Research paper
Effect of concentration change of 0.1% triton added 25 wt% TMAH during fabrication of deep cavities with mesa structures in SOI wafer

https://doi.org/10.1016/j.mee.2020.111323Get rights and content

Highlights

  • Etching characteristics of 0.1% Triton-added 25 wt% TMAH used for long duration (>20 h).

  • Quantification of change in TMAH concentration during etching.

  • A simple method to regain the etching characteristics by replenishing lost water content.

  • Fabrication of 380 μm height mesa structures with protected convex corners in SOI wafer.

Abstract

Tetramethylammonium hydroxide (TMAH) is the widely used CMOS compatible anisotropic etchant in silicon wet bulk micromachining to fabricate MEMS structures. 25 wt% TMAH is commonly used because it provides the best etch selectivity between silicon and silicon dioxide with smooth etched surface morphology. If Triton X-100 is used as an additive, it results in significant decrease in the corner undercut while maintaining the good etch rate which is very useful to create mesa structures. This paper mainly focuses on the changes in etching characteristics of 0.1% Triton added 25 wt% TMAH solution when used to fabricate 380 μm deep cavities involving mesa structures in SOI wafer. We observed that the changes in etching characteristics are due to change in the concentration of etching solution which primarily arises because of loss of water content. By replenishing the etching solution with lost water content of same temperature, etching characteristics comparable to that of fresh solution have been obtained. The change in the etchant concentration during the course of etching has been studied. Further, corner undercut and etch rate of silicon have been investigated in both fresh etching solution and the solution replenished with lost water content. This study is especially useful for the control of etching characteristics like the etch rate and undercut during the fabrication of bulk-micromachined structures involving long duration of etching by replenishing the etching solution with the lost water content of same temperature at prefixed intervals of time during the course of etching to regain the etching properties of fresh solution.

Introduction

Wet bulk micromachining is an easy and cost effective method for the fabrication of standard microstructures (e.g. cantilever, diaphragm, cavity, proof mass suspended by beams, etc.) for a wide variety of applications in microelectromechanical systems (MEMS) [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Anisotropic etching of silicon can be carried out using inorganic alkaline solutions (e.g. Potassium hydroxide (KOH), Sodium hydroxide (NaOH), etc.) and organic alkaline solutions (e.g. Tetramethylammonium hydroxide (TMAH), Ethylenediamine pyrocatechol (EDP), etc.) [[10], [11], [12], [13]]. Among all, KOH and TMAH are the most commonly used etchants. The etching characteristics of both etchants have been thoroughly studied by researchers under various etching conditions [[11], [12], [13], [14], [15], [16], [17], [18], [19]]. TMAH is preferred over KOH when CMOS compatibility and etch selectivity between silicon (Si) and silicon dioxide (SiO2) are major concerns. The effect of IPA on the etching characteristics of TMAH solution has thoroughly been investigated [[20], [21], [22]]. The etching characteristics of TMAH are largely affected by the addition of surfactant (e.g. NCW, NC-200, PEG, Triton, etc.) and a lot of studies have already been completed in this area [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. With decreasing TMAH concentration from 25 to 10 wt%, there is an increase in the etch rate of silicon [37]. The etch rate, undercut and etched surface morphology of Si{100} and Si{110} wafers in pure and Triton-added TMAH solutions are also studied [2,4,[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Furthermore, using optimized etching characteristics of pure and Triton-added TMAH solution new fabrication methods were developed for the realization of conventional and unconventional shaped structures in single and bonded silicon wafers [5,9,38]. The etching characteristics of Si{100} and Si{110} wafers in TMAH, TMAH+Triton, TMAH+IPA, TMAH+butanol as well as ternary TMAH solutions containing surfactants and alcohols are studied [35]. With addition of alcohol, especially butanol-2, there is a good improvement in the etched surface morphology. TMAH+Triton+butanol is a best etching solution where high quality etched surface is required over substantially large area [35]. The effect of Triton ranging from ppb to ppm level on the etching characteristics of 25 wt% TMAH is also studied. It is observed that Triton concentration equal to or more than 10 ppm should be added to suppress undercut at convex corners [4]. Recently, the etching characteristics of silicon are investigated in different concentrations of hydroxylamine (NH2OH) added 5 wt% TMAH solution [39,40]. With addition of hydroxylamine in TMAH, the etch rate and undercut at convex corners increase more than three times compared to those observed in pure TMAH [39,40]. The effect of Triton is highly dependent on the TMAH concentration [4,21,30,41]. Researchers have studied this effect in 5–25 wt% TMAH concentration. Although the addition of Triton in low concentration TMAH reduces undercut significantly, the etch rate is also decreased. Whereas in the case of 25 wt% TMAH, it is reported that the incorporation of Triton reduces undercut considerably without affecting Si{100} etch rate noticeably. Thus, Triton-added 25 wt% TMAH is the best choice for fabricating the structures containing convex corners on Si{100}. If 25 wt% TMAH is used for long duration of etching (>20 h), the concentration is found to change considerably which in turn affects the etching characteristics of the etchant significantly, especially undercutting at convex corners. The effects due to this concentration change have never been quantified before in any literature to the best of our knowledge. This is because of the fact that for many reported studies, the etch duration is small, generally less than 5 h. In this duration, the concentration change due to water loss is very less and hence the undercutting at convex corner is not influenced meaningfully. We can reduce this water loss to some extent by the use of reflux condenser, but it may be insufficient when we need to do etching for long duration with additional requirements of reduced corner undercutting and smooth etched surface.

In this work, we have investigated the effect of variation in concentration of 0.1% Triton X-100 added 25 wt% TMAH solution over long duration of etching on the etching characteristics of Si{100}. The etch rate and undercut rate are found to be influenced significantly by the concentration change over long period of etching. Here we have proposed a simple method of replenishing the etching solution with the lost water content of same temperature at prefixed intervals of time during the course of etching to regain the etching characteristics of fresh solution. This ensures that the solution behaves like original fresh solution during the entire duration of etching. We used this technique to fabricate 380 μm deep cavities in silicon-on-insulator (SOI) wafer with mesa structures by carrying out etching for about 22 h.

Section snippets

Experimental details

4-in. {100} oriented SOI wafer with 100 μm device layer thickness, 2 μm buried oxide thickness and 380 μm handle layer thickness is used for the fabrication of cavities with mesa type structures. The handle layer has a resistivity of 5–10 Ω-cm. The etching is carried out from backside (handle layer side) of the SOI wafer. Thermal silicon dioxide of 0.9 μm thickness is used as the masking layer. The oxide is patterned using photolithography and then followed by etching in buffered hydrofluoric

Volume and concentration change

The change in volume of 0.1% Triton-added 25 wt% TMAH etching solution is observed at every time interval t = 5 h. The results are tabulated in Table 1. It can be seen that the volume change of the fresh solution and the solution replenished with lost water content remains almost the same as the temperature remains constant in both cases. It may also be noted that the etching is performed in the same etching system under the same etching condition. The total volume change after 45 h of etching

Conclusions

In the present work, concentration dependent etching characteristics of Si{100} are studied using 0.1% Triton added 25 wt% TMAH solution. Over a long duration of etching, the changes in volume of etching solution and its concentration are determined. Further, variation in etch rate and undercut rate with time over the entire duration of etching are studied in detail. Increase in the concentration of etching solution and undercut rate, decrease in the etch rate of silicon are observed with

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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