The control of SiO2-like interface between high-κ dielectrics and silicon substrate pays more and more important, since the device performances and reliability characteristics are strongly affected by the interface quality. Nitridation of the Si surface using N2O prior to the deposition of high-κ gate dielectrics has been shown to be effective in achieving the low EOT (equivalent oxide thickness) and preventing boron penetration [13], [14]. However this technique results in higher interface charges [15], which leads to higher hysteresis and reduced channel mobility.
Ozone-formed oxide (ozone oxide) has superior characteristics. Even when the formation temperature is less then 400°C, ozone oxide has a high film density comparable to that of the device-grade oxide film formed at higher temperature, a low interface trap density, and a much thinner structural transition layer near the SiO2/Si interface [16]. The ozone surface treatment was employed to improve the interface quality between HfAlO and silicon substrate.
2-5 Interface Treatments After HfAlO Deposition
The interface treatment can be performed with the surface treatment before metal deposition and it can be plasma treatment or reactive gas annealing. The interface treatment can change the texture bonding, and reduce defects such as dangling bonds at the interface between metal gate and dielectric as well as incorporation of extra impurities. The changes of interface and incorporation will upset a balance within the primary interface. According the past research the direct contact of high-k materials
and Si-substrate will have many issues. We need high quality interface of dielectric/Si or metal/dielectric with low density of interface states (Dit~2x1010 states/cm2) arising from unsaturated surface bonds and other electrically active imperfections. Interface states will lead to low on-current because of carrier mobility is limited by scattering at the interface with the vertical electric fields present in the channel. Therefore in this thesis we try two surface treatment including ozone plasma and NH3 plasma after HfAlO deposition to make the higher quality interface between metal gate and high-k dielectric, or change the texture bonding of HfAlO.
2-6 Electrical Measurement
The Capacitance-Voltage (C-V) and Current-Voltage (I-V) characteristics were measured by Hp-4284 and Hp-4156. The dielectric constant (k-value) is then calculated from the measured capacitance at accumulation mode. The equivalent oxide thickness (EOT) was extracted by fitting the measured high-frequency (100 kHz) capacitance-voltage (C-V) data under accumulation condition. UCLA CVC simulation program was utilized to obtain the accurate flat band voltage (VFB). The C-V hysteresis phenomenon was measured by sweeping the gate voltage from accumulation to inversion then back. The tunneling leakage current density-electric field (J-E) and the reliability characteristics of MOS capacitors were measured by semiconductor parameter analyzer HP 4156C.
2-7 Reliability Measurement
We observe the reliability from stress-induced leakage current (SILC),, measurements. SILC were measured at room temperature, gate bias=--4 for RTA after
surface treatment and -3V for RTA before surface treatment, the J-E curves were measured at 10 sec 20sec 50sec 100sec 200sec 1000sec respectively during stress.
Fig 2-1 The leakage current of ALD 30nm of HfO2 and HfAlO after rapid thermal annealing of 800000℃℃℃℃ and 950000℃℃℃ ℃
Gate voltage
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Gate current (A/cm2 )
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
PDA 800 for HfAlO PDA 950 for HfAlO PDA 800 for HfO2 PDA 950 for HfO2
Define Active region
RCA Clean with HF-last
Surface Treatment
High_k Deposition
Post Deposition Annealing
Poly Gate Deposition
Define Gate & Patterning
Ion implant
Define by Lithography and pattered
One is Ozone surface treatment and the other is N2O treatment.
RTA 800oC 60seconds was used.
A 200nm poly silicon was deposited by LPCVD.
Define by Lithography and pattered
Implant As 20KeV dose:5e15#/cm2
Activation RTA 950oC 30seconds was used and
S/D was formed.
Fig 2-2 The diagram of process flow of pMOS transistor
Define Contact hole
& Patterning
Aluminum Deposition
Define by Lithography then etching passivation oxide and high_k.
Define metal and etching meatal
500nm aluminum was deposited by thermal evaporation system.
Backside contact
Define by Lithography and pattered
500nm aluminum was deposited by thermal evaporation system.
Figure 2-3 The cross section of pMOS transistor.
