Abstract:
A vapor deposition system includes a filter-diffuser device connected to a vapor inlet within a vacuum chamber for simultaneously filtering inflowing vapor to remove particulate matter while injecting vapor containing perfluordecanoic acid (PFDA) into the chamber through radially arranged porous metal filters to enable the deposition of a uniform monolayer of PFDA molecules onto the surfaces of a micromechanical device, such as a digital micromirror device.

Description:
PRIORITY STATEMENT &amp; CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 11/136,922, entitled “Vapor Deposition of Anti-Stiction Layer for Micromechanical Devices” filed on May 25, 2005, and issued on Apr. 19, 2011 as U.S. Pat. No. 7,927,423, in the name of Kenneth A. Abbott; which is hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to vapor deposition equipment and methods for depositing thin films, and more particularly to equipment and methods for vapor deposition of ultra-thin passivation layers on the surfaces of micromechanical devices. 
     In 1987, Larry J. Hornbeck, a scientist with Texas Instruments Incorporated (TI), invented a remarkable micromechanical device, which he initially called a deformable mirror device, but today is called a digital micromirror device or simply a DMD. The DMD is fabricated on a semiconductor chip and includes an array of hinge-mounted microscopic mirrors, each overlying an addressable memory cell whose binary state determines the ON or OFF position of its micromirror. The DMD chip is the basis for various imaging systems, including TI&#39;s amazing Digital Light Processing technology, which is used in digital home TV systems and motion picture projectors for movie theaters. 
     An early generation hinge-mounted DMD is described in Hornbeck U.S. Pat. No. 5,331,454, which discloses a solution to a sticking problem in which a special passivation layer is deposited on the metal surfaces of the DMD elements that repeatedly contact each other. The Hornbeck &#39;454 patent is hereby incorporated by reference.  FIGS. 1   a  and  1   b  of the Hornbeck &#39;454 patent, which are reproduced herein with the same figure designations and reference numerals, show one micromirror  12  of a DMD chip in which the micromirror (referred to as a deflection element) is positioned first in its undeflected position ( FIG. 1   a ) and then in its deflected position ( FIG. 1   b ) under the electrostatic influence of an underlying address electrode  10 . The micromirror  12  rotates on a hinge  14 , which is secured in a support layer  16  disposed above a substrate  20 . In the deflected position depicted in  FIG. 1   b , a corner of the micromirror  12  comes into contact with a landing electrode  18 , which stops the micromirror&#39;s rotation at a precise angle of deflection from its undeflected position. The micromirror and electrodes of the device consist essentially of aluminum. 
     Attractive inter-molecular forces, known as Van der Waals forces, tend to cause the contacting surfaces to stick together. These forces gradually increase as the repeated contacting action causes the area of the contacting surfaces to gradually increase. Eventually, the Van der Waals forces exceed the restorative forces, leaving the micromirror  12  stuck in its deflected position. When this occurs, image quality is degraded, requiring replacement of the DMD chip in the imaging system. The term “stiction,” which is short for “static friction,” generally is used to refer to this sticking phenomenon. 
     The Hornbeck &#39;454 patent explains how the deposition of a passivation layer on the surfaces of the micromirror and the landing electrode helps to prevent the build up of Van der Waals forces and the resulting sticking problem. The preferred passivant for the passivation layer is perfluordecanoic acid (PFDA).  FIGS. 3   a ,  3   b , and  3   c  of the Hornbeck &#39;454 patent are also reproduced herein.  FIG. 3   a  shows the molecular structure of a molecule of PFDA, which is a long-chain aliphatic halogenated polar compound having a COOH group at its polar end  34 . Following a plasma surface-activation step, a PFDA deposition step deposits an ultra-thin “monolayer” of PFDA on the activated surfaces, typified schematically in  FIG. 3   b . The deposited single-molecule thick layer has each molecule oriented with the polar end  34  strongly bonded to the contacting surfaces of the micromirror  36  and the landing electrode  38 , as depicted in  FIG. 3   c , in which the PFDA molecules are shown greatly exaggerated in relative size. The free end of each molecule terminates in a CF 3  group that is responsible for low Van der Waals surface forces. The deposited PFDA monolayer effectively eliminates performance-degrading stiction. 
