Circuit board

Abstract: Disclosed is an aluminum nitride thin film cir­ cuit board having an aluminum nitride substrate (1) and a conductive thin film pattern (3, 4) formed on the sub­ strate (1). The conductive thin film pattern (3, 4) has a multi-layer structure selected from the group con­ sisting of Ti/Ni/Au, Ti/Pd/Au, Ti/Pt/Au, Ni/Au, Cr/Au, and Cr/Cu/Au, and a boundary layer (2) of Aℓ-N-M-O (M is Ti, Ni, or Cr) is formed between the substrate (1) and the conductive thin film pattern (3, 4). Since the boundary layer (2) is formed, bonding properties between the substrate (1) and the conductive thin film pattern (3, 4) are improved. In particular, when the boundary layer (2) contains 0.02 to 30 atomic% of oxygen, a higher bonding strength can be obtained.

Description: The present invention relates to a thin film circuit board in which a conductive film pattern is formed on the surface of an aluminum nitride substrate. Conventionally, as a substrate material of a thin film circuit board used in a semiconductor module, alumina is normally used. However, a heat amount from an operating element tends to be increased along with an improvement in performance of an active element formed on a circuit board. For this reason, a circuit board using a substrate material such as alumina having a low thermal conductivity limits the number of elements to be packed. For this reason, a BeO thin film circuit board having a high thermal conductivity is used in place of an alumina board. However, since BeO has considerable toxicity, it poses problems during manufacture or polishing, and an application range is limited. For this reason, AℓN has been widely used as a material replacing BeO. Since AℓN has no toxicity, it does not poses problems in the manufacture, polishing, and disposal. In particular, since the thermal conductivity of AℓN can be adjusted within a wide range of 70 to 280 W/m·K, i.e., a range 3.5 times that of alumina, and in some cases, to a level superior to that of BeO, a higher packing density than a thin film circuit board of alumina can be realized. In addition, a circuit board having both a high packing density of active elements and a desired thermal conductivity can be obtained. As a thin film circuit board using alumina, a board in which a conductive thin film having a three layer structure of Ti/Pt/Au, Ti/Pd/Au is formed on an aluminum nitride substrate is known (see D.M. Mattox, Thin Solid Films, 18 (1973), pp. 173-186). However, in the circuit board with the above structure, since bonding properties between the conductive thin film and the aluminum nitride substrate are insufficient, the conductive thin film may often be peeled from the aluminum nitride substrate or disconnection of the conductive thin film may occur. Since the etching rate of the aluminum nitride substrate is varied depending on crystal orientations, a step is formed at a crystal boundary between different crystal orientations. Therefore, when the bonding properties between the conductive thin film and the aluminum nitride substrate are insufficient, disconnection of a conductive thin film pattern occurs on the step. The conventional thin film is patterned by etching using an etchant, e.g., aqua regia or fluoric acid having strong corrosive properties. In this case, the aluminum nitride substrate itself is also etched by such an etchant. That is, since aluminum nitride has poor anti-corrosion properties with respect to strong acids or strong alkalies as compared to aluminum or silicon, pits or trenches are formed in a substrate surface around the pattern when the conductive thin film pattern is formed. Thus, it is difficult to stably and reliably form a conductive thin film into a predetermined pattern. NEC RESEARCH AND DEVELOPMENT No. 85, April 1987, discloses an aluminum nitrate substrate having a boundary layer made of Ti, NiCr, and Ta₂N formed thereon, and a low-resistance layer made of Pd and Au etc., which is placed on the boundary layer. Ti, NiCr, and Ta₂N are usually used to form a thin film resistor layer. The low-resistance layer formed on the thin film layer made of Ti, NiCr, and Ta₂N is partly removed by etching, the exposed portion functions as a resistor. Although having an advantage in that it possesses an excellent thermal property compared to NiCr, Ta₂N, it has the drawback that when Ta₂N is oxidized by oxygen in air, its property deteriorates in such a way as to change; for example, its resistivity. Therefore, it is necessary to form a protective layer on it. In fact, in a Ta₂N/Ni/Cr/Pd/Au system shown in Table 2 of the NEC R & D, an NiCr layer functions as a protective layer for forming a thin film resistor layer. On the other hand, in order to allow the Ta₂N layer to function as a thin film resistor, a high level of purity of Ta₂N is required. However, Ta₂N having high purity is low in reactivity to a substrate with the result that a diffused region cannot occur in an aluminum nitride substrate. Actually, bonding strength between the thin film layer and the substrate shows 2.5 kg/mm². It has been desired that the bonding strength is improved. It is an object of the present invention to provide an aluminum nitride thin film circuit board