Abstract:
An imprint lithography system that pressurizes and depressurizes an air cavity behind a retained imprint template or substrate so as to deflect the template or substrate to aid in filling the template pattern with fluid resist and/or separating the template from the cured resist on the substrate. The system includes a controller, pressure sensors, and an impedance valve for modulating the air cavity pressure so as to reduce pressure wave oscillations within the cavity that otherwise negatively impact overlay accuracy control, fluid spread control and separation control.

Description:
BACKGROUND INFORMATION 
       [0001]    Nano-fabrication includes the fabrication of very small structures that have features on the order of 100 nanometers or smaller. One application in which nano-fabrication has had a sizeable impact is in the processing of integrated circuits. The semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate; therefore nano-fabrication becomes increasingly important. Nano-fabrication provides greater process control while allowing continued reduction of the minimum feature dimensions of the structures formed. 
         [0002]    An exemplary nano-fabrication technique in use today is commonly referred to as nanoimprint lithography. Nanoimprint lithography is useful in a variety of applications including, for example, fabricating layers of integrated devices such as CMOS logic, microprocessors, NAND Flash memory, NOR Flash memory, DRAM memory, or other memory devices such as MRAM, 3D cross-point memory, Re-RAM, Fe-RAM, STT-RAM, and the like. Exemplary nanoimprint lithography processes are described in detail in numerous publications, such as U.S. Pat. No. 8,349,241, U.S. Pat. No. 8,066,930, and U.S. Pat. No. 6,936,194, all of which are hereby incorporated by reference herein. 
         [0003]    A nanoimprint lithography technique disclosed in each of the aforementioned U.S. patents includes formation of a relief pattern in a formable (polymerizable) layer and transferring a pattern corresponding to the relief pattern into an underlying substrate. The substrate may be coupled to a motion stage to obtain a desired positioning to facilitate the patterning process. The patterning process uses a template spaced apart from the substrate and a formable liquid applied between the template and the substrate. The formable liquid is solidified to form a rigid layer that has a pattern conforming to a shape of the surface of the template that contacts the formable liquid. After solidification, the template is separated from the rigid layer such that the template and the substrate are spaced apart. The substrate and the solidified layer are then subjected to additional processes, such as etching processes, to transfer a relief image into the substrate that corresponds to the pattern in the solidified layer. The patterned substrate can be further subjected to known steps and processes for device fabrication, including, for example, oxidation, film formation, deposition, doping, planarization, etching, resist removal, dicing, bonding, and packaging, and the like. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0004]    So that features and advantages of the present invention can be understood in detail, a more particular description of embodiments of the invention may be had by reference to the embodiments illustrated in the appended drawings. It is to be noted, however, that the appended drawings only illustrate typical embodiments of the invention, and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0005]      FIG. 1  illustrates a simplified side view of a nanoimprint lithography system having a template and a mold spaced apart from a substrate. 
           [0006]      FIG. 2  illustrates a simplified view of the substrate illustrated in  FIG. 1 , having a solidified patterned layer formed thereon. 
           [0007]      FIG. 3  illustrates a cross-sectional view of a portion of a template and template chuck assembly including an air cavity that can be pressurized to impart shape modulation to the template. 
           [0008]      FIG. 4A-4D  illustrates a portion of a nanoimprint process employing template shape modulation. 
           [0009]      FIG. 5  illustrates a cross-sectional view of a portion of a substrate and substrate chuck assembly including a plurality of air cavities that can be pressurized to impart shape modulation to the substrate. 
           [0010]      FIG. 6  illustrates a schematic view of an air cavity pressure control system. 
           [0011]      FIG. 7  illustrates a schematic view of an air cavity pressure control system according to an embodiment of the invention. 
           [0012]      FIG. 8  illustrates a graphical representation of air cavity pressure change over time of air cavity pressure control systems according to  FIGS. 6 and 7 . 
           [0013]      FIG. 9  illustrates a graphical representation of valve and air cavity pressure variations over time of an air cavity pressure control system according to FIG. 
           [0014]      7 . 
