Abstract:
A process for fabricating monolithic membrane structures having air gaps is disclosed, comprising the steps of: providing a wafer; depositing and patterning a protective layer on the wafer; providing a trench in the wafer; depositing and patterning a metal in the trench; depositing and patterning a sacrificial layer on the metal; depositing and patterning a membrane pad on the sacrificial layer; providing a polymeric film on the protective layer and sacrificial layer, wherein part of the polymeric film has a tensile stress; and releasing part of the polymeric film from the protective layer and sacrificial layer, wherein the tensile stress of a portion of the polymeric film releases the portion of the polymeric film from the wafer and generates the air gap.

Description:
CLAIM OF BENEFIT OF PROVISIONAL APPLICATION  
       [0001]     This application claims the benefit of U.S. provisional application Ser. No. 60/460,524 filed on Apr. 4, 2003, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a process for fabricating a monolithic structure comprising a polymeric membrane on a patterned substrate and to a monolithic membrane-substrate structure. The monolithic membrane-substrate structure has a well-controlled air gap in between.  
         [0004]     2. Description of the Related Art  
         [0005]     Prior fabrication processes for making structures with well-controlled air gaps have used sacrificial layers to form the gap. However, the use of sacrificial layers to form release membranes or cantilever type structures suffers from certain limitations. The size of the air gap obtained from these processes is determined entirely by the height of the sacrificial layer. Since deposition of any sacrificial layer (SiO 2 , SiN, etc.) is time consuming and costly, the layer thickness is not much greater than about 1 μm. This severely limits the gap size, and also the lateral dimensions that the released structures can have.  
         [0006]     Other related techniques that are useful for achieving large air gaps consist of bonding together two separate sections either directly or by means of an adhesive. One of the pieces is recessed or contains a cavity that gives the required gap. Such a process is not monolithic in nature, and hence involves a more tedious and costly assembly procedure. Also, the size of the air gap may not be as uniform due to the difficulties associated with the bonding process.  
         [0007]     Numerous examples exist in the prior art that use the concept of sacrificial layers to release membranes and cantilevers, and realize air gaps. For example, U.S. Pat. No. 5,738,799 discloses using a sacrificial layer for an ink-jet printhead fabrication technique. Furthermore, N. S. Barker and G. M. Rebeiz also discuss use of sacrificial layers for phase shifters and wide-band switches in the publication “Distributed MEMS True-Time Delay Phase Shifters and Wide-Band Switches” (IEEE Transactions on Microwave Theory and Techniques, vol. 46, no. 11, November 1998).  
         [0008]     However, none of the prior art documents relies on the tensile stress intrinsically present in certain membranes to form released structures with air gaps much larger than the thickness of the sacrificial layer. The present invention achieves this, and also provides a means of ensuring that the tensile stress in the released membranes is retained.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention discloses a process for fabricating a monolithic structure having an air gap and consisting of a preferably polyimide membrane on a patterned silicon substrate. The process and resulting device do not require bonding of two separate sections to achieve the air gaps. The structure is easy to fabricate, reliable, and can be miniaturized using standard cleanroom processing techniques. Therefore, the size of the gap can be controlled with extreme precision. Furthermore, any desired layer can be patterned on the membrane or substrate surfaces forming the micro-chamber. The metal layers can easily be used as electrodes for electrostatic actuation of the membrane, providing tuning capability for any application. The process and resulting device relies on the use of an intermediate sacrificial layer to release the membrane. However, unlike conventional techniques, the height of the sacrificial layer does not determine the size of the air gap that is eventually obtained. This represents a significant advantage in fabricating tunable electronic components that require relatively large air gaps that would be impossible or extremely cost prohibitive to realize using conventional methods. The process also ensures that the polyimide membrane retains the desirable tensile stress.  
