Abstract:
A method and apparatus for performing vapor phase deposition to form a monolayer coating on the surface of an article. A liquid coating reagent is provided in a flow passageway extending into the process chamber. A carrier gas is flowed through the flow passageway to form a gas mixture including a vaporized coating reagent. The gas mixture is directed into the process chamber to contact the article. The vaporized coating reagent is deposited onto the article to form a coating thereon.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of U.S. application Ser. No. 09/007,989, filed Jan. 16, 1998, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to vapor phase deposition, and more particularly to vapor phase deposition of coatings. 
     Uniform and monolayer coatings, such as silanes, on silicon based surfaces are desired for a number of applications. In the micromachining of microelectromechanical system (MEMS), a final hydrophobic coating on the device is needed to prevent adhesion of adjacent surfaces due to capillary forces in water. 
     In contrast, a hydrophillic coating is desired for silicon based medical devices, such as filters or capsules, that are in contact with protein solutions to regulate hydrophilicity and minimize unspecific protein adsorption. 
     Currently, the coating methods most often used typically involve the assembly of a silane “monolayer” onto a silicon surface in an organic solution. It is known that alcohol groups, being hydrophilic and neutral, can drastically reduce protein adsorption on the surface of contact lenses, glass membranes, and porous silica. To assemble a monolayer of alcohol groups onto a silicon filter surface for protein ultrafiltration, one step is to coat silicon with vinyltrichlorosilane (VTS) or γ-glycidoxy-propyltrimethoxysilane (GPTMS), then convert the vinyl or epoxide resulting from the initial coating step to alcohol groups. Typical precursor molecules are alkyltrichlorosilanes (denoted as RSiCl 3 ) or alkyltrimethoxysilanes (denoted as RSi(OCH 3 ) 3 ), where R is any desired functional group to be introduced into the coating. However, trichlorosilanes and trimethoxysilanes are very sensitive to moisture. Even trace amounts of water in the organic solution could lead to polymerization. This causes the formation of multilayers with variable thicknesses, and submicron aggregates or islands on the silicon surface. To avoid this polymerization problem, an alternative method involves the use of monochlorosilane, which is incapable of polymerization. However, monochlorosilanes form a less stable coating than alkyltrichlorosilanes or alkyltrimethoxysilanes. 
     Another method calls for coating the silanes in a high vacuum. This approach, however, is more expensive than solution coating. 
     SUMMARY 
     One aspect of the invention is directed to an apparatus for forming a coating on an article. The apparatus includes a process chamber in which the article is supported; a storage region to contain a liquid coating reagent; an inflow assembly to flow a carrier gas through the storage region to produce a gas mixture including a vaporized coating reagent and the carrier gas, and to direct the gas mixture into the process chamber and onto the article to deposit the vaporized coating reagent on the article; and an outflow assembly to remove the gas mixture from the process chamber. 
     In another aspect, the invention is directed to a method of forming a coating on a surface of an article placed in a process chamber. The method includes providing a liquid coating reagent in a flow passageway extending into the process chamber; flowing a carrier gas through the flow passageway to produce a gas mixture including a vaporized coating reagent and the carrier gas; directing the gas mixture into the process chamber; and depositing the vaporized coating reagent on the surface of the article to form a coating thereon. 
     In yet another aspect, the invention is directed to a method of forming a coating on a surface of an article wherein, the method includes placing a porous article between an upper housing portion and a lower housing portion of a process chamber. The article spans a cross-section of the process chamber and the upper housing portion is sealed to the lower housing portion. A liquid coating reagent is introduced into a flow passageway extending into the process chamber, and a carrier gas is flowed through the flow passageway to produce a gas mixture including a vaporized coating reagent and the carrier gas. This gas mixture is directed into the process chamber to contact the article and form a coating thereon. The gas mixture is then exhausted from the process chamber. 
     Features of the just described method include the following. The carrier gas is flowed through the flow passageway before introducing the liquid coating reagent in the flow passageway. The flow rate of the carrier gas is measured to provide a first flow rate. After the gas mixture has been exhausted from the process chamber, the flow rate of the carrier gas is measured again to provide a second flow rate. The first flow rate is compared to the second flow rate to determine if the coating procedure has been performed successfully. 
