Fabrication of Multiscale 1-Octadecene Monolayer Patterned Arrays Based on a Chemomechanical Method

Abstract

A controlled and self-assembled micromachining system was built to fabricate a mico/nanoscale monolayer patterned array on a silicon surface using a diamond tip. The process was as follows: (1) we preprocessed a silicon wafer to obtain a hydrogen-terminated silicon surface; (2) we scratched three rectangular arrays of 10 μm × 3 μm with a spacing of 2 μm on the silicon surface with a diamond tip in 1-octadecene solution; the Si-H bonds were broken, and silicon free radicals were formed; (3) the 1-octadecene molecules were connected with silicon atoms based on Si-C covalent bonds, and the 1-octadecene nano monolayer was self-assembled on the patterned arrays of the silicon surface. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Sessile water contact angles were used to detect and characterize the self-assembled monolayers (SAMs). The XPS results showed that the Si2p peak and the O1s peak were significantly decreased after self-assembly; however, the C1s peak was successively significantly increased. Sessile water contact angles showed that the hydrophilicity was weakened after the formation of 1-octenecene SAMs on the silicon substrate. The nanofriction of the sample was measured with AFM. The change in nanofriction also demonstrated that the SAMs were formed in accordance with the patterned array. We demonstrated that, by using this method, self-assembled multiscale structures on silicon substrate can be formed quickly and conveniently.

1. Introduction

Silicon has become the most important material in the microelectronics industry [1,2,3,4]. When exposed to air, the silicon surface can be quickly oxidized. This oxide layer induces electron defect states and electron flow resistance, which jeopardizes the material’s performance [5]. Removing the oxide layer of the silicon surface and reforming a new Si-H surface can significantly enhance its electrical performance [6]. However, the Si-H surface can also be quickly oxidized when exposed to air, which leads to rapid performance degradation. Therefore, many attempts have been made to graft organic molecules onto a silicon surface, in order to stabilize, improve, and control the properties of the silicon surface. Khung et al. [7] studied the thermal grafting properties of nucleophilic aniline on the surface of planar Si (111) through XPS analysis and AFM characterization. FTIR and Sessile water contact angles were used to verify the proposed theory and results. Eihadj et al. [8] grafted organic molecules to modify the silicon surface. The process was carried out by electrochemical reduction of 4-nitrobenzene diazonium tetrafluoroborate in an aqueous medium containing HF and H₂SO₄. It appears that the cathodic grafting led to the formation of a polymeric layer, but the same grafting also occurred spontaneously within a few tens of seconds at the open circuit potential, an expected phenomenon in view of the reduction potential of 4-nitrobenzene diazonium. Eihadj et al. [8] modified the Si by grafting organic molecules. In an aqueous saluting containing HF and H₂SO₄, 4-nitrobenzene diazonium was electrochemically reduced. Squillace et al. [9] showed that a monolayer was bound to various cheap molecules through the ends of -OH, -COOH, or -NH₂ chains to form mixed coating. The method used wet chemistry in aryldiazonium salt to obtained fixation of the biofilms. Sabbah et al. [10] grafted linear olefin molecules onto amorphous carbon and hydrogen-terminated silicon surfaces. XPS analysis showed that the grafted a-C surface was more stable in terms of environmental oxidation than the grafted Si surface. Tran et al. [11] proposed a new method. Polyisoprene was covalently grafted onto hydroxylated and silicone-containing surfaces from natural or recycled rubber. Under strong mechanical stirring conditions, the polyisoprene coating had strong covalent grafting ability and was resistant to long-term impregnation with water and corrosive organic solvents. Yuan et al. [12] analyzed the oxidation resistance of different passivated Si surfaces by molecular dynamics simulations. An Si surface was oxidized by H₂O₂. The growth of different terminal groups of silicon oxides was vividly described as a sandwich Si/SiOx/SiOH structure. Yao et al. [13] proposed a novel approach to directly bind π -conjugated organic molecules on Si by a wet chemical reaction between N-vinylcarbazole and a hydrogen-terminated Si basement. The contact angle measurements through XPS determined that the 9-vinylcarbazole was covalently bonded to the silicon surface. Liu et al. [14] obtained a thermally stable structure of an aromatic amide ring by the heat treatment of a free radical-grafted poly methacrylic acid membrane. After heat treatment, the new structure had good thermal stability and a heat resistance index up to 226 °C. These works also open up broader application prospects in the fields of chemistry, biochemistry, and nanotechnology [15,16,17].

