Abstract:
A method for fabricating a signal controller, e.g., a filter or a switch, for a coplanar waveguide during the LIGA fabrication process of the waveguide. Both patterns for the waveguide and patterns for the signal controllers are created on a mask. Radiation travels through the mask and reaches a photoresist layer on a substrate. The irradiated portions are removed and channels are formed on the substrate. A metal is filled into the channels to form the conductors of the waveguide and the signal controllers. Micromachined quasi-lumped elements are used alone or together as filters. The switch includes a comb drive, a spring, a metal plunger, and anchors.

Description:
STATEMENT OF GOVERNMENT SUPPORT 
   The United States Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. 

   CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application incorporates by reference the U.S. patent application entitled “Waveguide Device and Method for Making Same”, Ser. No. 11/149,404, and filed on the same day as the present application. 
   FIELD OF THE INVENTION 
   The present invention relates to monolithic micromachined coplanar devices for controlling transmission of microwave signals, and a method for making the same. 
   BACKGROUND OF THE INVENTION 
   Waveguides are critical components for radar and communications systems. Waveguides operate by guiding the propagation of an electromagnetic wave so that the wave is forced to follow a path defined by the physical structure of the guide. Types of waveguides may be divided by the type of energy that is transmitted, including optical, microwave, and radio frequency transmissions. The present invention is concerned with the propagation of microwave energy, or energy in or near the microwave region of the electromagnetic spectrum. 
   Switches, couplers, splitters, filters, and other components are often used in conjunction with the waveguides to control the signal transmission. 
   The size of the passive filter typically dominates the volume of commercial transmitting and receiving components. Substantial miniaturization of the transmitting and/or receiving components can be achieved through the reduction in volume of microwave bandpass filters. Although conventional methods could fabricate filters during the fabrication process of transmission lines, such methods utilize large resonant structures as filters. 
   The U.S. patent application entitled “Waveguide Device and Method for Making Same,” (herein incorporated by reference) filed on the same day as the present application discloses a method for making high-aspect ratio waveguide by a manufacturing process known as “LIGA” (an acronym for the German: “Lithographie, Galvanoformung, and Abformung”). Thus, it is desirable to fabricate smaller filters in the waveguide when fabricating the waveguide by a LIGA process. 
   The manufacture of conventional RF switches is time, labor and capital intensive undertaking. It is also desirable to fabricate RF switches for the waveguide during the LIGA fabrication process of the waveguide. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide filters with high Q and reduced volume in coplanar microwave waveguides. Micromachined quasi-lumped elements are used alone or together as filters embedded inside of a waveguide during the LIGA fabrication process of the waveguide. Furthermore, these so-called quasi-lumped elements are herein taken to mean one or more capacitor, inductor, or serial and/or parallel combination of these elements. 
   It is another object of the present invention to provide a lateral RF switch with low-loss and high-isolation for coplanar waveguides. According to the present invention, the RF switch comprises a comb drive, a spring, a metal plunger, and anchors. When an actuating voltage is applied to the anchors, the comb drive draws the metal plunger into a gap on the coplanar waveguide to short the lines and shut off the transmission. When the actuating voltage is removed, the spring will return the plunger to its normal position, and the signal transmission starts. 
   It is a further object of the present invention to provide a method for fabricating a signal controller, e.g., a filter or a switch, for a coplanar waveguide during the LIGA fabrication process of the waveguide. Both patterns for the waveguide and patterns for the signal controllers are created on a mask. Radiation travels through the mask and reaches a photoresist layer on a substrate. The irradiated portions are removed and channels are formed on the substrate. A metal is filled into the channels to form the conductors of the waveguide and the signal controllers. 
   For the switch, a sacrificial layer is applied in the channels for the movable parts of the switch before filling of the metal, and is removed after filling of the metal. In addition, a gap is formed in the waveguide to allow the plunger to move in and out. 
   Because the signal controllers can be fabricated during the LIGA fabrication process, they are more cost effective. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, both as to its organization and manner of operation, may be further understood by reference to the drawings that include  FIGS. 1-8 , taken in connection with the following descriptions. 
       FIG. 1  is a side-view illustration of a portion of a manufacturing process used to create an embodiment of the invention. 
       FIG. 2  is a side-view illustration of an additional portion of the manufacturing process. 
       FIG. 3  is a side-view illustration of an additional portion of the manufacturing process. 
       FIG. 4  is a side-view illustration of an additional portion of the manufacturing process. 
       FIG. 5  is a side-view illustration of an embodiment of the present invention including a fully formed waveguide. 
       FIGS. 6A ,  6 B and  6 C show quasi-lumped elements fabricated in a waveguide according to one embodiment of the present invention. 
       FIG. 7  illustrates the Q factor of filters of the present invention. 
       FIG. 8A  shows a waveguide and its RF switch according to one embodiment of the present invention, with the switch in the open position. 
       FIG. 8B  shows a waveguide and its RF switch according to the embodiment  FIG. 8A , with the switch in the closed position. 
       FIGS. 9A and 9B  show two embodiments of the RF switch of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   The following description of illustrative, non-limiting embodiments of the invention discloses specific configurations and components. However, the embodiments are merely examples of the present invention: thus, the specific features described below are merely used to describe such embodiments to provide an overall understanding of the present invention. One skilled in the art readily recognizes that the present invention is not limited to the specific embodiments described below. Furthermore, certain descriptions of various configurations and components of the present invention that are known to one skilled in the art are omitted for the sake of clarity and brevity. 
   An embodiment of the present invention utilizes a LIGA manufacturing process. The main steps of the LIGA process are deep lithography; electroforming, and plastic molding. The LIGA process has typically been used to create micromachinery components such as gears or levers for use in microelectromechanical (“MEMS”) systems. In contrast, the present invention utilizes the LIGA process to create monolithic integrated signal control devices, e.g., filters and switches, for the waveguide during the LIGA fabrication process of the waveguide. The creation of signal control devices for the waveguide according to an embodiment of the present invention utilizing the LIGA process is described in greater detail below. 
   Prior to the process depicted in  FIG. 1 , a mask  120  is created using, for example, an Computer Aided Design (“CAD”) program such as AutoCAD®. While the embodiments of the invention described herein are illustratively created using a gold-plated mask as part of the process, the mask may be chrome or chrome plated, or may be of another material, so long as it is x-ray resistant. Typically, the mask  120  is a singular wafer or disk, but could be created in multiple pieces. The purpose of the mask is to shadow particular portions of the surface  110  to create a pattern thereon. As described below, a mask is used to create a pattern for a waveguide and filters fabricated on the internal surface of the waveguide in one embodiment. It is then used to create a waveguide and a RF switch for controlling signal transmission in the waveguide in another embodiment. In all embodiments the mask  120  includes slots or holes that allow radiation  130  to pass to the surface  110  below the mask  120 . The ordinarily skilled artisan readily comprehends that there are various methods of mask creation, including the inventors&#39; use of a CAD program to create a mask. A side view of mask  120  is depicted in  FIG. 1 . 
     FIG. 1  illustrates a portion of the manufacturing process used to produce a waveguide and its signal control device according to one embodiment of the present invention. As shown, the mask  120  possesses slots or holes through which radiation  130  is allowed to travel, thereby reaching the surface  110 . In the embodiments described herein, x-rays are used as the radiation  130 . However, other forms of energy may be used, including laser light, electron or ion beams, or ultraviolet light, as those of ordinary skill in the art readily comprehend. 
   The x-rays  130 , which may be created with a synchrotron, are painted in a predetermined pattern on the surface  110  through the slots in the mask  120 . Surface  110  is a photoresist that has previously been applied to substrate  100 , and typically is composed of a high molecular weight polymethylmethacrylate (“PMMA”) that has been glued or polymerized to the substrate  100 . The thickness of the applied PMMA is determined relative to an upper limit of the height of the waveguide of the present invention created through the LIGA process, and is generally on the order of hundreds of microns or up to two or three millimeters in depth. The type of photoresist used as the surface  110  depends upon the type of irradiation utilized to paint the surface  110  with the desired pattern. For instance, while PMMA works well with x-rays, other resists could be utilized for photolithographic UV painting, such as SU-8. 
   The substrate  100  may be a metallized or metallic substrate. For instance, the is substrate  100  may be a metallized silicon wafer of about 2 mm in thickness, or the substrate  100  may be a metallic plate of about the same thickness. However, the embodiment set forth herein utilizes a quartz substrate  100  which has been metallized with a titanium/copper/titanium layer (a waveguide of the present invention including copper on a Ti/Cu/Ti-covered quartz substrate achieved a measured attenuation of 0.064 dB/cm at 15.5 GHz). The ordinarily skilled artisan readily understands that additional metals such as aluminum or copper also could be used for this purpose. 
   Following irradiation, the device is transferred to a wet bench/developer so that the portions of the surface  110  that have been irradiated may be removed through application of a chemical reactive process. The reactive process may be an acid, solvent, or a base bath. The embodiment described herein utilizes a custom chemical mixture in three tanks, including an initial bath of a solvent mixture of ethylene glycol, butyl ether, morpholine, and ethanolamine, followed by an intermediate rinse which may be of tetramethylammonium hydroxide, potassium hydroxide, alcohol or ionized water, followed by a final rinse of ionized water. The developer and intermediate rinse tanks include a megasonic agitation unit, and all tanks include a filter with a membrane of 0.2 microns or less. The device is dipped into the baths beginning with the developer bath, where the irradiated portion of the surface  110  is allowed to dissolve. The device is then rinsed in the intermediate bath and with a final rinse of ionized water. Notably, since the cross-absorption and scattering rates of the x-rays into the non-irradiated portions of the PMMA is so low there is virtually no undercut as the PMMA is developed, resulting in extremely linear vertices. 
   The artisan of ordinary skill comprehends that other processes could be used to remove the irradiated portions, including an acid bath or a base bath, or a different solvent bath. 
   The device emerging from the developer process is depicted in  FIG. 2 . 
     FIG. 2  illustrates a substrate  100  on which the PMMA  110  is shown with channels therein. The channels illustrate the previously irradiated portions of the PMMA  110  that have been removed through chemical reaction and are now capable of being used as a mold to facilitate the application of a metal to the substrate  100 . In the embodiment of the invention described herein, copper is applied to the channels, as described below. However, other metals could be used. A non-inclusive list includes silver, gold, nickel, or even alloys such as nickel-cobalt, nickel-iron or bronze. 
     FIG. 3  illustrates a further step in the manufacturing process of the device of the present invention wherein the channels depicted in  FIG. 2  are filled with metal  140  through use of an electroplating (also known as electrodeposition) process. The device is connected to an electrical circuit thereby forming an anode and a cathode, and is then placed in a bath with free-floating copper ions. Electrical current passing through the circuit causes the copper ions to be attracted to the device. Over time, the channels in the surface  110  are filled with an even layer of copper  140 . 
   The ordinarily skilled artisan comprehends that a non-electrical deposition process could also be utilized, such as an electrochemical mechanical deposition or an electroless plating deposition using a chemical process. For instance, a chemical vapor deposition (also known as “CVD”) process could be employed utilizing copper (II) hexafluoroacetylacetonate (Cu(HFA) 2 ). The Cu(HFA) 2  could be mixed with pure hydrogen or a hydrogen/argon mixture (typically in a 1:3 balance) in a cold wall type vertical flow deposition reactor as a function of measured total pressure (typically with a pressure of 2-10 Torr) with a deposition temperature of about 310-390° C., and an inlet precursor mole fraction of about 0.008-0.09. Temperatures of about 310-360° C. result in selective copper deposition under the above conditions. 
   Once the channels are filled with copper vertices  140 , the device is planarized to remove the remaining PMMA. Alternatively, the PMMA may be removed using an acid, solvent or base bath or other mechanical and/or chemical process wherein the remaining PMMA is reactively or abrasively removed. 
     FIG. 4  illustrates the device with all of the PMMA removed. Copper vertices  140  surround channel(s)  150 . Channel(s)  150  constitute the hollow portion of the waveguide upon addition of a capping layer (as discussed herein). As disclosed below, signal control devices, such as filters and switches, can be fabricated in or out of the waveguide to control the signal transmitted in the waveguide. The copper vertices  140  possess surface finishes of optical-quality smoothness and are substantially perfectly vertical, as resulting from the extremely low cross-absorption and scattering rates of the x-rays into the non-irradiated portions of the PMMA. When x-rays and PMMA are utilized, the channel(s)  150  may have an approximate aspect ratio as high as 0.2/3000, wherein the channel  150  may be much less than a micron in width (or approximately as small as 0.2 microns) while having a height of many hundreds of microns (for example, 3 millimeters). When ultraviolet light and SU-8 are employed, the channel(s)  150  may have an approximate aspect ratio as large as 0.2/1000. 
