Abstract:
A method of dispensing a lubricant into a micromechanical device package and a micromechanical device package containing the lubricant. The method comprises the steps of mixing ( 102 ) the lubricant, typically a perfluoroalkanoic acid such as perfluorodecanoic acid with a suitable solvent, typically an ether solvent such as tetrahydrofuran or tert-butyl methyl ether. The mixture is allowed to equilibriate ( 104 ) before being filtered ( 106 ) to remove solid particles. The filtered solution is applied ( 108 ) to a surface that will be on the interior of the package, typically the ceramic substrate. The deposited mixture is then cured ( 110 ) to remove most, if not all, of the solvent, and the package is sealed ( 112 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC 119(e)1 of provisional application No. 60/108,619 filed Nov. 16, 1998. 
     The following patents and/or commonly assigned patent applications are hereby incorporated herein by reference: 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                 Patent No. 
                 Filing Date 
                 Issue Date 
                 Title 
               
               
                   
               
             
             
               
                 5,061,049 
                 Sept. 13, 1990 
                 Oct. 29, 1991 
                 Spatial Light Modulator 
               
               
                   
                   
                   
                 and Method 
               
               
                 5,331,454 
                 Jan. 16, 1992 
                 July 19, 1994 
                 Low Reset Voltage 
               
               
                   
                   
                   
                 Process for DMD 
               
               
                 5,583,688 
                 Dec. 21, 1993 
                 Dec. 10, 1996 
                 Multi-Level Digital 
               
               
                   
                   
                   
                 Micromirror Device 
               
               
                 5,939,785 
                 April 4, 1997 
                 Aug. 17, 1999 
                 Micromechanical Device 
               
               
                   
                   
                   
                 Including Time-Release 
               
               
                   
                   
                   
                 Passivant 
               
               
                 08/971,810 
                 Nov. 17, 1997 
                   
                 Monolayer Lubricant 
               
               
                   
                   
                   
                 for Micromachines 
               
               
                   
                   
                   
                 (now abandoned) 
               
               
                 09/406,386 
                 Sept. 27, 1999 
                   
                 Surface Treatment 
               
               
                   
                   
                   
                 Material Deposition 
               
               
                   
                   
                   
                 and Recapture 
               
               
                 09/416,910 
                 Oct. 13, 1999 
                   
                 Getter for Enhanced 
               
               
                   
                   
                   
                 Micromechanical Device 
               
               
                   
                   
                   
                 Performance 
               
               
                   
               
             
          
         
       
     
