Abstract:
A method of selectively activating a chemical process using a DC pulse etcher. A processing chamber includes a substrate therein for chemical processing. The method includes coupling energy into a process gas within the processing chamber so as to produce a plasma containing positive ions. A pulsed DC bias is applied to the substrate, which is positioned on a substrate support within the processing chamber. Periodically, the substrate is biased between first and second bias levels, wherein the first bias level is more negative than the second bias level. When the substrate is biased to the first bias level, mono-energetic positive ions are attracted from plasma toward the substrate, the mono-energetic positive ions being selective so as to enhance a selected chemical etch process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related to plasma processing systems and, more specifically to plasma processing systems and methods for substrate etching. 
       BACKGROUND OF THE INVENTION 
       [0002]    During semiconductor processing, plasma is often utilized to assist etch processes by facilitating the anisotropic removal of material along fine lines or within vias or contacts patterned on a semiconductor substrate. Examples of such plasma-assisted etching include reactive ion etching (“RIE”), which is in essence an ion-activated chemical etching process. 
         [0003]    Although RIE has been in use for decades, its maturity is accompanied by several negative features, including: (a) broad ion energy distribution (“IED”); (b) various charge-induced side effects; and (c) feature-shape loading effects (i.e., micro loading). For example, a broad IED contains ions that have either too little, or too much, energy to be useful, the latter of which is susceptible to causing substrate damage. Additionally, the broad IED makes it difficult to selectively activate desired chemical reactions, where side reactions are often triggered by ions of an undesired energy. Further, positive charge buildup on the substrate may occur and repel ion incident onto the substrate. Alternatively, the charge buildup may produce local charge differences that affect damaging currents on the substrate. Charge buildup may be due, in part, to the RF energy used to produce a negative bias on the non-conductive substrate or on the chuck, or table, used to support the substrate and attract positive ions from the plasma. Such RF frequencies are typically too high to allow positive or near neutral potential to exist for a sufficient time to attract electrons to neutralize the positive charges accumulated on the substrate. Non-uniform accumulation of charge across the surface of the substrate may create potential differences that can lead to currents on the substrate that can be damaging to devices being formed. 
         [0004]    One known, conventional approach to addressing these problems has been to utilize neutral beam processing. A true neutral beam process takes place essentially without any neutral thermal species participating as the chemical reactant, additive, and/or etchant. The chemical etching process at the substrate, on the other hand, is activated by the kinetic energy of the incident, directionally energetic neutral species. The incident directional, energetic, and reactive neutral species also serve as the reactants or etchants. 
         [0005]    One natural consequence of neutral beam processing has been the absence of micro-loading. That is, because of the process in which the thermal species that serve as etchants in RIE, there is relative little flux-angle variation in the incident neutral species. However, the lack of micro-loading results in an etch efficiency, or maximum etching yield, of unity, in which one incident neutral nominally prompts only one etching reaction. But with RIE, the abundant thermal neutral etchant species may all participate in the etching of the film, where the activation by one energetic incident ion may achieve an etch efficiency of 10, 100, and even 1000, while being forced to live with micro-loading. 
         [0006]    The separation of ionization and chemistry may be achieved if the voltage applied to the RF electrode is on the order of 1.5 kV and self-bias voltage on the order of −700 V. However, many processes, and devices, are intolerant of high ion-energy. 
         [0007]    While many attempts have been made to cure these shortcomings, i.e., etch efficiency, micro-loading, charge damage, etc., there still remains, and the etch community continues to explore, novel, practical solutions to this problem. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention overcomes the problems and other shortcomings of the prior art plasma etching systems set forth above. 
         [0009]    According to one embodiment of the present invention, a method of selectively activating a chemical process using a DC pulse etcher is performed in a processing chamber having a substrate therein for chemical processing. The method includes coupling energy into a process gas within the processing chamber so as to produce a plasma containing positive ions. A pulsed DC bias is applied to the substrate, which is positioned on a substrate support within the processing chamber. Periodically, the substrate is biased between first and second bias levels, wherein the first bias level is more negative than the second bias level. When the substrate is biased to the first bias level, mono-energetic positive ions are attracted from plasma toward the substrate, the mono-energetic positive ions being selective so as to enhance a selected chemical etch process. 
         [0010]    Another embodiment of the present invention includes a plasma processing method in which a substrate is supported on a substrate support within a plasma processing chamber. The substrate support is positioned at a first end of the plasma processing chamber. A plasma is electrically energized by a plasma generating electrode, which is positioned proximate a second end, opposite the first end, of the plasma processing chamber. The plasma is formed between the plasma generating electrode and the substrate. A pulsed DC waveform is applied to the substrate so as to bias the substrate at a first voltage and a second voltage. When the substrate is pulsed at the first voltage, positive ions are attracted from the plasma toward the substrate. Periodically, and when the substrate is pulsed at the second voltage, being less negative than the first voltage, electrons are attracted from the plasma toward the substrate. 
         [0011]    Still another embodiment of the present invention is directed to a plasma etching apparatus that includes a plasma processing chamber and a substrate support positioned within and at a first end of the same. A plasma generating electrode is positioned proximate to a second end of the plasma processing chamber, which opposes the first end. The plasma generating electrode is operably coupled to a plasma generating electrode that is configured to energize the plasma generating electrode, which capacitively couples power into the plasma processing chamber to form a plasma. The plasma is positioned between the plasma generating electrode and the substrate. The substrate support is operably coupled to a DC pulse generator, which is configured to apply a pulsed DC bias voltage to a substrate positioned on the substrate support. The DC pulse generator periodically applies first and second voltages to the substrate such that during the first voltage, positive ions are attracted to the substrate and during the second voltage, electrons are attracted to the substrate. 
         [0012]    While the present invention will be described in connection with certain embodiments, it will be understood that the present invention is not limited to these embodiments. To the contrary, this invention includes all alternatives, modifications, and equivalents as may be included within the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0013]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention. 
           [0014]      FIG. 1  is a schematic view of a chemical processing system in accordance with one embodiment of the present invention. 
           [0015]      FIG. 2  is a graphical representation of a DC voltage waveform and an RF voltage waveform suitable for use in driving DC and RF voltage source of the system of  FIG. 1  in accordance with one embodiment of the present invention. 
           [0016]      FIG. 3  is a schematic view of a chemical processing system in accordance with another embodiment of the present invention. 
           [0017]      FIG. 4A  is a schematic view of a chemical processing system in accordance with another embodiment of the present invention. 
           [0018]      FIG. 4B  is a schematic view of an alternative to the chemical processing system of  FIG. 4A . 
           [0019]      FIG. 5A  is a schematic view of a chemical processing system in accordance with still another embodiment of the present invention. 
           [0020]      FIG. 5B  is a schematic view of an alternative to the chemical processing system of  FIG. 5A . 
           [0021]      FIG. 6  is a schematic view of a chemical processing system in accordance with still another embodiment of the present invention. 
           [0022]      FIG. 7  is a schematic view of a chemical processing system in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following description, to facilitate a thorough understanding of the invention and for purposes of explanation and not limitation, specific details are set forth, such as a particular geometry of the plasma processing system and various descriptions of the system components. However, it should be understood that the invention may be practiced with other embodiments that depart from these specific details. 
         [0024]    Nonetheless, it should be appreciated that, contained within the description are features which, notwithstanding the inventive nature of the general concepts being explained, are also of an inventive nature. 
         [0025]    According to one embodiment, a method and system for performing plasma-activated chemical processing of a substrate is provided, inter alia, to alleviate some or all of the above identified issues. Plasma-activated chemical processing includes kinetic energy activation (i.e., thermal charged species) and, hence, it achieves high reactive or etch efficiency. However, plasma-activated chemical processing, as provided herein, also achieves monochromatic or narrow band IED, mono-energetic activation, space-charge neutrality, and hardware practicality. 
