Abstract:
Mixtures of an oxidizer and a high pressure fluid are produced by adsorbing an oxidizer in an adsorption bed and then desorbing the oxidizer with a high pressure fluid. The same steps can simultaneously occur in a second adsorbing bed but in reverse order. The oxidizer may be ozone and the high pressure fluid may be high pressure C0 2   including supercritical C0 2 . Such mixtures can be used for applications such as cleaning semiconductor wafers, food disinfection and water disinfection.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to method and apparatus for producing a mixture of an oxidizer and a high pressure fluid useful for cleaning objects such as integrated circuit wafers and for disinfecting food or water and particularly to method and apparatus for producing a mixture of ozone and supercritical or high pressure carbon dioxide (SCCO 2   or HPCO 2 ) useful for cleaning objects and for disinfecting food or water.  
       BACKGROUND OF THE INVENTION  
       [0002]     Cleaning objects prior to performing work on them is an essential step in many manufacturing processes. One manufacturing process will be discussed in detail. For example, semiconductor integrated circuit manufacture has many steps in which a pattern is transferred from a mask to a substrate. The pattern is typically transferred by selective exposure of the substrate to radiation through a mask. The substrate is coated with a radiation sensitive material, termed a resist, whose solubility when exposed to an appropriate developer is altered by the radiation. After selected portions of the resist are removed, the now exposed portions of the substrate are modified by, for example, ion implantation, etching as well as other processes. After the modification is complete, the resist is removed and the process repeated until integrated circuit fabrication is complete.  
         [0003]     As can be readily appreciated, the pattern must be accurately transferred from the mask to the substrate and this requires complete removal of the resist, as well as any unwanted material remaining from the process step, before the resist for the next process step is deposited and covers the substrate. Resists have typically been removed, that is, stripped, by either a wet technique, such as a HF rinse or a dry technique such as ashing. The latter technique essentially burns off the resist in an oxygen plasma. Although adequate for many purposes, these techniques have been found to possess drawbacks now that device dimensions are in the submicron regime. There are at least two potential problems. First, there may be unwanted debris remaining with dimensions comparable to device dimensions. Second, resist removal may be incomplete. It has been found that some process steps, for example, dry etching, may harden a portion of the resist and render it impervious to conventional stripping techniques. Accordingly, techniques other than the wet and dry techniques previously mentioned have been examined to determine their suitability for use in integrated circuit manufacture.  
         [0004]     Another cleaning technique uses supercritical fluids as a solvent for unwanted particles. A supercritical fluid is a material that is above both its critical temperature, T c , and critical pressure, P c . These values define the highest temperature and highest pressure at which the vapor and liquid phases of the material can exist in equilibrium and thus define the critical point. The critical point can be understood by considering what happens physically along the line separating the liquid and vapor phases as both pressure and temperature are increased. The gas density increases and the liquid density decreases due to thermal expansion. When the two densities are equal, a supercritical fluid is present. Both temperature and pressure may be further increased from the critical point with the material remaining a supercritical fluid.  
         [0005]     One supercritical fluid that has been examined for cleaning processes is supercritical carbon dioxide (SCCO 2 ). This material is attractive for use as a cleaning agent because it has a solubility comparable to those of light hydrocarbons without their environmental problems, and it has a relatively low surface tension. The latter attribute facilitates cleaning of small dimension features, such as holes in a semiconductor substrate, because the SCCO 2   can enter and clean the hole more easily than can high surface tension fluids.  
         [0006]     The literature describing the use of SCCO 2   for cleaning is now extensive. For example, U.S. Pat. No. 6,602,349 describes the use of SCCO 2 , with or without additives including solvents and surfactants, in cleaning semiconductor wafers to remove photoresist. U.S. Pat. No. 6,602,351 also teaches the use of SCCO 2   together with a solvent or surfactant for cleaning semiconductor surfaces. In addition to semiconductor integrated circuit wafers, mention is made of cleaning other devices such as micro-electro-mechanical and opto-electronic devices.  
         [0007]     A further cleaning technique uses ozone, a strong oxidizing agent, to remove unwanted resist. The use of ozone for cleaning semiconductor wafers is described in United States Patent Application Publication 2002/0157686, wherein a layer of heated liquid, for example, water or HF, covers the wafer, then ozone is provided and diffuses through the liquid. The ozone reacts with unwanted material, such as photoresist, and thus facilitates its removal.  
