1063-7397/03/3204- $25.00 © 2003 MAIK “Nauka /Interperiodica”
Russian Microelectronics, Vol. 32, No. 4, 2003, pp. 214–218. Translated from Mikroelektronika, Vol. 32, No. 4, 2003, pp. 271–276.
Original Russian Text Copyright © 2003 by Rudenko, Sukhanov, Bazaev.
The optical emission spectroscopy (OES) of plas-
mas is widely used to monitor etching in the fabrication
of very-large-scale-integration (VLSI) and ultra-large-
scale-integration (ULSI) circuits . The emission
intensity of reactive species is a function of their den-
sity in the bulk of the plasma. Their rate of decay in sur-
face reactions is speciﬁc to the material (SiO
icon, or metal), so that the bulk density and the emis-
sion intensity change as etching progresses from layer
to layer. For example, during the ﬂuorine-plasma etch-
ing of a SiO
/Si structure, a decrease in the emission
intensity of F* in passing the interface permits the end-
point detection (ED) of etching.
The endpoint change in emission intensity depends
on both the material and the etch area. As contact win-
dows decrease in size, the ED signal weakens to
approach the noise level; for this reason the industry
employs OES monitoring only if the etch area is above
3–5% of the wafer surface . However, the SiO
areas of ULSI circuits are often less than 1%. The
absence of reliable
monitoring at this stage can
lead to errors in ED .
The ED problem for OES monitoring in the context
of small etch areas, noise, and drift has been addressed
by artiﬁcial neural networks [2, 4] and discrete Fourier
images [5, 6]. These approaches are designed to make
the etching endpoint in the useful signal more detect-
able. Despite the progress in this avenue, the ED prob-
lem has yet to be solved.
Below we report an experiment in which phase
detection  was used to extract an ED signal from an
OES signal modulated by the driving signal of a
55-kHz ﬂuorine-plasma source in the presence of dis-
charge noise. Etching was performed on a SiO
structure; it was monitored with respect to F* lines.
Oscillations of emission intensity in steady-state
etching (with a process gas of constant composition)
may be caused by many factors. One of them is dis-
charge intrinsic noise, as shown by probe measure-
ments of low-frequency (LF) discharge in molecular
gases . It may be due to electron attachment and
detachment under periodically varying ionization. Sim-
ilar noise occurs in radio-frequency (RF) and micro-
wave discharges, especially at low levels of applied
power. External causes of noise in the emission are
power-supply instability and the drift of process param-
eters: applied power, pressure, and gas ﬂow rates. No
controller can prevent deviations of process parameters
because the feedback concept implies that any devia-
tion will be corrected with a delay. Thus, it is hardly
possible to eliminate noise from the optical emission.
For the above reasons we addressed the extraction of
a useful signal from a corrupted one . Our method
takes advantage of the fact that an OES signal is inevi-
tably modulated by the driving signal of the plasma
source. We used a phase detector to compare an OES
signal with the driving signal, the latter serving as the
reference. A discriminator was thus obtained for an ED
signal. The method was tested on an LF plasma source
because the percentage modulation of optical emission
may be as high as 100% in this case.
Figure 1 is a diagram of the experimental apparatus
for OES monitoring based on the phase-detection con-
cept, including a spectrometer, a signal-extraction sys-
tem, and a plasma etcher itself.
We used a Kvarts-4M high-speed optoacoustic
spectrometer, which consists of a photodetector and a
signal-processing unit controlled by a computer. The
output signal passes to a Unipan 232B phase detector
via a Unipan 233 bandpass ﬁlter. The output signal of
the phase detector goes to the signal-processing unit
through a specially designed interface.
Phase Detection Used in the Optical-Emission Monitoring
/Si Plasma Etching
K. V. Rudenko, Ya. N. Sukhanov, and N. I. Bazaev
Institute of Physics and Technology, Russian Academy of Sciences, Nakhimovskii pr. 36, Moscow, 117218 Russia
Received December 8, 2002
—An experiment is carried out to evaluate phase detection as a method for the optical-emission mon-
itoring of SiO
/Si ﬂuorine-plasma etching, the driving frequency serving as the reference frequency. It is shown
that the output signal-to-noise ratio can thus be increased several times, so that the minimum percentage etch
area for which endpoint detection is possible will be reduced from 2.1 to 0.4%. It is concluded that the new
method may be practical in the etching of small windows.