       
    
    
       [0015]    Those of skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the invention. 
       DETAILED DESCRIPTION 
       [0016]    Referring to the figures, and particularly to  FIG. 1 , illustrated therein is nanoimprint lithography system  10  used to form a relief pattern on substrate  12 . Substrate  12  may be coupled to substrate chuck  14 . As illustrated, substrate chuck  14  is a vacuum chuck. Substrate chuck  14 , however, may be any chuck including, but not limited to, vacuum, pin-type, groove-type, electrostatic, electromagnetic, and/or the like. Exemplary chucks are described in U.S. Pat. No. 6,873,087, which is hereby incorporated by reference herein. 
         [0017]    Substrate  12  and substrate chuck  14  may be further supported by stage  16 . Stage  16  may provide translational and/or rotational motion along the x, y, and z-axes. Stage  16 , substrate  12 , and substrate chuck  14  may also be positioned on a base (not shown). 
         [0018]    Spaced-apart from substrate  12  is template  18 . Template  18  may include a body having a first side and a second side with one side having a mesa  20  extending therefrom towards substrate  12 . Mesa  20  may include a patterning surface  22  thereon. Further, mesa  20  may be referred to as mold  20 . Alternatively, template  18  may be formed without mesa  20 . 
         [0019]    Template  18  and/or mold  20  may be formed from such materials including, but not limited to, fused-silica, quartz, silicon, organic polymers, siloxane polymers, borosilicate glass, fluorocarbon polymers, metal, hardened sapphire, and/or the like. As illustrated, patterning surface  22  comprises features defined by a plurality of spaced-apart recesses  24  and/or protrusions  26 , though embodiments of the present invention are not limited to such configurations (e.g., planar surface). Patterning surface  22  may define any original pattern that forms the basis of a pattern to be formed on substrate  12 . 
         [0020]    Template  18  may be coupled to chuck  28  as detailed further herein. Chuck  28  may be configured as, but not limited to, vacuum, pin-type, groove-type, electrostatic, electromagnetic, and/or other similar chuck types. Exemplary chucks are described in U.S. Pat. No. 6,873,087. Further, chuck  28  may be coupled to imprint head  30  which in turn may be moveably coupled to bridge  36  such that chuck  28 , imprint head  30  and template  18  are moveable in at least the z-axis direction. 
         [0021]    Nanoimprint lithography system  10  may further comprise a fluid dispense system  32 . Fluid dispense system  32  may be used to deposit formable material  34  (e.g., polymerizable material) on substrate  12 . Formable material  34  may be positioned upon substrate  12  using techniques such as drop-dispense, spin-coating, dip-coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), thin film deposition, thick film deposition, and/or the like. Formable material  34  may be disposed upon substrate  12  before and/or after a desired volume is defined between mold  22  and substrate  12  depending on design considerations. For example, formable material  34  may comprise a monomer mixture as described in U.S. Pat. No. 7,157,036 and U.S. Pat. No. 8,076,386, both of which are herein incorporated by reference. 
         [0022]    Referring to  FIGS. 1 and 2 , nanoimprint lithography system  10  may further comprise energy source  38  that directs energy  40  along path  42 . Imprint head  30  and stage  16  may be configured to position template  18  and substrate  12  in superimposition with path  42 . Camera  58  may likewise be positioned in superimposition with path  42 . Nanoimprint lithography system  10  may be regulated by processor  54  in communication with stage  16 , imprint head  30 , fluid dispense system  32 , energy source  38 , and/or camera  58 , and may operate on a computer readable program stored in memory  56 . 
         [0023]    Either imprint head  30 , stage  16 , or both vary a distance between mold  20  and substrate  12  to define a desired volume therebetween that is filled by formable material  34 . For example, imprint head  30  may apply a force to template  18  such that mold  20  contacts formable material  34 . After the desired volume is filled with formable material  34 , energy source  38  produces energy  40 , e.g., ultraviolet radiation, causing formable material  34  to solidify and/or cross-link, conforming to a shape of surface  44  of substrate  12  and patterning surface  22 , defining patterned layer  46  on substrate  12 . Patterned layer  46  may comprise a residual layer  48  and a plurality of features shown as protrusions  50  and recessions  52 , with protrusions  50  having a thickness t 1  and residual layer having a thickness t 2 . 
