Abstract:
A method for micro-machining a varactor that is part of a membrane suspended MEMS tunable filter. In one non-limiting embodiment, the method includes providing a main substrate; depositing a membrane on the main substrate; depositing and patterning a plurality of sacrificial photoresist layers at predetermined times during the fabrication of the varactor; depositing metal layers that define a fabricated varactor structure enclosed within photoresist; coupling a carrier substrate to the fabricated structure opposite to the main substrate using a release layer; etching a central portion of the main substrate to expose the membrane; removing the carrier substrate by dissolving the release layer in a material that attacks the release layer but does not dissolve the photoresist; and removing the photoresist layers to provide a released varactor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for fabricating a varactor suspended over a suspended membrane and, more particularly, a method for micro-machining a varactor filter including a varactor suspended over a suspended membrane on a semiconductor substrate. 
     2. Discussion of the Related Art 
     While communications systems and radar are advancing to higher frequencies of operation, there is a clear need to allow propagation of multiple channels in relatively narrow frequency bands. This can be accomplished by large waveguide filter banks. However, such filter banks are difficult to deploy in space and are not ideal for error or ground based operations. What would help advance high frequency applications are affordable, tunable filters that allow for operation over a wide frequency bandwidth. Silicon micro-machining is an ideal approach for developing tunable filters for high frequency applications by combining high-Q resonant structures with mature RF micro-electromechanical switches (MEMS) varactor technologies. 
     Due to low-loss characteristics, micro-machined transmission line resonators provide an excellent method to realize high-Q filters. The microstrip line can be suspended on a thin dielectric membrane that consists of 3000/4000 Åof SiO 2 /Si x N y  layers deposited on a thermally grown 7000 Å SiO 2  layer. This membrane can be formed by completely etching silicon after patterning the microstrip conductor. Cavities are formed on top and bottom silicon wafers and are metallized. These metallized cavities provide microstrip ground planes and shields when the three wafers are assembled together. The complete elimination of the substrate reduces the effective permittivity (∈ eff ) nearly to that of air (∈ r =1), therefore minimizing the dielectric loss and enabling the high-Q characteristics. The resonator discussed herein can be fabricated by deep reactive ion etching (DRIE) processes that allow precise control of the shape and size of the etched structures. The RF performance of the resonator depends on the geometric characteristics of the etched structures, and therefore avoiding the shortcomings of wet anisotropic etching, undercutting of convex corners, pyramidal shape of etched structures, etc. is advantageous. 
     Important parameters for designing such a microstrip resonator include that the conductor width and the ground plane distance have a direct effect on the characteristic impedance Z 0  of the line and the quality factor Qu of the resonator, provided that the shield height and the side wall distances are large enough. First, for the resonator to be loaded with MEMs varactors, the conductor width can be 600 μm. Then, to minimize their effects, the shield height can be 920 μm, and the sidewall distance from the center can be 1.8 mn. The quality factor Q u  can be calculated as 
               Q   u     =       π       λ   g     ⁢   x       =     β     2   ∝               
where, γ=∝+jβ is the complex propagation constant and λ g  is the guided wavelength.
 
     In most cases, numerical analysis tools over-estimate the quality factor Q. This is due to various factors that cannot be modeled accurately in numerical simulations. For example, surface roughness of the metal layers in cavity walls has an important factor on the overall loss, but is not considered in the simulations. 
