Abstract:
The present invention provides a method and apparatus for achieving conformal step coverage on a substrate by ionized metal plasma deposition. A target provides a source of material to be sputtered and ionized by a plasma maintained by a coil. The ionized material is deposited on the substrate that is biased to a negative voltage. A power supply coupled to the target supplies a modulated or time-varying signal thereto during processing. Preferably, the modulated signal includes a negative voltage portion and a positive voltage portion. The negative voltage portion and the positive voltage portion are alternated to cycle between a center-strong sputter step and an edge-strong sputter step. The film quality and uniformity can be controlled by adjusting the frequency and amplitude of the signal, the duration of the positive portion of the signal, the power supplied to each of the support member and the coil, and other process parameters.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an apparatus and method for processing substrates. Specifically, the invention relates to a method for depositing a conformal layer of material on a substrate using an ionized metal plasma process.  
           [0003]    2. Background of the Related Art  
           [0004]    Sub-quarter micron multi-level metallization represents one of the key technologies for the next generation of ultra large-scale integration (ULSI) for integrated circuits (IC). In the fabrication of semiconductor and other electronic devices, directionality of particles being deposited on a substrate is important to improve adequate filling of electric features. As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to 0.18 μm or less, while the thickness of the dielectric layer remains substantially constant. Thus, the aspect ratio for the features, i.e., the ratio of the depth to the minimum lateral dimension, increases, thereby pushing the aspect ratios of the contacts and vias to 5:1 and above. As the dimensions of the features decrease, it becomes desirable to obtain deposition uniformity and conformal step coverage on substrate as well as achieve acceptable particle performance.  
           [0005]    To obtain deposition in the high aspect ratio (HAR) features, one method uses a medium/high pressure physical vapor deposition (PVD) process known as an ionized metal plasma (IMP) process or high-density plasma physical vapor deposition (HDP-PVD). The plasma density in such high density plasma processes is typically between about 10 11  cm −3  and 10 12  cm −3 . Generally, IMP processing offers the benefit of highly directional deposition with good bottom coverage in HAR features. High density plasma sputtering processes have been successfully implemented for obtaining conformal coverage for titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), copper (Cu), tungsten (W), and tungsten nitride (WN). In one high density plasma deposition configuration, a typical chamber includes a coil, or other electromagnetic field generating device, for maintaining a high density, inductively-coupled plasma between a target and a susceptor on which a substrate is placed for processing. Initially, a plasma is generated by introducing a gas, such as helium or argon, into the chamber and then coupling energy into the chamber via the target to ionize the gas. The coil is positioned proximate to the processing region of the chamber and produces an electromagnetic field that induces currents in the plasma resulting in an inductively-coupled medium/high density plasma between the target and the susceptor. The ions and electrons in the plasma are accelerated toward the target by the negative bias applied to the target causing the sputtering of material from the target. At least a portion of the sputtered metal flux is then ionized by interaction with the plasma. An electric field due to an applied or self-bias, develops in the boundary layer, or sheath, between the plasma and the substrate and electrically attracts and accelerates the metal ions towards the substrate in a direction parallel to the electric field and perpendicular to the substrate surface. The bias energy is preferably controlled by the application of power, such as RF or DC power, to the susceptor to attract the sputtered target ions in a highly directionalized manner to the surface of the substrate to fill the features formed on the substrate.  
           [0006]    One difficulty with IMP processes is producing uniform film thickness over the entire substrate. In practice, the resulting film in IMP processes exhibit a greater thickness toward the center of the substrate. Center-thick films are undesirable because the increasingly smaller features of devices require good thickness uniformity to produce reliable devices.  
           [0007]    Therefore, there is a need for a method of depositing materials on a substrate in an inductively-coupled plasma environment wherein the resulting layers exhibit good uniformity and step coverage.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention generally provides an apparatus and method for depositing a conformal layer on a substrate in a plasma chamber using a high density plasma. In one aspect of the invention, a chamber having a target, a first power supply coupled to the target, a substrate support member, a second power supply connected to the substrate support member, and a coil to generate an electromagnetic field is provided. The target comprises a material to be sputtered by a plasma formed adjacent to the target during processing. A time-varied signal supplied by the first power supply preferably comprises a negative voltage portion and a positive voltage portion. Preferably, the second power supply connected to the substrate support member supplies a substantially constant negative bias to the substrate. A power supply is also connected to the coil, which is also sputtered during deposition.  
           [0009]    In another aspect of the invention, a plasma is formed in or supplied to a chamber to sputter a material from a target. A coil is energized in the chamber to enhance ionization of the sputtered material. During processing, a signal having a desired waveform is provided to the target. In one embodiment, the signal is varied between a negative voltage portion during which the target material is sputtered onto a substrate and a small positive voltage portion during which the coil alone is sputtered. A bias is provided to the substrate to influence the direction of ions in the chamber during processing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.  
         [0011]    It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0012]    [0012]FIG. 1 is a cross-section of a simplified processing chamber having a coil disposed therein.  
         [0013]    [0013]FIG. 2 is a graphical illustration of a signal applied to a target.  
         [0014]    [0014]FIG. 3 is a graphical illustration of a signal applied to a substrate.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    The embodiments described below are implemented using an ionized metal plasma (IMP) process that can be carried out using process equipment, such as an ion metal plasma (IMP) processing chamber, known as an IMP ELECTRA™ Chamber mounted on an Endura® platform, both of which are available from Applied Materials, Inc., located in Santa Clara, Calif. The equipment can include an integrated platform having a preclean chamber, an IMP-PVD barrier layer chamber, a PVD chamber, an IMP-PVD seed layer chamber, and a CVD chamber.  
         [0016]    [0016]FIG. 1 is a schematic cross-sectional view of an IMP chamber  100  according to the present invention. The chamber  100  includes walls  101 , lid  102 , and bottom  103 . A target  104  comprising the material to be sputtered is mounted to the lid  102  and disposed in the chamber  100  to define an upper boundary to a processing region  107 . Magnets  106  are disposed behind the lid  102  and are part of a rotatable magnetron that provides for magnetic field lines across the IS face of the target about which free electrons in the plasma spiral, and thus increase the density of a plasma adjacent to the target  104 .  
         [0017]    A substrate support member  112  supports the substrate  110  and defines the lower boundary to the processing region  107 . The substrate support member  112  is movably disposed in the chamber  100  and provides an upper support surface  105  for supporting a substrate  110 . The support member  112  is mounted on a stem  109  connected to a motor assembly  114  that raises and lowers the substrate support  112  between a lowered loading/unloading position and a raised processing position. An opening  108  in the chamber  100  provides access for a robot (not, shown) to deliver and retrieve substrates  110  to and from the chamber  100  while the substrate support member  112  is in the lowered loading/unloading position.  
         [0018]    A coil  122  is mounted in the chamber  100  between the substrate support member  112  and the target  105  and when energized by an AC power source provides electromagnetic fields in the chamber  100  during processing to assist in generating and maintaining a plasma between the target  104  and substrate  110 . The electromagnetic fields produced by the coil  122  induce currents in the plasma to density the plasma which, in turn, ionizes at least a portion of the sputtered target material flux. At least a portion of the positively charged ionized material is then attracted toward the negatively biased substrate  10  and deposits thereon. The coil  122  is made of a similar materials as the target and is also sputtered during processing.  
         [0019]    The chamber  100  optionally includes a process kit comprising a process shield  128  and a shadow ring  129 . The process shield  128  is an annular member suspended from the lid  102  between the coil  122  and the body  101 . An upwardly turned wall  131  of the process shield  128  is adapted to support the shadow ring  129  while the support member  112  is in a lowered position. The process shield is preferably coupled to ground to provide a return path for RF currents in the chamber  100 .  