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of lubricating compounds, particularly to the deposition of lubricating compounds for micromechanical devices. 
     BACKGROUND OF THE INVENTION 
     Electronic devices such as integrated circuits are often packaged in hermetically sealed enclosures. These enclosures protect the device from contaninants, particles, and water vapor that would otherwise enter the package and mechanically damage or electrically disrupt the device. The hermetic packages, however, do not perfectly seal out these elements over the life of the device. Additionally, some water vapor and debris is present in the enclosure cavity when the enclosure is sealed, or evolved by the packaging materials as the materials cure. 
     Getters, compounds that capture contaminants, moisture vapor, and particles, are included inside the device enclosures to trap these species and preclude degradation of device performance, thereby increasing the operational lifetime of the device. Various getter compounds are available depending on the environment to which the getter will be subjected. 
     Existing getter compounds are unsatisfactory for use with many modem micromechanical devices. Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits. Typical micromechanical devices include accelerometers, pressure sensors, micro-motors, and microrirrors. 
     Because of their small size, often less than 100 microns, micromechanical devices are very susceptible to damage from debris inside the micromechanical device package. For example, debris can easily obstruct the motion of, or electrically short-circuit, micromirror elements which are often no larger than 17 microns. 
     Many micromechanical devices include moving components that place unique demands on surface lubrication and passivation systems. For example, the deflectable element of a micromirror device rotates about a torsion beam hinge axis and is stopped by contact with a landing zone or spring structure. The contact point experiences metal-to-metal contact and some scrubbing action. This metal-to-metal contact can create sticking and friction (stiction) between the moving components. Stiction is caused by the capillary action of water vapor present on the surface, van der Waals attraction, and intermetallic bonding of the metals. Stiction becomes worse as the contacting surfaces wear against each other since the contact area is increased. 
     Passivation coatings on micromirror devices reduce stiction and wear between the contacting surfaces. One passivation material that is especially useful for micromirror devices is perfluorodecanoic acid (PFDA). PFDA, as taught by U.S. Pat. No. 5,331,454, issued Jul. 19, 1994 and entitled Low Voltage Reset for DMD, provides an oriented monolayer on the surfaces of the DMD. The oriented monolayer provides a chemically inert surface that reduces the stiction between adjacent metal parts. 
     Unfortunately, the PFDA forms a relatively weak bond between with the aluminum surfaces on which it is deposited. Because of the weak bond between the PFDA and the aluminum surfaces of the micromirror device, the scrubbing action between contacting parts wears the oriented monolayer and exposes the underlying aluminum. Without replenishment, exposed aluminum regions grow and eventually create unacceptably large stiction forces—ruining the device. 
     As taught in U.S. Pat. No. 5,939,785 entitled Micromechanical Device Including Time-Release Passivant, and U.S. patent application Ser. No. 60/105,269, entitled Getter for Enhanced Micromechanical Device Performance, a getter compound is included in the sealed enclosure containing the device. A getter compound is chosen that reversibly combines with the lubricant, generally a perfluoroalkanoic acid such as perfluorodecanoic acid (PFDA). Thus, the getter acts as a reservoir to source and store PFDA while maintaining a PFDA vapor within the package. The PFDA vapor condenses onto exposed surfaces of the device to maintain the monolayer. 
     PFDA is presently placed in the device package by a remote vapor deposition process described in U.S. patent application Ser. No. 60/102,438, entitled Surface Treatment Material Deposition and Recapture. As described therein, the lubricant is evaporated into a carrier gas and transported by the carrier gas to a heated deposition chamber. Once in the deposition chamber, the lubricant condenses out of the carrier gas forming a monolayer on the devices in the deposition chamber. The carrier gas also forms a monolayer on the surfaces of any ductwork and the deposition chamber itself. 
     While an improvement over prior systems, the vapor deposition and recapture process can be wasteful of the lubricant and require extensive periodic maintenance. For example, if the ductwork and deposition chamber are not kept sufficiently warm, large quantities of the lubricant will condense out of the carrier gas, clogging the deposition system and requiring the system to be disassembled and cleaned. What is needed is an improved method of delivering lubricant to the micromechanical device package that is not wasteful of the lubricant and does not require complex delivery mechanisms. 
     SUMMARY OF THE INVENTION 
     Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for the delivery of a lubricant for micromechanical devices. One embodiment of the claimed invention provides a method of applying a lubricant to a micromechanical device package comprising the steps of: mixing said lubricant with a suitable solvent, applying the lubricant and solvent mixture to an interior package surface, and sealing the micromechanical device package. 
     Another embodiment of the claimed invention provides a method of applying a lubricant to a micromechanical device package comprising the steps of: mixing a carboxylic acid lubricant, such as a perfluoroalkanoic acid or perfluorodecanoic acid, with an ethereal solvent; applying the lubricant and solvent mixture to an interior package surface; curing the lubricant and solvent mixture to remove at least some of the solvent; and sealing the micromechanical device package. 
     According to yet another embodiment of the disclosed invention, a display system is disclosed. The display system comprises a light path, a controller for generating image signals, and a micromirror device. The micromitror device is disposed along said light path and is operable to selectively reflect portions of light traveling along said light path in response to the image signals. The micromirror device is in a package containing a carboxylic acid lubricant, such as perfluoroalkanoic acid, more specifically perfluorodecanoic acid, deposited from solution of an ethereal solvent. 
    