         [0010]     According to a first embodiment, the present invention discloses a process for fabricating membrane-substrate structures comprising: providing a substrate; depositing a protective layer on said substrate; forming a trench in said substrate, said trench having a trench depth; depositing a first intermediate layer in said trench; depositing a sacrificial layer on said first intermediate layer, said sacrificial layer having a sacrificial layer depth; depositing and patterning a second intermediate layer on said sacrificial layer; depositing a layer of polymeric material on said second intermediate layer and said sacrificial layer, said layer of polymeric material contacting said second intermediate layer; removing said sacrificial layer; and forming an air gap between the layer of polymeric material and the substrate by release of said layer of polymeric material upon removal of said sacrificial layer, whereby said air gap has an air gap depth greater than said sacrificial layer depth.  
         [0011]     According to a second embodiment, the present invention discloses a process for fabricating membrane-substrate structures comprising the steps of: providing a substrate having a one side and an other side; depositing a first protective layer on said one side and a second protective layer on said other side; forming a trench in said substrate, said trench having a trench depth; depositing a third protective layer in said trench; depositing a first intermediate layer on said third protective layer; depositing a sacrificial layer on said first intermediate layer and said third-protective-layer, said sacrificial layer having a sacrificial layer depth; depositing a second intermediate layer on said sacrificial layer; depositing a layer of polymeric material on said first protective layer, said second intermediate layer, and said sacrificial layer; removing said sacrificial layer; and forming an air gap between the layer of polymeric material and the substrate by release of said layer of polymeric material from said sacrificial layer upon removal of said sacrificial layer, whereby said air gap has an air gap depth greater than said sacrificial layer depth.  
         [0012]     According to a third embodiment, the present invention discloses a process for restoring tensile stress to a monolithic membrane-substrate structure comprising the steps of: a) fabricating a membrane-substrate structure, said step of fabricating the membrane-substrate structure comprising: a1) providing a substrate having a one side and an other side; a2) depositing a first protective layer on said one side and a second protective layer on said other side; a3) forming a trench in said substrate, said trench having a trench depth; a4) depositing a third protective layer in said trench; a5) depositing a first intermediate layer on said third protective layer; a6) depositing a sacrificial layer on said first intermediate layer and said third protective layer, said sacrificial layer having a sacrificial layer depth; a7) depositing a second intermediate layer on said sacrificial layer; a8) depositing a layer of polymeric material on said first protective layer, said second intermediate layer, and said sacrificial layer; a9) removing said sacrificial layer; and a10) forming an air gap between the layer of polymeric material and the substrate by release of said layer of polymeric material from said sacrificial layer upon removal of said sacrificial layer, whereby said air gap has an air gap depth greater than said sacrificial layer depth; b) providing a wafer; c) disposing said membrane-substrate structure on said wafer; d) heating said wafer and said membrane-substrate structure; and e) cooling said wafer and said membrane-substrate structure to room temperature.  
         [0013]     According to a fourth embodiment, the present invention discloses a process for restoring tensile stress to a monolithic membrane-substrate structure comprising the steps of: a) fabricating a membrane-substrate structure, said step of fabricating the membrane-substrate structure comprising: a1) providing a substrate; a2) depositing a protective layer on said substrate; a3) forming a trench in said substrate, said trench having a trench depth; a4) depositing a first intermediate layer in said trench; a5) depositing a sacrificial layer on said first intermediate layer, said sacrificial layer having a sacrificial layer depth; a6) depositing and patterning a second intermediate layer on said sacrificial layer; a7) depositing a layer of polymeric material on said second intermediate layer and said sacrificial layer, said layer of polymeric material contacting said second intermediate layer; a8) removing said sacrificial layer; and a9) forming an air gap between the layer of polymeric material and the substrate by release of said layer of polymeric material upon removal of said sacrificial layer, whereby said air gap has an air gap depth greater than said sacrificial layer depth; b) providing a wafer; c) disposing said membrane-substrate structure on said wafer; d) heating said wafer and said membrane-substrate structure; and e) cooling said wafer and said membrane-substrate structure to room temperature.  
