Abstract:
A micromachined movable shuttle is constructed in III-V material to allow in situ placement of VCSELs on the shuttle for a solid state image scanner in optical display and printing applications. A comb drive is used to actuate the shuttle for scanning purposes. Hundreds of VCSELs may be fabricated on a large movable shuttle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of application Ser. No. 09/173,329, entitled “Monolithic Scanning Light Emitting Devices” filed Oct. 15, 1998, which claims priority to Provisional Patent Application, Serial Number 60/069,674, entitled “Monolithic Scanning VCSELs Using III-V Micromachining” filed on Dec. 12, 1997. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates generally to the field of optical imaging, and more particularly to optical scanning for high resolution, full width page scan, xerographic printing applications. 
     Xerographic exposure systems are important for printing. In xerographic printing, lasers or LEDS may be used to expose tiny dots on a photoreceptor surface. The photoreceptor has the property of holding an electrical charge in the absence of light. Illumination of a spot on the photoreceptor by a laser or LED causes the loss of charge at the exposed spot. In a typical xerographic system, charge left on the photoreceptor attracts toner that is then transferred to paper which has a greater charge than the photoreceptor. 
     Desirable features for xerographic exposure printing systems include full page-width printing, high resolution addressability, elimination of moving mechanical parts, and low power consumption. These features are important for achieving performance comparable to offset lithography and occupy a parameter space that lies beyond the speed and width capabilities of polygon raster output scanning (ROS) print engines. Polygon ROS printers typically consist of a laser light source, a polygon scanning beam deflector, an optical system of lenses and mirrors, a xerographic marking engine and the electronics to control the printer operation. 
     Solid state semiconductor light emitters are important devices in such diverse applications such as optoelectronic communication systems and high-speed printing systems. It is well-known in the proven art of silicon to provide suspension and actuation schemes, for example, comb drives using bending springs or parallel plate actuation using torsion springs. For optical beam steering applications, these silicon steering elements are typically combined with a light source in a separate package, or even with a light source ‘glued’ or bonded onto the silicon steering chip. 
     U.S. Pat. Nos. 5,536,988, 5,640,133, 5,025,346, “Fabrication of Submicron High-Aspect-Ratio GaAs Actuators” Zhang et al., Journal of Microelectromechanical Systems Vol. 2, No.2, p. 66-73, June 1993, “Laterally Driven Polysilicon Resonant Microstructuren” Tang et al., IEEE Micro Electro Mechanical Systems pp. 53-59, February 1989 (reprint), “Electrostatic-comb Drive of Lateral Polysilicon Resonators” Tang et al., Transducers &#39;89, Proceedings of the 5th International Conference on Solid-State Sensors and Acutators and Eurosensors III, Vol. 2, pp. 328-331, June 1990 (reprint) and “Comb-drive actuators for large displacements”, Legtenberg et al., Journal of Micromechanics and Microengineering, Vol. 6, pp.320-329, 1996 show the state of the art of micro-electromechanical systems (MEMS) actuators and methods of fabricating these devices. U.S. Pat. Nos. 5,747,366 and 5,719,891 show the state of the art of semiconductor light emitting assemblies. 
     U.S. patent application Ser. No. 08/761,681, entitled “Raster Output Scanner with Pivotal Mirror for Process Direction Light Spot Position Control” filed on Dec. 6, 1996 and assigned to the same assignee as the present invention teaches a MEMS torsional control device. 
     U.S. patent application Ser. No. 08/940,867, entitled “Highly compact Vertical Cavity Surface Emitting Lasers” filed on Sep. 30, 1997 and assigned to the same assignee as the present invention teaches the formation of highly compact and well-defined VCSELs. 
     All of the above references are hereby incorporated by reference. 
     SUMMARY OF INVENTION 
     In accordance with this invention, a solid state scanning imager is used to replace the polygon ROS used in optical display and printing applications. A VCSEL shuttle having a large movable stage carrying hundreds of scanning vertical cavity surface emitting lasers (VCSELs) is suspended by mechanical beams. Pairs of comb-drive structures are used to electrostatically actuate the movable stage. One set of comb-drive fingers (fixed fingers) is attached to the stationary mechanical beams while another set of comb-drive fingers (movable fingers) is attached to the movable stage. Alternating application of a bias voltage to selected fixed fingers causes the movable stage to travel back and forth in a scanning motion. The comb-drive structures need to possess enough flexibility to allow the movable stage to move sufficiently to allow scanning over half of the pitch of the VCSEL array. 
