Abstract:
A gas mixing system for a semiconductor wafer processing chamber is described. The mixing system may include a gas mixing chamber concentrically aligned with a gas transport tube that extends to a blocker plate. The gas mixing chamber and the transport tube are separated by a porous barrier that increases a duration of gas mixing in the gas mixing chamber before processes gases migrate into the transport tube. The system may also include a gas mixing insert having a top section with a first diameter and a second section with a second diameter smaller than the first diameter and concentrically aligned with the top section. The processes gases enter the top section of the insert and follow channels through the second section that cause the gases to mix and swirl in the gas mixing chamber. The second section extends into the gas mixing chamber while still leaving space for the mixing and swirling around the sidewalls and bottom of the mixing chamber.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit under 35 USC 119(e) of U.S. provisional Application No. 60/986,923, filed on Nov. 9, 2007, entitled “GAS MIXING SWIRL INSERT ASSEMBLY,” the entire content of which is incorporated herein by reference for all purposes. 
         [0002]    This application also relates to U.S. Pat. Nos. 6,068,703 and 6,303,501 to Chen et al, both of which are titled “Gas Mixing Apparatus and Method.” The entire contents of both patents are herein incorporated by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Today&#39;s wafer fabrication plants are routinely producing sub-100 nm feature size devices, and tomorrow&#39;s plants soon will be producing devices having even smaller feature sizes. 
         [0004]    One of the primary steps in fabricating modern semiconductor devices involves the formation of a dielectric, metal, or insulating layers over a semiconductor substrate. As is well known, such layers can be deposited by chemical vapor deposition (CVD). CVD processes are particularly suitable for use with high integration devices because CVD layers provide superior step coverage and post-annealing qualities to those layers formed by sputtering or other conventional deposition methods. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a plasma enhanced chemical vapor deposition (PECVD) process, the flowing gas may be excited to a plasma state. A controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. The process of depositing layers on a semiconductor wafer (or substrate) usually involves heating the substrate and holding it a short distance from the source of a stream of deposition (or process) gas flowing towards the substrate. In general, reaction rates in thermal and plasma processes may be controlled by controlling one or more of the following: temperature, pressure, and reactant gas characteristics. 
         [0005]    In the quest to achieve ever smaller devices, increasingly stringent process requirements are being imposed on integrated device manufacturing processes. One such requirement is the thorough mixture of process gases prior to introduction of the gases into a CVD chamber. A thorough mixture of the process gases is typically necessary to achieve a uniform deposition pattern on the semiconductor substrate. If the quality of the mixing achieved by the plurality of gases is insufficient, the CVD process using the gases will provide an uneven deposition pattern, which may result in variance of the sheet resistance of the deposited film, delamination during annealing, or other undesirable qualities which may degrade device performance. 
         [0006]    Unfortunately, CVD processes are becoming more sensitive to gas flow and mixture parameters as device sizes shrink and device performance increases. Conventional gas mixers adapted to provide adequate levels of gas mixing are costly to manufacture and sensitive to minor flaws associated with manufacturing. Conventional mixers typically only use one mixing step and rely on a mixer that is difficult to test prior to actual use in a semiconductor system. Hence, it would be desirable to provide an improved gas mixing apparatus that would provide reliable and thorough gas mixing. These issues and others are addressed by the present invention. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Gas mixing equipment is describe to homogenously mix process gases before they enter a reaction zone of a semiconductor processing chamber. The equipment includes a gas mixing insert with fluid channels shaped and oriented to cause the process gases flowing through it to collide and mix after leaving the insert. The mixing space around the insert is also partially enclosed to enhance the extent of mixing before the mixed gas escapes into a conduit that supplies gas to the showerhead or gas nozzles for distribution into the reaction zone. 
         [0008]    Embodiments of the invention include a gas mixing system for use with a semiconductor wafer processing chamber. The mixing system may include a gas mixing chamber concentrically aligned with a gas transport tube that extends to a blocker plate. The gas mixing chamber and the transport tube are separated by a porous barrier that increases a duration of gas mixing in the gas mixing chamber before processes gases migrate into the transport tube. The system may also include a gas mixing insert having a top section with a first diameter and a second section with a second diameter smaller than the first diameter and concentrically aligned with the top section. The processes gases enter the top section of the insert and follow channels through the second section that cause the gases to mix and swirl in the gas mixing chamber. The second section extends into the gas mixing chamber while still leaving space for the mixing and swirling around the sidewalls and bottom of the mixing chamber. 
