Effect of silicon wafer surface stains on copper-assisted chemical etching

doi:10.21203/rs.3.rs-2354085/v1

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Effect of silicon wafer surface stains on copper-assisted chemical etching

https://doi.org/10.21203/rs.3.rs-2354085/v1

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Silicon wafer slicing is a crucial process of solar cell fabrication, this process often stains the silicon wafer surface, Thus, this work systematically investigated the composition, source, cleaning method of typical white spot stains on silicon wafer surface. The EDS and XPS results showed that the white spot stains contained CaCO₃ and SiO₂ that are basically consistent with the filler components in sticky stick glue. Meanwhile, The effects of stains on copper deposition and copper-assisted chemical etching were systematically studied, the white spot stain was still attached on silicon after the deposition and the etching process, we have fond that the stains can affect the uniform deposition of copper particles on the surface of the silicon wafer, it also impede the catalytic etching process of copper particles. Finally, the KOH solution combined with the ultrasonic field was proposed to remove the surface stains of the silicon wafer. This study has important guidance on knowledge and cleaning of silicon wafer contamination for fabricating high-efficiency solar cells.

Silicon wafer

white spot stains

silicon wafer texturization

silicon wafer cleaning

Since the Industrial Revolution, fossil energies such as coal and petroleum have been consumed in large quantities, but these have also produced environmental pollution and climate change. Therefore, renewable energy sources such as photovoltaics and wind have been rapidly developed. The global photovoltaic installed capacity increased by 21 times from 2010 to 2021 and reached 965 GW at the end of 2021. It is expected that the installed photovoltaic capacity will enter the TW regime in 2022 and reach almost 5 TW by 2030 [1–3].

Silicon materials are the market leader for the entire photovoltaic industry [4]. In the silicon-based photovoltaics industry chain, the fabrication of silicon wafers is the most basic step. Diamond slicing is the main silicon wafer slicing technology in which high-hardness diamonds on steel wire are used to slice silicon into thin sheets by high-speed linear friction [5, 6]. During silicon wafer fabrication, manufacturing equipment, auxiliary materials, and negligent operations may produce silicon wafer surface stains [7–9]. During the slicing of silicon wafers, due to the repeated rubbing of diamond wires and silicon wafers, a large amount of brittle damage and plastic damage occurs on the silicon wafer surface. Due to this, the density of the dangling bonds must be very high, and these dangling bonds have high energy and strong activity. They often adsorb various surrounding molecules or atoms that stain the surface of the silicon wafer [10–12]. These stains are mainly produced via physical adsorption and chemical adsorption. Physically-adsorbed stains mainly form due to silicon waste, dust particles, glue, and other substances that do not react with silicon wafers. Silicon wafers combine with this type of stain through van der Waals forces. Chemical stains mainly form due to contaminants in the processing equipment and due to the shedding of small metal particles and metal ions in the slicing fluid. This type of stain combines with silicon wafers via chemical bonds such as ionic bonds and covalent bonds [13–15].

Stains on the surface of the silicon wafer affect the texture and appearance of the silicon wafer. Some stains may spread to the Si substrate to form carrier recombination centers that decrease a device’s performance and reliability [16–18]. There are many forms of stains on the surface of the silicon wafer. Fumitoshi Sugimoto et al. studied the adsorption behavior of organic pollutants on the surface of silicon wafers and found that low-boiling organic contaminants tended to be adsorbed immediately, while high-boiling organic contaminants required a longer exposure time [19]. Sunho Choi et al. found that red-colored contaminants on the surface of a silicon wafer were derived from the copper-plated diamond wire. By analyzing the particles in the slurry before and after slicing, it was found that the pulp contained a large amount of super-small Cu, SiC, and Si particles that increased the slurry viscosity. By providing additional coolant to dilute the slurry, Cu-contamination was eliminated [20]. M. Watanabe et al. analyzed the mechanism by which watermarks formed on the surface of a silicon wafer. They formed when silicon oxide (maybe H₂SiO₃) under the water droplets on the surface of the silicon wafer dissolved in the water drop. After the water droplets dried, H₂SiO₃ remained on the surface [21].

Currently, most of the stains will be easily removed during the silicon wafer cleaning process. However, white spot stains remain on the silicon wafer surface after cleaning, and there have been few studies on these stubborn stains in detail. Silicon wafers need to be textured to reduce surface reflectance to improve photoelectric conversion efficiency. Copper-assisted chemical etching has received widespread attention due to its high efficiency and low cost. The inverted pyramid structure generated by etching has an excellent light-trapping ability [22–24], but there have been no studies of how stains affect this method.

In this paper, the chemical composition of white spot stains on a silicon wafer surface was characterized by X-ray energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). To determine the origin of the white spot stains, copper-assisted chemical etching was used to etch white spot stains on the silicon wafer in which copper was deposited via chemical deposition. The effects of dirt on etching and deposition were studied. Finally, the silicon wafer was cleaned by ultrasonication in KOH solution for 5 min.

2.1 Testing and analysis of white spot stains silicon wafers

White stains on a silicon wafer are often encountered during the production of solar cells. The size of the silicon wafer was 210 mm × 210 mm, and the thickness was 150 µm. The resistivity was 1–3 Ω. Plastic board, sticky stick glue, and diamond wire were obtained from the enterprise that produced the solar cells. A tungsten filament scanning electron microscope equipped with Oxford EDS (SEM, TESCAN VEGA3, Czech) was used to characterize the morphology and elemental distribution of white spot stains, plastic board, sticky stick glue, and diamond wire in detail. The surface state of white spots was analyzed using X-ray photoelectric spectroscopy (XPS, K-Alpha+, Thermo Fisher Scientific, UK).

2.2 White spot stains silicon wafers deposits and etching experiments

The etching solution consisted of 2.76 M HF, 1.2 M H₂O₂, 0.05 M Cu(NO₃)₂, and deionized (DI) water. This was placed in a Teflon beaker and heated in a water bath to 40 ^oC. The white spot-stained silicon wafer was put into the etching solution and reacted for 7 min. After etching, the samples were thoroughly rinsed with distilled water and dried using nitrogen. The deposition solution consisted of 2.76 M HF, 1.2 M H₂O₂, 0.05 M Cu(NO₃)₂, and DI water. It was placed in a Teflon beaker at room temperature. A stained silicon wafer was placed into the deposition solution and reacted for 10 s. After deposition, the sample was thoroughly rinsed with distilled water and dried using nitrogen. After the experiment, the morphology of white spot stains before and after etching and deposition was studied by SEM.

2.3 White spot stains silicon wafers cleaning experiments

The cleaning solution consisted of KOH (10 wt%). At room temperature, the stained silicon wafer was placed into the cleaning solution with an ultrasonic field for cleaning. Stain shedding was observed with a metallographic microscope every minute of cleaning.

Fig. 1a shows a photo of the white spot-stained silicon wafer. Visible white spot stains are shown in the three yellow boxes in the photo. The white spot stain’s irregular morphology was observed by SEM, as shown in Fig. 1b. The spot was about 144 μm in length and 155 μm in width, The surface of the material was distributed with different-sized highlights. EDS element maps are shown in Fig. 1c-f, which show that the white spot stain contained C, Ca, O, and Si elements. C was present in the highest amount, so it is speculated that the white spot stains may be organic matter.