     Advantages of the invention include the following. The coating has a surface that is extremely smooth and without any detectable submicron aggregates. A uniform coating of about 1 nanometer (nm) in thickness can be consistently achieved. Use of the invention is particularly advantageous whenever it is necessary to coat irregular shapes or channels in microdevices. No solvent is needed in the coating step. The invention is applicable to a wide range of surfaces, including silicon based surfaces, glass based surfaces and metal oxide based surfaces. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a coating apparatus according to the present invention. 
     FIG. 2 shows an inner chamber of a coating apparatus including a platform. 
     FIG. 3 shows the inner chamber of a coating apparatus including a raised holder. 
     FIG. 4 shows the inner chamber and an input tube which has a coiled part with a depression for storing a coating reagent. 
     FIG. 5 shows the inner chamber and an input tube which has a pocket for storing a coating reagent. 
     FIG. 6 shows an inner chamber having an upper housing section and a lower housing section. 
    
    
     DESCRIPTION 
     FIG. 1 shows an apparatus  50  for performing vapor phase deposition according to the present invention. The apparatus includes an outer chamber  100 , an inner or process chamber  200  wherein an article to be coated is placed, an inflow assembly  300  which introduces a carrier gas and a coating reagent into the inner chamber, and an outflow assembly  400  which exhausts the inner chamber. The inner chamber, the inflow assembly and the outflow assembly are made of a material that is resistant to chemical attacks, such as Teflon®, because the coating reagent is a highly active chemical. 
     The outer chamber  100  encloses the inner chamber  200 . Although the outer chamber and the inner chamber are shown as having a rectangular shape in FIG. 1, they may alternatively be cylindrical in shape. The outer chamber  100  defines an enclosed space with a minimum of openings to provide a stable, insulated environment in which the process chamber is located. 
     The inner chamber  200  includes a bottom wall  202 , side walls  204 , a top wall  206 , an inlet  210 , and an outlet  220 . The inlet  210  is provided at the top wall  206 , and the outlet  220  is provided at the bottom wall  202 . The inlet  210  is coupled to a first end  312  of an input tube  310  of the inflow assembly  300 , which serves as a passage for introducing the carrier gas into the inner chamber  200 . (The structure of the input tube  310  will be explained in more detail below.) The outlet  220  of the inner chamber  200  is coupled to an end  412  of an output tube  410  of the output assembly  400  to exhaust the inner chamber  200 . The output tube  410  extends from the outlet  220  and passes through the outer chamber  100  via a bottom opening  120  of the outer chamber  100 . The opposing end of the output tube  410  is coupled to a vacuum pump  420  to exhaust the inner chamber. In an alternative embodiment, the output assembly does not include a vacuum pump, in which case the inner chamber  200  is exhausted by diffusion. 
     The inner chamber  200  may further include a base  230  at the bottom wall  202 . The base  230  may be placed over the outlet  220 , substantially covering the outlet. An article  60 , such as a silicon wafer or a glass substrate, may be placed on the base  230  for coating. The base  230  may be a porous material to allow the carrier gas to flow therethrough and exit the inner chamber  200  via the outlet  220 . 
     Alternatively, as shown in FIG. 2, the inner chamber may include a platform  232  provided above the bottom wall  202 , on which the article  60  may be placed. In yet another embodiment, as shown in FIG. 3, the inner chamber  220  may include a raised holder  234  provided above the bottom wall  202  to hold the sides or edges of the article  60  during the coating process. 
     Referring back to FIG. 1, the inflow assembly  300  includes the input tube  310 , a carrier gas supply  320  and a coating reagent supply  330 . The coating reagent supply  330  contains a coating reagent, such as an alkyltrichlorosilane or alkyltrimethoxysilane, in liquid form. The carrier gas supply  320  is connected to the input tube  310  through a desiccant tube  340  and a flow meter  350 . The carrier gas supply  320  contains an inert gas, such as nitrogen or argon, which will not react with the coating reagent. The desiccant tube  340  dries the carrier gas to substantially eliminate moisture from it. The flow meter  350  measures the carrier gas flow rate. 
