Abstract:
A method of forming free standing microstructures includes providing a substrate and forming a sacrificial layer on the substrate. A thin-film structural layer is then formed around and over the sacrificial layer. The sacrificial layer may be formed from an electrically conductive or non-electrically conductive material in certain embodiments of the invention. Nanometer-scale pores are then introduced through the thin-film structural layer by a non-lithographic method, such as anodic etching. Via the pores, at least a portion of the sacrificial layer is etched away or otherwise removed from underneath the thin-film structural layer. The free standing microstructures may be sealed by application of a sealing layer on top thereof. The microstructure may form an encapsulating cavity and provide integrated on-wafer packaging if separate microdevices are disposed inside the cavity. The entire process may be done at or near room temperature in some cases.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Application claims priority to U.S. Provisional Patent Application No. 60/686,713 filed on Jun. 2, 2005. U.S. Provisional Patent Application No. 60/686,713 is incorporated by reference as if set forth fully herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0002]     The U.S. Government may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. DAAH01-99-C-R220 and W31P4Q-05-P-R012 awarded by DARPA. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The field of the invention relates to thin-film microstructures formed by surface micromachining processes in the field of Micro-Electro-Mechanical-Systems (MEMS).  
       BACKGROUND OF THE INVENTION  
       [0004]     Although MEMS-based products are increasingly being used in commercial and research applications, the packaging of the MEMS microdevices is usually developed on a case-by-case basis in-house and remains as a significant obstacle to large scale commercial production. Due to the sensitive and fragile nature of the free standing microstructures formed in many MEMS devices, the packaging process often amounts to a significant portion (e.g., as much as 80-90%) of the cost of a MEMS-based product. The so called “on-wafer packaging” (also known as zero-level or device-level packaging) of MEMS devices on a single wafer, i.e., packaging the delicate devices in a protective housing on the wafer before the wafer is ready for dicing, has long been recognized as a promising approach, because it allows the use of packaging procedures similar to those used for regular electronics manufacturing in producing a large numbers of MEMS-based devices.  
         [0005]     In general, the on-wafer packaging (or encapsulation) approaches fall into two categories: (1) wafer bonding packaging and (2) integrated thin-film packaging. In the wafer bonding approach, a separate substrate is bonded to the MEMS wafer to cap the MEMS components using a wide variety of bonding techniques. While wafer bonding has a proven track record and is being widely used in industry, integrated thin-film packaging has long been considered to be a potentially more cost effective approach for mass production.  
         [0006]     In the so-called integrated thin-film packaging approach, the packaging process is carried out on the same wafer where the MEMS devices are fabricated by adding extra steps to the surface micromachining process used to construct the device. For example, an additional sacrificial layer is deposited on top of an unreleased microdevice and then covered by a thin-film structural layer that will eventually form a cavity and encapsulate the microdevice inside. The device is released after the sacrificial layer is removed through the etch holes opened in the structural layer (e.g., encapsulating shell). One known approach is to use wet or gas etchants that pass through a limited number of micrometer-sized etch holes that are lithographically opened in the encapsulation shell. The MEMS package is then sealed by conformal deposition of a thin-film on top of the encapsulation layer in an appropriate pressure condition.  
         [0007]     Compared with wafer-bonding packaging techniques, integrated thin-film packaging has several advantages including: (1) the use of surface-micromachining batch fabrication processes, thereby avoiding the need for aligning two wafers and the challenges of bonding on “processed” (i.e., not smooth) surfaces; (2) the elimination of the seal ring, allowing much smaller volume cavities, therefore increasing the number of available dice per wafer; and (3) a lower topography. Thin-film encapsulation processes even allow the post-encapsulation processes for additional MEMS or IC steps, if desired.  
         [0008]     Despite the anticipated advantages of integrated thin-film packaging, existing encapsulation methods suffer from a few drawbacks for on-wafer packaging. First, because of the lithography and etching techniques employed, the etch holes patterned in the encapsulation shell have a typical size of a few micrometers. Opening vertical etch holes in the encapsulation layer right above the device area is not desirable, because a significant amount of sealing material can diffuse through the etch holes and deposit on the MEMS device surfaces inside the cavity, thereby changing the device characteristics.  
         [0009]     While this issue can be alleviated by utilizing laterally directed etch channels, such channels require relatively long times to remove the sacrificial materials out of the cavity, lowering the process throughput and even potentially degrading the mechanical properties of the structure material. Improperly designed lateral etch channels can also lead to excessive gas evacuation time during the sealing process. Consequently, despite more recent advances, the parasitic deposition of sealing material inside the cavity has not been fully prevented.  
         [0010]     Polycrystalline silicon (polysilicon) thin-films have been found permeable if made very thin (nanometers) and potentially useful for integrated thin-film encapsulation. However, this thin-film is too thin and weak to serve as an encapsulating structural layer for typical MEMS devices. Thus, this method uses an additional layer of regular thin-film with etch windows, somewhat defeating the purpose of using permeable encapsulation layer.  
