Abstract:
Disclosed herein are exemplary embodiments of an improved Inductively Coupled Plasma (ICP) chamber which is externally coupleable to a processing chamber to monitor processes gases therefrom. The disclosed ICP chamber design is beneficial because it allows for the porting of reference gases for the purpose of performing actinometry, and/or allows for the introduction of plasma probes into the plasma within the ICP chamber, both of which improve the reliability of process gas concentration determinations. Also disclosed is a processing system for interfacing the ICP chamber to the processing chamber and for controlling both.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisonal of U.S. patent application Ser. No. 10/814,985, filed Mar. 31, 2004, which is incorporated herein by reference in its entirety and to which priority is claimed. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to an improved inductively coupled plasma chamber externally coupleable to a processing chamber for the analysis of process gases.  
       BACKGROUND  
       [0003]     A need exists in the art of semiconductor processing to accurately analyze the components and concentration of process gases used in etching and deposition processes. For example, by analyzing etch byproduct gases in an etching chamber as a function of time, it may be determined when one layer on a semiconductor wafer has been completely etched and another underlying layer of a different composition has started to be etched, a so-called “end point detection” technique. In another example, by analyzing the gases in a deposition chamber, it can be determined whether the deposition chemistry is optimal or perhaps needing adjustment.  
         [0004]     In many cases, analysis of such gases is performed “downstream.”—i.e., at some point along the exhaust line from which gases exit the processing chamber. Such an exemplary system is shown in  FIG. 1 . In this system, a process chamber  10  is ultimately connected to an Inductively Coupled Plasma (ICP) chamber  18  along the chamber&#39;s exhaust line. The ICP chamber  18 , as is known, induces a plasma in the exhaust gases using a low power RF generator  20 . As such, the exhaust gases are ionized (i.e., excited) and eventually relax, thereby emitting photons (i.e., electromagnetic radiation or light). A given gaseous species will emit photons at certain known wavelengths, and therefore by looking at the location of peaks in the emission spectrum for the exhaust gases and their magnitudes, the component species in the exhaust gases and their quantities can be inferred.  
         [0005]     In the system of  FIG. 1 , such optical analysis is accomplished by coupling some portion of the photons in the excited plasma through an optical window in the ICP chamber  18  to an optical fiber  26 , which carries the photons to an optical spectrometer  24  for analysis. A computer  22  is used to assess the location and intensity of peaks in the emission spectra provided by the optical spectrometer  24 , and if necessary to control the process chamber  10  in response. For example, if the computer  22  determines that the species have changed appreciably, it may determine that an end point has been reached in an etch and that the process chamber  10  should be turned off or modified accordingly (e.g., by introducing new etchant gases or by providing purging gases to the process chamber).  
         [0006]     One example of an ICP chamber  18  usable within the context of  FIG. 1  is the Candela™ Downstream Plasma Monitoring System, produced by Lightwind Corporation of San Francisco, Calif., which is illustrated in the following article, submitted herewith, and which is incorporated herein by reference: Bladimiro Ruiz, Jr. &amp; Herbert E. Litvak, “Investigation of Silicon Trench Etch Chemistry Using a Downstream Chemical Monitor,” 4th AVS Int&#39;l Conference on Microelectronics and Interfaces (2003).  
         [0007]     Traditional ICP chambers  18 , however, are not optimal and are potentially subject to providing an erroneous optical analysis of the processing gases, in no small part because factors other than gas concentrations can affect the magnitudes of spectral peaks of the analyzed gases. For example, the “temperature” of the excited electrons in the plasma, indicative of the electron&#39;s kinetic energy, will also affect spectral peak magnitudes. Electron temperature is ultimately affected by factors other than gas concentrations, such as variations in pressure. Thus, if pressure inadvertently increases, the electron temperature may decrease, which can influence the relative magnitudes of individual peak intensities. Absent knowledge of the decrease in electron temperature, the system of  FIG. 1  might erroneously conclude that the relative concentrations of gases had changed when in fact they had not.  
         [0008]     In short, knowledge of electron temperature, or similar variables, can improve the accuracy of the analysis of the composition and quantities of gases present in a sample, as is well known. One such means for measuring electron temperature is the use of a probe (e.g., a Langmuir probe). Such probes come in many forms, but in one embodiment, shown in  FIG. 2 , the probe  30  consists of a metal wire  34  (usually tungsten or platinum) which has an exposed tip. The bulk of the wire  34  is covered by an insulating material  32 , which is usually ceramic. The surface area of the exposed tip of the wire  34  is known, and the wire is biased using a DC power supply  36 . By placing a positive voltage on the power supply  36 , the electron density in the gas (and hence, its temperature or energy) will register as a current on ammeter  38 . Likewise, by placing a negative voltage on the power supply  36 , the density of positively charged species (i.e., the positively ionized gas components in the plasma, be they atoms or molecules) can similarly be measured, which like electron temperature can also be used to improve the accuracy of the characterization of the gases.  