         [0020]    One or more plasma gases are supplied to the chamber  100  through a gas inlet  136  from gas sources  138 ,  140  as metered by respective mass flow controllers  142 ,  144 . One or more vacuum pumps  146  are connected to the chamber  100  at an exhaust port  148  to exhaust the chamber  100  and maintain the desired pressure in the chamber  100 . Preferably the vacuum pumps  146  include a cryopump and a roughing pump and are capable of sustaining a base pressure of about 10 −8  mTorr.  
         [0021]    Three power supplies are used in the chamber  100 . A first power supply  130  delivers modulated or time-varied power to the target  104  to generate a plasma of the one or more plasma gases. By modulated or time-varied is meant that the voltage applied to the target varies with time, preferably on a periodic basis. The power supply  130  is adapted to vary at least the magnitude of the applied voltage to the target  104  and preferably is capable of changing the charge, i.e., negative and positive. Preferably, the first power supply  130  is a modulated direct current (DC) power supply capable of providing a modulated signal to the target  104 . However, the particular arrangement used to provide a modulated signal is not limiting of the present invention and may include any conventional components known in the art, such as switches, pulse generators, microprocessors and the like. A second power source  132 , preferably a RF power source, supplies electrical power in the megahertz range to the coil  122  to control the density of the plasma. A third power source  134 , preferably a RF or a DC power source, biases the substrate support member  112  with respect to the plasma and provides an electric field adjacent a substrate to attract the ionized sputtered material toward the substrate  110 .  
         [0022]    In operation, a robot delivers a substrate  110  to the chamber  100  through the opening  108 . After depositing the substrate  110  unto the upper surface  105  of the support member  112  the robot retracts from the chamber  100  and the opening  108  is sealed. The substrate support member  112  then raises the substrate  110  into a processing position. During the upward movement of the support member  112  the shadow ring  129  is lifted from the process shield  128 . During processing, the shadow ring  129  covers a perimeter portion (usually less than 3 millimeters) of the substrate  110 . Preferably, the space between the target  104  and the substrate support member  112  in a raised processing position is between about 100 mm and 190 mm preferably 130 mm-140 mm.  
         [0023]    One or more gases are then introduced into the chamber  100  from the gas sources  138 ,  140  to stabilize the chamber  100  at a processing pressure. A high negative voltage is then imposed on the target  104  from its power supply  130 , to strike a plasma in the chamber  100 . The coil power supply  132  is also activated to pass an RF signal through the coil  122 , which creates inductive coupling with the plasma region. The coil  122  will quickly establish a negative self-bias, which also causes sputtering of the coil surface.  
         [0024]    The coil  122  operates to induce electrical currents in the plasma between the target  104  and substrate  110  to create a more dense plasma, thereby enhancing the ionization of the sputtered material from the target  104  and the coil  122  which occurs as a result of interaction with the plasma ions. A portion of the ions formed from the sputtered material traverse the space between the processing region  107  and deposit on the substrate  110  which is biased by the third power supply  134 . The biases to the target  104  and support member  112  are controlled according to the processes described in detail below.  
         [0025]    Following the deposition cycle, the substrate support member  112  is lowered to a loading/unloading position. The robot is then extended into the chamber  100  through the opening  108  and the substrate  110  is received on the robot for removal from the chamber  100  and delivery to a subsequent location. Subsequent locations include various processing chambers, such as electroplating chambers, where the substrate  110  undergoes additional processing.  
         [0026]    The present invention controls the rate of deposition at the center and edge portions of the substrate to affect overall film uniformity. By modulating the RF coil/DC target power ratio over a well-controlled time scale, an increase in film uniformity across the surface of the substrate can be achieved. The proportions of coverage are controlled by adjusting the application of the waveform applied to the target  104 .  
         [0027]    During the deposition process, the power supply  130  delivers a modulated signal to the target  104 . The signal  200 , shown in FIG. 2, includes a negative voltage portion  202  and a positive voltage portion  204 . Although shown here as a square wave, any waveform oscillated between a negative voltage portion and a positive voltage portion may be used to advantage. Additionally, in another embodiment, the signal  200  is modulated between two negative voltages or between a negative voltage and no voltage (no signal).  
