Patent Publication Number: US-2011062110-A1

Title: Method for Fabricating a Micromirror with Self-Aligned Actuators

Description:
PRIORITY CLAIM 
     The present application claims priority under 35 U.S.C. §119(e)(1) to provisional application No. 61/243,012 filed on Sep. 16, 2009, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to optical scanning devices in general, and in particular to a method for fabricating micromirrors to be utilized in optical scanning devices. 
     2. Description of Related Art 
     Conventional electrostatic combdriven micromirrors do not offer perfect linear transformation between input voltages and mechanical scan angles. In addition, conventional electrostatic combdriven micromirrors often experience scanning instabilities due to pull-in phenomena. Thus, self-alignment procedures have been adopted in the micromirror fabrication process in order to mitigate the above-mentioned problems. However, such self-alignment procedures can be overly complicated. 
     Consequently, it would be desirable to provide an improved method for fabricating combdriven micromirrors. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, a set of coarse features is initially formed in a low-temperature oxide (LTO) layer deposited on a front side of a wafer. The wafer includes a substrate, a first and second silicon device layers separated from each other by a first and second silicon dioxide layers. A set of fine features is then formed in a photosensitive material layer deposited on top of the LTO layer, and the fine features are constrained laterally within the coarse features. Next, a portion of the LTO layer is removed to align the width of the coarse features with the width of the fine features. The first silicon dioxide layer and the first and second silicon device layers are subsequently etched to form stator comb fingers and rotor comb fingers. Finally, a rotatable mirror is formed by removing a portion of the substrate on a back side of the wafer, and the silicon dioxide layers from the front and back sides of the wafer. 
     All features and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a diagram of a laser-scanning confocal microscope in which a preferred embodiment of the present invention is applicable; 
         FIG. 2  is a detailed diagram of a micromirror within the confocal microscope from  FIG. 1 , in accordance with a preferred embodiment of the present invention; 
         FIG. 3  shows a set of combdrive actuators within the micromirror from  FIG. 2 , in accordance with a preferred embodiment of the present invention; and 
         FIGS. 4   a - 4   i  illustrates a method for fabricating the micromirror from  FIG. 2 , in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to the drawings and in particular to  FIG. 1 , there is depicted a diagram of a laser-scanning confocal microscope in which a preferred embodiment of the present invention is applicable. As shown, a laser-scanning confocal microscope  100  includes a diode laser  166 , an avalanche photodetector  188 , a stationary mirror  172 , a movable micromirror  174  and an objective system  111  having a 3× Keplerian beam expander  176  and a high-numerical aperture aspheric objective lens  178 . 
     A linearly-polarized laser beam from diode laser  166  is initially coupled into a single-mode polarization maintaining (PM) fiber  168 . Light exiting PM fiber  168  is then collimated by collimators  169  to a 1 mm diameter beam through a zero-order quarter wave-plate  170  whose axis is oriented at 45° to the incident polarization angle in order to convert the illumination to a circular polarization. After reflection off stationary mirror  172 , the illumination is incident on micromirror  174  at 22.5° to micromirror  174  normal. Micromirror  174  scans the illumination across objective system  111 , providing an effective numerical aperture of about 0.48 at a tissue sample  180 . Reflected light is subsequently converted into a linear polarization that is orthogonal to the initial illumination polarization, which is isolated using a walk-off polarizer  182  and an offset mirror  184 , and directed through a spatial filter  186  into avalanche photodetector  188 . 
     Higher values of numerical aperture of objective system  111  can be used to obtain better optical sectioning with high contrast in highly scattering tissue sample  180 . The resolution, field of view, and contrast of confocal microscope  100  is largely determined by micromirror  174 . There is, however, a trade-off in selecting between resolution and field of view. The product of micromirror  174 &#39;s size and its optical deflection angle determines the number of resolvable points in the final image, which translates into a given field of view and resolution according to the numerical aperature of objective system  111 . 
     The number of resolvable points, N, for micromirror  174  in a one-dimensional scan is given by 
     
