3. Experimental facilities and measurement methods
3.2 Electric property measurement methods
3.2.3 Electronics and sample chip
The measurement system for four-probe and self-heating method includes three main parts; sample temperature control and electronic measurement system.
For the sample temperature control, a sample holder from THERMODYNAMIC INSTRUMENTS CORP. with low magnetic field NiCr-heater was mounted onto the commercial cryostat (OXFORD He3 refrigerator), which provide a cooling power to cool sample to low temperature. The heating power was controlled by the temperature controller LakeShore-340 of Lake Shore Cryotronics, Inc., which control the temperature of sample in the range form 0.3 to 350 K. In the holder, there were three resistance-temperature-detector (RTD) used to detect the temperature of sample.
They are PT100 and Cernox from Lake Shore Cryotronics, Inc., and SA-1400 from THERMODYNAMIC INSTRUMENTS CORP. PT100 perform an accuracy temperature sensing in the calibrated range from 30 to 550 K, however, Cernox and SA-1400 give the accuracy temperature from 0.3 to 100 K. Especially, the calibrated SA-1400 temperature sensor is a high sensitivity and low magnetic influence sensor, which provide a double checking to prevent the missing working of Cernox.
Furthermore, this whole set was installed to a He4 dewar, which includes a superconductor magnetic with maximum field up to 9 Tesla.
The electronic instruments for resistance measuring include an alternating current source, preamp and lock-in amplifier, and power transformer. The model of current source was KEITHLEY 6221, which provide sine wave current from 1 nA to 100 mA and frequency range from 1 mHz to 100 kHz. The voltage signal were amplified by pre-amplifier 5113 and pick up with lock-in amplifier AMetek model 7265. All instruments operating were performed by a computer with GPIB and LABVIEW program. Meanwhile, all data were collected and transfer to computer
for recording and calculating by GPIB interface. The characterization and operating detail of cryostat probe was described in session 3.1-D. The detail schema of the temperature control devices and electronic instruments show in figure 3.5.
Trigger AC current source
LOCK-IN amplifier
Heat sink
High thermal
conductive substrate
4- probe electrodes
Figure 3.5 The schematic sketch of measurement setup and settling of specimen.
Chapter 4
Nanowires fabrication
Introduction
First of all, the fabrication of nano-scale materials is a challenge and much important to research of low-dimensional systems. This chapter introduces the experimental tools and techniques involved in fabricating nanowires. In order to synthesis nano-materials, several methods were reported to form nanostructure.
Some of them, the methods are used usually for some special materials. This chapter introduces two methods to fabricate nanowires. The first part is a Bottom-up method, which the nanostructure formed self-assembly (Self-assembly method, SAM). Section 4.1 describes the detail procedures to fabricate nano-porous template and how to form nanowires within these anodic aluminum oxide (AAO) templates. The second part is a Top-down method, which need a lot of patterning and etching processes. Sections 4.2 will give the description to the lithographic (Optical Lithography, OL and Electron Beam Lithography, EBL), film deposition and etching techniques.
4.1 Bottom-up method
In this method, we combined two techniques to form nanowires. First, we use the anodization process to form porous template, it’s so called anodic aluminum oxidation technique. This technique is a self-assembly reaction, the acid solution will oxidize aluminum (Al) foil under a specific applied electric field to form amorphous alumina (Al2O3) with high aspect ratio and uniform nano-pores. Second, a deposition procedure is used to grow materials into these nanopores in templates by so called electrodeposition.
4.1.1 Fabrication of nano-scale porous template
In recent years, there has been increasing interest in the fabrication of nanometer-sized fine structures because of their potential utilization in electronic, optical, and micromechanical devices. Although, Several techniques have been proposed to synthesis high-density and regular nano-pore arrays, such as e-beam and x-ray lithography, proton beam writing (PBW) and AAO [59-65]. The AAO is one approach to the fabrication of nanometer-sized structures, and the AAO has been considered as a naturally occurring structure as a host for the fabrication [64-71].
