High frequency semiconductor device

Abstract: A semiconductor device includes a silicon substrate (1) on which a circuit having a predetermined function is formed and a high frequency circuit chip (6) which is mounted on the silicon substrate (1) and operates at high frequencies, and operates with functions of the silicon substrate (1) and the high frequency circuit chip (6), wherein a thin film tape having a microstrip structure including an insulating film (3), a signal line (4) on a surface of the insulating film (3) and a grounding layer (8) on a rear surface of the insulating film (3), is disposed on the silicon substrate (1), whereby the silicon substrate (1) is electrically connected to the high frequency circuit chip (6). As a result, a conventional silicon substrate is employed and a production cost is reduced.

Description: FIELD OF THE INVENTION The present invention relates to a semiconductor device and, more particularly, to a transmission line of high frequency signal on so-called GaAs chip-on Si IC, namely, a silicon substrate on which high frequency GaAs chips are mounted. BACKGROUND OF THE INVENTIONS Figures 6(a) and 6(b) are diagrams showing a prior art semiconductor device disclosed in "Integration and Packaging of GaAs MMIC Subsystems Using Silicon Motherboards", IEEE 1990 GaAs IC Symposium, Technical Digest pp. 85-87, in which high frequency GaAs chips are mounted on a silicon substrate on which an IC pattern is formed. While it is advantageous to constitute a circuit by employing a GaAs substrate in view of high speed operation, it is disadvantageous in view of production cost because a GaAs substrate is expensive. Therefore, in general, after only a high frequency circuit in which high speed operation is generally required is produced with the GaAs substrate, and other circuits are produced with employing silicon substrate, these circuits are fabricated. More particularly, as shown in Figure 6(a), on the surface of a silicon substrate 1 on which IC patterns such as memory circuit and wirings are formed, chip die bonding grooves 11 are formed so that the surface part of a GaAs chip 6 and the surface part of the silicon substrate 1 are of the same level at die bonding the GaAs chip 6. This GaAs chip 6 is die-bonded in the die bonding groove 11 by solder or the like, and this GaAs chip 6 is connected with a wire bonding 20 to a transmission line 19 formed on the silicon substrate 1. As described above, by packaging the GaAs chip 6 in a concaved part of the substrate 1, the length of the wire bonding 20 connecting the transmission line 19 on the side of the silicon substrate 1 to the GaAs chip 6 is shortened, reducing reflection of high frequency signals. Figure 6(b) is a portion of a cross section taken along a line in the longitudinal direction of Figure 6(a). A grounding line 21 is formed on the rear surface of the substrate 1 in which the die bonding groove 11 is formed, and the grounding line 21 is electrically connected to the rear surface of the GaAs chip 6 through a via hole 22 which is formed in the substrate 1. Characteristic impedance of the transmission line 19 produced on the surface of the silicon substrate 1 is determined by such parameters as dielectric constant ∈r and thickness t of the substrate 1, width W of the transmission line 19 or the like. Thus, a semiconductor device (GaAs chip-on Si IC) 80 is constructed, that makes the function of ICs formed on the silicon substrate 1 and the function of the GaAs chip 6 complex. Further, when sealing a semiconductor device operating at high frequencies such as a GaAs chip or a GaAs chip-on Si substrate, if a dielectric resin material adheres onto the pattern of the chip, deterioration in high frequency characteristics occurs, in that the wavelength of a high frequency signal generated in the device should vary and deviate from the designed value or the dielectric loss should increase. In order to prevent this deterioration, the chips are sealed employing a hollow package, as illustrated in Figure 7. In other words, the semiconductor device 80 is encapsulated in a package body 23 which is designed according to the configuration of the semiconductor device 80, an external lead 25 fixed to the package body 23 is connected to an electrode of the semiconductor device 80 with a wiring or the like, so as to exchange signals between the semiconductor device 80 and the outside of the package, and the semiconductor device 80 is sealed by fixing a cap material 24 to the package body 23. In the prior art semiconductor device is constructed as described above, the silicon substrate mounting high frequency IC chips thereon, is required to have a high insulating property to obtain the resistivity over several KΩ /cm and to reduce the loss in transmitting high frequency signals. Therefore, a more expensive silicon substrate having a higher purity than a conventional silicon wafer must be employed, resulting in a high cost. In addition, the hollow package employed for packaging the high frequency IC chip is not widely used, and is specially oriented for each chip, resulting in extraordinarily high production cost with relative to a case of employing plastic sealing. US-A-5 049 978 discloses a semiconductor device which includes a high frequency circuit chip operating at high frequencies and being mounted on a silicon substrate, wherein a thin film tape having a microstrip structure including an insulating film, a signal line on a surface of said insulating film and a grounding layer on a rear surface of said insulating film, is disposed on said silicon substrate, whereby said silicon substrate is electrically connected to said high frequency circuit chip. EP-A-0 491 161 discloses a substrate for accomodating integrated circuit chips, wherein the substrate comprises an upper portion on a base made of silicon, and wherein on the upper surface of the substrate there are disposed circuit elements. US-A-5 087 530 discloses a thin film tape for the interconnection of integrated circuit chips on a substrate, wherein said TAB tape has a plurality of electrically conductive circuit patterns on a flexible insulative film, the latter having window regions, a ground layer formed on a back surface and a plurality of through holes for connecting said signal lines and said ground metallization. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor device capable of mounting a high frequency chip on an IC chip comprising a conventional silicon substrate. It is another object of the present invention to provide a semiconductor device which is cheap without employing a hollow package, and has a sealing structure in which high frequency characteristics do not deteriorate. Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the appended claims will become apparent to the those skilled in the art from this detailed description. In a semiconductor device in accordance with the present invention, which is specified in appended claims 1 or 2, a thin film tape having a microstrip structure comprising an insulating film, a signal line on a surface of the insulating film and a grounding layer on a rear surface of the insulating film, adheres to a silicon substrate, and a high frequency circuit chip and a circuit on the side of the silicon substrate are electrically connected by the thin film tape. As a result, the impedance of the signal line is not determined dependent on the dielectric constant of the substrate. In addition, an insulating layer having an aperture is disposed between the thin film tape and the silicon substrate and a high frequency circuit chip is encapsulated in the aperture, or a concaved part is disposed on the surface of the silicon substrate and a high frequency circuit chip is encapsulated in the concaved part, and a portion over the high frequency circuit chip is covered with a cap. As a result, a space is produced over the high frequency circuit chip and plastic sealing is employed. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a) to 1(c) are cross sectional views showing a semiconductor device in accordance with a first embodiment of the present invention. Figure 2 is a perspective view showing a GaAs chip-on Si IC having the structure of the first embodiment. Figure 3 is a diagram explaining a setting of impedance of a Tape Automated Bonding (TAB) tape of the semiconductor device of Figure 2. Figures 4(a) to 4(c) are cross sectional views showing a semiconductor device in accordance with a second embodiment of the present invention. Figure 5 is a perspective view showing a GaAs chip-on Si IC having the structure of the second embodiment. Figures 6(a) and 6(b) are a perspective view and a portion of a cross section showing a prior art semiconductor device. Figure 7 is a cross sectional view showing the prior art semiconductor device which is encapsulated in a package. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A first embodiment of the present invention will be described in detail with reference to Figures 1(a) to 1(c). In Figure 1(a), the same reference numerals as those of Figure 6 designate the same or corresponding parts. A Tape Automated Bonding (TAB) tape 100 has a structure in which a lower insulating film 10 comprising such material as polyimide to connect with a Si IC pattern, a transmission line 4, an upper insulating film 3 and a grounding layer 8 are successively laminated from the bottom and a microstrip line comprises the grounding layer 8, the upper insulating film 3 and the transmission line 4. Actually, as described above, the insulating film 3 equipped with the grounding layer and the transmission line is installed in the lower insulating film 10 by thermocompression bonding or the like with employing TAB technique. The lower insulating film 10 has the approximately same thickness as the GaAs chip 6, and high frequency signals are transmitted by connecting an electrode pad on the surface of the GaAs chip 6 to a projection part 4a of the transmission line 4. In addition, the GaAs chip 6 is connected to a grounding pad 17 formed on the silicon substrate 1 with solder or the like and is fixed. A cap 15 covering an aperture 100a of the TAB tape 100 comprises a metal layer 15a connected to the grounding layer 8 and an insulating material 15b such as polyimide formed on the metal layer 15a to cover the aperture 100a. This keeps a space over the GaAs chip 6 and shields the space electrically. In addition, as shown in Figure 1(b), the grounding layer 8 is electrically connected to a grounding line 16 which is formed on the same plane as the transmission line 4 via a through hole 18. Figure 2 is a perspective view showing a GaAs chip-on Si IC having the structure of Figures 1(a) to 1(c). Here, an IC pattern region 27 in which an IC pattern 2 is formed is produced on the silicon substrate 1. And a GaAs amplifier comprising an inductor 7 and a GaAs chip is constructed on the silicon substrate 1. As illustrated in the figure, the lower insulating film 10 having an aperture in a region in which the GaAs chip is disposed is