         [0024]    The above-mentioned system and process may be further employed in imprint lithography processes and systems referred to in U.S. Pat. No. 6,932,934, U.S. Pat. No. 7,077,992, U.S. Pat. No. 7,179,396, and U.S. Pat. No. 7,396,475, all of which are hereby incorporated by reference in their entirety. 
         [0025]    Referring further to  FIGS. 1 and 3 , template  18  is coupled to the template chuck  28 . The template chuck  28  includes opposed sides  61  and  63 , with first side  61  including interior recess  62  defined by inner support region  64 , and outer recess  66  and defined by inner support region  64  and outer support region  68 . That is, outer support region  68  cinctures outer recess  66 , inner support region  64  and inner recess  62 , and inner support region  62  cinctures inner recess  62 . In a particular embodiment, the outer support region  68  has a square shape, and the inner support region  64  has a circular shape; however, in other embodiments, the support regions  62  and  68  can include any geometric shape desired. A portion  68  of template chuck  28  is in superimposition with the inner recess  62  and can be transparent to radiation having a predetermined wavelength or a range of wavelengths. The portion  68  can include a thin layer of transparent material, such as glass. However, the material of the portion  68  may depend upon the wavelength of radiation emitted by the energy source. The portion  68  extends from side  63  and terminates proximate to the recess  62 , with portion  68  having an area at least as large as an area of mold  20  of retained template  28  so that mold  20  is in superimposition with portion  68 . 
         [0026]    The template chuck  28  includes throughways  78  and  80 . In an alternative embodiment, the template chuck  28  may have a different number of throughways. The throughway  78  places the recess  62  in fluid communication with the surface  63 , however, in other embodiments, the throughway  78  places the recess  62  in fluid communication with any surface of template chuck  28 . The throughway  80  places the recess  64  in fluid communication with the side  63 , however, in other embodiments, the throughway  80  places the recess  64  in fluid communication with any surface of template chuck  28 . The throughways  78  and  80  can facilitate placing the recesses  62  and  64 , respectively, in fluid communication with a pressure control system, such as a pump system  86 . 
         [0027]    The pump system  86  may include one or more pumps to control the pressure proximate to the recesses  62  and  64 . To that end, when the template  18  is coupled to the template chuck  28 , the template  18  rests against the support regions  64  and  68 , covering the recesses  62  and  64 . A flexible region  72  of the template  18  may be in superimposition with the recess  62 , defining an inner chamber or cavity  83  and a thicker region  74  of the template  18  may be in superimposition with recess  64 , defining an outer chamber or cavity  85 . The pump system  86  operates to control a pressure in the chambers or cavities  83  and  85 . Chamber or cavity  85  can be held or maintained at suitable vacuum pressures to retain template  18  against template chuck  28 , while chamber or cavity  83  can be subjected to positive pressure and/or negative pressure so as to impart desired shape modulation to the flexible region of the template  18 . Such shape modulation provides important advantages to nanoimprint lithography processes, including (a) improving the speed within which polymerizable material fills the pattern features of the template while also minimizing fill-related defects and (b) improving separation quality (i.e., separating the template from the polymerized material with reduced separation-related defects). With particular reference to  FIG. 4A-D , an example of such shape modulation is shown. In  FIG. 4A , cavity  83  is pressurized to bow or flex the flexible region  72  and mold  20  of template  18  toward substrate  12  and the deposited formable material  34 . In  FIG. 4B , the patterned surface of mold  20  has contacted formable material  34  and the material has begun filling the patterned surface of mold  20  in a center-to-perimeter direction. By  FIG. 4C , the patterned surface fill is almost complete, if not full complete, and the pressure in cavity  83  being reduced such that the flexible region  72  is approaching a parallel condition, if not fully parallel, with the substrate. Any necessary overlay alignment adjustments are also being completed at this stage. By  FIG. 4D , the patterned surface has been completely filled, overlay adjustments have been made, and formable material  34  has been solidified to form the patterned layer on substrate  12 . The template  18  is then separated from the formed pattern layer (not shown) by processes that can likewise include modulating the template shape, e.g., by pressurizing and/or applying vacuum to the cavity  83 . 
