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<reference>
Directional atomic layer etching of MgO-doped lithium niobate
using sequential exposures of H2 and SF6 /Ar plasma
,1 Jennifer Solgaard,2 Ryoto Sekine,2 Azmain A.

Ivy I. Chen

arXiv:2310.10592v2 [cond-mat.mes-hall] 20 Jan 2024

Hossain,1 Anthony Ardizzi,1 David S. Catherall,1 Alireza Marandi,2
James R. Renzas,3 Frank Greer,4 and Austin J. Minnich
1

1, ∗

Division of Engineering and Applied Science,

California Institute of Technology, Pasadena, California 91125, USA
2

Department of Electrical Engineering,

California Institute of Technology, Pasadena, CA, USA
3

Oxford Instruments Plasma Technology,

North End, Bristol BS49 4AP, United Kingdom
4

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
(Dated: January 23, 2024)

Abstract
Lithium niobate (LiNbO3 , LN) is a ferroelectric crystal of interest for integrated photonics owing
to its large second-order optical nonlinearity and the ability to impart periodic poling via an external
electric field. However, on-chip device performance based on thin-film lithium niobate (TFLN) is
presently limited by optical loss arising from corrugations between poled regions and sidewall surface
roughness. Atomic layer etching (ALE) could potentially smooth these features and thereby increase
photonic performance, but no ALE process has been reported for LN. Here, we report a directional
ALE process for x-cut MgO-doped LN using sequential exposures of H2 and SF6 /Ar plasmas. We
observe etch rates up to 1.01 ± 0.05 nm/cycle with a synergy of 94%. We also demonstrate ALE can
be achieved with SF6 /O2 or Cl2 /BCl3 plasma exposures in place of the SF6 /Ar plasma step with
synergies above 90%. When combined with a wet post-process to remove redeposited compounds,
the process yields a 50% decrease in surface roughness. With additional optimization to reduce
the quantity of redeposited compounds, these processes could be used to smoothen surfaces of
TFLN waveguides etched by physical Ar+ milling, thereby increasing the performance of TFLN
nanophotonic devices or enabling new integrated photonic capabilities.

∗

aminnich@caltech.edu

1

I

Introduction

Lithium niobate (LiNbO3 or LN) is a ferroelectric crystal of interest for a variety of integrated
photonics applications ranging from electro-optic modulators in fiber-optic communications
to quantum optics. [1] LN is a trigonal crystal characterized by a threefold rotational
symmetry about the crystallographic z axis. Because x-cut electro-optic modulators have
fewer processing requirements compared to their z-cut counterparts, [2] the x-cut surface is
the relevant surface LN nanophotonic circuits. The crystal structure of LN is described in
Refs. [3–5].
LN exhibits a number of desirable properties for photonics, including a large transparency
window, wide electro-optic bandwidth, ferroelectric properties, and high second-order nonlinear susceptibility, [6–10] making it an attractive platform compared to those based on
other materials like silicon nitride. [11] MgO-doped LN is used in photonics because the
dopant raises the optical damage threshold, allowing for high-intensity photonic applications
[12]. The lattice parameters for 5% mol MgO-doped x-cut LN are within 0.002 Å of the
values of undoped LN. [3]
Early efforts to create on-chip photonic devices involved Ti ion diffusion or protonexchange on bulk LN wafers to provide the necessary refractive index contrast. [13–18]
However, the relatively small refractive index contrast from this approach resulted in weak
optical confinement, imposing limitations on the types of devices and nonlinear phenomena
that could be observed. With the development of ion-slicing and wafer bonding processes
for LN onto silicon dioxide, [19–21] thin-film lithium niobate (TFLN) wafers have become
commercially available, allowing for the realization of dense circuits with tightly-confining
waveguides. Devices that have been fabricated on TFLN include record large squeezed
states on-chip, [22] > 100 GHz electro-optic modulators with CMOS compatible voltages,
[23] broadband frequency comb sources, [24–26] and on-chip mode-locked lasers. [27, 28]
Wet and dry etching methods are widely used for pattern transfer, but dry etching is the
most common owing to the need for anisotropic etching. [11, 13] Process development for
dry etching of LN is more challenging compared to that for other photonic materials such
as SiN because LN is a ternary compound. Fluorine- [29] or chlorine- [30] based reactive
ion etching (RIE) processes have been reported, but they suffer from redeposition of stable
Li compounds such as LiF, leading to an increase in sidewall roughness and scattering loss.
2

