Abstract:
Methods and apparatus for sputtering a target material, such as PZT, can include positioning a conductive grid between a target and a substrate. The target, the substrate, and a sputtering gas can be contained in a chamber, and power of a first RF source can be applied so as to maintain a plasma in the chamber. Power of a second RF source can be applied to the conductive grid. Target material can be sputtered from the target onto the substrate. Positioning of the conductive grid and application of power by the second RF source can affect properties of sputter deposition of the target material. For example, the second RF source and the conductive grid can be part of a capacitive circuit configured such that voltage change in the capacitive circuit affects properties of the sputtering gas and, in turn, properties of a sputter deposition process.

Description:
TECHNICAL FIELD 
       [0001]    This description relates to depositing thin layers of material onto a substrate. 
       BACKGROUND 
       [0002]    Physical vapor deposition (PVD) is a vacuum deposition process for depositing thin films onto a substrate, such as a silicon wafer. In a PVD sputtering process, the substrate and a target formed of the material to be deposited (or precursor) on the substrate are contained in a vacuum chamber. The target is bombarded with high energy ions to vaporize the target material. The vaporized material is then transported to the substrate, and this transport is typically along a line of sight between the target and the substrate. The sputtering gas that provides the ions may be an inert gas, or may include a reactive gas, in which case chemical reactions of the target material may occur during transport. The target material (or material resulting from the reaction) condenses on a surface of the substrate to form a layer. During PVD, it can be desirable to control properties of the deposited thin film. 
       SUMMARY 
       [0003]    In one aspect, the methods and apparatus disclosed herein feature sputtering a target material, such as lead zirconium titanate oxide (PZT). A conductive grid is positioned between a target and a substrate. The target, the substrate, and a sputtering gas are contained in a chamber. Power of a first RF source is applied so as to maintain a plasma in the chamber. Power of a second RF source is applied to the conductive grid, and material can be sputtered from the target onto the substrate. 
         [0004]    In another aspect, the methods and apparatus disclosed herein feature a chamber configured to contain a target, a substrate, and a sputtering gas. A first RF source is configured to apply power within the chamber. A conductive grid is positioned between the target and the substrate, and a second RF source is electrically connected to the conductive grid. 
         [0005]    Implementations can include one or more of the following features. The second RF source and the conductive grid can be part of a capacitive circuit configured such that voltage change in the capacitive circuit affects properties of the sputtering gas. A distance between the conductive grid and the substrate can be adjustable and can be between about one fourth and about three fourths a distance between the target and the substrate. The second RF source can include a DC bias, and power output of the second RF source can be adjustable. The conductive grid can include lead and can include at least 90% open space. The conductive grid can be configured to substantially cover a path between the target and the substrate. A third RF source can be configured to apply power to the substrate. The sputtering gas can include oxygen, and the target can include PZT. 
         [0006]    Implementations can provide none, some, or all of the following advantages. Adjusting a position of the conductive element, as well as an amount and frequency of RF power applied thereto, can facilitate control of the deposition process, such as by influencing properties of plasma in the deposition chamber. As another example, applying a DC bias to the conductive element and adjusting the DC bias can facilitate regulating an energy level at which target material contacts the substrate, which can further improve control of the deposition process. Improved control of the deposition process can facilitate achieving a desired target material layer on the substrate. Uniformity of target material deposition on the substrate can be improved. Thickness distribution, crystalline orientation, and internal stress of a target material layer deposited on the substrate can be controlled and improved. By applying power to plasma through the conductive element a deposition rate of the target material onto the substrate may be increased. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1A  is a cross-sectional elevation view schematic representation of a deposition apparatus. 
           [0008]      FIG. 1B  is a cross-sectional plan view schematic representation of the deposition apparatus of  FIG. 1A . 
           [0009]      FIG. 2  is a cross-sectional elevation view schematic representation of an alternative deposition apparatus. 
           [0010]      FIG. 3  is a flow diagram of a deposition process. 
       
    
    
       [0011]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0012]    Deposition of a material, such as lead zirconium titanate oxide (PZT), onto a substrate, such as a silicon wafer, can be implemented in a reaction vacuum chamber. The reaction vacuum chamber can include a target containing PZT and a conductive grid positioned between the target and the substrate. The conductive grid can be capacitively coupled to a radio frequency (RF) circuit, and RF power can be applied to the grid to affect a process of depositing material onto the substrate. A DC bias can also be applied to the grid. The deposition process can be a PVD sputtering process. 
         [0013]      FIG. 1A  is a cross-sectional elevation view of a deposition apparatus  100 . A deposition chamber  110  can enclose and seal a chamber space  114 .  FIG. 1B  is a cross-sectional plan view schematic representation of the deposition apparatus  100  of  FIG. 1A . Referring to  FIGS. 1A and 1B , the deposition chamber  110  can be composed and constructed sufficiently strong to resist an atmosphere of pressure (i.e., about 760 torr) as well as relatively high temperatures, such as about 500 degrees Celsius. A magnetron  120  can be attached to the deposition chamber  110  and configured to generate magnetic fields within the deposition chamber  110 . The magnetron  120  can be positioned at or near an end of the deposition chamber  110 . 