         [0028]    Substrate chucks can likewise be configured to provide for shape modulation of retained substrates. This can be advantageous for, among other things, separation of the template from the formed patterned layer. Turning to  FIG. 5 , wafer chuck  14  is shown retaining substrate  12  in a chucked condition. Wafer chuck  14  includes an outer zone  202 , an intermediate zone  204 , and a central zone  206 , where the intermediate zone  204  is disposed between the outer zone  202  and the central zone  206 . Each of the zones  202 ,  204 , and  206  is defined in part by a recessed land  208  and full-height lands  210  and  212 . In an embodiment, each of the lands  208 ,  210 , and  212  are continuous, and thus, the lands  208 ,  210 , and  212  are concentric. The outer zone  202  is laterally defined by the lands  208  and  210 , the intermediate zone  204  is laterally defined by the lands  210  and  212 , and the central zone  206  is laterally defined by the land  212 . In another embodiment, the recessed land  208  can be replaced by a full-height land. A pressure control system (not shown) can be connected to zones  208 ,  210  and  212  to independently apply positive pressure and/or negative pressure to each zone, such that each zone can be considered to be an air cavity similar to air cavity  83  of template  28 . Different pressures can be advantageously applied to the different zones during the imprinting, curing, and separation processes. 
         [0029]    Important considerations and requirements in nanoimprint lithography include high overlay accuracy between the imprint layer being formed and a previously formed patterned layer within the substrate. For certain nanoimprint lithography applications, overlay accuracy of 6 nm or less is required while maintaining high throughput (e.g., 1 second or less total time budget in step-and-repeat processes with 22×33 mm imprinted field size per step). With respect to a template chuck and template system as described above, the pressure and/or vacuum applied to inner cavity  83  needs to be tightly controlled to achieve these objectives. However, under high throughput conditions conventional pneumatic systems can produce pressure instability, including pressure wave oscillations, which can adversely affect overlay accuracy as well as separation. More specifically, while such systems can be effective in adjusting pressure to the inner cavity  83 , and thereby effect the desired shape modulation of template  18 , in doing so pressure wave oscillations are created within cavity  83  as the pressure system operates to increase or decrease pressure within the cavity. These pressure wave oscillations in turn can produce undesired movement of the template  18  at the nanometer scale, including lateral movement (i.e., movement in the x-y plane). Such nanometer scale lateral movement during the fluid fill and alignment step interferes with and disrupts overlay accuracy, leading to overlay defects. Further, such nanometer scale lateral movement during separation can impart undesirable shear stress on the formed pattern features of the formed patterned, leading to separation defects (e.g., feature distortion and/or feature shearing). In theory, the intensity and subsequent deleterious effects of such pressure wave oscillations can be ameliorated to some extent by slowing down the rate of pressure change, but such an approach would come at the cost of drastically reduced throughput rate. Further, while the above concerns have been expressed with reference to template  18  and cavity  83 , it is to be understood that similar concerns and disadvantages of pressure wave oscillations also apply, e.g., when adjusting pressures in zones  202 ,  204 , and  206  of substrate chuck  14 . 
