Ceramic substrate used for an electric or electronic circuit

Abstract: A ceramic substrate is used for an electric or electronic circuit, and comprises a ceramic foundation (31) and at least one conductive island (32/32a) formed of aluminum or an aluminum alloy and bonded to one surface of the ceramic foundation for providing conductive paths to circuit components (35a/35b) connected thereto, wherein the aluminum or aluminum-alloy island is brazed to the ceramic foundation with a brazing alloy selected from the group consisting of aluminum-silicon alloy, aluminum-germanium alloy, aluminum-silicon-magnesium alloy and aluminum-silicon-germanium alloy so that thermal stress between the ceramic foundation and the conductive island is taken up.

Description: This invention relates to a ceramic substrate used for an electric or electronic circuit and, according to the introductory portion of claim 1. BACKGROUND OF THE INVENTION A typical example of the ceramic substrate is illustrated in Fig. 1 and comprises a ceramic plate 1 of aluminum oxide sandwiched between two copper plates 2 and 3, and a heat sink 4 of copper. The copper plates 2 and 3 are bonded to both surfaces of the ceramic plate 1 through a liquid phase bonding phenomenon. Namely, the bonding surfaces of the copper plates 2 and 3 are firstly oxidized, and the ceramic plate 1 is laminated with the copper plates 2 and 3 on both surfaces thereof. While the ceramic plate laminated with the copper plates 2 and 3 are heated at 1065 to 1085 degrees in centigrade, the copper and the copper oxide are melted, and the copper plates 2 and 3 are bonded to the ceramic plate 1 through a copper-and-copper oxide eutectic phenomenon. One of the copper plates 2 and 3 provides a conduction path between circuit components, and the other is soldered to the heat sink member 4 at lower than 450 degrees in centigrade. A problem inherent in the prior art ceramic substrate is a small resistance against stress of thermal fatigue. When the electric or electronic circuit is activated for a task, a large amount of heat is produced in the circuit components, and the ceramic substrate conducts the heat for radiating. Since the copper plates 2 and 3 are different in thermal expansion coefficient from the ceramic plate 1, the ceramic plate is subjected to thermal stress. Upon being inactivated, no heat is produced, and the ceramic plate is released from the thermal stress. Thus, heat cycle takes place in the electric or electronic circuit. Since the aluminum oxide plate 1 and the copper plates 2 and 3 are rigidly coupled through the copper-and-copper oxide eutectic phenomenon, the ceramic substrate is repeatedly subjected to the thermal stress. The thermal stress is causative of cracks in the ceramic plate 1, and the ceramic substrate is broken in the worst case. A ceramic substrate used for an electric or electronic circuit, which comprises a ceramic plate formed of a substance mainly composed of aluminium nitride and conductive islands formed of aluminium or an aluminium alloy and bonded to one surface of said ceramic plate for providing conductive path to circuit components, which aluminium or aluminium alloy islands decrease the total weight of the ceramic substrate and enhance a resistance against repitition of thermal stress, is the subject of the earlier application EP-A-0 422 558. Further ceramic substrates used for an electric or electronic circuit, comprising a ceramic plate mainly formed of aluminium nitride are known from JP-A-01-012559, JP-A-02-094 652 and JP-A-63-318 760. In EP-A-0 097 944 a method of directly bonding a ceramic member and a metal member is disclosed, whereby an oxidation treatment process is used. US-A-4 865 877 discloses that the adhesion between a ceramic substrate and metallic plates is influenced by a certain surface roughness. It is an important object of the present invention to provide a ceramic substrate which is large in resistance against the reptition of thermal stress. In accordance with the present invention said object is obtained with the features of the characterizing portion of claim 1. The features and advantages of a ceramic substrate according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a side view showing the structure of the prior art ceramic substrate; Fig. 2 is a cross sectional view showing the structure of another ceramic substrate Fig. 3 is a cross sectional view showing the structure of a ceramic substrate according to the present invention, which is a modification of the ceramic substrate shown in Fig. 2; and Figs. 4A to 4C are views showing grains of aluminum nitride forming a surface portion of a ceramic plate of the ceramic substrate according to the present invention shown in Fig. 3. Turning to Fig. 2 of the drawings, another ceramic substrate comprises a ceramic plate 51 of aluminum nitride, ceramic plates 52a and 52b of silicon carbide provided on both surfaces of the ceramic plate 51, metallic plates 53a and 53b of aluminum respectively brazed to the ceramic plates 52a and 52b with brazing alloy films 54a and 54b, and nickel films 55a and 55b covering the metallic plates 53a and 53b, respectively. The nickel films 55a and 55b are replaceable with copper films, and the metallic plates 53a and 53b may be formed of an aluminum alloy such as, for example, an aluminum manganese, an aluminum nickel or an aluminum boron alloy. Electric or electronic circuit components (not shown) are provided on at least one nickel film 55a or 55b, and a heat sink may be bonded to the other of the nickel films 55a and 55b. The brazing alloy films 54a and 54b are formed of a substance selected from an aluminum-silicon alloy, an aluminum-germanium alloy and aluminum-copper alloy. Aluminum nitride has heat conductivity ranging from 50 watt/ mK to 270 watt/ mK, and silicon carbide is as large in heat conductivity as 35 to 270 watt/ mK. These substances are larger in heat conductivity than aluminum oxide the heat conductivity of which is as small as 15 to 20 watt /mK. Thus, the ceramic substrate shown in Fig. 4 is fabricated from the high-heat conductive plates, and, for this reason, the ceramic substrate can radiate a large amount of heat due to the electric or electronic circuit. Silicon carbide is affinitive to aluminum silicon alloy, and the brazing alloy films 54a and 54b are perfectly brazed to the ceramic plate 51. The density of aluminum is as small as 2.7 to 2.8 gram /cm 3 , however, copper is 8.9 to 9.0 gram/ cm 3 in density. The ceramic substrate shown in Fig. 2 is lighter than the prior art ceramic substrate using copper. The brazing films 54a and 54b are sufficiently deformable to take up a difference in the thermal expansion coefficient between the silicon carbide and the aluminum. For this reason, the ceramic substrate according to the present invention can well withstand against thermal stress due to heat cycle. Fig. 3 shows a ceramic substrate according to the present invention which is a modification of the ceramic substrate shown in Fig. 2, and the same reference numerals designate component layers and films corresponding to those of the ceramic substrate shown in Fig. 2. In the modification, the surface of the ceramic plate 51 of aluminum nitride is eroded by acid solution or alkaline solution for forming a rough surface, and the rough surface is oxidized in a high-temperature oxidation ambience. The aluminum oxide film ranges from 0.05 micrometer to 30 micrometers. In the roughening process, the original surface of the ceramic plate 51 shown in Fig. 4A is attacked by an acid solution or an alkaline solution, and small recesses 57 take place in the rough surface as will be seen from Fig. 4B. The recesses 57 penetrate 1/2 time to 3 times larger than the size of grain 51a. After the oxidation, grains 56a, which are hatched in Fig. 4C, are oxidized in the high-temperature oxidation ambience, and the oxidized grains 56a form the aluminum oxide film 56. Silicon carbide is deposited on the aluminum oxide film 56 by using a sputtering or a chemical vapor deposition to form ceramic plates 52a and 52b, and the ceramic plates 52a and 52b strongly adhere to the aluminum oxide film 56 by virtue of the rough surface. If the recesses 57 are shallower than a half of the grain size, the increment of surface area due to the roughening is too small to increase the adhesion. If the recesses 57 exceed the critical value three time larger than the grain size, spike-like grain clods tends to be crashed due to erosion of the grain boundaries, and the adhesion between the aluminum oxide film 56 and the ceramic plates 52a and 52b is decreased. The thickness of the aluminum oxide film 56 deeply concerns the adhesion as well as the heat radiation. If the aluminum oxide film 56 is less than 0.05 micrometer, non-oxidized surface is exposed to the boundary between the aluminum oxide film 56 and the ceramic plates 52a and 52b, and decreases the adhesion. The aluminum oxide film 56 greater than 30 microns deteriorates the heat conduction characteristics of the ceramic substrate, because the aluminum oxide is smaller in heat conductivity than the aluminum nitride and the silicon carbide.

