Chapter 1 Introduction
1.3 Organization of the thesis
This thesis is comprised of four chapters. Chapter 1 describes the background and motivation for the high-κ dielectrics and application of the MIM structure.
In Chapter 2, we first describe the experimental procedure and then, show the basic characteristics of HfO2 gate stacks on the metal of Tantalum (Ta), including basic analysis of the C-V, I-V and AFM characteristics. The HfO2 thin film in this thesis is deposited by Reactive sputtering (RS), and let us compare these results with the thin film of Al2O3 on TiN metal is deposited by metal-organic chemical vapor deposition (MOCVD).
In Chapter 3, we analyze the PDA effect of different temperatures with HfO2 on Ta metal in oxygen (O2), nitrogen (N2). However, MIM structure performance is not as good as we expected. High leakage current density is fatalness of MIM structure after PDA over 500°C in nitrogen (N2). In addition leakage current density reduced after PDA in oxygen (O2). We found out plasma N2O or NH3 surface treatment for bottom electrode can improve value of Voltage coefficient of capacitance (VCC).
Finally, in Chapter 4, the conclusions are made and the recommendation describes the topics which can be further researched.
Figure 1-1 The expected equivalent oxide thickness (EOT) trends from the published 2003- ITRS roadmap.
Figure 1-2 Power consumption and gate leakage current density comparing to the potential reduction in leakage current by an alternative dielectric exhibiting the same equivalent oxide thickness [10].
Material
Dielectric Constsnt(k)
Band gap Eg(eV)
△Ec(eV) to Si
Crystal Structure(s)
SiO2 3.9 8.9 3.2 Amorphous
Si3N4 7 5.1 2 Amorphous
Al2O3 9 8.7 2.8 Amorphous
Ta2O5 26 4.5 1-1.5 Orthorhombic
TiO2 80 3.5 1.2 Tetrag.
HfO2 25 5.7 1.5 Mono.,Tetrag.,Cubic
ZrO2 25 7.8 1.4 Mono.,Tetrag.,Cubic
Table 1-1 Basic properties for many high-κ candidates [10].
(a)
(b)
(c)
Figure 1-3 DRAM cell structures for (a) planar type capacitor, (b) trench capacitor, and (c) stacked capacitor [11].
CHAPTER 2
Characteristics of HfO
2Gate Dielectrics Deposited on Tantalum Metal
2.1 Introduction
A high capacitance density is important for a MIM capacitor to increase the circuit density and reduce the cell area and cost. Therefore, adoption of high-k material like Al2O3 or HfO2 is a very efficient way to increase the capacitance density.
Silicon oxide and silicon nitride are dielectrics that are commonly used in conventional capacitors, but their capacitance densities are limited due to low dielectric constants. It is expected to be one solution to enhance the capacitance density by using higher dielectric constant materials. Among various high-k dielectric candidates, HfO2 has been investigated as a promising material in gate dielectric of MOSFETs due to its high dielectric constant, excellent thermal stability, and high band gap. In addition, excellent MOS capacitors with HfO2 have also been demonstrated. Therefore, it seems that HfO2 is a promising candidate for the above applications.
The most commonly reported high-k materials are ZrO2, HfO2, La2O3, and Al2O3 and so on. The dielectric constants of reported high-k materials are listed in
Table 2-1[17]. Among these candidates, HfO2 attracts much more attention from recent researches. The reasons are briefly listed as follows.
(1) Suitable high dielectric constant：
The reported dielectric constant κ of HfO2 is about 25 ~30. This magnitude of κ-value is higher than that of Si3N4 (κ~7) and Al2O3 (κ= 8~11.5). It is not high enough to induce severe FIBL effect.
(2) Wide bandgap：
In general, as the dielectric constant increases, the bandgap decreases. The narrower bandgap would increase leakage current. The energy band offset of HfO2 is about 5.68eV, which is higher than the other high-κ materials such as ZrO2, Si3N4, and Ta2O5.
(3) Acceptable band alignment：
Band alignment determines the barrier height for electron and hole tunneling from gate or Si substrate. For SiO2, the band offset of conduction band and valence band is ~9eV, and the barrier height for electrons is 3.5eV and the barrier height for holes is 4.4eV. The high band offset for both electron and hole has the benefit of low leakage current. Figure 1-1 shows the calculated band offsets for most high-k dielectrics. For HfO2, barrier height for electron and hole is 1.6ev and 3.4eV, respectively. This band alignment is acceptable and better than other high-k materials such as Ta2O5.
(4) High free energy of reaction with Si：
For HfO2, the free energy of reaction with Si is about 47.6 Kal/mole at 727
℃,(see Table 2-1) [17] which is higher than that of TiO2 and Ta2O5. Therefore, HfO2
is a more stable material on Si substrate as compared to TiO2 and Ta2O5. [18]-[20]
(5) High heat of formation:
Among the elements in IVA group of the periodic table (Ti, Zr, Hf), Hf has the highest heat of formation (271 kcal/mole). Unlike other silicides, the silicide of Hf can be easily oxidized. That means that Hf is easy to be oxidized to form HfO2
[18]-[20]
According to these profits above discussions, we choose HfO2 as the major high-k material and in our investigation. The measurement is performed by MIM capacitor structures.
2.2 Experimental details
Four inches diameter (150-mm) p-type (100) Si wafers with nominal resistivity of 15 to 25 Ω-cm were used as substrate. Prior to the growth of Ta metal, the native oxide was cleaned by the conventional RCA cleaning and diluted HF etching in sequence for the removal of particles and native oxides. The capacitor area in this measurement about is 5.3×10–4 cm2. After standard RCA cleaning, a 550 nm SiO2
film was grown on Si substrate by wet oxidation, the 100 nm Ta layers were deposited sequentially by dc sputtering to obtain a Ta/SiO2/Si structure for the deposition of
HfO2 thin-film capacitors. After that, the HfO2 thin film of approximately different thickness for 5 nm, 6 nm, and 9 nm was deposited on Ta electrode. In addition, Bottom electrode is defined by mask. After HfO2 deposition, Ta gate was defined directly by metal mask. All these processes are performed at room temperature. Figure 2-2 shows the flow chart of HfO2 thin film fabrication.
The physical gate oxide thickness was determined by n&k analyzer 1280. The equivalent oxide thickness (EOT) was extracted by fitting the measured high-frequency capacitance-voltage (C-V) data from Hewlett-Packard (HP) 4284LCR meter under zero-biased. Moreover the capacitance was measure using 4284LCR meter at frequencies varied from 1 kHz to 1 MHz. In order investigate the thermal stability of high-k dielectric film. Thermal stress was performed with measurement temperature varied from 25°C to 125°C. The tunneling leakage current density-voltage (J-V) was measured by semiconductor parameter analyzer HP4145A.
2.3 Results and Discussions
2.3.1 Basic Characteristics of HfO2 Dielectrics Deposited on Ta Metal
Figure 2-3 reveals The C-V characteristics of the as-deposited HfO2 gate dielectrics for 5nm, 6nm, and 9nm were deposited difference thickness. The capacitance density deposited at 9nm of film thickness was lower than 5nm deposited sample with similarly condition. When gate oxide film reduces, the capacitor density significantly shows high desired results. The Figure 2-4 clearly display the capacitance density at 5nm film thickness has 25.67fF/cm2. The capacitor dielectrics
with higher dielectric constant κ offer an attractive path to achieving enhanced capacitance per area. It should be pointed out here that high-κ use as a gate oxide is distinct from high-κ use in DRAM capacitors. As mentioned previously, HfO2 gate dielectrics exhibit high capacitance density and replace conventional SiO2.
Since thinner HfO2 dielectric films would expect to have higher capacitance density than thick dielectric films. The corresponding capacitance-voltage (C-V) characteristics at the frequencies from 1 kHz to 1 MHz curves were presented in Figure 2-5 (a) (b). Figure 2-5 (a) (b) shows the frequency-dependence of the voltages of HfO2 dielectric films at 5nm samples. The capacitance densities reduced from 40.65fF/cm2 at 1 kHz to 25.67fF/cm2 at 100 kHz. However, the capacitance densities are very low at 1 MHz. The capacitance density decreases to about 0.47 fF/µm2 at 1 MHz compared to 40.7 fF/µm2 at 100 kHz. However, more serious frequency dispersion effect is shown in MIM. The capacitance densities decrease with increasing frequency in the range from 100 Hz to 1 MHz. It can be explained that at lower frequencies, we have different types of polarizations such as electronic polarization, orientation polarization, space charge polarization, and atomic polarization. Figure 2-2 shows the four types of polarizations.
These four compositions are illustrated as following.
(1) Electronic polarizability, αe.
