Abstract:
A plasma processing system and plasma monitor therefor is provided in which a plasma monitor housing is coupled to a plasma processing chamber such that a line-of-sight monitoring path extends through the housing to an optical sensor outside of a window. A separate reference signal path extends through the housing from a reference light source on one side of the housing to a reference optical sensor on the other side of the housing. The housing is configured so that deposits from the chamber affect all of the windows equally, and to retard the flow of contaminating film forming material onto the windows, using, for example, baffles, gas counterflow, and a balanced radial-leg housing. A processor uses the reference signal to determine window contamination and compensate for signal attenuation along the monitoring path caused by window coating, in the making of a measurement of plasma emissions. The measurement can be used by the processing system to control the plasma.

Description:
This application is related to U.S. patent application Ser. No. 11/082,223, filed Mar. 17, 2005, which is a continuation of International Application No. PCT/US03/26208, filed Aug. 21, 2003, which claims priority to U.S. Provisional Application No. 60/414,348, filed Sep. 30, 2002, the contents of all of which are hereby expressly incorporated herein by reference. 
   This application is also related to U.S. patent application Ser. No. 11/082,246, filed Mar. 17, 2005, and abandoned Mar. 6, 2008, which is a continuation of International Application No. PCT/US03/30051, filed Sep. 25, 2003, which claims priority to U.S. Provisional Application No. 60/414,349, filed Sep. 30, 2002, the contents of all of which are hereby expressly incorporated herein by reference. 
   FIELD OF INVENTION 
   This invention relates to plasma processing, and particularly to plasma monitoring of the plasma in vacuum chambers of plasma processing systems. 
   BACKGROUND OF THE INVENTION 
   Plasma processes are widely used for applying and etching thin films, particularly in the manufacture of semiconductors where micron and sub-micron thick layers of conductor, semi-conductor and insulator material are deposited, stacked and etched, in repeated cycles. In such manufacture, the properties of the plasma must be carefully controlled to maintain the quality, uniformity and consistency of the processed films. Precise control of the plasmas requires exact knowledge of the state of the plasma being controlled. 
   Optical methods are widely used to monitor the properties of plasmas and states and courses of plasma processes, particularly in the semiconductor manufacture industry. Observable radiation emitted from a plasma contains much information regarding the nature of the plasma present in a vacuum chamber that can be used by an operator or a programmed controller in the control of the plasma process. For the information to be used, the radiation emitted from the plasma must be accurately measured. 
   Optical emission spectroscopy is a measurement technique used in plasma processes to measure a full spectrum of radiation emitted from a plasma over a given range of wavelengths, particularly those that include visible and near visible light. This measurement technique involves some optical element such as a transparent lens or window through which spectroscopy sensors can observe the plasma and read emissions from the plasma for measurement and analysis. A light transmissive window is typically used to separate the plasma process chamber from the optical detection system and to allow the spectroscopy equipment to be situated in an ordinary atmosphere environment away from the vacuum environment within the chamber containing the plasma. 
   In the course of plasma vacuum processes that involve deposition and etching, vapors of various coating materials, reactants, etch byproducts, and other materials tend to fill the vacuum space within the chamber. Eventual deposition of coatings on the view windows through which optical detectors observe the plasma is common. Such deposits affect the window transparency and the radiation being monitored, and can affect the accuracy of the optical measurements if the change in window transparency is not considered. 
   When the windows through which optical measurements of plasma processes become clouded with film, they can and ultimately must be cleaned or replaced for the plasma process to continue. Such window maintenance involves processing system down-time, which is expensive. 
   In United States Patent Application 2005/0173375, a related application identified above, an apparatus and method for use of an optical system with a plasma processing system is disclosed. In this published application, the disclosed system and method are provided in conjunction with a plasma processing system, and the system is constructed and arranged to detect a plasma process condition through the window as well as the transmission condition of the window. The method includes detecting an optical emission from the plasma processing region and monitoring contamination of the window through which the plasma is being observed by the optical system. 