Claims: A ceramic substrate used for an electric or electronic circuit comprising a) a ceramic foundation (51/56/52a/52b) implemented by a ceramic plate (51) essentially composed of aluminum nitride (AlN), an aluminium oxide film (56) covering the surface of said ceramic plate (51) and having a thickness of about 0.05 micrometers to about 30 micrometers; b) metallic plates (53a/55a) of a substance selected from the group consisting of aluminium and aluminium alloys bonded to one surface of said ceramic foundation and providing a conductive path for at least one circuit component connected thereto, said metallic plates being brazed to said one surface of said ceramic foundation with a brazing alloy (54a) selected from the group consisting of aluminium-silicon alloy, aluminium-germanium alloy and aluminium-copper alloy, characterized in that said surface of said aluminium nitride plate (51) is a rough surface with recesses (57), which penetrate into the surface 1/2 time to 3 times deeper than the size of a grain (51a) of said plate (51), which grain size is in the range of 1 micrometers to 3 micrometers, and ceramic plates (52a, 52b) of Sic are CVD or sputter deposited onto said aluminium oxide film (56) and thus arranged between said aluminium oxide film (56) and said brazing alloy.

Inventors: KUROMITSU YOSHIROU, TANAKA HIROKAZU, TORIUMI MAKOTO, UMEZAWA MASAO, YOSHIDA HIDEAKI, YUZAWA MICHIO, KUROMITSU, YOSHIROU, TANAKA, HIROKAZU, TORIUMI, MAKOTO, UMEZAWA, MASAO, YOSHIDA, HIDEAKI, YUZAWA, MICHIO, Kuromitsu, Yoshirou, c/o Mitsubishi Materials Corp, Tanaka, Hirokazu, c/o Mitsubishi Materials Corp., Toriumi, Makoto, c/o Mitsubishi Materials Corp., Umezawa, Masao, c/o Mitsubishi Materials Corp., Yoshida, Hideaki, c/o Mitsubishi Materials Corp., Yuzawa, Michio, c/o Mitsubishi Materials Corp.
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1991
Main classification: H01L23
IPC codes: H01L23, H01L21, C04B37, H05K3, H05K1