         [0014]    A target  130  is positioned in the deposition chamber  110 , such as at an end of the deposition chamber  110  near the magnetron  130 . In some implementations, the target  130  includes PZT. An RF power source  132  can be coupled to the target  130  to apply RF voltage to induce a self-bias on the target. The RF power source can provide, for example, between about 500 watts (W) and about 5000 W, such as about 2000 W to about 4000 W, such as about 3000 W at a frequency of about 13.56 megahertz (MHz). 
         [0015]    A substrate  140  can be positioned within the deposition chamber  110 , such as within line of sight of the target  130  near an end of the deposition chamber  110  that is opposite the target  130 . The substrate  140  can be a semiconductor wafer, such as a silicon wafer. As an example, the substrate  140  can have a diameter D of about 300 millimeters (mm). The substrate  140  can be supported by a substrate support  142 . In some implementations, the substrate support can adjust a position of the substrate  140  in the deposition chamber  110  relative to the target  130 . Optionally, the substrate  140  can be electrically connected to a substrate power source  144 . In some implementations, the substrate power source  144  applies a direct current (DC) voltage bias to the substrate  140 . Alternatively or in addition, the substrate power source  144  can apply RF voltage to the substrate  140 . 
         [0016]    Gas can be evacuated from the chamber space  114  through an outlet  152 , which can be fluidically connected to a vacuum pump  154 . A sputtering gas  150  can be introduced to the chamber space  114  by an inlet  156 , which can be fluidically connected to a gas supply  158 . In some implementations, the sputtering gas  150  includes both a reactive gas and an inert gas. For example, the sputtering gas  150  can include about 1% to about 4% reactive gas and the remaining sputtering gas  150  can be an inert gas. In some implementations, the reactive gas is oxygen and the inert gas is argon. The sputtering gas  150  can be present in the deposition chamber at a relatively low pressure, such as an absolute pressure of between about 2 millitorr and about 10 millitorr, and this pressure can be adjustable. 
         [0017]    The sputtering gas  150  is ionized to produce positive ions, and the self-bias voltage on target  130  in conjunction with the magnetic field causes bombardment of the target  130  by the energetic positive ions. 
         [0018]    The deposition apparatus  100  can also include a conductive element through which the vaporized target material can pass, such as a conductive grid  160 , that can be positioned between the target  130  and the substrate  140 . For example, the conductive grid  160  can be positioned midway between the target  130  and the substrate  140 . Position of the conductive grid  160  relative to the target  130  and the substrate  140  can be adjustable. For example, the conductive grid  160  can be positioned at a distance G from the substrate  140  between about one fourth and about three fourths a distance T between the target  130  and the substrate  140 . As an example, the distance G can be between about 20 mm and about 50 mm. The conductive grid  160  can be generally planar and parallel to the substrate. The conductive grid  160  can be, for example, a grid composed of wires  161 , e.g., a wire mesh. In some implementations, an area of the conductive grid  160  can include at least about 90% open space. In some implementations, the conductive grid  160  substantially covers a path between the target  130  and the substrate  140 . That is, the conductive grid  160  can be configured so that any straight, line-of-sight path between the target  130  and the substrate  140  passes through the conductive grid  160 . Although some vaporized target material may be blocked by the conductive grid  160 , some of the vaporized target material will pass through, e.g., between wires  161  of the conductive grid  160 . In some implementations, an area spanned by the conductive grid  160  can be substantially larger than a surface area of the substrate  140 . 
         [0019]    A grid power source  164  can be electrically connected to the conductive grid  160 . The grid power source  164  can be configured to apply an RF signal to the conductive grid  160 . That is, for example, the grid power source  164  can apply to the conductive grid  160  an oscillating voltage with reference to a ground  165 . In some implementations, the conductive grid  160  and the grid power source  164  form a predominantly capacitive circuit. That is, the grid power source  164  can cause voltage of the conductive grid  160  to vary with respect to a reference voltage while little or no current flows through the conductive grid  160 . As an example, the grid power source  164  can apply about 100 W to about 500 W to the conductive grid  160  at a frequency of about 13.56 MHz. Power output of the grid power source  164  can be adjustable. Power applied to the conductive grid  160  can create a magnetic field within the deposition chamber  110 . Such a magnetic field can be desirable to affect properties of plasma within the deposition chamber, and some such properties are described below. Optionally, a grid DC bias circuit  166  can also be electrically connected to the conductive grid  160  and configured to apply a DC bias thereto. 
         [0020]    Applying power or a DC bias to the conductive grid  160  can, for example, alter properties of a plasma in the deposition chamber  110 , which can affect an amount of energy of target material  134  arriving at the substrate  140 . This may be desirable, for example, because target material  134  may form a thin film on the substrate more readily or more uniformly at some energy levels than at others. The power or DC bias supplied to the conductive grid  160  can be adjusted to optimize or otherwise control deposition rate, uniformity of deposition, or some other deposition property. In some implementations, the grid DC bias circuit  166  can include a capacitor (not shown), a capacitor and a resistor (not shown), or some other suitable circuit. 