The XPS spectrum was used to further analyze the valence states and composition of the white spot stains. The XPS spectrum in Fig. 2a shows that the white spot stains mainly contained C, Ca, and O, and a small amount of Si. Based on the XPS spectrum in 2a, Ca 2p was deconvoluted into two peaks. The peaks of 347.07 eV and 350.6 eV correspond to Ca 2p_3/2 and Ca 2p_1/2, respectively (Fig. 2b) [25]. The peaks at 531 eV in the O 1s spectrum and 347.07 eV and 350.6 eV in the Ca 2p spectrum show that the white spot stains contained CaCO₃ particles (Fig. 2d). The C 1s spectrum of the white spot stains was decomposed into three components: the peak of C-C was located at 284.7 eV, the peak of C-O-C was at 286.4 eV, and the peak of O-C=O (carbon in carbonate) was located at 289 eV (Fig. 2c) [26]. The Si 2p spectrum presented two characteristic peaks at 99 eV and 103.3 eV, which corresponded to monatomic silicon and SiO₂, respectively (Fig. 2e) [27]. During the production of some polymer materials, to ensure their mechanical properties and flame retardancy, CaCO₃, SiO₂, and other inorganic fillers are often added to the glues, rubbers, and other polymeric materials. The results further confirmed that the white spot stains were an organic polymer [28, 29].

During the slicing of the silicon wafer, the surface of the silicon wafer will only contact the diamond wire. Therefore, the diamond wire was characterized using EDS, to determine the reason for the stains. Fig. 3a shows that the diamond along the EDS line scan changed from a sharp structure to a flat structure, with very serious wear. This indicates it is a used diamond wire. There was a circular substance with a diameter of about 20 μm on the surface of the diamond wire. The EDS analysis of the substance is shown in Fig. 3b, which shows it contained C, Ca, O, and Si. However, the diamond wire was nickel plated, and the main elements were Fe, Ni, and C. This shows that the substance was not derived from the diamond wire. The elements in this substance are consistent with the elements in the white spot stain, showing that it is also a polymeric material.

To determine where the substance on the diamond wire was introduced, the entire slicing process was analyzed. When slicing silicon wafers, sticky stick glue is used to hold the silicon ingots and plastic boards together. The only polymeric materials that can contact the diamond wire are sticky stick glue and plastic board. These two substances were analyzed by EDS. Fig. 4a shows an SEM image of the plastic sheet covered with slice marks caused by diamond wire slicing. The element maps in Figs. 4b and 4c show that it only contained C and O, indicating that the substance was not introduced by the plastic board. Fig. 5a shows the SEM image of sticky stick glue, in which several areas in the figure were brighter than others. The element map in Fig. 5b-e shows that it contained C, Ca, O, and Si. The bright spots in the SEM image are Ca. Sticky stick glue contains the same elements as the white spot stain and diamond wire, indicating that the substance is sticky stick glue.

The process of restoring the white spot stains is shown in Fig 6. Yellow is a salver, red is sticky stick glue, blue is a plastic plate, and gray is a silicon wafer. During the slicing process, the diamond wire slices the silicon ingot from the bottom to the top. Due to the uneven stress at each point of the wire, a line bow phenomenon occurs [30] in which the outer slicing depth is greater than the inner slicing depth. Therefore, the outer side of the silicon ingot is sliced before the inside. To ensure that the diamond wire has enough slicing depth, the silicon ingot can be fully sliced. A plastic plate was added in the middle of the silicon ingot and the crystal bead. When the diamond wire slice the plastic plate, the silicon ingot slice through and contacts sticky stick glue. As a result, a large amount of sticky stick glue adhered to the diamond wire, so when the diamond wire slice the next batch of silicon ingots, sticky stick glue adhered to the silicon wafer and formed stains.

Fig. 7a and 7b show the SEM diagram before and after the deposition of the silicon wafer in the HF/Cu(NO₃)₂ solution. Fig. 7b shows that stain edges near the red dotted edges were darker than the area outside the red dotted line. Compared with before deposition, the area outside the red dotted line was much brighter, showing that this area was deposited with copper. There were no significant differences in the stain morphology before and after deposition. The stain did not dissolve in the deposition solution. The EDS results show that the distribution of copper outside the red dotted lines was denser than within the dotted line. There were almost no copper deposits on the surface of the stain. At the edges of the red dotted area, only a very small amount of copper was deposited because CaCO₃ in the stain reacted with HF in the deposition solution via the equation 2HF + CaCO₃ à CaF₂↓ + CO₂↑ + H₂O. The reaction generated CaF₂ precipitate, resulting in the rapid reduction of F^-around the stain. The pH increased, and the conductivity of the solution decreased. Therefore, F ^- in the area outside the red dotted line was not added to the reaction interface outside the red solid line. Thus, concentration polarization occurred [31]. F^– in the HF solution easily reacted with Si atoms to form Si-F bonds, which greatly reduced the Si-Si bond strength. This made it easier for Cu²⁺ ions to obtain electrons and be reduced to elemental copper. The F^- concentration around the stain decreased, which decreased the rate of Cu²⁺ deposition, which resulted in uneven copper deposits [32].

