1. Introduction
1.4 Organization of this dissertation
In this thesis, the well-known next generation lithography techniques includes:
electron-beam lithography, focused ion beam lithography, extreme ultraviolet lithography, x-ray lithography, microcontact printing, and scanning probe lithography will introduce in the chapter 1.
Chapter 2 describes the mechanism of electrical-field local oxidation of scanning probe nanolithography. In this section, we have investigated that the nanostructures fabrication on the silicon substrate. We have successfully demonstrated the ability of nanostructure patterns transfer of linearity lithography, and accurately defined the linewidth of nanostructure patterns by using SPL technique with anisotropic wet etching process. In this study, well-defined nano structures are fabricated by scanning probe lithography technique under properly control on tip bias, tip force, scanning speed and tip humidity of patterning environment will also be discussed.
Chapter 3 focused on the characterization and applications of the dip-pen nanolithography (DPN). We utilized an AFM-tip as a “nano-pen” to transport the “organic molecular ink” onto a substrate surface via the water meniscus. In this section, we report a successful technique for using DPN technique to write molecule ink (HAuCl4) structures directly onto oxidized surfaces of silicon nanowires (SiNWs). This novel method was performed based on scanning probe lithography technique and anisotropic wet etching process. Dip-pen nanolithography (DPN) was adopted to provide HAuCl4 ink (7.5 mM) on the surface of SiO2/Si samples and followed by 365 nm UV light-induced reduction of gold ions on the area of interest such that gold nanoclusters formed selectively. Then, X-ray photoelectron spectroscopy (XPS) was adapted to analysis the surface chemical state of the sample surface. We proposed this 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.
On the other hands, we also describe a novel platform to perform the selective deposition of gold nanoparticles on dip-pen nanolithographic patterns of SiO2 surfaces. We report an “inked” atomic force microscope (AFM) tip was adopted to deposit 2.2 mM organic N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS) molecules in
And, the molecules act as linkers for the selective deposition of gold nanoparticles on the SiO2 surface. X-ray photoelectron spectroscopy (XPS) was then used to evaluate the presence of gold nanoparticles on the SiO2 surface. Lateral force microscopy (LFM) was utilized to differentiate the surface between oxidized semiconductors and patterned areas with monolayer of AEAPTMS. Linewidths down to 60 nm have been successfully achieved by this method.
Chapter 4 describes a new approach that has successfully demonstrated for the facile patternwise deposition of AuNPs onto a SiO2 surface with nanometer scale resolution by SPL field-induced bond breaking of AEAPTMS SAMs. Patterning of AEAPTMS SAMs is realized by local filed-induced bond breaking using scanning probe lithography on the thin SiO2 surface. AuNPs with negative-charged citrate surfaces were selectively anchored on the unpatterned area via electrostatic force between AEAPTMS SAMs and AuNPs. Different tip/sample biases were investigated for the bond breaking efficiency of AEAPTMS SAMs.
In this section, SPL bond breaking effect on amino-functional silanation modified SiO2 surfaces of SiNWs was successfully demonstrated. Following SPL bond breaking of AEAPTMS, gold nanoparticles were selectively anchored on the unexposured area. It is found that the bond breaking efficiency is limited by the tunneling current through the thin SiO2 filmso that both the tip bias and tip scanning speed play the important roles. Single digit numbers of gold nanoparticles anchored onto unpatterned AEAPTMS SAMs were demonstrated. Moreover, we have also successfully demonstrated the binary and gray-level patterning with gold nanoparticles selective depositon by using the novel scanning probe lithography bonds breaking technique. Electrical characteristics of SiNWs after different surface modifications were illustrated. Also, optical responses of SiNWs with selective binding of gold nanoparticles were demonstrated. We believed that the SPL bond breaking technique is provide a pathway to modify surface to link different functional group, enabling diverse and exciting application in bio-sensing technology and nanoelectronics.
This thesis would be summarized in Chapter 5, where recommendations for future research are proposed.
Reference
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Chapter 2.
Scanning Probe Nanolithography
When the lithography skills of the semiconductor industry continues to shrink its design rules, the novel metrology and advanced lithography tools become increasingly important. While the dimensions of the very large scale integration devices are scaled down towards the nanometer region, traditional lithography techniques employing visible light and ultraviolet light confront a serious barrier due to the far-field limitation of the wavelength of light. Scanning probe lithography technique is a direct-writing and resistless novel method with a high resolution from sub-micrometers to nano-meters producing patterns on variety of materials [1, 2], where a conductive AFM probe tip biased with a negative electrical bias relative to the sample surface is used to provide an electric-field local oxidation of the sample surface and modify the chemical characteristics of the sample surface.
2.1 The History of SPM Invention
Dr. G. Binnig and Dr. H. Rohrer have successfully built a scanning probe microscopy (SPM) unique technique, first of the probe microscopy local, called scanning tunneling microscopy (STM) with tunnel effect, which maintains the probe-sample distance with constant tunneling current was invented in 1982 [3, 4]. It was demonstrated that STM could explore the three-dimensional topography of surfaces on atomic scale in air. Its principle consists primarily with scanner a surface with a metal point with current constant as show in Figure 2.1. The tunneling current is an exponential function of the SPM tip-sample surface distance, so it is very sensitive to SPM tip-sample surface distance. The tunneling current is dominated by the single or few atoms at the top of SPM-tip. Therefore, atomic resolution can be reached by STM technique. Beyond the revolution that this invention caused in the field of microscopy and of the physics of surfaces, this new instrument brought the bases for the development of the local probe microscopes. Dr.
Binnig and Dr. Rohrer opened a new field in surface science and changed the way to observe the material surface.
Therefore, in 1986, Dr. G. Binnig et al. introduced another apparatus for the surface
characterization microscope with atomic force (AFM) which presents the large one favour to be able to also study insulating materials. [5] Since atomic force microscopy can be applied to any types of material and environment, AFM has thus been used widely in surface characterization. Owing to its atomic scale resolution capability, to probe them mechanical properties, electric, magnetic, electrostatic of one surface, like carrying out local modifications as the lithography, AFM is also the powerful equipment for nano- structure fabrication. Due to Dr. Binning, Dr. Rohrer and Dr. Ruska they created the remarkable achievements, they were awarded the Nobel Prize in physics in 1986.
Figure 2.1 Principle of operation of the scanning tunneling microscope from the original article on the STM [3]. The piezodrives Px and Py scan the metal tip M to traverse the surface. It describes the operation of the microscope for purpose tunnel effect. The control unit (CU) applies the appropriate voltage Vp to the piezodrive Pz for constant tunnel current JT at constant tunnel voltage VT.
Afterward, the developments and applications of the novel SPM technique began rapidly. In 1990, Dr. J. A. Dagata et al. [6-8] reported that thechemical modification of hydrogen-passivated n-Si (111) surfaces by a scanning tunneling microscope (STM) operating in air, and direct writing offeatures with 100 nm resolution was demonstrated.
interface in the presence of oxygen.
In 1994, Dr. E. S. Snow and Dr. P. M. Campbell reported that an electrically conducting AFM-tip isused to oxidize regions of size 10 ~ 30 nm of theH-passivated Si (100) surface at write speeds up to 1μm/sec. This oxide serves as an effective mask for patterntransfer into the substrate by selective liquid etching.
Afterwards, in 1995, Dr. Dawen Wang et al. [10] demonstrated that writing of nanostructures on thin chromium films using atomic force microscopy(AFM). Protruded patterns of various shapeswere formed only on the water-adsorbed chromium surface when applyinga negative bias on the tip. The smallest feature size obtained is about20 nm.
In 1996, Dr. E. S. Snow et al. [11] also demonstrated that thefabrication of atomic point contacts by using anodic oxidation of thin aluminum films with an atomic force microscope.
Therefore, scanning probe lithography (STM / AFM) technique provides a powerful ability of patterning nano-structure on the surface of semiconductor or on the surface of conductor. SPL technique also offered a novel research theme of lithography process of the integrated circuits.
2.2 General Principle of Scanning Probe Microscopy (SPM)
The scanning probe microscopy (SPM) consists of an extremely sharp tip mounted or integrated on the end of a tiny cantilever spring which is moved by a mechanical scanner over the surface to be observed. Every variation of the surface height varies the force acting on the tip and therefore varies the bending of the cantilever. This bending is measured by an integrated stress sensor at the base of the cantilever spring and recorded line by line in the electronic memory. The interaction between tip and sample surface can be compared with the systems pickup.
Most AFM currently detect the bending position of the cantilever with optical techniques as shown in Figure 2.2. The laser beam bounces off the back of the cantilever onto a position-sensitive photodetector (PSPD). As the cantilever bends, the position of the laser beam on the detector shifts. The PSPD itself can measure displacements of light as
small as 10 Å. The ratio of the path length between the cantilever and the detector to the length of the cantilever itself produces a mechanical amplification. As a result, the system can detect sub-angstrom vertical movement of the cantilever tip. The interatomic forces acting on the SPM tip will cause the cantilever to deflect.
Figure 2.2 Schematic of the position-sensitive photo detector (PSPD)
Figure 2.3 shows the relationship curves of force vs. distance between SPM tip and sample surface. It was observed that a force is attractive force when the SPM tip-to-sample surface distance was provided with a few micrometers. However, when the SPM tip-to-sample surface distance decreased to small that the force becomes repulsive force, and variated very violently with decreasing very small distance. In the contact regime, the cantilever is held less than a few angstroms from the sample surface, and the interatomic force between the cantilever and the sample is repulsive. In the non-contact regime, the cantilever is held on the order of tens to hundreds of angstroms from the sample surface, and the interatomic force between the cantilever and sample is attractive (largely a result of
Laser
PSPD Detector
Sample Surface
Cantilever Mirror
SPM Tip
Feedback Circuits System
PZT Scanner
(Piezoelectronic Scanner)
the long-range Vander Waals interactions). Both contact and non-contact imaging techniques are described in detail in the following sections.
Contact Mode:
In contact-AFM mode, also known as repulsive mode, an AFM-tip makes soft
“physical contact” with the sample surface. The AFM-tip is attached to the end of a cantilever with a low spring constant, lower than the effective spring constant holding the atoms of the sample together. As the scanner gently traces the tip across the sample (or the sample under the AFM-tip), the contact force causes the cantilever to bend to accommodate changes in topography.
Figure 2.3 The relationship curves of interatomic force vs. distance between SPM tip and sample surface.
At the right side of the curve the atoms are separated by a large distance. As the atoms are gradually brought together, they first weakly attract each other. This attraction increases until the atoms are so close together that their electron clouds begin to repel each other
electro-statically. This electro-static repulsion progressively weakens the attractive force as the interatomic separation continues to decrease. The force goes to zero when the distance between the atoms reaches a couple of angstroms, about the length of a chemical bond.
When the total Vander Waals force becomes positive (repulsive), the atoms are in contact.
A capillary force exerted by the thin water layer often present in an ambient environment, and the force exerted by the cantilever itself. The capillary force arises when water wicks its way around the tip, applying a strong attractive force (about 10-8 N) that holds the tip in contact with the surface. Typically, the interatomic force operating range from 10-6 N to 10-8 N. Contact AFM can record topographic, frictional and elasticity variations with close to atomic resolution, allowing surface measurements and images on a scale beyond the capabilities of conventional microscopes.
Non-Contact Mode:
Non-contact mode AFM (NC-AFM) is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and the sample for NC-AFM is on the order of tens to hundreds of angstroms as shown in Figure 2.3. NC-AFM is desirable because it provides a means for measuring sample topography with little or no contact between the tip and the sample surface. Like contact AFM, non-contact AFM can be used to measure the topography of insulators and semiconductors as well as electrical conductors.
The total force between the tip and the sample in the non-contact regime is very low, generally about 10-12 N. This low force is advantageous for studying soft or elastic samples.
A further advantage is that samples like silicon wafers are not contaminated through contact with the tip. Because the force between the tip and the sample in the non-contact regime is low, it is more difficult to measure than the force in the contact regime, which can be several orders of magnitude greater. The small force values in the non-contact regime and the greater stiffness of the cantilevers used for NC-AFM are both factors that make the NC-AFM signal small, and therefore difficult to measure. Thus, in non-contact mode, the system vibrates a stiff cantilever near its resonant frequency (typically from 100 to 400 KHz) with the amplitude of a few tens of angstroms. NC-AFM does not suffer from the tip or sample degradation effects that are sometimes observed after taking numerous
scans with contact AFM. So, the NC-AFM is also preferable to contact AFM for measuring soft samples.
2.3 The mechanism of SPL field-induced local oxidation
Since the AFM-baesd field induce oxidation is performed under ambient conditions, a water bridge layer is always present on the sample surface. Sample surface passivation by surface hydrogenation impedes oxidation during the sample exposure to ambient air.As the hydrogen passivation layer can be locally removed by the additional electric field between the tip and the surface. And, the AFM-tip is biased negatively with respect to the sample surface; the sample surface starts to be oxidized when the addition bias exceeded a threshold voltage. The high growth rates occur at extreme electric field strengths near the apex of the AFM-tip of up to ~108 V/m. Field enhanced thin film oxidation can be modeled by Cabrera and Mott [12].
Figure 2.4 Distribution diagram of electric field exists between the apex of AFM-tip and the substrate surface.
The electric-field within the AFM-tip area can be calculated, assuming that the AFM-tip is considered as a charged sphere above can infinite conducting plane as is done
b a AFM Tip
R
Bias V
Sample surface
for a STM tip. Figure 2.4 shows that the distribution diagram of electric field exists between the apex of AFM-tip and the substrate surface. The value of the electric field on the substrate surface located at a distance R from the projection on the sample of the apex of the AFM tip, can be expressed as follows [13]:
for a STM tip. Figure 2.4 shows that the distribution diagram of electric field exists between the apex of AFM-tip and the substrate surface. The value of the electric field on the substrate surface located at a distance R from the projection on the sample of the apex of the AFM tip, can be expressed as follows [13]: