Diamond has excellent properties such as extremely high hardness, good thermal conductivity, extremely high electron mobility, broadband transparency and high chemical stability, which makes diamond widely used in the fields of microelectronic components, field emitters, high-power semiconductor equipment and cutting tools. High-aspect-ratio diamond microgrooves have more usable area in the vertical dimension, so that they can meet the heat dissipation requirements of high-power microelectronics devices. However, due to the extremely high hardness and chemical stability of diamond, it is difficult to process diamond microstructures with traditional mechanical and chemical methods. As a result, the laser processing and ion beam etching have become mainstream methods for diamond machining. However, ion etching equipment has high cost, low etching efficiency, and requires a vacuum environment, which is not suitable for industrial production. Laser processing has been widely used in diamond processing because of its advantages of high processing efficiency, simple processing technology, low cost and easy automation. In this study, the influences of laser processing parameters such as laser pulse energy, scanning speed, scanning times, repetition frequency and defocus distance on diamond microgrooves are systematically studied. The research results in this paper can provide technical support for the fabrication of high-aspect-ratio diamond microstructures and diamond related devices.

The experimental material is chemical vapor deposition (CVD) single-crystal diamond. First, a series of microgrooves are directly processed on the surface of CVD diamond plates using an ultraviolet nanosecond laser. Then, a scanning electron microscope is used to observe the surface and internal microscopic morphologies of diamond microgrooves, the width and depth of diamond microgrooves are measured using a three-dimensional video microscope, and the ablation products of ultraviolet nanosecond laser processing of diamond are analyzed using a Raman spectrometer.

The diamond microgrooves processed by ultraviolet nanosecond laser have no cracks and chippings, but a large number of ablation particles with diameters of 200-1000 nm are observed around and within the microgrooves (Fig. 2). Raman spectroscopy shows that the ablation products on the surface of diamond microgrooves are mainly graphite (Fig. 3), indicating that the diamond material removal is through surface graphitization of diamond induced by ultraviolet nanosecond laser. The width, depth, and ratio of depth-to-width of diamond microgrooves increase rapidly with the increase of pulse energy, and then stabilize (Fig. 5). The width(D), depth (H) and ratio of depth-to-width (S)of diamond microgrooves gradually decrease with the increase of laser scanning speed (Fig. 7). As the number of scannings increases, the depths of diamond microgrooves first increase rapidly and then gradually stabilize (Fig. 9). The laser repetition frequency has little effect on the width of diamond microgroove. With the increase of repetition frequency, the H and S of microgroove first increase and then tend to be stable (Fig. 11). The defocus distance also has a great influence on the width, depth and ratio of depth-to-width of the microgrooves processed by ultraviolet nanosecond laser. As the diamond sample moves upward, the defocus distance first decreases and then increases, resulting in the widths of diamond microgrooves first decrease and then increase, and the depths of the microgrooves first increase and then decreases. When the defocus distance is -1 mm, the ratio of depth-to-width of the microgrooves is the largest (Fig. 13). Microgrooves with ratio of depth-to-width of over 12 can be obtained by machining diamond with optimized parameters.

In this study, an ultraviolet nanosecond laser is used to process microgrooves on the diamond surface, and the effects of laser pulse energy, scanning speed, scanning times, repetition frequency and defocus distance on the width, depth and ratio of depth-to-width of diamond microgrooves are studied. With the increases of laser pulse energy and scanning times, the ratio of depth-to-width of the microgrooves increases rapidly and then tends to be saturated as the pulse energy increases to 200 μJ and the number of scannings reaches 30. As the scanning speed increases, the ratio of depth-to-width of the microgrooves gradually decreases. As the laser repetition frequency increases, the ratio of depth-to-width of the microgrooves gradually increases and then tends to be stabilized when the repetition frequency reaches 60 kHz. The ratio of depth-to-width of the microgrooves first increases with the decrease of negative defocus distance and then it decreases with the increase of positive defocus distance. When the negative defocus distance is -1 mm, the ratio of depth-to-width of diamond microgroove is maximum. Therefore, the laser parameters used for processing diamond microgrooves are as follows: the laser pulse energy is 200 μJ, the scanning speed is 5 mm/s, the number of scannings is 30, and the repetition frequency is 60 kHz, the defocus distance is -1 mm. The experimental results show that the ratio of depth-to-width of diamond microgrooves is greater than 12 and the diamond microgrooves have no cracks and chipping using the optimized processing parameters. Raman spectroscopy shows that the ablation products of diamond microgrooves are graphite, indicating that the laser processing of diamond is carried out by surface graphitization. There are obvious graphite residues in the microgrooves because it is difficult to remove the graphite from the bottom of diamond microgrooves with high aspect ratio.