Abstract:
A capacitance coupled, transmission line-fed, radio frequency MEMS switch and its fabrication process using photoresist and other low temperature processing steps are described. The achieved switch is disposed in a low cost dielectric housing free of undesired electrical effects on the switch and on the transmission line(s) coupling the switch to an electrical circuit. The dielectric housing is provided with an array of sealable apertures useful for wet, but hydrofluoric acid-free, removal of switch fabrication employed materials and also useful during processing for controlling the operating atmosphere surrounding the switch—e.g. at a pressure above the high vacuum level for enhanced switch damping during operation. Alternative arrangements for sealing an array of dielectric housing apertures are included. Processing details including plan and profile drawing views, specific equipment and materials identifications, temperatures and times are also disclosed.

Description:
CLAIM OF PRIORITY 
   This application claims the benefit of U.S. Provisional Application No. 60/573,892 filed May 24, 2004. The contents of this provisional application are hereby incorporated by reference herein. 

   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 DOCUMENT 
   The present document is related to the and commonly assigned patent application document “MEMS RF SWITCH”, AFD 729, Ser. No. 10/901,314, now U.S. Pat. No. 7,145,213; filed of even date herewith. The contents of this related even filing date document are hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   MEMS technology has numerous applications in both commercial and military electrical systems. MEMS switches, for instance, can be used in routing radio frequency and microwave frequency signals in high frequency circuits. Some advantages of MEMS switches used in this manner over other active devices such as field effect transistors (FETS) and positive intrinsic negative (PIN) diodes include lower signal loss, higher signal isolation, and lower power consumption for switch activation (In this regard see for example E. R. Brown, “RF-MEMS Switches for Reconfigurable Integrated Circuits,”  IEEE Trans. On Microwave Theory and Techniques , Vol. 46, No. 11, November 1998, p. 1868-1880; J. Lee, et al. “Monolithic 2-18 GHz Low Loss, On-Chip Biased PIN Diode Switches,”  IEEE Trans. On Microwave Theory and Techniques , Vol. 43, February 1995, p. 250-255; M. Shokrani and V. J. Kapoor, “InGaAs Microwave Switch Transistors for Phase Shifter Circuits,”  IEEE Trans. On Microwave Theory and Techniques , Vol. 42, May 1994, p. 772-778.) 
   A MEMS package ideally should be economical in materials cost, space requirements, and incorporation technique. A MEMS packaging arrangement must protect the enclosed switch from structural damage and contaminants, allow handling, conform to the RF requirements of the host system, be low cost, and not impede the performance of the switch or circuit. Some estimates attribute more than 70% of overall device costs to packaging (see for example M. Madou,  Fundamentals of Microfabrication , CRC Press, Boca Raton, Fla., 1997, p. 378). 
   Several approaches exist for packaging MEMS switches. The “chip-in-a-box” approach entails dicing of un-released switch wafers, die attachment, interconnection, switch release, and lid seal. This process requires die level handling and release of switches inside the packages. A second approach is a wafer bonding arrangement that requires a capping wafer and a bonding ring around the switch (see e.g., U.S. Pat. No. 6,452,238, J. W. Orcutt, et al., “MEMS Wafer Level Package,” Sep. 17, 2002). Bonding arrangements may incorporate solder, eutectic, and epoxy materials. These arrangements involve low temperature processes and may result in a high aspect ratio device due to the combined thickness of the switch and the capping wafer. 
   The on-wafer encapsulation approach of the present invention encapsulates the switches during the fabrication process, thus eliminating die handling issues and bonding ring requirements. In principle, the encapsulation of the present invention is similar to transistor passivation and requires no additional footprint or special wafer handling. The present invention-encapsulated switches, may be diced, integrated and packaged along with other circuits of the system. The encapsulation approach is scalable to any size wafer. 
   The U.S. Pat. No. 5,589,082 of Liwei Lin et al. discloses a MEMS device of the electromechanical filter type that appears of interest with respect to the present invention. In  FIGS. 7Q ,  7 R and  7 S of the Lin et al. patent there is shown a sequence of three fabrication views for a filter in which capping, releasing and sealing of the MEMS enclosure are accomplished. Although several aspects of this capping, releasing and sealing sequence may appear closely related to the present invention it is interesting to note distinctions in at least the fabrication materials used, the fabrication temperatures used and the ambient pressure established in the completed MEMS enclosure. 
   The U.S. Pat. No. 5,589,082 of Qing Ma et al. discloses an assemblage of semiconductor components into a solder-seal-ring-closed package. These components include film bulk acoustic resonators and MEMS switches. The emphasis of the Ma et al. disclosure centers around packaging semiconductor devices (referred to as microelectromechanical systems) by solder sealing two separate structures along a sealing ring extending around a cavity containing the microelectromechanical system. Ma et al. also teach use of surface mount techniques, including application of solder bumps to package the electrical components. Interconnection to the cavity is through via holes in the thinned Ma et al. wafer. The present invention however includes packaging of individual RF MEMS switches using a wafer scale approach built on surface micromachining procedures consistent with the fabrication of MEMS switches. No change in a normal fabrication technique is needed for the present invention. The present invention also does not require the use of vias or wafer thinning. 
   SUMMARY OF THE INVENTION 
   The present invention provides an integrated multi-step wafer-scale fabrication and packaging process for realizing individual RF MEMS switches. The packaging is directly integrated into the switch surface micromachining process used to build the RF MEMS switch. The achieved packaging is compatible with both capacitive and metal-to-metal contact switches. 
   It is therefore an object of the present invention to provide an encapsulated MEMS switch process. 
   It is another object of the invention to provide an encapsulated MEMS switch process allowing for post processing operations such as wafer dicing, die pick-and-place, and die attach. 