         [0021]    In some implementations, including elemental lead, e.g., substantially pure elemental lead, in the conductive grid  160  can improve deposition of PZT on the substrate  140 . Lead may tend to evaporate off of the substrate  140  during a deposition process. Without being limited to any particular theory, using a conductive grid  160  that includes lead can increase a concentration of lead atoms near the substrate  140 , thereby increasing an amount of lead available for formation of PZT on the substrate  140 . The wires of the conductive grid can be formed entirely of lead, or a layer of substantially pure lead could be deposited as a coating on the wires of the grid. In some implementations, PZT composition on the surface of the substrate  140  can be adjusted by adjusting power or DC bias applied to the conductive grid  160  or by adjusting an amount of lead in the conductive grid  160 . 
         [0022]      FIG. 2  is a cross-sectional elevation view of an alternative deposition apparatus  100 ′. A conductive coil  260  can be positioned between the target  130  and the substrate  140 . As an example, the conductive coil  260  can have a diameter A of between about 300 mm and about 350 mm. Position of the conductive coil  260  relative to the target  130  and the substrate  140  can be adjustable. For example, the conductive coil  260  can be positioned at a distance C from the substrate  140  between about one fourth and about three fourths a distance T between the target  130  and the substrate  140 . As an example, the distance C can be between about 20 mm and about 50 mm. In some implementations, the conductive coil  260  is electrically connected to a coil RF source  264 . For example, the coil RF source  264  and the conductive coil  260  can form a predominantly inductive circuit. In such implementations, the coil RF source  264  can cause current flow through the conductive coil  260 , which can induce an electromagnetic field within the deposition chamber  110 . This electromagnetic field can influence properties of a plasma in the deposition chamber  110  and can influence deposition of the target material  134  on the substrate  140 . In some implementations, the coil  260  is positioned inside the deposition chamber  100 . In some alternative implementations, the coil  260  is positioned outside of and around the deposition chamber  110 . Such implementations may be feasible where the deposition chamber  110  is composed of non-conductive materials, such as ceramics. 
         [0023]      FIG. 3  is a flow diagram of a PVD sputtering process  300 . The conductive grid  160  can be positioned between the target  130  and the substrate  140  (step  320 ). The target  130 , the substrate  140 , and the sputtering gas  150  can be contained within the deposition chamber  110  (step  330 ). 
         [0024]    The target  130  can be bombarded with ions as part of a PVD sputtering process so that the target  130  releases atoms or molecules of target material  134  (step  340 ). For example, the sputtering gas  150  can be ionized, and the magnetic field can concentrate plasma near the target  130 . Positive ions of the sputtering gas  150  can impact the target  130 , and momentum transfer can cause atoms or molecules of target material  134  to be ejected from the target  130 . The target material  134  can move in many or all directions away from the target  130 , including toward the substrate  140  in a direction of the arrows in  FIGS. 1 and 2 . 
         [0025]    RF power can be applied to the conductive grid  160  or the conductive coil  260  to affect properties of the sputtering process  300  (step  350 ). Deposition process properties can include, for example, density of plasma, plasma potential, sheath wide re-distribution, electron temperature, and ion flux distribution. Other deposition properties can include thickness distribution, crystalline orientation, and internal stress of material deposited on the substrate  140 . Additional deposition properties can include the properties of coverage of surface protrusions and depressions and areas therebetween on the substrate  140 , such as step coverage of surface topography of the substrate  140 . It may be desirable to control properties of the deposition process, for example, to improve uniformity of a layer of target material  134  deposited on the substrate  140 . Without being limited to any particular theory, deposition properties can be affected because power applied to the conductive grid  160  or conductive coil  260  can influence, for example, energy of target material  134  contacting the substrate  140 . Applying RF power or DC bias to the conductive grid  160  or the conductive coil  260  can also be used to increase plasma density in the chamber space  114 . Increasing plasma density may be desirable to increase a rate of vapor deposition. 
         [0026]    The sputtering process  300  can be implemented to deposit PZT from the target  130  onto the substrate  140  (step  360 ), as described above. 
         [0027]    The above-described implementations can provide none, some, or all of the following advantages. Adjusting a position of the conductive element, as well as an amount and frequency of RF power applied thereto, can facilitate control of the deposition process, such as by influencing properties of plasma in the deposition chamber. As another example, applying a DC bias to the conductive element and adjusting the DC bias can facilitate regulating an energy level at which target material contacts the substrate, which can further improve control of the deposition process. Improved control of the deposition process can facilitate achieving a desired target material layer on the substrate. Uniformity of target material deposition on the substrate can be improved. Thickness distribution, crystalline orientation, and internal stress of a target material layer deposited on the substrate can be controlled and improved. By applying power to plasma through the conductive element a deposition rate of the target material onto the substrate may be increased. 
         [0028]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. For example, instead of using a grid or a coil, a conductive element in some other form can be used, such as an expanded metal mesh, a perforated foil, or some other suitable conductive element. Accordingly, other embodiments are within the scope of the following claims.