High frequency signal transmission tape

Abstract: A high frequency signal transmission tape for connecting a plurality of high frequency IC chips to each other or connecting a high frequency IC chip to a signal transmission line disposed on a package, includes an insulating thin film (7) having a surface, a conductive signal line (8) disposed on the surface, and conductive grounding lines (9) disposed on opposite sides of and parallel to the signal line (8) with prescribed spaces apart from the signal line (8). Thus constituted signal transmission tape provides small reflection and attenuation of signals in a extremely high frequency band, i.e., a millimeter wavelength band higher than 30 GHz. Therefore, the high frequency IC chips arbitrarily arranged on a package are easily connected using the high frequency signal transmission tape.

Description: The present invention relates to a high frequency signal transmission tape for connecting semiconductor devices operating at frequencies higher than several tens of MHz to each other or connecting the semiconductor device to an external circuit element, such as a package. Figure 11 is a perspective view illustrating a conventional high frequency IC package. In the figure, a package body 1 comprises a dielectric part la comprising ceramic or the like and a chip bonding part 1b comprising metal or the like. A signal transmission line 6 is disposed on the dielectric part la. The chip bonding part 1b is grounded. High frequency IC chips 2a and 2b are disposed on the chip bonding part 1b. Each of the IC chips includes input-output pads 3 through which the IC chip is connected to the adjacent IC chip or to the signal transmission line 6 of the package 1. The IC chips 2a and 2b are fixed on the chip bonding part 1b using solder, conductive resin, or the like. Thereafter, as shown in figure 12, a metal wire 4 comprising Au, Al, or the like is pressed on the pad 3 of the IC chip 2 using a wedge type blade 5 of a bonding apparatus (not shown) and adhered to the pad 3 by thermocompression bonding or ultrasonic bonding· Figure 13 is a perspective view illustrating the package 1 after the wire-bonding process. In figure 13, a metal wire 4b connects the pads 3 of the adjacent IC chips 2a and 2b to each other and a metal wire 4a connects the pad 3 of the IC chip 2a to the signal transmission line 6 of the package 1. The length of each metal wire is about 300 µm. In this structure, high frequency signals are transferred through the IC chips and the signal transmission line via the metal wires. Such a metal wire for connecting the high frequency IC chips has the following drawbacks. Since the characteristic impedance of a metal wire significantly varies according to its length and shape, a large signal reflection occurs at the boundary between the metal wire and the bonding pad of the IC chip. In addition, the large inductance of the metal wire increases the attenuation of signal. For example, when an Au wire having a length of 1 mm and a diameter of 25 microns is employed, the attenuation increases with the increase in the frequency as shown in figure 14, adversely affecting the device characteristics. When the frequency exceeds 26 GHz, the attenuation exceeds 10 dB (power ratio of 1/10 or below), which makes the practical use of the device difficult. Meanwhile, JP-A-3-120736 discloses a semiconductor device in which a high frequency IC chip is connected to a coplanar transmission line on a package via bump electrodes. This structure is illustrated in figures 15(a) and 15(b). In these figures, the coplanar transmission line comprising a signal conductor 31 and grounding conductors 32 is disposed on a ceramic substrate 33. The ceramic substrate 33 is disposed on a metal module frame 30. The bump electrodes 34 are disposed on the conductors 31 and 32. When mounting the high frequency IC chip 2 on the substrate, the input-output pads 3 of the IC chip 2 are pressed onto the bump electrodes 34 while applying heat, whereby the pads 3 are electrically and physically connected to the bump electrodes 34. Figure 16 is a perspective view, partly broken away, illustrating an impedance-matched film carrier described in "20GHz Band MMIC Modules Assembled by Impedance-Matched Film Carrier" on page 29 of 1989 Autumn National Convention Record, The Institute of Electronics, Information and Communication Engineers. In the device of figure 16, grounding conductors 41 are disposed on the rear surface of a polyimide film 40, and a signal conductor 42 is disposed between the grounding conductors 41, providing a coplanar transmission line. Portions of the polyimide film 40 are opened to expose the coplanar transmission line. When mounting a high frequency IC chip on the film carrier, input-output pads of the IC chip (not shown) are connected to the coplanar transmission line via the openings 43. The document "IEEE Transactions on Microwave Theory and Techniques", vol.17, no.12 (December 1969), New York, US, discloses on pages 1087 through 1090 a coplanar waveguide in the form of a "surface strip transmission line". This known transmission line comprises a dielectric substrate on which one center strip and two ground planes are arranged in parallel. Therefore, this known device basically corresponds to the prior art already discussed with reference to figure 15. In EP-A-0 337 485 it is suggested to provide a flexible substrate on which a plurality of signal and ground planes as well as an IC chip to be supplied thereby are mounted. This document, therefore, suggests to provide a device wherein the flexible substrate forms the basis of the final circuit. Consequently, this known device is comparable to the device shown in figure 16 of the present invention and, thus, also has the disadvantage that the circuit design of the whole electronic device has to be completed in advance. In JP-A-61 245 544, finally, there is disclosed a tool for gang bonding. A transmission tape as claimed in the present invention is however not shown in this document. In the above-described prior art tapes, however, it is necessary to previously form the coplanar transmission line on the package or the film carrier according to the shape or the arrangement pattern of the high frequency IC chip to be connected thereto, resulting in an increased production cost and a restricted pattern layout of the IC chip. It is the object of the present invention to provide a high frequency signal transmission tape that reduces reflection and attenuation of signals in the extremely high frequency band, i.e., the millimeter-wave frequency band over 30 GHz, and that connects high frequency IC chips arbitrarily disposed on a package to each other with no increase in the production cost and no restriction of the pattern layout. This is solved by the advantageous features indicated in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a) and 1(b) are a perspective view and a plan view illustrating a high frequency signal transmission tape being a prestage of the present invention; Figure 2 is a perspective view illustrating the high frequency signal transmission tape of figures 1(a) and 1(b) during the bonding process; Figure 3 is a perspective view illustrating the high frequency signal transmission tape of figures 1(a) and 1(b) after the bonding process; Figure 4 is a graph illustrating attenuation vs. frequency characteristics of the high frequency signal transmission tape of figures 1(a) and 1(b); Figure 5 is a perspective view illustrating a high frequency signal transmission tape in accordance with a further prestage of the present invention; Figure 6(a) is a perspective view illustrating a high frequency signal transmission tape in accordance with a still further prestage of the present invention and figure 6(b) is a perspective view illustrating the high frequency signal transmission tape during the bonding process; Figures 7(a) to 7(c) are a perspective view and plan views illustrating a high frequency signal transmission tape in accordance with a still further prestage of the present in accordance with a fourth embodiment of the present invention; Figure 8 is a perspective view illustrating the high frequency signal transmission tape of figures 7(a)-7(c) during the bonding process; Figures 9(a) and 9(b) are perspective views illustrating a high frequency signal transmission tape in accordance with an embodiment of the present invention; Figure 10 is a perspective view illustrating the high frequency signal transmission tape of figures 9(a) and 9(b) during the bonding process; Figure 11 is a perspective view illustrating a package on which high frequency IC chips are mounted; Figure 12 is a perspective view illustrating a prior art metal wire during the bonding process; Figure 13 is a perspective view illustrating the package of figure 11 after the bonding process; Figure 14 is a graph illustrating attenuation vs. frequency characteristics of the metal wire of figure 12; Figures 15(a) and 15(b) are a plan view and a crosssectional view illustrating a prior art high frequency IC chip mounted on a coplanar transmission line formed on a package; and Figure 16 is a perspective view, partly broken away, illustrating a prior art impedance-matched film carrier having a coplanar transmission line. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1(a) and 1(b) are a perspective view and a bottom view illustrating a high frequency signal transmission tape which constitutes a prestage of the present invention. In these figures, reference numeral 7 designates an insulating thin film having a thickness of 30µm and a width of 340µm and comprising a hard dielectric material, such as Teflon (TM) or ceramic. A signal line 8 and grounding lines 9 are disposed on a surface of the insulating thin film. The grounding lines 9 are disposed at opposite sides of and parallel to the signal line 8 with prescribed spaces apart from the signal line. In production, a low resistance and soft metal, such as Au, Cu, or Al, is plated to the surface of the insulating thin film 7 to a thickness of about 20µm. Then, the plated metal film is formed in a desired pattern using conventional photolithography, resulting in the signal line 8 and the grounding lines 9. The width of each line is about 100µm and the space between adjacent lines is about 20µm. In the following, a description is given of the bonding process. First of all, as shown in figure 2, the signal line 8 of the high frequency signal transmission tape is applied to an input-output pad 3 of a high frequency IC chip 2a mounted on a package (not shown) while the grounding lines 9 are applied to grounding pads 18 of the IC chip 2a. In this state, a wedge type blade 14 is applied to a portion of the insulating thin film 7 opposite the pads 3 and 18 of the IC chip 2a, and heat or ultrasonic wave is applied through the thin film 7 to adhere the lines 8 and 9 to the pads 3 and 18, respectively. When the lines 8 and 9 comprise Au, heat and ultrasonic wave are applied and, when the lines comprise Al, only ultrasonic wave is applied. Since convex parts 14a corresponding to the signal line 8 and grounding lines 9 are formed at the tip of the blade 14, these lines are adhered to the pads with high reliability. In addition, the blade 14 is provided with a cutter 17. After the bonding, the cutter 17 slides downward along the blade 14 and cuts the signal transmission tape. In this way, as illustrated in figure 3, the high frequency IC chip 2a is electrically connected to the adjacent IC chip 2b and to the package 1 via the high frequency signal transmission tape. When the IC chip 2a is connected to the package, the signal line 8 and the grounding lines 9 of the signal transmission tape are in contact with the signal transmission line 6 and the external grounding pads 9 of the package 1, respectively. According to this prestage of the present invention, the high frequency signal transmission tape comprises the insulating thin film 7, the signal line 8 disposed on the surface of the thin film 7, and the grounding lines 9 disposed on opposite sides of and parallel to the signal line 8 with prescribed spaces apart from the signal line 8. The thus constituted signal transmission tape offers an improved impedance matching and a low inductance. Therefore, when the tape is used for connecting the IC chips 2a and 2b to each other or connecting the IC chip 2a to the signal transmission line 6 of the package 1, the connection is carried out with low loss and low reflection in the extremely high frequency band, i.e., the millimeter-wave frequency band over 30 GHz, as shown in figure 4. Further, even if the high frequency IC chips are arbitrarily arranged on the chip bonding part 1b of the package 1, these IC chips are easily connected to each other using the signal transmission tape without increasing the production cost. Figure 5 is a perspective view illustrating a high frequency signal transmission tape in accordance with a further prestage of the present invention. The fundamental structure of this signal transmission tape is identical to that of figure 1. In this tape, however, marks 10 are printed on a surface of the insulating thin film 7, where the conductive lines 8 and 9 are absent, at regular intervals in the longitudinal direction of the thin film 7. When the bonding is carried out with referring to the marks 10, the length of the signal transmission tape is controlled, improving the repeatability and uniformity of electrical characteristics, such as impedance. Figure 6(a) is a perspective view illustrating a high frequency signal transmission tape in accordance with a further prestage of the present invention. In figure 6(a), small openings 15, about 0.1µm along each edge, are