Al Pad
Al Backside Contact Si Substructure
FOX FOX
Surface treatment High k Gate dielectric
S D
Poly Gate Passivation Oxide
Surface treatment by UV-ozone or NH3
Fig 2-4 The diagram and structure of process flow of capacitance Si-Substract
Chapter 3
Result and Discussion
3-1
The Electrical Characteristics of Surface Treatment Before HfAlO Deposition
Nitridation of the Si surface using N2O prior to the deposition of high-κ gate dielectrics has been shown to be effective in achieving the low EOT and preventing boron penetration [17], [18]. However this technique results in higher interface charges [19], which leads to higher hysteresis and reduced channel mobility. So we use UV ozone to oxidation, ozone and atomic oxygen, produced by exposure of atmospheric oxygen to ultraviolet radiation, Figure 3-1 showed the schematic diagram of UV ozone system. The ozone generator (AnserosPAP-2000) decomposed the oxygen molecular to generate ozone gas by high electrical field. The ozone gas was mixed with UV lamp in chamber. By changing the oxygen flow and ozone generation power, the ozone concentration in the chamber could be adjusted.
Ozone-formed oxide (ozone oxide) has superior characteristics. Even when the formation temperature is less then 400°C, ozone oxide has a high film density comparable to that of the device-grade oxide film formed at higher temperature [20], a low interface trap density [21], and a much thinner structural transition layer near the SiO2/Si interface [22]. The ozone surface treatment was employed to improve the interface quality between HfAlO and silicon substrate. Figure 3-2 shows the drain
current (Id) versus the drain voltage (Vd) characteristic for UV ozone surface treatment and N2O surface treatment. The drain current is larger for the UV ozone surface treatment. Fig 3-3 shows the transconductance characteristics, we observe that transconductance peak value of UV ozone treatment is higher than samples with N2O rsurface treatment. Figure 3-4 and Figure 3-5 shows the drain current (Id ) versus the gate voltage (Vg) of pMOSFETs with and UV ozone treatment and N2O treatment. We can see that the substhreshold swing (S.S.) of UV-ozone treatment is 75.26 mV/decade which is better than the 91.2mV/decade of N2O treatment.
In this section, we also research the relationship between charge pumping and mobility through devices in different surface treatment. Unlike SiO2 films, high-k films are more susceptible to charge trapping. Charge trapping is arguable one of the most important issue in CMOS devices with high-k gate dielectrics, because of the large amount of bulk traps present in the high k films [23]-[26]., and they may cause mobility degradation [27]-[30] and Vth instability [31]-[40], it used to use charge pumping to measure interface state density in MOSFET devices by utilizing the exclusion of gate leakage to calculate interface state density in high-k dielectric. We consider an p-channel device of gate length L and width W Evaluate mobility by Spilt-CV technique. Figure 3-7 shows electron mobility with UV ozone and N2O surface treatment measured by split-CV method. From the mobility we can find that UV ozone treatment have higher mobility, it may be due to the UV ozone treatment have lower Icp and Nit shows in Fig 3-6 and from Fig 3-8 we can make a conclusion that high interface states lead to low mobility. . We compare to this two samples on electrical characteristic above. We found that UV ozone treatment effectively improve the electrical characteristics, such interface state density, electric mobility, transconductance and etc.
3-2 Electrical Properties with Surface Treatment After HfAlO uuuDeposition
We use uv-ozone, NH3 two surface treatment after HfAlO deposition. Figure 3-9 shows the comparison of the C-V curves of the samples with UV ozone or NH3 surface treatment , both ozone and NH3 surface treatment will reduce leakage current , but samples with NH3 post treatment accompany annealing reduce more, in Figure 3-10, the comparison of J-V curve of the samples with UV ozone or NH3
surface treatment , the NH3 treatment samples have lower leakage with thinner EOT , to sum up, samples with NH3 treatment have large capacitance and lower leakage current with thinner EOT. It may be due to RTA after NH3 treatment will make N diffuss into the HfAlO and form HfAlOxN and make partial change of the local coordination from O-Hf-O to O-Hf-N. The increase of O-Hf-N component drastically degraded the gate leakage current in HfAlOx(N) film [41]. Fig 3-11 shows the leakage current after gate injection stress with UV ozone, and NH3 treatment , trapping is observed obviously in UV ozone treatment samples. It may be due to there are large fixed charge and interface states at interface, therefore the trapping is observed after gate injection stress. We also look from the other side ,Figure 3-12 shows the normalize C-V curve, The C-V curve of NH3 surface treatment was similar to that of HfAlO, indicating that no additional fixed charge was generatede.
