CERAMIC SUBSTRATES AND METHODS OF MANUFACTURING SAME

Abstract: A ceramic circuit substrate can be manufactured by a method comprising the steps of (i) providing an electrically insulating ceramic substrate; and (ii) irradiating a predetermined region of said ceramic substrate with an energy beam such that at least a portion of said region is rendered conductive.

Description: This invention relates to ceramic circuit substrates and to the manufacture thereof. In particular, the invention is concerned with the forming of conductors of low or high resistivity on a ceramic material. With progress in the miniaturization of electronic instruments, ceramic circuit substrates have been used actively. Ceramic circuit substrates are used as hybrid substrates for mounting ICs or ceramic packages. Two methods, namely, the thick film technique and the thin film technique have been principally used in forming conductors on such a substrate. Each of these methods is capable of effecting miniaturization of about 1/10 to 1/3, relative to what is possible using a printed wiring substrate made of resin. The thick film technique, which uses the printing method, is suited for manufacturing substrates in large quantity at low cost. However, the thick film technique is not suited for the formation of conductors, such as wires, that have a fine line width. On the other hand, the thin film technique is useful for forming lines having a width as small as about 1 µm, since it employs the photoetching method. However, a thin film technique forms thin films by vapor deposition and sputtering, so that it is difficult to manufacture circuit substrates in large quantity at low cost. Hence, it is desired to develop a new method that is capable of forming relatively fine conductive lines while satisfying the desire to manufacture circuit substrates in large quantity at low cost. Further, both the thick and thin film techniques require a long manufacturing process to form the conductive lines, so that problems frequently arise in connection with the expeditions supply of substrates. Moreover, these techniques involve a liquid process, such as the use of pastes or developing solutions in the photoetching method. The administration of liquid processes is complicated from another point of view, in that it may create environmental problems. Additionally, there is a need to open holes in order to connect both sides of the circuit substrate. Accordingly, it is an object of this invention to provide an improved method of manufacturing ceramic circuit substrates. FR-A-2 332 839 discloses a process for polishing a substrate, which may be an electrically insulting ceramic substrate in which a predetermined region of the surface of the substrate is irradiated with a laser beam. The beam is focussed on to a plane and scanned so that the elements of the surface projecting above a nominal plane are fused and therefore tend to flatten out under the influence of gravity. Surprisingly it has been found that apparatus similar to that disclosed in FR-A-2 332 839 can be used in a novel process to produce a ceramic circuit substrate. Accordingly there is provided a method of manufacturing a ceramic circuit substrate in which a predetermined region of an electrically insulting ceramic substrate is irradiated with an energy beam, characterised in that the region is irradiated at an intensity and for a duration sufficient to render the region conductive. In one preferred embodiment, the conductive portion is irradiated to provide a region of low resistance, while in another preferred embodiment, the conductive portion is irradiated to have sufficient resistance to function as a resistor. The ceramic substrate may be composed of either a nitride or a carbide and has an electrically conductive region consisting of substantially a metallic element formed by irradiation of said substrate with an energy beam. A method of manufacturing a ceramic circuit in accordance with the present invention, and a variety of constructions of ceramic circuit capable of being manufactured by the method, will now be described, by way of example only, with reference to the accompanying drawings; in which, Figure 1 is an illustration of an apparatus for use in a method of irradiating a ceramic substrate with a laser beam. Figure 2 is a perspective view of a metal region formed on a ceramic substrate by the method of the invention; Figure 3, is a perspective view of a ceramic substrate, in which a metal region is produced at the edge of the ceramic substrate by the method of the invention; Figure 4a is a plan view of a ceramic circuit in which a plurality of metal layers and a resistor formed therebetween are fabricated on a ceramic substrate, Figure 4b is a diagram showing an enlarged plan view of part of Figure 4a, and Figure 4c is a similar plan view but with a GaAs IC chip attached on to the ceramic circuit substrate; Figure 5 is a cross-sectional view of a resistor being formed in a ceramic circuit substrate; and Figure 6a and Figure 6b are cross-sectional view of other constructions of a ceramic substrate and depict the steps of manufacturing a contact region connecting both sides of a ceramic circuit substrate. In the present invention, an electrically insulating ceramic substrate -- used as a hybrid substrate for mounting ICs or compassing a ceramic package -- is exposed to an energy beam so as to form conductors in the irradiated regions alone by metallization affected