Abstract:
Described are a method and apparatus for evaluating a least one characteristic of a plasma. The described method uses photons to raise the excitation state to or past the point of ionization of atoms which will traverse the plasma to be evaluated. The ionization of the atoms, followed by the measurement of the energy of any resulting secondary ions, facilitates the determining of one or more characteristics of the plasma. In one example, the photons are provided by a laser which directs a beam to intersect, and in some examples to be collinear with, a beam of atoms directed through the plasma.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to methods and apparatus that may be used for evaluating a plasma, and more specifically relates to such methods and apparatus providing improved evaluating of plasmas, particularly those for which previous measurement techniques have offered less than optimal results. 
         [0002]    A number of systems have been used or considered for evaluating of plasmas formed in chambers or similar devices. In many circumstances the systems have been configured to measure relatively high temperature and/or high density plasmas. For example, systems such as Heavy Ion Beam Probes have been used for such purposes. While different configurations of Heavy Ion Beam Probe systems are known for evaluating different types of plasma devices, in general, such systems operate by directing an ionized beam through the plasma, where the ions will become “heavy ions” through electron impact within the plasma, thereby becoming “doubly ionized.” The doubly ionized particles will then be deflected by the magnetized plasma, and detected by an energy analyzer which can then discern the energy gained by the ions, and from that energy data identify the electric potential of the plasma at the point of ionization. 
         [0003]    One limitation of such Heavy Ion Beam Probe systems is that the incident electron impact energy must be equal to or greater than the second stage ionization potential of the ions in the beam. Thus, a significant limitation on such Heavy Ion Beam Probe systems is that they are not suitable for use with relatively low temperature plasmas, for example, plasmas operating at approximately 1-2 eV. Such plasmas typically do not have enough sufficiently hot electrons to achieve significant second stage ionization in the ion beam, and thus signal levels are generally very low. 
         [0004]    This limitation on such probe systems is significant particularly to industries such as the semiconductor manufacturing industry, which typically uses “cold plasmas,” that is plasmas with electron temperatures of approximately 2 electron volts or less. However, measurement of plasma characteristics is very important in the semiconductor industry because the plasma may have a significant impact on the semiconductor manufacturing process. Because of this need to measure these cold plasmas, the most common techniques currently used to measure the plasma in semiconductor systems have used Langmuir probes inserted into the plasma. With such probes, however, the measurements are less than optimal because the mere presence of the Langmuir probe disrupts the plasma to at least some degree. Additionally, the measurement may only be made at a single point in the plasma, and is often believed to be inaccurate. 
         [0005]    An additional concern arises in some applications, and is exemplified in the semiconductor manufacturing industry, where the real need is to evaluate the plasma potential (voltage) and density across a geometrical dimension. For example, in semiconductor manufacturing, the vast majority of such manufacturing is done by depositing or otherwise forming a succession of patterned layers on a circular substrate such as a thin silicon wafer. Many forms of deposition operations and patterning operations, such as etching, involve the use of plasma mechanisms. The current conventional technology forms such layers on a 300 mm diameter wafer, and a critical factor in such manufacturing is the consistency of deposition or etching operations across the entire dimension of that wafer. Additionally, a varying plasma gradient across the wafer may create localized plasma charging resulting in damage to the structures formed, such as the gate oxide layer. Accordingly, for that industry, the currently-available techniques do not provide either a system capable of measuring the relatively cold plasmas that are typically of interest, or a mechanism for identifying any irregularities or discontinuities in plasma characteristics across the wafer dimension. While similar needs are believed to be experienced in other industries, the semiconductor manufacturing industry provides an accessible and understandable example of where currently-known plasma measurement methods and systems fail to provide capabilities that would be beneficial to the industry. 
