Abstract:
A method and apparatus for processing a substrate in a capacitively-coupled plasma processing system having a plasma processing chamber and at least an upper electrode and a lower electrode. The substrate is disposed on the lower electrode during plasma processing. The method includes providing at least a first RF signal, which has a first RF frequency, to the lower electrode. The first RF signal couples with a plasma in the plasma processing chamber, thereby inducing an induced RF signal on the upper electrode. The method also includes providing a second RF signal to the upper electrode. The second RF signal also has the first RF frequency. A phase of the second RF signal is offset from a phase of the first RF signal by a value that is less than 10%. The method further includes processing the substrate while the second RF signal is provided to the upper electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention is related to the following applications, all of which are incorporated herein by reference: 
         [0002]    Commonly assigned application entitled “Method and Apparatus for Processing a Substrate Using Plasma”, filed on even date herewith by the same inventors herein (Attorney Docket Number LMRX-P119) 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    In the processing of semiconductor substrates, plasma processing is often employed. Plasma processing may involve different plasma-generating technologies, for example, inductively-coupled plasma processing systems, capacitively-coupled plasma processing systems, microwave-generated plasma processing systems, and the like. Manufacturers often employ capacitively-coupled plasma processing systems in processes that involve the etching of materials using a photo resist mask. 
         [0004]    Important consideration for plasma processing of substrates involves a high etch rate and a high photo resist selectivity. A high etch rate refers to the rate at which the target material is etched during plasma processing. Generally speaking, the faster the underlying layer can be etched, a greater number of wafers can be processed per unit of time. All things being equal, manufacturers desire to process more wafers per unit of time to increase wafer processing efficiency. Photo resist selectivity refers to the discrimination between the photo resist mask and the underlying target layer during etching. 
         [0005]    As circuit density increases, manufacturers are required to etch or to form a greater number of devices per unit area on the wafer. The higher device density requires a thinner photo resist layer. The thinner photo resist layer, in turn, tends to be more susceptible to being inadvertently etched away. As a result, manufacturers constantly strive to create processing recipes that can etch the underlying layer at a high etch rate while avoiding damage to the photo resist mask. 
         [0006]    One way to increase the etch rate is to increase the plasma density during plasma processing. In a capacitively-coupled plasma processing system, plasma density may be increased by increasing the power of the higher frequency RF signals. To facilitate discussion,  FIG. 1  shows a prior art multi-frequency capacitively-coupled plasma processing system  100 , representing the plasma processing system typically employed to process substrates. As seen in  FIG. 1 , multi-frequency capacitively-coupled plasma processing system  100  includes a chamber  102  which is disposed in between an upper electrode  104  and a lower electrode  106 . 
         [0007]    In the implementation of  FIG. 1 , lower electrode  106  is provided with multiple RF frequencies, such as 2 Megahertz, 27 Megahertz, and 60 Megahertz. Upper electrode  104  is grounded in the implementation of  FIG. 1 . Multi-frequency capacitively-coupled plasma processing system  100  also includes a plurality of confinement rings  108 A,  108 B,  108 C, and  108 D. The confinement rings  108 A- 108 D function to confine the plasma within chamber  102  during plasma processing. 
         [0008]    There is also shown in  FIG. 1  a peripheral RF grounded ring  10 , representing the RF ground for the plasma generated within chamber  102 . To isolate peripheral RF ground  110  from upper electrode  104 , an insulating ring  112  is typically provided. A similar insulating ring  114  is also provided to insulate lower electrode  106  from an RF ground  116 . During plasma processing, the RF power provided to lower electrode  106  excites etching gas provided into chamber  102 , thereby generating a plasma within chamber  102  to etch a substrate that is typically disposed on lower electrode  106  (substrate is not shown to simplify  FIG. 1 ). 
