Abstract:
A method is disclosed for making shaped optical moems components with stressed thin films. In particular, stressed thin films are used to make mirror structures.

Description:
BACKGROUND 
     Passive optical components can play an important role in the refinement and optimization of an optical signal in the MEMS/MOEMS (micro-electromechanical systems/micro-opto-electromechanical) regime. Passive optical devices are often used to control the qualitative properties of light in printing, laser scanning operations or data communications where optical signals are modulated and optical mode quality is integral to system performance. Hence, there is a need to provide passive optical devices for use in optical MEMS/MOEMS systems. 
     SUMMARY 
     Stress control in MEMS (micro-electromechanical systems) is important since uncontrolled stress may cause a MEMS component to bow or buckle. However, the ability to control stress in a MEMS context can be used to desirable effect. Stress gradient materials may be used to make three dimensional structures utilizing controlled stress release. Controlled stress in thin films can be used to accurately shape the optical surface of MEMS components. For example, tensile or stress gradient materials can be used to make cylindrical and spherical MEMS mirrors as well as tunable MEMS blaze gratings for use in the MEMS/MOEMS regime. Applications include the areas of optical communications, beam scanning and optical spectroscopy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows an embodiment of a cylindrical MEMS mirror in accordance with the invention. 
     FIG. 1 b  shows an embodiment of a cylindrical MEMS mirror in accordance with the invention. 
     FIG. 2 shows an embodiment of a cylindrical MEMS mirror in accordance with the invention. 
     FIG. 3 a  shows a metal pattern for a spherical MEMS mirror in an embodiment in accordance with the invention. 
     FIG. 3 b  shows an embodiment of a spherical MEMS mirror in accordance with the invention. 
     FIG. 4 shows an embodiment of a MEMS blaze grating in accordance with the invention. 
     FIGS. 5 a - 5   f  show the steps for making an embodiment of a cylindrical MEMS mirror in accordance with the invention. 
     FIG. 6 a  shows the mask used in the step shown in FIG. 5 b.    
     FIG. 6 b  shows the lift-off mask put down in the step shown in FIG. 5 d    
     FIGS. 7 a - 7   e  show the steps for making an embodiment of a spherical MEMS mirror in accordance with the invention. 
     FIG. 8 a  shows the lift-off mask put down in the step shown in FIG. 7 b.    
     FIG. 8 b  shows the lift-off mask put down in the step shown in FIG. 7 d.    
     FIGS. 9 a - 9   f  show the steps for making an embodiment of a MEMS blazed grating in accordance with the invention. 
     FIG. 10 shows the lift-off mask put down in the step shown in FIG. 9 d.    
     FIGS. 11 a - 11   e  shows the steps for fabrication of a spherical mirror in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Cylindrical reflection mirrors can be used for focusing diffused light into a line for applications such as optical communications. FIGS. 1 a  and  1   b  show an embodiment in accordance with the invention of a MEMS structure for cylindrical reflection mirror  100 . Conventional surface MEMS design and fabrication including polysilicon deposition and etching or silicon-on-insulator wafer material together with conventional lithography steps for pattern definition may be used for making cylindrical reflection mirror  100 . 
     Stress gradient layer  110  typically has a thickness of from about 500 nm to 1000 nm and is typically made of MoCr which is deposited as described in Table 1 below. Stress gradient layer  110  is deposited on top of structural layer  530  which is, for example, either polysilicon or a single crystal device layer if silicon-on-insulator wafer material is used. Stress gradient layer  110  has a built in stress gradient in the thickness direction varying from compressive on one side of layer  110  to tensile on the other side of layer  110  next to structural layer  530 . The stress gradient can be as large as 3.0 Gpa or more. After structure layer  530  is released from substrate  510  (see FIG.  5 ), the stress gradient in stress gradient layer  110  causes released structure layer  530  to bend. 
