NOTCH-FREE ETCHING OF HIGH ASPECT SOI STRUCTURES USING A TIME DIVISION MULTIPLEX PROCESS AND RF BIAS MODULATION
Cross References to Related Applications
This application claims priority from and is related to commonly owned U.S. Provisional Patent Application Serial No. 60/512933 filed
October 21, 2003, entitled: Notch-Free Etching of High Aspect SOI
Structures Using a Time Division Multiplex Process and RF Bias
Modulation, this Provisional Patent Application incorporated by reference
herein.
Field of the Invention
The present invention relates generally to the manufacture of
silicon based micro-electro-mechanical-systems. More particularly, the
present invention relates to the manufacture of high aspect ratio silicon
structures using alternating deposition and etching steps with a
modulated RF bias.
Background of the Invention
The fabrication of high aspect ratio features in silicon is used
extensively in the manufacture of microelectromechanical (MEMS)
devices. Such features frequently extend completely through the silicon
wafer and may require etching in excess of 500μm into the silicon
substrate. Even "shallow" features require etch depths up to 30μm with
feature widths as small as lμm, requiring the definition of structures with
aspect ratios (depth/width) in excess of 30:1. To ensure manufacturability,
these processes must operate at high etch rates to maintain reasonable
throughputs.
Conventional, single step, plasma etch processes cannot
simultaneously meet these needs, and alternating deposition etching
processes have been developed. Such processes are frequently referred to
as Time Division Multiplexed (TDM) processes, which more generally
consist of at least one group containing two or more process steps, where
the group(s) is periodically repeated. These processes (see for example
U.S. Patent 4,985,114 and U.S. Patent 5,501,893) are typically carried out
in a reactor configured with a high-density plasma source, such as an
Inductively Coupled Plasma (ICP), in conjunction with a radio frequency
(RF) biased substrate electrode. The most common process gases used in
the TDM etch process for silicon are sulfur hexafluoride and
octofluorocyclobutane. Sulfur hexafluoride (SFβ) is typically used as the
etch gas and octofluorocyclobutane (C F8) as the deposition gas. During
the etch step (Figure 1(b)), SFβ facilitates spontaneous and isotropic
etching of silicon (Si); in the deposition step (Figure 1(c)), C4F8 facilitates
protective polymer deposition onto the sidewalls as well as the bottom of
etched structures. Upon energetic and directional ion bombardment,
which is present in etch steps, the polymer film coated in the bottom of
etched structures from the previous deposition step will be removed to
expose silicon surface for further etching. The polymer film on the
sidewall will remain because it is not subjected to direct ion bombardment,
inhibiting lateral etching. The TDM process cyclically alternates between
etch and deposition process steps enabling high aspect ratio structures to
be defined into a masked silicon substrate (Figures 1(d) & 1(e)). Using the
TDM approach allows high aspect ratio features to be defined into silicon
substrates at high Si etch rates. A complex TDM process may incorporate
more than one etch step, and more than one deposition step that are
cyclically repeated. Certain MEMS devices require that the silicon substrate be etched
down to a buried insulating layer such as silicon dioxide (Siθ2), which acts
as an etch stop (Silicon On Insulator, SOI structure), or which is required ( for functionality of the final device. When such structures are etched
using a TDM process a well-documented phenomenon, commonly referred
to as "notching", occurs. This is evidenced as a severe undercutting of the
silicon, localized at the silicon/insulator interface (Figure 2). It is
generally understood that this is caused by electrical charging effects
during the etching. Because of the different angular distributions of ions
and electrons in the plasma, ions tend to accumulate at the bottom of the
feature, and electrons at the top. During the bulk etch, because the silicon
substrate is sufficiently conductive, current flow within the substrate
prevents any charge separation (Figure 3A). However, when the etch reaches the silicon/insulator interface, the insulator is exposed and the
conductive current path is broken, which allows charge separation to
occur. The resultant electric field is strong enough to bend the trajectories
of arriving ions into the feature sidewall where lateral etching (notching) occurs (Figure 3B). Note, for a full discussion see KP Giapis,
Fundamentals of Plasma Process-Induced Charging and Damage in
Handbook of Advanced Plasma Processing Techniques, RJ Shul and SJ
Pearton, Eds, Springer 2000. The notching effect is more prevalent in high density plasma,
because the ion density and therefore the charging effect due to the ions, is
greater. The effect can therefore be reduced by the use of a low density
plasma (conventional reactive ion etching (RIE)) which is employed only
after the insulator has been exposed (Donohue et al. U.S. Patent
6,071,822). The major drawback of such an approach is the low etch rate
attainable, which is a serious shortcoming when features with various
depths must be etched. This is a necessary consequence of etching devices
with various feature sizes, which will etch to different depths due to
Aspect Ratio Dependent Etching (ARDE).
