Process for manufacturing metal-ceramic composite circuit substrates

Abstract: A metal-ceramic composite circuit substrate having a ceramic substrate and a metal plate joined to at least one main surface of the ceramic substrate, the rate of voids formed on at least a joint surface at a semiconductor mounting portion of the metal plate per unit surface area being not more than 1.5 %. The diameter of void formed on at least the joint surface at a semiconductor mounting portion of the metal plate is not larger than 0.7 mm. The surface undulation of the ceramic substrate is not more than 15 µm/20mm measured by the surface roughness tester in case that the ceramic substrate is joined directly to the metal plate. The metal plate is joined to the ceramic substrate through an brazing material containing at least one active metal selected from a group consisting of Ti, Zr, Hf and Nb. The ceramic substrate is at least one kind of ceramic substrate selected from a group consisting of Al 2 O 3 , AlN, BeO, SiC, Si 3 N 4 and ZrO 2 .

Description: The present invention relates to a process of manufacturing metal-ceramic composite circuit substrate and, more particularly, relates to a process of manufacturing an improved metal-ceramic composite circuit substrate wherein heat generated in a semiconductor element can be dissipated therefrom efficiently. Several methods are known in the art of joining metal members to ceramic members. An typical example is the direct joining method in which the metal member is directly joined to the ceramic member. The other example is the intermediate material method in which the metal member is joined to the ceramic member by inserting an intermediate material therebetween. As the direct joining method, a method wherein a copper plate is directly joined to an alumina substrate in the atmosphere of inert gas and heated and cooled is described in USP 4811893 etc. As the intermediate material method, an active metal method and a metalizing method are known. In the active metal method, an intermediate material containing an element of fourth group of the periodic table such as Ti or Zr etc. or an alloy containing said element is inserted between the metal member and the ceramic member. For example, in case that the stainless steel and the silicon nitride are joined, Ag-Cu-Ti base alloy is used as the intermediate material, and in case that the copper and the alumina are joined, Cu-Ti base alloy is used as the intermediate material. Recently, however, it is required to use semiconductor elements of a high electrical power and high integration to be mounted in the electronic equipment including the semiconductor device according to the miniaturization of said electronic equipment, and thus the heat quantity generated in the energized semiconductor elements becomes large. US 4,860,939 discloses a method for the direct bonding of a copper sheet to a substrate made of an electrically insulating material, with the heating of the copper/substrate set in order to obtain a eutectic mixture. The method consists in cleaning the copper to eliminate traces of oxidation on its surface before placing the copper on the substrate, in heating the copper/substrate set under a neutral atmosphere until a temperature greater than the temperature for the formation of the eutectic mixture is reached, and in applying the oxidating reactive gas only after the temperature greater than the temperature for the formation of the eutectic mixture is reached, in such a way that this temperature is reached before any oxidation of the surface of the copper. JP 7162105 discloses a circuit board where a metal circuit is joined to one surface of a ceramic substrate and a metal heat radiation plate is joined to the other surface of the ceramic substrate. The occupancy ratio of voids present in the joint layer, whose diameters are 1mm or more, is 0.3-3% of the volume of the entire joint layer. GB 2 099 742 discloses a ceramic substrate directly bonded between two copper members by placing the substrate between the copper members and heating in an appropriate atmosphere. To avoid blistering, channels and/or holes are provided in the substrate or the copper members to allow gas to escape from between the components during bonding. US 5,672,848 discloses a ceramic circuit board where a copper circuit plate is directly bonded at a predetermined position on a ceramic substrate and heat is applied; or the copper circuit plate is integrally bonded through a brazing material containing an active metal, such as Ti, Zr and Hf; and a semiconductor element is bonded onto a semiconductor element mounting integrally bonded onto a surface of a grooved or holed side of the copper plate element through a solder layer. By the above structure, the ceramic circuit board prevents solder voids when soldering the semiconductor element onto the copper circuit, thereby reducing the dispersion of the thermal resistance values between the semiconductor element and the ceramic substrate, and reducing or mitigating the thermal stress applied or acting on the mounting portion of the semiconductor element. An object of