Abstract:
An solid state scanning system having a single crystal silicon deflection mirror and scanning mirror is integrated with a light source. Separation of the micro-electro-mechanical systems and light emitters on separate substrates allows the use of flip-chip and solder bump bonding techniques for mounting of the light sources. The separate substrates are subsequently full wafer bonded together to create an integrated solid state scanning system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to “METHOD AND APPARATUS FOR AN INTEGRATED LASER BEAM SCANNER” by Floyd, Sun and Kubby (Attorney Docket No. D/98706). Ser. No. 09/201738, filed on the same day and assigned to the same assignee which is hereby incorporated by reference in its entirety. 
     BACKGROUND AND SUMMARY OF INVENTION 
     The present invention relates generally to the field of laser beam scanning systems, and more particularly to micro-electro-mechanical systems (MEMS) for laser beam scanning. Miniature laser beam scanning systems are important for applications such as barcode scanning, machine vision and, most importantly, xerographic printing. The use of MEMS to replace standard raster output scanning (ROS) in xerographic print engines allows simplification of printing systems by eliminating macroscopic mechanical components and replacing them with large arrays of scanning elements. Advanced computation and control algorithms are used in managing the large arrays of scanning elements. Such MEMS based printing systems are entirely solid state, reducing complexity, and allowing increased functionality, including compensation of errors or failures in the scanner elements. 
     An important step in constructing solid state scanning systems is integration of the semiconductor light emitter directly with MEMS actuators to gain the desired optical system simplification. Integrated scanners, which have lasers and scanning mirrors in the same structure, have been demonstrated using manual placement of laser chips onto MEMS wafers with micromachined alignment parts and adhesives by L. Y. Lin et al in Applied Physics Letters, 66, p. 2946, 1995 and by M. J. Daneman et al in Photonics Technology Letters, 8(3), p. 396, 1996. However, current techniques do not allow for wafer-scale integration of the light-emitter and MEMS device. 
     In accordance with the present invention a laser beam scanner consisting of a single crystal silicon deflection mirror and a torsional mirror is integrated with a laser diode in the same structure. Details of creating a torsional mirror and actuating it magnetically or electrostatically are detailed in U.S. Pat. No. 5,629,790 by Neukermans and Slater which is incorporated herein by reference in its entirety. 
     Using solder bump bonding methods, completed and tested laser diodes are bonded to a glass or a silicon carrier substrate. The carrier substrate is aligned and bonded to a Si or SOI wafer containing the MEMS layers. Bonding of the lasers to a carrier substrate completely partitions the bonding process from the MEMS. This complete partition eliminates possible conflicts between the conditions needed for solder bump bonding, such as the use of solder flux, and preserves the integrity of the MEMS layers. 
     The substrates are heated in a non-oxidizing environment to join the two substrates. High surface tension of the solder aligns the wettable metal bonding pads on each substrate with each other. The ability of the reflowed solder to self-align the substrates because of surface tension simplifies assembly. 
     The use of the SCS layer of a SOI wafer, rather than a polysilicon film provides for the introduction of very flat and smooth mirrors and high reliability torsion bars. The device is scalable to arrays of lasers and scanning mirrors. 
     Integration of the scanner and light source eliminates the need for external, manual alignment of light sources and scanning mirrors. Simplified post-processing steps such as interconnect metallization can be realized because the use of an etched recess results in nearly planar surfaces. In addition, pick and place technologies used for multi-chip module assembly can be adapted for wafer scale assembly and bonding of light sources to the carrier substrate. 
     Thus, the present invention allows the integration of lasers, electrical interconnects, and electrodes on a single glass or Si wafer for actuation of MEMS devices. The glass or Si wafer is aligned and bonded to the MEMS wafer, forming an integrated, three dimensional structure. 
    
    
     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 is shows MEMS layers and VCSEL for a laser beam scanner in accordance with an embodiment of this invention. 
     FIG. 2a shows a laser beam scanner with optical path having an electrostatically actuated torsion mirror in accordance with an embodiment of this invention. 
     FIG. 2b shows a top view a laser beam scanner in accordance with an embodiment of this invention. 
     FIG. 3a shows a laser beam scanner with optical path having a magnetically actuated torsion mirror in accordance with an embodiment of this invention. 
     FIG. 3b shows a laser beam scanner with optical path having a magnetically actuated torsion mirror using an external magnetic field in accordance with an embodiment of this invention. 
     FIGS. 4a-4j show process steps for fabricating MEMS components in accordance with an embodiment of this invention. 
