An optimization study on the anisotropic TMAH wet etching of silicon (100)

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1 International Journal of Material Science Innovations (IJMSI) 1 (3): , 2013 ISSN: Academic Research Online Publisher Research Article An optimization study on the anisotropic TMAH wet etching of silicon (100) Jalal Rouhi a,b, Shahrom Mahmud c, Nima Naderi c, Mohamad Rusop Mahmood a,b a Centre of Nanoscience and Nanotechnology (NANO-SciTech Centre), Institute of Science, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia b NANO-ElecTronic Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia c Nano-Optoelectronic Research (NOR) Lab, School of Physics, Universiti Sains Malaysia, Pulau Pinang, Malaysia * Corresponding jalal.rouhi@gmail.com ARTICLE INFO Accepted:15June2013 Keywords: Wet etching Local anodic oxidation Response surface methodology application A b s t r a c t A response surface methodology application was used to determine the optimum conditions for anisotropic TMAH wet etching of silicon (100). Local anodic oxidation process using atomic force microscopy lithography was used to transfer silicon oxide pattern as a mask on the silicon substrates. The effects of simultaneous changes in the two independent variables on the etching depth were investigated. The interaction of these factors on the etching depth was analyzed by the parameter of the interaction effect. The experimental results obtained were fitted to a quadratic equation model using multiple regression analysis of the response variables. Etching time and temperature were found to significantly contribute to the etching rate. The observed etching rates at various conditions were exactly consistent with theoretical studies. Academic Research Online Publisher. All rights reserved. 1. Introduction Several anisotropic silicon etchants exist, for example, potassium hydroxide (KOH), ethylenediamine pyrocatechol (EDP), and tetramethylammonium hydroxide (TMAH) [1]. Each etchant has its benefits and issues. The famous KOH etchant prepares the best selectivity for {111} planes against {100} planes to create well-controlled and defined cavities and very smooth etched surfaces. However, KOH is not completely CMOS-suitable because it includes alkali ions, which able to present charges under MOS transistor gates and corrodes aluminum, reason of threshold voltage shifts, and is not chosen against silicon dioxide [2, 3]. EDP would be more suitable because it does not include potassium or sodium ions and has a high selectivity against dielectrics [4]. This etchant is not aluminum selective and has other important disadvantages: it is mutagenic, toxic, and very hard to handle. Therefore, in

2 P Si J. Rouhi et al. / International Journal of Material Science Innovations (IJMSI) 1 (3): , 2013 cases where CMOS adaptability, low toxicity, and simplicity of handling are the major concerns, TMAH appears to be the best selection. TMAH is recognized in most clean rooms because it has previously been utilized in the IC process as a solution that usually used to develop positive photoresists. The silicon etch rate of TMAH is reasonable, and it features great selectivity against dielectrics. TMAH is also CMOS-suitable because it does not combine alkali ions. Thus, if additives are dissolved in the etchant solution, aluminum is passivated and can be protected. Although TMAH is an anisotropic etchant, the structures etched in TMAH are typically formed by some specific planes of silicon. With the intention of better understanding TMAH etching, the silicon crystal structure must first be recognized [5]. The silicon crystal structure is of the diamond type, with lattice constant (a) equivalent to 5.43 A. The structure is face centered cubic but with two atoms in the unit cell. In this paper, 25% TMAH solution was used to optimize etching process of (100) silicon. 2. Experimental procedures 2.1 Materials P-type Si (100) wafers having a thickness of 500 μm to 550 μm and resistivity of 0.75 Ω cm to 10 Ω cm were used for the etching process. AFM nanolithography with a Cr/Pt coated tip was used to transfer silicon oxide mask on the substrates at room temperature using SPI3800N AFM. TMAH with concentration of 25% was used in the wet etching process. 2.2 Mechanism of TMAH etching The mechanism of TMAH wet etching surveys alkaline silicon etching and can be shortened into three steps [6]. (CH 3 ) 4 NOH (CH 3 ) 4 N + + OH Si + 2OH 2+ Si (OH) 2 + 4e (1) Si (OH) H 2 O + 4e (OH) H 2 In the first step (Eq. 1), TMAH is decreased to create hydroxyl ions. Then, the silicon atoms at the 2+ surface react with the hydroxyl ions to create four electrons and oxidized silicates Si (OH) 2 are injected from each silicon atom into the conduction band. At the same time, water is decreased to prepare more hydroxyl ions, which are bonded to the silicate produced in second step. In final step, soluble silicic acid creates with hydrogen gas as a by-product. 2.3 Preparation of Silicon Wafers A P-type Si (100) wafer having a thickness of 500 μm to 550 μm and resistivity of 0.75 Ω cm to 10 Ω cm was cut using a diamond-shaped scriber into a smaller size (10 10 mm) to fit the circular sample 116 P age

