Chapter 1 Introduction
1.2 Motivation
To achieve a low temperature process on Ge-MOS device, high-k material is a good candidate to be gate dielectric for Germanium substrate. There are least four requirements to form gate dielectric on Germanium. First, the dielectric constant must be high (>20). Second, must be thermodynamic stable with Ge, the high-k material does not react with the Ge during depositing, to avoid a low-k interfacial layer formed during depositing, and make the dielectric constant of high-k material decrease. Third,
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large enough band offset with Ge (>1eV), enough barrier high between Ge and gate dielectric can prevent the high leakage by carriers get thermal energy to overcome the barrier between Ge and gate oxide and to create leakage. Forth, form a good interface with Ge. The hafnium oxide (HfO2) and the zirconium oxide (ZrO2) are meeting the above four conditions, and have been widely studied. For high-k metal gate, HfO2 is widely used in 45nm processing. Because of it has better thermodynamic stability than ZrO2 on silicon. However, for germanium as the channel material, ZrO2 is more compatible than HfO2, because of less interfacial layer which is low-k layer formed after post-deposition annealing due to Ge intermixing in ZrO2 [21]
. In addition, very high-k (k~37) ZrO2 have been proposed via Ge incorporation into ZrO2[22]. Therefore, ZrO2 is a good high-k material deposited on Ge, we choose ZrO2 as our research high-k material.
Among several metal oxide films formations, in general, low temperature deposition is prefer, because of the low thermal budget and low costs. However, the low-temperature deposition films may cause poor interfacial properties with substrate and larger leakage current due to numerous traps inside the bulk metal oxide film.
Proper annealing can reduced leakage and remove oxide charges and interface traps in the ZrO2. But for germanium substrate, the thermal stability of GeO2 is a critical problem to form a good Ge-MOS. Because PDA or following high-temperature
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processes (like S/D annealing) could induce Ge decomposition into gate dielectric and increase leakage source enhance the leakage current after annealing. On my thesis, we use the low-temperature (100℃) technique supercritical fluid (SCF) to transport the oxidant and penetrate the dielectric layer for trap passivation and interface oxidation at low temperature. And by leakage current fitting to see how leakage mechanism transfers after SCF treats. Next, to repair the interface of ZrO2/Ge after SCF treatment we use the Water Vapor Annealing. And then we combine these methods to form a good interface of ZrO2/Ge.
1.3 Organization of the Thesis
In chapter 2, we introduce the process flow of High-k on Germanium substrate Metal-Insulator-Semiconductor Capacitor (MIS-C) fabrication first. Second, the process instruments are introduced about RF-sputter, Vacuum Annealing Furnace, Rapid Thermal Annealing, Supercritical Fluid (SCF), Thermal coater. Third, electrical characteristics analysis instrument and material characteristics analysis instrument about X-ray Photoelectron Spectroscopy (XPS) and High-Resolution Transmission Electron Microscopy (HRTEM) are all introduced. Finally, the parameters extraction and transportation mechanism are also discussed in this chapter.
In chapter 3, there are two parts, we first study the effect of two steps PDA on
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the ZrO2 film which deposited by sputter on Germanium substrate, then discussing the thermal stability and the quality of high-k film. Then we accede Supercritical Fluid treatment(SCF) to our work and analyze in various electrical analysis techniques, such as capacitance-voltage (CV) and current density-voltage (JV) by Agilent 4980 and Keithley 4200 were perform to characteristic the device performance and analysis the interface and bulk quality of gate dielectric. For material analysis, such as x-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) to analyze variation of the interface after PDA and SCF.
On the second part, this section is continued from the preceding paragraph. First, we deposit the ZrO2 high-k gate dielectric on Ge substrates, and then anneal by using rapid thermal annealing with wet nitrogen (with water vapor). The annealing in water vapor can suppress growth of unstable low-k GeOx interlayer in Ge metal-oxide-semiconductor capacitor with high-k gate dielectric [23] by reducing the GeOx to Ge and through the low thermal budget process can repair the defect which produced by reduction of GeOx in the interlayer. Second, to compare annealing with and without water, we use the rapid thermal annealing with nitrogen to compare with water vapor annealing. By analyzing C-V and J-V curve which help us to understand the recovery of the defect in interlayer after GeOx reduction whether it need oxygen or not. And various material analysis techniques, such as high-resolution transmission
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electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), were performed to characteristic the cross section of device and surface morphology. In the end, we combine the Supercritical Fluid treatment and water vapor annealing to form the device with good interface and low defect in the bulk high-k dielectric.