3-3 Electrical Properties of Post Dielectric Annealing (PDA)
Fig 3-13 shows the capacitance-voltage curves of the as deposited HfAlO sample after NH3 surface treatment. There is clear that the scan from inversion to accumulation has a little shift with the scan from accumulation to inversion scan, this suggest that the interface quality is poor, and the film may be a mixing phase of amorphous and polycrystalline at 400℃ deposition, it may be have many imperfect bonds and defects due to incomplete crystal structure and it will lead to plentiful interface states exist at interface to be the trap centers and reduce the mobility.
Therefore we try post dielectric annealing at different temperature to see weather can be improving the quality of interface and lower the interface states. Fig 3-14 shows the C-V curve of HfAlO samples with 800℃ 900℃ 1000℃ PDA, and it is clear that the shift is reduced compare to the samples without PDA. PDA treatment at higher temperature after NH3 treatment improved the flat-band voltage shift. Besides improve the quality of interface, we also observe the PDA 1000℃ capacitance at accumulation is smaller than PDA 800℃, It suggests that the increasing thickness of interfacial layer would raise the CET and reduce the dielectric constant. Fig 3-15 shows the current density- voltage curve of the HfAlO samples with different PDA temperature 800℃ 900℃ 1000℃. Higher annealing temperature as 800℃ 900℃
lead to higher leakage due to high annealing temperature trigger small grains to merge into a large grain, and it will provide short leakage path with the boundaries around the large grains which lead the carries tunnel from top electrode to bottom electrode, Therefore the leakage current would increase. But for 1000℃ the leakage
therefore the curve will drop. Fig 3-16 shows the current- electrical field curves of the HfAlO with no PDA, We can observe that samples with no post dielectric annealing have more trap and the interface quality is not good lead to the obvious different with first scan and second scan, and electron trapping is saturated after the second injection. Fig 3-17 , shows the current-electrical field curves of the HfAlO with different PDA temperature, 800℃ 900℃ 1000℃, it is clear that the result of first scan and second scan are almost the same, therefore trapping is eliminated after annealing, it may due to high temperature can reduce defects in high-k, so PDA can suppress the trapping.
3-4 The Trapping Phenomenon of Observation Under SILC Stress
For Figure 3-18 ~ Figure 3-20 shows the J-E curves of as deposited samples and with, O3 and NH3 surface treatment under SILC stress. The result we compare samples with treatment and without treatment, that the trapping phenomenon after surface treatment is not severe as the as deposited HfAlO, therefore surface treatment can reduce trap generation under stress .In the other aspect we observed NH3 treatment is superior to UV ozone treatment in suppressing both electron, hole trappings, and interface trap creation under high-field stress. Interface hardness against hot-carrier bombardment and neutral electron trap generation are also improved.
3-5 Surface Composition Analysis by X-ray PPPPhotoelectron
SS
SSpectrometer
We use X-ray photoelectron spectrometer (XPS) to analysis composition of surface films, prior to and after periods of UV/ozone treatment, Fig 3-21 shows the Si(2p) binding energies before and after exposure to UV ozone, and after surface exposure to UV ozone, the XPS Si(2p) binding energy shifted toward that of SiO2
(103.6ev) [42],consistent with the formation of silicon that is coordinated to four oxygen atoms. The sampling depth of XPS, however, is only of the order of 7 Å and thus indicates near complete conversion to form a SiO2 surface layer that is at least 7 Å thick, and Figure3-22, Figure 3-23 show the electron binding energies of O(1s) and C xps spectra, The main O 1s binding energy is the Si-O type with a binding energy of 534.1 eV we can see the reduction in the amount of carbon and an increase in the amount of oxygen within the converted surface film, the reason for reduction of carbon is may be due to UV/ozone treatment removes up to 89% of the carbon from the resultant surface film [43], leading to an overall stoichiometry close to that of SiO2. Fig 3-26 shows the Hf (4f) binding energy with NH3 treatment accompany with RTA and w/o NH3 treatment, there is shift after NH3 treatment. This result suggests a partial change of the average local coordination structure from O-Hf-O to O-Hf-N after high temperature annealing with NH3 treatment because the elecronegativity of nitrogen is relatively low compared to that of oxygen. The results of the N 1s and O 1s XPS spectra also support this interpretation, shown in Fig 3-24, Fig 3-25. The main O 1s binding energy is the Hf-O type with a binding energy of
523.3 ev , therefore the film may turn to the HfAlOx(N) after NH3 treatment and RTA .
3-6 The Instruction of Leakage Current Conduction Mechanism
For the micro electrical devices, the goal for using insulating thin film is hope this film can insulate completely and without any leakage current, including TiO2 HfO2 SiO2, etc has larger band gap and maintain the structure of amorphous or polycrystalline to reduce leakage current, but on reality there are many physical mechanism can make carriers move in the insulting dielectric and form the leakage current, especially in the very thin film or electrical field is large.
There are two classes of leakage current mechanisms in the insulating thin film, one is electrode-limited conduction mechanism, it is determined by the character of emission electrode, for example Schottky emission [44] [45] field emission, it is also called tunneling, and thermionic field emission. Among them, tunneling is divided into direct tunneling and Fowler-Nordheim tunneling (F-N tunneling). The other is Transport-limited conduction mechanism, it is determined by the character of material, for example Ohmic conduction, Frenkel-Poole emission [46][47], Hopping conduction and ionic conduction. In this thesis we discussion and analysis the Schottky emission and F-P emission.
3-6.1 Schottky Emission
Under the function with electric field, the electron of metal cross the potential energy barrier from metal electrode to the conduction band of insulator, it is called Scottky emission. Consider a electron at x place from metal surface, use the law of virtual image, it will produce equal positive charge at –x place from metal surface,
the gravitation between electron and positive charge is called image force, the image force would lower energy barrier, it is Schottky effect Fig 3-27 Shows the diagram of Schottky emission, therefore electrons across the potential energy barrier easier via field-assisted and increase leakage current. The leakage current equation is showed in (eq.3-1)
[eq 3-1]
Where β= (e3 /4Πε0ε)1/2 ,A* is effective Richardson constant, ε0 is the permittivity of free space,εis the high frequency relative dielectric constant, KB is the Boltzmann constant(1.38 × 10-23 J/K), ψs is the contact potential barrier. We can find the slope of the leakage current equation. (eq 3-2), so we can draw the log(J/T2) versus E1/2 curve to see whether can receive a straight line to judge whether is the Schottky emission mechanism.
[eq 3-2]
3-6.2 Frenkel-Poole Emission
The theorem of Frenkel-Poole effect is similar to Schottky-emission, the different is that electron is excited from trap center to the conduction band of dielectric by way of thermal excitation via field-assisted, Fig 3-28 shows the diagram of
Frenkel-Poole emission Usually the dielectric with bigger energy band gap have larger probability to happen Frenkel-Poole emission.
In Fig 3-29, Fig 3-20 the conduction mechanism of the UV ozone-treated and NH3-treated HfAlO was extracted from fine Frenkel-Poole fitting. The current from the Frenkel-Poole
emission is of the form. where B is a constant related to the trapping density and carrier mobility in the HfAlO film, φB is the barrier height, Eeff is the effective electric field in the SiO2 film, ε0 is the free space permittivity, εHfAlO is the dielectric constant of HfAlO, k is the Boltzmann constant (1.38 × 10-23 J/K), Eact is a field-dependent effective activation , and T is the temperature (K)., From an Arrhenius plot of Eact [i.e., ln(J/Eeff) vs. (q/kT)], we can obtain Eact and B. The barrier height φB and dielectric constant εHfAlO of HfAlO can then be calculated from the intercept of the y axis and the slope of the fitting curves in the plot of Eact vs. Eeff , according
toEact =q aqπεkε0( Eeff )−qφB. As indicated in Fig.3-29, Fig 3-30 we obtained excellent linearity for each current characteristic. This tendency indicates that the Frenkel-Poole conduction mechanism is dominant in the Ozone-treated and
O2
N2
O3 Generator
O3 Destructor
UV Lamp PUMP
O3
Wafer O3
Hot plate PUMP
NH3-treated samples. It may be due to there are large tunneling effect in the interface.
