Abstract:
The present invention provides a method for controlling pressure in a chamber during a time division multiplexed process. A throttle valve is positioned based on an open-loop pressure control algorithm within at least one step of the time division multiplexed etch process. A pressure response of the step is evaluated and compared to a desired pressure response. The throttle valve is then positioned through a proportional, integral and derivative controller step to step of the time division multiplexed etch process based on the evaluation to the desired pressure response.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and is related to commonly owned U.S. Provisional Patent Application Ser. No. 60/460,932, filed Apr. 7, 2003, entitled: A Method and Apparatus for Process Control in Time Division Multiplexed (TDM) Etch Processes, this Provisional Patent Application incorporated by reference herein. This application is a continuation-in-part of application Ser. No. 10/815,965 filed on Mar. 31, 2004, now U.S. Pat. No. 7,115,520, entitled: A Method and Apparatus for Process Control in Time Division Multiplexed (TDM) Etch Processes, the contents of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of semiconductor wafer processing. More particularly, the present invention is directed to a method and apparatus for controlling the reaction chamber pressure during a time division multiplexed etching and deposition process. 
     BACKGROUND OF THE INVENTION 
     The fabrication of high aspect ratio features in silicon is used extensively in the manufacture of micro-electro-mechanical (MEMS) devices. Such features frequently have depths ranging from tens to hundreds of micrometers. To ensure manufacturability, the etching processes must operate at high etch rates to maintain reasonable throughputs, along with other performance requirements such as smooth etch profiles. 
     Conventional, single step, plasma etch processes cannot simultaneously meet these needs, and time division multiplex etch processes have been developed. Time division multiplexed (TDM) approaches for etching silicon have been described by Suzuki et al. (U.S. Pat. No. 4,579,623), Kawasaki et al. (U.S. Pat. No. 4,795,529) and Laermer et al. (U.S. Pat. No. 5,501,893). TDM etch processes typically employ alternating etching and deposition steps. For example, in etching a silicon (Si) substrate, sulfur hexafluoride (SF 6 ) is used as the etch gas and octofluorocyclobutane (C 4 F 8 ) as the deposition gas. In an etch step, SF 6  facilitates spontaneous and isotropic etching of silicon (Si); in a deposition step, C 4 F 8  facilitates protective polymer passivation onto the sidewalls as well as the bottom of the etched structures. In the subsequent etching step, upon energetic and directional ion bombardment, the polymer film coated in the bottom of etched structures from the preceding deposition step will be removed to expose the silicon surface for further etching. The polymer film on the sidewall will remain, inhibiting lateral etching. TDM processes cyclically alternate between etch and deposition process steps to enable high aspect ratio structures to be defined into a masked silicon substrate at high etch rates. 
     In each process step, gases (for example, SF 6  and C 4 F 8 ) are introduced into the reaction chamber through a gas inlet at flow rates specified in the process recipe. TDM etch process are typically performed in high density plasma reactors (e.g., inductively coupled plasma (ICP), electron cyclotron resonance (ECR), etc.). 
     TDM process recipes consist of a series of process loop(s) and steps. Each loop consists of two or more process steps controlling the process variables (e.g., gas flow rates, chamber pressure, RF powers, process step times, chamber temperature, wafer temperature, etc.). The steps within a loop are repeated a number of times before executing the next step or loop in the overall process recipe. It is known to make changes to the process step parameters as a loop repeats to improve etch performance, this is known in the art as process morphing (see Teixeira et al. U.S. Pat. No. 6,417,013). 
     Pressure control is an important component of etching and deposition processes. The flow rate and pressure of the process gases present in the chamber must be carefully controlled to provide the desired deposition and etch characteristics for a repeatable manufacturing process. 
     A TDM plasma reactor evacuation system typically comprises a turbo pump separated from the reaction chamber by a throttle valve. A pressure controller uses reactor chamber pressure data from a manometer to control a throttle valve. The controller opens or closes the throttle valve to increase or decrease the vacuum supplied from the turbo pump to the reaction chamber. In this manner, the controller maintains the desired pressure in the reaction chamber. During the TDM process chamber pressure set points and gas flow rates cyclically alternate within the process loops. The gas flows can be either single component or mixtures of multiple components. The pressure controller must adjust the throttle valve position to compensate for these changing flow and pressure conditions. Ideally, the pressure controller adjusts the throttle valve position to instantly achieve the pressure set point without pressure set point overshoot or undershoot. 
     Throttle valves and controllers, currently available, typically operate in either Pressure Control mode or Position Control mode. In the Pressure Control mode the controller monitors the pressure in the reaction chamber and maintains the set point pressure by adjusting the position of the throttle valve (i.e., closed loop pressure control). In position control mode the controller positions the throttle valve to a set point position without monitoring the chamber pressure (i.e., open loop pressure control). 
     A number of groups have looked at means for process control in plasma chambers. Kessel et al. (U.S. Pat. No. 4,253,480) describes a pressure regulator that employs an adjustable solenoid valve to control pressure. Kessel teaches the fundamental mechanism dictating the operations of many throttle valves used in vacuum chambers. The actual pressure in a container is measured and converted to electrical signals. A comparator generates a regulation signal that represents the difference between the actual pressure and a command pressure. A regulator uses the regulation signal to direct the valve in such a manner that the valve member is adjustable between intermediate positions within a range between the open and closed positions of the valve. In fact, the throttle valves used in TDM process tools are operated following such principles. However, as described earlier, the inability to control pressure during the transition of the constantly alternating TDM process steps is the real issue, and cannot be addressed by Kessel&#39;s technique. 
     Kaveh et al. (U.S. Pat. No. 5,758,680) and McMillin et al. (U.S. Pat. No. 6,142,163) describe the use of a ballast port for inserting gas into the evacuation system to compensate the pressure fluctuations in the reaction chamber so as to minimize throttle valve movement between different process steps. They further disclose a method to reduce the time for gas stabilization in a vacuum chamber. A throttle valve is first pre-positioned to the desired position. The desired position is estimated using pre-determined estimation curves. Then for a specified period of time, proportional and derivative (PD) control is enabled to control throttle valve movement. Then proportional, integral and derivative (PID) control is enabled to regulate throttle valve movement. The examples taught in the disclosure show that the time period for pressure stabilization is reduced from ˜20 seconds to at least 3-5 seconds. While Kaveh and McMillin contemplate the change of gas flow rates and pressures when process steps change from one to the next, the use in cyclical and alternating TDM processes is not taught. Additionally, many TDM processes employ alternating process steps which last only a few seconds or shorter, which makes pressure control impractical using the disclosed technique. 