[11, 29] Proton-exchanged LN has been noted to have lower LiF redeposition during plasma
etching due to lower surface Li content. Deep (> 1 µm) fluorine-based etches with less LiF
redeposition have been accomplished with proton-exchanged LN. [13, 31–33]
In the device community, physical Ar+ milling remains the preferred dry etch method
used for pattern transfer. However, this method has its own limitations such as low etch
selectivity with common lithography resists, non-vertical sidewalls, redeposition of LN, and
variations of etch depth across a single chip. [11, 34] Various approaches are available to
remedy some of these limitations; for instance, redeposited LN is typically removed using an
HF dip and RCA clean. However, the wet process also introduces micron-sized corrugations
in periodically-poled LN (PPLN) due to differential wet etch rates between poled domains,
[35] leading to optical loss which dominates the overall loss in TFLN devices. [36] As
a result, various device figures of merit such as resonator quality factors are at least an
order of magnitude from their intrinsic upper limits. Decreasing losses associated with
corrugations and sidewall roughness in PPLN circuits will enable system-level integration of
on-chip nonlinear optics and allow for quantum information processing. [11]
These challenges could be addressed with improved nanofabrication techniques which
offer sub-nanometer-scale etch depth control and surface smoothing. In particular, thermal
or plasma-enhanced atomic layer etching (ALE) has demonstrated etch depth control on
the angstrom scale and ability to smooth surfaces to the sub-nanometer scale. [37, 38] ALE
consists of sequential, self-limiting surface chemical processes that lead to etch per cycles
ranging from fractional monolayers to a few monolayers in crystalline materials. ALE can be
anisotropic (directional) or isotropic (thermal or plasma-thermal). [38–40] Anisotropic ALE
is based on surface modification by adsorption of a reactant followed by low-energy ion or
neutral atom sputtering. [39, 41, 42] The self-limiting nature of anisotropic ALE is defined
by the thickness of the modified surface and the difference in sputtering threshold between
the modified and unmodified surface. Thermal (isotropic) ALE is based on a cycle of surface
modification and volatilization reactions. Recent developments in ALE have also employed
a pulsed-bias approach, where the flow of gases is held constant and the DC bias is turned
on and off, resulting in faster ALE cycle times. [43] Thermal and anisotropic ALE recipes
have been developed for various semiconductors and dielectrics such as SiO2 , [44, 45] InP,
[46–48] GaAs, [49–52] and Si3 N4 . [53–58] Surface smoothing due to ALE has been observed
for various materials, [37, 58–64] a feature which has been attributed to conformal layer-by3

layer removal and curvature-dependent surface modification. [65] Despite the potential to
smooth step pattern corrugations and sidewall roughness in PPLN, no ALE processes have
been reported for LN.
Here, we report an ALE process for MgO-doped x-cut LN. Using sequential exposures of
H2 and SF6 /Ar plasmas, various etch per cycle values (EPC) from 5.4±0.3 Å/cycle to 10.1±
0.5 Å/cycle are achieved with synergies ranging from 88 − 94%. We observe the saturation
of both half-steps of the process. A 50% reduction in surface roughness is observed when the
ALE process is followed by a post-process RCA clean to remove redeposited products. In
addition, we demonstrate that the SF6 /Ar plasma step can be replaced with an O2 /SF6 or
Cl2 /BCl3 plasma and achieve EPCs of 0.88 ± 0.05 and 0.93 ± 0.06 and synergies of 96% and
94%, respectively. With further optimization to reduce the amount of redeposited Li and
Mg compounds, the process could be used to smoothen sidewall roughness and corrugations
in periodically-poled TFLN devices and thereby enhance their photonic performance.