3 holder in the AFM. The wafer was then cleaned using the Radio Corporation of America (RCA) method. First, the hot plate was switched on and the Pyrex beaker holding the H 2 O/NH 4 OH/H 2 O 2 solution with a ratio of 50:10:10 ml was put on the hot plate until the temperature reached 75 C. The wafer was then placed into the heated beaker for 10 min, after which it was taken out and rinsed with deionized (DI) water [7]. This step is done to remove any organic contamination and to dissolve the particles produced by the chemical oxidation of the silicon. Next, the wafer was placed in an HF/H 2 O solution with a ratio of 2:100 ml for 20 sec to remove the thin oxide layer and other ionic contaminants. Fig. 1. AFM image of silicon surface (a) before (b) after RCA cleaning The Si wafer was then washed with DI water and subsequently immersed in the H 2 O/HCl/H 2 O 2 solution with a ratio of 60:10:10 ml and temperature of 75 C for 10 min to remove any remaining traces of metallic contaminants. The wafer was then immediately taken out of the solution and kept in DI water for 5 min. Finally, the sample was dried under a nitrogen shower. Figure 1 shows the Si surface before and after RCA cleaning. 117 P age

4 2.4 Design of experiments to optimize etching process of (100) silicon with TMAH 25% DOE was used to create the regression model and perform statistical and graphical analyses [8]. The variables in this survey included two numerical factors etch temperature (A) and etch time (B). The ranges of these variables and the experimental design levels are shown in Table 1 Thirteen experimental runs were determined from the CCD according to four factorial points, four (axial) star points, and five center points. Table.1: The ranges of variables and experimental design levels Parameters Levels Etch temperature ( C ) A Etch time (s) B The axial distance (Alpha) from the center point was set to 0.5 in the coded units. Anisotropic wet etching experiments were performed on (100) oriented p-type silicon with a resistivity of 0.75 Ω cm to 10 Ω cm. A silicon oxide mask with a square shape was used as etching mask material and transferred onto the Si wafers using LAO (Figure 2) [9]. Fig. 2. AFM image of oxide mask fabricated by LAO 118 P age

5 3. Results and discussion 3.1 Experimental results for etching depth Table 2 shows the actual etching depth responses and the complete design matrix of the experiments applied in this study. Table.2: Experimental design and results for etching depth Runs Variable in coded levels Etch depth, nm Etch temperature, C Etch time, S ± ± ± ± ± ± ± ± ± ± ± ± ±3 The etching temperature for the etching process of the silicon layer was maintained at a desired temperature with an accuracy of ± 1 C. The etching depth of the silicon was obtained using AFM profiles. 3.2 Coded experimental model equations for etch depth The optimum levels of key parameters and the effect of their interactions on the etching depth of silicon wafer were determined by the CCD of RSM. The ANOVA for the etching depth showed that the model was significant because the values of P-value were less than 0.05, indicating that the model terms were significant. In contrast, values of P-value greater than 0.05 showed that the model terms were insignificant [10]. These insignificant model term (B 2 ) was subsequently removed to improve the model. The ANOVA results obtained after removing the insignificant terms for etching depth are given in Table P age

6 Table.3: ANOVA for the etching depth as the desired response (reduced models) Source Sum of squares DF Mean square F-value P-value Comments Quadratic 3.022E < significant A* 1.349E E < significant B* 1.081E E < significant A < significant AB < significant Residual Lack of Fit Not significant Pure Error R-Squared Adj. R-Squared: *A and B are the etching temperature and etching time, respectively. Figure 3 compares the data predicted by the quadratic model and those obtained from experiments. Fig. 3. Parity plots between the predicted and actual data of the etch depth The actual responses clearly agreed with the predicted responses, indicating the reliability of the improved model for predicting the etching depth under various conditions. A suitable orrelation to the linear regression fit with an R-squared of for etching depth was also obtained. The resulting model equation based on coded factors after eliminating the insignificant terms is presented in Eq P age