     The VCSEL shuttle is typically fabricated using III-V micromachining technology. This allows in situ manufacturing of III-V based light emitters which have highly desirable optical properties. For example, VCSELs can be made in the movable stage material as opposed to being attached separately. Undoped III-V material is used for the substrate for better enabling the electrical isolation of different parts of the VCSEL shuttle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale. 
     FIG. 1 shows a top view of a light emitting assembly movably supported on a semiconductor substrate; 
     FIG. 2 shows the cross-section of the semiconductor substrate in FIG. 1 prior any processing; 
     FIG. 3 shows a cross-sectional view of the light emitting assembly in FIG. 1 along line  3 — 3 ; 
     FIGS. 4 and 5 show a cross-sectional view of the movable light emitting assembly along line  4 — 4  of FIG. 1 using an RIE/RIE fabrication process; 
     FIG. 6 shows a cross-sectional view of the movable light emitting assembly along line  4 — 4  of FIG. 1 in using ODE/RIE fabrication process;. 
     FIGS. 7 and 8 show a cross-sectional view of the movable light emitting assembly along line  4 — 4  of FIG. 1 using an RIE and sacrificial layer fabrication process; 
     FIG. 9 shows a top view of a light emitting assembly movably supported on a doped semiconductor substrate; 
     FIG. 10 shows the cross-section of the semiconductor substrate in FIG. 1 prior to any processing; 
     FIG. 11 shows a cross-sectional view of the light emitting assembly in FIG. 9 along line  11 — 11 ; 
     FIGS. 12-18 show top views of the processing steps used in fabricating the light emitting assembly in FIG. 9; 
     FIG. 19 shows a top view of a 1 by 2 light emitting assembly array using a plurality of the light emitting assembly in FIG. 9; 
     FIG. 20 shows a top view of a light emitting assembly array with a plurality of light sources; 
     FIG. 21 shows a top view of an electrode actuated movable light emitting assembly supported on a substrate; and 
     FIG. 22 shows a cross-sectional view of the electrode actuated movable light emitting assembly along line  22 — 22  of FIG.  21 . 
     FIG. 23 shows a top view of a light emitting assembly array suspended using folded beams in accordance with an embodiment of this invention. 
     FIG. 24 shows a top view of a unit of the light emitting assembly array in FIG.  23 . 
     FIG. 25 shows a top view of staggered light emitting assembly arrays in accordance with an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a top view of a micromachined movable stage with a light emitter, actuation mechanism and suspension system. The light emitter preferably takes the form of any III-V based emitters, for example, vertical cavity surface emitting lasers (VCSELs), light emitting diodes (LEDS), and edge emitting light emitters. In the particular embodiment shown in FIG. 1, VCSEL shuttle  1  has movable VCSEL stage  10  which supports VCSEL  12 . Stage  10  has one mechanical degree of freedom (x) and is suspended with springs  16 - 19 . Springs  16 - 19  may take the form of straight bending springs (as shown), and may also include other configurations known from the silicon art, for example, folded beams. Mechanical designs such as ‘folded spring’ suspensions, tapered or stepped comb fingers, x/y folded spring suspensions, archimedian spiral springs for a rotational degree of freedom, etc., which are well-known in silicon structures, may be implemented in the III-V structures. The in-plane shape of structures (stage, springs, combs) is relatively arbitrary within fairly wide limits. 
     Movement of stage  10  is actuated with electrostatic comb drives  20  and  21 , all anchored to substrate  50 . Electrostatic comb drives may be replaced with any actuation system as is well known in the microelectromechanical systems (MEMS) art, for example, magnetic, thermal and piezoelectric systems. As shown, stage  10  is moved by applying voltage V 1  between fixed fingers  22  and movable fingers  24  of comb drive  20  and applying a voltage V 2  between fixed fingers  23  and movable fingers  25  of comb drive  21 . 