         [0009]     However, while the use of plasma probes  30  are a known way of characterizing the physics of a plasma, such probe measurements are believed in the prior art to have been taken only within the processing chamber  10  itself, i.e., within a plasma struck in the chamber that processes a semiconductor wafer or other workpiece. Articles disclosing the use of such intra-processing chamber probing techniques can be found in the following articles, all of which are incorporated herein by reference: Freddy Gaboriau et al., “Langmuir Probe Measurements in an Inductively Coupled Plasma: . . . ,” J. Vac. Sci. Technol., Vol. A20(3), pp. 919-27 (May/June 2002); V. Kaeppelin et al., “Ion Energy Distribution Functions and Langmuir Probe Measurements in Low Pressure Argon Discharges,” J. Vac. Sci. Technol., Vol. A20(2), pp. 526-29 (March/April 2002); M. V. Malyshev et al., “Diagnostic Studies of Aluminum Etching in an Inductively Coupled Plasma System: . . . ,” J. Vac. Sci. Technol., Vol. A18(3), pp. 849-59 (May/June 2000); D. M. Manos et al., “Characterization of Laboratory Plasmas With Probes,” J. Vac. Sci. Technol., Vol. A3(3), pp. 1059-66 (May/June 1985); and S. M. Rossnagel et al., “Langmuir Probe Characterization of Magnetron Operation,” J. Vac. Sci. Technol., Vol. A4(3), pp. 1822-25 (May/June 1986). Of course, such intra-processing chamber plasma probing techniques are only useful when the process being run in the chamber  10  is a plasma-based process, such as a plasma-based or -enhanced etch or deposition. (For example, it would have no utility to non-plasma-enhanced chemical vapor deposition (CVD) techniques). In any event, traditional ICP chambers  18  like those disclosed in  FIG. 1  are not believed to have previously incorporated the use of a plasma probe.  
         [0010]     Another technology that can further improve the optical characterization of a plasma is known as actinometry. In actinometry, a gas not otherwise useful in the process (a “reference gas”) is introduced into the plasma at a known rate and in known quantities. A suitable reference gas is preferably inert as concerns the process at issue and has a similar ionization cross-section or excitation cross-section to the gas species that are to be measured, as is known. For example, if Fluorine chemistries are to be characterized, Argon works well as a reference gas. Using Argon, the optical intensities of the peaks in the emission spectrum can be analyzed to more accurately understand the quantities of Fluorine species. If it is seen that the intensity of Argon peaks in the spectrum changes as the intensity of Fluorine peaks change, then it can be inferred that the change in fluorine intensity is not indicative of a change in concentration of the Fluorine, but instead that something else is occurring having the propensity to affect all emission intensities simultaneously (such as a change in electron temperature, a point which can be verified through the use of a plasma probe such as those noted above). However, if the intensity of Fluorine peaks change while the intensity of the Argon peaks stay constant, then it can be accurately inferred that the quantities of Fluorine are in fact changing.  
         [0011]     However, while actinometry is a known way of characterizing the physics of a plasma, actinometry, like plasma probing, is believed in the prior art to have been performed only within the processing chamber  10  itself, i.e., as applied to a plasma struck in the chamber that processes a wafer or other workpiece. Articles disclosing the use of such intra-processing chamber actinometry can be found in the following articles, all of which are incorporated herein by reference: Terry A. Miller, “Optical Emission and Laser-Induced Fluorescence Diagnostics,” J. Vac. Sci. Technol., Vol. A4(3), pp. 1768-72 (May/June 1986); V. M. Donnelly, “A Simple Optical Emission Method for Measuring Percent Dissociations of Feed Gases in Plasmas: . . . ,” J. Vac. Sci. Technol., Vol. A14(3), pp. 1076-87 (May/June 1996); A. D. Kuypers et al., “Emission Spectroscopy and Actinometry in a Magnetized Low Pressure Radio Frequency Discharge,” J. Vac. Sci. Technol., Vol. A8(5), pp. 3736-45 (September/October 1990); and Zhimin Wan et al., “Electron Cyclotron Resonance Plasma Reactor for SiO 2  Etching: . . . ,” J. Vac. Sci. Technol., Vol. A13(4), pp. 2035-43 (July/August 1995). Of course, intra-processing chamber actinometry is only useful when the process being run in the chamber  10  is a plasma-based process. In any event, traditional ICP chambers  18  like those disclosed in  FIG. 1  are not believed to have incorporated the technique of actinometry, despite its ability to improve the accuracy of optical gas analysis.  