     FIG. 5  illustrates an embodiment of the present invention wherein a cap layer has been added to cover the waveguide, thereby creating rectangular propagating portions  150  between copper vertices  140 , lower layer  100 , and upper layer  160 . Upper layer  160  may be metallic or metallized, and may be glued or brazed to connect to the copper vertices  140 . 
     FIG. 6A  is a top view of a further embodiment of the invention including a filter  610  constituted by quasi-lumped elements and located within propagating channel(s)  650 . Quasi-lumped elements are electrically small discontinuities implemented with uniplanar transmission lines. Uniplanar refers to a topology where devices exist only on a single plane. Examples include microstrip transmission lines and coplanar waveguides. 
   Uniplanar fabrication techniques are desirable over multilayer or three-dimensional designs due to their ease of fabrication and low cost. Quasi-lumped series and/or parallel capacitors and inductors are implemented as interdigitated microstrip discontinuities and as meandering lines.  FIGS. 6A through 6C  show various combinations of quasi-lumped capacitors and quasi-lumped inductors. In particular  FIG. 6B  shows series inductor  620   a  comprising a meandering line coupled to parallel capacitor  630   a  comprising a plurality of interdigitated microstrips while  FIG. 6C  shows parallel inductor  620   b  comprising a plurality of parallel microstrips coupled to series capacitor  630   b  which comprises a plurality of interdigitated microstrips. 
   The quasi-lumped series capacitors and inductors are fabricated simultaneously with the copper vertices  140  of the waveguide by the LIGA process described above. Specifically, on the mask  120 , in addition to positions corresponding to the copper vertices  140  for the waveguide, positions corresponding to conductors of the capacitors and inductors are also slots and holes so as to allow the radiation  130  to travel through the mask  120  and reach the surface  110  during radiation. 
   Then portions of the surface  110  that have been irradiated are removed through the chemical reactive process, and metal, such as copper, is applied to the channels on the substrate  100  and forms an even layer  140 . Finally, an upper layer  160  is added. 
   High-aspect ratio waveguides provide significantly higher inter-conductor capacitances than the traditional thin-film microstrip lines. It is this enhanced inter-conductor capacitance that provides the mechanism for creating high-Q filters, which would be otherwise unrealizable with standard fabrication techniques. 
   The high-aspect ratio waveguide and the high-Q filters in the waveguide are fabricated using the LIGA process, as described above. The LIGA process enables the creation of a waveguide with aspect ratios of or better than 7:1 and conductor sidewall slopes greater than 89.9°. These high-aspect ratio waveguides provide the required inter-conductor capacitances necessary to create quasi-lumped elements of high-Q filters. As shown in  FIG. 7 , these quasi-lumped elements fabricated on high-permittivity dielectric can provide quality factors similar to those of combline and interdigitated filters but in a smaller volume. 
   The high-aspect ratio waveguides created with the LIGA fabrication process are of a well-defined, predictable geometry, which allows accurate 3-D numerical modeling. As a result of the characteristics of LIGA-fabricated coupled-line geometries, new filters have unprecedented minimal insertion-loss, transition bandwidths, and in turn, smaller fractional bandwidths. 
   There are three key parameters for dielectric materials in filter applications: the dielectric constant ∈ r , the quality factor Q, and the temperature coefficient of the resonant frequency τ cf . Larger dielectric constants ∈ r  concentrate the electric field and reduce the physical size of the filter. Low-loss dielectrics provide higher quality factors Q resulting in lower insertion loss and steeper filter roll offs. A τ cf  close to zero is is required for stability against temperature changes. 
   To further reduce the size of the quasi-lumped elements and increase the Q of the filter, in one embodiment a high-permittivity, low-loss ceramic dielectric is used as the substrate. 
   In another embodiment, the substrate of the filters is alumina, a very pure dielectric with one stable phase, a high relative permittivity (∈ r =10), a very low intrinsic dielectric loss (1/Q=0.001), and an acceptable temperature coefficient (τ cf =−60 ppm/K). 
   In another embodiment, the finite-width conductor-backed coplanar waveguide (“CPW”) transmission lines and filters are fabricated on a 1 mm thick, 95 mm diameter quartz wafer. 
   While the reduction of substrate loss does contribute to the realization of high-Q filters, the dominant source of loss in CPW transmission lines is the conductor loss, which is due to high current densities at conductor edges. By increasing the thickness of the conductor, current densities and ohmic losses drop. Simulations show a significant reduction in conductor loss from −15.28 dB/m for 9 μm CPW to −3.95 dB/m for 500 μm CPW at 20 GHz. 
