Abstract:
We disclose a method for stabilizing against a drift of a deflection of a micromirror device having an electrostatic actuator, including the actions of: providing an actuator including at least two members beneath said micromirror and at least one electrode beneath said micromirror, at least one of said at least two members being formed of a semiconducting material, providing a surface layer on said at least one semiconducting member facing towards said other member of said actuator, said surface layer having a density of carriers being 10 17  cm 3  or higher.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority as a continuation-in-part of PCT Application No. PCT/SE2004/001963, entitled “SLM Structure Comprising Semiconducting Material” by inventor Torbjorn Sandstrom filed on 21 Dec., 2004, designating the United States and submitted in English. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to spatial light modulators (SLMs). In particular it relates to multivalued SLMs actuated with an analog voltage where said SLM comprising a semiconducting material in its structure.  
       BACKGROUND OF THE INVENTION  
       [0003]     SLMs with micromirror are well known in the art; for instance, see U.S. Pat. No. 6,747,783 by the same applicant as the present invention. SLMs can be said to be actuated in two distinct ways, analog actuation and digital actuation. In analog actuation an electrostatic force between an electrode and the mirror element is used to deflect the mirror element to a plurality of deflection states larger than two. The mirror position, or the degree of deflection, during actuation is determined by a balance between the actuation force and a spring constant of a support of the mirror element, for instance a hinge. Said mirror element is preferably set to a number of states between a fully deflected state and a non deflected state, where said fully deflected state is not determined by a fixed stop.  
         [0004]     In digital actuation, there are only two distinct deflection states of the mirror, fully on or fully off. The fully on state may be determined by a fixed stop, i.e., a high enough actuation force is applied in order to drive the mirror element to a fixed stop. Such a structure is sometimes referred to as a DMD structure (Digital Micromirror Device) and in such devices there are no deflection states in between the fully on and fully off states.  
         [0005]     Traditionally, said SLM is manufactured in an aluminum alloy, i.e., the actuator as well as the mirror element and the hinge element are made of said aluminum alloy. Said aluminum alloy has been shown to have some anelastic behavior, i.e., it has certain memory effects that makes the deflection of the mirror element for a specific driving voltage dependent not only on said voltage value but also on the history of applied voltage values. It could be thought of as a hysteresis effect, although it is generally more complex in its time dependence. It seems most metals show some amount of anelastic behavior, not only the traditionally used aluminum alloy. A material that does not show any measurable anelastic behavior is monocrystalline silicon. Silicon has several attractive properties, including perfect elastic behavior at room temperature, well-developed technology for etching, conduction of electricity, and a reasonable reflection of DUV electromagnetic radiation.  
         [0006]     However, one problem with the use of mono-crystalline silicon in actuators and/or mirror elements in high precision analog SLMs is that the surface potential is not stable. Said surface potential has been shown by experiments to vary as much as 1 V due to charges sitting on the surface, e.g., ionized molecules from air or electrons trapped at or in the native oxide of the silicon surface. Such a difference in surface potential gives a shift in actuating voltage for the same deflection, i.e., a drift in the characteristics of the actuator. Said shift may vary with time, temperature, electromagnetic radiation exposure, purging and an applied voltage history. All this together makes an SLM manufactured partly or completely of a semiconducting monochrystalline material, such as monochrystalline silicon very difficult to use for high precision applications.  
         [0007]     Thus, it is desirable to develop an SLM structure manufactured at least partly of a semiconducting material, which does not have the above mentioned problem with the drift in characteristics.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, one objective of the present invention is an SLM structure manufactured at least partly of a semiconducting material with no drift in characteristics, or one with a drift that is hardly measurable.  
         [0009]     This objective, among others, is attained by a method for stabilizing against a drift of a deflection of a micromirror device having an electrostatic actuator, including the actions of: providing an actuator including at least two members beneath said micromirror and at least one electrode beneath said micromirror, at least one of said at least two members being formed of a semiconducting material, providing a surface layer on said at least one semiconducting member facing towards said other member of said actuator, said surface layer having a density of carriers being 10 17  cm −3  or higher. By “beneath said micromirror” we refer to a specific orientation of a micromirror device. The function of an inverted micromirror device, or any other orientation of the same device, is of course independent of the geometrical orientation and “beneath” should be interpreted in this context.  
         [0010]     Further characteristics of the invention and advantages thereof will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying  FIGS. 1-8 , which are given by way of illustration only, and thus are not limitative of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  depicts schematically a top view of three mirrors in a micromirror array.  
         [0012]      FIG. 2  depicts a side view of the micromirrors along A-A in  FIG. 1  with one mirror in an addressed state.  
