High speed silicon wet anisotropic etching for applications in bulk micromachining: a review

Wet anisotropic etching is extensively employed in silicon bulk micromachining to fabricate microstructures for various applications in the field of microelectromechanical systems (MEMS). In addition, it is most widely used for surface texturing to minimize the reflectance of light to improve the efficiency of crystalline silicon solar cells. In wet bulk micromachining, the etch rate is a major factor that affects the throughput. Slower etch rate increases the fabrication time and therefore is of great concern in MEMS industry where wet anisotropic etching is employed to perform the silicon bulk micromachining, especially to fabricate deep cavities and freestanding microstructures by removal of underneath material through undercutting process. Several methods have been proposed to increase the etch rate of silicon in wet anisotropic etchants either by physical means (e.g. agitation, microwave irradiation) or chemically by incorporation of additives. The ultrasonic agitation during etching and microwave irradiation on the etchants increase the etch rate. However, ultrasonic method may rupture the fragile structures and microwave irradiation causes irradiation damage to the structures. Another method is to increase the etching temperature towards the boiling point of the etchant. The etching characteristics of pure potassium hydroxide solution (KOH) is studied near the boiling point of KOH, while surfactant added tetramethylammonium hydroxide (TMAH) is investigated at higher temperature to increase the etch rate. Both these studies have shown a potential way of increasing the etch rate by elevating the temperature of the etchants to its boiling point, which is a function of concentration of etch solution. The effect of various kinds of additives on the etch rate of silicon is investigated in TMAH and KOH. In this paper, the additives which improve the etch rate have been discussed. Recently the effect of hydroxylamine (NH2OH) on the etching characteristics of TMAH and KOH is investigated in detail. The concentration of NH2OH in TMAH/KOH is varied to optimize the etchant composition to obtain improved etching characteristics especially the etch rate and undercutting which are important parameters for increasing throughput. In this article, different methods explored to improve the etch rate of silicon have been discussed so that the researchers/scientists/engineers can get the details of these methods in a single reference.


Introduction
Etching is one of the major processes employed in the fabrication of microstructures. It is classified into two categories: wet etching and dry etching as presented in Fig. 1 [1][2][3][4]. Wet etching is further subdivided into two parts namely anisotropic and isotropic etching. Silicon wet anisotropic etching is extensively used in silicon Open Access *Correspondence: prem@iith.ac.in 1 MEMS and Micro/Nano Systems Laboratory, Department of Physics, Indian Institute of Technology Hyderabad, Hyderabad, India Full list of author information is available at the end of the article Pal et al. Micro and Nano Syst Lett (2021) 9:4 bulk micromachining for the fabrication of microelectromechanical systems (MEMS) and the surface texturing for solar cell applications [1][2][3][4][5][6][7][8][9][10]. Figure 2 presents the examples of a few microstructures which are fabricated using wet anisotropic etching for applications in MEMS. The etch rate of silicon is usually the strong function of the orientation, etchant type, concentration, and etching temperature. Moreover, the etch rate is significantly affected by the impurities (or additives) in the etchant. Different kinds of anisotropic etchants such as potassium hydroxide solution (KOH) [11][12][13][14][15][16][17][18][19][20][21][22][23][24], tetramethylammonium hydroxide (TMAH) [24][25][26][27][28][29][30][31][32][33][34][35][36], ethylenediamine pyrocatechol water (EDP or EPW) [37][38][39], hydrazine (N 2 H 4 ) [40,41], ammonium hydroxide (NH 4 OH) [42,43], cesium hydroxide (CsOH) [44], etc. have been investigated and employed for the fabrication of microstructures. However, TMAH and KOH are most widely used for the formation of microstructures in silicon wet bulk micromachining [1-8, 19, 28, 29, 34, 36, 45-48]. Both etchants have their own pros and cons. KOH solution provides higher etch rate and better anisotropy between {100}/{110} and {111} planes, whereas TMAH is a solution compatible for CMOS (complementary metaloxide-semiconductor) process and exhibits high etch selectivity between silicon and silicon dioxide. Silicon wafers are available in a variety of sizes from 25.4 mm (1 inch) to 300 mm (11.8 inches) as shown in Fig. 3. The size has gradually increased to improve throughput and to reduce the production cost of semiconductor devices. Presently 300 mm (~ 12 inch) diameter is the largest size wafer used in fab industry. Different wafer manufacturing companies are involved in the development of 450 mm (or 18-inch) diameter wafer [49]. The thickness of the wafers depends upon the size of the wafer to provide adequate mechanical support during device fabrication. It increases with increase of wafer diameter. The standard thickness versus wafer diameter as per semiconductor equipment and materials international (SEMI) is given in Table 1. In the wet chemicalbased bulk micromachining, the industrial throughput is a strong function of the etch rate. Wafer thickness versus etching time for Si{100} in 5 wt% TMAH and 20 wt% KOH at 70 ºC is shown in Fig. 4. The etch rate of silicon at 70 ºC in 5 wt% TMAH and 20 wt% KOH is about 29 µm/h and 40 µm/h, respectively [50,51], which is moderate (or low) if larger thickness wafers are etched to form through holes and diaphragms. Slower etch rate not only limits the throughput but also affects the frequently used masking layer (e.g. SiO 2 ) due to increased time of exposure with the etchants (say KOH), The etching of 775 µm thickness (standard thickness of 300 mm diameter wafers) of Si{100} in 5 wt% TMAH and 20 wt% KOH at 70 ºC requires about 27 h and 19 h, respectively. Further increases in the thickness and the size of the wafer will lead to an increase in the etching time. Hence, there   9:4 has been a strong demand to develop a solution or technique to attain high etch rate to reduce etching time that will eventually help for enhancing the industrial production. Several methods have been reported to improve the etch rate including the addition of certain additives [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65], ultrasonic agitation [66], microwave irradiation [67,68] and etching at (or near) the boiling point of the etchant [18,69]. The addition of certain kinds of additives to the etchant solution introduces other species which can enhance the etch rate. Recently, NH 2 OH-added TMAH and KOH solutions, which provide very high etch rate in comparison to pure KOH and TMAH, are explored for applications in wet bulk micromachining for the formation of MEMS structures [50,51,[59][60][61][62][63][64][65].
In this article, several methods employed to improve the etch rate of silicon have been reviewed and discussed in detail. It is aimed at providing a single reference to understand different methods explored to increase the etch rate of silicon and the underlying etching mechanism behind each technique. The advantages and disadvantages of each approach are briefly explicated. This paper is intended to serve as a guide on silicon etch rate enhancement for applications in silicon wet bulk micromachining for the fabrication of microstructures.