     Brown et al. (U.S. Pat. No. 6,119,710) describes the use of adjustable gas flow into the reaction chamber to compensate the pressure variations in the chamber. However, in many TDM processes, changing process gas flow rate during a process step is undesirable. 
     Beyer et al. (U.S. Pat. No. 5,944,049) describes regulating chamber pressure by controlling either the exhaust pressure at the exhaust side of a vacuum pump or the internal pressure at a compression stage of the first vacuum pump. Adjustments on vacuum pumping speed or injection of inert gas into the exhaust side or the compression stage of a vacuum pump are used to control reaction chamber pressure. Beyer does not teach how to use this technique in TDM processes. 
     Puech (U.S. Patent Application 20020168467) describes a way to control pressure by injecting passive control gas at a complementary flow rate into an area near the evacuation port. The flow rate of the controlled passive gas is regulated so as to maintain the total gas flow into the vacuum enclosure at a substantially constant rate. While Puech teaches the control of pressure in the TDM processes that employ process steps on the order of one second, the method does not teach the use of actively regulating throttle valve in pressure control. 
     The current methods of pressure control for TDM processes, Pressure Control and Position Control, have limitations. One problem with pressure control mode in a TDM process is that, in practice, there is typically a trade off between achieving fast pressure response time while minimizing set point deviations. Fast response times are possible at the expense of a period of pressure set point overshoot. Optimizing available Pressure Control mode algorithms to minimize set point overshoot results in slow response times. As the TDM step durations decrease, the time spent trying to reach the recipe specified set point becomes a significant fraction of the processing time. 
     A problem with the current method of Position Control mode in a TDM process is unacceptably long pressure response times. While position mode minimizes process overshoot, the slower response times result in the chamber pressure spending a large fraction of the process time approaching the requested set point value (i.e., out of compliance with the recipe specified set point). 
     Another problem with the position control mode method is that it is an open loop pressure control algorithm. Therefore, there is not any correction for perturbations in gas flow or pumping efficiency. These perturbations tend to cause the process pressure, and subsequent process performance, to vary with time. 
     Therefore, there is a need for a pressure control means for TDM processes, preferably for those processes that employ process steps that are a few seconds or less in duration. 
     Nothing in the prior art provides the benefits attendant with the present invention. 
     Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement of the semiconductor processing art. 
     Another object of the present invention is to provide a method for anisotropically etching a feature in a substrate comprising the steps of: subjecting the substrate to an alternating cyclical process within a plasma chamber, said alternating cyclical process having an etching step and a deposition step; introducing into said plasma chamber a first process gas for depositing a film onto the substrate during the deposition step of said alternating cyclical process; introducing into said plasma chamber a second process gas for etching the substrate during the etching step of said alternating cyclical process; regulating pressure of said plasma chamber by setting a throttle valve at a predetermined position set point for a predetermined period of time during at least one step of said alternating cyclical process; igniting a plasma for a recipe period of time for the deposition step of said alternating cyclical process and the etching step of said alternating cyclical process; enabling a closed loop pressure control algorithm after said predetermined period of time expires; and controlling pressure at a recipe specified pressure set point in said plasma chamber through a closed loop pressure control for a period that lasts the remaining time of the step. 
     Yet another object of the present invention is to provide a method of pressure control in a time division multiplex process comprising the steps of: regulating a process pressure in a vacuum chamber in at least one step of the time division multiplex process by setting a throttle valve at a predetermined position set point for a predetermined period of time; introducing into said vacuum chamber at least one process gas; enabling a closed loop pressure control algorithm after said predetermined period of time expires; and controlling pressure at a recipe specified pressure set point through a closed loop pressure control for a period that lasts the remaining time of said step of the time division multiplex process. 
     Still yet another object of the present invention is to provide a method for controlling pressure in a vacuum chamber, the method comprising the steps of: regulating a process pressure in the vacuum chamber by setting a throttle valve at a predetermined position set point for a predetermined period of time; introducing into said vacuum chamber a gas; enabling a closed loop pressure control algorithm after said predetermined period of time expires; and controlling pressure at a recipe specified pressure set point in said vacuum chamber through a closed loop pressure control. 
     Another object of the present invention is to provide a method of controlling pressure in a chamber during an etch process, the method comprising: placing a substrate in the chamber; executing a time division multiplexed etch process within the chamber; positioning a throttle valve based on an open-loop pressure control algorithm within at least one step of said time division multiplexed etch process; positioning said throttle valve based on a closed-loop pressure control algorithm from step-to-step of said time division multiplexed etch process; and removing said substrate from the chamber. 
     Yet another object of the present invention is to provide a method of controlling pressure in a chamber during an etch process, the method comprising: placing a substrate in the chamber; executing a time division multiplexed etch process within the chamber; positioning a throttle valve to a predetermined position within at least one step of said time division multiplexed etch process; maintaining said predetermined position of said throttle valve for a predetermined period of time; repositioning said throttle valve to a recipe specified position for the remainder of said step of said time division multiplexed etch process; and removing said substrate from the chamber. 
     Still yet another object of the present invention is to provide a method of controlling pressure in a chamber during an etch process, the method comprising: placing a substrate in the chamber; executing a time division multiplexed etch process within the chamber; controlling a throttle valve through a throttle preset parameter within at least one step of said time division multiplexed etch process; evaluating a pressure response of said step compared to a desired pressure response; applying a control algorithm to modify said throttle preset parameter step to step of said time division multiplexed etch process based on said evaluation step; and removing said substrate from the chamber. 
     Another object of the present invention is to provide a method of controlling process execution during an etch process, the method comprising: placing a substrate in the chamber; executing a time division multiplexed etch process within the chamber; modifying an execution timing of a subset of recipe parameters in at least one step of said time division multiplexed etch process; and removing said substrate from the chamber. 
