Method for producing a metal-ceramic composite substrate

Abstract: In the present invention, a metal-ceramic composite substrate is produced by joining a metal plate and a ceramic substrate by using a brazing alloy material of which the solid content comprises Ag in an amount of 90.0%-99.5%, and an active metal in an amount of 0.5-4.0% on a weight basis. The brazing alloy material may further comprise Cu in an amount of up to 9.5% and titanium oxide in an amount of up to 0.9%.

Description: This patent application is a divisional application of European Patent Application No. 98301799.7 which claims a process for producing a metal-ceramic composite substrate by joining a metal plate to a ceramic substrate. The present invention relates to a method for producing metal-ceramic composite substrates. Several methods are known in the art of joining metal such as copper plates to ceramic substrates such as aluminum nitride (AIN) and two typical examples are the active metal brazing process described in Japanese Patent Laid-Open No. 166165/1985 entitled "A method of Joining Nitride-Base Ceramics to Metals" and the process in which a copper plate is directly joined to a modified surface of an aluminum nitride substrate (as typically described in Japanese Patent Laid-Open No. 163093/1981). The active metal brazing process provides a higher strength of joint than the direct joining process and the resulting joint has the desired characteristics such as high durability to repeated heat cycles. Hence, the active metal brazing process currently finds extensive use in the joining of copper plates to non-oxide-base ceramic substrates, say, nitride ceramic substrates. Two brazing materials are commercially used in the practice of the active metal brazing process; one of them is composed of Ag, Cu and an active metal selected from among Ti, Zr and Hf (as described in Japanese Patent Laid-Open No.166165/1985) and the other is an active metal paste material composed of Ag, Cu and titanium hydride (as described in Japanese patent Laid-Open No.101153/1991). Circuit substrates produced by joining copper plates to both sides of an aluminum nitride substrate with the aid of these brazing materials have already been commercialized. In many brazing materials, an active metal is added to a structure comprising 72% of Ag and 28% of Cu, and accordingly the joining process is carried out at a. temperature higher than the eutectic point of 780 °C, preferably, at a temperature of around 850°C European patent no. 0368126 discloses an alloy for brazing ceramics having the composition in weight percent of 0.5 to 3% titanium, .25 to 2% aluminium, silicon or tin, 2 to 6% copper, balance silver. Japanese patent publication no. 09188582 discloses a brazing material comprising 60-94.25 wt.% Ag, 5-30 wt.% Cu, 0.5-4.5 wt.% of an active metal and titanium dioxide in a specific range of 0.25-0.9 wt.% made into a paste state and applied to an AlN substrate. A copper plate is piled on the coated brazing material and is heat-treated to give a bonded material. However, most recent circuit substrates are required to perform on larger electrical power and to meet this requirement, it has been desired to develop circuit substrates that are not only provided with good heat dissipating and electrical insulating properties but which also exhibit higher strength and thermal impact resistance. An object, therefore, of the invention is to provide a process for producing metal-ceramic composite substrates using a brazing material that is suitable for use in the manufacture of power module composite substrates having no non-contact portions and which are capable of improving their characteristics over the conventional brazing materials. Another object of the invention is to provide the brazing material. The present inventors conducted intensive studies in order to attain the stated objects and found that joining processes could be carried out at a temperature of 780 °C~ 870 °C, i.e. lower than the melting point of 875 °C by using active metal brazing materials added with 90% of Ag and 9.5% of Cu and 0.5% of an active metal (TiO), and that the jointed composite substrates had no non-contact portions. The present invention has been accomplished on the basis of this finding. The invention provides a process for producing a metal-ceramic composite substrate in accordance with claim 1 of the appended claims. The invention further provides a process for producing a circuit on a metal-ceramic composite substrate in accordance with claim 2 of the appended claims. The ceramic substrate is at least one kind of ceramic substrate selected from a group consisting of Al 2 O 3 , AlN and Si 3 N 4 . The heating temperature of the assembly is in the range of 780 °C to 870 °C which is not higher than the temperature of the melting point of the brazing material. The brazing material to be used in the invention has an Ag content of 90.0 ~ 99.5 wt%. If the Ag content is less than 90 wt% unsuitable composite substrates having non-contact portions will be produced. The Cu content of the brazing material is less than 9.5 wt%. The reason why the Cu content is smaller than that of the conventional Ag-Cu brazing material is that when the assembly is heated the Ag component and the contact surface of the copper plate are reacted to create the eutectic crystalization. The active metal that can be used in the invention is at least one element of group IVa of the periodic table as exemplified by Ti, Zr or Hf. These active metals may be added either in an elemental form or as a hydride and the amount of their addition ranges from 0.5 to 4.0 wt%. Below 0.5 wt%, a nitride layer will form in an insufficient amount to provide the necessary adhesive strength; beyond 4.0 wt%,the adhesive strength is increased, but, on the other hand, cracking is prone to occur after joining a metal plate to a ceramic substrate. Titanium oxide is added as TiO or TiO 2 in an amount of up to 0.9 wt%. The titanium oxide may be amorphous or crystalline. The present inventors confirmed by experiments that adding these amount of titanium oxide to a brazing material of the composition described above contributed to improvements in various characteristics of the resulting composite substrates such as resistance to repeated heat cycles, flexural strength, deflection and resistance to the passage through a furnace. A probable reason for these contributions would be that the TiO 2 or TiO added to the brazing material is dispersed uniformly to reduce the concentration of stresses. An alloy having the above-defined composition may be directly used as the brazing material of the invention. If a metal member is to be simply joined to a ceramic member, the brazing alloy material in the form of a plate or a foil may be used. The present invention is characterized in that the assembly is heated in the furnace at a temperature lower than the melting point of the brazing material, preferably at 780 °C~ 870 °C according to the composition of the brazing material for the reasons mentioned below. In case that the conventional brazing material formed of 72% of Ag and 28% of Cu added with an active metal Ti is used, the joining is intiated at a temperature higher than the eutectic melting point of Ag-Cu, that is, of 780 °C. Accordingly, in practice, the joining is carried out at a temperature of around 850 °C . In the present invention, the amount of Ag is more than 90 wt%. If Ag is 90 wt%, Cu is 9.5 wt% and Ti is 0.5 wt%, the melting point should be a temperature more than 875 °C in consideration of the phase diagram of silver-copper, so that the joining have to be carried out at a temperature of around 900 °C in order to obtain a perfect joint in the conventional process. If the brazing material of the present invention only is heated at a temperature lower than said melting point (875 °C), it may not be molten but merely sintered. However, in case of using a copper plate as a substrate for the power module as in the present invention, said brazing material and the copper plate are brought into contact with each other, so that when the assembly is heated Ag in the brazing material and Cu in the copper plate are reacted eutectically gradually from said contact portion in the micron order, Ag is diffused gradually into the copper plate started from said reaction and the eutectic structure is formed on the whole contact surface. As described on the foregoing pages, the metal-ceramics composite substrates produced by using the brazing material of the present invention are sufficiently improved in the reduction of the non-contact portions, and can be manufactured efficiently 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 scope of the ivention as defined by the appended claims.