Silicon etching mechanism in alkaline solution
The chemical reaction governing the etching process in alkaline solution is investigated by many researchers and several models have been proposed [70][71][72][73][74][75][76]. Among the different models, Gosalvez et al. model is more appropriate to explain the etching mechanism in alkaline solution [76]. Anisotropic etching consists of two alternating reactions, namely, (i) oxidation (ii) etching with the peculiarity that oxidation is very slow as compared to etching. Generally, silicon surface atoms are terminated by hydrogen. Hydroxide (OH − ) ions and water (H 2 O) molecules are the chemically reactive species in the alkaline solution (KOH, TMAH). Figure 5 presents the etching mechanism in pure alkaline solution. The oxidation step can occur through either chemical oxidation and/or electrochemical oxidation. In the chemical oxidation step, the H atoms of Si-H surface bonds are replaced by OH − ions due to the higher electronegativity of oxygen compared to hydrogen. Here, OH ‫-‬ ion acts as a catalyst while H 2 O is the reactive molecule. As a result, the surface Si atoms are oxidized. The chemical reaction for the process on Si{100}, which has two dangling and two back bonds, is given by the following equation. (1) In electrochemical oxidation, the surface Si-H bond is broken due to thermal dissociation. This leaves two electrons in the conduction band of Si. The OH ‫-‬ ions, being negatively charged, cannot be adsorbed on to the Si surface. H 2 O molecules, which are near the Si surface will accept that two electrons resulting in its dissociation into H + and OH ‫-‬ ions. These OH ‫-‬ ions are adsorbed on the Si surface forming Si-OH bonds and lead to the weakening of Si-Si back bonds (Fig. 5b). The chemical reaction for the same is shown in Eq. (2) The higher electronegativity of oxygen on the surface Si atom leads to the weakening of Si-Si back bonds. The polar water molecules in the solution attack the weakened Si-Si back bonds and self-dissociated into H + and OH − ions. The OH − ions formed get attached to the Si atom that is being etched out forming orthosilicic acid (Si(OH) 4 ). The H + ions form a Si-H bond with the Si atoms in the next layer of the substrate (Fig. 5c). The etching reaction is as shown in following equation The chemical/electrochemical reactions (1), (2) and (3) need to take place in a short time period in order to achieve high etch rate. In addition, reactants in chemical process must be sufficiently supplied to etching front, and etched products must be smoothly removed from etched surface. When any of them meet difficulties to advance in the etching process, etch rate decreases as a result. In other words, etch rate can be enhanced in many ways, such as by accelerating the chemical reactions themselves, by enhancing the supply of reactants, or by removing etched products from etching front. Several methods have explored to increase the etch rate which are discussed in subsequent sections.

Etching with ultrasonic agitation
In anisotropic wet etching of silicon, ultrasonic agitation applied to etching solution is effective to enhance the etch rate. In this etching method, ultrasonic vibrational force helps to remove the etched products from Si surface. As discussed in previous section, the etched products are hydrogen ion (H + ), hydrogen gas (H 2 ), and Si(OH) 4 . H + makes a new Si-H bond with a Si atom on the next layer of the crystal surface waiting for the next chemical reactions to start. H 2 can dwell on the etched surface and can be an obstacle for the etching front exposed to fresh reactants. Si(OH) 4 is ionized as dihydroxidodioxidosilicate(2-) (SiO 2 (OH) 2 2− ) in alkaline solution according to the following sequence.
(2) 4  However, silicate Si(OH) 4 still can stay on Si surface in the shape of polymer [41,42] in diluted alkaline solutions. Ultrasonic vibration applied to the solution enhances the detachment of H 2 bubbles as well as silicate Si(OH) 4 from Si surface allowing a fresh etch solution get contact to the surface.
Chen et al. [66] measured the average etch rate as a function of ultrasonic agitation power for 33 wt% KOH and 5 wt% TMAH water solutions. The results are shown in Fig. 6. Average etch rate increases according to the increase of ultrasonic power input in a range of 0 to 400 W with a single frequency of 40 kHz. Etch rate is saturated at the power of 350 W. The ratio of increase of etch rate between with and without agitation for 33 wt% KOH at 80 ℃ is 1.2, while for 5 wt% TMAH at 70 ℃ it is (4) Si(OH ) 4 + 2OH − → SiO 2 (OH ) 2− 2 1.6. The results are not much attractive from a viewpoint of the etch rate enhancement compared to the effect of etching temperature and the addition of oxidizers as will be discussed in next sections.
Most attractive effect of the ultrasonic agitation to etching solution is an improvement of etched surface morphology due to the conquest of micro-pyramids emerging on the etched surface. The study of the etched surface morphology is mostly done on Si{100} wafers as this orientation is most widely used in MEMS and semiconductor industries. In pure alkaline solution, the etched surface morphology primarily depends on etchant concentration and etching temperature. In the case of KOH solution, when the concentration is diluted less than 8 M (30 wt%), micro-pyramids start to appear on Si{100} as shown in Fig. 7 [77]. Various models have been proposed to explain the main reasons behind the formation of hillocks during etching process. Palik et al. [78] argued that the micro-pyramids are originated due to the attachment of etched products (e.g. H 2 gas and silicate) on silicon surface during etching process. Both can dwell on the Si surface acting as micro-mask sitting on every apex of the pyramids. Schiffrin's group investigated the suppression of micro-pyramids assuming that the H 2 bubbles are the source of micro-pyramids [79,80]. They added oxygen gas as an oxidizer to a saturation level, and isopropylalcohol (IPA) to 3.6 M KOH solution for weakening surface tension of the H 2 bubbles. They found the decrease in the generation of H 2 bubbles and consequently the number of hillocks [79]. They further applied ultrasonic agitation to 2 M KOH solution (again with saturated O 2 and IPA), for minimizing the dwelling time of H 2 bubbles on Si surface [80]. They obtained a pyramidfree etched surface on Si{100} as shown in Fig. 8. They obtained a root mean square roughness less than 20 nm.  Elwenspoek's group separately investigated the minimization of micro-pyramidal structures and found that the effective conditions are correlated to etch rate enhancement by means of adding oxidation agent (e.g., ferricyanide K 3 Fe(CN) 6 ) in etchant solution, or applying biased potential slightly higher than open circuit potential (OCP) to Si substrate [77]. They concluded that the silicon oxidation enhancement is effective to minimize the pyramidal structures and also for etch rate enhancement, though excessive oxidation resulted in etch stop by full passivation of Si surface with an oxide film.