which has a high bonding strength of a conductive thin film pattern to an aluminum nitride substrate, and is free from peeling or disconnection of the conductive thin film pattern from the aluminum nitride substrate. According to the first aspect of the present invention as set out in claim 1, there is provided an aluminum nitride thin film circuit board comprising an aluminum nitride substrate ; a boundary layer of AlNMO, where M is one of Ti, Ni, or Cr, said boundary layer being formed on said substrate ; and a conductive thin film pattern having a multi-layer structure formed on said boundary layer wherein said multi-layer structure consists of: a bonding layer formed on said boundary layer, said bonding layer consisting of at least one metal selected from the group consisting of Ti, Ni or Cr; an optional barrier layer of Ni, Pd, Pt and Cr formed on said bonding layer; and a low-resistance layer of Au formed on said bonding layer, or on said barrier layer if present, wherein said multi-layer structure is selected from the group consisting of Ti/Ni/Au, Ti/Pd/Au, Ti/Pt/Au, Ni/Au, Cr/Au, and Cr/Cu/Au. A method for producing such a circuit board is set out in claim 7. The oxygen content in the boundary layer preferably falls in the range of 0.02 to 30 atomic%. According to the first aspect of the present invention, an aluminum nitride thin film circuit board which has a high bonding strength of the conductive thin film pattern to the aluminum nitride substrate, and is free from peeling or disconnecticn of the conductive thin film pattern from the aluminum nitride substrate can be obtained. In the first aspect of the present invention, the average linear surface roughness (Ra) of the aluminum nitride substrate is preferably set to be 800 nm or less. The average crystal grain size of the aluminum nitride substrate preferably falls within the range of 0.5 to 20 µm. 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 sectional view showing a thin film circuit board according to a first embodiment of the present invention; and Fig. 2 is an enlarged view of a surface of an AℓN substrate on which a conductive thin film pattern is formed. In a circuit board according to the first aspect of the present invention, a boundary layer is formed between a conductive thin film pattern having a multilayer structure and an aluminum nitride substrate. The boundary layer serves to improve bonding properties between the substrate and the conductive thin film pattern. The thickness of the boundary layer preferably falls within the range of 5 to 500 nm. If the thickness is smaller than 5 nm, it is difficult to a uniform bonding strength; if it exceeds 500 nm, it is difficult to perform patterning by etching. The multi-layer structure of the conductive thin film pattern can be a two- or three-layer structure. In the case of a two-layer structure, the lower layer serves as a bonding layer, and an upper layer serves as a low-resistance layer. In the case of a three-layer structure, a lower layer serves as a bonding layer, an intermediate layer serves as a barrier layer for preventing a reaction between the lower and upper layers, and an upper layer serves as a low-resistance layer. The thickness of the bonding layer of the conductive thin film pattern preferably falls in the range of 10 to 900 nm. If the thickness is smaller than 10 nm, it is difficult to obtain a sufficient bonding strength; if it exceeds 900 nm, the bonding layer tends to be peeled from the substrate due to its internal stress. The thickness of the barrier layer preferably falls within the range of 50 to 1,000 nm and the thickness of the low-resistance layer preferably falls in the range of 50 to 9,000 nm. In this invention, the average linear average surface roughness (Ra) of the AℓN substrate is preferably selected to be 800 nm or less, more preferably 150 nm or less. The average linear surface roughness (Ra) of the Aℓ substrate is limited to a value equal to or less than 800 nm for the following reason. When the surface roughness exceeds 800 nm, disconnection of a conductor occurs or line widths become nonuniform in a patterning process of forming the conductor. In addition, bonding properties of the conductor with respect to the AℓN substrate are degraded, and reliability of the thin film circuit board is impaired. The above-mentioned surface roughness can be achieved by sintering using an AℓN grain raw material on the submicron order or by polishing the surface of the sintered substrate. More specifically, the surface of the AℓN substrate having predetermined dimensions may be polished by a known method, and thereafter, polishing powders having small grain sizes may be used as the surface roughness is decreased. As this AℓN substrate, a sintered body prepared by adding 20 wt% or less of a sintering agent such as a rare earth element, e.g., Y, an alkali earth element, or the like to an AℓN powder may be used. The AℓN crystal grain size of the AℓN substrate preferably falls within the range of 0.5 to 20 µm. If the AℓN average crystal grain size is 0.5 µm or less, warp or deformation of a substrate caused by sintering may exceed an allowable range capable of forming a conductive thin film. If