Coating boron carbide on aluminum

Details Coating boron carbide on aluminum Coating boron carbide on aluminum

2000-01-21

2004-10-26

Lund, Jeffrie R (Department: 1763)

Coating processes

Coating by vapor, gas, or smoke

Carbon or carbide coating

C427S249100, C427S255110, C427S255280, C118S715000

Reexamination Certificate

active

06808747

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to a composite stock piece of a coated metal substrate and to the resulting product, in particular aluminum coated with a protective layer of boron carbide, which is particularly useful for chamber walls and other parts facing a corrosive plasma.
BACKGROUND ART
Dry plasma etching is the preferred process for etching features on a silicon wafer having semiconductor integrated circuits partially developed in it. Typically, one or more planar layers are deposited over the previously defined substrate, and a layer of photoresist mask or a hard mask is deposited over the planar layers and patterned to leave apertures exposing portions of the planar layers. An etching gas admitted into the etching reactor is then excited into a plasma state, and it acts on the portions of the planar layers exposed by the mask to remove those exposed portions. The plasma etching process has proved to be very effective at defining extremely small features with low production of deleterious particles.
The field of plasma etching is typically divided among silicon etching, oxide etching, and metal etching. Each uses its preferred chemistry and presents its own problems. However, many problems are common among them, and the etching chambers dedicated to different ones of the uses tend to resemble each other. Such commonality of design offers an opportunity for savings.
The most prevalent use of metal etching is to define interconnects (and accompanying contacts or vias) in a layer of aluminum or aluminum alloy deposited over an interlayer dielectric. Once the generally planar aluminum layer has been deposited over the interlayer dielectric and into the contact or via holes, a photomask is deposited and defined over the aluminum layer. Then, an etching gas is admitted into the plasma etch chamber and excited into the plasma state. It has long been known that a chlorine-based chemistry is effective at etching aluminum. See, for example, U.S. Pat. No. 5,387,556 to Xiaobing et al. Gaseous hydrochloric acid (HCl) is the prototypical chlorine-based etchant. However, HCl is no longer considered the optimum aluminum etchant.
Aluminum quickly forms an overlying layer of a native oxide of alumina (Al
2
O
3
) and related materials forming a residue over the metallic aluminum being etched. Alumina is a very stable material and resistant to reductive breakdown, even by HCl. For these reasons, a plasma etch of BCl
3
, often in conjunction with HCl or Cl
2
, is often used for etching aluminum and its alloys. Wang et al. in U.S. Pat. No. 5,219,485 use a similar chemistry to etch suicides in order to avoid residues from the silicide etch.
However, the use of a powerful etchant like BCl
3
introduces a problem originating from the fact that the chamber is most economically made of aluminum, for example the alloy Al6061-T6. The seminal problem is that a chamber having an aluminum body and which is used for etching aluminum must balance the etching of the aluminum portion of the substrate against the etching of the chamber body. The physical integrity of the aluminum chamber is not as important as the fact that the etching of the aluminum chamber is likely to produce aluminum- based particles that deleteriously fall on the wafer and reduces its yield of functioning integrated circuits. That is, the chamber wall in a plasma reactor intended for aluminum etching must not be composed of raw aluminum.
For these reasons, it has been known to coat the wall of a plasma reactor for metal etching with an etch-resistant coating. Steger describes such an approach in U.S. Pat. No. 5,268,200 in which a protective coating of an electrically conductive hydrogen-containing layer is deposited on the aluminum wall. Another more typical approach is to coat the aluminum body with a surface layer of alumina. This surface coating of alumina is usually achieved by anodization of the underlying aluminum. Raw aluminum quickly forms with a native oxide of Al
2
O
3
to thickness of about 2.5 nm. However, further increases in the oxide thickness are quickly inhibited by the robustness of the aluminum oxide layer. Electrolytic anodization of the aluminum body easily increases the alumina thickness to 25 to 75 &mgr;m. Anodization of aluminum and aluminum-based alloys is well known. Typically, the aluminum body is submerged in a bath of electrolyte, for example, of 15 vol % solution of H
2
SO
4
, and the aluminum body is connected as an anode to one terminal of an electrical power supply while a cathode submersed in the electrolyte is connected to the other terminal. Thereby, the aluminum is electrolytically oxidized by applying DC current. The first layer of a few tens of nanometers of anodization presents a relatively dense barrier. However, further increases in the anodization thickness produces a relatively porous material. Pore size can be reduced by reducing the temperature of the anodization bath, but inevitably the thicker anodizations lack the robustness of a native aluminum oxide layer or the initial barrier layer.
Anodized aluminum has been an object of much development for its use in plasma reactors, particularly metal etch reactors. The fundamental objective has been to reduce the etching of the anodized aluminum chamber wall relative to the etching of the alumina-based residues resulting from the etching of the aluminum lines. Etching of the anodized aluminum wall in a metal etch reactor is a particular problem since anodized aluminum is fundamentally alumina and BCl
3
is being used for its effective removal of alumina
The reaction of BCl
3
and Al
2
O
3
is exothermic following the reaction
Al
2
O
3
+2BCl
3
→2AlCl
3
↑+B
2
O
3
, &Dgr;G°
f
(−15.19 kcal/mol),  (1)
where &Dgr;G°
f
is the reaction Gibbs free energy and the stated value is at 100° C.
A first observation has been that anodized aluminum tends to be relatively light and porous. A denser alumina layer would perhaps be more resistant to etching relative to a residue on the aluminum being etched and would further reduce water uptake in the alumina, which complicates its use inside a vacuum chamber. Many attempts have been made to improve the quality of the anodized layer. One such approach uses hot deionized (DI) water in the anodization process so as to seal the anodized layer with a layer of boehmite (AlOOH), which forms according to the reaction
Al
2
O
3
+H
2
O→2AlOOH↓, &Dgr;G°
f
(−468.68 kcal/mol).  (2)
Boehmite is chemically stable in the presence of BCl
3
, as seen by the reaction
2AlOOH+2BCl
3
→2AlCl
3
+B
2
O
3
+H
2
O, &Dgr;G°
f
(+398.03 kcal/mol),  (3)
but boehmite is not a hard material and is not stable due to dehydration. While the reaction is useful for filling the pores of the anodized aluminum, the hot deionized water sealing after anodization is insufficient for the present needs.
The requirements for a corrosion-resistant coating have intensified recently with the introduction of plasma etch reactors utilizing high-density plasmas. High-density plasma (HDP) reactors have been recently developed for a number of processes. Generally, a high-density plasma is defined as one having an ionized plasma density of greater than 10
11
cm
−3
. An example of an HDP metal-etch reactor is the Decoupled Plasma Source (DPS) Metal Etch Chamber available from Applied Materials, Inc. of Santa Clara, Calif. Tepman et al. have described an early version of the DPS reactor chamber in U.S. patent application, Ser. No. 08/296,043, filed Aug. 23, 1994 and incorporated herein by reference. The corresponding European application has been published as European Patent Application, 698,915-A1. The mechanical structure has changed somewhat in the commercial version of the DPS chamber.
A schematic representation of the commercial DPS chamber is illustrated in the cross-sectional view of
FIG. 1. A
upper, main processing compartment
10
is bounded by a curved ceramic dome
12
, an upper housing
14
to which the ceramic dome
12
is se

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