Fabrication Techniques for Lithium Niobate on Insulator Photonic Structures

Introduction

Lithium niobate on insulator (LNOI) has emerged as a promising platform for integrated photonics due to its excellent electro-optic, nonlinear optical, and piezoelectric properties. However, fabricating high-quality photonic structures on LNOI has long been challenging. This article provides an overview of key fabrication techniques that have enabled recent breakthroughs in LNOI photonics, including ion milling, focused ion beam milling, electron beam lithography, chemomechanical polishing, and mechanical dicing. We discuss the strengths and limitations of each approach, highlighting how they have overcome obstacles to produce low-loss waveguides and high-Q optical resonators.

LNOI Wafer Fabrication

The starting point for LNOI photonics is the fabrication of LNOI wafers themselves. This is typically done using crystal ion slicing, as illustrated in Figure 1. The process involves:

1. He+ ion implantation into a lithium niobate (LN) wafer

2. Bonding the implanted wafer to a substrate with a silica buffer layer

3. Thermal treatment to exfoliate a thin LN film

4. Thinning and polishing the LN film

This technique can produce LNOI wafers with sub-micrometer thick LN films and surface roughness below 0.5 nm. Both Z-cut and X-cut orientations are possible. The development of high-quality commercial LNOI wafers has been crucial for advancing LNOI photonics.

Ion Milling Techniques

Ion milling was one of the first techniques explored for patterning LNOI. It involves bombarding the sample with energetic ions (typically Ar+) to physically etch away material. Key challenges include:

• Redeposition of sputtered material, causing rough surfaces

• Mask erosion limiting etch depth

• Low etch selectivity between LN and typical mask materials

Early attempts using ion milling produced waveguides with propagation losses around 4 dB/cm due to sidewall roughness. However, optimization of the process has enabled dramatic improvements.

Focused Ion Beam Milling

Focused ion beam (FIB) milling uses a tightly focused beam of ions (often Ga+) for high-resolution patterning. Advantages include:

• Maskless, direct-write patterning

• Nanoscale resolution (< 10 nm)

• Ability to mill arbitrary 3D shapes

However, FIB is a serial technique with low throughput. It also suffers from redeposition issues similar to broad-beam ion milling.

To mitigate redeposition, a two-step FIB process was developed:

1. Coarse milling at high current

2. Fine polishing at low current

This enabled fabrication of microdisk resonators with Q factors >10^6. FIB has been particularly useful for creating photonic crystal structures with nanoscale features.

Electron Beam Lithography and Ion Milling

Combining electron beam lithography (EBL) with optimized ion milling has produced excellent results. The process typically involves:

1. EBL patterning of a hard mask (e.g. chromium)

2. Ar+ ion milling to transfer the pattern

3. Removal of the mask

4. Undercutting of structures by selective etching of the silica layer

Key factors for achieving high quality:

• Use of hard masks resistant to ion milling

• Careful optimization of ion energy and current

• Multi-step etching to control sidewall angle

This approach has achieved waveguide losses as low as 0.027 dB/cm and resonator Q factors >10^7.

Chemomechanical Polishing

Chemomechanical polishing (CMP) combines mechanical abrasion with chemical etching to produce ultra-smooth surfaces. While traditionally used for planarization, it has recently been adapted for fabricating LNOI structures in a technique called photolithography-assisted chemomechanical etching (PLACE).

The PLACE process flow:

1. Deposit a hard mask (e.g. Cr) on LNOI

2. Pattern the mask (e.g. by laser ablation)

3. CMP to selectively remove exposed LN

4. Remove mask and undercut structures

Key advantages:

• Ultra-low surface roughness (<0.5 nm RMS)

• Smooth sidewalls with controllable angle

• Compatibility with standard photolithography for large-area patterning

PLACE has achieved record-low propagation losses of 0.027 dB/cm in LNOI waveguides and Q factors up to 4.7x10^7 in microdisk resonators.

Mechanical Dicing and Cutting

An alternative approach avoiding lithographic patterning is precision mechanical dicing or cutting. This can produce extremely smooth sidewalls but is limited in the geometries it can create.

Blade dicing process:

1. Create LN thin film stripes by ion slicing

2. Use precision dicing saw to cut ridges

Achieved waveguide losses as low as 1.2 dB/cm.

Ultraprecision ductile-mode cutting:

• Uses single-crystal diamond tool

• Can create curved structures

• Nanoscale positioning accuracy

While mechanical methods can produce very smooth surfaces, they lack the resolution and flexibility needed for many integrated photonic applications.

Comparison of Techniques

Each fabrication approach has its own strengths and limitations:

Ion Milling:

+ Compatible with standard lithography

+ Can create arbitrary 2D patterns

- Redeposition issues

- Limited etch depth

FIB Milling:

+ Highest resolution

+ Maskless, flexible patterning

- Low throughput

- Ga+ contamination concerns

EBL + Ion Milling:

+ High resolution

+ Good process control

- Expensive, low throughput

- Multi-step process

PLACE:

+ Ultra-smooth surfaces

+ Compatible with photolithography

+ Single-step etching

- Limited to relatively thick structures

Mechanical Cutting:

+ Extremely smooth sidewalls

+ Simple, cost-effective

- Limited geometry options

- Challenging to integrate with other processes

The choice of technique depends on the specific application requirements. For large-scale production of integrated photonic circuits, PLACE currently offers the best combination of quality and scalability. For prototyping or specialized nanostructures, FIB or EBL-based approaches may be preferred.

Future Outlook

While great progress has been made, there is still room for improvement in LNOI fabrication technology. Key areas for future development include:

1. Further reducing optical losses, especially for thin (<200 nm) waveguides

2. Improving control over mode profiles and dispersion

3. Increasing fabrication throughput and yield

4. Developing techniques for 3D photonic structures

5. Integrating electrodes and other materials with LNOI

Conclusion

The development of advanced fabrication techniques has been crucial for realizing the potential of LNOI photonics. From early demonstrations with high losses, the field has progressed to ultra-low loss waveguides and resonators with Q factors rivaling those of bulk optics. This enables a wide range of applications in nonlinear optics, quantum photonics, and integrated optical systems.

Each fabrication approach offers unique capabilities, and hybrid processes combining multiple techniques may yield further advances. As fabrication technology continues to mature, LNOI is poised to become a mainstream platform for next-generation photonic integrated circuits.

By understanding the strengths and limitations of different fabrication methods, researchers and engineers can choose the most appropriate techniques for their specific applications. Continued innovation in LNOI fabrication will be essential for pushing the boundaries of integrated photonics and enabling new optical technologies.

Reference

[1] Y. Cheng, "Lithium Niobate Nanophotonics," Jenny Stanford Publishing, 2021.