1. Field of the Invention
This invention relates to monolithic integrated circuit devices and more particularly to microwave monolithic integrated circuit devices.
In the field of monolithic microwave integrated circuit devices of the high power type which are used in microwave systems, such devices are typically implemented using a metallized backside on a GaAs substrate, with the metallized backside being utilized as the ground. Via holes are opened from the backside through the substrate to provide a low inductance connection from the sources of the active devices (FET's) on the front side of the wafer to the metallized backside. In addition, the metallized backside also forms the contact plane to the heat sink in the final assembled package. In addition to the active devices on the front side of the wafer, microwave transmission lines are also included thereon, which are implemented in microstrips, using the metallized backside as the ground plane. Accordingly, there are two areas of such a device which are critical and have different required characteristics. First, it is desirable to have a thin substrate in the area of the active devices to permit good heat transmission to the metallized backplane so that the heat from the active devices will be transmitted to the heat sink in the assembled package. On the other hand, utilizing a thin substrate, or body, in the region above the transmission lines brings certain difficulties which will be described more fully hereinafter in the description of the prior art.
2. Description of the Prior Art
In the prior art, one of the attempted solutions to the heat flow and heat dissipation problem with GaAs devices has been to provide a layout in the active device area which provides for spacing out the individual gate fingers in an interdigitated FET layout. In so doing, the sources of heat are spread over the surface of the substrate which decreases the thermal resistance between the fingers and the backplane, however the thermal resistance does not decrease linearly with increasing gate finger spacing due to the two-dimensional nature of heat flow. More particularly, thermal resistance is inversely proportional to the cross-sectional area of the heat source and is proportional to the distance between the heat source and the heat sink. With this solution, although the thickness of the body may be maintained such that the microwave transmission line characteristics are maintained to avoid microstrip problems, the spacing of the gate fingers brings with it a problem that at high frequencies, the amount of acceptable phase variation on the gate feed and the required output level dictate that the gate finger spacing be as small as permitted by processing considerations. The gate finger spacing solution is more fully described in an article entitled "A Packaged 20-GHz 1-W GaAs MESFET With A Novel Via-Hole Plated Heat Sink Structure" by Y. Hirachi, Y. Takeuchi, M. Igarashi, K. Kosemura and S. Yamamoto, appearing in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 3, pages 309-316, March, 1984. FIG. 1 in this article is particularly relevant to the gate finger spacing consideration. The attempted solution of spacing the gate fingers to provide adequate heat conduction through a substrate adequately thick to avoid transmission line problems is impractical and is not effective because of the phase variation on the gate feed.
A second attempted solution is to provide a thinner GaAs substrate, to accomplish the reduction of the heat transfer resistance. Substrate thicknesses as small as 30 microns have been used for discrete power FETs, this approach having been described in a paper entitled "A Ka-Band GaAs Power MMIC" in the IEEE International Microwave Symposium Digest, MTT-S 1985, pages 31-34. This approach is not suitable for monolithic microwave integrated circuit devices because of two serious disadvantages, the first of which is that the yield is low due to the increased probability of breakage during die separation and die attachment. Since the number of dies per wafer in the monolithic microwave integrated circuit devices is at least fewer by a factor of four, the yield issue is much more important for these devices than it is for discrete FETs. Secondly, when a thin substrate is utilized in the microwave frequency, this requires a very narrow microstrip, which appears on the front side of the device, must be used for a high impedance microstrip line. As mentioned earlier, in the transmission line area the metallized backside functions as the ground plane and the distance between the microstrip transmission lines on the front side of the device and the metallized backplane on the back side of the device are critical in determining the impedance of the microstrip transmission line. The impedance of a microstrip is uniquely determined by the ratio of the microstrip width to the substrate thickness for a given substrate material. This is more fully described in Chapter 1 of the book entitled Microstrip Lines and Slotlines, by K. C. Gupta, Ramesh Garg and L. J. Bahl, published by Artech House, Inc., 1979. The closer the microstrip is to the backplane, i.e. a thinner substrate between them, the narrower the microstrip width must be for implementing a given impedance. For example, a 150 ohm line, which is a common value for matching stubs utilizing a 30 micro thick GaAs substrate requires that the width of the microstrip must be 0.8 microns. Such a narrow width is not only difficult to implement, but is very lossy at high frequencies.
Thus it can be seen from the foregoing that the prior art has not provided a high frequency device which has the advantages of good heat conduction, which for a high power and high frequency device requires a relatively thin substrate, and a sufficiently thick substrate to provide achievable microstrip widths to avoid the problems mentioned above.