     Yet another object of the present invention is to provide a method of controlling pressure in a chamber during an etch process, the method comprising: placing a substrate in the chamber; executing a time division multiplexed etch process within the chamber; controlling a throttle valve through a throttle preset parameter within at least one step of said time division multiplexed etch process; evaluating a pressure response of said step compared to a desired pressure response; applying a first control algorithm to modify said throttle preset parameter step to step of said time division multiplexed etch process based on said evaluation step; positioning said throttle valve based on a second control algorithm from step-to-step of said time division multiplexed etch process; and removing said substrate from the chamber. 
     The foregoing has outlined some of the pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     For the purpose of summarizing this invention, this invention comprises a method and an apparatus for controlling the pressure in a vacuum chamber during a TDM process. 
     A feature of the present invention is to provide a method for anisotropically etching a feature in a substrate. The method comprising the following steps. The substrate is placed within a plasma chamber and subjected to an alternating cyclical process having an etching step and a deposition step. The pressure of the plasma chamber is regulated by setting a throttle valve at a predetermined position set point for a predetermined period of time to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. A first process gas, such as octofluorocyclobutane, is introduced into the plasma chamber for depositing a film onto the substrate during the deposition step of the alternating cyclical process. A plasma is ignited for a recipe period of time for the deposition step of the alternating cyclical process. A closed loop pressure control algorithm is enabled after the predetermined period of time expires. Then, the pressure of the plasma chamber is controlled at a recipe specified pressure set point through a closed loop pressure control for the remaining time of the deposition step. Next, the pressure of the plasma chamber is again regulated by setting the throttle valve at a predetermined position set point for a predetermined period of time to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. A second process gas, such as sulfur hexafluoride is introduced into the plasma chamber for etching the substrate during the etching step of the alternating cyclical process. A plasma is ignited for a recipe period of time for the etching step of the alternating cyclical process. A closed loop pressure control algorithm is enabled after the predetermined period of time expires. Then, the pressure of the plasma chamber is controlled at a recipe specified pressure set point through a closed loop pressure control for the remaining time of the etching step. 
     The predetermined position set point can either be set or derived from the following: 
     1. A throttle valve position of a preceding like step of the alternating cyclical process; 
     2. An average valve position of a plurality of preceding like steps of the alternating cyclical process; or 
     3. Prior calibration experiments. 
     The predetermined position set point can be adjusted by an offset from the throttle valve position of the preceding like step of the alternating cyclical process. The predetermined position set point can change using a predefined function for the duration of the predetermined period of time. The predetermined position set point can be modified based on pressure performance of a preceding like step of the alternating cyclical process such as minimizing the time to reach the recipe specified pressure set point or minimizing the deviation from the recipe specified pressure set point. 
     The predetermined period of time is about 0.05 to 0.5 seconds long. The predetermined period of time can be modified based on pressure performance of a preceding like step of the alternating cyclical process such as minimizing the time to reach the recipe specified pressure set point or minimizing the deviation from the recipe specified pressure set point. 
     Yet another feature of the present invention is to provide a method of pressure control in a time division multiplex process. The method comprising the following steps. The process pressure in a vacuum chamber is regulated in at least one step of the time division multiplex process by setting a throttle valve at a predetermined position set point for a predetermined period of time to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. At least one process gas is introduced into the vacuum chamber for processing a substrate according to the time division multiplex process. A closed loop pressure control algorithm is enabled after the predetermined period of time expires. Then, the pressure of the vacuum chamber is controlled at a recipe specified pressure set point through a closed loop pressure control algorithm for a period that lasts the remaining time of the processing step of the time division multiplex process. 
     Still yet another feature of the present invention is to provide a method for controlling pressure in a vacuum chamber. The method comprising the following steps. The process pressure of the vacuum chamber is regulated by setting a throttle valve at a predetermined position set point for a predetermined period of time to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. A gas is introduced into the vacuum chamber. A closed loop pressure control algorithm is enabled after the predetermined period of time expires. Then, the pressure is controlled at a recipe specified pressure set point in the vacuum chamber through a closed loop pressure control algorithm. 
     Another feature of the present invention is to provide a method of controlling pressure in a chamber during an etch process. The method comprising the following steps. A substrate is placed in the chamber. A time division multiplexed etch process is executed within the chamber. The time division multiplexed etch process having a step of depositing a passivation layer onto the substrate by means of a plasma and a step of etching a material from the substrate by means of a plasma. A process loop is performed where the deposition step and the etch step are repeated. A throttle valve is positioned based on an open-loop pressure control algorithm within at least one step of the time division multiplexed etch process. The open loop pressure control algorithm can be a throttle valve position control which can hold the throttle valve in a constant position within a step of the time division multiplexed etch process. The throttle valve position control can be based on a function of time which can be linear or nonlinear. The throttle valve is then positioned based on a closed-loop pressure control algorithm from step-to-step of the time division multiplexed etch process. The closed-loop pressure control can be a proportional, integral and derivative controller. Finally, the substrate is removed from the chamber. 
     Yet another feature of the present invention is to provide a method of controlling pressure in a chamber during an etch process. The method comprising the following steps. A substrate is placed in the chamber. A time division multiplexed etch process is executed within the chamber. The time division multiplexed etch process having a step of depositing a passivation layer onto the substrate by means of a plasma and a step of etching a material from the substrate by means of a plasma. A process loop is performed where the deposition step and the etch step are repeated. A throttle valve is positioned to a predetermined position within at least one step of the time division multiplexed etch process. The throttle valve is maintained in the predetermined position for a predetermined period of time to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. The predetermined position can be derived from a recipe position set point, be proportional to the recipe position set point, or is offset from the recipe position set point. The predetermined period of time can be less than a period of time for the step of the time division multiplexed etch process. Then, the throttle valve is repositioned to a-recipe specified position for the remainder of the step of the time division multiplexed etch process. Finally, the substrate is removed from the chamber. 