   In addition to the monitoring the contamination of the window through which a plasma is being observed, the simplicity and reliability of the instrumentation, the length of the time between window servicings, and the accuracy and usefulness of the monitored information, affect the quality and efficiency of the plasma process. Accordingly, constant improvement is needed in optical monitoring and control systems for plasma processes, particularly in semiconductor manufacture. 
   SUMMARY OF THE INVENTION 
   An objective of the present invention is to improve the simplicity and reliability of the instrumentation, the length of the time between window servicing, and the accuracy and usefulness, of optical emission spectroscopy in the monitoring of plasma processes, particularly those used in the manufacture of semiconductors and other related products. 
   A more particular objective of the present invention is to provide a method and apparatus for monitoring the conditions of a plasma in a vacuum processing chamber while also determining the condition of the view window through which emissions from the plasma are being measured. 
   A further objective of the invention is to provide for the calibration or correction of a optical emission spectrographic measurement of plasma conditions and for the ability to accurately control a plasma process. 
   According to principles of the present invention, an optical emission spectrography system is provided for monitoring the condition of a plasma in a plasma vacuum processing chamber and having a housing connectable to the chamber such that emitted radiation from the chamber traverses a monitoring signal path from the plasma, into and through the housing, and out a window on the opposite side of the housing to a monitoring signal sensor that measures light from the spectrum emitted from the plasma. A separate reference signal path is provided that also passes through the housing from a reference source on one side to a reference signal sensor outside a separate window on the side of the housing opposite the reference source. 
   In certain embodiments of the invention, a processor receives the signals from both sensors and corrects a measurement of the signal emitted by the plasma with information from the reference signal sensor, to compensate for attenuation of the signals caused by deposits on the windows. The reference signal from the reference signal source is knowable, either from known source settings or preferably by providing a separate sensor at the source. From this knowledge, attenuation of the reference signal received by the reference signal sensor can be determined and appropriately attributable by the processor to the condition of the window through which the reference signal is measured. 
   According to embodiments of the present invention, the system is configured to ensure that the deposits onto the windows through which the plasma emission and reference signals pass are substantially the same. The processor can then use the determined attenuation of the reference signal to adjust the measured emission from the plasma to compensate for the attenuation due to coating on the window through which the monitored emission from the plasma passes. 
   In accordance with the illustrated embodiments of the invention, a set of baffles, wall structure and openings therein are provided within the housing to facilitate the randomization of material propagating from the chamber that can deposit onto the windows. The baffles are arranged in substantially identical configurations within substantially identical legs that radiate from a central cavity and lead to each of the respective windows. The legs contain separate line-of-sight paths for the monitoring and reference signals. 
   Potential deposits on the windows are impeded from flowing toward the windows by provision of counterflow flow of gas through a counterflow tube through which the emissions to be measured pass. The tube extends between the center of the central cavity to an isolation chamber located between the central part of the housing and the plasma chamber. Purge gas is fed into housing proximate the legs and flows through the counterflow tube to the isolation chamber, from which it is exhausted, preferably by a dedicated vacuum pump separate from those used to maintain vacuum in the processing chamber. Additional baffle surfaces along the monitoring signal and counterflow path collect coating material deposits, thereby removing them from the gas within the housing. These and other techniques that are known to limit the propagation of contaminating vapor in a vacuum system can be used in combination with the optical system configuration. 
   The optical system measurement of plasma emission can be used to monitor the process in the chamber and to facilitate the control of deposition, etch, chamber cleaning and other processes within the clamber. The optical system processor and the processing system controller can be coordinated or combined to accomplish such control. 