The growth of micro-pyramids and their stability in size was discussed by Elwenspoek [81]. There was no direct evidence that the H 2 worked as a micro-mask because he noticed that the pyramidal structures in KOH are less than that in TMAH, though H 2 bubbles are generated much more with KOH than with TMAH. Hence it cannot firmly be claimed that H 2 bubbles are the only source of micro-pyramids. Hence there is a room here to consider another etched product silicate Si(OH) 4 as a source of micro-pyramid at etching front throughout deep etching process. The presence of silicate on etching front was recognized during Si etching in KOH, NH 4 OH, N 2 H 4 (Hydrazine), and TMAH solutions. Schnakenberg et al. [42] suggested that the etched product Si(OH) 4 became polymerized and worked as a protection mask in NH 4 OH solution. Gajda et al. [41] further claimed that the polymerized Si(OH) 4 takes a shape of silica gels in N 2 H 4 solution. Veenendaal et al. [82] proposed a model of silicate micro-mask assuming the presence of Si(OH) 4 particles on the etching front. Two models of micro-pyramid growth due to micro-masking by H 2 and Si(OH) 4 are compared in Fig. 9. Difficulty in H 2 model lies on the instable presence of bubbles on the apex of micro-pyramids. On the other hand, silicate model allows Si(OH) 4 cluster persistently staying on an apex during etching process. The cluster may be semi-permeable against etching species. It is easier for the silicate model to explain stable shape and size of pyramid during deep etching of Si.
Mechanical agitation means for suppressing micropyramids was studied by Chen et al. [66]. They compared the effects of mechanical stirring and ultrasonic agitation, and concluded that the latter is superior for suppressing micro-pyramids. Figure 10 compares the SEM images of Si{100} surfaces etched in 5% TMAH solution. In the case of etching without any agitation, Si{100} etched surface is fully covered by pyramidal structures (i.e. hillocks). The number of hillocks are significantly reduced when the etchant is mechanically agitated using magnetic stirrer and ultrasonic waves. However, it cannot be claimed unquestionably whether the attachment of H 2 bubbles is the main origin of the micro-pyramids or it is caused by silicates. Hence it can be concluded that the ultrasonic agitation mechanically helps to detach the etched products from etching front, and suppresses the emergence of pyramids.
The emergence of micro-pyramids does not directly decrease average etch rate, because footprint of the micro-pyramids is much less than the total etched surface area. However, when micro-pyramids densely emerges and fully covers the Si{100} surface (see Fig. 10 Sample A), substrate orientation (i.e. {100}) disappears from the top surface and pyramidal facet and its ridge orientations start to dominate the average etch rate of the substrate. Physical detachment of etched products (i.e. H 2 and/or silicate) in the etching process can contribute to enhance the etch rate, by indirectly enhancing the access of reactants to etching front.
In MEMS device fabrication, ultrasonic agitation needs more issues to consider than that mentioned above. As MEMS requires the fabrication of a varieties of 3D shapes such as deep narrow grooves and the large volume cavities etched from small-sized apertures on a Si wafer, etch rates can be much less than that expected from the etch rate obtained from flat wafers having mask apertures of large area. Use of ultrasonic agitation is one of the solutions to this issue. Ohwada et al. etched arrayed deep grooves on a Si{110} wafer [83]. As the sidewalls of the grooves are perpendicular to the wafer surface and the groove aperture width is narrow down to 1 μm, H 2 bubbles as an etched product easily clog up in the etched groove cross-section, restricting fresh etching solution flowing into the groove. In order to let the bubbles out from the groove before growing in size, they applied ultrasonic agitation in the etching bath. The cross-sectional views of etched grooves are shown in Fig. 11. The grooves formed without ultrasonic agitation could not achieve uniform etch depth, while the etch depth of the grooves with wider spacing fabricated with ultrasonic agitation increases. Although the ultrasonic agitation of a single frequency of 28 kHz with an output power of 600 W had improved groove depth to some extent, but non-uniformity in etch depths among grooves is remained there. They argued that it is due to a standing wave developed in the silicon wafer by a single frequency of ultrasonic waves. They further employed a new ultrasonic oscillator sequentially generating three different frequencies of 28, 45, and 100 kHz with time duration of 1 ms each and succeeded in obtaining grooves with uniform depth as shown in Fig. 11c. This is an example of ultrasonic agitation employed to remove H 2 bubbles (etched products) from narrow apertures on a wafer surface for fabricating semi-closed etched cavities. The optimization of ultrasonic agitation power is not yet quantitatively investigated. Schiffrin's group just claimed "a moderate power" of ultrasound [79]. Though Ohwada et al. [83] and Chen et al. [66] reported the output power of the oscillators, those values are just input power applied to the etching bath walls. Effective sonic energy applied to the etching front needs a modelling tailored for each etching facility in relation to bath size, liquid level and wafer positions.
While the ultrasonic agitation of etchant solution helps to remove the etched products, in case of microfabrication of MEMS structures, there is a risk that the excessive ultrasonic agitation causes a fracture of thin film structure such as diaphragm. This is one of the main reasons that the ultrasonic agitation did not become popular in MEMS industry.

Microwave irradiation during etching
It was first reported by Dziuban that the etch rate of silicon in KOH water solution is enhanced by the assistance of microwave irradiation [67]. He designed an etching apparatus equipped with a microwave generator having Dziuban admitted that the mechanism of etch rate enhancement is not clear. He speculated that the microwave irradiation dissociated water molecules, and increased an amount of active OH − in the solution, which can enhance the attachment of OH to Si surface atoms. The most interesting results with this work are that the Si was etched by DI water at 180 ℃ and 4 MPa pressure. It is not clear if microwave irradiation contributed to change in the state of water, because no evidence was presented.
The major weakness of the work was that the author neither measured nor estimated the local temperature rise at solid-liquid interface during etching. It is concerned that the microwave had induced quite local heat generation on the surface of conductive solid Si, i.e. the etching front, in addition to heat bulk liquid solution which is measured during etching process. The results are shown in Fig. 12a can be understood that the temperature of solid-liquid interface already reached to a boiling point of 10 M KOH solution regardless of the controlled temperature measured apart from the etching front in a range 70-100 ℃. Obtained constant etch rates are equivalent to the values at boiling point of the 10 M KOH solution. There is a concern that the etching temperature measured by the etching microwave silicon (EMSi) system is not the temperature of solid-liquid interface but of the bulk solution. Real etching front temperature seems much underestimated. In addition, we think that an increase in etching rate with the increase of KOH concentration shown in Fig. 12b is not an effect of The etch rate increase depending on the increase of concentration is quite reasonable, assuming that etching front temperature always reached a boiling point in the EMSi system. The rise of boiling point with highly concentrated solutions utilized in etch rate enhancement is quantitatively discussed in the next section. We think that the reported etch rates enhancement argued in Fig. 12 is not due to the decomposition of water molecule, but mainly due to the local heating at etching front to the boiling point of etching solutions accelerating oxidation reaction described by Eq. (1) in previous section.
