Abstract:
Apparatus for removal of material in reactions having limited solubility and diffusion. An exemplary system removes unwanted material from the surface of a semiconductor wafer. 
     A flow apparatus is provided for removal of material from a work piece having at least one reaction region containing removable material. The apparatus may include first and second assemblies positionable in spaced-apart relation to form a zone extending between the two assemblies for movement of gaseous material. The first assembly may include a fixture positioned to receive the work piece with the reaction region of the work piece disposed in the zone to allow movement of the gaseous material thereover. A flow assembly is configured to transfer into the zone a gas comprising a condensable material and a reacting species. 
     In another embodiment a system for removal of material from a workpiece includes a chamber, a flow component and a support apparatus. The flow component is configured to pass gaseous material into the chamber at a selectable rate and allow exit of liquid and gaseous materials from the chamber. The support apparatus is configured to position the workpiece thereon with the surface region of the work piece oriented for contact with gaseous material passed into the chamber by the flow component. The system may further include a thermal control configured to provide a differential temperature between a portion of a work piece and gaseous material passed into the chamber.

Description:
RELATED APPLICATIONS 
     This is a conversion of provisional application Serial No. 60/165,542 filed Nov. 15, 1999. This application is related to Ser. No. 09/712,517 [Higashi 14] filed on even date herewith. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to cleaning processes of the type incorporating soluble oxidants and, in preferred embodiments, relates to removal of photo-resist (PR) and associated by-products during the cleaning stages of integrated circuit manufacture. 
     BACKGROUND 
     It is well known that gas solubilities in liquid media such as water can decrease with rising temperature. In particular, this has been problematic for cleaning systems based on dissolved ozone. When ozone functions as an oxidant in water, e.g., to effect removal of material from a surface, it can be counter productive to increase the intrinsic reaction rate of the ozone by increasing temperature. That is, when the intrinsic rate increases, the overall speed at which the system operates to remove material does not increase and may decrease, due to reduced solubility of the oxidant at elevated temperature. 
     In the semiconductor industry many manufacturing steps involve photolithography. Photoresist is commonly patterned and etched on an exposed surface of a partially fabricated semiconductor wafer in order to transfer a feature from a photomask to the surface. The feature, defined in the photoresist, is then transferred into the wafer material through, for example, a selective etch process. By way of illustration, this technique is commonly used to define zones for ion implantation, shallow trench isolation, polysilicon interconnect and Damascene trenches for metallization schemes. 
     After the PR is patterned and etched, and after the patterned photoresist is used to etch the feature, it is necessary to remove photoresist or byproducts of the reactions. Conventionally, this has been accomplished with a cleaning process based on reaction of ozone with the material to be removed. For example, in a batch process, a group of wafers may be dipped in a solution of water and ozone, possibly under agitation, to effect the removal. 
     It is generally recognized that the reaction rate for ozone removal of PR and associated byproducts from a wafer surface is limited by diffusion of O 3  to near the water/PR interface. Recently it has been observed that the rate of removal can be increased by first placing the wafers in a chamber of elevated temperature. With the wafers positioned above a pool of heated water, water vapor condenses upon the wafer surfaces while O 3  is introduced into the chamber. In contrast to performing the reaction while the wafers are immersed in a bath of ozonated water, the wafers above the pool have a limited thickness of water on the surfaces. Since the PR strip rate is limited by diffusion of O 3  through the DI water, providing a relatively thinner layer of water for a given partial pressure of O 3  gas increases the net amount of O 3  diffusion to the water/PR interface; and this will result in an increased reaction rate relative to the reaction rate which would result when the ozone diffuses into a bath of water to reach the PR. 
     Nonetheless, with both the solubility and the diffusion of the reactant species so limited, the approach of placing wafers over a pool of water to diffuse the species through condensate, at best, provides a rate-limited reaction. If the diffusion rate and overall rate of material removal could be further increased, significant economies may be made available. Specifically, PR removal in a single wafer processing system would become more cost effective. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the invention, the removal of material in reactions having limited solubility and diffusion of gaseous material can be increased by controlling the thickness of the layer into which the reactant species diffuses. In another embodiment, the thickness of the layer and the diffusion of the gaseous material can be individually controlled. Apparatus and method are provided. 
     A flow apparatus is provided for removal of material from a work piece having at least one reaction region containing the removable material. The apparatus may include first and second assemblies positionable in spaced-apart relation to form a zone extending between the two assemblies for movement of gaseous material. The first assembly may include a fixture portion positioned to receive the work piece with the reaction region of the work piece disposed in the zone to allow movement of the gaseous material thereover. A flow assembly is configured to transfer into the zone a gas comprising a condensable material and a reacting species. 
     According to another embodiment a system is provided for removal of material from a workpiece having at least one surface region containing the removable material. The system includes a chamber, a flow component and a support apparatus. The flow component is configured to pass gaseous material into the chamber at a selectable rate and allow exit of liquid and gaseous materials from the chamber. The support apparatus is configured to position the workpiece thereon with the surface region of the work piece oriented for contact with gaseous material passed into the chamber by the flow component. The system may further include a thermal control configured to provide a differential temperature between a portion of a work piece positioned on the support apparatus and gaseous material passed into the chamber by the flow component. 
     A method for chemical processing is also provided wherein the thickness of a layer of condensate on a surface is actively controlled and a reactant species is diffused from a gaseous region overlying the surface into the layer. According to an exemplary embodiment for removal of material from a work piece having at least one surface region containing the removable material, a workpiece is placed in an atmosphere comprising a condensable gas and a reacting species. The partial pressure of the condensable gas is controlled to limit the formation of liquid condensation on the surface region. 
     According to a method for removing unwanted material from the surface of a semiconductor wafer, the wafer is placed in a zone having a controllable atmosphere and gaseous materials including a gaseous reactant and a condensing vapor are passed into the zone for contact with a wafer surface having the unwanted material thereon. The partial pressure of the condensing vapor in the zone is controlled to condense the vapor on the wafer surface at a selectable rate and cover the unwanted material with a condensate layer of desired thickness. The gaseous reactant is allowed to dissolve in the condensate. 
     In another embodiment for removing unwanted material from the surface of a semiconductor wafer, the wafer is placed in a chamber, gaseous materials including an oxidant and a condensing vapor are passed into the chamber for contact with a surface of the wafer having the unwanted material thereon, and vapor is condensed on the wafer surface at a predeterminable rate to cover the unwanted material with a layer of fluid. The oxidant is allowed to diffuse into the fluid at a selectable rate. 
     The foregoing background and summary have outlined general features of the invention. Those skilled in the art may acquire a better understanding of the invention and the preferred embodiments with reference to the drawings and detailed description which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention will be acquired from the detailed description which follows. When read in conjunction with the accompanying drawing, in which FIG. 1 illustrates in schematic form an exemplary embodiment of a system for removal of material from a work piece. Features presented in the drawing are not to scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of the invention are schematically illustrated with reference to FIG. 1, wherein a flow system  10  is configured to remove material from a work piece. In this example the work piece is a partially fabricated semiconductor wafer  12  which may be of the type, for example, on which high density integrated circuits are formed. The wafer  12  includes at least one reaction region  14  on a surface  16  thereof. The region  14  contains material which is to be removed as part of a conventional photoresist pattern and etch sequence during semiconductor manufacture. It should be understood that several of the design features now illustrated are exemplary and only specific to the type of work piece and the type of material to be removed from the region  14 . Further, the chemical materials chosen to perform the operation are exemplary and numerous variants will be apparent based on, for example, the desired reaction rate, preferred values of reaction variables, the material to be removed and features specific to the work piece. In the illustrated example the material being removed from the reaction region  14  is photo resist and byproducts thereof, e.g., CO 2 . The system  10  may be used for removal of other materials, including atmospheric contaminants and various hydrocarbons, which may form on the surface  16 . 
     In the system  10  the work piece wafer  12  is placed in a zone  20  which receives a flow of gaseous material  22  (indicated with arrows) injected toward the central portion  24  of the wafer surface  16 . The reaction is based on dissolution of gaseous ozone in deionized water. Preferably, the ozone is delivered near the reaction region  14  by condensing water vapor (e.g., ten monolayers of H 2 O under steady state conditions) on the wafer surface  16  and allowing ozone to diffuse into the H 2 O. Although the invention is not limited by any specific theory on the mechanisms of the desired reaction, it is believed that the presence of sufficient O 3  in water effects a reaction which results in dissolution of photo resist material and associated byproducts. The system  10  facilitates such removal in a rapid and controllable manner. 
     The system  10  includes a rotatable upper assembly  30  having a plane surface  32  and a rotatable lower assembly  34  including a fixture portion  35  for holding the wafer  12  along a plane  36 . The plane  36  is indicated with a hatched line. The surface  32  and the plane  36  are in a spaced-apart relation. Preferably the distance between the plane surface  32  and the plane  36  is adjustable as notationally indicated by the arrow  38 . With the wafer  12  mechanically secured to the fixture portion  35  in a conventional manner, the zone  20  largely corresponds to a volume between the surface  32  and the plane  36 , extending, more or less, to the periphery  38  of the mounted wafer. 
     A wafer heating assembly provides thermal control of the wafer temperature. The heating assembly includes a thermal sensor  42  positioned to monitor the wafer temperature. The illustrated sensor is positioned for contact with the wafer in order to provide a calibrated electrical signal indicative of the wafer temperature. The temperature dependent signal generated by the sensor  42  is transmitted to a controller  44 , e.g., by wire or radio signal, and the controller  44  regulates thermal input to the wafer  12  from one or more energy sources  46 . Sources  46  may, for example, be an array of thermal elements which radiate infrared energy to the wafer  12 , or they may be diode lasers with appropriate lens systems to direct the radiation to the wafer  12 . 
     The gaseous material  22  is generated by an ozonating supply system  50 . From the system  50  a mixture of water vapor and ozone is selectively transmitted through a feed line  52  for injection through the surface  32  to direct the gaseous material  22  into the zone  20  at or near the center portion  24  of the wafer  12 . Movement of the material  22  into the feed line is through a valve  56  whose position is set by the controller  44 . After entering the zone  20 , a portion of the water vapor condenses on the wafer surface  16  and, wafer surface  16  toward the wafer periphery and exits the zone  20 . Although not illustrated, texture or flow-enhancing paterns maybe formed along the surface  32  to facilitate desired movement of the mixture  22  along the wafer surface  16 . 
     During operation of the flow system  10  a wafer  12 , containing photoresist material to be removed from the reaction region  14 , is positioned in the lower assembly  34  along the plane  36  so that the wafer surface  16  is spaced apart from the surface  32  of the upper assembly  30 . With this arrangement the distance between the wafer surface  16  and the surface  32  may be adjusted by movement of one or both assemblies  30  and  34  to select an appropriate volume for the zone  20 . Preferably, the volume of the zone  20  is determined for a prescribed flow rate, temperature and mix of the gaseous material  22  being injected into the zone  20 . The flow rate should be selected so that the partial pressures of ozone and water vapor over the reaction region  14  are predominantly a function of the flow rate and properties of the injected gases, including any carrier gases incorporated in the material  22 . Under these conditions the partial pressures of the ozone and the water vapor are relatively independent of pressure and temperature conditions outside the zone. Thus the zone  20  can develop conditions different from those of the adjoining atmosphere. 
     The temperature of the wafer surface and the resulting thin layer of condensed water formed on the surface  16  may be largely influenced by the temperature of the wafer. On the other hand, the temperature of gases in the zone  20  and above the condensed water will predominantly result from adjustments imparted to the material  22  by the heater assembly  54  as directed by the controller  44 . 
     Under these conditions the zone  20  provides an atmosphere comprising a condensable gas, e.g., water vapor, wherein the temperature of the gas relative to the wafer surface  16  is controllable to influence the rate of condensation on the surface  16 . Ozone or another gaseous reactant may diffuse into the condensation at a rate controllable by the partial pressure of the gaseous reactant. With sufficient ozone dissolved in the condensation, photoresist material is chemically removed from the reaction region  14 . Although this removal process is understood to include diffusion of an oxidant such as ozone into the condensed layer, operation of the flow system  10  to effect such removal may not validate or depend upon any specific theory to describe a removal mechanism for the unwanted material. 
     The rate of reaction may be enhanced with radiation. For example, as illustrated in FIG. 1, with ozone as the oxidant, the wafer surface  16  may be irradiated with one or more ultra violet sources  62  positioned above the plane surface  32 . To effect transmission of the radiation to the ozonated condensate on the surface  12 , the surface  32  and other components may be formed of fused silica or other material having a suitably high transmission for the radiation. 
     Spinning of the wafer  12  provides another means of controlling the thickness of the fluid layer and facilitates movement of condensate (carrying reaction products) off the wafer. This in turn accelerates the development of new condensate near the reaction site and diffusion of more ozone therein. The overall effect is to further improve the removal rate of material at a temperature and speed suitable for volume manufacture of semiconductors and other products. 
     Once the reaction is under way or substantially complete, the flow velocity and condensation rate of the gaseous mixture can be modified to increase the rate at which material is rinsed off the surface  16 . For example, removal of such may be enhanced by initially increasing the rate of condensation on the surface and increasing the spin rate of the wafer  12 . The volumetric flow rate of the gaseous mixture from the center portion  24  to the wafer periphery  38  may be increased by decreasing the distance between the plane surface  32  and the plane  36 . The temperature of the wafer surface  16  can be lowered relative to the temperature of the gaseous material  22  in order to increase the condensation rate. That is, the controller  44  may cut off thermal input to the wafer  12  from the energy source  46  while increasing the temperature of the gaseous material so that the surface  16  is at a lower temperature than the temperature of the gaseous material  22 . 
     The wafer may be dried by altering the composition of the gaseous material  22  provided by the supply system  50  from water vapor and ozone to a relatively inert gas such as nitrogen. The nitrogen may be passed over the wafer surface at a high flow rate while both the wafer  12  and the upper assembly surface  32  spin. The flow rate of the nitrogen can be readily increased by bringing the surface  32  closer to the plane  36 . The nitrogen may be heated under the direction of the controller  44  to enhance vaporization of the condensate. The controller can also turn on thermal input to the wafer  12  from the energy source  46  to increase the rate of evaporation. 
     A feature of the invention is the ability to control and increase the rate of a diffusion limited reaction which heretofore has been limited by the thickness of the condensate layer into which the gaseous reactant is dissolved. By actively controlling the thickness of the condensate layer the concentration of gaseous reactant in the condensate may be rapidly increased. An exemplary means of effecting this increased diffusion rate has been illustrated, e.g., by effecting independent control of wafer temperature and gas temperature over the wafer, or by controlling pressure in the zone  20 , thereby modifying the relative humidity that controls the thickness of the layer of condensate (e.g., water on the wafer surface). The condensation rate can be controlled and limited to maximize the reaction rate. 
     In the disclosed embodiment the gaseous reactant is an oxidant. Several variables controlling the state conditions of the oxidant, e.g., O 3 , may be modified to adjust the rate at which the oxidant reacts with the unwanted material, e.g., PR. The variables include temperature at the point of reaction (which may be varied by warming or cooling the wafer surface) and partial pressure of O 3  (which may be varied by modifying the net gas concentration or the pressure in the zone  20 ). 
