Paper ID ICLASS CONSTRUCTION OF NEW CVD EVAPORATION SUPPLY BY FLASHING SPRAY WITH TWO COMPONENT SOLUTION

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1 ICLASS-2006 Aug.27-Sept., 2006, Kyoto, Japan Paper ID ICLASS06-7 CONSTRUCTION OF NEW CVD EVAPORATION SUPPLY BY FLASHING SPRAY WITH TWO COMPONENT SOLUTION Motohiro OSHIMA, Akiko KOMEDA 2, Koji TOMINAGA 3, Jiro SENDA 4 and Kozo ISHIDA 5 Doshisha Univ. Graduate School, etf306@mail4.doshisha.ac.jp 2 Doshisha Univ. Graduate School, dtf0330@mail4.doshisha.ac.jp 3 Horiba. Ltd., koji.tominaga@horiba.com 4 Doshisha Univ., jsenda@maildoshisha.ac.jp 5 Horiba. Ltd., kozo.ishida@horiba.com ABSTRACT The authors proposed the novel chemical vapor deposition (CVD) by use of the flash boiling spray CVD(FS-CVD) to improve several kind of problem such as the supply line of raw materials and the formation of thin film when the mixed liquid with high volatily and low one is injected into the atmosphere with low pressure the flash boiling phenomena takes place, the fine, lean and homogeneous is formed and the vaporization promotes. This is just the point of the way proposed. In the experiment, tetraethyl orthosilicate (TEOS) with high volatility was used as the raw material of the thin film. The other liquid with low volatility was n-pentane by considering the mutual solubility, the vapor pressure, the toxicity and the explosiveness. The best injector for the proposal was the slit type due to its from vapor formation speed and distribution of which were taken simultaneously by the image of LIF with high magnification and that of Mie scattering also with high magnification vapor concentration. Keywords: Flash boiling, Spray, Two phase region, chemical vapor deposition, LIF. INTRODUCTION The technique of the low pressure CVD is one of the technology of thin film formation in the production process of semiconductor because it has the good stepcoverage and the good reproducibility of film formation. Figure. shows the schematic image of conventional CVD process. The liquid material is used in CVD process due to its easy handling. The liquid is evaporated at a vaporizer and is supply to a CVD chamber through the supply line. The line is surrounded by the heater to keep the condition of gas phase. When the vapor pressure of liquid is low, it is necessary to keep the highest temperature in the supply line and the vaporizer. However, there is some disadvantage against this kind of technique. The liquid as the raw materials is decomposed by the super heating and it adhered on the inside wall of the line as the particle[]. To avoid the disadvantage, it is required to be clean the wall of line and the vaporizer. Thus, the authors propose the new technique by use of the flash boiling spray[2]. 2. CHARACTERISTICS OF FS-CVD () Application of Pulsed Supply of Raw Material The liquid material is supplied directly to a CVD chamber through an injector is installed at its top side. Figure. 2 is the schematic image of the CVD by use of flash boiling spray (FS-CVD). It is capable of achieving the stable supply of raw material precursor and shutting down it by usage of an intermitted spray. The CVD chamber is more compact than that illustrated in Figure.. The distinguished matter is the application of an intermittent spray and the merit is the more remarkable flexibility for the film formation process than the conventional one because it is possible to control the injection pressure and the injection quantity[3]. Heater Vaporizer Liquid material Injector Flashing spray Mixing part Shower head Wafer Susceptor Wafer Susceptor Heater Exhaust CVD Chamber Mixed Material CVD chamber Fig. Schematic diagram of conventional technique of CVD Flashing spray CVD Fig.2 Schematic diagram of FS-CVD

