5.7 Patterning: lithography, masks, and etching

L1 Phenomenon

LN is hard to etch: fluorine chemistries form non-volatile LiF, so most low-loss TFLN waveguides are cut by argon ions that knock atoms off physically. Physical etching removes the mask too, and some of the removed LN lands back on the sidewalls. Mask choice, ion energy, and chamber pressure decide how steep and smooth the sidewalls are, and the sidewalls decide the loss.

Pattern writing time per wafer: EBL raster vs DUV stepper100 mm, 8 mm edge exclusion

EBL preset

100 nA

Exposed area (template) 10 %

Wafer, axis

t_EBL 21.1 h/cm²wafer 4.9 dderivedt_DUV < 1 min/waferreported

site map: # field, + field edge, x edge exclusion

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fields 622 × 22 mmderived

t = D A n / I. Usable area π(D/2 − 8 mm)² times the exposed fraction; step size, settling and stage moves are not included, so EBL times are lower bounds.

Elapsed time (both machines)60 s

E-beam preset

Wafer

E-beam, whole wafer: 4.9 d
Stepper, whole wafer: under 1 min

Usable area: wafer minus 8 mm edge exclusion; exposed area 10% of it (template). Step size, settling and stage moves are not included, so e-beam times are lower bounds. 22 × 22 mm fields: 6 per wafer derived. Stepper fields fill uniformly over 60 s, the reported upper bound (order template). E-beam path drawn at 4 mm pitch; its length fraction equals the written fraction derived.

Argon etching: sidewall angle and redepositionmodel = upper bound

profile: d mask, L LN, ~ redeposition, o SiO₂; 1 col = 1 row = 0.1 µm

                                            
                                            
                                            
                                            
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Selectivity S3.00

Mask angle85.0°

S 3.00a_mask 85.0°a_LN 88.3° (model)derivednearest measured 80°P

redeposition regime, ICP Ar (Kaufmann et al. 2023): points and bands only

Bias swept 100–800 V at 1 mTorr, 20 °C, 10 sccm Ar; pressure swept 1–15 mTorr (400 V) and 0.3–13 mTorr (600 V) P

tan(a_LN) = S tan(a_mask): S = 1 (SiO₂) and up to 3 (DLC) P; HSQ after development 85° P. Measured angles sit below the model because sputter yield depends on incidence angle and redeposition builds a second slope.

Selectivity LN : mask3.00

Etch progress100%

Redeposition

Ar⁺ flux from the top; mask dark, LN straw, redeposition grey (template thickness); not to scale.

Model sidewall: 88.3°
Nearest measured: DLC mask, 80°

Green curve: model upper bound for the 85.0° mask; dots: measured sidewalls P P.

Process viewer stepsopen in the Process viewer

#	STEP	ROUTE
13	PECVD SiO2 hard mask	ridge
14	Spin ARC and DUV photoresist	ridge
15	DUV stepper exposure, step-and-repeat	ridge
16	Develop; etch ARC (Ar/O2) and SiO2 (C3F8)	ridge
17	Ar ICP-RIE of LN (ridge)	ridge
18	Strip resist and SiO2 hard mask in HF	ridge
13	PECVD DLC hard mask (from CH4) with Si3N4 intermediate mask	strip
14	DUV stepper lithography	strip
15	Transfer into Si3N4 (fluorine plasma), then into DLC (oxygen plasma)	strip
16	Ar ion beam etching, full etch to the BOX	strip
17	SC-1 redeposition removal; O2 plasma DLC strip	strip

                                                
                                                
                                                
                                                
                                                
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Step progress t0.70

Step

Step progress0.70

17/23Ar ICP-RIE of LN (ridge) (ridge route)

