Abstract:
A method and apparatus are disclosed for optimizing a rinsing and drying process in semiconductor manufacturing. The optimization seeks to maximize processing throughput while maintaining low defect counts and high device yields, and utilizes simulation and experimental data to set the optimal process parameters for the rinsing and drying process. Improved methods of rinse liquid and purge gas nozzle movement are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of and priority to U.S. Provisional Patent Application No. 61/783,060, entitled “METHOD AND APPARATUS FOR SUBSTRATE RINSING AND DRYING”, filed on Mar. 14, 2013, the entire contents of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method and apparatus for rinsing and drying of a substrate in semiconductor manufacturing. More particularly, it relates to an improved method for rinsing and drying a substrate, in which defect counts are reduced. 
     Description of Related Art 
     A rinsing and drying step is commonly used in many processes in semiconductor manufacturing. In photolithography, this step is used after development of photoresist, to rinse the developer along with developed photoresist, and dry the substrate prior to removal from the processing tool. These steps are also used in other wet processing of substrates, for example, following substrate wet cleaning, or following electrochemical deposition. 
     In a typical rinse and dry step, the substrate is mounted on a spin chuck and rotated at a set rotation speed. Rinse liquid is dispensed from a nozzle, or plurality of nozzles onto the substrate, the rinse liquid displacing the contaminant that needs to be removed from the substrate. The contaminant may be, for example, a developer solution, cleaning solution, or electrolyte for electrochemical deposition. In a typical process, the rinse solution is introduced at the center of the substrate, initially radially displacing the contaminant from the center of the substrate. The radial displacement of the contaminant along with the rinse liquid is assisted by substrate rotation and by subsequently starting a flow of purge gas, where the flow of purge gas is used to remove the last droplets and traces of contaminant and rinse liquid from the substrate. The rinse liquid dispense nozzle and the purge gas nozzle, or plurality of nozzles, are typically moved across the surface of the substrate in a direction generally outwards from the center of the substrate, towards the substrate edge, leaving a cleaned and dried portion behind. The instantaneous boundary between the still-wet and dry portions of the substrate is defined by a circular rinse liquid meniscus. As the nozzles are moved radially outwards, so does the meniscus. Once the meniscus reaches the edge of the substrate, the entirety of the substrate has been rinsed and dried, and it can be removed from the spin chuck and processing tool for subsequent processing steps. 
     In semiconductor processing, the desire exists to increase process throughput, i.e. the number of substrates processed in a set amount of time. This desire to increase throughput leads to the use of aggressive processing conditions (i.e., substrate rotation speeds, rinse liquid and purge gas flows and velocities, nozzle movement velocities, etc.) which may lead under certain conditions to the disruption of the meniscus by the purge gas flow, or splashing of still-adhered rinse liquid and contaminant from the wet portion of the substrate to the dry portion of the substrate, both of which can lead to increased device defect counts and reduced processing yield. Under typical processing conditions, the defect count density increases with radius and is highest at the substrate periphery. Improvements are therefore needed to address these high defect counts, particularly at the substrate periphery, while maintaining low processing times and thus high process throughputs. 
     Specifically, the need exists for a method of optimizing the rinsing and drying process so as to maximize throughput without causing meniscus disruption and splashing, and thus increased defect counts. 
     SUMMARY OF THE INVENTION 
     An aspect of the invention includes a method for substrate rinsing and drying, comprising: loading the substrate into a rinse module, the substrate being mounted horizontally on a rotatable spin chuck within the rinse module; starting a first flow of rinse liquid from a dispense nozzle onto the substrate; moving the dispense nozzle horizontally on a dispense nozzle path from the center of the substrate towards the edge of the substrate; starting a second flow of purge gas from a first purge gas nozzle initially located proximate the center of the substrate, to establish a meniscus of the dispensed rinse liquid on the substrate; moving the first purge gas nozzle horizontally towards the edge of the substrate, so as to radially displace the meniscus and dispensed rinse liquid towards the edge of the substrate, wherein moving the first purge gas nozzle comprises maintaining an optimum position of the first purge gas nozzle relative to the dispense nozzle and the meniscus, so as to prevent disruption of the meniscus during movement of the first purge gas nozzle or disruption of the first flow of rinse liquid from the dispense nozzle, or both. 
     Another aspect of the invention includes further starting a third flow of purge gas from a second purge gas nozzle; moving the second purge gas nozzle horizontally towards the edge of the substrate, wherein moving the second purge gas nozzle comprises maintaining an optimum position of the second purge gas nozzle relative to the dispense nozzle and the meniscus, so as to prevent disruption of the meniscus during movement of the second purge gas nozzle towards the edge of the substrate or disruption of the first flow of rinse liquid from the dispense nozzle, or both. 
     Further aspects of the invention include various configurations of nozzles and scanning arms that allow moving the nozzles across the substrate and adjusting the height of the nozzle above the substrate. In accordance with embodiments of the invention, multiple nozzles can be mounted on a single scanning arm, or each nozzle can have its own scanning arm. Further in accordance with embodiments of the invention, scanning arms can be actuated in a linear or arcuate manner, and can include means for changing the distance between multiple nozzles mounted on a same scanning arm. 
     A yet further aspect of the invention includes a method for maintaining optimum position of purge gas nozzles with respect to the rinse liquid dispense nozzle, so as to prevent disruption of the dispensed rinse liquid flow, or meniscus formed on the substrate, or both. Other process parameters of the rinsing and drying process can also be optimized in accordance with further embodiments of the invention. 
     In a further aspect, the optimization of the rinsing and drying process may include setting conditions to ensure an optimum shear stress is exerted upon the liquid dispensed onto the substrate, and that the air flux over the substrate is set at a value that ensures maximum shear stress without splashing or meniscus disruption. 
     A further aspect of the invention includes using simulation results for shear stress and air flux, for process control. Furthermore, experimental measurements of the meniscus position, compiled in a library, can be used along with simulation results for process design and control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows the dependence of the simulated shear stress on the distance from a purge gas nozzle. 
         FIG. 2  shows the dependence of the simulated shear stress on azimuthal angular location and substrate radial position. 
         FIG. 3  shows the dependence of simulated average shear stress on radius and purge gas flow, with experimentally determined conditions where disruption of the meniscus and splashing occur. 
         FIG. 4  shows an air flux graph with experimentally determined conditions where splashing does and does not occur. 
         FIG. 5A  shows a map of operating conditions for a rinse module in which conditions satisfying both average shear stress criteria and air flux criteria cannot be found. 
         FIG. 5B  shows a map of operating conditions for a rinse module in which conditions satisfying both average shear stress criteria and air flux criteria exist. 
         FIG. 6  shows the dependence of air flux on dispense to purge gas nozzle distance and purge gas flow. 
         FIG. 7  shows the dependence of the rinse liquid dispense nozzle to purge gas nozzle distance on the instantaneous meniscus radius, for two nozzle configurations. 
         FIG. 8  shows a schematic of a rinse module in accordance with embodiments of the invention. 
         FIGS. 9A-9H  show schematics of various configurations of nozzles and scanning arms in accordance with embodiments of the invention. 
         FIG. 10  shows a flowchart of an exemplary process for rinsing and drying a substrate in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the present invention relate to design and control of a process, apparatus, and system for rinsing and drying a substrate, in semiconductor manufacturing. 
     In the following description, in order to facilitate a thorough understanding of the invention and for purposes of explanation and not limitation, specific details are set forth, such as particular geometries of a lithography, coater/developer, and gap-fill treatment system, and descriptions of various components and processes. However, it should be understood that the invention may be practiced in other embodiments that depart from these specific details. 
     In the description to follow, the terms radiation-sensitive material and photoresist may be used interchangeably, photoresist being only one of many suitable radiation-sensitive materials for use in photolithography. Similarly, hereinafter the term substrate, which represents the workpiece being processed, may be used interchangeably with terms such as semiconductor wafer, LCD panel, light-emitting diode (LED), photovoltaic (PV) device panel, etc., the processing of all of which falls within the scope of the claimed invention. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but does not denote that they are present in every embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Various operations will be described as multiple discrete operations in turn, in a manner that is most helpful in understanding the invention. However, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. Operations described may be performed in a different order than the described embodiment. Various additional operations may be performed and/or described operations may be omitted in additional embodiments. 
