Abstract:
A method and apparatus for minimizing or eliminating arcing or dielectric breakdown across a wafer during a semiconductor wafer processing step includes controlling the voltage across the wafer so that arcing and/or dielectric breakdown does not occur. Using an electrostatic clamp of the invention and by controlling the specific clamp voltage to within a suitable range of values, the voltage across a wafer is kept below a threshold and thus, arcing and/or dielectric breakdown is reduced or eliminated.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a method and apparatus for processing semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     A phenomenon which can reduce the yield of useful die from wafers during a semiconductor wafer processing step is the occurrence of arcing, also known as microarcing. Generally, during wafer processing, arcing can occur across the semiconductor wafer and in particular can be concentrated at material defects such as a crack, or at prominent feature of the wafer, which has been processed into the wafer, such as for example pillars. When such arcing occurs, part or all of the wafer can be irreparably damaged. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to minimizing or eliminating arcing across a semiconductor wafer during a semiconductor wafer processing. 
     The invention describes a methodology and apparatus used to eliminate and/or substantially decrease the arcing or dielectric breakdown which may occur on a semiconductor wafer or substrate. The invention includes using a chuck and preferably an electrostatic chuck to control the electrostatic clamp voltage applied to the wafer to within a suitable range of values, such that arcing or dielectric breakdown is substantially reduced or eliminated. Such controlling can occur dynamically as process values change during the process steps. Further, by way of example only, such invention is of particular value with wafers containing film having a high dielectric constant or wafers containing films of ferroelectric material. However, such invention is useful for etching all types of standard and conventional films where arcing can also be a problem. 
     In particular, the apparatus and method are particularly useful for reducing or eliminating arcing or dielectric breakdown during etching in a plasma reactor. 
     Further, the invention includes a reactor for processing a semiconductor wafer which includes a reactor chamber and a chuck, and preferably an electrostatic chuck, which can accept a wafer for processing. The reactor includes a power supply associated with the reactor chamber, which the power supply is capable of generating a first voltage at the surface of the wafer adjacent to the plasma during the processing of the wafer. The invention further includes a control mechanism that can control a second voltage that the electrostatic chuck applies to the wafer in order to hold the wafer to the chuck during wafer processing. The control mechanism is capable of adjusting the second voltage so that the difference between the first voltage and the second voltage or, in other words, the potential across the wafer, is kept below a threshold in order to minimize arcing across the wafer. Such adjustments can be made dynamically, if desired throughout the wafer fabrication process. 
     Accordingly, one aspect of the invention includes apparatus that controls the voltage applied to the surface of a wafer in contact with an electrostatic chuck in order to minimize the difference between the applied clamping voltage and the voltage built up on the other side of the wafer which is in contact with, for example, a plasma generated in an etch reactor. 
     A method of the invention includes the steps of placing a semiconductor wafer into a reactor and onto an electrostatic chuck, and generating a plasma in the reactor. The method further includes controlling the voltage across the wafer in order to minimize arcing. 
     In an aspect of the invention, the controlling step includes controlling the difference between the voltage at the first surface of the wafer in contact with the plasma, and the voltage at a second surface of the wafer in contact with the chuck. 
     In another aspect of the present invention, the plasma is generated by at least one of a high frequency power supply and a low frequency power supply. 
     In a further aspect of the present invention, both the high frequency power supply and a low frequency power supply are applied to the chuck. 
     In a further aspect of the present invention, the method includes applying a semiconductor processing step to one of high dielectric constant film on a substrate and a ferroelectric film on a substrate. 
     Accordingly, it can be seen that the present invention is effective in reducing or eliminating arcing across a wafer and in particular a wafer which has a high dielectric constant film and/or ferroelectric film. Such invention is advantageous in that it increases the yield of die. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematical representation of an embodiment of a reactor of the invention which can be used to carry out a method of the invention. 
     FIG. 2 depicts an enlargement of the wafer positioned on an embodiment of the electrostatic chuck of the invention of FIG.  1 . 