         [0011]     There thus is a need for a thin-film encapsulation layer that is permeable yet structurally strong enough to freely stand as an encapsulation shell. The need for structural strength means that the use of very thin layers should be avoided. The need for permeability suggests that the pores should be very small so they are sealed quickly before the sealing material passes through them. Yet, the sacrificial material needs to be removed through the tiny pores. The two seemingly conflicting requirements can be met, if the pores are very small but highly populated. Considering all the requirements, it is desired to have a relatively thick (i.e., on the order of micrometers) encapsulation layer with highly populated nanometer-scale pores formed through the layer preferably in a normal orientation.  
         [0012]     Moreover, because many MEMS-based devices use metals, which cannot withstand high processing temperatures, there is a need for thin-film encapsulation methods that avoid high-temperature processing steps. Metallic structures (e.g., gold, aluminum) are currently most commonly used in radio-frequency (RF) MEMS devices. These devices, however, cannot be packaged by integrated thin-film packaging if the processing includes high temperature steps.  
       SUMMARY  
       [0013]     In a first embodiment of the invention, a method of forming a free standing microstructure (e.g., a shell or encapsulation structure) includes providing a substrate and forming a sacrificial layer over the substrate. A thin-film structural layer is then formed over and around the sacrificial layer. Nanometer-scale pores are then introduced in the thin-film structural layer. For example, non-lithographic methods may be used to form an array of highly populated, directional pores having diameters in the nanometer range. Via the pores, at least a portion of the sacrificial layer is etched away or otherwise removed from underneath the thin-film structural layer. The thin-film structural layer may be sealed by application of a sealing layer on top thereof.  
         [0014]     The free standing structural microstructure or encapsulation layer can be used to enclose one or more microdevices (e.g., MEMS devices). The microdevice may include, for example, an RF-based MEMS device. The process described herein may also be used to liberate or initiate free standing of one or more portions of the MEMS device contained beneath the thin-film structural microstructure or encapsulation layer.  
         [0015]     In one aspect of the invention, the sacrificial layer is formed from a ceramic material such as phosphosilicate glass (PSG). In another aspect of the invention, the sacrificial layer may be formed from a polymer material such as, for instance, a photoresist material. In yet another embodiment of the invention, the sacrificial layer may be formed from a metallic material such as aluminum. The thin-film structural layer may be formed using, for example, a ceramic material (e.g., silicon), a metal (e.g., aluminum), or a polymer in combination with an appropriate sacrificial layer of material underlying the same.  
         [0016]     In one aspect of the invention, the sacrificial layer is formed at a temperature at or below 300° C. In another aspect of the invention, the sacrificial layer is formed at a temperature that is at or around room temperature. Similarly, in one aspect of the invention, the structural layer may be deposited at or below 300° C. In yet another aspect, the structural layer may be formed at a temperature that is at or around room temperature. Again, similarly, in one aspect of the invention, the sealing layer may be deposited at or below 300° C. In yet another aspect, the sealing layer may be formed at a temperature that is at or around room temperature. In one aspect of the invention, the sacrificial layer, structural layer, and sealing layer are all formed at a temperature at or below 300° C. In yet another aspect, the sacrificial layer, structural layer, and sealing layer are all formed at a temperature that is at or around room temperature.  
         [0017]     In one embodiment of the invention, a sacrificial layer is formed on the substrate, and a polymer structural layer is then deposited. The sacrificial layer can be either electrically conductive or non-conductive. Highly populated, highly directional nanopores can be introduced into the polymer layer via ion irradiation followed by etching. The sacrificial layer is then at least partially etched away or otherwise removed. Optionally, the structural layer may be sealed with a sealing layer. The sealing layer may be substantially impermeable to fluids.  
         [0018]     In another embodiment of the invention, a sacrificial layer is formed on the substrate, and an aluminum structural layer is then deposited. The sacrificial layer can be either electrically conductive or non-conductive. Highly populated, highly directional nanopores can be introduced into the aluminum layer via anodization etching, which turns the aluminum into alumina at the same time. The sacrificial layer is then at least partially etched away or otherwise removed. If the sacrificial layer is electrically non-conductive, a seed layer is formed on the sacrificial layer before the structural layer and removed before the sacrificial etching. Optionally, the structural layer may be sealed with a sealing layer.  
         [0019]     In still another embodiment of the invention, a sacrificial layer is formed on the substrate, and silicon structural layer, such as polysilicon, is then deposited. The sacrificial layer can be either electrically conductive or non-conductive. However, if a non-conductive material, such as a glass layer (e.g., phosphosilicate glass (PSG)) is used for the sacrificial layer, the silicon structural layer should be doped to be conductive. Highly populated, directional nanopores can be introduced into the structural layer via anodization etching. The sacrificial layer is then at least partially etched away or otherwise removed. Optionally, the structural layer may be sealed with a sealing layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIGS. 1A-1D  illustrate a fabrication process for forming a porous alumina microstructure.  
         [0021]      FIG. 2A  schematically illustrates the progression of pore morphology changes in an aluminum thin-film subject to anodization etching.  
         [0022]      FIG. 2B  illustrates a panel of scanning electron microscope (SEM) cross-sectional images illustrating the progression of pore morphology changes in an aluminum thin-film subject to anodization etching.  
