Recently, many technologies have been used to prepare various high-κ dielectrics, such as atomic layer deposition (ALD), chemical vapor deposition (CVD) and physical vapor deposition (PVD). Nevertheless, the sol-gel spin coating method also catches much attention; it is utilized to form the high-κ dielectric films [17] and memory charge trapping layers [18][19].
In the sol-gel processes, hydrolysis, condensation, and polymerization, the step-by-step formation leads to a metal-oxide network. And there is an arresting character of sol-gel spin coating method and which is an ability to synthesize new types of high-κ materials, called “inorganic-organic hybrid” [20].
The sol-gel spin coating method could be executed in the normal pressure environment rather than high vacuum system. And thin film formation with spin coating is simpler than ALD, PVD, or MBE to deposit an insulator because of its
cheaper precursors and tools.
1.5 Electron Spectroscopy for Chemical Analysis (ESCA)
In this section it is worth introducing the fundamental principle of ESCA because the most following analysis for material and physical properties are performed by ESCA technique.
ESCA, also known as XPS, is used to characterize the chemical bonding and film composition. Since the photon energy range of interest for material analysis corresponds to the x-ray energy (1-10 keV), photoelectron spectra with specific binding energies produced by x-ray radiation of a sample present chemical bonding information about given elements. Figure 1.7 shows relevant energy levels for ESCA measurements [21]. Binding energies of photoelectrons can be obtained from Eq.
(1-7), based on Figure 1.7.
Ekin =hν −Eb−φspec, (1-7) where Ekin is kinetic energy of the photoelectron, h is Plank’s constant, ν is the frequency of he photon, Eb is the binding energy, and Øspec is the work-function of the spectrometer.
However the ejected photoelectrons from ESCA analysis undergo inelastic energy losses due to collective oscillations (plasmon) and single particle excitation (electron-hole band to band transitions). More importantly, the excitation of a single electron from the valence band to the conduction band can also be detected at the onset of plasma energy loss, as illustrated in Figure 1.8 [22]. This onset point of plasma energy loss can be utilized to determine the band gap energy [23-25].
Photo-excited electrons lose their kinetic energies due to collective oscillations of outer shells, resulting in a plasmon spectrum. Single electron excitation from valence
band edge to conduction band edge also takes place at the onset of a plasmon spectrum. Ec, Ev, Eg, and hν denote conduction band minimum, valence band maximum, band gap energy, and photon energy of X-ray irradiation, respectively.
Figure 1.2 Calculated（lines）and measured（dots）results for tunneling currents from inversion layers through oxides [2].
Figure 1.1 Moore’s law for microelectronic industry. The exponential increase of transistors count as a function of time for distinct generations of microelectronics has been realized [1].
Ba
Fig. 1.4 The symmetric perovskite crystal structure (a) is not polarized when there is no applied electric field. And the applied field polarizes the structure (b).
Figure 1.3 The frequency dependence of the real (εr’) and imaginary (εr”) parts of the dielectric permittivity. In CMOS devices, ionic and electronic contributions are present.
(a) (b)
Fig. 1.5 XRD pattern of Ta thin film oxided at 700 ﾟ C for 1, 1.5, 2,3 and 4 min [11].
Fig. 1.6 The evolution of surface morphology resulted from thermal oxidation at 700 ﾟ C for 1(a), 3(b) and 4(c) min [11].
Fig. 1.8 Illustration of band gap energy by O 1s or N 1s photoelectron energy loss spectrum.
Fig. 1.7 Schematic of the relevant energy levels for XPS binding energy measurements. Note that a conducting specimen and spectrometer are in electrical contact and thus have common Fermi levels.
Schottky
Table 1.1 Basic conduction processes in insulators [6].
⎟⎟⎠
Chapter 2
Experimental Procedures
In this chapter we will illustrate the device fabrication process with figures and list the instruments for material and physical properties measurements.
2.1 Device Fabrication
In this investigation, CoTiO3 films were prepared by sol-gel spin coating method in a controlled environment, where was maintained at 22oC and 43 % RH. The process flow was illustrated in figure 1. First, n-type single crystal Si wafers with resistivity 4-7 Ωcm underwent standard RCA cleaning followed by a dilute-HF dip to remove the native SiO2. Then, the sol-like precursor for CoTiO3 was directly spun on the Si substrates at about 3000 revolutions per minute, and the spin speed was maintained for 30 seconds. However the precursor for cobalt and titanium elements were cobalt acetate tetrahydrate Co(OOCCH3)2．4H2O and titanium isopropoxide Ti(OCHC2H6)4, respectively. These two precursors were dissolved in 2-methoxyethanol for spin coating method. After the spin-coating of the precursor, in order to remove the solvent, the samples were baked at 90oC for 1.5 min on a hotplate.