the grain size exceeds 20 µm, deep pits may be formed due to removal of crystal grains, and cannot be repaired by polishing. Examples of a polishing powder include alundum, green carbon (major component = SiC), cerium oxide, and the like. After surface roughness Ra ≦ 300 nm is achieved by polishing using these materials, polishing may be continued using the same material as that of the AℓN substrate until a desired surface roughness is attained. Before the AℓN substrate is polished using a powder consisting of the major component of the substrate, pre-polishing is preferably performed using a powder having a hardness larger than that of the substrate. For example, green carbon or diamond powder is preferably used for pre-polishing. These powders can flatten a rough substrate surface to some extent, and the surface roughness can be easily reduced by the subsequent polishing. When this treatment is not performed, a group of deep pits due to removal of grains may be formed, and the above-mentioned defects cannot be easily removed by the subsequent polishing. Then, AℓN powder may be used as an abrasive in the final stage of polishing. If polishing in this stage is performed using a material having a larger hardness than that of AℓN, e.g., green carbon, diamond, or the like, the number of pits due to removal of crystal grains and the number of trenches formed by polishing may be increased. When cerium oxide having a smaller hardness than that of AℓN is used as the abrasive, trenches formed by polishing in the immediately preceding stage cannot be easily removed. Thus, a target surface roughness cannot be obtained. For this reason, the same material as that of the AℓN substrate or a material having hardness equivalent to that of AℓN is preferably used as the final abrasive. In this case, AℓN used as the abrasive preferably has a grain size of 2.0 µm or less. If the grain size of the abrasive exceeds 2.0 µm, it is difficult to obtain a desired surface roughness. In order to achieve Ra ≦ 60 nm, a grain size of 2.0 µm or less is preferable. The lower limit of the grain size is determined by the manufacturing limit of the powder. The target surface roughness is preferably selected to be Ra = 60 nm and Rmax = 600 nm or less. If the substrate has a surface roughness exceeding Ra = 800 nm, a pattern cannot be precisely formed by etching. In addition, a nonuniform thickness of the conductive thin film due to an uneven surface causes etching anisotropy, and leads to a decrease in reliability of the conductive thin film. According to the present invention, the surface of the AℓN substrate is preferably polished to obtain a mirror surface in order to improve patterning precision of the conductor. The "mirror surface" is a macroscopic expression. In the microscopic sense, as shown in Fig. 2, heterogeneous interface 14 such as a grain boundary, grain boundary phase, and the like of crystal grains 13, and an unevenness surface caused by an impurity are formed in AℓN substrate 12 on which conductor 11 is formed, and discontinuous surface (step) 15 may be formed at the polished position of each crystal grain 13. If an etchant having a large influence on the AℓN substrate is used to pattern a conductive thin film like in a conventional method, the etchant promotes an etching rate difference of the crystal grains having different etching properties depending on orientation surfaces and enlarges the step of discontinuous surface 14. As a result, it is difficult to reliably form a conductor having a stable pattern. The boundary layer preferably has an oxygen content falling within the range of 0.02 to 30 at%. If the oxygen content is selected to be less than 0.02 at%, it may be difficult to sufficiently enhance chemical reaction properties with respect to the AℓN substrate and a Ti, Ni, or Cr layer as the bonding layer, and to obtain a strong bonding strength. Meanwhile, if the oxygen content exceeds 30 at%, an Aℓ₂O₃ content between the AℓN substrate and the Ti, Ni, or Cr layer as the bonding layer may be increased, and the conductive thin film pattern tends to be peeled from the substrate. Note that when the circuit board employs a multilayer conductive thin film structure, a dielectric layer consisting of AℓN having the same composition as that of AℓN of an AℓN substrate may be formed on the substrate including a first conductive thin film layer, and a second conductive thin film layer may be formed thereon with a boundary layer formed therebetween. A method of manufacturing an AℓN thin film circuit board according to the first aspect of the present invention will be briefly described below. An AℓN substrate having a desired thermal conductivity and an appropriate surface roughness suitable for forming a boundary layer and a conductive thin film layer is prepared. The surface roughness can be controlled by polishing a sintered AℓN substrate or using a sintered AℓN substrate manufactured using submicron grains as a raw material. A TiN, NiN, or CrN layer is formed on the substrate by a conventional film formation method such as vacuum deposition method, a sputter deposition method, or the like, thereby forming a AℓN(Ti, Ni, Cr)O boundary layer. In this case, a