Chapter 1 Introduction
1.2 Motivation
In the early age of DRAM development, two dimensional cell structure was widely applied on the DRAM cell. Memory cell capacitance plays a important role, which can determine the data retention time, store charge, writer and erase speed. In the design bit generation, the minimum cell capacitance should be 25fF/cell. The measure of capacitance capacity can be described as the following equation:
C = ε
0ε
rA/d
Where
C:
the dielectric capacitanceε
0:
the vacuum permittivityε
r:
the relative dielectric constant of dielectric materialA:
the surface area of capacitord:
the dielectric thicknessIn order to get high memory density and high capacitance in the DRAM chip,increasing the surface area of capacitor and the relative dielectric constant of dielectric material can be used to increase capacitance. Three-dimensional cell structure was introduced to 4MB-above generation, trench capacitor cell and stacked capacitor cell (STC). Both of the trench and the stacked cell techniques are still used in commercial DRAM products now. Three-dimension cell structure of DRAM increases the surface area of capacitor to get high capacitance; nevertheless, it is not
dimension shrinking with high-k materials.
As memory trend develop in future. The low-power nonvolatile memories control are important trend in communication products. In the recent flash memory technologies, short program/erase times and operating voltage reductions are the most important issues to realize high speed/low power operation [29], [40]-[42]. For EEPROM and flash memory devices, the IPD requires a high charge-to-breakdown (QBD), high breakdown field and low leakage current to obtain good data retention characteristics [43]-[45]. It is not sufficient to meet the stringent data retention requirement of IPD while applying fluorine passivated silicon surface technologies due to the unavoidable leakage current [46]. In order to accomplish this without a trade-off between low power and high speed operations, high coupling ratio should be achieved by increasing the floating gate capacitance [47]-[54].
There are three different approaches can be used to increase coupling ratio. First, decrease the IPD thickness. Oxide/nitride/oxide (ONO) multi-layered films had been extensively investigated and frequently used as the dielectric layer in the flash memory devices and other applications [55]-[57]. However, decreasing the thickness of the IPD to increase the coupling ratio may cause serious leakage and reliability problems which are fatal in the retention time of flash memories. Secondly, increase the area of the IPD capacitor. High capacitive-coupling ratio cell [47]-[49], 3-dimension inter-poly dielectric [51], and hemisphere grain [52], [53] had been proposed to effectively increase the capacitance area and lower the control gate bias.
Therefore, it is straightforward and effective to incorporate alternative high dielectric constant (high-κ) materials on nonvolatile memories to replace oxide/nitride/oxide IPD for increasing floating gate capacitance without increasing cell area and complexity of fabrication while suppressing charge loss. By increasing the floating gate coupling ratio, high-κ IPDs can lead to a high electric field across tunnel oxide even at very low control gate voltage.
Recently, aluminum oxide (Al2O3) [18], [66]-[68] and hafnium oxide (HfO2) [21], [69]-[72] had been proved as promising candidates for the gate dielectrics of sub-0.1 µm device due to their higher κ, relatively high ϕB and superior thermal stability, shown in Table 1.1. Thanks to the high dielectric constant and high thermal stability, Al2O3 and HfO2 are suitable to be integrated into stacked-gate flash memories.
Nonetheless, the effects of these kinds of high-κ dielectrics on flash memories are seldom investigated. To further realize the dielectric properties of these high-κ dielectrics, some reliability issues such as breakdown field, charge trapping and temperature-dependence behaviors are extensively studied for both gate dielectric and flash memories applications.
Many deposition methods such as physical vapor deposition (PVD), metal-organic chemical vapor deposition (MOCVD), atomic layer chemical vapor deposition (ALCVD) [73], [74], and molecular beam epitaxial method (MBE), etc.