the present invention is to solve the technical problems as mentioned above. Another object of the present invention is to obtain an improved metal-ceramic composite circuit substrate in which voids formed on a joint surface between the circuit metal plate and the ceramic portion of the copper circuit plate, and the semiconductor element is substrate are limited as small as possible, so that the heat quantity generated in the semiconductor can be dissipated efficiently, the heat radiating ability of the semiconductor is not reduced and the weight of the substrate is not increased, and that the heat resistance of the substrate can be reduced without varying the pattern size and increasing the surface area of the substrate. The present inventors conducted intensive studies in order to attain the stated objects and found that the heat generated in the semiconductor could be dissipated with no problem by controlling the rate of voids and the diameter of void on the joint surface between the metal plate and the ceramic substrate. The present invention has been accomplished on the basis of this finding. According to a first aspect of the invention, there is provided a process of manufacturing a metal-ceramic composite circuit substrate as claimed in Claim 1. In the present invention, at least one kind of member selected from among Al 2 O 3 , AlN, BeO, SiC, Si 3 N 4 and ZrO 2 is used as a ceramic member, and a member having good electrical conductivity such as copper, aluminum or the like is used as a metal plate. In case that an alumina substrate is used as a ceramic member in the present invention, the surface undulation of the alumina substrate should not be more than 25 µm/20mm measured by the surface roughness tester, because if the surface undulation is more than said value, the maximum diameter of the void and the number of the voids each having the diameter not smaller than 100 µm will be increased, as a result that the rate of voids per unit surface area will be increased in proportion thereto. Further, in case that as a ceramic member an aluminum nitride substrate or a silicon carbide substrate etc. is used instead of an alumina substrate, the ceramic member is joined to a metal plate by using Ag-Cu base brazing material containing at least one active metal selected from among Ti, Zr, Hf and Nb. In this case, the brazing material in a paste form is used and applied. In order to prepare the brazing material in the form of paste, an organic solvent such as terpineol, BCA, DBP, methyl cellosolve etc., or an organic binder such as ethyl cellulose etc. of a suitable amount is added to a mixed powder of the brazing material. Such past of brazing material is screen printed on the ceramic substrate and the metal plate is superposed on the past, and then heated in the furnace. The amount of binder, especially, the amount of carbon contained in the binder to be eliminated is varied according to the heating temperature and time. The experiments about the void were carried out by varying the temperature in the furnace to 550°C, 600 °C and 650°C . The experiments show that the voids of large diameter exist at the binder elimination temperature of 550°C, however, the diameter of void becomes small according to the increase of the binder elimination temperature. Also, if the heating time is prolonged at constant temperature to eliminate the carbon in the bender, the diameter of void becomes small and the rate of voids per unit surface area is reduced. The foregoing and other objects, features, and advantages of the present invention will become apparent from the following more particular description of preferred embodiments of the invention. (Embodiment 1) As metal members, a copper plate of 0.3mm in thickness for the circuit side and a copper plate of 0.25mm in thickness for the heat think side were prepared. Also, as ceramic members, almina substrates of 30× 50× 0.635mm in dimension each having a different value of the surface undulation were prepared. A required metal-ceramic composite circuit substrate was obtained by contacting the copper plates with both main surfaces of the alumina substrate, heating and cooling in the atmosphere of inert gas to form a joint member, and etching the joint member to form a predetermined circuit. In each of these metal-ceramic composite circuit substrates, the maximum diameter of the void, and the number and the rate of voids each having a diameter not smaller than 100 µm per 15cm 2 formed on the joint surface between the alumina substrate and the copper plate for circuit side at a portion where the semiconductor element such as Si tip etc. is mounted were measured by using the supersonic crack detection device (mi-scope-i) manufactured by Hitachi Kenki in Japan. The results are shown in Table 1. In said measurements, the supersonic crack detecting conditions were set such that the image of voids detected by the supersonic crack detection device is in accord with the void diameter obtained by the actual observation of the cutting surface at the void portion. RATE OF VOIDS OF DIERCT JOINING SUBSTATE SAMPLE SURFACE WAVENESS OF ALUMINA