         [0006]    Accordingly, the present invention provides new methods and apparatus for evaluating plasmas which is capable of evaluating relatively cold plasmas, as well as hotter plasmas; and which in some examples provide additional capabilities of profiling plasmas across the dimension of the plasma. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention utilizes a photon source to assist ionization of atoms used for evaluating at least one characteristic of a plasma. In the described examples of the invention, a beam of neutral atoms (“neutrals”) will be directed toward the plasma, and some portion of those neutrals will be excited, and in some preferred examples ionized, through interaction with photons from a photon source such as, in some examples, a laser. Through the combination of the photon interaction increasing the kinetic energy of the neutrals, and the interaction of the plasma on those neutrals or ions, the ions will experience a net energy increase. The subsequent measurement of these results of the energy of the ions allows evaluating one or more characteristics of the plasma. In some examples, the ionized beam neutrals will be evaluated by an energy analyzer, as described in more detail later herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  schematically depicts an example of one system for monitoring plasmas in accordance with the present invention. 
           [0009]      FIG. 2  depicts a hypothetical waveform of plasma energy as may be expected from the performing of the evaluating method as described herein. 
           [0010]      FIG. 3  depicts an example of an alternative system for monitoring plasmas in accordance with the present invention. 
           [0011]      FIG. 4  schematically depicts the energy analyzer from the system of  FIG. 1  in greater detail. 
           [0012]      FIG. 5  schematically depicts one configuration of a magnetic steering device for an ion beam suitable for use with the systems of  FIGS. 1 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0013]    The following detailed description refers to the accompanying drawings that depict various details of embodiments selected to show, by example, how the present invention may be practiced. The discussion herein addresses various examples of the inventive subject matter at least partially in reference to these drawings and describes the depicted embodiments in sufficient detail to enable those skilled in the art to practice the invention. However, many other embodiments may be utilized for practicing the inventive subject matter, and many structural and operational changes in addition to those alternatives specifically discussed herein may be made without departing from the scope of the inventive subject matter. 
         [0014]    In this description, references to “one embodiment” or “an embodiment,” or to “one example” or “an example” mean that the feature being referred to is, or may be, included in at least one embodiment or example of the invention. Separate references to one or more examples or embodiments in this description are not intended to refer necessarily to the same embodiment; however, neither are such embodiments mutually exclusive, unless so stated or as will be readily apparent to those of ordinary skill in the art having the benefit of this disclosure. Thus, the present invention can include a variety of combinations and/or integrations of the embodiments described herein, as well as further embodiments as defined within the scope of all claims based on this disclosure, as well as all legal equivalents of such claims. 
         [0015]    Referring now to the drawings in more detail, and particularly to  FIG. 1 , therein is depicted in block diagram form, one example embodiment of a system  100  for evaluating a plasma in accordance with the present invention. For purposes of the present description, the term “evaluating” is used in its broadest context, and encompasses any measuring, determining or estimating. Similarly, each of those terms are used in their broadest scope. For example, “measuring” a plasma includes making any type of comparative, qualitative or quantitative measure of any property or indicator of a plasma, whether such measure is a single measurement, or represents monitoring over some time interval, whether such monitoring is continuous, periodic or intermittent. For purposes of the present description, the example will be discussed in the context of evaluating a plasma within a chamber  102  generally of a type suitable for use in a semiconductor manufacturing operation, either for deposition or etching. Although such chambers for these different purposes will not necessarily be interchangeable, for purposes of illustrating an environment of present invention, they may be considered similarly. Accordingly, chamber  102  will be discussed for purposes of the present example, as a chamber meant to operate with a plasma having an electron temperature (T e ) of approximately 2 eV, and existing over a diameter of approximately 300 mm (the diameter of the majority of current semiconductor wafers). As will be apparent to those skilled in the art, a plasma of this dimension typically requires a chamber having an internal diameter of at least 14 to 18 inches, and even larger in many cases. 
