The effects of fluorine dosage on the electrical properties and reliability characteristics of the Al2O3 inter-poly capacitors with surface fluorine passivation are evaluated in this chapter. It was found that the electrical properties of Al2O3 IPD capacitors strongly depend upon the fluorine passivation effect. The floating gate with 5e13cm-2 and control gate with 5e15cm-2 are respective the best condition for the Al2O3 IPD capacitors electrical characteristics in terms of leakage current, temperature, QBD and electron trapping rate of control gate. The capacitance as increase as fluorine dosage that the consequences indicate closely related to the fluorine passivation effect and fluorine dosage when changing fluorine dosage concentration. The results apparently demonstrate Al2O3 IPD capacitors with surface fluorine passivation effect 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 fluorine passivation thus appears to be very promising for future flash memory devices. Table 2.1 lists several physical and electrical parameters, including EOT, κ-value, interfacial layer thickness, and effective breakdown field and 63%-failure QBD values of the Al2O3 IPD capacitors with surface fluorine passivation under positive and negative CVS at various fluorine dosage effects.
Table 2.1 EOT, κ-value, interfacial layer thickness, effective breakdown field and 63%-failure QBD values of the Al2O3 capacitors with surface fluorine passivation under positive and negative CVS at various fluorine dosage effects.
EBD
(MV/cm)
Interfacial Layer
63% QBD (C/cm2) F
Dosage (cm-2)
κ EOT (Å)
positive negative Thickness (Å) positive negative
0 8.5 45.9 16.8 16.3 15 2.05 1.91
FG 5E13
9.1 42.8 19.2 18.9 10.5 2.11 2.16
CG 5E15
15.8
24.6 30.1 28.9 7.9 10.23 4.32
P-type Si Substrate 2000Å Buffer Oxide
2000Å n+Poly-I 2000Å n+Poly-II
TEOS 5000Å Al Metal Pad
SiNx~10Å 100Å Al2O3
Fig. 2.1 Cross-sectional view of Al2O3 inter-poly capacitors with surface fluorine passivation . The fluorine was implanted on Poly-I (Floating Gate) and Poly-II (Control Gate).
N2 950ºC PDA 30sec RCA cleaning and
2000Å buffer oxide 2000Å Poly-II deposition 2000Å Poly-I
deposition Poly-II Phosphorous 5E15 20KeV Poly-I Phosphorous
5E15 20KeV Fluorine 5E13 to 5E15 20KeV and activation Fluorine 1E12 to 1E14
10KeV and activation TEOS 5000Å deposition STD cleaning and
HF-last Gate and metal line
patterning 100Å Al2O3 IPD
deposition
Fig. 2.2 Key process steps of Al2O3 inter-poly capacitors with surface fluorine passivation.
- 2 . 0 - 1 . 5 - 1 . 0 - 0 . 5 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0
Fig. 2.3 (a) Floating Gate C-V curves. C-V characteristics of Al O floating gate
- 2 0 - 1 5 - 1 0 - 5 0 5 1 0 1 5 2 0
Fig. 2.4 (a) J-V characteristics of Al2O3 floating gate inter-poly capacitors with surface fluorine passivation effect is beneficial in suppressing low-field leakage current density that fluorine dosage ranging from 1e12cm-2 to 1e14cm-2 at 10keV.
Fig. 2.4 (b) J-V characteristics of Al2O3 control gate inter-poly capacitors with surface fluorine passivation effect is beneficial in suppressing low-field leakage current density that fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV.
1 1 0 1 0 0 1 0 0 0
Fig. 2.5 As-fabrication trap densities evaluation at 2 V constant voltage stress (CVS) of Al O floating gate inter-poly capacitors with surface fluorine passivation effect
1 1 0 1 0 0 1 0 0 0 0 . 1
1 C V S @ V
G = + 2 V
A l2O3 C o n t r o l G a t e
Current Density J/J 0
w /o F F :1 E 1 4 F :1 E 1 5 F :5 E 1 3 F :5 E 1 5
T im e ( s e c )
(c)
1 0 1 0 0 1 0 0 0
0 . 1 1
C V S @ VG = - 2 V
A l2O3 C o n t r o l G a t e
Current Density J/J 0
w / o F F : 1 E 1 4 F : 1 E 1 5 F : 5 E 1 3 F : 5 E 1 5
T im e ( s e c )
(d)
Fig. 2.5 As-fabrication trap densities evaluation at 2 V constant voltage stress (CVS) of Al2O3 control gate inter-poly capacitors with surface fluorine passivation effect that fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV under (c) positive 2V (d) negative 2V trap densities. Al2O3 inter-poly capacitors with fluorine passivation can reduce As-fabrication trap densities.