     Hornbeck and TI gradually brought DMD technology from early generation prototypes to a commercial DMD chip by the mid-1990&#39;s. Hornbeck U.S. Pat. No. 5,535,047 describes a later generation DMD structure in which each micromirror is elevated above a supporting yoke. The yoke is hinge-mounted and includes landing tips that contact landing sites of a stationary electrode when the yoke is rotated to a fully deflected position. TI&#39;s present commercial DMD chips use such elevated-mirror, hinged-yoke architecture with each micromirror representing one pixel in a very large array of pixels. The Hornbeck &#39;047 patent is hereby incorporated by reference. 
     FIGS. 2, 6 and 7 of the Hornbeck &#39;047 patent, which are reproduced herein, illustrate one pixel  18  in an exploded perspective view ( FIG. 2 ), and in schematic cross-sections in an undeflected position ( FIG. 6 ) and a deflected position ( FIG. 7 ). The pixel  18  is multi-level structure constructed above a substrate  64  that includes addressable memory cells, such as conventional SRAM cells (not shown), which change their binary states to determine the changing positions of each associated micromirror  30 . Each mirror  30  is supported by a post  34  that is mounted on a yoke  32 . The yoke  32  rotates on a pair of torsion hinges  40  ( FIG. 2 ). The other end of each hinge  40  is attached to a cap  42 , which is supported by a post  44 . The position of the yoke  32 , and thus also the mirror  30 , is determined by voltages applied to address electrodes  26  and  28  and a reset/bias bus  60  on the bottom level, and to address electrodes  50  and  52  supported at the intermediate level by posts  54  and  56 . The yoke  32  is shown with cross-hatched portions  74  and  78  in  FIG. 2  that are attracted to the respective underlying address electrodes  26  and  28 . Similarly, the cross-hatched portions  82  and  84  of the mirror  30  are attracted to the respective underlying address electrodes  50  and  52 . The reset/bias bus  60  has extensions that define landing sites  62 . The yoke  32  has landing tips  58  that contact respective landing sites  62  when the yoke is deflected to either one of two deflected positions. The contacting action between respective landing tips  58  and landing sites  62  can give rise to stiction forces, which are lessened by the deposition of a PFDA anti-stiction layer. 
     The Hornbeck &#39;454 patent describes methods for depositing a PFDA monolayer on the aluminum contacting surfaces of the device. For example, a solid source of PFDA is heated to its melting temperature to produce a vapor, which then forms the PFDA monolayer on the exposed aluminum surfaces of the device. 
       FIG. 4  schematically illustrates a prior art system  100  for depositing PFDA on DMD chips. The system includes a deposition chamber  110 , which is a box-like configuration having vertical sidewalls  112  and  114 , a bottom wall  116 , and a ceiling wall  118  that define a sealed enclosure. A base plate  120 , which is suspended by the sidewalls, serves as a support for a shelved cassette  122 . The cassette  122  holds multiple wafers  124  that contain DMD chips. It will be appreciated by those skilled in the semiconductor art that such wafers each have a large number of chips that are later separated from the wafer and packaged as individual DMD chips. Although  FIG. 4  shows only five wafers  124  held in a stacked arrangement in the cassette  122 , it will also be appreciated that a typical cassette can carry many more wafers in practice. The cassette  122  is open on its front and rear sides to allow gas vapor to flow through and react with the surfaces of the wafers  124 . 
     The chamber  110  has a front door (not shown) through which the cassette  122  passes at the beginning of a deposition process. The cassette may be robotically loaded into the chamber  110 , as is conventional with deposition equipment used in semiconductor processing. After loading of the cassette  122 , the door is closed and sealed so that a partial vacuum can be pulled inside the chamber. A heater (not shown) precisely controls the temperature within the chamber  110 . The walls  112 ,  114 ,  116 , and  118  of the chamber provide a sealed enclosure against the outside atmosphere. A sealed fitting  126  in the ceiling wall  118  provides a connection point for a gas input line  128 . Gas flowing in the line  128  enters the chamber  110  through a nozzle  130  retained in the fitting  126 . The nozzle  130  defines a gas inlet to the chamber  110 . A gas outlet for gas exiting the chamber  110  is provided by a sealed fitting  132 , which may be in a back wall (not shown) or in the sidewall  114 , where connection is made to an effluent line  134 . 
