Abstract:
A method of lubricating MEMS devices using fluorosurfactants  42 . Micro-machined devices, such as a digital micro-mirror device (DMD™)  940 , which make repeated contact between moving parts, require lubrication in order to prevent the onset of stiction (static friction) forces significant enough to cause the parts to stick irreversibly together, causing defects. These robust and non-corrosive fluorosurfactants  42 , which consists of a hydrophilic chain  40  attached to a hydrophobic fluorocarbon tail  41 , are applied by nebulization and replace the more complex lubricating systems, including highly reactive PFDA lubricants stored in polymer getters, to keep the parts from sticking. This lubrication process, which does not require the use of getters, is easily applied and has been shown to provide long-life, lower-cost, operable MEMS devices.

Description:
This application claims priority under 35 USC§ 119(e)(1) of provisional application No. 60/301,984 filed Jun. 3, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to micro-electro-mechanical systems (MEMS) devices and particularly to lubricating the surfaces of the moving parts in such devices. 
     BACKGROUND OF THE INVENTION 
     Micro-machined or micro-electro-mechanical systems (MEMS) devices, where there is repeated physical contact between moving parts, require lubrication to prevent the onset of stiction (static friction). This stiction can be strong enough to cause the parts to stick irreversibly together, making the devices inoperable. 
     For example, in the digital micromirror device (DMD™) of  FIG. 1 , which is one type of a MEMS device, the mirror/yoke  10 / 11  assemblies rotate on torsion hinges  12  attached to support posts  13  until the yoke tips  14  contact (land on) landing pads  15  located on a lower substrate layer  16 . It is this mechanical contact between the yoke landing tips  14  and the landing pad sites  15  that is of particular relevance to this invention. In some cases the mirror/yoke assemblies become slow in lifting off the landing pad, affecting the response of the device and in other cases the assemblies become permanently stuck to the landing pads. One of the primary causes of stiction has been shown to be that of the landing tips scrubbing into the metal landing pads. 
     This “sticking” problem has been addressed by lubricating or passivating the metal surfaces of the devices to make them “slick.” Getters have been used in the device package to store the lubricants in order to maintain a low stiction environment for all moving parts over long device lifetimes. One lubricant used is powdered perfluordecanoic acid (PFDA), which tends to decrease the Van der Waals forces associated with the mirror assemblies in the DMD™ or any moving parts in a MEMS device, and thereby reduces the tendency for the mirrors to stick to the landing pads. However, PDFA has a very reactive molecule, which can react with other package constituents, causing severe damage to the device. 
       FIG. 2  is a drawing of a typical DMD™ device package. This shows the DMD™  21  mounted in a package frame  20  with attached cover glass (package lid)  22 . The cover glass  20  is usually made opaque  23  on the underside with a transparent aperture for optical interfacing with the device. As mentioned, this stiction problem has normally been addressed by attempting to control the environment inside the packages. For example,  FIG. 3  illustrates how PFDA getters  31  and/or moisture gathering getters are attached to the underside of the glass cover  30  by means of an adhesive  32 . The getters are used to both collect moisture in the package and provide lubrication (PFDA) to the moving parts in the DMD™. These getters may also be installed in empty areas within the package cavity. However, the PFDA lubricants are reactive and difficult to handle in a manufacturing environment and as a result, tend to drive up the cost of packaged MEMS devices. The moisture getters are used to control rather large amounts of moisture in the package, which should only be present if the getters themselves are misprocessed. 
     What is needed is a robust, non-corrosive lubricant that can be easily applied, without the use of getters, to the surface of MEMS devices to prevent the moving parts from sticking together. The fluorosurfactant lubricant of the present invention meets these requirements. 
     SUMMARY OF THE INVENTION 
     This invention discloses a method of lubricating MEMS devices using fluorosurfactants. Micro-machined devices, which make repeated contact between moving parts, require lubrication in order to prevent the onset of stiction (static friction) forces significant enough to cause the parts to stick irreversibly together. These fluorosurfactants, which are applied by nebulization and therefore do not require the use of getters, replaces complex lubricating systems in conventional devices, including highly reactive PFDA lubricants stored in polymer getters, to keep the parts from sticking. Although fluorosurfactants are overall stickier than the commonly used PFDA lubricants, they are easy to apply, tend to be non-reactive to MEMS surfaces, and have been shown to be effective in providing longer-life, lower-cost, devices with no additional sticking parts after an initial burn-in period. 