through reduction. Suitable ceramics for this purpose include, for example, nitride ceramics that contain AlN, Si₃N₄, BN, or other nitrides as the main constituents and; carbide ceramics that contain SiC or other carbides as the main constituents. In particular, AlN and Si₃N₄, substances that can be reduced to metals of Al or Si in the regions irradiated with an energy beam, are preferred. Figure 1 shows a method for irradiating a ceramic substrate with a laser beam, pursuant to the present invention. A laser oscillator 11 generates a laser beam 12. Then the beam is scanned by controlling the angle of a mirror 13. At the irradiated portion of a ceramic substrate 14, a conductor 15 is formed. The reaction as shown in Eq. (1) when the ceramic material is a nitride and in Eq. (1) when the ceramic material is a carbide, is generated, respectively, to deposit a metal by reducing the ceramic material. MeN --> Me + (1/2) N₂(g) (1) MeC --> Me + C (2) "Me" in these equations denotes a metal. By scanning the laser beam 12, the reduced metal is rendered in a continuous form, which functions, for example, as a conductive line 15 on the ceramic substrate 14, producing a ceramic circuit substrate. For example, if a ceramic substrate made of AlN is irradiated with a laser beam, a reaction according to Eq. (4) will take place, by which AlN is reduced to generate, theoretically, 80.7% per volume Al, or 65.8 weight percent, from AlN. AlN --> Al + (1/2) N₂(g) (4) Moreover, by generating compounds of Al, or by depositing a large number of fine Al particles that are generated in a composite manner through control of the energy of the laser beam, the affected part can be made to possess a specific resistance that is larger than that of metallic Al, to form a conductor that functions as a resistor. Various preferred embodiments of the present invention will now be described by reference to the following, illustrative examples. Example 1 . With reference to Figure 2, an AlN substrate 21, with dimensions of 20 X 10 X 0.635(mm), was prepared by the hot press sintering of raw high purity AlN powder. The AlN substrate was then irradiated with an Nb³⁺ - YAG laser (Q-switch) beam having a spot diameter of 400 µm. The irradiation was performed in air. The irradiation conditions were 1 ms-width pulses at 10pps (pulses per second), and the energy per pulse was 5J. By scanning the laser beam at 5 mm/s, metallic Al, substantially resistance-free conductive path 22 was formed in a groove 23 that was formed during irradiation of AlN substrate 21 as shown in Figure 2. Example 2 . According to this embodiment of the present invention, the same laser beam as in Example 1 was employed, but with the energy raised to 10J/pulse so that, by moving the beam, the laser was made to cut. By means of this irradiation, metallic Al 31 was reduced to form a planar conductive layer on the cut surface, as shown in Figure 3. Such a planar conductive surface 31, for example, can be used for the connection between a ground pad (now shown) formed on the front surface and a ground electrode layer (not shown) formed on the back surface of the ceramic circuit substrate. Example 3 . An AlN substrate having the dimensions of 60 X 30 X 0.635(mm) and containing 3 weight-percent of Y₂O₃ was formed by sheet molding and atmospheric-pressure sintering. The AlN substrate was then irradiated by scanning with CO₂ laser beam at 2.5mm/s in air, with a beam spot diameter of 100 µm under the condition of 10J/pulse and 1000pps, to form reduced metallic Al, as conductive paths with small specific resistance, in the laser beam scanned portions of the AlN substrate. The resistivity of the paths thus obtained was determined via the four-probe method to be about 2.2Ω cm, i.e., the paths were substantially resistance-free. The substrate was then given a heat treatment at 650°C in a N₂ atmosphere (the value of oxygen is 20 ppm). Consequently, the resistivity was reduced to 2.0 Ω cm. Example 4 . First, an AlN substrate having dimensions of 60 X 30 X 0.635(mm) and containing 5 weight-percent of Y₂O3 was formed by sheet molding and atmospheric pressure sintering. The AlN substrate was then scanned in air, at a speed of 4mm/sec, with a beam (50mm spot diameter) from a YAG laser (Toshiba: LAY 711), which is conventionally used for resistor trimming. The conditions of the laser beam were: an exiting current of 14A, a frequency of 2KHz and a bite size of 2 µm. As a result, a groove was formed on the AlN substrate, as well as a conductive path (resistor) of 500Ω/cm at the at the bottom portion of that same groove. Furthermore, the depth and the width of the groove was measured with a scanning electron microscope to be 70 µm, respectively. In this embodiment of the present invention, the conductive path of reduced, metallic material served as a resistor because of the relatively low power of the laser. Example 5 . After soaking an AlN substrate containing 3 weight percent of Y₂O₃ in an aqueous solution of 10 weight percent of KOH, a Nb³⁺ - YAG laser (Q-switch) beam with spot diameter of 100 µm was scanned at 10mm/sec. A resistor of 1KΩ cm was thereby formed on the AlN substrate. The conditions of laser irradiation were: 1ms pulse width, 10pps and 5J/pulse. Example 6 . By scanning a Nb³⁺ - YAG laser (Q-switch) characterized by 5J/pulse, a 1ms pulse width, a spot diameter of 500 µm and a pitch of 300 µm, while Q-switching, in air onto an AlN substrate (prepared by hot press sintering), a