2 (2) Formation of Rapid Evaporation Spray due to Flashing and Lean Homogeneous Mixture Figure. 3 shows the pressure-temperature diagram relating to the flash boiling process. The process should generated when the liquid material pressure is depressed below its saturated vapor pressure, p v. The spray angle increases remarkably with decrease p v in the flash boiling spray. As a result, It is possible to realize the homogeneous vapor in CVD chamber due to this kind of flashing spray[4][5]. (3) Improvement of Evaporation Characteristics Mixed Solution with Two Components due to Formation of Two Phase Region The saturated vapor pressure of a number of materials is very low. Normally, it is necessary to require the high energy for evaporation of them. The technique proposed is to the mixed material having the low volatility material and the high volatility one use. Figure. 4 the P-T diagram of the solution with two components whose saturated vapor pressure is different each other. The critical point depends on the critical pressure, the critical temperature and the molar fraction of each component. The peculiarity is the existence of the two phase region shown in this figure where the liquid and vapor phases are mixed, whereas only the saturated vapor line exists in the case of pure liquid substance. Thus, it is able to reform the state of vaporization due to the mixing the material with lower volatility with the higher volatile material. As a consequence, there is the capacity that the more kinds of materials are applied to produce the thin film. 3. SELECTION OF HIGH VOLATILE MATERIAL TEOS which is one of the major raw materials for the formation of SiO 2 film, was used in the experiments, the seven kind of organic materials were the objective as the candidate of the high volatile material. They were accetone, benzene, cyclopentane, methyl formate, i-hexane, n-hexane, and n-pentane. Figure. 5 is the saturated vapor pressure of them[6]. The ordinate is the pressure and the abscissa is the temperature. Table. list the saturated vapor pressure of the two phase region at 320[K]. The molar fraction, X of high volatile material for TEOS is 0.2, 0.4 and 0.6, respectively. The two phase region is calculated by the modified BWR equation[7]. The criterion is shown below: It is able to actualized the mutual solubility with TEOS The saturated vapor pressure of two phase region is high enough. The saturated vapor pressure of TEOS at 320K is 0.660[KPa]. Their vapor pressure is around from two to three times as high as that of TEOS in the case of X=0.2. At Pressure [MPa] Methyl formate n-pentane Acetone Cyclopentane i-hexane n-hexane Benzene Temperature [K] Fig.5 Saturated vapor pressure of High volatile material Table. Saturated vapor pressure of mixture of TEOS and high volatile material Fig.3 Schematic image of flash boiling process TEOS+Low b.p.material x=0.2 [kpa] x=0.4 [kpa] x=0.6 [kpa] accetone benzene cyclopentane methyl formate i-hexane n-hexane Fig.4 Schematic diagram of two phase region n-pentane

3 X equal to 0.6 it shows about from three times to seven as much as the case TEOS. Especially, the saturated vapor pressure of the case of n-pentane is the highest comparing with the others. As a result, n-pentane is selected. Figure. 6 shows the effect of the molar fraction of n-pentane, X n-pentane, on the formation of two phase region. The critical temperature is calculated by Chueh-Prausnitz method[8]. The critical pressure is obtained by Kreglewski-Kay method[9]. The evaporation property increases with increase in X n-pentane. X n-pentane was kept to be 0.6 in the experiments. 4. EXPERIMENTAL APPARATUS AND CONDITION 4.. Magnified Photography of Flash Boiling Spray at near Nozzle Hole Figure. 7 illustrates the optical system of photography magnified. The chamber is the model of FS-CVD system. Solution of mixed materials is injected through an injector. The chamber is vacuumed by a vacuum pump whose exhaust speed is 50[l/min]. The chamber pressure is measured by a pilani pressure gauge. The second harmonic of is the Nd:YAG laser (Spectra-Physics : PIV400), intsnsity : 30[mJ/pulse].The laser beam comes through a pinhole whose pin diameter is 8[mm] and goes to a convex lens (focal length:40[mm]) in order to form the diffusion light. The scattered light is taken by a still camera with Nikkor lens (focal lengh:200mm, f:4) and bellows. The scanning area is 20[mm] 20[mm] from nozzle outlet. The magnification ratio is twice on the film Measurement of Distribution of Vapor Concentration Figure. 