   It is another object of the invention to provide a MEMS processing arrangement inclusive of the three major portions of switch fabrication, dielectric switch encapsulation and package sealing using a liquid or gaseous phase sequence. 
   It is another object of the invention to provide a MEMS switch packaging arrangement that is usable with either a metal-to-metal contact or a capacitive coupled switch arrangement. 
   It is another object of the invention to provide a MEMS switch packaging arrangement inclusive of a new encapsulation attachment materials combination. 
   It is another object of the invention to provide a MEMS switch packaging arrangement in which the environment within the MEMS enclosure can be freely selected to be that most favorable for switch operation. 
   It is another object of the invention to provide a MEMS switch process in which switch fabrication and frozen switch capping can be achieved in a single sequence. 
   It is another object of the invention to provide a MEMS switch in which switch package sealing can be accomplished by a plurality of different arrangements. 
   It is another object of the invention to provide a MEMS switch packaging process employing relatively low temperature materials, materials having processing temperatures compatible with the MEMS switch. 
   It is another object of the invention to provide a MEMS switch fabrication and packaging process in which temperatures not exceeding 270° C. are used. 
   It is another object of the invention to provide a MEMS switch enclosure process in which dielectric materials are used in order to avoid signal transmission line perturbations attending metallic material enclosures. 
   It is another object of the invention to provide a MEMS switch enclosure process in which internal pressures above vacuum level are achievable in order to provide permanent physical damping for moving switch components. 
   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 limited temperature organic photoresist coating materials based MEMS switch realization method comprising the steps of: 
   fabricating metallic elements of said switch in a sequence of photoresist coating, masking, exposing and etching steps ending with MEMS switch elements being held captive on an insulating substrate in a sacrificial layer of said photoresist coating materials; 
   enclosing said captive switch elements in a dielectric shell using additional of said photoresist coating, masking, exposing and etching steps compatible with both said fabricating step photoresist coating, masking, exposing and etching steps and with structure formed during said fabricating step photoresist coating, masking, exposing and etching steps; 
   said enclosing step including forming in said dielectric shell a plurality of apertures communicating from outside to inside thereof; 
   wet releasing said switch elements from captivity within said dielectric shell by said sacrificial layer of said photoresist coating materials by way of reagent received through said plurality of apertures communicating from outside to inside of said dielectric shell; 
   covering said plurality of apertures communicating from outside to inside of said dielectric shell with a coating material temperature compatible with said switch elements and with said dielectric shell. 

   
     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  in the drawings includes the views of  FIG. 1A  through  FIG. 1P  and shows a lower layer metal fabrication sequence for a MEMS switch according to the present invention. 
       FIG. 2  in the drawings includes the views of  FIG. 2A  through  FIG. 2P  and shows a radio frequency dielectric fabrication sequence for a MEMS switch according to the present invention. 
       FIG. 3  in the drawings includes the views of  FIG. 3A  through  FIG. 3L  and shows a switch sacrificial layer metal fabrication sequence for a MEMS switch according to the present invention. 
       FIG. 4  in the drawings includes the views of  FIG. 4A  through  FIG. 4L  and shows a bridge metal fabrication sequence for a MEMS switch according to the present invention. 
       FIG. 5  in the drawings includes the views of  FIG. 5A  through  FIG. 5N  and shows a cap sacrificial layer process for a MEMS switch according to the present invention. 
       FIG. 6  in the drawings includes the views of  FIG. 6A  through  FIG. 6L  and shows a capping layer fabrication sequence for a MEMS switch according to the present invention. 
       FIG. 7  in the drawings includes the views of  FIG. 7A  through  FIG. 7J  and shows a PECVD sealing sequence for the hole and tunnel interior to exterior communication paths of a MEMS switch according to the present invention. 
       FIG. 8  in the drawings includes the views of  FIG. 8A  through  FIG. 8J  and shows a spin on glass sealing sequence for the hole and tunnel interior to exterior communication paths of a MEMS switch according to the present invention. 
       FIG. 9  in the drawings includes the views of  FIG. 9A  through  FIG. 9J  and shows an epoxy sealing sequence for the hole and tunnel interior to exterior communication paths of a MEMS switch according to the present invention. 
       FIG. 10  in the drawings shows a present invention switch in unenclosed die condition. 
       FIG. 11  in the drawings shows a MEMS switch dielectric shell member according to the present invention. 
       FIG. 12  in the drawings shows an enclosed MEMS switch according to the invention. 
       FIG. 13  in the drawings shows a switch of the  FIG. 12  type in a ripped-open condition. 
       FIG. 14  in the drawings shows an interior view of an enclosed and released from captivity switch according to the invention. 
       FIG. 15  in the drawings shows tunnel structure usable with the enclosure of the present invention. 
       FIG. 16  in the drawings shows a MEMS switch with a silicon nitride cap in place. 
       FIG. 17  in the drawings shows electrical performance of a capped switch according to the invention. 
       FIG. 18  in the drawings shows sealing of tunnels of the  FIG. 15  type for the present invention. 
       FIG. 19  in the drawings shows a PECVD sealed switch according to the invention. 
       FIG. 20  in the drawings shows electrical performance for a capped and sealed switch according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description of the invention is divided according to the major steps in fabricating a MEMS radio frequency switch device according to the invention. These major steps are generally identified as switch metal fabrication, dielectric switch encapsulation and switch sealing using a liquid or gaseous phase sequence as have been heretofore identified herein. The process described herein accomplishes a capacitance operated MEMS switch; the process is however equally relevant to a metal contact switch. 
