Chemo-Mechanical Etching (PLACE) Technique

Updated 22 February 2026

Chemo-Mechanical Etching (PLACE) is a fabrication methodology that combines lithographic mask patterning with chemo-mechanical polishing to produce sub-nanometer rough features in integrated photonic devices.
The process uses a Cr mask patterned via femtosecond laser writing and colloidal silica slurry CMP to achieve propagation losses below 0.01 dB/cm and uniform etch depths.
PLACE outperforms traditional dry etching by offering enhanced uniformity, scalability, and optical quality with microring resonators reaching Q-factors exceeding 10⁷.

Chemo-mechanical etching, most prominently exemplified in the photolithography-assisted chemo-mechanical etching (PLACE) technique, is a fabrication methodology combining lithographic mask patterning with chemo-mechanical polishing (CMP) to realize high-resolution, ultra-low-loss features in integrated photonic devices. PLACE is predominant in the thin-film lithium niobate (TFLN) platform, where it enables waveguides, resonators, and other photonic components with sidewall roughness approaching the sub-nanometer regime and propagation losses orders-of-magnitude lower than conventional dry-etching processes. Through synergistic chemical softening and mechanical abrasion, PLACE overcomes the challenges of traditional ion-beam or plasma etching for dense, large-scale, high-performance photonic circuits.

1. Fundamental Principles and Process Workflow

PLACE exploits high-resolution patterning of a hard mask—typically chromium—followed by selective material removal of the underlying crystalline oxide (e.g., LiNbO₃) through chemo-mechanical interactions. The workflow comprises:

Substrate Preparation: The process commonly employs x-cut or z-cut TFLN (thickness: 300–700 nm) on an SiO₂/Si or multi-layered handle wafer (Chen et al., 2023, Gao et al., 2024). The substrate may be further cleaned in piranha (H₂SO₄:H₂O₂) and dried.
Mask Deposition: A chromium (Cr) layer—thickness ranging from 50 nm (modulators) up to 600 nm (microrings)—is deposited by e-beam evaporation or magnetron sputtering; occasional use of Cr/Ti adhesion stacks is reported (Gao et al., 2023, Li et al., 2023).
Mask Patterning: High-repetition-rate femtosecond laser direct writing (pulse width 200–500 fs, repetition rate 200 kHz–1 MHz) defines features as small as 200 nm in Cr, achieved by balancing fluence slightly above the Cr-ablation threshold and scan speed with sub-micron spots (Chen et al., 2023, Wu et al., 2022, Zhang et al., 2020). Typical top ridge widths are 1–2 μm, etch depths 50–300 nm.
Primary Chemo-mechanical Polishing: CMP uses colloidal silica slurries (5–15 wt% SiO₂), alkaline pH 9–11 (KOH-adjusted), and polyurethane pads. Material removal occurs in exposed regions (not protected by Cr), at rates from 10 to 100 nm/min under contact pressures 0.1–0.3 MPa and platen rotations of 40–200 rpm. The process delivers sidewall roughness <0.5 nm RMS and etch-uniformity ≤5 nm across 2"–4" wafers (Chen et al., 2023, Wu et al., 2022, Gao et al., 2023).
Mask Removal and Secondary CMP: The Cr hard mask is stripped using ceric ammonium nitrate/HCl or commercial Cr etchants, followed by a final low-pressure, short-duration CMP to further diminish top-surface and sidewall roughness (Li et al., 2023).
Optional Post-processing: Crystalline order is restored and scattering loss further suppressed by high-temperature annealing (e.g., 450°C for 2 h in air), reducing propagation loss below 0.01 dB/cm and boosting intrinsic Q to ≈4 × 10⁷ in microrings (Li et al., 2023).

2. Chemo-Mechanical Material Removal Mechanisms and Etch Rate Modeling

Material removal in PLACE is governed by two concurrent mechanisms:

Chemical Softening: The alkaline slurry reacts with the oxide surface (e.g., LiNbO₃ + 2OH⁻ → Li⁺ + NbO₂⁻ + H₂O), forming hydrated layers or amorphous gel that facilitate subsequent abrasive removal (Chen et al., 2023, Zhang et al., 2020).
Mechanical Abrasion: SiO₂ particles (20–50 nm) in the slurry shear off the chemically altered surface under controlled pressure imposed by the polishing pad.