     Fluorosurfactants contains a hydrophilic (water like) chain attached to a hydrophobic fluorocarbon. The molecules spontaneously align with the hydrophillic regions pointed towards the surfaces of the device&#39;s moving parts with the hydrophobic fluorocarbons pointing into the air between the parts. When the moving parts touch, they are lubricated by the fluorocarbons. The material is a liquid, and if scraped away by the contacting parts, it will spontaneously flow back into the contact area, restoring lubrication around the point of contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a drawing of a DMD™, one type of MEMS device, with moving part that requires lubrication to prevent the parts from sticking, thereby making the device inoperable. 
         FIG. 2  is a drawing of a typical DMD™ package with getters added to provide a controlled environment to help prevent stiction between the MEMS device&#39;s moving parts. 
         FIG. 3  is a drawing showing getters used for both storing PFDA lubricants and for collecting moisture inside a typical MEMS package to help prevent the moving parts from sticking. 
         FIG. 4  is a sketch showing the hydrophillic and hydrophobic sides of the fluorosurfactant used in the present invention. 
         FIG. 5  is a sketch illustrating how the fluorosurfactant lubricant molecules spontaneously align with the hydrophillic regions pointed towards the surface of the parts and the hydrophobic fluorocarbon tails pointing into the air between the moving parts. 
         FIG. 6  is a sketch illustrating the effect of surfactant lubrication of the present invention, showing the molecules spontaneously aligning with the hydrophillic regions pointed towards the surface of the moving parts in the MEMS device and the hydrophobic fluorocarbon tails pointing into the air between the moving parts, providing lubrication to the moving parts. 
         FIGS. 7   a  and  7   b  are graphs of life test data for DMD™ devices lubricated with the fluorosurfactant of the present invention, showing that after an initial burn-in period there are very few new defects at normal operating voltages, which indicates the effectiveness of this lubrication.  FIG. 7   a  shows over 10,000 micro-mirrors landed in the positive direction and then the mirrors being released as the applied energy is reduced.  FIG. 7   b  shows over 10,000 micro-mirrors landed in the negative direction and then the mirrors being released as the applied energy is reduced. 
         FIG. 8  is a process flow diagram for the back-end fabrication of a MEMS device that is lubricated with the fluorosurfactant of the present invention. 
         FIG. 9  is a cut-away drawing of a wafer of DMD™ devices showing the rotating yoke/mirror assembly (moving part), where the surfaces are nebulized with the fluorosurfactant of the present invention to prevent sticking. 
         FIG. 10  is a block diagram of a MEMS based projection display system where the lifetime of the system is considerably extended through the nebulization of the MEMS moving parts with the fluorosurfactant of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention uses a fluorosurfactant to lubricate the moving parts of a MEMS device. These fluorosurfactants are readily available materials that can be applied to the surfaces of the devices by a nebulization procedure, which is akin to spraying it on but with very small particles of the material. These lubricants, which are non-corrosive and very compatible with the MEMS fabrication process, remain robust over long extended periods of time. 
       FIG. 4  is a sketch describing the fluorosurfactant  42  of the present invention. The surfactant  42  contains a hydrophillic  40  (water like) chain or tail attached to a hydrophobic  41  fluorocarbon tail. While the hydrophillic  40  regions are capable of uniting with or taking up water, the hydrophobic fluorocarbon  41  regions do not take up water, but contain non-reactive halo-carbons; i.e., such as carbon, fluorine, and in some cases hydrogen. 
     When applied to a MEMS device, the surfactant molecules will display some degree of local order at the surface the device, as described in  FIG. 5 . Generally, molecules of the fluorosurfactant will spontaneously align with their fluoronated hydrophobic tails  52  pointing towards the air interface  51  and the hydrophillic tails  53  pointing towards the surface  50  of the device. For some surfactants, 100 percent of the molecules will align this way to give a crystalline order. However for most surfactants there will be a few stragglers that will align in the opposite direction with the hydrophillic tail  54  pointing towards the air interface  51  and the hydrophobic tail  55  pointing towards the surface  50 , although most of the molecules will align in the correct way, as shown. 