Fig. 8 shows an SEM image of a white spot-stained silicon wafer etched in HF/Cu(NO₃)₂/H₂O₂ solution at 40 ^oC for 7 min. There was oval-shaped material at the center of the picture, and the EDS results showed that the material was a stain adhered to the surface of the silicon wafer. After reacting for 7 min in a strongly-oxidizing solution, the stain did not dissolve, and many nanopores were formed by the etching solution in areas not covered by the stain.

Sticky stick glue is an epoxy resin, whose hydrolytic degradation was accelerated by OH^- in the KOH solution. This reduced the bonding performance. To understand the effect of KOH solution on stain removal, a metallographic microscope (MO) was used to observe changes in the stain morphology at different cleaning times. Figure 9a shows the MO image of the uncleaned silicon wafer. A large amount of black material stained the surface of the silicon wafer. Fig. 9b shows the stain morphology after cleaning for 1 min. The amount of stain shedding was very small, which exposed the silicon substrate. When cleaned for 3 min, the stain shedding was accelerated, and the stained area gradually shrank. Stains in the upper-left and lower-right corners were removed, and those in the middle part became sparser. After 4 min of cleaning, only a few spots remained on the wafer. At 5 min, no stains were visible under the microscope, and the stains were completely removed.

In this study, EDS and XPS were used to investigate the component and source of white spot stains on the silicon wafers surface. The results showed that the stains contained CaCO₃ and SiO₂, which is consistent with the component analysis of the fillers of sticky stick glue. Thus, we inferred that the white spot stain originated from the glue pollution on diamond wire surface during the wire cutting process. The white spot stain remained on the silicon wafer surface after the metal copper catalytic etching treatment in HF/Cu(NO₃)₂/H₂O₂ system, which show it has a good acid corrosion resistance. the existence of the stain can affected the uniform deposition of copper particles and hindered surface texturing of silicon wafer. Finally, the KOH solution combined with the ultrasonic field was proposed to remove the surface stains of the silicon wafer, which was completely removed within 5 min. This study has important guidance on knowledge and cleaning of silicon wafer contamination for fabricating high-efficiency solar cells.

Acknowledgments

The authors are grateful for financial support from the National Natural Science Foundation of China (Grant No.52274408, 51974143, 52204314, 52164050, 51904134); Major Science and Technology Projects in Yunnan Province (No. 202202AB080010, 202103AA080004, 202102AB080016, 202202AG050012); Yunnan Fundamental Research Projects (grant NO. 202201AW070014); Yunnan Provincial Rural Energy Engineering Key Laboratory (Grant No. 2022KF012), Yunnan High-level Talent Project (YNQR-GCC-2019-010) and the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT_17R48).

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Authors' contributions

Liang Ma: Data curation, Writing—original draft. Xiuhua Chen: Funding acquisition, Conceptualization. Chenggui Tang: Software. Shaoyuan Li: Funding acquisition, Visualization, Investigation. Fengshuo Xi: Funding acquisition, Supervision, Writing -review & editing. Huayuan Lan: Funding acquisition, Supervision, Writing -review & editing. Wenhui Ma: Funding acquisition, Supervision. Yuanchih Chang: Writing—review & editing.

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Effect of silicon wafer surface stains on copper-assisted chemical etching

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Abstract

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1. Introduction

2. Experimental

2.1 Testing and analysis of white spot stains silicon wafers

2.2 White spot stains silicon wafers deposits and etching experiments

2.3 White spot stains silicon wafers cleaning experiments

3. Results And Discussion

4. Conclusions

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