Abstract:
An electrophotographic printer includes an exposure unit having a MEMS scanner operable to scan a beam of light across a photoconductor. The MEMS scanner includes a mirror having an aspect ratio similar to the shape of the facets of a conventional rotating polygon scanner. In a preferred embodiment, the scan mirror has a length of about 750 microns in a dimension parallel to its axis of rotation and a length of about 8 millimeters in a dimension perpendicular to its axis of rotation. The MEMS scanner is operable to scan at a frequency of about 5 KHz and an angular displacement of about 20 degrees zero-to-peak mechanical scan angle.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application claims benefit from the U.S. Provisional Patent Application Ser. No. 60/542,896, entitled MEMS SYSTEM ADAPTED TO A LASER PRINTER, invented by Wyatt O. Davis et al., applied for on 9 Feb. 2004. 
   This application relates to material in the co-pending U.S. Patent Applications entitled HIGH PERFORMANCE MEMS SCANNER, invented by Wyatt O. Davis et al., application Ser. No. 10/986,640, applied for on 12 Nov. 2004; METHOD AND APPARATUS FOR MAKING A MEMS SCANNER, invented by Kelly D. Linden et al., application Ser. No. 10/986,635, applied for on 12 Nov. 2004; and METHOD AND APPARATUS FOR SCANNING A BEAM OF LIGHT, invented by Gregory T. Gibson et al., application Ser. No. 10/988,155, applied for on 12 Nov. 2004. 
   FIELD OF THE DISCLOSURE 
   The present invention relates to microelectromechanical system (MEMS) scanners and particularly to their application to laser printers. 
   BACKGROUND 
   Electrophotographic, computer controlled printers have become pervasive in the office, factory, print shop, copy center, and home. An electrophotographic printer operates by transferring toner to plain paper and fusing the toner by means of heat, pressure, and/or other fixing technologies. The pattern of the transferred toner may form characters, graphic images, etc. 
   The term electrophotography refers to the use of modulated light, frequently a scanned laser beam, to create an electrostatic latent image on a photoconductive carrying medium such as a drum or belt. The latent electrostatic image is formed by momentary electrical conductivity of the photoconductor in response to exposure to the modulated light. The momentary conductivity allows a surface charge to discharge through the photoconductor to a conductor held at a bias voltage at locations corresponding to the modulated light exposure. 
     FIG. 1  is a diagram illustrating the principal features of an electrophotographic printer. A photoconductive drum  102  is rotated past a charging or sensitization station  104  that deposits a static charge substantially uniformly over the surface of the drum  102 . An imaging module  106  modulates light selectively over the surface of the photoconductor  102 . This causes the static charge in those spots receiving light to discharge through the photoconductive layer to a conductive layer on the backside of the photoconductor surface. The pattern of discharged and non-discharged spots is referred to as a latent electrostatic image or latent image. 
   Electrophotographic printers may be made to write-white or write-black. In a write-black system, the toner charge is selected to be attracted to the photoconductor backside conductive layer bias voltage and repelled from the sensitization static charge deposited on the photoconductor surface. Thus, the spots “written” by the modulated light correspond to black areas of the printed page. 
   Once the electrostatic latent image is formed, the photoconductor  102  is further rotated to a developer  108 , where oppositely charged toner, most often in the form of fine, dry particles, is attracted to and deposited on the surface of the photoconductor in a pattern corresponding to the latent image. The photoconductor  102  is further rotated to a transfer point, where the patterned toner is then transferred to the paper  112 , often using an electrostatic attraction element  110  such as a corona wire in the form of a corotron or scorotron. 
   The paper  112 , with toner loosely adhered thereto, is fed forward through a fusing station  114  that, generally through a combination of heat and pressure, causes the thermoplastic toner particles to permanently adhere to the paper, thus forming a robust image. 
   Following transfer of the toner, the photoconductive medium  102  is rotated past a discharge lamp  116  and a cleaner  118 , and then repeats the process as it is rotated to the sensitizer or charger  104 . 