Claims: A process for producing a metal-ceramic composite substrate by joining a metal plate to a ceramic substrate, which comprises the steps of: forming a brazing alloy material in the form of a plate or a foil, the content of the alloy comprising Ag in an amount of 90.0%-99.5%, Cu in an amount of up to 9.5%, and an active metal selected from Ti, Zr and Hf in an amount of 0.5-4.0% on a weight basis; applying the brazingalloy material to the ceramic substrate; and superposing a metal plate on the applied brazing alloy plate or foil and heating the assembly at a temperature not higher than the melting point of the brazing alloy material to form a joint of the metal plate and the ceramic substrate. A process for producing a circuit on a metal-ceramic composite substrate comprising the steps of: producing a metal-ceramic composite substrate in accordance with claim 1; and applying an etching resist on the metal plate of the joint to form a circuit pattern and etching the metal plate to form a metalized circuit. A process according to claim 1 or claim 2 characterized in that the brazing alloy material further comprises an amount of titanium oxide of up to 0.9% on a weight basis.

Inventors: KIMURA MASAMI, NAKAMURA JUNJU, SAKURABA MASAMI, TAKAHARA MASAYA, KIMURA, MASAMI, NAKAMURA, JUNJU, SAKURABA, MASAMI, TAKAHARA, MASAYA, KIMURA, MASAMI, C/O DOWA MINING CO., LTD., Nakamura, Junju, 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: 1998
Main classification: B23K35
IPC codes: B23K35, H05K3, C04B35