Thin film circuit substrate and processes for its manufacture

Abstract: A thin film circuit substrate comprising multilayer conductor layers (1,4) formed via insulating layers (2,3), wherein a signal transmission path (1) in at least one of the conductor layers is embedded in a low dielectric constant insulator (2), and the low dielectric constant insulator (2) is embedded in an insulator layer (3) with a good adhesiveness. The thin film circuit substrate has a low dielectric constant, and thus an improved signal propagation velocity, but since the low dielectric constant insulator (2) which intrinsically has a low adhesiveness, is entirely surrounded by the insulator layer (3) with good adhesiveness the interlayer adhesiveness is not lowered overall.

Description: The present invention relates to a thin film circuit substrate and a process for the manufacture thereof. In recent years, the demand for an acceleration of the processing speed of electronic devices, including computers, has led to a requirement for an acceleration of a signal propagation velocity of a microstrip line on a circuit substrate. As is widely known, the signal propagation velocity of a circuit line is decided by the dielectric constant of an insulator thereof, and thus various circuit substrates using an insulator with a low dielectric constant have been proposed. As an insulating material used for a thin film circuit substrate for accelerating the signal propagation velocity thereof, there have been proposed fluorinated polyimides (dielectric constant = 2.7), fluoropolymers such as PTFE (dielectric constant = 2.1) or amorphous fluoropolymers such as Teflon AF (dielectric constant = 2.0 or less), having a dielectric constant lower than those of the usual, widely-used polyimides (dielectric constant = 3.3). The above-described low dielectric constant materials, however, have a defect in that the adhesion thereof to a substrate is low, and further, these materials cannot be applied to a substrate in a large thickness. Furthermore, since Teflon AF materials are damaged by a fluorine organic solvent, these materials have a drawback in that serious flaws arise when they are washed after having been soldered, or when a VPS (vapor phase soldering) is applied thereto. To solve the above-described problem, a method as illustrated in Fig. 26 may be considered, in which a signal transmission path 1 is sandwiched between an insulator layer 3 made of a normal polyimide with a high adhesion and an insulating layer made of an insulator with a low dielectric constant, whereby an insulating layer is formed. Even by a constitution such as described above, however, a reduction of the dielectric constant can be obtained only to a degree such that the dielectric constant of the finally obtained insulator is reduced at most to about the average value of the dielectric constants of the insulator layer 3 and the insulator 2, and further, such a constitution has problems with the adhesion strength thereof. In Fig. 26, reference numeral 4 indicates a ground layer. In US-A-4,931,354 it has been proposed to form a conductor onto a first insulation layer, and to cover this with a second insulation layer, leaving either a cavity or a porous layer, between the conductor and second insulator. Such a cavity or porous layer therefore extends over part of the periphery of the conductor. This cavity or porous layer may have a low dielectric constant, and therefore will improve the overall speed of the device. This is similar to the prior art device shown in Figure 26, except the low dielectric insulator shown in this example is much larger, and does not require a further insulating layer as required in US-A-4,931,354. According to this invention a multilayer thin film circuit substrate comprises a plurality of conductor layers separated by an insulator, and a conductor for the transmission of signals, the conductor being embedded in a first insulator with the first insulator covering the top and side surfaces of the conductor and at least partly covering the base of the conductor, the first insulator being solid and having a low dielectric constant compared to that of a second insulator, the conductor and the first insulator being surrounded entirely by the second insulator, the second insulator having a good adhesiveness to the conductor layers compared to that of the first insulator. The present invention also includes processes for the manufacture of a multilayer thin film circuit substrate as described above. A first method comprises the steps of:- forming a conductor on the surface of an insulator layer made of the material of the second insulator; eliminating, by isotropic etching, the surface of the insulator layer except for a portion thereof supporting part of the base of the conductor; coating the entire surface of the etched insulator layer with another insulator layer made of the material of the