Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2000-07-31
2001-02-27
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C712S029000, C712S029000, C712S029000, C156S345420
Reexamination Certificate
active
06194322
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates to an apparatus for plasma processing of semiconductor wafers, and more particularly, to an electrode assembly wherein the electrode is bonded to a support member. The invention also relates to a process of assembling the electrode and processing of a semiconductor substrate with the electrode assembly.
2. Description of the Related Art
Electrodes used in plasma processing reactors for processing semiconductor substrates such as silicon wafers are disclosed in U.S. Pat. Nos. 5,074,456 and 5,569,356, the disclosures of which are hereby incorporated by reference. The '456 patent discloses an electrode assembly for a parallel plate reactor apparatus wherein the upper electrode is of semiconductor purity and bonded to a support frame by adhesive, solder, or brazing layer. The soldering or brazing layer can be low vapor pressure metals such as indium, silver and alloys thereof and the bonded surfaces of the support frame and the electrode can be coated with a thin layer of metal such as titanium or nickel to promote wetability and adhesion of the bonding layer. It has been found that metallurgical bonds such as In bonds cause the electrode to warp due to differential thermal expansion/contraction of the electrode and the part to which the electrode is bonded. It has also been found that these metallurgical bonds fail at high plasma processing powers due to thermal fatigue and/or melting of the bond.
Dry plasma etching, reactive ion etching, and ion milling techniques were developed in order to overcome numerous limitations associated with chemical etching of semiconductor wafers. Plasma etching, in particular, allows the vertical etch rate to be made much greater than the horizontal etch rate so that the resulting aspect ratio (i.e., the height to width ratio of the resulting notch) of the etched features can be adequately controlled. In fact, plasma etching enables very fine features with high aspect ratios to be formed in films over 1 micrometer in thickness.
During the plasma etching process, a plasma is formed above the masked surface of the wafer by adding large amounts of energy to a gas at relatively low pressure, resulting in ionizing the gas. By adjusting the electrical potential of the substrate to be etched, charged species in the plasma can be directed to impinge substantially normally upon the wafer, wherein materials in the unmasked regions of the wafer are removed.
The etching process can often be made more effective by using gases that are chemically reactive with the material being etched. So called “reactive ion etching” combines the energetic etching effects of the plasma with the chemical etching effect of the gas. However, many chemically active agents have been found to cause excessive electrode wear.
It is desirable to evenly distribute the plasma over the surface of the wafer in order to obtain uniform etching rates over the entire surface of the wafer. For example, U.S. Pat. Nos. 4,595,484, 4,792,378, 4,820,371, 4,960,488 disclose showerhead electrodes for distributing gas through a number of holes in the electrodes. These patents generally describe gas distribution plates having an arrangement of apertures tailored to provide a uniform flow of gas vapors to a semiconductor wafer.
A reactive ion etching system typically consists of an etching chamber with an upper electrode or anode and a lower electrode or cathode positioned therein. The cathode is negatively biased with respect to the anode and the container walls. The wafer to be etched is covered by a suitable mask and placed directly on the cathode. A chemically reactive gas such as CF
4
, CHF
3
, CClF
3
and SF
6
or mixtures thereof with O
2
, N
2
, He or Ar is introduced into the etching chamber and maintained at a pressure which is typically in the millitorr range. The upper electrode is provided with gas holes which permit the gas to be uniformly dispersed through the electrode into the chamber. The electric field established between the anode and the cathode will dissociate the reactive gas forming a plasma. The surface of the wafer is etched by chemical interaction with the active ions and by momentum transfer of the ions striking the surface of the wafer. The electric field created by the electrodes will attract the ions to the cathode, causing the ions to strike the surface in a predominantly vertical direction so that the process produces well-defined vertically etched side walls.
A showerhead electrode
10
in an assembly for a single wafer etcher is shown in FIG.
1
. Such a showerhead electrode
10
is typically used with an electrostatic chuck having a flat bottom electrode on which a wafer is supported spaced 1 to 2 cm below the electrode
10
. Such chucking arrangements provide temperature control of the wafer by supplying backside He pressure which controls the rate of heat transfer between the wafer and the chuck.
The electrode assembly is a consumable part which must be replaced periodically. Because the electrode assembly is attached to a temperature-controlled member, for ease of replacement, it has been conventional to metallurgically bond the upper surface of the outer edge of the silicon electrode
10
to a graphite support ring
12
with indium which has a melting point of about 156° C. Such a low melting point limits the amount of RF power which can be applied to the electrode since the RF power absorbed by the plasma causes the electrode to heat up. The electrode
10
is a planar disk having uniform thickness from center to edge thereof. An outer flange on ring
12
is clamped by an aluminum clamping ring
16
to an aluminum temperature-controlled member
14
having water cooling channels
13
. Water is circulated in the cooling channels
13
by water inlet/outlet connections
13
a
. A plasma confinement ring
17
comprised of a stack of spaced-apart quartz rings surrounds the outer periphery of electrode
10
. The plasma confinement ring
17
is bolted to a dielectric annular ring
18
which in turn is bolted to a dielectric housing
18
a
. The purpose and function of confinement ring
17
is to cause a pressure differential in the reactor and increase the electrical resistance between the reaction chamber walls and the plasma thereby confining the plasma between the upper and lower electrodes. A radially inwardly extending flange of clamping ring
16
engages the outer flange of graphite support ring
12
. Thus, no clamping pressure is applied directly against the exposed surface of electrode
10
.
Process gas from a gas supply is supplied to electrode
10
through a central hole
20
in the temperature-controlled member
14
. The gas then is distributed through one or more vertically spaced apart baffle plates
22
and passes through gas distribution holes (not shown) in the electrode
10
to evenly disperse the process gas into reaction chamber
24
. In order to provide enhanced heat conduction from electrode
10
to temperature-controlled member
14
, process gas can be supplied to fill open spaces between opposed surfaces of temperature-controlled member
14
and support ring
12
. In addition, gas passage
27
connected to a gas passage (not shown) in the annular ring
18
or confinement ring
17
allows pressure to be monitored in the reaction chamber
24
. To maintain process gas under pressure between temperature-controlled member
14
and support ring
12
, a first O-ring seal
28
is provided between an inner surface of support ring
12
and an opposed surface of temperature-controlled member
14
and a second O-ring seal
29
is provided between an outer part of an upper surface of support ring
12
and an opposed surface of member
14
. In order to maintain the vacuum environment in chamber
24
, additional O-rings
30
,
32
are provided between temperature-controlled member
14
and cylindrical member
18
b
and between cylindrical member
18
b
and housing
18
a.
The process of bonding the silicon electrode
10
to the support ring
12
requires heating of the electrode to a bondin
Hubacek Jerome S.
Kennedy William S.
Lilleland John
Burns Doane Swecker & Mathis L.L.P.
Lam Research Corporation
Umez-Eronini Lynette T.
Utech Benjamin L.
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