     Electrostatic forces cause movable fingers  24  and  25  of comb drives  20  and  21  to ‘pull in’ to minimize the energy stored in the system. Movable fingers  24  and  25  are attached to stage  10  with its integrated VCSEL  12  and stage  10  is suspended with flexible springs  16 - 19 . Springs  16 - 19  are anchored to substrate  50  at anchor points  30 — 33 , while fixed combs  22  and  23  are anchored to substrate  50  at anchor points  34  and  35 . In a first order approximation, the stage position is proportional to the force, which is proportional to the square of the applied voltage. Conductive heat transfer through the suspension springs sinks the waste heat from the VCSEL into the substrate. The low threshold current, characteristic for VCSELs, makes it possible to keep the temperature of the stage under control. 
     An important metallization /contacting issue is how to get the electrical connections for the VCSEL(s)  12  onto the mechanically suspended stage  10 . In the embodiment shown, conductive line  40  is run from contact pad  42  on substrate  50  to stage  10  and to VCSEL  12 . Conductive line  40  runs over the neutral fiber of suspension spring  18 , and is electrically isolated from suspension spring  18 . The ‘neutral fiber’ is the stress free centerline of the flexing spring. The width of conductive line  40  is chosen sufficiently small relative to the width of the spring and runs along the centerline of the spring in order to minimize mechanical stress, which allows conductive line  40  to survive the mechanical scanning motion of the beam. The electrical isolation of conductive line  40  from suspension spring  18  may be accomplished in many ways, for example, with a dielectric layer underneath conductive line  40  or insulating the suspension beam surface using shallow ion implantation. 
     FIG. 2 is a cross-sectional view of substrate  50  having a VCSEL structure prior to any etching with substrate back side  52  and substrate front side  54 . In this embodiment, the VCSEL structure includes layers of undoped III-V substrate  56 , etch stop layer  58 , GaAs buffer layer  60 , N-type quarter wave distributed Bragg reflector (DBR) mirror layers  62 , active region  64 , P-type DBR mirror  66  and P+ contact layer  68 . In a preferred embodiment layer  56  is an undoped GaAs substrate, etch stop layer  58  is AlAs, buffer layer  60  is N-GaAs, n-DBR layer  62  is n-Al x Ga 1−x As/Al y Ga 1−y As, active layer  64  is a quantum well layer sandwiched by AlGaAs potential confinement layers, P DBR layer is P− Al x Ga 1−x As(x˜0.05)/Al y Ga 1−y As (y˜0.95) and P+ contact layer is GaAs. 
     The VCSEL structure may be formed using any well-known etching processes to form the VCSEL cavity. The semiconductor laser structure then undergoes ion implantation process for isolation and metallization. Holes flows through the p-DBR region confined by the ion implanted area and into that portion of the active layer which lies below the p-DBR cavity and recombine with electrons flowing upwards from the N-DBR below and producing photon emission and optical amplification. At sufficiently high current flow, this optical amplification in combination with feedback from the DBR mirrors will result in laser oscillation and emission within the VCSEL cavity. Regions formed by the ion implantation isolation process are highly resistive. 
     FIG. 3 is a cross sectional view along line  3 — 3  of FIG. 1, and shows the VCSEL structure described in FIG. 2 after the VCSEL structure has been processed or etched. VCSEL anode  42  has been deposited on top of the VCSEL  12  (not shown) and ground  44  has been deposited on buffer layer  60 . The region under VCSEL anode  42  is ion implanted for electrical isolation to direct current flow into the active region of the VCSEL. 
     FIG. 4 is a cross-sectional view of VCSEL shuttle  1  taken along line  4 — 4  of FIG. 1 prior to the shuttle release etch step in which an RIE/RIE process has been used. A deep reactive ion etch from the substrate back side  52  to layer  58  is used to produce a III-V membrane with a thickness equal to the desired thickness of the stage  10 , springs  16 - 19  and combs  20  and  21 . The III-V membrane forms a bridge structure between unetched substrate portions of substrate  50  with layer  56  having substrate etched portion  70 . In a second RIE step shown in FIG. 5, from substrate front side  54 , the III-V membrane is perforated in the shape of stage  10 , springs  16 - 19  (not shown) and combs  20  and  21  (not shown) with membrane etched portions  72  and  74 . This step releases the unanchored structures from substrate  50  and makes them movable. The anchor points are defined in the areas where the front side etch intersects the sidewalls of the backside etch. The two etches can be post processing steps, performed after the regular VCSEL process. 