arranged on a half surface of the silicon substrate 1, and an upper insulating film is provided, which has a transmission line 4, a grounding line 16, DC bias lines 12 and outer lead bondings 13 on a first principal plane, formed on the same surface by etching a metal thin film formed by, for example, metalling copper of 35 µm thickness with gold of several microns and a grounding layer 8 on a second principal plane. The grounding layer 8, which is the uppermost layer, is connected to a grounding pad 17 on the surface of the silicon substrate 1 via a through hole 18 and the grounding line 16. In the Si IC pattern region 27, elements of the IC pattern 2 are connected by wirings formed on the rear surface of the lower insulating film 10. The GaAs chip and the Si IC pattern region 27 are connected through the outer lead bonding 13, and the GaAs chip and the inductor 7 are connected through the outer lead bonding 13. Numeral 26 designates an external connecting pad of the IC pattern 2. Although a difference in level equivalent to a thickness of the lower insulating film 10 is generated by connecting the grounding line 16 to the grounding pad 17, it is possible to reduce the difference by forming the grounding pad 17 into a bump configuration or the like. Figure 3 is a cross sectional view showing the proximity of a polyimide tape (upper insulating film) 3 constructing the microstrip line. In order to fix the characteristic impedance of the transmission line 4 at, for example, 50Ω, supposed that dielectric constant of the polyimide tape 3 be 3.5 and the thickness thereof be 50 µm, a line width 14 of the transmission line 4 is set up at 95 µm. Figure 1(c) is a cross section showing the proximity of the DC bias line 12 or the transmission line 4 in performing plastic sealing of the GaAs chip-on Si IC of Figure 1(b). In this structure, even if the chip is sealed by a mold resin 28, deterioration of high frequency characteristics does not occur because the portion over the GaAs chip 6 is space state covered with the cap 15. Then, because the aperture 100a is covered with the insulating material 15b, even employing a thin metal layer 15a for the cap 15, there is no case that the metal layer 15a would be destroyed by a pressure in molding and the mold resin 28 would adhere to the GaAs chip 6. As described above, in accordance with this embodiment, the TAB tape 100 is disposed on the silicon substrate 1 on which IC patterns are formed, the GaAs chip 6 is encapsulated in the aperture 100a of the TAB tape 100, the rear surface of the chip 6 is connected to the grounding pad 17, and the transmission line 4 formed on the first principal plane of the upper insulating film 3 is electrically connected to the electrode pad on the surface of the chip 6. Therefore, the characteristic impedance of the transmission line 4 is determined dependent not on dielectric constant of the silicon substrate 1 but on such parameter as dielectric constant and thickness of the polyimide tape 3 or a width of the transmission line 4, resulting in a low cost without receiving such constraints as material of the silicon substrate 1. In addition, the GaAs chip 6 is encapsulated in the aperture 100a of the TAB tape 100 and the aperture 100a is sealed with the cap 15. Therefore, the space is kept over the GaAs chip 6 without employing a specially oriented hollow package and packaging is conducted with conventional plastic sealing, reducing production cost. Secondly, a second embodiment of the present invention will be described in detail with reference to Figures 4(a) to 4(c). In this embodiment, as illustrated in Figure 4(a), a chip bonding groove 11 having approximately same thickness as that of the GaAs chip 6 is formed in the silicon substrate 1, the GaAs chip 6 is encapsulated in this groove 11 and the grounding pad 17 disposed in the bottom part of the groove 11 is electrically connected to the rear surface of the chip 6. A TAB tape 101 which is constructed by laminating successively the transmission line 4, such insulating film 3 as polyimide and the grounding layer 8 and has an aperture 101a in a predetermined region, adheres to the silicon substrate 1, so that the transmission line 4 is in the lower side. The projection part 4a of the transmission line 4 is connected to an electrode pad on the surface of the GaAs chip 6. Further, as illustrated in Figure 4(b), the grounding layer 8 is electrically connected to the grounding line 16 formed on the same plane as the transmission line 4 via the through hole 18. A cap 15 covers the aperture 101a of the TAB tape 101, having the same construction as that of the first embodiment. This keeps a space over the GaAs chip 6 and shields the space electrically. Figure 5 is a perspective view showing a GaAs chip-on Si IC having the structure of Figures 4(a) to 4(c). As shown in the figures, the polyimide tape 3 having the aperture 101a in a region including the GaAs chip 6 is arranged on the silicon substrate 1. The transmission line 4, the grounding line 16, the DC bias lines 12 and the outer lead bondings 13 which are formed on the same plane by etching a metal thin film metalled copper with gold are disposed on a first principal plane of the tape 3. In the IC pattern 2, elements are connected by wirings, which are formed in the same plane as the transmission line 4, the grounding line 16 and the DC bias lines 12 and are electrically isolated with these wirings. The grounding layer 8 is formed on a second principal plane of the polyimide tape 3 and is