ALUMINUM NITRIDE CIRCUIT SUBSTRATE

Abstract: Disclosed is an aluminum nitride circuit substrate comprising an aluminum nitride plate and a conductive material bonded to the aluminum nitride plate through a metallized layer formed on the bonding surface of the aluminum nitride plate, the conductive material being of a metallic material which has a thermal expansion coefficient of 2 × l0⁻⁶ to 6 × l0⁻⁶/°C. The aluminum nitride circuit substrate according to this invention is free from the generation of crack caused by the difference of the thermal expansion coefficients of AlN plate and a conductive material bonded to the AlN plate to improve the reliability of the elements.

Description: This invention relates to an aluminum nitride circuit substrate, more particularly, to an aluminum nitride circuit substrate being free from generation of crack due to, for example, a large bonding area between a lead frame and a substrate. In the prior art, as a circuit substrate for a semiconductor, there has been generally employed an inexpensive alumina (Al₂O₃) substrate, on which a circuit is formed and various elements including a semiconductor device are mounted on the circuit. However, in recent years, with the tendency that the semiconductor device shall have a larger output, the heat release value of such devices increases. Therefore, the above-mentioned Al₂O₃ substrate is not sufficient in the heat dissipating characteristic. Also, with the above tendency, lead frames have increased in size. For this reason, the present inventors have tried to obtain a circuit substrate employing an aluminum nitride (AlN), having an excellent heat dissipating characteristic instead of Al₂O₃ However, the circuit substrate of AlN tends to cause the following new problem. That is, on a predetermined position of a surface of the circuit substrate, the so-called lead frame functioning as an electrode is also bonded. In the prior art a conventional metallized layer is formed on the surface of the Al₂O₃ substrate and then an electroconductive substance to be used as the lead frame is bonded through the metallized layer. In this case, a Fe-Ni-Co alloy was used for hermetic sealing to improve the soldering property. However, in such an AlN circuit substrate prepared by forming the conventional metallized layer on the surface of the AlN circuit substrate and bonding the lead frame, sometimes the generation of crack is observed at the bonded portion with the increase of a temperature. US-A-45 40 673 describes an aluminium nitride substrate comprising an aluminium nitride plate and a conductive material bonded to said aluminium nitride plate through a metallized layer formed on the bonding surface of the aluminium nitride plate, wherein the conductive material is a metallic material like copper. The preamble of claim 1 corresponds to the state of the art according to said document. In Patent Abstracts of Japan, Volume 9, No. 3 (M-349) [1726], 9th January 1985 and 35th Electronic Components Conference Washington, D.C., 20th-22nd May 1985, pages 26-31 there is disclosed a lead frame which is formed with a porous sintered layer of a copper material on the surface by laminating powder of copper or a copper alloy on the surface of a metallic base material of Mo, W, Fe-Ni-Co-Ti alloy etc. having a coefficient of thermal expansion in the range from ordinary temperature to about 500°C of 4 to 6 times 10⁻⁶/°C and a high performance aluminium nitride substrate with for instance Mo or W based metallisations having such a coefficient of thermal expansion, respectively. The object of the invention to provide an aluminium nitride circuit substrate and a metallic lead frame thereon which is free from generation of crack at the bonded portion. The invention is directed to an aluminum nitride circuit substrate comprising an aluminum nitride plate (1) and a metallic lead frame (5, 6), bonded to a surface of said aluminum nitride plate through a conductive metallized layer (3) formed on the bond surface of the aluminum nitride plate, characterized in that the bonding area of the aluminum nitride plate and the lead frame is not less than 4 mm², said lead frame is of a metallic material having a thermal expansion coefficient of 2 x 10⁻⁶ to 6 x 10⁻⁶/°C, and said conductive metallized layer contains: i) at least one element selected from a first group consisting of molybdenum, tungsten and tantalum; and ii) at least one element selected from a second group consisting of IIa group elements, IIIa and IIIb group elements, IVa group elements, rare-earth elements, and actinide elements in the periodic system. Fig. 1 is a cross-sectional view showing an example of the structure of the aluminum nitride circuit substrate of this invention. Fig. 2 is a plan view of an embodiment of this invention employing a plate-like lead frame. Fig. 1 is a cross-sectional view of the substrate when this invention is applied to a flat package. In Fig. 1, a semiconductor element 2 being made of, for example, Si is provided to a predetermined portion of the surface of the AlN plate 1. The Si semiconductor element 2 is bonded to the plate 1 through a metallized layer 3 and a soldering layer 4. As the soldering material, there may be employed the conventional soldering material such as a silver solder and an ordinary solder. Incidentally, since