Dziuban's group further developed a new apparatus named extended etching microwave silicon (E2MSi) aiming at separating the etching chamber from microwave irradiation processing of solution as shown in Fig. 13 [84]. It has two vessels connected in series by tubing. One vessel is for microwave irradiation processing of etching solution, and the other is for Si etching using the activated solution. Microwave power was in a range of 0-3000 W, and frequency was 2.45 GHz. Etching solution was firstly processed by microwave irradiation for 30 s, and then quickly introduced to etching vessel in a short time delay. Etch rate of Si{100} in 0.5 M KOH solution at 60 ℃ was measured as a function of applied microwave power. Etch rate increment, roughly proportional to the input power, is 0.44 um/min when microwave power was 3000 W. The obtained etch rate was 5 times higher than that without agitation. Etch rate enhancement effect of microwave was quickly decayed in less than 10 s after microwave irradiation as shown in Fig. 14 [84]. Time delay for the irradiated solution reaches to the wafer in the etching vessel was adjusted by the length of connecting tubes between two vessels. When the time delay was 10 s, the etch rate enhancement was almost disappeared. Dziuban' group illustrated a model of microwave irradiation effects in the vicinity of Si surface as shown in Fig. 15. They claimed that the H 2 O molecules in normal state are making clusters each containing hundreds of molecules bound to each other by weak hydrogen bonds. This makes the number of H 2 O molecules effectively contacting Si surface far less than that in case all the H 2 O molecules randomly approach to Si surface. Microwave irradiation destroys the water cluster in pieces and many of the H 2 O molecules should be isolated from cluster as shown in Fig. 15b. Such an activated state of water should have an increased number of contacting points on Si surface. This enhances the adsorption of water molecules ready for reaction. It is regarded as one of the etch rate enhancement means, by enhancing adsorption of reactant H 2 O to Si surface. This could be a reason for an increase in etch rate. When microwave irradiation stops, the finely randomized water molecules gradually rebuild larger sized clusters as shown in Fig. 15c, losing the activeness. Dziuban's explanation seems acceptable on the activation and its decay of the water molecules by  switching the microwave irradiation. He measured electric conductivity of water with and without microwave irradiation, and found that the irradiated water shows a conductivity 40% less than that for non-irradiated water. Recovery in conductivity was observed with water after microwave irradiation. These results together with FTIR absorption spectrum change, stimulation of water cluster may be possible for enhancing etching rate to some extent, though time decay of irradiation effect still could not be explained.
We think that Dziuban's experimental proofs seem insufficient in terms of accuracy and repeatability of experimental data. Firstly, the way they evaluated the time decay in etch rate (Fig. 14) is questionable. They plotted etch rate values at t = 3 and 10 s showing a time decay with E2MSi apparatus. As the etch rate measurement needs to etch Si typically for more than minutes, it seems hard to distinguish the etch rates taken by time difference of 7 s. Secondly, Fig. 14 contains a data taken by their first apparatus EMSi [67] which had a problem underestimating etching temperatures as mentioned before. Hence the mechanism of etch rate enhancement by microwave irradiation is still on a level of speculation. If the etch rate enhancement works according to Dziuban's model, it can be categorized as an enhancement of reactants supply to the etching front mentioned in previous section. It may be emphasized here that this study was not carry forwarded by any other research group. Hence the etching mechanism behind the increase in etch rate due to microwave irradiation presented by Dziuban cannot be corroborated.

Etching at boiling point of the etchant
It is known that the anisotropic etching is a time-consuming reaction because the oxidation reaction in Eq. (1) is the slowest reaction throughout the etching process. The oxidation reaction demands high activation energies [13,25]. It is reasonable to etch silicon at high temperature to accelerate the process by the aid of thermal energy. Seidel et al. reported the temperature dependence of silicon etching rate in KOH aqueous solutions in a concentration range of 10-50 wt% [13]. The etching rate followed the Arrhenius equation, and the activation energies for the etching reaction calculated for 20 wt% KOH are 0.57 eV (55 kJ/mol) on Si{100} and 0.59 eV (57 kJ/mol) on Si{110}. Considering the band model, density of state diagram for silicon, and the value of activation energy obtained from the Arrhenius equation, it was claimed that the etching rate is determined by the transfer of electrons in the case of KOH aqueous solution. Similarly, the temperature dependent etch rates were also investigated with TMAH solutions. Tabata et al. [25] reported the etching rates of silicon in TMAH aqueous solution in a concentration range of 5-40 wt%. The activation energies of the etching rate are 0.5 to 0.7 eV (48 to 67 kJ/mol) on Si{100} and 0.4 eV (39 kJ/mol) on Si{110}. These results consistently support the etching mechanism proposed by Seidel et al. [13]. Such large values of activation energy with KOH and TMAH solutions suggest that the etching at high temperature is effective to enhance the etch rate. In wet etching system, achievable maximum temperature is limited by the boiling point of the etching solution.
Tanaka et al. investigated the etch rates in high concentration KOH solutions at the temperature near to the boiling point of the solution aiming to obtain high etching rate with smooth etched surface [18,85]. Figure 16 shows the etch rates of Si{100} and Si{110} at different temperatures as a function of KOH concentration ranging from 20 to 50 wt%. The etch rate for each concentration decreases with increase of KOH concentration, Fig. 16 Etch rate of Si{100} and Si{110} as a function of KOH concentration. Highly concentrated solutions can reach higher temperatures allowing high speed etching [18,85], © 2004, Elsevier which is due to the shortage of the reactant H 2 O in the concentrated solution. The boiling point of the KOH solution increases with its concentration. Henceforth, the etching temperature of concentrated KOH can go much higher than the less concentrated KOH. As a matter of fact, boiling point of 50 wt% KOH solution is 145 ℃, while that of 28 wt% KOH solution is 110 ℃. Achievable etching rate for Si{100} shown in Fig. 16a is 10 μm/ min with 50 wt% KOH solution, while that with 28 wt% KOH solution is 5 μm/min. Similarly, for Si{110} shown in Fig. 16b, 50wt% KOH solution achieved etching rate of 21 μm/min, while 28 wt% KOH solution achieved 8 μm/ min. If the etch rates are compared with 80 ℃ etching temperature, the etch rates in 50 wt% KOH are increased by 5-9 times for Si{100} and 4-20 times for Si{110} when the etching temperature increase upto the boiling point of the etchant. Figure 17 shows the Arrhenius plots of Si{100} and Si{110} etching rates for different KOH concentrations ranging from 10 to 50 wt%. The Arrhenius plots are nearly linear from 80 ℃ to near the boiling point. This means that the activation process in the high temperature region is almost the same as that in the lower temperature region. Figure 18 shows the etched surface roughness of Si{100} and Si{110} in KOH with a concentration range of 15-50 wt%. It is apparent that the Si{100} etched surfaces are consistently smooth when etched with KOH solutions whose concentration is higher than 28 wt% as presented in Fig. 18a. On the other hand, the etched surface roughness results of Si{100} presented in Fig. 18b do not show any concentration range where smoother surface can be obtained. Optical microscope images of the etched Si{110} surface presented in Fig. 19 indicate that the smooth etched surface without hillocks can be obtained in 45 wt% KOH at 135 °C. Therefore, the results presented in Figs. 16,17,18,19 should be used as reference to find the optimal etching condition to obtain highspeed etching with smoother etched surface.
Tang et al. studied the etch rate of Si{100} in surfactant added TMAH at high temperature [69]. Etching temperature is varied from 80 to 115 °C, which is close to the boiling point of the etchant solution. Figure 20 shows the temperature dependence etch rate of Si{100} in Triton-X-100 added 25 wt% TMAH. It is reported that the etching rate above 100 ℃ is increased 3 to 4 times that in 80 ℃ solution. The effect of etching temperature on the etched surface roughness is presented in Fig. 21. The surface roughness is improved as the etching temperature approaches towards the boiling point of the etchant solution.
It is concluded that the etching at the temperature near to the boiling point of the etchant solution is an effective way of enhancing the etching rate. In addition, the etched surfaces are smoother compared to that etched at lower temperatures. However, when applying the high temperature etching technologies to real production, it is necessary to keep the solution composition constant by compensating selective evaporation among components. When looking at Figs. 20 and 21, there is a significant shift in experimental curves between 100 and 105 ℃. Such an event might be caused by a change in the composition during etching due to selective evaporation or decomposition of any component in the solution, such as Triton-X-100 which is a component to suppress etching rate.
A bottle neck step dominating the etching rate of silicon is the oxidation of silicon among all steps in the  [85] etching system. This is the reason why high temperature etching is so effective to achieve high speed etching. In comparison, stirring motion of the solution is less necessary for enhancing etching rate, but necessary only for improving uniformity in temperature of the etching solution in the bath. This is quite a contrast to chemical isotropic etching of silicon using hydrofluoric acid (HF) and nitric acid (HNO 3 ) water solution that aggressively reacts with Si. The isotropic etching is a typical diffusionlimited reaction process whose etched shape is strongly influenced by a weak flow and diffusion of the solution. In actual high-speed etching processing, swiftness and stability of wafer handling is also required until etching process is carried out at high temperature. The etch selectivity between silicon and silicon dioxide decreases with etching temperature. Hence it is necessary to use a etch mask which has high etch selectivity with silicon such as silicon nitride (Si 3 N 4 ), Cr-Au, etc.
Yang et al. studied the effect of three ion-typed surfactants including anionic SDSS, cationic ASPEG and non-ionic PEG on the etch rate of Si{100} in 30 wt% KOH and 10 wt% TMAH solutions [56]. The etch rate of Si{100} at different temperatures in pure and surfactant added 30 wt% KOH is shown in Fig. 23a, while Fig. 23b presents the same for pure and surfactant added 10 wt% TMAH. It can easily be noticed from Fig. 23 that the anionic SDSS is the most effective additive to increase the etch rate. The effect SDSS on etch rate increases as the etching temperature increases.
Yan et al. studied the modified TMAH by addition of ammonium persulfate (APS or AP, (NH 4 ) 2 S 2 O 8 ) to protect the aluminium layer during the etching silicon [106]. They achieved zero etching rate of Al when adequate amounts of AP and silicon are added to TMAH solution. Figure 24 shows the etch rate of aluminium as a function of AP concentration at 85 °C. The amount of dissolved silicon in 5 and 10 wt.% TMAH are 1.6 and 3.2 wt.%, respectively. The etch rate of Al decreases to zero when the concentration of AP reaches 0.4 wt% and 1.2 wt% in 5 wt% and 10 wt% TMAH, respectively. Figure 25 shows the etch rate and surface roughness of Si{100} as a function of AP concentration in 10 wt% TMAH with dissolved 3.2 wt% silicon at 85 °C. The etch rate of silicon increases from 0.4 μm/min to 0.85 ± 0.9 μm/min when AP concentration is in the range between 0.6 and 2.0 wt% [106]. Fujitsuka et al. studied different kinds of additives in 10 wt% TMAH to protect the Al during silicon anisotropic etching [108]. Figure 26 shows the etch rates of Al-Si and Si{100} as a function of ammonium persulfate (AP) concentration in 3.2 wt% Si dissolving 10 wt.% TMAH at 80 °C [108]. Si anisotropic etching without Al etch is achieved by dissolving Si and AP in TMAH. Jun et al. investigated the effect of ammonium persulfate (AP), pyrazine, ammonium hydrogen sulfate (AHS), and isopropyl alcohol (IPA) in 10 wt% TMAH at 70 °C [109]. The addition of AP, pyrazine, AHS increases the etch rate as shown in Fig. 27. It can be easily noticed that AP and pyrazine are more effective additives to increase the etch rate. Hence it can be stated that the ammonium wt% TMAH and 20 wt% KOH. In addition, the etch rate of SiO 2 and its selectivity with silicon are investigated. 5 wt% TMAH is selected because the lower concentration TMAH (2-5 wt%) provides greater etch rate in comparison to higher concentration TMAH (20-25 wt%). Similarly, 20 wt% KOH is chosen to obtain higher etch rate. In both etchants, the concentration of NH 2 OH is varied from 5 to 20% in step of 5% to obtain the optimum concentration for achieving improved chemical etch rate. Figures 28 and 29 present the etch rates of different orientation silicon surfaces in 5 wt% TMAH and 20% KOH without and with addition of different concentrations of NH 2 OH, respectively. Standard deviation in etch rate is determined by taking four measurements on the same chip at different locations. In both cases, the etch rate of silicon increases significantly as the concentration of NH 2 OH increases. In the case of TMAH, 10% NH 2 OH is an optimal concentration to obtain the highest etch rate, while 15% NH 2 OH is an appropriate concentration for KOH to achieve maximum etch rate of silicon.