     Still yet another feature of the present invention is to provide a method of controlling pressure in a chamber during an etch process. The method comprising the following steps. A substrate is placed in the chamber. A time division multiplexed etch process is executed within the chamber. The time division multiplexed etch process having a step of depositing a passivation layer onto the substrate by means of a plasma and a step of etching a material from the substrate by means of a plasma. A process loop is performed where the deposition step and the etch step are repeated. A throttle valve is controlled through a throttle preset parameter within at least one step of the time division multiplexed etch process to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. The preset parameter can be a throttle position or a preset hold time duration. A pressure response of the step is evaluated and compared to a desired pressure response. The desired pressure response can be a step function. A control algorithm is applied to modify the throttle preset parameter step to step of the time division multiplexed etch process based on the evaluation step. The control algorithm can be closed-loop pressure control which can be a proportional, integral and derivative controller. Finally, the substrate is removed from the chamber. 
     Another feature of the present invention is to provide a method of controlling process execution during an etch process. The method comprising the following steps. A substrate is placed in the chamber. A time division multiplexed etch process is executed within the chamber. The time division multiplexed etch process having a step of depositing a passivation layer onto the substrate by means of a plasma and a step of etching a material from the substrate by means of a plasma. A process loop is performed where the deposition step and the etch step are repeated. An execution timing of a subset of recipe parameters in at least one step of the time division multiplexed etch process is modified. The modification to the execution timing can be an initiation of set point transition or a set point duration. The modification of the execution timing can change the execution order of at least two process variables. Finally, the substrate is removed from the chamber. 
     Yet another feature of the present invention is to provide a method of controlling pressure in a chamber during an etch process. The method comprising the following steps. A substrate is placed in the chamber. A time division multiplexed etch process is executed within the chamber. The time division multiplexed etch process having a step of depositing a passivation layer onto the substrate by means of a plasma and a step of etching a material from the substrate by means of a plasma. A process loop is performed where the deposition step and the etch step are repeated. A throttle valve is controlled through a throttle preset parameter within at least one step of the time division multiplexed etch process to ensure that the chamber pressure does not overshoot or undershoot the desired operating level while minimizing the time required to reach the set point value. The preset parameter can be a throttle position or a preset hold time duration. A pressure response of the step is evaluated and compared to a desired pressure response. The desired pressure response can be a step function. A first control algorithm is applied to modify the throttle preset parameter step to step of the time division multiplexed etch process based on the evaluation step. The first control algorithm can be a first closed-loop pressure control which can be a first proportional, integral and derivative controller. Then, the throttle valve is repositioned based on a second control algorithm from step-to-step of the time division multiplexed etch process. The second control algorithm can be a second closed-loop pressure control which can be a second proportional, integral and derivative controller. Finally, the substrate is removed from the chamber. 
     The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing the major assemblies of a plasma processing machine; 
         FIG. 2  is a graph of the desired pressure versus time response for a two step TDM process operated in pressure control mode; 
         FIG. 3  is a graph of the prior art pressure versus time response for a two step TDM process operated in a pressure control mode; 
         FIG. 4  is a graph of the prior art pressure versus time response for a TDM process operated in position control mode; 
         FIG. 5  is a graph of the prior art pressure versus time response for a TDM process operated in position control mode over longer time scales; 
         FIG. 6  is a graph explaining the control system of an embodiment of the present invention; 
         FIG. 7   a  is a block diagram explaining the control system of an embodiment of the present invention; 
         FIG. 7   b  is a continuation of the block diagram of  FIG. 7   a  explaining the control system of an embodiment of the present invention; 
         FIG. 8  is a graph of pressure versus time for experimental examples when the process control method of the present invention is implemented for various input values; 
         FIG. 9  is a graph showing optimization of one of the input values of the present invention; 
         FIG. 10  is a graph of pressure versus time for experimental examples when the process control method of the present invention is implemented for various input values; and 
         FIG. 11  is a graph of pressure versus time for experimental examples when the process control method of the present invention is implemented where etch hold times are held at 0.25 seconds and deposition hold times are held at 0.40 seconds with a constant position offset for the etch step of α=0.88 and a constant position offset for the deposition step of β=1.25. 
         FIG. 12  is a block diagram explaining the control system of another embodiment of the present invention; 
         FIG. 13  is a graph of pressure versus time for experimental examples when the process control method of the present invention is implemented for various input values; 
         FIG. 14  is a graph of pressure versus time re-plotting the data of  FIG. 13 ; 
         FIG. 15  is a graph of the prior art pressure versus time response for a TDM process operated in throttle valve position control mode; 
         FIG. 16   a  is a block diagram explaining the control system of another embodiment of the present invention; 
         FIG. 16   b  is a continuation of the block diagram of  FIG. 16   a  explaining the control system of another embodiment of the present invention; 
         FIG. 17  is a graph of pressure versus time illustrating the process control method of the present invention as shown in  FIGS. 16   a  and  16   b  is implemented for various input values; 
         FIG. 18  is a graph of pressure versus time for experimental examples when the process control method of the present invention as shown in  FIGS. 16   a  and  16   b  is implemented for various input values; 
         FIG. 19  is a block diagram explaining the control system of another embodiment of the present invention; 
         FIG. 20  is a graph of hold time versus TDM iteration for experimental examples when the process control method of the present invention as shown in  FIG. 19  is implemented for various input values; 
         FIG. 21  is a graph of pressure versus time for the data in  FIG. 20 ; 
         FIG. 22  is a graph of pressure versus time for the data in  FIG. 20  at a later time than the data in  FIG. 21 ; 
         FIG. 23  shows the set point and output responses for two different variables at a step transition for a two step TDM process; and 
         FIG. 24  shows an example where the set point change has been delayed in relation to the change in set point in order to synchronize the output response. 
     