       FIG. 8  shows a schematic of a rinse module  100  in accordance with embodiments of the invention. The rinse module  100  comprises a spin cup  110  equipped with baffles  120  and fluid outlet  130 , all configured to facilitate removal of rinse liquid and contaminants that have been spun off from substrate  150  which is attached to rotating spindle  140 . Rinse liquid is dispensed via dispense nozzle  170  onto the upper surface of substrate  150 . The rinse liquid, typically deionized water, is supplied from rinse liquid reservoir  200 . Dispense nozzle  170  is mounted on a scanning arm  180 , which can be moved using motion system  250 , to scan the dispense nozzle  170  along a path that starts substantially at the center of the substrate  150  towards the edge of the substrate  150 . The combination of motion of dispense nozzle  170  and supplying the rinse liquid ensures the creation of a uniform rinse liquid film  152  of dispensed rinse liquid on the substrate  150 . A certain amount of rinse liquid is removed from the substrate  150  by centrifugal force, but the process is further assisted by providing one or more purge gas flows that ensure that the last traces of rinse liquid are removed from the substrate. 
     In an embodiment, a purge gas nozzle  160  is provided to introduce a purge gas, typically dry nitrogen, to the substrate surface, to assist in displacing the rinse liquid and drying the substrate  150 . Purge gas nozzle  160  can be mounted onto scanning arm  180 , together with the dispense nozzle  170 , and is supplied with purge gas from purge gas source  210 . With the purge gas nozzle  160  initially aligned substantially at the center of substrate  150 , the purge gas flow impinging on the dispensed rinse liquid film  152  causes the initial creation of a dried portion of the substrate  150 , surrounded by a still-wet portion covered with rinse liquid film  152 . The two portions are separated by a substantially circular meniscus  154  formed at the inner edge of the dispensed rinse liquid film  152 . The simultaneous movement of both the dispense nozzle  170  and the purge gas nozzle  160  towards the edge of the substrate  150  causes the purge gas to dry a progressively larger portion of the substrate  150 , enlarging thus the meniscus  154 , until the meniscus  154  reaches the edge of substrate  150 , at which point the substrate has been completely dried and is ready for transfer to subsequent processing steps. 
     In this simplest example of operation of rinse module  100 , the driving force for the rinse liquid film  152  is provided by both the centrifugal force exerted on the rinse liquid film  152  by substrate rotation, and by the shear stress exerted on the rinse liquid film  152  by the flow of purge gas, which flow follows a typical stagnation point flow pattern, i.e., downwards from purge gas nozzle  160  to the substrate  150 , and then turning radially outwards in all directions, with the purge gas flow velocity component parallel to the substrate  150  causing a shear stress, also known as aerodynamic drag, on the rinse liquid film  152 , to push it outwards and over the substrate edge. Counteracting the centrifugal and shear stress are forces of viscous drag as the rinse liquid film  152  slides across the substrate, and the surface tension forces resulting from the configuration of the meniscus  154  and wetting angles formed by the rinse liquid film  152  against the surface of substrate  150  in the presence of the purge gas and/or ambient air. 
     From the foregoing discussion it is immediately apparent that the relative alignment of the purge gas nozzle  160  with respect to the instantaneous position of the meniscus  154  is of utmost importance if the process is to proceed at a maximum possible rate, thus maximizing throughput, yet keeping defect counts low. A typical defect scenario occurs if the purge gas nozzle  160  “jumps” over the meniscus  154 , causing splashing back of rinse liquid droplets onto the already dried portion of the substrate  150 , thus increasing defect count. Other causes may include too small a height of the purge gas nozzle or too large a purge gas flow, both of which can also lead to disruption of meniscus  154  without the purge gas nozzle  160  necessarily “jumping” ahead of the meniscus  154 . In practice, the defect counts tend to cluster in the peripheral portions of the substrate, which indicates that the largest challenge is maintaining a stable meniscus  154  when its circumference is large and the purge gas stagnation flow patterns emanating from one or multiple purge gas nozzles are unable to maintain a sufficient and relatively uniform shear stress along the entire circumference of meniscus  154 . 
     The inventors have discovered that the solution to the problem of maximizing throughput while keeping defect counts low is amenable to the use of insights provided by computer simulation and targeted experiments, the results of which can yield useful data for both system design, e.g. nozzle selection, and for in-line process control, e.g. to actively control the positions of one or more purge gas nozzles, to prevent disruption of the meniscus and/or splashing of droplets back into the dried portion of substrate  150 . Other conditions that can be also accounted for and prevented include avoidance of disruption of the rinse liquid flow from dispense nozzle  170  by a nearby purge gas flow from purge gas nozzle  160 , when the separation distance between the two is small. 
     Computer codes such as FLUENT, provided by ANSYS Inc., of Southpointe, 275 Technology Dr., Canonsburg Pa. 15317, or COMSOL Multiphysics, provided by COMSOL Inc., of 1 New England Executive Park, Burlington Mass. 01803 are nowadays capable of handling the multi-physics multi-phase simulations involved in modeling the movement of rinse liquid film  152  and meniscus  154  across substrate  150  under the influence of centrifugal, shear, viscous, and surface tension forces. 
       FIG. 1  shows an exemplary graph of simulated shear stress vs. radial location, distance from the centerline of the purge gas jet for four different purge gas flows ranging from 8 to 30 liters per minute (LPM), for a fixed purge gas nozzle diameter and fixed height of the nozzle above the substrate. In the close vicinity of the jet centerline but not at the centerline itself, the shear stress curves  10 ,  12 ,  14 ,  16  all reach a peak value, which depends on the purge gas flow. The magnitude of the peak is highest for the largest purge gas flow. Thereafter, as the purge gas flow spreads radially outwards from the purge gas jet centerline, the shear stress drops off rapidly. Simulations of this kind can be used to determine the required purge gas flow rate to achieve a certain required magnitude of shear stress. 
       FIG. 2  shows another exemplary graph of simulated shear stress along the circumference of a rinse liquid meniscus, where the circumferential coordinate is expressed in terms of angle. The instantaneous radial location of the meniscus was varied, and plots  20 ,  22 ,  24  for three radii are shown, each at a progressively larger meniscus instantaneous radius. What can be seen is that when the meniscus radius is small, the peak of shear stress imposed on the meniscus and the rinse liquid film is spread out over a relatively large angle, portion of the circumference of the meniscus. As the radius is increased, the peak becomes sharper, i.e. it affects a progressively smaller portion of the meniscus circumference. At some very large radii, the peak may become so sharp as to disrupt the meniscus, so the inventors have explored using multiple purge gas nozzles and flows to “spread” the shear stress along a larger portion of the meniscus, even at large radii. An exemplary plot  26  of shear stress in this scenario is indicated with a dotted line in  FIG. 2 . The spacing between the peaks is determined by the spacing of the purge gas nozzles, and more generally their location with respect to the meniscus, and not just their azimuthal angular location along the circumference of the meniscus. Simulations of this kind can be used along with simulations of  FIG. 1  to determine e.g. the need for transition to multiple purge gas flows in order to maintain as uniform a shear stress acting on the rinse liquid film, as possible. 
     In similar fashion, simulations can be used to determine dependence of shear stress and other variables on e.g., nozzle height above the substrate  150 , or nozzle diameter. 
     The simulation results can now be combined with experimental data on conditions under which meniscus disruptions occur to map out the parameter space inside which the rinsing and drying process can safely operate without undue increase of defect counts. For example,  FIG. 3  shows an average shear stress (i.e., averaged circumferentially) for a centrally injected purge gas flow, for varying purge gas flow rates, and for varying instantaneous locations, radii of the meniscus. Plotted over the simulated shear stress curves  30 ,  32 ,  34 ,  36 , and  38  are radii at which meniscus disruption occurs. At this time, simulations are unable to accurately predict the exact meniscus location, so experimental determination of meniscus location, and particularly the radii at which disruption occurs have to be determined experimentally, for example by conducting experiments over a range of conditions and studying acquired video camera footage of the experiments to correlate disruption events with instantaneous conditions. The region  40  in  FIG. 3  indicates the point at which a single centrally-located purge gas flow is unable to maintain a stable meniscus, and additional purge gas flows need to be introduced. Furthermore, region  42  indicates locations at which splashing occurs of rinse liquid droplets from the rinse liquid film back onto the dried portion of substrate  150 . With both these failure modes mapped into the shear stress plot, it can be seen that a safe value of shear stress for the conditions used in this example is about 1 N/m 2 . Increasing the shear stress beyond this point has little benefit because of increased defects. 
     The inventors have discovered that besides the shear stress, another quantity plays a key role in determining the stability of the meniscus.  FIG. 4  shows a plot of simulated air flux caused by the component of purge gas velocity parallel to the substrate  150 . The plot shows also relative counts of defects caused by splashing vs. the air flux, determined experimentally in a manner similar to that employed in generating data for  FIG. 3 . What can be seen is that the plot shows two distinct regions,  50  and  52 , respectively. In region  50 , no splashing events are seen for relatively low values of air flux. When the air flux is increased above approximately 0.028 m 2 /s, there is a sudden increase of the number of defects caused by splashing droplets of rinse liquid. Therefore, the plot of  FIG. 4  is convenient for determining a second criteria that the rinsing and drying process conditions need to satisfy in order to keep the defect count low. 
     These aforementioned simulation and experimental results were found useful, by the inventors, for a number of different purposes including but not limited to: (1) designing the rinsing module hardware, e.g., nozzles, scanning arms, etc., (2) determining a narrowed space of conditions in which optimum rinsing and drying conditions are located, so as to reduce the number of experiments needed to arrive at an optimum process condition, and (3) the results can be directly used for in-line control of the rinsing and drying process. For example, the simulation and experimental results, such as e.g., meniscus locations determined experimentally for different process conditions, can be stored in look-up tables, or libraries, for fast look-up by an in-line controller that would use the data to maintain a precise location of one or more purge gas nozzles with respect to the instantaneous meniscus, to prevent disruption thereof. 
     Examples of using the aforementioned simulation and experimental data in designing rinse module hardware are shown in  FIGS. 5A and 5B . As was previously stated, the inventors have discovered that both the shear stress and air flux criteria need to be satisfied in order to enable processing at maximum rate while minimizing defects. Depending on the geometry, purge gas nozzle diameter, etc., and other fixed conditions imposed on the hardware design, such an optimum may exist or may not exist. For example, in  FIG. 5A  is shown an exemplary map of operating conditions for a fixed purge gas nozzle diameter, in which the height of the nozzle above the substrate and the purge gas flow rate are varied. What can be seen is that when the previously-determined criteria of average shear stress less than 1 N/m 2 , and air flux less than 0.028 m 2 /s are mapped into the graph, they create two disjoined islands of conditions,  60  and  62 , indicating an absence of optimum conditions for the given hardware constraints. 
     However, for another chosen exemplary nozzle diameter, an optimum set of conditions  68  can be found between the mapped areas satisfying shear stress and air flux criteria,  64  and  66 , respectively. Therefore, graphs such as  FIGS. 5A and 5B  are useful for both designing hardware and for determining a narrower set of processing conditions where the optimum processing conditions are located. The latter allows a significant reduction of time and cost involved in arriving at optimum production processing conditions for a rinsing and drying process. 
     So far in our description of simulation and experimental results, only single purge gas nozzles were used. It has been shown before, however, that in order to maintain a stable meniscus, particularly at large radii, a plurality of purge gas flows may be needed to maintain the optimum average shear stress and air flux conditions. The inventors have discovered that for conditions typical of rinsing and drying processes, a simple model can be built wherein superposition of results from multiple simulations of single purge gas flows processes is used to arrive at a very good estimate of conditions when multiple purge gas nozzles and purge gas flows are employed. The use of superposition eliminates the need for doing experiments over huge process condition spaces associated with multiple purge gas flow situations. For example, to simulate the plot  26  for two purge gas nozzles, of  FIG. 2 , inventors have applied superposition of two data sets simulated for a single purge gas nozzle. A similar approach can be taken in the case of air flux, where a simple vector addition of air flux contributions to a total air flux, from multiple individual air flux simulations can be used instead of simulating and storing results for multiple purge gas flow situations. In general, the use of multiple nozzles has a highly beneficial effect on the shear stress condition, and a lesser effect on the air flux condition. 
     A further aspect of the invention involves the use of fan nozzles for maintaining a meniscus, as opposed to an array of circular nozzles. Fan nozzles have the benefit of spreading the shear stress and air flux over a larger portion of the meniscus circumference, thereby reducing the likelihood of meniscus disruption for otherwise similar process conditions. Fan nozzles comprise a flattened nozzle exit causing the emerging purge gas jet to be elongated in the direction of the circumference of the meniscus. 
     Yet a further aspect of the aforementioned simulation and experimental processes is the optimization of impingement angle of the purge gas jet against the substrate surface. The change of impingement angle alters the stagnation flow pattern and can favorably alter the process conditions of the rinsing and drying process. 
     Further with respect to hardware and process optimization,  FIG. 6  shows a plot of air flux from a single purge gas nozzle plotted against the distance between the dispense nozzle for varying purge gas flows. What can be seen from the plots  72  and  74  is that they can be used to determine a required distance between the dispense nozzle  170  and purge gas nozzle  160  such that a required air flux (line  70 ) is maintained at the meniscus location. For example, for a purge gas flow rate of 15 liters per minute, the required distance from the dispense nozzle  170  to the purge gas nozzle  160  is in the neighborhood of 30 mm (condition  76 ). A larger distance of 55 mm (condition  78 ) can be used with a larger purge gas flow rate of 20 liters per minute. These plots can be used to minimize the purge gas flow consumption while ensuring that the air flux criteria are met. 
     Lastly,  FIG. 7  shows the distance between the dispense nozzle  170  and purge gas nozzle  160  plotted as a function of instantaneous meniscus radius for two purge gas nozzles in a rinse module  100  with two purge gas nozzles, of which nozzle  1  is closer to the dispense nozzle and nozzle  2  is located farther out from the dispense nozzle. The plot is generated such that aforementioned criteria of meniscus stability are satisfied, and clearly shows that for optimum conditions to be maintained, the distances of nozzle  1  and nozzle  2  to the dispense nozzle need to be varied as the nozzles are scanned across the substrate. Therefore, in both single purge gas flow and multiple purge gas flow rinse modules, provision needs to be made to vary these distances during the across-substrate nozzle scanning. 
     With reference again to  FIG. 8 , wherein a schematic of a rinse module  100  is shown, the aforementioned addition of a second purge gas nozzle  220  is achieved by adding a second scanning arm  230  to the rinse module  100 . The second purge gas nozzle  220  is supplied with purge gas from purge gas source  240 . In an alternative embodiment, the same purge gas source can be used to supply purge gas to both nozzles  160  and  220 . Furthermore, to enable a variable distance between the dispense nozzle  170  and the purge gas nozzle  160 , a motorized mount  190  can be utilized on scanning arm  180  to vary this distance. In addition to varying the distance, motorized mount  190  can also be used to vary the angle of purge gas nozzle  160  with respect to the substrate  150 , as indicated by reference  162 . A similar arrangement can be made (not shown) for second purge gas nozzle  220 , on scanning arm  230 , for variation of angle of second purge gas nozzle  220 . In addition to scanning nozzles  160 ,  170 , and  220  across the substrate, scanning arms  180  and  230  can be lowered and elevated using motion systems  250  and  260 , respectively, to vary the distance from the nozzles to the substrate. With all these provisions made for changing the relative positions and angles of nozzles, plus their heights, the aforementioned methods can be used to maintain optimum process conditions throughout a rinsing and drying process without causing disruption of the meniscus  154 , disruption of the flow of rinse liquid from the dispense nozzle  170 , and/or splashing. In further embodiments (not shown), separate scanning arms can be used for all three nozzles  160 ,  170 , and  220 , for example. 
       FIGS. 9A-D  show various possible scanning arm configurations for a rinse module  100  with a single rinse liquid dispense nozzle  170  (indicated with a dark dot) and a single purge gas nozzle  160  (indicated with a light dot).  FIG. 9A  shows a configuration where nozzles  160  and  170  are mounted on separate scanning arms  180  and  230  wherein the scanning arms are configured to move the nozzles radially outwards along radially opposing paths.  FIG. 9B  shows a configuration in which a single scanning arm  180  carries both nozzles  160  and  170  with provision made for independent adjustment of location of both nozzles, which can move in opposing directions or together towards the edge of substrate  150 .  FIG. 9C  shows a configuration wherein the scanning arms  180  and  230  of  FIG. 9A  are mounted azimuthally displaced at an angle other than 0 degrees and 180 degrees therebetween. Lastly, the configuration in  FIG. 9D  uses parallel scanning arms  180  and  230  to move nozzles  170  and  160 , respectively. In this configuration, the directions of motion are substantially parallel to a radius of the substrate  150 , and the path of one scanning arm may coincide with the radius of substrate  150 , while the other scanning arm moves along a parallel secant path. The movement along a secant path allows for an effective scanning speed of a nozzle across substrate  150  (in this case purge gas nozzle  160 ) to be different than the actual scanning speed of the nozzle itself. 
     Similar to  FIGS. 9A-D ,  FIGS. 9E-H  show scanning arm configurations wherein two purge gas nozzles  160  and  220  are provided, similar generally to the configuration shown in  FIG. 8 .  FIG. 9A  shows two radially opposing scanning arms  180  and  230  just like shown in  FIG. 8 .  FIG. 9B  shows a configuration where all three nozzles are mounted on the same scanning arm with provision made for independent movement thereof. In the embodiments of  FIGS. 9A and 9B , the purge gas nozzle  160  can be used to initially establish a meniscus  154  on substrate  150 , upon which the purge gas nozzle  160  follows the dispense nozzle  170  and meniscus outwards towards the edge of the substrate  150 . At some point during the rinsing and drying process, the second purge gas nozzle  220  can be activated and moved along a radially-opposing path, to maintain a stable meniscus  154  throughout the rinsing and drying process. 
     It is understood that in all embodiments shown in  FIGS. 9A-H , an alternative scanning method (not shown) can be used whereby the scanning arms pivot around their mounting points outside the periphery of substrate  150 , rather than being linearly scanned across the substrate  150 . Which configuration is used depends on space available in the rinse module  100 , the needed scanning speed, optimum processing conditions, etc., and it is possible to combine rotationally- and linearly-scanning scanning arms in the same rinse module  100 . Furthermore, the concepts outlined in  FIGS. 9A-H  can be extended to any number of scanning arms including any number of dispense nozzles and purge gas nozzles. Lastly, in many processing systems, the rinse module serves at the same time as a developing module, so additional scanning arms and nozzles are provided for dispense of developer liquid, in photolithography. In cleaning system, cleaning agents may be dispensed in the same rinse module via additional nozzles and scanning arms. 
     In all disclosed embodiments it is understood that simulation and experimental results, or both, discussed previously, are used by a controller (not shown) to achieve in-line control of the process. The controller can be preprogrammed to execute a rinsing and drying process in accordance with previously disclosed embodiments of the invention, i.e. using optimum settings determined from simulation and experimental results by ensuring optimum shear stress and air flux conditions, and the controller may use the same simulation and experimental results for in-line control as well, in a feedback or feedforward manner, for process correction on a wafer-to-wafer and lot-to-lot basis, for example. 
     With reference now to  FIG. 10 , therein is shown a flowchart of an exemplary rinsing and drying process  1000  in accordance with embodiments of the invention. In step  1010 , a substrate  150  is loaded into rinse module  100 , of  FIG. 8 , for example. Once rotation of substrate  150  is established, at step  1020  the first flow of rinse liquid is started from dispense nozzle  170  onto the substrate  150 , at a location substantially at the center of the substrate  150 . Once the rinse liquid film  152  is formed and the dispense nozzle  170  has advanced a predetermined optimum distance from the substrate center, at step  1030 , a second flow of purge gas is started from a first purge gas nozzle  160 , also at the center of the substrate  150 . This second flow of purge gas initially establishes a meniscus  154  and dry portion of the substrate  150 . Thereafter, in step  1040 , the dispense nozzle  170  and first purge gas nozzle  160  are moved towards the edge of substrate  150  until the meniscus  154  is pushed off the substrate  150 . During this movement, the flow rates of rinse liquid and purge gas, nozzle positions and distances therebetween, nozzle heights above the substrate, nozzle angles, etc., are all maintained at values optimized for maximum throughput with minimum defect counts using simulation and experimental data as disclosed before, i.e., ensuring optimum shear stress and air flux conditions. In rinse modules with a single purge gas nozzle, e.g., as described in  FIGS. 9A-D , the rinsing and drying process  1000  concludes at this point. 
     In rinsing modules with a second or more purge gas nozzles, the rinsing and drying process  1000  proceeds with the additional step  1050  of activating a third flow of purge gas from the second purge gas nozzle  220 . This step can be started at a predetermined meniscus position, which position is determined using simulation and experimental data in accordance with previously disclosed embodiments of the invention. At step  1060 , the second purge gas nozzle  220  is itself moved across substrate  150  towards the substrate edge. The third flow of purge gas and second purge gas nozzle position are used to assist the already-established second flow of purge gas in maintaining an optimum average shear stress and air flux across the rinse liquid film  152  and meniscus  154 , so as to maintain stable rinsing and drying conditions through the end of the rinsing and drying process  1000 . 
     Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teaching. Persons skilled in the art will recognize various equivalent combinations and substitutions for various components shown in the figures. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.