first insulator; eliminating, by etching, the other insulator layer remote from the conductor; and, forming a further insulator layer made of the material of the second insulator on the exposed insulator layer and remaining other insulator layer. A second method comprises the steps of: forming a first insulator layer made of the material of the first insulator on the surface of a second insulator layer made of the material of the second insulator; forming a conductor on the surface of the first insulator layer; coating at least the surface of the first insulator layer and the conductor with a third insulator layer made of the material of the first insulator; selectively eliminating, by etching, the first and third insulator layers remote from the conductor; and, depositing a fourth insulator layer made of the material of the second insulator on the exposed second insulator layer and remaining first and third insulator layers. The present invention has the advantage that it eliminates the above-described defects, and provides a thin film circuit substrate having a greater dielectric constant reduction, without a lowering of the interlayer adhesion. Particular embodiments of multilayer thin film circuit substrates and processes for their manufacture will now be described with reference to the accompanying drawings; in which:- Figure 1 is a schematic illustration showing an embodiment of the thin film circuit substrate according to the present invention; Figs. 2 through 10 are schematic illustrations showing a process for the manufacture of the substrate as shown in Fig. 1; Fig. 11 is a schematic illustration showing another embodiment of the thin film circuit substrate according to the present invention; Figs. 12 through 19 are schematic illustrations showing a process for the manufacture of the substrate as shown in Fig. 11; Figs. 20 through 25 are schematic illustrations showing another process for the manufacture of the substrate as shown in Fig. 11; and, Fig. 26 is an illustration showing an example of a conventional thin film circuit substrate. The thin film circuit substrate according to the present invention, as shown in Fig. 1, can be obtained by a process as illustrated in Figs. 2 through 10, comprising the steps of: eliminating, by an isotropic etching means, a surface layer of an insulator layer 3 with a good adhesiveness and having a signal transmission path 1 formed thereon, except for at least a portion supporting the signal transmission path 1 in an undercut state, and then coating the thus-etched insulator layer with a low dielectric constant insulator 2; and subsequently eliminating, by etching, said low dielectric constant insulator 2, except for the circumference of said signal transmission path 1, and then coating the thus-etched insulator 2 with a layer 3 of an insulating material with a good adhesiveness. According to the above-described constitution, the signal transmission path 1 is embedded in the low dielectric constant insulator 2, whereby a substantially low dielectric constant is obtained. Further, the low dielectric constant insulator 2, which generally has a poor or inferior adhesiveness, is embedded in an insulator with a good adhesiveness, whereby these degrees of adhesiveness compensate each other. A process for the manufacture of the thin film circuit substrate as shown in Fig. 1 is illustrated in Figs. 2 through 10. First, a ground layer 4 is coated with a polyimide having a good adhesiveness, and in general, a high dielectric constant, whereby an insulator layer 3 is formed as illustrated in Fig. 2. The coated polyimide is a polyimide generally utilized in usual thin film formation processes. Subsequently, on the surface layer of the above-described insulator layer 3 is formed a conductor layer, through a known step such as sputtering or etching. This conductor layer is composed of a plurality of signal transmission paths 1 and lands 5, for an interlayer contact. Fig. 2 illustrates a signal transmission path 1 on the left side, and a land 5 for interlayer contact on the right side. Thereafter the insulator layer 3 having a conductor layer formed on the surface layer thereof as described above, is subjected to an isotropic etching by a wet etching method using, e.g., hydrazine, or a dry etching method such as plasma etching, using as a mask the signal transmission path 1 and the land 5 for interlayer contact. The isotropic etching, by which the etching proceeds in all the directions, causes the above-described insulator layer 3 to be cut out inward from the outer circumferential edges of the signal transmission path 1 and the land 5 for interlayer contact, and accordingly, as illustrated in Fig. 3, a conductor layer in an undercut state, and on the reverse side of which the central portion alone is supported by the insulator layer 3 and the ends thereof are not supported, is obtained. Subsequently, the low