     FIG. 6 is a cross-sectional view of VCSEL shuttle  1  taken along line  4 — 4  of FIG. 1 in which an isotropic wet etch process has been used. This process is used on the substrate back side  52  to produce the III-V membrane supported by unetched layer  56  with etched portion  80 . The front side  54  etch is still RIE to form etched portions  82  and  84 . In this approach, the wet etch step on the back side can be done before the VCSEL process, leaving the front side of the wafer unaffected. After the regular VCSEL process, the additional RIE step from the front side would again release the moving parts. 
     FIG. 7 is a cross-sectional view of VCSEL shuttle taken along line  4 — 4  of FIG. 1 which uses a ‘sacrificial layer’ fabrication process. This process is based on an RIE step after the VCSEL process, defining the stage, combs and springs, and with etched portions  92  and  94  exposing etch stop layer  58 . FIG. 8 shows an underlying portion of etch stop layer  58  which is subsequently etched away (‘sacrificed’) from underneath the membrane forming etched portion  96 . This releases the moving parts from substrate  50 . The sacrificial etch is typically done with a wet etchant which etches the sacrificial layer material but does not etch the top layer material. 
     FIG. 9 shows a top view of VCSEL shuttle  100  which provides for electrical isolation of comb fixed fingers  122  and comb movable fingers  124  of comb drive  120  and comb fixed fingers  123  and comb movable fingers  125  of comb drive  121 . Since the fingers of each comb are of opposite polarities coupled by capacitance only, electrical isolation is needed which has to stand over 100 V electrical bias without breakdown. In one embodiment, this is realized by oxidizing layer  158  of about 500 to 1000 nm thick underneath the VCSEL structure, combined with etched isolation grooves  170  and  171  etching down to the oxidized layer. The oxidation of the AlAs layer is done through oxidation grooves  172  and  173  etched in the middle of the regions to be oxidized. 
     FIG. 10 is a cross-sectional view of substrate  150  having a VCSEL structure prior to any processing with substrate back side  152  and substrate front side  154 . VCSEL structure includes layers of doped III-V substrate  156 , etch stop layer  158 , buffer layer  160 , quarter wave distributed Bragg reflector (DBR) mirror layers  162 , active region  164 , PDBR mirror  166  and P+contact layer  168 . 
     FIG. 11 is a cross-sectional view of the fixed comb drive region along line  11 — 11  shown in FIG.  9 . The AlO x  layer  158  extends to the tips of static fingers  123 . Electrical contact pad  145  for comb drive fixed fingers  123  is deposited on top of nGaAs buffer layer  160 , so that the n-DBR layers  162  and the n-GaAs buffer layer  160  above the AlO x  layer  158  can couple the electrical static force between finger sets  123  and  125 . Comb movable fingers  125  are connected to the electrical ground  142  of VCSEL  112 ,so that no separate electrical contact is needed, which simplifies the fabrication process. When a bias is applied by V 1  between the comb drive fingers  122  and  124 , VCSEL shuttle  100  is scanned in a linear motion as indicated by the x arrow and when a bias is applied by V 2  between comb drive fingers  123  and  125 , VCSEL shuttle  100  is scanned in the opposite direction. 
     The VCSEL shuttle shown in FIG. 9 may be produced by many different processes, for example a process using the following steps: 
     FIG. 12 shows a top view of substrate  150  with oxidation grooves  172  and  173  etched from the front side  154  of layered substrate to reach the AlAs layer  158  beneath the n-GaAs buffer layer. 
     FIG. 13 shows the next step which includes oxidation of the AlAs layer to an AlO x  layer to cover desired areas  174  and  175 , for example, 200 um by 200 um. The oxidation area may be controlled by timing the length of the oxidation process. 
     FIG. 14 shows the steps of patterning and etching to buffer layer  160  at contact areas  146  and  147  in preparation for the deposition of electrical contacts  144  and  145  for fixed comb drives. 
     FIG. 15 shows the deposition of electrical contacts  144  and  145  onto exposed buffer layer  160 . 
     FIG. 16 shows the next step in which VCSEL  112  is fabricated with conductive line  140  connected to VCSEL contact pad  142 . VCSEL  112  has VCSEL cavity  113  formed by an ion implantation process isolation process. 