         [0016]    As depicted in  FIG. 1 , system  100  includes a laser  104  and an ion gun  106  that will each be at least partially housed within a pressure housing  108  in fluid communication with the interior of chamber  102 , and therefore maintained at the same pressure as that within chamber  102 . As will be apparent to those skilled in the art, it is not essential that laser  104  and ion gun  106  each be entirely retained within pressure housing  108 . However, at least the output of each device will be within pressure housing  108 . Pressure housing  108  may be of any appropriate form sufficient to house the identified components in an operative configuration, as described herein, and to support a vacuum as intended within chamber  102 . 
         [0017]    Several types of lasers may be suitable for use in various examples of systems constructed in accordance with the principles described herein, depending on the specific operating parameters for which those systems are intended. For the system intended for operation in the example environment identified for this example, and with the parameters identified above, laser  106  will preferably be a pulsed excimer laser operating at a wavelength (λ) of 193 nm, with an example pulse duration (ΔT p ) of approximately 12 ns, and a pulse repetition rate f rep  of approximately 1 kHz. In one presently-defined embodiment, the laser will have a pulse energy (u p ) of approximately 11 mJ, and a beam cross-sectional area of approximately 18 mm 2 . 
         [0018]    Ion gun  106  may again be of a number of different possible structures. In one currently-contemplated structure ion gun  106  will be a multi-stage Pierce-type extractor, having an Einzel lens. In one currently-identified structure, the ion gun will provide sodium (Na) ions with a beam energy (E b ) of approximately 10 keV, and a beam ion velocity (v) on the order of 2.9×10 5  m/s. As will be described later herein, in some environments it may be desirable to use ions of a different element. 
         [0019]    As can be seen in  FIG. 1 , laser  104  is placed and oriented to direct the beam along a path, indicated generally at  112 , through additional equipment, as will be described below, through chamber  102 , and toward energy analyzer  110 . As shown in  FIG. 1 , in this example, embodiment laser path  112  extends across the diameter of chamber  102 , and thus laser  104  and energy analyzer  110  are on essentially diametrically opposite sides of chamber  102 . While the directing of this path will in some cases be a matter of design choice, a laser path extending across the approximate diameter of chamber  102  will be a preferred structure for many applications. 
         [0020]    As can be seen in  FIG. 1 , in this example system the preferred configuration will also direct the ion beam from ion gun  106  generally along path  112 . Thus, in this example configuration, the laser beam, when energized, is coincident with a portion of the ion beam along path  112 . Although multiple configurations are possible to achieve this result, because it is relatively straightforward to steer the ion beam, system  100  includes ion gun  106  oriented at an angle relative to path  112  but directed toward the input side of a beam steering mechanism  116  configured to re-direct the ion beam through either magnetic or electrical interaction with the beam. As depicted in system  100 , beam steering mechanism  116  includes a first pair of deflection plates that will re-orient the ion beam from its original trajectory indicated generally at  114 , to along path  112 . The deflection plates may be of any suitable configuration as may be determined by those skilled in the art, but for example may be approximately 3 inches square, and spaced approximately 3 inches from one another, each charged to a voltage of approximately 3 kV between them. As will be described later herein, other mechanisms may be used to establish a magnetic field to provide the needed steering of the ion beam. 
