Adhesive bonding and miscellaneous chemical manufacture – Differential fluid etching apparatus – With radio frequency antenna or inductive coil gas...
Reexamination Certificate
2000-11-06
2003-04-22
Dang, Thi (Department: 1763)
Adhesive bonding and miscellaneous chemical manufacture
Differential fluid etching apparatus
With radio frequency antenna or inductive coil gas...
C118S7230IR, C118S7230IR
Reexamination Certificate
active
06551447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the present invention relates in general to plasma reactors and processes typically used for wafer processing or the like. More particularly, the field of the invention relates to a wafer processing plasma reactor in which the plasma is generated primarily by inductively coupled power.
2. Background
A variety of approaches are currently in use for deposition, etching, resist removal, and related processing for semiconductor wafers and other substrates. A typical process may involve the placement of photoresist, the etching of areas unprotected by the resist, and the removal of the residual resist. One method for etching and resist removal involves the immersion of the substrate in a chemical bath that attacks the substrate or the resist. This method is generally known as wet chemical etching or stripping. While wet etching is conceptually straightforward, it has significant disadvantages. For instance, wet etching requires the storage, handling, and disposal of toxic chemicals. In addition, wet chemical etch is isotropic, that is, it occurs in all directions relatively equally. This leads to undesired lateral etching under the photoresist, called undercutting, which limits the dimensional control that can be achieved with wet etch.
An alternative processing technology involves the use of a plasma for deposition, etching, resist removal, or the like. A plasma is a nearly electrically neutral ionized gas. It contains a substantial density of free electrons and positively charged ions. To remain ionized, a gas must constantly receive energy to offset the recombination of charged particles which occurs mostly on the walls of the reactor chamber. In conventional anisotropic plasma processing, the pressure must also be kept low to reduce the collision rate of the ions. Plasmas for etching are typically formed by applying a radio-frequency (RF) electric field to a gas held at low pressure in a vacuum chamber.
Anisotropic plasma etching in the fabrication of integrated circuit devices is desirable because it can much better control feature dimensions and sidewall profiles by processing under conditions far from thermodynamic equilibrium. This enables the production of integrated circuit features having sidewalls with a precisely defined location that extend substantially vertically from the edges of the masking layer. This is important in devices that have a feature size and spacing comparable to the death of the etch.
In
FIG. 1
is shown a typical wafer processing plasma reactor used for etching. This reactor includes a metal wall
1
that encloses a plasma reactor chamber
2
. Wall
1
is grounded and functions as one of the plasma electrodes. Gases are supplied to chamber
2
from a gas source
3
and are exhausted by an exhaust system
4
that actively pumps gases out of the reactor to maintain a low pressure suitable for a plasma process. An RF power supply
5
is connected to a second, powered electrode
6
that capacitively couples power into the chamber
2
to form a plasma. A wafer
7
is positioned on or near powered electrode
6
for processing. Wafers are transferred into and out of the reactor chamber
2
through a port such as slit valve
8
or the like.
RF power at 13.56 MHz is predominantly used in plasma reactors because this frequency is an ISM (Industry, Scientific, Medical) standard frequency for which government mandated radiation limits are less stringent than at non-ISM frequencies, particularly those within the communication bands. The substantially universal use of 13.56 Mhz is further encouraged by the large amount of equipment available at that frequency because of this ISM standard. Other ISM standard frequencies are at 27.12 and 40.68 MHz, which are first and second order harmonics of the 13.56 MHz ISM standard frequency.
This energy is applied to a gas in the reactor chamber to form a plasma. The plasma consists of two qualitatively different regions: a quasineutral, equipotential conductive plasma body
9
and a boundary layer
10
called the plasma sheath. The plasma body consists of substantially equal densities of negative and positive charged particles as well as radicals and stable neutral particles. RF power coupled into the reactor chamber couples energy into the free electrons, imparting sufficient energy to many of these electrons that ions can be produced through collisions of these electrons with gas molecules. In addition to this ionization, dissociation and excitation of molecules occur within the plasma body. In dissociation, a molecule, such as O
2
, breaks down into smaller fragments, such as atomic oxygen. In excitation, the molecule holds together, but absorbs energy and enters an excited electronic state. Control of the relative amounts of ionization, dissociation, and excitation depends upon a variety of factors, including the pressure and level of energy applied to the plasma.
A plasma comprises a plurality of mobile charge carriers and thus is a conductive medium.
Therefore, the interior of the plasma is at a roughly uniform electric potential. However, the plasma cannot exist for long in contact with a material object, and is separated from objects by a so called sheath. The plasma sheath is an electron deficient, poorly conductive region in which the electric field strength is large. The plasma sheath has a perpendicular electric field between the plasma body and any interface with a material object such as the reactor walls and the electrodes. The electric field at the interface with the wafers causes ions to accelerate perpendicularly into the surface of the wafers. This perpendicular bombardment makes anisotropic etching possible. Typically, a wafer
7
is positioned on or slightly above the powered electrode
6
where there is a strong “self” bias which enhances ion impact energy and thus the etching process so commercially viable etch rates may be obtained.
However, many modern IC structures are sensitive to ion bombardment by high energy (>100 eV) ions such as in the conventional plasma etch apparatus of FIG.
1
. Other etching processes require even lower ion energies to maintain selectivity. Because wafer damage decreases with decreasing ion energy and associated sheath voltage, it would be advantageous to operate at smaller discharge power levels and voltages. Unfortunately, for capacitively coupled power at 13.56 MHz, this reduction of voltage results in a proportionately lower rate of creation of reactive species and ions and therefore results in a slower etch rate for many processes, which significantly degrades process throughput.
Etch rates depend upon the ion current density at the wafer surface and the sheath voltage at the powered electrode (which determines the energy of the ions bombarding the surface for etching). With decreased sheath voltage the ion current density at the wafer must be increased to maintain a substantially constant etch rate. This requires that the plasma ion density near the wafer be increased. Unfortunately, in a conventional plasma etcher, both the sheath voltage of the powered electrode and the ion density near that electrode increase or decrease together and are monotonically increasing functions of the amplitude of the RF voltage applied to the powered electrode. Thus, if the sheath voltage is decreased by decreasing the voltage of the RF signal, the ion current density at the wafer also decreases thereby producing an even greater decrease in the etch rate than would be the case from a decrease in the sheath voltage alone. It is desirable, therefore, to independently control the sheath voltage and ion density at the wafer so that a soft etch process (i.e., an etch process with reduced sheath voltage at the wafer) can be implemented that has a commercially adequate etch rate.
Induction coils surrounding a reactor chamber may be used to inductively couple RF energy into a plasma to control the composition and density of the plasma for semiconductor processing. A separate powered electrode adjacent to a wafer is used t
Hammond Martin L.
Mattson Brad S.
Savas Stephen E.
Selbrede Steven C.
Dang Thi
Mattson Technology Inc.
Wilson Sonsini Goodrich & Rosati
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