         [0009]    Embodiments of the invention also include a gas mixing apparatus used in a semiconductor processing chamber. The mixing apparatus may include a gas mixing chamber comprising a gas inlet to receive process gases, a gas outlet to flow mixed gas out of the chamber, and an insert recess. The apparatus may also include a gas mixing insert slidably fitted within the insert recess, and a fluid flow channel at least part of which is formed in the gas mixing insert, and fluidly coupled to the gas inlet. The fluid flow channel includes one or more fluid separators, each comprising a first carrier channel having a channel surface separating the process gases into a plurality of gas portions flowing away from each other in the channel. A fluid collection space is formed between the gas mixing insert and the periphery of the gas mixing chamber. The separated gas portions exiting the fluid separators approach from substantially opposite directions and collide with each other to combine the gas portions into the mixed gas. The apparatus may further include a gas transport conduit to receive the mixed gas from the gas outlet and further mix and transport the mixed gas to a blocker plate of a showerhead. 
         [0010]    Embodiments of the invention also include a semiconductor fabrication processing chamber with a gas mixing system. The chamber may include an enclosure housing a processing chamber with a process gas inlet, a gas supply line fluidly coupled to the process gas inlet, and a support disposed within the processing chamber and having a support surface for supporting a semiconductor wafer. The chamber may also include a gas manifold fluidly coupled to the process gas inlet to distribute process gases across the semiconductor wafer, and a gas mixing apparatus fluidly coupled between the process gas inlet and the gas manifold. The gas mixing apparatus may include a gas mixing chamber concentrically aligned with a gas transport tube that extends to the gas manifold, where the gas mixing chamber and the transport tube are separated by a porous barrier that increases a duration of gas mixing in the gas mixing chamber before processes gases migrate into the transport tube. In addition the gas mixing apparatus may include a gas mixing insert having a top section with a first diameter and a second section with a second diameter smaller than the first diameter and concentrically aligned with the top section. The processes gases enter the top section of the insert and follow channels through the second section that cause the gases to mix and swirl in the gas mixing chamber. The second section extends into the gas mixing chamber while still leaving space for the mixing and swirling around the sidewalls and bottom of the mixing chamber. 
         [0011]    Embodiments of the invention further include a gas swirl mixing device for semiconductor processing chamber. The mixing device may include a transport plate having a plurality of holes with a first angle to guide a mixture of process gases to flow through the holes toward a first direction. The mixing device may also include a transport tube having a plurality of holes with a second angle near bottom of the transport tube, wherein the holes are connected to a recess on an inner sidewall of the transport tube to guide the mixture to flow toward a second direction, where the second direction is opposite to the first direction, and the second angle is different from the first angle. The mixing device also includes a mixing chamber concentrically aligned with the transport tube, where the plate is coupled to the transport tube that is coupled to the mixing chamber to prevent a secondary process gas path. 
         [0012]    Embodiments of the invention also include a chamber system for semiconductor processing. The system includes a processing chamber, a first substrate supporting member to support a first substrate within the processing chamber, a second substrate supporting member to support a second substrate, where the second substrate supporting member is positioned near the first substrate supporting member within the processing chamber. The system also includes a first swirl mixing device being located above the first substrate supporting member for providing mixed flow of process gases toward the first substrate. The system further includes a second swirl mixing device located above the second substrate supporting member for providing mixed flow of process gases toward the second substrate; where a first flow direction generated from a first transport tube of the first swirling mixing device being opposite to a second flow direction generated from a second transport tube of the second swirling mixing device. 
         [0013]    Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings and appendix wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sub-label is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components. 
           [0015]      FIG. 1A  shows a cross-sectional view of standard mixing insert design (prior art). 
           [0016]      FIG. 1B  shows a cross-sectional view of swirl mixing insert design according to embodiments of the invention. 
           [0017]      FIG. 2A  shows a first assembly of swirling mixing inserts according to the embodiment of the invention. 
           [0018]      FIG. 2B  shows a second assembly of swirling mixing inserts according to embodiments of the invention. 
