1 Introduction

In silicon bulk micromachining, wet anisotropic etching is a key technology for the fabrication of different types of microstructures (e.g. cantilever, diaphragm, cavity, etc.) for applications in microelectromechanical systems (MEMS). Silicon anisotropic etching has been performed in inorganic [e.g. Potassium hydroxide (KOH), Sodium hydroxide (NaOH), etc.] and organic [e.g. Tetramethylammonium hydroxide (TMAH), Ethylenediamine pyrocatechol (EDP), etc.] alkaline solutions (Seidel et al. 1990; Tabata et al. 1992; Dutta et al. 2011; Shikida et al. 2000; Gosalvez et al. 2011). Among all wet anisotropic etchants, KOH and TMAH are most commonly used. The etching characteristics of both etchants have been thoroughly investigated with respect to various etching conditions (Seidel et al. 1990; Tabata et al. 1992; Shikida et al. 2000; Gosalvez et al. 2011; Tang et al. 2014; Zubel and Kramkowska 2001; Tanaka et al. 2013; Rola and Zubel 2013; Pal et al. 2015). Both etchants have their own set of advantages and disadvantages. KOH provides high etch selectivity between {100} and {111} (i.e. R{100}/R{111}) and the significant etch rate of Si{100}, but potassium ionic contamination and poor selectivity between Si and SiO2 are its major drawbacks. On the other hand, TMAH involves no ionic contamination and provides high etch selectivity between Si and SiO2, however low etch rate ratio of R{100}/R{111} is a main disadvantage. Since TMAH does not involve alkali ions, it is compatible with complementary metal–oxide–semiconductor (CMOS) fabrication process. In wet chemical based silicon micromachining, etch selectivity between silicon and mask/structural layer, etched surface morphology, undercutting at convex corners, and CMOS compatibility are the major factors considered while selecting an etchant. These factors majorly depend on the type and concentration of the etchant. TMAH is preferred over KOH when CMOS compatibility is a concern and the silicon dioxide is used as structural/mask layer (Tabata et al. 1992; Chen et al. 2001). The etching characteristics of TMAH are considerably affected by the addition of surfactant and therefore it is widely studied in last two decades (Sekimura 1999; Sarro et al. 2000; Resink et al. 2005; Yang et al. 2005; Gosalvez et al. 2009, 2010; Pal et al. 2009a, b, 2010, 2012; Pal and Sato 2010; Xu et al. 2011; Zubel et al. 2012; Rola et al. 2014). In these studies, 0.001–0.1 vol% surfactant is added in TMAH to modify its etching characteristics. A systematic study of undercutting, etch rate and etched surface morphology in the wide concentration range of surfactant (i.e. ppb to ppm level) added TMAH is needed to know the surfactant concentration which affects the etching characteristics significantly.

In the present work, we have studied the etching characteristics of 25 wt% TMAH without and with addition of surfactant (Triton X-100) over a wide range of concentrations. The effect of surfactant concentration is investigated from ppb to ppm level. The major aim of this research work is to find out the surfactant concentration at which the etching characteristics are significantly affected.