Two groups have taught the use of TDM processes and novel RF
bias configurations' (U.S. Patent 4,579,623 and U.S. Patent 4,795,529).
Neither of these groups contemplate modulating the RF bias frequency in
conjunction with a TDM process.
The use of a low frequency (below 4 MHz) RF substrate bias with a
TDM deposition / etch process has also been described by Hopkins et al.
(U.S. Patent 6,187,685). The authors describe the use of amplitude-pulsed
RF bias (Figure 4) in a TDM process. Hopkins does not teach modulating the frequency of the RF bias.
U.S. Patents 5,983,828, 6,253,704 and 6,395,641 by Savas teach the
use of a pulsed ICP to alleviate surface charging and subsequent notching.
However, none of the Patents by Savas teach the modulation of the
frequency of the RF bias to eliminate or reduce notching.
Ogino et al. (U.S. Patent 6,471,821) teach frequency modulation of
the RF bias power as an effective means of reducing charging of the wafer
surfaces during a plasma etch process. Ogino et al. consider frequency
modulation between two discrete frequency values as well as continuous
frequency modulation. Ogino et al. do not consider the application of
frequency modulated RF bias to a TDM process.
Arai et al. (U.S. Patent 6,110,287) also teach frequency modulation
of the RF bias power in order to relax charge formation on the substrate
during an etch process. Amplitude modulation of the frequency modulated
RF bias power is also disclosed, including the case of pulsing between
some power level and zero. Arai et al. do not consider the application of
frequency and/or amplitude modulated RF bias to a TDM process.
Otsubo et al. (U.S. Patent 4,808,258) also teach frequency or
amplitude modulation of the RF bias power to improve etch rate and
selectivity of plasma processes. Frequency modulation between 1 MHz
and 13.56 MHz is disclosed. Amplitude modulation of the RF bias
between two discrete levels is also discussed. Otsubo et al. do not consider
the application of frequency and/or amplitude modulated RF bias to a
TDM process.
Therefore, there is a need for an alternating deposition and etch
process that reduces and/or eliminates notching.
Nothing in the prior art provides the benefits attendant with the
present invention.
Therefore, it is an object of the present invention to provide an
improvement which overcomes the inadequacies of the prior art devices
and which is a significant contribution to the advancement of the
semiconductor processing art.
Another object of the present invention is to provide a method for
etching a feature in a substrate comprising the steps of: placing the
substrate on a substrate support in a vacuum chamber, said substrate
support being a lower electrode; an etching step comprising introducing a
first process gas into the vacuum chamber, generating a first plasma from
said first process gas to etch the substrate; a passivation step comprising
introducing a second process gas into the vacuum chamber, generating a
second plasma from said second process gas to deposit a passivation layer
on the substrate; alternatingly repeating the etching step and the
passivation step; applying a modulated bias to the substrate though said
lower electrode; and removing the substrate from the vacuum chamber.
