Abstract:
A method for damping electro-mechanical ribbon elements over a channel defining a bottom surface and having a bottom conductive layer formed below the bottom surface includes the steps of: providing at least one constant amplitude voltage pulse to at least one ribbon element wherein the actuation pulse causes the ribbon element to contact the bottom surface of the channel; and providing at least one braking pulse to the ribbon elements wherein the braking pulse is separated by a narrow temporal gap from the constant amplitude voltage pulse. Furthermore, the method can also provide a sophisticated damping for ribbon elements which are actuated so that they do not contact the bottom of the channel.

Description:
FIELD OF THE INVENTION 
     This invention relates to modulating an incident light beam with a mechanical grating device and more particularly to a method for actuating a mechanical grating device that functions to diffract a light beam. 
     BACKGROUND OF THE INVENTION 
     Electro-mechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing and printing. Electro-mechanical gratings for spatial light modulation are well known in the patent literature; see U.S. Pat. No. 5,311,360, issued May 10, 1994, to Bloom et al., entitled “Method and Apparatus for Modulating a Light Beam”. This device, which is also known as a grating light valve (GLV), was later described by Bloom et al. with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces see U.S. Pat. No. 5,459,610, issued Oct. 17, 1995 to Bloom et al., entitled “Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate”. 
     According to the prior art, for operation of the GLV device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. Modulation of the diffracted optical beam is obtained by appropriate choice of the voltage waveform. The voltage needed to actuate a ribbon a certain distance depends on several factors including the stress in the ribbon material, the distance between the ribbons and substrate, and the ribbon length. 
     It is well known that the ribbon elements of the GLV device possess a resonant frequency which depends primarily on the length of the ribbons and the density and tension of the ribbon material; see for example “Silicon Microfabrication of Grating Light Valves,” Ph.D. Thesis, Stanford University 1995, Chapter 3, by F. S. A. Sandejas. When a ribbon is actuated or released, it rings at its resonant frequency, which is typically between 1 and 15 MHz. The mechanical response of the ribbon elements is damped by the surrounding gas as described in “Squeeze Film Damping of Double Supported Ribbons in Noble Gas Atmospheres,” Proc. Of Solid-State Sensor and Actuator workshop, Hilton, Head, SC, June 8-11, 198, pp. 288-291. This damping depends on the type of gas present and pressure, and determines the width of the resonant peak associated with the resonant frequency of the ribbons. As a result of this resonant ringing, the maximum frequency at which the GLV device can be operated is limited, and the diffracted light intensity contains undesirable temporal variations. There is a need therefore for a GLV device having increased operating speed and reduced temporal light intensity variations. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for actuating the ribbon elements of an electromechanical grating device so that the diffracted light intensity does not contain any undesirable temporal variations caused by the resonance of said ribbon elements, thereby producing a more ideal output that will allow higher frequency operation of the device. The above object is accomplished by a method for damping electro-mechanical ribbon elements suspended over a channel defining a bottom surface and having a bottom conductive layer formed below said bottom surface, the method comprises the steps of: providing at least one constant amplitude voltage pulse to at least one ribbon element to cause the ribbon element to contact the bottom surface of the channel; and providing at least one braking pulse to said ribbon elements wherein said braking pulse is separated by a narrow gap from said constant amplitude voltage pulse. 
     According to a further aspect of the invention, the above object is also accomplished by a method for damping electro-mechanical ribbon elements suspended over a channel defining a bottom surface and having a bottom conductive layer formed below said bottom surface, the method comprises the steps of: providing at least one constant amplitude voltage pulse to at least one ribbon element wherein said constant amplitude voltage pulse causes said ribbon element to be drawn into the channel wherein said ribbon element is in a suspended actuated state above the bottom of the channel; and providing at least two braking pulses to said ribbon elements wherein an initial braking pulse immediately precedes said constant amplitude voltage pulse and a final braking pulse immediately follows said constant amplitude voltage pulse. 