   RF Metal Process 
   RF metal defines the bottom contact in a capacitive switch arrangement according to the invention. The metal thickness used determines the power handling capability of the switch. High power switches require thick metal (greater than 1 micrometer) that in turn requires planarization processing. Fabrication of an RF metal layer is preferably accomplished according to the steps represented in the  FIG. 1  drawing, including the steps of  FIG. 1A  through  FIG. 1P . In this  FIG. 1  drawing and each other  FIG. 1  through  FIG. 9  drawing herein the left hand or odd-lettered steps represent a top view of the work piece and the right hand or even-lettered steps a side or profile view. Thick metal planarization is discussed in connection with the steps identified with the letters h and p in the sequence disclosed below. 
   By way of explanation, in the following description each fabrication step is provided with a plurality of identifications and correlation keys with an associated drawing of this document. For example each step in this description includes one or more references to a drawing FIG. This FIG. reference in most instances also includes use of a identifier having a numerical value in accordance with the drawing number involved—including a hundreds digit corresponding to the drawing number (e.g. the numerical identifier  802  appears in an individual drawing of  FIG. 8 , i.e., in  FIG. 8D ). Moreover the included identifications and correlation keys also include step identifications using the lower case alphabetical letter combinations between “a” and “Aae” as appear generally in the right hand lower portion of most drawing FIGs. These lower case alphabetical letter combinations between “a” and “Aae” do not however directly correlate with the lower case alphabetic letters identifying individual steps in the following discussion sequence. 
   The MEMS switch fabrication sequence may include the following steps. 
   Fabrication begins in  FIGS. 1A , and  1 B with a bare high resistivity wafer substrate  100 , such as sapphire. 
   Spin coat the wafer substrate  100  with one coat of a photo-imagable PolydiMethylGlutarImide (PMGI) polymer photoresist  102  such as Micro Chem. Corp. NANO PMGI SF-11 photoresist,  FIGS. 1C and 1D , cure at 270° C. on a hot plate or in an oven. 
   Spin coat the wafer substrate with one coat of a photo-imagable positive photoresist such as Shipley Microposit S-1813 photoresist  104 ,  FIGS. 1E and 1F , cure at 110° C. on a hot plate or in an oven. 
   Using an I-line stepper or contact lithography system, expose the coated wafer substrate  100  to an appropriate RF Metal mask and develop the S-1813 resist  104  using a diluted sodium hydroxide based developer such as Shipley Microposit 351 developer,  FIGS. 1G and 1H . 
   Expose the patterned wafer substrate  100  to Deep Ultra Violet (DUV) light and develop the SF-11 resist  102  using a tetraethylammonium hydroxide solution such as Micro Chem Corp. Nano-PMGI 101 developer,  FIGS. 1I and 1J . 
   Coat the wafer substrate  100  with evaporated metal such as titanium/gold (TiAu), 200 Å Ti/3000 Å Au  106 . The titanium is used as an adhesion layer and could be replaced with chromium (Cr),  FIGS. 1K and 1L . The gold is used as the conduction layer. 
   Lift-off the excess metal using tape and dissolve the S-1813 resist  104  using acetone, followed by an isopropyl alcohol rinse and De-Ionized (DI) water rinse,  FIGS. 1M and 1N . The DI water rinse is desirable to minimize cracking of the PMGI photoresist  102 . 
   Strip the SF-11 photo resist  102  using a hot (90° C.) 1-methyl-2-pyrrolidinone stripper such as Shipley 1165 remover, followed by a De-Ionized (DI) water rinse and nitrogen dry,  FIGS. 1O and 1P . For thicker metal, omit the SF-11 strip to achieve the thick metal planarization described in the initial sentences of the RF Metal process. The completed RF metal step is shown in  FIG. 1O  and  FIG. 1P  and includes the isolated conductors  101 ,  103  and  105 . 
   RF Dielectric Process 
   The RF dielectric defines the capacitance of the switch in the “closed”-state. Processing steps involving the RF dielectric appear in the  FIG. 2  drawings including the steps shown in  FIG. 2A  through  FIG. 2P . 
   Coat the  FIG. 1O  and  FIG. 1P  metallized wafer  100  with a thin dielectric material such as 2000 Å alumina Al 2 O 3    200  using RF sputtering,  FIG. 2A  and  FIG. 2B . Alternative dielectrics include silicon nitride Si 3 N 4  and silicon dioxide SiO 2 . 
   Spin coat the wafer with one coat PMGI photoresist (SF-11) 202, cure at 270° C.,  FIG. 2C  and  FIG. 2D . This step is also omitted for the thick metal planarization option. 
   Spin coat the wafer with one coat of positive photoresist (S-1813)  204 , cure at 110° C.,  FIG. 2E  and  FIG. 2F . 
   Using an I-line stepper or contact lithography system, expose the coated wafer to an appropriate RF Dielectric mask and develop the S-1813 resist  204  using a diluted developer (351:DI),  FIG. 2G  and  FIG. 2H . 
   Expose the patterned wafer to deep ultraviolet light and develop the SF-11 resist  202  using Nano-101 developer,  FIG. 2I  and  FIG. 2J . 
   Etch the exposed thin-film dielectric film  200  using a dry or wet chemical etch,  FIGS. 2K and 2L . 
   Strip the S-1813 resist  204  using an acetone rinse followed by an isopropyl alcohol and DI water rinse  FIGS. 2M and 2N . 
   Strip the SF-11 resist  202  using hot (90° C.)  1165  remover,  FIGS. 2O and 2P . For thick RF metal, delete this step to maintain planarization. The completed RF dielectric sequence is shown in the top and side views of  FIG. 2O  and  FIG. 2P . 
   Sacrificial Layer Process 
   In the present invention a sacrificial post determines the gap height of the switch and its capacitance in the movable member-up-state. To explain in more detail, fabrication of a MEMS switch (i.e., having a bridge or cantilever beam) requires a sacrificial layer to support the suspended portion of the beam during processing. This sacrificial layer is herein referred-to as the post layer. The completed post is shown as layer  312  in  FIG. 3K  and  FIG. 3L . Post height is determined by the thickness of the photoresist used during post formation. Spinning the photoresist at a slow speed results in a thicker film and a thinner film at higher speed. The achieved switch gap spacing can be varied from 1 to 5 micrometers with this process 
   Spin coat the wafer with one coat of PMGI photoresist (SF-11),  FIG. 3A , cure at 270° C. Repeat this process for three total coats of resist  300 ,  302  and  304  in  FIG. 3B  to form a three-micron stack thickness. The PMGI coats determine the gap height of the switches. SF-11 PMGI photoresist provides about 1.0 μm of thickness per layer. For thicker gap spacings, PMGI SF-19 resist could be used to achieve a 5.0 μm thickness per layer. 
   Spin coat the wafer with one coat of positive photoresist (S-1813),  306  in  FIG. 3C  and  FIG. 3D , cure at 110° C. 
   Using an I-line stepper or a contact lithography system, expose the coated wafer to the Sacrificial Layer mask and develop the S-1813 resist  306  using a diluted developer (351:DI),  FIG. 3E  and  FIG. 3F . 
   Expose the PMGI (SF-11) resist  300 ,  302  and  304  to deep ultraviolet light and develop the SF-11 resist  300 ,  302  and  304  using Nano-PMGI-101 developer,  FIG. 3G  and  FIG. 3H . 
   Strip the S-1813 resist  306  using acetone followed by an isopropyl alcohol rinse and a DI water rinse;  FIG. 3I  and  FIG. 3J . The DI water rinse is desirable to minimize cracking of the PMGI photoresist  304 . 
   Reflow the PMGI coating layers  300 ,  302  and  304  in a 250° C. hot air oven. The reflow step achieves a uniform sloped sidewall,  310  in  FIG. 3L , for the layers  300 ,  302  and  304  to ensure continuous metal coverage in the Bridge Metal process. The completed sacrificial layer process is shown in  FIG. 3K  and  FIG. 3L  where the photoresist layers  300 ,  302  and  304  appear in merged condition at  312 . 
   Bridge Metal Process 
   Bridge metal defines the top, movable portion of the present invention switch. The careful choice of bridge metallization minimizes curling of the switch. (See for example K. Leedy, et al, “Metallization Schemes for RF MEMS Switches”, J. Vacuum Science Technology A 21(4) July/August 2003, pp. 1172-1177.) 
   Spin coat the  FIG. 3K  and  FIG. 3L  wafer with one coat of a PMGI Lift-Off Resist such as Micro Chem. Corp. LOR-10 photoresist  400 , cure at 170° C. The resist chosen for this step should not interact with the existing PMGI sacrificial post resist at  312 . The cure temperature should also be lower that the 250° C. reflow temperature of the previous  FIG. 3K  and  FIG. 3L  step. 
   Spin coat the wafer with one coat of a positive resist (S-1813)  402 , cure at 110° C.,  FIG. 4C  and  FIG. 4D . 
   Using an I-line stepper or contact lithography system, expose the coated wafer to the Bridge Metal mask and develop the S-1813 resist  402  using a diluted developer (351:DI),  FIG. 4E  and  FIG. 4F . 
   Develop the LOR-10 photoresist  400  using a tetramethylammonium hydroxide developer such as Shipley Microposit developer LDD-26W. This developer should not interact with the existing PMGI sacrificial post resist  312 ,  FIG. 4G  and  FIG. 4H . 
   Aa. Coat the patterned wafer with a thin metal film such as 7000 Å of evaporated Au  404  in  FIG. 4I  and  FIG. 4J . Notably this step uses gold that adheres to the exposed gold of the RF metal process without requiring an adhesion layer. Such an adhesion layer may produce a stress gradient within the film resulting in curling of the switch being fabricated. A thin adhesion layer (of Ti or Cr) may be used on the bridge metal top surface to promote cap adhesion. 
   Ab. Lift-off the excess metal using tape and remove the S-1813 resist  402  using acetone, followed by an isopropyl alcohol rinse and a DI water rinse. The completed Bridge Metal step is shown in  FIG. 4K  and  FIG. 4L . 
   Cap Sacrificial Layer Process 
   The cap sacrificial layer defines the region to be covered by the encapsulating shell. The thickness of the sacrificial layer determines the inner shell height over the switch. This process covers the captive switch and should not compromise the existing structures. 
   The ability to stack sacrificial layer materials such as photoresist on the  FIG. 4  metal  404  of the movable switch element without causing harmful distortion to the metal  404  is in fact believed to be a notable aspect of the present invention. Often it is found that the curing of newly stacked organic materials is so permanently disruptive to an underlying metal layer or an underlying oxide layer as to preclude such procedures. The materials, sub layer thickness measurements, temperatures and other details recited for the  FIG. 5  sequence are therefore of special interest with respect to the present invention. Although this  FIG. 5  sequence accomplishes the addition of a second sacrificial layer on top of a first sacrificial layer and its exposed metal, it is believed the disclosed sequence is applicable to the fabrication of more than two such layers as accomplished herein and can be extended to three or more layers without significant difficulty where needed. 
   Ac. Spin coat the  FIG. 4K  and  FIG. 4L  wafer with one coat of PMGI photoresist (SF-11), cure at 200° C. Repeat this step for three total coats of resist  500 ,  502  and  504  as appear in  FIG. 5B . Three coats of SF-11 resist provide 3.0 μm gap spacing. This is the same PMGI photoresist used for the switch gap spacing  312  and should be cured at the indicated lower temperature to minimize impact on the existing films. PMGI SF-19 photoresist could be used for thicker gap spacing. 
   Ad. Spin-coat the wafer with one coat of positive photoresist (S-1813)  506 , cure at 110° C.,  FIG. 5C  and  FIG. 5D . 
