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High-Quality Dry Etching of LiNbO3 Assisted by Proton Substitution through H2-Plasma Surface Treatment - PMC

The exceptional material properties of Lithium Niobate (LiNbO3) make it an excellent material platform for a wide range of RF, MEMS, phononic and photonic applications; however, nano-micro scale device concepts require high fidelity processing of LN ...

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Nanomaterials (Basel)
. 2022 Aug 18;12(16):2836. doi:
10.3390/nano12162836

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High-Quality Dry Etching of LiNbO
3
Assisted by Proton Substitution through H
2
-Plasma Surface Treatment

Arjun Aryal

Arjun Aryal

1
Center for High Technology Materials (CHTM), University of New Mexico, MSC01 04-2710, 1313 Godard St. SE, Albuquerque, NM 87106-4343, USA

Find articles by
Arjun Aryal

1
,
Isaac Stricklin

Isaac Stricklin

1
Center for High Technology Materials (CHTM), University of New Mexico, MSC01 04-2710, 1313 Godard St. SE, Albuquerque, NM 87106-4343, USA

2
Electrical and Computer Engineering (ECE), University of New Mexico, MSC01 11001, Albuquerque, NM 87131-0001, USA

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Isaac Stricklin

1,
2
,
Mahmoud Behzadirad

Mahmoud Behzadirad

1
Center for High Technology Materials (CHTM), University of New Mexico, MSC01 04-2710, 1313 Godard St. SE, Albuquerque, NM 87106-4343, USA

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Mahmoud Behzadirad

1
,
Darren W Branch

Darren W Branch

3
Sandia National Laboratories, 1515 Eubank Blvd SE, Albuquerque, NM 87123, USA

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Darren W Branch

3
,
Aleem Siddiqui

Aleem Siddiqui

3
Sandia National Laboratories, 1515 Eubank Blvd SE, Albuquerque, NM 87123, USA

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Aleem Siddiqui

3
,
Tito Busani

Tito Busani

1
Center for High Technology Materials (CHTM), University of New Mexico, MSC01 04-2710, 1313 Godard St. SE, Albuquerque, NM 87106-4343, USA

2
Electrical and Computer Engineering (ECE), University of New Mexico, MSC01 11001, Albuquerque, NM 87131-0001, USA

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Tito Busani

1,
2,
*

Editor:
Tzi-yi Wu

Author information

Article notes

Copyright and License information

1
Center for High Technology Materials (CHTM), University of New Mexico, MSC01 04-2710, 1313 Godard St. SE, Albuquerque, NM 87106-4343, USA

2
Electrical and Computer Engineering (ECE), University of New Mexico, MSC01 11001, Albuquerque, NM 87131-0001, USA

3
Sandia National Laboratories, 1515 Eubank Blvd SE, Albuquerque, NM 87123, USA

*
Correspondence:
busanit@unm.edu

Roles

Tzi-yi Wu
:
Academic Editor

Received 2022 Jul 12; Accepted 2022 Aug 11; Collection date 2022 Aug.

© 2022 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (
https://creativecommons.org/licenses/by/4.0/
).

PMC Copyright notice

PMCID: PMC9415737 PMID:
36014702

Abstract

The exceptional material properties of Lithium Niobate (LiNbO
3
) make it an excellent material platform for a wide range of RF, MEMS, phononic and photonic applications; however, nano-micro scale device concepts require high fidelity processing of LN films. Here, we reported a highly optimized processing methodology that achieves a deep etch with nearly vertical and smooth sidewalls. We demonstrated that Ti/Al/Cr stack works perfectly as a hard mask material during long plasma dry etching, where periodically pausing the etching and chemical cleaning between cycles were leveraged to avoid thermal effects and byproduct redeposition. To improve mask quality on X- and Y-cut substrates, a H
2
-plasma treatment was implemented to relieve surface tension by modifying the top surface atoms. Structures with etch depths as deep as 3.4 µm were obtained in our process across a range of crystallographic orientations with a smooth sidewall and perfect verticality on several crystallographic facets.

Keywords:
thin films, Lithium Niobate, microstructures, fabrication, plasma etching, silicon integration
1. Introduction

Lithium Niobate (LiNbO
3
or LN) has proven to be the material of choice for a wide range of applications due to its exceptional piezoelectric, electro-acoustical, electro-optical, and non-linear optical properties [
1
]. The different crystallographic orientations of LN are heavily utilized for applications in surface-acoustic-wave (SAW) resonators [
2
], optical filters [
3
], optical sensors [
4
], modulators [
5
,
6
,
7
], transducers [
8
,
9
], optical waveguides [
10
,
11
], Q-switch lasers [
12
,
13
], oscillators [
14
], etc. For example, high temperature (1000 °C) annealing Ti-doped X cut LN can be utilized as an optical waveguide [
15
]. At the nano- and micro-scale, device performance is often constrained by the fabrication quality of processed LN films [
16
]. However, unlike many semiconductors and dielectric materials, LN substrates are complex for processing and are notoriously difficult to etch [
16
]. Typically, a long plasma dry-etch is required to obtain high-aspect ratio or deep etching profiles [
17
] in LN substrates, and, hence, a successful LN etching process needs to manage factors including substrate heating, redeposition of the etching byproducts and mask materials, and durability of the mask materials over the etching process.

Dry etching of LN substrates was studied by different research groups [
18
,
19
,
20
,
21
,
22
,
23
,
24
,
25
,
26
] and different plasma etch conditions have been investigated to optimize the quality of the resulting structures. Fluorine (F
2
) based plasma sources have most commonly been used to etch LN [
24
]; however, the use of this source is accompanied by the redeposition of LiF and its byproducts, which reduce the etching rate and create unfavorable features due to local micro-masking effects. The byproducts from fluorine-based dry-etch evaporate at only 800 °C, thus remaining on the surfaces and the sidewalls of the structures. This not only reduces the etch rate of the target material, but also prevents obtaining vertical etching profiles [
27
]. Additionally, typical mask material delaminates or are otherwise consumed during the etching process, resulting in roughness on the etched structure, which can degrade performance (e.g., increased optical losses due to surface roughness). Adding Argon (Ar) gas to the chemistry of the plasma etching has been reported to enhance the physical sputtering process and obtain a more uniform etching profile [
17
], thus mitigating a portion of these challenges during the dry etch.

To overcome the problem of redeposition of product compounds, the lithium concentration on the top surface needs to be minimized. This can be achieved by employing a Proton Exchange (PE) process, where some of the lithium ions are replaced by protons in the LN crystal structure [
28
]. It has been shown that the PE process will significantly reduce the amount of LiF redeposition, and therefore increase the etching rate and improve the etching profiles as well [
29
]. Ti in diffused waveguides has been fabricated on Z cut LN. The proton exchange process was applied by immersing the substrate in benzoic acid at T = 240 °C for 3.5 h [
30
]. However, the PE process is expensive, not available for all processes and it is time consuming. Therefore, an alternative is needed to replace PE process with a cost-effective and a more available method.

Here, we report our recent progress in etching different crystalline LN substrates using Inductively Coupled Plasma (ICP). By comparing different combinations of the mask materials on different LN orientations (cuts), we present optimal dry etch conditions to obtain deep etching profiles on all orientations with smooth and vertical sidewalls. LN thin films are commonly used in phononic and photonic devices such as Surface Acoustic Waves (SAW), resonators, transducers, optical waveguides, optical filters, etc. To ensure the propagation of the acoustic and optical waves with minimum losses, a smooth and vertical etching profile is required. The scalloping, lateral etching and surface roughness can then severely affect the performance of the device [
31
,
32
]. Indeed, the Q-factor is directly impacted by the roughness [
33
,
34
,
35
].

As described by the Ruze’s formula in Equation (1) of the [
35
], the loss due to scattering can be described as,
Δ
G
=
−
685.81

(
ϵ
λ
)
2
(
dB
)
. In other words,
Q

~

π
ϵ
λ
, where
ϵ
is the surface roughness and
λ
is the wavelength. Roughness at the hard edges of acoustic (or optical) devices can limit device Q-factors [
32
] and overall device performance. This effect is further aggravated as the frequency is scaled and can be a bottleneck for device performance. We also demonstrate that employing H
2
-plasma treatment prior to the processing step, as an alternative for the PE process, overcomes delamination of the metal mask on X- and Y-cuts LN samples and improves the quality of the etching profile.
2. Experiments and Results

A standard cleaning procedure: such as piranha clean (H2S04 and H202) followed by Acetone, IPA, DI water, and N
2
dry air blow was carried out prior to the lithography process [
36
]. Then, a negative photoresist (AZnLOF 2035) was applied to pattern the LN samples with different cuts through a contact lithography process. We utilized single crystal LN substrates with common orientations of 128YX, 128Y, 64Y, and thin film LN on Si substrates with the orientations of X, and Y. These different types of LN wafers were supplied from NANOLN vendor. After patterning the samples, they were cleaned in an acid solution (HCl:H
2
O, 1:3) and metallized in a metal evaporator. The metals mask deposition was carried out around 1 ×
10
−
6
torr without heating the substrate (nominal room temperature < 60 °C).

The whole fabrication procedure is schematically shown in
Figure 1
a. A hard mask consisting of Titanium (Ti), Aluminum (Al), and Chromium (Cr) was deposited. The process was followed by lift-off using 1165 (n-methyl pyrrolidinone) solution. Then, the samples were etched in a F
2
-based in Inductively Coupled Plasma (ICP). After etching, metal masks were removed using a diluted HF solution. Scanning Electron Microscopy (SEM) was applied to inspect and analyze the samples through the processing steps.
Figure 1
b,c show top and side view SEM image of the patterned photoresist after the lithography step, respectively. As seen, a perfect vertical sidewall was achieved to ensure formation of a high-quality metal mask after metallization process.

Figure 1.

Open in a new tab

(
a
) Schematic of the fabrication process to make pattern on LN samples. (
b
) Top view SEM image of a sample after lithography. (
c
) Side view SEM image of patterned photoresist after lithography.
Identifying the right metal material for the hard mask for the etching purpose is a crucial step for metal depositions. Some dry etch challenges, such as redeposition of the mask material and the reaction products, can be avoided using the right metal for the processing. These challenges are more severe if the process needs to be designed to achieve deep structures on LN substrates, as are desired for optical and photonic device applications.

To find an optimum metal mask for a deep dry etch on LN samples, we tried different metal combinations; Ti/Al (20 nm/1.3 µm), Ti/Al/Cr (20 nm/750 nm/750 nm).
Figure 2
compares the quality of the metallization on different LN cuts. As seen in
Figure 2
, the Ti/Al stack works perfectly on all types of LN cuts, while Ti/Al/Cr did not work perfectly on X and Y cut LN on the Si substrate, as shown in
Figure 3
; peeling off of the metal film was observed at the edges of the microstructures. In order to explain this observation, we hypothesize that both crystal orientation and the stress due to the transferring LN film on Si substrate play an important role in atomic bonding between metals and top surface of LN. Therefore, a surface treatment seems necessary to improve the quality of the Ti/Al/Cr metal mask on X- and Y-cuts.

Figure 2.

Open in a new tab

Comparison of Ti/Al mask quality on different LN cuts.
Figure 3.

Open in a new tab

Comparison of Ti/Al/Cr mask quality on different LN cuts. Inset shows peeling off the metal mask X- and Y-cuts at the edge of the structures.
A similar treatment was carried out by [
24
] to engineer crystal properties of the LN using proton implantation. Alternatively, the PE process can also be employed with molten benzoic acid. Quenching effects were studied on H
x
Li
1−
x
NbO
3
layers on Z cut LN waveguides. The hydrogen-doped
x
∼ 0.47 in (001) LiNbO
3
layers were created by proton exchange reaction in melt benzoic acid [
26
]. Here, we employed H
2
-Plasma (H2: 30 sccm, pressure: 15 mTorr, RF power: 150 W, ICP power: 300 W, DC bias: 53 V) in an inductively coupled plasma (ICP) chamber prior to the lithography and metallization steps in order to imitate the effect of proton treatment on the LN substrate.
Figure 4
shows the metal mask quality on X- and Y-cuts after H
2
-plasma treatment. As seen, a remarkable improvement is obtained using plasma treatment.

Figure 4.

Open in a new tab

Metal mask quality on (
a
) X-cut and (
b
) Y-cut LN after employing H2-plasma treatment.
After obtaining a high-quality metal mask on LN cuts, we proceeded to plasma dry etch using F
2
-based chemistry ICP. Previous research on etching LN revealed that formed Niobium Fluoride (NbF
5
) becomes volatile at temperatures around 200 °C, while the LiF byproducts will remain on the sidewalls and the surfaces that decreases the etching rate of the LN surfaces [
25
]. In our experiment, we used CHF
3
/Ar plasma treatment (CHF
3
: 30 sccm, Ar: 30 sccm, pressure: 5 mTorr, RF power: 100 W, ICP power: 150 W, DC bias: 40 V) to achieve high selectivity and higher etch rate with a Cr mask. The Argon gas increased the etching rate by enhancing the sputtering process during plasma etching [
24
]. To prevent heating effect on etching process (due to low thermal conductivity of LN), we implemented a periodic step process; 20 min etching followed by 4 min cooling (plasma is off). Then, after each 1 h of etching, the samples were cleaned in standard cleaning solvent (Acetone, IPA, DI water, and N
2
dry air blow)
Table 1
provides information on etching rates of different orientations of LN.

Table 1.

Etching rates of different cuts of LN using the CHF
3
/Ar plasma.

Orientation

Etching Depth (μm)

Etch Rate of LN (μm/h)

128YX (Bulk LN)

3.40

0.70

128Y (Bulk LN)

2.95

0.65

64Y (Bulk LN)

1.70

0.55

X with H+ treatment (Thin Film)

2.00

0.70

Y with H+ treatment (Thin Film)

2.00

0.75

Open in a new tab
In
Figure 5
, we present the results using the described etching process for (a) the Ti/Al mask and (b) the Ti/Al/Cr mask. The presence of Al exposure to the plasma results in re-deposition of Al by-products, which are visible in
Figure 5
a as “pyramids” or “cones”. In order to prevent the re-deposition, we protected the Al with a layer of Cr. CrF
4
by-products are more volatile compared to AlF
3
[
37
,
38
], so that the formation of cones or pyramids is prevented. It is clear that the etching profile with Ti/Al/Cr metals worked as a better mask, since the surface roughness and redeposition underneath the sidewalls are prevented and, also, the verticality is nearly maintained, as illustrated in
Figure 5
b. Unfortunately, we were not able to achieve successful results using the Ti/Cr mask. Ti is necessary for ensuring adhesion between LN and the Al, while Al is needed to ensure adhesion with Cr. The thickness of the Al also seems to be a critical parameter, which is still under study. We think that Al, thanks to its electrical conductivity and thermal conductivity, prevents the accumulation of electrons during the etching process.

Figure 5.

Open in a new tab

128YX LN etching using CHF
3
/Ar and metal mask of (
a
) Ti/Al and (
b
) Ti/Al/Cr.
As observed in
Figure 6
, the right-angle verticality is nearly reached for the metal masks defined on a microstructure. There is only ~3° offset for the etched LN with the stacks of metal masks. The offset might have arisen from the lateral etching occurring during the plasma dry etching process. So far, our results are satisfying in terms of overall etching profile considering verticality (
Figure 6
) and redeposition free sidewalls. Indeed, other research groups tried to etch lithium niobate material for ridge and rib structures, using fluorine dry etching. This typically shows less verticality, between 60° and 78°, with a visibly rough side wall [
39
,
40
]. Unfortunately, the literature on LN tends to not report the quantitative values of the smoothness of the side walls. In
Table 2
, we present the roughness in terms of root mean square (rms) and arithmetic average, applying ImageJ software to our SEM images. For the samples presented in
Figure 6
b, the rms is ~10, while for the samples presented in
Figure 7
, the rms is an order of magnitude lower (~1.5). Based on image analysis and qualitative comparison with published SEM images of side walls, we can affirm that we achieved state-of-the-art smooth side walls and the most vertical profiles.

Figure 6.

Open in a new tab

Side wall features on LN (
a
) Ti/Al/Cr metal mask and (
b
) 128Y LN etched using CHF
3
/Ar gas.
Table 2.

Roughness Measurements (Arbitrary Units).

128YX

128Y

64Y

X- or Y Cut Thin LN Film on Si

R
q
(
Root mean square roughness
)

1.669

9.511

1.273

10.262

R
a
(
Arithmetic average roughness
)

1.274

7.496

1.044

9.254

Open in a new tab
Figure 7.

Open in a new tab

ICP Etching of a micro-structure using CHF
3
/Ar gases with Ti/Al/Cr metal masks.
Table 1
lists the etch rates and the obtained depths for all the cuts using CF
3
/Ar (1:1) plasma dry etch. The etching rate may vary depending on the crystal orientation that may include properties of local neighborhood atoms, and role of the dangling and surface bonds. As seen in
Table 1
, the etching rates are nearly the same, but 64Y and 128Y cuts present a slightly lowering etch rate compared to the other cuts. Our etching rate results correspond similarly to the findings (
Table 1
, Ref. [
20
]), with minimum backside cooling of 10 sscm He flow. The etch rate with CHF3/Ar chemistry for PE bulk LN was reported to be 650 nm/h, which is close to our etch rate for bulk LN.