     It is an advantage of the inventive method that the light intensity diffracted by the electromechanical grating device that any undesirable temporal variations caused by the resonance of the ribbon elements are substantially reduced. The method allows high-frequency operation of the device which is especially important in applications such as displays, photofinishing printers and optical communications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is described with reference to the embodiments shown in the drawings. 
     FIGS. 1 a - 1   b  are illustrations of light diffraction by a prior art two-level electro-mechanical grating device in the un-actuated and actuated state respectively; 
     FIG. 2 is a view perpendicular to the illustration of FIG. 1 showing a suspended unactuated ribbon element above the channel with supporting layers according to the prior art; 
     FIGS. 3 a - 3   c  are idealized graphs depicting: a) a constant-amplitude voltage pulse that causes the ribbon elements to come into contact with the bottom of the channel; b) the mechanical response of the ribbon elements; and c) the intensity of the light diffracted into the first order as observed in the prior art devices; 
     FIGS. 4 a-b  are experimental results showing a) an actuation voltage pulse and b) the corresponding intensity of the light reflected into the zeroth order as observed in the prior art device; 
     FIGS. 5 a-   5   c  are idealized graphs depicting: a) a constant-amplitude voltage pulse that causes the ribbon elements to come into contact with the bottom of the channel followed by a braking pulse; b) the mechanical response of the ribbon elements; and c) the intensity of the light diffracted into the first order according to the present invention; 
     FIGS. 6 a-   6   b  are experimental results showing a) an actuation pulse followed by a braking pulse of opposite polarity and b) the corresponding intensity of light reflected into the zeroth order according to the present invention; 
     FIGS. 7 a-   7   c are idealized graphs depicting: a) a constant-amplitude voltage pulse that causes the ribbon elements to come into contact with the bottom of the channel followed by two braking pulses; b) the mechanical response of the ribbon elements; and c) the intensity of the light diffracted into the first order according to the present invention; 
     FIG. 8 is an idealized graph of a sequence of actuation voltage pulses each followed by a braking pulse of the same polarity according to the present invention; 
     FIG. 9 is an idealized graph of a sequence of actuation voltage pulses each followed by a braking pulse of opposite polarity according to the present invention; 
     FIGS. 10 a-   10   c  are idealized graphs depicting: a) a constant-amplitude voltage pulse that causes the ribbon elements to remain suspended above the bottom of the channel; b) the mechanical response of the ribbon elements; and c) the intensity of the light diffracted into the first order as observed in the prior art; and 
     FIGS. 11 a-   11   c  are idealized graphs depicting: a) a constant-amplitude voltage pulse that causes the ribbon elements to remain suspended above the bottom of the channel wherein the constant-amplitude voltage pulse is preceded and succeeded by a braking pulse; b) the mechanical response of the ribbon elements; and c) the intensity of the light diffracted into the first order according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The structure of a typical Grating Light Valve (GLV) device is shown in the cross-sectional views in FIGS. 1 a ,  1   b  and  2 . FIG. 1 a  depicts the ribbon structure of the device in the un-actuated and FIG. 1 b  in the actuated state. FIG. 2 is the view of the same device (as shown in FIG. 1 a ) in the un-actuated state but rotated 90 degrees to provide an insight into the layer build-up of the GLV. Referring to FIG. 2, typically a substrate  20  is provided which may be a single crystal silicon wafer or glass. In the case of a single crystal silicon wafer, a bottom conductive layer  22  is generated by heavily doping the silicon near the surface  23  of the substrate  20 . If glass is used as a substrate, the increased conductivity is achieved by depositing a bottom conductive layer  22  on the surface of the glass substrate  20 . The conductive layer  22  is covered by a protective layer  24 , which can be for the example of a silicon substrate, thermal oxide. A dielectric spacer layer  26  is formed atop the protective layer  24  and contains a channel  28  where the active region of the GLV device is located. The channel  28  defines a depth which is governed by the deposited thickness of the spacer layer  26 . The spacer layer  26  defines an upper surface level  27 . A plurality of ribbon elements  31  is patterned from a ribbon layer  30  formed atop the spacer layer  26 . The ribbon layer  30  comprises a dielectric material, which may be silicon nitride, covered by a conductive and reflective layer  32 . The conductive and reflective layer  32  of every other ribbon element  31  is connected to the bottom conductive layer  22  through an opening  34  that is filled with a thick layer of conducting material  36 . The thickness and tensile stress of the ribbon layer  30  is chosen to optimize performance by influencing the electrostatic force required for actuation and the returning force, which affects the speed, resonance frequency and voltage requirements of the ribbon elements  31  of the GLV. 