   These and other objects and advantages of the present invention will be more readily apparent from the following detailed description of illustrated embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE is a diagrammatic representation of a plasma processing apparatus having an optical emission spectroscopy system according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The FIGURE diagrammatically illustrates a plasma processing system  10  of the type used for processing semiconductors and other related substrates. The system  10  includes a vacuum chamber  12  in which is situated a substrate holder  14  on which may be mounted a semiconductor wafer  15  for processing. A plasma source is provided within the chamber  12  to energize a plasma  16  in process gas maintained under vacuum within the chamber  12 . The plasma source can include a pair of electrodes, which can include the holder  14  and a sputtering target or other electrode  18 , or can include some other plasma generating system, for example an inductively coupled plasma (ICP) source that typically includes an RF inductive or capacitive electrode coupled to the plasma from inside or outside of the chamber  12 . A programmed controller  20  is typically provided to maintain reliable and repeatable operation of the apparatus  10  and to precisely control the process. 
   To monitor the condition of the plasma  16 , and to provide the controller  20  with information for control of the plasma and the process performed with the plasma  16  in the chamber  12 , a self-calibrating optical emission spectroscopy (OES) system  30  is provided. The system  30  is mounted to the outside of the chamber  12  and coupled to the inside of the chamber  12  through a gate valve  24 . The valve  24  is situated to provide an observation path for the OES  30 , preferably a straight line-of-sight path to the plasma  16 , represented by arrows  31  and  32  in the FIGURE. The OES includes a plasma measurement device or optical sensor  35  that lies on the path  31 - 32 . The sensor  35  is capable of receiving and measuring a spectrum of emitted radiation from the plasma  16 , or at least a set of selected wavelengths of radiation within a spectrum, that preferably includes the wavelengths of visible light, or selected wavelengths of radiation in or outside of the spectrum of visible light, that contains the information of interest in monitoring the condition and properties of the plasma  16 . 
   The OES system  30  is provided with housing  36  that isolates the vacuum interior thereof, which, during operation, is in communication with the vacuum within the chamber  12  through the gate valve  24 , from the surrounding atmosphere in which the sensor  35  is situated. The sensor  35  is isolated from the plasma environment by a window  38  in a main branch or leg  39  of the housing  36 . The window  38  lies in line with and perpendicular to the path  31 - 32 . A central spherical cavity  40  is provided inside of and at the center of the housing  36  with its center lying on the path  31 - 32 . A pair of holes  41  and  42  are provided in the wall of the cavity  40 , on a diameter thereof that is in line with the path  31 - 32 . Additionally, a series of baffles  43 - 46  are provided on the opposite sides of the cavity  40 , each having a hole therein lying on the path  31 - 32 . 
   A purge gas supply  50  is connected to a space  49  inside of the housing  36 . The supply  50  may contain the same gas as the processing gas within the chamber  12 , a neutral gas or other gas compatible with the process within the chamber. Usually, the active chemicals are a small part of the processing gas, with a neutral gas such as argon making up most of it. Typically a suitable purge gas would be helium or argon, or the same gas as the processing gas or a neutral gas that is a carrier gas component of the processing gas. The purge gas is supplied to the space  49  within housing  36  at a pressure that is at least slightly greater than that within the chamber  12  so that the purge gas propagates generally from the housing  36  toward the chamber  12 , or at least to an isolation chamber  51  within the housing  36  to which an exhaust vacuum pump  52  is coupled through an exhaust shut-off valve  53 . The isolation chamber  51  communicates with the chamber  12  through the hole in baffle  43  and through gate valve  24 , so that process gas from chamber  12  is predominantly exhausted from the isolation chamber  51  through the exhaust pump  52  and a minimum amount of the process gas from the chamber  12 , which contains the substances that would tend to coat and contaminate the window  38 , are minimized. 
   The isolation chamber  51  is separated from the space  49  by the baffle  44 , which is sealed to the wall of the housing  36  at its outer edges and to the wall of the cavity  40  around the edge of the hole in the baffle  44 . In this way, purge gas from the supply  50  passes is injected into the space  49 , propagates generally into the cavity  40  and from there into the isolation chamber  51 . The propagation of gas into the cavity  40  from the space  49  within the housing  36  is through the hole  42  in the wall of cavity  40  and also through additional holes  47  and  48  in the wall of the cavity  40 , which are provided taking reference measurements, as explained below. 
