Laser micromachining in the most popular consumer

2022-07-31
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Laser micromachining in the field of consumer electronics in recent years, the battery life of notebook computers has been tripled, the memory capacity has become larger and the cost has become lower, and computers, intelligent and other digital devices have faster speed and stronger performance. There may be many reasons for these advances, but the use of laser micromachining is a recognized factor. Therefore, the demand for laser micromachining in the electronics industry has never been so strong as now

bright LED (light emitting diode) makes the battery life longer

the backlight of LCD uses high-efficiency led to replace the low-efficiency cold cathode tube bulb, which significantly increases the battery life of notebook computers and reduces the energy consumption of TV sets. Therefore, the LED industry is experiencing unprecedented growth in history

the LEDs used in flat panel displays are based on gallium nitride (GAN), which is cultivated and processed into thin layers (only a few microns in total) on sapphire wafers. Sapphire is an ideal choice because it can provide a lattice suitable for gallium nitride and is transparent. This is very important because some light can escape from the led by partially penetrating the edge of the sapphire substrate. Sapphire is also a good thermal conductor, which helps to dissipate heat of LED. However, sapphire has a well-known feature - it is difficult to cut, second only to diamond

in actual production, LED is processed in batch on a sapphire wafer with a diameter of 2 feet and a thickness of 100 microns. As the most car interior is an important source of air pollution in the car, the final LED chip is only 0.5mm × 0.5mm, or even smaller, so each wafer can produce thousands of LEDs. Then the LED is physically divided by a single cutting process

traditionally, single cutting is performed by rotating the diamond circular saw for engraving (local cutting), and then physical screwing. But now, most LED manufacturers have switched to laser engraving and then physical crimping through blank holder (see Figure 1). In the picture, a focused ultraviolet pulse beam is cutting sapphire locally. Generally, about 30% of the wafer thickness is cut in multiple passes (see Figure 2). Then carry out the traditional physical crimping

laser marking has become the preferred method for several reasons. First, by focusing the beam to a spot size of only a few microns or less, laser scoring can be much narrower than saw marks and significantly reduce edge damage (cracking and spalling). This means that led devices can be arranged more densely with smaller gaps between them (called chip spacing). Moreover, high-quality edges can avoid post-processing, which is impractical on such a small device. The above advantages can lead to higher output and lower unit cost. In addition, tight focusing can perform rapid marking with lower laser power, thereby reducing the cost of laser operation

what are the requirements of laser characteristics for engraving? The most common laser single cut method is to use a 266nm Q-switched semiconductor pumped solid-state laser for front-end (device end) marking. One of the most important laser parameters is beam quality, because a low M2 value ensures good edge quality and minimizes led segmentation. Basically, M2 value is used to describe the compactness of laser beam focusing. The theoretical minimum value of focusing spot size of perfect Gaussian beam is defined as M2 equal to 1. In fact, the M2 value of all lasers is usually greater than 1. (the main reason why many LED manufacturers use the coherent Avia laser is that its M2 rating is less than 1.3.) Other key laser parameters include reliability, pulse fluctuation stability and an average power of at least 2.5 watts to achieve a predetermined processing speed. Some manufacturers also use 355 nm laser to carve from the back of sapphire. This wavelength will produce three kinds of materials to repair the grinding effect. Observation will produce tiny fragments. Therefore, cutting from the back can keep the fragments away from the LED. This method requires higher beam quality, because sapphire is very transparent to 355 nm wavelength, and high intensity focused beam must be used for processing with this wavelength. Insiders pointed out that in recent years, the treatment and recycling of waste plastics have promoted nonlinear absorption. The commonly used laser models using this method are Avia and Avia, and the M2 value is less than 1.3. In addition, some LED manufacturers are investigating the use of hybrid picosecond lasers, such as Talisker of coherent, which can make 532nm wavelength produce the same effect as 266nm nanosecond pulse

larger memory capacity and smaller size

in recent years, the capacity of SD and microSD memory cards has increased steadily, and the physical size and shape of these cards can remain unchanged. Moreover, the unit cost per megabyte (MB) has decreased significantly. The main reasons for the above progress are as follows: first, the development of microlithography has brought about the increase of circuit density; Second, by using physically thinner wafers, more wafers can be vertically stacked in the same package size

at present, the thickness of memory wafers is usually 80 microns or thinner. 50 microns is a cutting-edge technology, while 20 microns are still in the R & D level. In terms of economies of scale, the diameter of these wafers can reach 300 mm. Silicon is a crystalline material, so a piece of 300 mm × A 50 micron wafer is very fragile, and mechanical contact can easily crack and damage the wafer. Moreover, the cost of post-treatment is usually much higher than US $100000, so damage must be avoided in the single cutting process

traditionally, single cutting with diamond circular saw rotation will be repeated many times. However, if the wafer thickness is 80 microns, the circular saw must slow down to a very uneconomical rotation speed and reduce the cutting pressure to avoid spalling, cracking and breakage (see Figure 3). This creates great opportunities for lasers. Now many chip manufacturers have switched to 355 nm Q-switched semiconductor pumped solid-state lasers. Similar to circular saws, laser cutting must use multiple passes to minimize thermal damage that requires post-treatment to eliminate. Therefore, the only most important laser parameter is the extremely high pulse repetition rate. More specifically, the scanning speed is usually 600 to 750 mm/s, so that the total cutting speed can reach 150 mm/s when doing about 5 passes. This application also requires very high edge quality, so there should be 50% pulse wave space superposition. Therefore, for this thin wafer application, coherent company has developed a laser with extremely high pulse repetition rate (Avia). The pulse repetition rate is 250 kHz, and the output power is greater than 8 watts, which can provide sufficient cutting energy for one pass. In addition, there is an increasing interest in using hybrid picosecond lasers in process development because shorter pulse duration produces smaller heat affected zone (HAZ), thus avoiding post-processing

make the computer and run faster

as the F1 (n) of the integrated circuit becomes smaller, the insulation gap between circuit wires becomes narrower and narrower. Traditionally, the insulating material used in the gap is silicon dioxide. However, the higher the circuit speed, the lower the impedance of the circuit is required, that is, the material with lower dielectric constant (such as higher resistance) must be used. Therefore, the so-called low-k (k means dielectric constant) materials have attracted people's interest

figure 4

traditionally, silicon dioxide has been used as a low dielectric constant material, but this reduces the porosity. Therefore, it is considered to use new materials to increase porosity by increasing air content, so as to reduce the dielectric constant value. Memory chips, which are fast processors, are produced by thin epitaxial layers that are closely distributed on large silicon wafers. The problem with single cut is that low dielectric constant materials are very soft. Therefore, the traditional diamond circular saw will cause great damage to the circuit, including delamination (see Figure 4). However, for thick wafers that do not produce memory devices, the cost of laser sawing is not very cost-effective, and it is not very practical at present

therefore, the preferred method now is the mixing process. In particular, 355 nm Q-switched semiconductor pumped solid-state lasers are used to cut soft epitaxial layers to eliminate cracking. Next, mechanical sawing is used to cut the wafer. Figure 5 shows the simultaneous use of the two processes. If the chip spacing between circuits in the wafer design is wide, the laser can make narrow marking along the edge of each chip spacing in one pass. If the chip spacing is narrow, multiple parallel beams need to be used for single wide scoring, and the width should be enough to accommodate the cutting of the saw blade. At the same processing speed, the former process requires less laser energy, that is, the process cost is lower, so it is often used. The key laser parameters of this process are beam quality and high repetition rate. The typical laser for this application is Avia, which can provide the required beam quality of 30 μ joules per pulse, and the M2 value is less than 1.3. In addition, its repetition rate is 250 kHz, and it supports a scoring speed of 200 mm/s when 50% pulse fluctuations are superimposed

conclusion

to sum up, with the smaller and smaller size of electronic components and the continuous progress of materials, the attraction of laser marking will continue to expand and gradually become an economically feasible process. Moreover, as laser manufacturers continue to improve the performance, reliability and cost of ownership of their products, the application fields of laser engraving will be more extensive. (end)

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