II

Experimental Methods

The samples consisted of bulk 3-inch 5% mol MgO-doped LN wafers (G & H Photonics). The
wafers were diced into 7 mm × 7 mm substrates using a Disco DAD 321 dicing saw and then
cleaned by sonication in AZ NMP Rinse, acetone, and isopropyl alcohol. The samples were
etched in an Oxford Instruments PlasmaPro 100 Cobra system. As shown in Figures 1(A)
to 1(D), the process consisted of sequential exposures to H2 and SF6 /Ar plasmas with purges
between each exposure. This process was inspired by the observation that proton-exchanged
LN can be etched with fluorine plasmas with reduced LiF redeposition [13, 18, 31–33] and
because the same plasmas successfully achieved quasi-ALE of SiN. [58]
The nominal ALE recipe consists of an 8-second H2 plasma exposure (300 W ICP power,
50 W RIE power, 205 V DC bias, 50 sccm H2 ) followed by a 30-second SF6 /Ar exposure
(300 W ICP power, 3 W RIE power, 47 V DC bias, 17 sccm SF6 , 35 sccm Ar), although
variations of the process parameters are possible around these values. 5-second purge times
with 40 sccm Ar were used between plasma half-steps. The chamber pressure was set at
10 mTorr and the substrate table temperature was set to 0 ◦ C as measured by the table
thermometer. The table temperature was set to 0 ◦ C rather than room temperature as more
vertical sidewall profiles have been reported at lower temperatures. [58]
To measure saturation curves, the chamber pressure and ICP power were kept constant
4

at 10 mTorr and 300 W, respectively, during half-steps, while the exposure time for each
half-step was varied. H2 exposure time was varied from 0 to 60 seconds with SF6 /Ar held
at 30 seconds, and SF6 /Ar exposure time was varied from 0 to 40 seconds with H2 exposure
held at 8 seconds. The substrate table temperature was kept constant at 0 ◦ C, as measured
by the substrate table thermometer. Prior to introducing the sample into the chamber for
etching, the chamber walls and blank Si carrier wafer were cleaned by a 30-minute Ar+
plasma with 1500 W ICP and 100 W RF power followed by a 15-minute O2 /SF6 plasma
with the same power parameters. When the sample was loaded into the chamber, a 3-minute
wait time was used before processing to allow the sample to equilibrate with the table. All
samples were etched for 50 cycles unless otherwise noted. After etching, the photoresist
was removed using room temperature AZ NMP Rinse for at least 30 minutes, followed by
sonication in acetone and isopropyl alcohol.
To enable etch depth measurements, step patterns consisting of periodic 400 × 400 µm2
squares were written into a resist using photolithography, as shown in Figure 1(E). The
pattern was transferred to the +x face of the samples using AZ5214 photoresist and a
Heidelberg MLA 150 Maskless Aligner with a dose of 150 mJ/cm2 , followed by development
with AZ 300 MF developer. Etch per cycle (EPC) was calculated by measuring the difference
in height from etch depth for a processed sample and dividing it by the total number of cycles.
The total etch depth was measured using 2.5 × 10 µm2 AFM scans on a Park Instruments
NX atomic force microscope with a non-contact cantilever tip. Scan rate was set to 1 Hz
and etch depths were averaged over 5 line profiles in the scan. RMS surface roughness of a
reference TFLN Ar+ milled waveguide sample and power spectral density (PSD) scans were
obtained using a Bruker Dimension Icon atomic force microscope (AFM) over a 50 × 50
nm2 area with a 0.5 Hz scan rate. Waveguide sidewall slope on measured TFLN samples
and sample tilt from all AFM scans were removed via linear plane fit.
X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos Axis
Ultra x-ray photoelectron spectrometer using a monochromatic Al Kα source. A 1.3 nm thick
layer of carbon, as measured by a quartz crystal monitor, was deposited using sputtering to
reduce charging effects during scans (Leica EM ACE600 Carbon Evaporator). The resulting
data was analyzed in CASA-XPS from Casa Software Ltd. For each sample, we collected
the carbon C1s, oxygen O1s, niobium Nb3d5/2 and Nb3d3/2, niobium Nb4s, lithium Li1s,
fluorine F1s, and magnesium Mg2p peaks. The carbon C1s peak was used as a reference to
5