Electronic polarization occurs in all dielectric materials. The electrons surrounding each nucleus are shifted very slightly in the direction of the positive electrode and the nucleus is very slightly shifted in the direction of the negative electrode. As soon as the electric field is removed, the electrons and nuclei return to their original distributions and the polarization disappears. The effect is
analogous to elastic stress and strain. The displacement of charge is very small for electronic polarization, so the total amount of polarization is small compared to the other mechanisms of polarization.
(2) Orientation polarizability, αo.
Orientation polarization involves nonsymmetrical molecules that contain permanent electric dipoles. An example is H2O. The covalent bonds between the hydrogen and oxygen atoms are directional such that the two hydrogens are on one side of the oxygen. The hydrogen side of the molecule has a net positive charge and the oxygen side has a net negative charge. Under an electric field, the molecules will align with the positive side facing the negative electrode and the negative side facing the positive electrode. Orientation polarization results in a much higher degree of polarization than electronic polarization. This is because large charge displacement is possible in the relatively large molecules compared to the spacing between the electrons and nucleus in individual atoms.
(3) Space charge polarizability, αs.
Space charges are random charges caused by cosmic radiation, thermal deterioration, or are trapped in the material during the fabrication process.
(4) Atomic or ionic polarizability, αi.
It involves displacement of atoms or ions within a crystal structure when an electric field is applied. A wide range of polarization effects is possible through this mechanism, depending on the crystal structure, the presence of solid solution, and other factors. Examples include pyroelectricity, piezoelectricity, and ferroelectricity, Figure 2-6 (a) shows the four types of polarizations. At higher frequencies, the capacitance densities have main contribution from the electronic polarization [21].
Just as we have a relaxed and an unrelaxed elastic modulus, we have a dependence of the capacitance densities on frequency which shown in Figure 2-6 (b). The electronic
polarization is the only process sufficiently rapid to follow alternative fields in the visible part of the spectrum. Ionic polarization processes are able to follow an applied high-frequency field and contribute to the capacitance densities at frequencies up to the infrared region of the spectrum. Orientation and space charge polarization have relaxation times corresponding to the particular system and process but, in general, participate only at lower frequencies. At Figure 2-5 (a) (b), the capacitance densities decrease with increasing frequency in the range from 100 Hz to 1 MHz. It can be explained that at lower frequencies, we have different types of polarizations such as electronic polarization, orientation polarization, space charge polarization, and atomic polarization. At higher frequencies, the dielectric constant has main contribution from the electronic polarization. Refer to Figure 2-6 (b), the capacitance densities decreases in the frequency ranging from 100 Hz to 1 MHz may be attributed to the decrease of space charge polarization.
Figure 2-7 (a) shows the normalized C-V curves (△C/Co) of MIM structure (Ta/HfO2/Ta). Voltage coefficient of capacitance (VCC) is one of the important parameters of MIM structure. It has been demonstrated that pure SiO2 MIM structures show negative parabolic curves in C-V relationship, but high-κ MIM structures exhibit strong positive parabolic curves in C-V relationship [40]. The mechanism of nonlinearity of C-V curves is unclear. It is supposed to relate with E-field polarization, carrier injections [23], high-κ thickness [24、25], frequency [26] and leakage current [27]. Theoretically, VCC decreases with measured frequency increases [24]. It is believed that the carrier mobility becomes smaller with increasing frequency, which leads to a higher relaxation time and a smaller capacitance variation [23]. From the equation below, where the voltage coefficients of capacitance (VCC) values of α and
β are listed in Table 2.2. The requirement of the quadratic coefficient of capacitance α is smaller than 100 ppm/V2, and the requirement of the linear coefficient of capacitance β is below 1000 ppm/V according to the ITRS roadmap [28].
) 1
Normalized capacitances (△C/Co) as a function of voltage with different thickness are shown Figure 2-7 (b) and Table 2.3. The decreasing of α with thickness is slower. This thickness effect is due to E-field reduction with increased thickness [47]. Thickness effect of the VCC is a negative impact for thinner film dielectric.
2.3.2 Thermal Stress on the MIM Capacitors
Figure 2-8 (a) depicts Capacitance density of the MIM capacitor with 5nm thickness at 100 kHz from 25°C to 125°C. Figure 2-8 (b) depicts Capacitance density of the MIM capacitor with 5nm thickness as a function of frequency after thermal stress from 25°C to 125°C. After thermal stress, the capacitance density decreases with temperatures at 100 kHz. The major reason is considered to be the interface defect density increasing during the thermal stress process. Moreover, the capacitance density decreases with frequency at all thermal stress. Especially, the capacitance density at 1 MHz is very lower than other frequency. As mentioned previously, the poorer frequency dispersion for MIM is probably decreased with space charge polarization. Figure 2-9 clearly confirms the results of Figure 2-8 (a) and (b).