     For operation of the device, an attractive electrostatic force is produced by a voltage difference between the bottom conductive layer  22  and the reflective and conductive layer  32  atop the ribbon layer  30 . In the un-actuated state (see FIG. 1 a ), with no voltage difference, all of the ribbon elements  31  in the GLV device are suspended above the substrate  20  at the same height. In this state, an incident light beam  14  is primarily reflected into the mirror direction as a reflected light beam  15 . To obtain the actuated state (see FIG. 1 b ), a voltage is applied to every other ribbon element producing a periodic grating. In the fully actuated state every other ribbon element  31  is in contact with the protective layer  24 . When the height difference between adjacent ribbons is ¼ of the wavelength of an incident light beam  16 ′ the light beam is primarily diffracted into a 1 st  order  17  and a −1 st  order  18 . Depending on the design, to obtain ¼ wavelength height difference the ribbon elements  31  may be brought into contact with the protective layer  24  or they may be suspended above the protective layer  24 . One or both of the diffracted orders can be collected and used by an optical system (not shown), depending on the application. Alternatively, the 0 th  reflected order may be used in certain optical systems. When the applied voltage is removed, the forces due to the tensile stress and the bending moment restore the ribbon elements  31  to their original un-actuated state (see FIG. 1 a ). 
     FIGS. 3 a-   3   c  show idealized plots of the actuation of a ribbon element  31  by a constant amplitude voltage pulse  40 . The constant amplitude voltage pulse  40  is a function of time as shown in FIG. 3 a  with a duration of 2 μsec at 10 volts. Here the abscissa shows the time in μsec and the ordinate shows the applied voltage in volts. The response of the mechanical position of the ribbon element  31  to voltage pulse  40  is depicted in FIG. 3 b , with the abscissa showing the time in μsec and the ordinate showing the mechanical position of the ribbon element  31  in nanometers. When the voltage pulse  40  is switched on, the ribbon element  31  is drawn into the channel  28  of the GLV device and comes into contact with the bottom of the channel  28 . The ribbon element  31  stays in the actuated position (drawn into the channel  28  and in contact with the bottom of the channel  28 ) for the duration of the constant amplitude voltage pulse  40 . As soon as the constant amplitude voltage pulse  40  is turned off (after 2 μsec), the ribbon element  31  releases from the bottom of the channel  28  and oscillates in a ringing motion  42  about the initial un-actuated position. The ringing motion has a resonant period t R    43  and is damped by the surrounding gas. The ribbon element  31  settles to its the initial un-actuated position after a certain time which is governed by a conventional damping function. The time-dependence of the light intensity of the diffracted into the first order resulting from the actuation of the ribbon element  31  is shown in FIG. 3 c . Here the abscissa shows the time in μsec and the ordinate shows the percent intensity of the incident light diffracted into the first order. The diffracted intensity  44  is constant during the duration of the voltage pulse  40 . After the constant amplitude voltage pulse  40  has been turned off, an intensity fluctuation  46  occurs that is associated with the ringing motion  42  of the ribbon elements  31 . 
     The ringing motion  42  of the ribbon elements  31  and associated light intensity fluctuations  46  have undesirable effects in certain systems, especially in systems that require high frequency modulation of the light intensity. For example, in data communication or data storage application where the light intensity is modulated in accordance to a high-frequency data stream, the residual ringing caused by one voltage pulse will affect the response to a subsequent voltage pulse. This effect, which is sometimes known as inter-symbol interference, has a negative impact on data integrity. 
