Method for etching a wafer in a plasma etch reactor
This invention is a hardware modification which permits greater uniformity of etching to be achieved in a high-density-source plasma reactor (i.e., one which uses a remote source to generate a plasma, and which also uses high-frequency bias power on the wafer chuck). The invention addresses the uniformity problem which arises as the result of nonuniform power coupling between the wafer and the walls of the etch chamber. The solution to greatly mitigate the nonuniformity problem is to increase the impedance between the wafer and the chamber walls. This may be accomplished by placing a cylindrical dielectric wall around the wafer. Quartz is a dielectric material that is ideal for the cylindrical wall if silicon is to be etched selectively with respect to silicon dioxide, as quartz it is virtually inert under such conditions.
This invention relates to ion-assisted plasma etching of semiconductor wafers in remote source plasma reactors with powered wafer chucks. More particularly, it relates to equipment improvements designed to improve etch uniformity over the surface of a wafer.
BACKGROUND OF THE INVENTIONIntegrated circuits are typically fabricated on a wafer of semiconductor material such as silicon or gallium arsenide. During the fabrication process, the wafer is subjected to an ordered series of steps, which may include photomasking, material deposition, oxidation, nitridization, ion implantation, diffusion and etching, in order to achieve a final product.
There are two basic types of etches: ion-assisted etches (also called reactive-ion, plasma or dry etches) and solution etches (also called wet etches). Solution etches are invariably isotropic (omnidirectional) in nature, with the etch rate for a single material being relatively constant in all directions. Reactive-ion etches, on the other hand, are largely anisotropic (unidirectional) in nature. Reactive ion etches are commonly used to create spacers on substantially vertical sidewalls of other layers, to transfer a mask pattern to an underlying layer with little or no undercutting beneath mask segment edges, and to create contact vias in insulative layers.
A plasma etch system (often referred to as a reactor) is primarily a vacuum chamber in which a glow discharge is utilized to produce a plasma consisting of chemically reactive species (atoms, radicals, and ions) from a relatively inert molecular gas. The gas is selected so as to generate species which react either kinetically or chemically with the material to be etched. Because dielectric layers cannot be etched using a direct-current-induced glow discharge due to charge accumulation on the surface of the dielectric which quickly neutralizes the dc-voltage potential, most reactors are designed as radio-frequency diode systems and typically operate at a frequency of 13.56 MHz, a frequency reserved for non-communication use by international agreement. However, plasma etch processes operating between 100 KHz-80 MHz have been used successfully.
The first ionization potential of most gas atoms and molecules is 8 eV and greater. Since plasma electrons have a distribution whose average energy is between 1 to 10 eV, some of these electrons will have sufficient energy to cause ionization of the gas molecules. Collisions of these energized electrons with neutral gas molecules are primarily responsible for the production of the reactive species in a plasma. The reactive species, however, can also react among themselves in the plasma and alter the overall plasma chemistry.
Since plasmas consisting of fluorine-containing gases are extensively used for etching silicon, silicon dioxide, and other materials used in VLSI fabrication, it is instructive to examine the glow-discharge chemistry of CF4. Initially, the only species present are CF4 molecules. However, once a glow discharge is established, a portion of the CF4 molecules dissociated into other species. A plasma is defined to be a partially ionized gas composed of ions, electrons, and a variety of neutral species. The most abundant ionic specie found in CF4 plasmas is CF3+, such ions being formed by the electron-impact reaction: e+CF4=>CF3++F+2e. In addition to CF4 molecules, ionic species, and electrons, a large number of radicals are formed. A radical is an atom, or collection of atoms, which is electrically neutral, but which also exists in a state of incomplete chemical bonding, making it very reactive. In CF4 plasmas, the most abundant radicals are CF3 and F, formed by the reaction: e+CF4=>CF3+F+e. Radicals are generally thought to exist in plasmas in much higher concentrations than ions, because they are generated at a faster rate, and they survive longer than ions in the plasma.