         [0014]     According to a fifth embodiment, the present invention discloses a monolithic membrane-substrate structure, comprising: a substrate having a trench, said trench having a trench depth; a protective layer located on the substrate; a layer of polymeric material located above the substrate and the protective layer; a first intermediate layer located in said trench; a second intermediate layer located under said layer of polymeric material and contacting said layer of polymeric material; and an air gap between the layer of polymeric material and the substrate.  
         [0015]     According to a sixth embodiment, the present invention discloses a monolithic membrane-substrate structure, comprising: a substrate having a first side; a first protective layer disposed on the first side of the substrate; a second protective layer disposed on the second side of the substrate; a third protective layer disposed on the first side of the substrate; a layer of polymeric material located above the first and third protective layer; a first intermediate layer located above said third protective layer and contacting said third protective layer; a second intermediate layer located under said layer of polymeric material and contacting said layer of polymeric material; and an air gap between the layer of polymeric material and the substrate.  
         [0016]     The process of the present invention can also realize large trench depths of about 50 μm or more. With large trench depths of about 50 μm or greater, the residual stress in the released polymeric, preferably polyimide, membrane may not be sufficient to keep the membrane taunt. This is not desirable from an application point of view, as the tuning ability and response time of the structure could be degraded. A further embodiment of the present invention solves this problem, by providing a process for restoring tensile stress to a monolithic membrane-substrate structure comprising the steps of: fabricating a membrane-substrate structure; providing a wafer; disposing said membrane-substrate structure on said wafer; heating said wafer and said membrane-substrate structure; and cooling said wafer and said membrane-substrate structure to room temperature.  
         [0017]     The main purpose of the air gap structure is to realize tunable RF capacitors with an air dielectric (for low loss), for use in tunable filters and phase shifters. In particular, the filters and phase shifters can be tuned by controlling the movement of the membrane. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:  
         [0019]      FIGS. 1-10  show a series of steps on a membrane-substrate structure, performed in accordance with a first embodiment of the process of the present invention;  
         [0020]      FIG. 11  shows a top plan view of the membrane-substrate structure obtained through the first embodiment of the process according to the present invention;  
         [0021]      FIGS. 12-22  show a series of steps on a membrane-substrate structure, performed in accordance with a second embodiment of the process of the present invention;  
         [0022]      FIG. 23  shows a top plan view of the membrane-substrate structure obtained through the second embodiment of the process of the present invention; and  
         [0023]      FIGS. 24A-24C  show a cross section of a device, in which large trench depths are realized, following sequential steps according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     In one embodiment of the current invention a silicon substrate or wafer  4  has protective layers  5  of Si 3 N 4  deposited on both sides of the silicon wafer by, for example, plasma enhanced chemical vapor deposition (PECVD), as shown in  FIG. 1 . Preferably, the layers  5  are about 0.5 μm thick.  
         [0025]     A window  6 , as shown in  FIG. 2 , is then patterned in one of the layers  5  so that the silicon wafer  4  lying underneath the layer  5  can be etched.  
         [0026]     Next, a trench  7 , shown in  FIG. 3 , is etched in the silicon wafer  4 . Preferably, the etchant used in this step is a KOH solution and the trench  7  has a depth of between about 5 and about 50 μm.  
         [0027]     A metal pad  8 , an example of which is shown in  FIG. 4 , is deposited and patterned in the trench  7 . Preferably the metal pad  8  consists of a Ti—Au film having a thickness of about 1 μm. The metal pad  8  can be, for example, an electrode for electrostatic actuation.  
         [0028]     The following figures show embodiments where the upper protective layer  5  is present, even after formation of the trench  7 . However, once the trench is formed, the upper protective layer is not necessary anymore and can be removed.  
         [0029]     A sacrificial layer  9 , shown in  FIG. 5 , is then deposited and patterned on top of the metal pad  8  to have a lateral dimension, in plan view, larger than the corresponding dimensions of the window  6  in the top layer  5 . In this embodiment, the sacrificial layer  9  is about 50 μm wider than the window  6 . The sacrificial layer  9  should preferably be larger than the lateral dimension of the window  6  to prevent the membrane from attaching to the trench sidewalls. Further, the sacrificial layer  9  is preferably 1 μm thick and composed of metal, PECVD SiO 2 , KCl, or the like. The material for the sacrificial layer  9  is selected such that the chemical or technique used to eventually dissolve away the sacrificial layer  9  does not attack a polymeric film, preferably a polyimide film.  
