selective laser etching (original) (raw)

Acronym: SLE

Definition: a process of laser material processing where one first exposes some material to ultrashort laser pulses and then etches away the irradiated areas

Alternative term: selecting laser-induced etching

Category: laser material processing

• laser machining

  • laser micromachining
    * selective laser etching

• laser applications

  • laser material processing
    * laser cutting
    * laser drilling
    * laser machining
    * laser ablation
    * laser welding
    * laser marking
    * laser additive manufacturing
    * selective laser etching
    * laser cleaning
    * laser coating
    * laser hardening
    * laser soldering
    * laser surface modification
    * (more topics)

Related: laser material processinglaser micromachining

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Contents

Introduction

Selective laser etching (SLE) — or somewhat more clearly selective laser-assisted etching — is a subtractive laser micromachining technology utilizing ultrafast lasers. It is especially suited for creating complex three-dimensional (3D) microstructures inside transparent materials — mostly in optical glasses such as fused silica or borosilicate, and also in crystalline materials like sapphire. It typically achieves high quality, even in fine structures which could hardly be produced with other methods, and thus benefits diverse application areas as explained below.

Selective laser etching should not be confused with laser etching, which is often understood in industry as a one-step process without chemical etching.

The Process of Selective Laser Etching

Selective laser etching is a two-step process:

Femtosecond Laser Irradiation

In the first step, intense ultrashort pulses are tightly focused into the volume of a transparent dielectric such as fused silica. The extremely high optical intensity causes multi-photon excitation, and subsequently avalanche ionization leads to laser-induced breakdown. That results in microscopic modifications of the material (e.g. density changes, nanopore generation), while neighboring material (which is not irradiated) is hardly affected at all.

By moving the laser focus in three dimensions (point-by-point), beginning with points which are deepest in the structure, sophisticated three-dimensional patterns can be generated inside the substrate (wafer), with minimal effect on adjacent unexposed material.

High-quality results typically require a rather short pulse duration (typically in the femtosecond region, sometimes picoseconds) and a small focus volume (few μm3). The pulse energy is normally on the order of 1 μJ.

Selective Chemical Etching

After selective irradiation, the sample is immersed in an etchant solution — commonly KOH or HF-based, sometimes augmented with organic solvents for increased selectivity and etch rate. The laser-treated regions are dissolved much faster than the untouched material around them, as the laser treatment greatly enhances the chemical reactivity. One thus obtains selective etching (removal) only of the irradiated parts. Of course, all of those regions must be accessible by the etchant; parts which are fully surrounded by unexposed material cannot be etched away. (This is a significant difference to femtosecond laser marking inside glasses.)

Achievable Results and Processing Speed

Selective laser etching enables the fabrication of intricate and high-quality microstructures in transparent materials, offering several distinctive advantages:

• Fine feature resolution: Structures with minimum feature sizes down to 1 μm are achievable, enabling the creation of highly detailed micro- and nanostructures.
• Excellent surface quality: Typical surface roughness after etching is on the order of 100 nm. This can be further reduced by subsequent processing steps, such as laser polishing. Importantly, the process reliably avoids micro-cracks or chipping — issues commonly encountered with mechanical or other subtractive methods.
• High aspect ratio: The process routinely produces channels with aspect ratios exceeding 100, and under optimized conditions, ratios well above this can be obtained, limited by the selectivity of etching. Besides elongated channels (which also can be tapered, i.e., have variable diameter), finely controlled spherical holes and arbitrarily shaped cavities can be fabricated as long as they remain accessible via connecting channels.
• Three-dimensional freedom: The method is compatible with a wide range of substrate thicknesses; the ultrafast laser focus can be precisely positioned even deep within the bulk material (e.g. 20 mm deep), enabling genuinely 3D embedded structures.

Figure 1: A device fabricated with selective laser etching. Source: Workshop of Photonics, Lithuania.

The method is applicable to a wide range of thickness values (at least up to several millimeters) of the substrate, as the laser radiation can be focused quite deep inside the substrate.

In terms of processing speed, when using a sufficiently powerful ultrafast laser, laser modification (“writing”) rates of tens of millimeters per second are feasible. The subsequent selective chemical etching is considerably slower, with typical etch rates on the order of 1 mm per hour of depth, depending on the chosen material, etchant, temperature and process optimization.

As with any advanced laser material processing technique, process outcomes depend critically on parameters such as pulse energy, pulse duration, repetition rate and scan speed. In addition, the composition of the etchant solution and the etchant temperature play significant roles in determining both etch rate and final structural quality.

Applications of Selective Laser Etching

Compared to conventional methods like wet etching after mask-based lithography, selective laser etching (SLE) offers unique advantages: it can create arbitrary 3D structures with high aspect ratios and micron-scale precision, often exceeding what is possible with other subtractive techniques. As a direct-write process, SLE eliminates the time and complexity associated with mask fabrication and allows for features both on the surface and embedded within the substrate.

These capabilities enable and accelerate progress in several modern technology areas, partially including established industrial applications:

Microfluidics

Microfluidics, which has become important e.g. in chemistry and biological research, requires fine channels and caverns as parts of a kind of “lab on a chip”. For such structures, SLE is well suited:

• It works with well suited (chemically inert) glass materials.
• It allows for complex structures, e.g. with substantial branching, forming whole networks of fluid channels, as would be difficult to produce with other methods.
• Apart from lab experiments, one may use generated structures also in things like implantable sensors or for local drug delivery.
• Biomedical micro-needles, are another example of a type of structure which is difficult to produce with other methods.

SLE-fabricated microfluidic structures are already used in commercial diagnostic devices and single-cell analysis.

Figure 2: A microfluidic device which has been fabricated with selective laser etching. Source: Workshop of Photonics, Lithuania.

Micro-optics

SLE enables direct fabrication of microlens arrays, waveguides, beam splitters and diffractive elements inside glass, supporting innovation in photonic circuits and optical sensing with true 3D geometries. Optical interconnects and integrated photonic chips benefit from this approach.

Micro-mechanics and MEMS

Complex freeform micromechanical components — gears, movable elements, sealed cavities, etc. — can be directly created in glass or sapphire. This extends to microelectromechanical systems (MEMS) with improved material stability and electrical insulation, compared with silicon technology.

Figure 3: Micro-mechanics made in glass with selective laser etching. Source: Femtika

Micro-electronics

Modern integrated electronics technology often requires precisely defined vertical electrical interconnections passing through insulating glass substrates. Such through glass vias (TGV) can serve as interposers in 3D integrated circuit packaging, MEMS, sensor applications and RF modules, for example. They can be well fabricated with SLE.

Alternative fabrication techniques for through glass vias are laser drilling and mechanical drilling. However, the possible high aspect ratios and the high quality of SLE (e.g. freedom from micro-cracks) are ideal.

Photonic Circuits and Photonic Crystals

Photonic integrated circuits may be produced, e.g. containing waveguides for optical interconnects. Like in photonic crystal fibers, waveguides can be formed by surrounding the core with a pattern of open channels. Such waveguides do not need to be straight, but can also form complex 3D structures.

Novel photonic crystals may also be fabricated with SLE.

Optical Traps

Optical traps for particles might benefit from true 3D parts fabricated with SLE. For example, one can generate 3D arrays of tiny traps for atoms or ions, also containing optical structures for manipulating the captured particles. Such traps may be used for modern quantum technology.

Laser Sources for SLE

A wide range of different laser wavelengths can be used:

• Values in the 1-μm region are most common and work well e.g. for silica glasses and other transparent materials. They are obtained from common laser sources, often based on ytterbium (Yb).
• Shorter wavelengths, e.g. in the 0.5-μm region (with frequency-doubled sources) or even in the ultraviolet region, can lower the threshold energy for material modification and allow finer structuring in some cases, though with differing absorption characteristics and typically shallower penetration depths.
• In the experimental phase, longer wavelengths, e.g. in the 1.5-μm or 2-μm region, extend the applicability of the technique to other materials (e.g. silicon), which would exhibit too strong linear absorption at shorter laser wavelengths. Erbium- or thulium-based sources can then be used.

SLE typically requires pulse energies on the order of 1 μJ. Most mode-locked lasers generate pulses with substantially less energy (often only a few nanojoules) but a high pulse repetition rate.

The pulse repetition rate for SLE is usually in the kilohertz to megahertz region, depending on the required writing speed. One thus generally uses a pulse picker to reduce the pulse repetition rate, followed by an ultrafast amplifier to reach the required pulse energy.

Different solutions for amplification are possible:

• Regenerative amplifiers are frequently used, often involving chirped-pulse amplification.
• Multipass linear bulk amplifiers may be an option where the required total gain is not very high because of a high pulse energy from the oscillator.
• Fiber amplifiers are also an option, where substantially gain is possible without the complexities of regenerative amplification. This approach definitely requires chirped-pulse amplification, as otherwise the peak power in the fiber would get too high.

The beam quality should usually be diffraction-limited (_M_2 factor near 1), as this is required for tight focusing over some distance.

For writing complex structures, the laser beam is sent through a computer-controlled laser scanner with a high-NA focusing objective. Additionally, computer-controlled translational stages may be employed to move the sample, allowing for an increased writing area and greater flexibility in the patterning process.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains five suppliers for selective laser etching. Among them:

⚙ hardware

FemtoTGV is a laser micromachining workstation optimized for fabricating Through Glass Vias (TGV).

The most used method for TGV fabrication in glass core substrates is laser irradiation, followed by a wet etching process (selective laser-assisted etching). For the laser processing step, WOP has designed FemtoTGV.

The achieved TGV parameters:

• Roundness: >95%
• Waist: borosilicate >90%, alkali-free >75% (with deviation of ±3%)
• Diameter tolerance: ±3%
• Accuracy: ±1 μm (<5 μm per 515 × 510 mm panel)
• Roughness: Rz < 1 μm
• Yield: 100%
• Taper: taperless, hourglass

⚙ hardware🧩 accessories and parts💡 consulting🧰 development

The Femtika Glass Laser Workstation is a user-friendly machine dedicated to perform high-precision glass micro-processing tasks. It is designed for fabricating complex 2D and 3D structures in transparent materials with sub-micron accuracy, employing selective laser etching (SLE) technology.

The machine is ideal for manufacturing TGVs, micromechanical components, microfluidic devices, photonic elements, and other complex glass structures ranging from micrometers to centimeters in scale.

Equipped with a dual-objective head, the system allows seamless transitions between fabrication modes, while the self-aligning optical system reduces maintenance and ensures consistent performance. Autofocus and high-sensitivity camera gives an opportunity to monitor processes in real-time and control fabrication operations.

Process specifications:

• Materials: fused silica, borosilicate glass, and other transparent substrates
• Smallest feature size: >1 μm
• Minimum surface roughness: <200 nm
• Maximum object height: 300 mm
• Aspect ratio: >1:200
• Minimum micro-hole diameter: 5 μm

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