    
     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 flow chart of the process of mixing, applying, and curing the lubricant material. 
     FIG. 2 is a cross-section view of a packaged micromechanical device showing the presence of a reversible source/sink for a lubricant and a quantity of the lubricant applied in FIG.  1 . 
     FIG. 3 is a schematic view of a micromirror-based projection system utilizing the improved micromechanical device of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Lubricating micromechanical devices is a challenge due to the very small clearances between moving parts and the very small forces used to operate the devices. Some micromechanical devices, such as micromirror devices, are easily destroyed by contact with moving fluids. Therefore even applying the lubricant to the micromechanical device in a manner that supports high-volume manufacture of the devices can be quite difficult. 
     A new method of applying a lubricant to micromechanical devices has been developed. The method allows the direct deposition of the lubricant to surfaces inside the micromechanical device package and is suitable for use in fully automated product flows capable of supporting high volume manufacturing. 
     FIG. 1 is a flowchart  100  of the new process of applying the lubricant to a micromechanical device package. In step  102  of FIG. 1, the lubricant is mixed with a suitable solvent. The lubricant generally is a carboxylic acid, more specifically a perfluoroalkanoic acid such as perfluorodecanoic acid (PFDA). The selection of a proper solvent is critical to this application. A suitable solvent evaporates without leaving harmful residue, and the lubricant remaining after evaporation of the solvent should be in the desired form. That is, the lubricant is mechanically robust against, and highly localized within, the small confines allowed it within the micromechanical package. Furthermore, the solvent must combine with the lubricant to form a mixture that is stable over an extended period of time. 
     There are many efficacious organic solvents for PFDA, but only a limited number that will form mixtures with PFDA that are suitable for this dispensing method. Since perfluoroalkanoic acids such as PFDA are strong acids, they are capable of catalyzing a wide range of reactions with unsuitable solvents, such as condensation and elimination reactions. These reactions lead to undesirable byproducts whose presence in the package can cause stiction. 
     Ethers are desirable solvents for use with perfluoroalkanoic acid lubricants, especially with PFDA. The particular ethers that are ideal for use with PFDA are tetrahydrofuran (THF) and tert-butyl methyl ether (MTBE). Both THF and MTBE have excellent solvating power for PFDA, are inert to long exposure to PFDA, evaporate quickly, and leave behind solid PFDA without undesirable contaminants. 
     Solvents that do not give good results are esters and ketones. Esters and ketones undergo acid catalyzed reactions under long exposure to PFDA and evaporate significantly slower than the ethers described above. 
     The chosen lubricant and solvent typically are mixed by adding lubricant to a fixed volume of solvent until no more lubricant dissolves. When using PFDA as the lubricant, the resulting mixture typically is 80% PFDA and 20% solvent by weight. Less concentrated solutions can be used, but the very low surface tension and viscosity of such mixtures makes it difficult to control the delivery process. 
     The dissolution process is typically endothermic, so time must be allowed for the saturated solution to reach equilibrium at room temperature as shown by step  104  of FIG.  1 . Thirty minutes or more are typically allowed for the solution to reach room temperature. 
     After the solution reaches equilibrium, it is filtered in step  106  of FIG.  1 . using a Teflon filter having 1.0 micron pores. The filter step  106  of FIG.  1 . removes fine particles of undissolved lubricant from the solution. The filtered solution is then applied to the package interior in step  108  of FIG.  1 . 
     The solution prepared by this process is adapted for the use of automated dispense equipment. The automated dispense equipment typically delivers a small amount, 5 to 20 mg, of the dissolved PFDA to an interior surface of the package by extruding the dissolved solution from a syringe. Alternate methods of applying the solution to the package interior include spraying the solution into the package interior and dipping part of the package interior into the solution. The solution can be dispensed onto any surface inside the package. For best results, the solution is deposited on the ceramic substrate. 
     After the solution is dispensed, it is cured by evaporating the solvent as shown in step  110  of FIG.  1 . At room temperature, the cure process takes about five minutes. The cure process can be accelerated by placing the device in a warm environment, typically 40° to 50° C. After the solvent is evaporated, the package is sealed as indicated by step  112  of FIG.  1 . Typical micromirror packages are sealed by seam welding a metal-framed window  210  shown in FIG. 2, onto a spacer ring  212  attached to a ceramic package substrate  208 . 
     After the package is sealed in step  112  of FIG. 1, the package is heated to distribute the lubricant. The pre-bake step  114  shown in FIG. 1 is typically a 150° C. bake for 12 hours to effect the evaporation of the lubricant. The evaporated lubricant forms a monolayer on the interior surfaced of the package and on the surfaces of the micromechanical device. As described in U.S. patent application Ser. No. 60/102,438 entitled “Surface Treatment Material Deposition and Recapture,” and U.S. patent application Ser. No. 60/105,269 entitled “Getter for Enhanced Micromechanical Device Performance,” the lubricant is also absorbed by a getter inside the package. The getter acts as a reversible source/sink of lubricant material to maintain a lubricant vapor within the package in order to effect repair of the monolayer as it is abraded by device operation. 
     FIG. 2 shows a typical micromechanical device  206  on a ceramic substrate  208 . A metal framed window  210  is separated from the ceramic substrate  208  by a spacer ring  212  to form a headspace  204  surrounding the micromechanical device  206 . The metal framed window  210  is typically seam welded to the spacer ring  212 , but the window can also be bonded to the ring using a suitable adhesive such as epoxy. 
     FIG. 2 shows the location of the lubricant  202  within the package as applied by the new process. Although the best results are obtained by applying the lubricant  202  to the ceramic substrate, the lubricant  202  can also be applied to the package lid, shown as glass window  210 , or even to some types of micromechanical devices  206 . 
     FIG. 3 is a schematic view of an image projection system  300  using a micromirror device  302  having a lubricant deposited according to the present invention. In FIG. 3, light from light source  304  is focused on the improved micromirror  302  by lens  306 . Although shown as a single lens, lens  306  is typically a group of lenses and mirrors which together focus and direct light from the light source  304  onto the surface of the micromirror device  302 . Image data and control signals from controller  314  cause some mirrors to rotate to an on position and others to rotate to an off position. Mirrors on the micromirror device that are rotated to an off position reflect light to a light trap  308  while mirrors rotated to an on position reflect light to projection lens  310 , which is shown as a single lens for simplicity. Projection lens  310  focuses the light modulated by the micromirror device  302  onto an image plane or screen  312 . 
     Thus, although there has been disclosed to this point a particular embodiment for a lubricant delivery method for micromechanical devices and a micromechanical device fabricated using the process, it is not intended that such specific references be considered as limitations upon the scope of this invention except insofar as set forth in the following claims. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims.