Electricity: electrical systems and devices – Electric charge generating or conducting means – Use of forces of electric charge or field
Reexamination Certificate
1998-11-19
2001-07-10
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Electric charge generating or conducting means
Use of forces of electric charge or field
C279S128000
Reexamination Certificate
active
06259592
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to an apparatus for retaining a workpiece within a semiconductor wafer processing system and, more specifically, to an improved composition of a polyimide based electrostatic chuck that maximizes electrostatic clamping ability without loss of material strength or modulus of elasticity.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece in various applications including retaining a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. The pedestal may form an electrode and a heat sink or heater as used in etching, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applications. For example,
FIG. 1
depicts a cross-sectional view of a reaction chamber used in semiconductor wafer processing. For a detailed understanding of the reaction chamber and its operation in processing the wafer, the reader should refer to the drawings and the detailed description contained in U.S. Pat. No. 5,228,501, issued Jul. 20, 1993, incorporated herein by reference. That patent teaches a PVD wafer processing chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif. Additionally, the operation of a conventional electrostatic chuck is disclosed in U.S. Pat. No. 5,350,479 issued Sep. 27, 1994 to the assignee hereof, and its disclosure is incorporated herein by reference.
The chamber
100
contains a pedestal
106
supporting an electrostatic chuck
104
. The electrostatic chuck
104
has at least one electrode
116
which is insulated from a wafer
102
placed upon an upper surface
105
of the electrostatic chuck
104
. Specifically, the electrode
116
is either embedded within the body of the electrostatic chuck
104
or encased in layers of dielectric material which comprise the electrostatic chuck. The electrode(s)
116
are coupled to a power supply (not shown) via electrical conductors
118
. The voltage from the power supply creates the electrostatic (or clamping) force which draws the wafer
102
to the chuck
104
. Additionally, a variety of components may circumscribe the pedestal
106
to protect the wafer
102
and chamber
100
from improper or excessive deposition, etching or the like. Specifically, a deposition ring
108
contacts the edges of the wafer
102
and a deposition shield
124
circumscribes the deposition ring
108
to define a reaction zone
126
. Lift pins
110
are mounted on a platform
112
. The platform is coupled to an actuator shaft
114
located below the pedestal
106
. The lift pins
110
engage the wafer and lift it off the pedestal
106
after processing is completed.
The mechanism of attraction in the electrostatic chuck used in these types of wafer processing systems is generally Coulombic force. That is, the increase of charges in the insulated electrode
116
induce opposite charges to gather on the backside of the wafer. The resultant force is generally weak per unit area i.e., 15 g/cm
2
at 1500V DC because of the composition of the chuck. For example, a commonly used type of dielectric material for fabricating electrostatic chucks is polyimide. Specifically, electrodes are usually sandwiched between two sheets of polyimide to form an electrostatic chuck. Among the beneficial characteristics of polyimide are its high strength and high modulus of elasticity. This material also has high volume resistivity (on the order of 10
14
ohm-cm) and surface resistivity (on the order of 10
14
ohm/cm
2
). Since the electrode(s) are insulated and a high resistivity dielectric is used, the charges creating the chucking force are not mobile i.e., the electrode and wafer are separated by the dielectric layer. As such, the wafer must come into contact with a large area of the chuck so that an adequate charge accumulation is established for wafer retention.
Additionally, the backside of the wafer
102
and the top surface
105
of the electrostatic chuck
104
are relatively smooth. However, imperfections in each of these surfaces create interstitial spaces when these surfaces come into contact. As such, not all of the wafer is in direct thermal contact with the chuck. Maintaining a uniform temperature across the entire wafer is essential to proper wafer processing. To maintain proper thermal transfer conditions at the wafer during processing, an inert thermal transfer gas is pumped into the interstitial spaces or specially formed grooves in the chuck surface when the clamping force is applied. More specifically, a feed-through pipe
122
in the pedestal
106
provides thermal transfer gas to an aperture
120
in the top surface
105
of the electrostatic chuck
104
. The gas, usually Helium or Argon, acts as a thermal conduction medium between the wafer
102
and the chuck
104
that has better thermal transfer characteristics than the vacuum it replaces. To further enhance thermal transfer conditions (i.e., cooling or heating of the wafer), the pedestal temperature is typically controlled using water-cooled conduits within a cooling plate (not shown) below the chuck
104
and/or with resistive heating elements buried in or clamped to the chuck
104
. This cooling technique is known as backside gas cooling.
Since the distribution of thermal transfer gas to the interstitial spaces and chuck groove is osmotic and the interstitial spaces may not all be interconnected, some spaces do not receive any gas. This condition can also lead to a non-uniform temperature profile across the backside of the wafer
108
during processing and result in wafer damage. As such, it is advantageous to have as large a gas aperture and groove width as possible to maximize thermal transfer gas flow and pressure beneath the wafer. However, the limited attractive wafer clamping (Coulombic) force establishes a limit on the size of this aperture and the gas pressure therein. Additionally chuck groove width is limited to approximately 1-2 mm. Specifically, if the thermal transfer gas pressure becomes greater than the Coulombic chucking force, the wafer may shift on the pedestal thereby causing a processing anomaly on the wafer. In an extreme situation, the wafer may even pop off the pedestal onto the chamber floor and likely break, rendering the wafer useless. Since effective and uniform heat conduction away from and/or into the wafer is an important aspect of the manufacturing process, different types of chucks are designed in an attempt to maximize clamping force and thermal transfer.
One example of an improved electrostatic chuck is one that employs the Johnsen-Rahbek (J-R) effect. In such a chuck, the dielectric material has an intermediate resistivity instead of a high resistivity. As such, there are mobile charges present in the dielectric material. These mobile charges create a small but effective current flow between the backside of the wafer and the top surface of the electrostatic chuck. Specifically, at points where these two surfaces come into contact, a zero potential exists. These contact points are extremely small in comparison to the total area of a wafer being retained on the chuck. As such, not all of the mobile charges are able to pass through the contact points. The resultant movement and accumulation of the mobile charges within the top surface of the electrostatic chuck and the backside of the wafer creates a very high electrostatic force across the interstitial spaces between the surfaces. This electrostatic force clamps the wafer to the chuck.
Electrostatic chucks using the J-R effect are usually fabricated from a ceramic ha
Applied Materials Inc.
Leja Ronald W.
Thomason Moser & Patterson LLP
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