Abstract:
A MEMS switch fabrication process and apparatus inclusive of a bulbous rounded surface movable contact assembly that is integral with the switch movable element and achieving of long contact wear life with low contact electrical resistance. The disclosed process is compatible with semiconductor integrated circuit fabrication materials and procedures and includes an unusual photoresist reflow step in which the bulbous contact shape is quickly defined in three dimensions from more easily achieved integrated circuit mask and etching-defined precursor shapes. A plurality of differing photoresist materials are used in the process. A large part of the contact and contact spring formation used in the invention is accomplished with low temperature processing including electroplating. Alternate processing steps achieving an alloy metal contact structure are included. Use of a subroutine of processing steps to achieve differing but related portions of the electrical contact structure is also included.

Description:
RIGHTS OF THE GOVERNMENT 
   The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 

   CROSS REFERENCE TO RELATED PATENT DOCUMENTS 
   The present document is somewhat related to the and commonly assigned patent document “RADIO FREQUENCY MEMS SWITCH CONTACT METAL SELECTION”, AFD 707, Ser. No. 11/047,344; now U.S. Pat. No. 7,235,750. The contents of this somewhat related application are hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   Radio frequency switches are used in many aspects of present day communication systems and radar systems including for example in cellular telephones and phased array radar antennas. Today, such radio frequency switching is often accomplished with the use of solid-state devices such as Field Effect Transistors and PIN diodes or with macro sized metal-to-metal contact switches or relays. Such solid state devices are often small and easily integrated with other radio frequency components but provide relative poor electrical performance. In contrast the larger in size Macro switches offer relatively good electrical performance including isolation measuring greater than 70 decibels, insertion losses near 0.07 decibels and contact resistances of less than one ohm however such switches are bulky and not easily integrated with many radio frequency components. 
   One solution to these difficulties is provided by the micro-sized or microelectromechanical or MEMS metal contact switch. Such MEMS switches may be fabricated using the same fabrication processes as is used in realizing solid-state devices. The size of these switches makes them easily integrated with radio frequency components and additionally, because they are mechanical devices, such switches provide relatively good radio frequency performance including isolation greater than 20 decibels and insertion losses near 0.1-0.5 decibels. Although the MEMS switches of the present invention are viewed as being primarily useful at radio frequencies the described structure and method are not limited to such usage and may indeed find application in any frequency range between direct current and signals in the gigahertz range. 
   Radio frequency MEMS metal contact switches have been fabricated and tested by industry, government laboratories, and academia. The upper electric contact area for previous switches has been “plug-shaped” with a flat bottom. Flat upper electric contacts are, however, not easily cleaned, have inconsistent wear patterns, and do not allow for switch operation at different areas on the contact surface. 
   This invention provides a way of implementing a “hemispherical-shaped” upper electric contact geometry into micro-switch fabrication and includes defining the upper contacts in a sacrificial layer using standard photolithography. The resulting electric contact geometry is then re-flowed in an oven to reform, by surface tension, the “plug-shaped” contact into a “hemispherical-shaped” contact. This allows for reliable contact cleaning (i.e. mechanical wiping) and consistent metal-to-metal contact with each switch actuation and also allows the switch to be operated at different areas on the contact surface by varying the switch actuation voltage. 
   SUMMARY OF THE INVENTION 
   The present invention provides a low cost easily fabricated metallic electrical contact assembly especially suited for use in a radio frequency microelectromechanical switch. 
   It is therefore an object of the present invention to provide an integral microelectromechanical switch element inclusive of both spring and electrical contact portions. 
   It is another object of the invention to provide a procedure useful in the fabrication of a microelectromechanical switch element having integral spring and electrical contact portions. 
   It is another object of the invention to provide an electrical contact shape for a radio frequency MEMS switch that is compatible with fabrication of the switching mechanism of the switch. 
   It is another object of the invention to provide an improved shape for the upper contact in a radio frequency MEMS switch. 
   It is another object of the invention to provide integrated circuit process-compatible fabrication of microelectromechanical switch elements. 
   It is another object of the invention to provide a photoresist-based process for fabrication of microelectromechanical switch elements. 