    
    
     Similar reference characters refer to similar parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     We disclose a means of controlling pressure in a TDM, or any alternating step process, through a “Hold and Release” method. A throttle valve is pre-positioned when a process step is switched to the next process step. A control system is implemented to automatically set the position value at which the throttle valve is pre-positioned. The set position is derived from the throttle valve position in the preceding process steps of the same type. For a pre-determined period of time the throttle valve is held at the set position. After the holding period, the throttle valve is released, and a closed loop feedback control algorithm (e.g., PID loop) is enabled for the throttle valve to regulate the pressure in a vacuum chamber in the pressure control mode. The control system and method are disclosed. 
     A plasma etching system according to the present invention is shown in  FIG. 1 . In an ICP reactor, a RF generator  100  delivers power to a coil  105  in the upper part of a reaction chamber  110 . This power is transmitted into one or more process gases that are introduced through a gas inlet (not shown) in order to ionize the process gas or gases and form a plasma  120 . A second RF generator  115  delivers power to a wafer support  130  so as to induce a DC bias on a wafer  125 , thereby controlling the direction and energy of ion bombardment to the surface of the wafer  125 . An evacuation system continuously removes the gaseous species (i.e., unreacted gases, volatile by-products, etc.) from the reaction chamber  110  through an exhaust manifold  150 . The pressure in the reaction chamber  110  is regulated through a throttle valve  145 . The throttle valve  145  is operated by a throttle valve controller  140 . The reaction chamber  110  pressure is measured by a manometer  135 . The output signal of the manometer  135  is fed as an input to the throttle valve controller  140 . 
       FIG. 2  illustrates the desired pressure response  200  of multiple loops  225  for a two step TDM process. The pressure set point  230  for the first step  205  requires a different throttle valve position than the pressure set point  235  for the second step  210 . A quick pressure response is desired in a TDM process due to the rapid rise time  215  between steps in conjunction with minimal deviation from set point  220  during the process steps. 
       FIG. 3  graphically illustrates a prior art solution with the throttle valve controller in pressure control mode. This figure shows a plot of pressure response  300  versus time with a corresponding throttle valve position  305  for a two step TDM process. During a TDM process, this control method results in pressure overshoot  325  from the recipe specified set point  320 . The pressure control performance degrades further as the TDM step  310  duration decreases. In addition, the corresponding throttle valve position  315  never realizes a steady state position. 
       FIG. 4  graphically illustrates an alternative prior art solution with the throttle valve controller in position control mode. This figure shows a plot of pressure response  400  versus time with a corresponding throttle valve position  405  for a two step TDM process. This control method uses throttle valve position set points  410  &amp;  415  to drive the throttle valve to set positions during the TDM process. In this example, a delay in pressure response  420  relative to position set point change is noticed and the desired pressure  402  is never achieved. 
       FIG. 5  graphically illustrates yet another problem encountered when position control mode is used to control reaction chamber pressure. This figure shows a plot of pressure response versus time for a two step TDM process with a fixed throttle valve position using two different gas flows in the corresponding steps. Pressure drift  500  over a long process run (100&#39;s of iterations) is observed. This pressure drift  500  may be due to the temperature increase in the reaction chamber wall. Consequently, additional measures must be taken in order to maintain process performance reliability and repeatability. 
       FIG. 6  shows a graph of a pressure response  605  versus time with a corresponding throttle valve position  600  for a two step TDM process. As demonstrated earlier, pressure control during the transition between process steps has proven to be difficult as pressure overshoot may occur. According to one embodiment of the present invention, position control mode is applied in the first segment  630  of a step  610 . The throttle valve is pre-positioned and held at a set position  635  that is derived from the throttle valve position  620  from the previous execution of the step. After the first segment  630 , the throttle valve is released from the position control mode. At that point a closed loop feed back control algorithm is enabled so that pressure control mode is applied for the remainder of the step  610 . 
     After the step  610  is completed, the process step is switched to a next step  615 . During the first segment  650  of this step  615  the throttle valve is switched to position mode. The throttle valve is held at another set position  655  that is derived from the known throttle valve position  640  of the previous execution of that step. Position control mode is applied for a set period  650  and the throttle valve is held at the set position  655  for this entire period. After the holding period  650  is expired, the throttle valve is released and a pressure control mode is applied for the remainder of the step  615  by enabling a closed loop pressure feedback control algorithm for the throttle valve. Alternatively, in the step described above, the throttle valve can be pre-positioned at a position that is derived from the average value of the throttle valve position measured in a number of previous steps of the same kind. This has the advantage of smoothing out step to step variations. 
     In many TDM processes, it is beneficial to assign different time lengths for the holding periods  630 ,  650  (see  FIG. 6 ). It is also beneficial to adjust the pre-positions  635 ,  655  flexibly—not just taking the throttle valve&#39;s last position from the previous process steps. According to another embodiment of the invention, the holding periods in the deposition step and the etch step are independently determined, and the pre-position values for the throttle valve in the holding periods are independently adjusted. One method of deriving an adjusted value for the pre-positions  635 ,  655  is to apply a multiplier to the throttle valve&#39;s last position from the previous execution of that step. As shown in  FIG. 6 , this multiplier will result in an offset  625 ,  645  of the pre-position values  635 ,  655  from the previous step value position  620 ,  640 . In this manner, the pre-positioned throttle valve position can be offset either greater than or less than the position from the previous step. 