         [0012]     Gas analysis chambers coupleable to production processing chambers  10 , such as ICP  18 , are beneficial in a production environment because they can provide some degree of analysis of gas composition and quantity in the processing chamber  10 . However, production processes continue to grow more sophisticated, and monitoring gas-based production processes within strict tolerances has become increasingly critical as the semiconductor industry pushes toward the fabrication of nanometer-sized structures. But traditional externally-coupleable ICP chambers  18  are relatively simple in design and are growing incapable of providing such needed accuracy. At the same time, it is difficult to employ actinometry and/or plasma probing in a production environment. For example, the gases used for actinometry may interfere with the process that is being run in the processing chamber  10 . Likewise, probing creates an impediment and complexity within the processing chamber  10 , and gives rise to problems of an additional contamination source, interference with the established processing plasma, increased maintenance, etc.  
         [0013]     Accordingly, the art would be benefited by the incorporation of additional gas analysis techniques into ICP chambers externally coupleable to the process chamber under analysis to improve the accuracy of optical measurements they provide.  
       SUMMARY  
       [0014]     Disclosed herein are exemplary embodiments of an improved Inductively Coupled Plasma (ICP) chamber which is externally coupleable to a processing chamber to monitor processes gases therefrom. The disclosed ICP chamber design is beneficial because it allows for the porting of reference gases for the purpose of performing actinometry, and/or allows for the introduction of plasma probes into the plasma within the ICP chamber, both of which improve the reliability of process gas concentration determinations. Also disclosed is a processing system for interfacing the ICP chamber to the processing chamber and for controlling both. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which:  
         [0016]      FIG. 1  illustrates a conventional ICP chamber coupled to a processing chamber in a processing system.  
         [0017]      FIG. 2  illustrates a conventional plasma probe.  
         [0018]      FIG. 3  illustrates a cross-sectional view of the improved ICP chamber.  
         [0019]      FIG. 4  illustrates the improved ICP chamber coupled to a processing chamber in a processing system.  
         [0020]      FIG. 5  illustrates the various locations where the improved ICP chamber can be utilized “downstream” from the processing chamber. 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 3  illustrates a cross-sectional view of the improved ICP chamber  50 , while  FIG. 4  illustrates the improved ICP chamber  50  coupled to a processing chamber  10  in the context of a production processing system. The ICP chamber  50  is cylindrical and has two flanges  54   a  and  54   b  on opposite ends, which are preferably KF40 flanges as are well known in the semiconductor processing arts. Flange  54   a  is coupleable using bolts and an O-ring (not shown) to a flange  53  ultimately coupled to the processing chamber  10  whose gases are being monitored. Flange  54   b  is similarly coupleable to a flange  66  which contains an end of the fiber optic cable  26  which sends photons from the ICP chamber  50  to the optical spectrometer  24  for analysis. The inside portions of the flange pieces  54   a ,  54   b  comprise end plates boltable to the circular ends of the cylindrical body of the ICP chamber  50 .  
         [0022]     Internal to the main body of chamber  50  is a cylindrical cavity  61  in which the gases from the processing chamber  10  are excited to form a plasma  58 . These gases are excited (ionized) by applying radio frequency (RF) power via RF generator  20  to coils  64  (shown in cross section), which may constitute a helical coil running along the length of the main body and around a dielectric  60 . The dielectric  60 , such as an aluminum oxide ceramic tube (e.g., alumina), quartz tube, or sapphire tube, etc., lines the cylindrical cavity  61 . The dielectric  60  is held in place by the ends plates of the flanges  54   a ,  54   b , and is sealed thereto using O-rings  95 . Such details concerning the construction of an ICP chamber are known. In any event, a plasma  58  can be excited in the ICP  50  in any number of ways known in the art, such as through the use of parallel plates. In other words, the plasma chamber  50  need not be cylindrical and its plasma cavity need not be cylindrical.  