Finally, in chapter 4, we give the conclusions and suggestions of the thesis for the future work.
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Fig. 1-1 Phase diagram for CO2.
Table 1-1 Critical temperature and pressure for some common fluids.
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Chapter 2
Experiment Instrument and Process
2.1 Experiment instrument of High-k on Germanium base Metal-Insulator-Semiconductor Capacitor
2.1.1 RF Sputtering
Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by sputtering, that is ejecting, material from a "target," that is source, which then deposits onto a "substrate," such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV (100,000 K). The sputtered ions (typically only a small fraction — order 1% — of the ejected particles are ionized) can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber (causing resputtering). Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight
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of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film. Charge build-up on insulating targets can be avoided with the use of RF sputtering where the sign of the anode-cathode bias is varied at a high rate, as Fig. 2-1. RF sputtering works well to produce highly insulating oxide films but only with the added expense of RF power supplies and impedance matching networks. Stray magnetic fields leaking from ferromagnetic targets also disturb the sputtering process. Specially designed sputter guns with unusually strong permanent magnets must often be used in compensation.
Fig. 2-1 RF sputtering.
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2.1.2 Water Vapor Rapid Thermal Annealing System
Rapid Thermal Anneal (RTA) is a subset of Rapid Thermal Processing as Fig.
2-2. It is a process used in semiconductor device fabrication which consists of heating a single wafer at a time in order to affect its electrical properties. Unique heat treatments are designed for different effects. Wafers can be heated in order to activate dopants, change film-to-film or film-to-wafer substrate interfaces, density deposited films, change states of depositing films, repair damage from ion implantation, move dopants or drive dopants from one film into another or from a film into the wafer substrate. Rapid thermal anneals are performed by equipment that heats a single wafer at a time using either lamp based heating, a hot chuck, or a hot plate that a wafer is brought near. Unlike furnace anneals they are short in duration, processing each wafer in several minutes. To achieve short time annealing time trade off is made in temperature and process uniformity, temperature measurement and control and wafer stress as well as throughput. And it can change the input gas to achieve different thermal treatment, even water vapor system, Fig. 2-3. Recently, RTP-like processing has found applications in another rapidly growing field — solar cell fabrication.
RTP-like processing, in which an increase in the temperature of the semiconductor sample is produced by the absorption of the optical flux, is now used for a host of solar cell fabrication steps, including phosphorus diffusion for N/P junction formation
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and impurity gettering, hydrogen diffusion for impurity and defect passivation, and formation of screen-printed contacts using Ag-ink for the front and Al-ink for back contacts, respectively.
Fig. 2-2 Rapid thermal annealing system.
Fig. 2-3 Water vapor system.
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2.1.3 Supercritical Fluid System
Supercritical fluid (SCF) is compound above their critical temperatures and
pressure, as shown in Fig 1-1. [15, 16] The attractiveness of supercritical fluid for commercial applications is their unique combination of liquid-like and gas-like properties. The supercriticality is a strange and intriguing state in which solids can dissolve in gases, and liquids can alternate between reflectivity and transparency. The critical temperature and pressure for some common supercritical fluids are displayed in Table 1-1. The CO2-based supercritical fluid is particularly attractive because CO2
is non-toxic, non-flammable, recyclable, and inexpensive and has a reasonably high solvent power for most organic components. Besides, its critical conditions are easily achievable with existing process equipment (31 °C, 1072 psi =72.8 atm).
Figure 2-5 shows the density-pressure-temperature surface for pure CO2. It can be discovered that relatively small changes in temperature or pressure near the critical point, resulting in large changes in density. Table 2-1 shows the comparison of several physical properties of typical liquid, vapor, and supercritical fluid state for CO2. It could be seen that supercritical CO2 (SCCO2) fluid possesses liquid-like density, so that SCCO2 fluid is analogous with light hydrocarbon to dissolve most solutes and own exceptional transport capability. [24, 25] On the other hand, SCCO2 fluid hold
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gas-like characteristic due to their viscosity and surface tension are extremely low, it allows SCCO2 fluid to keep superior diffusion capability than liquid to enter the nano-scale dimension, as shown in Fig. 2-7. These properties are the reasons for SCCO2 fluid to employ in many commercial applications, including the extraction of caffeine from coffee, fats from foods, and essential oils from plants for using in perfumes. Furthermore, in recent years, many records were investigated with SCCO2
fluid to apply in semiconductor fabrication by means of its high mass transfer rates
and infiltration capabilities to clean wafer, strip photoresist, repair low-k material
[25-29]
. Figure 2-8 is the Scanning Electron Microscope (SEM) image of removing photoresist and photoresist residue from ion implant wafers. To put it briefly, SCCO2 fluid is one of the green solvents and suitable for the fabrication of nano-structure devices.
