selective etch of Si3N4
a single-wafer wet-etch technology
Martin Knotter, Philips Semiconductors; and Nigel Stewart,
Ian Sharp, and Dana Scranton, Semitool
silicon nitride (Si3N4)
films on semiconductor wafers is and will continue to be an essential
part of microelectronics manufacturing for the foreseeable future. The
properties of Si3N4 are
desirable for a variety of well- known reasons, including its ease of
deposition, etch-masking properties, and stability. The stability of
Si3N4 films allows the material
to be used for such elements as gate spacers, etch and CMP stops, antireflective
coatings, barriers, and hard masks. In next-generation devices, Si3N4
will continue to be a critical material for device fabrication.
devices approach the 65- and 45-nm design rules, maintaining good etch
selectivity between Si3N4
and other films, such as Si and SiO2, will become
increasingly critical to the integrity of the finished product. Additionally,
the capability of adjusting selectivity to achieve process goals will
introduce potential enabling processes for next-generation devices.
This article discusses a single-wafer wet-etch technology for selective
are two conventional ways to etch Si3N4
films. Wet etching is accomplished by immersing wafers in a mixture
of H3PO4 and H2O
heated to 150°–180°C. Water in solution with H3PO4
is important for maintaining etch stability and performance. The nominal
etch rate depends on the properties of the deposited Si3N4
film, bath loading with silica or other etch by-products, and the temperature
and percentage of water in solution.
Typical etch rates are on the order of 4 to 11 nm/min, with selectivity
on the order of >10:1 for SiO2 and 50:1 for
silicon. Figure 1 shows typical etch rates for hot H3PO4
solutions as a function of water concentration at 1 atmosphere of pressure.
1: Typical etch rates as a function of water concentration in hot
phosphoric acid solutions.
second method for etching Si3N4 uses plasma. Plasma
etching using various gas species, such as CHxFy,
is a relatively fast process, with etch rates on the order of 20 nm/min.
With Si3N4:SiO2 selectivity on the
order of 2:5, the selectivity of plasma etching is generally inferior
to that of wet etching. However, because plasma etching has the advantage
of isotropy, it produces nearly vertical etch profiles. It generally
creates a passivation polymer to reduce lateral etching, which generally
requires a wet clean to remove the polymer. The high energies of plasma
etching can increase local temperatures, consuming thermal budget, and
cause device damage from the impact of high-energy particles or charging.
wet-etch processes have generally been performed in batch immersion
platforms, while plasma etching has been performed in single-wafer platforms.
This article describes a new approach in which Si3N4
is etched at elevated pressure and temperature in a single-wafer wet-process
tool. This approach produces etch rates and selectivity suitable for
advanced processes. The productivity that can be achieved using a high-volume
single-wafer tool makes the process feasible, especially for 300-mm
Selectivity and Rate
rate and selectivity are two critical parameters in the Si3N4
etching process. Solutions composed of hydrofluoric acid (HF) etch Si3N4,
but they also etch SiO2 at rates that are higher
than those for Si3N4, resulting
in poor selectivity. However, because of the characteristics of dilute
HF mixtures and the kinetics of the reactions that take place between
HF, water, Si3N4, and SiO2
at elevated temperature and pressure, the etch selectivity between Si3N4
and SiO2 can be adjusted to deliver selectivity
better than 10:1.
reactive species used to etch Si3N4 are monofluoride
species (F–, and HF), while the reactive species used
to etch SiO2 are difluoride species (HF2–
and H2F2). The reactivity and selectivity of HF
solutions can be altered by introducing high concentrations of protons,
resulting in a shift of the HF equilibrium from ionic fluoride species
(HF2– and F–) to
neutral fluoride species (H2F2 and HF). This shift
diminishes the reactivity of Si3N4 by 150 times
and SiO2 by approximately 2500 times. The addition of acidic
organic solvents can also alter the reactivity.1 In such
cases, the solution becomes less polar, and the HF species prefer to
be in a neutral form. While the decrease in reactivity is most pronounced,
the nonlinear nature of the shift in reactivity between Si3N4
and SiO2 is the key to attaining desirable etch selectivity.
suggested by the reaction kinetics model in Figure 2, diluting HF with
water causes the solution to contain more monofluoride species. This
gives rise to a trend toward higher etch selectivity ratios, but at
the expense of the etch rate. The etch rate can be increased by elevating
the temperature of the solution. Heating an HF:H2O
solution also results in the creation of higher concentrations of monofluoride
species as a consequence of entropy effects. Hence, by heating the HF
solution, the Si3N4 etch
rate increases at a faster rate than the SiO2
etch rate.2 The objective then becomes to select an appropriate
HF dilution and temperature for performing the process to achieve a
suitable etch rate and selectivity.