         [0023]      FIG. 3  illustrates a SEM cross-sectional image of a porous alumina thin-film microstructure (or encapsulation structure).  
         [0024]      FIGS. 4A-4D  illustrate a vacuum encapsulation process for the fabrication of a metal Pirani gauge.  
         [0025]      FIG. 5  is a graph illustrating the resistance vs. current characteristics of a Pirani gauge sealed in a thin-film encapsulation structure. Calibration data at different pressures is also shown.  
         [0026]      FIG. 6  is a graph illustrating the leak rate of two sealed cavities.  
         [0027]      FIG. 7A  is a top view of an encapsulated coplanar waveguide CPW device as viewed using an optical microscope. The gold (Au) signal line is visible through the transparent porous alumina shell.  
         [0028]      FIG. 7B  is cross-sectional schematic representation of the encapsulated CPW device taken along the line A-A′ in  FIG. 7A .  
         [0029]      FIG. 7C  is cross-sectional schematic representation of the encapsulated CPW device taken along the line B-B′ in  FIG. 7A .  
         [0030]      FIG. 8  is a graph illustrating the insertion loss difference between packaged and unpackaged CPW devices.  
         [0031]      FIGS. 9A-9D  illustrate a fabrication process for forming a free standing porous polysilicon shell.  
         [0032]      FIG. 10  is a schematic cross-sectional view of process of subjecting a polysilicon thin-film encapsulating structure to electrochemical etching.  
         [0033]      FIG. 11  illustrates a graph showing electrode potential as a function of time during electrochemical etching when a constant current is applied.  
         [0034]      FIGS. 12A-12H  illustrate a fabrication process for the formation of a Pirani gauge beneath a free standing porous polysilicon shell.  
         [0035]      FIGS. 13A-13F  illustrate a panel of SEM images of a polysilicon Pirani gauge encapsulated by a porous polysilicon shell that was sealed in vacuum.  
         [0036]      FIG. 14  illustrates a graph of the resistance vs. current characteristics of a Pirani gauge in known pressures after the cavity is broken.  
         [0037]      FIG. 15  illustrates a graph of the thermal impedance of a Pirani gauge at different pressures. The arrow indicates the thermal impedance of the sealed Pirani gauge.  
         [0038]      FIG. 16  illustrates the leak rate of two sealed cavities, inside each of which is a Pirani gauge of different designs.  
         [0039]      FIGS. 17A-17B  illustrate a process for encapsulating a micro-bridge device in a porous polysilicon shell using a Multi-User MEMS Process (MUMPS) foundry service. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]      FIGS. 1A-1D  illustrate a fabrication process for forming a free standing porous alumina microstructure  10 . From a thin-film, the microstructure  10  may be formed as a beam, bridge, plate, membrane, or the like. With reference to  FIGS. 1A-1D , the fabrication process includes providing a substrate  12 . A sacrificial layer  14  is then formed on the substrate  12 . The sacrificial layer  14  may be formed from a non-conductive material such as, for example a polymer material such as a photoresist. Alternatively, the sacrificial layer  14  may be formed from a ceramic material such as phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and boron silicate glass (BSG). In still other embodiments (e.g., where aluminum is used for the free standing microstructure), the sacrificial layer  14  may be formed from an electrically conductive material.  
         [0041]     A thin-film structural layer  16  is formed over the sacrificial layer  14 . Pores are then introduced into the thin-film structural layer  16  through the use of, for example, an anodized etching process. The pores preferably are nanometer-sized pores. In certain embodiments, the structural layer  16  may be formed from a polymer, in which case the pores are introduced by a different method, for example, an ion-irradiation followed by etching. Following pore formation, at least a portion of the sacrificial layer is then etched or otherwise removed from underneath the thin-film structural layer  16 .  
         [0042]     With respect to anodized porous alumina, the typical pore structure is a hexagonal array of cylindrical-shaped pores with a bottom Al 2 O 3  barrier layer. The pore diameter generally ranges from around 10 nm to around 300 nm. To form free standing microstructures, the bottom Al 2 O 3  barrier layer needs to be removed to allow the diffusion of etchant(s) through the pores to etch away the sacrificial material.  
         [0043]     With reference to  FIG. 1A , a thin-film stack was formed on a silicon substrate  12 . Going from the bottom to the top in  FIG. 1A , the stack consisted of a 0.3 μm thick PECVD oxide layer  18  deposited for insulation purposes, a 1.5 μm thick amorphous silicon (a-Si) sacrificial layer  14 , 1000/100 Å thick evaporated titanium/gold (Ti/Au) layers  22 ,  24 , and a 1 μm thick evaporated aluminum (Al) layer  26 .  FIG. 1B  illustrates the aluminum layer  26  undergoing anodized etching. The anodization etching was performed at 40 V constant bias in a 0.3 M oxalic acid solution at room temperature. During the anodization etching process, the current stabilized for a long period, indicating a process of stable pore growth, and then suddenly started to increase steadily accompanied with gas bubble generation. The generation of gas bubbles signified that the etching front had reached the gold (Au) layer  24  and, due to the existence of H 2 O in the electrolyte, electrolysis generated O 2  gas. At the same time, the color of the aluminum surface changed from opaque (i.e., the color of aluminum), to translucent and finally to transparent.  
