Abstract:
A method for controlling the non-uniformities of plasma-processed semiconductor wafers by supplying the plasma with two electrical signals: a primary electrical signal that is used to excite the plasma, and a supplemental electrical signal. The supplemental signal may be composed of a plurality of electrical signals, each with a frequency harmonic to that of the primary signal. The phase of the supplemental signal is controlled with respect to the phase of the primary signal. By adjusting the parameters of the supplemental signal with respect to the primary signal, the user can control the parameters of the resultant plasma and, therefore, control the non-uniformities induced in the semiconductor wafer.

Description:
FIELD OF THE INVENTION  
       [0001]    This is a Continuation of International Application No. PCT/US01/47487, which was filed on Dec. 17, 2001, which, in turn, claims the benefit of U.S. provisional application No. 60/259,861, which was filed Jan. 8, 2000, the contents of both of which are incorporated herein in their entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    In the processing of semiconductor wafers, such as silicon wafers, many techniques are known for modifying the surface of the wafer. These surface modifications may involve, for example, adding material, as in the case of a metal deposited on the wafer, or removing material, as is done during etching.  
           [0003]    A particularly popular set of techniques for adding or removing material from the surface of a silicon wafer involves the use of plasma. A plasma is a gas (or gas mixture) which is energized so that it partially decomposes and forms a mixture of charged and uncharged particles. Plasmas may be generated by the application of an alternating current electrical signal (AC current) to the gas mixture. This generating current typically has a high frequency, usually in the radio frequency (RF) range, and is applied to the gas mixture by an electrode placed in the gas-containing vessel. The frequency of the electrical signal applied to the gas is called the RF drive frequency of the plasma processing system.  
           [0004]    It has long been recognized in the art that the electrical response of plasmas generated by the application of AC power is nonlinear, that is, at typical applied power levels, the impedance (resistance to electrical current flow) of the plasma is not directly proportional to the applied voltage. The nonlinear response of the plasma to the applied power results in the generation of harmonic power frequencies in the plasma. Harmonic power frequencies are electrical signals with frequencies that are some integral multiple of the RF drive frequency of the plasma processing system. In a typical plasma processing system, hundreds of watts of power may be associated with harmonic power frequencies related to the RF drive frequency. These harmonics lead to a plasma that is not uniform in its characteristics.  
           [0005]    When a non-uniform plasma is applied to a silicon wafer, for example, to etch the wafer, the non-uniform plasma results in a non-uniform wafer etch whose characteristics vary with the distance from the plasma electrode. FIG. 1 is an illustration depicting the non-uniformity of a plasma-etched wafer as a function of the distance from the center  16  of the wafer. In FIG. 1, the surface profile  12  of the wafer  10  is non-uniform and roughly sinusoidal. This non-uniform wafer processing may be undesirable, and considerable effort has been expended in attempts to control the power harmonics present in the plasma to produce a more uniform plasma, and thus a more uniformly processed wafer.  
           [0006]    Processes to control the power harmonics present in a plasma are predicated on the wave nature of the AC electrical signal used to excite the plasma. In addition to the frequency of the AC signal described above, the signal has an amplitude, or strength, and a phase, or timing difference relative to other waves. The combination of several AC electrical signals of different frequencies, amplitudes or phases is governed by the principle of superposition. This principle states that the sum of two waves of differing amplitudes results in a wave that, in general, has an amplitude different from either of its addends. When waves are to be added, differences in their frequencies, phases or amplitudes can change the characteristics of the resultant wave. Thus, if AC electrical signals with different phases and frequencies are used in combination in a plasma processing system, the resulting excitatory waveform could demonstrate constructive or destructive interference.  
           [0007]    Previous attempts to control the non-uniformity present in a plasma have included the use of a plurality of RF drive electrodes or alternately, a plurality of segments of a segmented electrode. The plurality of electrodes or segments are excited at a single RF frequency by means of a single RF oscillator and a plurality of separate RF amplifiers and phase shifters. An example of this technique is seen in U.S. Pat. No. 5,932,116 (Matsumoto).  
           [0008]    Another approach to controlling the non-uniformity present in a plasma is found in U.S. Pat. No. 6,043,607. In this approach, a plurality of RF sources operating at a corresponding plurality of RF frequencies are used to generate a complex power waveform and excite a plasma within a semiconductor processing system. In the above-cited reference, the frequencies in the complex excitation waveform are not precisely controlled, so there are constantly varying phase differences between the plurality of RF sources.  