     The deposition system  100  has a gas input line  136  for receiving N 2  gas from a source  140  of dry nitrogen. After the cassette  122  has been loaded into the chamber  110  and the chamber has been sealed, the chamber is purged with nitrogen. This sets the stage for the deposition process. A vacuum pump  142  pulls a partial vacuum in the chamber  110  and draws gas out of the chamber through intermediate devices, which are described below. Nitrogen flows into the chamber from the source  140  through a mass flow controller  144  and a valve  146 , which are connected in series to a line  148  that is connected to the input line  128 . A second mass flow controller  150  controls nitrogen flow through an alternate path during vapor deposition. Electrically driven solenoid devices (not shown) precisely operate the mass flow controllers  144  and  150 . Such equipment is well known. The valve  146  and similar valves in the system  100  are pneumatically operated on/off valves. 
     A vaporizer  152  is used to heat powdered PFDA to a vapor. To initiate vapor deposition, valve  146  is turned off. Nitrogen gas, which serves as a carrier for the PFDA vapor, is provided to the vaporizer  152  through the mass flow controller  150  and a valve  154 . PFDA vapor is carried in the nitrogen gas stream into the chamber  110  from the vaporizer  152  through a valve  156  and a step-motor driven throttle  158 , which precisely controls the vapor flow rate. A second step-motor driven throttle  160  connected to the effluent line  134  cooperates with the first throttle  158  to provide uniform vapor flow through the chamber  110 . Excess PFDA that does not react in the chamber flows out through effluent line  134 , the throttle  160 , a valve  162  and into a trap  164 , where it solidifies. Nitrogen gas that is essentially free of PFDA flows out of the trap  164  through a valve  166 , and then through the vacuum pump  142  to an exhaust line  168 , where it leaves the system  100 . Pure nitrogen from the source  140  is also supplied to the trap  164  through a valve  170 . During cleaning and maintenance, the chamber  110  can be isolated from the vaporizer  152  by turning off the valve  156 . The vaporizer  152  can be purged through the trap  164  by opening a connecting valve  172  and passing nitrogen through the vaporizer and the trap. 
     Despite precise control of the PFDA vapor flow rate through the chamber  110 , the system  110  did not provide uniform PFDA deposition on the DMD surfaces of the wafers  124 . It was found that small particles of PFDA tended to form in the gas lines as the vapor flowed from the vaporizer  152  to the chamber  110 . Such particles would deposit on the mirror surfaces resulting in defective DMD chips. The invention addresses this problem. 
     SUMMARY OF THE INVENTION 
     A principal object of the invention is to provide a system for the simultaneous filtering and multi-directional injection of reactant vapors into a deposition chamber. 
     A further object of the invention is the provision of a filter-diffuser device connected to a vapor inlet within a vacuum chamber to remove particulate matter while injecting vapor containing a passivant into the chamber in a uniform manner. 
     A further object of the invention is the formation of an ultra-thin passivation layer on the surfaces of a micromechanical device to substantially reduce stiction forces. 
     A further object of the invention is the provision of multiple porous metal filters interconnected near a vapor inlet to a deposition chamber to enable formation of a uniform monolayer of PFDA on the surfaces of digital micromirror devices disposed within the chamber. 
     The novel features that characterize the invention are set forth in the appended claims. The nature of the invention, however, as well as its advantages, may be understood more fully upon consideration of the following illustrative embodiments, when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  are schematic prospective views of a prior art micromirror structure with the micro mirror in an undeflected position and a deflected position; 
         FIG. 2  is an exploded perspective view of one DMD pixel of a prior art array of such pixels showing the three-level structure of the pixel; 
         FIG. 3   a  shows the molecular structure of a perfluordecanoic acid (PFDA) molecule; 
         FIG. 3   b  schematically illustrates a monolayer of PFDA molecules bonded to a substrate; 
         FIG. 3   c  schematically illustrates a first monolayer of PFDA molecules on the tip of a micromirror and a second monolayer of PFDA molecules on the surface of an underlying electrode, the molecules being shown greatly exaggerated in relative size; 
         FIG. 4  is a diagram showing the interconnected components of a prior art vapor deposition system; 
         FIG. 5  is a schematic vertical cross-section through an experimental filter and connected components for introducing vapor into a chamber, a wall of which is shown broken away; 
         FIGS. 6 and 7  are cross-sectional views of the prior art pixel of  FIG. 2  showing the micromirror in an undeflected position and a deflected position; 
         FIG. 8  is a plan view of a filter-diffuser according to the present invention; and 
         FIG. 9  is a center cross-section of the filter-diffuser of  FIG. 8  taken through line A-A of  FIG. 8 , the cross-sectional view of the filter-diffuser being shown juxtaposed with a broken-away portion of a nozzle, shown in phantom outline, to which the filter-diffuser is connected in a deposition system. 
