Atomic layer deposition of zirconium silicate films using zirconium ...

Atomic layer deposition of zirconium silicate films using zirconium tetrachloride and tetra-n-butyl orthosilicate Won-Kyu Kim, Sang-Woo Kang, and Shi-Woo Rheea) Department of Chemical Engineering, Laboratory for Advanced Molecular Processing, Pohang University of Science and Technology, Pohang 790-784, Korea

Nae-In Lee, Jong-Ho Lee, and Ho-Kyu Kang Preceding Process Development Team, Samsung Electronics Co., Ltd. San #24, Nongseo-Ree, Kiheung-Eup, Yongin-City, Kyonggi-Do 499-900, Korea

共Received 7 May 2002; accepted 9 September 2002兲 Atomic layer chemical vapor deposition of zirconium silicate films with a precursor combination of ZrCl4 and tetra-n-butyl orthosilicate 共TBOS兲 was studied for high dielectric gate insulators. The effect of deposition conditions, such as deposition temperature, pulse time for purge and precursor injection on the deposition rate per cycle, and composition of the film were studied. At 400 °C, the growth rate saturated to 1.35 Å/cycle above 500 sccm of the argon purge flow rate. The growth rate, composition ratio (共Zr/Zr⫹Si)), and impurity contents 共carbon and chlorine兲 saturated with the increase of the injection time of ZrCl4 and TBOS and decreased with the increased deposition temperature from 300 to 500 °C. The growth rate, composition ratio, carbon, and chlorine contents of the Zr silicate thin films deposited at 500 °C were 1.05 Å/cycle, 0.23, 1.1 at. %, and 2.1 at. %, respectively. It appeared that by using only zirconium chloride and silicon alkoxide sources, the content of carbon and chlorine impurities could not be lowered below 1%. It was also found that the incorporation rate of metal from halide source was lower than alkoxide source. © 2002 American Vacuum Society. 关DOI: 10.1116/1.1517998兴 I. INTRODUCTION As the thickness of the gate oxide in field effect transistors approaches 2 nm, direct tunneling will become predominant and it will be necessary to replace the silicon dioxide layer with a material possessing a higher dielectric constant. Many of the alternatives, such as Ta2 O5 , 1 TiO2 , 2 and SrTiO3 , 3 are not thermodynamically stable in direct contact with the silicon substrate and the interfacial quality is degraded. Although some oxides such as Al2 O3 , 4 HfO2 , 5 and ZrO2 , 6 have good stability with Si and a high dielectric constant, the diffusion coefficient for boron or oxygen through these materials is high. Zirconium silicate has recently attracted interest due to its outstanding properties as a gate dielectric. It is thermodynamically stable with Si and is a good barrier against oxygen diffusion.7 Zirconium silicate thin films have been deposited by sputtering8 and atomic layer deposition 共ALD兲.9,10 The sputtering process has some drawbacks such as poor step coverage and the damaging effect of the plasma to the channel region of the complementary metal–oxide– semiconductor devices. ALD is promising because it enables accurate control of film thickness and composition at one atomic layer level through self-limiting surface reactions. But in this case, the selection of chemical precursors is important and chemical reactions associated with the ALD process must be well understood and controlled. Ritala et al.9 and Gordon et al.10 suggested a different ALD concept. Instead of using an additional oxygen source such as water, hydrogen peroxide, ozone, or oxygen radical, a兲

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they used two metal compounds such that at least one of them is an alkoxide in which the oxygen of the ligand is directly bonded to a metal. Due to the strong bond between metal and oxygen, metal alkoxides are less oxidizing toward silicon than common oxygen sources, thereby making it possible to create sharp silicon–metal oxide interface with no interfacial silicon oxide layer. Ritala et al. obtained very uniform Si-rich Zr silicate films with sharp interface using ZrCl4 and silicon alkoxides such as Si(OEt) 4 and Si(OnBu) 4 . Gordon et al. used amido compounds as the Zr precursor and t-butoxy silanol as the Si precursor with high growth rate and low impurity contents. In the ALD process, an inert gas purge is necessary to achieve less than 1 monolayer chemisorption in each cycle by purging physisorbed molecules in the top layer. Also there should be no chemical reaction that leads to the nucleation on the surface before the second reactant is introduced to induce the self-limited surface reaction. It is important to find out the ALD process window and optimized process conditions. To develop a successful ALD process, optimization of the process variables such as deposition temperature, purging gas flow rate, and the cycle time of the precursor injection is needed and their effect on the deposition rate and film composition should be clearly identified. In this article, we demonstrate the optimization of operating parameters for ALD for deposition of Zr silicate using the combination of ZrCl4 and Si(OnBu) 4 tetra-n butyl orthosilicate 共TBOS兲. II. EXPERIMENT The films were grown in a cold-wall flow-type ALD reactor on 共100兲 oriented p-Si single crystal substrates in the temperature range of 300–500 °C. Figure 1 shows the sche-

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©2002 American Vacuum Society