         [0030]     An intermediate layer  10 , shown in  FIG. 6 , is then patterned and deposited on the sacrificial layer  9 .  
         [0031]     A layer of polymeric material or film  11 , shown in  FIG. 7 , is then spun onto the silicon wafer  4  and cured at a temperature of about 300° C. The polymeric film  11  used should preferably shrink by about 20-40% upon final curing. The polymeric film is under tensile stress.  
         [0032]     A metal mask  12 , shown in  FIG. 8  and preferably composed of Al, is then preferably deposited on the polymeric film  11  and patterned to have mask holes  13 . The function of the mask holes  13  is that of allowing etching of the underlying portions of the polymeric film  11 . The polymeric film  11  is dry etched, preferably using reactive ion etching (RIE), in correspondence of the mask holes  13 , leaving film holes  13   a  in the polymeric film  11 , as shown in  FIG. 9 .  
         [0033]     The metal mask  12  is then removed, for example by a metal etchant such as Al etchant for an Al metal mask. The metal etchant used will depend on the composition of the metal mask used.  
         [0034]     As shown in  FIG. 10 , the sacrificial layer  9  is then removed by immersing the device in a solution such as BOE for SiO 2 , or hot DI water for KCl. This last step, as shown in  FIG. 10 , releases the polymeric film  11  from the substrate. Since the polymeric material is under tensile stress, the air gap height obtained is determined by the original depth of the trench  7 , and not only by the height of the sacrificial layer. The depth of the trench  7  is, in general, much larger than the depth or thickness of the sacrificial layer  9 .  
         [0035]      FIG. 11  shows a view from the top of  FIG. 10 , thus better showing a preferred position of the holes  13   a  in the polymeric film  11 . Also shown are the metal layers  8  and  10 , in dotted lines.  
         [0036]     In another embodiment of the process of the present invention, a protective layer  15  of Si 3 N 4  is deposited on the top of a silicon wafer  14  and a protective layer  16  of Si 3 N 4  is deposited on the bottom of the silicon wafer  14 , preferably by plasma enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD) as shown in  FIG. 12 . Preferably, the layers  15 ,  16  are about 0.5 μm thick.  
         [0037]     The following figures show embodiments where the protective layer  15  is present, even after formation of the trench  17 . However, once the trench is formed, the protective layer is not necessary anymore and can be removed.  
         [0038]     A window  17 , shown in  FIG. 13 , is then patterned in the top layer  15  so that the silicon wafer  14  lying underneath the top layer  15  can be etched.  
         [0039]     Next, a trench  18 , shown in  FIG. 14 , is etched in the silicon wafer  14 . Preferably, the etchant used in this step is a KOH solution and the trench  18  has a depth of between about 5 and about 50 μm.  
         [0040]     Further to this, another protective layer  19  of Si 3 N 4 , shown in  FIG. 15 , is deposited in the trench  18  to act as an etch stop layer  19  later.  
         [0041]     A first metal layer  20 , shown in  FIG. 16 , is deposited and patterned in the trench  18  and covers the etch stop layer  19 . The patterned metal layer  20  is preferably composed of a Ti—Au film and has a thickness of about 1 μm. The metal layer  20  can either form an electrode or reinforce the etch stop layer  19 .  
         [0042]     A sacrificial layer  21 , shown in  FIG. 17 , is then deposited and patterned on top of the first metal pad  20 . The sacrificial layer  21  has a lateral dimension which is larger than a corresponding lateral dimension of the window  16  in the layer  19 . In this embodiment, the lateral dimension of the sacrificial layer  21  is about 50 μm larger than the lateral dimension of the window  17 . The sacrificial layer  21  should preferably be larger than the lateral dimension of the window  16  to prevent the membrane from attaching to the trench sidewalls.  