         [0021]    System  100  also includes a charge exchange chamber, or neutralizer,  118  configured to neutralize the charges on the sodium ions in the beam from ion gun  106 . A neutralizer  118  may be of any suitable configuration to facilitate charge exchange of the ions. In one example, a source of neutral sodium gas may be provided within the chamber, whereby the neutral sodium atoms will facilitate a charge transfer with a substantial portion of the sodium ions. Downstream of neutralizer  118 , there will be another steering mechanism  120 , again such as a set of charged deflection plates. The purpose of this second steering mechanism is to deflect any remaining ions from the beam toward an ion dump, thereby leaving neutrals in the beam that will pass into chamber  102 , and thus through the plasma therein. Where deflection plates  120  are used, again each plate may be approximately 3 inches square, with positively and negatively-charged plates placed to steer remaining charged ions to ion dump  122 . Ion dump  122  may be just a grounded plate placed to collect the deflected positively-charged ions. As a result, the beam of atoms entering chamber  102  is generally limited to neutrals, except to the extent that the neutrals are ionized by the photons from the pulsed laser. As will be described in more detail later herein, because such laser energy is known, the energy gained by the neutrals by intersecting the plasma within chamber  102  may also be determined. 
         [0022]    On the opposite side of chamber  102  is found energy analyzer  110 , as discussed earlier herein. Again, a pressure housing  126  will couple energy analyzer  110  to chamber  102 , such that energy analyzer  110  is also at a common pressure and in fluid communication with chamber  102 . Energy analyzer  110  is preferably a Proca-Green-type analyzer, as is well-known in the art. Such analyzers use a pair of spaced, charged plates, indicated generally at  132   a - b , to deflect ions in the beam in functional relation to the energy of those ions (also schematically depicted in  FIG. 4 , in greater detail). Energy analyzer  110  also includes a pair of charge collection, or detector, plates ( 134   a - b  in  FIG. 4 ) which are coupled to appropriate circuitry (not depicted) configured to determine the relative current on each plate. The ratio of the currents will indicate the energy of the beam. The determination of the specific configuration and operating parameters of the energy analyzer will be highly dependent on the configuration of the entire system  100 , as well as of the analyzer  110  itself. The identification of an appropriate configuration, and of appropriate operating voltages and other parameters, is considered to be within the level of one skilled in the art having the benefit of the present disclosure. As will be apparent to those skilled in the art, energy analyzer  110  may include, or may be coupled to, a suitable recording mechanism configured to capture the data from energy analyzer  110 . 
         [0023]    The described current ratio measurement at any selected time interval represents a measurement of the ion energy at that time interval, and is thus directly representative of the plasma energy at that time interval. The correlation of that time-based measurement to a distance measurement across the plasma may be accomplished as described below. 
         [0024]    As is well known to those skilled in the art, energy analyzer  110  will require calibration. This is typically performed with the neutralizer turned off and with no plasma present. For the calibration and for the measurement itself, the currents detected on the two detector plates are monitored. As with an actual measurement as noted above, the incoming beam of ions will straddle the two plates, and the ratio of the currents will indicate the energy of the beam. For calibration, the gun voltage will be fixed, and the voltage of the analyzer will be varied. This will allow the determining in situ of two analyzer parameters known in the literature as “G” and “F,” representative of the geometric characteristics of the analyzer, as are empirically determined relative to the specific analyzer configuration at issue. The determination of such parameters are well-known in the art, and as one example, may be determined in accordance with the teachings of L. Solensten and K. A. Connor, “Heavy Ion Beam Probe Energy Analyzer For Measurement of Plasma Potential Fluctuations,” Rev. Sci. Instrum. Vol. 58, No. 4, 1987; which is hereby incorporated herein by reference to demonstrate the state of the prior art. 
         [0025]    Once energy analyzer  110  is calibrated, the potential (V) at any point in time may then be determined by a relation such as the following: 
         [0000]    
       