     A block diagram illustration for this embodiment is shown in  FIGS. 7   a  and  7   b  for a two-step TDM silicon etch process. In  FIG. 7   a , the holding period is denoted as “t etch hold ”  630  and as “t dep hold ”  650 . The step time periods are denoted as “t etch ”  610  and as “t dep ”  615 . These time lengths can be part of the process recipe at the beginning of the execution of the process. Further more, the throttle valve position from the previous step is denoted as “Etch Position”  620  and as “Dep Position”  640 . The pre-position for holding the throttle valve in the etch period  630  is taken from a preceding etch step and adjusted by a factor of α. Likewise, the pre-position for holding the throttle valve in the deposition period  650  is taken from a preceding deposition step and adjusted by a factor of β. The values of α and β can be set manually in the process recipe or automatically by a feedback control loop (e.g., PID) that measures the pressure and uses that information to adjust the pre-position value to minimize overshoot and minimize the time to reach setpoint as the process proceeds. The parameters α and β typically have a value between 0.5 and 2.0 which translate to 50 percent to 200 percent of the prior position. For example, in the case where α and β are set to 1.0, the invention will use the throttle valve position value  620  from the previous execution of the etch step as the pre-position value during the etch hold period. 
     It will be obvious to one skilled in the art that the pressure control scheme need not be applied to all steps within a TDM process loop. Setting the values of the etch hold length period  630  to zero allows the method to revert to the prior art method of closed loop pressure feedback control. Similarly, in the case where α and β are set to unity and the length of the etch hold period  630  is set to the etch step time  610  allowing the method to revert to the prior art method of position control mode (open loop pressure control mode). 
     PRESSURE CONTROL EXAMPLES 
     For any step in a TDM process using the current invention, two parameters need to be specified for the throttle valve; i.e., the duration of the hold time, and the magnitude of the offset. 
       FIG. 8  graphically exhibits experimental examples when the process control method of the present invention is implemented. The effect of various length of etch holding periods are displayed. As can be seen, if the etch holding period is less than about 0.1 seconds long, the pressure overshoot occurs during the deposition-to-etch transition. If the holding period is about 0.2 to 0.3 seconds long the overshoot is nearly eliminated and the deviation from the pressure set point  805  is minimized. As the etch holding time gets longer, the pressure overshoot reappears in the scope of the experiment. This result demonstrates that the hold and release method of the present invention indeed improves pressure control capability significantly. 
     In another embodiment of the invention, the pre-position hold time can be automatically adjusted as the process proceeds to minimize set point overshoot.  FIG. 9  graphs the pressure set point overshoot versus the pre-position hold time from the data of  FIG. 8  for a two step TDM silicon etch process. A feedback control loop (e.g., PID) that measures the pressure overshoot uses that information to adjust the pre-position hold time to minimize overshoot as the process proceeds. Likewise a feedback control loop (e.g., PID) that measures the time to reach setpoint can use that information to adjust the pre-position hold time so that the time to reach setpoint is minimized as the process proceeds. 
       FIG. 10  graphically exhibits experimental examples when the process control method of the present invention is implemented. The effect of various values of pre-positioning offsets for a fixed duration are displayed. At a negative 5% etch hold position adjust (α=0.95) the pressure overshoot is minimized. This result demonstrates that the hold and release method of the present invention indeed improves pressure control capability significantly. 
     It will be apparent to one skilled in the art that the described embodiments can be applied to multi-step looping processes that contain two or more process steps per loop. The invention can also be applied to looped processes where the pressure set-point or other recipe specified step parameters are changed within a loop during the course of the process, (e.g., morphed TDM processes). 
     It is important to note that the invention does not require a hold time and position offset for each step type within the alternating process. Another embodiment of the invention would include introducing a position hold time for at least one of the step types within a TDM process. 
       FIG. 11  graphically presents an example in which optimized control of pressure in a two step TDM silicon etch process is attempted. In  FIG. 11 , “t etch hold ” is 0.25 seconds, “t dep hold ” is 0.4 seconds, α=0.88 and β=1.25. Compared with previous examples (see  FIGS. 3 ,  4  and  5 ), the resultant pressure profile during the cyclical TDM process operation is significantly improved as it is nearly “squared”. The pressure approaches the set point values more rapidly and pressure overshoot and undershoot are almost eliminated. 
       FIG. 12  shows a block diagram illustration of yet another embodiment of the invention. This embodiment addresses the short coming of throttle valve position control through the application of a closed loop pressure control algorithm from step-to-step while maintaining throttle valve position control (open loop pressure control) within a TDM step. The embodiment is described below as applied to a two step TDM process. The method may also be applied to TDM processes that have more than two steps within a loop. Similarly, the method can be applied to any subset of steps within a TDM process. 
     Once the TDM process parameters have been loaded, initial values must be determined to achieve the desired process pressure set point. These initial values can be determined in a number of ways (e.g., offline calibration experiments prior to running the process, or performing the first few (or one) loop of the TDM process using the throttle valve in closed loop pressure control and using that value as a starting point for subsequent steps). 
     For the example two step TDM case, once the throttle valve value has been initialized, the process executes the first step within the loop (e.g., a deposition step). During the execution of this step, the throttle valve is set to position mode and directed to move to the initialized position. The valve is held at this position for the remainder of the step. 
     During the step, the pressure that corresponds to the position set point is recorded and compared to the process requested pressure set point. This information is then processed by a closed loop control algorithm (e.g., PID, feed forward, etc.) which results in an improved throttle valve position for the next occurrence of that step type within the loop. 
     The example above illustrates the throttle valve being held at a constant value (position) within a step. The throttle position within a step may be varied during the course of the step as a function of step time. This function can be non-linear. 
     The recorded step pressure that corresponds to a position set point input may be determined in a number of ways (e.g., derived from all or some subset of pressure readings from the step). In particular, it is useful to use a sample subset near the end of the step as these values can be more indicative of the steady state pressure for that step. These values can be smoothed or filtered within a step or over loop iterations (step-to-step) to improve the control of the closed loop algorithm. 
       FIG. 13  graphically exhibits an experimental example when the step-to-step pressure control method is implemented. The TDM process consisted of three steps, including an etch step with an associated pressure set point  1305 , and a deposition step with a different pressure set point  1310 . The early region of the etch pressure response  1315  shows the actual pressure is less than the pressure set point  1305 . A closed loop feed back control algorithm (e.g., PID, etc.) was used to adjust the etch throttle position  1330  step-to-step to eliminate the etch pressure offset. Similarly, the early region  1320  of the deposition pressure response shows the actual pressure is less than the process specified set point. A second closed loop feedback control algorithm (e.g., PID) was used to adjust the deposition throttle position  1325  step-to-step to eliminate the deposition pressure offset. In this example, the throttle position for at least one step was adjusted from loop-to-loop by holding the throttle position constant for a step within a loop. In another embodiment of the invention, the throttle position can be varied within a loop in a predetermined manner, sampled at some point within the step, and a closed loop control algorithm applied to the throttle position from loop to loop. 
       FIG. 14  re-plots the data from  FIG. 13  for the deposition step of the TDM silicon etch process from loop to loop. The lower curve  1405  shows the pressure error as a function of loop number. The pressure error in this case was the difference between the actual pressure recorded at the end of the deposition step and the recipe specified pressure set point. The throttle valve position  1410  is controlled by a feed back loop in response to the value of the pressure error function  1405 . In this example, the recipe specified set point for the deposition step was held constant from loop to loop. It is known in the art to change the pressure set point from loop to loop to maintain profile control. Changing the pressure set point from loop-to-loop for a particular step will induce an error signal for that step which in turn will be corrected by the closed loop controller. 
       FIG. 15  shows a plot of the prior art method of throttle valve position control for a two step TDM process (similar to  FIG. 4 ). While the position control method may be used to prevent the pressure response  1505  from exceeding the pressure set point  1510 , there is a significant period of time  1515  where the pressure response  1505  is significantly different from the recipe specified set point  1510 . 
       FIG. 16  is a block diagram of yet another embodiment of the current invention. This method overcomes the shortcomings of the prior art shown in  FIG. 15 . The block diagram illustrates the invention as applied to a two step TDM silicon etch process. Note that the same method can be applied to a TDM process with two or more steps. In addition, the method may be applied to any subset of steps within a TDM process. 
       FIG. 17  is a graphical representation of the method of  FIG. 16 . The method makes the transition between two different set points shorter by over driving the input variable at the beginning of the transition. In the case of pressure control, the throttle valve position is the input variable that is driven past the recipe specified set point for a period of time, before being released to the recipe specified value in order to optimize the pressure response  1705 . 
     For pressure control of a two step TDM silicon etch process consisting of a deposition step  1710  and an etch step  1715 , the method requires an etch preset throttle valve position  1730 , an etch preset hold time  1740 , an etch throttle valve position  1745 , a deposition preset throttle valve position  1725  a deposition preset hold time  1720 , and a deposition throttle valve position  1735 . The etch preset throttle valve position  1730  is the throttle position during the beginning of the etch step  1715 . The etch preset hold time  1740  is the length of time from the beginning of the etch step to hold the etch preset throttle valve position  1730 . The etch throttle valve position  1745  is the throttle position that corresponds to the recipe specified pressure or throttle position for the etch step. 
     Similarly, the deposition preset throttle valve position  1725  is the throttle position during the beginning of the deposition step  1710 . The deposition preset hold time  1720  is the length of time from the beginning of the deposition step  1710  to hold the deposition preset throttle valve position  1725 . The deposition throttle valve position  1735  is the throttle position that corresponds to the recipe specified pressure or throttle position for the deposition step. 
     At the beginning of the etch step  1715  of a TDM process, the throttle valve is set to the etch preset position  1730 . This position is held until the etch preset hold time  1740  expires. The throttle position is then moved to the etch throttle position  1745 . Similarly, at the beginning of a deposition step  1710  of a TDM process, the throttle valve is set to the deposition preset position  1725 . This position is held until the deposition preset hold time  1720  expires. The throttle position is then moved to the deposition throttle position  1735 . Note, instead of maintaining the throttle position at a constant position during the hold duration, the valve position may be varied in a predetermined manner as a function of time. 
     In order to shorten the duration of the pressure response  1705  to a set point transition, the throttle position preset values are chosen to “overshoot” the set point position. For example, if the throttle position of the previous step is less than the throttle position corresponding to the current step&#39;s throttle position or pressure set point, the preset throttle position for the current step will be greater than the throttle position set point for the current step. Similarly, if the throttle position of the previous step is greater than the throttle position corresponding to the current step&#39;s throttle position or pressure set point, the preset throttle position for the current step will be less than the throttle position set point for the current step. 
     The preset throttle positions (both deposition and etch) can be determined by a number of methods, including characterization experiments prior to running the TDM process, or through the use of a closed loop feed back algorithm that adjusts the preset throttle position value during the process. The preset throttle position may also be derived from the recipe specified position (e.g., applying a multiplier and/or offset to the recipe specified throttle position). 
     In the example above, the preset position is held constant for the time period of the position preset hold time. In another embodiment of the invention, the preset position can vary during the preset hold time (e.g., preset position is a function of time, monotonically increase or decrease, etc). 
     The preset hold times can also be determined by a number of methods, including characterization experiments prior to running the TDM process, or through the use of a closed loop feed back algorithm that adjusts the preset time value during the process. 
       FIG. 18  shows an example of the method from  FIG. 17  applied to a two step TDM silicon etch process. Note, the duration of the transition  1815  for the pressure response  1805  to reach set point  1810  is significantly shorter than the prior art method of position control (compare the duration  1515  in  FIG. 15  to the duration  1815  in  FIG. 18 ). The process parameters for  FIGS. 15 &amp; 18  were identical. The process was a two step TDM silicon etch process. 
                                                                   Deposition   Etch                                        SF 6     sccm   &lt;5   635           C 4 F 8     sccm   150   &lt;5           Pressure   mtorr   30   80           Time   seconds   2   2                        
The pressure control parameters for the data in  FIG. 18  are shown below:
 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Deposition 
                 Etch 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Throttle Hold Position 
                 % 
                 20% 
                 20% 
               