     Reinforcing beams  130  parallel to a common axis and spaced about 40 μm apart are present underneath structure layer  530  to prevent cylindrical reflection mirror  100  from bending in the direction perpendicular to the common axis. Typical dimensions for reinforcing beams  130  are a width of about 10 μm and a height of no more than about 5 μm. Typical dimensions for cylindrical reflection mirror  100  are about 200 mm by 250 mm. 
     Reflective layer  140 , typically of aluminum or gold, is deposited on top of stress gradient layer  110  to a thickness of about 200-500 nm by either thermal deposition or RF sputtering techniques in order to enhance the optical reflection characteristics of cylindrical reflection mirror  100 . Cylindrical reflection mirror  100  flatness is achieved by chemical and mechanical polishing structural layer  530  prior to deposition of stress gradient layer  110  and reflective layer  140 . Note that polishing is not needed if an SOI wafer is used. The curvature of cylindrical reflection mirror  100  is determined by the stress gradient in stress gradient layer  110  and the thickness of structural layer  530 . Increasing the stress gradient in stress gradient layer  110  and decreasing the thickness of structural layer  530  increases the curvature of cylindrical reflection mirror  100 . A typical thickness for structural layer  530  is about 100 nm to provide the mechanical support required while still avoiding the transfer of stress in structural layer  530  to cylindrical reflection mirror  100  which occurs if structural layer  530  is thinner than about 100 nm. If structural layer  530  is thinner than about 100 nm, an unacceptable level of anisotropic stress is present in cylindrical reflection mirror  100 . For a thickness above about 100 nm and below 500 nm anisotropic stress is not significant and the added thickness still allows for adequate bending of cylindrical reflection mirror  100 . 
     An embodiment in accordance with the invention of cylindrical reflection mirror  100  is shown in FIG.  2 . Cylindrical reflection mirror  100  is supported by torsion bar  220 . The angular position of cylindrical reflection mirror  100  is adjustable with sliding actuator  210  or an electrostatically driven comb drive actuator (not shown) such as described by M. J. Daneman et al. in “Linear Microvibromotor for Positioning Optical Components”, IEEE J. MEMs, vol. 5, no. 3, September 1996, pp. 159-165 which is incorporated by reference in its entirety. 
     A MEMS spherical mirror can focus light in two dimensions and is desirable for applications such as, for example, beam scanning or optical spectroscopy where a focused beam of light increases the light intensity for optimum results. The ability to control the stress of a metal thin film results in a semi-spherical reflecting surface. Stress gradient layer  110 , typically MoCr, with a controlled stress gradient is deposited on substrate  510  coated with sacrificial layer  520  (see FIG. 7 b ). In an embodiment in accordance with the invention, FIG. 3 a  shows metal pattern  310  for spherical mirror  320  and release window  315 , typically having dimensions of about 400 μm by 400 μm. FIG. 3 b  shows spherical mirror  320  upon release from substrate  510  typically having a radial extent of about 175 μm. The surface of spherical mirror  320  is typically coated with an aluminum or gold reflective layer by either thermal deposition or RF sputtering techniques. Sacrificial layer  520  is etched through release window  315  to allow release and lift of metal pattern  310  to form spherical mirror  320 . Single cantilever  325  anchors spherical mirror  320  to substrate  510 . After release, metal pattern  310  (see FIG. 3 a ) will conform to a spherical surface in the presence of biaxial stress. 
     The total lift and resulting radius of curvature can be designed using conventional micro-spring design recipes such as disclosed in U.S. Pat. No. 5,914,218 which is incorporated by reference in its entirety. For example, sputter conditions for forming stress gradient layer  110  for pattern  310  in MoCr with a thickness of about 500 nm and with an internal stress gradient of about 3.0 Gpa are as shown in Table 1 below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Sputter Deposition Conditions 
                 Time (sec) 
               