Figure 1. (a-d) ALE process for MgO-doped LN. A hydrogen plasma exposure (pink)
leads to a proton-exchanged modified layer (pink circles) at the top of the sample (blue
dots), followed by a purge. A subsequent SF6 /Ar plasma exposure (yellow dots) yields
volatile Nb, Li, and O species. A final purge completes the cycle. (e) Microscope image
(10× magnification) of the developed lithography pattern on the LN wafer. The dotted
line indicates the direction of AFM scan for etch depth measurements.

calibrate peak positions. We fit the data using a Shirley background subtraction and peak
fitting routines from Refs. [66, 67].
Two alternate recipes were also investigated. The first alternate recipe consists of a 20second H2 plasma exposure of the same parameters as the SF6 /Ar recipe followed by a
20-second O2 /SF6 exposure (300 W ICP power, 3 W RIE power, 38 V DC bias, 35 sccm
O2 , 15 sccm SF6 ). The second alternate recipe uses the same 20-second H2 plasma exposure
followed by a 20-second Cl2 /BCl3 exposure (300 W ICP power, 2 W RIE power, 40 V DC
bias, 20 sccm Cl2 , 40 sccm BCl3 ). The second alternate recipe was motivated by reports
of ALE processes for metal oxides based on BCl3 , [42] and the Cl2 :BCl3 gas flow ratio was
selected based on an RIE recipe of LN using chlorine. [30] Etch depth measurements and
500 × 500 nm2 surface roughness scans over 20 cycles from these alternate processes were
compared with those of the original nominal ALE recipe consisting of a 20-second H2 plasma
exposure followed by a 20-second SF6 /Ar exposure.
6

Figure 2. (a) EPC versus cycle number with 8 second H2 plasma exposures only(blue
triangles), 30 second SF6 /Ar plasma exposures only(red squares), and both half-cycles
(purple circles). All processes occur at 0 ◦ C. The dashed lines are guides to the eye. (b)
EPC versus H2 gas exposure time with SF6 /Ar exposure time fixed at 30 s. (c) EPC versus
SF6 /Ar exposure time with H2 exposure time fixed at 8 s. The etch rates are observed to
saturate with exposure time, demonstrating the self-limiting nature of the process.

III

Results

Figure 2(A) shows the thickness change of LN versus cycle number for individual half cycles
and the overall process. An etch rate of 0.6 Å/cycle is observed for the SF6 /Ar half step.
For the H2 step, a thickness increase was observed, which may be attributed to a volume
expansion of the crystal due to H2 exposure. Such thickness increases for one half-step have
been reported in other processes. [38] On the other hand, when using both steps sequentially,
an etch rate of 5.4±0.3 Å/cycle is observed. We note that this EPC corresponds to two
interatomic layer distances of x-cut LN based on the cationic interatomic distance of 0.26
nm in bulk LN. [4]
To gain more insight into the process and verify its self-limiting nature, we measured
7

