Adhesive bonding and miscellaneous chemical manufacture – Differential fluid etching apparatus – With microwave gas energizing means
Reexamination Certificate
1999-05-03
2001-07-10
Mills, Gregory (Department: 1763)
Adhesive bonding and miscellaneous chemical manufacture
Differential fluid etching apparatus
With microwave gas energizing means
C118S7230ER, C118S7230AN
Reexamination Certificate
active
06258204
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the design of electrode covers used for plasma etch and plasma-enhanced chemical vapor deposition reactors. Plasma etching of semiconductor wafers and other substrates relies on the production of ionized gaseous species using a radio frequency (rf) discharge at pressures in the range from about 0.01 to 10 Torr, commonly referred to as a glow discharge. The charged species react with molecules at the surface of the substrate, resulting in volatile reaction products which are carried away. Several types of etchers are commonly employed in semiconductor fabrication including wet chemical (barrel) reactors, vertical dry chemical plasma reactors, and horizontal dry chemical plasma reactors. Of interest herein are dry chemical parallel plate plasma reactors. Such parallel plate reactors are typically characterized by a reactor volume defined by a pair of vertically spaced-apart horizontal electrode plates, although other orientations may also be employed. An etchant gas is typically fed through at least one of the electrode plates, and rf energy is applied across the electrodes to induce the desired plasma. Parallel plate reactors may be configured to process either single or multiple wafers. Single wafer parallel plate reactors, because of their highly symmetric electrical field and gas flow characteristics, are able to provide highly uniform etching across the surface of the wafer. Parallel plate reactors are described in a number of U.S. Pat. Nos. 4,612,077; 4,534,816, 4,595,484, 4,590,042; 4,407,708; and 4,158,589, the disclosures of which are incorporated herein by reference.
Parallel plate plasma reactors usually operate by introducing a low pressure etchant gas through the upper electrode plate and placing a single wafer or multiple wafers over the lower electrode. The plasma is uniformly generated as the etchant gas flows downward and the rf energy is applied to the reactor, typically across the two electrodes.
The upper electrode must meet a number of requirements in order to achieve desired performance characteristics. Foremost, the electrode must have defined electrical properties, such as impedance, current capacity, and the like, in order to couple rf energy into the plasma in combination with the lower electrode. Additionally, the upper electrode material must be able to withstand prolonged exposure to the generated plasma, and interaction between the electrode material and the plasma should not have a deleterious effect on any of the desired plasma properties. In particular, the upper electrode should not generate large particles or large quantities of particles and should not release heavy metals or other contaminants into the zone between the opposed electrodes. Transition group metals severely degrade minority carrier lifetimes and significantly increase junction leakage. Alkali metals, particularly sodium, cause instability in MOS threshold voltages. The temperature characteristics of the plasma are also critical to system performance, and it is desirable that the electrode be able to be maintained at a uniform, stable temperature across its entire surface. Finally, it is often desirable that the etchant gas be introduced through the upper electrode. In that case, the electrode material should be machinable in order to form the necessary passages and other features for delivering a uniform flow of gas therethrough.
Heretofore, upper electrodes for parallel plate plasma reactors have generally been formed from a single (or coated) material, such as polycrystalline silicon, graphite, aluminum, flame sprayed silicon powder on aluminum, or the like. While each of these materials enjoys certain advantages, e.g. polycrystalline silicon is compatible with many plasma chemistries, anodized aluminum is relatively inexpensive and easy to fabricate, and graphite is readily machined and can be purified to semiconductor purity, no one material has been found to meet all electrode requirements.
Thus, it would be desirable to provide improved electrode cover constructions used in plasma etching. Such electrode covers should possess desirable electrical and thermal properties, and should be compatible with many or all plasma chemistries. In particular, it would be desirable if such electrodes were relatively easy and inexpensive to fabricate.
SUMMARY OF THE INVENTION
According to the present invention, an electrode assembly suitable for use in a plasma etcher comprises a plate, usually in the form of a disk, composed of a “semiconductor purity” material having a substantially uniform thickness. One face of the plate is bonded to a support frame composed of an electrically and thermally conductive material, leaving the other face substantially flat and free from protuberances. Usually, the support frame will be in the form of a ring which is bonded about the periphery of a plate in the form of a disk. Preferably, a plate and support frame are bonded together with a relatively ductile bonding layer formed by brazing, soldering, or the like. The bonding material should be composed of a thermally and electrically conductive material, such as metals, conductive epoxies, or the like preferably being formed from low vapor pressure materials which will have less tendency to contaminate low pressure reactor environments.
The present invention comprises the electrode assemblies themselves as well as improved plasma plate. The plasma side of the plate is counter bored in the area of the gas inlet holes to an appropriate depth. On the opposite side of the plate, another set of bores are placed around the outsides of the gas inlet feed throughs. These bores are machined to incorporate a set of metallic sleeves. The counter bore on the plasma side of the ceramic is used as the first step in removing the plasma from the ceramic surface. The metallic sleeves are utilized to prevent the plasma from invading the counter bore and touching the surface and to allow the surface to remain electrically uniform and planar to the substrate being etched. The sleeves create a negative charge close to the surface of the ceramic surface but not exposed to the plasma to create a dark space. The dark space mimics an electrically planar surface for the substrate while keeping the hot plasma from direct contact with the ceramic surface.
The plate portion of the composite electrode which is exposed to plasma can be formed from material which is most suitable for the processing conditions with less concern for the cost of the material or the ability to machine the material. Thus, the material of choice can be dictated primarily by plasma chemistry and the desirability to minimize formation of particles and release of other contaminates. Similarly, the support frame can be composed of a material which has desired electrical, thermal, and structural properties and which can be relatively easily machined or otherwise formed into a desired geometry, e.g. a ring. In particular, the material of the support frame should not be brittle and should allow attachment within the reactor by bolting or other conventional fasteners. The material of the support frame will usually be chosen to have a thermal expansion coefficient which is generally compatible with that of the electrode plate, but a certain amount of mismatch can be tolerated when the bonding layer is formed from a ductile material. In a preferred embodiment, the support frame is chosen to have a slightly greater coefficient of thermal expansion. By then joining and/or curing the bonding layer at a temperature above the expected operating temperature of the electrode, the electrode plate will be maintained under compression, enhancing the durability of the plate. In this way, the support frame can be reliably connected to an electrical power source as well as a heat sink intended to control the temperature of the electrode. By properly configuring the contact area between the support frame and the electrode plate, the rf fields produced by the electrode as well as the temperature profile maintained ac
Hassanzadeh P.
Mills Gregory
Oppenheimer Wolff & Donnelly LLP
Philips Semiconductors Inc.
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