How to use laser to process diamond efficiently and with high quality?

With the continuous development of the field of artificial intelligence, the application of new materials in high-power, high-frequency, high-temperature and low-power loss electronic devices has received increasing attention. The bandgap width of diamond is 5 eV, which is the widest bandgap material among the current single-element semiconductor materials. It also has excellent electrical properties such as high breakdown electric field, large saturated carrier velocity, high carrier mobility and low dielectric constant. It is expected to become the fourth-generation semiconductor material. However, due to its extremely high hardness, traditional processes cannot meet the processing accuracy and processing efficiency at the same time, so it is also called the “most difficult to process” material. At present, the main processing methods for diamond include electrospark machining, abrasive water jet machining, mechanical machining and laser machining. Among them, laser processing has low cost and good repeatability. It can process diamond efficiently and controllably, and the processing accuracy can reach the micron level or even nanometer scale.

Principles of Laser Processing Diamond

Diamond is a crystal composed of pure carbon elements, which has extremely strong stability and high hardness. Therefore, during the laser processing, diamond will not sublimate or chemically etch directly, but will first undergo a transformation from diamond to graphite phase to reduce its processing difficulty. Finally, the material absorbs laser energy to heat and evaporate (or sublimate). In this process, the pulse length has a great influence on the quality of diamond processing. Generally, it can be divided into “hot processing” and “cold processing” according to the relationship between the laser pulse length and the size of the atomic lattice collision of the processed material.

When laser interacts with diamond, heat transfer occurs between electrons and their lattices. The so-called “hot processing” is when a laser with a longer pulse length (i.e., a longer pulse duration) is used. The laser energy deposited in the electrons is transferred to the lattice within the time when the laser pulse irradiates the material, causing the material to heat up and reach a thermal equilibrium state, with a significant thermal effect. Cold processing is the opposite. Its laser pulse width is smaller than the time scale of the electron-phonon interaction. The laser energy deposited in the electrons does not have time to be transferred to the ions, and the laser pulse irradiation ends. At this time, the temperature of the ions is relatively low, and there is no significant thermal effect.

The relaxation time of electrons and holes in diamond is 1.5 ps and 1.4 ps respectively. Usually, microsecond lasers and nanosecond lasers will have a large heat-affected zone, which is usually suitable for rough processing. Picosecond and femtosecond lasers are cold processing and can be used for precision processing of diamond, but their processing efficiency is relatively low. Therefore, in the actual processing process, it is necessary to optimize the laser process parameters so that a high material removal rate can be guaranteed during the processing while reducing the generation of heat-affected zones in the processed samples and improving the quality of the processed surface.

Laser processing requirements for diamond

At present, the application research of laser in diamond material processing mainly focuses on laser cutting, laser drilling, micro-groove processing and laser flattening. Different processing applications have different requirements for laser technology.

1.Laser cutting:

The formation of highly collimated kerf and small kerf taper, processing of ultra-thick diamond plates, heat-affected zones, defects, etc. are the key issues that need to be addressed in diamond laser cutting. Therefore, the use of short pulse and ultra-short pulse laser technology, while ensuring the precise control of the laser beam focus position and the laser beam movement position and the development of new laser processing methods are the key points of the future development of diamond laser cutting technology.

2. Laser drilling

Controlling micro-hole accuracy (hole diameter, hole depth, hole wall quality) and avoiding material damage are the key to diamond laser drilling. Therefore, more accurate laser focusing technology is the most basic requirement for diamond laser drilling.

3. Microchannel processing

Directly machining microchannels on the surface of diamond-based materials can quickly transfer heat from the heat source to the coolant, which is a research hotspot in the field of ultra-high heat flux heat dissipation. The key to the diamond microchannel structure is to machine grooves with high collimation and good surface quality. Among various removal processing methods of diamond materials, laser processing has been widely used. In addition, when performing large-area microgroove processing, ensuring the consistency of the overall microgroove depth is a key point that should be paid attention to during the processing process.

4. Laser planarization

Laser planarization uses laser to irradiate the diamond surface at a certain angle and scan along a specific path, which can achieve rapid removal of the diamond surface. Generally speaking, when the laser irradiates the diamond surface vertically, the material removal efficiency is low, while a larger incident angle can obtain better surface roughness. However, in order to reach the threshold energy of diamond processing, as the laser incident angle increases, the required incident laser energy should also be further increased. The current angle range is generally selected between 75° and 85°.