         [0028]    During the negative voltage portion  202 , the positively charged ions supplied by the plasma gas, such as Ar, bombard the target  104  causing ejection of material therefrom. The energy of the Ar ions can be controlled by adjusting the bias to the target  104 . Preferably, the power supplied to the target  104  is sufficient to induce a negative voltage portion  202  between about −100V and about −300V, with increasing voltage resulting in increased sputtering from the target  104 . The resulting metal flux is then ionized under the influence of the plasma and deposits on the substrate  110 . During the negative voltage portion  202  of the signal  200 , the bulk of the material being deposited on the substrate  110  is produced by the target  104 , as opposed to the coil  122 . As a result, the deposited film exhibits a center-thick profile.  
         [0029]    During the subsequent positive voltage portion  204  of the signal  200 , sputtering from the target  104  is minimized or even terminated and sputtering from the coil  122  dominates the resulting deposition onto the substrate  110 . Deposition will therefore occur primarily at the edge of the substrate. It is believed that by providing increased deposition at the substrate edge for a predetermined period of time, better film uniformity will be obtained. Preferably, the positive voltage portion  204  is between about 0V and +50V. Additionally, during the positive portion  204  the electron temperature of the plasma is increased because the total flux of material is less than during the negative voltage portion. Accordingly, the plasma is able to ionize more of the sputtered material.  
         [0030]    The negative voltage portion  202  and the positive-voltage portion  204  are sequentially alternated to result in a series of target/coil sputtering steps (or center strong deposition steps), and coil sputtering steps (or substrate deposition steps). The frequency and duty cycle of the signal  200  can be adjusted to control the target/coil and coil sputtering steps to achieve the desired results. Preferably, the frequency of the signal  200  is between about 1 kHz and 200 kHz. As defined herein, the duty cycle is the ratio of the pulse width, t 1 , of the negative voltage portion  202  to the signal period T 1 , shown in FIG. 3. Preferably, the duty cycle is between about 50% and about 90% with a pulse width t 1  between about 1 μs and 1 ms.  
         [0031]    Although the voltage applied to the substrate  110  may be modulated in a manner similar to the signal  200  provided to the target  104 , preferably the voltage is maintained at a substantially constant value throughout a deposition cycle. Accordingly, a voltage drop is continuously maintained across a region between the plasma and the substrate  110  known as the sheath or dark space. Due to the resulting voltage drop in the sheath, an electric field is generated substantially perpendicular to the substrate  110 , thereby causing the ions to accelerate toward the substrate. FIG. 3 shows an RF signal  201  provided to the substrate  110  by the third power supply  134 . In the presence of a plasma, the signal  201  is shifted downward into the negative voltage region resulting in an induced DC bias (Vdc) on the substrate  110 . The Vdc, shown in FIG. 3 as a signal  206 , is maintained at a substantially constant value. In one embodiment, the power from the third power supply  134  is sufficient to produce an applied bias  153 , on the substrate  110  between about 0V and −300V. The particular values for power and voltage may be adjusted to achieve the desired result.  
         [0032]    The modulation of the target bias with periodic positive pulses has resulted in various additional findings. For example, it was discovered that in another embodiment of the process, modulation of the applied DC voltage to the target with waveform  200  minimized or prevents deleterious target conditions. One such condition is known as target poisoning. Target poisoning occurs during reactive sputtering when the reactive species saturates the surface of the target. Sputtering of a poisoned target produces an unusable film. For example, in TaN and WN deposition, the resulting film exhibits significantly increased resistivity. Another undesirable target condition, is the formation of nodules on the target surface which can occur during reactive sputtering. The nodules are buildup of dielectric material that occurs as a result of the interaction between the target materials and the gases in the chamber. Over time, the nodules can result in micro-arching and other deleterious effects capable of damaging substrates.  
         [0033]    The present invention mitigates the problems of target poisoning and nodule formation by reverse biasing the target periodically. The positive pulse is believed to “clean” the surface of the target by discharging the charged particles that adhere to the surface and ultimately result in target poisoning and nodule formation if left undisturbed for a sufficient period of time.  
         [0034]    While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.