connected to the grounding pad 17 via the through hole 18 and the grounding line 16. Figure 4(c) is a cross section showing the proximity of the DC bias line 12 or the transmission line 4 in performing plastic sealing of the GaAs chip-on Si IC of Figure 4(b). In this structure, similarly as in the first embodiment, even if the chip is sealed by the mold resin 28, high frequency characteristics do not deteriorate. As constructed as described above, the same effect as the first embodiment is obtained with employing less components and difference in level is reduced in connecting the grounding line 16 and the grounding pad 17. While the cap 15 comprises the metal layer 15a and the insulating material 15b in the first and the second embodiments, the insulating material 15b need not be employed if the metal layer 15a has sufficient intensity. As described above, in a semiconductor device in accordance with the present invention, since a TAB tape in which a grounding layer and a signal line are produced on both sides of a polyimide tape as a microstrip line is employed for a line transmitting a high frequency signal on a silicon substrate, and the TAB tape is electrically connected by adhering to the silicon substrate, the impedance of the signal line is determined dependent not on the dielectric constant of the silicon substrate but on the dielectric constant of the TAB tape and the width of the transmission line. Thus, a conventional silicon substrate is employed, reducing a production cost. In addition, since an insulating layer having an aperture is disposed between the thin film tape and the silicon substrate and a high frequency circuit chip is encapsulated in the aperture, or a concaved part is disposed on the surface of the silicon substrate and a high frequency circuit chip is encapsulated in the concaved part, and a portion over the high frequency circuit chip is covered with a cap, a space is kept over the high frequency circuit chip and plastic sealing is performed. As a result, an expensive hollow package is not required, reducing a production cost.

Claims: A semiconductor device including a silicon substrate (1) on which a circuit having a predetermined function is formed and a high frequency circuit chip (6) which is mounted on said silicon substrate (1) and operates at high frequencies, and operating with functions of said silicon substrate (1) and said high frequency circuit chip (6), wherein a thin film tape having a microstrip structure including an insulating film (3), a signal line (4) on a surface of said insulating film (3) and a grounding layer (8) on a rear surface of said insulating film (3), is disposed on said silicon substrate (1), whereby said silicon substrate (1) is electrically connected to said high frequency circuit chip (6), and wherein an insulating layer (10) having an aperture in a predetermined region is formed between said thin film tape and said silicon substrate (1), said high frequency circuit chip (6) is encapsulated in said aperture, a surface of said thin film tape having a said signal line (4) adheres to said insulating layer (10), said signal line (4) is connected to said high frequency circuit chip (6), an aperture (100a) is formed in a region corresponding to said aperture of said insulating layer (10) and said aperture (100a) of said thin film tape is covered with a cap (15a) comprising a metal layer (15a) connected to said grounding layer (8) and an insulating material layer (15b) formed on said metal layer (15a) to cover said aperture (100a). A semiconductor device including a silicon substrate (1) on which a circuit having a predetermined function is formed and a high frequency circuit chip (6) which is mounted on said silicon substrate (1) and operates at high frequencies, and operating with functions of said silicon substrate (1) and said high frequency circuit chip (6), wherein a thin film tape having a microstrip structure including an insulating film (3), a signal line (4) on a surface of said insulating film (3) and a grounding layer (8) on a rear surface of said insulating film (3), is disposed on said silicon substrate (I), whereby said silicon substrate (1) is electrically connected to said high frequency circuit chip (6), and wherein a concaved part (11) is formed in the surface of said silicon substrate (1), said high frequency circuit chip (6) is encapsulated in said concaved part (11), a surface of said thin film tape having said signal line (4) adheres to said silicon substrate (1), said signal line (4) is connected to said high frequency circuit chip (6), an aperture (101a) is formed in a region of said thin film corresponding to said concaved part (11) of said silicon substrate (1) and said aperture (101a) of said thin film tape is covered with a cap (15) comprising a metal layer (15a) connected to said grounding layer (8) and an insulating material layer (15b) formed on said metal layer (15a) to cover said aperture (101a). A semiconductor device according to Claim 1 or 2, wherein a through hole (18) is formed in said insulating film (3) constituting said thin film tape, and said grounding layer (8) of said thin film tape is connected to a grounding line (16) formed on the surface of said thin film tape. A semiconductor device according to Claim 1 or 2, wherein plastic sealing of said silicon substrate (1) including said cap (15) is carried out.

Inventors: NOTANI YOSHIHIRO, NOTANI, YOSHIHIRO, NOTANI, YOSHIHIRO, MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L23
IPC codes: H01L21, H01L23