Techniques for improving etch rate uniformity

Details Techniques for improving etch rate uniformity Techniques for improving etch rate uniformity

1999-06-30

2002-02-05

Dang, Thi (Department: 1763)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With microwave gas energizing means

Reexamination Certificate

active

06344105

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fabrication of semiconductor integrated circuits and, more particularly, to improved methods and apparatus for ion-assisted etch processing in a plasma processing system.
2. Description of the Related Art
In the fabrication of semiconductor-based devices, e.g., integrated circuits or flat panel displays, layers of materials may alternately be deposited onto and etched from a substrate surface. As is well known in the art, the etching of the deposited layers may be accomplished by a variety of techniques, including plasma-enhanced etching. In plasma-enhanced etching, the actual etching typically takes place inside a plasma processing chamber of a plasma processing system. To form the desired pattern on the substrate surface, an appropriate mask (e.g., a photoresist mask) is typically provided. A plasma is then formed from a suitable etchant source gas, or mixture of gases, and used to etch areas that are unprotected by the mask, thereby leaving behind the desired pattern.
To facilitate discussion,
FIG. 1A
depicts a simplified plasma processing apparatus
100
suitable for fabrication of semiconductor-based devices. The simplified plasma processing apparatus
100
includes a wafer processing chamber
102
having an electrostatic chuck (ESC)
104
. The chuck
104
acts as an electrode and supports a wafer
106
(i.e., substrate) during fabrication. An edge ring
108
borders the edge of the chuck
104
. In the case of etch processes, a number of parameters within the wafer processing chamber
102
are tightly controlled to maintain high tolerance etch results. Process parameters governing etch results may include gas composition, plasma excitation, plasma distribution over the wafer
106
, etc. Since the etch tolerance (and resulting semiconductor-based device performance) is highly sensitive to such process parameters, accurate control thereof is required.
The surface of the wafer
106
is etched by an appropriate etchant source gas that is released into the wafer processing chamber
102
. The etchant source gas can be released through a showerhead
110
. The etchant source gas may also be released by other mechanisms such as via a gas ring disposed inside the chamber or via ports built into the walls of the wafer processing chamber
102
. During ion-assisted etch processes, Radio Frequency (RF) power supplied to showerhead
110
ignites the etchant source gas, thereby forming a plasma cloud (“plasma”) above wafer
106
during etch processes. It should be noted that other means of plasma excitation may also be used. For example, the application of microwave energy, the use of inductive coils, the introduction of a wave excited by an antenna, or capacitive coupling to the showerhead
110
can also be used to excite the plasma. In ion-assisted etch processes, chuck
104
is typically RF powered using a RF power supply (not shown).
In an ion-assisted etch process, the local etch rate is dominated by ion concentration. Ion-assisted etch processes are typically used to perform oxide etches or polysilicon etches. In other words, ion driven/assisted etch processes generally refer to etching processes wherein the etching is predominately facilitated by the physical reaction of the accelerated plasma ions (“ions”) with the wafer (substrate). Ion-assisted etching applications include, for example, sputtering, Reactive Ion Etching (RIE), chemical sputtering, chemically assisted physical sputtering, and physically assisted chemical sputtering.
With ion-assisted etching, application of RF power to the chuck
104
(as well as the showerhead
110
) results in the formation of an electric field and in turn a sheath
112
above the wafer
106
. The electric field associated with the sheath
112
promotes the acceleration of ions toward the top surface of the wafer
106
. Ideally, the accelerated ions collide at an angle that is substantially perpendicular (i.e., substantially normal or about 90 degrees) with the respect to the surface of the wafer
106
during etch processes. The accelerated ions that collide with the wafer
106
operate to “physically” etch the wafer
106
.
The edge ring
108
is an insulator material that is electrically floating (i.e., not RF powered). Edge ring
108
is used to shield the edge of the chuck
104
from ion bombardment such as during etch processes. Edge ring
108
can also help focus the ion bombardment with respect to the wafer
106
. As shown in
FIG. 1A
, the chuck
104
can be surrounded by an inner surface
114
of the edge ring
108
. The inner surface
114
is also within the outer edge of the wafer
106
.
An outer surface
116
of the edge ring
108
extends beyond the outer edge of the wafer
106
. An upper portion of the inner surface
114
of the edge ring
114
is adjacent to not only the chuck
104
but also the wafer
106
. Conventionally, a top surface
118
of the edge ring
108
is below or about the same level as a top surface of the wafer
106
.
One major problem associated with ion-assisted etch processes using a convention plasma processing apparatus is that the etch rate is not uniform across the wafer
106
. More specifically, etch rate at locations near the edges of the wafer is significantly higher than the etch rate for points near the center of the wafer.
FIG. 1B
illustrates a cross-section of the wafer
106
following etch processes where the etched depth is greater at a perimeter portion
120
of the wafer
106
than at a middle portion
122
of the wafer
106
.
The non-uniform etch rate is attributed primarily to the non-uniform thickness of the sheath
112
above the surface of wafer
106
. As depicted in
FIG. 1A
, the thickness (or the plasma density at the sheath boundary) of the sheath
112
at the middle portion
120
of the wafer
106
is significantly thicker than the thickness (density) of the sheath
112
at the perimeter portion
11
6
of the wafer
106
. That is, in the vicinity of the electronically floating region above the edge ring
108
the sheath “curves” near the perimeter of the wafer
106
. The sheath curvature around the perimeter of the wafer
106
causes relatively more ions to collide near the perimeter of the wafer
106
during ion-assisted etch processes. A higher collision rate near the perimeter results in relatively higher etch rates near the perimeter of the wafer (see FIG.
1
B).
An additional problem is caused by the sheath curvature. In particular, the sheath curvature near the perimeter of the wafer
106
induces the ions to collide at angles that are not substantially perpendicular (i.e., not substantially normal or about 90 degrees) with respect to the surface of the wafer
106
. In ion-assisted etch processes, ion collisions at such non-perpendicular angles also contribute to higher etch rates. Furthermore, the non-perpendicular angles of ion collision near the edges can have an undesired “tilting” effect on the etched features (e.g., trenches, vias or lines) on the wafer
106
. Tilting generally refers to an undesired effect during etching whereby one or more sides of a feature are not substantially perpendicular with the surface of a wafer. Here, at the perimeter of the wafer
106
, the “tilting” effect produces an asymmetric feature. Features are intended to be symmetric, so asymmetric features are undesired and can cause severe problems that render a fabricated integrated circuit essentially defective.
One potential solution to address some of the problems associated with non-uniformity of etch rates in ion-assisted etching processes is to enlarge the chuck so that it extends beyond the edges of wafer. Enlarging the chuck would effectively shift the sheath curvature beyond the edges of the wafer. This may be a feasible solution for purely chemical etching applications. However, this solution would not be feasible for ion-assisted etch processes since the extended portion of the chuck would also be exposed to ions and the etching process. Exposing the chuck can cause particulate and/or heavy metal co

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