Abstract:
A chamber for transitioning a semiconductor substrate between modules operating at different pressures is provided. The chamber includes a base defining an outlet. The outlet permits removal of an atmosphere within the chamber to create a vacuum. A substrate support for supporting a semiconductor substrate within the chamber is included. A chamber top having an inlet is included. The inlet is configured to allow for the introduction of a gas into the chamber to displace moisture in a region defined above the substrate support. Sidewalls extending from the base to the chamber top are included. The sidewalls include access ports for entry and exit of a semiconductor substrate from the chamber. A method for conditioning an environment above a region of a semiconductor substrate within a pressure varying interface is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 10/113,014, filed on Mar. 28, 2002, and entitled “RAPID CYCLE CHAMBER HAVING A TOP VENT WITH NITROGEN PURGE.” The disclosure of this related application is incorporated herein by reference for all purposes. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates generally to semiconductor fabrication and more specifically to a semiconductor processing chamber designed to remove and preclude moisture from a region above a semiconductor substrate within the chamber in order to rapidly transition between pressure states within the chamber.  
           [0003]    Semiconductor manufacturing systems are constantly being designed with an eye towards improving throughput. For example, processing chambers have been configured in the form of a cluster tool in order to permit the integration of multiple steps of a process. These systems generally include other chambers for handling and transporting wafers between modules operating at atmospheric pressure and vacuum to ensure a clean process environment and improve throughput by eliminating the need to vent the processing chamber during wafer transfer steps. During the processing of a semiconductor substrate, the substrate is transported into and out of a load lock, which is a chamber that cycles between vacuum and atmospheric pressure. When the substrate is placed in the load lock from an atmospheric transfer module (ATM), the load lock will be at atmospheric pressure. The air in the load lock is then pumped out to provide a vacuum in the load lock chamber. The substrate is then transported to the processing chamber via a vacuum transfer module by a robotic arm. The processing operation (e.g., etching, oxidation, chemical vapor deposition, etc.) is then performed in the processing chamber.  
           [0004]    After the substrate has been processed, the robotic arm in the vacuum transfer module moves the substrate back to the load lock, which is in a vacuum condition from the transfer discussed above. Once the substrate is placed in the load lock, the pressure in the load lock is brought back to atmospheric pressure by venting in a gas such as nitrogen (N 2 ). When atmospheric pressure has been achieved, the processed substrate is transported to a substrate cassette for other processing steps, if necessary. In semiconductor processing, the value of a process system depends to a large extent on the rate at which substrates can be processed. That is, a process system with a higher throughput will produce more processed substrates in a given amount of time than a system with lower process rate. Thus, the process system with the highest throughput is the more desirable system with all other features being equal.  
           [0005]    However, the throughput of semiconductor process systems depends largely on the speed with which chambers, such as a load lock, can be cycled between low and high pressure. For the cluster architecture described above, the load lock is the chamber transitioning between different pressure states, therefore, the time to cycle the load lock is crucial to the system throughput. Unfortunately, the cycle speed of a load lock chamber in conventional process systems is generally limited by the rate at which the load lock can be cycled between a vacuum state and an atmospheric state without depositing particles on a substrate. In particular, the transition from an atmospheric pressure to a vacuum state inside the chamber is limited by the rate at which vacuum is pulled in the chamber. That is, condensation of moisture is avoided by limiting the rate at which a vacuum is pulled in the chamber. Moisture, in the form of airborne water vapor, will condense as the temperature drops below the dew point temperature as the vacuum is pulled. The individual water droplets can nucleate about a particle entrapped in the air and, because of the weight of the nucleated mass, fall onto the substrate if the vacuum is pulled at too fast of a rate. The water is eventually boiled off as vacuum is pulled, however, the particle is left on the surface of the substrate as a contaminate which may eventually lead to device failure. The contaminated substrate can negatively impact semiconductor yields.  
