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

Ivy I. Chen

arXiv:2310.10592v3 [cond-mat.mes-hall] 5 Oct 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, California 91125, USA
3

Oxford Instruments Plasma Technology,

North End, Bristol BS49 4AP, United Kingdom
4

Jet Propulsion Laboratory, California Institute

of Technology, Pasadena, California 91109, USA
(Dated: October 8, 2024)

1

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 propagation losses arising from surface roughness and corrugations. 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 an isotropic ALE process for
x-cut MgO-doped LN using sequential exposures of H2 and SF6 /Ar plasmas. We observe an etch
rate of 1.59 ± 0.02 nm/cycle with a synergy of 96.9%. 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 of
99.5% and 91.5% respectively. The process is found to decrease the sidewall surface roughness of
TFLN waveguides etched by physical Ar+ milling by 30% without additional wet processing. Our
ALE process could be used to smooth sidewall surfaces of TFLN waveguides as a post-processing
treatment, thereby increasing the performance of TFLN nanophotonic devices and enabling new
integrated photonic device capabilities.

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 for 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
other materials like silicon nitride. [11] By incorporating > 5% molar concentration MgO
into the melt during the Czochralski crystal growth process, the optical damage threshold
is raised, allowing for high-intensity photonic applications [12].
∗

aminnich@caltech.edu

2

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 on 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 squeezed quantum states on-chip,
[22] >100 GHz electro-optic modulators with CMOS compatible voltages, [23] broadband
frequency comb sources, [24–26] and on-chip ultra-fast lasers. [27, 28]
A necessary step in LN device fabrication is pattern transfer, typically using a dry etching
process. 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 non-volatile Li compounds such as LiF, leading to an increase in sidewall
roughness and scattering loss, which is the dominant loss mechanism. [11, 29] Protonexchanged 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 after Ar+ milling is typically removed
using an RCA clean. However, the wet process also introduces corrugations in periodicallypoled 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
3

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 an 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-bylayer 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 isotropic ALE process for MgO-doped x-cut LN. Using sequential
exposures of H2 and SF6 /Ar plasmas, we measure an etch per cycle (EPC) of 1.59 ± 0.02
Å/cycle with a synergy of 96.9%. We observe the saturation of both half-steps of the process.
While surface roughness is observed to increase on flat surfaces, a 30% reduction in surface
roughness on waveguide sidewalls is observed after 50 cycles of ALE. 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 2.24 ± 0.02 and 1.65 ± 0.03 and synergies of 99.5% and 91.5%, respectively.
The process could be used as a post-processing step after Ar+ milling to smoothen sidewall
roughness and corrugations in periodically-poled TFLN devices and thereby enhance their
photonic performance.
4

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 configured for
ALE. 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 motivated 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 40-second H2 plasma exposure (300 W ICP power,
52.5 W RIE power, 209 V DC bias, 50 sccm H2 ) followed by a 40-second SF6 /Ar exposure
(300 W ICP power, 3.5 W RIE power, 50 V DC bias, 17 sccm SF6 , 35 sccm Ar). The effect
of EPC on RF bias power was not studied. 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 was cooled to 0 ◦ C using liquid nitrogen as measured by the table thermometer.
To measure saturation curves, the chamber pressure and ICP power were kept constant
at 10 mTorr and 300 W respectively, while the exposure time for each half-step was varied.
H2 plasma exposure time was varied from 0 to 50 seconds with SF6 /Ar plasma held at 30
seconds, and SF6 /Ar plasma exposure time was varied from 0 to 50 seconds with H2 plasma
exposure held at 30 seconds. Prior to introducing the sample into the chamber for etching,
the etching chamber was cleaned with a blank Si wafer and 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. After the sample was loaded into the chamber, a 3-minute wait time
was used before processing to allow the sample to thermally 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 to ensure complete
removal of the resist, 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
5