         [0013]      FIG. 3  depicts a side view of the micromirrors along A-A in  FIG. 1  with no applied voltage.  
         [0014]      FIG. 4  depicts a band diagram where the voltage shift is created by charges on the surface of the semiconductor.  
         [0015]      FIG. 5  depicts the same band diagram as in  FIG. 4 , but with a degenerated “metallic” layer facing the gap.  
         [0016]      FIG. 6   a  depicts a band diagram of a near degenerated inverted P silicon.  
         [0017]      FIG. 6   b  depicts a band diagram of an n-silicon which is driven to create a conductive layer at the surface by a perpendicular electric field.  
         [0018]      FIG. 6   c  depicts a band diagram of a metal film shielding the semiconductor from charges on the surface.  
         [0019]      FIG. 6   d  depicts a band diagram of a degenerated semiconductor throughout its volume.  
         [0020]      FIG. 6   e  depicts a band diagram of a near-degenerated conducting surface layer created by a thin film with a high concentration of fixed ions.  
         [0021]      FIG. 7  depicts a side view of the inventive micromirrors along A-A in  FIG. 1 .  
         [0022]      FIG. 8  depicts a side view of another inventive embodiment of a micromirror.  
         [0023]      FIG. 9  depicts use of a phase step to reduce stray reflections from space between micromirrors. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows.  
         [0025]     A micromirror device may in at least one example embodiment of the invention be an SLM. For instance, said SLM may be used in lithography formation of patterns, digital or analog actuation, according to well known techniques to a person skilled in the art and therefore needs no further clarification in this context.  
         [0026]      FIG. 1  depicts a top view of three mirrors  100  in a micromirror array  10 , only three mirrors  100  are illustrated for reason of clarity, in a real micromirror array the number of mirrors may be as many as several millions.  
         [0027]     The micromirrors illustrated in  FIG. 1  are hinged mirrors which may be deflected clockwise or counterclockwise. The micromirror  100  may be rotated around a hinge  120  supported at an anchor or post  110 .  
         [0028]      FIG. 2  depicts the same three mirrors as in  FIG. 1 . In the illustrated embodiment both the mirrors  100  and electrodes  130  and  140  are made of silicon. The flexure hinge and the anchors or posts, as well as the reflective surface of the mirror, may be made of silicon. The mirrors may be tiltable when voltage is applied, as is illustrated by central mirror in the example embodiment of the invention in  FIG. 2 .  
         [0029]      FIG. 3  depicts the same three mirrors as depicted in  FIG. 1 , but with no voltage applied. Even in the absence of voltage some mirrors will tend to tilt due to the difference in surface potential created by electrostatic charges at the silicon surface, as illustrated by the slightly tilted leftmost and middle mirrors in  FIG. 3 .  
         [0030]      FIG. 7  depicts an embodiment of a micromirror array according to the present invention. Here the electrodes  130  and  140  are provided with a surface layer with a high density of carriers. The surface resistance may be at most 1000 Ω/square. The mirrors  100  are also provided with a surface layer with a high density of carriers. Said surface of the mirrors are facing the electrodes  130  and  140 , i.e., the gap between the mirrors  100  and the electrodes  130  and  140 . Electrostatic forces may still form on the surface of the semiconducting material in an actuator structure comprising of said mirror element and at least one electrode in the inventive embodiment as illustrated in  FIG. 7 . However, the resulting surface potential drift may be much smaller, thus the minor deflection may be much smaller.  
         [0031]     In at least one example embodiment of the present invention, one or more electrodes and said mirror may be manufactured of a semiconducting material. Said semiconducting material may further be provided with a surface layer in Which a Fermi level falls at an electron energy where it creates a high density of carriers, i.e., inside an allowed band (conduction or valence bands) or in the band gap but close to a band edge. This may in most cases be equivalent to creating a conductive surface layer. In one example embodiment of the invention, a certain level of density of carriers may determine the location of said Fermi level. A high density of carriers may be accomplished in a number of ways, such as by high doping, coating with a conductive layer, inversion or accumulation of the surface by means of doping in the semiconductor, creation of fixed charges in a film, or by electric fields.  
         [0032]      FIG. 8  depicts another embodiment of the present invention. In a case where an electric field direction towards the semiconductor can be fixed to always have one sign, the doping of the semiconductor surface may be such that it will always be in accumulation. In  FIG. 8  the actuator (the mirror  100  and the electrodes  130  and  140 ) comprise a silicon side and a metal side. Here, the metal side is the metal electrodes  130  and  140  and the silicon side is the mirror made of silicon or another type of semiconducting material. If the mirror  100  is always negative in relation to the electrode, the semiconducting mirror should be n-doped. Furthermore, the electric field during operation should not approach zero, since a finite field may be needed to assure accumulation even in the presence of charges.  