have been employed to prepare high-κ IPDs. The pros and cons of each deposition techniques are demonstrated in Table 1.2. For industrial application, PVD and MBE are not appropriate tools for high-κ film deposition. Since MOCVD has the advantage of superior step coverage, high deposition rate, good controllability of composition, excellent uniformity of film thickness over large area, we, therefore, choose the
structure is shown in Fig. 1.3. The MOCVD chamber is equipped with a turbomolecular pump and a liquid injection system, which has four independent-controlled injectors. The latter is consisted of a liquid pump to pump the precursors through a hot nickel frit with a proper rate because the pump is unreliable at low pump rates. The vapors are carried with a 200sccm flow of Ar to a gas distribution ring which is located at a proper distance from the substrate. In contrast to the conventional bubble system, the liquid injection is with sufficient temperature window to alleviate the thermal aging of the precursor. This is because the precursor remains in liquid state at room temperature until it is pumped into the vaporizer and injected into the deposition chamber. However, the precursor should be kept at long-term chemical stability in solvent and non-reactive with other precursors solvent [75], [76]. The components of the vaporizer, the gas ring and the connecting tube are maintained at a temperature of 190ºC with heating tapes and blankets, while the substrate temperature is controlled at 500ºC with quartz-halogen lamps and a thermocouple. A rotating suspensor is used for uniform heating during processing. A flow of 100sccm N2 is maintained throughout the deposition cycle. The base pressure of the MOCVD chamber is ~10-8Torr. The deposition pressure of the deposition is at the 5mTorr where the gas-phase collisions are scarce.
As many reports indicated, the direct contact of high-κ materials and Si-substrate will be imperfect and debatable. The dominance of the Si MOSFETs over competing technologies has largely been attributed to the high quality of thermally grown SiO2
interface due to the strong vertical electric field present in the channel. For maintaining the excellent transport properties at the Si interface, a possible method to suppress the interfacial layer thickness is to passivate the Si surface before the high-κ IPD deposition. One of the methods is to passivate the Si surface with fluorine implant surface, then nitrogen-contained ambient activation. Passivation of the Si surface using fluorine treatment after the deposition of polysilicon has been shown to be effective in achieving the low EOT and preventing the boron penetration [78], [79].
However, this technique results in higher interface charges which leads to higher hysteresis and reduced channel mobility [80]. The Si-F is a superior barrier for H2O and oxygen, and it can suppress oxygen to diffuse into Si substrate [78]. After the fluorine passivation treatment, a thin interlayer layer ( ﹤20 Å ) was deposited and measured by optical measurement system ( Ellipsometer ). As reports, passivation of the Si surface is prior to the deposition of high-κ gate dielectrics and it shows the result to achieve the low EOT and increase reliability by making the interface smoother [81].
1.3 Organization of This Thesis
There are five chapters in this thesis. In chapter 1, we present a conceptive introduction to describe the background of the semiconductor technology and discuss the possible issues that we may meet during the dimension scaling down. In addition, we would concern about the hopeful solutions to overcome the physical limits in the ITRS, discuss and explain the reasons for high-κ IPD application in the nonvolatile
In chapter 2, the effects of fluorine passivation on inter-poly characteristics of MOCVD Al2O3 dielectrics are examined. The basic electrical properties, electric field, leakage current, and reliability characteristics are presented and discussed.
In chapter 3, the effects of fluorine passivation on inter-poly characteristics of MOCVD HFO2 dielectrics are examined. The basic electrical properties, electric field, leakage current, and reliability characteristics are presented and discussed.
Finally, in chapter 4, the conclusions are made and the recommendations describe the topics which can be further researched .
Table 1.1 Materials properties of high-κ dielectrics, Al2O3, ZrO2 and HfO2.
High-κ Dielectrics
Al2O3 ZrO2 HfO2
Bandgap (eV) 8.3 5.82 6.02
Barrier Height to Si (eV) 2.9 1.5 1.6
Dielectric Constant 9 ~ 25 ~ 25
Heat of Formation
(Kcal/mol) 399 261.9 271
∆G for Reduction
(MOx + Si → M + SiOx) 63.4 42.3 47.6
Thermal expansion
coefficient (10-6oK-1) 6.7 7.01 5.3
Lattice Constant (Å)
(5.43 Å for Si) 4.7 - 5.2 5.1 5.11
Oxygen Diffusivity
at 950oC (cm2/sec) 5×10-25 1×10-12 ~10-12
Table 1.2 Comparisons of deposition techniques: sputtering, ALD, MOCVD and
Fig. 1.1 Scaling limits of various gate dielectrics as a function of the technology specifications for low stand-by power technologies [Ref. 7].