dielectric constant insulator 2 made of a material such as a fluorinated polyimide, amorphous fluoropolymer or perfluorinated polyalkylamine is coated over the entire surface of the thus-obtained conductor layer. The above-described formation of the low dielectric constant insulator layer 2 is achieved by utilizing the known spin coating method, etc., whereby the undercut portions 6 are filled with the low dielectric constant insulator 2 as illustrated in Fig. 4. Subsequently, as illustrated in Fig. 5, a resist 7 is applied to the low dielectric constant insulator layer 2, at a portion thereof corresponding to the signal transmission path 1, and thereafter, the low dielectric constant insulator layer 2 is etched by RIE (Reactive Ion Etching) and eliminated except for the circumference of the signal transmission path 1 and the portion of the insulator layer 2 penetrating the undercut portions 6, following which only the portion of the resist 7 above the signal transmission path 1 is peeled off, as illustrated in Fig. 6. In this state, a signal transmission path 1 embedded in the low dielectric constant insulator 2, as illustrated in Fig. 7, is obtained. Thereafter, an insulator with good adhesiveness is coated over the entire low dielectric constant layer 2, whereby an insulator layer 3 is formed, and further, the thus-formed insulator 3 is selectively etched to form a contact hole 9 as shown in Figs. 8 and 9. Reference numeral 7' in Fig. 8 indicates the resist to be applied for the etching. In this case, preferably the insulator is a photosensitive polyimide, to improve the etching efficiency for the contact hole 9. By this process, the circumference of the signal transmission path 1 is encircled by the low dielectric constant insulator 2, at almost all parts thereof, and the low dielectric constant insulator 2 having a poor adhesiveness is embedded in the insulator layer 3 with a good adhesiveness, whereby the adhesiveness of the combination of the insulator 2 and the insulator layer 3 is improved. Subsequently, a conductor layer is laminated on the entire surface of the insulator layer 3, and thereafter, a via 8 and an upper conductor layer 10 are formed by etching the conductor layer as illustrated in Fig. 10. Another embodiment of the thin film circuit substrate according to the present invention is schematically shown in Fig. 11. In this embodiment, the signal transmission path 1 is completely embedded in low dielectric constant insulators 2 and 2'. The thin film circuit substrate as shown in Fig. 11 may be prepared by a process as illustrated in Figs. 12 through 19. First, a polyimide is coated on a ground layer 4, to form an insulator layer 3, and then a low dielectric constant layer 2 is coated on the surface of the insulator layer 3, as illustrated in Fig. 12. Thereafter, a resist 7 is applied as a pattern to the surface of the low dielectric constant layer 2, as illustrated in Fig. 13, and the low dielectric constant insulator layer 2 is etched to form a pattern as illustrated in Fig. 14. Then a metal layer 1 is formed over the overall surface, e.g., by sputtering, as illustrated in Fig. 15, and a resist pattern 7" is applied to the surface of the metal layer as illustrated in Fig. 16, whereby the metal layer is etched so that the signal transmission path 1 and the land 5 are formed as illustrated in Fig. 17, and then the resist 7" is removed. Thereafter, a low dielectric constant layer 2', which may be a same or different material to the material of the low dielectric constant layer 2, is formed over the overall surface, a resist 7 is applied to the surface of the low dielectric constant layer 2' as illustrated in Fig. 18, and then the low dielectric constant layer 2' is etched so that the signal transmission path 1 completely embedded in the material of the low dielectric constant layers 2 and 2', and the land 5 mounted on the low dielectric constant layer 2, are formed as shown in Fig. 19, which corresponds to the step shown in Fig. 6 in the above-mentioned first process embodiment. Thereafter, the steps illustrated in Figs. 8 through 10 are repeated. The thin film circuit substrate as shown in Fig. 11 may be prepared by another process as illustrated in Figs. 12 through 19. First, a polyimide is coated on a ground layer 4, to form an insulator layer 3, and then a low dielectric constant layer 2 is coated on the surface of the insulator layer 3 and a metal layer 1 is formed on the surface of the low dielectric constant layer 2, as illustrated in Fig. 20. Then, a resist pattern 7" is applied to the surface of the metal layer as illustrated in Fig. 21, the metal layer is etched to form the signal transmission path 1 and the land 5, and the resist 7" is removed, as illustrated in Fig. 22. Thereafter, a low dielectric constant layer 2', which may be a same or different material to the material of the low dielectric constant layer 2, is formed