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                       D 
                        
                       
                           
                       
                        
                       θ 
                     
                     λ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where θ is the mechanical scanning half-angle of micromirror  174 , λ is the operating wavelength, and D is the diameter of micromirror  174 . 
     Preferably, confocal microscope  100  can be used to provide images of a 200×125 μm field of view at 3.0 frames per second. The number of resolvable points (408×255) in the images is proportional to the product of the diameter of micromirror  174  and the optical scan angle, as stated in Equation (1). Micromirrors with larger diameters (˜1 mm) capable of providing the same deflection angles can be designed within the limits set by the maximum driving voltage and at the cost of increased energy consumption. 
     With reference now to  FIG. 2 , there is depicted a detailed diagram of micromirror  174  from  FIG. 1 , in accordance with a preferred embodiment of the present invention. The size of a chip  200  containing micromirror  174  is approximately 2.8×2.8 mm 2 , and the diameter of rotatable minor  210  is approximately 1,024 μm. As shown, micromirror  174  has two axes, and electrostatic vertical combdrives can be utilized to provide fast, high-torque rotary actuation about the two axes of micromirror  174 . For example, two sets of staggered vertical combdrive actuators  260 ,  262  can be utilized to rotate rotatable mirror  210  along each of the two axes. The movements of combdrive actuators  260 ,  262  can be controlled by the application of appropriate electrical biases on chip  200  via pads V 1   inner , V 1   outer , V 2   inner , V 2   outer  and Ground. Combdrive actuators  260 ,  262  include rotor and stator comb fingers. The thickness and spacing between rotor and stator comb fingers are preferably fixed at approximately 8 μm. 
     The performance of micromirror  174  is characterized by its response to various electrical signal inputs. For example, one input can be a sinusoidal variable-frequency voltage with suitable offset (to ensure the applied voltage was always positive) between ground and one of combdrive actuators  260 ,  262  of each rotation axis. Optical scan angles of 22° and 12° on the inner and outer axes are achieved for frequency values around 2.81 kHz and 670 Hz on the inner and outer rotation axes, respectively. On the other hand, for a static voltage applied between ground and one of combdrive actuators  260 ,  262  on each rotation axis, off-resonance actuation using only one combdrive actuator results in single-sided deflection. The total optical deflection angle can be doubled by making use of both combdrive actuators  260 ,  262  on either side of the torsion bars forming the rotation axis. In this respect, off-resonance operation differs significantly from driving at resonant frequency. Optical scan angles of about 5° and 4.5° can be achieved by applying static voltages up to 240 V on the inner and outer axes, respectively. 
     Referring now to  FIG. 3 , there is illustrated a detailed diagram of combdrive actuators  260  from  FIG. 2 , in accordance with a preferred embodiment of the present invention. As shown, combdrive actuators  260  include rotor comb fingers  306  and stator comb fingers  308 . Preferably, each of stator comb fingers  308  has a width between about 0.5 μm and 50 μm, each of rotor comb fingers  306  has a width between about 0.5 μm and 50 μm, and a target gap spacing g ranges between 0.5 μm and 50 μm. As their names imply, rotor comb fingers  306  are capable of being rotated, while stator comb fingers  308  remain stationary throughout. 
     In response to a voltage being applied at stator comb fingers  308 , rotor comb fingers  306  rotate about a torsion bar  304 . Specifically, when a voltage is applied at stator comb fingers  308 , an electrostatic torque is experienced by rotor comb fingers  306 , which subsequently rotates rotor comb fingers  306  because they are constrained primarily to rotary motion by torsion bar  304 . Rotor comb fingers  306  are capable of being rotated to a maximum rotation angle of θ max . As rotor comb fingers  306  are being rotated, a shear stress is developed within torsion bar  304  due to twisting, and the shear stress offers a mechanical restoring torque against such twisting. The rotation of rotor comb fingers  306  reaches an equilibrium at a rotation angle at which the electrostatic torque exactly matches the mechanical restoring torque. 
     With reference now to  FIGS. 4   a - 4   i , there are illustrated a method for fabricating a micromirror, such as micromirror  174  from  FIG. 2 , in accordance with a preferred embodiment of the present invention. The process begins with a &lt;100&gt; double silicon-on-insulator (SOI) wafer  400 . Wafer  400  includes a substrate  420  having two &lt;100&gt; silicon device layers  412 ,  414  separated from each other by two silicon dioxide layers  416 ,  418 , as shown in  FIG. 4   a . Each of silicon device layers  412 ,  414  is approximately 30 μm thick, and each of silicon dioxide layers  416 ,  418  is approximately 1 μm thick. 
     Before further processing of wafer  400 , pre-fabrication of any complementary-metal-oxide semiconductor (CMOS) circuitry can be performed at this point, if necessary. For example, CMOS circuitry may include control electronics and sensors to adaptively correct for aberrations in a micromirror. 
     Following the CMOS circuitry pre-fabrication (if performed), wafer  400  is cleaned by immersing wafer  400  in a 9:1 solution of H 2 SO 4 :H 2 O 2  for approximately 8 minutes. After rinsing with de-ionized water, wafer  400  is spun dry. The above-mentioned cleaning process is commonly known as Piranha clean. 
     Next, wafer  400  is placed into a furnace in which a low-temperature oxide (LTO) layer  422  is deposited on top of silicon device layer  412  via a low-pressure chemical vapor deposition (LPCVD) process at a low temperature (450° C.) in order to reduce thermal budget. LTO layer  422  is preferably a silicon dioxide layer having a thickness between about 50 nm and about 1.5 μm. LTO layer  422  serves to protect any CMOS circuitry and to act as a hard mask for the deep trench etching to be performed to create vertical comb finger structures. 
     A first photolithography step is performed on LTO layer  422  to etch a set of coarse features  424 ,  426  of vertical comb finger structures on top of silicon device layer  412 . The photolithography step involves coating a layer of hexamethyldisilazane (HMDS) on LTO layer  422 , which serves as an adhesion promoter between LTO layer  422  and a photosensitive material to be added. Coarse features  424 ,  426  are etched in LTO layer  422  via a reactive ion etching (RIE) step using CHF 3  and O 2  gases, as shown in  FIG. 4   b.    
     A photosensitive material layer  428 , such as Shipley SPR 220-3 positive photoresist, is then spun on LTO layer  422 . A second photolithographic step is then performed on photosensitive material layer  428  to etch a set of fine features  430 ,  432  of vertical comb finger structures on top of LTO layer  422 . Fine features  430 ,  432  are constrained laterally within respective coarse features  424 ,  426 , as shown in  FIG. 4   c.    
     The misalignment tolerance for the second photolithography step, which includes a self-alignment step, is half of the gap spacing between stator comb fingers and rotor comb fingers. A significant advantage of the second photolithography step is that if the alignment is deemed to be unsatisfactory on inspection after the second photolithography step, the photoresist can be removed by a Piranha clean, and the self-alignment step can be repeated as many times as necessary. This flexibility eliminates the uncertainty in determining whether or not self-alignment has been achieved, as may happen when the self-alignment is performed to a layer buried deep within a material stack. The minimum comb gap spacing achievable can be determined by the maximum aspect ratio that a silicon deep reactive ion etching (DRIE) tool used in subsequent steps can achieve. 
     Next, a second RIE step is utilized to remove exposed LTO layer  422  in order to trim coarse features  424 ,  426  within LTO layer  422  to match the widths of corresponding fine features  430 ,  432  within photosensitive material layer  428 , in order to complete the self-alignment process, as illustrated in  FIG. 4   d.    
     Using coarse features  424 ,  426  within LTO layer  422  and fine features  430 ,  432  within photosensitive material layer  428  as masks, a DRIE is utilized to remove a portion of silicon device layer  412  (stopped on silicon dioxide layer  416 ) to form stator comb features  438  and rotor comb features  440  on top of silicon dioxide layer  416 , as shown in  FIG. 4   e . The DRIE is preferably performed in an inductively-coupled plasma generator using SF 6 /O 2  and C 4 F 8  gases in a pulsed scheme (commonly known as a Bosch process). 