Yet another object of the present invention is to provide an
apparatus for etching a feature in a substrate comprising: a vacuum chamber; at least one gas supply source for supplying at least one process
gas into said vacuum chamber; an exhaust in communication with said
vacuum chamber; a lower electrode positioned within said vacuum
chamber; a substrate holder connected to said lower electrode; a plasma
source for generating a plasma within said vacuum chamber; a control
system for alternately etching the substrate and depositing a passivation
layer on the substrate; and a modulation signal generator for providing a
modulated bias to said lower electrode.
The foregoing has outlined some of the pertinent objects of the
present invention. These objects should be construed to be merely
illustrative of some of the more prominent features and applications of the
intended invention. Many other beneficial results can be attained by
applying the disclosed invention in a different manner or modifying the
invention within the scope of the disclosure. Accordingly, other objects
and a fuller understanding of the invention may be had by referring to the
summary of the invention and the detailed description of the preferred
embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.
Summary of the Invention
For the purpose of summarizing this invention, this invention comprises an improved method and an apparatus for deep silicon trench
etching using an alternating cyclical etch process or time division
multiplexed (TDM) process to eliminate the notching observed on SOI structures.
In most plasma vacuum treatment processes (etching or deposition),
the main parameters that can be altered to optimize etching or deposition
performance are: the flow rates of the various gases, the working pressure,
the electromagnetic power coupled to the plasma to generate it, and the
energy with which the substrate is bombarded. As a general rule, to
optimize a deposition or etching process, the flow rates of the various
gases, the electromagnetic power coupled to the plasma and the substrate
bombardment energy are optimized at precise and constant values
throughout the treatment. Whereas, the present invention provides for an
improved method apparatus utilizing a modulated RF frequency o reduce
or eliminate notching during an alternating deposition and etch process.
A feature of the present invention is to provide a method for etching
a feature in a substrate. The substrate can be a semiconductor substrate
such as Silicon, Gallium Arsenide or any known semiconductor, including
compound semiconductors e.g., Group II and Group VI compounds and
Group III and Group V compounds. The substrate may also be a
conductor or a dielectric material such as glass or quartz. The method
comprising the steps of placing the substrate on a substrate support in a
vacuum chamber, generating a high density plasma using a first source of
RF energy. The substrate support is a lower electrode to which is
connected a second source of RF energy which provides a bias voltage to the substrate. An alternatingly and repeating process is performed on the
substrate. One part of the process is an etching step which is carried out
by introducing a first process gas, such as Sulfur hexafluoride, into the
vacuum chamber. A first plasma is generated from the first process gas to
etch the substrate. The other part of the alternatingly and repeating
process is a passivation step which is carried out by introducing a second
process gas, such as octofluorocyclobutane into the vacuum chamber. A
second plasma is generated from the second process gas to deposit a
passivation layer on the substrate. The passivation layer consists of a
polymer or a fluorocarbon polymer, or can be silicon, carbon, nitride or any
other known passivating materials that can be deposited via a plasma. During the alternatingly and repeating process, one or more process
parameters can vary over time within the etching step or the passivation
step. In addition, during the alternatingly and repeating process, one or
more process parameters can vary over time from etching step to etching
step or from passivation step to passivation step. During the alternatingly
and repeating process a modulated bias is applied to the substrate through
the lower electrode. The bias can be voltage controlled. The bias can be
frequency modulated and it can be applied at or below the ion transit
frequency. The bias can be switched between two distinct frequencies
such that the switching is defined by a switching rate and a switching
duty cycle. The switching rate can be less than about 10kHz and the
switching duty cycle less than 50%. The RF bias frequency can be
continuously modulated and be defined by a mathematical function such as exp(k*sin(t)). The RF bias can additionally be amplitude modulated.
Further, the RF bias can be phase modulated or wave shaped modulated.
Finally, upon completion of the etch process, the substrate is removed
from the vacuum chamber.
