The Texturing Process by TMAH…

TMAH is non-pollutant and non-toxic, and is widely used to manufacture a pyramidal structure. In many research, the Si wafer with texturing surface etched by TMAH is often used to produce solar cell and other optoelectronic devices ^[51-52]. In our study, we adjust the etching temperature, the concentration of TMAH solution, and add appropriate surfactant to grow the optimized pyramidal structure, follow by characterizations of surface morphology and reflectivity.

Anisotropic etching of silicon is a major way to fabricate the three-dimensional pyramidal structures with low reflectivity and high surface area. All etching experiments are carried out using the single-crystalline [100] p-type silicon wafer as source materials. In this experiment, the texturing process is approached by using TMAH solution due to the low contamination and good etching characteristics that have been mentioned before. Because the etching rate depends on the composition, temperature and the properties of silicon surface, the optimized setup of etching conditions with respect to the concentration of solution and the environment effect needs to be considered. The results of silicon wafers etched with TMAH solutions with different concentrations and varying temperature are investigated. Before the etching process, the wafers are immersed in Buffered Oxide Etch (BOE) solution to remove oxide contaminations. Here we induce the characterization of the surface morphology based on the scanning electron microscope (SEM) and atomic force microscope (AFM). To determine the influence of etching temperature, experiments are carried out at temperatures ranging from 60 °C to 80 °C in the water batch and agitate the etching solution at the seed of 100 rpm. The SEM and AFM morphology of the wafers etched in 2.38 % TMAH solution at 60 °C, 70 °C and 80 °C are shown in figure 3-2, 3-3 and 3-4.

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Figure 3-2 The morphology of silicon anisotropic etched in 2.38 % TMAH solution at 60 °C for 60 min. (a) shows the AFM morphology in 10 μm × 10 μ m area, and (b) shows the SEM morphology.

Figure 3-3 The morphology of silicon anisotropic etched in 2.38 % TMAH solution at 70 °C for 60 min. (a) shows the AFM morphology in 10 μm × 10 μ m area, and (b) shows the SEM morphology.

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Figure 3-4 The morphology of silicon anisotropic etched in 2.38 % TMAH solution at 80°C 60 min. (a) shows the AFM morphology in 10 μm × 10 μ m area, and (b) shows the SEM morphology.

According to figure 3-2 to 3-4, we can see that there are many bubbles sticking on the surface, and these bubbles affect the growing of pyramidal structure. When the temperature increases, more hydrogen bubbles form on the silicon substrate. It has been known that the general texturing reaction could be described in figure 3-5 ^[53].

Figure 3-5 The reaction of general texturing silicon surface ^[53].

It is obvious that the reaction will generate hydrogen gas bubbles, which may stick on the silicon surface in the process of texturing. By adding isopropyl alcohol (IPA) as a surfactant, we find a tendency of bubble-adhering diminishment on the etched surface. IPA molecules clad the gas bubbles around and lift them off, thus the formation of big hydrogen bubbles can be avoided. Moreover, the higher temperature kept during texturing process, the faster bubbles been lift off ^[51].

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Here we prepare solutions containing TMAH and IPA for avoiding the formation of big hydrogen bubbles on the surface of silicon wafer. Figure 3-6 to 3-8 show the AFM and SEM morphology of silicon anisotropic etched in 1.67 % TMAH and 30 % IPA at 60 °C, 70 °C, and 80 °C, respectively.

Figure 3-6 The morphology of silicon anisotropic etched in 1.67 % TMAH and 30 % IPA at 60 °C for 60 min. (a) shows the AFM morphology in 10 μm × 10 μm area, and (b) shows the SEM morphology.

Figure 3-7 The morphology of silicon anisotropic etched in 1.67 % TMAH and 30 % IPA solution at 70 °C for 60 min. (a) shows the AFM morphology in 10 μm × 10 μm area, and (b) shows the SEM morphology.

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Figure 3-8 The morphology of silicon anisotropic etched in 1.67 % TMAH and 30 % IPA solution at 80 °C for 60 min. (a) shows the AFM morphology in 10 μm × 10 μm area, and (b) shows the SEM morphology.

Figure 3-9 The relationship of the average height of the etched silicon as a function of temperature.

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From figure 3-6 to 3-8, we can conclude the influence of etching temperature.

The etching rate of (100) and (110) crystallographic planes increases more rapidly than that of the (111) crystallographic plane at increasing temperature. When the temperature is increased, the difference in etching rate results in higher pyramids ^[36].

According to figure 3-6 to 3-8, the addition of IPA will diminish the adherence of hydrogen bubbles to the etched surface. However, there are still bubbles sticking on the surface leading to incomplete pyramidal structure formation during etching process. Here we prepare the TMAH and IPA solutions with distinct concentrations respectively, and the operation temperature is set at 80 °C, which brings to higher etching rate in figure 3-8. In order to prevent the hydrogen bubbles from sticking onto the silicon surface to form the pyramidal structure perfectly, we put the silicon wafer in vertical direction rather than in horizontal direction.

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Figure 3-10 The AFM and SEM morphology of etched silicon etched by TMAH and IPA with different concentrations at 80 °C for 60 min. The concentrations of TMAH and IPA are 1.67 % and 30 % respectively in (a) (b), 1.19 % and 50 % respectively in (c) (d) and 0.714 % and 70 % in (e) and (f).

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Figure 3-11 The resulting height of pyramidal structure etched in different concentration of TMAH and IPA at 80 °C.

Figure 3-10 and 3-11 indicate that the higher IPA content in the solution the more completely diminishment of hydrogen bubbles adherence on the etched surface, thus the growing of three-dimensional pyramidal structures can be improved. However, the solution with 0.714 % TMAH and 70 % IPA brings about lower average etching height and lower distribution of pyramidal structures. That reason may result from the lacking in the etching ability to silicon due to fewer TMAH concentrations. Figure 3-10 and 3-11 shows the uniform of growing pyramidal structures morphology and higher average etching height under the solution with 1.19 % TMAH and 50 % IPA.

In this experiment, we have optimized the texturing solution contains 1.19 % of TMAH and 50 % of IPA under the temperature of 80 °C. The optimization process of growing the three-dimensional pyramidal structures is for the sake of decreasing

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reflectivity and the increasing surface area of silicon substrate ^[51]. The analysis method of N&K analyzer is discussed in chapter 4.