         [0044]     The etching process was stopped when the thin film became transparent. The structure of the bottom barrier layer during and after the anodization etching process was completed is shown in the SEM pictures in  FIG. 2B  (upper, middle, and lower SEM images) and associated schematic representations illustrated in  FIG. 2A . As seen in the middle image of  FIGS. 2A and 2B , a very thin arched barrier layer (around 10 nm in thickness) with a small void underneath was observed at the bottom of each pore. The arched barrier layer was then removed by a 5 wt % H 3 PO 4  wet etching solution for 25 minutes, which also thinned down the pore wall to a diameter of 50 nm. The widening of the pore (or thinning of the pore wall) is best seen in the bottom SEM image shown in  FIG. 2B  (and illustrated schematically in  FIG. 2A ).  
         [0045]     The 100 Å Au layer  24  was needed to form the numerous thin arches. Without the existence of the Au layer  24 , the Ti adhesion layer  22  would have been turned into an oxide layer by the electrolyte with a bottom barrier layer, similar to the typical pore morphology of porous alumina. Next, as best seen in  FIG. 1C , the Au layer  24  and Ti layer  22  beneath the porous alumina layer  16  were removed with an Au etchant and a Ti etchant (NH 4 OH:H 2 O 2 :H 2 O=1:1:8 on a volume basis) in the area defined by a photoresist mask  28 . As seen in  FIG. 1D , after the photoresist mask  28  was stripped away, the a-Si sacrificial layer  14  was etched away through the now-formed pores using XeF 2  gas etchant (not shown). The cross-section schematic of free-standing porous alumina structure is displayed in  FIG. 1D .  
         [0046]     A SEM cross-sectional image of the porous alumina layer  16  is displayed in  FIG. 3 , where a 1.5 μm thick air gap is visible below the porous alumina layer  16 . A magnified view of the cross section of the layer  16  is presented in the insert. The transparent porous alumina layer  16  exhibited a very good quality in terms of mechanical and structural purposes. Porous alumina layers  16  as large as 2 mm a side were successfully obtained without any cracks or wrinkles.  
         [0047]     The hermeticity of the encapsulation with the porous alumina thin-film layer  16  was studied by monitoring the pressure change inside the formed package through an encapsulated metal Pirani gauge  30 . See  FIGS. 4A-4D . The Pirani gauge  30  is a free standing device representing the typical surface-micromachined metal structure(s) that may be positioned inside the encapsulation structures contemplated herein. Moreover, the Pirani gauge  30  can read vacuum level in situ.  
         [0048]     Referring now to  FIGS. 4A-4D , a schematic illustration of a process for encapsulating a Pirani gauge  30  in an encapsulation package  40  is shown. After the fabrication of a metal Pirani gauge  30 , a 4 μm photoresist sacrificial layer  14  was deposited and patterned to define the gap between the Pirani gauge  30  and the alumina thin-film layer  16 . The photoresist  14  was hard baked at 120° C. in an oven for 20 minutes to reduce the outgassing during the subsequent processes, followed by an O 2  plasma etching for 2 minutes to roughen the surface for the purpose of improving the adhesion of subsequently deposited metal layers. The thin-film alumina cap  16  above the photoresist sacrificial layer  14  consisted of sputtered 3200 Å Ti adhesion layer  22 , evaporated 100 Å Au layer  24 , and 15000 Å aluminum layer  16 . The anodization etching of the aluminum layer  16  was performed on a 2 cm by 2 cm chip as shown in  FIG. 4B . The current compliance was set below 100 mA to reduce the amount of gas bubbles generated at the end of anodization etching process. Next, by using a photoresist mask (not shown), the pores within the thin-film alumina layer  16  over the cavity area were widened by subjecting the sample to a wet etching process using 5 wt % H 3 PO 4  etching solution (25 minute exposure). The Ti layer  22  and Au layer  24  (e.g., seed layers) were also removed through the now formed pores. The photoresist mask (not shown), along with the photoresist sacrificial layer  14  below the porous alumina layer  16 , was removed by O 2  plasma etching as shown in  FIG. 4C . Afterwards, the a-Si sacrificial layer  32  located underneath the Pirani gauge  30  was removed by XeF 2  plasma dry etching.  
         [0049]     Vacuum sealing of the package  40  was performed by depositing a sealing layer  34 . In a preferred aspect of the invention, the sealing layer  34  is substantially impermeable to fluids (e.g., liquids and gases). In this case, the sealing layer  34  was a PECVD low stress nitride layer of 2.5 μm thickness at 300° C. It should be understood, however, that the entire packaging process may be carried out at or below 300° C. For example, the entire packaging process may be carried out at or around room temperature if a room temperature sacrificial layer  14 ,  32  and sealing layer  34  are used. As seen in  FIG. 4D , the contact pads  36  for electric access to the Pirani gauge  30  were opened.  