     FIG. 17 shows a cross-sectional view along line  17 — 17  of FIG. 9 with the stage and electrostatic combs not yet etched. This figure shows the step of patterning the back  152  of substrate  150  to open a hole  180  using, for example, a spray etcher. The spray etchant of H 2 O 2  and NH 4 OH mix may be used. As the pH of the mixed solution is over  6 , the etchant becomes selective of GaAs over oxidation layer  158  so the etching will stop at the oxidation layer  158 . Deep dry etching may be used to remove the substrate. Implantation region  141  extends underneath conductive line  140  and is insulating so that the current can only flow into the active region of the VCSEL from pad  142 ). Electrical contact  143  is deposited on the comer of the substrate back side  152 . 
     The last step is shown in FIG. 18 where patterning and etching substrate front side  154  to define finger (not shown) and shuttle features including stage  110 . Preferably the etching is done with a high aspect ratio dry etching process to form the electrostatic combs. Stage  110  is attached to substrate  150  by suspension beams  116 ,  117 ,  118  and  119  as shown in FIG.  9 . 
     The current of VCSEL  112  flows through the n-GaAs layer in the long suspended beam  118  to the n-GaAs substrate  160 . To estimate the series resistance of a 30 μm wide and 450 μm long beam with a 10 μm thick n-GaAs buffer layer  60 , with doping density of 5×18/cm 3  and the electron mobility of 1000 cm 2 /Vs, the resistance per beam is about 19 ohms. Considering two beams in parallel connection, the total electrical resistance is 9.5 ohms. Comparing to the series resistance of the VCSEL which is about 200 ohms typically, the added series resistance from the long suspended beam is quite small. 
     Since there is built-in compressive strain in the VCSEL structure due to lattice mismatch between AlGaAs and GaAs, the stage may buckle after release from the GaAs substrate. Increasing the thickness of the GaAs buffer layer will not only reduce the series resistance of the VCSEL, but will also increase the mechanical stability of the shuttle stage. Another approach to prevent the stage from buckling is depositing a layer in tensile strain on top of the stage to compensate for the built-in compressive stress. Tensile strained dielectric layers such as SiN x  or SiO 2  may be used to accomplish this. 
     FIG. 19 shows a 1×2 scanning VCSEL array using the VCSEL structure described in FIG.  9 . This paftem of multiple VCSELs can be repeated to make large linear or two-dimensional arrays. The numbers in FIG. 19 refer to the same structures as those described in FIG.  9 . 
     The VCSEL scanners can be packaged in TO-type packages (e.g. 1 VCSEL per package, or more if desirable) with built-in short focal length lens. A ‘small’ stage scan length (e.g. 5 μm) can, within limits, be considerably magnified by placing the VCSEL scanner close to a short focal length lens and using a comparatively large throw distance (e.g. 1″ optical scan length feasible for a 50 μm mechanical scan amplitude using a 10 inch or 20 inch throw distance (250×, 125×respectively). 
     FIG. 20 shows a segment of a III-V VCSEL scanning array with scanning stage  210  carrying multiple VCSELs  212 . Only one set of comb actuators  220  and  221  will be described, however, as shown, multiple sets of combs are used to move stage  210 . Comb actuator  220  includes fixed finger sets  222  and  226  and movable finger set  224  while comb actuator  221  includes fixed finger sets  223  and  227  and movable finger set  225 . Movable finger sets  224  and  225  support stage  210  with suspension beams  216  and  217 . Fixed finger sets  222 ,  226  and  223 ,  227  are attached to substrate  250 . Voltages V 1  and V 2  supply fixed finger sets  222  and  223  with the power to actuate the comb drives  220  and  221  in the y direction shown. Each VCSEL  212  has conductive line  240  which runs from VCSEL contact pad  242  on substrate  250  over movable finger sets  224 ,  225  along suspension beams  216  and  217  to supply the VCSEL electrical connection. The pitch of VCSEL arrays is on the order of a hundred microns. The resonant scanning frequency is on the order of tens of kilohertz, with a displacement of over tens of micrometers. When combined with a projection optical system of seven (7) times magnification, the VCSEL array of 4 cm width is capable of covering a page width scan of 12 inches. 