2: Reaction kinetics of Si3N4 and SiO2
with HF species.
assess the effect of HF concentration on etch selectivity and rate,
a batch spin-spray system was used to perform a series of tests at different
HF dilutions. Spray tools, because they tend to have a high mass-transport
rate by virtue of their relatively turbulent and rapid boundary-layer
flow, are suitable for examining the limiting case of high mass transport.
preparation for the experiments, a 160-nm-thick layer of Si3N4
was deposited on 200-mm wafers using SiH2Cl2
and NH3 in a low-pressure chemical vapor deposition
(LPCVD) horizontal furnace from ASM (Bilthoven, The Netherlands). Process
temperature was 785°C and process pressure was 300 mTorr. The SiO2
film was thermally grown to a thickness of 600 nm on silicon at a temperature
of 950°C in oxygen. Nine-point measurements using a UV1250 spectroscopic
ellipsometer from KLA-Tencor (San Jose) determined layer thicknesses.
spin-spray etch experiments were performed at 85°C under conditions
of constant flow and time using a solution without HF and solutions
containing 0.01, 0.02, 0.03, and 0.04% HF, respectively. The test results
in Figure 3 show the Si3N4
etch rate and selectivity to SiO2. SiO2
was also etched, facilitating the presentation of selectivity.
3: Selectivity of Si3N4:SiO2 and
Si3N4 etch rate in four HF dilutions at 85°C.
results of this test validate the kinetics theory advanced above. Increasing
the HF dilution improves selectivity while lowering the etch rate. For
example, the selectivity ratio for Si3N4:SiO2
is approximately 6:1 at an HF concentration of 0.04% and approximately
85:1 at an HF concentration of 0.0075%. Meanwhile, an etch rate of approximately
2.0 nm/min is attained at an HF concentration of 0.04%, while an etch
rate of 0.6 nm/min is attained by reducing the HF concentration to 0.0075%.
The etch rate decreases by a factor of 3.3 as HF concentration decreases
by a factor of 5.3. Hence, between the higher and the lower HF concentration,
selectivity increases by a factor of 14 while the etch rate decreases
by a factor of 3.3. While these etch rates are too slow to be viable
for most production processes, the quantification of the theory and
the demonstration of the substantially nonlinear selectivity trend is
valuable for understanding the mechanics of the etch process.
a Production-Worthy Process
achieve a production-worthy process for advanced devices or 300-mm wafer
production, an etch rate of at least 10 nm/min and a selectivity for
of at least 10:1 is desirable. Extrapolating from the test data summarized
in Figure 3 and considering the effects of temperature, it was determined
that an HF concentration of approximately 0.08% in combination with
an elevated temperature would achieve a desired etch rate and selectivity.
4: Elevated pressure and temperature process chamber used for HF
analysis of the target process conditions suggested that an 0.08% HF
solution would have to be run at approximately 125°C to achieve
an etch rate at or above 10 nm/min. However, dilute HF at 125°C
results in a boiling solution. To etch at 125°C, it is necessary
to maintain pressure above ambient. To that end, a process chamber based
on the Capsule design from Semitool (Kalispell, MT) was developed. Pictured
in Figure 4, this chamber can operate at approximately 2 bar and 125°C
without boiling the solution.
shown in Figure 5, three HF dilutions were tested in the elevated pressure
chamber to derive a correlation for etch rate as a function of HF concentration.
From the data, it is apparent that increasing the temperature to increase
the etch rate was a viable option and that it was more beneficial than
only increasing the HF concentration.
5: Si3N4 etch rates at 125°C and 2 bar
pressure as a function of HF concentration.
at 125°C, the etch rate fell short of the desired target of 10
nm/min. Thus, a process was tested in which the temperature was increased
to 130°C. At 130°C, the Si3N4 etch rate
was found to be 13 nm/min. Moreover, SiO2 etched under the
same conditions resulted in an etch rate of 1.4 nm/min, resulting in
selectivity of approximately 10:1. The etch-rate data in Figure 6 for
both Si3N4 and SiO2 processed at 130°C
indicate that the rates are essentially constant over time.
etch processes generally tend to be diffusion limited, the mass transport
of reaction by-products and the replacement of etchant species is critical
to maintaining a sustained and uniform process. In diffusion-limited
processes, the hydrodynamics of flow can play a significant role in
etch uniformity across the wafer and in microscopic features.