           [0019]      FIGS. 3A-3B  are plots of thickness uniformity for the first assembly of swirl mixing inserts shown in  FIG. 2A  and the second assembly of swirl mixing inserts shown in  FIG. 2B , respectively. 
           [0020]      FIG. 4  is a simplified schematic of an automatic flow splitter according to embodiments of the invention. 
           [0021]      FIG. 5  is a plot of thickness vs motor step for a micrometer shown in  FIG. 4 . 
           [0022]      FIG. 6  is a simplified schematic of a dual-pressure heater lift according to embodiments of the invention. 
           [0023]      FIG. 7  includes plots of thickness uniformity versus compressed dry air (CDA) pressure in the Pneumatic cylinder as shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Chamber Left-to-Right matching issues for dielectric film depositions in processing, such as a two-step boron-phosphate silicate glass (BPSG) deposition within a sub-atmospheric chemical vapor deposition (SACVD) chamber, may cause significant wafer side-to-side variation in thickness. This uneven side-to-side matching may be caused by multiple factors, such as variation in gas mixing, uneven delivery of gas/vapor, and heater leveling or lift to sides. These problems may be addressed with swirl mixing inserts, automatic flow splitters, and/or dual-pressure heater lifts designed for thorough gas mixing and uniform distribution in vapor delivery, as well as accurate spacing in heater leveling. 
         [0025]      FIG. 1A  illustrates a cross-sectional view of a standard mixing insert. The standard mixing insert  100 A includes a mixing block  106 A coupled to a gas box  108 A, a mixing insert  124 , and a blocker plate  110 . A first process gas such as O 3  may flow into the mixing block  106 A from a pipeline  112  and enter into a mixing insert  124  through an inlet  126 A. On the other hand, a second process gas such as TEOS may flow into the mixing block  106 A from a pipeline  114  and enter into the mixing insert  124  through an inlet  126 B. The first and second process gases get mixed in the mixing insert  124  and flow down toward the blocker plate  110  as pointed by arrow  116 . The blocker plate is positioned below the mixing block  106 B and the transport tube  104 . 
         [0026]    The mixing insert  124  and the mixing block  206 A may be of cylindrical shape. External diameters  130 ,  132  of the mixing insert  124  are smaller than inner diameters of the mixing block  106 A. Therefore, a gap is formed between an inner sidewall of the mixing block  106 A and an external sidewall of the standard mixing insert  124  to provide a secondary gas flow path as pointed by arrows  118 A and  118 B. As shown in  FIG. 1A , the mixture of the first gas and second gas flow through the mixing insert  124  as pointed by arrow  116 . However, some of the first gas from the pipeline  112  may flow through the gap as pointed by arrow  118 A, while some of the second gas from the pipeline  114  may flow through the gap as pointed by arrow  118 B. 
         [0027]      FIG. 1B  illustrates a cross-sectional view of a swirling mixing insert according to the embodiment of the invention. The swirling mixing insert  100 B includes a mixing block  106 B coupled to a gas box  108 B, a mixing plate  102 , a transport tube  104 , a top cover  122 , and a blocker plate  110 . A first process gas such as O 3  may flow into the mixing block  106 B from the pipeline  112 , while a second process gas such as TEOS may flow into the mixing block  106 B from the pipeline  114 . The first and second gases flow through the mixing plate  102  and get mixed in the transport tube  204  and flow toward the blocker plate  110  as pointed by arrow  116 . 
         [0028]    The transport tube  104  may have a collared end  120  that has an external diameter approximately equal to an external diameter of the mixing plate  102 . An external diameter of the transport tube  104  is approximately equal to an inner diameter of the mixing block  106 B. The mixing block  106 B, the mixing plate  102 , the transport tube  104  may be made of a metal, such as aluminum. 
         [0029]    Referring to  FIG. 1B  again, the mixing plate  102  contacts the collared end  120  of the transport tube  104  at a spot  142  that contacts the mixing block  106 B at a spot  140 . Therefore, a metal-to-metal contact is formed between the mixing plate  102  and the collared end  120  of the transport tube  104 . Also, a metal-to-metal contact is formed between the transport tube  104  and the mixing block  106 B. Such metal-to-metal contacts help prevent a secondary gas path  118  as shown in  FIG. 1A  for the standard mixing insert  100 A. 