     FIG. 21 shows a z-, or Θ-degree of freedom of VCSEL motion. VCSEL  312  is supported on stage  310 . The micromachined semiconductor substrate  350  has VCSEL  312  on stage  310  suspended from suspension springs  314  and  315 . The actuation is done with parallel plate capacitors formed between stage  310  and electrodes  316  and  317  on glass cover  320 . “Raster Output Scanner with Pivotal Mirror for Process Direction Light Spot Position Control”, U.S. patent application Ser. No. 08/761,681, filed on Dec. 6, 1996 cited earlier discloses the manufacture and operation of a pivoting mirror, the operation of moving stage  310  being similar in operation to that of the pivoting mirror. Differential actuation of electrodes  316  and  317  produces a Θ motion; common actuation of electrodes  316  and  317  produces a z-motion. The dimensions and geometry of the suspension springs can be optimized to either favor a Θ or z-degree of freedom. Conductive line  340  electrically connects VCSEL  312  with contact pad  342 . FIG. 22 is a cross-sectional view of FIG. 21, along line  22 — 22 . Glass cover  320  is supported above substrate  350  by seal  322 . 
     An embodiment of a scanning VCSEL shuttle in accordance with the present invention is shown in FIG.  23 . VCSEL shuttle  400  has a large movable stage  410  carrying several hundred VCSELs  415  that is suspended by folded mechanical beams  420  to allow for a large scan range, driven by electrostatic actuation using comb drive fingers  425  and  426 . 
     Folded beams  420  are anchored to substrate  401  at anchor points  402 . Electrostatic comb drives may be replaced with other actuation systems as is well-known in the MEMS art, for example, magnetic, thermal and piezoelectric systems. Pairs of comb drive fingers  425  and  426  are used to provide the electrostatic driving force. Fixed fingers  425  are attached to rigid stationary beam  430  while movable fingers  426  are attached to movable stage  410 . When a bias voltage is applied to odd numbered electrodes  431 ,  433 , and the other odd numbered electrodes not shown, movable stage  410  is pulled to the right in FIG.  23 . When a bias voltage is applied to even numbered electrodes  432 ,  434 , and the other even numbered electrodes not shown, movable stage  410  is pulled to the left in FIG.  23 . Movable stage  410  is made to scan back and forth by alternating the voltage bias between the odd numbered electrodes such as electrodes  431  and  433  and the even numbered electrodes such as electrodes  432  and  434 . The dimensions of comb drive fingers  425  and  426  and folded beams  420  are chosen so that the maximum scan range is greater than half of the pitch of VCSELs  415  on movable stage  410 . For example, if the pitch of VCSELs  415  is 60 μm, the maximum scan range needs to be at least 30 μm. 
     In accordance with an embodiment of this invention, folded mechanical beams  420  are used as shown in FIG. 23 because the folded beam approach gives a much larger scan range than a straight clamped-clamped beam of the same length. The layers making up folded beams  420  are the same as the layers making up clamped-clamped beams  216  and  217  in FIG. 20. A folded beam flexure design for the same material composition reduces the development of axial forces and possesses a much larger linear deflection range. Folded mechanical beams  420  also function to allow conductive heat transfer from VCSELs  415  into substrate  401 . 
     Electrical connections to VCSELs  415  are established via conductive lines  435  which run from contact pads  480  on substrate  401  to movable stage  410  in accordance with an embodiment of this invention. Conductive lines  435  run over and are electrically isolated from the neutral fiber of folded beams  420  to VCSELs  415 . The ‘neutral fiber’ is the stress free centerline of folded beams  420 . Conductive lines  435  are chosen sufficiently small relative to the width of folded beams  420  and run along the stress free centerlines of folded beams  420  to allow conductive lines  435  to survive the flexing of folded beams  420 . Typical thickness for conductive lines  435  is 2 μm. The electrical isolation of conductive lines  435  from folded beams  420  may be accomplished in many ways, for example, with a dielectric layer underneath conductive lines  435  or insulating the surface of folded beams  420  using shallow ion implantation. 
     Folded beams  420  establish the return current flow from VCSELs  415  which reside at electrical ground. Return current flows through the active region of VCSEL  415  to n-DBR layer  62  (e.g., see FIG. 2) to n-GaAs buffer layer in the lower part of folded beam  420 , to undoped GaAs substrate  401  and out to ground. 
     FIG. 20 shows clamped-clamped beams  216  and  217 . The deflection of clamped-clamped beams  216  and  217  only obeys linear deflection theory up to a quarter of beam thickness, H. In contrast, folded beam flexure has the same beam stiffness ratio as the clamped-clamped beam flexure while obeying linear deflection theory up to approximately 10% of beam length, L. The beam stiffness ratio, s r  is defined as: 
     