         [0043]     The material for the sacrificial layer  21  is chosen such that the chemical used to eventually dissolve away the sacrificial layer  20  does not attack a polymeric film, preferably a polyimide film. For example, the sacrificial layer  21  is composed of metal, SiO, KCl, or the like, and is preferably 1 μm thick.  
         [0044]     Next, a second metal pad  22 , shown in  FIG. 18 , is deposited and patterned on the sacrificial layer  21 .  
         [0045]     A polymeric film  23 , shown in  FIG. 19 , is then spun onto the top layer  15  and cured at a temperature of about 300° C. The polymeric film  23  should preferably shrink by about 20-40% upon final curing.  
         [0046]     The bottom protective layer  16  is then patterned to form an etch mask as shown in  FIG. 20 . The silicon wafer  14  is mounted onto a holder (not shown) to protect the top layer  15 , and etched, preferably in KOH solution. This etching step opens access holes  24  in the bottom layer  16  as shown in  FIG. 20 .  
         [0047]     In  FIG. 21 , the portions of the silicon wafer  14  directly above the access holes  24  are etched to the etch stop layer  19 .  
         [0048]     As shown in  FIG. 22 , the layer  19  is then dry etched or wet etched, and the sacrificial layer  21  is then removed by immersion in a solution, such as BOE for SiO 2 , or hot deionized water for KCl. The tensile stress in the polymeric membrane  23  releases the membrane and a gap height equal to the original trench  18  depth is obtained as shown in  FIG. 22 .  
         [0049]      FIG. 23  shows a view from the top of  FIG. 12 , thus better showing a preferred position of the holes  24  below the polymeric film  23 . Also shown are the metal layers  20  and  22 , in dotted lines.  
         [0050]     In a third embodiment of the process of the present invention, large trench depths, preferably of about 50 μm or greater, can be realized. This is shown in  FIGS. 24A-24C  below. A released membrane-substrate structure  25 , like for example the structure of  FIG. 22 , is placed on a flat, rough surface  26 , preferably the unpolished side of a silicon wafer, with the polymeric, preferably polyimide, film  23  facing down as shown in  FIG. 24A . In a different embodiment, also the structure shown in  FIG. 10  can be used.  
         [0051]     The membrane-substrate structure  25  and the unpolished silicon wafer  26  are then placed in an oven (not shown) and heated to a temperature that is higher than the glass transition temperature of the polymeric material  23 . Preferably, this temperature is about 300° C. Heating the polymeric film  23  to a temperature higher than its glass transition temperature makes the polymeric film  23  more compliant with the unpolished side of the silicon wafer  26  as shown in  FIG. 24B . The surface roughness of the unpolished side of the silicon wafer  25  prevents the polymeric film  23  from adhering to the silicon wafer  26 .  
         [0052]     The released membrane structure  25  and silicon wafer  26  are allowed to cool down to room temperature, following which the polymeric film  23  will contract more than the silicon substrate  26  and the tensile stress in the polymeric film  23  of the released membrane structure  25  is restored as shown in  FIG. 24C .  
         [0053]     As an alternative to the Si 3 N 4  used in the above disclosed embodiments, SiO 2  can be used. The method of deposition for SiO 2  includes thermal deposition. With SiO 2  being deposited on both sides of the silicon wafer, etching is accomplished by using ethylene diamine pyrocatechol (EDP). The disadvantages of EDP are that it etches high resistivity silicon wafers slowly or not at all and releases byproducts that tend to get deposited on other parts of the device; the device must be cleaned more rigorously following etching steps using EDP.  
         [0054]     In addition, as an alternative to the silicon wafers  4  and  14  disclosed in the previous embodiments, GaAs can be used as a substrate. With GaAs wafers, Si x N y  deposited using PECVD or LPCVD is preferred.  
         [0055]     Although the present invention has been described with respect to specific embodiments thereof, various changes and modifications can be carried out by those skilled in the art without departing from the scope of the invention. It is intended, therefore, that the present invention encompass changes and modifications falling within the scope of the appended claims.