This approach is promising, especially for the preparation of large-area, nanometer-sized structures with high aspect ratios, which are difficult to form by a conventional lithographic process. Over all techniques, AAO is the better choice by most material researchers, because of its simple and rapid fabrication and cost effectiveness. The technique of porous formed in electrolytes under anodic bias has been studied and reported in 1953. [72] In 1970s, O’Sullivan and Wood [73]
presented a model based on the electric field distribution to explain why pores grow at
all and why their size distribution is quite narrow. The further refined models [74-76]
can give microscopic explanations for the dependence of, e.g., pore diameters and pore distances on applied voltage or electrolyte composition. In recent decade, the attractive issue about low-dimensional science is giving rise to a lot of research on growing porous anodic aluminum oxide (AAO) [77-85]. Anodic porous alumina, which is prepared by the anodic oxidation of aluminum in an acidic electrolyte, is one of the typical self-organized fine structures with a nanopore array [86, 87]. Anodic porous alumina has a packed array of columnar hexagonal cells with central, cylindrical, uniformly sized holes ranging from 7 to 400 nm in diameter. Such nanopore arrays of alumina (Al2O3) are known to exhibit hexagonally ordered pores and considerable structural strength on the nanoscale. Many types of nano-composites have been fabricated with anodic porous alumina used as a host material; when used for the preparation of magnetic recording media [88, 89], optical devices [90], functional electrodes [91, 92], and electrochromic [93] and electroluminescence display devices [94], the holes in these materials are filled with metals or semiconductors.
Self-organized formation of hexagonal pore arrays in anodic alumina provides a conventional tool to fabricate the low-dimension materials. Porous oxide growth on aluminum under anodic bias in various electrolytes has been studied for several decades [86]. But highly regular polycrystalline pore structures occur only for a quite small processing window. Recently reported self-organized pore growth, leading to a densely packed hexagonal pore structure for certain sets of parameters.
Especially, within an oxalic acid, the pore size is a linear relationship to anodic voltage [95]. Figure 4.1 shows this published result revealing this dependence.
According to these parameters and follow the process, it is easy to select the parameter for getting suitable size of pores.
Figure 4.1 Relationship between pore diameter and growth rate of anodic alumina membrane and anodic voltage.
Experiment
The chemical reactions of aluminum oxidation are [96]
Anodic reaction: 2 Al → 2 Al3+ + 6 e
-Oxide-electrolyte interface: 2 Al3+ + 3H2O → Al2O3 + 6 H+ Cathodic reaction: 6 H+ + 6 e- → 3H2
Overall reaction: 2 Al + 3 H2O → Al2O3 + 3 H2
This reaction occurs spontaneously until the compact barrier layer is formed.
Chemical reactions in aqueous are always complex, so that the diameter of pores in
AAO template well depend on temperature, electrolyte composition, and electrical potential. In order to obtain the high regular, uniform pore diameter, and thick alumina foil, a low temperature experimental set-up is designed to provide all components at low temperature. Figure 4.2 shows the experimental set-up.
Temperature of the electrolyte and components are maintained at low temperature around 0 °C using a commercial refrigerator. A resistance temperature detector (RTD) was used for the temperature sensing, and a 400 mL beaker was used to contain the electrolyte.
Temperature controller and Stirrer
Re fr ig era to r
AnodeCathodeAluminum foil Sourcemeter
Figure 4.2 The setup scheme for the fabrication of nano-porous template. sThe materials of anode and cathode electrode are copper and platinum, respectively.
This whole set were placing on a temperature and stirrer controller in a refrigerator.
A pure Aluminum foil (purity ~ 99.9999%) is degreased by ultrasonic cleaner with acetone and ethanol in sequence. The pure and cleaned aluminum foil was mounted on a copper plate which was served as the anode electrode, and subjected to electro-polishing in a H3PO4: H2SO4: H2O solution with weight ratio 4:4:2 for obtaining smooth surface. The applied potential is about 20 Volt to flatten the surface of foil. Then, a two-step anodic oxidization process is performed to fabricate AAO template. The flattened foil is anodized in an acid aqueous at a suggested applied voltage about 0 ~ 3 °C. Pretreated foil then goes through the second anodizing with the same condition of first step. To this step, sample formed three layers, they are pore-layer, barrier-layer, and aluminum-layer. The barrier-layer is a thin alumina layer, which is covering and stoking nanochannels to stop the passing of electrolyte. For the goal to remove the barrier layer, CuCl2 and 2 wt% NaOH are used to etch the aluminum and barrier layer in sequence. Figure 4.3 shows some SEM images of AAO templates by the above condition. This detail processes will show in Figure 4.4.
Figure 4.3 Three top-views and one side-view of anodic alumina membrane with pore size ~60, 20, and 10 nm.
Figure 4.4 The scheme of a whole procedure to deposit nanowires in AAO template.
Aluminum
Aqueous electrolyte
Aluminum
Electropolish
Anodize
Soaking of CuCl2, NaOH
Depositing of electrode
Electro-deposition
4.1.2 Fabricate nanowires by electodeposition
Metal Electrochemical deposition also called electrodeposition for short is the branch of electrochemistry that deals with the chemical action of electricity and production of electricity by chemical reaction. Electrodeposition is an important processing technique for depositing thin films and nanostructures due to its low cost, high yield, low energy requirements, and capability for generating complex and high aspect ratio features. Electrodeposition has been widely used in the microelectronics industry for interconnects, chip packaging, and magnetic storage. Especially, the thin film deposition or growth is important in these fields. Thin film science and technology play a crucial role in the high-tech industries all over the world today.
For the first half of the past century, interests in thin films were centered on optical applications. Here, we will use this technique to grow iron and Bi2Te3 nanowires.
The experiment setup were shown as Fig. 4.6
Figure 4.5 The scheme of experimental arrangement of electrodeposition setup.
4.2 Top-down method
The typical Top-to-Down technique for fabricating devices or sample is lithography. Roughly, there are two techniques used to make devices and samples, one is the Optical lithography, the other one is so called Electron-beam lithography.
The following sections will introduce these two technique and related techniques.
4.2.1 Lithography
Introduction to patterning
Optical lithography also called Photolithography is a process used in microfabrication to selectively remove parts of a thin film. Usually, it uses ultraviolet light to transfer a pattern from a photomask to a light-sensitive photoresist (or say resist) on the substrate. The exposed sample will go through a series of chemical treatments then cut the exposure parts into the material underneath the photoresist.
Photolithography shares some fundamental principles with photography, in that the pattern in the etching resist is created by exposing it to light, either using a projected image or an optical mask. This step is like an ultra high precision version of the method used to make photography. Subsequent stages in the process have more in common with etching than to lithographic printing. It is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously.
The steps involved in the photolithographic process are wafer cleaning;
photoresist application; soft baking; mask alignment; exposure and development; and
hard-baking. The typical sequences of process steps are given in Figure 4.6 is typical for most silicon substrate fabrication steps. The cleaned substrate is covered with a homogeneous metal layer, which is subsequently coated with suitable photoresist. Illumination and development of the resist through a mask exposes some areas of the metal layer, while others are protected by the resist. The illumination is usually carried out with ultraviolet light or with electrons. An etch step follows, which selectively removes the free metal surfaces. Here, the resist acts as an etch mask. Finally, the resist gets removed, and a patterned metal layer on the substrate results. However, the much nonmetal material is not performed by this processing, since essentially all suitable metal etchants attack those materials as well.
Therefore, fabrication scheme Figure 4.7 is typically used. Here, the substrate is first covered by resist, which gets illuminated and developed. Now, the metal is evaporated on the substrate, with the patterned resist acting as evaporation mask. The lift-off step follows. i.e., the resist is removed with the metal film on top. The final result is identical to that one of scheme Figure 4.6. For selective etching of the substrate Figure 4.8, steps 1 to 3 are identical to Figure 4.7. Then, the patterned resist is used as an etch mask for the substrate. We now discuss these fabrication steps in following sections.
Thin film deposition
Resist spin coating
Figure 4.6 The typical sequences of process steps are given as above. It is typical for most silicon substrate fabrication steps.
UV light exposure
Development
Etching and resist remove Metal mask
UV light or e-beam
Resist spin coating
UV light or e-beam Metal mask
Figure 4.7 The much nonmetal material is not performed by procedure as figure 4.6, since essentially all suitable metal etchants attack those materials as well.
Therefore, fabrication scheme above is typically used.
Thin film deposition UV light exposure
Development
Resist remove
Resist spin coating
Metal mask
Figure 4.8 For that of high temperature deposition process, the substrate etching procedure usually choosing to solve the difficulty.
Substrate etching UV light exposure
Development
Resist remove and film deposition UV light or e-beam
Defining patterns in resists
Optic lithography
By this we mean illumination of a photoresist by visible or ultraviolet light. The sample is coated with a thin and homogeneous photosensitive resist. This is done by dropping some resist solution onto the sample, which is then rotated for about one minute at high speed, typically a few thousand rpm. The spinning speed and the viscosity of the solution determine the thickness of the resist layer, which is of the order of 1 μm. After baking the resist the sample is mounted into a mask aligner, a device designed for adjusting the sample with respect to a mask that contains the structure to be illuminated. The mask aligner is equipped with a strong light source that illuminates the resist film through the mask see Figure 4.9a. The pattern sizes are Doppler limited, which means that the smallest feature sizes are about half the wavelength (~ 150 nrn), divided by the index of refraction of the resist (~ 1.5), which limits the resolution to roughly 100 nm. The mask can be a quartz plate Coated With a thin chromium film, which contains the pattern to be illuminated. In the contact illumination scheme, the Cr film is in mechanical contact with the resist and blocks the light, such that the resist underneath the Cr remains unexposed. During contact illumination the mask suffers contaminations due to dust particles on top of the resist, as well as by resist adhesion. This can be avoided by projection illumination, where the mask pattern is transferred into the resist via lenses. This technique is widely used in industry, but somewhat unusual in research labs. The photoresists can be classified as positive and negative. The solubility of the exposed areas increases for a positive resist, while it decreases in negative resist; see Figure 4.9a. Immersing the sample into a suitable developer removes the corresponding sections of the resist film. Both
types of resists have in common that their solubility as a function of the illumination dosage is a step-like function. This ensures high resolution and sharp edge profiles.