     Generally the invention as disclosed may be performed in a chamber. In the illustrated embodiment of the flow system  10 , the chamber is a zone  20  open to atmospheric conditions, but capable of accommodating state conditions different from the adjoining atmosphere. In a conventional closed chamber, e.g., one wherein the enclosed environment is sealed from the atmosphere, pressure can be varied by controlling the rate at which gas exits the chamber relative to both the volume of gas entering the chamber and the net volume increase due to by-products formed in the chamber. However, for the zone  20 , as described with reference to FIG. 1, the distance between the upper assembly surface  32  and the wafer  12  can be adjusted to vary state conditions such as pressure in the zone without having to modify the temperature, composition or volumetric flow rate of gaseous material injected to the zone  20 . 
     Thus vapor condensation rate can be controlled independent of predominant state conditions in the chamber vessel, e.g., zone  20  to vary the thickness of the fluid layer and maximize the diffusion rate of the reactant, e.g., ozone, into the condensed fluid. Another means of establishing conditions under which the rate of condensation is controlled is to inject a liquid  72 , such as water, from a reservoir  70  into the feed line  52 . A valve  74 , permitting movement of the liquid into the feed line  52 , is governed by the controller  44 . The controller  44  elevates the liquid to a desired temperature as the liquid passes through the heater assembly  54  so that the liquid  72 , upon entry to the zone  20 , undergoes thermal interaction with the wafer surface  16  while the wafer spins. Once the surface  16  reaches a desired temperature the controller closes the valve  74  and opens the valve  56  so that gaseous material is injected through the feed line  52  at a different, e.g., higher, temperature than that of the liquid  72 . This process could be cyclic in order to sustain optimal temperature differentials and periodically wash reacted material off the surface  16 . 
     The rate at which the wafer  12  is heated can be further increased with a supplemental feed assembly  80  comprising a feed line  82 , a heater  84  in the feed line  82 , under direction of the controller  44 , and a reservoir  86  providing a liquid  88  into the feed line for transmission through the heater  84  and to the wafer  12 . The liquid, e.g., water, is sprayed upon the undersurface  90  of the wafer  12  for thermal transfer. The controller  44  regulates movement of the heated liquid  88  with a valve  90 . Various liquids (including condensate containing removed material, liquid  72  and liquid  88 ) are permitted to collect in a drain region  92  for removal. 
     Generally the diffusion rate is controllable by the relative pressures of the vapor and the reactant gas as well as the overall chamber pressure. Further, with the heater assembly  54  placed exterior to the chamber, e.g., outside of the zone  20 , the temperature of the gaseous material  22  and wafer temperature are individually controllable. Although not illustrated, further control can be had over the reaction by individually injecting the condensable vapor and the gaseous reactant into the chamber in order to individually control their respective temperatures. 
     It should also be noted that the condensate need not be pure water. Other fluids such as methanol may be suitable and various mixtures, including those formed with inert materials, may be most useful in this application. 
     A system and method have been provided wherein the condensation rate of a vapor such as water can be controlled while separately controlling the reaction rate of an oxidant which dissolves in the condensed vapor. The vapor pressure of the reacting species can be modified relative to atmospheric pressure as well as the vapor pressure of other gaseous material in the chamber, e.g., the zone  20 . The chamber gas concentration of the condensing vapor and the reacting species, and the temperature of the work piece surface, are all adjustable to control the overall rate at which the species, e.g., ozone, reacts with material on the surface of the work piece. The condensable vapor and the reacting species can be selectively directed to the work piece. In the case where the work piece is a semiconductor wafer, the gaseous materials are directed toward a center portion of the work piece such that they may flow toward the periphery as they exit the chamber. 
     The invention has been described with only a few illustrative embodiments while the principles disclosed herein provide a basis for practicing the invention in a variety of ways. Although the disclosed systems and methods have been illustrated for semiconductor manufacturing applications, the concepts are generally applicable to removal of materials from surfaces with reactant species that diffuse through a layer. Other constructions, although not expressly described herein, do not depart from the scope of the invention which is only to be limited by the claims which follow: