Patent Publication Number: US-3880723-A

Title: Method of making substrates for microwave microstrip circuits

Description:
United States Patent 1 1 Lane [ Apr. 29, 1975 1 1 METHOD OF MAKING SUBSTRATES FOR MICROWAVE MICROSTRIP CIRCUITS [75] Inventor: Clyde H. Lane, Rome, NY.  
 [73] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, D.C.  
 [22] Filed: Aug. 28, 1973 [21] Appl. No.: 392,378  
 [52] U.S. Cl 204/12; 204/15 204/38 R [51] Int. Cl C23b 5/48; C23b 7/02; C23f 17/00 [58] Field of Search 204/12, 15, 38 C, 38 E,  
 [56] References Cited UNITED STATES PATENTS 2/1959 Brietzkc 204/12 5/1961 Schumpelt et a1. 204/281 3,181,986 5/1965 Pritikin 156/233 3,324,014 6/1967 Modjeska 204/15 3,414,487 12/1968 Helms et a1 204/13 Primary Examiner-T. M. Tufariello Attorney, Agent, or Firm-Henry S. Miller 1 1 ABSTRACT 3 Claims, 9 Drawing Figures METHOD OF MAKING SUBSTRATES FOR MICROWAVE MICROSTRIP CIRCUITS BACKGROUND OF THE INVENTION This invention relates generally to the fabrication of miniature microwave systems and more particularly to a method of making an encapsulated microstrip line or microwave circuit.  
  Due to the performance required from currently available microwave circuits, the industry has had to rely upon hybrid circuits. The planar fabricating techniques presently known have been unable to meet these stringent requirements and as a result there has developed a need for a method of fabricating a completely miniaturized microwave circuit.  
  One recent advance in the art is the utilization of an arc-plasma spray process whereby a metal or other material may be placed with great accuracy on top of a dielectric substrate. The process lends itself to masking, a technique well known in the art, and as a result circuit geometry may be provided on a&#39;substrate with relative ease. Also such a process is capable of building a device of different material layers with a high degree of reli&#39; ability. The method is characterized by its ability to function with a variety of materials and produce a high quality produce.  
  The arc-plasma spray process has been limited in the past however. to its ability to provide high-quality thin film deposition. No known attempt has been made to utilize the advantages of this process for fabricating encapsulated microwave apparatus whereby devices for TEM propagation made may be obtained or alternatlvely. the advantages of conformal heat sink qualities in an active miniature microwave device.  
 SUMMARY OF THE INVENTION The invention involves an improved utilization of the arc-plasma spray process for the fabrication of miniature microwave systems and like devices. In one aspect of the invention, the process consists of placing a miniature circuit upon a specially constructed substrate and arc-plasma spraying the circuit. The constructed substrate is subsequently removed. leaving the miniature circuit embedded in an arc-plasma sprayed substrate. The novelty in the process involves the removal of the constructed substrate without damaging the miniature circuit.  
  In another aspect of the invention. the additional step of arc-plasma spraying the embedded circuit to provide a totally encapsulated circuit is taught. Prior to the application of the dielectric, active or passive devices may be attached to the circuit and included in the encapsulation.  
  A further aspect of the invention would include plating one or more sides of an encapsulated circuit. The material chosen would allow for various functions such as a ground plane for allowing a TEM propagation mode for microwaves or alternatively a complete conformal heat sinking of active devices.  
  It is therefore an object of the invention to provide a new and improved process for providing substrates for microwave microstrip circuits.  
  It is another object of the invention to provide a new and improved process for fabricating substrates for microwave microstrip circuits that will provide greater flexibility in substrate properties and geometry than hitherto known.  
  It is a further object of the invention to provide a new and improved. microstrip fabrication process ,that will provide a planar substrate combined with the embedding of microstrip lines therein. a  
  It is still another object of&#39;theinvention to provide a new and. improved microstrip fabrication process that will allow for the heat sinking of active devices.  
  It is anotherobject of the invention to provide a new and improved fabrication process that provides for the complete ceramic encapsultation of microwave circuits employing TEM mode.  
  It is another object of the invention to provide a new and improved fabrication process for electronic devices that allows for greater selection and control of material properties of substrates.  