         [0009]    As discussed earlier, it is highly desirable to etch the target layer on the substrate while the substrate is disposed in chamber  102  without unduly damaging the overlying photo resist mask. In the prior art, increasing the etch rate of the target layer may be achieved by increasing the plasma density within chamber  102 . Generally speaking, the plasma density may be increased by increasing the power level of the higher frequency RF signals that are provided to lower electrode  106 . In the context of the present invention, a high frequency RF signal is defined as signals having a frequency higher than about 10 Megahertz. Conversely, RF signals with frequencies below 10 Megahertz are referred to herein as Low Frequency Signals. 
         [0010]    However, by increasing the power level of the higher frequency RF signals (e.g., the 27 Megahertz RF signal or the 60 Megahertz RF signal of  FIG. 1 ), it may be challenging to confine the generated plasma within chamber  102 . Even if the plasma can be satisfactorily confined, electron loss to the upper electrode during plasma processing places an upper limit on the plasma density within chamber  102 . It has been found that as the plasma density increases; electrons are lost to the grounded upper electrodes or other grounded surfaces of the multi-frequency capacitively-coupled plasma processing system  100 , thereby causing the plasma density within chamber  102  to reach a saturation point. Beyond this saturation point, increasing the RF power of the higher frequency RF signal does not increase the plasma density since the electron loss outpaces the generation of ions. 
         [0011]    Furthermore, increasing the RF power to the higher frequency RF signals has been found to adversely affect the photo resist selectivity. At a high RF power level, the photo resist mask is damaged to a greater extent due to increased bombardment, which causes the photo resist mask to erode away at a faster rate, thereby negatively impacting the etching process. 
         [0012]      FIG. 2  shows a prior art implementation whereby one or more high frequency RF signals  202  (e.g., the 60 Megahertz RF signal of  FIG. 2 ) are provided to upper electrode  104  in order to provide additional control over the generation of ions within chamber  102 . However, the implementation of  FIG. 2  still does not solve the aforementioned problem of plasma density saturation point effect. When the RF power level of the higher frequency signal provided to upper electrode  104  is increased, the aforementioned saturation point effect is also observed, limiting the plasma density and consequently, the etch rate through the target layer irrespective of the increase in the RF power level to the higher frequency RF signals. 
         [0013]    Additionally, other prior art implementation has tried to control the photo resist selectivity by controlling the temperature of the electrodes. It has been found that the approach of controlling the temperature of the electrodes is minimally effective in controlling the photo resist selectivity. Furthermore, the approach of controlling the temperature of the electrodes does not address the aforementioned problem of plasma density saturation point effect. 
         [0014]    Therefore, various aforementioned prior art implementations have proven ineffective in increasing etch rate without adversely affecting or maintaining high photo resist selectivity in capacitively-coupled plasma processing system in processes that involve the etching of materials using a photo resist mask. In the prior art implementation of  FIG. 1 , the increase in RF power to the lower electrode may lead to unconfinement of plasma, saturation point of plasma density, and adversely affect the photo resist selectivity. Whereas in the prior art implementation of  FIG. 2 , the increase in RF power to the upper electrode may lead to saturation point of plasma density. Furthermore, the prior art implementation of controlling temperature of the electrodes is minimally effective in controlling the photo resist selectivity while providing no solution for the plasma density saturation effect. 
       SUMMARY OF INVENTION 
       [0015]    The invention relates, in an embodiment, to a method for processing a substrate in a capacitively-coupled plasma processing system, which has a plasma processing chamber and at least an upper electrode and a lower electrode. The substrate is disposed on the lower electrode during plasma processing. The method includes providing at least a first RF signal to the lower electrode. The first RF signal has a first RF frequency. The first RF signal couples with a plasma in the plasma processing chamber, thereby inducing an induced RF signal on the upper electrode. The method also includes providing a second RF signal to the upper electrode. The second RF signal also has the first RF frequency. A phase of the second RF signal is offset from a phase of the first RF signal by a value that is less than 10%. The method further includes processing the substrate while the second RF signal is provided to the upper electrode. 
