Chapter 2 Application of Supercritical Fluid Technology on Silicon-Oxide
2.4 Summary
2.2.4 Auger Electron Spectroscopy (AES) Analysis
In order to analyze the composition of the silicon oxide film after various post-treatments, including pure SCCO2 fluids, H2O vapor and 3000 psi-SCCO2
treatment, we performed the Auger electron spectroscopy analysis. The first group labeled as pure SCCO2 fluids treatment, was designed as the control sample, and was only treated in pure SCCO2 fluid at 150 °C for 2 hrs. The second group labeled as H2O vapor treatment, was immersed into a pure H2O vapor ambience at 150 °C for 2 hrs in a pressure-proof stainless steel chamber with a volume of 30cm3. The third group marked as 3000psi-SCCO2 treatment, was placed in the supercritical fluid system at 150°C for 2 hrs, where was injected with 3000psi of SCCO2 fluids mixed with 5 vol.%
of propyl alcohol and 5 vol.% of pure H2O. The ethyl alcohol plays a role of surfactant between nopolar-SCCO2 fluids and polar-H2O molecules, so that the H2O molecule uniformly distributes in SCCO2 fluids and be delivered into the SiOx film for passivating defects. As shown in Figure 2-8, the pure SCCO2 fluids-treated and H2O vapor-treated films has oxygen composition lower than that of the silicon oxide film after 3000 psi-SCCO2 treatment. The ethyl alcohol plays a role of surfactant between nopolar-SCCO2 fluids and polar-H2O molecules, so that the H2O molecule uniformly distributes in SCCO2 fluids and be delivered into the SiOx film for passivating defects.
2.3 Analysis of Electrical Characteristics and Discussion
2.3.1 The current density-electric field (J-E) characteristics
The plot of leakage current density of SiOx films versus electric field is displayed in Fig2-10. to realize the influence of different treatment on dielectric characteristics.
Among various post-treatments, the pure SCCO2 fluids SiOx film exhibits the most serious leakage current, inferentially due to its poor dielectric characteristics with numerous traps inside the SiOx film. The improvement of electrical characteristics is observed by using H2O vapor process, however, a high leakage current density still appears at larger applied voltages. It could be inferred reasonably dependent on the defect passivation efficiency. The most indicating that H2O vapor can passivate the traps (or defects) and alter dielectric properties of the low-temperature-deposited SiOx
film. After H2O vapor treatment, effective improvement of electrical characteristic is obtained by the 3000 psi-SCCO2 treatment, exhibiting the lowest leakage current density among all samples. Low leakage current density (less than 10-7 A/cm2). is kept constantly, even biased at an electric field of 3 MV/cm. The electrical performance agrees with FTIR analysis, in which 3000 psi-SCCO2 treatment modified SiOx
dielectrics even effectively.
2.3.2 Conduction Mechanism
There may be different conduction mechanisms in the insulator thin film, including Schottky-Richardson emission [23], Frenkel-Poole emission [23,24], Fowler-Nordheim tunneling [23,24], and trap assisted tunneling [25,26] illustrated in Fig 2-9. The Schottky-Richardson emission generated by the thermionic effect is caused by the electron transport across the potential energy barrier via field-assisted lowering at a metal-insulator interface. The leakage current governed by the Schottky-Richardson emission is as following:
A T exp E k T
JSR * 2 SR 12 SR B
where
0
12 find the slope of the leakage current equation.
The Frenkel-Poole emission is due to field-enhanced thermal excitation of trapped electrons in the insulator into the conduction band. The leakage current equation is:
k is the Boltzmann constant. We can find the slope of the leakage current equation. B
potential barrier. Tunneling is a quantum mechanical process similar to throwing a ball against a wall often results that the ball goes through the wall without damaging the wall or the ball. It also loses no energy during the tunnel event. The probability of this event happening, however, is extremely low, but an electron incident on a barrier typically several nm thick has a high probability of transmission. The Fowler-Nordheim tunneling current IFN is given by the expression [27]:
FN ox
where m is the effective electron mass in the oxide, m is the free electron mass, ox q is the electronic charge, and is the barrier height at the silicon-oxide interface B given in units of eV in the expression for BFN. is actually an effective barrier B height that take into account barrier height lowering and quantization of electrons at the semiconductor surface. Rearranging IFN formula gives by:
FN
FN ox through the oxide is pure Fowler-Nordheim conduction [27].In the trap assisted tunneling model, it is assumed that electrons first tunnel through the SiOX interfacial layer (direct-tunneling). Then, electrons tunnel through traps located below the conduction band of the high-k thin film and leak to substrate finally [25]. The equation of leakage current density is [26]:
ox
The plot of the nature log of leakage current density versus the square root of the applied electric field was observed. It is found that the leakage current density is linearly related to square root of the applied electric field. The linear variations of the current correspond either to Schottky-Richardson emission or to Frenkel-Poole
conduction mechanism. For trap states with coulomb potentials, the expression is virtually identical to that of the Schottky-Richardson emission. The barrier height, however, is the depth of the trap potential well, and the quantity FP is larger than in the case of Schottky-Richardson emission by a factor of 2.
