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Diagnostics of the radio frequency magnetron discharge plasma used for TiO2 thin film sputtering deposition
Surface & Coatings Technology 187 (2004) 358 – 363 www.elsevier.com/locate/surfcoat
Diagnostics of the radio frequency magnetron discharge plasma used for TiO2 thin film sputtering deposition Lucel Sirghi a,*, Toru Aoki b, Yoshinori Hatanaka c a
Faculty of Physics, ‘Al. I. Cuza’ University, blvd. Carol I, no. 11, Iasi 700506, Romania b Research Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan c Aichi University of Technology, 50-2 Manori, Nishihazama, Gamagori 443-0047, Japan Received 27 June 2003; accepted in revised form 30 January 2004 Available online 20 April 2004
Abstract The density, temperature and energy distribution function of the electrons of the plasma nearby the deposition substrate of a radio frequency magnetron discharge used for sputtering deposition of TiO2 thin films were measured by a cylindrical Langmuir probe. In the lowenergy domain (0 – 15 eV), the electron energy distributions resembled a Maxwell distribution with a temperature value that decreased from 5 to 3 eV by the increase of the argon pressure from 1 to 20 mTorr. In the high-energy domain the electron energy distribution showed a depletion. At low values (approx. 0.5 sccm) the flow rate of argon affected the plasma density. This effect was attributed to the gas composition change induced by the atoms sputtered from the cathode titanium dioxide target, change that was proved by optical emission spectroscopy measurements. D 2004 Elsevier B.V. All rights reserved. Keywords: Radio frequency magnetron discharge; Titanium oxide; Langmuir probe; Electron density; Optical emission spectroscopy
1. Introduction Recently, titanium dioxide (TiO2) has received much attention as a coating material because of its good bio compatible [1], photocatalytic and hydrophilic [2] properties. The UV light induced hydrophilicity of this material is related to its photocatalytic property, which is due to the strong oxidizing power of UV light generated holes in the valence band. Studies of the photoinduced hydrophilicity of different crystal faces revealed that this property depends on the surface structure, particularly on the density of oxygen bridging sites [3]. Recent studies revealed the importance that the charge carriers transport phenomena, which depend on the thickness [4] and microscopic structure [5] of the TiO2 films, have for the UV induced hydrofilicity [6] or photocatalytic activity. Therefore, a strong dependence of surface properties on the preparation method for TiO2 coatings is expected. A variety of deposition techniques have been developed to produce TiO2 films with good surface proper-
* Corresponding author. Tel.: +40-232-201186; fax: +40-232-201150. E-mail address: [email protected] (L. Sirghi). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.035
ties [7,8]. Among these methods, the deposition by r.f. magnetron sputtering of a pure ceramic TiO2 target is known to produce films with good hardness and adhesion to the substrate, but with relatively low photocatalytic activity [8]. The low photocatalytic activity of these TiO2 thin films has been attributed to their heterogeneous, crystalline and amorphous, mesostructure [9]. The crystalline structure of TiO2 thin films obtained by r.f. magnetron sputtering of a pure titanium target in argon – oxygen mixture gas has been reported to depend strongly on the gas pressure [10]. This discharge parameter affects the plasma density, electron temperature and the flux and energy of positive ions bombarding the growing film [11]. The crystalline structure of films deposited on heated substrates by r.f. magnetron sputtering of a sintered TiO2 target in argon– oxygen mixture gas has been also found to depend on gas pressure, amorphous films being deposited at relatively large pressure values [9,12]. Mass spectroscopy measurements showed that the development of the rutile phase in the TiO2 films deposited by reactive r.f. magnetron sputtering is favored by the bombardment of the growing film by energetic Ar+ ions [11]. Generally, because of the large mobility of the electrons, the surface of a growing film is biased negatively
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with respect to the nearby plasma, which results in the occurrence of an ionic sheath that accelerates the positive ions towards the film surface. The temperature and density of the plasma electrons nearby the ionic sheath determine the positive ion flux towards the film surface. Therefore, the temperature and density of the electrons of the plasma in the vicinity of the deposition substrate are relevant plasma parameters in determining the structure and surface properties of the deposited film. This paper reports results concerning the effect of the gas pressure, gas flow rate and r.f. discharge power, on the temperature, density and energy distribution function of the plasma electrons nearby the deposition substrate of a TiO2 deposition system that used the r.f. magnetron sputtering of a sintered TiO2 target in an argon plasma. The results and discussion presented here are based on measurements of the Langmuir probe current – voltage characteristics and optical emission spectra of the plasma nearby the deposition substrate. The results concerning the dependence of the deposited film properties on the deposition parameters has been published elsewhere [9].