on the overall surface as illustrated in Fig. 23, a resist 7 is applied to the surface of the low dielectric constant layer 2' as illustrated in Fig. 24, and then the low dielectric constant layer 2' is etched to form the signal transmission path 1 completely embedded in the material of the low dielectric constant layers 2 and 2', and the land 5 mounted on the low dielectric constant layer 2, as shown in Fig. 25, which corresponds to the step shown in Fig. 6 in the above-mentioned first process embodiment. Thereafter, the steps as illustrated in Figs. 8 through 10 may be repeated. As preferred examples of the low dielectric constant layer material, there may be mentioned amorphous fluoropolymers such as Teflon AF available from Du-Pont and having a dielectric constant of 1.89 to 1.93, and perfluorinated polyalkylamine polymers such as Cytop available from the Asahi Glass Company and having a dielectric constant of 2.1 to 2.2. It was found that, when these amorphous fluoro polymers or perfluorinated polyalkylamine polymers are used as the low dielectric constant material according to the present invention, a thin film circuit substrate having a dielectric constant of 2.5 to 2.7 can be obtained. As is clear from the foregoing explanation, according to the present invention, the circumference of a signal transmission path is surrounded with a low dielectric constant insulator, whereby a substantially low dielectric constant can be obtained and signal propagation velocity can be improved. Further, since an insulating layer with a good adhesiveness is interposed between the upper and lower conductor layers, a lowering of the interlayer adhesiveness can be prevented.

Claims: A multilayer thin film circuit substrate comprising a plurality of conductor layers (4,10) separated by an insulator (2,3), and a conductor (1) for the transmission of signals, the conductor (1) being embedded in a first insulator (2) with the first insulator covering the top and side surfaces of the conductor (1) and at least partly covering the base of the conductor (1), the first insulator (2) being solid and having a low dielectric constant compared to that of a second insulator, the conductor (1) and the first insulator (2) being surrounded entirely by the second insulator (3), the second insulator (3) having a good adhesiveness to the conductor layers (4,10) compared to that of the first insulator (2). A multilayer thin film circuit substrate according to claim 1, further comprising a conductive land (5) connected to the conductor (1) for inter-layer connections. A multilayer thin film circuit substrate according to claim 1 or 2, in which the first insulator (2) is formed from a fluorinated polyimide, an amorphous fluoro-polymer, or a perfluorinated polyalkylamine. A multilayer thin film circuit substrate according to any one of the preceding claims, in which the conductor (1) is supported on only a part of its base by the second insulator (3) and all other parts of its surface is in contact with the first insulator (2). A multilayer thin film circuit substrate according to any one of claims 1 to 3, in which the conductor (1) is entirely surrounded by the first insulator (2). A method of manufacturing a multilayer thin film circuit substrate according to claim 4, comprising the steps of: forming a conductor (1) on the surface of an insulator layer (3) made of the material of the second insulator; eliminating, by isotropic etching, the surface of the insulator layer (3) except for a portion thereof supporting part of the base of the conductor (1) ; coating the entire surface of the etched insulator layer (3) with another insulator layer made of the material of the first insulator (2); eliminating, by etching, the other insulator layer (2) remote from the conductor (1); and, forming a further insulator layer made of the material of the second insulator on the exposed insulator layer (3) and remaining other insulator layer (2). A process of manufacturing a multilayer thin film circuit substrate according to claim 5, comprising the steps of: forming a first insulator layer made of the material of the first insulator (2) on the surface of a second insulator layer made of the material of the second insulator (3) ; forming a conductor (1) on the surface of the first insulator layer (2); coating at least the surface of the first insulator layer (2) and the conductor (1) with a third insulator layer (2') made of the material of the first insulator; selectively eliminating, by etching, the first and third insulator layers (2, 2') remote from the conductor (1); and, depositing a fourth insulator (3) layer made of the material of the second insulator on the exposed second insulator layer and remaining first and third insulator layers.

Inventors: TAKESHITA SHUJI, TAKESHITA, SHUJI, Takeshita, Shuji, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01L23
IPC codes: H05K3, H01L23, H05K1