     One design challenge in transitioning a co-planar waveguide (CPW) to a microstrip line is the transition of the ground plane to a different layer. For the case of a resonator, this transition occurs with the use of two rectangular metallized posts. The formation of a rectangular post inside a silicon cavity has an intrinsic difficulty because the post is a combination of four convex corners. A convex corner is defined as the corner bounded by the fastest etching crystal planes in the silicon. The etching of rectangular convex corners in anisotropic etching solutions by KOH or TMAH leads to a deformation of the edges due to cornering undercutting. It is clear that by using wet anisotropic etching it is very difficult to control the shape and size of the final etched structure. However, a DRIE process allows control of the etched structures at an expense of the surface roughness. By utilizing DRIE in contrast to anisotropic etching, very accurate membrane suspended filters can be fabricated whose measured response very closely matches the theoretical expectations. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method for micro-machining a varactor that is part of a membrane suspended MEMS tunable filter is disclosed. In one non-limiting embodiment, the method includes providing a main substrate; depositing a membrane on the main substrate; depositing and patterning a first sacrificial photoresist layer on the membrane; depositing a first metal seed layer on the patterned first photoresist layer; depositing and patterning a second sacrificial photoresist layer on the first metal seed layer to expose a central portion of the seed layer; depositing a first varactor beam on the first metal seed layer within the central portion of the second photoresist layer; removing at least a portion of the first photoresist layer, second photoresist layer and the first metal seed layer; depositing and patterning a third sacrificial photoresist layer over the first varactor beam and the membrane so as to expose a central portion of the first varactor beam; depositing a second metal seed layer over the third photoresist layer and the exposed portion of the first varactor beam; depositing and patterning a fourth sacrificial photoresist layer over the second metal seed layer so as to expose a central portion of a second metal seed layer; depositing a second varactor beam on the second metal seed layer so that the second varactor beam has a T-shape that is electrically coupled to the second seed layer and the first varactor beam; coupling a carrier substrate to the fourth photoresist layer and the second varactor beam opposite to the main substrate using a release layer; depositing and patterning a masking layer on the main substrate; etching a central portion of the main substrate to expose the membrane; removing the carrier substrate by using a material that attacks the release layer but does not attack the photoresist material; and removing any remaining portions of the first, third and fourth photoresist layers to provide a released varactor. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a top-via transition between a top wafer and a substrate wafer, according to an embodiment of the present invention; 
         FIG. 2  is a top view of the top wafer shown in  FIG. 1 ; 
         FIG. 3  is a bottom view of the top wafer shown in  FIG. 1 ; 
         FIG. 4  is a top view of the substrate wafer shown in  FIG. 1 ; 
         FIG. 5  is a perspective view of a filter employing a microstrip line on a membrane, according to another embodiment of the present invention; 
         FIG. 6  is a bottom view of a membrane substrate of the filter shown in  FIG. 5 ; 
         FIG. 7  is a top view of one end of the membrane substrate of the filter shown in  FIG. 5 ; 
         FIG. 8  is a top view of a membrane suspended MEMS tunable filter, according to an embodiment of the present invention; and 
         FIGS. 9(   a )- 9 ( i ) are cross-sectional views of a structure for fabricating a membrane suspended MEMS tunable varactor filter, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a method for micro-machining a varactor filter including a varactor suspended over a suspended membrane on a semiconductor substrate is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a cross-sectional view of a vertically integrated wafer package  150  including a top wafer  152  and a substrate wafer  154  showing a top-via transition. In one non-limiting embodiment, the top wafer  152  is a silicon wafer that is about 100 μm thick and the substrate wafer  154  is a silicon, GaAs, InP, GaN, etc. substrate wafer. The package  150  provides a hermetically sealed package for an RF device  156 , such as a cavity filter, MEMS, etc., positioned within a cavity  158  formed through a bottom surface of the wafer  152 , as shown.  FIG. 2  is a top view of the wafer  152 ,  FIG. 3  is a bottom view of the wafer  152  and  FIG. 4  is a top view of the substrate wafer  154 . 
     A short finite input co-planar waveguide (CPW)  160  and a short finite output CPW  162  are formed on a top surface  164  of the top wafer  152 . The input CPW  160  includes a signal line  180  having a flared and widened portion  182  electrically coupled to a signal via  184  extending through the wafer  152 . The input CPW  160  also includes a first input ground plane  186  electrically coupled to a ground via  188  extending through the wafer  152  and electrically separated from the signal line  180 . The input CPW  160  also includes a second input ground plane  190  electrically coupled to a ground via  192  extending through the wafer  152  and electrically separated from the signal line  180  opposite to the first input ground plane  186 . Likewise, the output CPW  162  includes a signal line  196  having a flared and widened portion  198  electrically coupled to a signal via  200  extending through the wafer  152 . The output CPW  162  also includes a first output ground plane  202  electrically coupled to a ground via  204  extending through the wafer  152 , and a second output ground plane  206  electrically coupled to a ground via  208  extending through the wafer  152 , where both the ground planes  202  and  206  are electrically separated from the signal line  196  on opposite sides thereof. 
     A transition CPW  210  is formed on a top surface  212  of the substrate wafer  154 . The CPW  210  includes a signal line  214  having a metallized transition  216  at one end and a metallized transition  218  at an opposite end. In one non-limiting embodiment, the transitions  216  and  218  are gold (Au) bumps. The CPW  210  also includes a first transition ground plane  220  and a second transition ground plane  222  on opposite sides of the signal line  214 , and electrically separated therefrom. The ground plane  220  includes a metallized transition  224  at one end and a metallized transition  226  at an opposite end. Likewise, the ground plane  222  includes a metallized transition  228  at one end and a metallized transition  230  at an opposite end. The transition  216  is electrically coupled to the signal via  184  and the transition  218  is electrically coupled to the signal via  200  so that the signal is transferred from the input CPW  160  to the transition CPW  210  and the RF device  156 , and from the RF device  156  to the output CPW  162 . The transition  224  is electrically coupled to the ground via  192 , the transition  226  is electrically coupled to the ground via  208 , the transition  228  is electrically coupled to the ground via  188  and the transition  230  is electrically coupled to the ground via  204 . 
     The design of the top-via transition shown in  FIGS. 1-4  requires a multi-phase process that involves both full wave simulation tools and a circuit simulator. There are five cascaded discontinuities that form the overall top-via transition. In order to assure optimum RF performance, any reflections from mismatches between these cascaded sections need to be eliminated. Therefore, each section is theoretically analyzed separately and is designed to have a 50Ω impedance. 
     The cascaded discontinuities include the CPW  160  on the top wafer  152  and the vias  184 ,  188 ,  192 ,  200 ,  204  and  208 . A third discontinuity is provided by the CPW  210 . A fourth discontinuity is the metallized transitions  216 ,  218 ,  224 ,  226 ,  228  and  230 . In one non-limiting embodiment, the transitions  216 ,  218 ,  224 ,  226 ,  228  and  230  are 1 μm of Cr/Au deposited on opposite sides of the wafer  152  and  154  to form the coplanar waveguide interconnects. Subsequently, 3 μm of Au is electroplated on both sides in order to form the transitions  216 ,  218 ,  224 ,  226 ,  228  and  230 . This creates bumps with 8 μm thickness sandwiched between the wafers  152  and  154 , therefore changing the capacitance, dielectric constant and characteristic impedance of the connections. Using a two-dimensional electrostatic simulation tool, the line geometry is modified in order to preserve the original 50Ω characteristic impedance. The CPW  210  within the cavity  158  is similar to a free-space coplanar waveguide because the micromachined cavity  158  is designed in such a way as to have no effect on the characteristic impedance of the CPW. 
       FIG. 5  is a perspective view showing this type of a filter  490 , according to another embodiment of the present invention. The filter  490  includes a ground plane wafer  492 , a membrane wafer  494  and a shield wafer  496 , where the wafers  492 ,  494  and  496  are silicon. A short finite input CPW  498  is provided at one end of the membrane wafer  494  and includes a signal line  500  and opposing ground planes  502  and  504  formed on a top surface of the wafer  494 . A short finite output CPW  506  is provided at an opposite end of the membrane wafer  494  and includes a signal line  508  and opposing ground planes  510  and  512 . 
     A membrane  514  is formed to the membrane wafer  494  and portions of the membrane wafer  494  are removed so that the membrane  514  is free to resonate.  FIG. 6  is a bottom view of the membrane wafer  494  showing a cavity  516  formed by removing silicon from the wafer  494  so that the membrane  514  is able to resonate. A microstrip line  518  is formed on the membrane  514  and is electrically coupled to the signal lines  500  and  508 . 
     The ground plane wafer  492  includes a ground plane  520  formed to bottom surface of the wafer  492  and provides the ground plane for the microstrip line  518 . Metallized posts are formed to the ground plane  520  and include a post  522  that makes electrical contact with the ground plane  502 , a post  524  that makes electrical contact with the ground plane  504 , a post  526  that makes electrical contact with the ground plane  510  and a post  528  that makes electrical contact with the ground plane  512 . 