         [0023]     A port  56  is also present for introducing an actinometric reference gas to the processing gases from the processing chamber  10 , hence improving the accuracy of the spectral analysis. Port  56  is coupled by an input line  83  to a mass flow controller  52  for introducing known quantities of the actinometric reference gas  76  ( FIG. 4 ). Port  56  can be located at many different locations on the ICP chamber  50 , but in a preferred embodiment ports into the flange  54   a  closest to the processing chamber  10 . In this way, gases from the processing chamber  10  will mix or diffuse with the actinometric reference gas (or gases) prior to introduction into the cylindrical cavity  61  where the plasma  58  is formed. However, port  56  may also port into the main body of the ICP chamber  50 , as shown in dotted lines in  FIG. 3 , although this may require milling a small hole into dielectric  60  to accommodate input line  83  which would then need to be pressure sealed. A gasket or line connection suitable to handle the chemicals and pressures at issue can be used to seal the input line  83  to the port  56 , and/or the input line  83  from the mass flow controller  52  may be directly welded to the flange  54   a  or to the main chamber body.  
         [0024]     The processing gases from the processing chamber  10  and the actinometric reference gas from input line  83  will preferably naturally diffuse into the cylindrical cavity  61  of the ICP chamber  50  where they can be excited and optically analyzed. However, an exhaust line  79  coupled to a pump (not shown) can be also used to move this mixture through the cylindrical cavity  61 . If gas used, exhaust line  79  is preferably present on the opposing flange  54   b , as shown in dotted lines in  FIGS. 3 and 4 .  
         [0025]     As best seen in  FIG. 4 , the mass flow controller  52  for the actinometric reference gas is preferably controlled by the computer  22  that controls the processing chamber  10  and receives spectral data from the spectrometer  24 . Accordingly, the computer  22  knows when it is an appropriate time in the process to start actinometric analysis (i.e., by signaling the mass flow controller  52  to introduce the actinometric reference gas), and knows by monitoring the spectral data from the spectrometer  24  whether the process being run in processing chamber  10  needs adjustment. (The mass flow controller  52  and the actinometric reference gas source  76  may be associated with various valves or purge lines as one skilled in the art will understand, which are not shown).  
         [0026]     Accordingly, the computer  22  at an appropriate step during the processing in processing chamber  10  can start actinometric analysis by activating the mass flow controller  52  to introduce the reference gas  76  into the cylindrical cavity  61 . Once actinometry has been performed to some end, e.g., improvement of the accuracy of detection of an etch end point, the computer  22  can shut off the mass flow controller  52  (and can possibly modify the process being run in processing chamber  10  if necessary). For example, assume that the ICP chamber  50  is monitoring a Fluorine-based etch occurring in processing chamber  10 , and that Argon is used as the actinometric reference gas. Suppose the computer  22  upon receipt of spectral information from the spectrometer  24  sees the magnitude of peaks in the Fluorine-based spectra rising, but also see the magnitude of Argon-based peaks rising. Absent the additional information provided by actinometry (namely, spectral information concerning the Argon reference gas), the computer  22  might erroneously conclude that the concentrations of Fluorine was rising, and accordingly might attempt to take corrective action by reducing input Fluorine gas flows to the chamber  10  (i.e., through processing chamber control line  80 ). But with the added benefit of the knowledge of the increase in the Argon peaks, the computer  22  can correlate this increase in Fluorine peaks with an increase in the Argon peaks, and perhaps come to the conclusion that the Fluorine concentration does not need reduction, but instead that the pressure in chamber  10  needs to be increased (or that electron temperature has increased).  
         [0027]     Although not shown, it should be understood that several ports  56  could be used for the introduction of several different actinometric reference gases. This would allow more than one reference gas to be used in the actinometric assessment of the processing gases, or can allow different reference gases to be used at different times in the process. However, the use of a plurality of ports  56  (and their associated mass flow controllers, etc.) are not shown for clarity.  
         [0028]     Also present in the improved ICP chamber  50  are plasma probes  62   a ,  62   b , which are preferably similar to the probe disclosed in  FIG. 2 , but which can comprise other plasma probes known in the art or hereafter developed and useful for analyzing plasmas. As shown, the probes can be introduced into the cylindrical cavity  61  in any number of different ways. For example, probe  62   a  enters the cavity  61  through a port hole in the flange  54   a . Alternatively, probe  62   b  directly enters the cavity  61  through the main body of the chamber  50 . For this orientation, it is important that the probe  62   b  not interfere with the coil  64  used to strike the plasma  58  or other necessary electronics. Additionally, probe  62   b  requires that a small hole be milled into the dielectric  60 . Both probes  62   a  or  62   b  are preferably seated within gaskets suitable to handle the chemicals and pressures at issue.  