         [0050]     The package  40  containing the Pirani gauge  30  was then intentionally ruptured to expose the free standing Pirani gauge  30  inside in addition to each layer of the structural layer  16 . The Pirani gauge  30  was observed to be free standing in SEM images. See He and Kim, “A Low Temperature Vacuum Package Utilizing Porous Alumina Thin Film Encapsulation,” IEEE Conference on Micro Electro Mechanical Systems held in Istanbul Turkey, January, 2006, which is incorporated by reference as if set forth fully herein.  
         [0051]     The pressure inside the sealed package  40  was obtained by matching the resistance-current curve of the sealed Pirani gauge  30  with the resistance-current curves of the Pirani gauge  30  calibrated at different known pressures.  FIG. 5  illustrates the resistance-current curves of the Pirani gauge  30 . The pressure inside the sealed package  40  was found to be around 8 Torr, a value larger than the deposition pressure of 0.5 Torr. This discrepancy is likely due to the outgassing of the photoresist  14  residue inside the package  40 . The hermeticity of the packages  40  was measured from the thermal impedance changes of two sealed Pirani gauges  30 . As displayed in  FIG. 6 , the pressure inside the sealed packages increased slightly (0.4 Torr) over the first 10 days, followed by no noticeable change for the next several days.  
         [0052]     Measurements were performed on test packages  40  to measure the extent to which the material of the sealing layer  34  was present inside the structural layer  16 . Substantial encroachment of the sealing layer  34  material inside the package  40  would have been likely if lithographically-defined etch holes in the structural shell  16  were to be sealed. The test packages  40  had porous alumina cavities (1.5 μm thick) formed on a bare silicon substrate. After a 5000 Å PECVD oxide deposition layer was formed, the cavity was ruptured using a probe tip and the thickness of oxide on top of the silicon substrate inside the cavity was measured by a NANOSPEC® thin-film measurement system (available from NANOMETRICS, INC., Milpitas, Calif.) using a thin oxide program (low limit: 20 Å). For all the tested packages  40 , a “less than 20 Å” result was obtained, indicating the porous alumina shell  16  effectively prevented the internal deposition of the sealing material  34  during the sealing process.  
         [0053]     To investigate the RF performance of the porous alumina thin-film package  40 , a CPW (Coplanar Waveguide) line (Cr/Au: 250/8000 Å) was packaged on a silicon substrate  12  with high resistivity (&gt;2000 Ω*cm). Following a similar fabrication process as that shown in  FIGS. 4A-4D , the porous alumina cavity was formed by removing the Ti/Au layers and the a-Si sacrificial layer sequentially. A PECVD deposition of 1 μm low stress silicon nitride sealed the cavity. The final step was etching away all the films above Au in the electrical contact area.  
         [0054]     Cross-sectional and optical microscopic views of the fabricated RF device  50  are shown in  FIGS. 7A-7C . The sealed cavity  52  is shown generally in the middle of the microscopic view shown in  FIG. 7A , measuring 160 μm by 300 μm, a typical size of a RF switch device. The Au signal lines  54  encapsulated inside the sealed cavity  52  is visible through the transparent structural shell, which is composed of a 1.2 μm thick porous alumina layer  16  and a 1 μm thick silicon nitride sealing film  34 .  
         [0055]     The insertion loss introduced by the packaging structure was extracted from the difference between the measured insertion loss of a packaged CPW line and an un-packaged CPW line. As seen in  FIG. 8 , the small difference in insertion loss (less than 0.1 dB up to 40 GHz) demonstrates that the encapsulation structure has a very small influence on the performance of the RF device  50 . The small amount of insertion loss that was measured was likely due to the silicon sacrificial layer left in the feed-through area  56  as seen in  FIG. 7C . The insertion loss of the RF device  50  may, however, be reduced by removing the sacrificial layer in the feed-through area  56  by adding an additional lithography and etching step.  
         [0056]     According to another embodiment of the invention, a free standing microstructure  70  is formed using porous polysilicon. Alternatively, the free standing encapsulation structure  70  may be formed from single crystal silicon.  FIGS. 9A-9D  illustrate a process for forming such a structure  70 . As seen in  FIG. 9A , a substrate  72  in the form of a silicon wafer was provided. The substrate  72  was then covered with a low-stress nitride (Si 3 N 4 ) layer  74  having a thickness of 0.6 μm. A sacrificial phosphosilicate glass (PSG) layer  76  having a thickness of 1.5 μm was deposited and patterned on the nitride layer  74 . In order to create an electrical contact between the silicon substrate  72  and the later-deposited polysilicon layer for electrochemical etching, openings  78  were made through the silicon nitride layer  74  to the silicon substrate.  
         [0057]     In certain embodiments, the thin-film structural layer  16 ,  70  may be formed from polysilicon or aluminum. In still other embodiments, the thin-film structural layer  16  may be formed using type III-V materials. In particular, the material may include compounds formed with at least one group III element and at least one group V element. These include, by way of example, gallium phosphide (GaP), gallium arsenide (GaAs), indium arsenide (InAs), gallium antimonide (GaSb), and indium antimonide (InSb).  