           [0006]    [0006]FIG. 1 is a schematic diagram of a load lock. Load lock  100  includes access ports  102 , bottom vacuum port  104 , and bottom vent port  106 . Within load lock  100 , is wafer support  110  having pads  112  on which a semiconductor substrate  108  rests on when inside the load lock. Of course, pads  112  can be pins. It will be appreciated by one skilled in the art, that load lock  100  transitions between differing pressure states. For example, if wafer  108  has been processed, then the wafer  108  is typically introduced into load lock  100  under a vacuum state. The vacuum state is then broken through the introduction of gas through bottom vent port  106 . Once the pressure in load lock  100  is brought to an atmospheric pressure, the wafer is then transferred out of load lock  100  to an atmosphere transport module. If wafer  108  is unprocessed, then the wafer is introduced into load lock  100  from an atmospheric transport module to the load lock while the load lock is at atmospheric pressure. Load lock  100  is then pumped out through vacuum port  104  to create a vacuum within the load lock.  
           [0007]    However, one of the shortcomings of the design of load lock  100  is that when either of access ports  102  are open, external moisture from outside the load lock will enter through either of the open access ports. Thus, when load lock  100  is pumped out to create a vacuum, moisture  116 , i.e., water vapor, that has entered the chamber through access ports  102  will reside in a region  114  over wafer  108 . As mentioned above, if a vacuum in the load lock is pulled too quickly, water vapor  116  will condense in region  114 . This condensation can nucleate around a particle in region  114  and eventually fall onto a surface of wafer  108 , thereby contaminating the wafer.  
           [0008]    An additional shortcoming with the design of load lock  100  is that when a gas is vented in through bottom vent port  106 , particulate matter which has fallen to the chamber bottom in the vicinity of the chamber inlet of bottom vent port  106  can be entrained in the gas flow. That is, any sufficiently light particulate matter on the bottom of chamber  100  can be kicked up during a venting operation. Thus, the entrained particulate matter can deposit on a substrate within the load lock thereby leading to lower yields.  
           [0009]    One attempt to solve the problem of the condensation falling on top of the surface of wafer  108 , is to restrict the rate at which a vacuum is pulled within load lock  100 . That is, a vacuum is pulled in two steps, with the first step at a slower rate, so as not to cross a dew point to avoid creating condensation. However, restricting the vacuum rate also restricts the throughput of the system.  
           [0010]    In view of the foregoing, there is a need to improve the cycling rate of the load lock between pressure states to allow for a higher throughput without exposing a substrate to contaminants.  
         SUMMARY OF THE INVENTION  
         [0011]    Broadly speaking, the present invention fills these needs by providing a chamber capable of rapidly cycling between differing pressure states without exposing a wafer inside the chamber to contaminates. The present invention also provides a method for conditioning an environment above the wafer inside the chamber.  
           [0012]    In accordance with one aspect of the present invention, a method for conditioning an environment in a region defined above a semiconductor substrate within a pressure varying interface is provided. The method initiates with a semiconductor substrate being introduced through an access port into a pressure varying interface. The pressure varying interface is at a first pressure. Then, moisture from a region defined above the semiconductor substrate is displaced. In one embodiment, the moisture is displaced by introducing a dry fluid through a top vent port of the pressure varying interface. Next, the access port is closed. Then, a pressure within the pressure varying interface is transitioned to a second pressure. Next, the semiconductor substrate is transferred from the pressure varying interface.  
           [0013]    In accordance with another aspect of the invention, a method for minimizing moisture in a region above a semiconductor substrate in a chamber is provided. The method initiates with providing a vent port extending through a top surface of a chamber. Then, a vacuum port extending through a bottom surface of the chamber is provided. Next, moisture is inhibited from entering a region defined over a semiconductor substrate positioned on a support within the chamber. Then, a pressure within the chamber is transitioned to a vacuum, wherein condensation forms outside of the region defined over the semiconductor substrate during the transition to a vacuum.  
