Advanced control for plasma process

Details Advanced control for plasma process Advanced control for plasma process

2002-12-19

2004-11-02

Chen, Kin-Chan (Department: 1765)

Semiconductor device manufacturing: process

Including control responsive to sensed condition

Optical characteristic sensed

C438S009000, C438S706000, C438S710000

Reexamination Certificate

active

06812044

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to plasma processes used in the fabrication of semiconductor integrated circuits on semiconductor wafer substrates. More particularly, the present invention relates to a method for monitoring plasma parameters during a plasma process and in-situ termination or modification of the process in the event that the plasma parameters fall outside plasma parameter specifications.
BACKGROUND OF THE INVENTION
Integrated circuits are formed on a semiconductor substrate, which is typically composed of silicon. Such formation of integrated circuits involves sequentially forming or depositing multiple electrically conductive and insulative layers in or on the substrate. Etching processes may then be used to form geometric patterns in the layers or vias for electrical contact between the layers. Etching processes include “wet” etching, in which one or more chemical reagents are brought into direct contact with the substrate, and “dry” etching, such as plasma etching.
Various types of plasma etching processes are known in the art, including plasma etching, reactive ion (RI) etching and reactive ion beam etching. In each of these plasma processes, a gas is first introduced into a reaction chamber and then plasma is generated from the gas. This is accomplished by dissociation of the gas into ions, free radicals and electrons by using an RF (radio frequency) generator, which includes one or more electrodes. The electrodes are accelerated in an electric field generated by the electrodes, and the energized electrons strike gas molecules to form additional ions, free radicals and electrons, which strike additional gas molecules, and the plasma eventually becomes self-sustaining. The ions, free radicals and electrons in the plasma react chemically with the layer material on the semiconductor wafer to form residual products which leave the wafer surface and thus, etch the material from the wafer.
In the fabrication of semiconductor devices, particularly sub-micron scale semiconductor devices, profiles obtained in the etching process are very important. Careful control of a surface etch process is therefore necessary to ensure directional etching. In conducting an etching process, when an etch rate is considerably higher in one direction than in the other directions, the process is called anisotropic. A reactive ion etching (RIE) process assisted by plasma is frequently used in an anisotropic etching of various material layers on top of semiconductor substrate. In plasma enhanced etching processes, the etch rate of a semiconductor material is frequently larger than the sum of the individual etch rates for ion sputtering and individual etching due to a synergy in which chemical etching is enhanced by ion bombardment.
To avoid subjecting a semiconductor wafer to high-energy ion bombardment, the wafer may also be placed downstream from the plasma and outside the discharge area. Downstream plasma etches more in an isotropic manner since there are no ions to induce directional etching. The downstream reactors are frequently used for removing resist or other layers of material where patterning is not critical. In a downstream reactor, radio frequency may be used to generate long-lived radioactive species for transporting to a wafer surface located remote from the plasma. Temperature control problems and radiation damage are therefore significantly reduced in a downstream reactor. Furthermore, the wafer holder can be heated to a precise temperature to increase the chemical reaction rate, independent of the plasma.
In a downstream reactor, an electrostatic wafer holding device known as an electrostatic chuck is frequently used. The electrostatic chuck attracts and holds a wafer positioned on top electrostatically. The electrostatic chuck method for holding a wafer is highly desirable in the vacuum handling and processing of wafers. An electrostatic chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure, in contrast to a conventional method of holding wafers by a mechanical clamping method.
Referring to the schematic of
FIG. 1
, a conventional plasma etching system is generally indicated by reference numeral
10
. The etching system
10
includes a reaction chamber
12
having a typically grounded chamber wall
14
. An electrode, such as a planar coil electrode
16
, is positioned adjacent to a dielectric plate
18
which separates the electrode
16
from the interior of the reaction chamber
12
. A second electrode
20
is provided in the bottom portion of the reaction chamber
12
. Plasma-generating source gases are introduced into the reaction chamber
12
by a gas supply (not shown). Volatile reaction products and unreacted plasma species are removed from the reaction chamber
12
by a gas removal mechanism, such as a vacuum pump (not shown).
The dielectric plate
18
illustrated in
FIG. 1
may serve multiple purposes and have multiple structural features, as is well known in the art. For example, the dielectric plate
18
may include features for introducing the source gases into the reaction chamber
12
, as well as those structures associated with physically separating the electrode
16
from the interior of the chamber
12
.
Electrode power such as a high voltage signal, provided by a power generator such as an RF (radio frequency) generator (not shown), is applied to the electrode
16
to ignite and sustain a plasma in the reaction chamber
12
. Ignition of a plasma in the reaction chamber
12
is accomplished primarily by electrostatic coupling of the electrode
16
with the source gases, due to the large-magnitude voltage applied to the electrode
16
and the resulting electric fields produced in the reaction chamber
12
. Once ignited, the plasma is sustained by electromagnetic induction effects associated with time-varying magnetic fields produced by the alternating currents applied to the electrode
16
and the electrode
20
. The plasma may become self-sustaining in the reaction chamber
12
due to the generation of energized electrons from the source gases and striking of the electrons with gas molecules to generate additional ions, free radicals and electrons. A semiconductor wafer
22
is positioned in the reaction chamber
12
and is supported by the electrode
20
. The electrode
20
is typically electrically-biased by a bias voltage
24
to provide ion energies that are independent of the RF voltage applied to the electrode
16
and that impact the wafer
22
.
In the etching of conductive and insulative layers on wafer substrates, chamber condition monitoring is important to prevent drifting of plasma parameters, particularly plasma electron density and electron collision rate, outside of preset parameter specifications. Traditional monitoring techniques include the use of monitor wafers to probe the etch rate and uniformity variation for a particular plasma process. However, this method is time-consuming and costly. Moreover, the data obtained through use of the monitor wafers may not be sufficiently sensitive to detect crucial deviations in process parameters from the specification. Accordingly, a cheaper and more accurate method of monitoring plasma parameters in a plasma process is needed.
As shown in
FIG. 2
, a strong correlation exists between the etch rate of a polysilicon layer (indicated by the connected circles) and the electron collision rate (indicated by the connected triangles). The plasma electron collision rate is proportional to the plasma electron density. Accordingly, it is proposed that maintaining the plasma electron density and electron collision rate within upper and lower limits during a plasma process can facilitate optimal plasma etching or plasma-mediated material deposition.
An object of the present invention is to provide a novel method for monitoring plasma parameters in a plasma process.
Another object of the present invention is to provide a novel and cost-effective method for monitoring plasma parameters in a plasma process.
Still another object of the present i

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