     FIG. 3 depicts a chart showing the controller step function of an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The method of the present invention can be performed in an etch reactor, such as the etch reactor of the invention depicted in FIG. 1, using the chuck configuration, such as the electrostatic chuck configuration shown in FIG.  2 . As is known in the art, electrostatic chucks apply an electrostatic force in order to clamp a wafer onto said chucks. It is to be understood that other reactors including, but not limited to other etch reactors, and other chuck configurations can be used and be within the scope and spirit of the invention. By way of example only, mechanical clamping chucks which have been modified to apply a potential to a wafer are within the spirit and scope of the invention. 
     The etch reactor of FIG. 1 is identified by the number  20  and is configurated as a tri-electrode reactor. The etching apparatus  20  includes a housing  22  and an etching chamber  24 . A wafer  26  is positioned on a bottom electrode  28 . The chamber  24  further includes a side peripheral electrode  30  and an upper electrode  32 . In a preferred embodiment, the side peripheral electrode  30  can be grounded or allowed to establish a floating potential as a result of the plasma developed in the chamber  24 . The upper electrode  32  is generally grounded. In typical operation, both the side peripheral electrode  30  and the upper electrode  32  are grounded as shown in FIG.  1 . 
     Preferably two A.C. power supplies, first power supply  34  and second power supply  36 , are connected to the bottom electrode  28  through a appropriate circuitry  38  which includes matching networks and a combiner. Further a controller  40  controls the sequencing of the first and second AC power supplies  34 ,  36 . Typically, the first power supply  34  operated in the kilohertz range and is optimally provided at about 450 KHz, and typically in the range of less than 500 KHz. The second power supply  36  operates in the megahertz range, and typically operates at about 13.56 MHz, although other frequencies above about 1 MHz and also multiples of 13.56 MHz can be used with the present invention. The power supply  34  is preferably powered at 200 watts and the second power supply  36  is preferably powered at 500 watts for this example. The low frequency KHz power supply can cycle up to about 500 watts if desired, and the high frequency MHz power supply can cycle up to about 1150 watts if desired during an etch process. Ion energy increases towards the kilohertz range while ion density increases towards the megahertz range. Additionally, reactor  20  includes gas inlet head  42  and a gas outlet port  44 . 
     The chuck  48  which is incorporated in the bottom electrode  28  is an electrostatic chuck. Electrostatic chucks are well known in the industry. This electrostatic chuck include an electrostatic clamp electrode  50 , to which is preferably applied a DC voltage from voltage source  52 . Controller  40 , in this preferred embodiment, can dynamically (overtime accordingly to changing process conditions such as changing power input to the electrodes) control the voltage applied to the electrostatic clamp electrode  50  by the source  52 . Such control depends on, for example, the ramping and cycling fo one or more of the other power supplier. 
     In this particular embodiment, the wafer  26  includes a film  54  which is comprised of one of a high dielectric constant material or a ferroelectric material. It is to be understood, however, that the invention can work successfully on any film and in any situation where arcing can be a problem. 
     It is to be understood that the above inventive structure can be modified such that one or more of the power supplies can be applied to electrodes  30  and/or  32  in addition to being applied to electrode  28  if desired. Further, it is to be understood that the invention can include only a single power supply applied to the lower electrode  28 . It is also to be understood that the electrode can be those used to establish both a capacitively coupled reactor and an inductively coupled reactor. 
