Abstract:
A method of treating silicon-based surfaces for reducing charge migration is disclosed. In accordance with the method, a silicon-based surface is treated with Nitrogen-rich pacifying gas environment, after the surface is actuated. The surface is actuated in a drying step, wherein residual water or moisture is removed from the surfaces at an elevated temperature and a reduced pressure. The method of the instant invention is particularly useful for the treatment of ribbon surfaces in grating light valve device, wherein after the ribbon surfaces are treated according to the current invention, surface charging remains low for several days, even in open air conditions.

Description:
FIELD OF THE INVENTION 
     The invention relates to micro-machine devices fabricated from silicon-based materials. Specifically, the invention relates to the surface treatment of silicon-based materials to reduce charge build-up and charge migration. 
     BACKGROUND OF INVENTION 
     Grating light valves have applications in display, print, optical and electrical technologies. A grating light valve is a device that is capable of constructively and destructively interfering with an incident light source. Exemplary grating light valves and methods for making grating light valves are disclosed in the U.S. Pat. No. 5,311,360, U.S. Pat. No. 5,841,579 and U.S. Pat. No. 5,808,797, issued to Bloom et al., the contents of which are hereby incorporated by reference. 
     Grating light valve devices are micro-fabricated from Si-based materials using lithographic techniques. Grating light valve devices are configured to have a plurality of reflective ribbons which are moved by applying an operating bias voltage across the ribbons and a coupled substrate structure. By alternating, or switching, the bias voltage the ribbons are alternated between positions for constructive and destructive interfere with an incident light source having a wavelength λ. 
     The ribbons of the grating light valves are preferably formed of Si 3 N 4  and the substrate structure is formed of Si or SiO 2 . The surfaces of the ribbons and the substrate tend to be strongly hydrophilic and, thus, readily adsorb, physisorb, or chemi-adsorb water or moisture. Adsorbed, physisorbed, or chemi-adsorbed water or moisture on the operating surfaces of the ribbons and the substrate facilitates surface charging. Charging refers to the undesirable collection and migration electrical charges on the insulating surfaces of the grating light valve. Adsorbed, physisorbed, or chemi-adsorbed water or moisture is a difficult parameter to control within the manufacturing process of grating light valves and can severely diminish the performance of grating light valves. 
     One application for grating light valves is in the field of imaging and display devices, wherein one or more grating light valves are used create a pixel of an image or a pixel of an image on a display device. The presence of surface charging on the operating surfaces of grating light valves can perturb or shift the switching bias voltages. Thus, some of the grating light valves within the display device do not shut off, turn on and/or produce the desired intensity when a bias voltage is applied. The result is the undesirable persistence of an image, portions thereof or the complete failure of the device to produce the image. 
     To help ensure that charging is minimized, grating light valve structure are handled and manufactured in moisture free or near moisture free environments. Further, grating light valve structures are hermetically sealed within a die structure, after manufacturing, to maintain a moisture free environment. Processing and storing grating light valve structures in moisture free environments is time consuming and expensive. Further, the steps required to seal grating light structures within a die structure adds several steps to the fabrication process. 
     What is needed is a method to produce micro-fabricated grating light valve structures that exhibit reduced surface charging. Further what is needed is grating light valve structures that exhibit reduced surface charging in open air environments with typical humidity levels. 
     SUMMARY OF THE INVENTION 
     Grating light valves of the instant invention generate the condition for constructive and destructive interference through a plurality of movable ribbons. The movable ribbons provide a first set of reflective surfaces that are movable relative to a second set of reflective surfaces. The second set of reflective surfaces are reflective surfaces on a substrate element or on a second set of ribbons. In operation, an incident light source having a wavelength λ impinges on the first set of reflective surfaces and the second set of reflective surfaces. The movable ribbons are displaced towards or away from the second set of reflective surfaces by λ/4, or a multiple thereof. The portion of light that is reflected from the first set of reflective surfaces and the portion of light that is reflected from the second set of reflective surfaces alternate between being in phase and being out of phase. Preferably, the first set of reflective surfaces and the second set of reflective surfaces are either in the same reflective plane or are separated λ/2 for generating the condition for constrictive interference. 