Still yet another feature of the present invention is to provide an
apparatus for etching a feature in a substrate. The apparatus comprising
a vacuum chamber having at least one gas supply source for supplying at
least one process gas into the vacuum chamber and an exhaust in
communication with the vacuum chamber. A lower electrode is positioned
within the vacuum chamber for applying a bias to the substrate that is
placed upon a substrate holder that is connected to the lower electrode. A
plasma source generates a plasma within the vacuum chamber. The
plasma that is generated is controlled through a control system, depending
on the plasma, alternately etching the substrate and depositing a
passivation layer on the substrate. A modulation signal generator
provides a modulated bias to the lower electrode. The bias can be powered
by RF or DC power. If the bias is powered by an RF source, it can be
frequency modulated and it can be provided at or below the ion transit
frequency. In addition, the frequency of the RF bias can be phase
modulated or wave shaped modulated in conjunction with the alternating
etching and deposition process.
The foregoing has outlined rather broadly the more pertinent and
important features of the present invention in order that the detailed description of the invention that follows may be better understood so that
the present contribution to the art can be more fully appreciated.
Additional features of the invention will be described hereinafter which
form the subject of the claims of the invention. It should be appreciated by
those skilled in the art that the conception and the specific embodiment
disclosed may be readily utilized as a basis for modifying or designing
other structures for carrying out the same purposes of the present
invention. It should also be realized by those skilled in the art that such
equivalent constructions do not depart from the spirit and scope of the
invention as set forth in the appended claims.
Brief Description of the Drawings
Figures l(a-d) is a pictorial example of one type of the TDM process
for deep silicon etching;
Figure 1(e) is a typical scanning electron microscopy photograph of
an etch performed using a TDM process for deep silicon etching;
Figure 2 is a scanning electron microscopy photograph of an etch
performed using a TDM process for deep silicon etching showing the notch
formation at the surface of the substrate;
Figures 3A and 3B are a pictorial of charge buildup at the surface of
the semiconductor substrate during a typical TDM process;
Figure 4 is a graph of amplitude versus time showing a pulsed
amplitude RF bias in a TDM process;
Figure 5 is a graph of amplitude versus time showing frequency
switched modulation using two discrete frequencies for a TDM process;
Figure 6 is a graph of amplitude versus time showing continuous
frequency modulation for a TDM process;
Figure 7 is a schematic of a plasma reactor configured for providing
a modulated bias to a substrate; Figure 8 is a schematic of a plasma reactor configured for providing
a modulated RF bias in conjunction with a modulated high density source
to a substrate for a TDM process; and Figure 9 is a scanning electron microscopy photograph of an etch
performed using a modulated bias as taught in the present invention for a
TDM process for deep silicon etching showing no notch formation at the
surface of the substrate.
Similar reference characters refer to similar parts throughout the
several views of the drawings.
Detailed Description of the Invention
We disclose an improved method and apparatus for reducing, or
eliminating, the notching observed when etching SOI structures, by using
an alternating deposition/etch process in conjunction with a modulated RF
bias.
The present invention provides a method and apparatus for
improved etching of silicon on insulator (SOI) structures through the use
of a frequency modulated RF bias, phase modulated RF bias, or a wave
shaped modulated RF bias in conjunction with a TDM process. By frequency modulation of the RF bias, for the present invention,
it is meant that bias voltage applied to the cathode is changed between at
least two frequencies, either discretely switched (Figure 5) or continuously
modulated (Figure 6) during the time division multiplex process.
By wave shaped modulation it is meant that the bias voltage
waveform to the cathode is changed between at least two shapes, either
discretely or continuously modulated. Such waveform shapes may be, for
example, a sine wave and a square- wave or any arbitrary waveforms.
By phase modulated bias it is meant that the bias waveform is
changed between at least two states, either discretely or continuously
modulated, where the difference between the two states is a phase
relationship.
