such a thickness as can absorb stresses resulting from the bimetal effect of the C.D.B. substrate and copper base plate. To attain the above-mentioned object, this invention provides a highly reliable semiconductor device, wherein the warping of a copper base plate concurrently acting as a heat sink plate can be reduced to a practically negligible extent, and the loss of a heat sink effect and the cracking of a semiconductor device can be prevented. This invention can provide a semiconductor device wherein the copper base plate bonded to the C.D.B. substrate is allowed to have a thickness at least over 35 times the extent to which the copper base plate is extended when thermally bonded to said C.D.B. substrate. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a partial lateral sectional view of a semiconductor device embodying this invention; Fig. 2 is an enlarged sectional view of the bonded sections of the C.D.BA substrate and copper base p#late; and Fig. 3 graphically shows the relationship between the length and the warp rate of the copper base plate used with the semiconductor device of Fig. 1. A description may now be made with reference to the accompanying drawings of a semiconductor device embodying this invention. Fig. 1 is a partial lateral sectional view of said semiconductor device. The subject semiconductor device is manufactured through the following steps. First, a C.D.B. substrate 2 is provided which is constructed by directly bonding a sheet 1A of, for example, copper or copper alloy to both planes of a nonmetallic refractory plate 1B. A semiconductor chip 3 is bonded to one plane of said C.D.B. substrate 2. A copper base plate 4, concurrently acting as a heat sink plate, is adhered to the opposite plane of said C.D.B. substrate 2 by means of, for example, solder 5. A silicone adhesive 6 is coated on the periphery of the copper base plate 4, thereby adhering the peripheral portion of the opening of a case 7 to that of the copper base plate 4. Silicone gel 8 is poured into the case 7. After the silicone gel 8 is cured, epoxy resin 9 is also poured into the case 7. After the cure of said epoxy resin 9, the whole mass is cast into a prescribed form. The contact portion of the semiconductor chip 3 is connected to electrode terminals 11 by means of an aluminium wire 10. Said copper base plate 4 had screw holes -12 at both the end portion. The fact that the copper base plate 4 used with the semiconductor device shown in Fig. 1 is warped is assumed to result from the bimetal effect of the C.D.B. substrate 2 and said copper base plate 4. Various materials were used to make experiments with respect to the thickness and shape of the C.D.B. substrate 2 and the composition, shape and thickness of said copper base plate 4. It was experimentally confirmed that a semiconductor device could be produced wherein the heat-releasing effect was little lost and the warp of the copper base plate could be reduced to a practically negligible extent. A description may now be made of the experiments which eventually resulted in the present invention. Throughout the experiments, the direction in which the central portion of the copper base plate 4 protruded toward the ceramic plate 1B was taken as plus +. (a) Thickness of the ceramic plate 1B Table 1 EMI5.1 <tb> Ceramic <SEP> plate <SEP> Maximum <SEP> warp <SEP> Heat <SEP> resistance <SEP> of <tb> thickness <SEP> (P) <SEP> <SEP> semiconductor <SEP> device <tb> <SEP> (mm) <SEP> (0C/w) <SEP> <tb> <SEP> 0.6 <SEP> 250 <SEP> 0.20 <SEP> ¯ <SEP> <SEP> 0.24 <tb> <SEP> 1.0 <SEP> 100 <SEP> 0.30 <SEP> ¯ <SEP> <SEP> 0.38 <tb> <SEP> 1.5 <SEP> 50 <SEP> 0.40 <SEP> ¯ <SEP> <SEP> 0.50 <tb> The ceramic plate 1B measured 65 mm (length) x 26 mm (width), and the copper base plate measured 85 mm x 30 mm x 3 mm. Table 1 above shows that an increases in the thickness of the ceramic plate 1B ensures a decline in the warp of the copper base plate 4. However, it was experimentally