         [0008]     U.S. Pat. No. 5,507,957 describes another use of ozone, namely, the treatment of fluids. Disinfecting water or food, for example, juice, may be considered to be a type of cleaning as unwanted entities are removed or rendered harmless. For example, enzymes, which cause spoilage, are destroyed. As a pure or purer product results, this process may also be thought of as a manufacturing or cleaning process. In the treatment described, ozone containing oxygen is passed through a first adsorbing bed which preferentially adsorbs ozone. The nonadsorbed oxygen rich gas and air are passed through a second adsorbing bed which preferentially adsorbs nitrogen. Subsequently, the adsorbed ozone and nitrogen are desorbed and the combined stream then contacts the material being treated.  
         [0009]     U.S. Pat. No. 6,242,165 describes a method for cleaning organic material from semiconductor wafers using an oxidizer in a supercritical state. Oxidizers include supercritical SO 3 , supercritical H 2 O 2 , supercritical O 2 , and supercritical O 3 . The cleaning composition optionally includes supercritical components such as CO 2  or inert gases that are mixed in a mixing manifold.  
         [0010]     While it is desirable to mix ozone from an ozone generator, the ozone being at a low pressure, with a fluid such as SCCO 2 , which is at high pressure, such mixing of fluids at different pressures is generally difficult and additional apparatus and methods for forming a mixture of SCCO 2  and ozone are desirable.  
       SUMMARY OF THE INVENTION  
       [0011]     One embodiment of the present invention relates to an apparatus comprising an adsorption bed, an oxidizer source connected to the adsorption bed wherein the oxidizer is at a first pressure, a high pressure fluid source connected to the adsorption bed wherein the high pressure fluid is at a second pressure, the second pressure being greater than the first pressure, a depleted oxidizer outlet, and a fluid mixture outlet comprising a mixture of oxidizer and high pressure fluid.  
         [0012]     According to another embodiment of the present invention, the apparatus includes a first and a second adsorption bed, an oxidizer source connected to the adsorption beds wherein the oxidizer is at a first pressure, a high pressure fluid source connected to the adsorption beds wherein the high pressure fluid is at a second pressure, the second pressure being greater than the first pressure, a depleted oxidizer outlet connected to the adsorption beds, and a fluid outlet comprising a mixture of oxidizer and high pressure fluid.  
         [0013]     One method according to the present invention comprises adsorbing an oxidizer in an adsorption bed, desorbing the oxidizer by adsorbing a high pressure fluid in the adsorption bed, producing an outlet fluid mixture of oxidizer and high pressure fluid, and directing the outlet fluid mixture to a device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]      FIG. 1  is a schematic representation of one embodiment of an apparatus according to the present invention for cleaning objects.  
         [0015]      FIG. 2  is a schematic representation of a further embodiment of an apparatus according to the present invention for preparing a mixture of oxidizer and high pressure fluid.  
         [0016]      FIG. 3  is a schematic representation of another embodiment of an apparatus according to the present invention for preparing a mixture of oxidizer and high pressure fluid using parallel adsorption beds. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  is a schematic representation of one embodiment of an apparatus according to the present invention for producing a mixture of oxidizer and high pressure fluid in a batch system. Depicted are fluid mixture source  101 , cleaning chamber  103 , and fluid outlet  109 . Line  102  connects fluid mixture source  101  and cleaning chamber  103 . “Line” is used to mean a pipe or other structure capable of conveying fluids. In a typical embodiment for cleaning of semiconductor wafers, cleaning chamber  103  is a single wafer post etch chamber. Within cleaning chamber  103  are substrate support  105  which supports the wafer  107  that is to be cleaned. Standard elements of the apparatus are not depicted for reasons of clarity. For example, fluid outlet  109  may go to a recycle apparatus that removes solvents and debris from the fluid and then recycles the fluid to fluid mixture source  101 . Further, the cleaning chamber  103 , fluid outlet  109  and line  102  represent standard components known in the industry. Fluid mixture source  101  will be described in more detail with respect to  FIG. 2  and  FIG. 3 .  
         [0018]      FIG. 2  is a schematic representation of one embodiment of a fluid mixture source according to the present invention. Depicted are adsorbing bed  201  containing an adsorbent, oxidizer source  205 , and high pressure fluid source  207 . The system of the present invention may include standard components such as valves and other flow control devices, such as flow controllers, to control the flow of oxidizer and high pressure fluid into adsorption bed  201 . As shown in  FIG. 2 , during operation, oxidizer flows from the oxidizer source  205  through port A of a three-way valve  229  and into the adsorption bed  201 , adsorbing onto an adsorbent. Depleted oxidizer flows through port B of a three-way valve  231  and through oxidizer outlet  225 . High pressure fluid flows from high pressure fluid source  207  through port A of three-way valve  231  and into adsorption bed  201 . The high pressure fluid adsorbs onto the adsorbent thereby desorbing the previously adsorbed oxidizer. This results in a mixture of high pressure fluid and oxidizer which then flows through port B of three-way valve  229  and exits the system through fluid mixture outlet  227 .  