           [0014]    In accordance with another aspect of the present invention, a chamber for transitioning a semiconductor substrate between modules operating at different pressures is provided. The chamber includes a base defining an outlet. The outlet permits removal of an atmosphere within the chamber to create a vacuum. A substrate support for supporting a semiconductor substrate within the chamber is included. A chamber top having an inlet is included. The inlet is configured to allow for the introduction of a gas into the chamber to displace moisture in a region defined above the substrate support. Sidewalls extending from the base to the chamber top are included. The sidewalls include access ports for entry and exit of a semiconductor substrate from the chamber.  
           [0015]    In accordance with yet another aspect of the present invention, a system for processing a semiconductor substrate is provided. The system includes a first transfer module configured to operate at a first pressure and a second transfer module configured to operate at a second pressure. A pressure varying interface in communication with the first and the second transfer modules is included. The pressure varying interface is capable of transitioning between the first and the second pressures. The pressure varying interface includes a top vent port and a bottom vacuum port. The top vent port is configured to introduce a fluid into the pressure varying interface, wherein the introduction of the fluid displaces moisture in a region defined above a semiconductor substrate in the pressure varying interface.  
           [0016]    It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0018]    [0018]FIG. 1 is a schematic diagram of a load lock used for semiconductor manufacturing operations.  
         [0019]    [0019]FIG. 2 is a schematic overview diagram of an exemplary semiconductor processing system with wafer handling automation including a load lock having a top vent port in accordance with one embodiment of the invention.  
         [0020]    [0020]FIG. 3 is a simplified schematic diagram of a load lock with a top vent port and a bottom vacuum port in accordance with one embodiment of the invention.  
         [0021]    [0021]FIG. 4 is a graph that compares a two stage pump down where the first stage is restricted against an unrestricted vacuum pull down rate.  
         [0022]    [0022]FIG. 5 is a schematic diagram of a load lock having a top vent port in communication with a diffuser in accordance with one embodiment of the invention.  
         [0023]    [0023]FIG. 6 is a schematic diagram of a load lock having a top vent port and a bottom vacuum port with multiple wafers inside the load lock in accordance with one embodiment of the invention.  
         [0024]    [0024]FIG. 7 is a flow chart diagram illustrating the method operations performed in conditioning an environment above a region of the semiconductor substrate within a pressure-varying interface.  
         [0025]    [0025]FIG. 8 is a flowchart diagram of the method operations for minimizing moisture in a region above a semiconductor substrate in a chamber in accordance with one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Several exemplary embodiments of the invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is discussed above in the “Background of the Invention” section.  
         [0027]    The embodiments of the present invention provide a method and apparatus allowing for the application of an unrestricted vacuum pull down rate by removing moisture from a region defined over a semiconductor substrate within the load lock. By locating a vent port above the wafer, rather than below the wafer, a gas purge is provided to remove moisture from above the wafer and to prevent moisture from flowing into the region of the wafer when the access ports to the external atmosphere of a pressure transition chamber, such as a load lock, are open. In one embodiment, the gas purge is dry, i.e., substantially free from moisture. As used herein, the terms wafer and substrate are used interchangeably. The vacuum pull down rate is no longer restricted since the condensation of moisture above the substrate is no longer a concern. That is, the gas purge forces any moisture to a region in the chamber that is below the substrate, and downstream of the evacuation flow relative to the substrate, thereby removing any concern associated with crossing the dew point. In addition, the substrate is positioned close to the vent port inlet defined in the top of the chamber to reduce the area between the top of the chamber and the substrate. Thus, the gas purge more effectively conditions the region above the substrate since the volume defined above the substrate and below the chamber top is minimized.  