1 E - 5 1 E - 4 1 E - 3 0 . 0 1 0 . 1 1 1 0 1 0 0
1 E - 5 1 E - 4 1 E - 3 0 . 0 1 0 . 1 1 1 0 1 0 0
Fig. 2.6 QBD Weibull plots of Al2O3 inter-poly capacitors with surface fluorine passivation effect under (c) positive CVS and (d) negative CVS that fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV. Al2O3 inter-poly dielectric with surface fluorine passivation effect can significantly increase QBD in Control Gate.
40 60 80 100 120 1E -7
1E -6
Al2O3 Floating Gate
Temperature
Current Density @ 3 MV/CM ( A/cm2 ) w /o F
F:1E 12 F:5E 12 F:1E 13 F:5E 13 F:1E 14
VG = +3V
(a)
(℃)
4 0 6 0 8 0 1 0 0 1 2 0
1 E - 7 1 E - 6 1 E - 5 1 E - 4
A l2O3 F lo a tin g G a te
T e m p e ra tu re
Current Density @ 3MV/CM( A/cm2 ) w /o F
F :1 E 1 2 F :5 E 1 2 F :1 E 1 3 F :5 E 1 3 F :1 E 1 4
VG = -3 V
(℃)
4 0 6 0 8 0 1 0 0 1 2 0
Fig. 2.7 Temperature dependence of gate current density at 3 MV/cm of Al2O3
inter-poly capacitors with surface fluorine passivation that fluorine dosage ranging from 5e13cm-2 to 5e15cm-2 at 20keV in control gate under (c) positive and (d) negative polarities.
Poly-I
-- --
Equivalent Oxide Thickness ( nm )
Al2O
Equivalent Oxide Thickness ( nm )
w /o F
-- --
Equivalent Oxide Thickness ( nm )
(c)
Fig. 2.9 The average EOT of Al2O3 IPD capacitors with surface fluorine passivation under(a) floating gate (b) control gate (c) floating gate and control gate compared.
6
6.2
Fig. 2.11 The dosage-average breakdown voltage images of Al2O3 capacitors with surface fluorine passivation under (a) Positive VG floating gate (b) Negative VG
floating gate (c) Positive VG control gate (d) Negative VG control gate.
-6.0
Fig. 2.12 The dosage-average breakdown voltage images of Al2O3 capacitors with surface fluorine passivation under (a) Positive VG floating gate and control gate compare (b) Negative VG floating gate and control gate compare.
- 3 0 - 2 5 - 2 0 - 1 5 - 1 0 - 5 0 5 1 0 1 5 2 0 2 5 3 0
1 E - 5 1 E - 4 1 E - 3 0 . 0 1 0 . 1 1 1 0 1 0 0
Fig. 2.14 The charge to breakdown (QBD) images of Al2O3 capacitors with surface fluorine passivation under (a) Positive VG (b) Negative VG floating gate and control gate compare.
4 0 6 0 8 0 1 0 0 1 2 0
Fig. 2.16 The FIB cross-sectional images of Al2O3 inter-poly capacitors.
Fig. 2.17 The FIB top images of Al2O3 inter-poly capacitors thickness must be smaller than 500Å to do HRTEM.
n+-poly FG
n+-poly CG p-Sub
interfacial layer
Wet oxide
pt Al2O3 IPD
Fig. 2.18 The TEM all cross-sectional images of Al2O3 inter-poly capacitors.