   It is another object of the invention to provide a process for fabrication of microelectromechanical switch elements that is based on use of a photoresist material that is heat responsive and solvent responsive. 
   It is another object of the invention to provide a photoresist reflow-based process for fabrication of microelectromechanical switch element. 
   It is another object of the invention to provide a deposited metal-based process for fabrication of microelectromechanical switch elements. 
   It is another object of the invention to provide a microelectromechanical switch arrangement having desirable immunity to mechanical stiction and other switch mechanism difficulties. 
   It is another object of the invention to provide a microelectromechanical switch arrangement providing desirably low electrical contact resistance. 
   It is another object of the invention to provide a microelectromechanical switch arrangement providing desirable contact resistance to mechanical wear. 
   It is another object of the invention to provide a MEMS switch upper contact that is more easily cleaned by normal contact use wiping action. 
   It is another object of the invention to enable use of a flat bottom upper contact in a MEMS switch. 
   It is another object of the invention to provide a MEMS switch upper contact that achieves a consistent wear pattern. 
   It is another object of the invention to provide a microelectromechanical switch arrangement inclusive of metal alloy materials in the spring and contact portions thereof. 
   These and other objects of the invention will become apparent as the description of the representative embodiments proceeds. 
   These and other objects of the invention are achieved by the method of making electrical contact elements for a radio frequency MEMS contact switch, said method comprising the steps of: 
   fabricating a metallic anchor member, a metallic contact actuation electrode and a metallic lower contact support element for said radio frequency MEMS radio frequency contact switch on a surface of an insulating substrate member; 
   covering said metallic anchor member, said contact actuation electrode and said metallic lower contact support element with a layer of sacrificial photoresist material; 
   forming selectively configured anchor member and rectangularly configured moveable contact member precursor perturbations in said layer of sacrificial photoresist material; 
   reflowing said layer of sacrificial photoresist material selectively configured anchor member and rectangularly configured movable contact member precursor perturbations into sloping sidewall and curving corner cross section shapes respectively by applying thereto an elevated temperature sequence having selected time and temperature magnitudes; 
   covering said layer of sacrificial photoresist material including said curving corner cross section precursor perturbations with a layer of intimately contacting movable contact-supporting metal of selected lateral extent; 
   said selected lateral extent layer of movable contact-support metal including both a contact metal anchoring portion received on said selectively configured anchor member and a bulbous movable contact portion each formed within said upper contact member precursor perturbations of said sacrificial photoresist material during said covering step; 
   releasing said layer of intimately contacting movable contact-support metal and said bulbous movable contact portion from said intimately contacting state with said layer of sacrificial photoresist material, said releasing enabling electrical movement control of said contact-support metal with said bulbous upper contact portion by influence of said contact actuation electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1   a  shows a substrate member usable in a repeating sequence part of a present invention process; 
       FIG. 1   b  shows a substrate with photoresist films usable in a repeating sequence part of a present invention process; 
       FIG. 1   c  shows a substrate with photoresist films and mask usable in a repeating sequence part of a present invention process; 
       FIG. 1   d  shows a substrate with a partially removed photoresist film usable in a repeating sequence part of a present invention process; 
       FIG. 1   e  shows a substrate with two partially removed photoresist films usable in a repeating sequence part of a present invention process; 
       FIG. 1   f  shows a substrate with photoresist films and deposited metal usable in a repeating sequence part of a present invention process; 
       FIG. 1   g  shows a substrate with selected deposited metal area usable in a repeating sequence part of a present invention process; 
       FIG. 2  shows a substrate with multiple deposited metal area pads usable in a present invention process; 
       FIG. 3  shows a substrate with multiple deposited metal areas pads and a contact usable in a present invention process; 
       FIG. 4  shows a substrate with multiple deposited metal areas pads, a contact and a photoresist film usable in a present invention process; 
       FIG. 5  shows a substrate with multiple deposited metal areas pads, a contact and a selectively modified photoresist film usable in a present invention process; 