     FIG. 1 a  illustrates a grating light valve with plurality of movable ribbons  100  that are formed in a spatial relationship over a substrate  102 . Both the ribbons  100  and the regions of the substrate between the ribbons have reflective surfaces  104 . The reflective surface are provided by coating the ribbons  100  and the substrate with any reflective material such as an aluminum or silver. The height difference  103  between the reflective surfaces  104  on the ribbons  100  and the substrate  102  is λ/2. When light having a wavelength λ impinges on the compliment of reflective surfaces  104 , the portion of light reflected from the surfaces  104  of the ribbons  100  will be in phase with the portion of light reflected from the surfaces  104  of the substrate  102 . This is because the portion of light which strikes the surfaces  104  of the substrate  102  will travel a distance λ/2 further than the portion of light striking the surface  104  of the ribbons  100 . Returning, the portion of light that is reflected from the surfaces  104  of the substrate  102  will travel an addition distance λ/2 further than the portion of light striking the surface  104  of the ribbons  100  , thus allowing the compliment of reflective surfaces  104  to act as a mirror. 
     Referring to FIG. 1 b , in operation the ribbons  100  are displaced toward the substrate  102  by a distance  105  that is equal to λ/4, or a multiple thereof, in order to switch from the conditions for constructive interference to the conditions for destructive interference. When light having a wavelength λ impinges on the reflective surfaces  104 ′ and  104  with the ribbons  100 ′ in the down position, the portion of light reflected from the surfaces  104 ′ will be out of phase, or partially out of phase, with the portion of light reflected from the surfaces  104  and the total reflected light will be attenuated. By alternating the ribbon between the positions shown in FIG. 1 a  and FIG. 1 b , the light is modulated. 
     An alternative construction for a grating light valve is illustrated in the FIGS. 2 a-b . Referring to FIG. 2 a , the grating light valve has a plurality of ribbons  206  and  207  that are suspended by a distance  205  over a substrate element  200 . The ribbons  206  and  207  are provided with a reflective surfaces  204  and  205 , respectively. Preferably, the surface  206  of the substrate  202  also are reflective. The first set of ribbons  206  and the second set of ribbons  207  are initially in the same reflective plane in the absence in the applied force. The first set of ribbons  206  and the second set of ribbons  207  are preferably suspended over the substrate by a distance  203  such that the distances between the reflective surfaces of the ribbons  206  and  207  and the reflective surfaces  208  of the substrate  202  are multiples of λ/2. Accordingly, the portions of light reflected from the surfaces  204  and  205  of the ribbons  206  and  207  and the reflective surface  208  of the substrate  202 , with a wavelength λ will all be in phase. The ribbons  206  and  207  are capable of being displaced relative to each other by a distance corresponding to a multiple of λ/4 and thus switching between the conditions for consecutive and destructive interference with an incident light source having a wavelength λ. 
     In the FIG. 2 b , the second set of ribbons  207  are displaced by a distance  203 , corresponding to a multiple of λ/4 of to the position  207 ′. The portion of the light reflected from the surfaces  205 ′ of the ribbons  207  will destructively interfere with the portion of the light reflected from the surfaces  204  of the ribbons  206 . 
     FIG. 3 plots and intensity response  307  of a grating light valve to an incident light source with a wavelength λ when and voltage  308  is applied across a selected set ribbons (active ribbons) and the underlying substrate. From the discussion above, the brightness value will be at a maximum when the ribbons are in the same reflective plane, separated by λ/2, or a multiple of λ/2, and brightness will be at a minimum when the ribbons are separated by λ/4, or a multiple of λ/4. 
     The curve  306  illustrates the initial intensity response of a grating light valve to an applied voltages without significant surface charging. The curve  309  illustrates the intensity response for the same grating light valve to an applied voltage after surface charging has occurred. The curves  306  and  309  are offset by a value  310 , which can be on the order of several volts. Such shifting in the intensity response curve is undesirable especially in display applications. 
