Abstract
The fabrication of micro-holes in silicon substrates that have a proper taper, higher depth-to-diameter ratio, and better surface quality has been attracting intense interest for a long time due to its importance in the semiconductor and MEMS (Micro-Electro-Mechanical System) industry. In this paper, an experimental investigation of the machining performance of the direct and chemical-assisted picosecond laser trepanning of single crystalline silicon is conducted, with a view to assess the two machining methods. The relevant parameters affecting the trepanning process are considered, employing the orthogonal experimental design scheme. It is found that the direct laser trepanning results are associated with evident thermal defects, while the chemical-assisted method is capable of machining micro-holes with negligible thermal damage. Range analysis is then carried out, and the effects of the processing parameters on the hole characteristics are amply discussed to obtain the recommended parameters. Finally, the material removal mechanisms that are involved in the two machining methods are adequately analyzed. For the chemical-assisted trepanning case, the enhanced material removal rate may be attributed to the serious mechanical effects caused by the liquid-confined plasma and cavitation bubbles, and the chemical etching effect provided by NaOH solution.
Highlights
The importance of micro through-holes can be represented by many applications, from solar cell and micro-devices to injection nozzles and turbine blades [1,2,3,4,5]
Taking the solar cell as an example, to avoid the shading effect of front electrodes, dense via holes that have a higher aspect ratio and small diameter are fabricated in solar panels to realize emitter wrap-through (EWT) or metal wrap-through (MWT) design, so that conventional top contact with front electrodes can be shifted to the rear through via holes, which increases the energy conversion efficiency [5]
The laser pulse frequency is adjustable between 0.2–1 MHz, in which range the output power stays nearly constant, leading to a maximum single pulse energy of around 260 μJ at a 0.2 MHz pulse frequency. For this ps laser machine, the high voltage (HV) level working on the HV modulator was employed to adjust the output power, and this power adjustment scheme has an advantage over the pump current control method, as the varied HV
Summary
The importance of micro through-holes can be represented by many applications, from solar cell and micro-devices to injection nozzles and turbine blades [1,2,3,4,5]. In terms of micro-drilling application, laser has shown good performance, and several drilling techniques have been proposed according to the relative movement between the laser beam and the workpiece, such as percussion drilling [2], trepanning [8], and laser helical drilling [9]. Among these drilling schemes, laser trepanning appears to be more competitive due to its flexibility with laser scanning path control, enabling it to make free-form holes with different contours at the hole entrance/exit [3,8]. The laser trepanning scheme has been widely employed for micro-hole production in various materials, such as superalloy [3,8], titanium alloy [10], tungsten carbide [4], polymers [11], and ceramics [12]
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