MLA 150 Maskless Aligner with a dose of 150 mJ/cm2 , followed by development with AZ
300 MIF 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.
AFM scans were performed on a Bruker Dimension Icon atomic force microscope (AFM) to
measure total etch depth and surface roughness. The total etch depth was measured using
2.5 × 10 µm2 AFM scan with the scan rate set to 0.5 Hz. The step profile was averaged
over the entire scan using Nanoscope Analysis 1.9 software to obtain the etch depth. RMS
surface roughness of a reference TFLN Ar+ milled waveguide sample and power spectral
density (PSD) scans were obtained 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 quadratic 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 1.69 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
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
40-second H2 plasma exposure of the same parameters as the SF6 /Ar recipe followed by
a 40-second O2 /SF6 exposure (300 W ICP power, 3.5 W RIE power, 39 V DC bias, 35
sccm O2 , 15 sccm SF6 ). The second alternate recipe uses the same 40-second H2 plasma
exposure followed by a 40-second Cl2 /BCl3 exposure (300 W ICP power, 5 W RIE power,
73 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] Whether the O2 /SF6
and Cl2 /BCl3 processes were at saturation was not determined. Etch depth measurements
and 500 × 500 nm2 surface roughness scans over 20 cycles from these alternate processes
were compared with 20 cycles of the original ALE recipe consisting of a 40-second H2 plasma
exposure followed by a 40-second SF6 /Ar exposure.
6

Figure 1. (a-d) ALE process for MgO-doped LN. A hydrogen plasma exposure (pink)
leads to a hydrogen-rich 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
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.

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 0.06 nm/cycle is observed for the SF6 /Ar plasma half
step. For the H2 plasma step, a thickness increase was observed, which might be attributed
to a volume expansion due to amorphization of the crystal during the H2 plasma 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 1.59±0.02 nm/cycle is
observed.
To gain more insight into the process and verify its self-limiting nature, we measured
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 plasma exposure time is varied from 0 to 50 seconds.
Saturation occurs at 1.46 ± 0.04 nm/cycle above 30 seconds H2 plasma exposure time. In
Figure 2(C), the H2 plasma exposure time is held constant at 30 seconds while the SF6 /Ar
plasma exposure time is varied from 0 to 50 seconds. The etch rate exhibits a soft saturation,
as the etch rate continues to increase with increasing exposure time. For SF6 /Ar exposure
7

Figure 2. (a) EPC versus cycle number with 40 second H2 plasma exposures only (blue
triangles), 40 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 plasma exposure time with SF6 /Ar plasma exposure time fixed at 30 s. (c)
EPC versus SF6 /Ar plasma exposure time with H2 plasma exposure time fixed at 30 s.
The etch rates are observed to saturate with exposure time, demonstrating the self-limiting
nature of the process.

times longer than 30 seconds, the etch rate continues to increase at a rate of 0.1 nm/cycle
per 10 seconds of additional SF6 /Ar plasma exposure, indicating that the half step exhibits
soft saturation. Soft saturation with SF6 /Ar plasma has been reported previously and
was attributed to the diffusion-limited fluorination of the surface [68]. In the present case,
soft saturation is hypothesized to occur due to the presence of a concentration gradient of
hydrogen into the LN film after H2 plasma exposure. By increasing the SF6 /Ar plasma
exposure time, more of the hydrogenated surface is removed, resulting in a soft-saturating
curve. At 50 seconds SF6 plasma exposure time, the etch rate is 1.59 ± 0.02 nm/cycle. The
observation of saturation for both half-steps indicates that the process is indeed atomic layer
8

Table I. Atomic concentrations for the fitted XPS data.
Sample

Nb (%)

O (%)

Li (%)

F (%)

Mg(%)

Untreated

18.92 ± 0.22

64.17 ± 0.76

13.68 ± 1.17

1.32 ± 0.56

1.91 ± 0.18

1 half cycle H2 plasma

14.69 ± 0.26

54.14 ± 0.97

21.17 ± 2.15

7.29 ± 0.21

2.72 ± 0.44

10 cycles ALE

16.40 ± 0.22

55.64 ± 0.76

18.79 ± 1.62

6.38 ± 0.19

2.79 ± 0.32

50 cycles ALE

18.04 ± 0.17

56.83 ± 0.55

15.17 ± 1.15

6.56 ± 0.13

3.40 ± 0.19

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 C) × 100, where α and β are the etch rate of the H2 plasma and SF6 /Ar
half-cycles, respectively; and EP C is the etch rate of the full cycle. For the present process
in which a thickness increase is observed after H2 plasma exposure, we take a conservative
approach and calculated the synergy assuming zero EPC for that step. We obtain a synergy
value of 96.9% for the nominal recipe. This synergy value is comparable 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 2.24 ± 0.01 nm/cycle
over 20 cycles. The half-step EPCs for the H2 and O2 /SF6 plasma step are -0.04 and 0.01
nm/cycle, respectively. The synergy for this process is 99.5%, with the H2 plasma half step
assumed to be 0 EPC for purposes of calculation as previously noted. The Cl2 /BCl3 ALE
process yielded an EPC of 1.65±0.03 nm/cycle over 20 cycles; the half-step EPCs for the H2
and Cl2 /BCl3 plasma step are -0.04 and 0.14 nm/cycle, respectively, and the synergy for this
process is 91.5%. While the reaction mechanisms of the three processes were not studied in
this work, the possible reactions are hypothesized to be fluorine or chlorine radicals forming
volatile compounds such as NbF5 , NbOF3 , OF2 , NbOClx , and BOClx as occurs in RIE of
LN [30, 69].
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, [70] complicating the interpretation
9

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 a 40 second H2 plasma exposure followed by a 40
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 normalized by carbon atomic concentration from XPS spectra for
each sample for untreated bulk LN, 1 cycle H2 plasma exposure, 10 ALE cycles, and 50
ALE cycles.

of the measurements. The C1s peak at 284.8 eV is used as a reference. Binding energy
values are reported in Table I. 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 (207.7 eV and 210.5 eV). [71–73] In Figure 3(B), we report the O1s spectra with two
subpeaks at 530.7 and 532.4 eV, corresponding to metal oxide and O-C bonds, respectively.
[74] In Figure 3(C), we report the F1s spectra with two subpeaks at 685.5 eV and 687.2
eV corresponding to LiF and F-C bonds, respectively. [73, 74] In Figure 3(D), we report
the Nb4s, Li1s, and Mg2p spectra at 61.0 eV, 55.7 eV, and 50.7 eV, respectively (values are
for bulk LN). [73, 74] The Li1s peak energy agrees well with reported binding energies for
LiF (55.7 ± 0.5 eV). [72, 74] After ALE, we observe a 0.3 eV shift for the Nb4s and Li1s
10

Table II. Comparison of metrics from LN ALE recipes with different removal step
plasma exposures on bulk LN. The bulk LN samples used have an initial surface roughness
of Rq = 0.2 nm. Values are from a 40 second H2 plasma exposure followed by a 40 second
plasma exposure indicated in the table, over 20 cycles. Whether the O2 /SF6 and Cl2 /BCl3
processes were at saturation was not determined.
Plasma Type EPC (nm/cycle) RMS Roughness (nm) Average Roughness (nm) Synergy (%)
SF6 /Ar

1.59 ± 0.02

0.57 ± 0.18

0.44 ± 0.11

96.9

O2 /SF6

2.24 ± 0.02

1.47 ± 0.52

0.99 ± 0.40

99.5

Cl2 /BCl3

1.65 ± 0.03

0.90 ± 0.46

0.59 ± 0.20

91.5

peaks, and a 0.6 eV shift for the Mg2p peaks towards higher binding energies, as expected if
fluoride bond formation occurred. [74] 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, and F obtained from
the XPS data at various stages of the process. The atomic concentrations are normalized by
the estimated carbon content for each sample, which is about 55% and is due to presence
of the conductive carbon coating. The uncertainties for all atomic concentration numbers
(including C) were estimated from the CasaXPS software using a Monte Carlo routine, and
the carbon-normalized uncertainties are propagated by adding uncertainties in quadrature.
Surface lithium content is observed to increase after 1 H2 plasma half-cycle. The presence of
fluorine is likely from residual fluorine on the chamber walls after the chamber clean. This
trend differs from that reported in previous studies in which surface lithium concentration
was found to decrease after a high power (500-2000 W), high temperature (170 °C), hour-long
H2 plasma exposure, forming proton-exchanged LN (c.f. Fig. 4 in Ref. [18]). These plasma
conditions in Ref. [18] are more similar in terms of substrate temperature and exposure
time to acid-based proton exchange, possibly accounting for the difference. After ALE, the
fraction of F, Li, and Mg increases compared to those of the untreated sample. Considering
as well the peak energy shifts mentioned earlier, we hypothesize that LiF and MgF2 are
formed and redeposited on the surface owing to their low volatility.
We characterized the effect of all three ALE recipes on surface roughness of bulk LN
samples. AFM scans over 500 × 500 nm2 were obtained on bulk LN subjected to 20 cycles
11

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) Averaged AFM line scans of the
TFLN waveguide side profile before and after 50 cycles ALE. The waveguide width
decreases by 50 nm on each side, yielding a lateral etch rate of 1 nm/cycle, which is
comparable to the vertical etch rate of 1.59 nm/cycle measured on bulk LN. (d)
Height-map PSD of the samples before ALE and after 50 cycles of ALE.