6: Si3N4 and SiO2 etch rates in
0.08% HF solution at 2 bar pressure under three rotational flow
data in Figure 6 demonstrate the influence of mass transport on the
etch rate. Usually, the wafer is rotated at an angular velocity sufficient
to achieve good mass transport while simultaneously delivering dilute
HF to the wafer at a flow rate of approximately 300 ml/min. Two cases
of rotational flow were examined, one with fluid delivery directly at
the center of the wafer and the other with fluid delivery 5 cm off-center.
By way of contrast, a third case involved wafers that were subjected
to neither rotation nor flow.
rotational flow was applied, the etch rate remained nearly constant,
although across-wafer etch uniformity was 3.5% in the case of the centered
nozzle and 2% in the case of the off-centered nozzle. The etch rate
for the wafers that were not subjected to rotation or flow decreased
substantially from 13 to 8.3 nm/min. These results clearly demonstrate
that the etch process is limited by diffusion or mass transport. In
fact, the etch process's dependence on mass transport means that uniform
fluid flow, or flow that maximizes mass transport across the entire
wafer surface, is essential for achieving good etch uniformity and selectivity
for Si3N4 and SiO2.
curve fit of the data in Figure 5 is relatively linear, highlighting
the important role played by water in the etching of Si3N4.
Water alone has a measurable impact on the etch rate of Si3N4
in the presence of low pH, as extrapolated from the etch data gathered
in this experiment and the literature.3 At an HF concentration
of 0, the approximate etch rate is predicted to be 2.4 nm/min. The mechanism
for etching Si3N4 in ultradilute
HF is essentially the same as etching it in H3PO4
solutions diluted with water. Figure 7 depicts the etch mechanism that
takes place in the presence of water in a low-pH solution.
7: Mechanism for determining the reaction that etches Si3N4
in H2O or HF. The reaction is driven by the protonation
of reactive sites to form [SiN*] and leads to the subsequent replacement
of NH3 by F– or H2O.
surface sites, [SiN*], are formed by the reaction of the reactive surface
site with a proton, transforming NH2 groups into a strong
leaving group, NH3, which are then replaced by F–
or H2O groups. The relative surface concentration of [SiN*]
is a function of pH. The acidic solutions discussed in this article
have a pH of <3. The concentration of [SiN*] will be independent
of pH if the ionization constant (pKa) value is on the order of 5 for
surface NH2 groups.4 Therefore, the etch rate
of Si3N4 in hot H3PO4 (with
water concentrations between 8 and 15%) and acidified water using, for
example, HF can be represented by the equation:
= k[H2O%][SiN*] = k´[H2O%]
k is the reaction rate constant, k0 is
the preexponential factor, Ea is the activation energy of the
rate-determining reaction step (J/mol), R is the molar gas
constant (8.314 J/mol K), and T is temperature in kelvin.
equation can be validated by curve-fitting published data on Si3N4
etch rates in boiling and nonboiling H3PO4
solutions to an Arrhenius relationship.3 The resulting curve,
shown in Figure 8, produces a strong correlation of etch rate for solutions,
normalized by water content, to etch temperature. The correlation coefficient,
R2, is greater than 0.99 for this relationship.
8: Published etch-rate data for Si3N4 etched
using LPCVD at 880°C in hot H3PO4 (normalized
for water concentration).3
the curve to the case of 100% water at 125°C yields an etch rate
of 4 nm/min, compared with the 2.4 nm/min rate extrapolated from the
experiments in the Capsule chamber. Furthermore, extrapolating the curve
to the case of 100% water and 85°C yields an etch rate of 0.2 nm/min,
compared with 0.3 nm/min. These findings support the etch model described
in this article and further explain the mechanisms of etch rate and
selectivity that favor a dilute acidic solution, such as ultradilute
HF, for etching Si3N4 films.
results of tests using ultradilute HF at an elevated temperature and
pressure demonstrate that Si3N4
etching can be performed with suitable selectivity to SiO2
at relatively high etch rates. These etch rates, coupled with modern
high-volume single-wafer platform technology, can provide suitable process
capability and productivity for advanced device manufacturing. Therefore,
it appears feasible to replace H3PO4
with dilute HF as an etchant for Si3N4.