     FIGS. 5a-5e show steps for fabricating substrate containing laser die and mirror actuation electrodes in accordance with an embodiment of this invention 
     FIG. 6a shows a completed laser beam scanner before release of deflecting mirror in accordance with an embodiment of this invention. 
     FIG. 6b shows a completed integrated solid state scanner after release of deflecting mirror in accordance with an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION 
     An embodiment in accordance with the present invention is shown in FIG.  1  and FIG. 2a. A laser beam scanner consisting of single crystal silicon (SCS) deflecting mirror  240  and torsional mirror  250  is integrated with laser diode or light emitting diode  105 . Using solder bump bonding methods, completed and tested laser diodes  105  are bonded to glass or silicon carrier substrate  101 . Carrier substrate  101  is aligned and bonded to MEMS substrate  130  containing the MEMS layers. Bonding of laser diode  105  to carrier substrate  101  completely partitions the bonding process from the MEMS layers. This complete partition eliminates possible conflicts between the conditions needed for solder bump bonding, such as the use of solder flux, and preserves the integrity of the MEMS layers. Typically, solders such as Pb/Sn, Au/Sn, or In/Sn are evaporated selectively onto wettable metal bonding pads  111  onto substrate  101  and reflowed to form hemispherical solder bumps  110 . Solder bumps  110  are contacted to wettable metal bonding pads  113  on laser substrate  106 . 
     Laser substrate  106  and carrier substrate  101  are heated in a non-oxidizing environment to join the respective substrates together. High surface tension of the solder aligns wettable metal bonding pads  11   1  with wettable metal bonding pads  113  on laser substrate  106 . The ability of the reflowed solder to self-align laser substrate  106  with carrier substrate  101  because of surface tension simplifies the assembly process. Additionally, very little pressure is required during the process of bonding laser substrate  106  to carrier substrate  101 . 
     Micromechanical elements (MEMS) are formed on MEMS substrate  130 , typically about 500 μm thick using conventional photolithography and the patterning of single crystal silicon (SCS) layer  118 , polysilicon layers  117  and insulating oxide layers  116 , which are typically PSG or thermal oxide, is performed using both dry and wet etching techniques. MEMS substrate  130  embodies SCS layer  118 , insulating oxide layer  116  and silicon substrate  115 . Typical thickness for each of layers  116 ,  117 , and  118  is on the order of several μm. VCSEL (vertical cavity surface emitting laser)  105  is solder bump  110  bonded to glass or dielectric-coated (typically SiO 2  or Si 3 N 4  coated) Si substrate  101 , typically about 500 μm thick. Additionally, two actuation electrodes  220  and two interconnects  125  are formed on glass or dielectric-coated Si substrate  101 . Interconnects  125  provide current to substrate  101  to power VCSEL  105  and to electrodes  220  for control of torsional mirror  250 . After solder bonding of VCSEL  105  to glass or dielectric-coated Si substrate  101 , substrate  101  is aligned and bonded to MEMS substrate  130 . 
     MEMS substrate  130  has deep reactive ion etching (RIE) and/or wet etched hole  135 , typically 3 mm in diameter, for emitted light  299  (see FIG. 2a) to pass through MEMS substrate  130  and onto deflecting mirror  240 . Deflecting mirror  240  reflects emitted light  299  onto torsional mirror  250 . As shown in FIG. 2a, polysilicon hinge  255  attaches deflecting mirror  240  to MEMS substrate  130 . Deflecting mirror  240  is etched from SCS layer  118 . Polysilicon hinge  255  allows deflecting mirror  240  to rotate clockwise about an axis perpendicular to the plane of FIG. 2a, out of MEMS substrate  130  to the position above via  135  as shown in FIG.  2 a. Deflecting mirror  240  is supported by support latch  268  controlled by a spring and latch assembly (not shown) in the manner described in the paper by Lin et al. in Photonics Technology Letters, 6(12), p. 1445, 1994 which is incorporated herein in its entirety by reference. Controlling the position and length of support latch  268  allows the angle of deflecting mirror  240  to be precisely fixed. Deflection of torsional mirror  250  in both directions is accomplished by charging alternately one of two actuator electrodes  220 . Torsional mirror  250  is electrically grounded and attracted to charged one of two actuator electrodes  220 . 
     FIG. 2b shows a top view of one combination deflection mirror/torsional mirror solid state element. Polysilicon hinges  255  and deflecting mirror  240  are shown along with hole  265  to receive the tab (not shown) on support latch  268 . The layout of torsional mirror  250  supported by torsion bar  270  with respect to hole  217  is also shown. 
