2-1 Background of Excimer Laser
2-1-1 General Feature of Excimer Laser
Excimer laser is the most powerful UV light source, and that has been widely applied on the semiconductor industry, such as lithography, thin film fabrication and post annealing [31-33]. The term excimer is short for 'excited dimer'. Under the appropriate conditions of electrical stimulation, a pseudo-molecule called a dimer is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range. An excimer laser typically uses a combination of an inert gas (Argon, krypton or xenon) and a reactive gas (fluorine or chlorine). Table 2-1 shows that the various wavelengths (λ), between 157 ~ 351 nm, can be obtained using different laser gas, and all excimer lasers are pulsed laser modes. In this dissertation, an equipment of KrF excimer laser (Lambda Physik Excimer Laser LPX 210i, λ=248 nm) was used as the light source of pulsed-laser deposition (PLD) and excimer laser annealing (ELA), which were proposed for PSrT film fabrication and post-annealing, accordingly. The mode of PLD or ELA can be adjusted by the modulation of optical lens of this laser equipment as shown in Fig. 2-1. The detail concepts of PLD and ELA are addressed as following.
Table 2-1Wavelength (λ) of an excimer laser depends on the gas molecules used.
Excimer
Laser Gas F2 ArF KrF XeBr XeCl XeF CaF2 KrCl Cl2
Wavelength
(λ) 157 nm 193 nm 248 nm 282 nm 308 nm 351 nm 193 nm 222 nm 259 nm
2-1-2 Concepts of Pulsed-Laser Deposition (PLD)
Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning.
Moreover, excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with less heating or change to the remainder of the material which is left intact. Hence, pulsed-laser deposition (laser ablation) is a superior technique for expressing the intrinsic properties of materials to
Focus Lens
Figure 2-1 Schematic drawings of the practical laser system for (a) PLD mode and (b) ELA mode conducted in this dissertation.
(a) PLD Mode
(b) ELA Mode
permit the formation of a high-quality laminate film.
Experimentally, PLD is extremely simple among all thin film growth techniques.
Fig. 2-1(a) shows a schematic diagram of an experimental setup of PLD. It contains a target tray and a sample holder with heater in this vacuum chamber. A high-power KrF laser is used as an external energy source and is focused to the target surface by optical lens. As for basic mechanisms for pulsed laser sputtering [31], five different sputtering mechanisms are described as
1. Collision sputtering
Collision sputtering in the sense of momentum transfer in direct beam-surface interactions cannot occur with laser pulsed. But indirect collision effects do, however, exist with photons.
2. Thermal sputtering
Thermal sputtering, in the sense of vaporization from a transiently heated target, may require temperatures well above the melting of boiling points.
3. Electronic sputtering
Electronic sputtering is not a unique process, but rather a group of processes having the common feature of involving some form of excitation or ionization.
4. Exfoliational sputtering
Exfoliational sputtering, as when flakes detach from a target owing to reaped thermal shocks, would show an obvious and characteristic topography which is dominated in our work.
5. Hydrodynamic sputtering
We use the term hydrodynamic to refer to processes in which droplets of material are form and expelled from a target as a consequence of the transient
melting, processes that have no analog in ion-surface interactions.
The secondary mechanism include various types of pulsed flow processes (outflow with reflection, effusion with reflection, and effusion with re-condensation) that differ both depending on whether the release is from the surface or from a reservoir , and also depending on whether particles that are backscattered toward the surface are reflected or absorbed (i.e. re-condensed).
Pulsed laser deposition is often described as a three-step process consisting of vaporization of a target material, transport of vapor plume, and film growth on a substrate. These steps are repeated thousands of time in a typical deposition run.
Therefore, the structural and the electrical characteristics of PLD ferroelectric films are strongly affected by the process parameters. In this dissertation, the most two important process parameters of PLD, ambient oxygen pressure and substrate temperature, were fully investigated.