SUBSTRATE MAXIMUM DIAMETER OF VOID (µm) VOID NUMBER/15cm 2 NOT LESS THAN 100 µm IN DIAMETER RATE OF VOIDS (%) No.1 4.8µm/20 mm BELOW 60 NOTHING 0 No.2 9.6 µm/20 mm 110 2 0.58 No.3 14.3 µm/20 mm 160 10 1.27 No.4 22.7 µm/20 mm 450 36 1.40 No.5 25.0 µm/20 mm 700 40 1.49 No.6 34.3 µm/20 mm 2,400 97 3.12 MESURED BY SUPERSONIC CRACK DELECTECTION DEVICE MUNUFACTURD BY HITACHI KENKI (MI-SCOPE-1) In said Table 1, the samples No.1~No.5 other than the sample No.6 are preferable. (Embodiment 2) As metal members, a copper plate of 0.3mm in thickness for the circuit side and a copper plate of 0.25mm in thickness for the heat sink side were prepared. Also, as ceramic members, nine sheets of aluminum nitride substrates of 30 × 50× 0.635mm in dimension each screen printed with the past of brazing material of Ag-Cu-Ti base and dried were prepared. The aluminum nitride substrates were subjected to the debinding process by varying the heating temperature and holding time, respectively, in the furnace to remove the carbon contained in the binder. A required metal-ceramic composite circuit substrate was obtained by contacting the copper plates with upper and lower surfaces of the aluminum nitride substrate and heating at a constant temperature of 850°C to from a joint member, and etching the joint member to form a predetermined circuit. In each of these metal-ceramic composite circuit substrates, the maximum diameter of the void, and the number and the rate of voids each having a diameter not smaller than 100 µm per 15cm 2 formed on the joint surface between the aluminum nitride substrate with the brazing material and the copper plate for circuit side at a portion where the semiconductor element such as Si tip etc. is installed were measured by using the supersonic crack detection device shown in the embodiment 1. The results are shown in Table 2. RATE OF VOIDS DUE TO ACTIVE METAL METHOD DEBINDING TEMPERATURE DEBINDING HOLDING TIMY MAXIMUM DIAMETER OF VOID (µm) VOID NUMBER/15cm 2 NOT LESS THAN 100µm IN DIAMETER RATE OF VOIDS (%) 550 °C 2 Hr 350 88 3.43 550 4 220 42 2.50 550 6 120 24 1.81 600 2 210 43 2.62 600 4 110 19 1.38 600 6 BELOW 60 2 0.34 650 2 120 20 1.47 650 4 BELOW 60 3 0.41 650 6 BELOW 60 NOTHING 0.10 MESURED BY SUPERSONIC CRACK DELECTECTION DEVICE MUNUFACTURD BY HITACHI KENKI (MI-SCOPE-I) After measurement, an electric power was applied on the composite substrate with Si tip as the semiconductor element mounted on the circuit surface of the substrate. It is found that the rate of voids in the substrate heated at a temperature of not less than 600°C for at least two hours is not more than 1.5% similar to that in the embodiment 1, and that the diameter of void is smaller than 0.7 mm. According to the metal-ceramic composite circuit substrate of the present invention, the rate of voids on the joint surface can be controlled to not more than 1.5%, and the diameter of void can be controlled to not larger than 0.7 mm even if the same materials are used to join to each other, so that the circuit substrate having a high thermal resistance and a high commercial value can be manufactured with low cost. While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the sprit and scope of the invention as defined by the appended claims.

Claims: A process of manufacturing a metal-ceramic composite circuit substrate comprising the steps of: joining a metal plate to at least one main surface of a ceramic substrate selected from a group consisting of Al 2 O 3 , AIN, BeO, SiC, Si 3 N 4 and ZrO 2 through a brazing material containing at least one active metal selected from a group consisting of Ti, Zr, Hf and Nb; and heating the substrate at a temperature of not less than 600°C to eliminate the carbon in the binder so that the rate of voids formed on at least a joint surface at a semiconductor mounting portion of said metal plate per unit surface area is not more than 1.5% said process being characterised in that the substrate is heated at a temperature of not less than 600°C for at least two hours to eliminate the carbon in the binder. A process of manufacturing a metal-ceramic composite circuit substrate as claimed in Claim 1 wherein the substrate is heated at a temperature of not less than 600°C for binder elimination so that said rate of voids in the substrate is not more than 1.5% and the diameter of said voids is not larger than 0.7mm. A process of manufacturing a metal-ceramic composite circuit substrate as claimed in Claim 1 wherein the substrate is heated at a temperature of not less than 600°C for binder elimination so that said rate of voids in the substrate is not more than 1.5% and the diameter of said voids is not larger than 0.5mm.

Inventors: KIMURA MASAMI, NAKAMURA JUNJI, SAKURABA MASAMI, TAKAHARA MASAYA, KIMURA, MASAMI, NAKAMURA, JUNJI, SAKURABA, MASAMI, TAKAHARA, MASAYA, KIMURA, MASAMI, C/O DOWA MINING CO., LTD., Nakamura, Junji, c/o Dowa Mining Co., Ltd., SAKURABA, MASAMI, C/O DOWA MINING CO., LTD., Takahara, Masaya, c/o Dowa Mining Co., Ltd.
Applicants: DOWA MINING CO, DOWA MINING CO., LTD.
Application year: 1997
Main classification: H01L23
IPC codes: C04B37, H05K1, H01L23, H05K3