     In display applications, the response of a grating light valve to an applied voltages is carefully calibrated to achieve a desired intensity level accurately. For example, eight bit voltage drivers subdivide the voltage curve, such as the voltage response curve illustrated in FIG. 3, into 265 grey levels. Clearly, a response curve shift of even a fraction of a Volt will seriously degrade the ability of the device to produce a desire intensity level. 
     Whether a grating light is constructed according to the principles illustrated in FIGS. 1 a-b , FIGS. 2 a-b , or any other construction utilizing ribbons moved by applying a bias across the ribbons and the substrate, there is the tendency for the ribbon surfaces and the substrate surfaces to exhibit charging. Charging on the surfaces of the ribbons and the substrate perturbs or the optical response causing the grating light valve to fail. Therefore, there is a need to provide grating light valve constructions which exhibit reduced charging. 
     According to the present invention, a micro-device which is fabricated from a silicon-based material and has silicon-based surfaces is treated with a pacifying gas to reduce surface charging. Preferably, the micro-device is a grating light valve with a plurality of movable ribbons comprising Si 3 N 4  surfaces coupled to a substrate element comprising SiO 2  surfaces, wherein the ribbons alternate between the conditions for constructive and destructive interference with an incident light source having a wavelength λ by apply the appropriate switching voltages across a selected portion of the ribbons and the substrate. 
     In accordance with the preferred method of the instant invention a grating light valve structure comprising silicon-based surfaces is placed in a vacuum environment with a pressure of 10 −6  Torr or less. The grating light valve structure is heated in the vacuum environment to temperatures of at least 250 degrees Celsius for a period of time sufficient to remove residual water or moisture form the surfaces of the structure; preferably 1 hour or more. The grating light valve is then allowed to cool to ambient temperatures and is exposed to a pacifying gas environment. Alternatively, the device is treated with the pacifying gas at elevated temperatures and is then allowed to cool to ambient temperatures. A cycling process of placing the grating light valve in the vacuum environment, heating the grating light valve, exposing the grating light valve to the pacifying gas environment and cooling the grating light valve is performed any number of times to achieve the intended goal of pacifying the surface and reducing charging of the surfaces. 
     Preferably, the surfaces of the grating light valve are pacified within isolation chamber where a vacuum environment with a pressure of 10 −7  Torr or less is achieved. Further, it is preferable that the pacifying gas used is substantially dried with a water content of less than 1 ppm. Further it is preferred that the pacifying gas contains a substantial amount of Nitrogen (50% or more) in combination with an noble Group VIII gas, such as Argon or Helium. Alternatively, the pacifying gas is approximately 100% dried Nitrogen. 
     After the grating light valve is cooled, the grating light valve is hermetically sealed within a die structure and installed in the intended device. Alternatively, the device is directly installed in the intended device and operates in an open air environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 a  is a cross-sectional view of a grating light valve with reflective ribbons in the constructive interference position. 
     FIG. 1 b  is a cross-sectional view of the grating light valve, shown in FIG. 1 a , with the active ribbons displaced to the destructive interference position. 
     FIG. 2 a  is a cross sectional view of the grating light valve with set of active ribbons and a set of bias ribbons in the same reflective plane. 
     FIG. 2 b  is a cross sectional view of the grating light in FIG. 2 a  with the active ribbons displaced from the bias ribbons to the destructive interference position. 
     FIG. 3 is a plot of the brightness response versus bias voltages applied to a grating light valve. 
     FIG. 4 is a block diagram for the method of making and grating light valve with treated surfaces, in accordance with the current invention. 
     FIG. 5 is a schematic representation of a isolation chamber used in the preferred method of the instant invention. 
     FIG. 6 a  is a plot comparing the open air charging of a treated silicon-based surface, in accordance with the current invention, and the same surface untreated. 
