Summary

Nanostructures and nanodevices fabrication of on single-crystal silicon have been demonstrated by electric-field-enhanced local oxidation on the semiconductor materials using the scanning probe lithography (SPL). We have successfully demonstrated accurate linear control of nanostructures fabrication for different linewidth by using the multipixel scanning method of SPL technique with ODE etching process, and feature size of desired nanopattern down to 20 nm can be easily obtained under control. We have successfully demonstrated accurate linear control of nanostructures fabrication for different linewidth from 25 nm to 80 nm by SPL technique. The standard deviation (1σ) for 25 nm nanostructures is 3.01 nm. It was indicated that the use of multipixel scanning method of SPL with more potential benefits to perform novel structures for fundamental studies of nanostructure, nano-patterning for linearity studies, and there is the possibility of using these techniques for practical applications, such as device nanofabrication and nanoelectronics.

Reference

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Chapter 3.

Dip-Pen Nanolithography

Dip-Pen Nanolithography (DPN) is a scanning probe nanopatterning technique in which an AFM-tip is used to deliver molecules to a surface via a solvent meniscus, which naturally forms in the ambient atmosphere. DPN is a direct-write soft lithography technique that is used to create nanostructures on a substrate of interest by delivering collections of molecules via capillary transport from an Atomic Force Microscope (AFM) tip to a surface as shown in Figure 3.1. This direct-write technique offers high-resolution patterning capabilities for a number of molecular and biomolecular ‘inks’ on a variety of substrates, such as metals, semiconductors, and monolayer functionalized surfaces. It is becoming a work-horse tool for the scientist interested in fabricating and studying soft-and hard-matter on the sub-100 nm length scale.

Figure 3.1 Schematic representation of Dip-Pen Nanolithography (DPN). Transport mechanism of molecules to the surface via the water meniscus forms between the AFM-tip and the sample surface in air. [1]

Using the conventional atomic force microscope it is possible to achieve ultra-high-resolution features as small as 15 nm line widths and ~ 5 nm spatial resolutions.

For nanotechnology applications, it is not only important to pattern molecules in high

resolution, but also to functionalize surfaces with patterns of two or more components. One of the most important attributes of DPN is that because the same device is used to image and write a pattern, patterns of multiple molecular inks can be formed on the same substrate in very high alignment.

3.1 Introduction of dip-pen lithography applications

In recent years, several lithography methods such as electron-beam lithography, photo-lithography, ion beam lithography [2], scanning probe lithography [3], micro-contact printing [4], and nano-imprint lithography [5] offer one the ability to build nano-structures on this scale of length, but none of these allow one to deposit molecule-based nanostructures directly on a surface especially on the oxide surface [6-8]. Dip-pen nanolithography (DPN) is a nano-writing procedure that utilizes the tip of an atomic force microscope as a “nano-pen” to transport an “ink” containing organic molecules onto a substrate surface via a water meniscus. Using the same AFM-tip to write and subsequently read patterns, it is possible to create nanoscale patterns with remarkable resolution (<100 nm) and simultaneously control the chemical functionality of the written regions [9].

Dip-pen nanolithography technique has emerged as a useful tool that allows one to make multi-component nanostructures on a surface with near-perfect registration capabilities.

This makes DPN a unique tool that can be used to fabricate both hard and soft structures with nano-scale precision on the semiconductor substrate [8, 10]. On the other hand, dip-pen nanolithography (DPN) allows one to fabricate one molecule thick nanostructures with feature size from micron to sub-100 nm dimensions on solid substrates [11].

Preparation of metal nano-clusters on the various substrates is a key issue in all fields of modern science and technology covering microelectronics, photonics, biochemical sensors, nano-technologic devices, and so forth.

In the field of nanoscience and technology, functionalized inorganic metallic nano-structures have been widely studied because of their potential applications in electronic devices [12, 13], photonics [14-16], and biodiagnostics [17-20]. Recently, one of the major themes of nanotechnology has been the search for techniques for self-assembling nano-particles to construct architectures on a solid-state surface for different applications.

Techniques to make low-dimensional assemblies of colloidal nanoparticles to fabricate

nanostructures are essential for developing and capitalizing upon the emerging field of nanoscience [7, 10]. In 2004, Dr. Y. Cui [13] et al. reported the effective assembly of very small nanostructures using the capillary force method; nano-particles were selectively forced into lithographically defined nano-fillisters but no particles were deposited on the surrounding areas. Dr. J. Zheng [21] et al. reported that an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) was formed on silicon by immersing the silicon substrate in OTS solution. Localized oxidation of the OTS-covered silicon substrate was carrier out with a commercial AFM-tip. An amino-terminated propyltriethoxylsilane (APTES) monolayer was selectively formed on lithographically created silicon oxide regions by soaking the substrate in APTES solution. Dr. Hun Zhang [8] et al. developed a method for fabricating arrays of gold nanostructures on a solid surface based on dip-pen nanolithography (DPN) and wet chemical etching. A 16-mercapto-hexadecanoic acid (MHA)-coated tip was prepared by immersing a commercial Si3N4 tip into the MHA solution. Each pattern was generated via DPN with a MHA-coated tip in contact with gold surface. Thiol-functionalized molecules, like 2-aminoethanethiol (HS–C2H4–NH2) (AET), were used to modify these nanopatterns. After the AET-modified nanostructures were immersed into a solution of citrate-stabilized gold nanoparticles, a monolayer of gold nano-particles localized on each of the AET-modified nanopatterns on the gold substrate surface. Except for micro-contact printing [22], many lithography methods, such as electron-beam lithography, photo-lithography, ion beam lithography [2], scanning probe lithography [5], and nano-imprint lithography[3], offer one the ability to build nano-structures on this scale of length, but none of these methods allow one to deposit molecule-based nanostructures directly on a surface, especially on an oxide surface.

DPN utilizes the tip of an atomic force microscope as a “nano-pen” to transport an

“ink” containing organic molecules onto a substrate surface via a water meniscus. DPN allows one to fabricate one-molecule-thick nanostructures with feature sizes from micron to sub-100 nm dimensions on solid substrates. This makes DPN a unique technique that can be used to fabricate both hard and soft structures with nano-scale precision [9] and it has also emerged as a useful tool that allows one to make multi-component nanostructures on the sample surface [11].

3.2 Impact of UV-induced production of gold nanoclusters on the SiO

2

surface of SiNWs by using DPN technique

Dip-pen nanolithography (DPN) was adopted to provide HAuCl4 ink (7.5 mM) on the surface of SiO2/Si samples and followed by UV light-induced reduction of gold ions on the area of interest such that gold nanoclusters formed selectively. X-ray photoelectron spectroscopy (XPS) was then used to evaluate the surface chemical composition before and after 365 nm UV light irradiation of gold salt on the surface of oxide/silicon. UV-induced reduction of the gold ions (Au³⁺) and aggregation of gold nanoclusters (Au⁰) on the SiO2/Si surface were observed.

UV irradiation resulted in decrease in the binding energy and the changes in the width of the Au4f peak corresponding to Au⁰. The silicon nanowires (SiNWs) with 100 nm wide and 2 μm long on the n-type (100) SOI substrate were fabricated by the scanning probe microscope (SPM)-based local oxidation technique and followed by wet etching with tetramethylammonium hydroxide (TMAH) solution. HAuCl4 ink was then selectively provided by DPN on the native oxide surface of SiNWs and followed by electrical property measurements. The thickness of the gold nanoclusters, estimated less than 1nm, was observed by lateral force microscopy (LFM). Id-Vds characteristics demonstrated that the conductance of SiNWs increased two times at 0.5 volts of Vds after DPN of gold nanoclusters. DPN of gold nanoclusters on the oxide surface of SiNWs enhanced carriers transport in the channel of n-type silicon nanowires. It is believed that the proposed nanofabrication technique can be further applied in both nanoelectronics and nanobio applications. In this 3.2 section, we report a successful technique for using DPN technique to write molecule ink (HAuCl4) structures directly onto oxidized surfaces of silicon nanowires (SiNWs). The silicon nanowires were performed based on scanning probe lithography (SPL) technique and anisotropic tetramethylammonium hydroxide (TMAH) wet etching were expressed. Utilizing the HAuCl4 solution as an ink in a conventional DPN procedure, we can directly generate monolayer nano-patterns onto oxidized silicon surface.

