3.1 Experimental procedures
In the experiment, the primary design is reveal in Fig.3-1.
(a) Characterization of carbon nanotubes density through Ni nanoparticle formation using hydrogen plasma treatment on the TiN buffer layer and nanoindentation:
The substrates used in the experiments were 6-inch p-type (100) orientated silicon wafers and cleaned using RCA cleaning procedures in order to remove chemical impurities and particles. A 7 nm layer of nickel and a 20 nm layer of tantalum nitride were deposited with a power of 800 watt at a sputtering pressure of 6.4 mTorr (0.85 Pa).
A 915 MHz microwave chemical vapor deposition (MPCVD) system was used to grow CNTs. The base pressure of the system was less than 2x10-3 torr. During the deposition of CNTs, the substrates were heated using a graphite heater. The nickel-coated substrates were first pretreated with hydrogen plasma at 550°C for 3, 5, 10 and 15 minutes, respectively. The pretreatment process is also illustrated in Fig.3-2.
The catalyst particles were examined by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HR-TEM). Also, the catalyst particles on the substrate surface were indented using a nanoindentation tester (MTS NanoIndenter XP System), a device for measuring the displacements and loads during indentation and the Young’s modulus and hardness. The indent was prepared by moving the indentation positions in arrays using the stage-scan function of the nano-indentation tester with the Berkovich indenter (tip radius ~50 nm), the loadings and unloadings were performed with an
approximately constant rate of 0.0166 mNs-1. In MPCVD system, the substrate heated directly by a resistively heated graphite stage. The substrate temperature was measured using a thermocouple attached directly to the upper surface of the stage. Gas flow rates were controlled independently using mass flow controllers. After nickel-coated catalysts were pretreated for 3, 5, 10 and 15 minutes, respectively, hydrogen (90 sccm) and methane (10 sccm) flowed into the chamber at 550°C for 10 minutes (The flow unit sccm means standard cubic centimeter per minute). The total pressure in the chamber was kept at 20 Torr. The resulting CNTs were characterized by FE-SEM, TEM, and Raman spectroscopy.
(b) Effects of hydrogen plasma pretreatment on the TaN buffer layer for growth of carbon nanotubes:
A 7 nm layer of nickel (Ni) and a 20 nm layer of tantalum nitride (TaN) were deposited with a power of 800 W at a sputtering pressure of 6.4 mTorr. MPCVD system was used to grow CNTs. The base pressure of the system was less than 2x10-3 Torr.
During the growth of CNTs, the substrates were heated using a graphite heater. The Ni-coated substrates were first pretreated with hydrogen plasma at 550 °C for 10 min with various H2 flow ratio (100, 200 and 300 sccm). Ni nanoparticles were examined by SEM and HR-TEM. In MPCVD system, the substrate heated directly by a resistively heated graphite stage. The substrate temperature was measured using a thermocouple attached directly to the upper surface of the stage. Gas flow rates were controlled independently using mass flow controllers. H2 (90 sccm) and CH4 (10 sccm) gases flowed into the chamber at 550 °C for 10 min. Prior to CNTs growth the catalyst films
were pretreated in H2 plasma for 10 min to promote the formation of catalyst particles and elemental Ni. The synthesis process is illustrated in Fig.3-3. The total pressure in the chamber was kept at 20 Torr while the gas flow rates were increased from 100 to 300 sccm. Synthesizing of aligned CNTs was investigated by using SEM, TEM techniques. In addition, Raman spectroscopy (Raman spectroscopy equipped, Condensed matter sciences (CCMS) in National Taiwan University (NTU)) with a change coupled device detector (Nd-YAG laser), operating at a wavelength of 532 nm and a power of 100 mW was employed in our experiment.
(c) Effects of fluorocarbon/oxygen plasma post-treatment on the surface performance of multiwalled carbon nanotubes:
A 7 nm layer of nickel and a 20 nm layer of titanium nitride were deposited with a microwave power of 800 W at a sputtering pressure of 6.4 mTorr. MPECVD system was used to grow the CNTs. The nickel catalysts were formed on titanium nitride/Si substrates. The base pressure of the system was less than 2 mTorr. During the deposition of CNTs, the substrates were heated using a graphite heater. The nickel-coated substrates were first pretreated with hydrogen plasma at 550°C for 10 min with hydrogen gas flow (200 sccm). After pretreated was finished, hydrogen (80 sccm) and methane (20 sccm) were flowed into the chamber at 550°C for 10 minutes. And, the surface characteristics on both of fluorination and oxidation are promoted by mixture of fluorocarbon and oxygen (CF4/O2) as the reaction gases (at 20 Torr for 2 and 10 min).
FE-SEM and HR-TEM were used to characterize the morphology and microstructures of CNTs. Fourier transform infrared spectroscopy (FTIR, ASTeX
PDS-17 System), thermal desorption atmospheric pressure ionization mass spectrometry (TDS-APIMS) and, X-ray photoelectron spectroscopy (XPS, VG Scientific Microlab 310F) was used to explore the changes in the chemical components of CNTs under various stages of CF4/O2 post-plasma treatment, as illustrated in Fig.3-4.
In addition, the as-grown and the plasma treated CNTs were studied with Raman spectroscopy by means of a Jobin Yvon micro-Raman LabRam system in a backscattering geometry. A 632.8 nm He-Ne laser was used as the light source and the power of the laser was adjusted by optical filters (Raman spectroscopy equipped, Department of materials science and engineering at NCTU)
. By using a 100× objective les, the illuminated spot on the CNTs samples was focused to approximately 2 µm in diameter. The resolution of the Raman spectra was 1 cm-1 with the typical acquisition time of 30 sec.
(d) Effect of fluorocarbon/oxygen plasma post-treatment on the lateral carbon nanotubes:
The wafers were cleaned by the standard procedures for removing the chemical impurities and particles on surface. The layer structure was prepared by stacking Ti-Ni-TiN-SiO2 layers on Si substrate. Though process steps varied according to the electrode structures, the basic sequences of experiment process are as follows:
(i) The SiO2 layer was grown on the p-Si (100) wafer by a wet oxidation process at 1100°C.
(ii) The stacking layer on the Ta electrodes, catalyst, and TiN buffer layer were deposited by sputtering using argon plasma.
(iii) The selected region follow patterned by photolithography and plasma etching.
The spacing of the two electrode pattern was 1 μm. The top Ta electrodes layer is introduced as a barrier layer for vertical growth, covering the top of the Ni catalytic layer.
(iv) The CVD chamber temperature was steadily raised to the process temperature of 750 °C within 10 min in atmosphere. During the synthesis of CNT, the total flow rate of process gases was maintained at 200 sccm. For instance, the flow rate of the hydrocarbon source, C2H4 (ethylene), was 10 sccm and the remaining 190 sccm was hydrogen gases.
After the CNT synthesis, the chamber was purged continuously with a mixture of N2 until the chamber temperature reached room temperature, as illustrated in Fig.3-5 (a).
(v) We subsequently exposed the CNT to 20 sec of CF4/O2 gas (200 sccm) treatment at 20 Torr pressure. FE-SEM, AFM was used to characterize the morphology and microstructures of CNT. The surface performance of CNT was investigated by XPS. The devices were measured using standard dc techniques with a semiconductor parameter analyzer (HP-4156B).The process is simple illustrated in Fig.3-5 (b).
(e) Characteristics of indentation on carbon nanotubes films:
The Ni layer (70Å) was deposited as catalyst, and then grown CNTs using Thermal CVD at 600°C and 15 minutes. The reaction gases were CH4, C2H4, N2, and H2, and flow rates were 200, 10, 500, and 500 sccm, respectively. The resulting morphologies were investigated by SEM. CNTs were subjected to nanoindentation, using a Nanoindenter with a Berkovitch indenter, on the top and side of the CNTs films, loads are increasing by 50, 100, 300, 500 mN, respectively. The crystallization was measured by Raman spectrum (Raman spectroscopy equipped, Department of materials science and
engineering at NCTU). The valuation of quality of CNTs was characterized using Gaussian curves fitting in order to obtain the ID/IG ratio. The Nanoindenter with a Berkovitch indenter process is also simple illustrated in Fig.3-6.
3.2 Deposition system
(a) Physical vapor deposition (sputtering) system
The system can provide the buffer layer of thin film deposition, such as TiN or TaN and Ni metal layer.
(b) Micro wave plasma chemical vapor deposition (MPCVD) system
A 915 MHz micro wave plasma chemical vapor deposition (MPCVD) system was used to pretreatment and grow CNTs. The nickel catalysts were formed metal substrates.
(b)Thermal chemical vapor deposition (Thermal CVD) system Thermal CVD system was used to grow lateral CNTs.
3.3 Measurement System
(a) Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM, Hitachi S-4000) was utilized to observe the, lengths, diameters, density and the morphologies of the CNTs arrays.
(b) Atomic force microscope (AFM)
Digital Instruments DI 5000 were utilized to observe the particles size, diameters, density and the morphologies of the catalysts after pretreatment.
(c) Raman spectroscopy
Raman's spectrum in the highest vibration of balanced location promptly, and offer the information between the crystallization and atom band.
(d) High-Resolution transmission electron microscopy (HR-TEM)
High resolution transmission electron microscope (JEM-2010F) at 300 kV was utilized to discover the microstructural analysis of the CNTs.
(e) An energy dispersive X-Ray (EDX)
An EDX analyzer was used to investigate the composition if Fe-Ni surface before and after plasma post treatment
(f) Fourier transform infrared spectroscopy (FTIR, ASTeX PDS-17 System) FTIR analyzer was used to explore the changes in the chemical components of CNTs under various stages of post-plasma treatment.
(g)Thermal desorption atmospheric pressure ionization mass spectrometry (TDS-APIMS)
TDS analyzer was used to measurement the changes in the chemical components.
(h) X-ray photoelectron spectroscopy (XPS, VG Scientific Microlab 310F) XPS analyzer was used to measurement the chemical bonding.
(i) I-V system: Semiconductor parameter analyzer (HP-4156B).
I-V system was used to measurement thelateral CNT device.
Fig.3-1. The primary experimental design
Fig.3-2 The nickel-coated substrates via pretreatment process with hydrogen plasma.
Fig.3-3 The CNTs growth from catalyst films were pretreated in H2 plasma to promote the formation of catalyst particles and elemental Ni.
Fig.3-4 The changes in the chemical components of CNTs under various stages of CF4/O2
post-plasma treatment.
(a)
(b)
Fig.3-5 (a) The layer structure was prepared by stacking Ta-Ni-TiN-SiO2 layers on Si substrate. (b) The design is simple illustrated.
Fig.3-6 The Nanoindenter with a Berkovitch indenter process.