2. Experimental apparatus Fig. 1 shows a sketch of the experimental apparatus used in the experiments. The magnetron chamber consisted of a stainless steel cylinder with the diameter of 39 cm and height of 19 cm, which was connected to a vacuum system (minimum pressure of 10 6 Torr). The working gas (high purity argon) was fed into the chamber through a mass flow controller (MFC). The gas pressure was controlled by the conductance of the chamber connection to the vacuum pumps. The magnetron target, which consisted of a sintered TiO2 (99.99% purity) disk of 10 cm in diameter, was installed at the bottom of the chamber and connected to the r.f. power supply through a matching box. The deposition substrate holder was mounted at a distance of 10 cm from the magnetron target. A Langmuir probe consisting of a tungsten wire with a length of 5 mm and a diameter of 0.15 mm was installed at 1 cm from the substrate holder and 4 cm from the chamber axis. Because of the relatively large distance between the target and the Langmuir probe (approx. 10 cm), the probe measurements were not affected by the magnetic filed of the target. The I(V) characteristic of the probe was digitally acquired by a data acquisition system comprised of a digital analog converter (DAC) that generated a rump biasing voltage ranged from 30 V to + 30 V, a differential operational amplifier (OPA) that collected the current signal across a resistor R installed in the probe biasing circuit, an analog digital converter (ADC) that converted the probe current signal into digital data, and a personal computer (PC). Three parallel LC circuits tuned on the fundamental and second harmonic components of the r.f. discharge current (13.56 MHz), respectively, were installed in series in the probe circuit in order to reject the r.f.
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Fig. 1. Sketch of the experimental setup.
component of the probe current [13] (for simplicity, only one LC filter is represented in Fig. 1). Before each measurement, the probe was biased to a high positive potential (200 – 300 V) and heated to incandescence by the electronic current drown from the plasma in order to prevent the probe surface contamination by the deposition of titanium oxide. Each probe characteristic, Ip(Vp), was digitally acquired as a sequence of 1200 probe current data (16-bit resolution) by increasing of the probe bias potential from 30 V to + 30 V in steps of 50 mV. The probe current was smoothed using a smoothing technique based on a convolution of the data with a Gaussian error distribution function with a half-width of 600 mV. To determine the electron energy distribution function (EEDF), plasma potential (Vs), electron temperature (Te) and electron density (ne), the second derivative of the probe current was computed by a method using the least square fitting of a sequence of 40 probe current data (which corresponded to 2 V biasing voltage interval) with a thirdorder polynomial function. Assuming an isotropic plasma [14], the EEDF, f(u)u1/2, was determined using the Druyvesteyn [15] formula: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 ! 2me Vs Vp d Ip 2 f ðuÞu1=2 ju¼eðVs Vp Þ ¼ 2 ; ð1Þ e Ap e dVp2 where me and e are the electron mass and electric charge, respectively, u is the electron kinetic energy, Vp, the probe biasing potential and Ap is the probe surface area. The plasma potential Vs was identified as the probe voltage at which the second derivative of probe current crossed through zero. Taking into account the level of noise in the Langmuir probe current data, a noise level approximately 10 4 in the normalized EEDF data has been estimated, so that the results at energies larger than 20 eV were discarded. The electron density was computed by the EEDF integral over the whole resolved energy spectrum: where umax = 20 eV. ne ¼ mu0max f ðuÞu1=2 du;
ð2Þ
To compare the electron energy distribution function obtained in various experimental conditions, the electron
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Fig. 2. Typical optical emission spectrum of the negative glow plasma (discharge power was 100 W and the argon flow rate was 6 sccm). The light intensity signal in the range 300 – 500 nm is represented multiplied by ten on the plot.
