Apr 01, 2020
Nowadays, ultrafast laser (femtosecond and picosecond pulse width) is an important part of the industrial production process. By virtue of its high-quality non-thermal material processing ability, coupled with the progress in laser technology, process development, beam control, and transmission, it further expands the application scope of ultrafast laser in the industrial market. However, in order to maintain the balance between input and output, the following conditions must be met simultaneously: first, it is necessary to prove its technical feasibility in the industrial processing process; because the interaction between ultrafast laser and matter is unique, it is necessary to have a fine scientific understanding of this process; second, the productivity of industrial production must ensure that the end-user can be brought with the investment matched the revenue, which is bound to promote the progress in beam control and transmission to make full use of the potential processing speed.
The field of consumer electronics clearly provides the most evidence. Mobile phones, microprocessors, displays, and memory chips are extremely complex components, which are composed of a large number of different materials, a very small size, and a very small thickness of multi-layer materials. So we need advanced, high precision processing capacity, and economically feasible mass production capacity. Here's an example of why we need to develop processing, laser technology, and new beam transmission technology simultaneously to meet the current and future challenges.
Making flat panel displays for mobile phones, tablets or televisions is one of the most complex technologies today, with similar or greater difficulties than the Apollo program of the 1960s. Different production steps involve a large number of different materials, which have the lateral resolution of the micron level and the thickness of tens of nanometers. Because of the difficulty of the whole process, it is not surprising that industrial productivity (the proportion of products that can pass strict quality testing) is regarded as a secret and a challenge. A key limitation is the existence of bad spots on the panel, which will hinder the commercialization of the screen. In the past few years, several different repair technologies have been developed, usually involving multi-wavelength nanosecond lasers. For example, a bright pixel is repaired by laser carbonizing or cutting the electrodes of a thin film transistor that controls the pixel (Figure 1).
Figure 1: thin film transistor electrode cutting, cutting width of 1.9μm.
Current technology has reached its limits. Because of the progress in the resolution of the high-definition screen, the size of pixels becomes smaller and smaller, and the thermal effect of nanosecond laser processing related to it limits the quality of the repair. In addition, new display technologies, including organic light-emitting diodes (OLEDs) and active matrix light-emitting diodes (AMOLED), have widely used organic and polymer materials, which are highly sensitive to heating and thus incompatible with heat treatment. Because the pulse duration is very short, the ultrafast laser is very suitable for non-thermal micromachining, and will not generate heat. They are widely used in the field of advanced screen repair processing, which promotes the development of a new generation of compact high-speed multi-wavelength ultrafast lasers.
Some industrial processes have begun to use high-precision ultrafast laser processing. This includes selective ablation, which is usually accurate to 30 nm/pulse, and high-precision thin film transistor electrode cutting with a cutting width of less than 2 μ M. These processes need to develop advanced and flexible beam shaping technology to obtain flat-top beam and ensure its uniform transmission, and to shape the sample with size as low as 2×2μm.
In another example, semiconductor circuits become more and more complex, and they require more functions to be integrated into smaller sizes. Therefore, the current wafer is composed of many layers of various materials, such as low dielectric constant materials suitable for rapid operation. An important process in the semiconductor manufacturing industry is wafer cutting and separation, that is, cutting a wafer into separate chips (Figure 2). Traditionally, the diamond saw is used, but the current technology has reached the limit. Because of the brittleness, the thickness, and the number of layers of the materials with low dielectric constant, the probability of negative effects such as crack and delamination is increasing.
Figure 2: semiconductor wafer cutting and dicing.
Although the use of UV nanosecond laser processing is promoted, the thermal effect of nanosecond laser processing still greatly limits the quality of processing results. On the other hand, ultrafast lasers show the ability to process silicon and high-quality multilayer materials. Until recently, the average power limitation of ultrafast laser is still a major problem, which seriously limits the total production efficiency. Today, the power of industrial femtosecond laser with high reliability is between 50-100w, which makes its production capacity match the industrial requirements.
The ultrafast laser is an important part of the advanced micromachining process, which plays an important role in quality control and measurement. Rudolph technologies recently launched a new tool for the semiconductor industry to measure the thickness of opaque films. The system is based on acoustic measurement, using a very short laser-generated ultrashort pulse. The reflection time of the ultrasonic pulse on the surface of each layer is measured by high-precision pump detection technology.
