Enhanced etching characteristics of Si{100} in NaOH-based two-component solution

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In this work, we have studied the etching characteristics of Si{100} (etch rate, surface morphology and undercutting) in pure and NH2OH-added 10 M NaOH to promote the applications of wet etching in MEMS fabrication. These etching characteristics are systematically presented in following subsections.

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Etch rate

Etch rate is a key parameter when etching is involved in silicon micromachining for the formation of microstructures for MEMS-based sensors and actuators. It is defined as the vertical distance etched per unit time (i.e., etch depth (d) per unit time (t) or d/t). The etch depth on the samples etched in pure and modified NaOH is measured using a 3D laser scanning microscope. Figure 1 presents the etch rate of the Si{100} surface in pure and modified NaOH at 70 ± 1 °C. The standard deviation is determined by taking six measurements on the same chip at different locations. It can easily be noticed that the etch rate increases remarkably by the addition of NH2OH to pure 10 M NaOH. The etch rate becomes double in 12% NH2OH + 10 M NaOH than that of pure 10 M NaOH.

Although our focus in this work is on the etching characteristics of silicon in NaOH based solutions, which is useful for academic and industrial applications. Yet we attempt to give an insight behind increase in etch rate in NH2OH-added NaOH solution. We speculate that the etch rate of silicon increases owing to high accessibility of OH− ions and H2O in NH2OH-added NaOH [49,50,51,52,53,54,55]. In wet chemical etching of silicon, the OH− ions and H2O plays an important role as a catalyzing and active etching species, respectively [2, 55, 60]. These extra OH− ions and H2O molecules might be produced from the chemical decomposition of NH2OH as intermediate and final products in the presence of alkaline solutions.

The effect of aging of 12% NH2OH-added 10 M NaOH on the etch rate of Si{100} is investigated. To perform this study, etching experiments are performed every day for next 5 days. After 5th day, etching experiments are carried out on 5 days interval. The results are shown in Fig. 2. It can obviously be noted that the etch rate significantly decreases up to two days, but there is a minor change after third day. Though the etch rate is decreasing with etchant age, it is more than that of pure NaOH solution presented in Fig. 1. It can be stated that the addition of NH2OH enhances the etch rate significantly and very useful for industrial application to improve the throughput. As stated earlier, the present work is focused on engineering applications of wet etching. However, we attempt to explain the reason behind reduction of Si{100} etch rate with etchant age. As the etchant age is increases, the availability of OH− and H2O may decrease, which leads to reduction in the etch rate [60].

Undercutting rate

Undercutting is the lateral etching which takes place under the masking layer. Undercutting at the convex corners is an essential parameter to fabricate freestanding structures like cantilever beams for applications in MEMS/NEMS-based sensors and actuators [61, 62]. However, convex corner undercutting is undesirable for fabricating the mesa structures for some applications such as accelerometers and other sensors. Undercutting length along the < 110 > direction (l<110>) and etch time (t) is used to define the undercutting rate (Urate = l<110>/t), which is a very important parameter to estimate the release time. Figure 3 presents the undercutting rate at the convex corners in pure and NH2OH-added 10 M NaOH. It clearly indicates that the undercutting rate increases significantly when NH2OH is added to 10 M NaOH. The undercutting rate in NH2OH-added NaOH becomes more than double to that in pure NaOH. The optical micrographs of the undercut convex corners for different etching duration are shown in Fig. 4. The similar explanation as the etch rate can be applicable behind the significant increase of the undercutting rate at convex corners on the addition of NH2OH. The availability of catalytic species OH− and H2O near the convex corner are more in modified NaOH solution, which improves the undercutting considerably.

Effect of etchant age on the undercutting rate is studied and presented in Fig. 5. The undercutting rate follows the same trend as the etch rate. It decreases with etchant age and becomes stable after two days of the etchant age. The reactive species in the etchant solution decrease with etchant age that results in the decrease of undercutting with etchant aging [55]. Although the undercutting rate decreases with the age of the solution, it is higher in comparison to pure NaOH. It can be concluded that the etchant should be used immediately after its preparation to obtain higher undercutting. However, in terms of undercutting rate, NH2OH-added NaOH solution is better than NH2OH-added TMAH/KOH as the effect etchant age on its etching characteristics is less.

To demonstrate the applications of high undercutting rate in NH2OH-added NaOH in the fabrication of MEMS components, various kinds of microstructures are released in this etchant. The SEM micrographs of the fabricated structures are exhibited in Fig. 6.

Etched surface morphology

Surface morphology is another important parameter to be considered in MEMS, especially for optical applications (e.g., MOEMS) [1,2,3, 37, 39, 63]. It plays a crucial role in silicon micromachining for the fabrication of components for micro-devices such as cavities, gratings, diaphragms, micro-mirrors, cantilevers, etc. The average surface roughness on the samples etched in pure and NH2OH-added NaOH is measured using 3D laser scanning microscope at different locations of the sample. The results are presented in Fig. 7. As can be seen in the figure, the surface roughness substantially decreases by the addition of NH2OH. The SEM micrographs of surface morphology etched at different times in pure and NH2OH added 10 M NaOH solutions are presented in Fig. 8. Based on the results presented in Figs. 7 and 8, it can be stated that the modified etchant is a good choice for the fabrication of microstructures where a smooth surface is needed. The surface roughness at the microscopic scale occurs due to the non-uniform removal of atoms from the surface or lattice defects which are present on the surface and extend into the bulk crystal. It is characterized by the formation of pyramids or hillocks on the surface. In wet etching, surface roughness is the result of various factors; one is the formation of a hydrogen bubble during etching which hinders the surface reactions and acts like a micro mask on the surface, and another is the deposition of etching by-products on the surface [2].

The effect of etchant age on the surface roughness is presented in Fig. 9. It can simply be noticed that the surface roughness is fluctuating with the etchant age. As the etched surface morphology depends on various parameters and can be influenced by any kinds of surface contamination on the silicon surface during etching. However, we can claim from the results that the etched surface roughness is not deteriorated with etchant age.

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