   In various printers, light emitting diode (LED), liquid crystal shutter (LCS), vacuum fluorescent, and other types of arrayed light modulator write heads have been used for modulating light onto the photoconductor. Generally though, scanned laser beam exposure or imaging modules have gained favor in the art due to an appropriate balance of cost, speed, performance, and durability. An electrophotographic printer that uses a scanned laser beam to provide light modulation onto the surface of the photoconductive medium may be conveniently referred to as a laser beam printer or LBP. 
     FIG. 2  illustrates the general construction of an LBP exposure unit  106  made according to the prior art with a rotating polygon beam scanner. A laser diode  202  having a wavelength matched to the sensitivity of the photoconductor (often infrared in the case of an organic photoconductor) is modulated with an image signal. Beam-forming optics  204  produce a laser beam having a desired shape and trajectory. The laser beam is reflected off a rotating polygon mirror  206  and is scanned across the photoconductor  102  through optical elements  208 . It may be noted that the design of the exposure module  106  is such that the reflective facets  210   a ,  210   b , etc. of the rotating polygon  206  are placed forward of the center of rotation such that the arriving beam sweeps over each mirror surface as it is deflected across its deflection angle, the deflection angle being sufficient to traverse the photoconductor  102 . 
   One difficulty encountered with scanned laser beam exposure modules relates to the technology used to scan the laser beam. Most frequently, rotating polygon mirrors have been used. Rotating polygon mirrors may suffer from relatively large mass, slow ramp-up to speed, large size, noise, bearing reliability issues, relatively high power consumption, and other shortcomings. 
   Overview 
   Various aspects according to the disclosure relate to microelectromechanical system (MEMS) scanners and the use of a MEMS laser beam scanner in an electrophotographic printer exposure unit. Such an approach can result in reduced mass and size, faster start-up, reduced noise, higher reliability, and other advantages, compared to rotating polygon exposure units. 
   According to several aspects of the invention, a MEMS laser beam scanner may be formed with various physical and operational attributes; including mirror size, scan angle, scan frequency, and mirror flatness; to be especially well-adapted to an electrophotographic printer exposure unit. A MEMS mirror with extended length transverse the scanning axis can be substituted for a rotating polygon without substantial modifications to the exposure module optical design. 
   Other aspects will become apparent to the reader through reference to the appended brief description of the drawings, detailed description, claims, and figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating the principal components of a typical electrophotographic printer. 
       FIG. 2  is a diagrammatic view of a LBP exposure unit with a rotating polygon scanner made according to the prior art. 
       FIG. 3  is a view of a MEMS scanner made according to an embodiment of the present invention. 
       FIG. 4  is a graph illustrating the dynamic response of the MEMS scanner of  FIG. 3 . 
       FIG. 5  is a view of a number of MEMS devices of  FIG. 3  showing how they can be arrayed on a silicon wafer during manufacture. 
       FIG. 6  is a view of a mounting clamp for mounting the MEMS scanner of  FIG. 3 . 
       FIG. 7  is a detailed view of a piezo-electric stack used to form the actuator of  FIG. 6 . 
       FIG. 8  is a front perspective view of a MEMS scanner package for use in a LBP. 
       FIG. 9  includes two additional perspective views of the MEMS scanner package of  FIG. 8 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  illustrates a MEMS scanner  302  that may be used in a LBP. The exemplary embodiment shown herein relates to a 40 page per minute (ppm), 1200 dot per inch (dpi) LBP. MEMS scanner  302  is photolithographically formed from single-crystal silicon using bulk micromachining as is known to the art. A scan plate  304  having a mirror surface is coupled to a pair of torsion arms  306   a ,  306   b  through respective suspension beams  308   a ,  308   b . Torsion arms  306   a ,  306   b  define a rotational axis  310  about which scan plate  304  and suspension beams  308   a ,  308   b  rotate. Suspension beams  308   a  and  308   b  help to keep the mirror surface relatively flat, typically within lambda/4, by spreading the torque loads induced by the torsion arms  306   a ,  306   b  across the surface of the scan plate  304 . 