   In one embodiment, electrically thick copper is used to form low-loss quasi-lumped elements and transmission lines. 
   Different combinations of fabrication materials can be used for the waveguide and the filters of the present invention, such as nickel on silicon, and copper on quartz. 
   Simulations show that the electrically thick, micromachined coplanar waveguide possesses decreased dielectric loss, significantly reduced conductor loss, and propagation characteristics conducive to the creation of high-Q filters. The uniplanar filter fabricated in a high-aspect ratio waveguide using the LIGA fabrication process provides miniaturized components for microwave telemetry transmitters and receivers. 
     FIG. 8A  shows a microwave waveguide and its RF switch fabricated by a LIGA process according to one embodiment of the present invention. The switch is used to control signal transmission in CPW (hereinafter “waveguide”)  801 , and includes a comb drive  802 , a spring  803 , a metal plunger  805 , and anchors  804   a , and  804   b . When an actuating voltage is applied to the anchors  804   a  and  804   b , the comb drive  802  draws the metal plunger  805  into a gap on the waveguide  801  to short the lines and shut off transmission, as shown in  FIG. 8B . When the actuating voltage is removed, the spring  803  will return the plunger  805  to its normal position, and signal transmission starts. 
   In one embodiment, in its normal position, the tip of the metal plunger  805  is in the same plane as the first internal vertical surface of the waveguide, so that there is no interference to signal transmission in the waveguide. When the actuating voltage is applied, the tip of the metal plunger  805  is moved to touch the other internal vertical surface of the waveguide to stop signal transmission in the waveguide, as shown in  FIG. 8A . The position of the tip is controlled by the comb drive  802  and the spring  803 . 
   The switch is fabricated simultaneously with the waveguide  801  via an x-ray, or UV, LIGA process, modified to include a sacrificial layer for releasing the moving parts. Specifically, on the mask  120 , in addition to positions corresponding to the copper vertices  140  for the waveguide, positions corresponding to parts of the switch, i.e., the comb drive  802 , the spring  803 , anchors  804   a , and  804   b , and the plunger  805 , are also slots and holes, so as to allow the radiation  130  to travel through the mask  120  and reach the surface  110  during radiation. At the same time, the slot for one of the vertices of the waveguide is cut off to leave a gap in the waveguide  801 . In one embodiment on the mask, the solid portion for the gap in the waveguide  801  is on the extension line of the opening for the plunger  805 , and has a width that allows the plunger to move in and out, but does not cause significant transmission loss. 
   Then, the portions of the surface  110  that have been irradiated are removed through a chemical reactive process. A sacrificial layer is applied to internal surface of the channels created on the substrate in which the moving parts—the comb drive  802 , the spring  803 , and the plunger  805 —are going to be formed. 
   Metal, such as copper, is applied to channels on the substrate  100 , those with the sacrificial layer and those without, and forms an even layer  140 . Finally, the sacrificial layer is removed to release the moving parts, and an upper layer  160  is added on the waveguide. Sacrificial layers can be fabricated from photoresist, oxide, or metal and selectively removed with appropriate etchants or a timed release for thin structures. 
   In one embodiment, the switch is fabricated from 50 μm thick, electroplated copper or nickel. To reduce the size of the comb drive  802 , a small spring constant, and thus thin lines, are required for the return spring  803 . A spring and comb drive fabricated with 5 μm lines will have a circuit area of approximately 2 mm 2 . 
     FIGS. 9A and 9B  show several embodiments of the RF switch of the present invention. 
   Single-pole single-throw (“SPST”) capacitive or DC-contact shunt switches are ideal for time-domain multiplexed systems (“TDM”), where the transmit power is shut off during the receiving operation. This standard is adapted in many wireless communications systems and virtually all pulsed radars operate in this mode. When the transmitting amplifier is active, the switch is in the lowest energy state and does not interfere with transmission. When the transmitting amplifier is shut off, the switch is activated and provides very high isolation with excellent reliability. 
     FIG. 9B  shows examples of single-pole double-throw (“SPDT”) switches for time-domain multiplexed systems. Conventional thin-film shunt switches are not acceptable for high-power operation. The RF switch of the present application can be placed in series with a high-power source, allowing for compact circuits. 
   The previous description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments without the use of inventive faculty. For example, some or all of the features of the different embodiments discussed above may be deleted from the embodiment. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope defined only by the claims below and equivalents thereof.