Silicon etching process takes place in three major steps: (1) diffusion of the reactants to the silicon surface, (2) chemical reaction between etchant and silicon atoms, and (4) diffusion/dissolution of reaction products into the bulk etchant [3,56,92]. Factors (1) and (3) are termed "diffusion-limited dissolution", while factor (2) is termed "reaction rate limited dissolution", which depend on the etching temperature, types and concentrations of etchants and additives and crystallographic directions. The chemical activities of the etching solutions induced by additive and types of etchant strongly affect the etching rate for a fixed etching temperature. If an additive improves the wettability of the etchant, the etchant can easily diffuse to the silicon surface and the reaction products can quickly diffuse to the bulk etchant that results in higher etch rate. The etch rate may increase if the chemical activities of the etchant are improved by the incorporation of the additive. So, it can be stated that an additive in alkaline solution can increase the etch rate if it increases diffusivity or reactivity or both. The addition of complexant increases OH − free ions in the solution, resulting an increase of the etch rate [52]. The addition of SDSS improve the wettability in KOH and chemical activity in TMAH that results in increase of the etch rate [56].
The behaviour of NH 2 OH in alkaline solutions is extensively studied [112][113][114][115][116]. To explain the possible etching mechanism in NH 2 OH-added alkaline solution a simple model was proposed by Swarnalatha et al. [117]. In this model, NH 2 O − and OH − ions are considered as catalysts, while H 2 O as the reactive molecule. When NH 2 OH is added to pure alkaline solutions, it reacts with OH − ions in the solution forming NH 2 O − ions and water molecules as shown in the following equations [112]: The overall reaction of decomposition of NH 2 OH in alkaline solution is given by, So, the reactive species, which are greatly accessible in NH 2 OH-added alkaline solution, are NH 2 O − ions, OH − ions and H 2 O molecules. As the number of reactive species in NH 2 OH-added alkaline solution is more in comparison to that in pure alkaline solution, the etch rate significantly increases in the presence of NH 2 OH. In addition to these species, other intermediate compounds (NH 2 NHOH, HNO and NH 2 O radical) may also participate in the etching reaction and might be helping to improve the etch rate. The etching mechanism proceeds through sequential oxidation and removal of surface silicon atoms by water molecules. The first step in the mechanism is the oxidation of surface silicon atoms. This can proceed through either chemical oxidation and/or electrochemical oxidation.   It may be emphasized here that the maximum etch rate is obtained when the concentration of NH 2 OH reaches to its optimal value. As per the experimental results, NH 2 OH concentration affects optimal balance of main active species in the etchant and therefore the etch rate decreases if NH 2 OH concentration is decreased/ increased below/above its most favorable value. A qualitative comparison of various kinds of additives that increase the etch rates of Si in TMAH and KOH solution is presented in Tables 2 and 3, respectively.
Hydroxylamine (NH 2 OH) is unstable in nature in the presence of alkaline solutions [115]. Therefore, it is important to study the influence of etchant age on the etch rate of silicon. 10% NH 2 OH + 5 wt% TMAH and 15% NH 2 OH + 20 wt% KOH are the optimal compositions to achieve the highest etch rate in comparison to pure TMAH and KOH, respectively. Hence these compositions are employed to study the aging effect. In order to know the effect of etchant age on the etch rate of Si{100}, Si{110}, and Si{111}, the same etchant solution is continuously used and the experiments are performed at different time interval in the same solution. Figure 30 shows the effect of etchant age for 10% NH 2 OH + 5 wt% TMAH, while Fig. 31 presents for 15% NH 2 OH + 20 wt% KOH. It can easily be noticed that the etchant age considerably affects the etch rate. It is reduced with the age of the etchant. Hence it can be recommended here that the NH 2 OH-added solution should be used immediately to achieve high etch rate. To fabricate the controlled depth cavities, etch depth must be measured in between etching process. The main cause of aging may be the selfdegrading behavior of NH 2 OH in TMAH/KOH solution [65].
Silicon dioxide, especially thermally grown oxide, is extensively used in silicon micromachining as etch mask layer to create various kinds of grooves and cavities. Moreover, it is employed as structural layer for the fabrication of MEMS structures such as cantilever. Figure 32 presents the etch rate of thermally grown SiO 2 and the etch selectivity between silicon and silicon dioxide. The etch selectivity is calculated using the etch rates of silicon and silicon dioxide in pure and different concentrations of NH 2 OH-added TMAH. The same data for pure  and NH 2 OH-added KOH are shown in Fig. 33. In both kinds of etchants, the etch rate of SiO 2 increases as the concentration of NH 2 OH increases. In the fabrication of MEMS structures, the etch selectivity between silicon (or substrate) and mask/structural layer (e.g. SiO 2 ) is an important concern and is defined as the ratio of the etch rate of silicon (substrate) to that of SiO 2. If an etchant provides high etch selectivity, mask/structural layer can be exposed in the etchant for a longer time, which is needed to fabricate deep cavities/grooves or freestanding structures (e.g. cantilever) on silicon wafer. It can simply be noticed that the etch selectivity increases appreciably on the addition of NH 2 OH, which is desirable to use oxide layer as a mask to form high depth cavities and/ or to fabricate suspended structures of silicon dioxide. It can be concluded that the addition of NH 2 OH not only increases the etch rate of silicon but also gives higher etch selectivity Si and SiO 2 as compared to pure TMAH/ KOH.