5. Laser peeling (slicing)

The high hardness of diamond makes it impossible to slice it using traditional wire cutting methods. Laser stripping uses pulsed lasers of a specific wavelength to penetrate the surface of the material and focus inside the material, generating a higher energy density in the focal area, forming multi-photon absorption, so that a modified layer is formed at the required depth in the material, which is conducive to forming a certain crystal fracture position in the stripping process, thereby improving the controllability of the stripping process and the thickness consistency of the wafer. In order to prevent the diamond substrate material from being damaged during the processing process, precise control of the laser energy is often required.

New Diamond Laser Processing Technology

In recent years, in order to meet the processing needs of transparent hard materials such as diamond, researchers have developed various hybrid laser processing technologies based on traditional laser processing methods, including water-guided laser processing, water-assisted laser processing, and hybrid laser processing methods.

1.Diamond processing using water-guided laser technology

Water-guided laser processing is a technology that uses a fine water jet to guide the laser for processing. When the laser passes through a pressure-modulated water cavity, the laser beam is focused on a very small nozzle, and a very fine high-pressure water column is ejected from the nozzle. Due to the total reflection phenomenon at the interface between water and air, the laser will be confined in the fine water jet, and the water jet is conducted and focused, so that the laser is guided by the high-pressure water jet to process the surface of the processing material.

Compared with dry laser cutting, most of the energy can be consumed in water when using water-guided laser processing, and it can also take away excess residue, effectively reducing the heat-affected zone and thermal residual stress, and preventing thermal damage inside the material. At the same time, since the laser is confined in the water beam, the focus of the laser is extended, and the processing efficiency of axial processing is improved. Therefore, water-guided laser has good applicability for diamond processing, especially for processing diamond microchannel structures. However, since the laser power will attenuate in water, water-guided laser is not very suitable for drilling deep holes. In addition, water-guided laser also requires a finer and more stable water jet to ensure processing accuracy.

2.Diamond Processing by Liquid Laser Ablation

Unlike water-conducting lasers, laser liquid phase ablation uses pulsed lasers to ablate target samples immersed in liquids, and directly prepares micro-nano structures in a liquid environment. The key is to reduce the heat generated during laser processing through the cooling effect of water. In addition, it has the advantages of isolating air and reducing debris accumulation. However, in this technology, the laser beam is easily scattered by debris and bubbles suspended in water, and the absorption of the liquid layer will also lead to a large loss of laser energy. Therefore, it is necessary to accurately control the thickness of the liquid layer, such as introducing water spray to form an ultra-thin high-speed flowing water film, to ensure the uniformity of the surface quality of the material structure and the processing accuracy.

3.Hybrid laser processing method

Most current laser processing methods are performed in the form of a single pulse width, which has various limitations determined by the intrinsic characteristics of the laser. As mentioned above, microsecond lasers and nanosecond lasers cause the material to have a large heat-affected zone, while femtosecond and picosecond lasers have the problem of low efficiency and are difficult to process efficiently. Therefore, it is possible to consider using two or more lasers with wavelengths or pulse lengths to process diamonds. For example, when using lasers to polish the surface of diamonds, nanosecond or microsecond lasers can be used for rough processing first, and then femtosecond or picosecond lasers can be used to further reduce the surface roughness.

In addition, diamonds can also be processed in combination with different laser methods, such as using CO2 lasers for local heating and then using water-conducting lasers to quickly quench the heated area, thereby completing the cutting of polycrystalline diamond plates, which can complete efficient processing at a speed higher than other cutting technologies. However, hybrid laser technicians are usually more complex and have greater restrictions on the processed samples. They are generally used for high-end applications with special needs.

Summary
The rapid development of artificial intelligence has led to the increasing application of diamonds, such as as a substrate or as a heat dissipation material. However, its extremely high hardness poses a great challenge to its processing, and laser technology, with its non-contact, strong controllability, and high precision, has shown unique advantages in the processing of diamond materials. Since laser polishing of diamonds is always accompanied by problems such as thermal stress, how to balance processing efficiency and processing quality and develop new hybrid laser processing technology is the key to solving the problem of processing damage.