Semiconductor device manufacturing: process – Including control responsive to sensed condition – Optical characteristic sensed
Reexamination Certificate
2002-08-28
2004-12-14
Goudreau, George A. (Department: 1763)
Semiconductor device manufacturing: process
Including control responsive to sensed condition
Optical characteristic sensed
C216S060000, C134S001200, C134S022110
Reexamination Certificate
active
06830939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor processing, and more particularly, the present invention relates to the etch process by which semiconductor material is etched out leaving well-defined features. Still more particularly, the present invention relates to a system, method and software program product for accurately determining the changes in a signal that are indicative of an endpoint of the etching process.
2. Description of Related Art
There are many steps involved in the processing of a wafer, of which etching is one of the crucial steps. Etching is a process whereby a selected area on a wafer surface is removed so as to make a desired pattern on the surface. Plasma etching can accurately remove patterns of very small dimension on the surface of a semiconductor wafer. Reactive Ion Etching (RIE) is an etching technique in which radio frequency radiation in a low pressure gas ionizes the gas and dissociates the gas molecules into more reactive species.
FIG. 1
is a cross-sectional illustration of an etcher in which the RIE process may be performed.
FIG. 1
is a diagram of an exemplary etcher intended only to aid in describing etching principles useful in understanding the description of the present invention and not intended to faithfully represent any actual etcher. Etcher
100
is a plasma etch reactor in which the RIE process is confined within chamber
102
. In operation, plasma
118
is produced in chamber
102
when etching gas
122
enters reaction chamber
102
and is ionized by the application of an electric field established between cathode
110
and anode
112
. As etching gas
122
is ionized into plasma
118
, the respective velocity of electrons and ions are significantly different due to the difference in their masses. A typical etcher
100
uses anode
112
at ground potential and cathode
110
connected to the RF generator
114
and biased above ground. Wafer
130
is placed on the platen. Gas molecules of etching gas
122
are accelerated to the substrate surface of wafer
130
toward cathode
110
due to the difference in potential across the electrodes. Wafer
130
is bombarded with reactive positive ions created in the plasma which causes atoms of the substrate to be sputtered and usually react chemically with etching gas
122
, thereby removing the top layer of material on wafer
130
. The newly-formed gases
126
are removed from chamber
102
by vacuum system
125
through exhaust port
124
.
The etching step of wafer production is an integral part of semiconductor manufacturing; however, equally important to accurate etching is detecting the precise point in time the etching process has ended, i.e. the “endpoint.” The endpoint of the etching process is where all etched feature patterns are fully delineated and undercutting of the substrate is held to a minimum. Typically, endpoint detection mechanisms determine the endpoint of an etch process by distance determinations or by optical emission. A laser interferometer (not shown in the figure) reflects laser radiation off wafer
130
during processing and the thickness of the etched layer is determined by the interference of the reflected light. Alternatively, the optical emissions of the reaction products of the etching process are used to determine an endpoint.
Emission collection and processing mechanism
150
receives and pre-processes light emitted by the plasma into a form usable by an endpoint detection mechanism (not shown). Initially, collimating and focusing optics
152
receive light
154
from inside chamber
102
and transmit it, usually through an optical fiber, to optical emission analyzer
156
. One type of optical emission analyzer
156
, a monochromator, monitors the intensity of a single wavelength of light
154
from the exhaust gases and outputs signal
158
based on the intensity of the light at the wavelength being monitored. Generally, it is expected that the intensity of the light at this wavelength will change at the endpoint of the etching process, and thus the output signal
158
indicates that transition.
Accurately detecting the endpoint of an etch process by analyzing the optical emission of the reaction products depends on identifying an optimal discrete wavelength, usually associated with a reactant species, that exhibits a quantifiable intensity change of the light associated with the endpoint transition. Although the intensity of light from many reactant species decreases at the etching process endpoint, the light from some other species increases in intensity. Many factors have a detrimental effect on signal detection that must be compensated for, e.g. low amplitude of transmitted light energy
154
due to a dirty view port, spurious noise from plasma fluctuations that masks the endpoint signal, unsteady electrical fields from the electrodes, electronics malfunctions, or inaccurate optical measurements.
As fine line patterning becomes more prevalent and the percentage of area to be etched becomes smaller, accurate endpoint detection becomes more difficult. As feature sizes decrease, the percent of the wafer open area also decreases, requiring plasma etch endpoint detection systems to be more sensitive and accurate. Traditional endpoint detection systems that monitor one or two wavelengths do not generate enough information for successful endpoint determination when open areas drop below a nominal surface percentage.
In another alternative, collimating and focusing optics
152
receive light from reactant species inside chamber
102
and feeds the entire spectrum of light over a substantially broad range of wavelengths
154
to optical emission analyzer
156
. Optical emission analyzer
156
is a spectrometer which is capable of monitoring multiple discrete wavelengths. Using processing functionality in optical emission analyzer
156
, the operator can then select the most appropriate sets of wavelengths for reactant species associated with a particular etching process and generate output
158
from the intensities of the selected wavelengths. Monitoring intensities of multiple wavelengths gives an operator increased flexibility. The disadvantage is that the complexity increases for endpoint detection.
FIG. 1
further depicts etcher
100
as having magnets
142
on rotating magnet housing
140
for magnetically enhanced RIE etching. Magnets
142
revolve around chamber
102
causing plasma
118
to follow the magnetic field associated with magnets
142
. This produces a more homogenous etch of the surface. However, magnets
142
induce one or more bright spots in plasma
118
that revolve with magnet housing
140
. Optical emission analysis of the etch process becomes more challenging because the intensity of the light
154
now is periodic with the rotation of a mechanical device and is not based solely on the reactant processes. Here it is necessary for the operator to have expert knowledge of plasma chemistry and/or spectroscopy. The operator must also understand how the rotating magnets degrade the endpoint signal. Understanding how to do this optical emission analysis to find the endpoint using multiple sets of wavelength from the output of a spectrometer has been heretofore unknown.
SUMMARY OF THE INVENTION
The present invention is directed to a system, method and software product for creating a predictive model of the endpoint of etch processes using Partial Least Squares Discriminant Analysis (PLS-DA). Initially, intensity readings for discrete wavelengths in a spectrum are collected from a calibration wafer using optical emission spectroscopy (OES). Intensity values in the OES data may represent a signal that is non-periodic or periodic with time. Periodic signals may be sampled synchronously or non-synchronously with the period of a signal. Initially, the OES data is arranged in a spectra matrix X having one row for each data sample.
The OES data is processed to remove transients that occur during the startup and shutdown of the etch process. Wavelength regions are selected with desirable endpo
Gallagher Neal B.
Harvey Kenneth C.
Hosch Jimmy W.
Wise Barry M.
Buchel, Jr. Rudolph J.
Goudreau George A.
Verity Instruments Inc.
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