Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
1998-07-20
2002-03-05
Bueker, Richard (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S7230AN, C118S7230IR, C156S345420
Reexamination Certificate
active
06352049
ABSTRACT:
BACKGROUND
1. Technical Field
The invention is related to a plasma reactor used for etching and/or deposition of a film on to a workpiece, and more particularly to such a reactor wherein the density of species within the primary processing chamber of the reactor can be independently controlled.
2. Background Art
A plasma reactor may be employed to perform various processes on a workpiece, such as a semiconductor wafer during the fabrication of microelectronic devices on the workpiece. The wafer is placed inside a vacuum chamber of the reactor and processing gases are introduced. The gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Depending upon the composition of the gases from which the plasma is formed, the plasma may be employed to etch a particular material from the workpiece or may be employed to deposit a thin film layer of material onto the workpiece.
In the case where the plasma reactor is used for etching, examples of typical processing gases employed as etchants include Cl
2
, BCl
3
, CF
4
, SF
6
, NF
3
, HBr, and various C
x
H
y
F
z
gases, among others. These gases, however, are not chemically reactive enough in these forms to satisfactorily etch the materials desired to be removed from the workpiece undergoing processing in the reactor. This is where the plasma comes into play.
Process gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Species of neutral and charged particles, as well as other particles and substances, are created in the plasma from the processing gases. For example, if Cl
2
is used as the process gas, the following species of neutral and charged particles may be present in the chamber:
Cl
2
→Cl
2
+Cl+Cl
+
+e+Cl
−
+Cl
2
+
Neutrals formed from the etchant gases, such as Cl, F and Br, are extremely unstable and reactive, and can be effectively used to chemically react with materials on the workpiece to produce gaseous substances, thereby in effect removing or etching the material from the workpiece. For example, in the case where it is desired to etch silicon from the surface of a semiconductor wafer, Cl
2
can be symmetrically dissociated to form Cl neutrals in the plasma. These Cl neutrals will react with the silicon of the wafer according to the following formula:
Si+XCl→SiCl
x
, X=2,4 (1)
The product, SiCl
x
is a gas that is eventually evacuated from the processing chamber.
The foregoing reaction, however, may not occur unless sufficient energy is added. This energy can be in the form of heat, but typically in a plasma assisted etch process, the majority of this energy comes from a physical bombardment of the surface of the wafer. The physical bombardment is the task performed by the charged particles also formed in the plasma from the processing gases. Typically, charged particles are drawn toward the wafer via a bias power applied to the wafer support to enhance the bombardment and create a desired directionality to it—usually normal to the upper surface of the wafer. These charged particles not only provide the energy that fuels the chemical etch process associated with the etchant gas neutrals, but also physically remove material from the surface of the wafer as a result of the particles impact with the wafer.
The charged particles need not be exclusively formed from etchant gases. Any charged particle can be made to bombard the wafer and create the desired effect regardless of whether it will chemically react with the material to be etched. For example, when more charged particles than can be obtained from the etchant gases alone are required for a particular etch process, a non-reactive gas such as argon may be introduced. The argon forms charged particles in the plasma. Although the introduced argon is not chemically reactive with the wafer materials, it provides the desired boost in the overall availability of charged particles used to bombard the wafer.
The concentration or density in the plasma of both the neutral particles formed from the etchant gases and the charged particles formed from all of the processing gases will play a significant role in the etching process and in determining the characteristics exhibited by the etched workpiece. For example, both act to etch material from the workpiece. Therefore, an increase in density of all will have the effect of increasing the overall etch rate—often a desirable effect.
It must be noted, however, that the physical bombardment of the workpiece by the charged particles will also etch materials that may not be intended to be removed. Thus, an increase in the charged particle density can result in damage to the devices being formed on the workpiece, even though the etch rate of the materials intended to be etched would increase. As a result, it can be more advantageous to increase the overall etch rate by increasing only the density of the etchant gas neutrals in the plasma.
The relative densities of the etchant gas neutral and the charged particle species formed in the plasma will also have profound effects, for example, on etch process characteristics such as etch selectivity, etch feature profile, and etch rate microloading.
The term etch selectivity refers to the ratio of etch rates of two different materials on a workpiece undergoing etching in the plasma reactor. To form features and patterns in the various layers of a workpiece, the etch process must be selective so that some materials are etched, while others are not. In one common scenario, it is desired that a silicon layer on a workpiece be etched much faster than photoresist or oxygen-containing layers of the workpiece so as to etch a pattern into the silicon. This is referred to as a high silicon-to-photoresist and silicon-to-oxide selectivity, respectively.
The following example of etching a hole through a silicon layer to an underlying gate oxide layer on a semiconductor wafer, illustrates one example of the importance of high selectivity. Prior to etching, a layer of photoresist material is formed over the surface of the silicon layer over those areas that are not to be etched. Accordingly, there is no photoresist formed in the area where the hole is to be etched. The desired result of the etching process is to quickly etch through the silicon layer where the hole is to be formed, but not to significantly etch the surrounding photoresist, or the underlying gate oxide layer. Thus, a high silicon-to-photoresist and silicon-to-oxide etch selectivity is desired. If an adequate level of selectivity is not maintained, a so-called “punch through” condition can result wherein the photoresist or oxide layer is etched through causing damage to the device being formed on the workpiece.
The densities of the plasma species have a significant impact on the selectivity exhibited during an etching process. For example, if the process chemistry is such that the etchant gas neutrals chemically react with the material to be etched (e.g. silicon) to a greater extent than other materials (e.g. photoresist and oxide), then having a high density of neutral species will help achieve the desired selectivity by causing an increased etch rate of the material being etched in comparison to the other materials. Conversely, since charged particles remove material from the workpiece surface through physical impact, they tend to etch all the various materials of the workpiece equally. Thus, a greater density of the charged particle species in the plasma can cause a greater etch rate of all the workpiece materials.
Accordingly, an increase in etchant gas neutral species under certain conditions can increase selectivity, while a decrease in the neutral species likewise can decrease selectivity. Whereas, an increase in the density of charged particle species under certain conditions can cause a decrease in selectivity, a decrease in charged particle species under certain conditions can cause an increase in selectivity. Therefore, one way of optimizing the desired selectivity of an etch process would be to increase the
Chen Arthur
Grimbergen Michael
Kolandenko Arnold
Lee Chii
Loewenhardt Peter
Bueker Richard
Michaelson and Wallace
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