           [0009]    Each of these previous attempts has focused on attenuating or accentuating the harmonic power frequencies present in a non-uniform plasma without independently controlling both the phase and amplitude of the input power at each selected harmonic frequency. In FIG. 1, the dotted-line surface profile  14  of the wafer  10  illustrates a potential advantage of a plasma processing system in which both phase and amplitude are controlled at each selected harmonic frequency. The nonuniform surface perturbations of the wafer  10  have been greatly attenuated in the dotted-line surface profile  14 , resulting in a more uniform wafer.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a method for controlling the electrical signals present in a plasma by providing two signals to a plasma drive electrode: a radio frequency signal and a supplemental signal. The supplemental signal is controlled separately from the radio frequency signal and is comprised of at least one signal harmonic to the fundamental frequency and having a controlled phase relationship with the fundamental frequency of the radio frequency signal. Likewise, a plurality of supplemental signals, each at a frequency harmonic to the fundamental frequency, may be employed.  
           [0011]    The method includes a means for determining the parameters of any electrical signals present in the plasma and correlating the parameters of these signals with those of the radio frequency signal and the supplemental signal(s). The two signals can then be adjusted to provide a time-independent phase difference therebetween. In one embodiment, at least the first, second and third harmonic frequencies of the radio frequency signal can be controlled. The parameters of the radio frequency signal and the supplemental signal can then be correlated with the electrical signals in the plasma to change the parameters of the plasma. The method is particularly suited to controlling plasma parameters such as: the etch rate and deposition rate of the plasma, the uniformity of the etch and deposition rates, the selectivity of the etch of one material relative to the etch of another material, the uniformity of the selectivity, feature profile (or anisotropy), the uniformity of the feature profile, the deposited film stress, and the uniformity of the deposited film stress. The method may also be suited to controlling other parameters as well.  
           [0012]    The present invention has one embodiment in a device that has three main components: a means for producing an electrical signal with a fundamental frequency, a means for producing a separately controlled supplemental electrical signal with a frequency harmonic to and in phase with the first electrical signal, and a plasma drive electrode driven by the electrical signal and the supplemental electrical signal. The means for producing the electrical signals may include signal generators as well as other means known in the art.  
           [0013]    Other objects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0014]    [0014]FIG. 1 is an illustration showing a non-uniform plasma processed substrate and the relationship between the substrate&#39;s uniformity and the distance from the substrate&#39;s center.  
         [0015]    [0015]FIG. 2 is a block diagram of the basic components of a plasma processing system.  
         [0016]    [0016]FIG. 3 is a block diagram of an exemplary embodiment of an apparatus by which this invention may be implemented.  
         [0017]    [0017]FIG. 4 is a block diagram of a second exemplary embodiment of an apparatus by which this invention may be implemented.  
         [0018]    [0018]FIG. 5 is a block diagram of a third exemplary embodiment of an apparatus by which this invention may be implemented.  
         [0019]    [0019]FIG. 6 is a block diagram of a fourth exemplary embodiment of an apparatus by which this invention may be implemented. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0020]    Referring now more particularly to the drawings, FIG. 2 shows a generalized block diagram of the most basic components of a plasma processing system. An electrical signal with a frequency in the radio frequency (RF) range is generated by RF source  50 . RF source  50  is controlled by source controller  62 . Match network  52  optimizes the transfer of RF power from RF source  50  to plasma generating electrode  58 , and is controlled by match network controller  64 . Disposed between match network  52  and plasma generating electrode  58  are two components: a band pass filter  54  and a V-I probe  56 . Band pass filter  54  protects match network  52  from the electrical signals present in the plasma  60 . V-I probe  56  records the properties of the RF signals present in the system and provides those properties to a system monitoring component, depicted generally in this example as  66 .  