Fig. 1.2 Leakage current density and EOT projection of nitrided oxides from ITRS roadmap 2004 update.
Fig. 1.3 A schematic diagram of typical MOCVD system structure.
Fig. 1.4 Band alignment of topical high-k dielectrics.
Barrier Height to Si
Eg of Si ( 1.12eV )
Ф
Be(eV)
4.0 3.0 2.0 1.0 Ec Ev 1.0 2.0 3.0 4.0
Ф
Bh(eV)
SiO
2Si
3N
4Al
2O
3ZrO
2HfO
2Ta
2O
5(3.9)
3.0 0.3 2.9
4.3
1.5
3.2
3.3 1.6
1.8 2.4 3.1
4.7
K-Value
(7) (9~11) (25~40) (25~40) (25~40)CHAPTER 2
Characteristics of Al
2O
3Inter-Poly Dielectrics With Fluorine Passivation
2.1 Introduction
Recently, Devices with high dielectric constant materials (high-k dielectrics materials) such as aluminum oxide (Al2O3)have been studied intensively. However, investigation of the materials shows that fluorine dopant penetration through dielectrics is a significant problem due to the improved characteristics and reliability.
In tradition, ultra-thin oxide will undergo tunneling effect and then cause gate leakage current, which cause reliability probes. High dielectric constant materials, Al2O3 are used to replace SiO2 has widely studied. Compare with SiO2 at the same equivalent oxide thickness (EOT), high dielectric constant materials have thicker physical thickness which can stop from tunneling effect, and avoid more leakage current. Using the high dielectric constant materials is expected to have reduced leakage current and increase breakdown field and the charge-to-breakdown (QBD) as
in the Al2O3 inter-poly dielectrics (IPD) film with the poly surface fluorine implantation (PSFI) method. The incorporation of fluorine atoms into the Al2O3
inter-poly dielectrics (IPD) film reduces not only interface dangling bonds but also bulk traps, which is responsible for the improvements in electrical properties. Among these materials, Al2O3 inter-poly dielectric is a promising candidate because of its compatibility with Fluorine implantation.
With the scaling down of thickness of the inter-poly dielectrics (IPD), the quality of dielectric becomes very critical for the application of the EEPROM and flash nonvolatile memories. Lower leakage of the dielectric means longer data retention time. As many reports indicated that high dielectric constant materials with fluorine implantation has been shown improved electrical properties [82]-[84]. It is found that the incorporation of fluorine on the bottom poly-Si surface can not only reduce leakage current by one order of magnitude, but also enhance the breakdown field and the charge-to-breakdown (QBD) as well. This is ascribed to the resultant smoother interface between the dielectric and the floating gate by surface fluorine passivation and less electron charge traps in the bulk. However, the QBD is better than other only sputter and NH3 nitridation method. Moreover, the effect of fluorine dosage on the electrical properties and reliability characteristics of Metal Organic Chemical Vapor Deposition (MOCVD) Al2O3 inter-poly capacitors with surface fluorine passivation are studied in this chapter. The electrical properties of the Al2O3 IPD are strongly influenced by the fluorine passivation. The optimum floating gate is 5e13cm-2 and control gate is 5e15cm-2 in terms of leakage current, electron trapping rate and QBD.
2.2 Experimental Details
The n+-polysilicon/Al2O3 IPD/n+-polysilicon capacitors were fabricated on 6-inch p-type (100)-oriented silicon wafers. Silicon wafer was thermally oxidized at 980oC to grow a 2000Å buffer oxide. 2000Å bottom polysilicon film (Poly-I) was deposited on the buffer oxide by low pressure chemical vapor deposition (LPCVD) system using SiH4 gas at 620oC and subsequently implanted with phosphorous at 5e15cm-2, 20keV and implanted with fluorine dosage ranging from 1e12cm-2 to 1e14cm-2 at 10keV. Then activated with N2 RTA at 950°C for 30s. Prior to the growth of Al2O3 IPDs, the native oxide covered Poly-I was cleaned by the conventional RCA cleaning and diluted HF etching in sequence for the removal of particles and native oxides. The surface of Poly-I prepared in this matter was known to be contamination free and terminated with atomic hydrogen. After being wet cleaned and dipped in HF solution, and 10nm Al2O3 IPD was deposited by Metal-Organic Chemical Vapor Deposition (MOCVD) system at 500°C with Ar/O2 ambient. Annealing of aluminum oxide (Al2O3) IPDs was carried out by rapid thermal annealing (RTA) at 950oC temperatures ranging in an N2 atmosphere for 30s. Subsequently, a 2000Å top polysilicon layer (Poly-II) was deposited by LPCVD and implanted with phosphorous at 5e15cm-2, 20keV. Then, implanted with fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV. Dopants were then activated with N2 RTA at 950°C for 30s.
