Abstract:
A MEMS-based mirror is provided with trenches between adjacent electrodes in order to be able to withstand relatively high applied voltages, and thus has a substantially reduced exposure to uncontrolled surface potentials. The MEMS-based mirror, thus avoids voltage drifts and has an improved mirror position stability. The trench dimensions are selected such that the voltage applied between each adjacent pair of electrodes stays within predefined limits. An insulating layer, such as silicon dioxide, electrically isolates each pair of adjacent electrodes. Each insulting layer extends partially above an associated trench and is characterized by the same height and width dimensions.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to micro-electro mechanical system (MEMS) devices, and more particularly, to MEMS mirrors adapted to operate safely under high applied voltages. 
   A MEMS device is typically formed, in part, using well known integrated circuit (IC) fabrication processes. One such device is a gimbaled mirror adapted to pivot in response to electrostatic actuation. The mirror&#39;s motion is used to redirect light beams. Electrostatic actuation is often achieved via capacitive action between electrodes formed on a planar surface of a substrate and the mirror positioned above the electrode. To obtain a relatively large range of angular motion, the mirror is suspended at a relatively large distance above the electrodes. Accordingly, a relatively large voltage is often necessary to cause the actuation. 
   In practice, the maximum voltage that can be applied is limited by the well understood voltage breakdown effects. For example, if two flat electrodes are separated by a uniform gap, the voltage at which the ambient gas breaks down is described by the well known Paschen&#39;s curve, which is shown in  FIG. 1 . As seen from  FIG. 1 , the exact curve depends on the type of ambient gas, but the general shapes are similar. Breakdown is characterized by an uncontrolled discharge of electrical energy, which can cause severe physical damage to any surfaces exposed to the discharge. 
   In addition to breakdown of the ambient gas, breakdown can also occur along surfaces. If two conductors are separated by an insulator, breakdown can occur along the surface of the insulator. Unlike Paschen breakdown, surface breakdown is not a simple function of separation, but is a more complex function of the linear path length of the insulator and is strongly influenced by surface contaminants. 
   Whether breakdown occurs through the gas or on the surface, safe operation of MEMS devices using electrostatic actuation must occur below voltages that would result in breakdown. This constraint places practical limits on the design of electrostatic MEMS devices such as the MEMS mirror mentioned above. 
   Another common problem encountered in electrostatic MEMS devices is control of the surface potentials between the electrodes and various insulating surface layers. Surface potentials on dielectric surfaces are prone to drift over time due to charge migration along dielectric surfaces between the electrodes. This can cause serious problems regarding repeatability of the mirror positioning. 
   The conduction characteristics of these surfaces are inherently unstable due to sensitivity to temperature, moisture and other environmental factors. They can also be affected by electromagnetic radiation (light), which can be time dependent, depending on the application, contributing to system crosstalk. The conductivity of these surfaces is also strongly affected by impurities and process steps and materials used in the deposition and etching of the surfaces. All of these factors combined contribute to a loss of control of the surface potentials that contribute to the forces and torques applied to the actuable elements resulting in an unreliable and uncontrollable device. 
   BRIEF SUMMARY OF THE INVENTION 
   A MEMS-based mirror, is adapted to withstand relatively high applied voltages and has a substantially reduced exposure to uncontrolled surface potentials by the provision of trenches between adjacent electrodes of the MEMS-based mirror. The MEMS-based mirror, accordingly, avoids voltage drifts and has an improved mirror position stability. The trench dimensions are selected such that the voltage applied between each adjacent pair of electrodes stays within predefined limits, as determined, for example, by the Paschen curve. 
   In one embodiment, the electrodes (conductive elements) are formed in a silicon substrate. An insulating layer, such as silicon dioxide is used to electrically isolate the electrodes from one another and from the rest of the substrate. Each insulting layer extends partially above an associated trench. In some embodiments, the height of the extension is 2 to 4 times the width of the extensions. The insulating layer extensions that protrude from a bottom surface of their associated trenches provide a greater path length along the insulator surfaces that separate the electrodes from each other and from the rest of the substrate. A greater path length mitigates surface breakdown effects and thus, in conjunction with the trench discussed above, allows for relatively large voltages to be applied to the electrodes. 