And the procedure (coating-and-baking) was repeated for 5 times. Afterward the films were oxidized at 400oC in an N2/O2 ambient for 10 min, in which both N2 and O2 flow were 50 sccm. In order to study the properties of CoTiO3 high-k dielectrics after high temperature treatment, rapid thermal annealing (RTA) was performed. The samples were annealed at 600oC, 700oC, 800oC or 900oC for 30s in N2 ambient.
Photolithography was used to define gate areas and then TaN metal was deposited on the top of samples by reactive DC-sputtering. Lift-off was performed to form the MIS capacitors. Thereafter, ohmic contacts were formed by thermal evaporation of 300-nm-thick aluminum (Al) electrode on the backside of the samples.
2.2 Material and Physical Properties Measurements
The microstructure of spin-on CoTiO3 film and Si substrate were studied by JEOL JEM-2100F field emission transmission electron microscopy (TEM) equipped with Link ISIS-300 energy dispersive X-ray analyzer (EDS). And the TEM EDS with a 5-nm electron beam probe was used to perform chemical analysis qualitatively.
The characteristic of crystallization of spin-on CoTiO3 films with different annealing temperature were identified by PANalytical X’Pert Pro X-ray diffraction system under normal atmosphere. Optical module with X-ray mirrors and a parallel plate collimator was used to perform gracing incident X-ray diffraction (angle of incidence θi ~1˚). The beam source originated from Cu Kα radiation with a 0.154-nm wavelength and this beam source was operating at 1.8 kW.
Surface morphology of spin-on CoTiO3 films with different annealing temperature was obtained by Veeco dimension 5000 scanning probe microscope (SPM) under normal atmosphere. The highest resolution in X-Y plane and Z direction were about 1.5 nm and few angstroms, respectively. And the tip curvature radius was about 2 nm.
A ULVAC-PHI Quantera high resolution X-ray photoelectron spectrometer (HR-XPS) with 180˚ spherical capacitor analyzer was used to analyze quantitatively the chemical composition of the dielectrics CoTiO3 prepared by sol-gel coating method.
The capacitance-voltage (C-V) curves and current-voltage (I-V) curves were measured in the same probe station by HP 4284 and Keithly 4200, respectively.
●
Spin coating (CoTiO3)
200oC baking Repeat 5 times 400oC annealing in N2 and O2
RTA (600oC / 700oC / 800oC / 900oC) in N2
Define patterns with photoresist
Lift-off and electrodes formation TaN/Al deposition
Chapter 3 Material Properties
In this chapter, we will report the material properties of spin-on CoTiO3 thin films analyzed by Transmission Electron Microscope (TEM), Energy Dispersive Spectrometer (EDS), Grazing Incident X-Ray Diffraction (GI-XRD), Scanning Probe Microscope (AFM), Auger Electron Microprobe (AEM) and Electron Spectroscopy for Chemical Analysis (ESCA).
3.1 Si-sub/CoTiO
3Interface Quality
In figure 3.1, the graph is the cross-section of Al-electrode/1-coated CoTiO3 thin film/bare Si substrate structure and the CoTiO3 thin film is annealed at 400oC. There are two interfacial layers astride the CoTiO3 thin film. The interfacial layer between Si-sub and CoTiO3 thin film is about 2.23 nm and the interfacial layer between Al-electrode and CoTiO3 thin film is about 1.88 nm. Furthermore, the thickness of the 1-coated CoTiO3 thin film on Si-sub is about 5.27 nm.
In order to qualitatively recognize the composition of the spin-on dielectric, EDS analysis is performed. As shown in figure 3.2 (a), the three principal elements, Cobalt, Titanium and oxygen, are detected. However, as shown in figure 3.2 (b), Al peak and Si peak maybe come from the interfaces beside the dielectric, and the Cu peak should be contributed to the Cu net which is used to hold the sample. Therefore there are three main elements, cobalt, titanium and oxygen in the spin-on dielectric.
3.2 Surface Morphology
As mentioned in section 1.3.4, it is desirable that the surface morphology of high-κ dielectric is still smooth though it undergoes high temperature treatment.