         FIG. 10  is a block diagram, similar to  FIG. 4 , showing the component parts of a vapor deposition system in which a filter-diffuser constructed according to the present invention is interconnected. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 5 , an experimental filter  180  is shown connected beneath the ceiling wall  118  of the previously described vapor deposition chamber  110 . The fitting  126  is shown secured in seal-forming relationship in the ceiling wall  118 , and the nozzle  130  is shown secured in seal-forming relationship in the fitting  126 . Gas-tight seals can be provided by using conventional o-rings (not shown) or by welding. A threaded opening  182  is provided in the nozzle  130  to enable connection to the gas input line  128  (shown in  FIG. 4 ).  FIG. 5  also shows a coupling  184  making a tapered threaded connection to the bottom of the nozzle  130 . The lower end of the coupling makes a standard threaded connection to a narrow neck portion  186  of the filter  180 . The filter  180  has a cylindrical sidewall  188  and a circular bottom wall  190  that define an interior cavity  192 . Cylindrical passageways  194 ,  196 , and  198  in the nozzle  130 , coupling  184 , and filter neck portion  186  provide fluid communication for gas flow from the opening  182  in the nozzle  130  down into the cavity  192 . 
     The preferred material for the filter  180  is porous metal with submicron pores suitable for trapping microscopic PFDA particles while allowing individual PFDA molecules to flow through into the interior of the vapor deposition chamber. Most preferably, the filter is formed from stainless steel particles that that are compacted into a mold and then sintered into a porous solid in the shape of the mold. U.S. Pat. No. 3,933,652 describes a technique of making such porous stainless steel filters. 
     Although the experimental filter  180  successfully removed PFDA particles from the vapor entering the chamber  110 , the uniformity of the PFDA deposition on the DMD chips was poor. This was attributed to a restricted flow rate of PFDA vapor into the deposition chamber  110  through the filter  180 . An alternative filter arrangement solved this problem and provided additional benefits, as well now be described. 
     In accordance with the invention, referring to  FIGS. 8 and 9 , a filter-diffuser is designated generally by reference numeral  200 . The filter-diffuser  200  includes a manifold  202  that has an axially oriented throat  204 , which includes a tapered threaded interior wall  206 . The interior wall  206  mates with the complementary end of the previously described nozzle  130 , the mating end of which is shown in phantom outline. Porous metal filters  208 , preferably eight in number as shown in  FIG. 8 , are arranged symmetrically in daisy-wheel fashion around the periphery of the manifold  202 . Preferably, the filters are porous stainless steel filters with submicron pores that readily pass individual PFDA molecules but filter out larger PFDA particles. Suitable filters of this type can be purchased from Mott Corporation of Farmington, Conn. 
     The manifold  202  has a wide cylindrical body portion  210  extending radially outward from the narrower throat portion  204 . The filters  208  have elongated cylindrical walls  212  terminating in threaded ends  214  that screw into threaded sockets  216  at the periphery of the manifold body  210 . The free end of each filter  208  terminates in a closed circular wall  218 . The manifold  202  has an interior cavity  220 , which is open at the upper end of the throat  204  where it receives gas inflow from the nozzle  130 . Each filter  208  has a cylindrical cavity  222 , which is open at its inner end and in fluid communication with the manifold cavity  220 . 
     It will be appreciated that the filter-diffuser  200  can be used in place of the combination of the filter  180  and coupling  184  shown in  FIG. 5 . When installed in the chamber  110  of FIG.  4 , an improved vapor deposition system is achieved. Even though the individual filters  208  in the preferred embodiment may be smaller in diameter than the experimental filter  180 , the effective surface area of all eight filters  208  combined greatly increases the gas flow-through volume by comparison to the single filter  180 . Additionally, the daisy-wheel arrangement of the filters  208  causes gas vapor to be injected into the chamber  110  in highly diffused manner, which results in a more uniform distribution of the PFDA molecules in the vapor, and consequently greater success in depositing a uniform monolayer on the exposed aluminum surfaces of the wafers being processed. 
     It is believed that the multi-directional flow of vapor through the walls of the filters  208  effectively agitates the vapor within the chamber to deliver PFDA molecules to the surfaces of the wafer in a continuous and uniform manner. Through experience, the deposition process is terminated after a predetermined time upon completion of the monolayer formation and to prevent over reaction. Five minutes has been found to achieve the desired results. 
     Although preferred embodiments of the invention have been described in detail, it will be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.