         [0028]    [0028]FIG. 2 is a schematic overview diagram of an exemplary semiconductor processing system with wafer handling automation including a load lock having a top vent port in accordance with one embodiment of the invention. To streamline wafer processing, one or more unprocessed wafers  122  are placed in a wafer cassette  124 , which is then placed in a load port  126 . A robotic arm  130  in an atmospheric transfer module (ATM)  128  picks up a wafer  122  from the cassette  124 . Both load port  126  and ATM  128  are at atmospheric pressure. The robotic arm  130  transfers unprocessed wafer  122  from ATM  128  to wafer support  134  within load lock  132 . It should be appreciated that load lock  132  is at atmospheric pressure. Then, a vacuum is pulled in load lock  132  by pumping the air out of the load lock chamber through vacuum port  136 . Of course, the access doors to load lock  132  are closed during vacuum pull down operations. Once a vacuum condition is established in load lock  132  an access port between vacuum transfer module  140  and the load lock is opened. Wafer  122  is then transported by robot arm  144  from wafer support  134  through vacuum transfer module  140  to processing chamber  142 . After wafer  122  has been processed, the wafer is removed from processing chamber  142  through vacuum transfer module  140  into load lock  132 . Load lock  132  is brought to atmospheric pressure by venting a gas into the load lock through top vent port  138  until atmospheric pressure is obtained in the load lock chamber. The processed wafer is then transferred to cassette  124  through ATM  128  by robotic arm  130 . One skilled in the art will appreciate that more than one wafer can reside in load lock  132  at the same time. For example, as a wafer is placed from vacuum transfer module  140  to load lock  132 , an unprocessed wafer may be inside the load lock. Thus, robotic arm  144  can place a processed wafer in load lock  132  and remove an unprocessed wafer for processing. In one embodiment, one semiconductor substrate is in load lock  132  during pump and vent sequences. It should be appreciated that throughput is optimized here as processing chamber  142  does not sit idle waiting for wafers. One skilled in the art will appreciate that the equalization of the load lock pressure to atmosphere is precisely controlled. That is, the sudden pressure adjustment may result in mixing, thereby causing a higher moisture content over the wafer. The precise control of the equalization of the load lock pressure is achieved through the venting process and avoids the turbulence inside the chamber so that particles are not kicked up.  
         [0029]    Still referring to FIG. 2, vent port  138  is located on the top of load lock  132  in order to create an environment that is substantially moisture-free above and upstream of the substrate support  134 . Additionally, by locating top vent port  138  on the top surface of load lock  132 , air entering load lock  132  is prevented from occupying a region defined above substrate  134  and below top vent port  138 , as will be explained in more detail below. While the embodiments discussed herein refer to transitioning the pressure within load lock  132  between a vacuum and atmospheric pressure, it should be appreciated that the load lock described herein can include any chamber for any system that operates between two pressures.  
         [0030]    [0030]FIG. 3 is a simplified schematic diagram of a load lock with a top vent port and a bottom vacuum port in accordance with one embodiment of the invention. Load lock  132  includes top vent port  138  and bottom vacuum port  136 . Access ports  146  allow a wafer  122  to transfer into and out of load lock  132 . When wafer  122  is inside load lock  132 , the wafer rests on substrate support  134  having pins  150 . For example, as a wafer is introduced into load lock  132  from an atmospheric transfer module, one of the access ports  146  will open to allow wafer  122  to enter the load lock. As one of the access ports  146  opens to allow entrance of wafer  122 , a fluid flow is provided through vent port  138  into load lock  132 . The fluid continues to flow as wafer  122  is moved into load lock  132  until access ports  146  are closed. In one embodiment, the fluid continues to flow for a short time period after access port  146  is closed. It should be appreciated that when access ports  146  are shut, load lock  132  becomes isolated, therefore, a slight positive pressure may build up in the load lock prior to a vacuum pump initiating to create a vacuum.  