         [0016]    The above summary relates to only one of the many embodiments of the invention disclosed herein and is not intended to limit the scope of the invention, which is set forth in the claims herein. These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
           [0018]      FIG. 1  shows a prior art multi-frequency capacitively-coupled plasma processing system, representing the plasma processing system typically employed to process substrates. 
           [0019]      FIG. 2  shows a prior art implementation whereby one or more high frequency RF signals are provided to upper electrode in order to provide additional control over the generation of ions within chamber. 
           [0020]      FIG. 3  shows a simplified diagram of an implementation wherein a mirroring circuit is employed to detect an RF signal from the lower electrode and to provide the upper electrode with a transformed RF signal that is in-phase with the RF signal of lower electrode during plasma processing. 
           [0021]      FIG. 4   a  shows an example plot of an RF signal from the lower electrode. 
           [0022]      FIG. 4   b  shows an example plot of an RF signal directed to the upper electrode running in phase with the RF signal from the lower electrode of  FIG. 4   a    
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0023]    The present invention will now be described in detail with reference to a few embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. 
         [0024]    Various embodiments are described herein below, including methods and techniques. It should be kept in mind that the invention might also cover articles of manufacture that includes a computer readable medium on which computer-readable instructions for carrying out embodiments of the inventive technique are stored. The computer readable medium may include, for example, semiconductor, magnetic, opto-magnetic, optical, or other forms of computer readable medium for storing computer readable code. Further, the invention may also cover apparatuses for practicing embodiments of the invention. Such apparatus may include circuits, dedicated and/or programmable, to carry out tasks pertaining to embodiments of the invention. Examples of such apparatus include a general-purpose computer and/or a dedicated computing device when appropriately programmed and may include a combination of a computer/computing device and dedicated/programmable circuits adapted for the various tasks pertaining to embodiments of the invention. 
         [0025]    In accordance with embodiments of the invention, there are provided methods and arrangements for controlling the electron loss to the upper electrode such that the plasma density can be increased without the need to unduly increase the power to the plasma. By increasing the plasma density without a concomitant increase to the RF power requirement, the target layer can be etched at a higher rate without unduly degrading the photo resist selectivity. In an embodiment, the upper electrode is configured such that the upper electrode is negatively biased, thereby allowing electrons present in the plasma chamber to be repelled from the upper electrode and trapped within the plasma volume for a longer period of time. As the negatively charged electrons are trapped for a longer period of time, the plasma density is increased. 
         [0026]    Generally speaking, during plasma processing the bombardment mechanism causes electrons to be emitted from the substrate. As discussed earlier, electron loss to the upper electrode limits the increase in plasma density since the electron loss creates saturation point effect which limits the plasma density increase irrespective of the RF power provided to the plasma. By driving the upper electrode more negatively, the electrons are thus repelled from the upper electrode instead of being quickly lost to the upper electrode, resulting in a greater number of electrons in the plasma, thereby increasing the plasma density. The higher plasma density then can more effectively etch the target layer to achieve the desired high etch rate. Since it is unnecessary to increase the RF power to achieve the high level of plasma density, photo resist selectivity is not adversely affected to the same degree as might have been in the prior art. 
         [0027]    The above summary relates to only one of the many embodiments of the invention disclosed herein and is not intended to limit the scope of the invention, which is set forth in the claims herein. These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
         [0028]      FIG. 3  shows, in accordance with an embodiment of the present invention, a simplified diagram of an implementation wherein a mirroring circuit is employed to detect an RF signal from the lower electrode and to provide the upper electrode with a transformed RF signal that is in-phase with the RF signal of lower electrode during plasma processing. As the term is employed herein, in-phase denotes the implementation wherein the phase difference between the RF signal to the lower electrode and the RF signal to the upper electrode is within about 1%. 