     A third RIE step is then utilized to remove silicon dioxide layer  416 , using coarse features  424 ,  426  within LTO layer  422  and fine features  430 ,  432  within photosensitive material layer  428  as masks. Photosensitive material layer  428  is subsequently removed, leaving rotor comb features  440  unprotected by any masking element, while stator comb features  438  are still protected by LTO layer  422 , as illustrated in  FIG. 4   f.    
     A second DRIE step is utilized to remove portions of silicon device layers  412  and  414  (stopped on silicon dioxide layer  418 ) to define rotor comb fingers  444  in silicon device layer  414 . After the completion of the second DRIE step, rotor comb features  444  reside only in silicon device layer  414 , while stator comb features  442  reside in both silicon device layers  412  and  414 , as illustrated in  FIG. 4   g.    
     The lower section of stator comb features  442  (portions located in silicon device layer  414 ) is redundant from an actuation perspective, but they do not affect the operation of a micromirror. 
     A third photolithographic step using a photoresist layer  446  is then performed on a backside of wafer  400  using a third photomask to align to the features on the front side of wafer  400 . Preferably, photoresist layer  446  is approximately 15 μm thick and is capable of protecting the underlying silicon through a substrate DRIE step. Photoresist layer  446  can be, for example, Shipley SPR 220-7 positive resist. The third photomask contains the outline of a rotatable mirror structure  445  and is used to remove all silicon directly beneath rotatable mirror structure  445 , as illustrated in  FIG. 4   h.    
     Since the feature on the third photomask is relatively large (comparable to the size of the entire device), a significant amount of misalignment can be tolerated. Wafer  400  is bound by photoresist to a second silicon substrate (not shown) serving as a mechanical handle in preparation for the backside substrate DRIE step on substrate  420 . The backside DRIE step releases the devices and creates dicing lines to facilitate cleaving of wafer  400  into individual chips. 
     Wafer  400  can be separated from its handle wafer by soaking wafer  400  in acetone, following which a fourth RIE is performed on the front and back sides of wafer  400  to remove any remaining exposed hard mask in LTO layer  422  and silicon dioxide layer  418 . The result is an optical scanning device having multiple bond pads  448 , stator comb fingers  450 , rotor comb fingers  452 , and a rotatable mirror  454 , as illustrated in  FIG. 4   i.    
     As a final step, metals, such as chromium/gold, can be evaporated on the surface of mirror  454  through a shadow mask to improve reflectivity. 
     As has been described, the present invention provides a method for fabricating micromirrors with self-aligned actuators. The method of the present invention includes the pre-fabrication of CMOS circuitry prior to the micro-electrode-mechanical system (MEMS) process sequence at a low thermal budget. The method of the present invention may utilize conventional silicon processing tools with low-operating temperatures in order to prevent diffusion of previously implanted dopants during the MEMS fabrication steps. The fabrication strategy is a “MEMS-last” strategy, where the micromachining of mechanical structural layers is performed after the completion of the CMOS back-end-of line (BEOL) process steps. This modular strategy offers the advantage of being compatible with any CMOS fabrication process. If the MEMS fabrication sequence is designed to have thermal budget similar to that of a BEOL process, it can be considered as an optional CMOS BEOL process, with no effect on CMOS front-end-of-line (FEOL) processes (especially dopant diffusion steps). If the materials used in the MEMS fabrication sequence are CMOS compatible, the MEMS fabrication can be done as an extension of the CMOS processing. In addition, the difficulties of performing photolithography on previously bulk micromachined substrates present in “MEMS-first” approaches are avoided, which is especially important where high aspect-ratio structures are used in MEMS structures. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.