   Suspension beams  308  are coupled to scan plate  304  by respective outer (lateral) connectors  316   a ,  316   b ,  316   c ,  316   d  and respective axial connectors  318   a ,  318   b . Taken together, suspension elements  308   a ,  316   a ,  316   b , and  318   a  form a first suspension coupling between first torsion arm  306   a  and scan plate  304 . Similarly suspension elements  308   b ,  316   c ,  316   d , and  318   b  form a second suspension coupling between second torsion arm  306   b  and scan plate  304 . 
   A mirror surface can be formed on the surface of scan plate  304  using metal, stacked dielectric, or other technologies known to the art. Aluminum can be used to form a mirror having greater than about 85% reflectivity at red and infrared wavelengths (having a local minimum at about 825 nanometers wavelength). Gold or silver can be used to form a mirror having greater than about 90% to 95% reflectivity at red and infrared wavelengths. Stacked (such as quarter-wave) dielectric reflectors can achieve very high reflectivity across a wide range of wavelengths. 
   Torsion arms  306   a ,  306   b  terminate at respective “T-bars”  312   a  and  312   b . T-bars  312   a  and  312   b , in turn connect to respective mounting pads  314   a ,  314   b  and  314   c ,  314   d  as illustrated. Taken together, T-bar  312   a  and mounting pads  314   a ,  314   b  constitute a first mounting structure for coupling torsion arm  306   a  to a support structure (not shown). Similarly, T-bar  312   b  and mounting pads  314   c ,  314   d  form a second mounting structure for coupling torsion arm  306   b  to a support structure (not shown). In alternative embodiments, mounting structures can take other forms, including for example a pair of rectangular mounting pads, each joined directly to a respective torsion arm, or other forms. Alternatively, a frame-type mounting structure may be formed peripheral to the scan plate  304  and torsion arms  306   a ,  306   b . The exemplary embodiment of  FIG. 3  may have certain advantages such as, for example, packing more devices per wafer, having reduced dynamic stress, allowing individual mounting pads to be coupled to actuators, and allowing the mounting pads  314  to “float” relative to one another, thereby reducing residual stresses in the MEMS scanner. 
   When mounting pads  314   a ,  314   b ,  314   c , and  314   d  are mounted to a housing, periodic application of power to an actuator (not shown) will cause mirror  304  to periodically rotate back and forth about the axis of rotation  310  defined by torsion arms  306   a ,  306   b.    
   Scan plate  304  is formed to be approximately 8 millimeters long (in the direction perpendicular to the axis of rotation  310 ) and 750 micrometers wide (in the direction parallel to the axis of rotation  310 ). Thus, for the exemplary embodiment, the scan plate (and mirror formed thereon) has a lateral dimension about 10.67 times its longitudinal dimension. 
   When driven with an appropriate signal, (such as a 5 kilohertz (KHz) sine wave varying between about 0 (zero) and 25-30 volts for a four actuator design) the mirror responds with a ±20° mechanical scan angle at a frequency of 5 KHz. 
   As illustrated, MEMS scanner  302  includes two torsion arms  306   a ,  306   b , each 8.76 millimeters long (including fillets), terminated on their proximal ends by a 400 micron by 200 micron elliptical fillet at respective suspensions (in particular at suspension beams  308   a ,  308   b ), and terminated on their distal ends at respective T-bars  312   a ,  312   b , again with a 400 micron by 200 micron elliptical fillet. The torsion arms  306   a ,  306   b  are 384 microns wide. As with the rest of MEMS scanner  302 , the torsion arms are etched to a full wafer thickness of 700 microns using DRIE processing. For a given scan plate mass and mass distribution, the width, depth, and length of the torsion arms and T-bars may be adjusted to produce alternative resonant scan frequencies and angles. 