     FIG. 6 b  is a plot comparing leakage current of a treated silicon-based surface, in accordance with the current invention, and the same surface untreated. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, the present invention is for a grating light valve capable of alternating between the conditions for constructive and destructive interference with an incident light source. The grating light valve switches between the conditions for constructive and destructive interference through at least one set of movable ribbons. In accordance with the invention, the silicon-based surfaces of the grating light valve are treated with a pacifying gas to reduce surface charging to and help achieve consistent performance. Further the invention seeks to pacify the silicon-based surface of grating light valve devices, such that they are capable of operating in an open air environment with typical humidity or moisture levels. Alternatively, a grating light valve structure is sealed within a die structure after treatment of the surfaces to further enhance the consistency and performance of the device. The invention is also used to the other micro-structures the silicone-based surfaces which exhibit charging leading to degraded performance. 
     In the preferred embodiment of the present invention the silicon-based surfaces of a grating light valve are treated to drying conditions and then exposed to a pacifying gas environment containing Nitrogen. The pacifying gas environment most preferably is dried Nitrogen or is a mixture of dried Nitrogen and noble Group VIII gas including He, Ne, Ar, Kr, Xe, Rn or mixture thereof. 
     FIG. 4 is a block diagram outlining the steps for treating the silicon-based surfaces of a grating light valve device to drying conditions and exposing the surfaces to a pacifying gas environment, in accordance with the instant invention. In the step  401 , and prior to the step  407 , the structure is placed in a vacuum with a pressure of 10 −6  Torr or less, and preferably 10 −7  or less. In the step  403 , the temperature of the structure is elevated to a temperature of at least 250 to 350 degrees Celsius for a period that is sufficient to drive off a portion of the adsorbed, physisorbed, or chemi-adsorbed water, moisture and other volatile materials form the surface of the device; preferably, one hour or longer. After the surfaces of the device are dried, in the step  403 , then in the step  405  the structure is allowed to cool to ambient temperatures between 20 and 30 degrees Celsius. After the structure is allowed to cool to ambient temperatures in the step  405 , then in the step  407  the structure surfaces are exposed to the pacifying gas environment. 
     It is understood that the step  401  of placing the device in a vacuum environment and the step  403  of heating the device can be performed concurrently, separately and in any order. What is important is that the elevated temperature is maintained while the structure in a vacuum to facilitate the removal of adsorbed, physisorbed, or chemi-adsorbed water, moisture and other volatile materials from the structure surfaces. Further, while it is preferable to allow the device to cool to ambient temperatures in the step  405  prior to the step  407  of exposing the structure to the pacifying gas environment, the structure may also be exposed to the pacifying gas environment in the step  407  without cooling the structure in the step  405 . Further, it is understood that the steps  401 - 407  can be repeated any number of time to achieve the desired result of pacifying the silicon-based surfaces of the structure. 
     FIG. 5 illustrates an apparatus  500  configured for use in the preferred method of the instant invention. A grating light valve structure  513  is placed within an isolation chamber  510 . The isolation chamber  510  is in communication with a vacuum device  407 , a pressure meter  509  and a pacifying gas source  505 . A vacuum is drawn through isolation chamber  510  with the vacuum device  507 , until the pressure meter  509  reads approximately 10 −6  Torr or less. The temperature of the grating light valve structure  513  is elevate with a heating element  408  to a temperature of 250 degrees Celsius or higher. The above stated conditions are maintained for a period of time that is sufficient remove a substantial portion of residual water from the surface of the structure. The period of time for drying varies depending on the structure, surface areas, pressure used and temperature used. But good results for treating the silicone-based surface of grating light valve structures has been observed within an hour of drying time. 
     Still referring to FIG. 5, the pacifying gas  505  is a Nitrogen-rich gas as previously described. After the grating light valve structure  513  is dried, the grating light valve structure  513  is preferably allowed to cool to ambient temperatures. The grating light valve structure  513  is then exposed to a pacifying gas environment  505  by back filling the isolation chamber  510  with the Nitrogen-rich gas source  505 . The pressure of the Nitrogen-rich pacifying gas within the isolation camber  510  is adjusted to any reasonable value but is preferably adjusted to a value between 0.5 and 6.0 Torr. The procedure described is cycles any number of times to pacify the silicon-bases surface of the grating light valve structure. 