The appearance of high power and high-reliability laser system has greatly improved laser processing and quality control. More specifically, ultrafast lasers with an average power of 50 to 200W can improve production efficiency and productivity, thus expanding their application in new fields. However, the beam control and transmission of such a high-power laser are not easy. In order to make profits, it is necessary to achieve a processing speed of 100M / s, while maintaining the positioning accuracy of the micron level. The current generation of galvanometer scanners has reached the limit and new methods are needed.
ESI company has launched a hybrid processing system combining galvanometer and Acoustooptic Technology. When operating at higher processing speed, the inertia of the scanning galvanometer means the lag of execution, such as a sharp turn, so the processed structure will not be the same as the designed shape. However, acoustooptic modulators show a very sensitive response, but in a very small range. The combination of galvanometer motion and acoustooptic deflection can achieve accurate synchronization and overcome this limitation. This technology is particularly useful in the graphics manufacturing of interconnected digital circuits because they are becoming more and more integrated and therefore require increased wiring density.
Researchers from Japan's DISCO company use the same laser to perform both micromachining and process control, thus combining the two.
In this case, an ultrafast laser is used to drill a blind hole on a double-layer substrate. The upper layer is 80 μ m thick transparent material and the lower layer is 20 μ m thick metal film. In order to precisely control the number of laser pulses, so that the ablation range is limited to the transparent substrate, it is necessary to use a spectrum analyzer to monitor the plasma emission, that is, using laser-induced breakdown spectroscopy (LIBS) technology.
Figure 3: core shape of kagome fiber.
Because the plasma emission has a unique emission spectrum according to the type of atoms ablated, it can timely and accurately detect when the transparent layer is completely ablated. Another method is that the polygon scanner can achieve a scanning speed of more than 100m / s. This kind of single mirror can rotate at high speed, and can completely replace the low inertia mirror which can only reflect the beam in X and Y direction. If the rotation of the pulse laser and the polyhedral mirror can be accurately synchronized, only one point on each surface may affect the processing of the sample. In this case, the micromachining process is more like a digital process, that is to say, the laser needs to be controlled to turn on and off to produce the required graphics. In order to obtain ideal results, it is necessary to achieve a very precise synchronization between the laser and the scanner, and the manufacturing accuracy of the polyhedral mirror is very high, and the processing needs to be carefully designed. In cooperation with amplitudesyst è MES and Nextscan company in Belgium, Professor beat neuenschwander of Bern University of Applied Sciences University in Switzerland has realized high-speed surface micro modeling with micron positioning accuracy by using 500 kHz ultrafast laser.
More innovations in beam propagation are still in the works. The fiber optic transmission system makes the laser processing industry a new look, and the industrial class ultrafast laser still can not benefit from this until recently. Due to the beam limitation of the small fiber core and the very high peak intensity of the ultrafast pulse, the serious nonlinear effect will be produced, which will eventually lead to fiber degradation. In order to get rid of this limitation, hollow micro structure fiber has been developed, but the core diameter is limited to a few microns, which is too small for practical application. The development of a hollow large mode area kagome micro structure fiber paves the way for the fiber transmission of high energy and high power femtosecond laser beam. This special hollow fiber core with the shape of a circular internal spinning wheel limits the laser mode prevents it from interacting with the fiber microstructure and combines low nonlinearity, large mode field area, and flexible decentralized control. By cooperating with Glo photonics in France, amplitude Syst è MES has been able to transmit milliJoule level pulses for a distance of several meters, while ensuring the pulse duration is less than 500fs. In another experiment with photonics tools, pulse laser with an average power of 100W can be transmitted, and pulse compression of less than 100fs can be realized. Other teams and laser manufacturers are also rapidly using kagome fiber to develop flexible transmission systems (as shown in Figure 4). We can expect more in-depth changes in ultrafast laser processing technology in the next few years.
With the further development of the principle of interaction between short-pulse laser and matter and the development of technology in beam control and transmission system, the ultrafast laser has entered our daily life. Through the most advanced industrial processing process, it changes the way we look at things, communicate, and work. It will be the key to successfully manufacture more complex consumer electronic equipment in the future.