In order to study the XPS spectra of Al2O3 dielectrics, the blanket wafers with Al2O3 deposition on top of the Poly-I annealed at different PDA temperatures are prepared. The detected binding energy of the Al 2p and O 1s signals are calibrated by C 1s signal (284.5eV) [130]. As shown in Fig. 7.8(a), the binding energy of the Al 2p signal is found to gradually increase, while the binding energy of the O 1s signal decreases as the PDA temperature increases to 900°C. This consequence suggests that the dielectric becomes more completely oxidized during high temperature O2
annealing (the electronegativity of Si, O and Al is 1.9, 3.44 and 1.61, respectively).
However, as annealed temperature reaches up to 1000°C, the O binding energy increases but the Al binding energy remains unchanged as compared to the 900°C-annealed IPD. After arranging the background signal to equivalent level, as seen in Fig. 7.8(b), O intensity gradually increases but Al intensity saturates when annealing temperature is higher than 900°C. With annealing temperature larger than 900oC, the oxidation of Al metallic atoms is nearly completed, excess O from annealing ambient turns to oxidize underneath Si substrate and form additional SiO2
interfacial layer, which is also proven by large EOT shown in Fig. 7.1(a). Both the turnovers of O binding energy and saturated Al intensity are strong evidence that large amounts of Si-O bonds are formed during 1000°C PDA.
Figure 7.9 presents the Al and O atomic concentrations extracted from XPS for various PDA temperatures. The O/Al ratio of as-deposited Al2O3 IPD is found to be
higher than 1.5 due to oxygen-rich dielectric deposition [131]. Initially, this non-stoichiometric value decreases as the PDA temperature increases [132]. The O/Al atomic ratio reduces to a nearly stoichiometric value of 1.52 at 900°C, but increases again at 1000°C due to Al loss [133]. Moreover, increased O/Al atomic ratio at 1000oC is partially ascribed to the interfacial SiO2 growth. According to the previous studies, the electrical properties of Al2O3 were reported to be intimately correlated to the film composition. To explain the variation of electrical properties when the annealing temperature increases from 800 to 900°C, including breakdown field and QBD, we resort to the changes in composition and surface roughness.
The Weibull distributions of breakdown fields for the samples are shown in Fig.
7.10. Sample with 800oC PDA has the largest effective breakdown field than 900oC- and 1000oC-annealed. Since the presence of excess free oxygen can suppress the aluminum rich defects and results in a larger breakdown field [134], the large breakdown field of 800oC-annealed Al2O3 IPD can be partially explained by high O/Al ratio. Furthermore, surface roughness also play a key role in determine breakdown field. Taking into account the increased surface roughness of Poly-I upon increasing annealing temperature, as indicated in Fig. 7.11, our result shows that the 800°C-annealed sample did have a higher breakdown field than the 900°C-annealed quite straightforwardly.
In contrast, the dependence of QBD on the annealing temperature shown in Fig.
7.5 is not consistent with the trend in breakdown field. This occurrence owes to the fact that breakdown is triggered once a critical electron trap density is reached based on the so-called percolation model [71]. The critical electron trap density is strongly dependent on the thickness and inherent physical properties of the dielectric.
Therefore, thicker IPDs coming from increasing the annealing temperature exhibit
higher Weibull slopes and QBD mean value. Moreover, an insufficient thermal annealing budget will result in excess oxygen presented in the bulk, which can act as efficient electron trap centers, making the dielectric more vulnerable to charge wear-out. As a result, the dielectric annealed at 900°C exhibits a better stoichiometric characteristic, smaller trapping rate, and lower trapped electron density, even though the effective breakdown is relatively small.