         [0026]    Referring now to the figures, and in particular to  FIG. 1 , a chemical processing system  10  according to one embodiment of the present invention is shown and described in detail. The chemical processing system  10  is configured to perform plasma-assisted or plasma-activated chemical processing of a substrate  12  positioned within a processing chamber  14  of the chemical processing system  10 . The chemical processing system  10  further comprises a gas feed supply  16  that is fluidically coupled to the processing chamber  14  and is configured to supply one or more processing gases to the processing space  18  within the processing chamber  14  and above the substrate  12  when positioned on a substrate support  20 . A vacuum pump  19  draws a vacuum on the processing space  18 . 
         [0027]    Three electrodes  22 ,  24 ,  26  reside within the processing chamber  14 . The first electrode  22  may be incorporated into, or comprise, the substrate support  20  while the second electrode  24  is positioned within the processing chamber  14  and opposing the substrate  12 . The third electrode  26 , being optional, may be positioned along one or more walls of the processing chamber  14  and may be grounded. 
         [0028]    The first electrode  22  is biased by a DC pulse from a DC pulse generator  28 , while the second electrode  24  is included in a plasma source  30  and is actively powered. More particularly, and as specifically shown, the first electrode  22  is electronically coupled to ground through a negative DC voltage source  32  via, for example, a relay circuit  34 , while the second electrode  24  is coupled to an AC voltage source  36  that may be an RF power supply. 
         [0029]    In use, the AC voltage source  36  may be electronically coupled to the second electrode  24  via an impedance matching circuit  38  and is configured to apply a continuous AC power to the second electrode  24 . For example, as shown in  FIG. 2 , a negative AC RF voltage  40  operating at 13.56 MHz, may be applied to the second electrode  24  for igniting a capacitively coupled plasma  42  within the processing space  18 . Generally, the plasma  42 , particularly the electrons within the plasma  42 , are retained within the processing chamber  14  proximate the grounded third electrode  26 . While the generic impedance matching circuit  38  is shown in this and other illustrative embodiments, one of ordinary skill in the art would readily appreciate that other manners of electrical connections may be used. 
         [0030]    At a particular time interval, such as in accordance with a desired waveform, the relay circuit  34  coupled to the first electrode  22  is switched so as to apply a pulsed DC bias to the first electrode  22 . For example, and as shown in  FIG. 2 , a pulsed negative bias  46  may be applied to the first electrode  22 , during which positive ions are drawn toward the substrate  12 . Pulsed periods of less negative bias  44  (even positive bias) applied to the first electrode  22  between the intervals of negative bias  46  draws electrons from the processing space  18 , proximate the third electrode  26 , toward the first electrode  22  and the substrate  12 . As a result, the DC pulse bias achieves a mono-energetic ion excitation of the substrate  12  during the negative bias  46  and an energetic electron dump via a more positive bias  44  onto the substrate  12  to neutralize positive charge on the substrate  12 . The waveform for the DC pulse (V RF (t)) may vary in DC pulse frequency (from about 1 Hz to about 1 GHz and, more particularly from about 100 kHz to about 1 MHz) and duty cycle (from about 1% to about 99%) in which the fraction of the total pulse interval in which the DC pulse is applied and which may be adjusted to a particular energetic electron dump need, and where the pulse duty cycle is defined as the ratio of time of applied negative bias (i.e. to attract ions), to the total pulse period. Varying the duty cycle may be used to control how mono-energetic the ion excitation of the substrate is. In general, the duty cycle should be kept large enough to maintain as mono-energetic ion energies, as possible, without generation of any performance-degrading charge-up effects on the substrate. Due to the high mobility of electrons in the plasma, a duty cycle of 90%, 95%, or even 99% may provide sufficient time for electrons to provide neutralization of charge built from ion impingement, in any high aspect ratio (“HAR”) features present on the substrate. 