L2 Model and data

Lithography

Item	Value	Source; status
EBL, HSQ (FOx-16) negative resist, 4-inch LNOI, 100 kV	about 500 nm HSQ; sleeve 150 nm wide, 4 passes at 1900 µC/cm², 8 nA, 5 nm step; bulk 7600 µC/cm², 100 nA, 25 nm step; develop 5 min in 1:3 AZ351B:DI water; resist sidewall 85°	P
EBL, ZEP-520A positive resist	about 1 µm, baked 170 °C for 2 min; 300 µC/cm² at 2 nA (JEOL 9500)	P
EBL, HSQ multi-pass	HSQ with multi-pass exposure	P
DUV stepper	248 nm; 22 mm × 22 mm reticle stepped across 4- and 6-inch wafers; 60 nm ARC + 600 nm resist on 650 nm PECVD SiO₂ hard mask	P
DUV vs EBL write time	DUV exposure under 1 min per wafer vs more than 8 days for EBL at 10 nA, 125 kV (authors' estimate)	P
EBL write time, bulk dose above, per cm²	21.1 h at 100 nA	derived t = D A / I = 7600 µC/cm² × 1 cm² / 100 nA
Stepper for poling electrodes	0.15 µm and 0.5 µm steppers for combs and pads	P
Femtosecond-laser direct writing (PLACE)	ablates a 600 nm sputtered Cr film into a hard mask	P

Hard masks

Mask	Reported use	Source; status
HSQ (spin-on oxide)	direct EBL resist and mask for Ar+ etching; microdisks with HSQ masks from 2014 P(title)	P, P, P
SiO₂ (PECVD, 650 nm)	DUV pattern transferred by C₃F₈ etch; removed with HF	P
a-Si (deposited Si)	EBL then Ar+ plasma etch of LN; also used for nanophotonic SHG waveguides P(title)	P; outside5sigma.com note: a-Si mask cracked and etched inhomogeneously on Y-cut reported
Cr (lift-off)	EBL lift-off Cr mask; F + Ar plasma; about 75° sidewalls, < 2 nm RMS roughness, 0.4 dB/cm	P
Cr (sputtered, 600 nm, fs-laser patterned)	mask for CMP-based pattern transfer (PLACE)	P
DLC (PECVD from CH₄, 300 nm)	DUV pattern into Si₃N₄ (F chemistry), then DLC (O₂ plasma); LN:DLC selectivity up to 3 vs about 1 for SiO₂ masks; 80° sidewalls	P
ZEP (polymer)	direct mask for Ar ion milling, slanted sidewalls	P

Argon etching: reported conditions

Tool and conditions	Result	Source; status
ICP, Ar 10 sccm, 1 mTorr, 20 °C, DC bias swept 100-800 V	redeposition falls and trenching rises with bias	P
ICP 300 W, 1 mTorr, 600 V	24 nm/min	P
Pressure sweep 1-15 mTorr (400 V, 600 W) and 0.3-13 mTorr (600 V, 300 W)	etch rate and sidewall angle fall with pressure; lateral etching dominates near 9 mTorr (400 V) and 7 mTorr (600 V); redeposition-free near 11 mTorr (400 V) and 5-7 mTorr (600 V); minimum rates 27 and 12 nm/min	P
ICP-RIE Ar+, bias power 112 W	350 nm etched, 250 nm slab; Q_i 10.0 ± 0.7 million	P
ICP-RIE Ar plasma	about 80 nm/min; 200 nm of a 400 nm film	P
Ion mill, 600 V beam	about 35 nm/min; smooth, slanted sidewalls	P
Ar IBE with DLC mask	full etch of 0.7 µm, base width 3.3 µm, 80°	P
IBE, incidence angles 0, 15, 25, 35°	used to test the redeposition argument	P

Redeposition removal

Method	Value	Source; status
SC-1 wet clean	NH₄OH:H₂O₂:H₂O = 1:1:5	P
KOH:H₂O₂ (LiTaO₃ analog)	3:1 KOH(40%):H₂O₂(30%)	P
Dry-only regimes	three methods: tuned bias and pressure; dense patterns or a retained slab; trapezoidal mask	P

Sidewall angles reported

Angle	Context	Source; status
about 60°	stated typical for ridge waveguides with SiO₂ masks	P
about 75°	Cr mask, F + Ar	P
80°	DLC mask, Ar IBE	P
85°	HSQ resist after development (before etch)	P

Alternatives to Ar etching

Method	Reported	Source; status
Fluorine RIE	LiF by-products are non-volatile and impede etching	P, P
PLACE (photolithography-assisted chemo-mechanical etching)	700 nm z-cut; Cr mask by fs laser; CMP transfers the pattern; Cr wet strip; second CMP; anneal 450 °C, 2 h, air; ring R = 200 µm, gap 3.8 µm; Q_L 4.29 × 10⁶, Q_i 4.04 × 10⁷, < 1 dB/m	P
PLACE microdisk	Q > 10⁸; OPO threshold 19.6 µW	P(abs)
PLACE origin (microdisks)	Q above 10⁷	P(title)

Loss and Q by fabrication route

Route	Best value found	Source; status
EBL + Ar ICP-RIE, ion-sliced, ring	Q_i 2.9 × 10⁷, 1.3 dB/m	P(abs)
EBL + Ar ICP-RIE, ring (2017 baseline)	Q_i about 1 × 10⁷, 2.7 dB/m	P
DUV + SiO₂ mask + Ar ICP-RIE, wafer-scale	0.27 ± 0.05 dB/cm across a 4-inch wafer	P
DUV + DLC + Ar IBE, strip	down to 4 dB/m	P
HSQ + tuned Ar ICP, redeposition-free	1.55 dB/cm	P
PLACE ring with bus	< 1 dB/m (Q_i 4.04 × 10⁷)	P(abs)
CMP film from bulk, ring	Q about 1 × 10⁸	P
Material limit after anneal	about 0.2 dB/m (material-limited Q 180 million in arXiv v1)	P

Propagation loss by fabrication route○ EBL+Ar □ DUV+Ar ◇ DUV+DLC △ PLACE × CMP from bulk

Loss axis

log

Zhang 2017-12-18 · EBL + Ar
ring, Q_i about 1e7, 2.7 dB/m
P doi:10.1364/OPTICA.4.001536

Zhang 2017-12, EBL + Ar	2.70 dB/m	P
Luke 2020-08, DUV + Ar	27 dB/m	P
Gao 2022, CMP from bulk	0.40 dB/m	template
Li 2023-08, DUV + DLC	4.00 dB/m	P
Kaufmann 2023-01, EBL + Ar	155 dB/m	P
Li 2023-09, PLACE	<1.00 dB/m	P(abs)
Zhu 2024-07, EBL + Ar	1.30 dB/m	P(abs)

Q axis (log only): Q = 2π n_g / (λ α), λ = 1550 nm, n_g = 2.3 (template). Arrow: anneal of ion-sliced LN from about 1.5 to 0.2 dB/m P. Arrow under a marker: upper bound.

Loss axis

Zhang 2017-12-18 · EBL + Ar
ring, Q_i about 1e7, 2.7 dB/m
P doi:10.1364/OPTICA.4.001536

Green staircase: lowest reported loss up to each date among the rows shown (Q-only rows excluded). Q axis (log only): Q = 2π n_g / (λ α), λ = 1550 nm, n_g = 2.3 template. Dashed arrow: anneal of ion-sliced LN from about 1.5 to 0.2 dB/m P. Arrow under a marker: upper bound. Hover or focus a marker for its paper.

L3 Details and open points

Etched vs etchless: bonding TFLN onto patterned Si or SiN avoids LN etching altogether; the trade-offs are reviewed in P(title).
Why the angle matters: a slanted ridge needs a wider base, raises bend radius, and reduces electrode overlap; vertical strips allow tighter integration P.
Sidewall model used in the etch figure (derived geometry, inputs template unless a row above supplies them): a mask edge of slope angle a_m receding vertically at rate R_m retreats laterally at R_m / tan(a_m); the LN etched at R_LN then takes slope tan(a_LN) = S tan(a_m) with S = R_LN / R_m. With S = 1 and a_m = 85° the model gives 85°; measured sidewalls are lower because sputter yield depends on incidence angle and redeposition builds a second slope P. The model is an upper bound.
Etch rates are tool-specific; Kaufmann et al. vary ICP power only to keep the endpoint system in range and report that power has only a minor effect on redeposition at fixed bias and pressure P.
Charging: a 5-10 nm Al discharge layer on dielectrics such as LN during EBL is reported in the author's blog (outside5sigma.com) reported.
Pyroelectric hazard: heating z-cut LN on a hotplate can expose resist by pyroelectric discharge and crack substrates; outside5sigma.com notes reported, reported. Pyroelectric coefficient magnitude 4 × 10⁻⁵ C/(m² K) in one compilation P-comp and 8.3 × 10⁻⁵ C/(m² K) from a vendor V.

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