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Kim et al.: Atomic layer deposition of zirconium silicate films

FIG. 1. Schematic diagram of the ALD system with computer-controlled solenoid valves.

matic diagram of the ALD reactor, where the susceptor is resistively heated. Prior to deposition, the Si substrate was cleaned using the modified RCA method. The pressure in the reactor was fixed at 1 Torr by using a throttle valve located between the pump and the reaction chamber. Argon 共99.99995%兲 was used as a carrier and purging gas. ZrCl4 共CERAC, 99.9%兲 and TBOS 共TCI, 98%兲 were used as reactant gases and the temperature 共vapor pressure兲 of the container of ZrCl4 and TBOS were 160 °C 共40 Torr兲 and 95 °C 共2 Torr兲, respectively. Source vapor was carried from each bubbler by Ar. Carrier gas and purge gas were introduced into the reactor for a fixed amount of time separately and sequentially by means of digitized on/off control of solenoid valves. To control source injection and purge time, we varied valve on/off time from 0.2 to 10 s. The flow rate of the purge gas was varied from 20 to 600 sccm. The feed line and shower head were held at 250 °C to prevent the condensation of the precursors. Deposition conditions are summarized in Table I. Film thickness and refractive index of the films were measured by ellipsometry, and the composition of the deposited film was analyzed by x-ray photoelectron spectroscopy 共XPS兲. III. RESULTS AND DISCUSSION It was reported that the reaction between TBOS and ZrCl4 took place even at 250 °C.9 TBOS has the lowest activation energy among tetra-alkoxysilanes and it can react with other metal compounds although the compound itself is not thermally decomposed below 650 °C.11 The reaction mechanism

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FIG. 2. Growth rate 共Å/cycle兲 as a function of the flow rate of Ar purging gas 共deposition temperature: 400 °C, ZrCl4 pulse: 5 s, TBOS pulse: 5 s, purge: 5 s兲.

was reported to be somewhat complicated and affected by temperature and other operating conditions.12 In the ALD process, the inert gas purge was important so that less than 1 monolayer was chemisorbed in the top layer. Argon purge was introduced after the injection of each precursor. To optimize purge conditions, we first varied the flow rate of the argon purging gas while keeping other parameters such as ZrCl4 and TBOS injection time and purge time constant at 5, 5, and 5 s, respectively. Figure 2 shows the dependence of the growth rate on the flow rate of the purge gas. At the substrate temperature of 400 °C, insufficient purging gave films with increasing thickness from center to edge. As the Ar flow rate was increased, the growth rate saturated at 1.35 Å/cycle and the thickness of the film was uniform above 500 sccm. No significant change in the growth rate was observed when Ar purge was greater than 500 sccm. Figure 3 shows the film thickness measured by ellipsometry as a function of the number of deposition cycles at 400 °C with 500 sccm of purge. One of the attractive features of the ALD process was that the film thickness was controlled simply by the number of reaction cycles used. In this figure, we found the ideal linear growth as a function of the cycle time resulting from the self-limited surface reaction. Figure 4 shows the dependence of the growth rate on the TBOS injection time at the substrate temperature of 400 °C. ZrCl4 injection time and purging time were fixed at 5 s. Insufficient TBOS injection resulted in a low growth rate due to the lack of chemisorbed TBOS molecules. As the TBOS injection time was increased, the growth rate saturated at 1.35 Å/cycle above 2 s. No significant change in the growth

TABLE I. Operating conditions for the deposition of zirconium silicate film. Parameter Purge gas 共Ar兲 Carrier gas 共Ar兲 Deposition pressure Substrate temperature Bubbler temperature Vapor pressure Showerhead temperature Solenoid valve on/off time

JVST A - Vacuum, Surfaces, and Films

Condition 20– 600 sccm 20 sccm 1 Torr 300–500 °C ZrCl4 : 160 °C, TBOS: 95 °C ZrCl4 : 40 Torr, TBOS: 2 Torr 250 °C ZrCl4 : 0.2–10 s, TBOS: 0.2–5 s, Purge: 0.5–5 s

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FIG. 3. Ellipsometric measurement of Zi silicate film thickness 共Å兲 deposited on Si共100兲 at 400 °C as a function of the number of cycles.

FIG. 7. Composition ratio and impurity content as a function of the TBOS injection time.

FIG. 4. Growth rate as a function of the TBOS injection time 共deposition temperature: 400 °C, ZrCl4 pulse: 5 s, purge: 5 s兲.

FIG. 8. Growth rate as a function of the purge time 共deposition temperature: 400 °C, ZrCl4 pulse: 5 s, TBOS pulse: 2 s兲.

FIG. 5. Composition ratio (Zr/(Zr⫹Si)) and impurity content 共at. %兲 as a function of the TBOS injection time.

FIG. 9. Growth rate as a function of substrate temperature (ZrCl4 pulse: 5 s, TBOS pulse: 2 s, purge: 2 s兲.

FIG. 6. Growth rate as a function of the ZrCl4 injection time 共deposition temperature: 400 °C, TBOS pulse: 2 s, purge: 5 s兲.