         [0031]    With reference now to  FIG. 3 , a chemical processing system  50  in accordance with another embodiment of the present invention is shown and described in detail. The chemical processing system  50  is similar to that of  FIG. 1 , having the gas feed supply  16  ( FIG. 1 , not shown in  FIG. 3A ) to supply process gas to a processing space  52  and a vacuum pump  19  ( FIG. 1 , not shown in  FIG. 3A ) to draw a vacuum on the same. A substrate support  54  supports a substrate  56  within the chamber  58 . Three electrodes  60 ,  62 ,  64  are also provided in the processing space  52  and oriented in the manner described previously with respect to the system  10  of  FIG. 1 . The second electrode  62 , as shown, is divided in two parts such that the second electrode  62  includes a circular central electrode  62   a  and an annular peripheral electrode  62   b  surrounding and insulated from the central electrode  62   a  by an annular insulating ring  66 . The second electrode  62  is coupled to an AC voltage source  68  via impedance matching circuit  70  and is configured to apply a separately controllable and continuous AC bias to the electrode parts  62   a ,  62   b . The second electrode  62  is further coupled to the plasma source  72 . 
         [0032]    The first electrode  60 , again shown as forming a portion of the substrate support  54 , is electrically coupled to a DC voltage source  74  via a relay circuit  76 , which is operable to be switched in the manner described in greater detail above. By segmenting the second electrode  62 , greater control of plasma formation and uniformity may result. That is, the distribution of plasma formation may be controlled radially outwardly toward the walls of the processing space  52 . 
         [0033]      FIGS. 4A and 4B  illustrate two related embodiments of the present invention. For illustrative convenience, like reference numerals having primes thereafter designate corresponding components of the embodiments. With specific reference to the embodiment of  FIG. 4A , a chemical processing system  80  is shown and includes a processing chamber  82  that is generally similar to those described previously, although not all components are shown for illustrative convenience. The chemical processing system  80  includes three electrodes  84 ,  86 ,  88 ; however, the first electrode  84  of the instant chemical processing system  80  is alternately coupled to ground through the negative DC voltage source  90  or a parallel positive DC voltage source  92 , via, a double throw relay circuit  94 . The relay circuit  94  is switched so as to alternately apply a DC voltage function, for example, a negative bias followed by a positive bias, to the first electrode  84  to attract mono-energetic positive ions onto the substrate  96  during negative pulses, while the positive bias draws electrons or negative ions to the substrate  96  between the negative pulses to neutralize positive charge that may have accumulated on the substrate  96  during the negative pulses. 
         [0034]      FIG. 4B  is similar to  FIG. 4A  except that the second electrode  86 ′ is divided into a central portion  86   a  and  a  concentric outer portion  86   b  with an insulating ring  87  therebetween, as was described previously. It would be understood that the plasma generation source  98  with impedance matching circuit  100  of  FIG. 4A  may be configured to apply a separately controllable and continuous AC bias to the electrode parts  86   a ,  86   b  in  FIG. 4B . 
         [0035]    The plasma generating electrode need not be RF based. Instead, and as is shown in  FIG. 5A , a chemical processing system  110  for processing a substrate  111  in accordance with yet another embodiment of the present invention, similar to that of  FIG. 1  but with the plasma source  30  ( FIG. 1 ) including a DC source  112  powering the second electrode  114  while the first and third electrodes  116 ,  118  electrically coupled to a DC voltage source  119  and ground, respectively, and has been discussed previously. With the DC source  112 , the grounded third electrode  118 , which is optional in embodiments wherein the plasma source applies an RF bias to the second electrode  24  ( FIG. 1 ), is generally required. The third electrode  118  may comprise, in part, a grounded wall of the processing chamber  120 , or may be a separately-constructed electrode that is then positioned inside, or in some configurations outside, the processing chamber  120 . 