substrate temperature, atmosphere, degree of vacuum, and film formation rate are adjusted as needed. Prior to formation of the TiN, NiN, or CrN layer, the substrate surface is sufficiently washed by a wet washing method, a reverse-sputtering method, or the like. Since the AℓN substrate is unstable in strong acids or strong alkalies, a washing solution must be carefully selected, and a neutral washing solution is preferably used. The oxygen content in the boundary layer is controlled according to a film formation atmosphere, purity of a film formation material, and the like. A Ti, Ni, or Cr layer is formed without changing a vacuum pressure. An Ni/Au, Pd/Au, Pt/Au, Au, or CuAu layer is subsequently formed still in the vacuum state. Thereafter, these layers are patterned by a photoetching technique using a resist, thereby forming boundary layer 2, bonding layer 3, and low-resistance layer (including a barrier layer) 4 on AℓN substrate 1, as shown in Fig. 1. These layers are then patterned to manufacture an AℓN thin film circuit board. Etching of the conductive thin pattern is performed using an etchant of KI + I₂ + deionized water for an Au layer, an etchant of CuSO₄ + HCℓ + ethyl alcohol + deionized water for an Ni layer, an etchant of HF + deionized water for Ti and TiN layers, and an etchant of H₂SO₄ + deionized water for Cr and CrN layers. According to the first aspect of the present invention, since the boundary layer is formed between the conductive thin film pattern and the substrate, the conductive thin film pattern can be formed on the substrate with an extremely high bonding strength. More specifically, when a thin film layer is normally formed on an AℓN substrate with a high bonding strength, the bonding strength depends on the lattice constants of the thin film layer and the substrate, a difference in linear expansion coefficient, and a chemical reactivity therebetween. Paying attention to the lattice constant, TiN, NiN, or CrN of the boundary layer directly contacting the AℓN substrate has an NaCℓ type cubic structure. Taking the closest packed surface in a [111] direction into consideration, however, TiN, NiN, or CrN has a hexagonal structure, and its lattice constant is approximate to that of AℓN. In this case, a rate of misfitting with the lattice constant of AℓN is low, and the TiN, NiN, or CrN layer can be strongly bonded to the AℓN substrate. The Ti, Ni, or Cr layer as the bonding layer has sufficient chemical reactivity with respect to the TiN, NiN, or CrN layer as the boundary layer, and provides good bonding properties with respect to the conductive layer formed thereon. Therefore, when a conductive layer is formed on the boundary layer and the bonding layer, the bonding strength of the conductive thin film pattern with respect to the AℓN substrate can be improved. Furthermore, when the boundary layer (TiN, NiN, or CrN layer) directly contacting the AℓN substrate contains 0.02 to 30 at% of oxygen, the bonding strength of the boundary layer with respect to the AℓN substrate can be remarkably improved. According to the present invention, since the conductive thin film pattern can be provided on the AℓN substrate with a high bonding strength, an AℓN thin film circuit board which can prevent peeling or disconnection of the conductive thin film pattern in use and can realize a high packing density of active elements can be obtained. The present invention will be described in detail below by way of its examples. Examples 1 - 5 First, an AℓN substrate having a thermal conductivity of 280 W/m·K was lapped and polished so as to have an average linear surface roughness of 800 nm, and the substrate surface was then subjected to wet washing and sputter Ar Ion etching. Subsequently, a boundary layer and a conductive thin film layer were formed on the surface of the AℓN substrate under the conditions shown in Table 1 below. A resist pattern was formed on the conductive thin film layer by a photoetching technique, and then, using the pattern as a mask, an Au layer was etched using an etchant of KI + I₂ + deionized water; an Ni layer, an etchant of CuSO₄ + HCℓ + ethyl alcohol + deionized water; a Cu layer, an etchant of HNO₃ + deionized water; a Pt layer, an etchant of aqua regia + deionized water; a Ti layer and a TiN layer, an etchant of HF + deionized water; and a Cr layer and a CrN layer, an etchant of H₂SO₄ + deionized water. Thus, a conductive thin film pattern was formed on the boundary layer, thereby manufacturing an AℓN thin film circuit board. For thin film circuit boards of Examples 1 to 5, the bonding strength of the conductive thin film pattern was tested by the tensile test, and the presence/absence of disconnection of the conductive thin film pattern after a 1,000-hour temperature cycle test (held at -50°C and 150°C each for 30 minutes) was examined. Table 1 also summarizes the test results. As can be apparent from Table 1, in the thin film circuit boards of Examples 1 to 5, the bonding strength of the conductive thin film pattern with respect to the AℓN substrate was sufficient, i.e., 2 kg/mm² or more, and no disconnection was observed after the 1,000-hour temperature cycle test, thus revealing their reliability.