Fig. 2-4 The supercritical fluid element.
CO2molecule
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Fig. 2-5 Density-pressure-temperature surface for pure CO2.[25]
Fig. 2-6 The supercritical fluid system.
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Tab. 2-1 Comparison of physical properties of CO2.[26]
Fig. 2-7 Schematic of cleaning high aspect ratio structures with liquids and supercritical fluids.
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2.2 Process Flow of Zirconium Oxide on Germanium Substrate Metal-Insulator-Semiconductor Capacitor (MIS-C) Fabrication
A 0.003 ohm-cm p-type (100) Ge wafer was cleaned with cycling DHF clean process and immediately loaded into the sputter chamber. As the chamber pressure reached to the 2×10-6 torr, about 5nm ZrO2 film was deposited by RF-sputter.
First, the sample was subjected to the post deposition annealing, first under 250°C for 30min in high vacuum furnace to be the STD sample, and then anneal with 300°C、
400°C and 500°C for 30sec by rapid temperature annealing (RTA) in nitrogen environment, separately. The SCF treatment was performed right after the 500°C RTA to repair the device performance. The sample was placed in a SCF system at 100°C for 1 hr, where was injected with 3000 psi of SCCO2 fluid that were mixed with 10 vol.% of propyl-alcohol and 10 vol.% of DI water. Finally, deposit 100nm tantalum-nitride electrodes by sputter and 500nm aluminum electrodes by thermal evaporation on the top surface of ZrO2 film with two electrode areas of 3.14×10-4 cm2 and 2.83×10-3 cm2, then BOE the back side of germanium wafer and deposit 500nm aluminum as back electrodes to fabricate MOS capacitors. The experiment flows of ZrO2/Ge capacitor with various treatments are exhibited in Fig. 2-8.
Second, a 0.003 ohm-cm p-type (100) Ge wafer was also cleaned with cycling
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DHF clean process and immediately loaded into the sputter chamber. As the chamber pressure reached to the 2×10-6 torr, about 5nm ZrO2 film was deposited by RF-sputter.
There are two environment for rapid thermal annealing, first, the sample was annealed by using rapid thermal annealing in wet nitrogen (with water vapor) environment with 300°C for 3mins. Another is annealing in dry nitrogen environment without oxygen with 300°C for 3mins. Finally, deposit 100nm tantalum-nitride electrodes by sputter and 500nm aluminum electrodes by thermal evaporation on the top surface of ZrO2
film with two electrode areas of 3.14×10-4 cm2 and 2.83×10-3 cm2, then use BOE to the back side of germanium wafer and deposit 500nm aluminum as back electrodes to fabricate MOS capacitors. The experiment flows of ZrO2/Ge capacitor with various treatments are also exhibited in Fig. 2-9.
In the end, we combine this two methods Supercritical Fluid treatment and water vapor annealing to form the device with good interface and low defect in the bulk high-k dielectric. First, we deposit about 5nm ZrO2 film on Ge substrate by RF-sputter, and then anneal with 500°C for 30sec by Rapid Temperature Annealing (RTA) in nitrogen environment. Second, we use the Supercritical Fluid treatment (SCF) with 100°C and 3000psi for 1hr, and then anneal with 300°C for 3min by Rapid Temperature Annealing (RTA) in wet nitrogen environment (water vapor and nitrogen). The last, we deposit 100nm tantalum-nitride electrodes by sputter and
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500nm aluminum electrodes by thermal evaporation on the top surface of ZrO2 film and bottom of substrate.
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Fig. 2-8 The experiment flows of ZrO2/Ge capacitor with different PDA treatment and SCF treatment.
Fig. 2-9 The experiment flows of ZrO2/Ge capacitor with wet N2 treatment.
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2.3 Analysis Methods
2.3.1 Methods of Electrical Characteristics
2.3.1.1 Parameter Description
There are three parameters represent the characteristics of MOS capacitors.
Effective Oxide Thickness (EOT)
(2-1)
(2-2)
Eq. (2-1) represents the gate oxide capacitanceequivalent thickness of the SiO2, Eq. (2-2) represents the effective oxide thickness (EOT) related to the dielectric constant of ZrO2. For the Eq. (2-2), the less EOT represents the value of k is higher.
Where εSiO2 is dielectric constant of SiO2, εZrO2 is dielectric constant of ZrO2, dthick is
thickness of ZrO2.