On the other hand, the situation with 1000°C annealing is more complex. There are several possible mechanisms responsible for the degradation. First, the amount of excess oxygen increases, acting as electron trap centers and degrades QBD. Second, the formation of extra grain boundaries from the dielectric crystallization during such high temperature annealing serves as additional electron trapping sites [135]. Based on X-ray diffraction (XRD) spectra shown in Fig. 7.12, we observe that after 900°C annealing, the Al2O3 film has crystallized. A diffraction peak associated with crystalline Al2O3 appears at about 32.9° after 900°C PDA, which corresponds to the
) 2 20
( plane for monoclinic Al2O3 [136]. Third, in Fig. 7.11, the measured surface roughness of Poly-I after selectively Al2O3 IPD etching was found to increase from 2.89nm for the 900°C annealed IPD to 3.26nm for the 1000°C annealed Al2O3 IPD. A rougher interface is more harmful due to the enhanced localized field which leads to higher leakage current, lower QBD and effective breakdown field [121]. Fourth, the Al2O3/Si system is not stable at an ambient of 1000°C, and severe intermixing has been observed in previous research [137]. Figure 7.13 demonstrates the AES depth profiles for Al, O, N and Si elements. Clearly, the inter-diffusion between the Al2O3
IPD and underlying Poly-I becomes more notable when annealed at 1000°C. The existence of these Al-Si-O mixtures may act as positive fixed charge centers [138] and facilitate the trapping of electrons. Fifth, phosphorous diffusing from poly-Si may act
as a network modifier and produce non-bridging oxygen, resulting in increased atomic ratio as the PDA becomes higher than 900°C [139]. Consequently, the IPD annealed at 1000°C not only exhibits a slightly larger leakage current, but also a higher electron trapping rate and a smaller QBD than when annealed at 900°C.
7.4 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. 900°C annealing is the best condition for the Al2O3 IPD electrical characteristic in terms of leakage current, electron trapping rate and QBD. The XPS and AES analyses indicate that this consequence is closely related to the compositional changes and excess oxygen concentration when changing annealing temperature. The results apparently demonstrate Al2O3 IPD with surface nitridation and optimized PDA temperature 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 and 900°C annealing thus appears to be very promising for future flash memory devices. Table 7.1 lists several physical and electrical parameters, including EOT, Poly-I surface roughness and the 63%-failure QBD values of the Al2O3 IPDs with surface NH3 nitridation annealed at various temperatures. κ-value, interfacial layer thickness and extracted ϕB of the Al2O3 inter-poly capacitors with surface NH3
nitridation at various PDA temperatures in O2 ambient are summarized in Table 7.2.
Table 7.1 EOT, Poly-I surface roughness 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 O2 ambient. Due to the extremely large leakage current, the corresponding EOT and 63%-failure QBD values of the as-deposited Al2O3
inter-poly capacitors can not be determinable.
Poly-I surface 63% QBD (C/cm2) PDA (°C) EOT (Å)
roughness (nm) positive negative
as-deposit N/A 2.66 N/A N/A
800 46 2.76 0.065 0.171
900 53 2.89 0.288 0.861
1000 63 3.26 0.257 0.728
Table 7.2 κ-value, interfacial layer thickness and extracted barrier heights of the Al2O3 inter-poly capacitors with surface NH3 nitridation at various PDA temperatures in O2 ambient.
ϕB (eV) PDA (°C) κ IL Thickness (Å)
positive negative
800 8.2 8 1.54 1.62
900 9.3 11 2.18 2.24
1000 9.4 20 2.10 2.15
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Effective Breakdown Field ( MV / cm ) 800oC
900oC 1000oC
(b)
Fig. 7.1 (a) C-V curves (b) J-E characteristics of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient. Al2O3 inter-poly capacitor with 900oC PDA in O2 ambient is beneficial in scaling EOT and suppressing low-field leakage current density.
1 10 100 Closed : Negative VG Normalized DR Current ( A / ( nFV ) -1 )
Fig. 7.2 (a) As-fabrication trap densities evaluation at 2 MV/cm constant voltage stress (CVS) (b) dielectric relaxation current of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient. Al2O3 inter-poly capacitor with 900oC PDA in O2 ambient can reduce as-fabricated trap densities.
0 10 20 30 40 50 capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient under constant current stress. Al2O3 inter-poly capacitors with optimized 900oC PDA can suppress electron-trapping and increase QBD.
0 2 4 6 8 10 12 14 16
Equivalent Oxide Thickness ( nm )
Optical Al2O3 Thickness ( nm )
Fig. 7.4 κ-value and IL thickness extraction of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient.
Closed : Negative VG
Current Density @ 6MV / cm ( A / cm2 )
Fig. 7.5 Temperature dependence of gate current density at 6 MV/cm of Al2O3
inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC.
0 1 2 3 4 5 6 7
Trapping Centroid from Cathode, X t / EOT
Charge Fluence ( 1016 cm-2 )
Trapped Charge Density ( 1012 cm-2 )
Charge Fluence ( 1016 cm-2 ) (b)
Fig. 7.6 (a) Centroid of trapped charges (b) trapped charge density of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient under constant current stress. Al2O3 inter-poly capacitors with optimized 900oC PDA can suppress electron trapping rate below 10-4.