We calculated the value the value of φB for electrons was 0.33 eV. and 0.46 eV
Figure 3-1 UV ozone system schematic diagram
Fig 3-3 The transconductance characteristic of HfAlO samples with UV ozone
a and N2O surface treatment
Fig 3-8 The relationship between mobility and Nit of HfAlO with UV ozone and N2O surface treatment
N2O O3
mobility (cm2 /v-s)
20 25 30 35 40 45 50
Nit(1011 )
5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2
Mobility
N it
Fig 3-9 The C-V curves of O3 and NH3 treatment with 800000℃℃℃℃ RTARTARTA after HfAlO RTA
Fig 3-11 The leakage current after a gate injection stress of UV ozone
and NH3 treatment. Electron trapping is observed in ozone treat samples
Gate voltage
-2.5 -2.0 -1.5 -1.0 -0.5 0.0
Gate current
10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4
O3 treatment first scan O3 treatment second scan O3 treatment third scan NH3 first scan
NH3 second scan
1
Fig 3-12 The normalize C-V curve of HFAlO with NH3 treatment and without treatment samples
Gate voltage (v)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
normalize capancitance
0.0 0.2 0.4 0.6 0.8 1.0
1.2
as deposited HfAlONH3 treatment first scan NH3 treatment second scan
Fig 3-13 The C-V curves of as deposited HfAlO
Fig 3-15 The J-V curves of HfAlO before PDA and after PDA Gate voltage
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Gate current (A/cm2 )
10-10 10-9 10-8 10-7 10-6
PDA 800 PDA 900 PDA 1000
EOT=18A
EOT~27A EOT~21A
Fig 3-16 The leakage current after gate injection stress of as-deposited HfAlO
Fig 3-18 The J-E curves of as deposited HfAlO samples under SILC stress
Fig 3-20 The J-E curves of HfAlO samples with NH3 surface treatment under SILC stress
under 16(MV/cm) stress
Electrical field (MV/cm)
-12 -10 -8 -6 -4 -2 0
Gate current (A/cm2 )
10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2
0sec 10sec 20sec 100sec 200sec 500sec 1000sec
Si (2p) Fig 3-21 The electron binding energies of Si 2p orbital before and after UV ozone surface treatment
Fig 3-22 The electron binding energies of O (1s) orbital before and after UV ozone surface treatment
C
Binding energy
275 280 285 290 295 300
intensity
uv-ozone treatment as deposited
Fig 3-23 The electron binding energies of C orbital before and after UV ozone surface treatment
Fig 3-25 The electron binding energies of N(1s) orbital with NH3 treatment with different RTA temperature
Fig 3-24 The electron binding energies of O (1s) orbital with NH3 treatment with different RTA temperature
O ( 1 s )
B i n d i n g e n e r g y ( e v )
5 2 0 5 2 5 5 3 0 5 3 5 5 4 0 5 4 5
intensity
a s d e p o s i t e d
w i t h N H3 t r e a t m e n t Hf-O
532.1~532.3 ev
N (1s)
X Data
380 385 390 395 400 405 410 415
as deposited with NH3 treatment
Fig 3-26 The electron binding energies of HF(4f) orbital with NH3 treatment with different RTA temperature
Binding energy
5 10 15 20 25 30
intensity
with NH3 treatment as deposited
16.4ev (O-HF-N)
O-Hf-O
Fig 3-27 Schematic band diagram of Schottky emission
Figure 3-28 Schematic band diagram of Frenkel-Poole emission
Metal Dielectric Substrate
Metal Dielectric Substrate
Figure
Fig 3-29 The conduction mechanism fitting of HfAlOsamples with UV ozone treatment .
Fig 3-30 The conduction mechanism fitting of HfAlOsamples with NH3
Fig 3-30 The conduction mechanism fitting of HfAlOsamples with NH3