Flat Band Voltage (Vfb)
(2-3)
Eq. (2-3) represents the number of charge exists inside the dielectric, that means the Vfb near the zero bias, the less oxide charges existing inside the dielectric. Where
2 2
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φms is the work function difference between gate and substrate, Q0 is the number of
oxide charges in the dielectric.
Hysteresis (△Vfb)
Hysteresis represents the quality of interface between the dielectric and substrate, smaller △Vfb indicate better interface quality.
2.3.2 Methods of Material analysis
2.3.2.1 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula,chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. XPS requires ultra high vacuum (UHV) conditions. XPS is a surface chemical analysis technique that can be used to analyze the surface chemistry of a material in its "as received" state, or after some treatment, for example: fracturing, cutting or scraping in air or UHV to expose the bulk chemistry, ion beam etching to clean off some of the surface contamination, exposure
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to heat to study the changes due to heating, exposure to reactive gases or solutions, exposure to ion beam implant, exposure to ultraviolet light.
XPS is also known as ESCA, an abbreviation for Electron Spectroscopy for Chemical Analysis.
XPS detects all elements with an atomic number (Z) of 3 (lithium) and above. It cannot detect hydrogen (Z = 1) or helium (Z = 2) because the diameter of these orbital is so small, reducing the catch probability to almost zero.
XPS is used to measure:
elemental composition of the surface (top 1–10 nm usually)
empirical formula of pure materials
elements that contaminate a surface
chemical or electronic state of each element in the surface
uniformity of elemental composition across the top surface (or line profiling or mapping)
uniformity of elemental composition as a function of ion beam etching (or depth profiling)
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2.3.2.2 High-Resolution Transmission Electron Microscopy
High-resolution transmission electron microscopy (HRTEM) is an imaging mode of the transmission electron microscope (TEM) that allows the imaging of the crystallographic structure of a sample at an atomic scale. Because of its high resolution, it is an invaluable tool to study nano-scale properties of crystalline material such as semiconductors and metals. At present, the highest resolution realized is 0.8 angstroms (0.08 nm) with microscopes. Ongoing research and development such as efforts in the framework of TEAM will soon push the resolution of HRTEM to 0.5 Å . At these small scales, individual atoms and crystalline defects can be imaged. Since all crystal structures are 3-dimensional, it may be necessary to combine several views of the crystal, taken from different angles, into a 3D map. This technique is called electron crystallography. One of the difficulties with HRTEM is that image formation relies on phase-contrast. In phase-contrast imaging, contrast is not necessarily intuitively interpretable as the image is influenced by strong aberrations of the imaging lenses in the microscope. One major aberration is caused by focus and astigmatism, which often can be estimated from the Fourier transform of the HRTEM image.
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Chapter 3
Results and Discussion
3.1 Effect of Supercritical Fluid and Post-Deposition Annealing on the ZrO
2/Ge MOS-Capacitor
This section we investigate the effect of Supercritical Fluid treatment (high-pressure H2O treatment) on the sputter-deposition of ZrO2/Ge stack. Then analysis of electrical characteristics can examine the quality of the treated device, and the material analysis of X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) reveal the situation of interfacial GeOx
between ZrO2 and Ge which is a redox of ZrO2 with H2O and GeOx with H2, including the bonding of Zr and Ge with O, and real thickness of GeOx. The suppression of GeOx interlayer between ZrO2 and Ge substrate can decreases the gate leakage current effectively.
3.1.1 Thermal Stability of ZrO2/Ge MOS-Capacitor
The thermal stability of MOS-C is the most important norm to referee quality of MOS-C. In the Fig. 3-1, it represents the variation of gate leakage current density (JG) for ZrO2 on Ge substrate after different thermal treatments. The symmetrical JG of
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ZrO2 on p-type Ge substrate at negative and positive electrical field is due to fast generation rate of minority carrier. Furthermore, the JG of ZrO2/Ge capacitor decreases with increasing annealing temperature until 400 °C and then increases at 500 °C, as shown in Fig. 3-2. The high and unstable JG of ZrO2/Ge capacitor may be related to formation, decomposition and amalgamation of the Ge oxide during thermal
ZrO2 on p-type Ge substrate at negative and positive electrical field is due to fast generation rate of minority carrier. Furthermore, the JG of ZrO2/Ge capacitor decreases with increasing annealing temperature until 400 °C and then increases at 500 °C, as shown in Fig. 3-2. The high and unstable JG of ZrO2/Ge capacitor may be related to formation, decomposition and amalgamation of the Ge oxide during thermal