       
         s r =k x /k y   (1) 
       
     
     where k x  and k y  are the spring constants of a suspension beam along the x and y axis, respectively. The y axis is taken to be in the scan direction. 
     FIG. 24 shows a unit cell of VCSEL shuttle  400  with folded mechanical beam  420 . For the folded beam flexure shown in FIG. 24, the system spring constant k y  for the unit cell is given by: 
     
       
         k y =8EHb 3 /3L 3   (2) 
       
     
     where E is the Young&#39;s modulus of the material, L is the beam length, H is the beam thickness, b is the beam width. The maximum deflection of movable stage  410  supported by folded mechanical beam  420  is given by: 
     
       
         d m =d(k x /2k y ) ½   (3) 
       
     
     where d is the gap between fingers and k x  is a function of the deflection in the y direction, δ y , from the neutral position. With increased deflection of folded mechanical beam  420  from the neutral position, the spring constant, k x , in the x direction decreases as 1/δ y   2 . 
     Maximum displacement of movable stage  410  is proportional to: 
     
       
         d m α(dLH/b) ½   (4) 
       
     
     Equation (4) shows that the maximum deflection without sticking or finger touching is proportional to the thickness, H, of beam with a proportionality constant of approximately 1.08. Hence, the thicker the folded beam, the larger the maximum deflection will be for a given width, b, folded beam length L and comb drive fingers gap, d. Referring to FIG. 24, if folded beam  420  has a length, L, of 500 μm, a thickness, H, of 25 μm, and a width, b, of 7 μm and comb drive fingers gap, d, of 2 μm, the maximum displacement, d m , is greater than 40 μm. 
     Increasing the thickness of folded beams  420  aids in preventing beam flexure and preventing shuttle  410  from buckling. Referring also to FIG. 2, slight lattice mismatch between AlGaAs layer  58  and GaAs substrate  56  in the structure of VCSELs  415  leads to compressive stress in AlGaAs layer  58  and promotes buckling of shuttle  410 . The lattice constant of an AlGaAs lattice is slightly larger than the lattice constant of a GaAs lattice. As the thickness of GaAs buffer layer  60  (see FIGS. 2-8) increases, the buckling-free length of shuttle  410  will increase as well. Assuming that a deep etching technique is developed for GaAs material, it is possible to make shuttle  410  over 100 μm thick (such etching processes exist for silicon material). Fold mechanical beams  420  and shuttle  410  in FIG. 23 may also be strengthened by depositing a tensile strained dielectric film such as SiN x  or SiO 2  to compensate for the built-in compressive strain. Typically, GaAs substrate  56  underneath beams  430  is not removed to increase the mechanical strength of beams  430  to which fixed fingers  425  are attached. 
     In order to electrically isolate fixed fingers  425  from movable fingers  426  and VCSELs  415 , VCSELs  415  are grown on semi-insulating substrate  56  with 18 μm thick n-GaAs buffer layer  60  as shown in FIG.  2 . VCSELs  415  are grown on top of n-GaAs buffer layer  60 . Referring to FIG. 23, fixed fingers  425  are isolated from movable fingers  426  by isolation grooves  499 . VCSELs  415  receive power from conductive pads  480 . Contact lines (not shown) to VCSELs  415  from conductive pads  480  are laid out over the centers of folded mechanical beams  420  where stress due to bending is a minimum. Isolation grooves  499  are etched into semi-insulating substrate  56  shown in FIG.  2 . 
     The driving voltage needed to displace VCSEL movable stage  410  supported by a folded beam flexure a distance of 30 μm is approximately 224 V in static (non-resonant) mode. The driving voltage, V, may be expressed as: 
     