   To reduce exposure to uncontrolled surface potentials, the top surface of the insulating layer is substantially recessed from the top surface of the electrodes. Any charges that accumulate on the insulator will be shielded by the conductive sidewalls of the trench thereby reducing the device susceptibility to surface potential drift on the insulators. 
   To further increase the range of voltages that may be applied to the MEMS-based mirror of the present invention, a multitude of trenches is disposed between the electrodes receiving these voltages. Each trench is characterized by the same depth and the same width. The trench heights may be different from the trench widths. The insulating layer electrically isolating each electrode extends upwardly from the bottom surface of each trench and it is substantially recessed from the top surface of the electrodes. The multitude of trenches electrically isolating each electrode allows one to cascade the applied voltage to an arbitrarily high value depending on the number of such trenches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the breakdown voltage as a function of the trench gap, as known in the prior art. 
       FIG. 2  is a cross-sectional view of a MEMS-based mirror, in accordance with one embodiment of the present invention. 
       FIG. 3  is a top view of the MEMS-based mirror shown in  FIG. 2 , in accordance with one embodiment of the present invention. 
       FIG. 4  is a perspective view of the MEMS-based mirror shown in  FIG. 2 , in accordance with one embodiment of the present invention. 
       FIG. 5  is a cross-sectional view of a MEMS-based mirror, in accordance with another embodiment of the present invention. 
       FIG. 6  is a top view of the MEMS-based mirror shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a cross-sectional view of a MEMS-based mirror  10 , in accordance with one embodiment of the present invention. As described below, MEMS-based mirror  10  is adapted to withstand relatively high applied voltages, and has a substantially reduced exposure to uncontrolled surface potentials, thereby avoiding voltage drifts and mirror position instability. 
   As seen from  FIG. 2 , electrodes  14  and  16  are electrically isolated from the rest of the substrate via insulators  20  and  24  and electrically isolated from one another via insulator  22 . In the exemplary embodiment 10 of the MEMS-based mirror, the electrodes as well as supporting structures  30 ,  32  that support mirror  34  are formed using silicon and thus have matching thermal expansion coefficient to the electrodes and base material. In such embodiments, insulating layers  20 ,  22  and  24  may be formed using silicon dioxide. Metal pads  26  and  28  are used to apply voltages to electrodes  14  and  16 . Although not shown, it is understood that similar metal pads are also formed and used to apply voltages to the base substrate  12 . 
   MEMS-based mirror  10  is characterized, in part, by the trenches disposed between the electrodes as well as the trenches disposed between the electrodes and the substrate. Shown in  FIG. 2  is trench  36  disposed between substrate  12  and electrode  14 ; trench  38  disposed between electrodes  14  and  16 ; and trench  40  disposed between electrodes  16  and substrate  12 . Insulating layers  20 ,  22  and  24  are formed so as to extend from the bottom surface of trenches  36 ,  38  and  40  respectively. The heights and widths of oxide extensions, respectively shown as H and W, are selected so as to increase the total path length along the insulator surfaces that separate the electrodes from each other and from the rest of the substrate. Furthermore, the width A, of each of the trenches, and dimensions H and W are selected to maximize the voltage that can be applied between each adjacent electrode pairs, as discussed further below. 
     FIG. 1  shows the breakdown voltage as a function of the trench width A, commonly referred to as Paschen curve, for both air  62  and nitrogen  64 . For example, when trench width A is approximately 80-90 μm, the breakdown voltage is approximately 800 volts. To provide additional operating margins, in some embodiments, a maximum voltage of, for example, 400 volts is applied between each adjacent electrode pairs when a trench width of 80 μm is selected. Another reason to add additional margin is that the curve in  FIG. 1  assumes that the electrodes are infinite in extent. Real electrodes are finite in size, and the fringe fields caused by sharp edges could significantly increase the local electric fields compared to the case for infinite electrodes. Such field enhancements could reduce the actual value of breakdown voltage compared to the Paschen curve. A safe rule of thumb is to operate with a factor of two safety margin from the breakdown voltage predicted by Paschen. It is understood that safety margin factors other than two may be used. 