         [0030]      FIG. 6  illustrates a pressure control system  300  for controlling pressure to air cavity  350 . The system includes a conventional pressure regulator  301  and conventional vacuum regulator  321  for controlling the amount of pressurized air delivered or amount of vacuum applied to air cavity  350 . Pressure regulator  301  includes pressure controller  302 . Pressure valve  304 , under the control of pressure controller  302 , connects to pressurized gas supply source  314  and supplies pressurized gas from pressurized source  314  to air cavity  350  through connecting tubing  316 , switch valve  340 , connecting tubing  342 , air filter  344  and tubing  346 , as shown. To depressurize the air cavity  350 , pressure valve  304  is closed and vent valve  306  is opened to vent air cavity to atmosphere  312 . Similarly, vacuum regulator  321  includes vacuum controller  322 . Vacuum valve  324 , under the control of vacuum controller  342 , connects to vacuum source  334  to apply vacuum to air cavity  350  through connecting tubing  336 , switch valve  340 , and connecting tubing  342 , air filter  344  and tubing  346 , as shown. To re-pressurize the air cavity  350  to atmospheric pressure, the vacuum valve  324  is closed and vent valve  326  is opened to vent air into cavity  350  from atmosphere  332 . Switch valve  340  is a toggle switch for switching between pressure and vacuum conditions. Switch controller  360  controls switch valve  340  in response to input from pressure controller  302  and vacuum controller  322 . In line pressure sensors  310  and  330  provide pressure feedback to controllers  302  and  322  respectively. System  300  suffers from at least the following three drawbacks. First, the transition time between pressure states in air cavity  350  is limited by the air flow rate between the pressure source  314  (or vacuum source  334 ) and the air cavity  350 . This flow rate is proportional to the pressure difference between the pressure source  314  or atmosphere  312  (or between vacuum source  334  or atmosphere  332 ) and the air cavity  350 . In many cases this pressure difference is insufficient to establish high flow rates needed for high throughput rates. Second, the mechanical toggling of switch valve  340  back and forth between pressure and vacuume in an effort to maintain at or near zero pressure conditions makes it difficult to maintain a smooth steady state under such conditions. Third, the in-line pressure sensors  310  and  330  are not configured to provide an accurate enough reading of air cavity pressure for high accuracy control purposes, both due to their physical displacement from the air cavity  350  and due to their susceptibility to variations in pressure readings to turbulent flow in lines  316  and  336 . 
         [0031]      FIG. 7  illustrates an embodiment of the invention which provides for a system and method for accurately controlling pressure within the cavity with reduced pressure wave oscillations, particularly when transitioning between steady states under high throughput conditions. Such system and method thus provides for high throughput while minimizing overlay error and separation defects. More particularly, system  400  is configured to control pressure within air cavity  450 , which is shown here as representative of e.g. inner cavity  83  defined by the template  18  and template chuck  28  assembly shown in  FIG. 3 , or of zones  202 ,  204  or  206  of the substrate  12  and substrate chuck  14  assembly shown in  FIG. 5 . System  400  includes controller  402  which is configured to control pressure valve  404 , vacuum valve  406  and impedance valve  408 . In certain embodiments, pressure valve  404 , vacuum valve  406  and/or impedance valve  408  are high flow rate, fast response servo valves. As further explained herein, impedance valve  408  is used to dynamically match pneumatic impedance to avoid pressure oscillation within cavity  450  during controlled transition between steady pressure states. Such transition control can use feedforward and/or feedback control. Pressure valve  404  connected to pressurized gas supply source  414  and supplies pressurized gas from source  414  to air cavity  450  through connecting tubing  405  and  407 , impedance valve  408 , and air filter  413  as shown. In an embodiment, the pressurized gas supply source  414  supplies clean dry air (CDA) to cavity  450 . Vacuum valve  406  is connected to vacuum supply source  416  and applies vacuum to cavity  450  through connecting tubing  407  and  409 , impedance valve  408 , and filter  413  as shown. Valves  404  and  406  are further configured in a symmetrical arrangement to minimize turbulent flow. In operation, gas supply  414  and vacuum supply  416 , as controlled by valves  404  and  406  respectively, can create a high pressure differential (either positive pressure or vacuum) in a push/pull mode to provide for rapid pressure adjustment to meet high speed operational requirements. Air cavity pressure sensor  410  is positioned at the dead end of air cavity  450  and provides input to controller  402 . Valve pressure sensor  412  is positioned at the intersection of connecting tubing  405 ,  407  and  409  in fluid communication with air cavity  450  and likewise provides input to controller  402 . In an embodiment, air cavity sensor  410  or valve sensor  412  or both are dead end sensors. Sensors  410  and  412  are used to measure pressure at the cavity  450  and at valves  404  and  406  and provide associated input to controller  402 . From such input, the controller  402  calculates flow rate, resistance, and leakage, and then applies a pressure control law based on such inputs and calculations to increase pressure control accuracy, minimize transition time from one steady state to another, and smooth such transition between states, as compared to system  300 . 
