Vapor deposition of anti-stiction layer for micromechanical devices

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.

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'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 '454 patent is hereby incorporated by reference. FIGS. 1 a and 1 b of the Hornbeck '454 patent, which are reproduced herein with the same figure designations and reference numerals, show one micromirror12of a DMD chip in which the micromirror (referred to as a deflection element) is positioned first in its undeflected position (FIG. 1a) and then in its deflected position (FIG. 1b) under the electrostatic influence of an underlying address electrode10. The micromirror12rotates on a hinge14, which is secured in a support layer16disposed above a substrate20. In the deflected position depicted inFIG. 1b, a corner of the micromirror12comes into contact with a landing electrode18, which stops the micromirror'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 micromirror12stuck 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 '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. 3a, 3b, and 3c of the Hornbeck '454 patent are also reproduced herein.FIG. 3ashows the molecular structure of a molecule of PFDA, which is a long-chain aliphatic halogenated polar compound having a COOH group at its polar end34. Following a plasma surface-activation step, a PFDA deposition step deposits an ultra-thin “monolayer” of PFDA on the activated surfaces, typified schematically inFIG. 3b. The deposited single-molecule thick layer has each molecule oriented with the polar end34strongly bonded to the contacting surfaces of the micromirror36and the landing electrode38, as depicted inFIG. 3c, in which the PFDA molecules are shown greatly exaggerated in relative size. The free end of each molecule terminates in a CF3group 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'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'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 '047 patent is hereby incorporated by reference.

FIGS. 2, 6 and 7 of the Hornbeck '047 patent, which are reproduced herein, illustrate one pixel18in an exploded perspective view (FIG. 2), and in schematic cross-sections in an undeflected position (FIG. 6) and a deflected position (FIG. 7). The pixel18is multi-level structure constructed above a substrate64that includes addressable memory cells, such as conventional SRAM cells (not shown), which change their binary states to determine the changing positions of each associated micromirror30. Each mirror30is supported by a post34that is mounted on a yoke32. The yoke32rotates on a pair of torsion hinges40(FIG. 2). The other end of each hinge40is attached to a cap42, which is supported by a post44. The position of the yoke32, and thus also the mirror30, is determined by voltages applied to address electrodes26and28and a reset/bias bus60on the bottom level, and to address electrodes50and52supported at the intermediate level by posts54and56. The yoke32is shown with cross-hatched portions74and78inFIG. 2that are attracted to the respective underlying address electrodes26and28. Similarly, the cross-hatched portions82and84of the mirror30are attracted to the respective underlying address electrodes50and52. The reset/bias bus60has extensions that define landing sites62. The yoke32has landing tips58that contact respective landing sites62when the yoke is deflected to either one of two deflected positions. The contacting action between respective landing tips58and landing sites62can give rise to stiction forces, which are lessened by the deposition of a PFDA anti-stiction layer.

The Hornbeck '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. 4schematically illustrates a prior art system100for depositing PFDA on DMD chips. The system includes a deposition chamber110, which is a box-like configuration having vertical sidewalls112and114, a bottom wall116, and a ceiling wall118that define a sealed enclosure. A base plate120, which is suspended by the sidewalls, serves as a support for a shelved cassette122. The cassette122holds multiple wafers124that 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. AlthoughFIG. 4shows only five wafers124held in a stacked arrangement in the cassette122, it will also be appreciated that a typical cassette can carry many more wafers in practice. The cassette122is open on its front and rear sides to allow gas vapor to flow through and react with the surfaces of the wafers124.

The chamber110has a front door (not shown) through which the cassette122passes at the beginning of a deposition process. The cassette may be robotically loaded into the chamber110, as is conventional with deposition equipment used in semiconductor processing. After loading of the cassette122, 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 chamber110. The walls112,114,116, and118of the chamber provide a sealed enclosure against the outside atmosphere. A sealed fitting126in the ceiling wall118provides a connection point for a gas input line128. Gas flowing in the line128enters the chamber110through a nozzle130retained in the fitting126. The nozzle130defines a gas inlet to the chamber110. A gas outlet for gas exiting the chamber110is provided by a sealed fitting132, which may be in a back wall (not shown) or in the sidewall114, where connection is made to an effluent line134.