               
                   
                 Hold Time 
                 ms 
                 1000 
                 800 
               
               
                   
                   
               
             
          
         
       
     
     The throttle hold position for the data shown in  FIG. 18  was derived from the recipe specified set point for that step. In both the deposition and etch steps, the throttle hold position was set for a 20% overshoot of the recipe specified throttle position set point. 
     In yet another embodiment of the current invention, the pressure control parameters (throttle valve hold position and throttle valve hold time) can be optimized as the process is executed through the use of a closed loop control algorithm.  FIG. 19  shows a block diagram outlining this method for a TDM process containing at least one process step that utilizes a throttle valve preset to improve pressure control. The method requires initial values for the throttle valve hold position and throttle valve hold time. These values may be determined from previous experiments or process history. As the process proceeds, the pressure response is compared to the recipe specified set point generating an error value. This error value is input to the closed loop controller which outputs an improved throttle hold position and or throttle hold time. 
     In another embodiment of the invention, the initial throttle valve hold position is held constant throughout the process. This simplifies the control architecture by allowing the closed loop controller to adjust only one variable, the position hold time, to optimize the pressure overshoot response. 
     Similarly, in another embodiment, the throttle hold time is held constant throughout the TDM process. This allows the closed loop controller to adjust only one variable, the throttle valve hold position, to optimize the pressure overshoot response. 
     It is important to note that if more than one TDM process step utilizes the method, each step may have its own closed loop control algorithms. Furthermore, if multiple closed loop control algorithms are used, each step may have a different algorithm. 
       FIG. 20  is a graphical representation of experimental data from the method shown in  FIG. 19 . In this experiment, a two step TDM silicon etch process was used. The parameters for the etch process are shown below: 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Deposition 
                 Etch 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SF 6   
                 sccm 
                 &lt;5 
                 635 
               
               
                   
                 C 4 F 8   
                 sccm 
                 150 
                 &lt;5 
               
               
                   
                 Pressure 
                 mtorr 
                 30 
                 80 
               
               
                   
                 Time 
                 seconds 
                 2 
                 2 
               
               
                   
                   
               
             
          
         
       
     
     In these experiments, the hold time value was adjusted by the control algorithm on a step to step basis.  FIG. 20  shows the applied deposition hold time as a function of TDM iteration. Initially, a zero value hold time was applied to the deposition step in order to illustrate the method. The throttle hold position was derived from the recipe specified throttle position in the step and set to overshoot the throttle set point by 12%. The graph shows the Deposition Hold time reaching a steady state value of 520 ms within 65 TDM loops (iterations). 
     The initial value for the hold time in a step can be set a number of ways, including values derived from off-line experiments or previous processes. The initial value can also be determined during the first few cycles by maintaining the throttle hold position until the actual pressure reaches the pressure set point, recording the corresponding time and using that hold time for the next occurrence. 
     The experimental data in  FIG. 20  was generated by applying a throttle hold position derived from the position or pressure set point and controlling the shape of the pressure response by varying the throttle valve hold time at that position. Similarly control algorithms could be used that use a recipe derived hold time or correct both throttle position hold time and hold position using a closed loop control algorithm. 
       FIG. 21  shows the pressure set point  2105 , throttle valve position  2110 , and pressure response  2115  corresponding to the data in  FIG. 20 . Due to the initial hold time of 0 msec, reducing to pure position control mode the pressure response  2115  reaches the pressure set point  2105  at the end of the step  2120 . 
       FIG. 22  shows the pressure response for the same process shown in  FIG. 21  at a later time. At this point the control algorithm has determined stable values for both the deposition  2220  and etch  2225  throttle position hold times. The pressure response  2215  more closely follows the pressure set point  2205  (compare to pressure set point  2105  and pressure response  2115  in  FIG. 21 ). 
     Note, the method of  FIG. 12  may be combined with the method of  FIGS. 16 and 19  to obtain a pressure control algorithm that results in closed loop pressure control step-to-step, open loop pressure control within a step, with a second control loop that decreases the time for the pressure response to reach the set point while minimizing pressure overshoot. 
     TDM processes consist of a series of process loops and steps. It is common for at least one recipe parameter set point to change from step-to-step. Typically during the transition from one step to the next, all set point changes are executed simultaneously at the beginning of the new step. Due to variable response times of the process outputs, the output responses will not be synchronized. As the timing of a TDM process is reduced, synchronization errors that were only a small fraction of the total step time become significant. These errors can arise from timing offsets incurred in passing the set points or timing errors introduced at the component level through the introduction of a dead time between when the set point information is received and the output responds.  FIG. 23  shows the set point and output responses for two different parameters at a step transition for a two step TDM process. While the output B response  2325  responds immediately after a set point change  2330 , the output of response A  2310  shows a significant dead time  2315  following a change in the set point  2305 . This results in a synchronization error  2320  between the output response of A and B. 
     Once the lag  2315  has been characterized, it is possible to correct it by applying a timing modification to (lagging) the set point of the leading response in order to resynchronize the output responses.  FIG. 24  shows an example where the set point change for output B  2430  has been delayed for duration  2420  in relation to the change in set point A  2405  in order to synchronize the output B response  2425  with the output A response  2410 . Conversely, it is also possible to correct the lag  2315  by leading the set point of the lagging response. 
     The timing modification can be applied to any subset of the recipe parameters that undergo a set point change during a step transition. Note that the timing of each recipe parameter can be modified either independently, or as a function of the timing modification of another parameter. 
     Note, in some cases it may be desirable to purposefully introduce a non-synchronization between the output responses. For example, when two recipe parameters have significantly different output response times, it can be beneficial to delay the set point change of the parameter with the faster response. 
     It is also important to note that for a TDM process that the set point changes that occur during the transition from one step to the next need not be simultaneous. In another embodiment, the method shown in  FIG. 23  can be used to maintain or modify the timing relationship between recipe parameters from step to step in a TDM process that contains recipe parameter set points that do not execute simultaneously at a step transition. In another embodiment, the system can monitor the timing performance of at least two recipe parameters (e.g., pressure, gas flow, RF power, etc.) and adjust the synchronization using a closed loop control algorithm during the TDM process. 
     The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. 
     Now that the invention has been described,