2-1-3 Concepts of Excimer Laser Annealing (ELA)
The low-temperature (≤ 200 oC) PLD PSrT films usually have poor crystallinity and unwilling ferroelectricities, so those films must be post annealed to improve the electrical properties. Figure 2-1(b) displays the schematic diagram for practical ELA setup. ELA can be carried out with low mechanical stress under low substrate temperature, but its process window is very hard to be controlled. The ELA technique can achieve local-high-temperature heating within very short duration time, which has been applied in the formation of ultra-shallow junction [34] and the production of low-temperature poly-silicon (LTPS) thin film transistor (TFT) [36-38].
However, the reports of ferroelectric films treated by ELA are very limited [39, 40].
Table 2-2 lists the comparisons of various post treatment technologies on
ferroelectrics. The advantage of ELA is obvious, except the concern of throughput.
This chapter describes how a novel excimer laser annealing (ELA) technique can achieve local-high-temperature heating and crystallize the amorphous PSrT films.
Physical and electrical properties are examined to study the detailed effects of excimer laser annealing on PSrT films. Furthermore, the novel laser-assisted two-step process, the combination of initial crystal seed induced by ELA and the grain growth carried out by subsequent RTA, which may be a potential technique to improve the crystallinity and electrical properties of whole PSrT films. The detail comparisons of various post-annealings are also presented.
2-2 Experimental Details
2-2-1 PSrT Films Post-Treated by Novel Laser-Assisted Annealing
A low-thermal budget treatment, laser-assisted annealing, is proposed to improve the poor crystallinity of PSrT films deposited at extremely low temperature (200 oC). Figure 2-2 shows the multi-layer structure of Pt/PSrT/Pt/Ti/SiO2/p-type
Table 2-2Summarized comparisons of various post-treatment technologies.
Thermal
Budget Uniformity Mechanical Stress Releaseing
Mass Production Throughput
Furnace Annealing High Δ Ο Ο
Microwave
Annealing High Ο Δ Δ
Rapid Thermal
Annealing (RTA) Low Δ Χ Δ
ELA with Optical
Scanning System Low Δ Ο Χ *
* The Throughput of ELA technique can be improved by equipment modifications.
Ο: Good Δ: Normal Χ: Worse
Si used in this work. The Pt/Ti films of 100/4 nm were subsequently sputtered onto the SiO2/Si as the bottom electrode/adhesion layer. The thinner PSrT film is adapted due to the limited depth of laser absorption in ferroelectrics. Table 2-3 describes the specific parameters of PSrT film preparation and post-annealings. The as-deposited PLD PSrT thin films were post treated by a rapid thermal annealing (post-RTA) technique (JetFirst Processor, Jipelec) with the conditions as following: the ambient oxygen gas flowing rate was 100 sccm, the process temperatures were 450 or 600 oC, the heating rate was programmed as 30 oC/sec or 50 oC/sec, respectively.
Furthermore, the as-deposited films were also annealed by a novel laser-assisted two-step process: post-excimer laser annealing (post-ELA) and subsequent RTA (post-ELA + RTA). After the modulation of optical lens of the KrF excimer laser system, the post-ELA was processed with the conditions: the substrate temperature, the ambient oxygen pressures (PO2), the laser pulsed rate, the average laser energy
fluence (LEF) and the number of laser pulses were 300 oC, 80 mTorr, 1 Hz, 47.6 – 105.6 mJ/cm2 per pulse, 40 - 180 pulses, respectively. The LEF of laser pulse was calibrated inside the vacuum chamber by a photodiode meter. The subsequent RTA was executed at 450 or 600 oC in oxygen ambience with 60 sec duration time. The detail analyses of physical properties and electrical characteristics are methodically discussed for PSrT films treated by RTA, ELA, and the novel laser-assisted two-step process. After the physical examinations, the Pt top electrodes, with a diameter of 165 μm, were deposited by sputtering and patterned by shadow mask process. Next, the voltage was biased on the top electrode, and the bottom electrode was grounded for electrical measurements. The design of experiment is listed as step by step in Fig.
