Abstract:
A method and apparatus to prevent charging of a substrate, retained by an electrostatic chuck in a plasma chamber, during ignition of a plasma. The method deactivates a voltage to the chuck electrodes (or other conductive element in a substrate support pedestal) and allows the chuck electrodes to float during ignition of the plasma. The method activates the chuck electrodes again following the ignition of the plasma.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/058,435, filed on Apr. 10, 1998, now U.S. Pat. No. 6,033,482. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     This invention generally relates to semiconductor wafer processing systems and, more particularly, to a process for igniting a plasma to process a substrate, such as a semiconductor wafer, in a plasma processing chamber of a semiconductor wafer processing system. 
     2. Description of the Background Art 
     Substrates such as semiconductor wafers are often processed by methods involving the use of plasma in the presence of the wafer or substrate. During wafer processing, the wafer rests on a pedestal containing an electrostatic chuck that can be a monopolar, or preferably, a bipolar electrostatic chuck. Electrostatic chucks contain one or more electrodes embedded within a dielectric material such as polyimide. Such a polyimide chuck is described in commonly assigned U.S. patent application Ser. No. 08/744,039, filed Nov. 5, 1996, now U.S. Pat. No. 5,885,469, and incorporated herein by reference. When a voltage is applied to the electrodes, charges on the wafer and charges on the electrodes electrostatically retain the wafer on the chuck surface. As such, the wafer is held in a stationary position while being processed. 
     To accomplish wafer processing, such as plasma precleaning, a wafer is supported in a process chamber upon a pedestal. The pedestal generally contains an electrostatic chuck for retaining the wafer while the wafer is processed, e.g., exposed to a plasma to sputter clean the surface of the wafer. The chuck has one or more electrodes embedded within a chuck body. The chuck body is fabricated of a dielectric such as polyimide, aluminum nitride, alumina, and the like. In a well known manner, a voltage, applied to the electrodes, retains the wafer against the support surface of the chuck by electrostatic force. 
     An anode electrode is disposed above the pedestal and the pedestal generally contains a conductive element (e.g., a pedestal base) that is used as a cathode. During plasma cleaning of a wafer, a gas such as argon, helium, hydrogen, or a combination thereof is supplied to the chamber and energy is applied between the cathode and anode to produce a plasma. The active gas atoms bombard the wafer and sputter clean its surface. Typically, energy from a direct current (DC) voltage ignites and sustains the plasma. However, a radio frequency (RF) voltage may also be used to sustain the plasma and/or bias the wafer. 
     More specifically, in a wafer cleaning process, a reactive cleaning gas (process gas) such as hydrogen is introduced into the chamber. The plasma is formed when electrons are stripped from a portion of the process gas atoms to form positive ions. Positive ions and electrons both leak out of the plasma; however, the electrons, being lighter, move faster and therefore leave more rapidly. As a result, the plasma is at an electric potential that is positive with respect to the chamber walls, which are usually at ground potential. Electron bombardment of the surface of the wafer makes the plasma positive with respect to the wafer. This self bias of the wafer causes ions to accelerate towards the wafer and bombard its surface. If the wafer is further biased with an electric potential from a power supply that is substantially negative with respect to the plasma, additional positive ions from the plasma are accelerated towards the wafer. 
     While the plasma exists in the chamber, voltages on the wafer and the chuck are generally defined by the potential of the plasma. For the particular case that the wafer is allowed to “float” relative to the chamber walls and the chuck, some charge accumulates on the wafer primarily due to bombardment of the surface by energetic electrons from the plasma. However, as the wafer charges, an electric field develops which acts to repel electrons from the plasma. Thus, the potential difference between the plasma and the wafer is self limiting. The potential difference between the plasma and the floating wafer is approximated by the formula:        ΔV   =         kT   e       2      e            ln        (       m   i       2.3        m   e         )                                
     where ΔV is the potential difference between the plasma and the floating wafer, k is Boltzmann&#39;s constant, T e  is the electron temperature, m i  is the mass of an ion and m e  is the mass of an electron. Typical values of ΔV are in the range of 1 to 10 volts. This is generally small enough that field emission from the wafer does not occur during plasma processing. However, during plasma ignition, transient bursts of high voltage can cause, under some circumstances, field emission from the wafer. 