2 Experimental details

In this work, P-type {100} and {110} oriented Czochralski-grown silicon wafers of 4-inch diameter with 5–10 ohm-cm resistivity are used. Thermally grown silicon dioxide of 1 µm thickness is used as etch mask. Photolithography is employed to pattern oxide layer. This step is followed by oxide etching in buffered hydrofluoric acid (BHF) solution. After patterning of the oxide layer, wafers are diced into small chips of size 15 × 15 mm2. Commercially available 25 wt% TMAH (99.999 %, Alfa Aesar) is employed to carry out the etching experiments. Triton X-100 (C14H22O(C2H4O)n, where n = 9–10) is used as surfactant and its concentration in the etchant (i.e. TMAH) ranges from 20 ppb to 1000 ppm. High concentration TMAH is preferred due to its favorable properties (e.g. smooth etched surface morphology and high undercutting) for the fabrication of MEMS structures (Gosalvez et al. 2009; Pal et al. 2010). All etching experiments are performed in a Teflon made cylindrical container equipped with reflux condenser to prevent evaporation of the solution (or to avoid concentration change) during etching process. Teflon container is partially inserted in constant-temperature water bath to heat the etching solution to the required temperature. Etching is carried out at 76 ± 0.5 °C without any agitation/stirring. In all experiments, the samples are held vertically in a PFA made chip holder containing multiple slots in order to etch many samples at a time to ensure the same etching conditions. Prior to immersion in the etchant, the samples are dipped in 2 % hydrofluoric acid (HF) for 1 min in order to remove native oxide and subsequently rinsed in deionized (DI) water. After etching process, the samples are thoroughly rinsed in DI water. Undercutting, etch depth and etched surface roughness are measured using 3D scanning optical microscope (Olympus, OLS4000). The etched surface morphology is inspected using scanning electron microscope (SEM, Zeiss EVO18). In order to investigate the effect of surfactant concentration on the wettability of silicon surface, the contact angles of TMAH drop with various surfactant concertation on the silicon surface are measured using a contact angle measurement system (AST, VCA Optima).

3 Results and discussion

As stated earlier, the influence of a wide range of surfactant concentrations on the etch rate, etched surface morphology and undercutting at convex corner are investigated in 25 wt% TMAH. These etching characteristics are discussed in the following subsections:

3.1 Etch rates of Si{100} and Si{110}

The etch rate is an important parameter to determine the etching time to create required etch depth in the substrate. In the fabrication of microstructures on/in silicon wafer, {100} and {110} wafers are commonly employed. Therefore, the study of the etch rate of these two orientations in pure and different surfactant concentrations added TMAH is very important from the application point of view. The etch rates of {100} and {110} surfaces in 25 wt% TMAH without and with surfactant concentrations ranging from ppb to ppm are presented in Fig. 1a, b, respectively. The etch rates are estimated from the etch depth measurement results.

Fig. 1
figure 1

Effect of surfactant concentration on the etch rates of a Si{100} and b Si{110} in 25 wt% TMAH at 76 °C (surfactant concentration: 20 ppb–1000 ppm)

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It can easily be observed that the etch rate of Si{100} is not affected significantly with increase in surfactant concentration, whereas Si{110} etch rate is influenced markedly by the addition of surfactant. From Si{100} etch rate data (Fig. 1a), it is very difficult to estimate critical surfactant concentration which alters the etching characteristics of TMAH solution. However, in the case of Si{110} (Fig. 1b), it can easily be noticed that the surfactant concentration equal to or greater than 1 ppm affects the etching characteristics dramatically. In the previous studies, it has been confirmed that the surfactant molecules adsorb at solid–liquid interface (i.e. silicon-etchant interface) (Gosalvez et al. 2009, 2010; Jeon et al. 1995; Paria and Khilar 2004). The density of adsorbed surfactant molecules depends on the orientation of the wafer surface. Some of the studies were focused on orientation dependent adsorption of surfactant molecules (Pal et al. 2009a, b; Tang et al. 2009). In these studies, it was confirmed that the surfactant molecules adsorb more strongly on Si{110} surface than Si{100}, as a result, a dense layer of surfactant molecules forms on {110} surface. It can be seen in Fig. 1a that the etch rate of {100} is not changing significantly when the surfactant is incorporated in the etchant. It indicates that the adsorbed layer of surfactant molecules on {100} surface is not able to protect the surface from the etchant. Therefore the reduction in the etch rate of Si{100} is much weaker. In the case of Si{110} (Fig. 1b), significant reduction in the etch rate indicates that the adsorbed layer of surfactant molecules partially protects the silicon surface from the direct attack of the etching agents (OH− ions and H2O) (Gosalvez et al. 2009; Pal et al. 2009a, b). The lack of etching agents at silicon surface results in the reduction of reaction rate and consequently the etch rate of Si{110} is suppressed considerably.