2-3.
(a) PLD Parameters for As-deposited PSrT Film
Laser KrF Excimer Laser (λ = 248 nm)
Target (Pb0.6Sr0.4)TiO3 Ceramic Bulk Distance between Target and Substrate4 cm
Substrate Pt/Ti/SiO2/Si
Substrate Temperature (Ts) 200 oC Ambient Oxygen Pressure (PO2) 80 mTorr
Laser Energy Fluence (LEF) 1.55 J/cm2 per pulse
Laser Pulsed Rate 5 Hz
PSrT Film Thickness ~ 120 nm
(b) Post-Annealing Condition of PSrT Film
Laser KrF excimer laser (λ = 248 nm)
Substrate Temperature (Ts) 300 oC Ambient Oxygen Pressure (PO2) 80 mTorr
Laser Pulses 40 － 180
Laser Energy Fluence (LEF) 47.6 － 105.6 mJ/cm2 per pulse
Laser Pulsed Rate 1 Hz
Ambient Oxygen Flowing Rate 100 sccm Annealing Temperature 450 － 600 oC
Heating Rate 30 － 50 oC/sec
Annealing Duration 60 sec
ELA ParametersRTA Parameters
Table 2-3The process conditions of PSrT film treated with post-annealings: (a) PLD parameters for as-deposited PSrT film, and (b) post-annealing parameters of ELA and RTA.
Figure 2-2 Schematic view of the multilayer structure conducted for PSrT films post-annealed in this dissertation.
Pt/Ti (100/4 nm)
MFM: Pt/Ti/SiO2/Si
PLD PSrT
AES (Depth Profile)
ESCA (Elemental ratio)
XRD, GIXRD (Crystallinity)
AFM (Surface Roughness)
Top Pt Electrode Patterning
SEM (Surface Morphology)
n&κ Analyzer (n, κ, Reflectivity)
Physical Analysis
TEM (Structure, EELS) 4156C
(I-V) Keithley KI-82 (C-V)
Post-Annealing
Breakdown Analysis (TDDB) Electrical Analysis
1. RTA 2. ELA 3. ELA + RTA
Figure 2-3 The experimental flow of PSrT films treated by post-annealing.
2-2-2 PSrT Films Prepared by Pulsed-Laser Deposition
A relatively low-temperature (≤ 450 oC) pulsed-laser deposition is conducted to prepare PSrT film for two architectures, metal/ferroelectric/metal (MFM) and metal/ferroelectric/semiconductor (MFS) (Fig. 2-4), which are used to simulate the devices of COB and MFS-FET, accordingly. PLD PSrT films (200 nm thick) were deposited on the Pt/SiO2/Si substrate or p-type Si (100) wafer. The deposition temperature (substrate temperature, Ts) varied from 300 oC to 450 oC, calibrated at the wafer upper surface. It means that a thermal couple, connected to a temperature controller, was used to touch the surface of samples and sense the temperature of Pt/SiO2/Si substrate, denoted as Ts. During the PLD process, the oxygen partial pressure, target to substrate distance, laser pulsed rate, and average laser power density (laser energy fluence) were 80 - 200 mTorr, 4 cm, 5 Hz, and 1.55 J/cm2 per pulse, respectively, as denoted in Table 2-4. The physical properties of PSrT films were analyzed before Pt top electrode deposition. Then, the Pt top electrodes, with a thickness of 100 nm and a diameter 75 μm, were deposited by sputtering and patterned by shadow mask process to form a MFM capacitor structure or a MFS configuration.
The relationships among process parameters, physical properties, and electrical characteristics were systematically studied to obtain the optimal process conditions.
The related mechanisms of conduction and fatigue will be thoroughly studied in this dissertation. Figure 2-5 reveals the detail experimental flow.