   Ae. Using an I-line stepper or contact lithography system, expose the coated wafer to the Cap Sacrificial mask and develop the S-1813 resist  506  using diluted developer (351:DI),  FIG. 5E  and  FIG. 5F . 
   Af. Expose the PMGI SF-11 resist  500 ,  502  and  504  to deep ultraviolet light and develop the SF-11 resist  500 ,  502  and  504  using Nano-101 developer,  FIG. 5G  and  FIG. 5H . 
   Ag. Strip the S-1813 resist  506  using acetone followed by an isopropyl alcohol rinse and DI water rinse,  FIG. 5I  and  FIG. 5J . 
   Ah. Strip the remaining lift-off resist (LOR-10) 400 using a tetramethylammonium hydroxide developer such as Shipley Microposit developer LDD-26W, followed by a DI water rinse and nitrogen dry,  FIG. 5K  and  FIG. 5L . 
   Ai. Reflow the PMGI coated wafer in a 250° C. hot air oven. This reflow provides a uniform sloped sidewall  508  required for the capping layer step coverage. The reflow process should not exceed 250° C. to minimize impact on the existing PMGI films  312 . The exposure time for this reflow temperature is preferably made somewhat short in the interest of damage avoidance to the underlying layers of a device; exposure times in the range of 60 to 300 seconds are thus found to be practical. No adverse impacts on the existing films have been observed. The completed second sacrificial layer step is shown in  FIG. 5M  and  FIG. 5N . 
   Capping Layer Process 
   The capping layer defines the dielectric shell that will enclose the RF MEMS switch. This step also defines access holes or tunnels within the dielectric shell allowing for removal of the sacrificial layer photoresist of the shell and the switch. Access holes are shown in  FIG. 11  of the drawings. Access tunnels are shown in the  FIG. 14  and  FIG. 15  drawings. Although a combination of access holes and access tunnels may be used in the invention one or the other of these communication paths from outside to inside of the dielectric enclosure is believed a satisfactory arrangement. Because of the larger aperture dimensions involved, the resulting improved flow of reactant materials achieved and the difficulty in fabricating the small holes shown in, for example,  FIG. 12  and  FIG. 13  we have found the use of tunnels to be the most desirable arrangement. 
   Aj. Coat the  FIG. 5M  and  FIG. 5N  wafer with a thin adhesion layer  600  composed of 0.01 μm oxygen rich sputtered alumina Al 2 O 3 , followed by a thick dielectric film, of nominally 1.67 μm sputtered silicon nitride Si 3 N 4    602 ,  FIG. 6A  and  FIG. 6B . The alumina layer provides adhesion of the silicon nitride cap  602  to the gold and sapphire substrate surfaces at  604  and  605 . Films deposited by PECVD at low temperatures (below 200° C.) could also be used. Possible alternative dielectric films at  602  also include alumina Al 2 O 3 . 
   The sputtered silicon nitride used in the cap at  602  has undergone extensive deposition development. Silicon nitride thin films may be fabricated by reactive RF sputtering using a 99.999% pure Si target with a Denton Vacuum Discovery-18 type of magnetron sputtering system and a base vacuum of 5×10 −6  Pa. A mass flow regulated Ar—N 2  sputtering pressure of 0.53 Pa and 400 watts of forward power result in a nominal deposition rate of 0.13 nm/s. The N 2  partial flow rate (the ratio of the nitrogen flow rate to the total flow rate of nitrogen and argon) is 50%. Deposited films for the shell cap at  602  are 1670 nm thick and have an intrinsic compressive stress of 102 MPa. 
   Ak. Spin coat the dielectric coated wafer with one coat of positive photoresist (S-1818) 606, cure at 110° C.,  FIG. 6C  and  FIG. 6D . A thick photoresist  606  is necessary to serve as an etch mask for the cap layer dielectric. S-1818 photoresist provides about 1.8 μm of film thickness per coat. 
   Al. Using an I-line stepper or contact lithography system, expose the coated wafer to the Capping Layer mask and develop the S-1818 resist  606  using a diluted developer (351:DI),  FIG. 6E  and  FIG. 6F . Access holes  804  (in  FIG. 8B ) and access tunnels  608  and  610  are also defined in this lithography step. 
   Am. Etch the exposed thick film dielectric at  602  using a dry or wet chemical etch,  FIG. 6G  and  FIG. 6H . Visual examination should ensure the access holes or tunnels  608  and  610  are cleared to allow complete removal of the PMGI sacrificial resists at  508  and  312 . 
   An. Strip the S-1818 resist  606  using acetone followed by an isopropyl alcohol rinse and a DI water rinse,  FIG. 6I  and  FIG. 6J . 
   Ao. Strip all the remaining PMGI SF-11 photoresist  508  and  312  using a hot (90° C.)  1165  stripper,  FIG. 6K  and  FIG. 6L . 
   Ap. Immediately rinse the wafer in a submersion bath of isopropyl alcohol. Repeat the isopropyl alcohol bath step 3-4 times. Rinse the wafer in a bath of methanol. Repeat the methanol bath rinse step 3-4 times. Complete the release step by using a carbon dioxide critical point dry. The completed Capping Layer step with released RF MEMS switch is shown in  FIG. 6K  and  FIG. 6L . 
   Although not expressly shown in the drawings the access tunnels of  FIG. 14  and  FIG. 15  in the drawings are preferably formed during the  FIG. 6  sequence. This may be accomplished by providing the capping layer mask with combination tooth-like extensions at its periphery followed by covering these extensions and removal of the mask material. 
   PECVD Sealing Process 
   The Plasma Enhanced Chemical Vapor Deposition (PECVD) process may be used to seal the access holes and tunnels of the encapsulation shell. 