   The suspension beams  308   a ,  308   b  are 396 microns wide, are slightly bent to make a slightly obtuse angle with respective torsion arms  306   a ,  306   b  of 91.6 degrees, and extend laterally to an extent equal to the lateral extent of the 8 millimeter lateral dimension scan plate  304 . Respective suspension center connectors  318   a ,  318   b  extend from the centerlines of suspension beams  308   a ,  308   b  to the centerline of the scan plate  304 , a distance of 500 microns (including fillets). The center connectors  318   a ,  318   b  are each 164 microns wide and include 100 micron radius fillets at both ends. Four suspension outer connectors  316   a ,  316   b ,  316   c , and  316   d  extend from the ends of suspension beams  308   a ,  308   b  to the scan plate  304 , one on each end of each suspension beam as indicated. The outer connectors  316   a ,  316   b ,  316   c ,  316   d  are each 250 microns wide (laterally) by 400 microns long (longitudinally) and do not have fillets. The respective suspensions thus each include a suspension beam  308 , a center suspension connector  318 , and two outer suspension connectors  316 ; and connect the torsion arms  306   a ,  306   b  to the scan plate  304  in a manner that reduces stress concentrations, spreads the torque load, and reduces dynamic deformation of the scan plate during operation. Alternative suspension configurations are possible and could be implemented by one skilled in the art. 
   The T-bars  312   a ,  312   b  are each 1.8 millimeters long (total lateral dimension inclusive of fillets) by 400 microns wide (longitudinal dimension) and extend symmetrically from and perpendicular to the axis formed by torsion arms  306   a ,  306   b . The outer ends of T-bars  312   a ,  312   b  connect to four respective mounting pads  314   a ,  314   b ,  314   c ,  314   d  with 200 micron radius fillets as shown. The mounting pads are each 5 millimeters square. The geometry of the T-bars and mounting pads may be adjusted to suit application requirements. 
     FIG. 4  shows graphs illustrating the dynamic response of the MEMS scanner of  FIG. 3  when a periodic drive signal is applied. Curve  402  indicates an amplitude response  404  as a function of periodic drive frequency  406 . Curve  408  illustrates scanner vs. drive phase  410  plotted against the same periodic drive frequency axis  406 . From inspection of curve  402 , one can see a peak in response at about 5 KHz corresponding to the resonance frequency of the MEMS scanner in the rotation mode. While the size of the peak is plotted on a relative basis, it is, for the exemplary embodiment, sufficiently high to produce a resonance response of ±20° mechanical scan angle at acceptable drive power. For a four-actuator embodiment, a drive waveform approximating a 5 KHz sine wave with amplitude of 0 (zero) to 25-30 volts results in ±20° mechanical scan angle. 
   The secondary peak at between 65 and 70 KHz corresponds to the resonant behavior of the piezo-electric stack actuators. 
   Curve  408  illustrates how the phase relationship of the drive signal to the MEMS scanner response inverts at the resonance points. Below 5 KHz, the phase relationship (drive to response) is 0°. Above 5 KHz but below the secondary peak, the phase relationship is −180°. At the primary resonant peak, the phase relationship inverts and passes through −90° (response lagging drive) as indicated. Above the secondary peak, the response of the system drops and the phase response again inverts, passing from −180° below the peak, through −270°(+90°) at the secondary resonance peak, to −360° (0°) at frequencies above the secondary resonance peak. To maximize efficiency, it has been found to be advantageous to operate the MEMS scanner at or very near the primary resonance peak. 
   For operation at 5 KHz, the resonance frequency of the MEMS scanner is trimmed to be a few hertz above 5 KHz, typically in the range of 5.001 to 5.005 KHz. Such trimming may be accomplished using methods described in U.S. Pat. No. 6,245,590, hereby incorporated by reference. It has been found to be advantageous to factory trim resonant frequency using a method of adding weight, in the form of epoxy applied to the scan plate. 