         [0030]    According to embodiments of the invention, the prevention of the secondary gas path helps improve mixing uniformity of process gases. The prevention of secondary gas path may also provide even splitting of the gas delivery to a first substrate  202  and a second substrate  204  as shown in  FIGS. 2A &amp; 2B . 
         [0031]      FIGS. 2A and 2B  show exploded views of swirl mixing insert assemblies ( 200 A and  200 B, respectively) to mix reactive gases that are introduced to exposed surfaces of substrates  202  and  204  in a dual-wafer processing chamber  201 .  FIG. 2A  shows a swirl mixing insert assembly  200 A, positioned above each of the substrate  202  and  204  in the processing chamber  201 . Assembly  200 A may include a mixing plate  206  placed over a collared end  218  of a transport tube  208 . The circular mixing plate  206  includes a plurality of holes  207  extending through the thickness of the plate  206  that are shaped and oriented to direct gas flow in a first direction (e.g., a clockwise direction). 
         [0032]    The process gases flow through a set of holes  214  formed through the sidewall of the transport tube  208  near the tube end opposite the collared end  218 . The set of holes  214  formed in the transport tube  208  have an orientation  224 A designed to direct the flow of the process gases in a second direction (e.g., a counterclockwise direction) that is opposite to the first direction. Mixing gases may be enhanced by entering the mixture of gases into the transport tube  208  in a first direction and exiting the mixture of gases from the transport tube  208  in a second direction that is opposite to the first direction. Such enhanced mixing may form a substantially uniform reactive gas mixture that supplies the reactants (e.g., ozone and TEOS) for a chemical vapor deposition of a dielectric film (e.g., a BPSG film) on the exposed surfaces of substrates  202  and  204 . 
         [0033]    In  FIG. 2A  the dominant flow direction of the mixed gases is the direction of the second process gas flowing through holes  214  (e.g., CCW). Because the dual-wafer process chamber uses a pair of swirl mixing insert assemblies  210 A to mix the gases above each of the substrates  202  and  204 , both substrates are exposed to a mixed gas flowing in the same direction (e.g., CCW). This can create an asymmetrical flow path for the pair of mixed gases that can result in differences in the deposition uniformity for each of the substrates  202  and  204 . For example, when both sets of mixed gases are circulating in a counterclockwise direction above the substrates, the dielectric film deposited on substrate  202  may have a higher uniformity than the film deposited on substrate  204 . 
         [0034]      FIG. 2B  shows the dual wafer processing chamber  201  using the swirl mixing insert assembly  200 A to provide a mixed gas to one substrate  202 , while using swirl mixing insert assembly  200 B for the second substrate  204 . The two assemblies  200 A &amp;  200 B are designed to provide mixed gases with different flow directions. In the example shown in  FIG. 2B , the mixed gases exiting the assemblies  200 A &amp;  200 B flow in opposite directions (e.g., CCW versus CW) to provide a symmetric flow of gases over substrates  202  and  204 . This provides a similar uniformity of the deposited dielectric films (e.g., a BPSG film) over both substrates. 
         [0035]    The assembly  200 B may include a mixing plate  210  placed over the collared end  218  of a transport tube  212 . The circular mixing plate  210  includes a plurality of holes  209  extending through the thickness of the plate that are shaped and oriented to direct process gases in the second direction (e.g. CCW) that is opposite to the first direction (e.g. CW) generated by the mixing plate  206  of assembly  200 A. The holes  209  are different from the holes  207  of assembly  200 A. The mixture of the first and second gas flows into the transport tube  212  through the set of holes  209  and exits the transport tube  212  through a set of holes  216  that have an orientation  216 A designed to direct the gas flow in the first direction (e.g. CW). As noted above the example shown in  FIG. 2B  has the mixed gases emerging from assemblies  200 A &amp; B circulating in opposite directions (e.g., CCW versus CW). 
         [0036]    The mixing plate  102  as shown in  FIG. 1B  may be the mixing plate  106  (CW) of the first assembly  200 A or plate  110  (CCW) of the second assembly  200 B as shown in  FIG. 2A and 2B , while the transport tube  104  as shown in  FIG. 1B  may be the transport tube  208  (CCW) of the first assembly  200 A or the transport tube  212  (CW) of the second assembly  200 B. 