found that an increase in the ceramic plate 1B raised the heat resistance of the resultant semiconductor device, the heat release at the fitting of a semiconductor device was reduced on the whole, and consequently, the attempt to increase the thickness of the ceramic plate 1B proved objectionable. (b) Composition of the base plate 4 Table 2 EMI6.1 <tb> Base <SEP> material <SEP> Maximum <SEP> warp <SEP> Heat <SEP> resistance <SEP> of <tb> <SEP> semiconductor <SEP> device <tb> <SEP> (0c/w)- <SEP> <tb> <SEP> Brass <SEP> 200 <SEP> 0.30 <SEP> - <SEP> 0.35 <tb> <SEP> Iron <SEP> 180 <SEP> 0.32 <SEP> - <SEP> 0.37 <SEP> <tb> The members used in the experiment (b) had the same measurements as those of the experiment (a). Table 2 shows that the warp of the base plate still occurred even when the base plate was prepared from brass or iron having a smaller thermal expansion coefficient than copper. The brass and iron having a lower heat conductivity than copper led to an increase in the heat resistance of the semiconductor device. It was found therefore that the application of a base plate prepared from any other material than copper was unsatisfactory. (c) shape of copper base plate 4 It was attempted to render the copper base plate extensible or contractible by application of honing or corrugation. Nevertheless, the copper base plate still warped to an extent of 200 to 300 microns, showing that said attempt proved quite ineffectual. It was further tried to warp the copper base plate crosswise in order to prevent the copper base plate from lengthwise bending and thereby reduce the warp caused by the bimetal effect. Still, the base plate warped to an extent of 170 to 230 microns, showing that said process was quite fruitless. (d) The amount of solder for effecting bonding between the C.D.B. substrate 2 and copper base plate 4 It was tried to apply a large amount of solder for the bonding between the C.D.B. substrate 2 and copper base plate 4 in order to effect the deformation of the solder, thereby, releasing the internal stress of the bonded mass. The application of the solder even to a thickness of 2 mm proved ineffective to reduce on warp of the base plate, which indicated as large as warp as 200 microns even at minimum. (e) Thickness of the copper base plate 4 Table 3 EMI7.1 <tb> Sample <SEP> C.D.B. <SEP> Copper <SEP> Maximum <SEP> warp <SEP> Heat <SEP> resistance <tb> number <SEP> substrate <SEP> Dlate <SEP> <SEP> (p) <SEP> <SEP> of <SEP> a <SEP> semicon <tb> <SEP> thickness <SEP> thickness <SEP> ductor <SEP> device <tb> <SEP> (mm) <SEP> (mm) <SEP> ( C/W) <SEP> <tb> <SEP> A <SEP> 1.2 <SEP> 3 <SEP> 200 <SEP> - <SEP> 250 <SEP> 0.20 <SEP> - <SEP> 0.24 <tb> <SEP> B <SEP> 1.2 <SEP> 4 <SEP> 50 <SEP> ¯ <SEP> <SEP> 80 <SEP> 0.19 <SEP> - <SEP> 0.24 <tb> <SEP> C <SEP> 1.2 <SEP> 5 <SEP> 30 <SEP> ¯ <SEP> <SEP> 60 <SEP> 0.19 <SEP> ¯ <SEP> <SEP> 0.23 <tb> <SEP> D <SEP> 1.2 <SEP> 6 <SEP> 0 <SEP> - <SEP> 30 <SEP> 0.18 <SEP> - <SEP> 0.22 <tb> <SEP> E <SEP> 1.2 <SEP> 7 <SEP> 0 <SEP> 0.17 <SEP> - <SEP> 0.22 <tb> The members provided in the above experiment (e) had the following measurements: Copper base plate 4 85 mm x 30 mm x t (thickness), Ceramic plate 1B 65 mm x 26 mm x 0.6 mm, and Copper sheet 1A 0.3 mm (thickness). Table 3 above shows that an increase in the thickness of the copper base plate 4 led to a decline in its maximum warp rate and prevented the semiconductor device from having a large heat resistance. The following is the reason for the occurrence of the above-mentioned advantageous event. When the solder melts at a temperature of about 1850C, the copper base plate 4 (thermal expansion rate of 177 microns) and ceramic substrate 1B (thermal expansion rate of 80 microns) are bonded together. When the bonded mass is cooled to the normal temperature 250C, an internal stress takes place in the bonded mass to an extent equal to a difference of 97 microns between the above-mentioned thermal expansion rates of 177 