         [0029]     It may be beneficial to use more than probe  62 , as the different orientations of the probe ( 62   a  is horizontal;  62   b  is vertical) may provide different data, or because it may be beneficial to probe the plasma  58  at more than one location to improve its accuracy. However, in the simplest embodiment, only one probe  62  is needed. Additionally the probes  62   a  and  62   b  in other embodiments can be made moveable within the cylindrical cavity  61  so that different locations of the plasma  58  can be monitored.  
         [0030]     The probe(s)  62  are accompanied in the processing gas analysis system by the use of a DC voltage power supply  70  and an ammeter  72 , as best shown in  FIG. 4 , and which function similarly to like devices in  FIG. 2 . As incorporated into the system, the computer  22  controls the voltage on voltage supply  70 , and receives current readings from ammeter  72  to better understand the influences (e.g., electron temperature) taking place in the plasma  58 . For example, suppose the probe(s)  62  register an increase in electron temperature, and the optical spectra from spectrometer  24  evidences an increase in the magnitude of the peaks for the processing gases received from processing chamber  10 . Absent knowledge of the increase in electron temperature, computer  22  might erroneously conclude that the concentrations of the gases were rising in the processing chamber  10 , and might attempt to take corrective action by reducing input gas flows to the chamber  10  (i.e., through processing chamber control line  80 ). But with the added benefit of the knowledge of the increase in electron temperature, the computer  22  can correlate this increase with an increase in the peaks, and perhaps come to the conclusion that the input gas flows do not need reduction, but instead that the pressure in chamber  10  needs to be increased.  
         [0031]     In short, modification of traditional ICP chambers coupled externally to the processing chamber to include the ability to perform actinometry and plasma probing offer significant advantages to the analysis of processing gases. For a given analysis application, perhaps only one of these techniques (actinometry, probing) would be beneficial or desirable, and hence perhaps only one would be used. In other applications, the benefits provided by both techniques might be necessary, and hence both would be used.  
         [0032]      FIG. 5  illustrates the various locations where the ICP chamber  50  can be utilized “downstream” from the processing chamber  10 . As shown, the ICP chamber  50  can be coupled directly to the processing chamber  10  ( 50   a ); can be coupled between the exhaust port on the processing chamber  10  and the throttle valve  90  ( 50   b ); can be coupled between the throttle valve  90  and the turbo pump  92  ( 50   c ); can be coupled between the turbo pump  92  and the rough pump  94  ( 50   d ); or can be coupled along the roughing line  96  ( 50   e ). The addition of actinometric and/or plasma probing techniques can be beneficial in any of these downstream locations, and preferably occurs at pressures ranging from 1 mTorr to 200 Torr.  
         [0033]     Additionally, and although not shown, the ICP chamber  50  can be used to analyze the processing gases before they are introduced into the processing chamber  10 , although in this circumstance it may be beneficial to ensure that the gases being tested are de-ionized before introduction into the processing chamber  10 . Additionally, care should be taken to ensure that any actinometric reference gases introduced “upstream” will not adversely affect the process which will take place in the processing chamber  10 .  
         [0034]     As noted earlier, the incorporation of actinometry and probing capability into the improved ICP chamber  50  has significant benefits. First, modification to the processing chamber  10  is not necessary, reducing potential sources of contamination and necessary maintenance of the chamber  10 . Second, the ICP chamber allows for the analysis of gases used in the processing chamber  10  even when those gases are not ionized (e.g., CVD deposition). Additionally, there is no need to introduce actinometry reference gases or probes into the process chamber, which removes factors from the processing chamber which could adversely affect the sensitive processes being run therein.  
         [0035]     “Processing gas” as used herein should be understood as including both gases introduced into the processing chamber  10  to perform a process on a workpiece as well as gaseous products or byproducts stemming from reaction of the introduced gases with the workpiece. Moreover, “processing gas” should not be understood as necessarily comprising only one type of molecule or species. For example, two etching gases introduced into a chamber, or one gas introduced into the chamber and another gas which results from interaction with the workpiece, constitutes a “processing gas,” even though that gas comprises a mixture of more than one type of molecule or species.  
         [0036]     Saying that two items are “coupled” does not necessarily imply that the items are in direct contact. Two items can still be functionally coupled even if an intermediary intervenes between them.  
         [0037]     It should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.