     The shield wafer  496  includes a cavity  530  and a metallized shield layer  532 . The cavity  530  combines with the cavity  516  within the membrane wafer  494  to allow the membrane  514  to resonate. 
     The microstrip line  518  includes a transition  536  and a transition  538  that transitions to the signal lines  500  and  508 , respectively, to provide impedance matching, such as 50Ω.  FIG. 7  is a top view of one end of the membrane wafer  494  at an end thereof that includes the input CPW  498 . The posts  522  and  524  are shown relative to the ground planes  502  and  504 .  FIG. 7  also shows sections A, B and C, and dimensions w and l, which will be described below. 
     The transition  536  occurs by a short section of low impedance CPW on silicon (section B) followed by a short section of high impedance microstrip on membrane (section C). The widths of the CPW signal line  500  and the microstrip line  518  are tapered to allow a smooth transition and to minimize the loss due to an abrupt discontinuity between the signal lines of the two 50Ω lines. The posts  522  and  524  allow the CPW ground planes  502  and  504  to transition to the microstrip ground plane that can be 250 μm from the microstrip line  518 . The 50ΩCPW (section A) has a conductor/gap dimension of 80/100 μm, and the 50Ωmicrostrip line  518  has a conductor width of about 1150 μm. The height of the microstrip shield, which is the thickness of the membrane wafer  494  plus the depth of the cavity  530  in the shield wafer  496  can be 925 μm. 
     The performance of such a transition depends on a number of conditions. These conditions include the length of the tapered transmission line sections (sections B and C) as well as the location of the via contacts (l) and the width (w) of the opening between them. The two dimensions l and w control the series inductance between the two 50Ω lines  500  and  518 , which increases as l and/or w increase. This series inductance created by the inductive loop formed by the CPW ground planes  502  and  504 , the metallized posts  522  and  524  and the microstrip ground can be varied between 70-160 pH. Therefore, the transition performance can be controlled by changing l and w, thus providing the main reason why the DRIE process provides clear advantages over anisotropic etching. 
     The fabrication process for realizing the transition can be provided as follows. For the ground plane wafer  492 , alignment keys are formed on the backside, 275 μm of silicon is DRIE etched in the parts where probing windows will be formed, and on the backside, by a second DRIE etch, 250 μm of silicon is etched. This etch forms the ground planes for the membrane suspended microstrip lines, the post contacts for the ground plane transitions, and the probing windows. Finally, the ground planes and the via contacts are metallized with 2 μm of sputtered gold. 
     For the membrane wafer  494 , 3000/4000 Åof LPCBD SiO 2 /Si x N y  layers are deposited on top of the 7000 Å SiO 2  layers on both sides. Alignment keys are formed on the backside. Circuit metallization is formed with 1 μm of gold using a standard lift-off process. Next, the wafer  494  is etched from the backside down to the thermal SiO 2  layer to release the membrane  514 . The final process step is thermal compression bonding of the two wafers. 
       FIG. 8  is a top view of a membrane suspended MEMS tunable filter  540  that includes the membrane wafer  494 , where the ground plane wafer  492  has been removed and the shield wafer  496  is not shown, and where common elements to the filter  490  are identified by the same reference numeral. The filter  540  includes a plurality of RF MEMS varactors  542  attached to the microstrip line  518  and suspended over the membrane  514 . The varactors  542  include main tuning varactors  544  for tuning the resonator frequency of the filter  540  and fine tuning varactors  546  for tuning the inter-resonator coupling between the varactors  542 . 