         
           
             
               
                 
                   V 
                   = 
                   
                     
                       
                         V 
                         A 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     i 
                                     U 
                                   
                                   - 
                                   
                                     i 
                                     L 
                                   
                                 
                                 ) 
                               
                               
                                 ( 
                                 
                                   
                                     i 
                                     U 
                                   
                                   + 
                                   
                                     i 
                                     L 
                                   
                                 
                                 ) 
                               
                             
                              
                             F 
                           
                           + 
                           G 
                         
                         ) 
                       
                     
                     - 
                     
                       V 
                       G 
                     
                   
                 
               
               
                 
                   eq 
                   . 
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
       Where: 
       [0026]    i U  represents the current detected on the upper plate of the energy analyzer,
 
i L  represents the current detected on the lower plate of the energy analyzer,
 
V A  represents the variable voltage of one deflection plate of the energy analyzer, and
 
V G  represents voltage of the ion gun.
 
         [0027]    In performing a plasma measuring operation, ion gun  106  and laser  104  will be actuated as described above to direct a beam of, in this example, sodium atoms through a plasma within chamber  102 , and the ions charged as a result of the laser and the plasma will be detected by energy analyzer  110 . In the course of that operation, the sodium ions in the accelerated beam will be neutralized by neutralizer  118 , and the remaining charged ions removed by the second steering mechanism, such as deflection plates  126 , before the atoms enter chamber  102  and encounter the plasma therein. The described laser pulse from laser  104  can excite, and preferably ionize, the entire beam of atoms, including within chamber  102 , and for all practical purposes, virtually instantaneously. As a result, after such excitation, and preferably ionization, the first detected ions will likely be ions outside of the plasma, having already have passed through the plasma before interacting with the laser beam photons (depending upon the configuration of the system surrounding chamber  102 ). Subsequently, a string of ions will be detected, coming from the plasma sheath proximate the output port of chamber  102 ; followed by ions across the dimension of the plasma; followed by ions from the sheath proximate the input port; and then potentially outside of chamber  102  on the input side. For example, a hypothetical detector signal may be expected to look something like the hypothetical curve  200  depicted in  FIG. 2 . By correlating the time scale of the x-axis of  FIG. 2  with the velocity of the ion beam, the measured plasma energy reflected in curve  200  can be correlated with the diameter of the plasma to provide an energy profile of the plasma within chamber  102 . Correlation of the time to the distance dimension of the measurement may be obtained by multiplying the time by the speed of the neutral beam. The speed of the neutrals is determined by the energy (E) and the mass (m) as in the following equation: 
         [0000]        v =√{square root over ((2 E/m ))}  eq. 2 
         [0000]    The energy of the neutral beam is known from the energy imparted to the ions out of the ion gun, and the mass is known from the species of ions from the gun. 
         [0028]    This type of profile can be very useful, for example, when designing a chamber, so as to identify irregularities or undesirable properties within the chamber, such as may be caused by various structures or configurations within the chamber. Additionally, it is contemplated that periodic measurements of the type described herein may be useful during the actual use of a chamber, such as in a semiconductor manufacturing process. As is well known to those skilled in the art, during such manufacturing operations a number of wafers will be processed, and the number of active and/or carrier gases may be introduced into a chamber, such as source gases for deposition processes and etching or otherwise reactive gases for etching or other removal processes. Typically, after multiple such cycles the chamber will experience build up of deposition products or byproducts on various surfaces, and over time this build-up can degrade performance of the chamber, in some cases by interfering with the generation or uniformity of the plasma that will be formed within the chamber. Accordingly, it is contemplated that at least for some types of chambers and/or operations, periodic measurements of the type described herein could identify when a chamber needs cleaning, repair or other remedial action. 
         [0029]    Referring again to the curve of  FIG. 2 , the described current ratio, ion energy measurement is directly representative of the plasma potential, as the plasma potential is equal to the ion energy gained (that is the difference between the ion energy measurement and the initial ion energy) divided by a constant. The constant is known as the elementary charge, which is the same as that of an electron and has a value of approximately 1.6022×10 −19  coulombs. The curve from  FIG. 2  will look exactly the same, but the vertical scale will change to the plasma potential (voltage) as mentioned. In some circumstances, it may be possible to evaluate, at least by estimating, the actual plasma density. For example, with certain parameters known or estimated, the plasma density can be inferred from Boltzmann&#39;s relation: 
         [0000]      n e =n 0 e (V/T     e     )   eq. 3 
       Where: 
       [0030]    V is the potential of the plasma in volts,
 
T e  is the electron temperature in electron volts, and
 
n is the density (here, the electron density (n e ) is equal to a reference density (n o ) multiplied by the exponential). The reference density can be determined where the following parameters are known or estimated: the plasma potential, the electron temperature, and the density at any point in the plasma. Even in the absence of these three parameters, Boltzmann&#39;s relation together with the plasma potential profile and the electron temperature can generate a plasma density profile identifying the relative value of the plasma density across the dimension of the plasma, although not reflecting the absolute value of the density. Such a relative plasma density is still a measurement of significant importance when evaluating the uniformity of a plasma within a chamber, such as one used for semiconductor processing.
 