         [0019]     The depleted oxidizer flowing through oxidizer outlet  225  may be recycled through a recycle system or exhausted to an exhaust waste treatment system. The high pressure fluid and oxidizer mixture flowing through fluid mixture outlet  227  may flow directly to a device or tool, such as cleaning chamber  103  shown in  FIG. 1 , or may be sent to a storage vessel for later use.  
         [0020]     The operation of the apparatus shown in  FIG. 2  can be described in greater detail as follows. Adsorption bed  201  first receives an oxidizer, such as ozone, from an oxidizer source  205 , such as an ozone generator. The oxidizer adsorbs onto the adsorbent, and a stream of depleted oxidizer flows through oxidizer outlet  225  and to a recycle or exhaust system (not shown). An oxidizer sensor (not shown) may be associated with the oxidizer outlet  225  to monitor the concentration of oxidizer exiting the adsorption bed  201 . Alternatively the oxidizer sensor may be associated with the ozone generator operating at low power, which reduces operating costs.  
         [0021]     When the oxidizer concentration, as measured by the oxidizer sensor, reaches a predetermined setpoint, oxidizer flow through the adsorption bed  201  will stop and a high pressure fluid from high pressure fluid source  207 , such as supercritical carbon dioxide, will begin to flow through adsorption bed  201 . The high pressure fluid adsorbs onto the adsorbent thereby displacing the previously adsorbed oxidizer. This in turn, creates a mixture of the oxidizer and high pressure fluid that flows through fluid mixture outlet  227  and to a device such as a semiconductor processing chamber or a storage vessel.  
         [0022]     A further sensor may be associated with the fluid mixture outlet  227  and connected to a programmable logic controller (PLC) to monitor the oxidizer concentration in the fluid mixture. In addition, a flow controller for controlling the flow rate of high pressure fluid into the adsorption bed  201  may be fluidly connected to high pressure fluid source  207  and electrically connected to the PLC. The operation of the apparatus in this configuration would enable monitoring of the oxidizer concentration in fluid mixture outlet  227  with the sensor and providing a signal indicative of oxidizer concentration in the fluid mixture to the PLC. The PLC would then send a signal to the flow controller to adjust the high pressure fluid flow rate based upon a predetermined setpoint for the desired oxidizer concentration in the fluid mixture exiting from the fluid mixture outlet  227 .  
         [0023]     As an optional step, following desorption of oxidizer, the bed is vented to the atmosphere, and high pressure fluid in the void space and any remaining in the adsorption bed  201  is removed by flowing a purge gas, such as oxygen, through the adsorption bed  201 . The oxidizer adsorption, desorption with high pressure fluid and high pressure fluid removal steps are repeated cyclically until cleaning is complete. The process as described with respect to  FIG. 2  is a batch process because during the ozone adsorption and high pressure fluid removal steps, the system does not produce a fluid mixture of oxidizer and high pressure fluid.  
         [0024]     In another embodiment of the present invention, shown in  FIG. 3 , the fluid mixture is produced in a continuous manner. Referring to  FIG. 3 , the system of the present invention includes adsorption beds  301  and  303 , and fluid sources  305  and  307 . Further shown is one possible valving system, wherein input valves  309  and  311  are used to direct oxidizer from source  305  to either adsorption bed  301  or  303 , respectively and valves  313  and  315  are used to direct the high pressure fluid from source  307  to either adsorption bed  301  or  303 , respectively. Valves  317  and  319  are used to control the high pressure fluid output from adsorption beds  301  and  303 , respectively, and valves  321  and  323  are used to control the output of the depleted oxidizer from adsorption beds  301  and  303 , respectively. In an alternative arrangement, any one of the two-way valve pairs  309  and  311 ,  313  and  315 ,  321  and  323 , or  317  and  319  can be replaced with three-way valves.  
         [0025]     The operation of the apparatus shown in  FIG. 3  will now be described in detail. At any time, one adsorption bed will adsorb the oxidizer while the other bed is purged of first the oxidizer and then of excess high pressure fluid. The oxidizer desorption is accomplished by passing the high pressure fluid through the adsorption bed containing the oxidizer. Following oxidizer desorption, the bed is optionally vented to the atmosphere and high pressure fluid in the void space and any high pressure fluid in the adsorption bed is removed by flowing a purge gas, such as oxygen, through the adsorption bed. The oxidizer adsorption, desorption with high pressure fluid and purge steps are repeated cyclically in both beds until cleaning is complete. The process described with respect to  FIG. 3  is a continuous process because one bed is adsorbing while the other bed is desorbing and the system operates continuously in creating the output fluid mixture.  