formed through the insulating thin film 7 at regular intervals in the longitudinal direction of the tape, and the bonding is carried out using the openings 15 as the marks for controlling the length of the tape. Figure 6(a) illustrates the bonding process. The wedge type blade 14 is provided with a tape guide 20 through which the high frequency signal transmission tape is applied to the surface of the IC chip, and a sprocket roller 16 having projections 16a arranged at the same intervals as the intervals of the openings 15. After bonding the signal line 8 and the grounding lines 9 onto the input-output pad 3 and the grounding pads 18 of the IC chip 2a, respectively, the tip of the blade 14 is moved backward and applied to the bonding pads of the IC chip 2b. While moving the blade 14, the projections 16a of the sprocket roller 16 detect the openings 15 in order to measure the length of the signal transmission tape. Since the length of the signal transmission tape is mechanically controlled in this way, the repeatability and uniformity of electrical characteristics are further improved. While in the above-described tape the openings 15 are mechanically detected by the sprocket roller, the openings may be detected using an electrical or optical method. Figures 7(a) to 7(c) are a perspective view and plan views illustrating a high frequency signal transmission tape in accordance with a further prestage of the present invention. In this tape, a soft dielectric material, for example, polyimide, is employed for the insulating thin film 11. Since such soft dielectric material easily absorbs ultrasonic wave, slits 11a are formed in the width direction of the thin film 11 to expose the signal line 8 and the grounding lines 9. The width of each slit is about 100 µm. When bonding this high frequency signal transmission tape onto an IC chip, as illustrated in figure 8, the tip of the blade 14 is directly applied to the signal line 8 and the grounding lines 9 exposed in the slit 11a so that ultrasonic wave or heat is directly applied to these lines. In this case, the length of the signal transmission tape is controlled by the slits 11a. In figure 7(c), the slits 11a are formed according to the intervals between a plurality of IC chips arbitrarily arranged, whereby the work efficiency in mass production is improved. Figures 9(a) and 9(b) are perspective views illustrating a high frequency signal transmission tape in accordance with an embodiment of the present invention. In this embodiment, a high frequency signal transmission tape comprises a signal conductor 8, dielectric layers 12 disposed on opposite sides of the signal conductor 8, and grounding conductors 9 disposed on opposite sides of the dielectric layers 12. Preferably, the dielectric layer 12 comprises ceramic and the conductors 8 and 9 comprise Au. Figure 10 illustrates the high frequency signal transmission tape of figure 9(a) during the bonding process. As shown in figure 9(b), three metal wires, such as Au wires, 4a to 4c may be connected via a dielectric resin 13 in order to form a high frequency signal transmission tape. In this case, the metal wire 4a serves as a signal line and the metal wires 4b and 4c serve as grounding lines.

Claims: High frequency signal transmission tape for connecting a plurality of high frequency IC chips to each other or for connecting a high frequency IC chip to a signal transmission line disposed on a package, comprising: [a] a signal line (8;4a) comprising a conductive material; [b] two dielectric strip lines (12;13); and [c] two grounding lines (9;4b) comprising a conductive material; [d] said two dielectric strip lines (12;13) being adhered to said signal line (8;4a) and each grounding line (9;4b) being adhered to a corresponding dielectric strip line (12;13) in such a way that all lines are arranged in parallel in a plane which forms said transmission tape. High frequency signal transmission tape according to claim 1, characterized in that said signal line (4a) and said grounding lines (4b) are formed of metal wires (4a, 4b), said dielectric strip lines (13) being formed of resin. High frequency signal transmission tape according to claim 1 or 2, characterized in that said signal line (8; 4a) and said grounding lines (9; 4b) comprise gold and said dielectric strip lines (12; 13) comprise ceramic.

Inventors: KATOH TAKAYUKI, NOTANI YOSHIHIRO, KATOH, TAKAYUKI, NOTANI, YOSHIHIRO, Katoh, Takayuki, c/o Mitsubishi Denki K.K., Notani, Yoshihiro, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L23
IPC codes: H01L23, H01P1, H01P5, H01L21