Figure 2-4 Schematic view of PSrT films applied in (a) MFM and (b) MFS structures in this dissertation.
Si
Table 2-4The PLD process conditions of PSrT film.
PLD PSrT Film Condition
Laser KrF Excimer Laser (λ = 248 nm)
Target (Pb0.6Sr0.4)TiO3 Ceramic Bulk Distance between Target and Substrate 4 cm
Substrate Pt/SiO2/Si or p- type Si
Substrate Temperature (Ts) 300 － 450 oC Ambient Oxygen Pressure (PO2) 50 － 80 mTorr Laser Energy Fluence (LEF) 1.55 J/cm2 per pulse
Laser Pulsed Rate 5 Hz
PSrT Film Thickness ~ 200 nm
MFM: Pt/SiO2/Si
Figure 2-5 The experimental flow of PSrT films fabricated by PLD technique.
2-3 Material characterization Techniques
1. n&κ Analyzer
The reflectivity, refraction index (n), and extinction coefficient (κ) of PSrT film were measured by n&κ analyzer (1280, n&κ Technology). The n and κ are influenced by the electronic structure and/or crystallinity of the film. The band gap of PSrT films, Efg, was also estimated by the optical investigation of n and κ.
2. Scanning Electron Microscope (SEM)
The thin film surface morphology, cross sectional profile, and film thickness were examined by field emission scanning electron microscopy (FESEM) (S4000, Hitachi).
3. Atomic Force Microscope (AFM)
The surface roughness and surface morphology were examined by AFM (DI Nano-Scope III, Digital Instruments). The root mean square value of the film roughness was calculated.
4. Electron Spectroscope for Chemical Analysis (ESCA)
An ESCA (ESCA 210, Fison (VG)) was performed to analyze the element ratios on the surface of PSrT films.
5. Auger Electron Spectrum (AES)
AES machines (Auger 670 PHI Xi, Physical Electronics, and Microlab 310D &
310F, Fison (VG)) were used to analyze the surface element ratios and element depth profile of PSrT films. The depth profiling was accomplished with incorporated ion guns that enable the specimen surface to be continuously
sputtered away while Auger electrons were being detected. Then the inter-diffusion of the elements was investigated.
6. X-Ray Diffraction (XRD) Analysis
Two X-Ray diffraction methods were generally used. For the theta-2 theta powder method, the detected X-ray beams are diffracted from the lattice-planes that are all parallel to the substrate. The powder method is suitable for comparison with standard X-ray powder diffraction data to find the preferred orientation in the films. Thus, the microstructures of the thin films can be characterized by a Siemens D5000 Diffractometer with Cu Kα (λ ~ 0.154 nm) radiation. However, for glancing angle method, the normals of diffracted lattice planes are not parallel to each other and incline at various angles to the substrate. The glancing method is good for phase identification and provides some "information" about orientation distribution. The crystallinity of the films with post-ELA was also analyzed by the glancing incident X-ray diffraction (GIXRD) machine (D/MAX2500, Rigaku, using Cu Kα, λ ~ 0.154 nm) with a fixed incident angle of 2 degrees.
7. Transmission Electron Microscope (TEM)
The crystallinity and nano-scale structure were examined by the TEM equipment (JEM-2000FX, JEOL, using the electron acceleration voltage of 200 kV). TEM samples were fabricated by standard sample preparation techniques with tripod polishing and ion milling using the Gatan PIPS system operated at 3 kV. The maximum resolution of this instrument is about 0.31nm. The diffraction pattern and bright/dark field image were performed in this investigation. The TEM was equipped with a Gatan image filter (GIF, Model 2000, Gatan) which provides the fingerprint of chemical bonding states with
high lateral/energy resolution in terms of electron energy loss spectroscopy (EELS) spectra, recorded with an energy resolution of 1.2 eV (full width at half maximum, FWHM, of zero-loss peak) and an energy dispersion of 0.3 eV per channel.