   Aq. Bake out the  FIG. 6K  and  FIG. 6L  encapsulated wafers in a 90° C., nitrogen oven for 1 hour. Coat the wafer with a thin adhesion layer composed of 0.01 micrometer of oxygen rich sputtered alumina, Al 2 O 3 , followed by a thick film of PECVD silicon oxide, nominally 2 μm,  700  in  FIG. 7A  and  FIG. 7B . PECVD SiO 2  is deposited at  270  degrees Centigrade and 900 millitorrs of pressure. Since such PECVD plugs the access tunnels or access holes the PECVD SiO 2  is not observed to deposit within the encapsulated shell. As an alternative, PECVD silicon nitride may also be used. 
   Ar. Spin coat the wafer with one coat of positive photoresist (S-1818)  702  in  FIG. 7C  and  FIG. 7D , cure at 110° C. A photoresist is necessary to serve as an etch mask for the sealing layer dielectric. 
   As. Using an I-line stepper or contact lithography system, expose the coated wafer to the Sealing Layer mask and develop the S-1818 resist  702  using a diluted developer (351:DI),  FIG. 7E  and  FIG. 7F . 
   At. Etch the exposed thick film dielectric at  704  in  FIG. 7E  and  FIG. 7F  using a dry or wet chemical etch,  FIG. 7G  and  FIG. 7H . 
   Au. Strip the S-1818 resist  702  using acetone followed by an isopropyl alcohol rinse and a DI water rinse,  FIG. 7I  and  FIG. 7J . The completed PECVD sealed switches are shown in  FIG. 7I  and  FIG. 7J . 
   Spin-on-Glass Sealing Process 
   The spin-on-glass process may also be used to seal the access holes in the encapsulation shell. The low viscosity of the spin-on-glass minimizes penetration into the access holes or tunnels. (See for example H. Elderstig and P. Wallgren; “Spin deposition of polymers over holes and cavities”, Sensors and Actuators A 46-46, 1995, pg. 95-97.) 
   Av. Spin coat the  FIG. 6K  and  FIG. 6L  wafer with a spin-on-glass film  800  such as 3 micrometers of Honeywell Accuflo-3025, cure the film at 160° C., 200° C., and 240° C.,  FIG. 8A  and  FIG. 8B . A three-step sequential cure cycle using progressively higher temperatures is desirable to completely cure this type of spin-on glass. A single layer is formed. The cap holes are formed in step Al in  FIG. 6E  and  FIG. 6F ; they should be at the sides ( 608  and  610 ) or can be on top as shown. 
   Aw. Spin coat the wafer with a thick positive photoresist,  802  in  FIG. 8C  and  FIG. 8D , such as Hoechst Celanese Corp. AZ-9260 resist, cure at 110° C. This photoresist will serve as an etch mask for the spin-on-glass. This resist provides a 5-6 μm film thickness. 
   Ax. Using an I-line stepper or contact lithography system, expose the AZ-9260 resist-coated wafer to the Sealing mask and develop the AZ-9260 photoresist with a diluted potassium hydroxide developer such as Hoechst Celanese Corp. AZ-400K,  FIG. 8E  and  FIG. 8F . 
   Ay. Etch the patterned spin-on-glass  800  using a dry or wet chemical etch,  FIG. 8G  and  FIG. 8H . 
   Az. Strip the AZ-9260 photoresist  802  using acetone followed by an isopropyl alcohol rinse. The completed spin-on-glass sealing process is shown in  FIG. 8I  and  FIG. 8J . 
   Epoxy Sealing Process 
   This alternate sealing process involves deposition of epoxy droplets onto individual switch caps. For this process, a dam can be fabricated around the switch to contain the epoxy however epoxy sealing without such a dam is also feasible. This process may also be used as an alternative after step An above. 
   Aaa. For the epoxy sealing process, the sacrificial layers at  312  and  508  are not initially removed. 
   Aab. Spin coat the wafer with a thick negative photoresist such as MicroChem. Nano SU-8-2007,  900  in  FIG. 9C  and  FIG. 9D , cure at 95° C. This resist provides ˜7.0 μm film thickness. 
   Aac. Using an I-line stepper or contact lithography system, expose the coated wafer to the Sealing Ring mask and develop the Nano SU-8-2007 using an ethyl lactate and diacetone alcohol developer such as MicroChem SU-8 Developer,  FIG. 9E  and  FIG. 9F . 
   Aad. Strip the remaining PMGI SF-11 photoresist  508  and  312  using 90° C. 1165 stripper. Immediately rinse the wafer in several baths (3-4) of isopropyl alcohol, followed by rinsing in baths of Methanol, and a carbon dioxide critical point dry,  FIG. 9G  and  FIG. 9H . 
   Aae. Coat the switch shells with an epoxy sealant  902  such as OptoCast 3401 or 3410 supplied by Electronics Materials Corp. Cure the epoxy using UV light followed by a 125° C. bake. The completed epoxy sealing process is shown in  FIG. 9I  and  FIG. 9J . 
   Comparative Discussion 
   Now that the foregoing formal description of a MEMS capacitance coupled radio frequency switch device and fabrication sequence has been disclosed, it may be helpful to an appreciation of the invention to consider several differences between the disclosed fabrication sequence and the fabricated device in comparison with the more conventional fabrication sequences and devices of similar general nature as are known in the art. 
   Readers familiar with the MEMS device art will for example appreciate that the device fabricated in the disclosed sequence is of an electrical switch nature as opposed to a transducer or other MEMS device and that such MEMS switches are attended by a somewhat unique collection of characteristics and susceptibilities. First among these characteristics and susceptibilities is a sensitivity to normal semiconductor device fabrication temperatures, temperatures in the 900° C. region for example. Temperatures of this magnitude and even lower (but especially higher temperatures) are found to be destructive to the metal components of a MEMS switch device in that they result in warping or distortion of previously fabricated switch metal components. 
