Surface Treatment Effect on Si (111) Substrate for Carbon Deposition using DC Unbalanced Magnetron Sputtering

A. S. Aji*, M. F. Sahdan, I. B. Hendra, P. Dinari, Y. Darma

Quantum Semiconductor and Devices Lab., Physics of Material Electronics Research DivisionDepartment of Physics, Institut Teknologi Bandung

*Email: aji.ravazes70@gmail.com

Abstract. In this work, we studied the effect of different surface treatment in silicon (111) substrate deposited with carbon. We focused on surface treatment of silicon substrate that utilizes fluoride acid to remove contaminants and native oxides on silicon (111) substrate. The effect of HF treatment is investigated by SEM, EDAX, and Raman spectroscopy. The SEM EDAX results show that the carbon islands grow not uniformly on the surface of substrate which uses the surface treatment than another one. Those results are clarified by Raman spectroscopy. But, the Raman spectroscopy intensity of sample treated by HF is lower than sample without treatment. Moreover, we also investigate the thermal stability of these samples by thermal annealed to 600 ºC and 900 ºC.

PACS: 81.15Fg, 81.15.Cd, 81.65.-b, 81.05.U-Keyword: film deposition, sputtering, surface treatments, carbon based materials.

INTRODUCTION

Nowadays, carbon-based materials are on the trending for material research, from carbon nanotube to graphene. Graphene is the latest structure of carbon-based materials known recently. Graphene and carbon nanotube have many promising utilities from solar cell to electronic devices. Usually that carbon-based material will be grown on a silicon substrate in order to be used [1].

As we know that surface treatment is one of many important things before depositing material on silicon substrate. Carbon layer on silicon substrate has been done from several groups by using CVD [2]. This method resulted great outcome. In this paper, we deposited the carbon layer on the silicon substrate by using DC unbalanced magnetron sputtering. Before growing the carbon layer, we perform the surface treatment on the substrate by dipping and cleaning in a HF solution.

In this paper, we will discuss the HF treatment on the surface of silicon (111) substrate for carbon growth. HF dip has several advantages, for example it can remove organic contaminant, native oxides, and increase surface contact angle up to 70º. Furthermore, we also investigate the effect of annealing on this material from 600 ºC to 900 ºC. The results can be seen on SEM images and Raman spectroscopy.

EXPERIMENTAL DETAILS

HF treatment is one of the wafer-cleaning techniques alongside RCA solutions that have been used from the 1970s [3]. RCA solution is divided into two types depend on its pH value. SC-1 (standard cleaning 1) has high pH value. SC-1 has the ability to remove organic contaminant and particle. Furthermore, SC-2 (standard cleaning 2) has low pH value. SC-2 has the ability to desorb metal contaminant by forming a soluble complex. Silicon wafer substrate that has been cleaned by both SC solutions still has a problem that the native oxide, organic, and metal contaminant are still remain. Native oxide is hard to be removed using RCA solutions. Native oxide is a very shallow layer (10 Å to 1 nm) on top of silicon substrate that formed when silicon substrate is exposed in the air. Native oxides arise from several causes and their main source is emerging from the air in the room growing on the silicon wafer itself, and they also emerge from DI water [4][5]. Organic contaminant usually arises from organic vapor in the ambient and wafer storage containers while metal contaminant usually arises from ion in chemical solutions. Organic contaminant usually arises from organic vapor in the ambient and wafer storage containers while metal contaminant usually arises from ion in chemical solutions. HF dipping is just a way to remove the native oxide from silicon substrate.

(a) (b)

(c) (d)

Figure 1. SEM images for no treatment and HF treatment on silicon substrate preparation and annealed at 900 ºC; (a) no surface treatment at x10000 magnification; (b) HF surface treatment at x10000 magnification; (c) no surface treatment at x25000 magnification; (d) HF surface treatment at x25000 magnification.

This technique actually scrapes the top from silicon that has a native oxide and other contaminants. After rinsing with HF solution, the native oxide along other contaminants and a small silicon layer will be lifted and detached from substrate.

Furthermore, HF dipping also makes the silicon substrate become hydrophobic. After the top silicon layer is removed by dip the silicon substrate to HF solution, it will form a silicon surface that has a large contact angle. A surface with a contact angle of 0° is completely hydrophilic, while a contact angle of equal to or greater than 90° (H2O which is beading on the surface can have an angle greater than 90°) defines a completely hydrophobic surface. Contact angle of silicon substrate with H2O after dipped on HF solution is 66º-85º [6,7].

EXPERIMENTAL SETUP

Samples used were prepared from silicon (111) substrate. The sample was cut to 1x1 cm2. In this work, we focused on substrate preparation using wet cleaning technique. We create two samples with varying sample preparation techniques using and without using HF dip on wet cleaning technique on silicon (111) substrate.

(a)

(b)

Figure 2. AFM Images of HF treated surface (a) and non-treated surface (b)

1μm

1μm

1μm

1μm

Figure 3. Raman spectroscopy for no treatment and HF treatment on silicon substrate.

The first sample is dipped using HF 1% diluted on DI water. Before the sample is dipped on HF solution, the samples have been cleaned using H2O2 1% and ultrasonic cleaned around 70 ºC sample. The difference is on the second sample we did not using HF solution after ultrasonic cleaning process. After the substrate preparation is done, the carbon is grown using DC unbalanced magnetron sputtering system. The target that used is Fe:C from BATAN. Pressure and temperature given is 2x10-2 mbar and 300 ºC for 1 hour. We used Ar gas for sputtered the target and accelerated the Ar gas with 500 V from power supply. After the carbon is grown with DC unbalanced magnetron sputtering, we annealed the samples on 600 ºC for 30 minutes and 900 ºC for 1 hour.

Characterization of these samples is performed by Raman Spectra to see the bound between silicon and carbon. The SEM images are also taken to see the surface of this sample. Furthermore, we also see the energy dispersion to emphasize the results obtained.

RESULTS AND DISCUSSION

SEM images of two samples are given on figure 1. Sample without HF dip treated substrate is given on figure 1(a) and 1(c). Figure 1(a) and 1(c) clearly demonstrate the presence of polluter (white part) that cannot be removed without HF dip treatment. The carbon itself is shown on figure 1(a) and 1(c) by the white and black island on silicon surface. Next, figure 1(b) and 1(d) show the result of HF dip treated substrate. The figure clearly shows the carbon island is completely spread. This is caused performed by the HF treatment on silicon (111) substrate. The HF completely removes the Si-H bond on surface and providing silicon arms to the place of carbon to be grown. From the AFM images in figure 2, it can be clearly seen that the substrate treated by HF has more

carbon on it than another one. The Raman spectroscopy in figure 3 emphasizes the AFM images that the carbon-silicon and carbon-carbon peak has higher intensity. Silicon (111) peak on 550 Raman shift is removed to see the difference silicon-carbon and carbon-carbon peak between HF treated surface and non treated surface.

EDAX values obtained shown on tabel 1 also confirm the presence of carbon in both samples. Although amount of carbon on HF treated substrate less than non HF treated substrate, but the SEM images shows that the carbon layer more uniform on HF treated substrate. Samples measured by EDAX are all composed of carbon and silicon 100%. There are two possibilities which can be deduced from these results. First, the small area measured by EDAX are consist only that two materials, but there is the possibility of oxides and other impurities on other area. Second, the resulting sample is completely clean from oxides and impurities, so there is no percentage other than carbon and silicon.

Table 1. The EDAX results from two samples with HF treatment and without HF treatment.

ElementHF treatment Non HF treatment

Mass (%) Atom (%) Mass (%) Atom (%)C 8.13 17.14 6.23 13.46Si 91.87 82.86 93.77 86.54

Total 100 100 100 100

Figure 4 shows the difference of Raman spectroscopy results from two samples. There are 3 peaks on figure 4(a) that indicated silicon and carbon bound. The highest peaks from the figure 1 lies on 500 – 550 cm-1

Raman shift. That peaks is resulted from the silicon on the substrate. The peaks became the highest peak than other two because of penetration depth of laser that used for Raman spectroscopy. Silicon substrate that penetrated is about 100 nanometers. Samples without

(a)

(b) (c)

treatment that annealed at 600 ºC have highest peak and samples without treatment that annealed at 900 ºC is higher than with HF treated at 900 ºC. Figure 4(b) show the difference of two samples on 220 – 240 cm-1

Raman shift. Raman spectroscopy studies demonstrate that the peak shows presence of carbon bond with another carbon. The peaks is similar as figure 4(a), the untreated sample preparation that annealed 600 ºC is the highest peaks followed by untreated sample preparation that annealed at 900 ºC and HF treated sample preparation that annealed at 900 ºC. Then, figure 4(c) lies on 220 – 240 cm-1 Raman shift. Samples without treatment that annealed at 600 ºC have highest peak and samples without treatment that annealed at 900 ºC is higher than with HF treated at 900 ºC. Raman spectroscopy studies demonstrate that the peak shows presence of carbon bond with silicon substrate [8].

CONCLUSION

The HF treatment on substrate makes carbon easily grew and dramatically reduces the polluter on top of silicon (111) substrate. Raman spectroscopy, AFM images, and EDAX results shows the carbon population with HF treatment on substrate is better than without HF treatment. Furthermore, the SEM images show the carbon with HF treatment is more uniform on top of silicon (111) substrate.

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