thin Al film was formed over the entire surface of the AlN substrate. The scanning speed was 5mm/s. This layer could be used, for example, as a ground electrode layer on the back surface of a ceramic circuit substrate. Example 7 . When a Si₃N₄ ceramic substrate prepared by hot press sintering was irradiated in air with a Nb³⁺-YAG laser (Q-switch) beam having a spot diameter of 400 µm, a groove was formed in the Si₃N₄ substrate. At the same time, an outermost layer of SiO₂, a Si polycrystalline intermediate layer and an inner layer of metallic Si wire was formed in the groove. The laser beam conditions were 1ms pulse width, 5J/pulse and 10pps. The scanning speed was 5mm/s. Example 8 . First, as shown in Fig. 4a, an AlN substrate 41 with dimensions of 15 X 15 X 0.5(mm) and a thermal conductivity of 120 W/m·k was prepared. A prescribed portion of the AlN substrate 41 was irradiated by being scanned at 10mm/S in air with an Nb³⁺ - YAG laser (Q-switch) beam, spot diameter of 400 µm, under the conditions of 5.2J/pulse, 1ms pulse width and 10pps. As a result, a resistor 42 was formed. Subsequently, a thick film of Au paste (Du Pont Co. 9791) was applied all over the back surface of the AlN substrate 41 (not shown). In addition, a plurality of conductor patterns 43 (width of about 0.6mm), each having a characteristic impedance of about 50Ω, were printed on the front surface using the same paste. After driving the substrate for 10 minutes at 120°C, it was sintered in air for 10 minutes at 850°C. By these treatments, as shown in Figure 4a, there was formed a substrate for the ceramic package which has a plurality of conductors 43 and a resistor 42 (45Ω; reflection coefficient of -0.048). More specifically, as shown in Figures 4a and 4b, resistor 42 is formed between two selected conductors 43; one of the two conductors 43 is for a ground voltage supply, the other for a clock supply. The ends P₁ of conductors 43 in the center region of the AlN substrate 41 were used as wire bonding pads, while the other ends P₂ were used as connecting pads for external contacts. A GaAs IC gate array 44 with 2K gates was die bonded to the front center of the substrate 41, as shown in Fig. 4c, and the bonding pads of the GaAs IC gate array 44 and the conductors 43 of the substrate 41 were bonded with wires 45. With this arrangement, it was confirmed that an operation with characteristics that are similar to those achieved via conventional external attachment of a matching chip resistor could be effected, even when the clock frequency was increased by about 10 percent. This is because the resistor was provided near the GaAs IC gate array 44. Example 9 . First; an AlN substrate 51 with the dimensions of 15 X 15 X 0.5(mm) and a thermal conductivity of 120 W/m·k was prepared, and a prescribed portion in the front of the AlN substrate was irradiated in air with an Nb³⁺ - YAG laser (Q-switch) beam having a spot diameter of 400 µm at 10J/pulse. The entire back surface of the AlN substrate 51 was then coated with a thick film 52 of Au paste, and a plurality of conductor patterns 53 (width of about 0.6mm), each having a characteristic impedance of 50Ω, were printed on the front surface. After drying for 10 minutes at 120°C, the substrate was sintered in air for 10 minutes at 850°C. With these treatments, as shown in Figure 5, a substrate for a ceramic package was manufactured that had a ground electrode layer 52 on the back surface of the AlN substrate 51, a plurality of conductors 53 on the front surface (corresponding to conductors 43 in Figures 4a to 4c), and a resistor 54 (with a resistance of abut 45Ω) that was formed on the inner peripheral surface of a fine hole 55 which connected one of the conductors 53, a clock input conductor, to the ground electrode layer 52. With the front center of the AlN substrate 51 thus obtained, a GaAs IC gate array 56 with 2k gates was die bonded, that is, the bonding pads of the GaAs IC gate array 56 and the conductors 53 of the substrate 51 were bonded with wires 57, as shown in Figure 5. As in Example 8, it was confirmed that an operation could be carried out which had characteristics similar to those achieved with external attachment of a matching chip resistor, even when the clock frequency was increased by about 10 percent. Example 10 . First, the front and the back surfaces of an AlN substrate 61 with a thickness of 0.635mm and a thermal conductivity of 170 W/m·k, as shown in Figure 6a, were irradiated three times in air with a Nb³⁺-YAG laser (Q-switch) beam 62 having a spot diameter of 400 µm. The conditions of the laser beam used were: 3J/pulse and 1ms pulse width. Upon irradiation with laser beam 62 as described above, a melted portion with a diameter of about 0.3mm was formed in the irradiated region of the AlN substrate 61, forming metallic Al. A connector 63 was thereby formed (see Figure 6a). The resistance of connector 63 in the vertical direction was measured and found to be about 0.3Ω. Next, after printing conductor patterns 64 and 65, respectively, on the front and back surfaces of the AlN substrate 61 by the use of a thick film of Au paste, the substrate was dried in air for 10 minutes at 120°C and sintered in air for 10 minutes at 850°C. As a result, a ceramic circuit substrate was manufactured having upper wires 64 and lower wires 65 contacted via the connector 63, as shown in Figure 6b. As described in detail in the foregoing examples, it is possible to manufacture ceramic