PANalytical X'Pert Pro (XRD) and Veeco Dimension 5000 Scanning
Probe Microscope (D5000) are used to analysis surface morphology of films with different high temperature annealing.
Figure 3.3 presents the GI-XRD spectra of the CoTiO3 thin films. No significant signals could be found for samples treated at temperatures below 600oC, indicating amorphous CoTiO3 films to begin with. When a sample was annealed at a temperature above 700oC, signals of crystallized CoTiO3 phases were found. This suggests the crystallization temperature of spin-on CoTiO3 films being 600~700°C. Furthermore, signals of Si substrates were also found in the GI-XRD spectra for samples annealed at temperatures beyond 800oC. It is speculated that the crack of CoTiO3 films has partially exposed the Si substrate after annealed at elevated temperatures.
Figure 3.4 shows the SPM images of CoTiO3 films with different high temperature treatments. From figure 3.4(a) to figure 3.4(f) are the flattened and 3-D images of samples baked or annealed at 200oC, 400oC, 600oC, 700oC, 800oC, and 900oC, respectively. The extended dark regions was found in the images for 600~900oC annealed samples. Serious cracks of the CoTiO3 thin film can be found after annealed at 900oC, as shown in figure 3.4(f). And figure 3.5 reports that the roughness of samples suddenly becomes serious when annealing temperature is higher than 600 oC.
3.3 Composition Analysis
3.3.1 Auger Electron Microscope Analysis
The incorporation of carbon element in a dielectric will decrease the effective dielectric constant [26]. Therefore Auger depth profile is used to analyze the elements in spin-on dielectric CoTiO3 with 600oC annealing. As shown in figure 3.6, it is obvious that carbon element signal only exists at the start of analysis, i.e. at surface.
The surface carbon may be resulting form the absorption of residual in the air. Hence
there is no carbon element in the dielectric formed by this sol-gel spin coating method.
In this analysis, because we didn’t have the reference sample to derive the relative sensitivity factor for evaluating atomic concentration in depth profile, there is an error in the atomic percentage in the figure 3.6. Despite this deviation, the qualitative composition result could be acceptable.
3.3.2 Electron Spectroscopy for Chemical Analysis
There is no doubt that the spin-on dielectrics only contains cobalt, titanium and oxygen elements, which is confirmed by EDS analysis in section 3.1 and Auger depth profile in section 3.1. However, we still don’t know the chemical properties and atomic concentration ratio of this dielectric prepared by sol-gel spin coating. In the section we use electron spectroscopy for chemical analysis (ESCA) to obtain further information.
Figure 3.7 shows ESCA results of the dielectric formed by sol-gel spin coating method and the dielectric under analysis is 1-coated and annealed at 600oC. From figure 3.7(a) to 3.7(d) are the spectrums of silicon 2p orbital, oxygen 1s orbital, cobalt 2p orbital and titanium 2p orbital, respectively. As shown in figure 3.7(a), two main peaks identify single crystalline silicon (99.3 eV) and silicon dioxide (103.3 eV) in the silicon 2p orbital spectrum. We also observe that the shift and the growth of the silicon-dioxide-peak increases, as the annealing temperature is increased, which means that more complete structure and thicker silicon oxide are formed after higher temperature annealing. The spectrum of oxygen 1s orbital shown in figure 3.7(b) reveals that there may be two kinds of metal-oxygen bonds with lower binding energy near 531 eV , e.g. Co-O and Ti-O for all samples. However, the broader binding energy distribution for the sample annealed at 200oC may be resulting from hydroxides in the dielectric [27]. Furthermore, the samples with 800oC and 900oC
annealing have silicon oxide bond with binding energy near 533 eV, which is consistent with the results in figure 3.7(a). We also notice that two shake-up peaks with higher binding energy than two main peaks (2p3/2 and 2p1/2) appear in the Co spectrum, as shown in figure 3.7(c).
In order to confirm the atomic concentration ratio of the dielectric prepared by sol-gel spin coating method, more detailed ESCA analysis for the spin-on dielectric annealed at 600 oC is executed. After background removal by Shirley method and curve fitting for oxygen-metal bonds (oblique line area) in oxygen spectra, we integrate the intensity from 775 to 810 eV for cobalt spectra, from 453 to 468 eV for titanium spectra and from 527 to 535 eV for oxygen spectra, as shown in figure 3.8.
The relative sensitivity factors for cobalt, titanium and oxygen are 3.529, 2.077 and 0.733, respectively. And the atomic concentration ratio is obtained as,
93175.67 53580.63 53828.65
Therefore, the atomic concentration ratio is almost close to 1:1:3.
5nm
Fig. 3.1 TEM image of Al-electrode/1-coated CoTiO3
thin film/bare Si structure.
Fig. 3.2 Electron dispersive spectra (EDS), associated with the TEM image showed in Fig. 3.1, of the CoTiO3dielectric annealed at 400oC.
1.88 nm 5.27 nm 2.23 nm
(a)
(b) Al
CoTiO
3Si-sub
Fig. 3.3 XRD spectra of spin-on CoTiO3 films. The marked peaks correspond to crystallized CoTiO3 phases.
(012) (104) (110) (113) (024) (116)
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3.4 SPM images of spin-on CoTiO3 films with various thermal treatments at (a) 200oC, (b) 400oC, (c) 600oC, (d) 700oC, (e) 800oC and (f) 900oC. The image size is 1 μm by 1 μm.
200 400 600 800 1000
Fig. 3.5 Surface roughness of spin-on dielectrics as functions of annealing temperature.
Fig. 3.6 Auger depth profile of the CoTiO3 dielectric annealed at 600oC.
543 540 537 534 531 528 525 (b)
810 800 790 780 770
(c)
470 465 460 455 450
(d)
810 800 790 780
468 465 462 459 456 453
0 1s(c) for the spin-on dielectric annealed at 600oC.
Chapter 4
Physical Property
In this chapter, we will report the physical properties of spin-on CoTiO3 thin films, such as dielectric permittivity, C-V and I-V characteristics, current transport mechanism, band energy gap and band alignment.
4.1 Dielectric Permittivity
According to section 3.1.1, it is easy to form an interfacial layer between Si-sub and a CoTiO3 film. However, it is imprecise and difficult to extract the κ value of CoTiO3 film from measuring capacitance. In order to estimate the dielectric constant of CoTiO3 thin films, a thermal oxidation is used to grow a high quality SiO2 thin layer before the CoTiO3 spin-coating. The C-V characteristics of both TaN/CoTiO3/SiO2/Si and TaN/SiO2/Si capacitors are demonstrated in Fig. 4.1. The well C-V characteristics can be observed for both two capacitors without flat-band voltage shift, as shown in Fig. 4.1. The capacitance effective thickness (CET) is extracted from C-V curves at 100 kHz without considering quantum effect. The CET of CoTiO3/SiO2 and SiO2 are 4.66nm and 4.27nm, respectively. However figure 4.2 shows HR-TEM image of the Si/SiO2/CoTiO3/TaN structure. Thicknesses of 1-coated high-k dielectric and thermal oxide are 4.02 and 4.27 nm, respectively, as shown in this TEM image. As a result, the exact dielectric constant of CoTiO3 thin film is found to be 40.2, which is matched the value of CoTiO3 films fabricated by direct oxidation of sputtered Co/Ti layers [28][29], indicating that the high permittivity CoTiO3 films can also been deposited by simple sol-gel spin coating method.
4.2 C-V and I-V Characteristics
Figure 4.3 shows the C-V characteristics of CoTiO3 gate dielectric with different
thermal treatments. The sample with 600oC RTA shows a steeper C-V slope in the depletion region, suggesting a better CoTiO3/Si interface. RTA temperatures beyond 600oC result into flatter C-V curves which may be due to the sub-stoichiometric interfacial-oxide growth and thermal stress.
Figure 4.4 shows the I-V characteristics of CoTiO3 gate dielectrics. The leakage current density increases with increasing RTA temperature (600~900oC), even though the C-V curves suggest a larger effective oxide thickness (EOT) for samples annealed at higher temperatures. This can be explained by the cracks and crystallization of CoTiO3 thin films, as discussed before. Finally we sum up the fundamental electrical behavior of spin-on CoTiO3 dielectric in figure 4.5, and the dielectric annealed at 600oC has the smallest EOT.
4.3 Current Transport Mechanism
Figure 4.6 shows the I-V curves measured at elevated temperatures. The CoTiO3
film under test was annealed at 600oC. The I-V curves were fitted by the Schottky emission model (inset), and the barrier heights of 0.74, 0.72, 0.70, and 0.69eV were extracted at room temperature, 40oC, 50oC, and 60oC, respectively. Fittings with the Frenkel-Poole (FP) conduction model were also carried out. Figure 4.7 demonstrates that the current conduction is not dominated by the FP conduction but by the Schottky emission, which shows a smaller Si/CoTiO3 barrier height.