               
                   
                   
               
             
             
               
                   
                 Pressure: 1.6 mT 
                 390 
               
               
                   
                 Voltage: 518 volts 
               
               
                   
                 Current: 1.13 A 
               
               
                   
                 Pressure: 2.2 mT 
                 330 
               
               
                   
                 Voltage: 470 volts 
               
               
                   
                 Current: 1.26 A 
               
               
                   
                 Pressure: 3.0 mT 
                 300 
               
               
                   
                 Voltage: 457 volts 
               
               
                   
                 Current: 1.30 A 
               
               
                   
                 Pressure: 3.9 mT 
                 330 
               
               
                   
                 Voltage: 453 volts 
               
               
                   
                 Current: 1.31 A 
               
               
                   
                 Pressure: 5.0 mT 
                 300 
               
               
                   
                 Voltage: 457 volts 
               
               
                   
                 Current: 1.30 A 
               
               
                   
                   
               
             
          
         
       
     
     Sputter conditions for stress gradient layer  110  for cylindrical mirror  100  and tunable blazed grating membrane structure  410  are also described by Table 1. 
     If the lift is such that single cantilever  325  is raised above substrate  390  on the order of a few tens of microns, spherical mirror  320  can be electrostatically actuated using a metal contact (not shown) buried under sacrificial layer  520  (see FIG. 7 c ) below cantilever  325  of spherical mirror  320 . Electrostatic actuation allows precise adjustment of the cantilever angle and the option of removing spherical mirror  320  out of the optical path in applications where light is collected from a moving or adjustable position source. Spherical mirror  320  typically has a thickness of 2-3 μm or from 5-10 μm if using a silicon on insulator device layer. 
     The fill-factor of spherical mirror  320  and the reflectivity may be increased by inserting webbing (not shown) between petals  321  of spherical mirror  320  in an embodiment in accordance with the invention. Dielectric or metal layers with no built in stress are deposited prior to deposition of stress gradient layer  110  and patterned using standard lithographic techniques. The dielectric or metal layers are then dry or wet etched to define shape. The webbing layer is released simultaneously with metal pattern  310  of stress gradient layer  110  and deformed into a spherical shape by the stress relaxation of metal pattern  310  on release. 
     MEMS tunable blazed gratings have applications for spectrophotometers. FIG. 4 a  shows tunable blazed grating membrane structure  410  in accordance with an embodiment of the invention. Stress gradient layer  110  typically 500-1000 nm thick is deposited on amorphous silicon or polysilicon layer  920  which is deposited on substrate  510 . After release of patterned amorphous silicon or polysilicon layer  920 , layer  920  is curled up due to the stress in stress gradient layer  110  to form blazed grating membrane structure  410 . Blaze angle  415  is adjustable by applying a bias voltage greater than about 100 volts across substrate  510  and each blazed grating membrane  945 . Each blazed grating membrane curls up on release by removal of dielectric layer  520  in a timed etchant, for example 49% hydrofluoric acid. 
     Equation (1) is the grating equation: 
     
       
           a  sin θ m   =mλ   (1) 
       