(a)
(b)
Fig. 7.7 Band diagrams of Al2O3 inter-poly capacitors with surface NH3 nitridation under (a) positive (b) negative gate voltage biased to the Poly-II. Al2O3 inter-poly capacitors at negative polarity show less electron trapping and gate voltage shift.
Poly-I e
-Xt
Poly-II +
∆VG
N N N N N
Al2O3 IPD
Poly-II e
-Poly-I Xt
-∆VG N
N N N N Al2O3 IPD
535 530 75 70 1000oC
900oC 800oC As-dep.
Al 2p O 1s
Al 2p Intensity ( arb. unit ) O 1s Intensity ( arb. unit )
XPS Binding Energy ( eV ) (a)
535 530 75 70
O 1s Intensity ( arb. unit )
XPS Binding Energy ( eV ) As-dep.
800oC 900oC 1000oC
Al 2p Intensity ( arb. unit ) Al 2p
O 1s
Increased PDA Temp.
(b)
Fig. 7.8 (a) XPS binding energy spectrum (b) corresponding XPS binding energy spectrum after arranging to the equivalent background signal for the O 1s and Al 2p signals as a function of PDA temperatures with C 1s calibration at 284.5 eV. The binding energy of O and Al signals is strongly dependent on PDA temperature.
25
Al, O Atomic Concentration ( % )
PDA Temperature ( oC )
O 1S / Al 2P Atomic Ratio
Fig. 7.9 Al and O atomic concentrations extracted from XPS as a function of PDA temperatures with C 1s calibration at 284.5 eV.
-3 Closed : Negative VG
Effective Breakdown Field ( MV / cm )
ln ( - ln ( 1-F ) )
800oC 900oC 1000oC
Fig. 7.10 Weibull plots of effective breakdown field of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient.
Fig. 7.11 AFM images (5µm×5µm) of the poly-I surface of Al2O3 inter-poly capacitors with surface NH3 nitridation for (a) as-deposited (b) 800oC (c) 900oC (d) 1000oC PDA in O2 ambient. Surface roughness becomes more severe as PDA temperature increasing.
As-deposited
RMS = 2.66 nm 800oC PDA
RMS = 2.76 nm
(a)
(c)
(b)
(d) 900oC PDA
RMS = 2.89 nm 1000oC PDA
RMS = 3.26 nm
20 30 40 50 60 800oC PDA
900oC PDA 1000oC PDA
( 111 ) ( 202 )
Intensity ( arb. unit )
2θ ( Degree )
Fig. 7.12 XRD spectra for aluminum oxide on Si(100). Al2O3 IPD is crystallized while PDA temperature larger than 900oC.
0 100 200 300 400 500
N Al Si
O
Counts ( arb. unit )
Sputtering Tim e ( sec )
800oC 900oC 1000oC
Fig. 7.13 AES depth profiles of Al2O3 IPD with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient. The signal of N is magnified by 5 times.
CHAPTER 8
Thickness Scaling and Reliability Improvement of High-κ IPD for Next Decade Stacked-Gate Flash Memories
8.1 Introduction
Recently flash market increases exponentially and especially mass storage applications have took-off in addition to code storage area. In the coming decade, we will be required to provide flash technologies that are compatible with embedded DRAM read/write speed with high retention. In order to meet the requirement, not only to reduce leakage current through inter-poly dielectric (IPD) and tunnel oxide, but also to improve electric field and charge-to breakdown of the dielectrics. Previous, we had presented the effects of surface NH3 nitridation and post-deposition annealing temperature on reactive-sputtered Al2O3 IPD characteristics in Chapter 6 and 7, respectively. However, even after process optimization, the breakdown charges of Al2O3 IPD deposited by reactive sputtering are relatively low [22], [23]. In this chapter, inter-poly dielectric thickness scaling and reliability characteristics are studied on the Al2O3 and HfO2 IPDs, and compared with TEOS IPD. Drastically leakage current reduction and reliability improvement has demonstrated by replacing TEOS IPD by high-κ IPDs, which are suitable to apply for mass production in the future.
8.2 Experimental Details
The n+-polysilicon/High-κ 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 5×15cm-2, 20keV, then activated with RTA at 950°C for 30s. Prior to the deposition of high-κ IPDs, Poly-I was cleaned by the conventional RCA cleaning and diluted HF etching in sequence for the removal of particles and native oxides, then subjected to ammonia (NH3) nitridation in the LPCVD furnace at 800°C for 1hour. Sub-10nm Al2O3 and HfO2 IPDs was deposited either by reactive sputtering (RS) in an Ar/O2
ambient at room temperature or by metal organic chemical vapor deposition (MOCVD) with O2 gas at 400oC. Post-deposition annealing of Al2O3 and HfO2 IPDs was carried out by rapid thermal annealing at 900oC and 800oC, respectively, for 30s.
Subsequently, a 2000Å top polysilicon layer (Poly-II) was deposited by LPCVD and implanted with phosphorous at 5×15cm-2, 20keV. Dopants were then activated with 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 Al2O3 inter-poly capacitor with surface NH3 nitridation and post-deposition oxygen annealing are shown in Fig. 4.1 and 5.2, respectively. Tera-ethyl-ortho-silicate (TEOS) IPD deposited at 700oC LPCVD followed by 900oC annealing was used for references.
The equivalent oxide thickness (EOT) was obtained from the high frequency (100kHz) capacitance-voltage (C-V) measurement using a Hewlett-Packard (HP) 4284 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. Table 8.1 summarized the deposition conditions and EOT of the various IPDs deposited either by LPCVD, RS or MOCVD.
8.3 Results and Discussions
Figure 8.1 plots leakage current density at VG = 5V for various IPD candidates.
Leakage current density is increasing as EOT of IPD scaling down. Comparing to TEOS IPD, RS Al2O3 IPD can reduce leakage current density larger than 1 order of magnitude in both polarities. Further leakage current reduction can be achieved by changing RS Al2O3 IPD to MOCVD Al2O3 and HfO2 IPD. Larger than 2 and 3 order of magnitude leakage current reduction comparing to TEOS IPD is expected by alternative deposition instrument.
Breakdown voltage (BV) comparison between high-κ IPDs and TEOS IPD is shown in Fig. 8.2. As EOT scaling down, BV decreases. RS Al2O3 IPD can provide larger than 1V BV increment than TEOS IPD in both polarities, although the improvement seems to vanish for EOT less than 5nm. Comfortingly, MOCVD Al2O3 IPD exhibits larger than 10V BV, which is nearly twice of TEOS IPD. While adopting to HfO2, BV can maintain larger than 6V as the EOT = 3.3nm regime. After eliminating thickness effect, effective breakdown electric field (BEeff) is still higher than TEOS IPD, presented in Fig. 8.3. BEeff is extended to larger than 2 MV/cm and 8 MV/cm for RS and MOCVD Al2O3 IPD, respectively. Both MOCVD Al2O3 and HfO2 IPD can stand for as high as 19 MV/cm breakdown field, which are enough to be
applied for next decade flash memory.
In the dielectric reliability point of view, high-κ IPDs also examine superior charge-to-breakdown (QBD), as drawn in Fig. 8.4. Our results indicate QBD-value of IPDs is strongly dependent on dielectric thickness. From percolation model [71], the critical electron trap density is strongly dependent on the thickness and inherent physical properties of the dielectric. Therefore, thicker IPDs exhibit higher breakdown voltage and QBD mean value, contribute to bulk-dominant dielectric breakdown in both polarities. RS Al2O3 IPD reveals near 1-order of magnitude QBD improvement than TEOS IPD in both polarities. On the other hand, albeit that RS Al2O3 IPD can tolerate higher QBD than TEOS IPD, the maximum QBD observed is only ~ 1 C/cm2, which may be a another major challenge for RS Al2O3 IPD to be used to replace conventional IPD. Insufficient QBD becomes more severe as EOT scaling down due to bulk-dominant dielectric breakdown, predicted by percolation model. Fortunately, QBD can be drastically improved by MOCVD tool. The 63% failure QBD of MOCVD Al2O3 IPD is about 5.5 C/cm2 in positive polarity, which is near 1 order of magnitude improvement comparing to RS Al2O3 IPD. Nevertheless, negligible QBD improvement is observed for negative polarity. Mechanism of significantly polarity-dependent QBD
of MOCVD Al2O3 IPD is unknown yet and need further inspection. For MOCVD HfO2 IPD, the calculated QBD is relatively poor partially from thinner EOT than Al2O3
IPD. Breakdown voltage, effective breakdown field and QBD values of the various IPDs deposited either by LPCVD, RS or MOCVD are listed in Table 8.2.