       FIG. 6  is a sketch illustrating the effect of fluorosurfactant lubrication on the moving parts in a DMD™ MEMS device, where the mirror yoke  60  rotates on torsion hinges  61  until the tip  62  of the yoke makes contact  64  (lands on) with the landing pad  63 . This illustrates the surfactant molecules properly aligned with the hydrophillic tails  65 , 66  pointing towards the surface of the landing pad  63  and yoke assembly  60 , respectively, and the hydrophobic tails  67 , 68  pointing into the air interface between the moving parts. In operation, when the moving parts touch, they are lubricated by the fluorocarbons. Since the material is a liquid, when it is scraped or scrubbed away during contact of the parts, it will spontaneously flow back into the contact area  64 , restoring lubrication and preventing sticking of the parts. 
       FIGS. 7   a  and  7   b  are graphs showing the results of life testing DMD™ devices lubricated with the fluorosurfactant of the present invention. This shows data taken after the surfactant was applied at 0 hours  70  before burn-in and then for multiple additional readings  71  taken up to 5000 hours later. This shows the effectiveness of lubricating the moving parts with the fluorosurfactant of the present invention. In  FIG. 7   a , sufficient energy (&gt;15 units) is applied to land over 10,000 micro-mirrors in the positive (+x degrees) direction. Similarly, in  FIG. 7   b , sufficient energy is applied to land over 10,000 micro-mirrors in the negative (−x degrees) direction. In both cases, as the operating energy is decreased from &gt;15 units (normal operating energy is &gt;15 units) to 0 units, more than 90% of the landed mirrors lift-off their landing pads. Below 0 energy, a reset voltage is applied to lift-off the remaining mirrors that are not permanently stuck. As shown, after an initial burn-in period  70 , the device stabilizes with the mirrors lifting-off consistently, without additional stuck mirrors, as the energy is decreased and the reset voltage is applied. 
       FIG. 8  is a process flow diagram for the back-end fabrication of a MEMS device that is lubricated with the fluorosurfactant of the present invention. The process is comprised of fabricating wafers  80  of a particular MEMS device having moving parts, partially sawing  81  the devices apart but leaving them slightly attached, testing  82  the individual chips on the wafer, completing the sawing  83  or separation of the chips, packaging the individual chips  84 , nebulizing  85  by spraying the surfaces of the chips with a fine mist of the fluorosurfactant of the present invention, and attaching lids  86  or cover glasses to the package. Although shown applied at the device level, the nebulization can also be applied at the wafer level. This process uses readily available surfactants, which are robust over time, to provide lubrication to all moving parts of the MEMS device. The process is non-corrosive relative to the typical materials found in MEMS devices. A range of various surfactants can be used to prevent stiction in MEMS devices, while at the same time eliminating the need for getters and thereby reducing the packaging costs of the devices substantially. 
       FIG. 9  shows a small portion of a DMD™  940  device that is built-up in four levels, these being a memory substrate level  90 , an address electrode/landing pad level  91 , a yoke/hinge level  92 , and a mirror level  93 , as indicated. 
     The substrate  90  contains an orthogonal array of CMOS address circuits over which a reflective micro-mirror superstructure with mechanical moving parts is fabricated. A thick oxide  900  isolation layer, which has vias for connecting to the CMOS address circuits, is placed on top of the CMOS array in the substrate. 
     The mirror superstructure is then fabricated on top of this isolation layer  900 , beginning with an Aluminum metal-3 layer  91 , which includes yoke address electrodes  910 , 911  and landing pads  912 . The address electrodes  910 , 911  connect through vias  913  to the respective binary outputs of the CMOS address circuits in the substrate  90 . 
