Chapter 1 Introduction
1.2 Motivation
Fast low-power nonvolatile memories are required for future wireless 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 [28], [34]-[36]. 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 [37]-[39]. It is not sufficient to meet the stringent data retention requirement of IPD while applying thermal or CVD oxynitride technologies due to the unavoidable leakage current [29]-[32], [40]. 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 [34], [35], [41]-[48].
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 [49]-[51]. 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 [41]-[43], 3-dimension inter-poly dielectric [45], and hemisphere grain [46], [47] had been proposed to effectively increase the capacitance area and lower the control gate bias.
Although the coupling ratio of above mentioned cell structure could be dramatically improved, they must be fabricated with many additional process steps for fabrication such complex structures and be difficult to control well. The final approach is to increase the dielectric constant (κ) of IPD materials [22], [23], [27], [52]-[59].
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) [17], [60]-[62] and hafnium oxide (HfO2) [20], [63]-[66] 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) [67], [68], 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 MOCVD technology as our tool to deposit thin high-κ IPDs. A detail schematic 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 [69], [70]. 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 and the resulting Si/SiO2 interface [71]. The Si/SiO2 interface is known to have a very low density of interface states (Dit~2×1010 ststes/cm2) arising from unsaturated surface bonds and other electrically active imperfections [71]. Interface states lead to degradation of on-current, since carrier mobility is limited by scattering at the 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. Generally, there are many methods to passivate the Si surface such as surface nitridation, nitrogen-contained ambient annealing, or nitride deposition as the bottom layer. Nitridation of the Si surface using NH3 treatment before the deposition of high-κ materials has been shown to be effective in achieving the low EOT and preventing the boron penetration [72], [73]. However, this technique results in higher interface charges which leads to higher hysteresis and reduced channel mobility [74].
The NH3 treatment would nitridize the Si surface to form a silicon nitride layer [75]-[77]. Silicon nitride is a superior barrier for H2O and oxygen, and it can suppress oxygen to diffuse into Si substrate [72]. After the NH3 treatment, a thin silicon nitride ( SixNy ) layer ( ~10 Å ) was deposited and measured by optical measurement system ( Ellipsometer ). As reports, nitridation 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 [78].
1.3 Organization of This Thesis
There are four 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 flash memories.
In chapter 2, the effects of post-deposition annealing (PDA) temperature 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 PDA temperature 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
2. High deposition rate.
3. Good controllability of composition. quality than PVD or CVD.
2. Excellent coverage and conformity.
3. Poor conformity, especially for high aspect ratio.
Cons：
1. Hard to deposit ultra-thin films.
2. Poorer conformity than ALCVD.
2. Poor throughput for ULSI
standard.
3. UHV tool and the cost of
maintenance.
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.
CHAPTER 2
Effects of PDA Temperature on the Electrical Properties of Al
2O
3IPD with NH
3Nitridation
2.1 Introduction
With the scaling down of thickness of the inter-poly dielectrics (IPD), the quality of the 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-κ IPDs with surface NH3
nitridation have been shown improved electrical properties [21]-[23]. Among those potential candidates, aluminum oxide (Al2O3) is the most attractive for IPD application in nonvolatile flash memories because of its higher conduction band offset with respect to the underlying poly-Si electrode and its higher permittivity with respect to Si3N4 [17], [21], [57], [71], [79], [80]. On the other hand, it is found that the incorporation of nitrogen 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 [23]. This is ascribed to the resultant smoother interface between the dielectric and the floating gate by surface nitridation and less electron traps in the bulk [23]. However, the QBD is unfortunately quite low. Moreover, the effects of post-deposition annealing (PDA) temperature on the electrical properties
and reliability characteristics of MOCVD Al2O3 inter-poly capacitors with surface NH3 nitridation are studied in this chapter. The electrical properties of the Al2O3 IPD are influenced by the PDA temperature. The optimum is 900ºC Al2O3 IPD 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 950ºC 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, then activated with RTA at 950ºC for 30s. Prior to the growth of Al2O3 IPDs, the native oxide covering 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, all samples were subjected to ammonia (NH3) nitridation in the LPCVD furnace at 800ºC for 1 hour. Then, 10nm Al2O3 IPDs were deposited by MOCVD system at 500ºC. Annealing of Al2O3 IPDs was carried out by rapid thermal annealing (RTA) at temperatures ranging from 800ºC to 1000ºC in an N2 atmosphere for 30s. Subsequently, a 2000Å top polysilicon layer (Poly-II) was deposited by LPCVD system and implanted with phosphorous at 5e15cm-2, 20keV. Dopants were then activated with RTA at 950ºC for 30s. Finally, 5000Å TEOS oxide passivation and
Al metal pads were defined. It is worthy to mention that we took one of the 900ºC PDA samples annealed again at 900ºC in N2 atmosphere followed by the dry etching step, called post-etching annealing (PEA). The cross-sectional view and key process steps of Al2O3 inter-poly capacitor with surface NH3 nitridation and post-deposition nitrogen annealing are shown in Figs. 2.1 and 2.2, respectively.