         [0031]    As indicated in FIG. 3, the fluid from vent port  138  creates a region  148  substantially free of moisture. Region  148  is defined between wafer  122  and the top of load lock  132 . In one embodiment, vent port  138  is in communication with a diffuser for dispersing a fluid from the vent port throughout region  148  as will be shown in more detail with reference to FIGS. 5 and 6. The flow of gas through vent port  138  purges away moisture entering load lock  132  when access ports  146  are open during wafer transport. That is, any moisture laden air entrained with the activity from the movement of a wafer into and out of load lock  132  is forced below wafer  122  as indicated by arrows  147 . In one embodiment the fluid vented into load lock  132  through vent port  138  is an inert non-toxic gas such as nitrogen, argon, helium, etc. Additionally, the flow of gas from vent port  138  forces any existing moisture from region  148  defined above wafer  122  to a region below wafer  122 . Thus, existing internal moisture is removed from region  148  while external moisture is prevented from permeating region  148  of load lock  132 .  
         [0032]    Still referring to FIG. 3, once wafer  122  has been placed into load lock  132  from an atmospheric transfer module, the robot arm carrying the wafer is removed and access port  146 , through which the wafer is introduced into the load lock, is closed. As mentioned above, the flow of gas through vent port  138  may continue for a brief time period after access ports  146  have been shut. Alternatively, the flow of gas through vent port  138  may stop as soon as access ports  146  are shut. In order for wafer  122  is to be transferred to a processing chamber through a vacuum transfer module, load lock  132  must be pumped out to create a vacuum. Thus, a vacuum pump, in communication with vacuum port  136 , creates a vacuum in load lock  132  in one embodiment. It will be apparent to one skilled in the art that since region  148  is substantially free of moisture, the rate at which vacuum is pulled in load lock  132  is no longer restricted. That is, there is no need to perform a two-step vacuum process because there is no moisture above the surface of wafer  122 . If the dew point temperature is crossed while pumping out load lock  132 , moisture outside of region  148  may condense. For example, any moisture forced below wafer  122  may condense. However, the condensation is no longer a concern, as it is no longer in region  148  over wafer  122 . Therefore, even if the moisture nucleates around a particle, the moisture and the particle will fall to the bottom surface of load lock  132 . Furthermore, any particles which have fallen to the bottom of load lock  132 , remain at the bottom of the load lock since vent port  138  is located at the top of load lock  132 . That is, any particles on the bottom of load lock  132  are not kicked up during venting operations since vent port  138  is located at the top of the load lock. One skilled in the art will appreciate that the gas used to vent into load lock  132 , such as nitrogen, is substantially free from moisture and highly filtered to avoid introducing particulate matter into the load lock.  
         [0033]    [0033]FIG. 4 is a graph that compares a two stage pump down where the first stage is restricted against an unrestricted vacuum pull down rate. The restricted vacuum pull down rate is represented by lines  152  and  154 . During a first stage of the restricted vacuum pull down rate, represented by line  152 , care is taken so as to not cross the dew point. Once point  156  is reached, the vacuum pull down rate may be increased, as shown by the initial slope of line  154 , since the dew point is avoided by the two stage process. Therefore, a condensation cloud will not form, but throughput suffers with this two stage process. Additionally, the slow pump for the first stage reduces turbulence that can stir up any particles as lower pressure atmospheres are less able to move particles. Therefore, a one stage process is configured to pump the load lock without stirring up particulate matter.  
         [0034]    On the other hand, line  158  represents an unrestricted vacuum pull down rate. For example, where moisture has been precluded or substantially removed from a region defined above a wafer, such as region  148 , as discussed with reference to FIG. 3, the unrestricted pull down rate can be applied. By providing a gas purge through top vent port  138 , the vacuum pull down rate can be increased, since the environment above the wafer is able to be conditioned to be substantially free from moisture. It should be appreciated that the gas used to purged the environment above the wafer is a dry gas. That is, crossing the dew point is no longer a concern due to the displacement of moisture above the wafer by a dry inert gas being vented through the chamber top. Thus, the time to reach the vacuum state within the load lock is less for the unrestricted vacuum pull down rate, as represented by time t 1 . One skilled in the art will appreciate that the valve system associated with the unrestricted pull down rate is less complex, therefore, the valve system associated with the unrestricted vacuum pull down rate will be less expensive. While the restricted vacuum pull down rate, utilizing a two stage process to avoid crossing the dew point, does not reach the desired vacuum level until time t 2 . The less time to pump out a chamber of a pressure varying interface, such as a load lock, translates to a higher throughput, since the pressure varying interface can be cycled quicker between differing pressure states.  
         [0035]    [0035]FIG. 5 is a schematic diagram of a load lock having a top vent port in communication with a diffuser in accordance with one embodiment of the invention. Here, top vent port  138  connects to diffuser  160 . Diffuser  160  directs the gas flow uniformly over a region defined above wafer  122  and below the diffuser, such as region  148  of FIG. 3. Thus, a sweeping fluid flow is created over wafer  122 , thereby conditioning the environment above the wafer. That is, the inert gas flow from diffuser  160  displaces any moisture above wafer  122  and in effect provides an inert gas environment above the wafer. Diffuser  160  is shown as having a diameter slightly larger than wafer  122 , however, it should be appreciated that the diameter of the diffuser can be smaller than the diameter of the wafer. Furthermore, the diffuser and be any shape suitable for creating a substantially moisture free environment above the wafer. Additionally, if any one of access ports  146  are opened, moisture-containing air that enters load lock  132  is directed below wafer  122 . The inert gas purge from vent port  138  prevents external moisture from entering the region defined above wafer  122  and below diffuser  160  in a similar manner as discussed with respect to FIG. 3. Hence, the vacuum pull down rate can be increased. While vacuum port outlet  136  is shown substantially centered under wafer  122 , it should be appreciated that the vacuum port outlet can be located anywhere on the bottom surface of load lock  132 . Additionally, edge  164  is shown as a rounded corner to assist in pumping out the chamber. Vacuum pump  162 , which evacuates load lock  132  to create a vacuum, can be any commercially available vacuum pump suitable for load lock  132 . Diffuser  160  is rigidly attached to the top inside surface of load lock  132  in one embodiment. It will be apparent to one skilled in the art that while vent port  138  is shown as centered over wafer  122 , the vent port can be located at any position of the chamber top, as long as a gas flow that provides an environment substantially free from moisture and particles above the wafer, can be delivered.  
         [0036]    Still referring to FIG. 5, distance  161  between a top surface of wafer  122  and a bottom surface of diffuser  160  is between about 3 millimeters (mm) and about 3 centimeters (cm) in one embodiment. In a preferred embodiment, distance  161  is between about 5 mm and 2 cm and more preferably distance  161  is about 1 cm. As mentioned above, vacuum pump  162  can start as soon as access ports  146  are shut or a short time period thereafter. In one embodiment, vacuum pump  162  starts between about 0 and about 2 seconds after access ports  146  are closed. Preferably, vacuum pump  162  starts between about 0 and about 0.5 seconds after access ports  146  are closed. One skilled in the art will appreciate that while load lock  132  is being pumped out, vent port  138  is shut so that a vacuum can be pulled inside the load lock. It should be appreciated that any suitable valve  139  can be used to close access to vent port  138  to allow for a vacuum to be pulled inside load lock  132 . Additionally, diffuser  160  can be any diffuser compatible with semiconductor operations, such as diffusers incorporating powdered metals, sintered nickel, expanded polytetrafluoroethylene (PTFE) membrane laminated to fabric and baffles, etc. It will be apparent to one skilled in the art that access ports  146  do not open at the same time as load lock  132  is a pressure varying interface between chambers at different pressures.  