         [0029]    In the implementation of  FIG. 3 , lower electrode  304  is provided with multiple RF frequencies signal  302  such as 2 Megahertz, 27 Megahertz, and 60 Megahertz. In an embodiment, the RF signal from lower electrode  304  may be detected by probe  306 , wherein probe  306  is a phase and amplitude detector designed to pick up low frequency RF signal, i.e., frequencies less than 10 Megahertz. 
         [0030]    In accordance with an embodiment of the present invention, the signal from the probe  306  is directed to a control circuit  308 . The control circuit  308  is provided with the capability for phase and amplitude adjustment allowing for the modification of the phase and/or amplitude of the RF signal depending on whether the RF signal is to be in-phase or out-of-phase with the RF signal from the lower electrode  304 . 
         [0031]    The control signal coming out of control circuit  308  is directed to an RF signal generator  310  for generating an RF signal. Thereafter, the RF signal generated by RF generator is optionally amplified (via amplifier  320 ) to the desired phase or amplitude. In the context of the embodiment of the present invention, the amplitudes of the RF signals from the upper and lower electrodes are considered to be the same when the values of the amplitudes are within about 1% of each other. 
         [0032]    In the implementation of  FIG. 3 , the amplified RF signal from the amplifier  320  is directed to the upper electrode  312 . Consequently, the RF signal being directed to the upper electrode is in-phase with the RF signal being supplied to the lower electrode in accordance with an embodiment of the present invention. 
         [0033]    The features and advantages of having the RF signal directed to the upper electrode running in-phase with the RF signal from lower electrode in-phase can be better understood through  FIGS. 4   a  and  4   b .  FIG. 4   a  shows an example plot of an RF signal from the lower electrode, in accordance with one embodiment of the present invention.  FIG. 4   b  shows an example plot of an RF signal directed to the upper electrode running in phase with the RF signal from the lower electrode of  FIG. 4   a , in accordance with one embodiment of the present invention. 
         [0034]    As mentioned previously, in-phase denotes the implementation wherein the phase difference between the RF signal to the lower electrode  304  and the RF signal to the upper electrode  312  is within about 1%. At the minimal points during the negative cycles of the implementation of  FIGS. 4   a  and  4   b , the RF signal  410  of the lower electrode and the RF signal  450  of the upper electrode are at the most negative voltage values with respect to the plasma. 
         [0035]    Referring back to  FIG. 3  when both RF signals are in-phase and at their minimal, as shown in  FIGS. 4   a  and  4   b , during plasma processing in the plasma chamber  314 , the upper electrode  312  and lower electrode  304  are at their most negative values. The positive charged argon particles (not shown) in the plasma chamber  314  will accelerate and bombard the upper electrode  312  and the substrate  316 , which is disposed above the lower electrode  304 , to generate primary electrons which are low energy electrons and secondary embedded electrons which are high energy electrons. 
         [0036]    Since both the substrate  316 , disposed atop the lower electrode  304 , and the upper electrode  312 , during this negative cycle of RF signals, are at their most negative values, the maximum potential between the upper electrode  312  and lower electrode  304  with the plasma creates the highest electron trapping. The electrons that come off of the upper electrode  312  or the substrate  316  tend to be trapped between the negatively biased upper electrode  312  and the negatively biased substrate  316 , which is disposed above the lower electrode  304 . Since the electrons are negatively charged, the electrons might repel in between the two negatively charged upper electrode  312  and lower electrode  304 . Instead of being immediately lost to upper electrode  312  (as may be the case if upper electrode  312  is grounded, for example) the negatively biased upper electrode  312  may repel the negatively charged electrons, thereby causing the electrons to be trapped in between upper electrode  312  and lower electrode  304  for a longer period of time. It is believed that eventually, through the mechanism of random collision, the negatively charged electrons are eventually lost to RF ground  318 . 
         [0037]    The longer residence time of the negatively charged electrons within plasma chamber  314  contributes to a higher plasma density without requiring a corresponding increase in the amount of RF power supplied to plasma processing chamber  300 . Note that the mechanism to increase the plasma density of  FIG. 3  does not require the increase in the RF power supplied to the RF signals. Consequently, the photo resist selectivity is not negatively impacted to the same degree that might have been impacted had the higher plasma density been achieved by increasing the RF power level. 