The deposition system100has a gas input line136for receiving N2gas from a source140of dry nitrogen. After the cassette122has been loaded into the chamber110and the chamber has been sealed, the chamber is purged with nitrogen. This sets the stage for the deposition process. A vacuum pump142pulls a partial vacuum in the chamber110and draws gas out of the chamber through intermediate devices, which are described below. Nitrogen flows into the chamber from the source140through a mass flow controller144and a valve146, which are connected in series to a line148that is connected to the input line128. A second mass flow controller150controls nitrogen flow through an alternate path during vapor deposition. Electrically driven solenoid devices (not shown) precisely operate the mass flow controllers144and150. Such equipment is well known. The valve146and similar valves in the system100are pneumatically operated on/off valves.

A vaporizer152is used to heat powdered PFDA to a vapor. To initiate vapor deposition, valve146is turned off. Nitrogen gas, which serves as a carrier for the PFDA vapor, is provided to the vaporizer152through the mass flow controller150and a valve154. PFDA vapor is carried in the nitrogen gas stream into the chamber110from the vaporizer152through a valve156and a step-motor driven throttle158, which precisely controls the vapor flow rate. A second step-motor driven throttle160connected to the effluent line134cooperates with the first throttle158to provide uniform vapor flow through the chamber110. Excess PFDA that does not react in the chamber flows out through effluent line134, the throttle160, a valve162and into a trap164, where it solidifies. Nitrogen gas that is essentially free of PFDA flows out of the trap164through a valve166, and then through the vacuum pump142to an exhaust line168, where it leaves the system100. Pure nitrogen from the source140is also supplied to the trap164through a valve170. During cleaning and maintenance, the chamber110can be isolated from the vaporizer152by turning off the valve156. The vaporizer152can be purged through the trap164by opening a connecting valve172and passing nitrogen through the vaporizer and the trap.

Despite precise control of the PFDA vapor flow rate through the chamber110, the system110did not provide uniform PFDA deposition on the DMD surfaces of the wafers124. It was found that small particles of PFDA tended to form in the gas lines as the vapor flowed from the vaporizer152to the chamber110. 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.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIG. 5, an experimental filter180is shown connected beneath the ceiling wall118of the previously described vapor deposition chamber110. The fitting126is shown secured in seal-forming relationship in the ceiling wall118, and the nozzle130is shown secured in seal-forming relationship in the fitting126. Gas-tight seals can be provided by using conventional o-rings (not shown) or by welding. A threaded opening182is provided in the nozzle130to enable connection to the gas input line128(shown inFIG. 4).FIG. 5also shows a coupling184making a tapered threaded connection to the bottom of the nozzle130. The lower end of the coupling makes a standard threaded connection to a narrow neck portion186of the filter180. The filter180has a cylindrical sidewall188and a circular bottom wall190that define an interior cavity192. Cylindrical passageways194,196, and198in the nozzle130, coupling184, and filter neck portion186provide fluid communication for gas flow from the opening182in the nozzle130down into the cavity192.

The preferred material for the filter180is 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 filter180successfully removed PFDA particles from the vapor entering the chamber110, 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 chamber110through the filter180. An alternative filter arrangement solved this problem and provided additional benefits, as well now be described.

In accordance with the invention, referring toFIGS. 8 and 9, a filter-diffuser is designated generally by reference numeral200. The filter-diffuser200includes a manifold202that has an axially oriented throat204, which includes a tapered threaded interior wall206. The interior wall206mates with the complementary end of the previously described nozzle130, the mating end of which is shown in phantom outline. Porous metal filters208, preferably eight in number as shown inFIG. 8, are arranged symmetrically in daisy-wheel fashion around the periphery of the manifold202. 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 manifold202has a wide cylindrical body portion210extending radially outward from the narrower throat portion204. The filters208have elongated cylindrical walls212terminating in threaded ends214that screw into threaded sockets216at the periphery of the manifold body210. The free end of each filter208terminates in a closed circular wall218. The manifold202has an interior cavity220, which is open at the upper end of the throat204where it receives gas inflow from the nozzle130. Each filter208has a cylindrical cavity222, which is open at its inner end and in fluid communication with the manifold cavity220.

It will be appreciated that the filter-diffuser200can be used in place of the combination of the filter180and coupling184shown inFIG. 5. When installed in the chamber110ofFIG. 4, an improved vapor deposition system is achieved. Even though the individual filters208in the preferred embodiment may be smaller in diameter than the experimental filter180, the effective surface area of all eight filters208combined greatly increases the gas flow-through volume by comparison to the single filter180. Additionally, the daisy-wheel arrangement of the filters208causes gas vapor to be injected into the chamber110in 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 filters208effectively 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.