(a) Without F
Al2O3 IPD
(b) FG 5E13
Al2O3 IPD
(c) CG 5E15
Al2O3 IPD
CHAPTER 3
Characteristics of HfO
2Inter-Poly Dielectrics with Fluorine Passivation
3.1 Introduction
In the recently, for pursuing the high speed, short program/erase times and low control power operation of nonvolatile flash memories products, the employment of high-permittivity (k) inter-poly dielectrics (IPDs) into flash memories has attracted much attention. For EEPROM and flash memories devices, by increasing the fluorine passivation effect, high-k IPDs can lead to a high electric field across tunnel oxide even at very low control gate voltage. The high-k IPDs of fluorine passivation can increase charge retention, because of the barrier height (φB) between Si and the adopted high-k dielectrics should be large than 1.5eV for effectively suppressing the loss of floating gate charges through electron thermal emission. Usually, high-k IPDs with fluorine passivation can repair only high-k IPDs defects in flash memories. The effects of fluorine passivation on the electrical properties and reliability characteristics of the HfO2 and Al2O3 inter-poly capacitors with polysilicom surface fluorine
property. The high dielectric constant (high-k) material shows that fluorine atom implantation and combination of silicon are significant problem due to the passivation effect.[87] In addition, to find suit fluorine dosage is important for less or more dosage resulted in bad electrical characteristic. For this reason, the HfO2 of fluorine passivation is expected to have higher capacitance, k-vale, EOT, effect breakdown field and reduced leakage current, breakdown voltage, electric trapping, improved charge-to-breakdown (QBD) and interfacial layer. Among these high-k materials, HfO2
with fluorine passivation is a promising candidate, because of its compatibility with HfO2 inter-poly dielectrics (IPDs)and tunnel dielectrics (TDs) on flash memories performance.
The most commonly reported high-k materials are ZrO2, Al2O3 and HfO2, that shown inTable1.1. Comparison of the high-k materials with HfO2, HfO2 has gained much attention as promising insulation. The reasons are briefly 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 6.02eV, which is higher than the other high-κ materials such as 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 1.4 shows the calculated band offsets for most high-κ dielectrics [88]. 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 [89].
(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：
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 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 HfO2 IPD was deposited by Metal-Organic Chemical Vapor Deposition (MOCVD) system at 500°C with Ar/O2 ambient. Annealing of hafnium oxide (HfO2) 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 1e15cm-2 at 20keV . Dopants were then activated with N2 RTA at 950°C 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 fluorine passivation effect are show the in Fig. 3.1 and 3.2, respectively.
The equivalent oxide thickness (EOT) was obtained from the high frequency
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.
3.3 Results and comparison between Al2O3 andHfO2
In this chapter, the effects of fluorine passivation on the MOCVD Al2O3 IPDs and HfO2 are compared in terms of EOT, κ-value, dosage-EOT, dosage-average breakdown current, effective breakdown field and 63%-failure QBD values and temperature under (a) Positive VG (b) Negative VG Floating Gate and Control Gate.
3.3.1 Basic Electrical Properties
A high capacitance density is important for using in memories store to increase the circuit density and reduce the cell area and lower cost. Therefore, adoption of high-k materials like Al2O3 and HfO2 are a very efficient way to increase the capacitance density. The evidence is showed in Figure 3.3 (a) and (b) in HfO2 IPD capacitors. By using silicon surface fluorine implantation (SSFI) method [93], the fluorine atoms inter polysilicon lattices and fluorine itself physical property, which causing increased capacitance. The EOT is decreased as raising Fluorine passivation
gate and control gate compared. In general, as the dielectric constant increases, the bandgap decreases. The narrower bandgap would increase leakage current through thermal emission. So, we know that HfO2 is higher k value than Al2O3, that Al2O3 is more capacitance density. On other hand in Figure 3.5 and Finger 2.10 Comparison of HfO2 IPD and Al2O3 IPD capacitors with surface fluorine passivation under floating gate and control gate. The coupling rate formula in Figure 3.6 as follows.
(1) control gate＞＞floating gate
(2) ( control gate / floating gate + control gate ) ～1
By the coupling rate formula, Al2O3 IPD capacitors have coupling rate more efficiently than HfO2 IPDcapacitors.