         [0058]     As best seen in  FIG. 9B , a 1.5 μm undoped polysilicon layer  80  was then deposited by LPCVD, followed by a 2000 Å PSG deposition layer  82 . The polysilicon  80  was symmetrically doped to 0.02 Ω*cm from the PSG layers  82  and  76  by annealing at 1000° C. for about 1 hour in nitrogen. The annealing step also helped release the intrinsic stress in the polysilicon layer  80 . Next, all the thin films deposited on the backside of the wafer (not shown) were etched away by reactive ion etching (RIE) to expose the silicon backside surface for electrical contact with the anode in an electrochemical etching device (described below).  
         [0059]     After dicing the wafer into 1 cm×1 cm dice, a photoresist mask layer  84  ( FIG. 9C ) (NR9-8000® negative photoresist) was patterned to define the area for electrochemical etching before each die was mounted in a custom-built TEFLON® cell for electrochemical etching. Details of the electrochemical etching device may be found in the publication entitled “Post-Deposition Porous Etching of Polysilicon: Fabrication and Characterization of Free-Standing Structures,” presented in the ASME International Mechanical Engineering Congress and Exposition in Anaheim, Calif., November 2004, which is incorporated by reference as if set forth fully herein.  FIG. 10  schematically illustrates the set up used for electrochemical etching of the polysilicon layer  80 . Liquid In—Ga was then painted on the backside of the sample to provide good electrical contact between the sample and the copper jig in the TEFLON® cell. The electrochemical etching was performed in the dark at room temperature in an electrochemical etching solution comprising 49% HF:ethanol in a 1:1 ratio (on a volume basis).  FIG. 9C  illustrates the formation of the porous polysilicon layer  80  after initiation of electrochemical etching. Once the pores are formed in the polysilicon layer  80 , the electrochemical etching solution reaches the interface of the now porous polysilicon layer  80  and the sacrificial phosphosilicate glass (PSG) layer  76 . The HF:ethanol etching solution then continues to attack or react with the underlying sacrificial phosphosilicate glass (PSG) layer  76  until the free standing porous polysilicon structure is formed as illustrated in  FIG. 9D . In order to alleviate the stiction of the free-standing porous polysilicon layer with the layer  74  when the liquid is evaporated, the device  70  may be dried in supercritical CO 2 .  
         [0060]     After 200 seconds of electrochemical etching at 4 mA/cm 2  the porous region in the upper part of the polysilicon layer  80  was visually distinguishable from the solid region underneath. After 250 seconds of electrochemical etching, many trenches were present in the PSG sacrificial layer  76  located right underneath the polysilicon layer  80 , signifying that the polysilicon layer  80  was turned porous through its entire thickness and thus HF in the electrochemical solution diffused through the porous polysilicon  80  to attack the PSG layer  76 . An irregular etching pattern in the PSG layer  76  was observed. This indicated that pore growth inside the polysilicon layer  80  was not uniform along the thickness direction. It was hypothesized that the electrochemical etching current flows mainly along the polysilicon grain boundaries, resulting in preferential etching and thus a higher pore growth rate at the grain boundaries.  
         [0061]     The electrochemical etching current was carefully adjusted to prevent the occurrence of electropolishing in the polysilicon layer  80  under the edge of the photoresist mask  84 . In electrochemical etching, when the current density is higher than that of the first peak in the current-potential curve, electropolishing will take place instead of pore formation. However, higher current density and hence higher pore growth rate is preferred in this process in order to prevent the photoresist mask  84  from peeling off in the HF-ethanol electrochemical etching solution and to minimize etching undercut. Generally, a high-enough current density to keep the photoresist mask  84  intact during electrochemical etching but low-enough to prevent the lateral electropolishing was found when the current density was around 4 mA/cm 2 . After 255 seconds of etching, no electropolishing was observed, while the partly etched PSG layer  76  indicated that pores are formed through the entire thickness of the polysilicon  80  in the unmasked area.  
         [0062]     Once the free standing encapsulation structure  70  was released, wrinkles and cracks were observed on most of the porous polysilicon structures  80 , indicating the presence of high compressive stress in the layer  80 . Prior to introduction of the pores, the polysilicon layer  80  was in a low stress condition. It is believed that the compressive stress was introduced in the porous polysilicon layer  80  due to large amount of H 2  generated during the electrochemical etching process. Excessive hydrogen atoms tend to bond to silicon atoms, resulting in a lattice expansion of the Si—Si bond length and thus introducing the compressive stress in porous silicon layer  80 . Although the hydrogen can be desorbed from the Si—H bond by annealing at medium temperature (e.g., above 400° C.), a challenge was presented because the porous polysilicon layer  80  starts to free-stand as a membrane or the like soon after the electrochemical etching process is complete, i.e., before annealing can be applied.  
         [0063]     In this process, the time window for annealing is thus after the electrochemical etching front reaches the interface of the polysilicon layer  80  and the sacrificial PSG layer  76  and before the HF-based etching solution attacks PSG layer  76  enough to free the polysilicon layer  80  into a free standing structure. It was found that this operating window (i.e., when the pores reached the interface) can be determined by the observation of a sharp increase in electrode potential during the electrochemical etching step.  