A reactor 10 configured for frequency modulated RF bias is shown
in Figure 7. The reactor 10 shown comprises a vacuum chamber 12, a gas
inlet 14, an exhaust 16, an inductively coupled plasma power source (RF
generator) 20 that is connected to a first impedance matching network 21
that provides power to a coil 22, a lower electrode 24 and a substrate
holder 26 for a substrate 28. Provided, as part of the reactor 10, is a
modulation signal generator 30 that is connected to a voltage control
oscillator 32 that is connected to a broadband amplifier 34 that is
connected to a second impedance matching network 36 that supplies
power to the lower electrode 24. It should be noted that applying a
modulated RF bias is not limited to this configuration and one skilled in
the art will appreciate that alternate approaches are possible. Thus the
waveform can be digitally synthesized and applied to the broadband
amplifier 34 using an arbitrary waveform generator. The RF Bias
frequency is preferentially modulated during the etch sub-cycle of the
deposition/etch process, since this is when the notching primarily occurs.
However, the RF bias frequency can also be modulated through both deposition and etch sub-cycles.
The duration of the RF Bias high-frequency state should be short enough that charge build-up on the feature surfaces has not reached a
steady state, or is at a steady state for only a short period of time. This
time scale is typically on the order of a few microseconds to a few milliseconds. The duration of the low-frequency RF bias state should be
long enough that charge bleed off can occur, but not so long that reduced etch rates are produced. The low-frequency duration should also be of the
order of a few microseconds to a few milliseconds. In the case of switching the RF bias frequency between two discrete values, the duty cycle, as
defined by the high frequency duration divided by the total modulation
cycle period, should be in the range of 5-90%, preferably 10-50%
The RF bias frequency for the high-frequency state should be below
the ion transit frequency to allow ions to follow the voltage on the cathode
over time.
The ion transit frequency, cϋ i, is described by: ωp = ( e n0 / ε0M )^ Where e - charge on an electron n0 - ion density ε0 - permittivity in a vacuum M - mass of the ion
For a typical high density plasma etcher for semiconductor applications, the ion transit frequency is approximately 2 MHz. Notching
performance is improved by using a high frequency state in the range of
50kHz to 2MHz. A preferred embodiment uses a high frequency state in the range of 50kHz to 300kHz and a low frequency state in the range of
1kHz to 10kHz which significantly improves notching performance.
In a time division multiplex process, the etch step proceeds in two
stages. During the 1st stage of the etch step, the passivating film from the
previous deposition step is removed from the horizontal surfaces. This passivation removal process typically follows an ion assisted etch
mechanism with the polymer removal rate being a function of the ion
energy. Ion energy (and passivation removal rate) increases with
increasing bias voltage. Thus it is important to control the magnitude of
the bias voltage. Once the passivation layer has been cleared, the exposed
Si then proceeds to etch by a primarily chemical etch mechanism.
As described above, the amplitude of the modulated bias is
maintained constant as the frequency is changed. It is also possible to
additionally change the amplitude of the waveform. This may be done in
such a manner that the amplitude is high when the frequency is high or
alternately such that the amplitude is low when the frequency is high. As
with the frequency, the amplitude can be switched discreetly between two
levels or continuously varied between two levels.
It is important to note for all the systems and methods described above that the TDM finish etch method can be implemented as a single
sequence or multi-sequence process. A sequence refers to a group of
deposition and/or etch steps. In the single sequence implementation,
modulated RF bias is used during the entire process. In the multi-sequence implementation, the first sequence can be
any suitable method (TDM or conventional plasma etch process) that
results in the required etch profile and etch rate, but which is terminated
before the underlying insulator is exposed. The RF bias need only be
modulated for the period when the insulator film is exposed, since this is
when the maximum benefit from charge reduction and, therefore, notch
reduction is expected. The etch is completed using a modulated RF bias
"finish" etch to avoid notching at the silicon/insulator interface.
Etch endpoint detection methods, such as laser reflectance and
optical emission spectroscopy (OES), are helpful in determining when the
insulating film has been exposed.
