APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 11
16 MARCH 1998
Asymmetric microtrenching during inductively coupled plasma oxide
etching in the presence of a weak magnetic field
Marc Schaepkens and Gottlieb S. Oehrleina)
Department of Physics, University at Albany, State University of New York, Albany, New York 12222
~Received 4 December 1997; accepted for publication 20 January 1998!
When fabricating microscopic features in SiO2 layers using low pressure, high-density fluorocarbon
plasmas, microtrenching has commonly been observed. Microtrenching has been explained either as
due to ion scattering from sloped sidewalls or negative charging of the sidewalls by electrons, and
the influence of the associated electric field on ion trajectories. In this work, we show that a weak
magnetic field produces a significant asymmetry in microtrenching. Our results demonstrate
unambiguously that electron-based sidewall charging is to a significant extent responsible for
microtrenching, and, more generally, that differential charging is an important effect in
microstructure fabrication using high-density plasmas. © 1998 American Institute of Physics.
@S0003-6951~98!04211-9#
When forming microstructures in thin films the formation of small trenches at the bottom corners of the feature
being etched is commonly observed. Microtrenching has frequently been explained by ion scattering from sloped feature
sidewalls.1–3 On the other hand, for etching of dielectric
films differential charging of microstructures has also been
suggested to produce microtrenching.4 Differential charging
has been proposed to occur as a result of the difference in
angular distribution for ions and electrons. The ion angular
distribution is highly anisotropic, whereas the electron angular distribution is nearly isotropic. On microscopic features
electrons will mainly arrive at the surface portions near the
top of the feature, and are prevented from reaching the bottom, whereas ions will reach the bottom of the feature. This
differential charging produces local electric fields inside the
feature, which will lead to changes in ion and electron trajectories until electron and ion currents eventually are balanced everywhere along the surface. Surface discharge
mechanisms have also been suggested to play a role in the
balancing of ion and electron currents.5 Microtrenching can
directly result from ion deflection by negatively charged
sidewalls, see Fig. 1~a!.
We propose here to use a weak magnetic field (;102 G!
to distinguish between these two mechanisms. If the differential charging mechanism is to a significant extent responsible for microtrenching, asymmetric microtrenching should
occur if a weak magnetic field is applied, see Fig. 1~b!. On
the other hand, if ion scattering is primarily responsible for
microtrenching, the etch profile should not be significantly
affected by the introduction of a weak magnetic field since
ions are too immobile to be influenced. Even if a perturbation of the ion trajectories should occur, the deflection should
be opposite to that predicted for electrons, allowing us to
determine the dominant mechanism. In this letter results obtained in the above experiment will be presented. Our results
strongly support the differential charging mechanism.
The inductively coupled plasma reactor used here has
a!
Electronic mail: [email protected]
been described by Rueger et al.6 Briefly, a planar coil, supplied with 1400 W inductive power at 13.56 MHz, generates
the plasma through a 19 mm thick quartz coupling window.
The plasma is fed with 40 sccm of trifluoromethane (CHF3)
at the operating pressure of 6 mTorr. A wafer is clamped
electrostatically to a wafer holder cooled to 10 °C. A 5 Torr
He backside pressure is applied between the chuck and the
wafer. The distance between the chuck and the induction coil
is 7 cm. The chuck is connected through a matching network
to a variable frequency ~0.4–40 MHz! rf power supply ~0–
250 W!, which allows for rf biasing of the wafer independently from the plasma generation.
For this work, a small magnet (l3w3h52.2cm30.5
cm30.4 cm! was placed in the center of the 125 cm diameter
wafer. Patterned samples located relative to the magnet as
shown in Fig. 2, were chosen for analysis. The samples consisted of a 900 nm thick patterned resist mask on top of 1600
nm thick oxide ~BPSG! film on silicon ~100!. The diameter
of the contact holes is 600 nm. The samples were etched for
FIG. 1. ~a! Schematic view of the differential charging mechanism in the
absence of a magnetic field. The local electric fields in the feature deflect
ions from the center towards the negatively charged sidewalls, resulting in
symmetric microtrenching. ~b! Schematic view of differential charging in
the presence of a magnetic field. The Lorentz force F L deflects electrons,
resulting in an asymmetric electron angular distribution, resulting in asymmetric microtrenching.
0003-6951/98/72(11)/1293/3/$15.00
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© 1998 American Institute of Physics
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1294
Appl. Phys. Lett., Vol. 72, No. 11, 16 March 1998
M. Schaepkens and G. S. Oehrlein
FIG. 2. Schematic view of the experimental geometry. On the right hand
side from the magnet, iron filings show the magnetic field line pattern. On
the left hand side from the magnet the lines are shown schematically. The
locations of the patterned samples that were analyzed by SEM are indicated.