circuit substrates that possess various properties, including the following: (1) It is possible to form substantially resistance-free conductive pathways, resistors and other elements with a fine width, directly onto an electrically insulating ceramic material; this makes it possible to obtain a high density circuit substrate. (2) It is possible to obtain circuit substrates in large quantity and inexpensively by means of a simple process. (3) The rapid supply of ceramic circuit substrate can be achieved. (4) It is possible to form conductors by a dry process, which simplifies carrying out the process and eliminates the possibility of creating environmental problems. (5) When applied to the manufacture of substrate for a given ceramic package, it is possible to form with ease a matching resistor, controlled to a predetermined value, which is located in the vicinity of the IC to be mounted and which has improved characteristics. (6) When the present invention is applied to the manufacture of a circuit substrate to be connected between both sides, it is possible easily to form fine connectors that do not penetrate the substrate and that have a size approximately equal to the spot diameter of the energy beam. Since the connector does not penetrate the ceramic substrate, a reduction in the strength of the ceramic substrate can be avoided during the formation of these connectors, even with high density. (7) A resistor formed in the direction of a substrate depth can be manufactured. The present invention is not restricted to the embodiments described above. For instance, other ceramic substrates, such as BN, SiC, BeO and so forth, can be used instead of AlN and SI₃N₄. Other energy beams, such as those produced by an Ar laser, a filament lamp light and a gas discharge light, electron beams and others can also be used for the above-described purposes. In the Examples 8 and 9, moreover, the resistor was formed between the ground voltage supply and the clock input. However, the resistor may be formed between the ground voltage supply and another signal input. Furthermore, the present invention can be applied when bonding pads, to which bonding wires are die bonded, are formed by the energy beam irradiation in the same manner described in the preceding examples. The scanning of the energy beam can also be altered or eliminated by moving the ceramic substrate itself. Other modifications and improvement of the invention will also become apparent to those of skill in the art, and the invention is intended to cover such modifications and improvements as defined by the appended claims.

Claims: A method of manufacturing a ceramic circuit substrate in which a predetermined region of an electrically insulating ceramic substrate is irradiated with an energy beam, characterised in that the region is irradiated at an intensity and for a duration sufficient to render the region conductive. A method as claimed in claim 1 characterised in that to irradiate said predetermined region there is relative movement between the beam and the substrate. A method as claimed in claim 1 or 2 characterised in that the energy beam is a laser beam (12). A method as claimed in any one of claims 1 to 3, characterised in that the ceramic substrate (14) is SiC. A method as claimed in any one of claims 1 to 3 characterised in that said ceramic substrate is of AlN. A method as claimed in any one of claim 1 to 3 characterised in that said ceramic substrate is of Si₃N₄. A method as claimed in any preceding claim, characterised in that said conductive portion is formed in a groove (23) in said ceramic substrate (14), said groove (23) being formed during said irradiation of said ceramic substrate (14). A method as claimed in any of the claims 1-6, characterised in that the energy beam cuts said ceramic substrate (14) to form a surface on which said conductive portion is provided. A method as claimed in any preceding claim characterised in that, said conductive portion is of low resistance. A method as claimed in any of the claims 1-8 characterised in that said conductive portion constitutes a resistor. A method as claimed in claim 10, characterised by the step of forming a plurality of conductors distinct from said conductive portion on said ceramic substrate (14), and wherein at least two of said conductors are connected by said conductive portion. A method as claimed in any of the claims 1-6, characterised in that a passage (55) through said ceramic substrate (51) is formed by said irradiation and said conductive portion (54) is provided on the inner wall of said passage. A method as claimed in claim 12, characterised by the step of forming conductors (53, 52) on the front and back surface of said ceramic substrate (51), such that the conductor (52) on the back surface of said ceramic substrate (53) is connected by said conductive portion (54) to the conductor (53) on the front surface. A method as claimed in any of the claims 1-6, characterised in that said conductive portion (63) penetrates the thickness of said irradiated region. A method as claimed in claim 14, characterised by the step of forming conductors (64, 65) on front and back surfaces of said ceramic substrate (61), respectively, said conductors (64, 65) being connected to each other by said conductive portion (63) which penetrates said region.

Inventors: IWASE, NOBUO C/O TOSHIBA CORPORATION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: C04B41
IPC codes: C04B41, H05K3, H01L23