It may seem irrelevant at first what kind of resist is used in a particular process.
There may, however be some process specific requirements which favor one type or the other. Most importantly negative resist predominantly produces an undercut profile which means that after development, the resist area in contact with the sample is smaller than the area at the resist surface, Figure 4.9b. This is a consequence of the approximately exponentially decreasing intensity of the illuminating light as it penetrates into the resist. An undercut profile is highly, desirable for subsequent metallization steps, in which the resist itself serves as mask. After the metallization the resist including the metal film on top usually has to be removed in a lift-off step, which is bound to fail for resists with an overeat profile since the metal on the sample and that one on top of the resist are connected. An undercut profile avoids this problem, provided the thicknesses of metal layer and resist are properly selected.
In principle, the resolution can be increased by using shorter wavelengths. In X-ray lithography resists, are illuminated with wavelengths in the 10 nm regime.
While significant progress, has been achieved over the past decade severe technological obstacles have to be overcome before this, version of optical lithography can he widely used. Photoelectrons limit the resolution to several 10 nm, and optical components as well as masks are difficult to fabricate since, metals get transparent in the UV. The ultimate limit of such lithographic techniques is set by the resolution of the resists, which contain organic polymers. The cross linking of the polymers is enhanced or reduced by the light, which modifies their solubility accordingly. Thus, the resolution cannot become better than the size of the corresponding monomers, which is of the order of 0.5 nm. For feature sizes below ~ 150 nm, electron beam lithography is· the current technique of choice.
(a)
UV light or e-beam Metal maskPositive resist Negative resist
(b)
Overcut Undercut
Figure 4.9 The selected resists and exposure devices may change the pattern resolution of by the typical sequences of process steps. Inset (a) is the resist cross section of positive and negative resist. The solubility of the exposed areas increases for a positive resist, while it decreases in negative resist. Inset (b) is the resist profile of different resist.
Electron beam lithography (EBL)
Instead of light electrons may be used as well for illuminating resists, which are in this case polymers like PMMA (poly-methyl metacrylate) with a well-defined molecular weight. In a positive resist, the electron beam breaks the bonds between the monomers, and an increased solubility results. In negative resists, on the other hand, the electron beam generates inter-chain cross linking, which deceases the solubility in that respect electrons have a very similar effect as U.V. light on the resist.
A focused electron beam is scanned in a predefined pattern across the, sample using deflection coils in the electron optics. In contrast to optical lithography, this is a serial and therefore a slow process. However structure sizes of 50 nm and even below can be fabricated. Many research groups use electron beam lithography in the lab for all feature sizes below 2 μm, because the technique gives very good and reproducible results. One type of electron beam lithography uses a high energy-beam of electrons (about 30 keV or larger), which produce extremely small spot sizes of about 1 nm only. However the illumination resolution is worse than this, since the spatial distribution of secondary electrons backscattered from the substrate actually illuminate the resist. Since the intensity of those electrons drops from the substrate towards the surface of the resist, an undercut profile is intrinsic to this process. The undercut is often enhanced by a two-layer electron beam resist
A focused electron beam is scanned in a predefined pattern across the, sample using deflection coils in the electron optics. In contrast to optical lithography, this is a serial and therefore a slow process. However structure sizes of 50 nm and even below can be fabricated. Many research groups use electron beam lithography in the lab for all feature sizes below 2 μm, because the technique gives very good and reproducible results. One type of electron beam lithography uses a high energy-beam of electrons (about 30 keV or larger), which produce extremely small spot sizes of about 1 nm only. However the illumination resolution is worse than this, since the spatial distribution of secondary electrons backscattered from the substrate actually illuminate the resist. Since the intensity of those electrons drops from the substrate towards the surface of the resist, an undercut profile is intrinsic to this process. The undercut is often enhanced by a two-layer electron beam resist