     After the silicon-based surfaces of the grating light valve structure have been treated, the grating light valve structure can be used in open air conditions. Alternatively, the grating light valve is sealed within a die structure  501  using a glass cap  502 . Because the die structure  501  and the glass cap  502  also have silicon-based surfaces, they are also treated to the drying conditions and exposed to the pacifying gas as described in detail above. 
     To seal the grating light valve  513  within the dies structure  501 , a preformed metallized gasket  504  is provided on a sealing edge  512  of the die structure  501 . The glass cap  502  is provided complementary preformed metallized gasket  506 . The performed gasket is preferably a Au/Sn eutectic solder preformed gasket. The glass cap  502  is placed on the die structure  501  with gaskets  504  and  506  aligned and overlapping. 
     Prior to sealing the grating light valve structure  513  within the die structure  501 , the gas environment  505 ′ within the isolation chamber  510  is modified with a second gas source  515 . The second gas source  515  is preferably a noble group VIII gas. The Nitrogen-rich pacifying gas environment is removed by applying a vacuum to the isolation chamber  510  with the vacuum device  507  and then backfilling the isolation chamber  510  with the second gas source  515 . Once the preferred gas environment is achieved within the isolation chamber  510 , then the temperature of the die structure  501  and the glass cap  502  are adjusted with the heating element  508  to a sealing temperature of approximately 300 degrees Celsius ±50, depending on the gasket materials that are used. The sealing temperature is maintained for approximately 10 minutes or a period of time that is sufficient to cause the metallized gaskets  504  and  506  to melt and solder the glass cap  502  to the die structure  501  and, thus, encapsulating a portion of the gas environment  505 ′ therein. The sealed die structure (not shown) is then cooled and removed from the isolation chamber  510  to be installed and used in the desired device. 
     FIG. 6 a  plots charging values observed for a silicon-based surface prior to and after treatment of the surfaces in accordance with the instant invention. In these measurements, a continues 12 Volt DC basis was placed across the Si 3 N 4  ribbons of a grating light valve and the offset voltage was periodically measured. The line  501  plots charging values of the silicon-based surface prior to treatment. The curve  501  is a typical charge curve for a silicon-based surface that is undried and that has been exposed to open air conditions. The surface accepts a charge quickly at first and then tappers off with time as the surface reaches charge saturation. Line  506  plots charging values for the same silicon-based surface after being treated in accordance with instant invention. It is clear form the curve  506  that the treated surface does not ready accept a charge as observed for the surface prior to the treatment. Further, even after 5 days in open air conditions the surface does not readily accept a charge as illustrated by the charging values plotted on the  603 . 
     FIG. 6 b  plots leakage current leakage values, which provides a signature for charge migration, measured on a silicon-based surface prior to treatment and after treatment in accordance with the current invention. The current leakage values plotted on line  607  for the untreated silicon-based surface are significantly higher than the current leakage values plotted on the line  609  for the same silicon-based surface after treatment. Further even after 7 day in open air condition the current leakage values of the treated silicon-based surface, plotted on the line  611 , remain low. In this example, the passivation process reduce the leakage currents measured by approximately 50 times. The low charging values and leakage current values observed for silicon-based surfaces after treatment in accordance with the present invention, indicate a potential for grating light valve structure that are capable of operating in open air environments with typical humidity and moisture levels. 
     The present invention has been described relative to a preferred embodiment. Improvements or modifications that become apparent to persons of ordinary skill in the art only after reading this disclosure are deemed within the spirit and scope of the application. Specifically, treatment of silicon-based surfaces as described above is not limited to grating light valves and may be used to treat other silicon-based surfaces where charging in a concern. Further, it is understood that practicing the instant invention is not dependent on a particular grating light valve construction chosen.