FIG. 10. Composition ratio and impurity content as a function of the substrate temperature.

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FIG. 12. Relationship between the composition ratio and the refractive index of the zirconium silicate film. FIG. 11. Dependence of the refractive index on the deposition temperature.

rate was observed when TBOS injection time was longer than 2 s. We obtained the composition ratio (Zr/(Zr⫹Si)) and impurity contents 共at. %兲 such as carbon and chlorine from XPS. Figure 5 shows that the composition and carbon and chlorine were saturated at 0.3, 3– 4 at. %, and 4 –5 at. % even at 1 s injection time of TBOS. Figure 6 shows the variation of the growth rate with respect to the ZrCl4 injection time. Purge time was fixed at 5 s. As the ZrCl4 injection time was increased, the growth rate saturated at 1.35 Å/cycle above 5 s. No significant change in the growth rate was observed for ZrCl4 injection times longer than 5 s. Figure 7 shows the composition ratio and impurity content. We found that the composition ratio (Zr/(Zr⫹Si)) and carbon and chlorine content were saturated at 0.3, 3– 4 at. % and 4 –5 at. % at 2 s injection time of ZrCl4 . It was surprising to see that excessive amount of ZrCl4 共5 s valve opening time with vapor pressure of 40 Torr兲 was needed compared with TBOS 共2 s with vapor pressure of 2 Torr兲 and still the composition ratio (Zr/(Zr ⫹Si)) of the film was only 0.3. The incorporation rate of the metal from the chloride source was obviously much lower than from the alkoxide source.13 The reaction pathway involved in this process that determined the stoicheometry of the film was not clear.10 According to the precursor-mediated adsorption model the physisorbed SiCl4 , which has a structure similar to ZrCl4 , could either desorb back to the gas phase or react with a hydroxyl group on the surface. With a reaction activation energy between SiCl4 and a hydroxyl group, the model predicts that only about 1 out of 106 physisorbed SiCl4 molecules should react with hydroxyl groups at 600 K.14 Figure 8 shows the dependency of the growth rate on the purge time. TBOS and ZrCl4 injection time were fixed at 2 and 5 s, respectively. When the purge time was longer than 2 s, the growth rate was saturated at 1.35 Å/cycle. No significant change in the growth rate was observed when Ar purge was over 2 s. The optimum shortest cycle time was 5–2–2–2 s, ZrCl4 injection time, first purge time, TBOS injection time, and second purge time, respectively. When the other process parameters such as source temperature, flow rate, time of purge, source injection time, and operating pressure were fixed, the growth rate decreased to 1.05 Å/cycle with an increase in substrate temperature from JVST A - Vacuum, Surfaces, and Films

300 to 500 °C as shown in Fig. 9. Suntola points out that saturation density decreases with increasing temperature, which often occurs on disordered surfaces such as surfaces of polycrystalline and amorphous materials.15 Alternatively it might have been due to the desorption of film forming species along with the change in the reaction pathway with increased deposition temperature.10,16 Figure 10 shows the composition ratio and impurity content of the films analyzed by XPS. In this figure, we found that the ratio decreased from 0.40 to 0.23 as the substrate temperature was increased from 300 to 500 °C. Carbon and chlorine impurities also decreased to 1.1 and 2.1 at. %, respectively. Figure 11 shows the dependence of the refractive index at a wavelength of 632.8 nm, along with the composition of the film, on the substrate temperature. The refractive indices of single crystalline ZrO2 and SiO2 were 2.2 and 1.46, respectively. As the zirconium content decreased, the refractive index also decreased. Figure 12 shows the relation between the composition ratio and the refractive index 共wavelength: 632.8 nm兲 of all deposited films. In all cases, the refractive index was less than the linear combination of the ZrO2 and SiO2 refractive indices. This was probably due to impurities in the film and the density difference between crystalline and amorphous phase. A similar tendency was also observed between composition and dielectric constant.17 IV. CONCLUSION Optimization of the ALD process to deposit zirconium silicate films was demonstrated with a precursor combination of ZrCl4 and TBOS. The optimum shortest cycle time was found to be 5–2–2–2 s (ZrCl4 injection time, first purge time, TBOS injection time, and second purge time兲. At the deposition temperature of 400 °C, the growth rate, composition ratio (Zr/(Zr⫹Si)), carbon, and chlorine content of the zirconium silicate thin film deposited at this optimum condition were 1.35 Å/cycle, 0.3, 3– 4 at. %, and 4 –5 at. %, respectively. The growth rate, composition ratio, and impurity content decreased with the increase of the deposition temperature from 300 to 500 °C. It appeared that by using only zirconium chloride and silicon alkoxide sources, the impurity content of carbon and chlorine could not be lowered below 1%. It was also found that the incorporation rate of metal from halide source was lower than that of the alkoxide

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source and an excessive amount of chloride source was needed. More study is needed to characterize the electric properties of the silicate film and properties of the silicate and silicon interface.

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