  It is another object of the invention to provide a new and improved process for constructing high quality microwave microstrip circuits without a circuit size restriction.  
  These and other advantages. features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings.  
 DESCRIPTION OF THE DRAWINGS FIGS. 1 through 9 show a crossectional view of a microstrip circuit during various steps of the invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 to 5, the process for embedding circuits in a dielectric involves depositing two to ten thousand angstroms ofa readily soluable or etchable substrate. Followed with a conductive layer 2 to 10,000 angstroms thick which is adherent to the first film. An example of the process would consist of depositing an aluminum film 12 on an aluminum foil substrate 10 by evaporation in a vacuum. Without breaking the vacuum then. deposit a film of gold I4 over the aluminum.  
  Upon this base. a photo resist 18, such as Waycoat. KMER or Shipley 1350H, is applied providing a pattern for electroforming circuit conductors (18) 0.0003 inches in thickness. Although gold would prove to be the best selected material, copper or silver could be utilized as an adequate substitute.  
  Subsequent to the electroforming of the circuit conductors, the photoresist material 16 is etched away. as shown in FIG. 3, along with the thin film 14 leaving the circuit conductor 18 resting on the aluminum film 12. The combination of materials shown in FIG. 3 is then arc plasma sprayed with a suitable dielectric substrate 20; approximately 0.010 to 0.020 inches thick as shown in FIG. 4.  
  In the final step the original substrate 10 and the first deposited film is removed by dissolving or etching. thereby leaving the circuit conductor 18 embedded in the dielectric 20. Subsequently, appropriate connectors may be applied to the conductor for utilization of the circuit in a microwave device.  
  A supplementary process for fabricating circuits is illustrated in FIGS. 6 and 7. A dielectric substrate 22, for example alumina (Algog), has deposited thereon a conductive layer 24. The conductive layer could con sist of 200 angstroms of chromium (26) and 2 to 10,000 angstroms of gold. The conductive layer would be formed by evaporation without breaking vacuum.  
  The rnethod would continue with the placement of a photo resist. electroform ing of a circuit structure through the resist and the subsequent removal of the resist andthin film conductors; as described heremhefore with regard to the aforementioned basic process.  
  The resultant&#39;product&#39;is&#39;arcj plasma sprayed and the original substrate 22 is removed leaving. the circuit 30 &#39;embedd ed&#39;in the dielectric&#39;substrate&#39;32 as shown in FIG. 7.  
  Following these steps active and passive miniature devices may be attached to the circuit thereby making it more adaptable for a specific microwave function. The substrate. circuit and attached devices are then subjected to the arc plasma spray process. The circuit 30 and attached devices are then buried in the dielectric 32 shown in FIG. 8.  
  Appropriate arrangements must be made to allow for connections to the circuit. however. these would he considered conventional to one skilled in the art.  
  A further advance in the art results from an additional step being added to the supplementary process. Concerning FIG. 9. the buried conductor 30. is held in the dielectric substrate 32 as described. lf conductive material 34. 36 is applied to opposite sides of the di electric. it is now possible to achieve a TEM mode of propagation in the circuit. The advantages ofsuch a device are obvious to those in the art lt should be understood. of course. that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous m\&#39; .&#39;lifications or alterations may be made therein u ithoat departing from the spirit and scope of the invention as set forth in the appended claims.  
 What is claimed is:  
  I. In a method of fabricating microwave. microstrip circuits. the steps of: depositing a first layer of soluable material on a smooth soluable substrate material; depositing a second layer of soluable conductive material on said first layer; applying a patterned photo resist to the second layer: electroplating a conductive material through said resist: removing the resist; arc-plasma spraying the resulting circuit and exposed second layer with a dielectric material and removing the original substrate and first and second layers.  
  2. A method of fabricating microwave. microstrip circuits according to claim 1 including the additional step of arc-plasma spraying the circuit and exposed dielectric surface with a dielectric material.  
  3. A method of fabricating microwave, microstrip circuits according to claim 2 including the additional step of electroplating a layer of conductive material on opposed exterior surfaces of the microcircuit.