3.2 Etched surface morphology

The etched surface morphology is a major concern for optical applications, fabrication of microstructures with uniform thickness, high efficiency silicon solar cells, etc. Analogous to etch rate, etched surface morphology primarily depends on the etchant type and its concentration. Moreover, it is affected by etching temperature and/or the agitation of etchant during etching process. The morphology of the etched surface can be modified by altering the etching characteristics of the etchant by adding different types of additives (e.g. alcohol, surfactant, etc.). Figure 2a, b shows the etched surface roughness of Si{100} and Si{110} in pure and surfactant-added 25 wt% TMAH at 76 °C, respectively and their corresponding etched surface morphologies are presented in Figs. 3 and 4, respectively.

Fig. 2
figure 2

Etched surface roughness of a Si{100} and b Si{110} in pure and a wide range of surfactant concentrations added 25 wt% TMAH at 76 °C (etching time: 2 h, surfactant concentration: 20 ppb–1000 ppm)

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Fig. 3
figure 3

Etched surface morphology of Si{100} in 25 wt% TMAH at 76 °C: a without and with (b) 20 ppb, c 100 ppb, d 1 ppm, e 10 ppm, f 100 ppm, and g 1000 ppm surfactant addition (etching time: 2 h)

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Fig. 4
figure 4

Etched surface morphology of Si{110} in 25 wt% TMAH at 76 °C: a without and with (b) 20 ppb, c 100 ppb, d 1 ppm, e 10 ppm, f 100 ppm, g 1000 ppm surfactant (etching time: 2 h)

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It can be noticed from Figs. 2a and 3 that the surface roughness of Si{100} is not affected remarkably by the addition of the surfactant as the pure 25 wt% TMAH itself provides smooth etched surface morphology. However, the etched surface morphology of Si{110} is remarkably improved by the addition of the surfactant as can be observed in Figs. 2b and 4. In general, the surface roughness deteriorates due to the hillocks formed during etching and the inhomogeneity (striped zigzag morphology) in the etched surface morphology (typically on Si{110}). The hillocks form due to micromasking by H2 bubbles and/or impurities (e.g. byproducts) sticking to the surface during etching (Yang et al. 2005; Pal et al. 2007a, b; Cheng et al. 2006; Baum and Schiffrin 1997). The formation and growth of hydrogen bubbles on silicon surface is strongly related to surface wettability which is directly correlated with the surface tension of the solution. The addition of surfactant changes the wetting capacity of the etchant which is evaluated by measuring the contact angle between the silicon surface and etchant drop. Figure 5 presents the contact angles of pure and surfactant-added 25 wt% TMAH on silicon wafer surface. It can be noticed here that the addition of surfactant affects the contact angle of pure TMAH, and it decreases with increase in the surfactant concentration which manifests improved wettability of the etchant. The increased wettability improves the hydrophilic ability between the hydrogen bubbles and silicon surface, as a result the H2 bubbles are hard to stick to the etched surface. The faster detachment of H2 bubbles from the silicon surface leads to reduced hillocks formation and consequently the etched surface roughness decreases (Gosalvez et al. 2009; Yang et al. 2005; Pal et al. 2007a, b; Cheng et al. 2006; Baum and Schiffrin 1997). It can be concluded here that the use of surfactant promotes the release of hydrogen bubbles from the silicon surface during etching process.

Fig. 5
figure 5

Contact angles of pure and surfactant-added 25 wt% TMAH on silicon wafer surface (surfactant concentration: 20 ppb–1000 ppm)

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3.3 Undercutting at convex corner on Si{100} surface