microns and 80 microns. In this case, the internal stress of the bonded mass progressively decreases from the bonded junction of the copper base plate 4 and ceramic substrate 2B to the opposite nonbonded edge of said bonded mass. If, therefore, the copper base plate 4 has a far larger thickness than the above-mentioned difference of 97 microns between the thermal expansion rates of said copper base plate 4 and ceramic substrate lB, then no warp appears on the opposite nonadhered side of the copper base plate 4. The warping rates of Samples A to E of the copper base plate 4, which appeared up to the finished assembly of the subject semiconductor device, were traced, the results being set forth in Table 4 below. It was found that the contraction stress resulting from the cure of the subsequently poured epoxy resin so acted as to offset the stresses caused by the aforementioned thermal expansion of the copper base plate 4 and ceramic substrate 1B. Therefore, the warping of the copper base plate 4 in the finished semiconductor device was appreciably reduced. The above-mentioned Samples A to E of the copper base plate 4 involved in the finished semiconductor device showed the warping rates listed in Table 4 below. Table 4 EMI9.1 <tb> Sample <SEP> Number <SEP> Maximum <SEP> warping <SEP> rate <SEP> (p) <SEP> <tb> <SEP> A <SEP> 130 <SEP> ¯ <SEP> <SEP> 160 <tb> <SEP> B <SEP> - <SEP> 10 <SEP> - <SEP> <SEP> 40 <tb> <SEP> C <SEP> - <SEP> 20 <SEP> - <SEP> 30 <tb> <SEP> D <SEP> ¯ <SEP> <SEP> - <SEP> 30 <SEP> ¯ <SEP> <SEP> 0 <tb> <SEP> E <SEP> - <SEP> 70 <SEP> - <SEP> - <SEP> 20 <tb> When a finished semiconductor device is attached to a fixing member, heat-releasing grease is applied to the fitting plane of said semiconductor device. If, therefore, the warping rate of the copper base plate 4 involved in the finished semiconductor device is less than 50 microns, no practical difficulties arise. When attached to the fixing member, the finished semiconductor device is screwed at both end portions. Therefore, the minus warp of the central portion of the copper base plate 4, which advances in a direction away from the ceramic substrate lB, is forcefully corrected so as to render said central portion substantially flat. Consequently, Samples B to E of the copper base plate 4 whose warping rates are indicated in Tables 3 and 4 above fall within the range of practical application. If the copper base plate 4 is rendered excessively thick, the cost of the component parts of the subject semiconductor device will be raised. Therefore, a finished semiconductor device involving, for example, Sample B of the copper base plate 4 proves practically satisfactory. Fig. 3 graphically shows the relations-hip between the length of the copper base plate (given on the abscissa) and to warping rate (indicated on the ordinate). In one section of Fig. 3, the thickness t (solid line) of the copper base plate is taken as a parameter. In the other section of Fig. 3, a rate A (broken line) of the thickness t of the copper base plate to the thermal expansion rate EQ of the soldered copper base plate is taken as a parameter. To minimize the heat resistance of a semiconductor device, therefore, it is advised that, assuming the application of a ceramic substrate 1B has a thickness of 0.5 to 0.6 mm, a copper base plate should be applied which has such a thickness as over 35 times the thermal expansion rate EQ of copper during soldering. Table 5 below sets forth the length of a copper base plate embodying this invention, the corresponding thickness the warping rate of the resultant semifinished semiconductor device, and the ratio of the thickness to the thermal expansion rate EQ of said copper base plate. Table 5 EMI10.1 <tb> Length <SEP> Q <SEP> <SEP> of <SEP> Thickness <SEP> t <SEP> Warping <SEP> rate <SEP> of <SEP> Ratio <SEP> of <SEP> the <tb> the <SEP> copper <SEP> of <SEP> the <SEP> copper <SEP> the <SEP> copper <SEP> base <SEP> thickness <SEP> t <tb> base <SEP> ate <SEP> <SEP> base <SEP> plate <SEP> plate <SEP> to <SEP> the <SEP> thermal <tb> <SEP> ('urn <SEP> <SEP> (mm) <SEP> (p) <SEP> <SEP> expansion <SEP> rate <tb> <SEP> AZ <SEP> <SEP> of <SEP> the <tb> <SEP> copoer <SEP> olate <SEP> <tb> <SEP> (t/##) <tb> <SEP> 15 <SEP> 0.9 <SEP> 60 <SEP> ¯ <SEP> <SEP> 70 <SEP> 40 <tb> <SEP> 30 <SEP> 1.8 <SEP> 65 <SEP> ¯ <SEP> <SEP> 75 <SEP> 40 <tb> <SEP> 50 <SEP> 3.0 <SEP> 75 <SEP> - <SEP> 85 <SEP> 40 <tb> <SEP> 65 <SEP> 3.9 <SEP> 70 <SEP> ¯ <SEP> <SEP> 80 <SEP> 40 <tb> <SEP> 100 <SEP> 5.2 <SEP> 65 <SEP> - <SEP> 75 <SEP> 35 <tb> The notation AZ given in Table 5 above represents the thermal expansion rate of the copper base plate 4 when soldered to the C.D.B. substrate 2 (temperature difference: 1600C). Table 5 above shows the warping rate of a semifinished semiconductor device constructed by soldering the copper base plate 4 to the C.D.B. substrate 2. If the warping rate of said semifinished product falls within the range of up to 85 microns, then it is possible to reduce said warping rate down to about 50 microns and prove practically applicable. This invention is characterized in that the warping stresses of the copper base plate and C.D.B. substrate, resulting from their bimetal effect, are allowed to act crosswise of the copper base plate, thereby deforming only that plane of the copper base plate which is bonded to the C.D.B. substrate and preventing the semifinished p#roduct as a whole from indicating any warp. Consequently, the present invention offers the advantages that the copper base plate can be finished substantially in the flat form. In the subsequent adhering step, the case can be smoothly and tightly adhered to the copper base plate; when a finished product is screwed to the fixing member, its flat fitting plane can be tightly attached to said fixing member, thereby minimizing losses in heat release; a highly reliable semiconductor device can be manufactured without any damage to the internal semiconductor chip and C.D.B substrate; and it is possible to resolve various difficulties occurring in the conventional semiconductor device-assembling step such as the unsatisfactory adhesion of the case, power waste resulting from noticeable losses in heat release, deterioration of the weather proof property of the semiconductor device, and separation between the case and copper base plate during heating. The copper base plate involved in the semiconductor device embodying this invention can be prepared from copper or copper alloy whose thermal expansion rate ranges between 16 x 106 and 22 x 10 6

Claims: Claims: 1. A semiconductor device which comprises: a substrate constructed by directly bonding a sheet of copper or copper alloy to at least one plane of a nonmetallic refractory plate; a semiconductor chip fitted to one plane of said substrate; and a copper base plate bonded to the other plane of said substrate to concurrently act as a heat-releasing plate, characterized in that said copper base plate (4) has a thickness at least 35 times the thermal expansion rate of said copper base plate when thermally bonded to said substrate (2). 2. The semiconductor device according to claim 1, characterized in that said copper base plate has a semiconductor chip (3) placed thereon, and an opening of a case (7) is fitted to the base plate with an epoxy resin (9) set therein. 3. The semiconductor device according to claim 1, characterized in that said copper base plate is prepared from copper or a copper alloy whose thermal expansion rate ranges from 16 x 10-6 to 22 x 10"6. 4. The semiconductor device according to claim 1, characterized in that said base plate is provided with a screw hole (12) at both end portions to effect the fitting of said base plate to a fixing member.

Inventors: SAITO, YASUMASA C/O PATENT DIVISION, TANI, KEIZO C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1984
Main classification: H01L23
IPC codes: H01L21, H01L23