         [0032]    As further detailed herein, system  400  and related methods of use achieves the following advantages over prior systems and/or methods: (1) increases pressure control accuracy within the cavity at steady state pressure, (2) minimizes the transition time from one steady state to another, and (3) smooths the transition from feed forward (FF) to feedback (FB) control. (As used herein, the term “feedback control” refers to a control method in which a feedback signal and a reference input are compared in order to control the output, so as to maintain operation within specified parameters, and the term “feedforward control” refers to a control method in which the control signal is based on a plant or system model and is applied before the output of a process. For system  400 , feedforward control is based on e.g. the air cavity volume, tubing length, and flow rate of servo valves, etc.) The above advantages in turn meet important imprint lithography requirements, including the following. First, the system allows for nm level overlay control accuracy during fluid filling and alignment. That is, overlay accuracy, also referred to as image placement error, is directly related to pressure/vacuum control accuracy. In certain applications, ±0.04 kPa pressure control accuracy is needed to achieve  6  nm overlay (i.e., image placement error ≦6 nm). The system provides for such pressure control accuracy. Second, during, for example, the fluid fill and alignment step, the system allows for rapid reduction of cavity pressure from e.g., 25 kPa to steady state value (±1 kPa) in less than 0.1 s. This maximizes available time for dynamic fluid spread and in-liquid alignment while still meeting total high throughput time budget requirements. Third, during, for example, the separation step, the system allows for further rapid ramp down of cavity pressure from e.g., 0 to a set point value of −30 kPa) in 0.1 s. An analogous system (not shown) is likewise able to ramp up wafer chuck vacuum pressure from e.g. −70 kPa to 20 kPa in 0.1 s. 
         [0033]      FIG. 8  shows air cavity pressure change over time under the pressure system  300  of  FIG. 6  and pressure system  400  of  FIG. 7 . Line  500  represents pressure change for system  300  and Line  502  represents pressure change for system  400 . In system  300 , standard pressure and/or vacuum regulators with associated sensors are used to control pressure change in the cavity. As previously, noted the provided sensors in the regulators are positioned in line near the pressure valves and the pressure is controlled using a feedback (FB) control scheme. As reflected by line  500 , the pressure change follows an asymptotic curve, taking at least 0.6 seconds to reach the 0 kPa target from the initial state of 25 kPa. Thus, this approach has the following deficiencies. First, overall throughput is limited by feedback (FB) control scheme, as such scheme necessarily requires time-dependent steps of obtaining and processing feedback signals. However, even if the a feedforward (FF) scheme were employed, performance would not be improved, as pressure oscillation at the air cavity and pressure valve, caused by pressure wave reflection, would continue significantly beyond 1 second without ever reaching true equilibrium at 0 kPa. Second, control accuracy is affected by sensor location, i.e., in-line sensors that are subjected to turbulence in the air flow path necessarily produce less accurate pressure readings. Third, two separate regulators are needed to control both pressure and vacuum in the air cavity. Finally, there is a long transition time when ramping down pressure to at or near zero due to the limited pressure difference between air cavity and atmospheric environment. By contrast, system  400  as reflected by line  502  shows a rapid yet controlled transition from 25 kPa to 0 kPa in less than 0.1 second. 