8 is the optical system of Mie scattering and LIF. The second harmonic of Nd:YAG laser (Spectra-Physics : PIV400) is set for the light source of excitation. The laser beam comes through a pinhole and two sheets of cylindrical lense to form the laser sheet. The forcal length of both lenses is 00[mm]and 000[mm], respectively. The laser sheet passes through the spray axis. The information of fluorescence and scattered light were recorded by two sets of ICCD camera with an image intensifier. A bandpass filter (center wavelength : 307[nm], half width : 0[nm]) is placed in front of the CCD camera(a) to capture the fluorescent The role of notch filter is to eliminate noize. The other one (center wavelength : 265.7nm, half width : 6.7nm) is set in front of the other CCD camera(b) to take the information of the scattered Experimental Condition Table. 2 summarizes the experimental condition. The molar fraction of n-pentane is constant at 0.6 as described above. The mixed solution is pressurized at 0.42MPa by the nitrogen gas. The injected quantity of all the nozzles is.8[mg]. The dopant,,2,4-trimethyl-benzene (C 9 H 2, b.p. : 432K) of 5.0[%] is added as the tracer for TEOS. Figure. 9 is the fluorescent spectra of the dopant excited at 266[nm]. The injection with multihole and that with slit nozzle were examined in the experiment. The ambient pressure, p amb, in the case of magnificated photography is 7, 40, 60 and 00[Pa], and that in the case of Mie scattering and LIF is 7, PC Pin hole Cylindrical lenses Nd:YAG laser (266nm) Control circuit Injector Pulse generator Fig.6 Two phase region of TEOS and n-pentane as a function of molar fraction of n-pentane CCD Camera CCD Camera with I.I.(A) with I.I. (B) Chamber Band pass filter Notch filter (λ=307[nm], FWHM=0[nm]) PC Band pass filter (λ=266[nm] FWHM =7[nm]) Fig.8 Optical system for Mie scattering and LIF Material Table.2 Experimental condition TEOS + n-pentane Mole fraction of n-pentane Dopant Nozzle type Ambient pressure[pa] Injection pressure [MPa] Injection quantity [mg/pulse] Injection period [ms] 0.6,2,4-Trimethyl-Benzene (5[vol%],T b =432[K]) Multi hole slit 7,40,60,00,250*,000*, 2000* Fig.7 Optical system of magnification photography * Only the case of Mie scattering and LIF condition

4 40, 60, 00, 250[Pa], and 2[kPa], respectively. Figure. 0 shows experimental condition on the P-T diagram. The condition of p amb =2[kPa] locates in two phase region, that p amb equal to [kpa] is in the vicinity of the line saturated vapor pressure, and the other p amb is as same as those of FS-CVD system. 5. SPRAY IMAGE Figure. displays the image of flash boiling spray taken by magnificated photography. The photographying is the dimensionless time t/t inj =0.4 where t inj is the injection duration. The flash boiling phenomena certainly breaks out in the case of multihole nozzle comparing with the spray injected into the atmosphere over the saturated vapor pressure[0]. However, there are many droplets at the distance, Z, from nozzle outlet equal to Z:20[mm]. The vaporization is not perfectly promoted while the ambient pressure, p amb, is below the saturated vapor pressure. The penetration of the case of the slit nozzle is much shorter than that of the multihole and there are little droplets. The atomization characteristics of the former nozzle is much different from those of the latter one as well known. Namely, the atomization of the former promotes due to the break up of the liquid jet and the latter proceeds due to that of the sheet, as a result, the sheet breakup occurs earlier than the liquid jet breakup. The reason of the trend is why Relative fluorescence intensity Excited wavelength (λ=266[nm]) Wavelength [nm] Fig.9 Fluorescence spectra for tracer excited at 266nm the contact area of the spray of the slit nozzle is wider than that of the multihole nozzle. 6. DISTRIBUTION OF VAPOR CONCENTRATION Figure. 2 displays the vapor distribution taken by LIF technique. The vaporization promote with decrease in the ambient pressure. Especially, when the ambient pressure is placed in the two phase region the phenomena are different from these under the saturated vapor pressure. The vapor distribution of multihole nozzle is denser than that of slit nozzle in each ambient pressure. Namely, the distribution of the latter is more homogeneous. The evaporation rate is defined as the ratio of the ga area, A g, to the summation of A g and the liquid area, A l. Figure. 