Micro-drilling of silicon wafer by industrial CO2 laser
© Subramonian et al; licensee Springer. 2015
Received: 1 September 2014
Accepted: 2 January 2015
Published: 26 February 2015
Laser micromachining is currently used in the MEMS production to replace the traditional etching process which consumes longer time to complete. The objective of this study is to investigate the drilling capability of industrial CO2 laser in processing of silicon wafer.
In this work, the holes were drilled on P-type silicon wafer with thickness of 525 μm. Geometrical characteristic of holes produce, which is diameter entrance that depends on laser parameter were investigated and analyzed. Analysis of Variance (ANOVA) was used to analyze the result and generated an appropriate model for the laser drilling processing.
The laser parameters involved were laser power, pulse frequency and duty cycle. The experimental results showed the entrance diameter of drilling holes was increase when the laser power and duty cycle increased.
The entrance diameter of drilling hole decreases when the pulse frequency increases.
Microelectromechanical system (MEMS) is a process technology applied to produce smaller devices or systems that combine together the electrical and mechanical components (Ku et al. 2011). Silicon wafer is the main material for MEMS, photonics, and semiconductor manufacturing industries (Ku et al. 2011; Wadhwa and Kumar 2014; Chen 2006). Nowadays, laser micromachining is used in the MEMS production to replace the traditional method such as the etching process which takes a longer time to finish the process. As a micromachining process, laser drilling is used to create through holes by using a laser beam where the principle of thermal removal of material occurred. There are many laser drilling techniques such as percussion drilling, single-pulse drilling, helical drilling, and trepanning drilling (Nayak et al. 2014). Nd:YAG laser becomes one of the most popular types of laser used in laser micromachining. The diameter used is the smallest value of laser wavelength (1.06 μm), which makes the Nd:YAG laser beam to be adsorbed by silicon wafer, but the initial cost is expensive. Compared with CO2 laser, the processing cost for this type of laser is economical, but due to the high wavelength value (10.64 μm), silicon wafer cannot absorb the CO2 laser beam. Silicon wafer will reflect more CO2 laser beam than absorb it. This phenomenon becomes a constraint in applying laser drilling process on silicon wafer by using CO2 laser.
A variety of lasers has been successfully used for silicon micromachining on experimental scale (Chung and Lin 2010; Vilhena et al. 2009), and these laser types have shorter wavelengths and shorter pulse lasers with high peak power. These lasers are not realistic for practical mass production due to being unstable, dangerous to the operator, and very high costs. Jiao et al. (2014) studied the effects of the hole geometry and the spatter which are around the drilled hole by femtosecond laser deep drilling with various temperatures and found 56% increasing drilling efficiency when temperature was raised from 27°C to 600°C. Recently, CO2 and Nd:YAG lasers were tried out for micromachining of metals and glass and gained success to certain extent (Jiao et al. 2013). This intended to carry out silicon micromachining with CO2 laser which is also relatively economical with better material removal rate. Chung and Wu (2007) discuss about silicon micromachining by using CO2 laser by placing a silicon wafer on the Pyrex glass. The morphology view and cross-sectional profile were studied using an optical microscope. As a result, the depth of silicon etching was increased when the pass number and laser power increase. Besides, the hole diameters produced also increase when the pass number, laser power, and scanning speed were increased. Jiao et al. (2009) investigated the integrated effect of several laser parameters (focus position, laser power, focus lens, and number of pulses) on the results, including micro-hole geometry and spatter characteristics of the laser micro-drilling process. In the proposed research, Clark-MXR fs laser (Clark-MXR, Inc., San Jose, CA, USA) was used, and its wavelength was 1.03 μm with a nominal frequency of 1 kHz. By using a DUV microscope (Olympus Corporation, Tokyo, Japan), the average value of the diameter in the two directions was taken as the hole diameter. The experimental results showed that the increase of laser power and the number of pulses caused the hole entrance diameters and hole exit diameter to also increase. Yan et al. (2012) use CO2 laser to conduct laser percussion drilling of thick-section alumina. The five controlled parameters of laser processing which are the pulse repetition rate, pulse duty cycle, laser peak power, pulse duration, and pulse energy were applied in the research. Spatter deposition and hole diameter (exit diameter and entrance diameter) of the drilled holes were investigated. Spatter deposition and hole diameter increase with increasing laser peak power and duty cycle. Wee et al. (2011) discussed four controlled parameters that affected the laser processing of silicon wafer in air and under water. The controlled parameters consist of scan velocity, laser pulse frequency, power level, and focal plane position that affect the laser spatter deposition (in air), irradiated areas (under water), and taper formation. The drilled hole on silicon wafer is observed with scanning electron microscopy (SEM).
Three controlled parameters involved in laser drilling of ceramic alumina are pulse frequency, laser power, and scanning speed (Bharatish et al. 2013). The processing performance, such as hole entrance diameter, hole exit diameter, hole taper, and hole circularity, was investigated. The laser parameters and response were correlated using response surface methodology (RSM) (Sivarao et al. 2013a). The laser parameters were feasible in the fabrication of MEMS structures on silicon. Nd:YAG laser (JK300HPS) was used with three controlled parameters which are width, pulse energy, and height of the laser power. The processing performance, such as the spot diameter produced, was investigated using the analysis of variance (ANOVA) method, and the model produced was discussed (Sivarao et al. 2013b). The experimental studies of the micro-laser drilling of silicon wafers by using CO2 laser are also highlighted. Laser parameters such as pulse frequency, laser power, and duty cycle will be set up to drill holes on 525-μm thickness of silicon wafers. The entrance diameter of drilling holes will be analyzed.
Complete specification of the Helius CO 2 laser machine
Helius 2513 laser cutting machine
FANUC Series 160 i-L
Laser beam wavelength
CO2 gas with the real ingredient mixture of N2 (55%), He (40%), CO2 (5%) with purity of 99.995%
5 to 7.5 in.
5 to 6 bar
Results and discussion
ANOVA generated a final equation in terms of the actual factors for the entrance diameter of the drilling hole. From the equation, the value of percentage error between the experimental and predicted values for hole entrance diameter can be obtained. The final equation is shown below:
Entrance diameter = 1.262 − (0.017 × laser power) + (2.938 × 10−4 × pulse frequency) − (0.015 × duty cycle) − (2.5 × 10−5 × laser power × pulse frequency) + (3.5 × 10−4 × laser power × duty cycle) + (8.75 × 10−6 × pulse frequency × duty cycle).
The micro-holes were successfully generated on the silicon wafer using CO2 laser and showed an overwhelming result. The experimental results showed that the increase of laser power and duty cycle in the laser drilling process significantly can increase the entrance diameter of the drilling hole. Otherwise, the increase of pulse frequency will reduce the entrance diameter of the drilling hole. The diameter of holes produced in this experiment (0.475 to 0.575 mm) is close to the diameter of the hole produced in the standard. A model was generated, and the percentage of error between the experimental data and predicted data was below 10% which is acceptable. The developed model was optimized by finding the best laser parameter to minimize the hole entrance diameter produced.
The authors are in debt to specially thank the top-level management of Universiti Teknikal Malaysia Melaka, Centre for Research and Innovation Management and Faculty of Manufacturing Engineering administrators for their continuous courage and support which brought this research work to successful findings benefiting industries and mankind. The authors and researchers are very much thankful to the Ministry of Higher Education Malaysia (MoHE) for awarding a research grant (Grant No.: PRGS/1/2014/TK01/FKP/02/T00007) which enabled the progress of this critical research.
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