Claims: An aluminum nitride thin film circuit board comprising: an aluminum nitride substrate (1); a boundary layer (2) of AlNMO, where M is one of Ti, Ni, or Cr, said boundary layer (2) being formed on said substrate (1); and a conductive thin film pattern (3, 4) having a multi-layer structure formed on said boundary layer (2), wherein said multi-layer structure consists of: a bonding layer (3) formed on said boundary layer, said bonding layer (3) consisting of at least one metal selected from the group consisting of Ti, Ni or Cr; an optional barrier layer of Ni, Pd, Pt and Cu formed on said bonding layer; and a low-resistance layer (4) of Au formed on said bonding layer, or on said barrier layer if present, wherein said multi-layer structure is selected from the group consisting of Ti/Ni/Au, Ti/Pd/Au, Ti/Pt/Au, Ni/Au, Cr/Au, and Cr/Cu/Au. A circuit board according to claim 1, characterized in that said boundary layer (2) contains 0.02 to 30 atomic% of oxygen. A circuit board according to claim 1, characterized in that said boundary layer (2) has a thickness of 5 to 500 nm. A circuit board according to claim 1, characterized in that said aluminum nitride substrate (1) has an average linear surface roughness of not more than 800 nm. A circuit board according to claim 1, characterized in that said aluminum nitride substrate (1) comprises aluminum nitride crystals having an average particle size of 0.5 to 20 µm. A circuit board according to claim 1, characterized in that said bonding layer (3) has a thickness of 10 to 900 nm, said optional barrier layer (3) has a thickness of 50 to 1,000 nm, and said low-resistance layer (4) has a thickness of 50 to 9,000 nm. A method for producing an aluminum nitride thin film circuit board, comprising the steps of: (a) polishing one surface of an aluminum nitride substrate (1) to have an average linear roughness of not more than 300 nm; (b) polishing the polished surface of the aluminum nitride substrate (1) by aluminum nitride ceramic particles to have an average linear roughness of not more than 150 nm; (c) forming a boundary layer (2) of AlNMO, where M is one of Ti, Ni, or Cr, said boundary layer (2) being formed on the polished surface of the aluminum nitride substrate (1); (d) forming a bonding layer (3) of at least one metal selected from the group consisting of Ti, Ni and Cr on said boundary layer (2); (e) forming an optional barrier layer of Ni, Pd, Pt and Cu on said bonding layer; (f) forming a low-resistance layer (4) of Au on said bonding layer, or on said barrier layer if present; (g) patterning said low-resistance layer (4), said barrier layer, if present, and said bonding layer (3) to form a conductive thin film pattern having a multi-layer structure, wherein said multi-layer structure is selected from the group consisting of Ti/Ni/Au, Ti/Pd/Au, Ti/Pt/Au, Ni/Au, Cr/Au, and Cr/Cu/Au. A method according to claim 7, characterized in that said aluminum nitride substrate (1) comprises aluminum nitride crystals having an average particle size of 0.5 to 20 µm. A method according to claim 7, characterized in that said aluminum nitride ceramic particles have a diameter of not more than 2.0 µm. A method according to claim 7, characterized in that said bonding layer (3) has a thickness of 10 to 900 nm, said optional barrier layer has a thickness of 50 to 1,000 nm, and said low-resistance layer (4) has a thickness of 50 to 9,000 nm. A method according to claim 7, characterized in that said polishing in step (b) is carried out to have an average linear roughness of not more than 60 nm.

Inventors: IWASE NOBUO, YASUMOTO TAKAAKI, IWASE, NOBUO, YASUMOTO, TAKAAKI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01L23
IPC codes: H01L23, H05K3, H05K1