     The next layer  92  consists of the yoke  920  and torsion hinge  922  structure, mirror address electrodes  925 / 926 , electrode post  927  and hinge posts  923  and post caps  924 . The yoke  920 , which supports a mirror assembly  930  on the top level  93 , is suspended in air above the metal-3 layer  91  and rotates about a diagonal axis, on the torsion hinges  922 , until the yoke landing tips  921  contact the landing pads  912  below. The geometry of the yoke  920  and the spacing between the metal-3 level  91  and the yoke/hinge level  92  determines the tilt angle of the yoke/mirror structure. The hinge posts  923  sit on top of and in contact with the metal landing pads  912  at the metal-3 level  91 , so that the yoke and landing pads are at the same electrical potential. The mirror address pads  925 / 926  are attached to the yoke addressing pads  910 / 911  by additional posts  927 . 
     The top level  93  consists of the reflective mirrors  930  and mirror posts  931 , which ride on top of the yoke  905 , tilting typically +/−10°. 
     In operation, electrostatic forces cause the mirror/yoke structure  930 / 920  to rotate on its torsion axis, defined along the torsion hinges. These electrostatic forces are established by the voltage potential difference between the yoke address electrodes  910 / 911  and the yoke  920  and between the mirror address electrodes  925 / 926  and the mirror  930 , respectively. In each case, these forces are a function of the reciprocal of the distance between the two plates; i.e.,  910 / 911  and  920  and  925 / 926  and  930 . As the rigid yoke/mirror structure rotates on its axis, the torsion hinges  922  resist deformation with a restoring torque that is an approximate linear function of the angular deflection of the structure. The structure rotates until either this restoring torsion beam torque equals the established electrostatic torque or until the yoke/mirror structure is mechanically limited in its rotation, i.e., the yoke tips  921  land on the landing pads  912 . It is at this point of contact between the yoke tips  921  and the landing pads  912  that stiction occurs, which can render a particular mirror permanently inoperable or slow to respond to the electrostatic forces, thereby causing a device defect. 
     By nebulization of the metal surfaces of the devices using the fluorosurfactant of the present invention, highly reliable, lower-cost, DMD devices can be produced. Although overall these parts have stickier surfaces than conventional PFDA lubricated parts, the mirrors have been shown to be effective in lifting off their landing pads without additional stuck mirrors, after an initial burn-in period. Test devices have been operated for a period of &gt;5000 hours with no additional stuck mirrors. This process is effective since the molecules of the fluorosurfactant spontaneously align with their fluoronated hydrophobic tails  52  pointing towards the air interface  51  and between the moving parts their hydrophillic tails  53  pointing towards the moving surfaces  50  of the device, thereby lubricating the areas of contact between moving parts. As the lubricant is scrubbed away during contact, the surfactant quickly flows back into the area to assure a lubricated contact, thereby eliminating sticking between the parts. Also, the fluorosurfactants are non-corrosive to the typical surfaces of MEMS device, thereby leading to a long operating life for the devices. 
       FIG. 10  is a block diagram of a MEMS based projection display system where the lifetime of the system is considerably extended through the nebulization of the MEMS device&#39;s moving parts with the fluorosurfactant of the present invention. One example of such a system is a DMD™ projection display. In the projector, light from a light source  100  is focused on to the MEMS DMD™ device  102  by means of a condenser lens  101 , placed in the path of the light. An electronic controller  103 , is connected to both the DMD™  102  and the light source  101  and used to modulate the DMD™  102  and to control the light source  100 . For all DMD™ pixels positioned towards the light source (ON pixels), the incoming light beam is reflected into the focal plane of a projection lens  104 , where it resized and projected on to a viewing screen  106  to form an image  107 . On the other hand, DMD™ pixels positioned away from the light source (OFF pixels), as well as any stray light reflected from various near flat surfaces on and around the DMD™, are reflected into a dark trap  105  and discarded. 
     In operation, if a DMD™ mirror sticks in either the ON or OFF binary state it causes a bright white or solid black defect on the display screen, respectively. For obvious reasons, more than just a handful of these defects are unacceptable in a display. By applying the process of the present invention where all surfaces of the MEMS device are nebulized with a fluorosurfactant, the moving parts are sufficiently lubricated to eliminate the sticking of mirrors over the required long lifetimes of the projector. 
     While this invention has been described in the context of preferred embodiments, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.