     FIG. 5  illustrates a prototypical layout of MEMS scanners  302   a ,  302   b ,  302   c ,  302   d ,  302   e , and  302   f  on a 100 millimeter silicon wafer  502 . As may be seen, the MEMS scanners are densely packed with interdigitated mounting pads and mirrors. This is done to maximize yield per wafer. Larger wafers would be similarly densely packed with devices. Rather than dicing the devices apart using a dicing saw, a photolithographic step such as deep reactive ion etch (DRIE) is used to almost completely release the scanners from the wafer. Very fine silicon “bridges” may be seen connecting the scanners to the wafer at intervals. To release the scanners, these bridges are simply broken and the scanners popped out. 
     FIG. 5  further illustrates an alternate T-bar  312  design wherein the ends of the “T” are offset toward the scan plate. This can result in shorter part length or better interdigitation of neighboring parts on the wafer, and may be useful for minimizing scanner size, maximizing yield from the wafer, etc. 
     FIG. 6  illustrates a clamp and actuator arrangement for the MEMS scanner. A pair of commercially-available piezo-electric stacks  602   a  and  602   b , set upon a common mounting base  604 , support respective mounting pads  314   a ,  314   b  of MEMS scanner  302  through respective first insulators  606   a ,  606   b . From their respective positions, the piezo-electric stacks  602   a ,  602   b  may be alternately electrically compressed and expanded to produce a periodic rotation of the mounting pads  314   a ,  314   b  about the axis of rotation  310  defined by torsion arms  306   a ,  306   b . Similarly, common mode activation of the piezo-electric stacks  602   a ,  602   b  may be used to rotate the MEMS scanner  302  about a transverse axis substantially parallel to the long axis of the mirror  304 . 
   To maintain contact between the MEMS scanner  302  and the piezo-electric actuator stacks  602   a ,  602   b , respective clamps or pressure assemblies  608   a  and  608   b  ( 608   b  not shown) press the mounting pads  314   a ,  314   b  down against the actuator stacks. Clamp  608   b  is omitted from  FIG. 6  for clarity. As shown, clamps  608  include (starting from the bottom of the assembly and contacting the mounting pad  314 ) a first pressure plate  610 , an optional series disk spring  612 , a second pressure plate  614 , a second insulator  616 , and a third pressure plate  618 . In one embodiment, an edge of first pressure plate  601  is extended out from the pressure assembly as shown. As will be explained below, this provides a bonding position for a heater wire or lead. Series disk spring  612  is of a commercially available type such as SPRINGMASTERS #D63203 and is selected to have relatively low stiffness but high (&gt;&gt;5 KHz) intrinsic resonant frequency. A series of two springs, a different number of springs, or no springs at all may be used, depending upon application requirements. First and second pressure plates  610  and  614  provide robust surfaces for series disk spring  612  to press against. Second insulator  616  provides for electrical insulation of the MEMS scanner  302 . First and second insulators  606 ,  616  are formed from a material with appropriate density, electrical insulating ability, and compressive strength such as PYREX glass. First and second pressure plates  610 ,  614  are formed from materials that are suitably electrically conductive and have appropriate physical properties including compressive strength, toughness, and density, such as steel. Third pressure plate  618  provides a mounting surface for second insulator  616  and connects the assembly to a housing (not shown). Third pressure plate  618 , preferably formed from steel, includes a bore  620  for receiving a mounting and adjustment screw (not shown). As may be appreciated by those skilled in the art, alternative or modified clamps may be used. 
     FIG. 7  is a view of a piezo stack actuator  602 . Such actuators are available from several sources including http://www.physikinstrumente.de model PICMA 885.10. 
     FIGS. 8 and 9  are views of a MEMS scanner housing  802  for use in a LBP. Two front plates  804   a ,  804   b  are fastened to a rear housing  806  with mounting screws  808   a ,  808   b ,  808   c ,  808   d . MEMS scanner  302  is held in a cavity therein that allows for an appropriate amount of rotation. Threaded adjustment screw holes  810   a ,  810   b ,  810   c , and  810   d  receive adjustment screws (not shown) that protrude into corresponding adjustment screw receiving bores  620  formed in third pressure plates  618  (shown in  FIG. 6 ). During assembly, adjustment screws are turned to provide an appropriate amount of preload on series disk spring  612  (not shown). Behavior of the MEMS scanner under actuation can be observed through MEMS observation port  812 , formed in the top of rear housing  806 . The MEMS scanner assembly  802  is secured to the exposure unit of a LBP via mounting tabs  814   a ,  814   b  formed in housing  806 . 
