Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
2001-12-18
2004-09-07
Hassanzadeh, Parviz (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S7230AN, C156S345340, C156S345470, C156S345430
Reexamination Certificate
active
06786175
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a showerhead electrode assembly for improved thermal uniformity in a semiconductor processing reactor. Specifically, the invention relates to an electrode plate removably clamped to a backing plate or heat sink plate using a compliant, thermally conductive electrostatic clamp. The invention also relates to a method of processing a semiconductor substrate such as a wafer with the showerhead electrode assembly.
BACKGROUND OF THE INVENTION
Vacuum processing chambers are generally used for chemical vapor depositing (CVD) and etching of materials on substrates by supplying process gas to the vacuum chamber and application of an RF field to the gas. For example, parallel plate, transformer coupled plasma (TCP™, also called ICP), and electron-cyclotron resonance (ECR) reactors are disclosed in commonly-owned U.S. Pat. Nos. 4,340,462; 4,948,458; and 5,200,232. During processing, the substrates are held in place within the vacuum chamber by substrate holders. Conventional substrate holders include mechanical clamps and electrostatic clamps (ESC), which clamp a wafer to a pedestal during processing. The pedestal may form both an electrode and a heat sink. Examples of mechanical clamps and ESC substrate holders are provided in commonly-owned U.S. Pat. Nos. 5,262,029, 5,740,016, and 5,671,116 and in U.S. Pat. No. 6,292,346. Substrate holders in the form of an electrode can supply radiofrequency (RF) power into the chamber, as disclosed in U.S. Pat. No. 4,579,618.
Electrostatic clamps secure a substrate to a pedestal by creating an electrostatic attractive force between the substrate and the clamp. A voltage is applied to one or more electrodes in the ESC so as to induce opposite polarity charges in the substrate and electrode(s), respectively. The opposite charges pull the substrate against the pedestal, thereby retaining the substrate.
In a monopolar design, the ESC comprises an electrode, a workpiece and a dielectric between them. In the normal mode of operation for a monopolar ESC the electrode is connected to the negative pole of a DC power supply. The workpiece is connected through the plasma to ground. Under this arrangement the workpiece is not securely clamped prior to exposing the workpiece to the plasma or other return path.
In a bipolar configuration the ESC can clamp the workpiece absent a plasma; a second pole of the ESC provides the return path. This configuration uses both the positive and negative potential of the DC power supply to electrostatically clamp the workpiece.
A tripolar ESC contains three poles. The inner two poles are similar to the bipolar configuration, and are used to clamp the workpiece. The outer pole can be used as either a plasma shield or a workpiece bias pickup point. U.S. Pat. No. 5,572,398 discloses a tri-polar electrostatic clamp using separate positive and negative electrodes housed on a non-polarized base.
Multi-pole ESC's use either AC or DC that is phased to each pole of the electrode. The phasing of the voltage applied to the electrode permits rapid clamping and release. The supply and control circuitry required for a multi-pole type of ESC is more complicated than for the monopole, bipolar or tripolar configurations. The multi-pole configuration is used to minimize charge build up on the workpiece and also helps reduce de-clamping issues.
The materials and processes used in processing a semiconductor substrate such as a wafer are extremely temperature sensitive. Excessive temperature fluctuations in the substrate may compromise system performance and the resulting properties of the semiconductor device. Thus, in order to adequately control the temperature of the substrate, good thermal contact between the substrate and the substrate holder is desired.
To facilitate heat transfer between the substrate and substrate holder, a very large electrostatic or physical force is commonly used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the substrate holder. Due to surface roughness of both the substrate and the holder, however, when the two surfaces are pressed against each other only point contacts are established; i.e., small interstitial spaces (voids) constitute the majority of the interface. Under low pressure processing conditions, this interface is evacuated and the voids comprise a vacuum, which is a very good thermal insulator. Thus, heat transfer between the two surfaces is limited mainly to the point contacts.
To improve temperature uniformity across the substrate during processing, an inert, high thermal conductivity gas such as helium is pumped into the interstitial spaces formed between the substrate and the support surface. This heat transfer gas acts as a more efficient thermal transfer medium between the substrate and the substrate holder than the vacuum it replaces.
Some ESC devices are designed to minimize escape of heat transfer gas into the surrounding low pressure atmosphere (i.e., the reaction chamber). The support surface in such a device can have a circumferential raised rim having a diameter that is approximately equal to the diameter of the wafer and a flex circuit covering the support surface of the underlying pedestal. The flex circuit is usually a conductive material encased in a flexible dielectric material. The conductive material is patterned to form the electrostatic electrode. The dielectric material insulates the conductive material from other conductive components and also acts as a gasket. Once the wafer is clamped, a gas tight seal is created between the wafer and the rim. As such, heat transfer gas leakage from beneath the wafer at the rim is minimized. An electrostatic substrate holder comprising a lip seal for clamping substrates is disclosed in commonly-owned U.S. Pat. No. 5,805,408. Electrostatic substrate holders comprising flex circuits are disclosed in U.S. Pat. Nos. 6,278,600 and 6,033,478.
As described above, adequate wafer temperature control can be obtained through backside He gas pressure and electrostatic clamping techniques. In addition to substrate temperature control, however, temperature control of all the reactor surfaces that come into contact with active process chemistry is desirable for process repeatability and uniformity. Various upper electrode heating and cooling designs have been developed and are being used in semiconductor processing apparatus, for example parallel plate dielectric etchers. A reaction chamber component having improved temperature uniformity is disclosed in commonly-owned U.S. Pat. No. 6,123,775.
As shown schematically in
FIG. 1
, in some prior art designs the upper electrode plate
150
is held against a temperature controlled heat sink plate
110
via a backing ring
120
. The electrode
150
is a planar disk having uniform thickness from center to edge thereof. The temperature of the heat sink plate is maintained by circulating a heated or cooled liquid through channels
114
within the plate. The heat sink plate is furnished with a process gas feed
134
for supplying gas to the process chamber. The gas then is dispersed through a plenum
140
and passes through gas dispersion holes (not shown) in the electrode to evenly disperse the process gas into the reaction chamber. According to this design, most of the thermal contact between the electrode
150
and the heat sink plate
110
is established at the periphery of the electrode where force from a peripheral mechanical clamp
160
is directly applied. The advantage of this technique is that the electrode plate, which is a consumable part, is separable from the heat sink plate. The disadvantage, however, is that the limited peripheral contact between the electrode and heat sink plate results in large center-to-edge temperature non-uniformity in the electrode.
As shown in
FIG. 2
, the center-to-edge temperature non-uniformity problem of limited thermal contact has been addressed in designs that use a backing plate
220
that can be bonded or brazed to the main electrode
250
. The electrode plat
Dhindsa Rajinder
Lenz Eric
Burns Doane , Swecker, Mathis LLP
Hassanzadeh Parviz
Lam Research Corporation
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