   In addition to temperature sensitivity it appears significant that the disclosed fabrication sequence enables the use of silicon oxide, silicon nitride and other dielectric materials in the fabrication of the switch device. These especially useful materials are excluded from possible employment in many MEMS devices that are dependent on hydrofluoric acid etching steps in achieving release of a stabilized transducer or other element for example or for other processing steps. A wet hydrofluoric acid etch removal would typically require a water rinse to remove all acid; any water rinse employed can however be catastrophic to a MEMS switch structure. Use of hydrofluoric acid also precludes the use of many dielectrics in the switch and shell including silicon dioxide, SiO 2 , silicon nitride, Si 3 N 4  and alumina, Al 2 O 3 ; this is especially true for a switch dielectric where precise thickness and integrity should be maintained. Thus in the above disclosed processing sequence a release of switch elements from their bound state by an organic material using an organic solvent rather than an acid, is for example employed. 
   A notable attribute of the present invention MEMS switch processing is the achieved seamless merging of switch formation, dielectric enclosure and dielectric enclosure sealing operations in a single integrated processing sequence, a sequence performable at the integral wafer level of MEMS fabrication as opposed to during individual die processing. Notably the photoresists and other materials involved in closure of the MEMS package and sealing of the closed package are either the same as those already employed in the fabrication of switch elements or of compatibility with these already employed materials. 
   Switch Operation 
   MEMS radio frequency switches are typically fabricated in a coplanar waveguide configuration as shown in the scanning electron microscope-produced microphotograph of  FIG. 10 . In this  FIG. 10  the bridge  1000  is anchored on the ground (GND) lines  1002  and  1004  and spans the center signal line  1006 . By way of explanation, the  FIG. 10  and several other of the microphotograph “drawings” herein originate in the form of mounted glossy photographic prints. In order to designate specific details in such “drawings”, where drawn-in lead lines are impractical on the glossy photographic paper surface, the usual number and single lead line procedure is replaced with vertical and horizontal drawing coordinate lines each bearing the appropriate reference number and each located in a margin adjacent the glossy print surface. The two numbers  1000  relating to the  FIG. 10  bridge structure provide an example of this arrangement in the  FIG. 10  drawing. Additionally, the lower margin line at  1008  in  FIG. 10  indicates the length of a 200 micrometer or 200 micron feature in the  FIG. 10  photograph. Other details regarding scanning electron microscope variables used for the photograph also appear in the lower margin of  FIG. 10  and each of the other microphotographs herein. 
   Operation of the  FIG. 10  switch may be understood from a consideration of the  FIG. 6L  drawing. An RF signal is applied to the RF metal conductor  612  in  FIG. 6L  and passes into the switch area un-attenuated by the overarching bridge metal  614 . To actuate the switch, a direct current (dc) voltage is superimposed on the RF signal, i.e. applied between the RF metal signal line  612  and the Bridge metal line  614 . Electrostatic attraction pulls the bridge  614  into contact with the RF dielectric film  616  covering the RF metal signal line; the dielectric film  616  prevents an electrical short circuit between conductors  614  and  612 . The resulting increased capacitance formed by the signal line  612 , dielectric  616 , and bridge  614  effectively shorts the RF signal between conductors  612  and  614 . When the dc voltage is removed, the elastic restoring force of the structure pulls the bridge  614  up and allows the RF signal to pass. 
   The purpose of the encapsulation layer  602  is to protect the switch from environmental contamination, i.e. dust and moisture. The encapsulation layer does not interfere with switch motion or degrade RF performance. 
   Process Demonstration 
   The Fabrication Process described herein has been exercised in a class  100  clean room device fabrication facility. The following microphotograph-representing images describe graphically some of the results reached in developing this fabrication process. Notably the fabrication process integrates RF MEMS switches with dielectric shells. The radio frequency test measurements included in these results show the presence of a dielectric shell does not degrade switch performance. 
     FIG. 11  in the drawings represents a scanning electron microscope micrograph showing a silicon nitride shell with the cylindrical holes at  804  in the  FIG. 8B  drawing appearing in the cap top. These holes are needed to remove the two sacrificial photoresist layers  508  and  312 . The  FIG. 11  nitride cap is disposed on a silicon substrate. The nitride cap brightness in the  FIG. 11  image is due to the non-conductive nature of the material when viewed in an electron microscope. Note the crispness of the cap sidewalls in contacting the substrate surface and the slope of the cap in the region where the access holes are present. The  FIG. 11  drawing additionally shows that the stress level in the silicon nitride film is well controlled (e.g. the cap is neither sagging nor buckled). Device dimensions and other details are shown along the lower edge of  FIG. 11 . 
     FIG. 12  in the drawings illustrates the cap  602  adhering to the ground and signal conductors and to the silicon substrate. The cap holes at  1204  are also clearly visible in  FIG. 12 . The  FIG. 12  sample is flash coated with approximately 100 angstroms of gold for electron microscope viewing. 
     FIG. 13  in the drawings shows the  FIG. 12  switch in a cut-away or torn-away condition. In this drawing the nitride cap is partially removed to reveal the switch underneath. The nick  1300  at the edge of the bridge structure is where an electrical probe tip was pushed into the bridge  1306  to verify switch integrity. Note that in addition to the gap between the MEMS switch bridge  1306  and the RF signal line  1304  and the substrate, a gap exists between the MEMS switch bridge  1306  and the normally located underside of the nitride cap  1302 . The  FIG. 13  sample is also flash coated with approximately 100 angstroms of gold for electron microscope viewing. Dimensions and other details also appear at the lower edge of  FIG. 13 . 