energy probability function (EEPF) was computed as f(u) of which integral Eq. (2) was normalized to unity. For the energy domain where the EEPF resembled the Maxwellian distribution, the electron temperature was computed from the slope of EEPF logarithm: TEEPF ¼
dln f ðuÞ 1 : du
ð3Þ
The optical spectroscopy measurements were performed with a resolution of 3 nm by an optical spectrometer (Hamamatsu Plasma Processing Monitor C6670-04) with an optical fiber probe that collected the light emitted by the negative glow plasma. Typical emission spectrum of the light emitted by the negative glow plasma showing the emission lines corresponding to Ar, O2+, Ti and Ti+ is presented in Fig. 2. Because of the relatively low resolution of the optical spectrometer, the emission lines corresponding to Ti and Ti+ (337.1, 337.3 and 336.1 nm) [16] are merged on the spectrum. Also, the lines of the second negative emission system of O2+ (312.3, 314.1, 321 and 323.1 nm) [17] are merged into a broad peak. From the strong emission lines of Ar spectrum corresponding to 4p ! 4s transitions in the 650 –950 nm range we have chosen the line at 750.4 nm.
Fig. 3. Dependence of the plasma density on the Ar flow rate at four values of the discharge power (Ar pressure was 4 mTorr).
flow rate, the electron density increased at small values (approx. 0.5 sccm) and remained constant at values larger than 1 sccm. However, this pattern changed for the case of a low discharge power (50 W), when the electron density remained constant for the whole range of the argon flow rate values. This dependence may be explained by the effect of the sputtered O and Ti atoms on the discharge plasma. At low argon flow rate and relatively high discharge power the gas composition in the discharge chamber is modified by the sputtering process and a large concentration of Ti and O atoms is expected. Because of their high electron affinity, the O atoms capture low energy electrons from plasma and form negative ions with the effects of decreasing of density and increasing of temperature of the plasma electrons. This phenomenon is negligible at low discharge power values ( < 50 W), when the sputtering rate is low. To prove the effect of sputtering on the gas composition, we performed optical emission spectroscopy measurements of the light emitted by the negative glow plasma. Fig. 4 presents the dependence on the argon flow rate of the emission intensities of some lines from Ar (line 750.4 nm), Ti and Ti+ (lines 337.1, 337.3 and 336.1 nm) and O2+ (lines 312.3, 314.1, 321 and 323.1 nm) emission spectra. The discharge power was kept at 100 W and gas pressure at 4 mTorr in these measurements. By the increase of the argon flow rate, the intensity of the lines corresponding to Ar
3. Results and discussion We studied the effect of gas flow rate, discharge power and gas pressure on the electron density, temperature and energy distribution function. Optical emission spectroscopy measurements were performed to support the explanation of plasma parameter dependence on the gas flow rate. 3.1. Dependence of plasma parameters on the r.f. discharge power and gas flow rate Fig. 3 presents the dependence of the plasma density near the deposition substrate on the flow rate of argon at four values of the discharge power. By the increase of the argon
Fig. 4. Intensity of lines corresponding to Ar, O+2,Ti+ and Ti emission in negative glow plasma as they changed by the increase of the argon flow rate (the discharge power was 100 W and the gas pressure was 4 mTorr).