The labels A, B, and C indicate the positions on the sample where the
dominant component of the magnetic field is perpendicular to the cross
section ~pointing into the surface!, parallel to the cross section, and perpendicular to the cross section ~pointing out of the surface!, respectively. The
labels correspond to the labels in Fig. 4~a!.
150 s at a self-bias voltage of 2125 V ~150 W rf bias power!
at 3.4 MHz, to a depth of around 1000 nm into the oxide.
The thickness of the resist mask after etching is 700 nm. The
etched structures were examined by scanning electron microscopy ~SEM!.
Figure 3~a! shows a micrograph from a feature for which
FIG. 4. ~a! Microtrenching asymmetry measured as a function of orientation
of the magnetic field to the cross section. ~b! Schematic top view and cross
section of a contact hole etched in the presence of a weak magnetic field.
The orientation of the magnetic field is also indicated in the figure.
the magnetic field runs parallel to the cross section. The symmetric microtrenching profile seen in this case is identical to
that seen in separate experiments where no magnetic field
had been applied. For comparison, Fig. 3~b! shows a scanning electron micrograph taken from the cross section of a
feature that was located close to the north pole of the magnet, see Fig. 2. The magnetic field line is perpendicular to the
cross section and pointing out of Fig. 3~b!. The feature
etched significantly deeper on the right hand side than in the
middle or on the left hand side of the feature. At this orientation of the magnetic field relative to the SEM cross section
electrons are deflected to the right, whereas positive ions are
either not affected or would be deflected to the left.
The microtrenching profiles change in a systematic fashion with the orientation of the magnetic field relative to the
SEM cross section. The microtrenching asymmetry will be
defined in this letter as the oxide depth measured in the right
microtrench d r minus the oxide depth measured in the left
microtrench d l . The difference is plotted as a percentage of
the depth measured in the middle of the trench d m :
Microtrenching Asymmetry5 @~ d r 2d l ! /d m # * 100%.
When moving along a sample that is cracked for SEM
analysis, the orientation of the magnetic field with respect to
the cross section changes, see Fig. 2. In this fashion the
microtrenching asymmetry was measured as a function of
the orientation of the magnetic field line with respect to the
cross section of the feature. Figure 4~a! shows the microtrenching asymmetry measured as a function of angle u
FIG. 3. Scanning electron micrographs from a cross section of a feature
etched in the presence of a 150 G magnetic field. ~a! The field is parallel to
the cross section. The microtrenching profile is identical to that observed for
experiments conducted without a magnetic field present. ~b! The field is
perpendicular to the cross section and points out of the surface.
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Appl. Phys. Lett., Vol. 72, No. 11, 16 March 1998
between the magnetic field B and the magnetic field component that is perpendicular to the cross section B' ,
u 5arccos~ B' /B! .
The magnetic field B and its components were calculated as
a function of position from the magnet, assuming that the
magnet acts as a magnetic dipole. For samples placed too
close to the magnet this assumption is only qualitatively correct, and Fig. 4~a! should be interpreted in this fashion. A
clear trend in the microtrenching asymmetry is observed in
Fig. 4~a!. When the magnetic field component perpendicular
to the cross section dominates, a severe microtrench asymmetry can be observed. If the magnetic field is parallel to the
cross section, symmetric microtrenching can be observed.
Figure 4~b! shows schematically how the results obtained from the cross sectional views need to be interpreted
in a three dimensional fashion. Dependent on the orientation
of the magnetic field, there will be a certain area on the
bottom of the feature that is flat, i.e., a plateau that extends
towards the sidewall on one side in the feature. Around the
plateau there is an area where microtrenching occurs. The
microtrench is deepest on the side opposite of where the
plateau extends towards the sidewall, and in a direction that
is expected for electron deflection by the magnetic field.
In summary, the results presented in this letter demonstrate that differential charging of microstructures contrib-
M. Schaepkens and G. S. Oehrlein
1295
utes significantly to microtrench formation. More generally,
we conclude that differential charging is an important effect
in microstructure fabrication using high-density plasmas and
should be included when explaining other microstructuring
phenomena, e.g., the slowdown of the etch rate with feature
size.7,8
The authors would like to thank Eric Sanjuan for his
supporting work and Marcus Doemling, Neal Rueger, and
Theo Standaert for helpful discussions. They gratefully acknowledge support of this work by Lam Research Corporation, Micron Technology, and by the Department of Energy
under Contract No. DE-FG02-97ER54445.
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