Femtosecond Laser Precision Drilling in Diamond: When the Sharpest 'Light' Meets the Hardest 'Stone'

Diamond, commonly known as diamond, is the hardest known substance in nature (Mohs hardness level 10), with its excellent thermal conductivity, chemical inertia, and wide spectrum light transmittance, it has become an ideal material in many high-precision micro-cutting tools, micro-electromechanical systems (MEMS), high-precision gratings and extreme environmental electronic devices.

However, the hardness of diamonds also makes it famous as the "hardest processing" material. This challenge becomes more complex as synthesis technology develops.

Diversity and processing challenges of diamond materials

Currently, the preparation methods of artificial diamonds are mainly: high pressure and high temperature method (HPHT) and chemical vapor deposition method (CVD). Although it is possible to create materials with the same chemical and physical properties as natural diamonds, different preparation methods lead to significant differences in material properties, which directly affects subsequent processing plans.

For example, CVD diamonds have higher growth purity and non-conductive properties, which makes traditional electric spark processing methods that rely on the conductivity of materials completely inapplicable. What's even more difficult is the thermal sensitivity of diamonds, which are prone to cracking during processing. Traditional processing methods have a great heat impact, are prone to microcracks, and are more difficult to process complex geometric features.

Femtosecond laser: disruptive technology to break through diamond drilling bottleneck

In 2013, ultra-short pulse laser drilling technology won the German Future Award for its scientific research work in diesel injection nozzles. Since then, the technology has been continuously developed and has been constantly opening up new application areas. The essence of femtosecond laser drilling is to use the nonlinear absorption effect of 10⁻¹⁵second ultra-short pulses to instantly focus energy on the surface of the material, and directly vaporize the substance through multiphoton ionization rather than traditional thermal melt removal.

Nanosecond/picosecond lasers have a thermal diffusion depth of microns due to long pulse duration (10⁻⁹, -10⁻¹² seconds), causing severe graphitization phase transformation and recasting layer in diamond. The action time of the femtosecond laser is much smaller than that of the electron-lattice heating time (10⁻¹² seconds), and the material removal is completed before energy diffusion. This mechanism minimizes thermal damage, thereby achieving high-precision and high-quality processing effects. The femtosecond laser precision hole making equipment developed by Monochrome Technology can realize the processing of straight through holes with a diameter of tens to hundreds of microns on diamond sheets, with an accuracy of ±2μm, a true roundness of 98%, and no traditional defects such as burrs and microcracks.

Core advantages of femtosecond laser drilling

With a deep understanding of femtosecond laser drilling method and technical application, we have taken advantage of femtosecond laser.

1. Nano-scale surface roughness:

The "cold processing" and "non-contact" characteristics of femtosecond laser can significantly reduce the risk of workpiece deformation, cracking or fragmentation. The processing hole walls are smooth and the surface roughness can reach the nano-level. At the same time, femtosecond laser is a relatively clean process, with very little debris generated after processing and almost no complicated post-treatment is required, which is particularly valuable for expensive diamond materials.

2. Excellent true roundness:

True roundness is a key indicator for measuring the quality of hole processing, which refers to the degree of deviation between the actual contour in the same cross-section of the workpiece and the ideal circle, and is a type of aperture tolerance. In semiconductor processes, whether it is the through hole of the lithography mask or the etching guide hole with a high aspect ratio, if the true roundness of the hole is insufficient, it will lead to uneven metal deposition and affect the circuit conduction accuracy.

The aperture accuracy of the monochrome technology five-axis femtosecond laser special-shaped hole forming equipment can reach ±1.5μm. Combining high-precision ring drilling and auger drilling technology, the laser beam is cut along the preset circumference of the hole until it is screwed into the drilling. During the entire drilling process, the incident angle of the beam is changed to compensate for the natural taper of the laser drilling. In this way, perfect through holes can be drilled to achieve extremely high true roundness.

(a) Impact laser micro-drilling and (b) Micro-drilling produced by ring drilling laser micro-drilling

Conclusion

Femtosecond laser precision drilling technology has successfully broken the processing bottleneck of hard and brittle materials such as diamond with its comprehensive advantages of low thermal influence, high accuracy, strong flexibility and non-reliance on material conductivity. It can be expanded to third and fourth generation semiconductor materials such as silicon carbide, gallium nitride, and sapphire, opening up a new application path for the development of precision devices, advanced optics, quantum technology and other fields.