Tunable cavity resonator and method of fabricating same

Abstract

A tunable cavity resonator for filtering radiation in the optical and IR wavelengths and a method for fabricating same. The resonator having a pair of reflectors, one in fixed relationship to a substrate and the other formed upon a suspended moveable membrane disposed a cavity length from the one reflector. The resonator also including a pair of spaced apart electrodes either constituted by the reflectors or juxtaposed therewith, which are electrostatically operable to move the membrane and other reflector relative to the one reflector. A first reflector layer is deposited on the substrate to form the one reflector. A sacrificial layer having a high etch selectivity for releasing the membrane in a suspended and spaced relationship from the one reflector is formed on the first reflector layer. The membrane is deposited on the sacrificial layer using a deposition technique characterised by providing the required intrinsic stress in the membrane. A second reflector layer is formed on the membrane to form the other reflector. The second reflector layer is patterned in accordance with a prescribed membrane geometry and then etched to achieve the same. The sacrificial layer is then initially etched to remove regions thereof down to the first reflector layer on the substrate exposed by the etching. Those regions of the sacrificial layer that are intended to function as the residual support structure of the membrane are then protected with photoresist and the remaining unprotected regions of the sacrificial layer are finally etched, removing the protection from the support structures to suspend the membrane in substantially parallel relation to the first reflector layer.

1 . A method for fabricating a tunable cavity resonator having a pair of reflectors, one being disposed in fixed relationship to a substrate material and the other being a suspended moveable membrane disposed a cavity length from the one reflector, and a pair of electrodes either being constituted by the reflectors or being juxtaposed therewith, one electrode with the one reflector and the other electrode with the other reflector, the method comprising: 
 depositing a first reflector layer on a substrate to form the one reflector of the cavity resonator;    forming a sacrificial layer of a prescribed material having a high etch selectivity for releasing the membrane in a suspended and spaced relationship from the one reflector;    forming the membrane on the sacrificial layer using a deposition technique characterised by providing the required intrinsic stress in the membrane;    depositing a second reflector layer on the membrane to form the other reflector;    patterning the second reflector layer in accordance with a prescribed membrane geometry;    etching the second reflector layer and the membrane to achieve said prescribed membrane geometry; and    etching the sacrificial layer to to release the membrane and suspend it in substantially parallel relation to the first reflector layer;    wherein all of the aforementioned steps are processes are performed at a low enough temperature and for a short enough period of time to maintain the form and integrity of the devices in the underlying substrate.    
     
     
         2 . A method as claimed in  claim 1 , including the step of forming recesses for support locations of the membrane in the sacrificial layer down to the substrate, after forming the sacrificial layer and prior to forming the membrane thereon; 
 extending the membrane layer to cover the sacrificial layer and the exposed substrate within the recesses during the forming of the membrane;    extending the second reflector layer to cover the membrane layer within the recesses during the forming of the second reflector layer; and    etching the sacrificial layer to remove it from around the membrane, so that the membrane and reflector layers are disposed with the recesses are revealed to constitute the support structure for suspending the membrane and the second reflector above the substrate.    
     
     
         3 . A method as claimed in  claim 1 , including initially etching the sacrificial layer to remove regions thereof down to said first reflector layer on the substrate exposed by said etching.  
     
     
         4 . A method as claimed in  claim 3 , including protecting those regions of the sacrificial layer intended to function as the residual support structure of the membrane.  
     
     
         5 . A method as claimed in any one of the preceding  claim 1 , including finally etching the remaining unprotected regions of the sacrificial layer to release the membrane and suspend it by the support structure in substantially parallel relation to the first reflector layer.  
     
     
         6 . A method as claimed in  claim 1 , including forming the substrate material as a semiconductor system that provides access to optical wavelengths necessary for resonance purposes in the resonant cavity.  
     
     
         7 . A method as claimed in  claim 6 , including forming the sacrificial layer of a thickness such that the resultant cavity length so formed allows filtering of radiation having optical wavelengths in the infrared region.  
     
     
         8 . A method as claimed in  claim 1 , including forming the suspended moveable membrane with sufficient resiliency so that it may be displaced to an extent that is commensurate to the full cavity length, but such displacement is controllable to be marginally less than the full cavity length to avoid the membrane contacting the one reflector.  
     
