Etching a substrate: processes – Gas phase etching of substrate – Application of energy to the gaseous etchant or to the...
Reexamination Certificate
1997-03-20
2001-10-16
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Gas phase etching of substrate
Application of energy to the gaseous etchant or to the...
C216S079000
Reexamination Certificate
active
06303045
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the manufacture of semiconductor integrated circuits. More particularly, the present invention relates to improved techniques for performing a nitride (Si
3
N
4
) etch in a variable-gap plasma processing system, which advantageously improves substrate throughput while minimizing particulate defect density.
In the fabrication of semiconductor devices, e.g, semiconductor integrated circuits (ICs) or flat panel displays, devices such as component transistors are typically formed on a substrate, e.g. a silicon wafer or a glass panel. The etching of a nitride, or Si
3
N
4
layer, is commonly performed in the manufacture of certain integrated circuit devices such as complementary metal oxide semiconductor (CMOS) transistors. Nitride layer etch, also known as well nitride etch or tank nitride etch in the case of CMOS transistors, is typically performed to define the n and p wells, for example.
To facilitate discussion,
FIG. 1
depicts a simplified layer stack
100
, representing the layers that may be formed above a semiconductor substrate during semiconductor IC fabrication. In
FIG. 1
as well as the figures herein, it should be noted that the layers shown therein are illustrated only; other additional layers above, below, or between the layers shown may be present. Further, not all of the shown layers need necessarily be present and some or all may be substituted by other different layers using knowledge commonly possessed by those skilled in the art.
Layer stack
100
generally includes a substrate
102
, which is typically formed of silicon. Above substrate
102
, there may be disposed an oxide layer (SiO
2
) layer
104
. A nitride (Si
3
N
4
) layer
106
is shown disposed above oxide layer
104
. To etch a desired pattern in nitride layer
106
, an overlaying photoresist (PR) layer
108
is then formed atop the blanket deposited nitride layer
106
. Photoresist layer
108
may then be patterned (e.g., through a conventional photoresist technique) to facilitate the etching of the underlaying nitride layer
106
. By way of example, one such photoresist technique involves the patterning of photoresist layer
108
by exposing the photoresist material in a contact or stepper lithography system, and the development of the photoresist material to form a mask to facilitate subsequent etching. Using an appropriate etchant, the areas of nitride layer
106
that are unprotected by the photoresist mask are then etched away, leaving behind a desired pattern on nitride layer
106
.
In the prior art, the etchant employed to etch through nitride layer
106
is typically a mixture comprising SF
6
and helium. When excited into a plasma (e.g., by radio frequency or RF energy), the fluorine species of the plasma etch through the unprotected areas of nitride layer
106
to form silicon fluoride, which is then evacuated away. The helium component in the prior art SF
6
/helium chemistry is employed typically to assist in the distribution of the plasma etchant throughout the substrate, thereby improving uniformity. Further, helium may also help in cooling the substrate during etching in order to, for example, prevent the protective photoresist features from burning up.
It has been found, however, that the use of prior art SF
6
/helium chemistry for etching nitride layer
106
typically requires a fairly narrow gap between the top surface of the substrate and the top electrode of the plasma processing chamber. When the prior art SF
6
/helium chemistry is employed, the narrow gap is required to ensure an acceptable etch result. However, the requirement of a narrow gap has several disadvantages.
To facilitate discussion,
FIG. 2
depicts a typical plasma processing chamber
200
, representing a plasma processing chamber typically employed in the prior art to etch through the nitride layer. In the present example, plasma processing chamber
200
represents a plasma processing chamber of a plasma processing system known by the brandname of RAINBOW 4400™, which is available from Lam Research Corporation of Fremont, Calif. Although the RAINBOW 4400™ is employed herein to facilitate discussion, it should be borne in mind that the technique disclosed herein is not limited to this particular configuration; the inventive and disclosed nitride etch technique may be adapted, using knowledge commonly possessed by those skilled in the art, to other plasma processing chamber configurations.
Plasma processing chamber
200
typically includes a lower electrode or chuck
202
, which is typically grounded. Substrate
204
, representing a substrate having thereon a nitride layer to be etched, is typically disposed above lower electrode
202
during etching.
An upper electrode
206
is disposed above substrate
204
and is separated therefrom by a gap
208
. Upper electrode
206
is mounted to a movable backing plate
210
, typically in the form of a large circular metal disk. Movable backing plate
210
and upper electrode
206
may be moved along the direction of the z axis by a gap drive assembly which includes a plurality of lead screws
212
, a chain
214
, and a gap drive motor
216
. By changing the direction of rotation of gap drive motor
216
, movable backing plate
210
and upper electrode
206
may be moved toward or away from electrode
202
, thereby varying the size of gap
208
.
During the etch, the pressure within plasma processing chamber
202
is typically maintained at a lower pressure than the ambient environment pressure. In one embodiment, the nitride etch is carried out at a chamber pressure of about
500
milliTorr (mTorr). To maintain the pressure differential between the interior of plasma processing chamber
200
and the ambient pressure, seals
220
are typically provided around the periphery of movable backing plate
210
. Seals
220
, of which there are two in
FIG. 2
, are typically formed of a relatively non-reactive sealing material such as a suitable rubber, e.g., VITON™ rubber. To reduce friction between seals
220
and the interior surface of chamber wall
224
as backing plate
210
is moved toward or away from the substrate, seals
220
are typically lubricated with a suitable lubricant.
To facilitate etching, an etchant source gas rapture, e.g., SF
6
/helium in the case of the prior art nitride etch, is typically flowed into chamber interior
226
. In the configuration of
FIG. 2
, upper electrode
206
has a showerhead configuration, i.e., upper electrode
206
is provided with a plurality of apertures for releasing etchant source gases into chamber interior
226
. However, the etchant source gases may also be provided through other mechanisms, e.g., via apertures in chamber wall
224
or a gas ring surrounding lower electrode
202
.
An RF power source
228
is then turned on to provide RF energy to upper electrode
206
. RF power source
228
is typically coupled to upper electrode
206
via an RF timing network
230
of a conventional design. RF tuning network
230
functions to minimize the impedance between RF power source
228
and plasma processing chamber
200
, thereby maximizing power delivery. The supplied RF power ignites or strikes the plasma from the supplied etchant source gases within chamber interior
226
to etch the unprotected areas of the nitride layer. Reaction byproduct gasses are then exhausted away through an exhaust port
240
. Exhaust port
240
may be coupled to an automatic pressure control (APC) system
242
, which automatically varies the rate of the gas exhausted through exhaust port
240
to maintain the desired chamber interior pressure.
As mentioned earlier, the prior art SF
6
/helium chemistry, which is employed to etch the nitride layer, typically requires a fairly narrow gap
208
, e.g., between 0.8 cm to 1.2 cm, to achieve an acceptable etch result. This gap clearance is typically insufficient to ensure proper loading and unloading of substrate
204
. By way of example the robotic arm that is typically employed to move substrate
204
from load lock
244
into chamber interior
226
Beyer Weaver Thomas & Nguyen, LLP
Gulakowski Randy
Lam Research Corporation
Olsen Allan
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