Plasma reactor

Details Plasma reactor Plasma reactor

1997-03-24

2001-09-18

Mills, Gregory (Department: 1763)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With microwave gas energizing means

C118S7230MR, C118S7230ER

Reexamination Certificate

active

06290806

ABSTRACT:

FIELD OF THE INVENTION
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 INVENTION
Integrated 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 CF
4
. Initially, the only species present are CF
4
molecules. However, once a glow discharge is established, a portion of the CF
4
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 CF
4
plasmas is CF
3
+
, such ions being formed by the electron-impact reaction: e+CF
4
=>CF
3
+
+F+2e. In addition to CF
4
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 CF
4
plasmas, the most abundant radicals are CF
3
and F, formed by the reaction: e+CF
4
=>CF
3
+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.
FIG. 1
is a diagrammatic representation of a typical parallel-plate plasma etch reactor. To perform a plasma etch, a wafer
11
is loaded in the reactor chamber
12
and precisely centered on a disk-shaped lower electrode
13
L, thereby becoming electrically integrated therewith. A disk-shaped upper electrode
13
U is positioned above the wafer (the number
13
* applies to either
13
L or
13
U). The flow of molecular gas into the chamber
12
is automatically regulated by highly-accurate mass-flow controllers
14
. A radio-frequency voltage
15
is applied between electrodes
13
L and
13
U. Chamber pressure is monitored and maintained continuously through a feedback loop between a chamber manometer
16
and a downstream throttle valve
17
, which allows reactions products and surplus gas to escape in controlled manner. Spacing of the electrodes is controlled by a closed-loop positioning system (not shown). At a particular voltage known as the breakdown voltage, a glow discharge will be established between the electrodes, resulting in a partial ionization of the molecular gas. In such a discharge, free electrons gain energy from the imposed electric field and lose this; energy during collisions with molecules. Such collisions lead to the formation of new species, including metastables, atoms, electrons, free radicals, and ions. The electrical discharge between the electrodes consists of a glowing plasma region
18
centered between lower electrode
13
L and upper electrode
13
U, a lower dark space
19
L between the lower electrode
13
L and plasma region
18
, and an upper dark space region
19
U between the upper electrode
13
U and plasma region
18
. Dark space regions
19
* are also known as “sheath” regions. Electrons emitted from the electrodes
13
* are accelerated into the discharge region. By the time the electrons have reached plasma region
18
, they have acquired sufficient kinetic energy to ionize some of the molecular gas molecules and raise the electrons of other molecular gas molecules to less-stable atomic orbitals of increased energy through a mechanism known as electron impact excitation. As each of these excited electrons “relaxes” and falls back to a more stable orbital, a quantum of energy is released in the form of light. This light gives the plasma region its characteristic glow. Free electrons may also collide with species already formed by collisions between free electrons and gas molecules, leading to additional subspecies. Because free electrons have little mass, they are accelerated much more rapidly toward the electrodes than are ionized gas molecules, leaving the plasma with a net positive charge. The voltage drop through the plasma is small in comparison to the voltage drops between the plasma and either of the plates at any given instant of an AC voltage cycle. Therefore, plasma ions which are accelerated from the plasma to one of the plates are primarily those that happen to be on the edge of one of the dark spaces. Acceleration of ions within the plasma region is minimal. Although ions are accelerated toward both electrodes, it is axiomatic that the smaller of the two electrodes will receive the greatest ionic bombardment. Since the ions are accelerated substantially perpendicularly between the two electrodes (parallel to the electric field), the ions

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