     A plasma in a preclean chamber is typically ignited by a transient burst of high voltage applied to the cathode electrode. The chamber, anode and chuck electrodes are typically grounded during ignition of the plasma. The transient high voltage can be larger than 1000 volts and typically lasts about 1 second, though it can last longer if there are problems achieving ignition. During this transient burst of high voltage, a similarly high transient voltage develops on the wafer. If the voltage on the wafer is large, a substantial electric field exists at the surface of the wafer. If the chuck electrodes are energized during the ignition of the plasma, the transient voltage on the wafer is even higher due to the high electric potential already existing between the chuck electrodes and the wafer. This high voltage can lead to substantial charging of the wafer through field emission. A similar effect occurs if other components within a pedestal are energized or grounded during plasma ignition including components such as a bias electrode or resistive heater. Charging of the wafer through field emission is undesirable because it leads to substantial charge imbalance between the wafer and the chuck. The charge imbalance results in a residual charge being accumulated on the chuck surface. After the electrodes are deactivated, the residual charge will retain the wafer with such force that the wafer cannot be removed from the chuck. Charging of the chuck surface can also lead to arcing between wafer and chuck that can damage the wafer and/or the chuck. 
     Therefore, a need exists for a method of protecting a wafer on an electrostatic chuck against charging during ignition of a plasma or other similar high voltage process. 
     SUMMARY OF THE INVENTION 
     To overcome the disadvantages associated with the prior art, the method of the present invention deactivates the chuck electrodes just prior to a high voltage process, such as ignition of a plasma, and leaves the electrodes deactivated during that high voltage process. It is critical that the chuck electrodes be floating, i.e., not grounded, during plasma ignition. Once the high voltage process has ceased, e.g., the plasma has been ignited, the chuck electrodes can be energized to retain the wafer without risk of charge build up upon the wafer. The invention is applicable to other components within the substrate support pedestal. As such, resistive heater elements and RF bias electrodes used for wafer biasing should be electrically “floating” during plasma ignition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a block diagram of a typical semiconductor wafer process chamber, illustratively, a “preclean” chamber; and 
     FIG. 2 depicts a flow diagram of the method of the present invention. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     The method of the present invention is applied to a semiconductor wafer processing system such as that depicted in FIG. 1 is best understood in terms of the flow diagram of FIG. 2 which shows the steps to be followed to sputter clean a wafer while the wafer is retained in the chamber using a bipolar electrostatic chuck. 
     FIG. 1 depicts a schematic, cross sectional view of a plasma processing system  50 , known as a “preclean” chamber, for “cleaning” a wafer prior to deposition processing. In the preclean chamber  100 , a wafer  102  is supported in the chamber  100  upon a pedestal  101  that contains a bipolar electrostatic chuck  104  and a cathode electrode  120 . The chuck  104  has a pair of electrodes  106  and  108  embedded within a chuck body  107  made of a dielectric such as polyimide, aluminum nitride, boron nitride, alumina, and the like. A voltage, from a chuck power supply  150 , applied to the electrodes  106  and  108 , holds the wafer  102  against the chuck  104  by electrostatic force. 