3.3.2 Electric Field and Leakage Current Density Characteristics
Figure 3.7 depicts the effective breakdown field of HfO2 inter-poly capacitors with surface fluorine passivation in different dosages under (a) floating gate (b) control gate. Both of floating gate and control gate are beneficial in suppressing low-field leakage current density, but floating gate is smaller than control gate. The factor is ascribed Hf-F than Si-F band slightly reduces interface dangling bonds and better performance. Figure 3.8 (a), (b), (c), (d) and Figure 3.9 (a), (b) show the dosage-average breakdown voltage images of HfO2 capacitors, these image can evidence 5e13cm-2 and 1e15cm-2 are optimal dosage in floating gate and control gate, respectively. Compared Figure 3.10 and Figure 2.13, Al2O3 IPD capacitor is lesser low-field leakage current density than HfO2 IPD capacitor. The result for two important focus. First, the HfO2 IPD capacitor narrower bandgap would increase leakage current through thermal emission. Second, the lower barrier is caused electron
3.3.3 Relation of Trapping Density and Defect
Effects of fluorine incorporation on the reliabilities of stack-flash memories with SSFI have been studied. In this chapter, fluorine was incorporated during the poly implant step and was diffused into the poly
/
HfO2 IPD interface during subsequent dopant activation. Figure. 3.11 (a) show the fluorine concentration and depth profiles with implantation [94]. Fluorine can easily penetrate the poly and HfO2 IPD film and react on their interface. For explain, Figure. 3.11 (b) showed F tends to segregate at HfO2/SiO2 interface after FGA [95]. In the reason, F diffuses toward Poly/HfO2interface as HfO2/SiO2 . By this SSFI method, interface was incorporated with fluorine, and reach to repair defect of crystal in the bulk of HfO2 IPD film. The effects include solving mobility degradation and threshold voltage instability, as well as reducing the number of dangling bonds and charge traps [96]-[99]. Figure 3.12 (c), (d) Control Gate adopted constant voltage stress (CVS) of HfO2 IPD under (c) positive 2V (d) negative 2V trap densities. Low traps slope can be explained fewer defect and better performance. The fluorine atoms into Hf-based films were caused Hf-F and Si-F bands so as to improve quality of the film. These stronger bonds improve Fowler Nordheim (FN) tunneling and smoother interface, which results in less interface states generation and reduced traps of electron and hole in both polarities. The results clearly reveal HfO2 IPD with polysilicon surface implantation, optimized control gate (CG) dosage was 1e15cm-2, which can significantly reduce trapping rate and less bulk
3.3.4 Reliability Characteristics
In this part, the effect of fluorine incorporation into the Poly and Poly/ HfO2
films on the charge-to-breakdown (QBD) distribution under Flowler-Nordheim (F-N) stressing was systematically studied. It was found that fluorine incorporation relaxes the lattices structure of interface, especially near Poly/ HfO2 interface. In addition, appropriate fluorine incorporation can improve the QBD distribution of stacked-gate flash memories device, too. A straight line symbolizes has better quality film. The charge-to-breakdown with Weibull slope can be explained fewer defect and better performance. We also discussed a possible mechanism for the QBD distribution improvement.
Figure 3.13 (a), (b), (c) , (d) were shown QBD Weibull distributions of HfO2
inter-poly capacitors with surface fluorine implantation at various fluorine dosages from FG and CG under constant current stress (CVS) in both polarities. Fluorine atoms were implanted into Poly and segregated into Poly and Poly/ HfO2 interface by activation, respectively. The most effectively of fluorine dosage is 5e13cm-2 and 1e14cm-2 in FG and CG, respectively, and more about one to two order magnitude of enhancement in QBD compared without fluorine passivation. Excess fluorine incorporation caused outdiffusion and degraded not only the reliability of Poly/ HfO2
interface but also dielectric-breakdown immunity, showed in floating gate 1e14cm-2. Figure 3.14 (a), (b), we found, that appropriate fluorine 5e13cm-2 incorporation into Poly/ HfO2 films (FG) could dramatically improve about one order QBD compared only poly (CG). HfO2 QBD as lower value as HfO2 leakage current density characteristics was compared to Al2O3 IPD film. The HfO2 IPD film lesser about two order in QBD, ascribed to two important focuses. First, HfO2 material is narrower
through electron thermal emission. Second, HfO2 IPD arising from unsaturated interface bonds and other electrically active imperfections [100].