       
         V=(d m dk y /nε 0 h) ½   (5) 
       
     
     where n is the number of fingers for one drive direction, h is the width of the comb fingers, d is the gap spacing between fingers and ε 0  is the dielectric constant in air. The driving voltage under resonance conditions is reduced to 22.4 V if the Q factor in air of movable stage  410  is 100. If narrower folded beams are used the driving voltage can be reduced further. The resonant frequency for movable stage  410  is calculated to be about 5.3 kHz. 
     Movable stage  410  needs to scan at least half of the pitch of VCSELs  415  to resolve all the pixels across a page width line. If VCSEL  415  pitch is 60 μm, then the maximum scan range of movable stage  410  has to exceed 30 μm. As discussed above, movable stage  410  in the configuration shown in FIG. 23 is capable of a displacement greater than 40 μm in either direction while maintaining linearity. 
     An alternative design for a scanning VCSEL shuttle is shown in FIG.  25 . Multiple movable stages  411  are arranged in parallel so that VCSELs  415  are staggered. For the same VCSEL pitch as shown in FIG. 23 of 100μm, the required displacement of multiple movable stages  411  is only half that of the single movable stage  410  shown in FIG. 23 increasing reliability and reducing the driving voltage. With a 7× projection system,  435  VCSELs and 145 stages are needed to achieve results similar to that of the configuration shown in FIG.  23 . 
     Electrical connections to VCSELs  415  are established via conductive lines  435  which run from contact pads  480  on substrate  401  to movable stages  411  as in FIG.  23 . Conductive lines  435  run over and are electrically isolated from the neutral fiber of folded beams  420  to VCSELs  415 . Return current from VCSELs  415  flows through folded beams  420 . The ‘neutral fiber’ is the stress free centerline of folded beams  420 . As noted above, conductive lines  435  are chosen sufficiently small relative to the width of folded beams  420  and run along the stress free centerlines of folded beams  420  to allow conductive lines  435  to survive the flexing of folded beams  420 . The electrical isolation of conductive lines  435  from folded beams  420  may be accomplished in many ways, for example, with a dielectric layer underneath conductive lines  435  or insulating the surface of folded beams  420  using shallow ion implantation. 
     For the folded beam flexure shown in FIG. 25, the system spring constant k y  is given by: 
     
       
         k y =4k 1 k 2 k 3 /[k 1 k 2 +k 3 (k 1 +k 2 )]  (6) 
       
     
     where: 
     
       
         k n =EHb 3 /L n   3  n=1, 2, 3  (7) 
       
     
     with E, H, and b as defined above. L n  is the length of the respective folded beam segments as shown in FIG.  25 . Taking L 2 ≈L 1 =L 3 /2 results in the expression for k y:   
     
       
         k y =2EHb 3 /5L 1   3   (8) 
       
     
     For the same length ratios, the system spring constant k x  is given by: 
     
       
         k x =EHb/L 1   (9) 
       
     
     Hence, for the folded beam structure shown in FIG. 25, with L 1 =250 μm, the maximum deflection is about 79 μm before sticking. The resonant frequency of VCSEL shuttle  411  is calculated to be about 11 khz. 50 movable fingers  426  are used for the comb drives in each scan direction. For a beam width, b, of 7 μm, the static driving voltage, V s =348 V and assuming a Q of 100, the resonant driving voltage V r =34.8 V. If the beam width, b, is taken to be 5 μm, the static driving voltage, V s , is reduced to 210 V and again assuming a Q of 100, the resonant driving voltage V s =21 V. 
     As those skilled in the art will appreciate, other various modifications, extensions, and changes to the foregoing disclosed embodiments of the present invention are contemplated to be within the scope and spirit of the invention as defined in the following claims.