     For effective cleaning, the wafer must be able to be chucked and temperature controlled prior to a plasma being ignited in the chamber. Temperature control is established by applying a heat transfer medium (a gas such as helium) between the wafer  102  and chuck  104  to fill the vacuum within the interstitial spaces beneath the wafer. Use of a heat transfer medium, generally known as backside gas, promotes uniform heat transfer between the pedestal assembly and the wafer. Chucking is necessary to ensure that a region of backside gas is maintained between the wafer  102  and the chuck  104  such that the wafer does not “float” off the chuck when backside gas is supplied. To allow for the wafer to be temperature stabilized prior to cleaning, bipolar chucks, which do not utilize the plasma to form a ground path to the wafer and thus do not need the plasma to ensure an electrostatic attraction between the wafer and the chuck, are generally used in preclean chambers because the plasma will not be ignited until the wafer is heated. As such, electrostatic chucks having two or more electrodes can be used in a preclean chamber as long as a bipolar chucking voltage is applied to at least two electrodes. Any chucking voltage may be used with the method of the present invention including DC and AC. Alternatively, although less common in preclean chambers, a monopolar chuck (also known as a unipolar chuck) having a single electrode within the chuck body and utilizing a wafer grounding electrode in contact with the wafer may be used to chuck the wafer  102  without the presence of the plasma  110 . By applying a voltage between the grounding electrode and the embedded electrode, the wafer  102  is electrostatically retained on a monopolar chuck without a plasma. 
     A heat transfer gas supply  130  provides gas for transferring heat between the wafer  102  and the chuck  104 . The gas flows through a passageway  109  in the chuck body  107  to the support surface  105  and disperses between the wafer and support surface to improve heat transfer between the pedestal and wafer. Alternatively, the wafer  102  can rest on a biased pedestal having one or more bias electrodes (not shown, but similar to the chucking electrodes) for applying a direct current (DC) or radio frequency (RF) bias voltage to the wafer  102 . Furthermore, the pedestal may contain a resistive heater  121  and/or biasing electrodes with or without electrodes for the electrostatic chuck. 
     An anode electrode  111  is disposed above the wafer  102  and the chuck  104 . The cathode electrode  120  is disposed immediately below the chuck  104  and supports the chuck  104  in the chamber  100 . Alternatively, the cathode electrode may be formed by additionally or alternatively biasing the walls of the chamber  100  relative to the anode electrode  111 . A cathode power supply  122  provides voltage to the cathode electrode  120 . 
     During plasma cleaning of the wafer, a gas such as argon, helium, hydrogen, or a combination thereof is supplied to the chamber, from a gas source  155 . Once the chamber has an appropriate gasIpressure, energy from a DC voltage supplied to the chamber by the cathode power supply  122  ignites and sustains the plasma  110 . The active gas atoms bombard the wafer  102  and sputter clean its surface  103 . Alternatively, An RF voltage may be used to produce the plasma  110 . 
     A system controller  160  includes hardware that provides the necessary signals to initiate, regulate, and terminate the processes occurring in the preclean chamber  100 . The system controller  160  includes a programmable central processing unit (CPU)  162  that is operable with a memory  164  (e.g., RAM, ROM, hard disk and/or removable storage) and well-known support circuits  166  such as power supplies, clocks, cache, and the like. By executing software stored in the memory  164 , the system controller  160  produces control outputs  159 ,  165 ,  167 ,  168 , and  169  that respectively provide signals for controlling the heater power supply  161 , the gas source  155 , the cathode power supply  122 , the chuck power supply  150 , and the heat transfer gas supply  130 . The system controller  160  also includes hardware for monitoring the processes through sensors (not shown) in the preclean chamber  100 . Such sensors measure system parameters such as temperature, chamber atmosphere pressure, plasma content, voltage and current. Furthermore, the system controller  160  includes at least one display device  170  that displays information in a form that can be readily understood by a human operator. The display device  170  is, for example, a graphical display that portrays system parameters and control icons upon a “touch screen” or light pen based interface. 
     The steps of the method of the present invention could be implemented by a suitable computer program running on the CPU  162  of the system controller  160 . The CPU  162  forms a general purpose computer that becomes a specific purpose computer when executing programs such as the embodiment of the method of the invention depicted in the flow diagram of FIG.  2 . Although the invention is described herein as being implemented in software and executed upon a general purpose computer, those skilled in the art will realize that the invention could be implemented using hardware such as an application specific integrated circuit (ASIC) or other hardware circuitry. As such, the invention should be understood as being able to be implemented, in whole or in part, in software, hardware or both. 