     These kinds of intensity fluctuations also occur in systems that make use of the 0 th  order reflected light. FIGS. 4 a  and  4   b  show an experimental result wherein a voltage pulse  47  has been applied to a GLV device of the type shown in FIGS. 1 and 2 in a system that collects 0 th  order reflected light. The voltage pulse  47  in FIG. 4 a  has a duration of approximately 0.5 μsec and an amplitude of approximately 16 volts. The signal from a photodetector measuring the 0 th  order intensity is shown in FIG. 4 b  (arbitrary units). With no voltage applied to the ribbon elements  31 , the majority of the light is reflected into the 0 th  order and the 0 th  order signal  48   a  is at a high level. During the voltage pulse  47 , the 0 th  order signal  48   b  is constant and at a low level. Ideally, in an optimized device, this low level would be close to zero. After the voltage pulse  47  has been turned off, an intensity fluctuation  48   c  occurs because of the ringing motion of the ribbon elements  31 . 
     The use of a braking pulse can significantly reduce the ringing of the ribbon elements  31  and the associated intensity fluctuations. FIGS. 5 a-   5   c  show idealized plots of the actuation of a ribbon element  31  by a constant amplitude voltage pulse  50  followed by a braking pulse  52 . The constant amplitude voltage pulse  50  is a function of time as shown in FIG. 5 a  with a duration of 2 μsec at 10 volts. Immediately after the constant amplitude voltage pulse  50 , a narrow braking pulse  52  is applied that is separated from the constant amplitude voltage pulse  50  by a narrow gap  54 . Here the width of the gap  54  is approximately the same as the width of the braking pulse  52 . Both widths are smaller than one-half the period of the oscillation of the ribbon elements. The response of the mechanical position of the ribbon element  31  to the applied voltage pulses (constant amplitude voltage pulse  50  and braking pulse  52 ) is shown in FIG. 5 b . The abscissa shows the time in μsec and the ordinate shows the mechanical position of the ribbon element  31  in nanometers. When the voltage pulse  50  is switched on, the ribbon element  31  is drawn into the channel  28  of the GLV device and comes into contact with the bottom of the channel  28 . The ribbon element  31  stays in the actuated position (drawn into the channel  28  and in contact with the bottom of the channel  28 ) for the duration of the constant amplitude voltage pulse  50 . As soon as the voltage pulse  50  is turned off, the ribbon element  31  returns to the initial un-actuated position. There are very small oscillations  55  about the initial position of the ribbon element  31 , but they do not significantly affect the intensity of the light diffracted into the first order as shown in FIG. 5 c . Here the abscissa shows the time in μsec and the ordinate shows the percent intensity of the incident light diffracted into the first order. The intensity is a function of time and the diffracted intensity is constant during the duration of the constant amplitude voltage pulse  50 . After the constant amplitude voltage pulse  50  has been turned off the intensity returns to zero. The duration of the 1 st  order diffracted light intensity is now limited to approximately the duration of the constant amplitude voltage pulse  50 . 
     FIGS. 6 a  and  6   b  show an experimental result wherein a voltage pulse  67   a  followed by a braking pulse  67   b  have been applied to a GLV device of the type shown in FIGS. 1 and 2 in a system that collects 0 th  order reflected light. The voltage pulse  67   a  in FIG. 6 a  has a duration of approximately 0.5 μsec and an amplitude of approximately 16 volts. The braking pulse  67   b  has a polarity opposite to that of the voltage pulse  67   b  and is approximately −14 volts. The polarity was reversed for the braking pulse  67   b  because the driver electronics could not slew fast enough to produce a well-formed gap with a positive polarity braking pulse. Unlike the idealized result in FIG. 5 a , in this case there is no well-defined gap because of the rise time associated with the driver electronics. The signal from a photodetector measuring the 0 th  order intensity is shown in FIG. 4 b  (arbitrary units). With no voltage applied the ribbon elements  31 , the majority of the light is reflected into the 0 th  order and the 0 th  order signal  68   a  is at a high level. During the constant amplitude voltage pulse  67   a , the 0 th  order signal  68   b  is constant and at a low level. After the voltage pulse  67   a  has been turned off and the braking pulse  67   b  has been applied, the ribbons elements return to their initial un-actuated state without ringing substantially and only a small intensity fluctuation  68   c  occurs. 