       FIG. 6  shows a substrate with multiple deposited metal areas pads, a contact and a twice selectively modified photoresist film usable in a present invention process; 
       FIG. 6   a  shows how alterations of the  FIG. 6  photoresist film occur; 
       FIG. 7  shows accomplished alterations of the  FIG. 6  photoresist film; 
       FIG. 8  shows addition of a metallic two layer thin film to the  FIG. 7   b  structure; 
       FIG. 9  shows addition of a modified photoresist layer to the  FIG. 8  metallic two layer thin film structure; 
       FIG. 10  shows addition of a limited metallic film to the  FIG. 9  structure; 
       FIG. 11  shows a completed MEMS switch achieved from the  FIG. 1-FIG .  10  sequence of steps; 
       FIG. 12  shows an alternative arrangement of the  FIG. 8  structure; 
       FIG. 13  shows a photoresist modification of the  FIG. 12  structure; 
       FIG. 14  shows a substitution of contact metal for the  FIG. 13  photoresist area; and 
       FIG. 15  shows a completed MEMS switch achieved from the  FIG. 12-FIG .  14  modification sequence of steps. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  in the drawings shows a sequence of steps usable to form a number of physical portions of a MEMS switch assembly and its electrical contacts in according with the present invention. In the  FIG. 1  drawings a plurality of steps as appear in the views of  FIG. 1A  through  FIG. 1F  are employed in a sequence that is used multiple times in a switch and contact formation process. In a certain sense therefore the  FIG. 1  sequence of steps may be likened to a subroutine in a computer program, a subroutine that is called into execution multiple times by a higher-level routine. 
   The  FIG. 1  sequence commences with a substrate  100  that is composed of for example sapphire and that has been cleaned with a known buffered oxide etch, isopropyl alcohol and acetone sequence. The substrate  100  may have a thickness in the range of 500 nanometers, a surface roughness of 100 angstroms and may be supported on a vacuum chuck fixture during the  FIG. 1  processing. This substrate  100  is covered with a first layer  102  of photoresist preferably of the PMGI type and a second layer  104  of photoresist preferably of the 1813 type as is represented in the  FIG. 1   b  drawing. These two different photoresist materials are desired because their use enables achievement of the overhang appearing in  FIG. 1   e . The photoresist layers may have layer thicknesses in the range of 1000 to 1500 angstroms and 2000 to 3000 angstroms respectively. Such thickness may be determined by spin rate control. Curing of the photoresist layers  102  and  104  may be accomplished with a hot plate bake. The PMGI and 1813 photoresist materials are commonly used in integrated circuit processing and are available from MicroChem Corporation of Newton, Mass. and Rohm and Haas (Shipley) of Philadelphia, Pa. respectively for example. 
   In the  FIG. 1   c  drawing a mask  110  having one or more apertures  108  disposed therein is used to define an area of the photoresist layer  104  to be exposed to ultraviolet light  106 . For the 1000 to 1500 angstroms thickness film of 1813 photoresist in the layer  104 , an exposure time of 4 to 5 seconds may be used for this  FIG. 1   c  step. By way of this exposure in  FIG. 1   c , and development of the 1813 photoresist with a type 351 developer in a 5 to 1 ratio mixture of distilled water to developer, an aperture  114  may be formed in the 1813 photoresist layer  104 . A subsequent exposure using deep ultraviolet light as shown in  FIG. 1   d  may be accomplished for the 2000 to 3000 angstroms PMGI photoresist in layer  102  using an exposure time of 400 to 600 seconds through the  FIG. 1   d  aperture  114 . Development of the PMGI photoresist may be accomplished using MicroChem  101  developer and a 45 second exposure to achieve the undercutting  116 . 
   In the  FIG. 1   f  step of  FIG. 1  an evaporated layer of Gold metal is shown to have been deposited over the exposed surfaces of the 1813 photoresist  104  at  120  and the exposed portion of the substrate  100  in the aperture  114  and  116  area; the metal  122  being in this latter position. This layer of Gold may have a thickness of about 2800 angstroms and is preferably preceded by a 200 angstroms film of chrome for substrate adhesion. The deposition is preferably accomplished by evaporation achieved under chamber pressure conditions of one to two millitorrs. As shown in the  FIG. 1   g  drawing the metal at  122  is the sought after portion of this metal layer. Removal of the photoresist supported portions of the metal layer, the unwanted portions at  120 , is preferably accomplished by way of a metal lift off sequence using standard adhesive coated tape that is followed by dissolution of the photoresist layers using acetone for the 1813 material of layer  104  and then using heated 1165 stripper for the PMGI material of layer  102 . The 1165 stripper material is preferably used at a temperature of 90 to 100 degrees Celsius and is available commercially from MicroChem Corporation. 