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emission spectrum increased, while the intensity of the lines corresponding to Ti, Ti+ and O2+ emission spectra decreased. The line intensities changed largely at low values of the argon flow rate and very little at large values of the argon flow rate. Since the Langmuir probe measurements have not showed significant changes of the electron temperature by the change of the gas flow rate, the change in the light emission intensity is attributed mainly to the changes of plasma density and gas composition. The optical emission spectroscopy measurements show that the important changes in the gas composition occur mainly at low values of the argon flow rate, when the yield of the sputtered atoms is comparable to the contribution of the argon atoms fed into the discharge. The change in discharge power and Ar flow rate did not significantly affect the electron temperature and the EEDF. At relatively low energy values ( < 12 eV) the EEPF resembled the Maxwellian distribution with a temperature value approximately 4.2 eV, but at large energy values the distribution showed a much steeper decrease that depended on the argon flow rate. The depletion of the EEPF in its high-energy tail could be the effect of the high-energy electron loss at the magnetron chamber wall [18]. The electrons with kinetic energy exceeding the retarding potential drop of the ion sheath at the magnetron chamber wall can reach the wall and recombine with positive ions. The height of the potential barrier for the low-energy electrons is determined by the voltage of the ionic sheath at the plasma – wall interface, which is given by the difference between plasma potential and wall potential. Since the magnetron chamber wall was grounded, the d.c. value of the plasma potential set a d.c. value of the height of the wall sheath potential barrier to a value less than 10 eV. Thus, the electrons with kinetic energy values larger than 10 eV may escape and recombine at the magnetron chamber wall. The inelastic collisions may also enhance the loss of high-energy electrons. Electrons with kinetic energy values larger than the first excitation energy threshold (11.54 eV) or ionization energy threshold (15.66 eV) are lost also by inelastic collisions with the gas atoms. Therefore, the electron loss rate should be larger in the high-energy part than in the low-energy part of electron kinetic energy spectrum. Because of the depletion of the EEPF in high-energy electrons, the effective values of the electron temperature (two thirds of the electron average energy) were smaller than the electron temperature computed by the slope of the EEPF logarithm. Thus at a discharge power of 100 W in Ar at 4 mTorr, the EEPF logarithm showed a slope corresponding to a temperature value approximately 4.2 eV, while the effective electron temperature had values approximately 3.7 eV (see Fig. 6). The plasma potential showed a very weak dependence on the discharge power and, by the increase of the argon flow rate, it decreased slightly from values approximately 8 V at 0.5 sccm to values approximately 6 V at 4 sccm. Since the d.c. component of the discharge current should be zero (the
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discharge is coupled to the r.f. power supply through a d.c. current blocking capacitor in the matching box), the voltage across the ionic sheath, which forms at the magnetron chamber wall should be close to the plasma floating potential. Since the chamber of the magnetron was grounded, the plasma nearby the wall and deposition substrate has positive values. The richer is plasma in highenergy electrons, the higher the plasma potential is. Therefore, the little higher values of the plasma potential at low argon flow rate indicate the presence of more high-energy electrons in the plasma nearby the wall of the magnetron chamber. 3.2. Dependence of the plasma parameters on the gas pressure It is well known that the gas pressure has a major effect on the discharge plasma parameters. Usually, the electronatom inelastic collision rates are increased by the increase of the gas pressure with a large effect on the plasma density, electron temperature and EEDF. From the point of view of plasma as a fluid, the gas pressure has a major impact on the plasma transport coefficients, which determine the particle and power balance of the plasma. We performed probe measurements at gas pressure values ranged between 1 and 20 mTorr and constant discharge power and argon flow rate. Fig. 5 shows the dependence of plasma density on argon pressure for three values of the discharge power. By increase of the gas pressure, the plasma density increased steeply at low gas pressure ( < 2 mTorr) while it increased little at higher gas pressure. The dependence of the plasma density on the gas pressure can be understood if the escape mechanism of the high-energy electron from the strong magnetic field near the cathode target is analyzed. The high-energy electrons, which are emitted by the cathode target and accelerated into the cathode sheath, are trapped in the transversal magnetic field of the target and produce the negative glow plasma through ionization collisions. They can escape from the region of high transversal magnetic filed through either
Fig. 5. Dependence of the plasma density on the argon pressure at three values of the discharge power (the argon flow rate was 4 sccm).