   Another type of breakdown process that can occur is surface breakdown. In this case, breakdown occurs along the surface of an insulator and is a more complex function of the separation between the conductors than that displayed in  FIG. 1 . In general, the breakdown voltage increases as the total path length along the surface of an insulator separating two conductors is increased. Accordingly, in some embodiments, the insulator is extended to a height H above the bottom of the trench to increase the total path length along the insulator between, for example, electrodes  14  and  12 . In one embodiment, the height and width of oxide extensions, respectively shown as H and W, are selected to have respective values of, for example, 20 μm and 10 μm. An insulating layer  66  on the back of the substrate prevents breakdown from occurring on the backside. 
     FIG. 2  shows that the top surface of the insulators  20 ,  22 , and  24  are disposed substantially below the surface of electrodes  14 ,  16 , and substrate  12 . By recessing the insulators below these surfaces, charges that accumulate over time on the insulators are effectively shielded by the conductive sidewalls of the trench thereby reducing the susceptibility of the device to such charging processes. In general, the greater the aspect ratio (B-H)/A is, the less sensitive the MEMS device will be to charging on the insulators. In some embodiments, the ratio of insulator depth to trench width, namely (B-H)/A may be two. For example, to sustain the application of 400 V between each adjacent pair of electrodes, e.g., electrodes  14  and  16 , or between the substrate and an adjacent electrode, e.g., substrate  12  and electrode  14 , in one embodiment, insulator depth (B-H) is selected to be 160 μm, and trench width A is selected to be 80 μm. If the insulator height is, e.g., 20 μm, then the trench depth is, e.g., 180 μm. 
     FIG. 3  is an exemplary embodiment of top view of MEMS-based mirror  10  whose cross-sectional view along lines AA′ is shown in  FIG. 2 . As is seen from  FIG. 3 , electrodes  14  and  16  are shown as being separated from one another, in part, by trenches  36 ,  38  and  40  each with a width of A. Each of insulating oxide extensions protruding from the bottom surface of their respective trenches  20 ,  22  and  24  has a width of W. The height of the oxide extensions is seen from cross-sectional view shown in  FIG. 2 .  FIG. 4  is a perspective view of MEMS-based mirror  10 . 
   To further increase the range of voltages that may be applied to a MEMS-based mirror of the present invention, a multitude of trenches is disposed between the electrodes receiving these voltages.  FIG. 5  is a cross-sectional view of a MEMS-based mirror  20 , in accordance with such embodiments. Disposed between electrodes  16  and  18  is trench  40 ; and disposed between electrode  18  and substrate  46  is trench  44 . Similarly, disposed between electrodes  14  and  12  is trench  36 ; and disposed between electrode  12  and substrate  46  is trench  42 . Trench  38  is disposed between electrodes  14  and  16 . 
   Each of the trenches shown in  FIG. 5  is characterized by a depth B and a width A. The trench dimensions are selected in accordance with the Paschen curve shown in  FIG. 2 . For example, in one embodiment, to sustain the application of 800 V between electrode  16  and substrate  46 , in one embodiment, trench depth B is selected to be 180 μm, and trench width A is selected to be 80 μm. In one embodiment, the height H and weight W of oxide extensions, are selected to have respective values of, for example, 20 μm and 10 μm. 
     FIG. 6  is an exemplary embodiment of top view of MEMS-based mirror  20  whose cross-sectional view along lines AA′ is shown in  FIG. 5 . As is seen from  FIG. 6 , for example, substrate  46  and electrode  16  are shown as being separated from one another, in part, by trenches  44 , and  40  each having a gap of A. Each of insulating oxide extensions  52 ,  24 ,  22 ,  20  and  50  protruding from the bottom surface of their respective trenches  44 ,  40 ,  38 ,  36  and  42  has a width of W. The height of the oxide extensions is seen from cross-sectional view shown in  FIG. 4 . It is understood that by providing more trenches with oxide or other insulating materials extending above these trenches, substantially higher voltages may be applied between the electrodes. For example, by providing three trenches each selected to safely support, for example, 400 V, applied voltages of 1200 volts can be sustained. 
   The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the height or width of the trenches disposed between the electrodes. Nor is the invention limited by the height or width of insulating extensions, e.g., oxide, nitride or otherwise, protruding from the bottom of the trenches. The invention is not limited by the number of trenches nor is the invention limited by the number of extensions protruding from each trench. The invention is not limited by the material used to form the electrodes. The present invention may also be used in other MEMS devices. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.