Surface roughness and morphology
Etched surface roughness and morphology are the important etching characteristics of an etchant. The requirement of smooth and textured surface morphology depends on the application. To fabricate uniform thickness suspended microstructure or uniform depth cavity/ groove or to achieve high reflectance for optical application smooth surface is needed, while in the case of solar cell application rough surface is required to reduce the reflectance of light. As discussed in "Etching with ultrasonic agitation" section, the main cause of surface roughness in the wet etching process is micromasking by the hydrogen bubbles and/or impurities on the surface during the etching process [2,3,7,19,27,[76][77][78][79][80][81][82][118][119][120]. With an increase in hydrogen bubbles (or impurities or etching products) and their attachment with silicon surface during etching, surface roughness increases. The precise origin of most morphologic features is still under debate. However, in general, four simultaneous conditions are required to form hillocks on an etched surface.  These conditions are the existence of a micromasking agent that stabilizes the apex atom/s, fast downward motion of the floor surface, stable side edges, and very stable lateral facets. The micromasking agent can be an insoluble reaction product, an etchant impurity, or an etchant inhomogeneity (such as a hydrogen bubble). The etchant inhomogeneities refer in general to the formation  . 32 a Etch rate of silicon dioxide and b the etch selectivity between silicon and silicon dioxide (R Si /R SiO2 ) in 5 wt% TMAH at 70 ± 1 °C without and with addition of NH 2 OH [50] of inhomogeneous patterns due to the slow transport of the reactants and/or products. Surface morphology can be explained based on etch rate variation of the crystallographic orientation of silicon, which depends on the local neighbourhood of the exposed surface atoms, dangling bonds, and surface bonds [2]. Etched surface morphology and the surface roughness are influenced by the addition of an additive/s. Surfactant and isopropyl alcohol (IPA) are most extensively studied to improve etched surface morphology [3,7,[27][28][29][30][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105]. The surfactant is commonly used as an additive in TMAH to obtain smooth etched surface morphology, while IPA is explored as most suitable additive in KOH to improve etched surface morphology. The present article is focused on the brief overview of etchants and the methods that provide higher etch rate in comparison to conventional etching method using pure alkaline solution. Hence the etched surface results obtained in the etchants providing higher etch rate have been included in this paper. Figure 34 presents the surface roughness obtained in pure and redox + complexant added 4.5 M KOH [52]. It can easily be noticed that the roughness is reduced by one order of magnitude when redox + complexant is incorporated to KOH. As discussed in previous section and Fig. 23, the addition of SDSS is more effective at higher temperature (i.e. 90-100 °C). Figure 35 shows the SEM micrographs and corresponding average roughness of Si{100} surface etched in pure and SDSS added 30 wt% KOH and 10 wt% TMAH at 90 °C. The addition of SDSS significantly affects the topology of the etched surface in KOH solution as can be seen in Fig. 35a, b, while it worsens the roughness when added to TMAH as can be observed in Fig. 35c, d. The etched surface roughness of Si{100} as a function of ammonium persulfate (AP or APS, NH 4 ) 2 S 2 O 8 ) concentration in 10 wt% TMAH is presented in Fig. 25 [106]. When AP concentration increases, etched surface roughness decreases. The surface roughness is less than 0.1 μm when AP concentration in the range between 0.6 and 2.0 wt.%. However, it becomes rougher if too much AP is incorporated. Figure 36 shows the SEM photographs of the silicon surface etched in 5 wt% TMAH without and with the addition of AP [106]. It can easily be noticed that the silicon surface improves significantly on the addition of AP. Figure 37 presents the etched surface morphology as a function of ammonium persulfate concentration in 3.2 wt% Si dissolving 10 wt.% TMAH at 80 °C [108]. It can straightforwardly be noted that the addition of AP suppresses the formation of micropyramids. The effect of AP, pyrazine, and AHS on the etched surface roughness of Si{100} in 10 wt% TMAH at 70 °C is presented in Fig. 27 [109]. The surface roughness is optimum when 2.5 g/500 mL AP is added. When 0.75 g of pyrazine is added, the surface roughness is improved to 35.3 nm. In the case of AHS, best surface roughness is obtained when 5.0 g/500 mL AHS is added to 10 wt% TMAH. Figure 38 show the SEM images of etched surfaces of Si{100} with the addition of various concentration ammonium persulfate (AP) in 10 wt% TMAH at 70 °C [109]. The number of hillocks decreased significantly when 1.0 g or 2.5 g/500 mL AP is added. The cleanest etched surface is obtained when 2.5 g of AP is added. It can be emphasized here that the addition of ammonium peroxodisulfate (AP) helps to reduce hillocks and at the same time increase the silicon etching rate. Figures 39,40,41 show the etched surface roughness and morphology of Si{100}, Si{110}, and Si{111}, respectively, in pure and different concentrations of NH 2 OH-added 5 wt% TMAH [50,[60][61][62]. The surface roughness of Si{100} gradually decreases as the concentration of NH 2 OH increases. It decreases by an order of magnitude for NH 2 OH concentration of 10% or more. The surface roughness of Si{110} and Si{111} is not influenced significantly when NH 2 OH is added to TMAH. As 10% NH 2 OH-added TMAH provides maximum etch rate, the same etchant composition is suitable to obtain a smoother surface in comparison to pure TMAH.   As discussed previously in etch rate "Etch rate" section, the NH 2 OH is an unstable solution and decomposes in the presence of alkaline solutions. Therefore, the effect of etchant age on the etched surface roughness and morphology is investigated in the optimal composition of NH 2 OH-added TMAH/KOH (i.e. 10% NH 2 OH + 5 wt% TMAH and 15% NH 2 OH-20 wt% KOH), which provides the highest etch rate of silicon with improved etched surface morphology. To observe the aging effect, the same etchant solution is continuously used for one week and the experiments are performed at regular time interval. Figures 45, 46, 47 show the average surface roughness and the corresponding morphology of Si{100}, Si{110} and Si{111}, respectively in NH 2 OH-added TMAH, whereas Figs. 48, 49, 50 present the same for Si{100}, Si{110} and Si{111}, respectively, in NH 2 OH-added KOH [50,51,[60][61][62][63][64][65]. Although the etchant age influences the etched surface roughness and morphology, it is not significant like etch rate presented in previous section. The etch rate decreases with the age of the etchant. Hence it is recommended that the NH 2 OH-added TMAH/KOH should be used immediately after preparation of the solution to achieve higher etch rate and better etched surface morphology.

Undercutting
The removal of bulk material at mask edges and convex corners is termed as undercutting. MEMS and NEMS (nanoelectromechanical systems)-based sensors and actuators usually employ freestanding structures like cantilevers, fixed-fixed beams, diaphragms, etc. To fabricate such structures, the phenomenon of undercutting is employed, which removes the underneath material of the masking layer and ultimately results in freestanding structures [1][2][3][4][5][6][118][119][120][121][122]. In the case of a microstructure with free end, undercutting at the convex corner is inevitable. At the same time, convex corner undercutting is undesirable to fabricate mesa structures for applications in accelerometer and other sensors [118][119][120][121][122][123]. Various methods have been proposed to recess undercutting which includes altering the etching characteristics by addition of surfactants, employing corner compensation methods where the additional mask is provided at the convex corners to delay the etching at the convex corners, etc. [118,119]. To decrease the release time of suspended structure to improve industrial production and/or to minimize the exposure of structural layer in the etching solution, high undercutting is inevitable. Therefore, an etchant composition providing high undercutting is greatly desirable in silicon wet bulk micromachining. Undercutting length at convex corners (l), etch depth (d), and etch time (t) are used to define the undercutting rate (l/t), and undercutting ratio (l/d), which are very important parameters to estimate the release time for the fabrication of overhanging structures using wet anisotropic etching [118,119].