         [0037]    [0037]FIG. 6 is a schematic diagram of a load lock having a top vent port and a bottom vacuum port with multiple wafers inside the load lock in accordance with one embodiment of the invention. Here, multiple wafers  122  and  123  are transitioning into and out of the load lock as depicted by arrows  166 . One skilled in the art will appreciate that a processed wafer  123  may be introduced from a vacuum transfer module to load lock  132  while an unprocessed wafer  122  is inside load lock  132 . Thus, the robotic arm depositing processed wafer  123  can then take wafer  122  to be processed. In one embodiment, one wafer is in load lock  132  during a venting in or pumping out operations. That is, two wafers are in load lock  132  when one of the access ports  146  are open. As discussed above, when an access port  146  is opened, gas flows through vent port  138  and diffuser  160  creating a region substantially free of moisture above top wafer  122 . As wafer  122  moves into load lock  132 , the flow of gas through diffuser  160  conditions the environment above the wafer. Additionally, the flow of clean dry gas radially fans out over the surface of wafer  122  in a sweeping motion. On the other hand, when processed wafer  123  enters load lock  132  when unprocessed wafer  122  is in the load lock, the flow of clean dry gas provides increased cooling for the processed wafer. Moreover, processed wafer  123  may off-gas, which can then condense on unprocessed wafer  122  and contaminate the unprocessed wafer. The flow provided by top vent  138  sweeps away any off-gassing and residues in a downward direction toward the bottom of load lock  132 . It should be appreciated that the residues will be eventually pumped out through vacuum port  136  by vacuum pump  162 . Another advantage of locating vent port  138  on top of the load lock  132  is that as a gas is vented into the load lock, particles resting on the bottom surface are not kicked up by the air flow into the load lock. When the vent port is located on the bottom of load lock  132 , the particles can become entrained in the flow from the bottom vent and deposit onto the surface of any wafer present in load lock  132 . However, by locating vent port  138  on top of load lock  132 , particles on the bottom surface remain there.  
         [0038]    Still referring to FIG. 6, the diameter of vent port  138  is about 100 mm in one embodiment. In one embodiment, where load lock  132  has a seven liter capacity, the flow rate of the clean dry gas, such as nitrogen, provided to the load lock through vent port  138  is between about 10 standard liters per minute (SLM) and about 100 SLM. The flow rate range for a seven liter chamber is preferably between about 40 SLM and about 60 SLM with a preferred flow rate of 50 SLM. While the flow rates are provided for a seven liter chamber it should be appreciated that the flow rate ranges can be scaled accordingly for larger or smaller chambers. One skilled in the art will appreciate that the embodiments discussed herein are temperature independent. In another embodiment, a vacuum state is achieved inside of load lock  132  in less than 10 seconds, preferably less than 6 seconds, from the initiation of the vacuum pull-down cycle. Of course, the vacuum pull down cycle can be initiated by starting vacuum pump  162  or by opening a suitable valve  164  between the suction side of the vacuum pump and load lock  132  when the vacuum pump is already running.  
         [0039]    [0039]FIG. 7 is a flow chart diagram illustrating the method operations performed in conditioning an environment above a region of the semiconductor substrate within a pressure-varying interface. The method initiates with operation  170  where a semiconductor substrate is introduced through an access port into a pressure-varying interface. Here, the pressure-varying interface is at a first pressure, such as atmospheric pressure when the semiconductor substrate is being transferred from an ATM. In one embodiment, the pressure varying interface is a load lock. The method then moves to operation  172  where moisture from a region defined above the semiconductor substrate is displaced. For example, a clean dry gas flow through a top vent port will displace the moisture from the region above the semiconductor substrate as discussed with reference to FIGS. 3, 5 and  6 . As mentioned above the gas is any suitable inert, non-toxic gas. In addition, the flow of gas will prevent any external moisture from entering the region defined above the semiconductor substrate as an access port to the pressure-varying interface is opened. The displaced moisture as well as the external moisture entering through an open access port is forced below the semiconductor substrate. The method then advances to operation  174  where the access port is closed. It should be appreciated that prior to closing the access port, the semiconductor substrate is placed on a substrate support and a robot arm is removed from the pressure varying interface. It will be apparent to one skilled in the art that the access port can be any suitable access port providing access into the chamber with the capability of sealing the chamber, such as a slot valve.  
