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Ultra-fast laser processing in the sapphire glass powerful

DATE:2016-11-25READ:

     In recent years, an exciting alternative process has been put to industrial use, that is, the use of ultrafast lasers in the near infrared wavelength range of sub-picosecond pulse. In this method, ultrashort pulses are closely focused on most or surfaces of the glass, and the power density per square centimeter exceeds several watts, triggering complex and diverse processes such as simultaneous multi-photon absorption, avalanche and collision ionization, The localized damage to the substrate, and almost no energy deposition (only a few microjoules or less). Since the energy used for each pulse is extremely modest, the thermal effect on the part (even the focusing volume) is negligible. This method is commonly referred to as "cold ablation" and can be used to create highly accurate 3D structures. Compared with other micro-fabrication techniques, femtosecond laser micro-fabrication of transparent materials has a unique advantage.
 
     Processing of tempered glass
 
     The rise of smart phones has increased the importance of the display. Smart touch screen (touch screen) has gone beyond the phone keypad to become the most important user interface. A typical smartphone consists of four glass plates: two on the display to hold the film transistor and the liquid crystal material; one to provide a touch function; and a chemical toughened glass cover to protect the bottom from scratching, impact damage and dirt. Because users want lightweight, slim smartphones, thinner glass panels are used. The typical glass display panel is 0.3 mm thick; the chemically tempered glass cover is 0.7 mm thick. This makes the traditional cutting process to the limit. Cutting wheels are no longer suitable for processing this glass because they have been specially chemically strengthened and milling requires a lot of rework on grinding and polishing.
 
     Ultra-short pulse lasers with infrared and green wavelengths are well suited for the processing of this material. Picosecond pulse can reduce the generation of cracks, cutting quality far more than ordinary milling. The laser beam is swept across the material to be cut many times. The speed, edge quality, and the angle of the edge can be determined by the machining strategy. Compared with other laser processes, the ablation process is more robust: for example, a slight deformation of the glass does not affect the processing results. In the test, the Corning Eagle XG had a flexural strength of 280 MPa using a green picosecond laser. Test results using an infrared picosecond laser showed a three-fold increase in velocity and a slightly lower bending strength at the edge.
 
    More powerful picosecond lasers give us a greater opportunity to improve the efficiency of the processed glass. This is particularly true when we compare the ablation rate of steel and glass as well as the machining efficiency. (Fig. 5) The ablation efficiency of the steel begins to decrease at a pulse energy density of 5 J / cm2 due to the plasma shielding, while the glass allows a higher pulse energy density until the processing efficiency is maximized. Therefore, when the glass is processed, a higher pulse energy can be converted into a higher ablation efficiency.
 
    When considering further reducing the weight and thickness of the glass, one might use ultra-thin glass. 50 microns thick glass processing is more subtle, more sensitive to mechanical processing. In fact, such a glass without laser processing is impossible.
 
    Sapphire for any tricky situation
 
    Sapphire is second only to the diamond second hard material, it is difficult to use mechanical methods to process. The use of lasers to cut sapphire is today's standard manufacturing method for LED manufacturing, where sapphire is used as a substrate substrate. Because of its scratch resistance and light transmission, sapphire will be used to produce watches and optical instruments to protect the mirror.
 
    Ultrashort pulsed lasers enable precise machining when working with small profiles. For example, when cutting circular parts and drilling micro-holes, flexible contour processing can be achieved by high-speed scanner. Ultrashort pulses remove thermal effects during machining, resulting in excellent edge quality. Figure 6 shows a 0.4 mm thick circular, square and triangular profile cut from sapphire using a TruMicro 5070 infrared picosecond laser. The minimum profile size is 0.2 mm. And the combination of ultrashort pulse intelligent processing can avoid the formation of the gap and cracking. Experience has shown that a pulse energy of about 100 micro-joules is optimal. For example, a laser beam with a pulse energy of 250 microfocus can be split into two 125-joules of independent laser beams to produce higher yields while processing two parts at the same time. Excellent processing quality depends on the processing methods, dust and workpiece between the appropriate fixture.