     
         9 . A method as claimed in  claim 1 , including forming the membrane of silicon nitride.  
     
     
         10 . A method as claimed in  claim 1 , including forming the sacrificial layer of zinc sulphide.  
     
     
         11 . A method as claimed in  claim 1 , including forming the substrate from an infrared sensitive material.  
     
     
         12 . A method as claimed in  claim 11 , including forming the substrate of mercury cadmium telluride (MCT).  
     
     
         13 . A method as claimed in  claim 1 , including forming the membrane using PECVD.  
     
     
         14 . A method as claimed in  claim 1 , including forming the electrodes separately of the reflective layers.  
     
     
         15 . A method as claimed in  claim 1 , including forming the reflective layers to function as electrodes.  
     
     
         16 . A method as claimed in  claim 1 , including etching the second reflector layer using an anisotropic etching process.  
     
     
         17 . A method as claimed in  claim 16 , including dry etching the second reflector layer.  
     
     
         18 . A method as claimed in  claim 17 , including plasma etching the second reflector layer.  
     
     
         19 . A method as claimed in  claim 18 , including reactive ion etching the second reflector layer.  
     
     
         20 . A method as claimed in  claim 1 , including initially etching the sacrificial layer using an isotropic etching process.  
     
     
         21 . A method as claimed in  claim 20 , including initially dry etching the sacrificial layer.  
     
     
         22 . A method as claimed in  claim 1 , including protecting the support structures with photoresist.  
     
     
         23 . A method as claimed in  claim 1 , including finally etching the remaining unprotected regions of the sacrificial layer using an isotropic etching process.  
     
     
         24 . A method as claimed in  claim 23 , including wet etching the remaining unprotected regions of the sacrificial layer.  
     
     
         25 . A tunable cavity resonator fabricated according to the method as claimed in  claim 1 .  
     
     
         26 . A tunable cavity resonator comprising: 
 a substrate material;    a moveable membrane disposed in substantially parallel relationship to said substrate and suspended relative thereto at the periphery of the membrane by a support structure;    a pair of reflectors, one being a first reflector layer disposed in fixed relationship upon the substrate material and the other being a second reflector layer disposed on the suspended deformable membrane to form a resonant cavity, the reflectors being disposed a cavity length from each other;    the membrane and one reflector being shaped in accordance with a prescribed membrane geometry; and    a pair of electrodes either being constituted by the reflectors or being juxtaposed therewith, one electrode with the one reflector and the other electrode with the other reflector;    wherein the suspended moveable membrane is of substantially uniform thickness and has an intrinsic stress to permit electrostatic displacement of the membrane over distances for tuning in the infrared band using voltages applied to the electrodes commensurate with read out integrated circuit electronics associated with the resonator.    
     
     
         27 . A resonator as claimed in  claim 26 , wherein said substrate material is a semiconductor system that provides access to the optical wavelengths necessary for resonance purposes in the resonant cavity.  
     
     
         28 . A resonator as claimed in  claim 27 , wherein said cavity length corresponds to optical wavelengths in the infrared region.  
     
     
         29 . A resonator as claimed in  claim 26 , wherein the displacement of the suspended moveable membrane can be up to the full cavity length, which is correspondingly larger to accommodate such displacement without the membrane contacting the one reflector.  
     
     
         30 . A resonator as claimed in  claim 26 , wherein said membrane is formed of silicon nitride.  
     
     
         31 . A resonator as claimed in  claim 26 , wherein said support structures are formed of zinc sulphide.  
     
     
         32 . A resonator as claimed in  claim 26 , wherein said substrate is formed from an infrared sensitive material.  
     
     
         33 . A resonator as claimed in  claim 32 , wherein substrate is formed of mercury cadmium telluride (MCT).  
     
     
         34 . A resonator as claimed  claim 26 , wherein said electrodes are formed separately of the reflective layers.  
     
     
         35 . A resonator as claimed in  claim 26 , wherein reflective layers are formed to function as electrodes.  
     
     
         36 . A method for fabricating a tunable cavity resonator for filtering incident radiation with longer photonic wavelengths from applications where the wavelengths may be in the order of less than 1 micron to applications where the wavelengths may be greater than 20 microns, the cavity resonator having a pair of reflectors, one being disposed in fixed relationship to a substrate material and the other being disposed on a moveable membrane suspended by a support structure and disposed a cavity length from the one reflector, to form a Fabry Perot (FP) cavity, and a pair of electrodes either being constituted by the reflectors or being juxtaposed therewith, one electrode with the one reflector and the other electrode with the other reflector, to control the movement of the membrane in response to a prescribed dc voltage applied across electrodes, the method comprising: 
 forming the membrane on a sacrificial layer using a plasma deposition technique at a sufficiently low temperature with a gas flow ratio and prescribed RF frequency and power to excite the plasma so that the resultant stress in the membrane when released from the sacrificial layer and suspended by the support structure is controllable and very low, and the pinhole density in the membrane is sufficiently low to provide the membrane with the requisite integrity and morphology to exhibit a near-ideal Fabry Perot response.    
     
     
         37 . A method as claimed in  claim 36 , including fabricating the cavity resonator to function as a filter applicable for detecting incident radiation of wavelengths in ranges, whereby the wavelength of resonance can be in the region of 100 nm to 1,000,000 nm.  
     
     
         38 . A method as claimed in  claim 36 , including forming the membrane of silicon nitride.  
     
     
         39 . A method as claimed in  claim 36 , including forming the sacrificial layer of zinc sulphide.  
     
     
         40 . A method as claimed in  claim 36 , including forming the substrate from an infrared sensitive material.  
     
     
         41 . A method as claimed in  claim 40 , including forming the substrate of mercury cadmium telluride (MCT).  
     
     
         42 . A method as claimed in  claim 36 , including forming the membrane using PECVD.  
     
     
         43 . A method as claimed in  claim 36 , including matching the temperature of the deposition to the tolerance of the materials constituting the sacrificial layer and the substrate.  
     
     
         44 . A method as claimed in  claim 36 , including selecting the gas flow ratio at the desired deposition temperature to achieve the requisite intrinsic stress at a prescribed RF frequency and power that still provides the membrane with the requisite integrity and morphology to maintain a high yield process.  
     
     
         45 . A method as claimed in  claim 44 , including setting the gas flow ratio to comprise an appropriate ratio of silane to ammonia to a diluting gas so as to achieve the required level of intrinsic stress that can result in virtually zero stress in the released membrane.  
     
     
         46 . A method as claimed in  claim 36 , including forming the electrodes separately of the reflective layers.  
     
     
         47 . A method as claimed in  claim 36 , including forming the reflective layers to function as electrodes.  
     
     
         48 . A method as claimed in  claim 36 , including etching the second reflector layer using an anisotropic etching process.  
     
     
         49 . A method as claimed in  claim 48 , including dry etching the second reflector layer.  
     
     
         50 . A method as claimed in  claim 49 , including plasma etching the second reflector layer.  
     
     
         51 . A method as claimed in  claim 50 , including reactive ion etching the second reflector layer.  
     
     
         52 . A method as claimed in  claim 36 , including etching the sacrificial layer to release the membrane and suspend it by the support structure in substantially parallel relationship to the first reflector layer.  
     
     
         53 . A method as claimed in  claim 52 , including the step of forming recesses for support structure locations of in the sacrificial layer down to the substrate, after forming the sacrificial layer and prior to forming the membrane thereon; 
 extending the membrane layer to cover the sacrificial layer and the exposed substrate within the recesses during the forming of the membrane;    extending the second reflector layer to cover the membrane layer within the recesses during the forming of the second reflector layer; and    etching the sacrificial layer to remove it from around the membrane, so that the membrane and reflector layers are disposed with the recesses are revealed to constitute the support structure for suspending the membrane and the second reflector above the substrate.    
     
     
         54 . A method as claimed in  claim 36 , including initially etching the sacrificial layer using an isotropic etching process.  
     
     
         55 . A method as claimed in  claim 54 , including initially dry etching the sacrificial layer to form the support structure.  
     
     
         56 . A method as claimed in  claim 55 , including protecting the support structure with photoresist.  
     
     
         57 . A method as claimed in  claim 36 , including finally etching the remaining unprotected regions of the sacrificial layer using an isotropic etching process to release the membrane and suspend it by the support structure.  
     
     
         58 . A method as claimed in  claim 57 , including wet etching the remaining unprotected regions of the sacrificial layer.  
     
     
         59 . (canceled)  
     
     
         60 . A tunable cavity resonator fabricated according to the method as claimed in  claim 36.