         [0038]    At the maxima points during the positive cycles of the implementation of  FIGS. 4   a  and  4   b , the RF signal  420  of the lower electrode and the RF signal  460  of the upper electrode are at the highest positive voltage values with respect to the plasma. In accordance with an embodiment of the present invention, secondary electrons are not being emitted during this time because the potential between the upper electrode  312  and the lower electrode  304  with respect to the plasma is low. 
         [0039]    Further, during the positive cycle, the plasma potential in the plasma volume is substantially higher than the potential of the peripheral ground plate. It is believed that secondary electrons ejected from these peripheral ground plates (e.g., ground plates  318  and  322 ) are also trapped in the plasma volume between the ground plates, resulting also in a longer residence time and a higher plasma density. Over the entire cycle (both negative and positive), the average plasma density is thus increased. 
         [0040]    In an embodiment, the phase difference between the RF signal from the lower electrode and the RF signal from the upper electrode can be used as a knob to control the uniformity of etching, i.e., better photo resist selectivity to the underlying layer being etched. In the implementation of  FIG. 3 , an arrangement where the phase of the RF signal directed to the upper electrode  112  can be adjusted to the phase of the RF signal of the lower electrode  304  during part of the cycle where the RF signals are at their most negative values. 
         [0041]    For example, it is known that lower energy electrons and higher energy electrons impact the etch process in different ways. Since a high density of higher energy electrons is believed to be beneficial for photo resist selectivity, it is desirable in many cases to negatively bias upper electrode  312  to cause more of the higher energy electrons to be trapped. It has been observed that unexpected beneficial etching uniformity may be achieved by adjusting the phase difference between the RF signal directed to the upper electrode  312  and the RF signal from the lower electrode  304  during the negative cycle. In accordance with an embodiment of the present invention, the phase shifting is found to be beneficial to etching uniformity for phase difference of less than about 10%. 
         [0042]    As can be appreciated from the foregoing, embodiments of the invention achieve a higher level of plasma density to improve etching through the target layer in the capacitively-coupled plasma processing chamber without unduly damaging the photo resist during etching. By providing a mechanism for increasing the plasma density without requiring a concomitant increase in the RF power level of the RF signals provided to the plasma processing chamber, plasma density is increased while PR photo resist is maintained the same or is minimally impacted. Furthermore, the uniformity of etching is further enhanced through the control of the phase difference between the RF signal directed to the upper electrode and the RF signal to the lower electrode. 
         [0043]    In an embodiment, the phase of the upper electrode RF signal may be adjusted to either lag or lead the phase of the lower electrode RF signal. When the upper electrode RF signal is out of phase with the lower electrode RF signal, it is observed that photoresist selectivity is reduced. For certain applications such as photoresist (PR) or polymer strip, controlling the relative phases between the upper electrode RF signal and lower electrode RF signal may improve the desired result of removing more PR or polymer. 
         [0044]    Alternatively or additionally, the amplitude of the upper electrode RF signal may be adjusted to either exceed or to be lower than the amplitude of the lower electrode RF signal. When the amplitude of the upper electrode RF signal is not equal to the amplitude of the lower electrode RF signal (defined herein as being different by more than 5%), it is observed that photoresist selectivity is reduced. As in the case with the phase difference, for certain applications such as photoresist (PR) or polymer strip, controlling the relative amplitudes between the upper electrode RF signal and lower electrode RF signal may improve the desired result of removing more PR or polymer. 
         [0045]    While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents, which fall within the scope of this invention. Also, the title, summary, and abstract are provided herein for convenience and should not be used to construe the scope of the claims herein. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. Although various examples are provided herein, it is intended that these examples be illustrative and not limiting with respect to the invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.