Plasma etches proceed by two basic mechanisms. The first, chemical etching, entails the steps of: 1) reactive species are generated in the plasma; 2) these species diffuse to the surface of the material being etched; 3) the species are adsorbed on the surface; 4) a chemical reaction occurs, with the formation of a volatile by-product; 5) the by-product is desorbed from the surface; and 6) the desorbed species diffuse into the bulk of the gas. The second, reactive-ion etching, involves ionic bombardment of the material to be etched. Since both mechanisms occur simultaneously, the complete plasma etch process would be better aptly identified as an ion-assisted etch process. The greater the chemical mechanism component of the etch, the greater the isotropicity of the etch.
Parallel-plate etch reactors have fallen into disfavor for certain applications. For example, the voltage required to sustain the plasma is far higher that is required to etch polycrystalline silicon or single-crystal silicon. In fact, the voltage is so high that ions are accelerated to energies sufficient to etch silicon dioxide. For this reason, it is very difficult to perform an etch of silicon that stops on a silicon dioxide layer using a parallel-plate reactor. For such applications, a new type of plasma reactor has been developed. In this type of reactor, the plasma is generated in a source chamber remote from the wafer (typically at the very top of the chamber, and the wafer chuck is powered separately from the plasma source generator. Such a reactor is generally called a high-density source plasma reactor. Examples of sources used to create the high-density plasma are: a Mori source, a helicon source, and an electron cyclotron resonance (ECR) source. A description of the operation of such sources is beyond the scope of this disclosure, and not particularly relevant thereto.
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One of the problems associated with high-density source plasma etch reactors is that etching is not uniform across the surface of the wafer. Nonuniform power coupling between the wafer and the walls of the etch chamber can be the dominant cause of nonuniform etching rates across the surface of the wafer. Nonuniform power coupling occurs because regions near wafer edge are physically closer to the grounded walls of the chamber than are regions closer to the wafer center. Thus, higher power is coupled to the walls through a unit area near the edge of the wafer than is coupled by a unit area located nearer the center of the wafer. This radially nonuniform coupling of the rf power to the chamber walls results in lower etch rates near the center of the wafer than near the edge. It can also adversely affect other process results such as feature profile and/or selectivity.
SUMMARY OF THE INVENTIONThis invention is a hardware modification which permits greater uniformity of etching to be achieved in a high-density-source plasma reactor (i.e., one which uses a remote source to generate a plasma, and which also uses high-frequency bias power on the wafer chuck). The invention addresses the uniformity problem which arises as the result of nonuniform power coupling between the wafer and the walls of the etch chamber. The solution to greatly mitigate the nonuniformity problem is to increase the impedance between the wafer and the chamber walls. This may be accomplished by placing a cylindrical dielectric wall around the wafer. Quartz is a dielectric material that is ideal for the cylindrical wall if silicon is to be etched selectively with respect to silicon dioxide, as quartz it is virtually inert under such conditions. Any dielectric material can be used, including those which are etchable, provided that they do not have a negative impact on the etch process.
BRIEF DESCRIPTION OF THE DRAWINGS
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Although only a single embodiment of the invention has been disclosed herein, it will be obvious to those having ordinary skill in the art of ion-assisted etching that changes and modifications may be made thereto without departing from the scope and the spirit of the invention as hereinafter claimed.
Claims
1-9. (canceled)
10. A method for etching a wafer in a plasma etch reactor having a main chamber with a sidewall and a wafer chuck, comprising:
- disposing a wafer on the wafer chuck, wherein the wafer is surrounded by a dielectric ring that prevents electrical coupling between the wafer and the chamber sidewall;
- generating a plasma in a source chamber; and
- supplying the plasma to the main chamber.
11. The method of claim 10, wherein the dielectric ring extends above a plane defined by an upper surface of the wafer.
12. The method of claim 10, wherein the plasma is high-density plasma.
13. The method of claim 10, generating the plasma in the source chamber comprises the use of a Mori source, a helicon source, or an ECR source.
14. The method of claim 10, wherein the dielectric ring comprises quartz.
15. The method of claim 10, further comprising establishing a voltage differential between the wafer chuck and the chamber sidewall.
Type: Application
Filed: Apr 8, 2005
Publication Date: Aug 11, 2005
Inventor: Kevin Donohoe (Boise, ID)
Application Number: 11/101,730