         [0031]    In applications in which it is desired to monitor a chamber actively used in semiconductor manufacturing, the use of sodium ions may be considered undesirable, as sodium, in many cases is considered a contaminant to the actual manufacturing operations. In such circumstances, therefore, it may often be preferable to use alternative ions, such as for example, calcium (Ca), germanium (Ge), magnesium (Mg) or aluminum (Al). When using these alternative ions, one point to be addressed will be the energy required to ionized the neutrals in the beam. In some cases, such as that of germanium, this may be possible with other commercially available lasers, such as, for example, a 157 nm laser. Other shorter-wavelength lasers would be suitable for use with these other ions. However, at the present time, those lasers are less cost-effective for general commercial practice of the described techniques. 
         [0032]    Referring now to  FIG. 3 , therein is depicted an alternative embodiment of a system  300  for evaluating a plasma. Components that are substantially similar to those described in reference to system  100  depicted in  FIG. 1  are numbered similarly here. As the basic function of the described components is similar to that previously discussed, only the primary differences will be addressed here. As is apparent from  FIG. 3 , the laser  104  is no longer directing a beam  302  along a path that is coincident with the beam of ions  304  from ion gun  106 . Accordingly, laser  104  is oriented to cause beam  302  to intersect beam  304  from ion gun  106 . In system  300 , the ions resulting from interaction with photons of a laser beam  302  are deflected by a magnetic field resulting from the plasma within chamber  102 . In the absence of the plasma generating such a magnetic field, alternative structures would have to be supplied in order to provide the necessary deflection of the secondary ions. Although not depicted in  FIG. 3 , and similar to the system depicted in  FIG. 1 , ion gun  106 , laser  104  and analyzer  110  are all within a pressure housing enabling fluid communication and a common pressure with chamber  102 . 
         [0033]    As will be appreciated from consideration of  FIG. 3 , the identified system only provides a measure at the point of ionization, the intersection of the photon beam with the neutral beam within the plasma. Because the measurement is only at a single point, with the described system, for this system to yield information across the plasma the laser must be scanned across the plasma, while allowing for passage of the resulting ions to the energy analyzer. 
         [0034]    Referring now to  FIG. 5 , the figure depicts an alternative steering mechanism in the form of a generally toroidal coil  400  that may be used in place of either or both of the previously-described sets of deflection plates  116 ,  118 . Coil  400  includes at least one conductor, such as the wire  404  wound around a generally circular core  406 , having a break therein, indicated generally at  402 , to facilitate the traverse of ions therethrough. Those skilled in the art will recognize that other mechanisms for establishing an appropriate magnetic field may also be used for the directing and/or focusing of charge particles originating with ion gun  106 . 
         [0035]    Many modifications and variations may be made in the techniques and structures described and illustrated herein, without departing from the spirit and scope of the present invention. For example, as is apparent from the preceding discussion, there may be other types of lasers that are suitable, and in some cases preferable, depending upon the atoms to be ionized and the specific configuration of a plasma monitoring system. Additionally, there may be alternative structures which may be assembled in order to produce atoms from ion gun  106  and the photons from laser  104  along a coincident path. Additionally, there may be additional excitation mechanisms used to elevate the energy state of certain atoms thereby further facilitating photon ionization of those atoms under appropriate conditions. Accordingly, it should be readily understood that the scope of the invention includes all of these and other variations which will be apparent to those skilled in the art having the benefit of the present disclosure.