   The housing  36  is provided with side legs or branches  55  and  56 . The insides of the branches  39 ,  55  and  56  are in communication with the space  49  into which the purge gas flows from the source  50 . Leg  55  is a reference source leg while leg  56  is a reference sensor leg. A reference source  60  is provided adjacent the reference source leg  55 , lying on a reference signal path represented by the arrows  61  and  62  that is perpendicular to the path  31 - 32  and intersects the path  31 - 32  at the center of the cavity  40 . The reference source  60  emits a signal along the path  31 - 32 , through a window  67  in the end of the leg  55  of the housing  36 . The holes  47  and  48  in the wall of cavity  40  lie on path  61 - 62 . A reference signal optical sensor  65  is provided adjacent the reference sensor leg  56  on the path  61 - 62 . The sensor  65  is directed toward the reference source  60  to receive emissions from the reference source  60  along the path  61 - 62 , and through a window  68  in the wall of the housing  36  at the end of the leg  56 . A pair of baffles  71  and  72  are positioned in the reference source leg  55  of the housing  36 . The baffles  71 ,  72  have holes at their centers that lie on the path  61 - 62 . Similarly, a pair of baffles  73  and  74  are positioned in the reference sensor leg  56  of the housing  36 . The baffles  73 ,  74  also have holes at their centers that lie on the path  61 - 62 . 
   Additionally, a counterflow tube  75  is provided in alignment with the path  31 - 32 . The interior of the tube  75  is lined with a series of baffles  76 , each having a hole therein aligned on the path  31 - 32 . The outside of the tube  75  is sealed to the edge of the opening  41  in the wall of the cavity  40  and to the edge of the hole in the baffle  44 . The tube  75  has a gas inlet  77  near the center of the cavity  40  and a gas outlet  78  within the isolation chamber  51 . The gas outlet is the inlet for radiation from the plasma that is being measured by the sensor  35  after it travels along path  31 - 32  through the tube  75  and out the gas inlet  77  thereof. 
   An optical calibration sensor  85  is provided adjacent the source  60  to adjust the emissions from the source  60  so that it essentially replicates at least a portion of the spectrum of the normal emission from the plasma  16  that is to be measured by the sensor  35 . The sensors  35 ,  65  and  85  have outputs connected to a processor  80 , which controls the source  60  and communicates with the controller  20  of the system  10 . 
   In time and over the course of extended operation, the window  38  will experience a mild window coating, which can be enough to affect the measurements made by sensor  35  of the emissions from the plasma  16  that propagate along path  31 - 32 . The coating is considered “mild” due to details of the system  30  that have been provided to reduce the rate of coating on all the windows  38 ,  67  and  78 . These details help to ensure that the coatings are the same or similar on all the windows  38 ,  67  and  68 . These details include the provision that all three legs  39 ,  55  and  56  with windows  38 ,  67  and  68 , respectively, of the optical system  30  contain optical baffles, which are baffles  45  and  46  in leg  39 , baffles  71  and  72  in leg  55  and baffles  73  and  74  in leg  56 . These baffles function in conjunction with the respective holes  42 ,  47  and  48  in the wall of the cavity  40  to achieve these goals, which include reducing the flow of reactants to the windows  38 ,  67  and  68 . 
   To impede the flow of process gases to the windows  38 ,  67  and  68 , the inert purge gas, preferably the carrier gas used in the process chamber, is fed into the space  49  of the housing  36  of the optical system  30  with a two-fold goal: to create a flow of gas that will reduce the flow of reactants to the reference and diagnostic parts of the optical system  30 , and to create a volume with many gas collisions that will tend to ensure that the reactant gas flow from the plasma  16  to all parts of the optical system  30  will be the same or similar. 
   The portion of the housing  36  that connects the optical system  30  to the process chamber  12  contains a baffle with the long slender counter-flow tube  75 , preferably having a diameter of about ⅛ inch. In the counterflow tube  75 , gas reactants will have to diffuse counter to the purge gas flow in order to reach the windows  38 ,  67  and  68 . The role of the counterflow tube  75  is three-fold: to collimate the light to the detector along the path  31 - 32 , to provide a large surface area on which the process reactants can deposit, and to define flow that will impede the reactants&#39; diffusion to the rest of the optical system. Thus to enhance this deposition, the tube  75  can be heated. It can also be designed to have a large surface area, which is what the baffles  76  within it can provide. 