LCR meter. Moreover, the physical thickness was estimated by high resolution transmission electron microscopy (HRTEM). The electrical properties and reliability characteristics of the inter-poly capacitors were measured using a HP4156C semiconductor parameter analyzer.
2.3 Results and Discussions
In this chapter, the Characteristics and Reliabilities of Fluorine Passivation Effect on the MOCVD Al2O3 capacitors with various Fluorine Dosage show to investigated in terms of store of capacitance, leakage current and dielectric reliabilities.
2.3.1 The Basic Electrical Properties on the MOCVD Al2O3 IPD
Figure 2.3(a) shows the high frequency C-V curves (1MHz) of the floating Gate (FG) corresponding EOT of Al2O3 inter-poly capacitors with surface Fluorine Passivation Effect that fluorine dosage ranging from 1e12cm-2 to 1e14cm-2 at 10keV.
Figure 2.3(b) shows the high frequency C-V curves (1MHz) of the control Gate (CG) corresponding EOT of Al2O3 inter-poly capacitors with surface fluorine passivation Effect that fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV. The EOT is decreased as raising Fluorine passivation effect, which can be ascribed to the fluorine itself physical factors. Because of fluorine atoms inter wafer lattices and to fill vacancy and cause AlF3 formation, therefore are able to increased capacitance. As the fluorine dosage of floating gate continually increases to 1e14cm-2 and the fluorine
defects and slightly increase permittivity, smaller EOT value is therefore obtained as compared effectively with 1e14cm-2 and 5e15cm-2 samples.
2.3.2 Electric Field and Leakage Current Density Characteristics
Figure 2.4(a) and Figure 2.4(b) that compares the J-E characteristics of the Al2O3 inter-poly capacitors with fluorine Passivation at various fluorine dosage. It is found that the sample with floating gate fluorine 5e13cm-2 and control gate 5e15cm-2 can effectively reduce the low-field leakage current about one to two orders of magnitude than other samples, which is helpful to increase effective breakdown field from the floating gate and control gate sample. The leakage current in negative polarity is smaller than that in positive polarity due to asymmetric band diagram. It can be explained by the reduced damage generated and assistance in the interface of the inter-poly dielectrics and polysilicon film. Because thermal stress and atoms impact on lattice of crystal cause various crystal defects that like interstitial impurity atoms, edge dislocation, self- interstitial, dislocation loop, vacancy etc. Using fluorine passivation not only can reduce defect but also increase electrical properties.
2.3.3Relation of Trapping Density and Defect
Figure 2.5(a) , Figure 2.5 (b) show the Positive and Negative trap densities
fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV. Presents the transient currents for the fluorine passivation effect of Al2O3 IPDs under a low field of 2 MV/cm in order to suppress the creation of stress-induced traps. Filling of the pre-existing electron traps in the high dielectric constant materials leads to the decrease of the current leakage magnitude over time for all samples [85], [86].
Moreover,the rate of leakage current reduction in either polarity is nearly identical, suggesting that the traps are distributed uniformly across the films. Incorporation of Fluorine atoms tends to segregate at Poly and Poly/ HfO2 interfaceafter activation.
The N2 950℃ can effectively assist the crystal lattice atoms arranges in order again, and the stress levels on the fluorine implanted case are consistently higher which result from the much stronger Al-F bonds relative to Al-Al, Al-O or even Al-Si bonds.