of ALE. Table II compares the EPC, surface roughness, and synergy of the three different
recipes. For reference, the bulk LN samples have an initial surface roughness of Rq = 0.2 nm.
The O2 /SF6 process yielded the roughest surface but the highest synergy, and the Cl2 /BCl3
process yielded a similar etch rate to the SF6 /Ar process with the lowest synergy of the
three recipes. It is hypothesized that the Cl2 /BCl3 process may be modified to have higher
synergy by lowering the RIE power on the Cl2 /BCl3 plasma half step. In comparison, the
SF6 /Ar process at saturation produced the smoothest surface after 20 cycles of ALE with
12

Rq of 0.57 ± 0.18 nm. The increase in surface roughness may be attributed to redeposition
of LiF and MgF2 .
Since sidewall roughness of TFLN waveguides are rougher than the surface of bulk LN
samples, 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 RCA clean, corresponding to the state-of-art process for TFLN device fabrication.
[11, 34] Figures 4(A) and 4(B) show the quadratic plane-fit height map of Ar+ etched sidewall
before and after 50 cycles of ALE, respectively. After ALE, the sidewall surface is visually
smoother. The sidewall surface smoothing may be attributed to the isotropic nature of the
etch.
To support this hypothesis, we measured the lateral etch rate of the waveguide using
AFM. Figure 4(C) shows an AFM profile averaged over the whole scanned image of the
TFLN waveguide sidewall before and after 50 cycles of ALE. From the decrease in width,
we infer the lateral etch rate to be 1 nm/cycle on each side of the waveguide compared to a
vertical etch rate of 1.59 nm/cycle previously measured on bulk LN, confirming the largely
isotropic nature 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 0.82 ± 0.25 nm and an average roughness of 0.65 ± 0.16 nm.
After 50 cycles, Rq and Ra are measured as 0.55 ± 0.13 nm and 0.44 ± 0.12 nm, respectively.
The PSD is observed to decrease over all measured spatial frequencies. Therefore, despite
the roughening observed on flat LN surfaces, sidewall smoothing is still observed owing to
the isotropic nature of the process.

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.
The etch rate of a typical RCA wet etch exhibits uncontrolled variability due to temperature and concentration fluctuations in solution. The reported ALE process has potential to
overcome these issues due to the self-limiting nature of the process with well-controlled etch
13

rates.
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. A post-ALE wet clean may be beneficial
to remove the redeposited compounds selectively, in contrast to the present approach using
an RCA wet etch which etches lithium niobate. Development of an in-situ gas-based removal
process or a process based on thermal cycling may enable redeposition-free ALE. For thermally cyclic processing, investigation of chemistries which produce more volatile products,
such as those based on Br, is of interest for further study. Directional ALE processes with
high anisotropy are also of interest as they could be employed for pattern transfer, yielding
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 150 mm diameter substrates, and therefore our process has the
potential to extend to wafer-scale applications.

V

Conclusion

We have reported an isotropic ALE process for x-cut MgO-doped lithium niobate consisting
of sequential exposures of hydrogen plasma and SF6 /Ar plasma that is compatible with
low-pressure ICP RIE systems. We observe an etch rate of 1.59 ± 0.02 Å/cycle with a synergy exceeding 96%. Both half-steps exhibited saturation with respect to exposure time,
though the SF6 plasma half step was observed to soft-saturate. The substitution of O2 /SF6
or Cl2 /BCl3 plasmas in place of SF6 /Ar plasma was also found to yield ALE with synergies
exceeding 90%. Finally, the process was found to smoothen the sidewalls of TFLN waveguides fabricated using the state-of-art process, suggesting the potential of ALE to enhance
the photonic performance of TFLN devices.

VI

Acknowledgements

This work was supported by Oxford Instruments and the NSF under Award #2234390. This
research was carried out, in part, at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under contract with the National Aeronautics and Space Administration
(NASA). We gratefully acknowledge the critical support and infrastructure provided for this
14

work by The Kavli Nanoscience Institute and the Molecular Materials Research Center of
the Beckman Institute at the California Institute of Technology.

VII
A.

Author Declarations
Conflict of Interest

I.I.C., J.S., R.S., A.M., F.G., and A.J.M. have submitted a United States provisional
patent on this technology.

VIII

Data Availability

The data that support the findings of this study are available from the corresponding author
upon reasonable request.

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19
</reference>

<statements>
1. ALE (atomic layer etching): H2 + SF6/Ar sequential exposures, 1.59 nm/cycle, [arXiv] 96.9% synergy;
2. 96.9% synergy; [arXiv] smooths sidewalls but redeposits LiF/MgF2 needing a wet clean; Br-based directional ALE is emerging (J. Vac. Sci. Technol. A 44, 022606, 2026).
</statements>

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