         [0034]    Turning to  FIG. 9 , both air cavity pressure and valve pressure of system  400  are depicted as the pressure is reduced from 25 kPa to 0 kPA. Line  700  represents the air cavity pressure and line  702  represents valve pressure. Here equilibrium is smoothly and rapidly achieved in about 0.1 seconds. This is aided by feedforward (FF) control in the first transition state indicated by section  704 . The feedforward control (FF) scheme is designed to minimize the steady state to steady state transition time to the limitations of the physical hardware itself, e.g., gas supply pressure, air cavity volume, resistance caused by tubing and fittings to connect servo valve and air cavity, and the like. Here, impedance valve  408  and one of pressure valve  404  or vacuum valve  406  is fully open with the other remaining valve fully closed, depending on whether increasing or decreasing pressure is being applied to air cavity  450 . The fully open time is calculated by a feedforward (FF) coefficient multiplied by changed pressure difference. The fully open time is a function of the volume of air cavity  450 , the pressure difference between current valve and set point valve, the supply source pressure (vacuum or pressure) and the impendence between the source (vacuum or pressure) and the air cavity which is determined by tubing length, tubing, connecter and fitting internal diameter (ID). The transition time can be minimized by increasing the pressure/vacuum source pressure and/or reducing the air cavity volume and impendence of the system. 
         [0035]    In the transition state, designated  706 , the pressure control quickly and smoothly transitions from a feedforward (FF) to a feedback (FB) scheme. Here, pressure valve  404  or vacuum valve  406  smoothly transitions from fully open to a steady state valve as the control law transitions from feedforward (FF) to feedback (FB). Here the concern is pressure oscillation at valve  404  or  406  and air cavity  450  caused by pneumatic impedance mismatch when quickly changing the valve from fully open (under FF control) to steady state value (under FB control). Therefore, control of impedance valve  108  by controller  402  is activated to actively control the impedance to match the source and load impedance and to reduce pressure oscillation as pressure control transitions between the FF and FB states. Inputs from both valve sensor  412  and air cavity sensor  410  are used by controller  402  to estimate flow rate and the control law is designed to control flow rate to smoothly reduce pressure to the steady state in one direction and to control pressure in order to reach the steady state value without pressure oscillation, or at the least with reduced pressure oscillation (e.g., oscillations having an amplitude of 0.04 kPa or less, or 0.01 kPa or less, or 0.001 kPa or less). 
         [0036]    In the steady state, designated  708 , the control accuracy is controlled by a feedback (FB) scheme and is mainly limited by measurement accuracy of pressure sensor and servo valve flow rate control resolution. The feedback control law is designed to control servo valve flow rate to balance the leakage rate of system. Both air cavity sensor  410  and valve pressure sensor  412  are used as the input of the feedback controller to estimate leakage rate and servo valve flow rate. Air cavity  450  (and similarly each individual wafer chuck zone on the wafer side) can be modeled as sealed cavity with minimum leakage. The volume of air cavity  450  can be modeled as a constant. So in steady state, the pressure in cavity  450  mainly depends upon the number of gas molecules in the air cavity as based on the ideal gas law, which in turn depends the leakage rate and controlled flow rate from servo valve. The impedance between output of servo valve and input of air cavity is adjusted to increase control resolution for given servo valve. So the control accuracy is only limited by sensor accuracy and servo valve control resolution. The dead end sensors at both the valves and air cavity provide for more accurate pressure signals by avoiding turbulence associated with the air flow path. The flow rate control resolution of the servo valve is limited by non-linear behavior such as dead zone, hysteresis of servo valve which directly affect control accuracy. This flow rate control resolution is improved by using both vacuum and pressure valve operating at the range of servo valve outside of the dead zone in order to dynamically balance the flow rate of air flowing into the air cavity, based on the flow rate as measured by the valve and air cavity sensor, in order to achieve high control accuracy. For example, when regulating pressure at steady state, the vacuum valve is opened to a value to extract air or gas molecules at a specified rate while the pressure valve is opened at a value to replenish the air or gas molecules at an equivalent rate, such that air cavity pressure remains dynamically balanced. This dynamic balancing approach is capable of achieving pressure control accuracy of at least 0.04 kPa or at least 0.01 kPa or at least 0.001 kPa or better. 
         [0037]    Further modifications and alternative embodiments of various aspects will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only. It is to be understood that the forms shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description.