     According to a second embodiment shown in FIG. 7 a , the actuation of a ribbon element  31  by a constant amplitude voltage pulse  70  is followed by a first and a second braking pulse  72  and  74 . This pair of braking pulses can be used in cases when a single braking pulse does not completely stop the ringing of the ribbon elements  31 . The constant amplitude voltage pulse  70  is a function of time as shown in FIG. 7 a  with a duration of 2 μsec at 10 volts. Immediately after the constant amplitude voltage pulse  70 , a narrow braking pulse  72  is applied separated from the voltage pulse  70  by a narrow gap  76 . The second braking pulse  74  is applied after the first braking pulse  72 . Here the two braking pulses  72  and  74  are similar in amplitude and duration. They are separated by a braking gap  78  whose width is approximately equal to the resonant period of the ribbon elements. The response of the mechanical position of the ribbon element  31  to the applied voltage pulses (constant amplitude voltage pulse  70  and first and second braking pulse  72  and  74 ) is shown in FIG. 7 b . When the voltage pulse  70  is switched on, the ribbon element  31  is drawn into the channel  28  and comes into contact with the bottom of the channel  28 . The ribbon element  31  stays at the bottom of the channel  28  for the duration of the voltage pulse  70 . Once the applied constant voltage pulse  70  is turned off, the ribbon element  31  begins to return to its initial un-actuated position. The application of the first braking pulse  72  reduces the velocity, but not sufficiently to bring the ribbon element  31  to rest. There is a first and second oscillation  71  and  73  about the initial position before the application of the second braking pulse  74 . The second braking pulse  74  brings the ribbon to rest. The effect of these additional oscillations  71  and  73  on the intensity of the light diffracted into the first order is to cause a first and a second minor intensity peak  71   a  and  73   a  (see FIG. 7 c ). 
     FIGS. 8 and 9 illustrate the use of braking pulses for an incoming data stream that is pulse width modulated. The data are applied to the mechanical ribbon elements  31  as a sequence of actuation pulses that are constant amplitude voltage pulses of various widths (duration). As shown in FIG. 8 each constant amplitude voltage pulse  80  is followed by a braking pulse  82  of the same polarity. FIG. 9 shows an alternative embodiment for the actuation of the mechanical ribbon elements  31 . A sequence of constant-amplitude voltage pulses  90  is applied to the ribbon elements  31 , wherein adjacent constant amplitude voltage pulses have the opposite polarity. In this embodiment, the polarity of braking pulses is opposite to that of the associated voltage pulses, i.e., opposite to the polarity of the voltage pulse that precedes it. This mode of operation is advantageous when the driver electronics cannot produce well-formed gaps between the constant amplitude voltage pulses and the braking pulses. Switching polarity has the further advantage of minimizing induced charge in the GLV layers. 
     According to a further embodiment of the present invention, the mechanical ribbon elements  31  are actuated so that they do not contact the bottom of the channel  28 . The ribbon elements  31  remain suspended above the bottom of the channel  28  because the constant amplitude voltage pulse does not generate enough electrostatic force to completely overcome the tensile force. This mode of operation is usually used with devices that have a deep channel  28  (approximately one wavelength deep). The amplitude of the voltage pulse is chosen to achieve ¼ wavelength deflection of the ribbon elements  31 . 
     FIGS. 10 a-b   10   c  show idealized plots of the actuation of a ribbon element  31  by a constant amplitude voltage pulse  100  for non-contact operation in which the actuated mechanical ribbon does not contact the bottom of the channel  28 . The constant amplitude voltage pulse  100  shown in FIG. 10 a  has a duration of 2 μsec and an amplitude of 10 volts. The response of the ribbon element  31  to the applied constant amplitude voltage pulse  100  is depicted in FIG. 10 b . When the constant amplitude voltage pulse is turned on, the ribbon element  31  is drawn into the channel  28  of the GLV device. Since the ribbon element  31  does not contact the bottom of the channel  28 , there is pulse onset ringing  102  and pulse shutoff ringing  104  in the response. The onset ringing  102  occurs for nearly the entire duration of the constant amplitude voltage pulse  100 . The shutoff ringing  104  occurs for approximately the same amount of time. This ringing affects the intensity of the light diffracted into the first order as shown in FIG. 10 c , generating light intensity fluctuations at onset  106  and shutoff  108 . 
     Braking pulses may be used to eliminate ringing in this non-contact mode of operation as well. FIGS. 11 a-   11   c  show idealized plots of the actuation of a ribbon element  31  by a constant amplitude voltage pulse  110  preceded by an initial braking pulse  112  and succeeded by a final braking pulse  114  (see FIG. 11 a ). The braking pulses  112  and  114  are separated from the constant amplitude voltage pulse  110  by an initial gap  116  and a final gap  117 . In this example, the two braking pulses  112  and  114  and the two gaps  116  and  117  are all of approximately the same equal width. The response of the mechanical position of the ribbon element  31  to the applied voltage pulses (initial braking pulse  112 , constant amplitude voltage pulse  110  and final braking pulse  114 ) is shown in FIG. 11 b . Compared to FIG. 10 b  there is no pulse onset ringing  102  or pulse shutoff ringing  104  observable. For the duration of the constant amplitude voltage pulse  110 , the element stays in a suspended position above the bottom of the channel  28  and the diffraction of the incident light beam takes place. As soon as the applied constant amplitude voltage pulse  110  is turned off, the ribbon element  31  returns into the initial position. There are no significant ribbon oscillations visible at the beginning or at the end of the constant amplitude voltage pulse  110 . The intensity of light  118  diffracted into the first order is shown in FIG. 11 c . The diffracted intensity is constant during the constant amplitude voltage pulse  110  with no intensity fluctuations observable at turn-on or turn-off. The duration of the intensity in the light diffracted into the first order is now approximately limited to the duration of the constant amplitude voltage pulse  110 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       14  incident light beam 
       15  reflected light beam 
       16  incident light beam 
       17  diffracted light beam 1 st  order 
       18  diffracted light beam −1 st  order 
       20  substrate 
       22  bottom conductive layer 
       23  surface of the substrate 
       24  protective layer 
       26  spacer layer 
       27  upper surface level 
       28  channel 
       30  riibbon layer 
       31  ribbon elements 
       32  reflective layer 
       34  opening 
       36  conducting material 
       40  constant amplitude voltage pulse 
       42  ringing motion 
       44  resonant period 
       44  light diffracted into the first order 
       46  intensity fluctuation 
       47  actuation pulse 
       48   a  light intensity before actuation 
       48   b  light intensity during actuation 
       48   c  light intensity after actuation 
       50  constant amplitude voltage pulse 
       52  braking pulse 
       54  gap 
       55  small oscillations 
       67   a  actuation pulse 
       67   b  braking pulse of opposite polarity 
       67   a  light intensity before actuation 
       68   b  light intensity during actuation 
       68   c  light intensity after actuation 
       70  constant amplitude voltage pulse 
       71  first oscillation 
       71   a  first intensity peak 
       72  first braking pulse 
       73  second oscillation 
       73   a  second intensity peak 
       74  second braking pulse 
       76  gap 
       78  braking gap 
       80  constant amplitude voltage pulses 
       82  braking pulses 
       90  constant amplitude voltage pulses 
       92  braking pulses 
       100  constant amplitude voltage pulse 
       102  pulse onset ringing 
       104  pulse shutoff ringing 
       106  onset intensity fluctuation 
       108  shutoff intensity fluctuation 
       110  constant amplitude voltage pulse 
       112  initial braking pulse 
       114  fmal braking pulse 
       116  initial gap 
       117  final gap 
       118  intensity of light diffracted into the first order