         [0021]    [0021]FIG. 3 shows a block diagram of an apparatus by means of which this invention may be implemented. The output amplitude of 60 MHz RF source  105  is controlled by 60 MHz RF source controller  100  which is connected by either mechanical or electrical means thereto. A frequency of 60 MHz is used here for the purpose of description, but another frequency, either higher or lower, might be used. The output power of 60 MHz RF source  105  is typically in the range from 1 to 5 kilowatts. The 60 MHz RF source  105  is electrically connected to RF coupler  110 . RF coupler  110  has two output terminals. Almost all of the power incident on RF coupler  110  emerges from it at the main output terminal  112 , which is electrically connected to 60 MHz match network  115 . A small fraction of the incident power, determined by the coupling factor of RF coupler  110 , flows through the lower power output terminal  111  and is directed to voltage amplitude leveler  145 . The output of the lower power output terminal  111  to the voltage amplitude leveler  145  will subsequently be considered below. The coupling factor of RF coupler  110  is typically in the range from −65 to −50 dB. The 60 MHz match network  115  optimizes the transfer of RF power from 60 MHz RF oscillator  105  to plasma  140 . Automatic control of 60 MHz match network  115  is provided by 60 MHz match network controller  120  by means that are well understood by contemporary practitioners of related art. The output of 60 MHz match network  115  is electrically connected to 60 MHz band pass filter  125  which, in turn, is electrically connected to plasma excitation electrode  135  by means of RF transmission line  130 . The purpose of 60 MHz band pass filter  125  is to prevent RF power due to the nonlinear electrical properties of plasma  140  and consequent harmonic generation therein from adversely affecting 60 MHz match network  115 . The pathway between the main output terminal  112  of the coupler  110  and the V-I probe  132  constitutes the first circuit branch  126  of the device. V-I probe  132  is located in RF transmission line  130  between 60 MHz band pass filter  125  and plasma excitation electrode  135 . V-I probe  132  facilitates measurement of the voltage and current in RF transmission line  130  at the location of V-I probe  132 . The outputs of V-I probe  132  are electrically transmitted to oscilloscope and network analyzer  185 , where they may be used for analysis and/or control of system operation. The use of a V-I probe as intended herein is well-known. See, for example U.S. Pat. No. 5,325,019.  
         [0022]    Return now to RF coupler  110 ; the power level output by the coupler&#39;s lower power terminal  111  is determined by its coupling factor. The pathway between the lower power output terminal  111  of the coupler  110  and the V-I probe  132  constitutes the second circuit branch  128  of the device. Power emerges from the lower power terminal  111  and is electrically directed to the input terminal of automatic voltage amplitude leveler  145 , which electronically assures that the 60 MHz RF voltage amplitude of the signal emerging therefrom is virtually independent of the amplitude of the input signal thereto for all input signals of practical interest for the purpose of this invention. This 60 MHz RF signal is electrically directed to harmonic generator  150  or to a plurality of harmonic generators  150   a ,  150   b , . . . ,  150   n . It should be readily apparent that the discussion that follows is applicable to additional harmonic generators simultaneously producing other harmonics (e.g., the fifth) of the signal originating from 60 MHz RF source  105  or to a single harmonic generator  150  that simultaneously provides at a plurality of output terminals a plurality of harmonics.  
         [0023]    In the discussion that follows, a single harmonic generator  150  generates the third harmonic of the signal originating from 60 MHz RF source  105 . That is, the frequency of the harmonic signal is 180 MHz. The output of harmonic generator  150  is electrically connected to RF voltage-controlled phase shifter  155 , which is, in turn, electrically connected to RF phase controller  160 . RF voltage-controlled phase shifter permits the phase of the RF harmonic signal to be adjusted with respect to the phase of the 60 MHz RF signal. This adjustment of the relative phase is meaningful because the harmonic RF signal is phase locked to the fundamental RF signal by virtue of the means by which it is produced. The preferred range of phase adjustment for the harmonic RF signal (i.e., the 180 MHz signal in the example discussed here) is ±180° or ±π radians.  
         [0024]    The output terminal of RF voltage-controlled phase shifter  155  is electrically connected to the input terminal of 180 MHz voltage-controlled amplifier  165 . Amplifier gain controller  170  controls the voltage gain of 180 MHz voltage-controlled amplifier  165  to which it is connected by electrical or mechanical means. The output terminal of 180 MHz voltage-controlled amplifier  165  is electrically connected to the input terminal of 180 MHz match network  175 . The 180 MHz match network  175  optimizes the transfer of RF power from 180 MHz RF voltage-controlled amplifier  165  to plasma  140 . Automatic control of 180 MHz match network  175  is provided by 180 MHz match network controller  185  by means that are well understood by contemporary practitioners of related art. The output of 180 MHz match network  175  is electrically connected to 180 MHz band pass filter  190 . The purpose of 180 MHz band pass filter  190  is to prevent RF power at any frequencies other than 180 MHz from adversely affecting 180 MHz match network  175 . The output terminal of 180 MHz band pass filter  190  is electrically connected to RF transmission line  130  between 60 MHz band pass filter  125  and V-I probe  132 . In this way RF power at both the fundamental RF frequency (60 MHz) and the selected harmonic frequency (180 MHz in this example) is directed to plasma excitation electrode  135 .  