         [0026]     The cycle time for the dual adsorption bed process is preferably in the range between 2 and 20 minutes. For example, time periods for the various steps according to one embodiment of the present invention are summarized in the following table.  
                                       Bed 301   Bed 303   Time (minutes)                   Pressurization   Oxidizer adsorption   0.25       Oxidizer adsorption   CO2 purge   4.00       Oxidizer adsorption   Depressurization   0.25       Oxidizer adsorption   Purge for CO2 removal   0.50       Oxidizer adsorption   Pressurization   0.25       CO 2  purge   Oxidizer adsorption   4.00       Depressurization   Oxidizer adsorption   0.25       Purge for CO 2  removal   Oxidizer adsorption   0.50                  
 
         [0027]     The cycles may be operated continuously with the appropriate valves being opened and closed as steps begin and stop during the cycle, as is known in the art.  
         [0028]     There are several alternatives available for valve control systems for operating the apparatus and performing the methods of the present invention. For example, valves may be controlled with a computer, mechanically or even manually. Further, different valves may be controlled in different manners.  
         [0029]     The oxidizer source for the system of the present invention can be an ozone generator, which produces a mixture of oxygen and ozone (O 2  and O 3 ) by partially converting a stream of oxygen into the ozone. The appropriate amount of conversion is set according to the desired outcome, and the highest ozone concentration is not always used because higher concentrations require more power to generate and thus have a higher cost. A practical maximum concentration is 20 percent O 3 . An oxygen rich feed gas for producing the oxidizer, such as ozone, may be produced from a pressure swing adsorption (PSA) facility. While ozone is the preferred oxidizer, other oxidizers may be used, such as hydrogen peroxide (H 2 O 2 ) or nitrogen trifluoride (NF 3 ).  
         [0030]     The high pressure fluid is typically chosen based on the material being cleaned. For semiconductor cleaning, SCCO 2  is usually preferred, while for disinfecting food products such as juice or drinking water, high pressure CO 2  including SCCO 2  may be used. The high pressure fluid may optionally contain co-solvents such as alcohols or disinfectants. The high pressure fluids and their generation are well known in the art.  
         [0031]     Suitable adsorbents for the adsorption beds include silica gel, high silica mordenites and other materials that do not destroy ozone to a significant extent during adsorption. The appropriate adsorbents for the adsorption beds may be chosen by the operator based on the high pressure fluid and oxidizer used.  
         [0032]     The operating parameters for the system according to the present invention can be readily set by the operator skilled in the art. For example, the adsorption beds are sized to adsorb the desired amount of fluid. A useful range for oxidizer adsorption pressures is from 5 psig to 50 psig (pounds per square inch gauge) because it approximately matches the pressure of the ozone/oxygen mixture from oxidizer. The desorption pressure using high pressure fluid is preferably in the range of 50 psia to 4000 psia (pounds per square inch absolute). In a more particular example, when treating water, the pressure range is typically between 50 psia and 200 psia. When using ozone as the oxidizer, the ozone concentration may be varied between 6 percent and 20 percent and the flow rate of the high pressure fluid may also be varied.  
         [0033]     There are several alternatives that may improve the cycle times. For example, the purge gas may be used at the same temperature as the oxidizer feed, however, a slightly higher temperature, for example, 10 to 30 degrees C. higher than the feed temperature, for the purge gas during part of the purge may reduce the amount of purge gas needed. Standard heaters may be used to heat the purge gas. Additionally, an ozone compatible vacuum pump, for example, a dry vacuum or a water ring vacuum, may be used to reduce the amount of purge gas required during the purge operation.  
         [0034]     While  FIG. 1  shows a semiconductor wafer being cleaned, it has previously been noted that the system of the present invention can be used to disinfect food or to clean water. In such systems, supercritical a high pressure CO 2  destroys enzymes that cause food to spoil. For water disinfection, ozone destroys microorganisms in water while CO 2  lowers the pH of water thereby suppressing the formation of unwanted disinfection byproducts.  
         [0035]     Moreover, while it is understood that the term oxidizer includes such standard oxidizers as ozone, hydrogen peroxide, and nitrogen trifluoride, in food and water disinfecting applications where ozone is used, the ozone may react with enzymes or microorganisms by mechanisms other than oxidation. Hence, the term oxidizer is here defined to embrace ozone when employed in food and water disinfecting applications.  
         [0036]     Other variations in the apparatus and operation are contemplated. For example, more than two adsorption beds may be used. Moreover, as noted above, additional solvents or disinfectants may be added to the high pressure fluid.  
         [0037]     It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description and examples, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the following claims.