   The use of clamps  608  to secure the MEMS scanner  302  in housing  802  results in a mount that “floats”, allowing the mounting pads  314  to move a bit with respect to one another. In some embodiments, slight twisting of the clamps  608  during assembly can result in slight in-plane twisting of the mounting pads  314 . This can result in undesirable residual stress in the T-bars and/or torsion bars of the MEMS scanner. Such twisting may be reduced or eliminated by running or “burning-in” the mounted scanner for a few hours at reduced scan angle. In an exemplary embodiment, the scanner is run at half amplitude for approximately four hours. The burn-in process can reduce the occurrence of “infant” failures associated with mechanical failure of the T-bars and/or torsion arms. Alternative, reduced twist clamp assembly designs may be substituted to reduce or eliminate the need for scanner assembly burn-in. 
   The MEMS scanner  302  may be driven by four piezo-electric stacks  602 , one juxtaposed against each mounting pad  314   a ,  314   b ,  314   c , and  314   d . Alternatively, one end of the MEMS scanner may be held in a fixed position, i.e. mounting pads  314   c  and  314   d  may be clamped against a solid mounting point, and the other end of the MEMS scanner may be driven by piezo-electric actuators, i.e. mounting pads  314   a  and  314   b  may each be clamped against piezo-electric stacks as shown in  FIG. 6 . In a third alternative, three of the mounting pads are each clamped to a fixed, solid mounting point and one piezo-electric stack actuator is used. Typically, the choice hinges on cost vs. actuator power requirements. 
   As indicated above, the MEMS scanner is trimmed to have a resonant frequency within a few hertz of the desired operational frequency. As may be appreciated from curve  402  of  FIG. 4 , small changes in resonant frequency can result in relatively substantial changes in rotation amplitude (for a given periodic actuation voltage). The inventors have discovered that controlled heating of the MEMS device further trims the resonance frequency, and hence the actuation amplitude, even though in the exemplary embodiment the MEMS scanner has no outer frame. Referring back to  FIG. 6 , the extended tab on the first pressure plate  610  of clamp  608   a  receives a heater wire as does the corresponding pressure plate of clamp  608   b  (not shown). Similarly, corresponding pressure plates adjacent mounting pads  314   c  and  314   d  (also not shown) also receive heater wires. The heater wires may be attached by soldering to the gold plated extended tab of first pressure plates  610 , by soldering to metallized bond pad formed, for example on the mounting pads  314 , or by other method as will be apparent to those skilled in the art. In use, the scan amplitude is monitored by sensors and the electric potential between the two ends of scanner  302  (mounting pads  606   a  and  606   b  forming one such end and mounting pads  606   c  and  606   d  forming the other end) is adjusted. Resistance of the silicon material to current flow, and particularly the torsion arms  306   a ,  306   b , causes joule heating. Higher temperatures cause a “softening” of the torsion arms and a corresponding reduction in resonant frequency. Thus, when the device is operated just below its nominal resonant frequency, heating may be increased to reduce the scan amplitude or heating may be reduced to increase the scan amplitude. It has been experimentally determined that 0 to 1.5 W of tuning power can provide a resonant frequency tuning range of about 8 Hz. This range may be somewhat lower at higher scanning frequencies and somewhat higher at lower scanning frequencies, presumably as a result of airflow over the scanner providing cooling during operation. 
   The preceding overview of exemplary embodiments of the invention, brief description of the drawings, and detailed description describe exemplary embodiments of the present invention in a manner intended to foster ease of understanding by the reader. Other structures, methods, and equivalents may be within the scope of the invention. As such, the scope of the invention described herein shall be limited only by the claims.