2.3.3 J-V Curves Measurement under Various Temperatures
Figure 2-10 shows J-V characteristics of MIM capacitor measured at various temperatures from 25°C to 125°C. The dependence of leakage current density and measured temperature is observed, i.e. leakage current density increases with measurement temperature increased. Conduction mechanism is found by fitting equation described as follows. Many conduction mechanisms are fitted, including Fowler-Nordheim Tunneling [29、30] , Frenkel-Poole Emission [29、30], Trap Assisted Tunneling [31、32], and Schottky Emission [29].
In the Fowler-Nordheim Tunneling model, leakage current occurs in the high field region. High electric field across on high-κ thin film inclines band diagram and electron can tunnel more easily. The equation of leakage current density is [33]:
( )
⎟The Fowler-Nordheim Tunneling plots were made for Jg (not shown in the thesis). In the Fowler-Nordheim Tunneling plots, Jg does not show a linearity relationship. The conduction mechanism is therefore not the Fowler-Nordheim Tunneling.
In the Schottky Emission model, the Schottky emission is generated by the thermionic effect and is caused by the electron transport across the potential energy barrier at a metal-insulator interface. The equation of leakage current density is [33]:
( )
The Schottky Emission plots were made for Jg (not shown in the thesis). Jg does not show a straight line in the Schottky Emission plots, therefore the conduction mechanism is probability not the Schottky Emission.
In the Trap Assisted Tunneling model, it is assumed that electrons first tunnel through the SiOx interfacial layer (direct-tunneling). Then, electrons tunnel through traps located below the conduction band of the high-κ thin film and leak to substrate finally [31]. The equation of leakage current density is [32]:
⎥⎦
The Trap Assisted Tunneling model plots were made for Jg (not shown in the thesis). Jg is not a straight line in the Trap Assisted Tunneling model plots, therefore the conduction mechanism is probability not the Trap Assisted Tunneling model.
In the Frenkel-Poole Emission model, a lot of traps exist in high-κ thin film and electrons which get enough thermal energy can leap and stay in these traps temporarily and leak to substrate in the end. The equation of leakage current density is [33]:
The Frenkel-Poole Emission model plots were made for Jg in Figure 2-11. Jg
shows a clear linearity in the Frenkel-Poole Emission model plots, therefore the conduction mechanism is probability the Frenkel-Poole Emission model.
From the conduction mechanism fitting, we speculate that the conduction mechanism of MIM structure is Frenkel-Poole Emission.
2.3.4 Compared to MIM (TiN/Al2O3/TiN) and MIM (Ta/HfO2/Ta) structure
Figure 2-12 (a) C-V curves depicts significantly the capacitance densities at TiN/Al2O3/TiN structure is lower than Ta/HfO2/Ta structure. Because the dielectric constants with HfO2 are higher than Al2O3, Ta/HfO2/Ta structure can increase the circuit density and reduce the cell area and cost. In addition, Figure 2-12 (b) shows Ta/HfO2/Ta structure performances better than TiN/Al2O3/TiN structure in leakage currents. In TiN/Al2O3/TiN structure, we found out that TiN might react with Al2O3
with high thermal budget resulting in inter-diffusion of Ti and Al. Lots of metal ion Ti in the Al2O3 thin film and incomplete structure of Al2O3 might be the reason of high leakage current density. Moreover, another reason is considered to be chlorine out-diffusion. In order to improve high leakage current density, we make attempt to different methods. e.g., we inserted AlN between Al2O3/TiN and deposited electrode thickness thinner. But these methods decrease defects insignificantly. Now we used another high-k material HfO2 and metal Ta instead of Al2O3 and TiN. CVD films have better step coverage than sputtered films, but produce films that have less desirable mechanical and electrical properties such as high leakage current. For some material we believe reactive sputtering is better than MOCVD, because reactive sputter product contamination less.
2.4 Summary
In this chapter, MIM capacitors have been successfully fabricated with HfO2 as the dielectric layer. We also discussed Characteristics of HfO2 Gate Dielectrics Deposited on Tantalum Metal. We found out the poorer frequency dispersion for MIM (Ta/HfO2/Ta) structure. Besides, the capacitance density under high frequency is very low, especially at 1 MHz. the voltage coefficients of capacitance (VCC) values of α and β are higher than 100 ppm/V2 1000 ppm/V according to the ITRS roadmap.