For all of the modulated RF bias systems and methods described
above, the ICP power can be maintained "on" continuously during the etch
cycle, or it to can be pulsed. This pulsing can be either in phase with the
RF bias modulation (i.e., the ICP is "on" when the RF bias frequency or
amplitude is high), can be out of phase with the RF Bias modulation (i.e.,
ICP is "on" when the RF bias frequency or amplitude is low) or can bear
some other phase relationship to the RF bias (i.e., can be phase shifted).
A reactor 10 configured for a modulated RF bias in conjunction with a modulated high density source is shown in Figure 8. The reactor 10
shown comprises a vacuum chamber 12, a gas inlet 14, an exhaust 16, an
inductively coupled plasma power source (RF generator) 20 that is
connected to a first impedance matching network 21 that provides power
to a coil 22, a lower electrode 24 and a substrate holder 26 for a substrate
28. Provided, as part of the reactor 10, is a modulation signal generator 30 that is connected to both the RF Generator 20 and an RF Bias
Generator 50 that is connected to a second impedance matching network
36 that supplies power to the lower electrode 24. The high density source
can be operated at a preferred frequency of 2 MHz or can be at higher
frequencies (e.g., 13.56, 27, 40, 60, 100 MHz, 2.45 GHz) or at lower
frequencies (e.g., 50 kHz — 2 MHz). While the examples below were
performed using an ICP source to generate the high density plasma, other
sources such as ECR, helicon, TCP, etc. could also be used.
Example
The result of etching an SOI structure using a standard TDM etch
process (prior art) with an approximate 2 minute over-etch (sufficient to
etch other smaller structures) is shown in the cross section of Figure 2.
The notch at the silicon-insulator interface is evident, and extends ~ 3μm
into the silicon. Other features with widths of ~ 4μm were undercut to an
extent that they were no longer attached to the substrate.
The preferred embodiment is a significant improvement over the
prior art in terms of notch performance. The reactor is a commercially
available Unaxis VLR modified according to the requirements of the present invention. Figure 7 represents the preferred embodiment of the
present invention, namely, frequency modulation of the RF bias for
improved notch performance. The modulated RF bias is amplified and is applied through an impedance matching network to the electrode.
The test pattern used to characterize the notch performance has ~
45μm of Si before a buried oxide layer of lμm. The oxide layer acts as an
etch stop layer. The lines are 4μm wide and spaces range from the
smallest opening of 2.5μm to the largest opening of lOOμm. The resist is ~
1.7μm thick and the percentage of exposed to unexposed Si on a 6" wafer is
~ 15%. The finish etch process recipe with ideal notch performance
corresponding to the preferred embodiment of the present invention is
detailed below. The Si etch before exposure of the oxide layer is non-critical for
notch performance. The transition between the standard TDM Si etch and
the TDM Si finish etch (present invention) is made using an optical
emission end point technique. Finish Etch with Notch performance - Present invention
1* - RF Bias modulation
High Frequency value / duration — 100 kHz / 660 μsec Low Frequency value / duration — 1 kHz / 1340 μsec Total time for 1 cycle - 2000 μsec
Results
The result of etching an SOI structure using a frequency modulated TDM
Si finish etch is shown in Figure 9. The over etch on the 15μm, lOμm, 9μm
and 8μm (from L-R in Figure 9) is sufficient to etch the smallest feature
(2.5μm opening) to the buried oxide layer. The absence of any notching at
the Si — oxide interface seen in the cross section is a significant improvement over the prior art.
The present disclosure includes that contained in the appended
claims, as well as that of the foregoing description. Although this
invention has been described in its preferred form with a certain degree of
particularity, it is understood that the present disclosure of the preferred
form has been made only by way of example and that numerous changes
in the details of construction and the combination and arrangement of
parts may be resorted to without departing from the spirit and scope of the
invention.
Now that the invention has been described,