Furthermore, it was demonstrated that UV light-induced reduction of gold ions on the region of interest such that gold nanoclusters formed selectively. It is believed that the

proposed nanofabrication technique combined DPN method with the self assembled monolayer (SAMs) process can be further applied in both nano-electronics and nano-biochemical sensors applications.

3.2.1 Experiment details of the UV-induced production of gold nano- clusters with DPN lithography

The main key of this research was a detailed investigation of the UV light-induced reduction of gold ions deposited from an aqueous HAuCl4 ink solution onto a silicon nanowire by using dip-pen lithography method. In our experiment, first, the silicon wafer was cleaned by using standard RCA cleaning process to remove surface organic contamination. After wafer cleaning, a thermal oxidized SiO2 layer of 30 nm thick was formed. Then, the sample was immersed into the HAuCl4 solution for 10 min, and baked at 120 °C for 30 min in a hot-plate. XPS analysis was then used to evaluate the surface chemical composition before and after 365 nm UV light irradiation of gold salt on the surface of oxide/silicon. Afterward, we have performed the silicon nanowire with 100 nm width and 2 μm length on an n-type (100) SIMOX SOI wafer with 50-nm-thick silicon and 150-nm-thick buried silicon dioxide by using the scanning probe microscope (SPM)-based local oxidation technique in ambient using PtIr tip (tip diameter =10 nm) for SiNW oxide patterns. SiNWs were generated by tetramethylammonium hydroxide (TMAH) anisotropic wet etching procedure. Then, HAuCl4 ink-coated tips was prepared by immersing a commercial PtIr tip (tip diameter =10 nm) into a 7.5 mM solution for 60 sec for following DPN procedure. DPN experiments were carried out under ambient conditions (set point = 1.0 V, 25 ± 1 °C, 85 % relative humidity) by using an commercial atomic force microscopy, and a commercial lithography software (NSCRIPTOR, DPN Writer^TM, NanoInk Inc.) with a HAuCl4 ink-coated tip. We are utilizing an inked 7.5 mM HAuCl4 AFM-tip to deposit gold nano-clusters on the surface of the silicon nanowire, it is in fact possible to write nano

-scopic patterns of reactive HAuCl4 production on the silicon nanowires (SiNWs) with proper control of tip scan speed, tip force, and relative humidity environment. After DPN writing procedure, the sample was baked at 120 °C for 30 min in a hot-plate. Influence of UV-induced production of gold nanoclusters on the oxide surface of SiNWs was demonstrated by both of the XPS analysis and electrical properties.

3.2.2 Relationship of DPN transport mechanisms

In the dip-pen nanolithography (DPN) procedure, water meniscus controls the effective tip-substrate contact regions, aqueous HAuCl4 molecules can be deposited on silicon oxide surfaces of SiNWs with precise control over pattern shape and feature size.

Figure 3.2 shows the relationship of AFM-tip writing speed and relative humidity of environment for dip-pen nanolithography of aqueous HAuCl4 ink on the surface of silicon dioxide.

Figure 3.2 Characteristic curves between speed of the dip-pen lithography and relative humidity of environment at room temperature (25 ºC).

It was observed that line widths increased with longer tip-substrate contact times, and the resolution of the patterning is dependent upon writing speed (v) and relative humidity. It was found that line widths depend on two coefficients, including: contact width I and diffusion coefficient (D). Contact width term further is assumed that the finite size of the tip apex results in a minimum contact width and dependent upon relative humidity. Diffusion is the process by which ink molecules spread radially onto the substrate from the tip. Diffusion coefficient is assumed proportional to the dwell time and inversely to DPN tip drawing speed.

It means that we can place nano-scale patterns directly on oxidized silicon surface via proper control of parameters in DPN technique. HAuCl4 monolayer nano-patterns on the surface of silicon dioxide were imaged by lateral force microscopy (LFM) immediately after writing. For aqueous HAuCl4 inks, line-widths are essentially dependent of the DPN pen-tip writing speed and dwell time. Under various relative humidity conditions, the diffusion coefficient and contact width were estimated, which are very important for line-width control in the advanced applications.

3.2.3 X-ray photoelectron spectra of UV-induced reduction of gold nanoclusters

In order to determine the influence of the UV-light irradiation for aqueous HAuCl4

solution, a silicon substrate that was used to immersion the 7.5 mM HAuCl4 solution for 10 mins and dry with N2 gas. Finally, the sample substrate was then irradiated in the 365 nm UV light for 20mins. The x-ray photoelectron spectroscopy (XPS) analysis results as show in the Figure 3.3. After 365 nm UV light irradiation process, it was clearly observed that the Au4f7/2 and Au4f5/2 spin-orbit components peak signal implicated that the UV-induced reduction of the gold ions (Au³⁺) and aggregation of gold nanoclusters (Au⁰) and gold salt on the SiO2 surface of silicon.

In this 3.2.3 section, we report a successful strategy for using DPN to write molecule monolayer structures directly onto the surface of silicon dioxide of SiNWs, as shown in Figure 3.4 (a). Utilizing the 7.5 mM aqueous HAuCl4 solution as the “molecule ink” in a conventional DPN procedure, we can directly write nano-scaled molecule patterns onto oxidized silicon surface of SiNWs.

180 160 140 120 100 80 60 40 20 0 0.0

2.0x10

⁵

4.0x10

⁵

6.0x10

⁵

8.0x10

⁵

1.0x10

⁶

Au4f_5/2=87.7eV

Si_2p

In te n s it y ( a .u .)

Binding energy (eV)

HAuCl₄/Si with the UV irradiation Si-Substrate

Au4f_7/2=84.0eV

Figure 3.3 X-ray photoelectron spectroscopy (XPS) analysis spectra of the 365 nm UV-light irradiation process on the SiO2 surface of the silicon substrate.

The silicon nanowires (SiNWs) with 100 nm wide and 2 μm long on the n-type (100) SOI substrate were fabricated by the scanning probe microscope (SPM)-based anodic oxidation technique and TMAH anisotropic wet etching as shown in Figure 3.4 (b). Inset AFM images of Figure 3.4 (b) shows the details of silicon nanowire (SiNW) structure with a line-width about 100 nm.

The x-ray photoelectron spectroscopy (XPS) analysis, which was then used to evaluate the surface chemical composition before and after 365 nm UV light irradiation of gold salt on the surface of oxide / silicon as shown in the Figure 3.5.

Figure 3.4 (a) Schematic diagram of the Dip-Pen Nanolithography directly patterning.

(b)AFM images of a silicon nanowire device on (100) SOI substrate fabricated by using scanning probe microscope (SPM) local oxidation method and TMAH wet etching.

Nanowire is about 100 nm wide and 2 μm long.

D S

Si Si

Al

SiO2(150nm) Si

HAuCl₄-coated Tip

Gate Al

DPN drawing @ RH= 85 %

(a)

100 nm

2 μm

Source

Drain

SiO

₂

Si

Si

UV-induced reduction of the gold ions (Au³⁺) and aggregation of gold nanoclusters (Au⁰) on the SiO2/Si surface were found. It was observed that UV irradiation resulted in decrease in the binding energy and the changes in the width of the Au4f peak corresponding to Au⁰ and the growth of gold nano-clusters can be evidenced. The similar result was reported by Dr. Ferdi Karadas et al. [23].

94 92 90 88 86 84 82

Figure 3.5 XPS spectra of the AuCl^- / native oxide / Si system as function of UV exposure time. It was found that UV irradiation induced production of gold nanoclusters on the surface of a silicon dioxide sample.

AFM images were recorded at a scan rate of 1 Hz. HAuCl4 molecule ink was then selectively provided by DPN on the native oxide surface of silicon nanowires (SiNWs) as shown in Figure 3.6 (a) and Figure 3.6 (b). Owing to the thickness of the HAuCl4 molecule monolayer patterns is too thin to identify by AFM analysis. Therefore, we utilized the lateral force microscopy (LFM) to identify and analysis the molecule monolayer patterns on the silicon dioxide of SiNWs. LFM images of the silicon nanowire after DPN of HAuCl4 inkingin area of 0.5 μm by 2.0 μm across the SiNWs at speed of 50 nm/s and RH of 85 % as shown in Figure 3.6 (b).

0 .0 0 .5 1 .0 1 .5 2 .0

0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4 1 .6

Thickness (nm)

S c an s ize (u m ) L F M

(A-A' sectio n )

Figure 3.6 (a) AFM images of a silicon nanowire. (b) LFM image of a silicon nanowire after DPN of HAuCl4 inkin area of 0.5 by 2 μm across the SiNW at speed of 50 nm/sec and RH of 85 %. (c) Depth profile of A-A’ section of a LFM image. A 2-μm LFM profile parallel to the SiNW and the thickness of gold nanoclusters is the less than 1 nm in thickness.

AFM A’ LFM

(c)

(a)

(b) ^A

SiNW

0.5μm

0μm 2μm 0μm 2μm

LFM images were recorded at a scan rate of 3 Hz. From Figure 3.6 (c) results, it was observed that the thickness of the gold nanoclusters, estimated less than 1nm, was by lateral force microscopy (LFM) measurement. The corresponding major oxidation product is determined as chlorine gas, which is initially is adsorbed onto the surface, but eventually diffuses out into the vacuum on electrical property measurement system.

3.2.4 Electrical Characteristics of Influence of UV-induced Reduction of Gold Nanoclusters

Figure 3.7 (a) shows the Ids–Vds characteristics of SiNWs when Vds was swept from -1.0 V to +1.0 V and the bias of the control bottom gate is +5 V. It was demonstrated that the conductance of SiNW increased two times at 0.5 volts of Vds after DPN of gold nanoclusters. I suggested that the UV-induced production of gold nanoclusters on the oxide surface of SiNWs could be induced electrons into n-type silicon nanowires (SiNWs), the top-side surface of SiNWs will be in electrons accumulation such that the conductivity of the SiNWs increased. From the Figure 3.7 (b), electrical characteristic curve (Id-Vg) shown that the change in threshold voltage (ΔVT) is about 0.3 V. It was shown that the gold ions (Au³⁺) and gold nanoclusters (Au⁰) on the silicon nanowire surface can enhance

Figure 3.7 (a) shows the Ids–Vds characteristics of SiNWs when Vds was swept from -1.0 V to +1.0 V and the bias of the control bottom gate is +5 V. It was demonstrated that the conductance of SiNW increased two times at 0.5 volts of Vds after DPN of gold nanoclusters. I suggested that the UV-induced production of gold nanoclusters on the oxide surface of SiNWs could be induced electrons into n-type silicon nanowires (SiNWs), the top-side surface of SiNWs will be in electrons accumulation such that the conductivity of the SiNWs increased. From the Figure 3.7 (b), electrical characteristic curve (Id-Vg) shown that the change in threshold voltage (ΔVT) is about 0.3 V. It was shown that the gold ions (Au³⁺) and gold nanoclusters (Au⁰) on the silicon nanowire surface can enhance

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