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.
(1) Suitable high dielectric constant：
The reported dielectric constant 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 fringing-induced barrier lowering effect.
(2) Wide bandgap：
In general, as the dielectric constant increases, the bandgap decreases. The narrower bandgap would increase leakage current through thermal emission. The energy bandgap 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 3.1eV and the barrier height for holes is 4.7eV. The high band offset for both electron and hole has the benefit of low leakage current. Figure 3.1 shows the calculated band offsets for most high-κ dielectrics [83]. For HfO2, barrier height for electron and hole is 1.6eV and 3.3eV, respectively. This band alignment is acceptable for nonvolatile memory requirement and better than other high-κ materials such as Ta2O5 [33].
(4) High free energy of reaction with Si：
For HfO2, the free energy of reaction with Si is about 47.6 kcal/mole at 727ºC (see Table 1.1), 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.
(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. And it means that Hf is easy to be oxidized to form HfO2 and the oxide of Hf is usually stable on Si substrate.
(6) Superior thermal stability with poly-Si：
Unlike ZrO2, HfO2 shows a good thermodynamic stability with poly-Si [84], [85]. The HfO2 would not react easily with poly-Si in high temperature as ZrO2 [86].
According to these profits discussed above, we choose HfO2 as one of the major high-κ IPDs in our investigation for next decade flash memories.
3.2 Experimental Details
The n+-polysilicon/HfO2 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 HfO2 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 HfO2 IPDs were deposited by MOCVD system at 500ºC. Annealing of HfO2 IPDs was carried out by rapid thermal annealing (RTA) at temperatures ranging from 600ºC 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. The cross-sectional view and key process steps of HfO2
inter-poly capacitor with surface NH3 nitridation and post-deposition nitrogen annealing are shown in Figs. 3.2 and 3.3, respectively.
The equivalent oxide thickness (EOT) was obtained from the high frequency (10kHz) 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.
3.3 Results and Discussions
3.3.1 Basic Electrical Properties
Figure 3.4 (a) and (b) show the high frequency C-V curves (10kHz) and the current density-effective electric field (J-E) characteristics of HfO2 inter-poly capacitors with surface NH3 nitridation annealed at 600ºC to 1000ºC, respectively.
The low capacitance and suppressed leakage current of 1000ºC PDA sample can be ascribed to its thicker EOT, shown in Fig. 3.5. Lower post-deposition annealing temperature won’t cause large difference in the oxygen diffusivity between HfO2 and Al2O3 film. When annealing temperature is high as up to 1000ºC, due to the high oxygen diffusion coefficient of HfO2, shown in Table 1.1, oxygen can easily penetrate the HfO2 film and react with the layer beneath HfO2 film to form thick interface layer.
This effect can be clearly observed in Fig. 3.6. Compared to the low oxygen diffusivity of Al2O3 film, oxygen can penetrate all the way through HfO2 film to form interface layer at high temperature. So, the higher the post-deposition annealing temperature is, the thicker the interface will be formed. As HfO2 is not a good oxygen diffusion barrier, the control of oxygen concentration and temperature would be very critical when we apply HfO2 film as the inter-poly dielectric in production line.
3.3.2 Electric Field and Leakage Current Density Characteristics
Figure 3.7 depicts the Weibull distributions of the effective breakdown field of
annealing under (a) positive and (b) negative polarities. In both polarities, the effective breakdown field of the 800ºC PDA annealed HfO2 IPD is obviously higher than those of as-deposited and 600ºC PDA samples. And we believe this is due to the improved thin film quality of 800ºC PDA sample. Since HfO2 IPD annealed at 1000oC will oxidize underlying Poly-I more effective than others, both large effective breakdown field and degraded Weibull slope can be explained by thicker interfacial layer and poor interface morphology. Figure 3.8 demonstrates the Weibull distributions of the leakage current of HfO2 inter-poly capacitors with surface NH3
nitridation at various PDA temperatures in both polarities as the gate bias is 6MV/cm.
The magnitude of the leakage current in both polarities is almost the same, about 10-7A/cm2 in spite of different PDA temperature. The leakage current of as-deposited and 600ºC annealed samples goes up enormously to 10A/cm2 when the gate bias increases as high as to 5V (about 15.6MV/cm), however there is only three order of magnitude enhancement in the leakage current of 800ºC PDA sample, as shown in Fig.
3.9. The extreme high leakage current density of as-deposited and 600ºC annealed samples stands for their breakdown, and this fact will retard their application to the flash memories [28]. The relatively low leakage current of 1000ºC PDA sample is due to its thicker EOT and thus lower electric field (about 11.3MV/cm) as compared with that of 800ºC PDA sample.
3.3.3 Reliability Characteristics
Figure 3.10 shows QBD Weibull distributions of HfO2 inter-poly capacitors with surface NH3 nitridation at various PDA temperatures under (a) positive and (b) negative polarities. The EOT of 800ºC PDA sample is almost the same as those of as-deposited and 600ºC annealed sample, however, a two order magnitude of
enhancement in QBD can be observed. This fact indicates that post-deposition temperature of 800ºC can effectively improve thin film quality of HfO2 bulk. On the other hand, according to percolation model [87], [88], stress-induced defects must get in a continuous line to form a leakage path. The 1000ºC PDA sample will have larger QBD and Weibull slope because of its thicker EOT. Figure 3.11 presents curves of gate current density shift of HfO2 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing under (a) positive and (b) negative constant voltage stress (CVS) of 1V gate voltage. Electron trapping is observed in both polarities. As PDA temperature increases, electron trapping rate can be suppressed, which means improved film quality. However, the reason for small gate leakage current shift of 1000ºC PDA sample is totally different with others. Voltage drop across HfO2 IPD will be decreased as the thickness of interfacial layer increasing, which will cause the reduction of the electric field across HfO2 IPD in 1000ºC PDA sample, reduced leakage path formation and therefore larger QBD is obtained.