Table 2
represents the roughness measurements for different crystallographic orientations on the etched structures with arbitrary units. The roughness values were calculated using the ImageJ version 1.53s, image processing program created by Wayne Rasband and other contributors in the National Institute of Health, USA. This is the Fiji implementation of ImageJ in combination with the SurfChaJ plugin [
41
], which roughly calculates the roughness of a surface based on an image provided. Comparing the results, 64Y bulk LN is etched with minimum surface roughness compared to other bulk LN and thin films on Si.
3. Conclusions

In summary, we have shown an optimum ICP dry etch process to obtain a deep etching profile in LN substrate with minimum roughness and vertical sidewalls. The combination of CHF
3
/Ar (1:1) gases was used to etch all LN substrates. The plasma etching characteristics of different orientations of LN samples were studied under various metal masks. The stack of Ti/Al/Cr metals was found to be the best mask material for all cuts, i.e., 128YX, 128Y, 64Y, X-, and Y-cuts, as it removed all the redeposition and micro-masking issues during the dry etch process. However, to achieve the same quality of metal mask, X- and Y-cuts samples were treated with H
2
-Plasma. It was assumed that the plasma treatment will mitigate surface stress on the transferred LN film due to the Si substrate and also modify the surface structures by replacing Li-ion with protons. Likewise, we included periodic interruption steps in our plasma dry etch method, followed by chemical cleaning between each cycle to avoid thermal effect and minimize byproduct redeposition during the long etching process. Microstructures with etch depths as deep as 3.4 µm were achieved using this method with a smooth sidewall and perfect verticality, which is promising for the fabrication of optical devices, where high-aspect ratio structures are required.
Acknowledgments

This work was performed, in part, at the Center for High Technology Materials (CHTM) and Center for Integrated Nanotechnologies (CINT), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
Author Contributions

A.A.: methodology, investigation and formal analysis, writing original draft., I.S.: SEM analysis, review and editing. M.B.; supervision, review and editing, D.W.B.: materials sponsor. A.S.: conceptualization; review and editing. T.B.: conceptualization, supervision, review and editing, project, funding acquisition. All authors have read and agreed to the published version of the manuscript.
Data Availability Statement

The data reported in this manuscript are available on request from the corresponding author.
Conflicts of Interest

The authors declare no conflict of interest.
Funding Statement

This research was funded by the Department of Energy, Sandia Laboratories Academic Alliance, grant number 22027.
Footnotes

Publisher’s Note:
MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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</reference>

<statements>
1. H2/proton substitution and hard masks address chemistry and mask-related damage
2. Fluorine or chlorine reactive-ion routes produce nonvolatile lithium-containing byproducts, especially LiF, which redeposit on sidewalls and cause micro-masking, rougher profiles, and scattering loss
3. Increasing ion energy or RF bias can clear redeposition but can also create trenches, pits, and jagged redeposition damage; LN's low thermal conductivity makes continuous etching prone to heating
4. Replacing near-surface Li with H reduces LiF formation; H2 plasma before lithography and metallization imitates proton exchange and improves mask quality, while Ar addition enhances physical sputtering and uniformity
5. Proton exchange itself reduces LiF redeposition but is costly and time-consuming, and as a waveguide route can degrade electro-optic and nonlinear coefficients
6. Durable metal hard masks, especially Ti/Al/Cr stacks, survive long deep etches and reduce redeposition and micro-masking
7. Conductive mask layers also reduce charging during e-beam lithography and LN etching, and Al may help dissipate charge and heat according to the authors’ hypothesis
8. Because LN conducts heat poorly, intermittent etch segments with cooldown and intermediate chemical cleaning are used to limit thermal effects and byproduct accumulation
9. The mask stack is not arbitrary: Ti/Cr alone failed, Al thickness remained under study, and Al can redeposit as cones or pyramids
10. For H2/proton substitution, proton-exchanged LN has lower LiF redeposition; H2 plasma before lithography/metalization improved mask quality on X- and Y-cuts
11. For H2/proton substitution, it is a pre-etch treatment; traditional proton exchange is costly/time-consuming and can degrade electro-optic/nonlinear coefficients in alternative device routes
12. For hard mask and cycling, Ti/Al/Cr masks enabled etch depths up to 3.4 µm with smooth, nearly vertical sidewalls; periodic 20 min etch/4 min cool and hourly cleaning were used to manage heat and byproducts
13. Quantitative sidewall smoothness data are also scarce in the LN etch literature
14. Second, use H2/proton substitution to limit LiF and thermal damage
15. Second, use robust conductive hard masks to limit LiF and thermal damage
16. Second, use cooldowns to limit LiF and thermal damage
</statements>

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