7 D = ( A) + ( B) + (52.91 A 2 ) + ( AB) (2) where D is the etching depth (nm), and A and B are the etching temperature and etching time, respectively. 3.3 Interaction effect between the etching temperature and time on the etching depth The optimal level of each variable and the effect of their interactions on the etching depth were considered by an interaction plot and a three-dimensional response surface plot (Fig. 3). Fig. 4. (a) Interaction and (b) surface plot of the effect of etch temperature and etch time on etching depth Figure 4(a) shows the interaction effects of the etching temperature and etching time on the etching depth of silicon (100). The lines of the two factors in the interaction plot are not parallel; therefore, there is an interaction between the etching temperature and etching time. Using the RSM theory, the interaction of these factors on the etching depth can be further analyzed by the parameter of the interaction effect. This parameter measures the interaction degree between the two factors under study. Figure 4(a) shows that the effects of the etching temperature at the minimum and maximum levels are nm (difference between the etching depths at 10 s) and nm (difference between 121 P age

8 the etching depths at 50 s), respectively. This result indicated that the etching depth significantly increased with increased temperature. As expected, higher etching temperatures allowed the increase in the etching rate of silicon (100), resulting in higher etching depth. Figure 5 compares the etching rates of a silicon (100) layer by TMAH 25% at various etching temperatures obtained by Shikida et al. [3], Chen et al. [11], Tabata et al. [12] and the present study. Fig. 5. Comparison of etching rate of (100) silicon surface among different studies 4. Conclusions A statistical system was used to determine the mutual and individual effects of etching temperature and time on the wet etching process, which then helps to provide a better understanding and more precise analysis of the process. The RSM and central composite design (CCD) were used to determine the optimum conditions for etching process. The interactions between the etching temperature and etching time on the etching depth showed that the etching depth and etching rate significantly increased with increased temperature at 25% TMAH. The etching rate of silicon (100) was compared with previous studies. These result indicated that the etching depth significantly increased with increased temperature. Acknowledgments The authors would like to acknowledge the collaboration of NANO-SciTech Centre in Universiti Teknologi MARA and Nano-Optoelectronic Research (NOR) Lab in Universiti Sains Malaysia. 122 P age

9 References 1. Van Veenendaal, E., et al., Micromorphology of single crystalline silicon surfaces during anisotropic wet chemical etching in KOH and TMAH. Sensors and Actuators A: Physical, (3): p Seidel, H., et al., Anisotropic etching of crystalline silicon in alkaline solutions I. Orientation dependence and behavior of passivation layers. Journal of the electrochemical society, (11): p Shikida, M., et al., Differences in anisotropic etching properties of KOH and TMAH solutions. Sensors and Actuators A: Physical, (2): p Shome, K., et al., Metallized ultrathin porous silicon membranes for biological sensing using SERS. Frontiers in Pathogen Detection: From Nanosensors to Systems, PM Fauchet and BL Miller, eds.(spie, 2010), : p F Rouhi, J., et al., Controlling the shape and gap width of silicon electrodes using local anodic oxidation and anisotropic TMAH wet etching. Semiconductor Science and Technology, (6): p Kovacs, G.T.A., N.I. Maluf, and K.E. Petersen, Bulk micromachining of silicon. Proceedings of the IEEE, (8): p Rouhi, J., et al. Fabrication of nanogap electrodes via nano-oxidation mask by scanning probe microscopy nanolithography. Journal of Micro/Nanolithography, MEMS, and MOEMS, (4): p Rouhi, J., et al., Optimization of nano-oxide mask fabricated by atomic force microscopy nanolithography: A response surface methodology application. Micro & Nano Letters, (4): p Rouhi, J., et al., Field emission in lateral silicon diode fabricated by atomic force microscopy lithography. Electronics letters, (12): p Hadeed, F. and C. Durkan, Controlled fabrication of 1 2 nm nanogaps by electromigration in gold and gold-palladium nanowires. Applied Physics Letters, : p Chen, P.H., et al., The characteristic behavior of TMAH water solution for anisotropic etching on both silicon substrate and SiO2 layer. Sensors and Actuators A: Physical, (2): p Tabata, O., et al., Anisotropic etching of silicon in TMAH solutions. Sensors and Actuators A: Physical, (1): p P age