     The RF MEMS varactors proposed herein are based on conventional RF MEMS fixed-fixed beam varactors suspended 2-3 μm over a printed transmission line. The fabrication process of the invention for the varactors  542  allows the formation of membrane suspended MEMS structures. The main motivation is that by removing the silicon substrate, or any other type of suitable substrate, under the CPW or microstrip interconnect, the loss is reduced. This result in a low-loss, high-Q structure for various devices, such as filters, phase shifters, etc. However, combining the substrate etching process, which creates the suspended membrane, with RF MEM switches is a challenging problem. The reason behind this is that in all conventional MEMs fabrication processes, a sacrificial layer, such as a photoresist or polyimide, is used to create the MEMS air bridges. This sacrificial layer needs to be removed, typically by a solvent, and subsequently the MEMS are released following a critical point dying. However, the nature of the sacrificial layer imposes significant restrictions on any post processing on the MEMS devices prior to their release. 
     The present invention proposes a technique that allows the wafer with non-released MEMS to be mounted on a carrier wafer, and then providing DRIE etching of the MEMS wafer. The outcome of this process is the fabrication and operation of MEMS devices on a released membrane wafer, although this technique can be used for multiple wafer thinning approaches. 
       FIGS. 9(   a )- 9 ( i ) illustrate a series of fabrication steps for a membrane suspended tunable filter structure  10  that has application for fabricating the varactors  542 , according to an embodiment of the present invention. The structure  10  includes a silicon substrate  12  on which has been deposited a membrane  14 , such as the membrane  514 , by a suitable process well known to those skilled in the art. In one non-limiting embodiment, the membrane  14  includes three layers, particularly a silicon oxide layer (SiO 2 ), a silicon nitride layer (Si 3 N 4 ) and a silicon oxide layer (SiO 2 ). A co-planar waveguide (CPW)  16 , such as the CPW  498 , is deposited on the membrane  14 . 
     In  FIG. 9(   b ), a sacrificial photoresist layer  18  has been deposited and etched on the membrane  14  and the CPW  16  to provide openings  40  in the photoresist layer  18  that expose the CPW  16 , as shown. A nickel seed layer  20  is deposited on the etched photoresist layer  18  to provide a layer that accepts electroplating. Nickel is being used herein for the varactor. However, as will be appreciated by those skilled in the art, other metals may be equally applicable. Another sacrificial photoresist layer  22  is then deposited and patterned on the seed layer  20  to provide a central opening  42 , as shown. Once the photoresist layer  22  is patterned, an electroplated nickel layer  24  is deposited on the nickel seed layer  20  within the opening  42  of the photoresist layer  22  so that legs  44  of the nickel layer  24  are electrically coupled to the CPW  16 , and a lower varactor beam  46  is provided above the microstrip line of the CPW  16 , as shown in  FIG. 9(   c ). 
     The photoresist layer  22 , the nickel seed layer  20  outside of the electroplated nickel layer  24  and the photoresist layer  18  of the lower beam  46  is removed by a suitable etching process. Another sacrificial photoresist layer  26  is then deposited over the structure  10 , and is patterned to expose a central portion of the electroplated nickel layer  24  through an opening  48 , as shown in  FIG. 9(   d ). Another nickel seed layer  28  is then deposited on the patterned photoresist layer  26 , and another sacrificial photoresist layer  30  is deposited and patterned on the nickel seed layer  28  to provide a central opening  50 , as shown in  FIG. 9(   d ). Another electroplated nickel layer  32  is deposited within the openings  48  and  50  on the nickel seed layer  28  to provide an upper varactor beam  52  of the varactor, as shown in  FIG. 9(   e ). 
     A layer of a release material  36 , such as AIT Cool Grease 7016, is deposited on the electroplated upper beam  52  and the photoresist layer  30 , and a carrier substrate  38  is attached to the release layer  36 , as shown in  FIG. 9(   f ). The structure  10  is then flipped over and a masking layer  40  is deposited and patterned to form an opening  54  relative to the substrate  12 . The substrate  12  is selectively etched by a DRIE process through the opening  54  to expose the membrane  14 , as shown in  FIG. 9(   g ). The carrier wafer  38  is detached from the structure  10  by using a suitable material, such as isopropyl alcohol (IPA), that attacks the release layer  36 , but does not attack the photoresist layers  26  and  30  so that the varactor made up of the lower beam  46  and the upper beam  52  are protected during the carrier substrate release process by the photoresist. The varactor is released by removing the photoresist layers  26  and  30  in a solvent bath that dissolves the photoresist, and subsequent critical point drying. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.