The process is quantitatively modeled by rate equations:

$R_\text{net} = k_\text{chem} \cdot [\text{OH}^-]^n + k_\text{mech} \cdot F^m$

where $[\text{OH}^-]$ is hydroxide concentration (slurry pH), $F$ is down-force, $k_\text{chem}, k_\text{mech}$ are empirical constants, and exponents $n,m \sim 1$ . Additional phenomenological models relate removal rate to chemical concentration $C$ , applied pressure $P$ , and pad speed $V$ :

$R_\text{CMP} = k \cdot C^\alpha \cdot P^\beta \cdot V^\gamma$

with typical fitting parameters $\alpha \sim 0.8$ , $\beta \sim 0.5$ , $\gamma \sim 1.2$ , $k \sim 0.1$ –$0.2$ (Li et al., 2023).

Surface roughness evolves as $\sigma(t) = \sigma_0 \exp(-\lambda t) + \sigma_\text{min}$ with $\lambda$ the smoothing rate constant, yielding sub-nanometer $\sigma_\text{min}$ after >10 min CMP (Li et al., 2023).

3. Process Parameters, Geometries, and Achievable Performance

PLACE enables precise control of critical geometrical parameters. Table 1 summarizes typical reported values.

Parameter	Value/Range	Source
Mask (Cr) thickness	50–600 nm	(Gao et al., 2023, Li et al., 2023)
Feature (top) width	1–2.35 μm	(Gao et al., 2024, Li et al., 2023)
Etch depth	50–300 nm	(Zhang et al., 2020, Li et al., 2023)
Sidewall angle	10–90° (geometry-dependent)	(Gao et al., 2024, Li et al., 2023, Wu et al., 2022)
RMS roughness (sidewall)	0.2–0.5 nm	(Gao et al., 2023, Chen et al., 2023)
Propagation loss	0.009–0.13 dB/cm	(Li et al., 2023, Zhang et al., 2020)
Modulator insertion loss	2.6–2.8 dB (fiber-to-fiber)	(Gao et al., 2024, Wu et al., 2022, Gao et al., 2023)
Loaded Q (microring)	$4.29 \times 10^6$	(Li et al., 2023)
Intrinsic Q (microring)	$4.04 \times 10^7$	(Li et al., 2023)

Precise control of CMP time and mask design achieves uniform depth and width across large die areas, with minimum etch depth non-uniformity better than ±5 nm. Feature minimum size is limited by laser spot (~200 nm) and mask quality; practical waveguide widths are typically ≥1 μm (Chen et al., 2023, Zhang et al., 2020).

4. Comparison to Alternative Etching Techniques and Impact on Photonic Performance

PLACE offers several advantages over conventional dry or purely chemical etching methods:

Roughness and Scattering Loss: CMP yields sidewall and surface RMS roughness as low as 0.2–0.3 nm, directly minimizing Rayleigh scattering loss ( $\alpha_\text{scatter} \propto \sigma^2/\lambda^3$ ). Dry etch (RIE/ICP) approaches yield roughness ≥1–2 nm and angle ≤80°, resulting in higher loss (~0.2 dB/cm) (Wu et al., 2022, Zhang et al., 2020).
Uniformity and Aspect Ratio: PLACE maintains large-area uniformity and supports vertical or shallow sidewalls. Dry-etch approaches can suffer from non-uniformity, mask charging, and "scalloping".
Throughput and Scalability: Femtosecond-laser mask writing and CMP scale to wafer-level processing with mask-writing speeds of 4.8 cm²/h at 200 nm resolution (Chen et al., 2023). Device yields >90% on 4" LNOI wafers are reported (Wu et al., 2022).
Propagation Loss and Q-factor: PLACE enables propagation losses below 0.01 dB/cm (with annealing) and microring resonator intrinsic Q exceeding $10^7$ , surpassing both ion-sliced and dry-etched TFLN (Li et al., 2023).
Design Flexibility: Ridge and rib geometries, tapers, spot-size converters, and complex coupled cavities are all accessible via mask design and CMP time tuning (Gao et al., 2023, Li et al., 2023).

5. Representative Applications: Modulators, Amplifiers, and Resonators

PLACE-fabricated TFLN structures support a diverse set of photonic devices:

Electro-Optic Modulators: Dual-arm phase modulator with 3 V half-wave voltage, 1 cm modulation length, and 2.8 dB fiber-to-fiber insertion loss; sideband generation for frequency combs (29 sidebands at 2 W input) (Gao et al., 2024).
Electro-optic Isolators: On-chip isolator with 39.5 dB isolation at 24 GHz RF, insertion loss 2.6 dB, and stable performance over 1510–1600 nm (Gao et al., 2023).
Microring Resonators: Monolithic integration of microring and bus waveguide with 3.8 μm coupling gap, loaded Q $4.29\times10^6$ , intrinsic Q $4.04\times10^7$ , and propagation loss 0.0091 dB/cm (post-annealing) (Li et al., 2023).
Integrated Amplifiers: Erbium-doped TFLN amplifiers, realized via PLACE and Ta₂O₅ cladding, yield >20 dB small-signal net gain over 10 cm at 1532 nm (Liang et al., 2021).
Single-Mode Waveguides: Index-contrast >0.6, losses 0.13 dB/cm, and efficient tapers for coupling (Zhang et al., 2020).

6. Optimization Strategies and Practical Guidance

Optimization of PLACE relies on controlled tuning of mask patterning and polishing parameters:

Cr Mask Quality: Pulse energy tightly above ablation threshold avoids substrate damage. Cr thickness should balance selectivity and pattern transfer fidelity; 400–600 nm typical for microrings, 50–100 nm for waveguides (Li et al., 2023, Gao et al., 2023).
Slurry Parameters: SiO₂ concentration, pH (9–11), and addition of minor oxidizers (e.g., 1 wt% H₂O₂) set etch selectivity and surface finishing (Li et al., 2023, Chen et al., 2023).
Pad Hardness and Pressure: Medium-hard polyurethane pads, pressures of 0.2–0.3 MPa, and platen speeds 50–100 rpm yield uniform removal. Softer pads and reduced pressure suppress nano-scratches and enhance finish (Wu et al., 2022, Gao et al., 2023).
CMP Endpointing: Use of optical interferometry for process termination ensures depth uniformity (Gao et al., 2024).
Post-etch Annealing: 450°C in air for 2 h recovers lattice damage and reduces scattering, crucial for Q factors above $10^7$ (Li et al., 2023).

7. Extensions: Chemo-Mechanical and Metal-Assisted Etching Beyond LN

While PLACE is primarily associated with TFLN, chemo-mechanical etching principles extend to other materials and pattern formation. In Ge/Cr/Au systems, metal-assisted chemical etching (MACE) under coupled mechanical (Euler buckling of metal film) and redox control produces self-organized spiral, radial, and labyrinthine nanostructures. The pattern selection results from interplay between redox-kinetics, diffusion constraints, and buckling-induced stress, with characteristic groove spacings (λ ~20 μm) tunable by film thickness and elastic parameters (Wong et al., 2024). This establishes a unified chemo-mechanical model where local reaction rates and evolving film stress dictate emergent micro- and meso-scale patterns relevant for template-free 3D microfabrication.

In summary, PLACE represents a paradigm for achieving sub-nanometer roughness and ultra-low propagation loss in high-index-contrast photonic platforms, with demonstrated performance surpassing traditional dry etching in yield, scalability, and optical quality. Quantitative modeling of removal rates, smoothing kinetics, and mechanical pattern transfer underpins continued advances in waveguide, resonator, and device engineering for integrated photonics (Gao et al., 2024, Li et al., 2023, Gao et al., 2023, Wu et al., 2022, Zhang et al., 2020, Chen et al., 2023, Liang et al., 2021, Wong et al., 2024).