To study the undercutting at Si{100} surface, usually, rectangular shape mask pattern formed by < 110 > directions is used as schematically presented in Fig. 51. To calculate the undercutting rate and undercutting ratio, the undercutting length is measured along < 110 > directions (i.e., l <110> ). As discussed in previous sections, the addition of Pyrazine, AP, and AHS in TMAH increases the etching rate effectively and improves the surface flatness. However, these additives do not affect undercutting significantly [109]. Figure 52 shows the undercutting at convex corners on Si{100} surface without and with addition of various concentrations of ammonium persulfate (AP) in 10 wt% TMAH at 70 °C [109]. The undercutting ratio at 2.5 g/500 mL, at which the optimal etching rate and the surface roughness are achieved, is almost identical to that of the pure TMAH solution. Figures 53 and 54 present the undercutting rate (l <110> /t) in pure and different concentrations of NH 2 OH-added 5 wt% TMAH and 20 wt% KOH, respectively. The undercutting rate increases with increase of NH 2 OH concentration and starts decreasing after certain concentration of NH 2 OH. In the case of TMAH, it becomes maximum when NH 2 OH concentration reaches 10% as can be seen in Fig. 53, while in KOH the highest value is achieved when NH 2 OH concentration reaches 15% as can be noticed in Fig. 54. The undercutting at convex corners occurs due to the appearance of high index planes during etching process [3,118,119]. Hence the increase in undercutting indicates that the etch rate of high index planes also dramatically improves when NH 2 OH is added into TMAH/KOH. The increment in undercutting is more than three times to that in pure TMAH/KOH, which is highly advantageous for the fast release of structure from substrate to minimize industrial manufacturing time. The  undercutting ratio (l/d) can be calculated by dividing the undercutting ratio by etch rate.
Si{110} wafer is a primary choice when the microstructures with vertical sidewalls formed by {111} planes are fabricated using wet anisotropic etching [124][125][126][127][128][129][130][131]. These vertical sidewalls appear at < 112 > directions which form a rhombus shape structure containing two types of convex corners (acute and obtuse corners) as illustrated in Fig. 55 for the fast release of suspended microstructures on Si{111} wafer. The fast (or more) undercutting at mask edges takes place mainly due to the emergence of high index planes during etching process. As the etch rate of high index planes improve when NH 2 OH is added into TMAH/KOH, the undercutting rate at < 112 > mask edges on Si{111} surface increases on the addition of NH 2 OH in TMAH/KOH. A solution providing high undercutting ratio is very useful to control the gap between freestanding structure and bottom surface such as bridges, combdrive structures, micro-gyroscope [140][141][142][143][144][145][146][147].
If the etch rate and undercutting rate results are considered together, 10% and 15% NH 2 OH are optimal concentrations to achieve improved etching characteristics in 5 wt% TMAH and 20 wt% KOH, respectively. The effect of aging is presented in Figs. 63 and 64 for TMAH and KOH based solutions, respectively. It can easily be observed from these results that the undercutting rate decreases with increase of etchant age. Hence, it is suggested to use the NH 2 OH-added TMAH/KOH immediately after preparation to obtain high undercutting. NH 2 OH-added TMAH/KOH provides high etch rate, high undercutting, and high etch selectivity between silicon and SiO 2. High etch rate is useful to form cavities/ grooves is less etching time, while high undercutting rate is extremely useful for the fast realization of suspended structures on silicon wafers. High etch selectivity

Summary and conclusions
Various methods to improve the etch rate of silicon have been reviewed and discussed systematically. Ultrasonic agitation during etching process improves the etch rate as well as etched surface morphology. However, due to mechanical sonic force applied to the device, the fragile structures may get damaged. Microwave irradiation significantly increases the etch rate, but experimental proofs are insufficient in terms of accuracy and repeatability of experimental data as no other research group confirmed (or carry forward) the effect of microwave irradiation on the etch rate of silicon. The etchant concentration significantly affects the etch rate. There are two ways to reach local maximum in etch rate, both having pros and cons. The first approach is to etch Si using a diluted KOH or TMAH solutions. Typical concentrations are in a range of 10-20 wt% for KOH and 2-5 wt% for TMAH. Pros are easiness to apply, cons are moderate increase of etch rate and inevitable etched surface roughness by an emergence of micro-pyramids. The second approach is etching at near boiling point typically at 145 ℃ using highly concentrated solution of 50 wt% KOH. Pros are available etch rate of the order of 10 μm/ min, cons are lower etch selectivity between Si and oxide mask demanding alternative mask materials. The etching characteristics are strongly influenced by different kinds of additives. Various additives have been investigated to improve the etch rate of silicon. The addition of redox system and complexant in KOH solution effectively increases the etch rate, but these additives are not studied further by other researchers and hence not very common in wet anisotropic etching.
The effects of various kinds of additives in TMAH solution to protect the Al layer during wet anisotropic etching step are investigated by several research groups. Ammonium peroxodisulfate (AP) is found as an optimal additive to obtain zero etch rate of Al. The addition of AP supresses hillocks/pyramids and at the same time increase the etch rate of silicon.
The addition of NH 2 OH significantly influences the etching characteristics of TMAH/KOH and provides highest etch rate and undercutting, which are the desirable properties to increase the industrial throughput when the structures are fabricated using silicon wet bulk micromachining. In this paper, the effect of NH 2 OH concentration on the etching characteristics of TMAH/KOH (e.g. etch rate, etched surface morphology and undercutting) is methodologically presented. Moreover, the influence of etchant age on the etching characteristics is systematically explained. The major conclusions pertaining to NH 2 OH-added TMAH/KOH are as follows: • The etch rate of silicon is increased when NH 2 OH is added to TMAH/KOH solution. • 10% and 15% are the optimal concentrations of NH 2 OH to be incorporated in 5 wt% TMAH and 20 wt% KOH, respectively, to achieve the high etch rate of silicon. • The etch selectivity between silicon and silicon dioxide (i.e. Si/ SiO 2 ) improves substantially with the addition of NH 2 OH. Therefore, SiO 2 thin film can be used as a mask/structural layer for the fabrication of microstructures. • The etch rate of Si{100}, Si{110} and Si{111} decreases significantly with etchant age. Hence it is recommended to use the etchant immediately after preparing the solution. • In pure TMAH/KOH, OH − and H 2 O are the active species, while the addition of NH 2 OH increases the active species which are NH 2 O − , OH − and H 2 O. The oxygen in OH − and NH 2 O − has high electronegativity and responsible for the high etch rate in NH 2 OH-added TMAH/KOH solution.
• The addition of NH 2 OH affects the surface roughness and morphology, but the variation is not significant. • The aging of etchant does not affect surface roughness considerably. • Analogous to etch rate, 10% and 15% are the optimal concentrations of NH 2 OH to be added in 5 wt% TMAH and 20 wt% KOH, respectively, to obtain the highest undercutting rate, which is greatly beneficial for the fast release of the microstructures for application to develop MEMS-based devices.    66 Freestanding SiO 2 microstructures fabricated on a Si{100}, b Si{110} and c Si{111} wafers in NH 2 OH-added KOH [51]