         [0036]      FIG. 5B  illustrates a chemical processing system  110 ′ that is similar to the chemical processing system  110  of  FIG. 5A  and in which like reference numerals having primes thereafter designate corresponding components of the embodiments. However, in  FIG. 5B  the second electrode  114 ′ is electronically coupled to ground through the negative DC voltage source  112 ′ via a relay circuit  122 . In that regard, a pulsed DC voltage may also be applied to the second electrode  114 ′. 
         [0037]    Additionally,  FIG. 6  illustrates a chemical processing system  130  in accordance with another embodiment of the present invention and in which like reference numerals having primes thereafter designate corresponding components of the embodiments. The illustrative chemical processing system  130  is again similar to the system  10  of  FIG. 1 , but with the first electrode  22  being segmented to include a central circular segment  22   a , an intermediate annular electrode segment  22   b  concentrically surrounding the central electrode segment  22   a , and an outer electrode segment  22   c  concentrically surrounding the central and intermediate electrode segments  22   a ,  22   b . The electrode segments  22   a ,  22   b ,  22   c  are separated by annular insulator rings  132 ,  134  and respectively biased by separate controllable DC bias voltage sources  74   a ,  74   b ,  74   c  via relay switches  76   a ,  76   b ,  76   c . The DC sources  74   a ,  74   b ,  74   c  each apply pulsed DC voltages to the electrode segments  22   a ,  22   b ,  22   c  of the first electrode  22 , typically at the same frequencies and in-phase, but adjusted, for example by varying pulse widths or duty cycle, to improve radial uniformity. 
         [0038]    The conductivity of the substrate  12 ′ for use with the chemical processing system  130  of  FIG. 6  having the electrically segmented first electrode  22 ′ should be less conductive than the substrates suitable for use with other embodiments. 
         [0039]      FIG. 7  illustrates a chemical processing system  140  in accordance with still another embodiment of the present invention. Again, three electrodes  142 ,  144 ,  146  are operably coupled to a processing chamber  148 . The first electrode  142  may support a substrate  150  within the processing chamber  148  while the second electrode  144  is positioned proximate a side of the processing chamber  148  that generally opposes the substrate  150 . 
         [0040]    The second electrode  144 , as shown, is segmented and includes a central portion  144   a , an intermediate portion  144   b  separated from the central portion  144   a  by a first annular insulator  152 , and an outer portion  144   c  separated from the intermediate portion  144   b  by a second annular insulator  154 . Each portion  144   a ,  144   b ,  144   c  of the second electrode  144  is respectively biased by separate controllable DC bias voltage sources  156   a ,  156   b ,  156   c  via relay switches  158   a ,  158   b ,  158   c.    
         [0041]    The first electrode  142  is electrically coupled to one or more AC voltage sources  160  having an RF power supply  162  therein. The AC voltage source  160  may be electronically coupled to the second electrode  144  via an impedance matching circuit  164  and is configured to apply a continuous AC bias to the second electrode  144 . 
         [0042]    The various embodiments of the present invention that are described in detail above provide a flux of ions onto a substrate having a narrow ion energy distribution. This is advantageous in many plasma processes, particularly in ion-activated chemical etching processes, where the energy of the ions is a factor in selecting the chemical process that will be activated. Chemical processes may therefore be selected and controlled by mono-energetic ions, i.e., if the energy distribution is narrow. With the present invention, this can be achieved by controlling the level of DC pulses used to bias the substrate. 
         [0043]    Additionally, the buildup of positive charge on the substrate during ion bombardment, which occurs when bias voltage is more negative, may be neutralized by pulsing the bias on the substrate and controlling the more positive, or less negative, level of the pulsed waveform. The establishment of the pulse width (or duty cycle) of the waveform controls the amount of negative charge attracted to the substrate to neutralize the substrate. The charge may be electrons or, where the pulse width is sufficiently wide enough, negative ions when they are present in the plasma. 
         [0044]    While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. The invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the present invention.