     MEMS components such as deflecting mirror  240  and torsional mirror  250  can be formed in MEMS substrate  130  by using a combination of well-known surface and bulk micro-machining techniques. Polysilicon hinges  255  may be formed as described by M. C. Wu, “Micromachining for Optical and Optoelectronic Systems,” Proceedings of IEEE, Vol. 85, p. 1833, 1997 and by Pister et al., “Microfabricated Hinges,” Sensors and Actuators, A: Physical v. 33 n. 3 pp. 249-256, June 1992 which are hereby incorporated by reference in their entirety. 
     As seen in FIG. 1, bonding of VCSEL  105  to glass or SiO 2  coated Si substrate  101  completely separates the bonding process from the MEMS components. The separation eliminates possible conflicts between conditions needed for solder bump bonding, such as the use of solder flux and the integrity of the MEMS layers. Full wafer bonding of glass or dielectric-coated Si substrate  101  to MEMS substrate  130  is done at low temperature to avoid damage to VCSEL  105 . Metallization on glass or dielectric-coated Si substrate  101  is achieved by use of adhesive bonding techniques requiring temperatures of between 20° C.-100° C. 
     Another embodiment in accordance with the present invention is shown in FIG.  3 A. VCSEL (vertical cavity surface emitting laser)  105  is solder bump  110  bonded to glass or dielectric-coated Si substrate  101 . Glass or dielectric-coated Si substrate  101  is aligned and bonded to MEMS substrate  130 . MEMS substrate  130  has deep RIE and/or wet etched via  135  for emitted light  199  to pass through the surface of MEMS substrate  130  and onto deflecting mirror  240  which reflects emitted light  299  onto torsional mirror  250 . Torsional mirror  250  contains ferro-magnetic thin film  330  with magnetization in the plane of torsional mirror  250 . Coil  380  on glass or dielectric-coated Si substrate  101  generates magnetic field  391  perpendicular to the magnetic field created by ferromagnetic thin film  330  contained on torsional mirror  250 . Hence, actuation of coil  380  turns torsional mirror  250 . Polysilicon hinge  255  attaches deflecting mirror  240  to MEMS substrate  130 . Polysilicon hinge  255  allows deflecting mirror  240  to rotate clockwise about an axis perpendicular to the plane of FIG. 3a, out of MEMS substrate  130  to a position above via  135  as shown in FIG.  3 a. Deflecting mirror  240  can be supported by support latch  268  controlled by a spring and latch assembly (not shown) in the manner shown by Lin et al. in Photonics Technology Letters, 6(12), p. 1445, 1994 and incorporated herein in its entirety by reference. Fixing the position and length of support latch  268  allows the angle of deflecting mirror  240  to be precisely fixed. 
     FIG. 3b shows an embodiment in accordance with this invention wherein torsional mirror  250  contains microfabricated coil  350  generating magnetic field  385  perpendicular to torsional mirror  250  but is otherwise similar to FIG.  3 a. Coil  350  is a conductive loop which may be formed by vapor depositing conductive material onto torsional mirror  250  and patterning into coil  350 . External magnetic field  370  is applied parallel to the plane of torsional mirror  250  to turn torsional mirror  250 . Application of current to coil  350  results in an angular deflection of torsional mirror  250  proportional to the current introduced into coil  350 . Hence, coil  350  behaves like a galvanometer coil. Direction of current flow in coil  350  determines the direction of the angular deflection of torsional mirror  250 . 
     Steps for fabricating deflecting mirror, supporting latch and VCSEL in accordance with this invention are shown in FIGS. 4a-4j and FIGS. 5a-5e. The starting material is MEMS substrate  130  which comprises a silicon on insulator material (SOI). MEMS substrate  130  includes silicon substrate  115 , thermally-grown SiO 2  layer  116  bonded to wafer  113 . MEMS substrate  130  is then thinned to the required thickness. MEMS substrates  130  are commercially available from, for example, Bondtronix, Inc. of Alamo, Calif. or Ibis Technology Corporation of Danvers, Mass. Typical thickness of SCS layer  118  is 2-20 μm depending on the required stiffness of the torsional spring elements and mirror surfaces to be constructed. Other MEMS layers are deposited on top of MEMS substrate  130  by well-known methods such as low pressure chemical vapor deposition (LPCVD). These MEMS layers include mechanical layers of polycrystalline silicon (polysilicon)  117  (not shown in FIGS. 4) and sacrificial oxide layer  119  that is phosphorus-doped glass (PSG). The embodiment in FIG.  4 a   4 c has PSG layer  119  deposited directly on top of SCS layer  114   118 . Polysilicon layer  117  (see FIG. 1) is subsequently deposited on PSG layer  119 . Typical thicknesses for polysilicon layer  117  and PSG layer  119  are 1-2 μm. 
