化学镀金工艺优化

Deep etching of silicon (Si) is very much desirable for wide variety of applications. Under the context, a cost effffective and reproducible through etching of ∼375 μm thick Si wafer is demonstrated through long hour metal assisted chemical etching (MACE) followed by short duration KOH etching. During MACE, apart from pH and temperature, metal catalyst size and coverage density during electroless plating plays an important role. Optimization of gold deposition in terms of plating solution concentration and deposition time during MACE is studied for effffective through etching. HAuCl₄ concentration of ∼5 mM for 30 s is found to be best suited for MACE and produces deep and highly dense pores in Si with threshold pore radius ∼250 nm and above. Following the MACE, KOH etching effffectively scoops out porous Si to realize through etching.

Silicon (Si) micromachining is extensively used for the fabrication of patterns and complex three dimensional structures. Introduction of new techniques like surface micromachining , anisotropic etching, LIGA process  and Bosch process  has revolutionized its application in microflfluidics  and micro-electro-mechanical systems (MEMS) devices  etc. Deep etching of Si by dry and wet anisotropic etching requires a hard mask of Si₃N₄/SiO₂. Many of the techniques and the processes like low pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD) and thermal oxidation are carried out in controlled environment where pressure, flflow rate of gases, temperature etc needs to be maintained for high quality growth of Si₃N₄/SiO₂ . Considering these many variable parameters in the growth process and the maintenance of instrument increases the cost of production. Thus, a simple wet etch techniques using polymer as a mask will be viable and useful for many researchers to practice without the need of such high end facilities. Silicon anisotropic wet etching is done in hydroxide containing solution, where crystal planes with difffferent hybridised SP3 orbital effffects the etch rate drastically. Isotropic wet etching of Si in HF solution is well known which happens in the presence of holes (h+), produced by oxidative chemical reaction or supplied externally under bias. Etch profifile can be restricted to certain region by trapping hole concentration in selected geometry. HF based metal assisted chemical etching (MACE) method was reported by Li and Bohn. Metals like gold (Au), silver, platinum and copper are patterned on Si in the desired region of etching. Due to catalytic activity of metal, concentration of holes in the metal vicinity is more, which intern increases Si rate of etching in these regions. MACE has received much attention in recent years due to its low cost and control over etched morphology. Chartier et al. used molar concentration ratio = + (ρ) [HF] / ([HF] [H₂O₂ ]) to characterize different etched morphologies. Effffect of catalyst shape and concentration, on etched direction was studied by Hildreth . Controlled 3D motion of insulator pinned metal catalyst was also demonstrated by Hildreth. Silicon micromachining using MACE produces, porous Si, Si nanowires, patterned structure and have application in trench capacitor, antireflflective surface, thermal conversion, and bio-mimic super-hydrophobicity.

Deep etching of Si by wet method requires hard mask or metal as catalyst. Fabrication of through wafer micro funnel has been reported previously. Dry etching in combination with anisotropic KOH etching is adopted for realization of high density devices and interconnects in packaging. Recently, MACE based deep etching of Si is reported, where metal is deposited by evaporation or sputtering technique. Implementation of these processes in large-scale manufacturing increase the production cost. Thus, a simpler and a cost effffective method entirely based on wet synthesis is required for deep or through etching of Si wafer, resulting in large scale production of devices.

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Fig. 3 shows change in morphology of single MACE etch pore after short duration of KOH etching and their evolution by merging with adjacent pores forming a larger pore. MACE of sample (C, 30) leads to formation of pores above threshold radius. The short KOH treatment widens the pore and will terminates at < 111 > facet making an angle ∼54.7° with < 100 > plane . The lateral widening (lshift) of the pore by KOH etching for time (t) min can be calculated using the formula given in Fig. 3a. The etch rates of the Si < 111 > plane (R < 111 > ) in 30 wt% KOH at 70 °C is ∼5 nm/min . For 15 min of KOH etching the calculated lateral shift is ∼90 nm. For MACE etched pores having inter-pore distance ∼180 nm, pore walls collapse after KOH treatment and they merge to form a bigger pore, as shown in Fig. 3b. Pores formed after KOH etching has straight edges due to anisotropic etching of Si.