saturation curves for each half-cycle. In Figure 2(B), the SF6 /Ar plasma half step is held
constant at 30 seconds while the H2 exposure time is varied from 0 to 30 seconds. Saturation
occurs at 10.1 ± 0.5 Å/cycle above 30 seconds H2 exposure time. In Figure 2(C), the H2
exposure time is held constant at 8 seconds while the SF6 /Ar exposure time is varied from 0
to 40 seconds. The etch rate saturates at 5.4 ± 0.3 Å/cycle for SF6 /Ar exposure times longer
than 30 seconds, indicating that the SF6 /Ar step preferentially etches the hydrogen-modified
layer. These observations indicate that both half-steps are self-limiting, implying that the
process is indeed atomic layer etching.
The synergy, S, as defined by Ref. [61], quantitatively compares the etch depth using
only individual steps of the ALE cycle to the etching obtained with the full etch cycle as
S = (1 − (α + β)/EP Ccycle ) × 100, where α is the EPC of the H2 half-cycle, β is the EPC of
the SF6 /Ar half-cycle, and EP Ccycle is the EPC of the full cycle. For the present process in
which a thickness increase is observed after H2 exposure, we calculated the synergy assuming
zero EPC for that step. We obtain a synergy value of 88% for the nominal recipe. If we
consider a variation of the recipe where the H2 half step is 30 seconds long, we achieve an
EPC of 10.1 ± 0.5 Å/cycle, corresponding to four cationic layer distances, with a synergy of
94%. These synergy values are compatible with typical synergy values reported in Ref. [37].
We also investigated alternate ALE recipes using O2 /SF6 or Cl2 /BCl3 plasma exposures
for the removal step. The O2 /SF6 ALE process yielded an etch rate of 0.88 ± 0.05 nm/cycle
over 20 cycles. The half-step EPCs for the H2 and O2 /SF6 step are 0 and 0.3 Å/cycle,
respectively, and the synergy for this process is 96%. The Cl2 /BCl3 ALE process yielded an
EPC of 0.93 ± 0.06 nm/cycle over 20 cycles; the half-step EPCs for the H2 and Cl2 /BCl3
step are 0 and 0.6 Å/cycle, respectively, and the synergy for this process is 94%.
We next characterize the chemical composition of bulk LN before and after 50 cycles
ALE for the SF6 /Ar plasma process using XPS. No depth-profiling XPS is reported due to
preferential sputtering of O over Nb with an Ar+ beam, [68] complicating the interpretation
of the measurements. The C1s peak at 284.8 eV is used as a reference. In Figures 3(A)
to 3(D), we show the core levels of Nb3d, O1s; F1s; and Nb4s, Li1s, and Mg2p, respectively.
For the Nb3d XPS spectra in Figure 3(A), we observe a single doublet peak consisting of a
3d5/2 and 3d3/2 subpeak corresponding to LN (207.5 eV and 210.3 eV). [69–71] In Figure 3(B),
we report the O1s spectra with two subpeaks at 530.5 and 532.3 eV, corresponding to metal
oxide and O-C bonds, respectively. [72] In Figure 3(C), we report the F1s spectra with
8