         [0033]     In another embodiment both the electrodes  130  and  140  and the mirror  100  are made of a semiconducting material. In this case the doping of the mirror  100  should be opposite to the electrodes, e.g., an n-doped mirror means a p-doped electrode. It is only during the active (deflection critical) phase that the field must have the specified direction, i.e., at instances in time when the field is used to modulate the light and needs high precision deflection. If the direction of the electrical field is opposite, i.e. a mirror that is always positive, the doping should be reversed, i.e., the mirror should be p-doped and the electrode n-doped if both the mirror and electrode are made of a semiconducting material.  
         [0034]      FIGS. 4 and 5  illustrate band diagrams explaining how the invention works. Band diagrams are described in many textbooks on semiconductor physics and MOS technology, for example, S. M. Sze: “Semiconductor Devices Physics and Technology”, John Wiley &amp; Sons Inc, New York (2001) (ISBN 0471333727).  
         [0035]      FIG. 4  illustrates the band diagram of an actuator (electrode  500  and minor  430 ) with metal on one plate (electrode) and an n-doped semiconductor on the other (mirror) separated by an air gap  420 . There may be one Fermi level in the metal electrode  410  and another Fermi level in the semiconducting mirror  470 . The voltage seen in an external circuit may be the difference in Fermi levels.  FIG. 4  illustrates the Fermi levels and various bands with and without surface charges on a surface of the semiconducting mirror  430 . When charges are built/added up at the surface, said charges must be balanced by opposite charges. Since an n-doped semiconductor may be depleted close to the surface  450 , as may often be the case, the nearest place where balancing charges can be found is on the inner side of the depletion layer. Balancing charges are formed by a change in the depth of the depletion layer  455 . Between plus and minus charges there may be an electric field that can be integrated to give the surface potential change on the semiconductor. A change in surface potential may be proportional to the separation of charges  490 . As can be seen from  FIG. 4 , the Fermi level in an n-doped semiconductor without charges  470  may be closer to the Fermi level in a metal  410  than the Fermi level in an n-doped semiconductor with charges  475 . It can also be deducted from  FIG. 4  that when comparing the bulk material of the mirror a valence band  480  without charges may be closer to the Fermi level in the semiconductor  470  than a valence band with charges  485 . Additionally, a conductance band without charges  460  may be further away to the Fermi level  470  in the bulk material of the semiconducting mirror than a conductance band with charges  465 .  
         [0036]      FIG. 5  illustrates a band diagram of an actuator, a metal electrode  500  and a semiconducting mirror  530  separated by an air gap  520  according to the present invention. A surface of the semiconducting mirror  530  facing the metal electrode  500  may be doped high enough to become degenerated, i.e., said mirror  530  may be said to have metallic properties. In this application metallic properties means that the Fermi level in an example embodiment of the invention is inside an allowed band, here for instance the valence band  580 .  
         [0037]     In case a conducting layer in an example embodiment of the invention is formed outside of a depleted region, e.g., in an inversion layer, a degenerated surface layer, or a metal layer, said layer can be contacted to the substrate or any other suitable point in order to keep it from electrically floating.  
         [0038]     There are movable charges at a surface of the semiconducting mirror  530 , and when some charges are added balancing charges can be found right at the surface of said mirror  530 . The separation of charges  590  may be much smaller, in the order of nanometers, compared to the separation of charges  490  in the state of the art actuator structure as illustrated in  FIG. 4 , and thus the surface potential may be much smaller. A smaller surface potential will lead to very small deflection of the mirror when no voltage is applied between the mirror and the electrode. Also, a voltage shift due to charges  540  may be more or less eliminated, due to the fact that the Fermi level in the mirror  570  without charges in one example embodiment of the invention is more or less equal to the Fermi level in the mirror with charges  575 . As also can be seen from  FIG. 5 , the valence band  580  coincides with the valence band with charges  585 , and the conductance band  560  coincides with the conductance band with charges.  
         [0039]     In  FIGS. 4 and 5  it may be assumed that a force between the mirror  430 ,  530  and the electrode  400 ,  500  may be constant, i.e., the electric field in the air gap  420 ,  20 , in the actuator is constant. The influence from added charges is shown as a change in Fermi levels, i.e., the external voltage, needed to keep force (deflection of the mirror  430 ,  530 ) constant.  
         [0040]      FIGS. 6   a - 6   e  illustrate other embodiments of the present invention. In  FIG. 6   a , a band diagram of a near degenerated inverted p-silicon is shown, The same band diagram would be applicable for a near degenerated n-silicon (inverted or non-inverted) or an enrichment layer. The semiconducting material may be en elemental semiconductor such as silicon, diamond-like carbon, or germanium, or it may be a mixed semiconductor or a semiconducting compound such as silicon-germanium, GaAs, or silicon carbide.  
