Patent Publication Number: US-6660174-B2

Title: Method of manufacturing a microstrip edge ground termination

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The following application is cross-referenced and incorporated herein by reference: 
     U.S. patent application Ser. No. 09/957,791, now issued U.S. Pat. No. 6,525,631, entitled “SYSTEM AND METHOD FOR IMPROVED MICROSTRIP TERMINATION,” inventor William W. Oldfield, filed concurrently herewith and incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a microstrip termination. More particularly, the present invention relates to a microstrip termination using a grounded thin film resistor. 
     BACKGROUND 
     Terminations are common components in most microwave systems. Microstrip terminations are easy to process using thin film technology, but the performance drops off rapidly with increasing frequency. Thin film technology typically uses an alumina substrate, with gold and resistor material sputtered onto the substrate, which is then patterned with photolithography techniques to define transmission line traces and resistors. Thick films could also be used, but they typically do not go to high frequencies (above 20 Ghz). 
     FIG. 1 illustrates a standard microstrip termination, known as an edge ground circuit. In the microstrip  100  of FIG. 1, a microstrip transmission line  104 , typically a metal line, is formed on the microstrip substrate  102 , made of a dielectric such as alumina. An area of resistive material  106  is formed on the substrate  102  along the transmission line  104  near an edge ground. The edge ground is formed with a transmission line  110  connecting the resistive material  106  to the metal plated edge  108  which connects to a metal ground plane  112  deposited on the bottom surface of the substrate below the trace and resistive material. The resistive material  106  is used to terminate a signal propagating along the transmission line by matching the impedance of the transmission line and preventing reflection of the propagating signal. 
     FIG. 2 illustrates another standard microstrip termination used when a termination is required away from an edge. This termination  200  also includes a microstrip substrate  202  typically having a metal bottom layer  212 , a transmission line  204 , and an area of thin film resistive material  206 . The substrate  202  also has an area of metal  208  between the resistive material  206  and the edge of the substrate  202 . The substrate  202  typically contains Monolithic Microwave Integrated Circuits (MMICs) connected to the transmission line  204 , and the substrate  202  is mounted on a carrier. A carrier is typically a thin metal plate, on the order of ½ to 1 mm thick, and provides the ground for the microstrip substrate and the MMICs thereon in addition to the metal bottom layer  212 . 
     The termination of FIG. 2 further uses a ground via  210 . The via  210  is formed from metal deposited in a hole in the substrate that extends from the area of metal  208  on the top surface of the substrate to the metal bottom layer  212 . The termination shown in FIG. 2 can be placed anywhere in a subsystem circuit, but the performance is generally worse than the edge ground circuit of FIG.  1 . The poor performance is due to the increased inductance to ground resulting from the small via. 
     FIG. 3 shows the typical performance of an edge grounded microstrip termination. One reason for the poor performance illustrated in the figure is that the “environment” of the resistor is not correct. 
     It is desired that a DC to microwave termination be a reflectionless transition from the transmission line impedance of Zo to ground. The reactive part of the transition should, therefore, match the resistive part from the Zo ohm line to ground. For instance, if the midway resistance of a 50 ohm termination resistor is 25 ohms, the surrounding reactive environment is preferably also 25 ohms. In a coaxial termination, the outer conductor diameter over the resistor is tapered down from the 50 ohm diameter to the diameter of the resistor at the ground end, to provide a smooth impedance transition “environment.” It is therefore desirable to provide a microstrip termination that sufficiently prevents signal reflection by providing a smooth impedance transition. 
     BRIEF SUMMARY 
     In accordance with the present invention, a method is provided for manufacturing circuits having a microstrip termination formed by a transmission line providing a signal through a thin film resistor on the top surface of a substrate through a metal coated tapered edge to a ground plane formed on a bottom of a substrate. Several circuit regions with tapered ground planes are first formed by first cutting holes in a substrate with a laser drill, and then cutting grooves in the substrate with a diamond saw along the holes. Edges of the grooves are sawed to form an angle with the surface of the substrate for creating the tapered edges. Conductive material is deposited on the substrate, then etched in order to form a transmission line pattern and ground regions. A portion of the transmission line is then etched away to form a thin film resistor of the top surface of the substrate. The substrate is then diced along the laser drill alignment markings in order to form individual circuits, each circuit having a transmission line pattern, an area of resistive material, and a tapered edge ground. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with respect to particular embodiments thereof, and reference will be made to the drawings, in which: 
     FIG. 1 is a front and side view of a prior art microstrip termination; 
     FIG. 2 is a front and side view of another prior art microstrip termination; 
     FIG. 3 is a graph showing the typical return loss for a standard microstrip termination of FIG. 2; 
     FIGS. 4A and 4B are front and side views of a microstrip termination in accordance with one embodiment of the present invention; 
     FIG. 5 is a graph showing the improved performance of a microstrip termination in accordance with one embodiment of the present invention; 
     FIGS. 6A and 6B show front and side views of a stage in the manufacturing process of a microstrip termination in accordance with one embodiment of the present invention; 
     FIGS. 7A and 7B show front and side views of a stage in the manufacturing process of a microstrip termination in accordance with one embodiment of the present invention; and 
     FIGS. 8A and 8B show front and side views of a stage in the manufacturing process of a microstrip termination in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 4A and 4B illustrate a microstrip termination  300  in accordance with one embodiment of the present invention. The termination  300  includes a microstrip substrate  302  having a metal transmission line  304  passing on the top surface of the substrate of the substrate. A resistor  306 , or area of a thin film resistive material, is positioned along the transmission line  304  near the edge of the substrate. A transmission line segment  310  may extend between the resistive area  306  and a tapered ground plane. The tapered ground plane comprises a metal plated edge  308  that connects the transmission line segment  310  to a metal ground plane region  312  deposited on the bottom of the substrate  302  below the transmission line  304  and resistor  306 . The edge  308  of the substrate is tapered, or angled, with respect to the top and bottom surfaces of the substrate. The taper is of an angle such that the proper impedance matching, or impedance environment, is obtained for a signal propagating through the termination to ground. The taper is between 0 degrees and 90 degrees, and preferably about 30 degrees. The taper may start before the left edge of the resistor  306  in FIG. 4, below the resistor, or after the right edge of the resistor, preferably starting just before the left edge of the resistor. 
     FIG. 5 illustrates the improved performance of a microstrip termination using a tapered ground plane as shown in FIG.  4 . Results may vary depending on a number of factors, including taper angle, substrate thickness, and signal strength. 
     Stages of a manufacturing process for a microstrip termination as illustrated in FIGS. 4A and 4B are shown in FIGS. 6-8. FIG. 6A shows a first stage  400  in a process, including a substrate  402 , preferably made from alumina, after an initial round of processing. In order to separate tapered ground plane regions so that metal may later be deposited to connect the tapered plane to ground, and provide a saw alignment for later cutting of tapered edges for circuits, a drill, laser, or other appropriate device is used to drill holes  406  in the substrate  402  along what will become the tapered edge of the circuits. Once the holes are drilled, a sawing device such as a diamond saw is used to form the tapered edge of the circuits. The saw creates a tapered edge  404 , preferably tapered about 30 degrees from the top surface of the substrate as shown in FIG. 6B. A second tapered edge  405  is also created. In one embodiment, the size of the holes  406  drilled in the substrate are slightly larger than the final saw kerf  409 . Saw kerf  409  is typically defined as the width of the path cut by a saw as the saw moves through the substrate. The drill holes allow for easier travel of the saw. 
     A second stage  500  in a process for manufacturing the tapered ground plane circuits is shown in FIGS. 7A and 7B. After the alumina has been drilled and grooves cut, the substrate is sputtered with gold or metal with standard processing techniques. A mask may then be positioned on the substrate so the sputtered metals may be etched to form the final pattern. The final pattern includes a transmission line  505 , bottom ground plane  508 , a tapered ground plane  504 , and in the area of the resistive material  510 . At this point, the substrate  502  is etched but yet not diced into individual circuits. 
     The circuits with tapered ground planes are shown after dicing to complete manufacturing in FIGS. 8A and 8B. A saw without a tapered edge may be used to divide the substrate into individual circuits. The saw may or may not align using laser drill holes  506 . Each circuit has a tapered plane  604  of about 30 degrees, an area of resistive material  606 , and a transmission line  608  as described above. 
     Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many additional modifications will fall within the scope of the invention, as that scope is defined by the claims which follow.