Controlled potential plasma source

Details Controlled potential plasma source Controlled potential plasma source

1995-03-10

2002-04-23

McDonald, Rodney G. (Department: 1753)

Etching a substrate: processes

Gas phase etching of substrate

Application of energy to the gaseous etchant or to the...

C216S063000, C216S065000, C216S069000, C216S070000, C156S345420, C427S569000, C427S570000, C427S571000, C427S572000, C427S575000, C427S585000, C427S586000, C427S595000, C427S596000, C427S598000, C118S7230AN, C118S7230MW, C118S7230MR, C118S7230MA, C118S7230ER, C204S192120, C204S298060, C204S298090, C204S298160, C204S298310, C204S298340, C204S298370, C204S298380

Reexamination Certificate

active

06375860

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to plasma processing apparatus and methods, and more particularly to plasma processing apparatus and methods that use a controlled potential plasma source in order to minimize particulate formation within the plasma.
In recent years, plasma processing has emerged as one of the most versatile and efficient techniques for the processing of materials in several of the largest manufacturing industries in the world. For example, in the electronics industry, plasma-based processes are indispensable for the manufacture of very large-scale integrated (VLSI) microelectronic circuits (or chips). Plasma processing is also a critical technology in the aerospace, automotive, steel, biomedical, flat-panel displays, solar cells, and toxic waste management industries. For an overview of the many and varied applications that rely on plasma processing for materials processing, see, e.g.,
PLASMA PROCESSING OF MATERIALS, Scientific Opportunities and Technological Challenges,
National Research Council (National Academy Press, Washington, D.C. 1991).
In general, plasma processing involves the creation and maintaining of a plasma, and the application of the plasma to a particular material that is to be processed by the plasma. A plasma is a partially or fully ionized gas containing electrons, ions, and neutral atoms and/or molecules. In a typical plasma processing application, the nonlinear collective interactions of the electrically charged particles with each other, with neutral atoms and molecules, and with electric and magnetic fields, are used to selectively process a particular material that is exposed to the plasma. For example, in a plasma deposition or etching application, the plasma is used to selectively process a semiconductor wafer on which VLSI microelectronic circuits are being formed.
In plasma deposition, and many other plasma processing applications, one of the technological challenges that must be addressed is the control of “particles” and “particulates” in the plasma. (Note, as used herein, the terms “particle ” and “particulate” are used as synonyms.) A “particle” is generally considered as a small piece of material that is larger than a cluster of a few molecules, but small enough to remain suspended in a fluid for a time. A “particle” may originate from a source external to the plasma, or may be formed within the plasma due to the physical and chemical processes occurring within the plasma.
While the presence of certain types of particles may be advantageous to some kinds of plasma processing operations, e.g., because the particles help promote a desired chemical or physical process carried out during the plasma processing operation, most particles are not advantageous. When a particle or particulate is not advantageous to the plasma process it is referred to as a “contaminant”. Dust particles are an example of contaminants that interfere with the delicate plasma etching operation used in making VLSI chips. See, e.g., Donovan,
Particle Control For Semiconductor Manufacturing
(Marcel Dekker, Inc. New York 1990). The presence of a dust particle having a size less than about 1 &mgr;m (where one &mgr;m is 10
−6
meters), for example, renders most VLSI processing impossible, where circuit traces and other component sizes and spacings on the VLSI chip may only be on the order of 0.35-1.0 &mgr;m. Hence, contaminants as small as 0.1 &mgr;m may present a problem with the precise deposition and/or etching that must be achieved in most VLSI processing applications. Thus, there is a critical need in the plasma processing art for a way to remove and/or control the presence and/or location of contaminants in the plasma so that such contaminants do not interfere with the plasma processing operation being performed.
Several techniques are known in the art for removing contaminants from the plasma that originate from sources external to the plasma. Generally, such techniques, e.g., filtering the gases used to form the plasma, have proven effective at reducing the density of such contaminants to manageable levels. A significant need still persists, however, for eliminating or minimizing the presence of internally-formed contaminants, i.e., contaminants that originate from particles formed within the plasma itself due to the physical and chemical processes occurring within the plasma. The present invention is directed to this need.
To conceptually understand how contaminants are formed within a plasma, reference is made to
FIG. 1A
, which schematically illustrates a plasma-formation device
20
. As seen in
FIG. 1A
, the plasma formation device includes opposing electrodes
22
and
24
, each of which is connected to a voltage potential source
26
. By introduction of appropriate gases into the region between the electrodes, and by application of an appropriate potential to the electrodes, a plasma
28
is formed. Such plasma
28
is made up of electrons and ions. At the same time that the ions and electrons in the plasma are spent, e.g., as they are attracted to and strike the electrodes and/or interact with other elements and/or a workpiece being processed by the plasma, new electrons and ions are also formed. Hence, the generation of the plasma, and the loss of the plasma, is a continual process. As this continual process proceeds, the plasma, as a whole, is maintained at a potential that is positive with respect to the chamber wherein the plasma is confined, yet the plasma itself tends to stay electrically neutral. To remain electrically neutral, the electron flux leaving the plasma must generally be equal to the positive ion flux leaving the plasma. Because electrons are smaller and lighter than ions, and have a significantly higher thermal velocity than do ions, the electrons tend to diffuse out of the plasma much faster than the heavier and slower ions. However, to achieve a charge balance within the plasma, an ambipolar electric field E
A
is created that tends to also drag the positive ions out of the plasma, as well as to retard the escape of electrons from the plasma. This causes the plasma potential to be positive relative to the vessel or chamber wherein the plasma is formed. Because the plasma potential is positive relative to the vessel or chamber wherein the plasma is formed, negative ions tend to get trapped within the plasma. Such negative ions thereafter serve as nucleation points for dust particles and other contaminants within the plasma.
Further, it is noted that any particulates in the plasma are also charged negatively. As the particulate grows from the nucleation site, due to its presence in the plasma, it begins to acquire a large negative charge. The plasma charges up the particulate negatively because there is an initial large net negative flux to the particle. This occurs because the electrons, being lighter, have a larger thermal velocity than the positively charged ions. As the particle charges up, it begins to repel the electron current and the charging process is slowed. The charging ceases in equilibrium when the negative current and positive current to the particle exactly cancel. What is left, though, in equilibrium is a large negative charge on the particle. This will occur even if the nucleation site is a neutral gas molecule, atom, or a positive ion. Thus, no matter what the initial nucleation site may be, the particulates quickly become trapped within the plasma. Once trapped, further particle growth occurs due to agglomeration and the particle can become large enough to cause damage to the workpiece.
Further, when negatively-charged particulates are present in the plasma, such particulates limit the rate at which certain plasma deposition processes can be carried out. There is thus a need in the art for a plasma source that removes such negatively-charged particulates so that the plasma deposition and other processes can be carried out at a more rapid rate.
The trapping of negative ions, and subsequent formation of contaminants within a plasma, has been demonstrated, both through model

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