The equivalent oxide thickness (EOT) was obtained from the high frequency (10 kHz) capacitance-voltage (C-V) measurement using a Hewlett-Packard (HP) 4284 LCR meter. The electrical properties and reliability characteristics of the inter-poly capacitors were measured using a HP4156C semiconductor parameter analyzer.
2.3 Results and Discussions
2.3.1 Basic Electrical Properties
Figure 2.3 (a) shows the high frequency C-V curves (10kHz) and the corresponding EOT of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800ºC to 1000ºC. The EOT increases as PDA temperature rising up to 900ºC, which can be ascribed to the thick interfacial layer (IL) growth. As the PDA temperature continually increases to 1000ºC, in spite of the thickest IL, Al2O3 film may partially crystallize and slightly increase permittivity, smaller EOT value is therefore obtained as compared to 900ºC PDA samples. However, the differences of the EOT among these samples are less than 3Å, which can be ascribed to both the effects of surface NH3 nitridation and extremely low oxygen diffusivity of Al2O3 film.
Figure 2.3 (b) presents the J-E characteristics of the Al2O3 inter-poly capacitors with NH3 nitridation at various PDA temperatures under both polarities. It is found that the
large leakage current of 1000ºC PDA sample may be the proof of crystallization. We also found that 900ºC PDA with additional 900ºC post-etching annealing (PEA) sample can effectively reduce the low-field leakage current than other samples, which is helpful to suppress charge loss from the floating gate. It can be explained by the reduced damage generated by ion bombardment during the Poly-II patterning. In addition, it is worthy to mention that polarity dependence can be observed in gate leakage current curves, the leakage current in negative polarity is smaller than that in positive polarity due to asymmetric band diagram.
2.3.2 Electric Field and Leakage Current Density Characteristics
Figure 2.4 shows the breakdown characteristics of Al2O3 inter-poly capacitors with NH3 nitridation at various PDA temperatures under both polarities. Effective breakdown field exhibits nearly independent on PDA temperatures. Figure 2.5 compares the Weibull distributions of the leakage current of Al2O3 inter-poly capacitors at various PDA temperatures with NH3 nitridation in both polarities as the magnitude of gate bias is 6MV/cm. Once again, 1000ºC PDA sample has large leakage current caused by partially crystallization, and 900ºC PDA with 900ºC PEA sample has better performance in preventing charge loss from floating gate.
2.3.3 Reliability Characteristics
Figure 2.6 demonstrates (a) QBD Weibull distributions and (b) the corresponding curves of gate voltage shift of Al2O3 inter-poly capacitors with surface NH3 nitridation at various PDA temperatures when constant current stress of 5mA/cm2 is applied in
positive polarity is attributed to its thicker interfacial layer. The interfacial layer becomes thicker when post-deposition annealing temperature rises, then the voltage drop across interfacial layer will increase and result in stronger electric field. The increase in the gate voltage indicates that the primary mechanism responsible for the long-term wear-out in Al2O3 film is the creation of electron traps under positive polarity, as shown in Fig. 2.6 (b). We also found that Al2O3 inter-poly capacitors annealed at 900ºC with additional 900ºC PEA exhibits smaller trapping rate than other conditions and this phenomenon indicates improved film quality ,which is consistent with the result of suppressed gate leakage current shown in Fig. 2.3. We believe that additional 900ºC post-etching annealing can reduce damage generated by ion bombardment during the Poly-II patterning and further improve Al2O3 inter-poly capacitors characteristics such as leakage current and stress-induced trapping rate.
However, it is totally different situation in negative polarity. Figure 2.7 shows (a) QBD Weibull distributions and (b) the corresponding curves of gate voltage shift of Al2O3
inter-poly capacitors with surface NH3 nitridation at various PDA temperatures when constant current stress of 5mA/cm2 is applied in negative polarity. In Fig. 2.7 (a), although thickness of interfacial layer increases as PDA temperature rising, there is no apparent difference in QBD for negative polarity. This fact reveals that the thickness of Al2O3 film dominate breakdown mechanism, i.e. bulk breakdown. In Fig. 2.7 (b), hole trapping is observed, which can ascribed to the electron-hole pairs generation caused by electron impact after injection from Poly-II to Poly-I under negative bias. Then holes jumped to valence band and were trapped in the Al2O3 bulk when they injected back to Poly-II. Such mechanism is called anode hole injection (AHI) [81], [82]. The other mechanism for hole trapping is injection of accumulation hole of Poly-I. The similar trapping rate in negative polarity is in agreement with the result of identical charge-to-breakdown as shown in Fig. 2.7 (a). Band diagrams of Al2O3 inter-poly
capacitors with surface NH3 nitridation under (a) positive and (b) negative gate voltage biased to the Poly-II are demonstrated in Fig. 2.8 (a) and (b) respectively.