3 is the evaporation rate as a function of the time after the end of injection. The case of the slit nozzle shows the distinguished evaporation fluorescent are, that is, almost the 00[%] of evaporation rate when the ambient pressure, p amb, is ranged from 7[Pa] to 500[Pa]. The range is limited up to 250[Pa] in the case of the multihole nozzle. At p amb equal to 500[Pa] of the latter nozzle, the evaporation ratio is about 0.85 at the injection end, it increases, it is around.0 from.0[ms] to 3[ms] after the injection end at it decreases because the remained liquid in the nozzle is com out. When Pressure [kpa] [Pa] 2[kPa] [kpa] Saturated vapor pressure line 250[Pa] 00[Pa] 40[Pa] 7[Pa] Temperature [K] Fig.0 Experimental condition Fig. Flash boiling spray pattern taken by magnificated photography

5 the ambient pressure is over [kpa], the trend is different that mentioned above in both injectors. It is found that the evaporation ratio in the slit nozzle has the minimum at about 2.0[ms] after the injection end and is the maximum at around 3.0[ms] after the injection since the heat of evaporation from ambient gas is low. The ratio in the multihole nozzle shows the maximum approximately at 3.0[ms] after the end of injection. The reason is why the small droplet at out of nozzle axis is rolled up. Generally speaking, the slit nozzle promotes the evaporation more than the multihole nozzle. The evidence is Figure 2 and Figure 3. The standard deviation, S a, is calculated by the following equation to take account of the distribution of vapor. multihole nozzle. REFERENCES. Miyazaki, S., Physics of film growth by chemical vapor deposition, JSAP Vol.69 No.6,pp , 2000, (in Japanese). S a = n ( I () i I ) 2 i= n where I(i) is the intensity of LIF signal, I is the average intensity and n is the total number intensities. Figure. 4 is the effect of the ambient pressure on the standard deviation of the vapor region from time after injection end. S a of the multihole nozzle decreases gradually from the time after injection end is in spite of the ambient pressure. It is marked matter that S a of the slit nozzle is much smaller than that of the multihole nozzle. Namely, the former nozzle has the great advantage in the formation of the more homogeneous vapor against the latter nozzle. 7. CONCLUSION The following conclusions are drawn from the experiments: () It is able to actualize the more homogeneous spray by use of the flash boiling phenomena. (2) The slit nozzle shows the great advantage to produce more homogeneous distribution of vapor against the Fig.3 Time history of evaporation rate Fig.2 Vapor distribution

6 2. Oshima, M., Tominaga, K., Matsuda, K., Senda, J. and Ishida, K., Proposal of Flashing Spray Process by Use of Mixed Material Solution, Proc. EMS-24, July 4-6, pp.37-40, Senda, J., Miyoshi, K., Yakawa, Y., Ishida, K., Matsuda, K. and Shimizu, T., CVD New Evaporation Supply Method by Flashing Spray, J. ILASS-JAPAN, Vol.2, No.38, pp.8-32, 2003, (in Japanese). 4. Fujimoto, H., Iwami, Y. and Senda, J., Atomization Characteristics of Liquefield n-butane Spray with Flash Boiling Phenomena, Proc. ICLASS- 97, (Seoul), pp.74-72, Senda, J., Asai, T., Kawaguchi, B. and Fujimoto, H., Characteristics of Gas-Dissolved Diesel Fuel Spray (Spray Characteristics and Simulating Flash Boiling Process), JSME Int. J., Series B, Vol.43, No.3, pp , N. B. Valgaftik, The thermophysical property of vapor and liquid, Nauka Publishing Corp.(Rossiya), 972, (in Japanese :trans. by Endo, K.) 7. Saito, S., Base of equilibrium physical properties prediction by statistical thermodynamics,baihukan, pp.43-78,985, (in Japanese). 8. Chueh, P.L., Prausnitz, J.M., Vapor-Liquid Equilibrium at High Pressure: Calculation of Critical Temperature, Volume and Pressure of Nonpolar Mixture, AICh J., Vol.3, No.6, pp.07-3, A.Kreglewski and W.B.Kay, "The Critical Constants of Conformal Mixtures", J. Phys. Chem., Vol.73, No.0, pp , Senda, J., Kawano, D., Hotta, I., Kawakami, K and Fujimoto, H, SAE Paper, , pp.-3, ACKNOWLEDGEMENT The authors express their thanks to the Ministry of Economy, Trade and Industry, japan who supported a part of experiment work. Fig.4 Effect of ambient pressure on standard deviation of vapor region