         [0037]    Extensive experiments have been performed by using the first assembly  200 A, the second assembly  200 B of swirl mixing inserts, Automatic Flow Splitter, and Dual-Pressure Heater Lift mechanism.  FIG. 3A  shows the thickness maps for the substrate  204  at two pressures 200 torr and 600 torr by using the first assembly  200 A of swirl mixing inserts, where a thickness map  302  is for the substrate  202  or left side at 200 torr, a thickness map  304  for the substrate  204  or right side at 200 torr, a thickness map  306  for the substrate  202  at 600 torr, and a thickness map  308  for the substrate  204  at 600 torr. 
         [0038]    A thickness uniformity is defined by: 
         [0000]      Thickness Uniformity=(maximum thickness-minimum thickness)/average thickness/2% 
         [0000]    In  FIG. 3A , note that the thickness uniformity is 5.99% for the thickness map  308 , which is significantly higher than for the other three thickness maps  302 ,  304  and  306  (ranging from 3.59% to 3.84%). 
         [0039]      FIG. 3B  shows the thickness maps for substrate  204  at two pressures 200 torr and 600 torr by using the second assembly  200 B of swirl mixing inserts according to the embodiments of the invention, where a thickness map  312  is for the substrate  202  or left side at 200 torr, a thickness map  314  for the substrate  204  or right side at 200 torr, a thickness map  316  for the substrate  202  at 600 torr, and a thickness map  318  for the substrate  204  at 600 torr. Note that the thickness uniformity is reduced to 4.75% for the thickness map  318  from 5.99%, and is closer to the ranges for the other three thickness maps  302 ,  304  and  306  (ranging from 3.49% to 4.09%). 
         [0040]    Results show that the thickness variation on both substrate  202  and substrate  204  have been reduced when using the second assembly  200 B of swirl mixing inserts to replace the first assembly  200 A of swirl mixing inserts. The second assembly  200 B allows uniform mixing for both substrates  202  and  204 , and improves thickness uniformity for substrate  204  from 6% to 4.75%. 
         [0041]    Processing gas/vapor may not be distributed evenly to chamber substrates  202  and  204  for 2-step BPSG due to different conductance in gas delivery hardware. As more gas flow results in thicker film, thickness may not be matched on the substrates  202  and  204 . In another set of the embodiments, Automatic Flow Splitter is designed with motorized micrometers that can be used to adjust conductance of process gas flow to the substrate  204  and substrate  204  to match thickness uniformity for both substrates  202  and  204 . This Automatic Flow Splitter helps solve the thickness match issue as described above. 
         [0042]      FIG. 4  illustrates a simplified schematic for an Automatic Flow Splitter. The Automatic Flow Splitter  400  includes a gas panel  402 , two step motors  406 A and  406 B, two micrometers  404 A and  404 B for adjusting conductance of gas flow through valve openings, a remote plasma source  408 , and a controller system with software  410 . The controller system  410  may determine how to adjust the step motors  406 A and  406 B. The two step motors  406 A and  406 B can then adjust the two micrometers  404 A and  404 B in order to change the relative gas delivery to the substrates  202  and  204  for balancing the thickness on the two substrates  202  and  204 . There may be 1500 steps for the motors  406 A and  406 B, each step is at 260 mils. 
         [0043]    In a specific embodiment, when the thickness on the substrate  202  is thinner than the substrate  204 , a signal may be sent from substrates  202  and  204  through communication lines  412  and  414  to the controller system  410 . The controller system  410  may send signals through lines  416 A and  416 B to the step motors  406 A and  406 B to adjust the micrometers  404 A and  404 B to increase the gas flow from gas panel  402  to the substrate  204  and decrease the flow to the substrate  204 . 
         [0044]    In an alternative embodiment, when the thickness on substrate  202  is thicker than the substrate  204 , a signal may be sent from substrates  202  and  204  through the communication lines  412  and  414  to the controller system  410 . The controller system  410  may send signals through the communication lines  416 A and  416 B to the step motors  406 A and  406 B to adjust the micrometers  404 A and  404 B to decrease the gas flow from gas panel  402  to the substrate  204  and increase the flow to the substrate  204 . 
         [0045]    Inventors have performed extensive experiments by using the Automatic Flow Splitter  400 .  FIG. 5  demonstrates the effect of motor step on thickness. Note that curve  502  is thickness for the substrate  204  versus motor step for the step motor  406 A, curve  504  is thickness for the substrate  204  versus motor step for the step motor  406 A, and curve  506  is delta thickness between curve  502  and curve  504  with negative values as shown in the vertical axis on the right side of  FIG. 5 . When the motor step for the step motor  406 A increases, the micrometer  404 A is adjusted to decrease the gas flow to the substrate  204  so that the thickness decreases on the substrate  204  as shown by curve  502 , while the thickness on the substrate  204  increases as shown by curve  504 . The change in thickness on the substrate  204  is substantially linear to the change in thickness on the substrate  204 . This example demonstrates that Automatic Flow Splitter design  400  adjusts conductance of process gas flow, and matches thickness uniformity of both substrates  202  and  204  at 1500 steps from fully open. 
         [0046]    For a 2-step SACVD BPSG process with chamber pressure requirement of 200 Torr and 600 Torr, a heater is usually leveled at a chamber pressure of 200 Torr. In case of depositing a multiple stacks of films, chamber pressure may be changed. For example, a chamber pressure of 600 Torr may be used to achieve a relatively low deposition rate, while a chamber pressure of 200 Torr may be used to achieve a relatively higher deposition rate. Parallelism between heater surface and faceplate are often leveled at 200 Torr. When the chamber pressure is changed to 600 Torr, the parallelism between heater surface and faceplate is altered due to the chamber pressure change. This problem may be resolved by using a Dual-Pressure Heater Lift design. 
         [0047]      FIG. 6  shows a schematic of a Dual-Pressure Heater Lift design  600 , including a heater  602 , a cantilever  604 , a pneumatic cylinder  606 , a main frame  608  that supports the cantilever  604 , the heater  602  through a support member  612 . The Dual-Pressure Heater Lift design  600  also includes a slider  610  that is coupled to the main frame  608 . The pneumatic cylinder  606  and the main frame  608  as well as the supporting member  612  are attached to a carrier member  614 . A carrier  620  is coupled to the slider  610  and the support member  612  through a hub  618  that is coupled to the carrier member  614 . The pneumatic cylinder  606  would provide a CDA pressure to lift the cantilever  604 . For example, when the chamber pressure is changed from 200 Torr to 600 Torr or other pressure, a pressure is pressed against the heater  602  to generate a downward movement in the slider  610  to cause the tilting of the heater  602 . A CDR from the Pneumatic cylinder may be provided to lift the heater  602  to adjust the spacing between the heater and a faceplate or showerhead (not shown) above the heater. 
         [0048]    One benefit of the Dual-Pressure Heater Lift design  600  is to allow the cantilever  604  to be lifted by Pneumatic pressure to counterbalance the effect of chamber pressure change from 200 Torr to 600 Torr. The Dual-Pressure Heater Lift design  600  incorporates the pneumatic cylinder  606  at an end of the cantilever  604  to counter balance additional force from chamber pressure difference of 400 Torr. 
         [0049]    In a further embodiment of the invention, different compressed dry air (CDA) pressures may be required for balancing heater lift of the substrates  202  and  204  to resolve the side-to-side matching issue, as geometrical tolerances may introduce inconsistency to heater surface tilting. 
         [0050]    Inventors have performed experiments to demonstrate that Dual-Pressure Heater Lift design  600  accommodates heater tilting at 600 Torr and improves thickness uniformity.  FIG. 7  show multiple thickness maps for multiple stacks of films, including a first set of thickness maps  702 ,  704 ,  706  and  708  for substrate  204  (left side) at CDA pressures of 0 psi, 10 psi, 15 psi and 20 psi, respectively, and a second set of thickness maps  712 ,  714 ,  716 , and  718  for substrate  204  (right side) at CDA pressures of 0 psi, 10 psi, 15 psi and 20 psi, respectively. Note that the thickness uniformity decreases from 6.25% to 4.95% for the substrate  204  with increasing CDA pressure from 0 psi to 20 psi. The thickness uniformity also decreases from 4.75% to 3.5% with increasing CDA pressure from 0 psi to 20 psi. 
         [0051]    Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention. 
         [0052]    Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included. 
         [0053]    As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the channel” includes reference to one or more channels and equivalents thereof known to those skilled in the art, and so forth. 
         [0054]    Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.