         [0025]    Oscilloscope and network analyzer  195  is used to monitor and facilitate control of (a) the output power of 60 MHz RF source  105 , (b) the RF voltage as measured by V-I probe  132 , (c) the RF current as measured by V-I probe  132 , (d) the mechanical or electrical output of RF phase controller  160 , (e) amplifier gain controller  170 . Additional inputs to oscilloscope and network analyzer  195  may be added to permit measurement and control of other parameters. Still another embodiment of the invention is depicted in FIG. 4. In this embodiment, a harmonic generator is unnecessary. In FIG. 4 it is assumed that the lower frequency signal is to be 60 MHz and the third harmonic (i.e., 180 MHz) of that lower frequency is also to be used, just as in the embodiment of FIG. 3. Refer now to FIG. 4. Clock  200  produces a 180 MHz square wave. Clock  200  is electrically connected to 180 MHz band pass filter  292 , the output of which is a 180 MHz sinusoid. Clock  200  is also electrically connected to frequency divider  210 , which in this example, converts the 180 MHz square wave to a 60 MHz square wave. Frequency divider  210  is electrically connected to 60 MHz band pass filter  211 , the output of which is a 60 MHz sinusoid. The description that immediately follows deals with the subsequent processing of the 60 MHz sinusoidal signal; the processing of the 180 MHz sinusoidal signal will be considered later. The pathway between the 180 MHz clock  200  and the V-I probe  232  as shown in the lower portion of FIG. 4 constitutes the first circuit branch  226  of the device. The pathway between the 180 MHz clock  200  and the V-I probe as shown in the upper portion of FIG. 4 constitutes the second circuit branch  228  of the device.  
         [0026]    The output of 60 MHz band pass filter  211  is electrically connected to the signal input terminal of voltage-controlled amplifier  212 . Amplifier gain controller  214  controls the voltage gain of 60 MHz voltage-controlled amplifier  212 , to which it is connected, by electrical or mechanical means. The output terminal of 60 MHz voltage-controlled amplifier  212  is electrically connected to the input terminal of 60 MHz match network  215 . The 60 MHz match network  215  optimizes the transfer of RF power from 60 MHz RF voltage-controlled amplifier  212  to plasma  240 . Automatic control of 60 MHz match network  215  is provided by 60 MHz match network controller  220  by means that are well understood by contemporary practitioners of related art. The output of 60 MHz match network  215  is electrically connected to 60 MHz band pass filter  225 . The purpose of 60 MHz band pass filter  225  is to prevent RF power at the 180 MHz RF harmonic frequency, specifically, but also at other harmonic frequencies from adversely affecting 60 MHz match network  215 . The output terminal of 60 MHz band pass filter  225  is electrically connected by means of RF transmission line  230  to V-I probe  232 . V-I probe  232  is located in RF transmission line  230  between 60 MHz band pass filter  225  and plasma excitation electrode  235 . V-I probe  232  facilitates measurement of the voltage and current in RF transmission line  230  at the location of V-I probe  232 . The voltage and current sensing outputs of V-I probe  232  are electrically connected to oscilloscope and network analyzer  295 , where they may be used for analysis and/or control of system operation. The power output terminal of V-I probe  232  is electrically connected to plasma drive electrode  235 , which excites plasma  240 .  
         [0027]    The output of 180 MHz band pass filter  292  is electrically connected to RF voltage-controlled phase shifter  255 , which is, in turn, electrically connected to RF phase controller  260 . RF voltage-controlled phase shifter  255  permits the phase of the 180 MHz signal to be adjusted with respect to the phase of the 60 MHz RF signal. This adjustment of the relative phase is meaningful because the harmonic RF signal is phase locked to the fundamental RF signal by virtue of the means by which it is produced. The preferred range of phase adjustment for the 180 MHz signal is ±180° or ±π radians.  
         [0028]    The output terminal of RF voltage-controlled phase shifter  255  is electrically connected to the input terminal of 180 MHz voltage-controlled amplifier  265 . Amplifier gain controller  270  controls the voltage gain of 180 MHz voltage-controlled amplifier  265  to which it is connected by electrical or mechanical means. The output terminal of 180 MHz voltage-controlled amplifier  265  is electrically connected to the input terminal of 180 MHz match network  275 . The 180 MHz match network  275  optimizes the transfer of RF power from 180 MHz RF voltage-controlled amplifier  265  to plasma  240 . Automatic control of 180 MHz match network  275  is provided by 180 MHz match network controller  285  by means that are well understood by contemporary practitioners of related art. The output of 180 MHz match network  275  is electrically connected to 180 MHz band pass filter  290 . The purpose of 180 MHz band pass filter  290  is to prevent RF power at the frequencies other than 180 MHz from adversely affecting 180 MHz match network  275 . The output terminal of 180 MHz band pass filter  290  is electrically connected to RF transmission line  130  between 60 MHz band pass filter  215  and V-I probe  232 . In this way RF power at both the lower RF frequency (60 MHz) and the selected harmonic frequency (180 MHz in this example) is directed to plasma excitation electrode  235 .  