   The metal  122  achieved by way of the  FIG. 1  sequence may be replicated in several locations across the surface of substrate  100  by way of providing a mask of appropriate size and configuration at  110  in  FIG. 1   c . Such a mask can provide simultaneous processing of multiple metal pads of similar or differing size and shape on the substrate. This metal may moreover be used for a variety of purposes in fabricating the sought-after MEMS switch. One collection of such metal areas is shown in the drawing of  FIG. 2  herein where metal areas or metal pads  200 ,  202  and  204  have been formed in order to provide switch anchor, switch bottom actuation electrode and switch bottom contact functions respectively. The chrome metal used as a precursor for the Gold metal of pads  200 ,  202  and  204  is represented at  206  in the  FIG. 2  drawing and by a similar showing in several subsequent drawings. 
     FIG. 3  in the drawings shows the addition of a film  300  of conformal additional metal on the surface of the bottom contact metal pad  204  of  FIG. 2 . This film  300  is preferably of about 500 angstroms thickness and is accomplished by way of a sputtering process using the above-described  FIG. 1  sequence of steps, i.e., the subroutine processing steps. During this use of the  FIG. 1  steps the metal evaporation of  FIG. 1   f  is replaced by the sputtering sequence. This use of the  FIG. 1  steps also includes the adhesive tape and solvent steps removal of portions of the metal  300  overlying the pads  200  and  202  and the intervening substrate  100  areas. The metal film  300  is preferably composed of a Gold alloy and is achieved under sputtering conditions involving chamber pressures of 2 to 5 millitorrs. 
   The drawing of  FIG. 4  shows the deposition of a photoresist film  400  over the  FIG. 3  substrate  100  and pads  200 ,  202  and  204  structure. This photoresist film  400  is preferably composed of PMGI material of 2 to 3 micrometers thickness and achieved with the use of a conventional spinning and baking sequence involving two to three steps and about one nanometer per step. The film  400  is a sacrificial element in the present process and is additionally processed as is described in the ensuing paragraphs herein. 
   Mask patterning, exposure to deep ultraviolet light and development of the PMGI photoresist  400  in the region overlying the anchor pad  200  is represented in the  FIG. 5  drawing. Notably the sidewalls  502  and  504  of the recess  500  in the developed photoresist film  400  are substantially vertical in disposition at this point in the processing. A similar mask patterning, exposure to deep ultraviolet light and development of the PMGI photoresist  400  in the region  600  overlying the lower switch contact pad  204  appears in the  FIG. 6  drawing. Notably the sidewalls  602  and  604  of the recess  600  thusly formed in the photoresist film  400  are also substantially vertical in disposition at the  FIG. 6  point in the switch processing. It is also significant to note that the exposure and the development for the recess  600  are each made to be of a partial nature and that the recess  600  is made to have a depth of about 0.7 to 1.2 micrometers as opposed to the greater substantially photoresist thickness depth of the recess  500 . As a result of these differences between the recesses  500  and the recess  600 , separate mask patterning, exposure to deep ultraviolet light and development of the PMGI photoresist  400  in the regions of these recesses is preferred. The partial nature of the exposure and the development for the recess  600  prepare for a subsequent processing step of this recess. 