     The input tube  310  has three ports or channels. As explained above, the first port  312  is coupled to the inlet  210  of the inner chamber  200 . A second port  314  is connected to the carrier gas supply  320 , and a third port  316  is connected to the coating reagent supply  330 . 
     The input tube  310  additionally includes a storage region  318  located between the port  312 , and the ports  314  and  316  to hold a liquid coating reagent  52  which is fed into the input tube  310  via the third port  316  from the carrier reagent supply  330 . 
     The storage region  318  may have various forms. As shown in FIG. 1, the input tube  310  can include a coiled portion  310   a  with a bottom part  311   a  functioning as the storage region  318 . In this embodiment, the liquid coating reagent  52  occupies no more than half the diameter of the input tube  310  to provide the carrier gas with sufficient space to flow through the input tube. 
     Alternatively, as shown in FIG. 4, a coiled portion  310   b  of the input tube  310 ′ may have a depression  311   b  which serves as the storage region  318 ′. Also, as shown in FIG. 5, the input tube  310 ″ may comprise a curved tube  310   c  including a pocket  311   c  which forms the storage region  318 ″. 
     As shown in FIG. 1, the outer chamber  100  includes heating coils  130  joined to its inner walls. The heating coils can be used as a secondary drying mechanism in addition to the desiccant tube  340  to ensure that the carrier gas flowing through the input tube  310  is substantially moisture free. The heating coils are connected to a thermal-couple or a thermal-set (not shown) such that their temperature can be controlled electrically. The temperature within the outer chamber  100  is usually maintained below the boiling point of the coating reagent  52  to prevent rapid evaporation of the coating reagent in the storage region  318 . 
     The heating coils  130 , as shown, may be wound more closely together at the bottom of the outer chamber  100 , where the inner chamber  200  is located, to provide a slightly higher temperature within the inner chamber than at the top of the outer chamber where the inflow assembly  300  is located. This temperature gradient prevents vapor condensation on the article  60 , such as capillary condensation in small channels of porous articles, such as silicon filters. 
     A coating operation which may be carried out in the coating apparatus  50  will now be described. The article  60 , such as a silicon wafer, having the dimensions of 1×2 cm (centimeters) is cleaned in 2:1 sulfuric acid and 30% hydrogen peroxide (piranha) at 80° C. for 20 minutes. Then the article is rinsed with deionized water and transferred into the inner chamber  200 . A carrier gas, such as nitrogen, from the carrier gas supply  320  is passed through the desiccant tube  340  to dry the carrier gas. The carrier gas is then directed into the inner chamber  200  where it is heated by the heating coils. After about 10-15 minutes of flowing the carrier gas, the article  60  becomes substantially moisture free and only hydroxyl groups bonded to the article remains. The hydroxyl groups bonded to the article are referred as silanol groups. 
     Then about 0.1 to 0.4 ml (milliliters) of a liquid coating reagent, such as alkyltrimethoxysilane, is injected into the port  316  of the input tube  310  from the coating reagent supply  330 . The liquid coating reagent flows through the input tube  310  and is held in the storage region  318 . As noted, the temperature within the outer chamber  100  is maintained below the boiling point of the coating reagent so it is in a liquid form in the storage region  318  for sufficient time to complete the coating process. Even at a temperature below the boiling point, trace amounts of the liquid coating reagent are vaporized. The vaporized coating reagents form a gas mixture with the flowing carrier gas and is directed into the inner chamber  200 . Eventually all available coating reagent is vaporized. Typically, 0.1 to 0.4 ml of liquid coating reagent takes about 20-30 minutes to completely evaporate. 
     The vaporized coating reagent carried into the inner chamber  200  reacts with the silanol groups on the surface of the silicon wafer and generally forms a monolayer coating. Once all the silanol groups react with the coating reagent and the monolayer coating has been formed, the coating thickness does not increase appreciably even when continuously exposed to the coating reagent since there is no hydroxyl groups to react with the coating reagent. Eventually all of the coating reagent in the storage region is carried away by the carrier gas and flushed out through the outlet  220 . If there had been excess hydroxyl groups in addition to the silanol groups, i.e., trace amounts of water on the wafer, the coating reagent would have reacted with the excess hydroxyl groups and formed aggregate or multilayers on the article. 
     Therefore, if baking is used to dry the article  60  rather than a gas, care must be taken not to over bake the article. Over baking would the eliminate the hydroxyl groups bonded to the surface of the article in the form of silanol groups, causing a shortage of hydroxyl groups needed for the coating process. 
     During the coating operation above, the heating coils  130  of the outer chamber  100  maintain a slight temperature gradient at the bottom of the outer chamber where the inner chamber is located. The temperature in the inner chamber  200  is maintained at about 10° C. higher than the top of the outer chamber where the input tube  310  of the input assembly is located. This temperature gradient prevents vapor condensation in the inner chamber  200 . 
     In the embodiment of FIG. 6, the outer chamber  100 , the inflow assembly  300 , and the outflow assembly  400  are substantially the same as in FIG. 1, and thus, they are not shown in FIG.  6 . This embodiment of a coating apparatus  50 ′ may be used for coating a porous article, such as a silicon filter  64  including micro-channels. The inner chamber  200 ′ having a rectangular shape is formed by joining together an upper clamp member  270  and a lower clamp member  272 . In another embodiment, the inner chamber  200 ′ has a cylindrical shape. The upper clamp member  270  includes a first cavity  274 , a first inner wall  276 , and a first outer wall  278 . The upper clamp member  270  further includes an inlet  210 ′ extending between the first inner wall  276  and the first outer wall  278 . 
     The lower clamp member  272  includes a second cavity  280 , a second inner wall  282  and a second outer wall  284 . The lower clamp member  272  further includes an outlet  220 ′ extending between the second inner wall  282  and the second outer wall  284 . A plurality of bolts  275  are used to join the upper and lower clamp members  270  and  272 . The bolts  274  are provided at each corner of the clamp members  270  and  272 . 
     The silicon filter  64  is placed on the lower clamp member  272  over the second cavity  280 . The silicon filter is sufficiently large to span the first and second cavities  274  and  280 . A sealant  286 , such as O-rings, is placed on the outer periphery of the silicon filter and on the upper surface of the lower clamp member  272 . The upper clamp member  270  is joined to the lower clamp member  272 , with the first and second cavities  274  and  280  being aligned. The bolts are tightened and the sealant seals the first and second cavities, forming the inner chamber  200 ′ which is hermetically sealed except for the inlet  210 ′ and the outlet  220 ′. 
     A gas mixture, including a carrier gas and a vaporized coating reagent, is directed into the inner chamber  200 ′ through the inlet  210 ′. The carrier gas and the coating reagent fill the inner chamber and form a monolayer coating on the surfaces of the silicon filter  64 , including the microchannels of the silicon filter. 
     Since the silicon filter is placed between the inlet  210 ′ and the outlet  220 ′, the carrier gas and the coating reagent must pass through the silicon filter to exit the inner chamber. This provides a convenient way of determining whether the coating operation has been performed properly. If properly performed, the coating would not block the channels of the silicon filter so that the gas flow rate before and after the coating procedure should be substantially similar. On the other hand, if the coating procedure has been improperly performed and the channels are blocked, the flow rate after the procedure would be substantially less than the flow rate before the procedure. Therefore, the success of the coating procedure may be determined by measuring the flow rate of the carrier gas through the silicon filter using the flow meter  350  before and after the coating operation and comparing the two flow rates. This test could be implemented as part of the coating operation simply by coupling the flow meter  350  to the coating apparatus  50 ′. 
     While the invention has been shown and described with reference to an embodiment thereof, those skilled in the art will understand that the above and other changes in form and detail may be made without departing from the spirit and scope of the following claims.