         [0040]    Still referring to FIG. 7, the method then proceeds to operation  176  where the pressure-varying interface is transitioned to a second pressure. For example, after an unprocessed semiconductor substrate enters the pressure-varying interface from an atmospheric transfer module, the pressure within the pressure-varying interface will be brought down to vacuum from atmospheric pressure, which allows for the unprocessed semiconductor substrate to be transitioned to a vacuum transfer module. During the transitioning of the pressure-varying interface to a second pressure, a rate at which a vacuum is pulled inside the chamber is not restricted. That is, the region defined above the semiconductor substrate and below an inlet to the top vent port to the load lock is substantially free of moisture. In light of the lack of moisture above the substrate, condensation falling onto the surface of the semiconductor substrate is not a concern. It should be appreciated that the gas flow described in reference to operation  172  substantially removes the moisture from a region defined above the semiconductor substrate, thereby removing concerns related to condensation forming and nucleating around a particle above the semiconductor substrate. Thus, the dew point can be crossed by a rapid transition from atmospheric pressure to a vacuum. In turn, the throughput is increased because the load lock can be transitioned from a positive pressure to a vacuum without restriction. The method then advances to operation  178  where the semiconductor substrate is transferred out of the pressure varying interface. Here, the semiconductor substrate may be transferred to the vacuum transfer module for eventual transfer into a processing module.  
         [0041]    [0041]FIG. 8 is a flowchart diagram of the method operations for minimizing moisture in a region above a semiconductor substrate in a chamber in accordance with one embodiment of the invention. The method initiates with operation  180  where a vent port extending through a top surface of the chamber is provided. Here, the vent port may be configured as those illustrated in FIGS. 3, 5 and  6 . The method then proceeds to operation  182  where a vacuum port extending from a bottom surface of the chamber is provided. The vacuum port may be located at any position on the bottom surface of the chamber. In one embodiment, a vacuum pump in communication with the vacuum port provides the suction necessary to evacuate the chamber. The method then moves to operation  184  where moisture is prevented from entering a region defined within the chamber over a semiconductor substrate and under the vent port. For example, a gas flow may be provided through a vent port located on a top surface of the chamber. As discussed with respect to FIGS. 3, 5 and  6 , the gas flow will prevent moisture from entering the region defined above a substrate within the chamber while an access port of the chamber is opened for the introduction or removal of a semiconductor substrate from the chamber. In particular, the gas flow creates a barrier to any air introduced into the chamber and forces the air to a region below the semiconductor substrate support in the chamber. In one embodiment, the gas is an inert, non-toxic gas, such as nitrogen.  
         [0042]    The method of FIG. 8 then advances to operation  186  where a pressure within the chamber is transitioned to a vacuum. As moisture is prevented from the region above the semiconductor substrate and forced to regions below or to the side of the semiconductor substrate, if any condensation forms, it will form in the regions below or to the side of the semiconductor substrate. Since condensation over the semiconductor substrate is not a concern, the vacuum pull-down rate can be increased without creating turbulence and stirring up particles. Thus, the throughput of the system is improved since the chamber can be cycled between the pressure states more efficiently without impacting the quality of the semiconductor substrate.  
         [0043]    In addition, during polysilicon etch operations, hydrogen bromide gas is swept away by the gas flow provided through the top vents in the embodiments described above. One skilled in the art will appreciate that the sweeping air flow from the top vent will prevent hydrogen bromide gas, given off by a processed wafer, from condensing on an unprocessed wafer. The nitrogen gas scrubs some of the hydrogen bromide off of the processed wafer, thereby minimizing the chances of cross contamination. Thus, locating the vent at the top of the chamber allows for blanketing the semiconductor substrate in a layer of an inert gas which protects the wafer from moisture condensation and from reactive species that off-gas from processed substrates. In one embodiment, the processed wafer is below the unprocessed wafer inside a load lock having multiple wafers.  
         [0044]    In summary, the present invention provides a clean substantially moisture-free environment in a region above a substrate within a chamber and at the same time increases throughput by allowing a pressure varying interface such as a load lock to more efficiently cycle between pressure states. The invention has been described herein in terms of several exemplary embodiments. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.