The Al-F bonds strength are 159Kcal/mole whereas the Al-Al bonds strength are only 45 Kcal/mole. This is perhaps manifest best in the relative melting points of AlF3 (1291℃) than Al (660℃) or other bonds samples. Therefore, we believe that the dependence of the IPD characteristic on fluorine passivation effect is closely related to the bulk defects in the high-k dielectric. Figure 2.5(c) , Figure 2.5 (d) show the 100Å Al2O3 gate dielectric under a 2V constant voltage stress (CVS) for 1000 sec, and good reliability evidenced to show fluorine passivation is better than without fluorine passivation from the small current charge and stress-induced leakage current (SILC) respectively. Moreover, Al2O3 inter-poly capacitors fluorine passivation effect exhibits small electron trapping rate than without fluorine passivation effect. But in Figure 2.5(a) , Figure 2.5 (b) the fluorine passivation effect is bad than without fluorine passivation. Because of the process was outdiffusion by rapid thermal annealing (RTA) three time in Floating Gate. The result of the process produced more defect and influenced electrical properties characteristics.
2.3.4 Reliability Characteristics
Figure 2.6(a), Figure 2.6 (b) show QBD Weibull plots of Al2O3 inter-poly capacitors with surface fluorine passivation effect in positive and Negative CVS that fluorine dosage ranging from 1e12cm-2 to 1e14cm-2 at 10keV in floating Gate. The Weibull distributions of the charge-to-breakdown (QBD) field in both polarities as the magnitude of gate bias is 5.7MV/cm. Effective charge to breakdown(QBD) field exhibits nearly independent in various fluorine dosage. Because the process was outdiffusion by rapid thermal annealing (RTA) three time in floating gate, so caused more dangling bond and defects in the crystal. The increase of most fluorine passivation effect was due to the increase of fluorine dosage magnitude in floating gate. But fluorine dosage 1e14cm-2 was too much, and 1e12cm-2 , 5e12cm-2 was not enough to reach to fit the process of floating gate. This was ascribed to the resultant not smooth interface between the high dielectric and the floating Gate polysilicon by more or less fluorine implantation caused stress defect. The fluorine dosage 5e13cm-2 sample had best performance in preventing charge loss from floating Gate. The lesser leakage current means that retaining data time longer and higher charge breakdown field.
Figure 2.6(c), Figure 2.6 (d) show QBD Weibull plots of Al2O3 inter-poly capacitors with surface fluorine passivation effect in positive and negative 5.7MV/cm
bonds. For positive and negative gate bias, capacitors with fluorine surface passivation effect can significantly increase charge-to-breakdown (QBD) characteristics one order compared without fluorine passivation effect at 5e15cm-2 dosage in control gate inter-poly capacitors.
We believe that the dependence of the IPD characteristic on fluorine passivation is closely related to the bulk defects in the high dielectric material. In control gate, fluorine passivation can effectively reduce dislocation, voied, vacancies, interstitial and dangling bond result in the less interface defect density. By the result of QBD can evidence that 5e15cm-2 dosage is the optimized dosage at 20keV in control gate.
2.3.5 Temperature Characteristics of the fluorine passivation Al2O3 IPD
Figure 2.7(a) , Figure 2.7 (b) show illustrates the temperature dependence of gate current density at 3 MV/cm of Al2O3 inter-poly capacitors with surface fluorine passivation that dosage ranging from 1e12cm-2 to 1e14cm-2 at 10keV in floating gate.
Figure 2.7(c) , Figure 2.7 (d) show illustrates the temperature dependence of gate current density at 3 MV/cm of Al2O3 inter-poly capacitors with surface fluorine passivation that dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV in control gate.
In general, floating gate and control gate samples reduce about one order leakage current density, exhibited considerably weak measuring temperature dependence on the leakage current density in either polarity. By the Figures implying that the tunneling mechanism for the Al2O3 IPD is Fowler Nordheim-like, rather than Frankel
In general, floating gate and control gate samples reduce about one order leakage current density, exhibited considerably weak measuring temperature dependence on the leakage current density in either polarity. By the Figures implying that the tunneling mechanism for the Al2O3 IPD is Fowler Nordheim-like, rather than Frankel