Leakage conduction mechanism is also investigated to support the comments on the electrical improvement of SiOx film. Fig. 2-11(a) plots ln (J/E) versus reciprocal of electric field variation for the pure SCCO2 fluids treated SiOx film, and a schematic energy band diagram accounting for leakage transport mechanism shown in the inset.
A good linear fitting explains Fowler-Nordheim (F-N) tunneling [28] occurs in the electric fields higher than 1.2 MV/cm. Also, it is consistent with the electrical behavior of pure SCCO2 fluids treated SiOx film in Fig. 2-10 that leakage current density sharply increases, while gate bias voltage larger than 1.2 MV/cm. This could be attributed to the trap-assisted tunneling due to numerous traps inside the 150°C- pure SCCO2 fluids treated SiOx film [29]. For the 3000 psi-SCCO2 treated SiOx film, a plot of leakage current density versus the square root of the applied field (E1/2) gives a good representation of the leakage behavior at high electric fields, as shown in Fig. 2-11(b).
The leakage current density of the 3000 psi-SCCO2 treated SiOx is linearly related to the square root of the applied electric field, demonstrating Schottky-Richardson emission transport mechanism [30]. The Schottky-type conduction can be verified by comparing the theoretical value of
0
123
SR q 4
with the calculated one obtained from the slope of the experimental curve ln J versus E1/2 [31], where q is the electronic charge, the dielectric constant of free space, 0 is the high frequency relative dielectric constant. The Schottky emission generated by the thermionic effect is caused by electron transport across the potential energy barrier via field-assisted lowering at a metal-insulator interface, shown in the insert of Fig. 2-11(b), and independent of traps. From the slope of ln J versus E1/2, the calculated value of relative dielectric constant ( ) is 3.5, and which is close to the determined value of 3.8 in capacitance-voltage (C-V) measurement (referring to table 2-3). This also proves, for 3000spi-SCCO2 treated SiOx film, the conduction mechanism is really Schottky
emission, but not trap-dependent Poole-Frenkel emission [31]. Additionally, the evolution of conduction mechanisms from trap-assisted tunneling to Schottky emission can confirm these defects inside low-temperature-deposited SiOx film is minimized effectively by implementing the proposed SCCO2 technology. The leakage current densities of SiOx films after different treatments are shown as a function of applied positive gate bias voltage in Fig. 2-12, and the lower leakage current still could be acquired after 3000 psi-SCCO2 and H2O vapor treatment, especially treated with SCCO2 fluids. This could be attributed to the influence of traps in the interface between parasitical SiOx and Si wafer. Generally, in positive gate bias, the sources of electron are (1) the interface states, (2) defects in depletion region, (3) back electrode of substrate, [33] and the later two source are negligible due to the p-type signal-crystal Si wafer is used in this work. For pure SCCO2 fluids treated SiOx film, the great quantity of interface states still exist which generate electron-hole pair and lead to higher leakage current, as described in the inset of Fig. 2-12 After 3000 psi-SCCO2 treatment, the interface states were deactivated, hence the leakage current is reduced. The reduction of interface states would be proved in capacitance-voltage measurement. [36]
2.3.3 The capacitance-voltage (C-V) characteristics
The capacitance-voltage (C-V) characteristics are also generally used to judge the quality of dielectric films. Figure 2-13 shows capacitance-voltage characteristics of SiOx films after different treatment, measuring at 1M Hz with gate bias swing from negative voltage to positive voltage (forward) and from positive voltage to negative voltage (reverse). The slope of C-V curve in transient region, i.e. from Cmax to Cmin, is relative to the interface states, for example, the sharp slope indicates fewer defects exist in the interface between SiOx and Si wafer. In Fig. 2-13, the pure SCCO2 fluids -treated SiOx film presents the worst C-V curve and lower capacitance. This expresses the number of interface states exist and lead to the smooth C-V curve. Additionally,
the lower dielectric constant, as shown in table 2-3, could be referred to the influence of defects in SiOx film. With H2O vapor treatment, the sharper C-V curve and higher capacitance are obtained, and it could be attributed to the reduction of defects in SiOx
film and the interface. Furthermore, the best improvement is achieved by 3000 psi-SCCO2 treatment. This exhibits that the SCCO2 treatment possesses excellent ability to passivate the defects, including Si dangling bonds and interface states.
Besides, from Fig. 2-13, the shift of C-V curve under forward and reverse swing is also appears in pure SCCO2 fluids and H2O vapor-treated SiOx films. It is resulted from the trapped carrier in defects of SiOx films, and that is not expected for gate insulator of transistors. Under negative gate bias, the electric inject from Al gate into SiOx films and trapped by defects, leading to the larger gate bias is required for inducing electron-inversion layer. For describing clear, These results conform to the tendency in current-voltage characteristics and again verify that the SCCO2 technology could effectively deactivate defects in SiOx films.The main reason could be referred to the positively charged Si dangling bonds are passivated as a matter of fact, upon reducing the oxide thickness, it is difficult to calculate the density of interface states by using the high-low frequency method because of the substantially increased gate leakage current.
2.3.4 Breakdown voltage measurement and gate bias stress
Figure 2-14 show the breakdown characteristic curves of SiOx films after various treatments at negative gate bias region, individually. The breakdown voltage is mainly relative to the qualities of dielectric films and the density of defects in the dielectric films. A large number of traps lead to the trap-assisted tunneling early occurs and a high leakage current appears at small electric field, such that the lower breakdown voltages of dielectric films comes up. In Fig. 2-15, whether at negative gate bias, the pure SCCO2 fluids -treated SiOx film presents the worst performance in breakdown voltage because of the high density of defects, and the improvements of
breakdown voltage are gradually achieved via H2O vapor and 3000 psi-SCCO2
treatment. This result exhibits clearly that the density of defects in SiOx films are effectively reduced, and the breakdown voltage of 5~7nm SiOx film thereby could be substantially ameliorated from 1 V to 10 V at negative gate bias. It also indicates that the SCCO2 fluids technology is greatly useful to enhance the low-temperature deposited SiOx films by passivating defects, and allows the treated SiOx film holding good reliability as the gate dielectric.
Another important property of dielectric films is the reliability under gate bias stress. Due to the gate dielectric is stressed at a high field when the transistors are operating, so that it is demanded for gate dielectric to have excellent resistance to the impairment under long time stress at operating electric field. During high electric field stress, the carriers of leakage current and high electric field would impact the weak bonding, leading to more defects, higher leakage current and the degradation of transistor [32]. Therefore, the reliability of dielectric under gate bias stress would judge whether agrees with the application of gate dielectric. Figure 2-15 shows the variation of leakage current of different-treated SiOx films as a function of stress time at a high electric field = 3.8 MV/cm, where I0 is the initial leakage density. As well as the tendency of the measurement of breakdown voltage, the pure SCCO2 fluids -treated SiOx film behaves the most rises in the degree of leakage current as the stress time increasing, because of the great amount of defects and weak bonding. However, after treating with 3000 psi-SCCO2 process, the e-gun-deposited SiOx film performs a fine reliability under high electric field stress, hence it is extremely suitable for the application of gate dielectric.
2.4 Summary
we have well improved the dielectric characteristics of e-gun deposited SiOx films at 150 °C. From experimental results, the H2O molecule is operative to react with Si dangling bonds, and the amount of S-O bonding in e-gun deposited SiOx film increased obviously after H2O vapor treatment. The preliminary improvement on electrical properties of SiOx film was achieved due to the passivation of traps. A further study also demonstrated that SCCO2 process mixed with co-solvent is optimum method to improve the dielectric characteristics of SiOx film. In virtue of the gas-like and liquid-like properties, it is allowed for SCCO2 fluid to transport H2O molecules efficiently into SiOx film and more of traps thereby were terminated. Additionally, the hysteresis-free in C-V curve was obtained, and it perhaps was a result of the removal of ion charges by the proposed SCCO2 process.
Chapter 3
Application of Supercritical Fluid Technology on Resistive Random Access Memory
3.1 Fabrication of Non-volatile Memories and Experiment Process
Since the first observation of bistable resistance states in the 1960’s, reversible and reproducible resistance switching phenomena caused by applied electric field have been investigated widely to be used as resistive random access memories (RRAM).
Recently, numerous metal oxides and perovskite oxides including Nb2O5, TiO2, and Nb-doped SrTiO3 have been reported for RRAM applications. Nevertheless, there is only few studies mention the process of resistance memory at low temperature. the SCCO2 fluid is proposed to fabricate the resistance memory at 150 °C, so it would be applicable to product RRAM on the substrates with low glass transition temperatures (Tg). The supercritical fluid technology is generally used for the impurity extraction, dehydration and drying of materials with a fine structure. It also had employed SCCO2
fluid to deliver oxidant into metal oxide film for terminating electric defects by the gas-like and liquid-like properties. According to the mechanisms of bistable resistance states are possibly dominated by some kind of charge trap, the we would employ SCCO2 technology to fabricate a resistance memory by varying the trap density in SiOx
film. The experimental works will focus on the effect of passivating traps by SCCO2
treatment and investigate the influence of varying trap density on resistance switching phenomena.
Using the e-gun evaporation deposition method, the average thickness of 5-7 nm SiOx films using pure SiO2 target were directly deposited on p-type (100) silicon substrates having a resistivity of 1–10 Ωcm. During the deposition process, the chamber pressure and the substrate temperature were maintained at 2 × 10-6 torr and 25 °C, respectively. These SiOx films were split into three groups and treated with different methods. The first group was placed in supercritical fluid system at 150 °C for 2 hrs, where was full of 3000 psi-SCCO2 fluid, and taken as the control sample.Because the un-treated SiOx simple and pure 3000 psi-SCCO2 fluid were the same in current density-electric field characteristics.The second group was immersed into a pure H2O vapor ambience at 150 °C for 2 hrs, in a pressure-proof stainless steel chamber. The third method was treated by 3000 psi-SCCO2 fluid mixed with 8 vol.%
ethyl alcohol and 2 vol.% H2O, where the H2O is applied as oxidant to passivate electric defects in SiOx film.The ethyl alcohol acted as a role of surfactant between nonpolar SCCO2 fluid and polar H2O molecule for making the uniform distribution of H2O molecule in SCCO2 fluids, so that the H2O molecules could be effectively delivered into SiOx layer by SCCO2 fluids. After different treatments, the circle-shaped electrode of Al were thermally evaporated onto the surface of treated SiOx films through a shadow mask to from the metal insulator semiconductor (MIS) structure, and the bottom electrode of Al were deposited onto the backside of p-type silicon substrates. The current-voltage (I-V) characteristics of MIS structure were measured by HP 4156-A semiconductor analyzer.
3.2 Analysis of Characteristics and Discussion 3.2.1 The current density-electric field (J-E) characteristics
Figure 3-1 shows the current density (J) of SiOx film treated by 3000 psi SCCO2
fluid as a function of bias voltage, the bias was applied on top electrode with grounded bottom electrode. In negative bias region, the plot of ln (J/E2) versus reciprocal of electric field (1/E) is displayed in top right inset of Fig. 3-2. A linear dependence indicates the trap-assisted tunneling dominates current transport mechanism while applying larger negative bias than –0.8 V, as the schematic band diagram in bottom left inset of Fig. 3-1. Due to a large number of traps is present in the e-gun deposited SiOx
film, the high leakage current reaching about 10-1 A/cm2 is observed at the bias of –2.5 V. In positive bias region, the electrons are generated mainly from (1) the interface states, (2) traps in depletion region, (3) bottom electrode of substrate. In this work, the generation of electrons from the bottom electrode of substrate or the traps in depletion region is negligible because of using p-type single-crystal Si as substrate. The saturate-like leakage current is caused from the interface states between SiOx and Si substrate (as illustrated in bottom right inset of Fig. 3-1) and limited by the amount of interface states or the carrier generation rate, so the leakage current is lower than that under positive bias voltage. From these electrical behaviors, it is revealed that the pure CO2 molecule is almost ineffective to passivate the traps originated from deposition process.
The current characteristic curves of SiOx film treated by H2O vapor is shown in Fig. 3-3, and it exhibits initially a high resistance state. Compared to the SiOx film treated by pure SCCO2 fluid, the leakage current is reduced obviously after H2O vapor
treatment. For further realize the reduction of leakage current, the leakage current density of high resistance state in negative bias region is analyzed according to Poole-Frenkel (P-F) emission, as shown in Fig. 3-4. The P-F emission is owing to field enhanced thermal excitation of trapped electrons in insulator onto the conduction band, and a schematic band diagram of P-F emission is drew in inset of Fig. 3-4. The linear trend in Fig. 3-4 expresses that the P-F emission dominates the conduction mechanism while applying larger negative bias than –1 V. The conversion of conduction mechanism from trap-assisted tunneling to P-F emission indicates parts of traps in e-gun deposited SiOx layer were passivated by H2O molecule during H2O vapor process. Therefore, the lower leakage current is obtained in either negative bias or
treatment. For further realize the reduction of leakage current, the leakage current density of high resistance state in negative bias region is analyzed according to Poole-Frenkel (P-F) emission, as shown in Fig. 3-4. The P-F emission is owing to field enhanced thermal excitation of trapped electrons in insulator onto the conduction band, and a schematic band diagram of P-F emission is drew in inset of Fig. 3-4. The linear trend in Fig. 3-4 expresses that the P-F emission dominates the conduction mechanism while applying larger negative bias than –1 V. The conversion of conduction mechanism from trap-assisted tunneling to P-F emission indicates parts of traps in e-gun deposited SiOx layer were passivated by H2O molecule during H2O vapor process. Therefore, the lower leakage current is obtained in either negative bias or