4.4 Band Energy gap and Band Alignment
As mentioned in Section 1.5, XPS technique can be utilized to determine the band gap energy. In this section we characterize the band gaps of SiO2 and the spin-on CoTiO3 dielectric annealed at 600oC. In order to align the band diagram, we also use high resolution XPS analyzer to detect the maximum valence energy band level of
thermally grown SiO2 and the spin-on CoTiO3 dielectric [30].
As mentioned in Section 1.5, the background rise below XPS core level peaks is due to inelastic scattering effects of the photon-electrons. More importantly, the excitation of the electrons from valence band to conduction band can also be detected at the onset of plasma energy loss. As a result, the onset of the background increase relative to the peak position corresponds with the band gap of the material. Figure 4.8 shows the spectrum of oxygen 1s orbital for SiO2, which is illustrating that the energy band gap of thermal SiO2 is about 9.0 eV. This value is almost close to the common results [6][7]. Furthermore, the high resolution core level and band gap spectra of spin-on CoTiO3 film is shown in Fig. 4.9. The energy band gap of spin-on CoTiO3 is about 2.2 eV, which is close to the value of CoTiO3 powders fabricated by a modified Pechini method [31].
On the other hand, the measurements are performed on thermal SiO2 (~15nm) and the 5-coated dielectric CoTiO3 (~20nm)/SiO2 (~15nm) stacks. The valance band spectrum for these layers contains the information about the density of states of both the CoTiO3 and SiO2 films. The maximum valence energy band and the valence band offset (ΔEv) between CoTiO3 and SiO2 films can thus be determined as about 4.0 eV, as indicated in Fig. 4.10.
As mention in figure 4.7, the energy barrier height between silicon substrate and spin-on CoTiO3 dielectric is about 0.74 eV at room temperature. Because the energy band alignment between Si and SiO2 is a well-known result, we can deduce the band alignment between Si, SiO2 and spin-on CoTiO3 dielectric, as shown in figure 4.11.
Figure 4.11 shows the energy band alignment between Si, SiO2 and high-k dielectric CoTiO3, which serves to summarize the key results we have obtained from the analysis of HR-XPS and Schottky emission characteristics. And this deduction is consistent with the result in figure 4.9.
Fig. 4.1 TEM micrograph of an ultrathin CoTiO3 film spin-coated on a high quality thermal SiO2 layer and annealed at 600oC.
Fig. 4.2 C-V curves of capacitors with TaN/CoTiO3/SiO2/Si and TaN/SiO2/Si stack structures.
Fig. 4.4 I-V curves of spin-on CoTiO3 films with different thermal treatments.
Fig. 4.3 C-V curves of spin-on CoTiO3 films with different thermal treatments.
200 400 600 800 1000 10
100 EOT
Jg@Vg=2V
Temperature (oC)
E.O.T.(nm)
1E-6 1E-5 1E-4 1E-3 0.01 0.1
J (A/cm 2
)
Fig. 4.5 Effective oxide thickness and current density of spin-on dielectrics as functions of annealing temperature.
Fig. 4.7 Effective barrier heights extracted from I-V curves by using Schottky-emission and Frenkel-Poole models.
Fig. 4.6 I-V curves measured at RT and elevated temperatures.
(Inset) Extracted Schottky-emission barrier heights.
550 545 540 535 530 525
SiO2
9.0 eV O 1s
Intensity (a.u.)
Binding Energy (eV)
550 545 540 535 530 525
9 12 15 18 21 24 27
Counts (x104 /s)
Binding Energy (eV)
2.2 eV CoTiO3
O 1s
Fig. 4.8 ESCA spectra of O 1s for thermally grown 15 nm-SiO2.
Fig. 4.9 ESCA spectra of O 1s for the spin-on 15 nm-CoTiO3
dielectric annealed at 600oC.
14 12 10 8 6 4 2 0 -2 -4 SiO2 CoTiO3
CoTiO3 SiO2
ΔVBmax=4.0 eV
Intensity (a.u.)
Binding Energy (eV)
SiO2 Si-sub CoTiO3
4. 0 eV 4. 4 eV
Eg=2.2 eV Eg=1.1 eV
Ec=0.7 eV
Fig. 4.11 Band alignment between Si, SiO2 and spin-on CoTiO3 dielectric with 600oC annealing.
Fig. 4.10 Maximum valence energy band spectra of thermally grown 15 nm-SiO2 and spin-on 20 nm CoTiO3 dielectric annealed at 600oC measured by high resolution ESCA.