Si has substantially the same thermal expansion coefficient as AlN, it is not particularly necessary to provide a buffering layer between Si and AlN plates. In Fig. 1 lead frames 5 and 6 are bonded to the both end portions of the plate 1 through a metallized layer 3, a plating layer 8 and a soldering layer 4 similarly as in the case of the Si element 2. The material constituting the lead frames 5 and 6 is required to have a thermal expansion coefficient substantially similar to that of AlN (4.6 x 10⁻⁶/°C) as mentioned above. Specifically, it is generally in the range of 2 x 10⁻⁶ to 6 x 10⁻⁶/°C, preferably 4 x 10⁻⁶ to 5 x 10⁻⁶/°C. In addition to the above requirement, the material of the lead frames may preferably have a higher thermal conductivity (preferably not less than 50 W/m·K) and a lower electrical resistance. As the specific examples of such a conductive material, there may be mentioned, for example, molybdenum (Mo); tungsten (W); a molybdenum series alloy such as Co-Sn added molybdenum, dope molybdenum (containing K, Si and alumina in the sum total of not more than 0.5 %); a tungsten series alloy such as dope tungsten (containing a slight amount of Al, Si and K) and thoriated tungsten (containing ThO₂ in the range of 0.5 to 1.5 % by weight); a nickel series alloy such as an Invar steel (containing 34 to 37 % of Ni) and a Fe-Ni alloy (trade name: "NSD" produced by Toshiba K.K., containing 42 % Ni and having a thermal expansion coefficient of 5 x 10⁻⁶); a chromium series alloy; a copper series alloy such as phosphor bronze, beryllium copper, brass; copper; a clad metal of an Fe-Ni-Co alloy for hermetic sealing and copper (e.g. KOV-Cu clad metal; "KOV" is a trade name of Toshiba K.K.); a Mo-Cu clad metal; a W-Cu clad metal. Among them, particularly preferred is molybdenum. As the metallized layer 3 on the AlN plate, employed is a conductive metallized layer containing: (i) at least one of element selected from a first group consisting of molybdenum, tungsten and tantalum; and (ii) at least one element selected from a second group consisting of IIa group elements, IIIa and IIIb group elements, IVa group elements, rare-earth elements, and actinoide elements. In the above elements of the conductive metallized layer, the elements belonging to the first group of molybdenum (Mo), tungsten (W) and tantalum (Ta) are excellent in heat resistance and further approximate to the AlN material in thermal expansion coefficient, so that these elements serve to improve the heat resistance characteristics and the cyclic heat resistance characteristics. These elements are included in a filler of the metallized layer as a single element or a combination of two or more elements. In more detail, these elements belonging to the first group exist in the metallized layer in the form of each element, a compound or solid solution including each element, or a mixture of two or more elements selected from the group consisting of these elements, compounds and solid solutions. The compounds are oxides, nitrides, carbides, acid nitrides, carbonic nitrides, carbonic oxides, carbonic acid nitrides, borides or silicides, of these elements. In addition to the above-mentioned elements, these compounds are composite compounds or solid solutions which include at least one of elements belonging to the second group. That is when taking the case of Mo as the element, Mo exists in the conductive metallized layer in the form of Mo or Mo-Al solid solution. Further, in the elements of the conductive metallized layer, the elements belonging to the second group comprising the IIIb group elements (B, Al, Ga, In, Tl), the IVa group elements (Ti, Zr, Hf), IIIa group elements (Sc, Y, Te), the rare earth elements (Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and the actinoid (Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr) are excellent in wetness relative to AlN, so that these elements serve to improve the adhesion between the conductive metallized layer and the AlN plate. Further, Al, Ti, Zr, Hf, Y, Ce, Dy, Th, Sm are particularly preferable as the elements belonging to the second group. In the same way as in the elements belonging to the first group, these elements belonging to the second group are included in the metallized layer in the form of a single element or a mixture of two or more elements. That is, these elements exist in the metallized layer in the form of each element, a compound or solid solution including each element or a mixture of two or more elements selected from the group consisting of these elements, compounds and solid solutions. In the case of Ti as the component phase, Ti exists in the conductive metallized layer in the form of e.g. TiN, TiO₂. In this invention, the composition ratio of elements belonging to the first group and those belonging to the second group is not defined specifically. The ratio is appropriately determined according to the kind of elements used or the combination. For instance, it is preferable that the ratio of the sum total of the elements belonging to the first group to that of the elements belonging to the second group is 90:10 to 10:90 in atomic ratio. It is effective to apply this invention to the following case. That is, in the case of employing a plate-like lead frame, not a pin-type, since an area to be bonded is large, the difference between their thermal expansion coefficients cause a problem. For solving this problem, the conductive material of this invention must be employed. The invention is effective when the area is not less than 4 mm², preferably 8 mm², more preferably 16 mm². Also, in the case of employing a plate-like lead frame, it is possible to provide a Si plate on the lead frame and further provide various elements on the Si plate, and therefore the present invention can contribute to the development of a circuit substrate having larger capacity. In Fig. 2, a plate-like lead frame 5 is bonded to the AlN plate through a metallized layer, a plating layer and a metallized layer 3 in this order viewed from the lead frame 5. Example The circuit substrate as shown in Fig. 2 was prepared in the following manner. That is, on the predetermined portion of the AlN plate 1 of 12.7 mm x 12.7 mm having a thermal conductivity of 150 W/m·K, provided were a metallized layer made of Mo-TiN alloy (the ratio of Mo:TiN is 2:1) at a temperature of 1,700 °C in an atmosphere of nitrogen for 1 hour. Then a nickel plating layer was formed by electroless plating on the metallized layer and a silver solder layer made of a silver-copper alloy was formed on the plating layer. Then, lead frames 5 and 6 of 3.0 mm x 3.4 mm were bonded to the AlN plate. The bonding area on which the lead frames were bonded was 4 mm², respectively. As the material for the lead frames 5 and 6, molybdenum was used having a thermal expansion coefficient of 5.1 x 10⁻⁶/°C. Then, the whole body of the circuit substrate was hermetically sealed to produce a flat package. On the other hand, the same packages were prepared as in the above except for forming a metallized layer of a Mo-Mn alloy containing 8 % of Mn at a temperature of 1,500 °C in an atmosphere of wet N-H₂ gas for 1 hour on the AlN plate. There were no differences between these packages in appearance. However, the tensile strength of the package employing the Mo-Mn alloy was as little as about 1 kg·f/mm² and all the packages employing the Mo-Mn alloy were revealed to cause generation of crack or peeling off at the metallized layer after 200 cycles of TCT (Thermal Cycle Test) by setting -50 °C x 0.5 hour and 125 °C x 0.5 hours as one cycle and after 500 cycles of TCT as in the above. The tensile strength test was conducted in a manner that a pin made of KOVAR (trade name; produced by Toshiba K.K.; Ni-Co-Fe alloy) was provided on the lead frame by soldering and a tensile strength was applied to the pin by use of a pullgauge in the vertical direction. As will be clear from the above-mentioned, the aluminum nitride circuit substrate of this invention is, in addition to have an excellent heat dissipating characteristic, free from the generation of crack, also because the lead frame is constituted of a metallic material having a thermal expansion coefficient substantially similar to that of aluminum nitride, an excessive stress is not applied to the circuit substrate if the temperature of the circuit substrate is raised by heat developed in an element is use. Accordingly, an reliability of the circuit substrate is very much improved and an industrial value becomes greater.

Claims: An aluminum nitride circuit substrate comprising an aluminum nitride plate (1) and a metallic lead frame (5, 6), bonded to a surface of said aluminum nitride plate through a conductive metallized layer (3) formed on the bond surface of the aluminum nitride plate, characterized in that the bonding area of the aluminum nitride plate and the lead frame is not less than 4 mm², said lead frame is of a metallic material having a thermal expansion coefficient of 2 x 10⁻⁶ to 6 x 10⁻⁶/°C, and said conductive metallised layer contains: i) at least one element selected from a first group consisting of molybdenum, tungsten and tantalum; and ii) at least one element selected from a second group consisting of IIa group elements, IIIa and IIIb group elements, IVa group elements, rare-earth elements, and actinide elements in the periodic system. The aluminum nitride circuit substrate according to claim 1, wherein said thermal expansion coefficient is in the range of 4 x 10⁻⁶ to 5 x 10⁻⁶/°C. The aluminum nitride circuit substrate according to claim 1, wherein the bonding area of the aluminum nitride plate and the lead frame is not less than 8 mm². The aluminum nitride circuit substrate according to claim 3, wherein the bonding area of the aluminum nitride plate and the lead frame is not less than 16 mm². The aluminum nitride circuit substrate according to claim 1, wherein the material of said lead frame is at least one selected from the group consisting of molybdenum, tungsten, a molybdenum series alloy, a tungsten series alloy, a nickel series alloy, a chromium series alloy, a copper series alloy, a clad metal of an Fe-Ni-Co alloy for hermetic sealing and copper, a Mo-Cu clad metal and a W-Cu clad metal. The aluminum nitride circuit substrate according to claim 5, wherein the material of said lead frame is molybdenum. The aluminum nitride circuit substrate according to claim 5, wherein the material of said lead frame is Invar steel.

Inventors: MIZUNOYA, NOBUYUKI, SATO, HIDEKI
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: H01L23
IPC codes: H05K1, H01L23, C04B37, H05K3