     Formation of MEMS elements occurs by conventional photolithography and patterning of SCS layer  114   118 , polysilicon layer  117 , and PSG layer  119  is performed using both wet and dry etching. In accordance with an embodiment of this invention, deflecting mirror  240  and deep recess  135  are required. 
     FIGS. 4a-4j show steps for fabricating deflecting mirror  240 , torsional mirror  250 , supporting latch  268 , and deep recesses  135  and  217 . Latch  268  has a tab (not shown) which inserts into corresponding hole  165  in the bottom of deflecting mirror  240 . The final configuration of deflecting mirror  240  and latch  255  are shown in FIG.  6 b. Typical sizes for deflecting mirror  240  are between 0.5 mm 2  to 1 mm 2 . 
     FIG. 4a has silicon nitride (SiN x ) deposited on substrate  130  using LPCVD. SiN x  layer (not shown) is patterned using CF 4 /O 2  RIE with a photoresist mask. Potassium hydroxide (KOH) is used to etch holes from the bottom of substrate  115 , stopping on layer  116 . Size of hole  217  is similar to torsional mirror  250  to allow free rotation. Hole  135  is simultaneously etched, for fitting VCSEL  105  which typically has dimensions of 500 μm by 500 μm. Alternatively, holes  217  and  135  may be defined by deep RIE using C 4 F 8  and SF 6  with a mask of SiN x  or photoresist. 
     FIG. 4b shows recess  135  (200-250 μm deep) etched into MEMS substrate  130  using a combination of CF 4 /O 2  RIE for etching SCS layer  114   118 and insulator layer  116  and a deep RIE of recess  135  using C 4 F 8  and SF 6 . 
     FIG. 4c shows CVD deposition of PSG layer  119 . 
     FIG. 4d shows the wet etch of windows  410  into PSG layer  119  down to SCS layer  114   118 . 
     FIG. 4e shows deposition of aluminum film  430  (typically 0.1-0.2 μm thick) as a high reflectivity layer. 
     FIG. 4f shows a wet etch (typically a mixture of phosphoric and nitric acid) of aluminum film  430  to remove aluminum in all but the mirror regions. The mirror region locations coincide with the locations of windows  410 . 
     FIG. 4g shows the etch of vias  433  using CF 4 /O 2  RIE with a photoresist mask. This step also serves to open laser die window  135 . 
     FIG. 4h shows formation of hinges  255  for deflecting mirror  240  from polysilicon layer  117  (not shown, see FIG. 1) that is deposited in this step. 
     FIG. 4i shows etch of PSG layer  119  and SCS layer  114   118 to pattern deflecting mirror  240 , hinges  255  and access holes  437 . A typical size for access holes  437  is 10 μm by 10 μm. Access holes  437  allow for the etchant used to release deflecting mirror  240  to reach insulating layer  116 . Deflecting mirror  240  size is typically from 1 mm 2 -2 mm 2 . Torsional mirror  250  is also defined in this step. 
     FIG. 4j shows release of deflecting mirror  240 , torsional mirror  250  and hinge  255  by etching PSG layer  119  and layer  116  using an HF based etch. 
     FIGS. 5a-e show the steps used to fabricate wafer  103  containing VCSEL  105  and mirror actuation electrodes  220  in accordance with an embodiment of this invention. 
     FIG. 5a shows starting glass or silicon substrate  101  for fabrication of wafer  103 . 
     FIG. 5b shows deposition of silicon nitride or silicon dioxide layer  502  by LPCVD or plasma-enhanced CVD process to provide electrical isolation from silicon substrate  101 . 
     FIG. 5c shows deposition of electrodes  220  and solder for solder bumps  110 . 
     FIG. 5d shows completed deposition of electrodes  220  for mirror actuation. Electrodes  220  are much thicker ˜200-300 μm) than solder bumps  110  (typically 50-100 μm) and are electroplated. 
     FIG. 5e shows alignment and solder bump bonding of VCSEL  105  to Si substrate  101  in the GaAs bonding step. Solder bumps  110  can be defined on metal bonding pads  113  of VCSEL substrate  106 . Si substrate  101  and VCSEL substrate  106  are heated to allow solder to flow and contact wettable metal bonding pads  111  on Si substrate  101 . 
     FIG. 6a shows integration of substrate  101  with MEMS substrate  130  using well-known procedures of adhesive bonding while FIG. 6b shows the finished assembly with raised deflecting mirror  240  locked into place with latch  168 . 
     Linear arrays of lasers can be bonded in a similar way; the extent of the array being perpendicular to the cross section shown in FIG.  6 a. 
     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.