2-4 Electrical Measurement Techniques
After the material characterizations, the Pt top electrodes were then deposited by sputtering and patterned by shadow mask process to form the MFM capacitor or MFS gate structure. Next, the testing voltage or input signal applied on the top electrode, and the bottom electrode was grounded.
1. Current-Voltage (I-V) Measurement
An automatic measurement system that combines IBM PC/AT, semiconductor parameter analyzer (4156C, Agilent Technologies) and a probe station was used to measure the leakage current (I-V) characteristics and breakdown properties.
The testing voltage biased on the top electrode, and the bottom electrode was grounded. The capacitors were stressed under various voltage to estimate the time depend dielectric breakdown (TDDB). Conduction mechanisms due to electrode-limited Schottky emission (SE) and bulk-limited Poole-Frenkel emission (PFE) in the PSrT film were investigated by analyzing current density versus electric field (J-E) curves.
2. Capacitance-Voltage (C-V) Measurement
Computer-controlled Keithley package 82 system was used to obtain high frequency C-V and quasi static C-V simultaneous curves. The package 82 system includes a model 590 CV analyzer for high frequency C-V measurement, a model 595 quasi static C-V meter along with the 595 remote coupler, lower
noise BNC cables and IEEE 488 bus. A model 230-1 voltage source and a model 5905 calibration source are included too. In this work, the high frequency C-V data at 100 kHz were taken on the capacitor to determine the capacitance value in the accumulation mode. The dielectric constant was calculated from C-V using the following relation:
d A ε ε
C
=
r×
0×
, (2-1)where A is the capacitor area, d is the thickness of ferroelectric film,
ε
0 is the vacuum permittivity,ε
r is the dielectric constant, and C is the ferroelectric capacitance.3. Tangent Loss Measurement
Capacitance and tangent loss are functions of frequency, so the impedance/gain-phase analyzer (4194A, Hewlett Packard) was applied to extract the capacitance-frequency (C-f) and loss tangent-frequency (tanδ-f) data.
The frequency of 4194A ranges from 100 Hz to 15 MHz. Tangent loss is an indicator of resistive leakage. The leakage path is parallel to the capacitance in the equivalent circuit. If the loss tangent increases, the impedance will decrease, then the leakage current will increase. The admittance and impedance spectra were measured as a function of frequency with 4194A impedance gain phase analyzer. The AC electrical data, in the form of parallel capacitance and conductance, were recorded in the frequency range of 100 Hz to 10 MHz at the AC signal amplitude of 0.1 V.
4. Polarization Measurement
The ferroelectric polarization-electric field (P-E) characteristics of the PSrT films were determined directly from virtual ground circuits (RT-66A, Radiant
Technologies). The pulse with a triangle-shaped wave at 10 kHz trains was conducted to measure the polarization. The P-E curve was obtained analytically by calculating the polarization P from the relative permittivity
ε
r (dielectric constant) versus electric field E using the following equation.( )
∫ −
×
=
dEP
ε
0ε
r1
, (2-2)5. Fatigue analysis Measurement
A pulse generator (8110A, Hewlett Packard) and a pulse/function generator (8116A, Hewlett Packard) were connected together with low noise BNC cables to generate +3 V/-3 V bipolar wave pulsed at 1 MHz, confirmed by an oscilloscope (54645A, Hewlett Packard), as an input signal for the measurement of polarization switching degradation (fatigue). Figure 2-6 gives the schematic equipment setup and bipolar switching waveform. The measurement of fatigue analysis consists of four steps, described as (i) J-E & P-E measurements before fatigue switching, (ii) capacitors are fatigued with 1 MHz +3 V/-3 V bipolar waveform, (iii) J-E & P-E measurements after fatigue switching, and (iv) the fatigue mechanism can be analyzed by J-E curve fitting.
Figure 2-6 The schematic equipment setup and switching waveform for the operation of polarization bipolar switching degradation (fatigue).
3V