Figure 3. Surface XPS spectra showing (a) Nb3d, (b) O1s, (c) F1s spectra, and (d) Nb4s,
Li1s, Mg2p. The spectra is shown for (top) original and (bottom) etched bulk MgO-doped
LN over 50 ALE cycles consisting of an 8 second H2 plasma exposure followed by a 30
second SF6 /Ar exposure. The measured (blue dots) and fit spectra (gray lines) intensity
are reported in arbitrary units (a.u.) against the binding energy on the x-axis. (e) Surface
atomic concentration obtained from XPS spectra for untreated bulk LN, 1 cycle H2
exposure, 1 full cycle, and 50 cycles. Due to the conductive carbon coating, roughly 55% of
the atomic concentration in the sample surface consists of carbon. The plot is truncated at
50% to highlight the concentration of non-carbon species.
two subpeaks at 685.8 eV and 688.1 eV corresponding to LiF and F-C bonds, respectively.
[71, 72] In Figure 3(D), we report the Nb4s, Li1s, and Mg2p spectra at 60.4 eV, 55.6 eV,
and 50.8 eV, respectively (values are for bulk LN). [71, 72] The Li1s peak energy agrees well
with reported binding energies for LiF (55.7 ± 0.5 eV). [70, 72] After ALE, we observe a
0.5 eV shift for the Nb4s, Li1s, and Mg2p peaks to higher binding energies, as expected if
fluoride bond formation occurred. [72] There is also an increased concentration of Mg after
ALE, suggesting that MgF2 is also formed.
In Figure 3(E), we report the atomic concentrations of Nb, Li, Mg, O, F, and C obtained
from the XPS data at various stages of the process. Due to the carbon coating used in
XPS, the carbon content of the surface is estimated to be about 55% in each sample. In
the untreated bulk sample, the atomic concentrations are found to be 8.1% Nb, 24.5% O,
9

Table I. Comparison of metrics from LN ALE recipes with different removal step plasma
exposures. Values are from a 20 second H2 exposure followed by a 20 second plasma
exposure indicated in the table, over 20 cycles.
Plasma Type EPC (nm/cycle) RMS Roughness (nm) Average Roughness (nm) Synergy (%)
SF6 /Ar

0.96 ± 0.05

1.34 ± 0.19

1.08 ± 0.20

94

O2 /SF6

0.88 ± 0.05

0.79 ± 0.28

0.56 ± 0.21

96

Cl2 /BCl3

0.93 ± 0.06

0.43 ± 0.06

0.34 ± 0.05

94

8% Li, 3.1% Mg, and 0.2% F. With one H2 cycle exposure, the atomic concentrations are
8.7% Nb, 22.8% O, 9.4% Li, 2.8% Mg, and 1.8% F. The presence of fluorine is likely from
residual fluorine on the chamber walls after the chamber clean. Surface lithium content is
observed to increase after 1 H2 half-cycle. This trend differs from that reported in previous
studies in which surface lithium concentration was found to decrease after an hour-long H2
plasma exposure, forming proton-exchanged LN (c.f. Fig. 4 in Ref. [18]).
After 1 cycle of ALE, atomic concentrations are measured to be 7.7% Nb, 20.8% O, 9.3%
Li, 3.6% Mg, and 2.2% F. After 50 cycles, the atomic concentrations are 5.9% Nb, 19.5% O,
11.3% Li, 4.4% Mg, and 5.5% F. These data suggest that the presence of fluorine compounds,
specifically LiF and MgF2 , increases with cycle number, while Nb and O are preferentially
etched.
We characterized the effect of Li and Mg compound redeposition on surface roughness for
all three ALE recipes. AFM scans over 500 × 500 nm2 were obtained on bulk LN subjected
to 20 cycles of ALE. The unetched polished bulk LN sample had a surface roughness of
0.27 ± 0.04 and 0.35 ± 0.05 nm for RMS (Rq ) and average (Ra ) roughness, respectively.
The O2 /SF6 process yielded Rq = 0.79 ± 0.28 and Ra = 0.56 ± 0.21 nm, and the Cl2 /BCl3
process yielded Rq = 0.43 ± 0.06 and Ra = 0.34 ± 0.05 nm. In comparison, the SF6 /Ar
process produced the roughest surface after 20 cycles of ALE with Rq (Ra ) of 1.34 ± 0.19
(1.08 ± 0.20) nm. Table I compares the EPC, surface roughness, and synergy of the three
different recipes.
We next characterized the effect of the SF6 /Ar ALE process on the sidewall surface roughness of TFLN waveguides. For these measurements, we used additional samples consisting
of TFLN with Ar+ milled waveguides that were smoothed post-etch with an HF dip and
10