9: Etch selectivity of Si3N4:SiO2
as a function of the molar concentration of HF and pH at 25°C.
discussion has focused on developing and demonstrating a process that
can provide a suitable etch rate and selectivity between Si3N4
and SiO2. The kinetic model described here also suggests
that selectivity can be adjusted based on HF concentration and temperature.
Figure 9 shows etch selectivity as a function of molar concentration
and pH for HF-based solutions. This graph demonstrates that selectivity
can be chosen based on molar concentration, providing the means to achieve
a wide range of process objectives.
scanning electron microscope (SEM) images in Figures 10a and 10b demonstrate
how a dilute HF process at an elevated temperature can be used. The
figures show a poly layer and an Si3N4/SiO2
hard mask with photoresist. In Figure 10a, the hard mask has been structured
in the dilute HF process using a solution that provides 1:1 selectivity.
In Figure 10b, the structure has undergone plasma etch, in which the
structuring of the Si3N4/SiO2
hard mask provides the desired poly etch profile. These images illustrate
the versatility and feasibility of using the dilute HF process to etch
10: SEM images showing (a) poly structure with Si3N4/SiO2
hard mask under resist, and (b) structured poly layer after resist
removal and plasma etch (process not optimized).
application of dilute HF solutions at elevated temperature and pressure
provides a means for adjusting the selectivity of Si3N4
to SiO2 to achieve various process objectives. In the era
of thinner films and challenging device architectures, such a process
will provide precise process performance at a low cost of ownership.
authors wish to acknowledge Nicole Wils from Philips Research (Eindhoven,
The Netherlands) for supplying the poly structure/hard-mask SEM images,
Rene Vroom from Philips Semiconductors' MOS34 for providing wafers with
Si3N4 layers, and James
Heffernan from Semitool UK (Cambridge) for providing logistics.
Deckert, "Pattern Etching of CVD Si3N4/SiO2
Composites in HF/Glycerol Mixtures," Journal of the Electrochemical
Society 127, no. 11 (1980): 2433–2438.
Harrap, "Equal Etch Rates of Si3N4
and SiO2 Utilizing HF Dilution and Temperature
Dependence," in Semiconductor Silicon 1973, ed. HR Huff and
RR Burgess (Pennington, NJ: Electrochemical Society, 1973), 354.
van Gelder and VE Hauser, "The Etching of Silicon Nitride in Phosphoric
Acid with Silicon Dioxide as a Mask," Journal of the Electrochemical
Society 114, no. 8 (1967): 869–872.
Knotter and TJJ Denteneer, "Etching Mechanism of Silicon Nitride in
HF-Based Solutions," Journal of the Electrochemical Society
148, no. 3 (2001): F43–F46.
Martin Knotter, PhD, is principal scientist for cleaning/wet
processes at Philips Semiconductors in Nijmegen, The Netherlands. When
he joined the company in 1990, he belonged to the surface chemistry
group. In 1995 he joined the process module group, working on advanced
cleaning processes. He received an MS in chemistry from the University
of Amsterdam and a PhD in organometallic chemistry from the University
of Utrecht, both in The Netherlands. (Knotter can be reached at +31
24 3532225 or email@example.com.)
Stewart is a mechanical design engineer at Semitool Europe
(Cambridge, UK), which he joined in 2001. From 1985 to 2001, he was
employed at Westcode Semiconductor. He received a higher national diploma
in mechanical engineering and a higher national certificate in electronics
engineering. (Stewart can be reached at +44 1223 518879 or firstname.lastname@example.org.)
Sharp is Semitool's European sales manager for the greater
European region. At Semitool, which he joined in 1989, he has held positions
as a process engineer, product manager, and general manager. In addition,
he has worked as a process engineer in the III-V sector. Sharp holds
multiple U.S. patents. He received a BS in chemistry from the University
of Glasgow in Scotland. (Sharp can be reached at +44 1223 518463 or
Scranton, PhD, is vice president of surface preparation technology
at Semitool (Kalispell, MT.) He has been associated with the semiconductor
industry for 19 years, 15 of which he has spent with Semitool. Scranton
has held various positions in engineering, marketing, sales, and general
management. He received BS and MS degrees in mechanical engineering
from the University of Wyoming in Laramie and a PhD in mechanical engineering
from LaSalle University in Philadelphia. (Scranton can be reached at
406/751-6360 or email@example.com.)