         [0029]    Oscilloscope and network analyzer  295  is used to monitor and facilitate control of (a) the output power of 60 MHz RF source at the gain controller  214 , (b) the RF voltage as measured by V-I probe  232 , (c) the RF current as measured by V-I probe  232 , (d) the mechanical or electrical output of RF phase controller  260 , (e) the mechanical or electrical output of amplifier gain controller  270 . Additional inputs to oscilloscope and network analyzer  295  may be added to permit measurement and control of other parameters.  
         [0030]    Still another embodiment is shown in FIG. 5. This embodiment differs from the embodiment of FIG. 4 only in the location of voltage-controlled phase shifter  255  and its associated phase controller  260 . Whereas these elements are located in the 180 MHz branch  228  of the embodiment of FIG. 4, they are located in the 60 MHz branch  226  in the embodiment of FIG. 5. If only two frequencies are used, either configuration may be used to advantage. However if more than two frequencies are used, a logical extension of the embodiment depicted in FIG. 4 may be used as described below.  
         [0031]    In some circumstances, it may be desirable to use RF power at a lowest frequency; e.g., 60 MHz, and RF power at two or more harmonics of the lowest frequency. For example, it may be advantageous to use RF power at 120 MHz and 180 MHz in addition to RF power at 60 MHz. In such a situation the embodiment shown in FIG. 6 may be used. This embodiment functions in essentially the same way as the embodiment shown in FIG. 4. Note, however, that a  120  MHz RF path  229  has been added, and that this 120 MHz RF path  229  comprises elements that correspond one-to-one with elements of the 180 MHz RF path  228 . The clock frequency may be chosen to be the lowest frequency that is divisible without a remainder by every one of the frequencies of interest. In the example considered here, the clock frequency is therefore 3 60 MHz. Note that in FIG. 6, the elements of the 120 MHz RF path  229  are denoted by a number and a subscript a; the corresponding elements of the 180 MHz RF  228  path are denoted by the same number and a subscript b. Because of the similarity of the operation of the embodiment of FIG. 5 and the embodiment of FIG. 6, no further discussion of FIG. 6 will be included.  
         [0032]    It should be apparent to the reader that the embodiment of FIG. 6 may be extrapolated to include more than three frequencies. For example, one could use 60 MHz, 120 MHz, 180 MHz, and 240 MHz. In such a case, a branch for each of the three higher frequencies would, of course, be required. The lowest suitable clock frequency would be 720 MHz, and division of the clock frequency by 3, 4, 6, and 12 for the several lower frequency branches would be necessary.  
         [0033]    Return now to the assertion made above that if more than two frequencies are to be used, an extension of the embodiment of FIG. 4 is preferable to an extension of the embodiment of FIG. 5. The reason for this statement is that the phase of each of the higher RF frequencies is independently controlled with respect to the lowest RF frequency, which is the frequency at which most of the RF power is typically delivered to the plasma  240 . In an extension of the embodiment of FIG. 5, a change in the phase of the lowest frequency RF signal (60 MHz in the examples considered herein) changes the phase of all of the other RF frequencies with respect to the lowest RF frequency signal. Consequently, optimum adjustment of all of the phase relationships may be more difficult to achieve than with the embodiment of FIG. 4 in which the phase of each higher RF frequency component can be individually adjusted with respect to the phase of the lowest RF frequency.  
         [0034]    While the principles of the invention have been made clear in the illustrative embodiments set forth above, it will be apparent to those skilled in the art that various modifications may be made to the structure, arrangement, proportion, elements, materials, and components used in the practice of the invention.  
         [0035]    For example, an entirely automatic embodiment of the invention has not been described, but it is entirely reasonable to suppose that such an embodiment will be possible after a data base that correlates V-I probe outputs to process uniformity will have been established.  
         [0036]    It will thus be seen that the objects of this invention have been fully and effectively accomplished. It will be realized, however, that the foregoing preferred specific embodiments have been shown and described for the purpose of illustrating the functional and structural principles of this invention and are subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.