     When a substrate is being etched in a plasma, the potential of the front surface  56  (FIG. 2) of the wafer assumes a time averaged negative potential with respect to the plasma potential. The time averaged negative potential or DC potential (commonly referred to as the “DC Bias”, V dc ) is generally dependent on the plasma conditions and the low and high frequency power applied to the wafer. In this particular situation, due to the presence of the high dielectric constant and/or ferroelectric material layer on the substrate, the back surface  58  of the wafer  26  is insulated from the front surface  56  of the wafer. The DC potential of the back surface  58  of the wafer is determined primarily by the clamping voltage from the chuck  48 . For example, in the case of using an electrostatic clamp, the DC potential of the back side  58  of the wafer is greatly influenced by the clamping voltage (V ESC ). By way of example only, in a representative etch process, V dc  can be for example −1000 volts, and V ESC  can be for example −700 volts. The difference would then be (−1000 volts)−(−700 volts), or −300 volts. The less negative the difference is the less likely that arcing will occur. In other words, the lower the absolute value of V dc −V ESC  is the less likely that arcing will occur. It is to be understood that arcing can occur at a potential of −200 volts or smaller negative potential values, but that it generally occurs at a potential of −300 volts and certainly at greater negative potential values. 
     For a system with a pure mechanical clamp, the potential of the back surface of the wafer is not generally well controlled and assumes a value somewhere between zero and the potential of the front surface of the wafer. Thus, due to the presence of the high dielectric constant and/or ferroelectric layer between the front and the back surface of the wafer, a potential difference between the two surfaces can exist. The potential difference between the front and the back surfaces of the wafer (across the high dielectric constant and/or ferroelectric layer) can be high (several hundred volts), especially when high dielectric constant materials such as strontium bismuth tantalate (Y−1) are being used. 
     Another high dielectric constant film material that can benefit from the invention include lead zirconium titanate (PZT). 
     The DC potential difference between the front and the back surface of the wafer can lead to very high electric fields, especially across thinner dielectric layers or material defects. The value of this electric field can far exceed the breakdown strength of the dielectric leading to an electrical breakdown. 
     It has been found that Y 1  films can have material defects caused during the Y1 film formation, which defects consists of “cracks” in the dielectric layer, where the dielectric layer is very thin. Electrical breakdown of the dielectric can occur in the “crack” area leading to an arc on the front side of the wafer. 
     When the ESC potential, V ESC , is set to a value which is close to the potential of the front surface of the wafer, the potential difference and hence the electric field in the dielectric layer and/or ferroelectric layer are substantially reduced. This prevents and/or substantially decreases the number and physical size of the arc spots on the wafer being etched. 
     Accordingly, the electrostatic clamp voltage is adjusted to eliminate and/or substantially reduce arcing or dielectric breakdown which may occur when a substrate containing a layer of, by way of example only, a high dielectric constant and/or ferroelectric material, is being etched in a plasma. 
     The invention thus makes use of the electrostatic clamp for a novel application of eliminating and/or substantially reducing arcing or dielectric breakdown which may occur when a substrate containing a layer of high dielectric constant and/or ferroelectric material is being etched in a plasma. Although electrostatic clamps are widely used in practice for clamping substrates without the need for any front side contact, the electrostatic clamp potential has never been used as a “knob” to control the occurrence of arcing and/or dielectric breakdown. 
     FIG. 3 demonstrates a dramatic step function jump from essentially little or no arcing or microarcing below about 900 watts from the MHz power supply  36  to damaging arcing at above about 900 watts. Accordingly, arcing increases with the increase in the MHz power and with an increase in V dc −V ESC . Further, increasing the KHz power can also provide an onset of arcing. Accordingly, the invention includes programming the control  40  to ensure that the V ESC  is appropriately set in order to dynamically keep the difference V dc −V ESC  during an etch operation in a desirable range, and below a arcing threshold value, no matter what the operating condition for power supply  34  and  36  are. Thus, as the power supplies are ramped and cycled, the control  40  can keep V ESC  and the difference V dc −V ESC  below an appropriate threshold to minimize or prevent arcing. 
     Industrial Applicability 
     Accordingly, the present invention is useful in successfully etching emerging films such as high dielectric constant and ferroelectric films. It can be seen that the present invention allows for improved throughput by minimizing or eliminating arcing which can destroy some or all of the wafer. 
     Other features, aspects and objects of the invention can be obtained from a review of the figures and the claims. 
     It is to be understood that other embodiments of the invention can be developed and fall within the spirit and scope of the invention and claims.