3.3.5 Temperature Characteristics of the fluorine passivation
In this part, the carrier transportation of poly gate with and without fluorine incorporation characteristics will by investigating. Figure 3.15 (a), (b), (c), (d) were shown temperature dependence of gate current density at 1.73 MV/cm of HfO2
inter-poly capacitors with surface fluorine passivation that under floating gate and control gate both polarities. The FG and CG leakage current increase with increasing measuring temperature under both poly-gate incorporation of fluorine, show obvious high temperature dependence. As the fluorine implant dosage increases to 5e13cm-2 and 1e15cm-2 during FG and CG, respectively. The increase in the effectively trapping level is easily observed, meaning that most of the shallow traps in HfO2 IPD film with fluorine passivation can effectively resist leakage current density in higher temperature. Besides, the low leakage current density can ascribe the bulk defect really perfect with fluorine repair. Comparing the with fluorine and without fluorine passivation samples, we find, the poly gate leakage current density of the SSFI HfO2
IPD films is about one to two orders less than that of samples without any fluorine implantation in over 125℃. The schottky emission current of poly gate depended on temperature variation. However, because the barrier height extracted from Schottky
is more about two orders thermal stability compared to HfO2 films. Table 1.1 show both of HfO2 and Al2O3 IPDs have good thermodynamic stability with poly-Si in high temperature. The high heat of formation, Al2O3 films has the highest heat of formation (399 Kcal/mole) than HfO2 films (271 Kcal/mole), the mean are not be oxidized easily.
Besides, Al2O3 films for free energy chemical reduction function is (63.4 Kcal/mole) higher than that HfO2 films (47.6 Kcal/mole), causing Al2O3 IPDs more thermal stable about one order compare with HfO2 IPD. By the result, the thermal stability of HfO2
IPDs and Al2O3 IPDs both are much improved owing to the incorporation of fluorine.
3.4 Summary
In this thesis, we demonstrate that incorporation of SSFI method by N2 950℃
RTA activation after HfO2 IPDsdeposition remarkably improves inter-poly dielectrics behavior such as significantly less EOT, charge trapping rate and interface states generation. We found that fluorine tends to segregation into the poly/ HfO2 and poly interface and fluorine atoms diffusion into lattice generation strong Si-F, Hf-F and Al-F bonds compared to Si-H, Hf-H and Al-H. Both floating gate and control gate compare, we evidence that fluorine more effective passivation at poly/ HfO2 (FG), beside C-V, EOT and k value characteristics. Flash memories with Al2O3 IPDsand HfO2 IPDs can clearly exhibit significant improvement in data retention, leakage current and charge-to-breakdown (QBD) to replace conventional silicon oxide or oxide/nitride/oxide (ONO) IPDs. Table 3.1 lists several physical and electrical parameters, including EOT, κ-value, interfacial layer thickness, effective breakdown field and 63%-failure QBD values of the HfO2 IPD capacitors with surface fluorine passivation under positive and negative CVS at various fluorine dosage. After understanding flash memories trend need, the thesis is adopted as Al2O3 IPDs and
proved as promising candidates for the gate dielectrics of 45nm and 32nm generation stacked-gate flash memories device.
As the HfO2 IPD capacitors process had completed the work. We used high-resolution transmission electron microscopy (HRTEM) to confirm HfO2 IPD thickness and interfacial layer thickness. Before HRTEM, We must be cutting and digging a hole with focused ion beam (FIB). Figure.3.17 shows the FIB
As the HfO2 IPD capacitors process had completed the work. We used high-resolution transmission electron microscopy (HRTEM) to confirm HfO2 IPD thickness and interfacial layer thickness. Before HRTEM, We must be cutting and digging a hole with focused ion beam (FIB). Figure.3.17 shows the FIB