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Silicon annealing is a crucial step in the semiconductors industry. It is used in the manufacture of silicon chips and wafers to change the characteristics of these devices.
Laser silicon annealing is used when manufacturing Low Temperature Poly Silicon (LTPS) displays. These displays a very versatile: they can be found in LCD, OLED, curved and flexible displays. LTPS displays have the highest resolution, brightness and colour depth many other advantages, providing an ideal user experience. In addition, their low power consumption prolongs the battery life of handheld devices. To create LTPS, the amorphous silicon from normal TFT silicon semiconductor junctions must be transformed into poly-crystalline silicone (see Figure 1).
Figure 1. Schematics of crystalline formations of Amorphous Silicon TFT semiconductor junction (left) and Low Temperature Poly Silicon TFT semiconductor junction (right).
Excimer lasers emitting in the UV were conventionally used for Silicon annealing, however 532 nm DPSS lasers are more cost-effective and rely on the high absorption rate of 532 nm radiation by silicon. Powerlase developed the Procyon g2000 laser system with this application in mind. The instrument creates several beams with a combined power of 2000 W. They are aligned into a line focus with a scalable line width between 750 and 1300 mm. The line focus is moved across the silicon surface and transforms the amorphous silicon into polycrystalline silicon as shown in Figure 2. The beam is actually nanosecond pulsed so that it maintains pulse fluence to necessary levels, and all pulses are synchronized within 1 ns trigger variation.
Figure 2.a. Green laser line focus irradiating the moving surface of the silicon layer, converting amorphous silicon to polycrystalline silicon.
Figure 2.b. Green laser annealed silicon layer on SiO2 substrate, with a thickness of 60 nm and average crystal grain size of 0.4 µm, comparable to excimer laser annealing.
In the Excimer Laser Annealing process, the speeds reach around 90 cm2/s, whereas with the Green Laser Annealing process using the Procyon g2000 laser source one can achieve speeds between 130 to 150 cm2/s. At the same time, the cost is less than 1/5th that of the excimer based system. In addition, the toxic gases required in excimer lasers need to be regularly exchanged along with the laser windows and chambers that hold these gases. The Procyon g2000 laser source eliminates these requirements, thus dramatically reducing the safety risks. At the same time, it increases the laser optics availability to over 95%.
Figure 3. The Powerlase Procyon g2000 laser system (laser system with control electronics).
Figure 4. The Powerlase Procyon g2000 laser system (green emission by several shelved laser cavity modules).
The Powerlase Procyon g2000 laser source has very low pulse to pulse energy variations, capped by specification at 6% 6 σ range. Therefore, a very stable average power is emitted by the system as a whole (Figure 3.), with typical variation in the 0.4% 6 s. The laser system is completely modular and can provide from 2 to 10 beams of 200 W average power each (Figure 4).
This information has been sourced, reviewed and adapted from materials provided by Andritz Powerlase Limited.
For more information on this source, please visit Andritz Powerlase Limited.