Furthermore, conduction mechanism of Ta/HfO2/Ta structure has been studied. The conduction mechanism of Ta/HfO2/Ta structure is Frenkel-Poole Emission.
The measurement results show high capacitance density compared to TiN/Al2O3/TiN structure. The Ta/HfO2/Ta capacitor exhibits the highest capacitance density value of 25.67fF/µm2. The leakage currents of Ta/HfO2/Ta capacitors are very small compared to TiN/Al2O3/TiN structure. These show that the HfO2 dielectric is very suitable for MIM applications. Thus indicates that it is very suitable for HfO2 dielectric to use in silicon IC applications.
High-κ Dielectrics
HfO2 ZrO2 Al2O3
Bandgap (eV) 6.02 5.82 8.3
Barrier Height to Si (eV) 1.6 1.5 2.9
Dielectric Constant ~30 ~25 9
Heat of Formation
(Kcal/mol) 271 261.9 399
∆G for Reduction
(MOx + Si → M + SiOx) 47.6 42.3 64.4
Thermal expansion coefficient
(10-6 K-1) 5.3 7.01 6.7
Lattice Constant (Å)
(5.43 Å for Si) 5.11 5.1 4.7 - 5.2
Oxide Diffusivity
@ 950oC (cm2/sec) 1x10-12 5x10-25
Table 2-1 Materials properties of high-κ dielectrics, Al2O3, ZrO2, and HfO2
3.5
Figure 2-1 Band alignment of topical high-κ dielectrics.
1. Silicon substrate, RCA clean and HF dip to remove native oxide.
2. SiO2 550 nm film deposited at furnace Wet Oxidation.
3. Bottom electrode Tantalum 100 nm deposited by Reactive Sputter (RS)
4. The HfO2 thin film 5 nm deposited by RS
5. Finally, Top Electrode deposited 100 nm are used to metal mask
Figure 2-2 Flow chart for the fabrication of HfO2 thin films.
Voltage (V)
Figure 2-3 The C-V characteristics of the as-deposited HfO2 gate dielectrics for 5nm, 6nm, and 9nm were deposited difference thickness
40 50 60 70 80 90 100
Figure 2-4 Capacitance density varied with 5nm, 6nm, and 9nm HfO2 dielectric film thickness
Voltage (V)
Figure 2-5 (a) Capacitance-voltage (C-V) and (b) Capacitance-frequency characteristics of HfO2 5nm thin film on MIM capacitors at the frequencies from 1 kHz to 1 MHz.
Figure 2-6 (a) Schematic representation of different mechanisms of polarization [22]
Figure 2-6 (b) Frequency dependence of several contributions to the polarizability [51
]
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Figure 2-7 (a) Normalized C-V curves (△C/Co) of MIM structure (Ta/ HfO2/Ta) with 5 nm thickness
Figure 2-7 (b) DC bias dependence of normalized capacitance (△C/Co) at 100 KHz for 5nm、6nm、9nm MIM capacitor
High-k
material Frequency α (ppm/V2) β (ppm/V)
1K 14215 17200
10K 8277 10536
HfO2(5nm)
100K 5139 7025
Table 2-2 Summary of α and β extracted from MIM structure (Ta/ HfO2/Ta)) with 5nm
Thickness Frequency (Hz)
α (ppm/V2) β (ppm/V)
5nm 5139 7025
6nm 4329 6649
9nm
100K
447 1483
Table 2-3 Summary of Quadratic VCC, α, and linear VCC, β, extracted from MIM structure (Ta/HfO2/Ta) at 100 KHz for 5nm、6nm、9nm MIM capacitor
Voltage (V)
Figure 2-8 (a) Capacitance density of the MIM capacitor with 5nm thickness at 100 kHz from 25°C to 125°C. (b) Capacitance density of the MIM capacitor with 5nm thickness as a function of frequency after thermal stress from 25°C to 125°C.
25 50 75 100 125
Capacitance (fF/um2 )
-10
0 10 20 30 40 50
1K Hz 10K Hz 100K Hz 1M Hz
Temperature (
oC)
Figure 2-9 Capacitance density of the MIM capacitor as a function of temperature at frequencies varied from 100Hz to 1MHz.
Voltae (V)
Figure 2-10 The J-V curves of MIM capacitor with 5nm thickness under various
Figure 2-10 The J-V curves of MIM capacitor with 5nm thickness under various