     
     where a is the grating pitch, and light is assumed to be normally incident to the grating. In an embodiment in accordance with the invention, for example, taking a=3 μm and λ=670 nm results in first order diffraction angle θ 1 =12.9° and second order diffraction angle θ 2 =26.5°. With blaze angle  415  adjusted to equal to 13.25°, the specular reflection of the blaze matches the positive second order of diffraction. Adjusting blaze angle  415  to 6.45°, the specular reflection matches the first order of diffraction. 
     FIGS. 5 a - 5   f  show the steps for fabrication of cylindrical mirror  100  in accordance with an embodiment of the invention. FIG. 5 a  shows bulk silicon substrate  510 . FIG. 5 b  shows deposition, typically by either sputtering or plasma enhanced chemical vapor deposition (PECVD) and patterning of sacrificial layer  520  on silicon substrate  510 . A typical composition for sacrificial layer  520  is SiO 2 , although other materials such as Si 3 N 4  may be used if silicon on insulator is not used for bulk silicon substrate  510 . Mask  610  is placed over sacrificial layer  520  for creation of reinforcing beams  130 . FIG. 5 c  shows silicon substrate  510  after etching with a 45% KOH (potassium hydroxide) solution. FIG. 5 d  shows deposition of sacrificial layer  525  and polysilicon layer  530 . Lift-off mask  620  shown in FIG. 6 b  is placed over polysilicon layer  530 . The open center of lift-off mask  620  indicates where stress gradient layer  110 , for example, a MoCr layer, is left on silicon substrate  510  when lift-off mask  620  is removed. FIG. 5 e  shows deposition of MoCr layer  110  as described in Table 1 above. Finally, FIG. 5 f  shows release of layer  530  using a 49% HF (hydrofluoric acid) wet etch to remove SiO 2  sacrificial layers  520  and  525 . Release of layer  530  results in release of cylindrical mirror  100 . 
     FIGS. 7 a - 7   e  show the steps for fabrication of spherical mirror  320  in accordance with an embodiment of the invention. FIG. 7 a  shows bulk silicon substrate  510 . Sacrificial layer  520 , typically SiO 2 , is deposited on silicon substrate  510  as shown in FIG. 7 b . Photoresist lift-off mask  710  is shown in top view in FIG. 8 a . Silicon substrate  510  is patterned using photoresist lift-off mask  710  followed by deposition of stress gradient layer  110 , typically MoCr as described in Table 1, shown in FIG. 7 c . Subsequently, lift-off mask  710  is removed along with excess MoCr associated with stress gradient layer  110  in an acetone soak lift-off process. Finally, photoresist mask  720 , shown in top view in FIG. 8 b , is deposited on stress gradient layer  110  using spin-on techniques to cover the sections of stress gradient layer  110  not to be released. Exposed regions of stress gradient layer  110  are released using a 49% HF (hydrofluoric acid) wet etch for sacrificial layer  520  removal. Duration of the HF etch is typically about 15 minutes for release of spherical mirror structure  320 . Photoresist mask  720  allows petals  321  of spherical mirror  320  to be underetched while the remainder of spherical mirror structure  320  is protected from etching. As noted above, the efficiency of spherical mirror  320  may be enhanced by introducing webbing material between petals  321 . 
     FIGS. 9 a - 9   f  show the steps for fabrication of tunable blazed grating membrane structure  410  in accordance with an embodiment of the invention. Sacrificial layer  520  is deposited on glass or bulk silicon substrate  510  to a thickness of about 5 μm as shown in FIG. 9 a . Sacrificial layer  520  is typically SiO 2  but sacrificial layer  520  may also be silicon nitride (Si 3 N 4 ) or silicon-oxynitride (SiON x ), for example. Sacrificial layer  520  is patterned using standard lithography as shown in FIG. 9 b  with mask  999  (see FIG. 10) to expose anchor positions  950  for each individual grating  988 . Polysilicon or amorphous silicon layer  920  is deposited using chemical vapor deposition over sacrificial layer  520  as shown in FIG. 9 c . Polysilicon or amorphous silicon layer  920  functions as the mechanical support layer for individual grating membranes  945 . Layer  920  is patterned using mask  999  shown in FIG. 10 with the exposed portions being dry etched to expose sections of sacrificial layer  520  and defining individual grating membranes  945  in polysilicon layer  920  as shown in FIG. 9 d . Layer  920  is again patterned using standard lithography for a MoCr lift-off process. As shown in FIG. 9 e , MoCr layer  110  is deposited using the process described in table 1 with excess resist being removed in the lift-off process which leaves MoCr layer  110  only on the tops of individual gratings  988 . Sacrificial layer  520  is removed using a wet etchant, typically 49% hydrofluoric acid. As FIG. 9 f  shows, individual grating membranes  945 , typically having a length of 100 μm, are left anchored to substrate  510  and grating membranes  945  curl up as shown in FIG.  4 . 
     FIGS. 11 a - 11   e  show the steps for fabrication of spherical mirror  320  in accordance with an embodiment of the invention. FIG. 11 a shows silicon on insulator wafer (SOI)  1100  with single crystal silicon (SCS) layer  1120  as the fabrication starting point. As noted earlier, SOI wafer  1100  may be substituted for silicon substrate  510  in accordance with the invention. Use of commercially available SOI wafers  1100  reduces the number of processing steps and provides SCS layer  1120  which provides higher optical and mechanical quality than polysilicon material. Single crystal silicon (SCS) layer  1120  is typically 100 nm thick with sacrificial layer  520  typically having a thickness of 2 μm. FIG. 11 b  shows lithographic patterning using the photographic negative of mask  710  (see FIG. 8 a ) and etching (etchant??) of SCS layer  1120 . Following etching of SCS layer  1120 , photoresist mask  710  (see FIG. 8 a ) is put over SCS layer  1120  as shown in FIG. 11 c  and stress gradient layer  110  is deposited as described in Table 1. Unwanted portions of stress gradient layer  110  are then removed in a lift-off process using acetone solvent. Finally, photoresist mask  720 , shown in top view in FIG. 8 b , is put on stress gradient layer  110  using spin-on techniques to cover the sections of stress gradient layer  110  not to be released as shown in FIG. 11 d . Exposed regions of stress gradient layer  110  are released using a 49% HF (hydrofluoric acid) wet etch for sacrificial layer  520  removal as shown in FIG. 11 e . Duration of the HF etch is typically about 15 minutes for release of spherical mirror structure  320 . Photoresist mask  720  allows petals  321  (see FIG. 3 b ) of spherical mirror  320  to be underetched while the remainder of spherical mirror structure  320  is protected from etching. Again, the efficiency of spherical mirror  320  may be enhanced by introducing webbing material between petals  321  as described above. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.