3.4 Summary
The effects of PDA temperature on the electrical properties and reliability characteristics of the HfO2 inter-poly capacitors with surface NH3 nitridation are evaluated in this chapter. It was found that the electrical properties of HfO2 IPD strongly depend upon the PDA temperature. 800ºC annealing is the best condition for the HfO2 IPD electrical characteristics in terms of EOT scaling, leakage current, electron trapping rate and QBD. On the other hand, the high oxygen diffusivity of HfO2 film will result in thick I.L. growth, retarding EOT scaling. As the result, the control of oxygen concentration and temperature would be very critical when we apply HfO
film as the inter-poly dielectric in flash memories. Table 3.1 lists several physical and electrical parameters, including EOT, breakdown electric field, 6MV/cm-biased leakage current density and 63%-failure QBD values of the HfO2 IPDs with surface NH3 nitridation annealed at various temperatures.
Table 3.1 EOT, effective breakdown field, 6MV/cm-biased leakage current density and 63%-failure QBD values of the HfO2 inter-poly capacitors with surface NH3 nitridation under positive and negative CVS 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 31.3 12.86 13.63 162 278 0.9 1.0
600 31.1 13.37 14.60 245 355 0.33 0.76
800 32.2 19.69 20.24 245 550 103 16
1000 42.3 20.14 21.75 110 309 172 11.5
Fig. 3.1 Band alignment of typical high-κ dielectrics.
P-type Si Substrate
Fig. 3.2 Cross-sectional view of HfO2 inter-poly capacitors with surface NH3
nitridation and post-deposition nitrogen annealing.
RCA cleaning and
Fig. 3.3 Key process steps of HfO2 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 ) 1e-9
Fig. 3.4 (a) C-V curves and (b) J-E characteristics of HfO2 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing.
Equivalent Oxide Thickness ( A )
Fig. 3.5 Equivalent oxide thickness of HfO2 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing.
as-dep 600℃ 800℃ 900℃ 1000℃ 900℃ PEA
Equivalent Oxide Thickness ( nm )
2.5
Fig. 3.6 Comparison of EOT between Al2O3 and HfO2 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. 3.7 The Weibull distributions of the effective breakdown field of HfO2 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 ) 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
Fig. 3.8 The Weibull distributions of the leakage current density of HfO2 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.
Gate Leakage Current Density@Vg=5V ( A/cm2 )
Gate Leakage Current Density@Vg=5V ( A/cm2 ) 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
Fig. 3.9 The Weibull distributions of the leakage current density of HfO2 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing under
Charge to Breakdown ( mC/cm2 )
Fig. 3.10 QBD Weibull distributions of HfO2 inter-poly capacitors with surface NH3
nitridation at various PDA temperatures under (a) positive and (b) negative polarities.
Time ( sec )
0 10 20 30 40 50 60 70 80 90 100 110
Gate Leakage Current Shift ( pA )
-3.0
Gate Leakage Current Shift ( pA )
0.0
Fig. 3.11. Curves of gate current density shift of HfO2 inter-poly capacitors with surface NH3 nitridation and post-deposition nitrogen annealing under (a) positive and (b) negative constant voltage stress.
CHAPTER 4
Conclusions and Recommendations for Future Works
4.1 Conclusions
According to SIA roadmap, oxide thickness small than 20Å is necessary for deep sub-quarter micron devices. However, pure SiO2 can’t meet the requirement due to the large tunneling current. In our study, it was found that the electrical properties of Al2O3 and HfO2 IPD strongly depend upon the PDA temperature. The PDA temperature up to 1000ºC may cause Al2O3 film crystallization and thus poor quality.
Additional 900ºC PEA is beneficial to improve thin film quality because it can reduce the damage generated during the Poly-II patterning. For HfO2 IPD, since HfO2 is not a good oxygen diffusion barrier, the control of oxygen concentration and temperature would be very critical when we apply it as the inter-poly dielectric in the flash memories. 800ºC and 900ºC annealing are the best condition for the HfO2 and Al2O3
IPD respectively. Table 4.1 lists the comparison of 800ºC PDA HfO2 IPD and 900ºC PDA with 900ºCPEA Al2O3 IPD samples in terms of EOT, effective breakdown field, 6MV/cm-biased leakage current density and 63%-failure QBD values.
4.2 Recommendations for Future Works
1. More HRTEM images to evidence thickness variation and interfacial layer reaction.
2. More physical analyses to quantitatively understand film composition.
3. Fully Fabricated stacked-gate flash memories with high-κ inter-poly dielectrics to study the device characteristics, including program/erase speed, retention time and charge loss mechanism.
Table 4.1 Comparison of 800ºC PDA HfO2 IPD and 900ºC PDA with 900ºCPEA Al2O3 IPD samples in terms of EOT, effective breakdown field, 6MV/cm-biased leakage current density and 63%-failure QBD values.
EBD
(MV/cm)
Jg@6MV/cm (nA/cm2)
63% QBD (mC/cm2) IPD
material
EOT (Å)
(＋) (－) (＋) (－) (＋) (－)
800ºC PDA HfO2 32.2 19.7 20.2 245 550 103 16
900ºC PDA with
900ºCPEA Al2O3 55.5 18.0 18.8 6.0 3.1 6570 610
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