         [0041]     In  FIG. 6   a  the actuator is comprised of an electrode  600  made of a metal, a mirror  630  made of silicon, and an air gap  620  between sad mirror  630  and said electrode  600 . The Fermi level  610  in the metal electrode  600  is in the example embodiment of the invention below the Fermi level  670  in the semiconductor. A conductance band  660  at the surface facing towards the metal electrode  600  is closer to the Fermi level  670  in the mirror  630  than to the conductance band  660  in the bulk material of the mirror, i.e., it is deeper into the mirror material. On the other hand, a valence band  680  is further away from the Fermi level  670  at the surface of the minor element  630  facing towards said metal electrode  600  than the valence band  680  in the bulk material is to the same Fermi level  670 .  
         [0042]      FIG. 6   b  illustrates a band diagram of an n-silicon mirror, which is driven to create a conductive layer at the surface facing the metal electrode by a perpendicular electric field. The Fermi level in the metal  610  is lower than the Fermi level  670  in the semiconducting mirror  630 . A conductance band  660  is closer to the Fermi level  670  at a surface of the semiconducting mirror  630  facing the metal electrode  600  than the Fermi level  670  is to the same conductance band deeper into the semiconducting mirror. However, a valence band  680  is further away from the Fermi level  670  at a surface of the semiconducting mirror  630  than the valence band  680  is to the same Fermi level  670  deeper into the mirror element  630 .  
         [0043]      FIG. 6   c  depicts a band diagram of a metal film  695  shielding the semiconducting mirror  630  from charges on the surface facing towards the metal electrode  600 . The Fermi level  6   1   0  in the metal electrode  600  is lower than the Fermi level  670  in the semiconducting mirror  630 . A conductance band  660  is further away from the Fermi level  670  at the metal film  695  than the conductance band  660  is to the same Fermi level  670  further into the semiconducting mirror  630 . The valence band  680  is closer to the Fermi level at the metal film  695  than the valence band  680  is to the Fermi level  670  further into the semiconducting mirror  630 .  
         [0044]      FIG. 6   d  illustrates a band diagram of a semiconducting mirror which is degenerated throughout its volume and not only on its surface facing towards the metal electrode. A Fermi level  610  in the metal electrode  600  is below a Fermi level  670  of the semiconducting mirror  630 . The Fermi level  670  of the semiconducting mirror  630  is above both a conductance band  660  and a valence band  680  throughout its volume. A distance between said Fermi level  670  and said conductance band  680  is constant throughout the volume as is the distance between said Fermi level  670  and said valence band  660 .  
         [0045]      FIG. 6   e  illustrates a band diagram of a near degenerated conducting surface layer generated by a thin film with a high concentration of fixed ions. A Fermi level  610  in the metal electrode  600  is lower than a Fermi level  670  in the semiconducting mirror  630 . In this embodiment, the Fermi level  670  at the thin film with high concentration of ions  697  is closer to the conductance band  660  than the Fermi level  670  is to the same conductance band  660  further into the semiconducting mirror  630 . The valence band is however further away from the Fermi level  670  at the thin film with high concentration of fixed ions than said valence band is to the same Fermi level further into the semiconducting mirror.  
         [0046]     With a density of carriers high enough to create a minimized surface potential of the semiconducting surface in the actuator, the balancing of charges can be done by small physical displacement of carriers. An accumulation or inversion layer should be able to absorb changes of 10 11  carriers/cm 2  without going into depletion. A field in the air gap  620  is typically 10-50 MV/m. This field corresponds to a necessary charge rearrangement of 5-25*10 10  carriers/cm 2 . To absorb this change there should be 10-50*10 10  carriers/cm 2  close to the surface. To have this amount of carriers within 0.01 μm there is a need for 1-5*10 17  carriers/cm 3  in the layer. This gives a rough estimate of the density of carriers needed. The limit for degeneracy which can be estimated around 10 19  carriers/cm 3  in silicon.  
         [0047]      FIG. 9  includes an area of  FIG. 7 , such as between electrodes  160  and  130 . It illustrates phase step configurations. A phase step has a difference in height between the substrate and the top of the phase step  902 ,  903 , equal to one-quarter wavelength of the illumination source. Stray light reflected from the substrate will be one-half wavelength out of phase with that reflected from the top of the phase step. This phase difference produces diffraction or destructive interference, which minimizes the projection of stray reflected light.  
         [0048]     While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art which modifications and combinations will be within the spirit of the invention and the scope of the following claims.