Figure 4. AFM scan showing height-maps of TFLN waveguide sidewall with linear-plane
tilt removal before (a) and after 50 ALE cycles (b). (c) AFM line scan of the step pattern
after 50 cycles of ALE, indicating the occurrence of microtrenching and supporting the
directional nature of the process. (d) Height-map PSD of the samples before ALE, after 50
cycles ALE, and after 50 cycles ALE and an RCA clean.
RCA clean, corresponding to the standard process for TFLN device fabrication. [11, 34]
Figures 4(A) and 4(B) show the plane-fit height map of Ar+ etched sidewall before and
after 50 cycles of ALE at 0°C, respectively. After ALE, the surface is visually rougher. Figure 4(C) shows an AFM line scan of the step pattern on bulk LN after 50 cycles of ALE. We
observe the occurrence of microtrenching at the base of the sidewall, where there is a deeper
etch at the base of the step compared to regions farther from the step. Microtrenching is
commonly found after directional etches originating from ion reflection from sidewalls and
is also observed in Ar+ milled TFLN. The profile is consistent with the directional nature
11

of the process.
To more quantitatively characterize the surface topology of the TFLN sidewalls, we computed the surface power spectral density (PSD) from the AFM scans. Figure 4(D) shows
the PSD curves before and after ALE on TFLN. Using AFM scans, the initial RMS sidewall
roughness is measured to be 1.75 ± 0.80 nm and an average roughness of 1.25 ± 0.60 nm.
After applying 50 cycles, Rq and Ra are measured to be 3.1 ± 0.55 nm and 2.61 ± 0.56
nm, respectively. The PSD is observed to increase over all spatial frequencies, particularly
at low frequencies ≲ 0.5 nm-1 . The enhancement at low frequencies indicates that longer
wavelength features are accentuated by the process compared to those of shorter wavelength.
Despite this increase in roughness after ALE, the sample roughness can be decreased
below its initial value by an RCA clean. The RCA clean consisted of a 5:1:1 ratio of
water:ammonium hydroxide:hydrogen peroxide solution at 80 ◦ C for 10 minutes. After an
RCA clean following ALE of the TFLN sample, the measured Rq and Ra decrease to 0.86 ±
0.50 nm and 0.59 ± 0.41 nm, respectively. Figure 4(D) shows that after the RCA clean,
the roughness at nearly all spatial frequencies is reduced below the original values. These
observations suggest that the roughness induced by the ALE process is from redeposition of
LiF and MgF2 .

IV

Discussion

Our ALE process may find applications in improving the photonic performance of TFLN
devices by reducing optical loss associated with corrugations in PPLN and sidewall roughness. A supercycle, consisting of ALE followed by the wet process, could facilitate even
larger decreases in surface roughness than have been reported here. However, to smooth the
corrugations, we note that a different wet chemistry will be needed to remove the lithium
and magnesium redeposition after ALE to avoid reforming the corrugations in PPLN.
Future topics of interest include investigating the mechanism for etch selectivity of the
hydrogen-exposed surface over the unmodified surface and identifying approaches to reduce
the quantity of redeposited Li and Mg compounds. For removal of redeposited compounds
by post-treatment, wet processes which selectively remove the redeposited products without
etching LN and thereby reintroducing corrugations in PPLN are of high importance. Development of an in-situ gas-based removal process or a process based on thermal cycling may
enable redeposition-free ALE without the need for a subsequent wet clean. For thermally
12

cyclic processing, further investigation of chemistries which produce more volatile products,
such as those based on Cl and Br, are important topics for further study. If the redeposition
can be decreased sufficiently, ALE could be employed for the entire pattern transfer process,
which would yield precise and uniform control of etch depth over the entire chip with precision of around the EPC (∼1 nm). This degree of control would permit scaling of TFLN
devices and circuits to the system level. The ALE system in our work (Oxford Instruments,
Plasma Pro 100 Cobra) is able to process 200 mm diameter substrates, and therefore our
process has the potential to extend to wafer-scale applications.