     Those skilled in the art would be readily able to devise a computer program suitable for implementing the present invention from the flow diagram of FIG.  2 . The routine  202  begins with step  204 , where the wafer  102  is placed on the chuck  104 . To retain the wafer, a chucking voltage is applied to the chuck electrodes  106  and  108 . Next, in step  206 , the wafer  102  may undergo pre-plasma processing such as heating or cooling by flowing backside gas from the heat transfer gas supply  130  through the passage  109  and/or activating a resistive heater  121  to heat the wafer  102 . Wafer cooling is generally provided by a water jacket (not shown) within the pedestal base  123 . 
     Once the wafer  102  is ready for plasma processing (cleaning), the chuck power supply  150  is turned off (or the chuck voltage is reduced) in step  208  and the electrodes  106  and  108  are allowed to float. If backside gas is in use, it must be turned off before the chucking voltage is turned off to prevent the wafer  102  from floating off the chuck  104 . In step  210 , the plasma  110  is ignited by a conventional process. For example, the plasma  110  could be ignited by applying a transient burst of high voltage to the cathode  120  in a manner well known in the art. Other methods of ignition may be used such as applying a transient burst of high gas pressure to the chamber  100  in conjunction with a RF voltage between cathode  120  and anode  111 . Those skilled in the art will be able to devise other methods of igniting the plasma  110  in other chambers such as a coil/microwave driven reactors. 
     It is critical to the practice of the invention that the chuck electrodes  106  and  108  be allowed to float during the plasma ignition step  210 . If the electrodes  106  and  108  are grounded, a path exists for charge to flow from the plasma  110  through the wafer  102  to ground via the chuck  104 . Such a current to ground could be quite large and cause damage to or field emission from the wafer  102  and/or the chuck  104 . 
     In step  212 , after the plasma  110  has been ignited and the transient high voltage is no longer applied, a chucking voltage can be applied to the chuck electrodes  106  and  108 . Backside gas, if required, can then be turned on. If an RF bias is to be applied to the wafer  102  during plasma processing (before or after the plasma  110  is struck) the chuck voltage applied to the chuck electrodes  106  and  108  should be turned off or reduced first. The chuck voltage can be raised after the RF bias has been turned on. The wafer  102  can now be processed by conventional plasma cleaning methods in step  214 . 
     In the present example, the wafer  102  is sputter cleaned by bombardment with ions from the plasma  110 ; however, any plasma process may occur in step  214 . Such processes include, but are not limited to plasma ashing, plasma etching, reactive ion etching (RIE), glow discharge sputter deposition, RF sputter deposition, magnetron sputtering, plasma enhanced chemical vapor deposition (PECVD) or any other similar process which utilizes a plasma to process a wafer or similar substrate. 
     A surge of voltage can also occur when the plasma is turned off. Therefore, the steps of the routine  202  should be performed in reverse order once the plasma cleaning is completed. For example, if the RF bias was turned on after striking the plasma, the backside gas would be turned off; then the chuck voltage would be turned off (or reduced); then the RF bias would be turned off; and finally the cathode power supply  122  would be turned off. 
     Although the invention is described in terms of a high voltage used to retain a wafer on a chuck during plasma ignition, the invention may be used with any other high voltage process that produces a large voltage on a substrate retained on a pedestal having one or more electrodes. For example, the method could be applied to a system having a wafer supported on a pedestal containing a conductive element such as a resistive heater  121  or biasing electrode proximate the wafer  102  that, if grounded or powered during plasma ignition, would produce a substantial electric field between the wafer and ground (i.e., via the grounded or powered element). Consequently, the pedestal is susceptible to arcing and the wafer may experience field emission during plasma ignition. As such, to protect the wafer and chuck from such damage, these elements should be floated during the plasma ignition period. 
     The present invention has the advantage of preventing charge accumulation on an electrostatic chuck and thereby preventing a loss of chucking force due to such charge accumulation. Consequently, wafer processing is more uniform, leading to higher yields, increased productivity and reduced cost per wafer. 
     Although various embodiments, which incorporate the teachings of the present invention, have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.