2.4 Summary
The effects of PDA temperature on the electrical properties and reliability characteristics of the Al2O3 inter-poly capacitors with surface NH3 nitridation are evaluated in this chapter. It was found that the electrical properties of Al2O3 IPD strongly depend upon the PDA temperature. 900ºC annealing is the best condition for the Al2O3 IPD electrical characteristics in terms of leakage current, trapping rate and QBD. Moreover, additional post-etching annealing is beneficial to improve Al2O3 thin film quality because it can reduce the defects generated during the Poly-II patterning.
The results apparently demonstrate Al2O3 IPD with surface nitridation, optimized PDA temperature and another 900ºC PEA can effectively reduce charge transfer between control gate and floating gate, better retention and disturb characteristics are expected by replacing ONO IPD to Al2O3 IPD. The Al2O3 dielectric with surface NH3
nitridation, 900ºC post-deposition and post-etching annealing thus appears to be very promising for future flash memory devices. Table 2.1 lists several physical and electrical parameters, including EOT, effective breakdown field, 6MV/cm-biased leakage current density and 63%-failure QBD values of the Al2O3 IPDs with surface NH3 nitridation annealed at various temperatures.
Table 2.1 EOT, effective breakdown field, 6MV/cm-biased leakage current density and 63%-failure QBD values of the Al2O3 inter-poly capacitors with surface NH3 nitridation under positive and negative CCS at various PDA temperatures in N2
ambient.
EBD
(MV/cm)
Jg@6MV/cm (nA/cm2)
63% QBD (mC/cm2) PDA
Temp.
(ºC)
EOT (Å)
positive negative positive negative positive negative
As-dep 55.6 18.4 19.1 20.4 24.5 5880 560
800 56.0 18.3 18.8 15.8 13.2 4300 570 900 57.0 18.3 18.6 13.8 13.2 6750 690
1000 56.2 18.5 19.0 490.0 2089 3800 700 900 with
900 PEA 55.5 18.0 18.8 6.0 3.1 6570 610
P-type Si Substrate
Fig. 2.1 Cross-sectional view of Al2O3 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing.
RCA cleaning and
Fig. 2.2 Key process steps of Al2O3 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing.
Gate Voltage ( V )
Electric Field ( MV/cm )
-20 -10 0 10 20
Current Density ( A/cm2 ) 10-11
Fig. 2.3 (a) C-V curves and (b) J-E characteristics of Al2O3 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing.
Breakdown Electric Field ( MV/cm )
Breakdown Electric Field ( MV/cm )
8 10 12 14 16 18 20 22 24
Fig. 2.4 The Weibull distributions of the effective breakdown field of Al2O3 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing under (a) positive and (b) negative polarities.
Gate Leakage Current Density@6MV/cm ( A/cm2)
Gate Leakage Current Density@6MV/cm ( A/cm2 ) 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3
Fig. 2.5 The Weibull distributions of the leakage current density of Al2O3 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing under (a) positive and (b) negative polarities as the gate bias is 6MV/cm.
Charge to Breakdown ( C/cm2 )
Fig. 2.6 (a) QBD Weibull distributions and (b) the corresponding curves of gate voltage shift of Al2O3 inter-poly capacitors with surface NH3 nitridation at various PDA
Charge to Breakdown ( C/cm2 )
Fig. 2.7 (a) QBD Weibull distributions and (b) the corresponding curves of gate voltage shift of Al2O3 inter-poly capacitors with surface NH3 nitridation at various PDA temperatures when CCS of 5mA/cm2 is applied in negative polarity.
Poly-I
Fig. 2.8 Band diagrams of Al2O3 inter-poly capacitors with surface NH3 nitridation under (a) positive and (b) negative gate voltage biased to the Poly-II.
CHAPTER 3
Effects of PDA Temperature on the Electrical Properties of HfO
2IPD with NH
3Nitridation
3.1 Introduction
Recently, HfO2 has gained much attention as promising insulator. The reasons are briefly listed as follows.
Recently, HfO2 has gained much attention as promising insulator. The reasons are briefly listed as follows.