V

Conclusion

We have reported an ALE process consisting of sequential exposures of hydrogen and SF6 /Ar
plasma for x-cut MgO-doped lithium niobate that is compatible with low-pressure ICP RIE
systems. We observe etch rates up to 10.1 ± 0.5 Å/cycle with a synergy of up to 94%. Both
half-steps of the recipe exhibited saturation with respect to exposure time. Despite the occurrence of fluoride and magnesium compound redeposition leading to increased roughness
after ALE, the sample roughness can be reduced below its original value by a post-process
RCA clean. We have also found that the use of O2 /SF6 or Cl2 /BCl3 plasmas in place of
SF6 /Ar plasma also yield ALE with synergies exceeding 90%. With further development to
reduce redeposited nonvolatile products, the ALE process has potential to smooth sidewall
roughness and corrugations in PPLN and thereby enable improved TFLN device performance.

VI

Acknowledgements

This work was supported by the NSF under Award #2234390. This research was primarily carried out at the Microdevices Laboratory (MDL) Jet Propulsion Laboratory (JPL),
California Institute of Technology, and in part, carried out at the California Institute of
Technology. We gratefully acknowledge the critical support and infrastructure provided for
this work by The Kavli Nanoscience Institute and the Molecular Materials Research Center
of the Beckman Institute at the California Institute of Technology.
13

VII

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</reference>

<statements>
1. Material damage in lithium niobate (LN) after plasma etching can be mitigated mainly by changing the etch chemistry and ion energy to reduce LiF formation and lattice disorder, combining the etch with well‑chosen wet cleans, and using pre‑/post‑treatments such as proton exchange, doping, annealing, and polishing to restore the surface and near‑surface region.
2. Fluorocarbon plasmas (CF₄, CHF₃, C₄F₈) generate LiF, which is involatile and accumulates on the surface and sidewalls, lowering etch rate and roughening profiles, so simply running “harder” often increases damage rather than depth. Ar‑based ICP‑RIE processes avoid LiF formation by relying on physical sputtering; with optimized pressure, low bias, and a post‑wet cleaning step they can produce smooth, vertical sidewalls while minimizing chemical damage. Directional atomic layer etching (ALE) using HBr/BCl₃ for modification followed by a low‑power Ar plasma for removal improves volatility of etch products relative to F‑ and Cl‑based plasmas, reducing surface roughening and premature etch termination and thus limiting damage per cycle.
3. LiF formation during CHF₃ and other F‑based etches has been directly observed as dense nanoscale precipitates that increase peak‑to‑peak roughness by over an order of magnitude compared to the polished starting surface. One successful strategy is cyclic processing: ICP etch in C₄F₈/He or similar chemistry for a few minutes, pause, then use an SC‑1 clean (H₂O/H₂O₂/NH₄OH) or HF to remove LiF and redeposited LN before resuming etching, which yields deeper etches with improved sidewall profiles. Post‑etch RCA cleans (e.g., 5:1:1 H₂O:NH₄OH:H₂O₂ at ~80 °C) after ALE or continuous plasma etching can actually reduce RMS and Ra roughness below the pre‑etch values by removing redeposited compounds and smoothing high‑frequency features.
4. MgO‑doped LN is widely used to suppress photorefractive damage, and ALE/dry‑etch processes have been demonstrated directly on MgO‑doped thin films. In MgO‑doped LN subjected to Br‑based ALE followed by RCA clean, the roughness after processing can be reduced to values below the original polished surface, indicating that a combination of suitable chemistry, low‑damage etch cycles, and post‑cleaning can both pattern and “heal” the near‑surface region. Adjusting stoichiometry and doping to reduce defect formation and photorefractive sensitivity therefore complements process‑level mitigation for LN‑based nonlinear photonics.
</statements>

Begin the assessment now. Output only the JSON list, without any conversational text or explanations.