     FIG. 14  in the drawings shows a microphotograph including a switch that has been released from sacrificial layer captivity. In the  FIG. 14  drawing there appears at  1400  and  1402  two metal conductors of the switch while at  1404  and  1406  the sapphire substrate is shown. A tunnel aperture for removal of sacrificial layer materials appears at  1408  in  FIG. 14 ; these tunnels are shown in even better perspective in the scanning electron microscope view of the  FIG. 15  microphotograph. 
     FIG. 16  in the drawings shows a microphotograph of a functional RF MEMS switch with a silicon nitride encapsulant on a sapphire substrate. RF probe marks as at  1600  appear on both sides of the ground and signal lines in  FIG. 16 . The result of RF test measurements made on the nitride capped devices and compared with switch performance measurements on non-capped capacitive RF MEMS switches are shown in the  FIG. 17  drawing. These measurements indicate no loss in RF performance with the cap being present. In the  FIG. 17  drawing the switch isolation of the lowermost curves relates to the left hand scale and switch insertion loss of the upper curves relate to the right hand scale. As indicated by the two  FIG. 17  curves, no measurable loss from switch cap presence is observed. Switch insertion loss in  FIG. 17  is less than 0.3 dB (at 26 GHz) and isolation is greater than 20 dB (at 26 GHz). 
   Several methods may be used to seal the access holes or tunnels in the switch package of the present invention. The primary sealing approach using Plasma Enhanced Chemical Vapor Deposition (PECVD) achieves the sealing configuration shown in the  FIG. 18  drawing where sealed access tunnels appear. Additional RF test measurements taken with PECVD sealed switches verify this sealing process also does not degrade switch performance. Test results for three sealed switches are shown in the  FIG. 20  drawing where again both switch isolation and switch insertion loss are indicated. In the  FIG. 20  drawing insertion loss is less than 0.3 dB (at 26 GHz) and isolation is 14-15 dB (at 26 GHz).  FIG. 19  in the drawings shows a switch of the  FIG. 16  type in an encapsulated and sealed condition. A dimension line in the lower right corner of  FIG. 16  and  FIG. 19  provides feature size indication. 
   ALTERNATIVES 
   Several alternatives in materials and processes may be employed in achieving the invention. These include the following: 
   Other high resistivity substrates, such as quartz, GaAs, or Si may be used. 
   Other materials may be used to seal the holes in the dielectric cap; materials such as Dow Corning Q1-4939 silicone; Honeywell Accuglass 512B; Electronic Materials Inc. OptoCast 3500 or 3600 series epoxies; Thermoset glob-top encapsulants; or solder shots. 
   A thin, stiff template similar to a shadow mask (such as made from stainless steel or other metal) could be made to include holes over the cap areas needing to be sealed. The template could be placed over the wafer containing the nitride caps and the sealing material, such as epoxy, could be flowed across the top of the caps with a squeegee or similar applicator. 
   A dry process can also be used to seal the access holes. Following an oven bake-out, a film is laminated over the encapsulated wafer and is heat cured. The sealed wafer is then patterned and the film removed from the contact pad areas. 
   A reflow process can also be used for access hole sealing. In this process, following an oven-bake out, glass or other frit beads are deposited on the wafer and reflowed to form a continuous film over the shells. The sealed wafer could then also be coated with photoresist, patterned, and etched to remove the glass film from the contact pad areas. 
   Once the RF MEMS switches are capped and access holes sealed additional process steps can be followed to hermetically seal the switch if required. For the case of the non-hermetic epoxy sealed cap, bake-out of the epoxy can be done in a controlled environment. A hermetic over-seal cap may then be placed on the individual switches as the next stage of the process. The individual devices can then be separated after wafer dicing and handled by conventional methods, such as by pick-and-place techniques. An attribute of the present invention is that it is multi-step in nature and allows for the possibility of hermetic sealing if needed. 
   ADVANTAGES AND NEW FEATURES 
   The present invention represents an integrated multi-step wafer-level process tailored to the fabrication and encapsulation of RF MEMS switches. The encapsulation arrangement is compatible with the switch fabrication process and utilizes the same sacrificial photoresist for both the device and dielectric shell. The sacrificial photoresist for the dielectric shell is cured at a lower temperature than the switch sacrificial layer to minimize secondary reflow of the switch sacrificial layer. The approach inherently protects the RF MEMS switch with sacrificial photoresist until the final process step when all the sacrificial photoresist is removed. Specifically, the dielectric encapsulant and RF MEMS devices are released simultaneously as a photoresist stripper penetrates cylindrical through-holes or tunnels patterned into the dielectric shell. A separate fabrication step seals the holes or tunnels in the dielectric shell to fully encapsulate each MEMS structure on the wafer. RF MEMS switches have been fabricated and released concurrently with a perforated silicon nitride shell covering them. The measured RF performance of suspended switches when tested up to 26 GHz does not show degradation due to the presence of the dielectric encapsulant. 
   The present invention involves a multi-step encapsulation method in which the shell is formed using a sputtered dielectric material such as silicon nitride or alumina. This shell has photo-lithographically defined access holes that are used to simultaneously release both the RF MEMS switch and the shell sacrificial photoresist. The access holes are sealed using silicon oxide or spin-on-glass or an epoxy layer. The resulting switches are sealed at atmospheric pressure or below atmospheric pressure and can thus provide sealed-in air for switch damping. In addition, the choice of photoresists and associated curing temperatures distinguish the present process. 
   The present invention concept is thus believed unique for the following reasons: (1) it allows for simultaneous release of both the MEMS switch and the dielectric encapsulating shell; (2) it provides options for sealing the dielectric shell access holes; (3) it is suitable for RF MEMS switch encapsulation, specifically the dielectric shell does not impede the RF performance of the devices; (4) the individual packaged switches can then be diced (or handled) and are suitable for further incorporation into an electronic circuit; (5) the sputtering technique used to deposit silicon nitride results in structurally sound cap shells; and (6) a multi-step concept has been demonstrated. 
   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.