   In addition, the isolation chamber  51  provided between the baffle plates  43  and  44  in the portion of the housing  36  of the optical system  30  that is connected to the chamber  12  is evacuated by means of the pump  52 . This is preferably a turbo-molecular pump. The role of this pump is to create a large pressure differential for gas flow along the tube  75 . 
   To ensure that reactant gas to all the windows  38 ,  67  and  68  is the same, the inside of the housing  36  of the optical system  30  has to be designed with care. The central space includes a spherical shell that forms the wall of the spherical cavity  40 . The shell has three gas inlet openings  42 ,  47  and  48  for optical access to the windows  38 ,  67  and  68 , plus one gas outlet opening  41  for the counterflow tube  75 . The counterflow tube  75  is located so that its inlet opening  77  is at the center of the spherical cavity  40 . The spherical shape of the cavity  40 , the small diameter of the counterflow tube  75 , and the central location of its inlet opening  77  all facilitate keeping substantially identical the flux of potential coating material to the windows  38 ,  67  and  68 . 
   Further, if after initial calibration tests, the film thickness on the windows is still not the same, compensation for unequal deposition rates on the windows can be made by adjusting the diameters of the holes in the baffles, preferably the baffles  71 - 74  in the reference legs  57  and  58 . 
   The purge gas from source  50  is fed into the space  49  surrounding the spherical shell surrounding cavity  40 . From there it flows into the cavity  40  through the three openings  41 ,  47  and  48 . From the cavity  40 , the gas flows through the counterflow tube  75  to the isolation chamber  51  from which it is exhausted by pump  52 . 
   To further insure that all the windows have the same coating, the windows  38 ,  67  and  68  and the branches of the optical system  30  containing them, should be kept at the same uniform temperature. This can include providing heating pads and temperature control circuitry (not shown) around the optical system  30 . 
   This invention depends on the light attenuation of all the windows to be the same or similar. After many hours of operation, the coating on the baffles becomes thick, and the coating may start to peel. This can result in the flakes falling on the windows. To prevent the flakes from falling onto the windows, the legs of the optical system containing the windows should be in the same horizontal plane. 
   In operation, before the plasma process in chamber  12  begins, the optical system  30  calibrates itself with the processor  80  causing the activation of the reference source  60 . The spectrum emitted from the source  60  is set to approximately that expected to be received from the plasma  16 . The output of the reference source  60  is sensed by the reference calibration sensor  85 , which communicates the reference signal to the processor  80 , which in turn compares the spectrum of the sensed signal to reference data stored at the processor  80  and readjusts a control signal to the source  60 . 
   As a result, a reference signal is emitted from the source  60  along the path  61 - 62 , through both windows  67  and  68 , to the reference signal sensor  65  on the opposite side of the housing  36 . The output of the sensor  65  is communicated to the processor  80 , which interprets the received signal to determine the condition of the windows  67  and  68 , from which it derives the condition of the window  38  through which the emissions from the plasma  16  are to be measured. The fact that the signal along path  61 - 62  passes through two windows, windows  67  and  68 , is taken into account by the processor  80 . 
   When the plasma  16  is ignited, a spectrum is emitted along the path  31 - 32  and received by the sensor  35  through the window  38 . The sensor  35  then delivers a measurement signal to the processor  80 , which compares the spectrum of the measurement signal with stored data to determine the condition of the plasma  16 . In comparing the measurement signal, the processor corrects the measured signal based on the received reference signal from the sensor  65  in order to compensate for the derived condition of the window  38 . The condition of the plasma  16 , so determined, is communicated from the processor  80  to the controller  20  of the processing apparatus  10 , which can control the operation of the apparatus  10  to adjust the plasma  16  or to take other action in response to the determined plasma condition. 
   Although only certain exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.