As discussed previously, the etch rate of Si{110} is reduced to a significantly low level in surfactant added TMAH. Therefore, the effect of surfactant concentration on the undercutting at convex corner is investigated only for Si{100} wafer. Two kinds of corners namely convex and concave are frequently encountered in bulk micromachined structures. The convex corners in the mask patterns encounters significant lateral undercutting, while no undercutting is observed at the concave corners (Pal et al. 2007a, b; Pal and Sato 2015). Undercutting at convex corner has its own benefits and problems in the fabrication of microstructures. Convex corner undercutting plays a major role in the fabrication of freestanding microstructures. Fast undercutting results in quick release of the structures. Therefore, in the fabrication of suspended microstructures, high undercutting rate is desirable. The undercutting rate mainly depends on the type and concentration of the etchant. TMAH etching yields strong undercutting, especially in high concentration TMAH (20–25 wt%) (Pal et al. 2007a, b; Pal and Sato 2015). On the other hand, in the fabrication of structures with convex corners, undercutting should be eliminated. It means, undercutting is disadvantageous when the microstructures with convex corners are to be fabricated. Several studies proposed that the undercutting of convex corners can be reduced by adding different kinds of additives in the etchant without employing any complex corner compensation structure at the convex corners in mask pattern (Sekimura 1999; Resnik et al. 2005; Pal et al. 2007a, b, 2010). Figure 6 shows the undercutting measured along <110> direction at the convex corner on Si{100} surface in pure and a wide range of surfactant concentrations added 25 wt% TMAH.

Fig. 6
figure 6

Undercutting at the convex corner along <110> direction in pure and surfactant-added 25 wt% TMAH at 76 °C (etching time: 2 h, surfactant concentration: 20 ppb–1000 ppm)

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Furthermore, the undercutting at convex corner on Si{100} surface in 25 wt% TAMH with different surfactant concentrations is qualitatively examined using SEM measurements of microstructures with convex corners. Figure 7 presents the SEM images of cantilever beam shape mask pattern etched in pure and surfactant added 25 wt% TMAH. It can easily be observed from Figs. 6 and 7 that the undercutting is reduced when surfactant is added in TMAH. Moreover, it can be seen that the undercutting reduces to a significantly low level when the surfactant concentration is equal to or greater than 1 ppm. As mentioned earlier, the undercutting is advantageously used for the fabrication of suspended microstructures. Figure 8 shows various shapes overhanging SiO2 microstructures fabricated in pure 25 wt% TMAH. These types of microstructures are widely used for the development of various kinds of sensing devices such as chemical, biological, pressure, acceleration, etc. (Peng et al. 2006; Chen et al. 2007; Tang et al. 2004). It is well known that undercutting at convex corners occurs due to the appearance of high etch rate planes (i.e. high index planes such as {221}, {331}, {441}, etc.) (Pal et al. 2007a, b; Pal and Sato 2015; Smiljanic et al. 2012). Undercutting reduces with the addition of surfactant into the etchant because the surfactant molecules form a dense layer on the high index planes at the convex corner. This surfactant layer inhibits the etching agents to react chemically with the silicon atoms, thus results in dramatic reduction in the undercutting (Gosalvez et al. 2009; Pal et al. 2009a, b; Pal and Singh 2013). The adsorbed surfactant layer at low surfactant concentration (20–100 ppb) is not able to hinder the reaction of etching agents with the silicon atoms of the high indexed planes, hence the undercutting is not significantly reduced as presented in Fig. 6.

Fig. 7
figure 7

SEM images of convex corner undercutting in pure and surfactant-added 25 wt% TMAH at 76 °C (etching time: 2 h, surfactant concentration: 20 ppb–1000 ppm)

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Fig. 8
figure 8

Different shapes SiO2 freestanding structures fabricated in 25 wt% TMAH at 76 °C

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4 Conclusions

The etching characteristics of 25 wt% TMAH without and with addition of a wide range of surfactant concentrations are thoroughly investigated. The etch rate and etched surface morphology of Si{100} surface are not significantly altered by the addition of the surfactant into TMAH solution, whereas the etch rate and surface roughness of Si{110} are changed. This change is considerable at the surfactant concentration equal to or greater than 1 ppm. Correspondingly the undercutting at convex corners on Si{100} surface is suppressed to a significantly low level when 1 ppm or larger amount of surfactant is added into TMAH solution. The effect of surfactant is saturated at the surfactant concentration equal to or greater than 10 ppm. This study reveals that a very small amount of surfactant (100 ppb) also affects the etching characteristics. In order to achieve optimal effect of the surfactant molecules on the undercutting at convex corners on Si{100} surface, etch rate and etched surface morphology of Si{110}, the amount of surfactant should be equal to or more than 10 ppm.