         [0064]      FIG. 11  illustrates a typical graph of electrode potential versus time for electrochemical etching at a constant current (in this case 4 mA/cm 2 ). The circled portion of  FIG. 11  (identified by the arrow) illustrates that the etching front has reached the interface of the porous polysilicon layer  80  and the sacrificial PSG layer  76 . As seen in  FIG. 11 , the electrode potential gradually increases and reaches a relatively constant value. However, when the measured potential (mV) increased sharply, this spike coincided with the moment when the porous etching front reached the interface of the porous polysilicon layer  80  and the sacrificial PSG layer  76 .  
         [0065]     After the free standing encapsulation structure  70  was taken out of the etching setup and thoroughly cleaned, annealing was then performed using a rapid thermal annealing (RTA) process. In particular, the device  70  was quickly heated at 700° C. for 5 minutes in a nitrogen environment. The effect of the annealing process was noticeable. Porous polysilicon membranes  80  that were not subject to the annealing process were formed with thicknesses of only 100 μm in size. In contrast, porous polysilicon membranes  80  subject to the annealing process were formed with thicknesses as large as 600 μm without any cracks.  
         [0066]     Because the HF-based electrochemical etching solution etches the sacrificial PSG layer  76  quickly after diffusing through the porous polysilicon layer  80 , the window for annealing is small and accurate determination of this point is needed to stop the etching process. While the process was successful using one die at a time, it may not be as practical for an entire wafer under production conditions. However, a barrier layer (not shown) resistant to the electrolyte (e.g., silicon nitride) may be placed between the polysilicon layer  80  and the PSG layer  76  to solve the problem. The barrier layer can be later removed during the device release process.  
         [0067]     With reference to  FIGS. 12A-12H , the thin-film encapsulation process was used to seal a micro Pirani gauge  90  that not only measures the vacuum pressure but also represents a free standing polysilicon microstructure. With reference to  FIG. 12A , the fabrication process started with a 5000 Å low-stress nitride deposition layer  92  as the insulation layer deposited on a substrate  94 , followed by an LPCVD deposition of 1.5 μm a PSG layer  96 , which was then patterned as the sacrificial layer between the Pirani bridge gauge  90  and the substrate  94 . Next, a 1 μm in situ doped polysilicon layer  98  was deposited by LPCVD and patterned to define the Pirani bridge structure  100  (see  FIG. 12H ).  
         [0068]     With reference to  FIG. 12B , a 5 μm PSG sacrificial layer  102  was then formed by two LPCVD depositions. Each deposition was followed by a 1 hour 1000° C. annealing process in the presence of Nitrogen to densify the PSG sacrificial layer  102 . The relatively thick PSG sacrificial layer  102  was patterned, and openings  104  were made through the nitride layer  92  to the silicon substrate  94  in order to allow for an electrical path between the polysilicon encapsulation layer (described in more detail below) and silicon substrate  94  for electrochemical etching. With reference to  FIG. 12C , a layer of 1.5 μm thick undoped LPCVD polysilicon was then deposited to form an encapsulation layer  106 , followed by a 3000 Å LPCVD PSG deposition layer  108 . The last polysilicon layer was also symmetrically doped to a resistivity of 0.02 Ω*cm from the PSG layers  102 ,  108  on both sides by annealing at 1000° C. in nitrogen.  
         [0069]     With reference to  FIGS. 12C and 12D , the top PSG layer  108  was then stripped off in buffered oxide etchant (BOE), and the insulating layers on the backside were removed by RIE (not shown). After the substrate  94  (e.g., wafer) was diced, each die was processed with a NR9-8000® negative photoresist to define the area of the polysilicon layer  106  into a porous polysilicon encapsulation layer  106 . The die was mounted in a TEFLON® cell for the electrochemical etching as describe herein. After stopping the electrochemical etching at the interface of the polysilicon layer  106  and the PSG layer  108  by monitoring the electrode potential as a function of time as described previously the sample was then taken out and cleaned in Piranha solution resulting in the structure illustrated in  FIG. 12D . Next, a short RTA annealing (700° C. for 5 minutes) was performed to release the stress generated during the electrochemical etching process.  
         [0070]     Then, as illustrated in  FIG. 12E , the PSG sacrificial layers  108  were removed by concentrated 49% HF, which obviously diffused through the pores in the 1.5 μm-thick porous polysilicon layer. The release time was approximately one minute regardless of the size of the cavity. On the electrical feedthrough line, the remaining PSG under the polysilicon layer  106  was used to isolate the feedthrough line from the polysilicon shell. The device was designed so that enough PSG is left by time-controlled etching.  
         [0071]     The sample was then thoroughly rinsed in DI water and methanol, followed by a supercritical CO 2  drying step. Next, as seen in  FIG. 12F , the device  90  was sealed in a vacuum by depositing a sealing layer  110  of polysilicon in LPCVD with a deposition pressure of 179 mTorr and a deposition temperature at 600° C. After sealing, as seen in  FIG. 12G , the electrical contact pads  109  were opened outside the cavity by etching away the polysilicon layers  106  and  110  in RIE and the PSG layer  102  in BOE. As seen in  FIG. 12H , a 1000 Å gold evaporation layer  112  was formed on the exposed polysilicon feedthrough lines  98  necessary for wire bonding and completed the fabrication process.  