Although the real mechanism for dielectric characteristics promotion of MOCVD IPDs is not clear yet, one possible explanation is speculated from less defect density and uniform thickness of MOCVD deposited dielectrics. Consequently, EOT of IPD can be further scaled down to 3nm by HfO2 IPD without degradation dielectric
characteristics significantly, which is the thinnest EOT of IPD for stacked-gate flash memory in our knowledge.
8.4 Summary
Previous, we had presented the effects of surface NH3 nitridation and post-deposition annealing temperature on reactive-sputtered Al2O3 IPD characteristics in Chapter 6 and 7, respectively, and found an optimized 900oC PDA with surface nitridation could significantly improve IPD reliability. However, even after process optimization, the quality of Al2O3 IPD deposited by RS still can not meet the stringent requirement due to relative low QBD. In this chapter, dielectric characteristics including, leakage current density, effective breakdown field and QBD of Al2O3 IPD are compared with LPCVD TEOS IPD and MOCVD Al2O3 and HfO2 IPD. The results clearly indicate high-κ IPDs, regardless of deposition tools, exhibits high potential to replace TEOS IPD. Moreover, MOCVD deposition demonstrates significant reliability improvement compared to RS deposition. The QBD can be significantly improved as well as reduced leakage current density, enhanced breakdown voltage and effective breakdown field by using MOCVD replacing RS. According to the 2003 ITRS roadmap, the required low-field leakage current should be less than 1 mA/cm2
@ 5V [28]. Our results clearly demonstrate that as thin as 5nm and 3nm EOT of MOCVD-deposited Al2O3 and HfO2 IPD is suitable to meet the requirement of 45nm and 32nm generation stacked-gate flash memories, respectively.
Table 8.1 Deposition conditions and EOT of the various IPDs deposited either by LPCVD, RS or MOCVD.
IPD Material Surface Nitridation PDA (°C) EOT (Å)
TEOS without 900 65
RS Al2O3 with 900 47
RS Al2O3 with 900 53
RS Al2O3 with 900 63
MOCVD Al2O3 with 900 55
MOCVD HfO2 with 800 32
Table 8.2 Breakdown voltage, effective breakdown field and 63%-failure QBD values of the various IPDs deposited either by LPCVD, RS or MOCVD.
BV (V) BEeff (MV/cm) 63% QBD (C/cm2) IPD Material
positive negative positive negative positive negative TEOS 8.5 7.5 13.08 11.54 0.103 0.067 RS Al2O347Å 5.6 5.9 11.9 12.55 0.159 0.619 RS Al2O3 53Å 6.9 7.6 13.0 14.3 0.287 0.861 RS Al2O3 63Å 8.2 8.9 13.0 14.1 0.554 1.151 MOCVD Al2O3 10.4 10.6 18.9 19.3 5.491 0.891
MOCVD HfO2 6.2 6.2 19.4 19.4 0.191 0.023
20 30 40 50 60 70 80 90 100
LPCVD TEOS Open : Positive VG Close : Negative V
Fig. 8.1 Current density at 5V as a function of EOT for various IPDs. High-κ IPDs can reduce leakage current larger than 1-order of magnitude.
20 30 40 50 60 70 80 90 100 Close : Negative VG Al2O3 : PDA 900oC HfO2 : PDA 800oC
Breakdown Voltage ( V )
EOT ( A )
Fig. 8.2 Breakdown voltage as a function of EOT for various IPDs. High-κ IPDs exhibits higher breakdown voltage than TEOS IPD.
20 30 40 50 60 70 80 90 100
Effective Breakdown Field ( MV / cm )
EOT ( A )
Fig. 8.3 Effective breakdown field as a function of EOT for various IPDs. High-κ IPDs exhibits higher breakdown field than TEOS IPD.
20 30 40 50 60 70 80 90 100 Close : Negative VG Al2O3 : PDA 900oC HfO2 : PDA 800oC Charge to Breakdown ( C / cm2 )
EOT ( A )
Fig. 8.4 Charge-to-breakdown as a function of EOT for various IPDs. High-κ IPDs exhibits higher QBD than TEOS IPD.
CHAPTER 9
Conclusions and Recommendations for Future Works
9.1 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, N atomic concentration is shown to
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, N atomic concentration is shown to