Nano-thin films are made using the chemomechanical method, which combines “top-down” mechanical scribing with “bottom-up” chemical self-assembly [6,18,19]. This process happens in a liquid environment; when the pretreated silicon surface is scratched with a hard scribe, for example, a diamond tool, the Si-O or Si-H bonds are broken. Then the silicon free radicals will bind covalently with organic molecules in the solution to form a monolayer. Yang et al. [20] prepared the SAMs with olefin molecules by mechanical engraving and chemical self-assembly. Niederhauser et al. [21] created an independent space with different monolayers in different and precisely controlled areas. Lua et al. [22] produced sharp and shallow functionalized features on silicon by wetting hydrogen-terminated silicon with a small tungsten carbide ball. Lua et al. [22] wetted end-hydrogen Si with small tungsten carbide spheres, resulting in sharp and shallow morphology. Shi et al. [23] scribed the silicon surface in the presence of 4-benzoic acid diazonium tetrafluoroborate with a diamond tool and modified the oxide-coated silicon surface using a combination of a chemical and mechanical method. Niederhauser et al. [24] mechanically carved the oxide-coated Si in 1-alkenes under ambient conditions and obtained an alkyl monolayer on the Si substrate. Niederhauser et al. [25] scribed silicon in 1-iodoalkanes, etc. The silicon surface was patterned and formed an organic monolayer. Yang et al. [26] considered chemomechanical methods as a simple and convenient tool for simultaneously functionalizing and picturing silicon. Cross-scale controllable self-assembly was realized on a silicon substrate. This technique had a wide range of potential applications. Hydrocarbon-based films grown on the surface of silicon were characterized by the chemomechanical method with good stability under X-ray, hot acid, water, and air conditions, which has proved to be the cheapest and fastest method to obtain both functionalized and formed silicon surfaces, simultaneously [27,28]. Shi et al. [29] scribed a hydrogen-terminated silicon surface with a diamond tool in 1-hexadecene solution. The silicon surface was modified and functionalized by a 1-hexadecene organic solution. The Si-H bonds were broken, and the silicon free radicals reacted with the unsaturated bonds (C═C double bond) of 1-hexadecene molecular to form Si-C bonds. The modification and the preparation of cross-scale micro/nano monolayer arrays can be realized by the chemomechanical method, which is reacted in an inert atmosphere. Jiang et al. [30] reported the monolayer was prepared by using a controlled diamond tip in a variety of organic solvents. At the same time, it was proved that the alkyl monolayer on a carved silicon surface had good stability for many kinds of harsh environments. Zhao et al. [31] prepared an octadecene reaction film on a hydrogen-terminated single crystal silicon substrate in the presence of ultraviolet irradiation. The formation of the ultrathin film was due to the formation of the unsaturated C=C bond and suspended Si-H bond through free radical reaction.

2. Materials and Methods

2.1. Experimental Materials

Silicon (100) wafers (n-boron, 500 ± 25 μm, resistivity 0.001~0.004 Ω·cm) were used. A monolayer material of 1-octadecene was the analytical reagent (Beijing Chemical Reagent Factory, imported and repackaged). Super pure water was obtained by the Mili-Q water system (electrical resistivity 18.2 M Ω). In addition, high purity nitrogen with a purity of 99.999% was used.

2.2. Establishment of Micromachining System

A controllable self-assembly micromachining system was built including a 3D high precision stage (produced by PI), a charge coupled device (CCD), a three-way dynamometer (MiniDyn9256A1, KISTLER instruments inc.), and a diamond tool. The motion of the tool was controlled by the 3D micromotion stage, the approximation process was observed by a CCD magnifying system, and the contact between the tool and the stage was detected by a dynamometer to control the cutting depth of the tip on the silicon surface. In the experiment, the piezoelectric ceramic drive (PZT) amplifier module of the micromotion stage was controlled by an external signal to drive its motion.

2.3. Experimental Method

In order to obtain the hydrogen-terminated silicon surface, it was necessary to pretreat the silicon according to certain steps [32]: (1) we cleaned the silicon ultrasonically with acetone, ethanol, and ultra-pure water twice in succession, each time for 5 min; (2) we etched the silicon in a 5% HF solution for 5 min to remove the oxide layer from the surface; (3) we heated the silicon substrate in a mixture of water, concentrated hydrochloric acid, and hydrogen peroxide with a volume ratio of 4:1:1 for 20 min; (4) nitrogen was injected into a semiconducting pure 40% NH₄F solution for 20 min to remove the oxygen; then, the substrate was placed in the NH₄F solution for 10 min to etch and remove the oxide layer from the surface again; (5) finally, we rinsed with plenty of ultra-pure water. After the above treatment, the hydrogen-terminated silicon surface was obtained.

Then, the sample of chemically pretreated silicon was immersed in a pool of 1-octadecene solution, which was placed on the established self-assembly system. The whole system was in a nitrogen airtight environment. Prewritten programs were run to drive the 3D micro workbench according to a certain trajectory. The diamond tool was used to scratch the Si surface in 1-octadecene solution, and the depth of the graph was controlled at about 10 nm. Some specific parameters of the graph were as follows: step 10 nm; length of rectangle 10 μm; and width of rectangle 2 μm. In order to process the rectangle array, the starting coordinates were transformed after finishing each rectangle.

3. Results and discussion

3.1. Subsection AFM Characterization

AFM (Dimension 3100, Digital Instruments) was used to record the surface morphology of the silicon sample before and after assembly [33,34]. For convenience, all morphology images were scanned in contact mode with the same tip (V-shaped Si₃N₄ micro cantilever, length 200 μm, elastic constant 0.12 N/m). The imaging was performed in atmosphere at 300 K and 60% relative humidity. The scanning range was 1 μm, and the scanning rate was 2.0 Hz.

Figure 1 shows the AFM surface morphology image of the array of the 1-octadecene monolayer on silicon prepared according to Section 2.3. It can be seen that the rectangular array was regular and clear. Further characterization is shown in Figure 2 in order to compare the difference between before and after the assembly in 1-octadecene solution. It shows that the roughness of Si was small before the assembly. After assembly, the undulation of the SAMs’ surface was quite obvious, the morphology was in the shape of a cluster, and the carved area was covered by the homogeneous lattice structure. It can be seen in the AFM figures that the morphology was different in the indented area. However, it could not be deduced that the 1-octadecene molecules connected with the covalent bond to the Si surface. On the basis of the morphology characterization, the bond formed between the organic molecules and silicon atoms was further determined by spectral analysis.

3.2. XPS Detection

XPS is used to analyze the chemical composition of samples, element state or binding state, and structure of the surface layer according to the electron binding energy of atoms and its changes [35]. XPS was performed using the PHI 5700 ESCA System (Physical Electronics). Figure 3 shows the XPS spectra of the sample before and after self-assembly. The element contents of C, O, and Si on the silicon surface before and after self-assembly are summarized in

Table 1. Element Content before Assembling and after Assembling on Silicon.

	Peak Position	Element Content before Assembly	Element Content after Self-Assembly
Peak ID	Center/eV	AT%	AT%
C1s	284.6	18.0	50.0
O1s	531.6	42.8	27.6
Si2p	103.4	39.2	22.4

, where the center represents the bond energy value corresponding to the highest characteristic element peak, and AT% represents the percentage of each element in all measured elements.

The characteristic peaks of all elements can be seen in Figure 3 and Table 1, C1s = 284.6 eV, O1s = 531.55 eV, and Si2p ≈ 103 eV [36]. The Si2p peak was significantly decreased after self-assembly, and the C1s peak was successively significantly increased. The increasing C1s/Si2p XPS signals were in good agreement with previous results [21]. The C1s peak before self-assembly was the reference peak, which was not the characteristic peak generated on the silicon surface. However, 1-octadecene SAMs contain an organic group; the C1s peak was significantly enhanced after self-assembly. The Si2p peak was obviously weakened due to the existence of the self-assembled monolayer covering part of the silicon atoms. The content of O1s was decreased, because the Si-O bonds were broken and replaced by the Si-C bonds. This indicates that the 1-octadecene molecules were assembled on the silicon substrate [24]. Figure 4 shows the differences in silicon peaks in the two forms of silicon substrate before and after the reaction, to compare the changes in silicon elements on the surface before and after the assembly in detail.

It can be seen that the Si2p peaks were divided into two groups; there were two forms of Si on the silicon surface. From the peak attribution table [36], it can be found that the Si at 100.125 eV was in an Si-Si bond, the Si at 103.375 eV was in Si-O, and the Si at 100.375 eV was in an Si-C bond [26,37]. After assembly, the peak value at 103.375 significantly decreased, the peak value at 100.125 almost disappeared, and the peak value at 100.375 obviously appeared. The ratio of the silicon peak area at 103.375 eV and 100.125 eV was about 4:1 before the reaction, indicating that the silicon surface was dominated by silicon dioxide, while the ratio of the silicon peak area at 103.375 eV and 100.375 was 1:4 after the reaction, indicating that the silicon bonded with carbon greatly increased. This was caused by the substitution of Si-Si and Si-O bonds for the Si-C bonds between the 1-octadecene molecules and the Si substrates. The above XPS spectrum analysis further proves that a self-assembled monolayer formed on the silicon substrate after the reaction, and the 1-octadecene molecules were covalently linked to the silicon surface.

3.3. Sessile Water Contact Angle Detection

Contact angle provides important data to measure surface wettability and surface energy. Different functional groups have different contact angles due to their different wettability. The SL series contact angle measuring instrument used in this experiment was established based on image analysis. The instrument included four parts: a CCD optical system, a sample table and fixed frame, a light source control, and software analysis.

Figure 5 shows the measurement results of the contact angles of the silicon surface, the hydrogen-terminated silicon surface, and the silicon surface after controllable self-assembly of the 1-octadecene monolayer, respectively. It can be seen that the contact angle of the silicon (100) surface to water was 25°, the silicon terminated by hydrogen to water was 77°, and the 1-octenecene reaction film terminated by methyl was 102°. The hydrophilicity was weakened after the formation of 1-octenecene SAMs on the silicon substrate [31,38]. This further proves that a 1-octadecene monolayer was formed on the silicon surface, and the modification of the silicon surface was realized by the mechanical-chemical method.

3.4. Nanofriction Analysis of Self-Assembled Film Based on AFM

The nanofriction of the samples before and after self-assembly was measured using AFM (Dimension 3100) [39,40]. When the tip was scanned within a certain small range, the cross section of the friction image was observed, and the friction signal under each load was recorded. The load increased by specific amounts, and the curve of the relationship between the friction signal and the load was obtained. The experimental parameters were as follows: the scanning speed of the tip was 2.0 Hz, the scanning range was 1 m, and the load increased from 1.0 V, 0.5 V, to 4.0 V. The friction value of the silicon surface before and after assembly under each load was recorded. Each experiment measured the friction value at ten different points. The values with a large deviation were removed, and the other values were averaged for a total of ten sets of experiments. Origin software was used to draw the diagram, and the corresponding data were represented by the same symbol in the two diagrams, as shown in Figure 6. Then, the three groups of data that were close to each other were respectively taken from the friction signal data of the sample surface before and after assembly for comparison in the same coordinates. The solid line represents the Si-H surface before assembly, and the dashed line represents the SAMs surface after assembly, as shown in Figure 7.

The following points can be seen from the relation curve of the friction signal and load:

(1)

The initial friction force under one load was different. This also proves that the chemical composition of the silicon surface changed before and after assembly.

(2)

The slope of the friction signal and the load relation curve on the surface of the sample after assembly was small; that is, the friction coefficient of the self-assembled sample was small, which indicates that the friction characteristic of the surface before and after assembly were obviously different. The friction signal of the SAM after assembly changed slightly with the increase in load, while the friction signal of the Si-H before assembly changed greatly with the increase in load.

(3)

The friction signal on the SAM surface after assembly was stronger than that on the Si-H surface before assembly; in other words, the friction on the SAM surface after assembly was greater.

Since the hydrophilicity of the silicon substrate was weakened after the formation of 1-Octadecene monolayer on the surface, we speculate that under high relative humidity, the thicker water film on the surface of Si-H had the lubrication effect of a fluid film, which made the friction less. With the increase in the load, the probe penetrated into the water film and directly came into contact with the surface of the SI-H.

4. Conclusions

A controllable self-assembly micromachining system using a diamond tool was established. Three rectangular arrays of 10 μm × 3 μm with a spacing of 2 μm were mechanically carved on the hydrogen-terminated silicon surface. At the same time, the inscribed place was chemically self-assembled with a 1-octadecene monolayer. Thus, the silicon surface was patterned and functionalized in one step by the chemomechanical method.

XPS, sessile water contact angles, and AFM were used to detect and characterize the self-assembled monolayer, which prove that 1-octadecene SAMs were generated on the silicon surface. The XPS results showed that the Si2p peak and the O1s peak were significantly decreased after self-assembly; however, the C1s peak was successively significantly increased. The Sessile water contact angles showed that the contact angle of the silicon (100) surface to water was 25°, and the 1-octenecene reaction film terminated by methyl was 98°. The hydrophilicity was weakened after the formation of the 1-octenecene SAMs on the silicon substrate. It also can be found from the AFM topography that the undulation of the SAMs surface was quite obvious after assembly; the morphology was in the shape of a cluster, and the scribed area was covered with a relatively uniform lattice structure.

In addition, AFM was also used to measure the nanofriction before and after the assembly sample. It is concluded that the friction of the 1-octadecene film prepared on the silicon surface did not change obviously with the increase in load. The surface activity and adhesion energy were low, and the corresponding friction force changed slowly.

We can not only prepare specific three-dimensional structures with special properties and functions on silicon substrate based on the chemomechanical method but also realize a highly controllable shape and position and greatly improve the efficiency of preparing self-assembled structures. The films and structures obtained by this method can be used as hydrophobic fences, masks in wet etching, and so on. They will have very broad application prospects.