Sapphire is widely used in smart phones, tablet computers, flat panel TVs and other electronic display industries cause the characteristics included abrasive resistance, high hardness, thermal conductivity, electrical insulation, and stable chemical properties. Since sapphire is a hard and brittle material, conventional machining has defects such as cracks, chipping, delamination, edge cracking, and tool wear.
Laser cutting technology is a high-speed, high-quality cutting method, so sapphire wafers are cut, not only with high processing speed, good cut quality, but also for cutting any pattern. Compared with Nd:YAG laser cutting, QCW fiber laser has better stability and beam quality and higher energy density, which has obvious advantages for hard and brittle material cutting. The QCW fiber laser is combined with the shielding gas to cut the sapphire wafer, the heat-affected zone is small, the surface of the processing area is not corrugated, and the processing equipment is low in cost, so it is widely used in the field of sapphire processing.
The principle of cutting sapphire with QCW fiber laser is shown in Figure 1. The laser is transmitted through the fiber to the collimating mirror and then passed through the focusing mirror. Finally, the focused spot is obtained in the focal plane and then applied to the sapphire surface for cutting. Optical grade sapphire substrate is cut by QCW fiber laser with thickness of 0.52mm, diameter of 5mm.
应用脉宽为0.15~0.18 ms、波长为1064 nm、光斑直径为15μm、重复频率为0~5 kHz、能量密度变化范围为0~2.4 × 105J/cm²、切割速度变化范围为0~50 mm/s。加工采用氮气作为辅助气体，喷嘴直径为0.5-1 mm。
The application pulse width of 0.15~0.18ms, wavelength of 1064 nm, spot diameter of 15μm, repetition frequency of 0~5 kHz, energy density varies from 0 to 2.4 × 105 J/cm², and the cutting speed varies from 0 to 50 mm/s. The process uses nitrogen as the auxiliary gas and the nozzle diameter is 0.5-1 mm.
Firstly, set the position of the cutting line in the sapphire cutting software. We change the angle of the starting line(the angle between the starting line and the edge of the material), as shown in Figure 3. Select the different angles to test, and confirm the position of the line, then test.
In order to eliminate the adverse effects at the beginning of the sapphire cutting, a section of the cutting line added before the cutting of the formal pattern is the starting line, also called the auxiliary line. The cutting angle of the cutting line is also an important factor affecting the cutting quality. It has been found through experiments that the sputter length is substantially eliminated when the angle is 30° or 90° .
Nitrogen was used as the auxiliary gas, and the nozzle diameter was 0.5 mm. The effect of cutting the sapphire round hole is shown in the figure below:
We can see from the final cut sample photo that:
1. From the microscope, the sapphire after cutting has a smooth edge, basically no dross, no burr.
2. The cutting surface has almost no slope and the taper are less than 100.
3. The sapphire front edge cutting effect is better, the collapse size of the chipping size is smaller than the back-surface, and the backside collapse amount is also controlled below 10μm.
Laser energy density is an important factor affecting the amount of sapphire collapse. The higher the laser energy density, the more obvious the chipping phenomenon on the back of the sapphire and the larger the size of the chipping. Therefore, while ensuring that the sapphire can be cut through, the laser power is reduced to the side of the sapphire, but the smaller the energy, the more serious the slag is on the back surface, and it is difficult to remove. If the laser energy is properly increased, the molten material on the back side of the sapphire can be made into a powder, which can change the cutting effect on the back side of the sapphire.
To obtain a good surface quality grade, it is necessary to optimize the process parameters such as laser power and cutting speed. Here are a few of the main factors that affect the quality of sapphire cutting.
Laser energy is the primary source of energy for laser-cut sapphire substrates. Laser power and cutting speed determine the energy input to the sapphire substrate. The amount of laser power has an important effect on the cutting. When other process parameters such as cutting speed and auxiliary gas pressure are constant, the slit width exhibits a linear proportional relationship with the increase of laser power. If the laser power is too small, it will not be cut through; if the power is increased, the slit width will increase and the end surface roughness will increase. The laser power is as large as possible during laser cutting, which can fully utilize the power advantage of the laser and improve efficiency.
For sapphire cutting, the cutting speed of the material is proportional to the laser power, that is, increasing the power can increase the cutting speed. The laser cutting speed can have a relative adjustment range while other process parameters remain unchanged.
If the cutting speed is small, the slit width is large and the slit roughness is large. If the cutting speed is large, the slit width and the roughness are small, and the roughness is slowly increased after exceeding the optimal cutting speed. If the cutting speed is too high, the incision scraping slag is more or not permeable; if the cutting speed is too low, the material is over-fired, and the slit width and the heat-affected zone are large. To improve the cutting quality, optimize the cutting speed when other process parameters are unchanged.
In addition to the auxiliary gas used to blow off the slag from the cutting zone to remove the inert gas and active gas of the slit, the use of nitrogen for the sapphire material can suppress the excessive combustion of the cutting zone.
The basic requirement for laser cutting for assist gas is that the gas flow rate is large so that there is sufficient nitrogen to react exothermically with the slit material and there is sufficient power to blow off the molten material at the slit. When cutting sapphire at high speed, increasing the gas pressure can increase the cutting speed and prevent the slag from being generated on the back of the slit. During the cutting process, the impurities contained in the nitrogen adversely affect the cutting quality, so the purity of the nitrogen is high.