Download Basic Study of the Electric Forces on a Glass Substrate in a Dry

Journal of the Korean Physical Society, Vol. 48, No. 5, May 2006, pp. 982∼984
Brief Reports
Basic Study of the Electric Forces on a Glass Substrate
in a Dry Etching System
HeeHwan Choe∗
School of Electronics, Telecommunications and Computer Engineering, Hankuk Aviation University, Goyang 412-791
(Received 22 February 2006)
Electrostatic chucks (ESCs) in dry etchers for thin film transistor-liquid crystal display (TFTLCD) fabrications were investigated theoretically. First of all, the behaviors of glass encountering
electric fields in the presence of a plasma were studied. It was shown that, in some conditions, the
ESCs should be used. By applying the simple ESC model, the voltage region for the stability of
glass substrate was obtained.
PACS numbers: 52.75.Rx, 81.05.Gc
Keywords: Glass, ESC, Plasma, Dry etching
The electric field with the negatively charged glass cause
an electric force. Besides this electric force, the gas flow
may cause a force with the air gap between the glass and
the electrode. The ESC is widely used to hold the substrate from these forces. However, the ESC may affect
the plasma, because the static field changes the boundary conditions of plasmas. The ESC should hold the
substrate against the electric force and have little effect
on the plasma state or the substrate. In this paper, as
a start for the study of glass-holding systems, we restric
ourselves to the dry etching system - the force on the
glass in a RIE mode.
I. INTRODUCTION
In microelectronics fabrication, substrate holding systems are used for some processes, such as dry etching,
photolithography, and chemical vapor deposition (CVD).
In such vacuum processes, although holding systems
must be used to protect the substrate from some forces,
vacuum holding systems cannot be used. For vacuum
processes, a mechanical system was used, until the ESC
(electrostatic chuck) technology was introduced, which
was used for XY recorders [1]. There were some reports
of handling problem in using ESC [2, 3] for GaAs or Si
wafers. However, no attempt has been made to investigate the force on the substrate caused by the ESC.
Glass substrates are widely used in display devices.
However, there have been few reports on the behavior of
glass in vacuum processes. In dry etching systems, capacitively coupled plasma (CCP) discharge systems are
commonly used. Two types of CCP etchers are widely
used - PE (plasma etching) and RIE(reactive ion etching). As is well known, in a plasma system, the electrons
from the plasma move to the chamber walls and the
substrate. If the substrate is a dielectric material, the
electrons are accumulated. For RIE, the RF (radio frequency) power is applied from the lower electrode via an
impedance matching box (IMB) under the glass. With
a process gas, the applied power generates the plasma.
Figure 1 shows a schematic diagram of the conventional
RIE system used in the modeling. In this case, the electrode has a surface charge due to the current from the
power, and that charge will produce the electric field.
∗ E-mail:
Fig. 1. Schematic diagram of the dry etching system for
the glass substrate.
[email protected]; Fax: +82-2-3159-9257
-982-
Basic Study of the Electric Forces on a Glass Substrate· · · – HeeHwan Choe
-983-
II. THEORY AND RESULTS
If the power is applied from the lower electrode, the
charge density at the electrode is determined from the
displacement current at the electrode. If the current at
the electrode is given by I = I0 cos ωt, the charge(Q) at
the electrode is
Q=
I0
sin ωt.
ω
If we assume the dimension of the electrode to be sufficiently large compared to that of the gap between the
upper and the lower electrodes, then the magnitude of
the electric field due to the charge at the lower electrode
is
Q
E=
,
2A0
Fig. 2. Electric force on the glass .
(1)
where A is the area of the lower electrode.
If Eq. (1) is utilized, the electric force on the glass
from the lower electrode can be obtained by calculating
the charge on the glass. To calculate the charge on the
glass, we used a simple homogeneous capacitive discharge
model [4]. At the lower electrode, the voltage from the
electrode to the plasma-sheath boundary is
en 3 2
1 2
2
V =
s − 2s0 sin ωt − s0 cos 2ωt ,
20 2 0
2
If an ESC is used to hold the glass, the force from
the ESC should be large enough to overcome the electric
force:
FESC ≥ qE − ρG AdG ,
(3)
where ρG is the density and dG is the thickness of the
glass. If low-frequency or high-current conditions are
used, the ESC voltage must be increased because the
force caused by the ESC is proportional to the ESC voltage (FESC ∝ VESC ).
where s0 = I0 /enωA, and the capacitance at the lower
electrode is
C = 0
III. CONCLUSIONS
A
,
s̄ − s0 sin ωt
with s̄ being the time-averaged sheath length. Therefore,
the charge in the sheath is V C. If we assume that the
electrons out of plasma are accumulated at the glass and
that the sheath thickness collapses to zero at some time
during the rf cycles [4], the charge at the glass can be
estimated as −V C, that is,
q = −V C = −
enAs0 3 − 4 sin ωt − cos 2ωt
.
4
1 − sin ωt
(2)
From Eqs. (1) and (2), we can calculate the electric force
acting on the glass.
2
1
I0
sin ωt(3 − 4 sin ωt − cos 2ωt)
qE =
8A0 ω
1 − sin ωt
Figure 2 shows the changing electric force on the glass.
For most of the time during the RF period of, the calculation shows that the electric force exerted on the glass
is attractive. However, a repulsive force exists for some
intervals. Although the repulsive force is small compared
to the attractive force, the repulsive force might be larger
than the gravitational force on the glass for some discharge conditions. In such a case, the glass begins to
part from the electrode if there are no other methods to
hold the glass.
Although there were many researches on plasma discharges [5,6] there have been rare studies on glass. There
have been rare studies on glass holding systems using
ESC. In thin film transistor (TFT) fabrication, if RIE
systems are used, the glass may experience an upward
force due to the electric field acting against its weight.
That electric force has been shown to depend on the frequency and the current, which are determined from the
RIE system configurations and the process conditions.
The calculation results explains why the ESC voltage
should have a minimum value to hold the glass. However, it should be noted that a large ESC voltage may
cause some problems with the thin film on the glass and
with the plasma boundary conditions.
ACKNOWLEDGMENTS
This work was supported in part by contract
M103BY010043-05B2501-04311 from the Ministry of
Commerce, Industry and Energy through the SYSTEM
IC 2010 Programs and by an additional contribution
from Hankuk Aviation University under contract 200501-023.
-984-
Journal of the Korean Physical Society, Vol. 48, No. 5, May 2006
REFERENCES
[1] G. A. Wardly, Rev. Sci. Intrum. 44, 1506 (1973).
[2] D. Tossel, K. Powell, M. Bourke and Y. Song, The International Conference on Compound Semiconductor Manufacturing Technology, ManTech Digest, 5d (2000).
[3] K. Asano, F. Hadakeyama and K. Yatsuzuka, IEEE Trans.
Ind. Appl. 38, 840 (2002).
[4] M. A. Lieberman and A. Lichtenberg, Principles of
Plasma Discharge and Materials Processing (Wiley, New
Jersey, 2005), p. 391.
[5] Shou-Zhe Li and Han S. Uhm, J. Korean Phys. Soc. 45,
409 (2004).
[6] D. C. Kwon, N. S. Yoon, J. H. Kim, Y. H. Shin and K.
H. Chung, J. Korean Phys. Soc. 47, 163 (2005).