     FIG. 7  in the drawings in fact shows the results of this subsequent processing step for both the recess  500  and the recess  600 . The  FIG. 7  changes represent in fact a significant aspect of the present invention. The processing represented in  FIG. 7  actually involves a reflowing of the  FIG. 6  PMGI photoresist  400  in order to enable the formation of a hinge element on the pad  200  in the recess  500  and a bulbous metal alloy contact in the recess  600  adjacent the pad  204 . This reflowing is preferably accomplished thermally and involves subjecting the  FIG. 6  photoresist and recesses to a bake at 250 degrees Celsius for a period of three to four minutes. One result of this thermal exposure is represented in the  FIG. 6   a  cutaway drawing of the recess  600  region where the corners  608  and  612  of the recess  600  are shown to soften or recede along the arrows  606  and  614  toward the interior of the recess  600 , to the extent of the lines  610  and  616 . This change forms a bulbous or rounded dimple shape from the original square cornered recess  600 . The nature of this bulbous or rounded dimple shape appears at  600  in the  FIG. 6   a  and  FIG. 7  drawings. Another result of this thermal exposure is represented by the sloping corners and the sloping walls  720  and  722  achieved in the recess  500  as is shown in  FIG. 7 . 
   The bulbous or rounded dimple shape at  600  and the sloping walls  720  in  FIG. 7  are reproduced in a continuous sputtered layer of metal  800  over the photoresist  400  as appears in the  FIG. 8  drawing. This metal preferably includes an initial layer  806  of Gold or Gold alloy of about 500 angstroms thickness, an intermediate thin layer of Gold  807 , and a covering layer  808  of Titanium of about 200 angstroms thickness. These layers are preferably achieved through use of a standard sputtering process involving the pressures, of 2 to 3 millitorrs, and the times of 30 seconds and 1 minute for the Gold and the titanium layers. The Gold or Gold alloy layer serve as the electrical contact metal and the metal layer of Gold and Titanium at  800  serve as the seed layer for the electrical structural layer by which the movable element of the switch being fabricated can be later separated from its underlying structure of formation to permit movement between contact open and contact closed conditions during switch actuation. The metal layer  800  follows the contours of the bulbous or rounded dimple shape at  600  and the sloping walls  720  in  FIG. 7  as appear at  802  and  804  in the  FIG. 8  drawing. 
     FIG. 9  in the drawings shows the results of adding another layer  900  of photoresist of some 6 micrometers thickness over the Titanium metal of layer  800  in order to prepare for an ensuing electroplating process. The layer  900  is made of type  9260  photoresist, a photoresist formulation having the especially desirable high viscosity characteristic. The 9260 photoresist is available from the Ulm Germany corporation Microchemicals GmbH. The representation in  FIG. 9  also shows the 9260 photoresist has been mask exposed, developed and etched away in the region overlying the anchor pad, actuation pad and contact pad  200 ,  202  and  204  respectively to leave the larger recess  902  in which an etching away of the now exposed and uppermost layer of Titanium  808  can be accomplished. Removal of the Titanium layer  808  can be accomplished with a buffered oxide etch diluted with de-ionized water (1:10) in a three to four minute etch. Following Titanium removal the Gold layer  806  remains in the recess  902  and together with the walls  904  and  906  of the etched  9260  photoresist forms a thick mold region in which electroplated metal may be received. 
     FIG. 10  in the drawings shows the receipt of Gold electroplate metal  1000  in a thickness of about 5 micrometers in the recess area  902 . The existing Gold layer  806  has in fact provided a plating electrode with the aid of which the plated metal area  1000  can be formed. Notably the metal  1000  does not extend over the surfaces  908  and  910  of the 9260 photoresist since no such plating electrode is present in this location to attract the metal particles moving through a plating bath. It is notable that the plated metal  1000  extends down to the Gold layer of the anchor pad  200  and thus is able to form an intimate rigid interface with the anchor pad at  1002 . It is notable also that the plated metal  1000 , which is actually the movable element of the sought-after MEMS switch, is of 5 micrometers thickness and is fully surrounded by (but not covered by) photoresist material structures. The bulbous region  1004  of the plated metal  1000  is of course the desired rounded contact area of the movable switch element  1006 . 
   Completion of the present process involves several additional steps including dissolving of the 9260 photoresist at each end of and adjacent the sides of the movable switch element  1006 ; this may be accomplished with use of acetone. Completion also involves etching away the  FIG. 8  Titanium film  808  from areas adjacent the movable switch element  1006 , i.e., from areas exposed by the just completed dissolution of the 9260 photoresist. This etching away may involve another use of a buffered oxide etch as accomplished in the cavity  902  in connection with the  FIG. 9  step. Completion also involves etching away the  FIG. 8  Gold film  806  from areas adjacent the movable switch element  1006 , i.e., from the areas exposed by the dissolution of the 9260 photoresist and the etching away of the Titanium film from areas adjacent the movable switch element  1006 . A final step in the process involves release the movable switch element  1006  by removing the underlying PMGI photoresist layer  400  using a 1165 stripper heated to 90 to 100 degrees Celsius in a gross dissolving step. A view of the completed MEMS switch appears in  FIG. 11 . 
   By way of slight modification of the thusly-described process it is possible to achieve a MEMS metal contact switch structure with metal alloy electrical contacts providing advantages in fabrication and performance as are somewhat described in the above identified companion patent document. Notably such an MEMS switch with alloy contacts is believed to be new to the MEMS switch art and provides longer switch lifetimes and better switch wear characteristics. This alloy related slight modification of the thusly-described sequence departs from the  FIG. 1  through  FIG. 11  process just after the  FIG. 7  drawing and involves the alternate steps shown in the  FIG. 12  through  FIG. 14  drawings herein before returning to the  FIG. 1  through  FIG. 11  sequence commencing with the  FIG. 9  step. 
   According to this modification of the described process a layer  1200  of alloy metal is deposited and patterned over the thermally reflowed photoresist  400  then the Gold and Titanium layers  806 ,  807  and  808  are deposited as described above in connection with  FIG. 8 . The gold alloy layer  1200  may be composed of Gold and Palladium, Gold and Platinum, Gold and Silver metals or of other alloy metal combinations as suggested in the companion patent document or composed of other herein unspecified alloys. As shown in the  FIG. 12  drawing a thin film  1200  of such metal alloy, a film of about 500 angstroms thickness, preferably also achieved in a sputtering step, is formed over the photoresist layer  400  and again includes recesses of the  802  and  804  configuration. Sputtering conditions similar to those specified earlier herein may be used to form the film  1200 . 
   As suggested in the  FIG. 13  drawing a second step series in the modified MEMS switch process involves the deposition, masking, developing and etching to achieve a limited photoresist area  1300  over the bulbous or rounded dimple area  804  in the thin film-covered photoresist  400 . The area  1300  may be composed of type 1813 photoresist. After forming the area  1300  the exposed portions of the alloy film  1200  may be etched away and the area of this etching covered with photoresist in order to form a cavity in which a size-limited layer of contact alloy  1400  may be formed. The size-limited layer of contact alloy  1400  may be made of Gold alloy material and may have a dimension of 8 nanometers in diameter and may be of 500 angstroms thickness for examples. 
   Following formation of the contact area alloy  1400  the photoresist deposition and configuring to form a  FIG. 9  cavity  902  for reception of plating metal  1000  and formation of the movable contact element  1006  may be accomplished. With presence of the cavity  902  the modified MEMS switch process may in fact be considered to have returned to the  FIG. 1  through  FIG. 11  sequence of steps for processing in the  FIG. 10  manner. A cross sectional view of the alloy contact modified MEMS switch appears in the  FIG. 15  drawing. 
   The MEMS switch described thus far in this document is of the single pole single throw normally open switch type. The present invention is not viewed as being limited to switches of this classification however and is believed to be extendable to double and triple throw arrangements also having the normally open characteristic. Extension of the described switch and process to a normally closed switch configuration is however viewed as entailing difficulties. 
   The thusly described switch formation sequence provides a switch having advantages over other procedures for MEMS switch formation; among these are: 
   the use of conventional integrated circuit materials and procedures, materials and procedures compatible with fabricating other solid state device elements of an electrical circuit, during switch formation; 
   the achievement of a rounded bulbous movable contact having desirably long operating life and low contact resistance; 
   the use of relatively low temperature processing steps such as photoresist baking and electroplating to accomplish switch formation; 
   the achievement of MEMS switches with an alloy electrical contact; 
   the achievement of MEMS switches with integral mechanical element and electrical contact portions. 
   the achievement of MEMS switches with desirable immunity to mechanical stiction and other switch mechanism difficulties. 
   While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention, which is defined in the appended claims.