         [0072]      FIGS. 13A-13F  illustrate SEM pictures of a polysilicon Pirani gauge  90  encapsulated by a porous polysilicon layer  106  or shell.  FIG. 13A  illustrates an open encapsulation shell  106  that was intentionally clipped to expose the Pirani gauge  90  inside the cavity.  FIG. 13B  illustrates the serpentine Pirani gauge structure suspended above the substrate by approximately 1 μm and free from the polysilicon shell  106 . The encapsulation shell  106 , composed of solid polysilicon sealing layer  110  on top of the porous polysilicon layer  106 , is shown magnified in  FIG. 13C . The porous and solid regions of the polysilicon layers  106 ,  110 , defined by the photoresist mask in the electrochemical etching step, are clearly distinguishable in  FIG. 13D . Pore size of the porous polysilicon layer  106  is estimated to be around 5 nm from  FIG. 13E .  FIG. 13F  illustrates an SEM cross-sectional image of the interface between the polysilicon sealing layer  110  and the porous polysilicon layer  106 . The transition appears abrupt, which suggests that penetration of the polysilicon sealing layer  110  into the pores is minimal.  
         [0073]     The pressure inside the sealed cavity was measured from the encapsulated Pirani gauge  90 . The resistance vs. current characteristics of a Pirani gauge  90  was first obtained while vacuum encapsulated. Without affecting the performance of the Pirani gauge  90 , the seal on the top empty cavity was then broken intentionally with a probe tip. The entire sample was then placed in a pressure-controlled chamber, where the Pirani gauge  90  was calibrated against known pressures. The pressure inside the sealed cavity, extracted by matching the resistance of the Pirani gauge  90  while sealed with the calibration data obtained above, was around 200 mTorr.  FIG. 14  illustrates the resistance vs. current characteristics of the Pirani gauge  90  after the cavity was broken. The extracted pressure of 200 mTorr was consistent with the deposition pressure of the sealing polysilicon thin film (179 mTorr). The residual gas inside the cavity could be H 2  byproduct produced during the polysilicon deposition or from the outgassing of the remaining PSG plug in the feed-through channel.  
         [0074]     A better interpretation of the data plotted in  FIG. 14  is to transfer the resistance vs. pressure curve into a curve of thermal impedance vs. pressure. It was found that even though the resistance of the Pirani gauge  90  drifts over time, the thermal impedance remains relatively constant at a given ambient pressure. The thermal impedance (T.I.) is defined as  
           T   .   I   .     =       T   avg       P   E         ,     
     ⁢       T   avg     =       1   ξ     ⁢     (         R   b       R   0       -   1     )             
 
         [0075]     where P E  is the electrical power, T avg  is the average temperature across the Pirani gauge, ξ is the temperature coefficient of resistance (1000 ppm/° C. for polysilicon), R b  and R 0  are the resistances of the microbridge at a given pressure and ambient pressure, respectively. After the thermal impedance of the Pirani gauge  90  was extracted by a linear curve fit applied to the power vs. temperature data measured at each calibration pressure, the data was plotted as shown in  FIG. 15 . The long-term hermeticity was monitored by reading the thermal impedance change of the Pirani gauge  90  over time. The thermal impedance changes of two sealed Pirani gauges  90  with different gauge dimensions over one year are shown in  FIG. 16 . The result shows no noticeable pressure change (&lt;30 mTorr) for a period of time in excess of one year.  
         [0076]     To demonstrate the usefulness of this technique for common surface micromachining processes, the Multi-User MEMS Processes, or MUMPS, was selected to fabricate a micro-bridge device  120  encapsulated by the porous polysilicon shell  122 . MUMPS is a popular commercial foundry service that provides cost-effective, proof-of-concept MEMS fabrication. One of the standard processes in the MUMPS program is PolyMUMPs, a three-layer polysilicon surface micromachining process, whose thickness data is listed in Table 1 below.  
                                         TABLE 1                                   Material layer   Thickness (μm)                                        Nitride   0.6           Poly 0   0.5           First Oxide   2.0           Poly 1   2.0           Second Oxide   0.75           Poly 2   1.5           Metal   0.5                      
 
         [0077]      FIGS. 17A and 17B  schematically represent a fabrication process for integration with MUMPS. Poly 1  and Poly 1  layers were used to construct the microbridge resonator  120  inside the Poly 2  shell  122 . Supporting posts  124  were designed to reinforce the polysilicon shell  122  of large size. A sacrificial oxide was used to isolate the polysilicon shell  122  from the electrical feedthrough  126 . The post process on the MUMPS chip started in-house with the removal of all the layers on the backside by RIE, a step necessary to create the electrical contact to the Poly 2  layer through the substrate for electrochemical etching. Using photoresist as a mask, part of the Poly 2  encapsulation layer was turned porous by electrochemical etching. The bridge structure  120  was then released in one minute in concentrated 49% HF, followed by rinsing and supercritical CO 2  drying.  
         [0078]     While embodiments of the present invention have been shown and described, various modifications may be made without departing from the scope of the present invention. The invention, therefore, should not be limited, except to the following claims, and their equivalents.