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collisions with neutral atoms or drift towards regions of low magnetic field intensity. At very low pressure ( < 2 mTorr) the momentum transfer collisions between the high-energy electrons and neutral atoms are rare and they do not contribute to the electron escape from the trapping magnetic field. In this case, the ionization probability in the negative glow plasma is proportional to the gas pressure and the plasma density allover the magnetron chamber increases steeply by the increase of the gas pressure. As the gas pressure increases towards higher values, the increase of the ionization probability is counteracted by the increase of the escape probability of the highenergy electrons from the trapping magnetic field of magnetron through momentum transfer collisions with neutral atoms. Therefore, the plasma density increases little by the increase of the argon pressure over 2 mTorr. By the increase of the gas pressure, the electron mean energy (effective temperature) decreases (see the Fig. 6). At low gas pressure, the electrons lose little energy in inelastic collisions with the atoms and their mean value of the kinetic energy (electron temperature) is high. As the pressure increases, the electron kinetic energy loss through electron inelastic collisions with neutral atoms increases and the electron temperature decreases. A difference between the electron temperature computed by the slope of the EEPF logarithm in the low-energy region and the effective temperature computed by the electron mean energy was observed for whole range of the gas pressure values and this difference was larger at low pressure values. This means that the departure of EEPF from the Maxwellian distribution is larger at low pressure. Semilogarithmic plots of EEPF measured at four values of the gas pressure and a discharge power of 100 W are presented in Fig. 7. The slope of EEPF logarithm in the low-energy region showed a strong dependence on the gas pressure. A depletion of electron energy distribution in the high-energy electrons was noticed for the whole range of gas pressure values. As it was discussed above, this depletion of EEDF in the high-energy region is due to the inelastic (excitation and ionization) electron-atom collisions and high-energy electron loss through recombination at the magnetron chamber wall.
Fig. 7. EEPF at four values of Ar pressure (the argon flow rate was 4 sccm and the r.f. discharge power was 200 W).
The microstructure and properties of the deposited films has been noticed to depend on the gas pressure [9]. Thus, the films deposited at pressure values lower than 4 mTorr presented a heterogeneous amorphous and crystalline (nano crystals of rutile) structure, while films deposited at pressure values larger than 10 mTorr were amorphous. The films and their substrates (silicon wafers) were grounded during the deposition, which fixed the voltage drop across the ionic sheath at the film-plasma interface to the d.c. value of the plasma potential. The ion sheath at the film-plasma interface generated a flux of positive ions of Ar+, Ti+, O+ and O2 + that impinged the film surface during the deposition. The positive ion bombardment of a film surface during deposition increases the mobility of the adatoms and leads to a more compact film microstructure [19]. However, it was observed that crystalline films were obtained at low values of the gas pressure, at which the flux density of low-energy ions coming from the nearby plasma was weak (due to the low values of the plasma density). This discrepancy was explained by taking into account that the kinetic energy of the positive ions coming from the nearby plasma is too small ( < 10 eV) to induce a mobility of the adatoms over a scale long enough to lead to the formation of a crystalline microstructure [9]. Growing of crystalline films is favored by the high-energy ion impingement more than the low-energy ion impingement. The change of the gas pressure resulted also in a change of the film surface bombardment by the energetic particles coming directly from the magnetron target [20] (sputtered O ions accelerated in the cathode sheath nearby the target, energetic backscattered Ar+, and electrons escaped from the magnetic trap of the magnetron cathode). Because of the scattering collisions, the increase of the gas pressure weakened the flux of high-energy particles going from the cathode region of the magnetron discharge towards the growing film, which may explain the observed dependence of the deposited film structure on the gas pressure.
4. Conclusion Fig. 6. Dependence of the electron temperature on the argon pressure at the argon flow rate of 4 sccm and two values of the discharge power.
A cylindrical Langmuir probe was used to measure the density, temperature and energy distribution function of the
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electrons of plasma nearby the deposition substrate of a radio frequency magnetron discharge used in deposition of TiO2 thin films. The dependence of these plasma parameters on the discharge power, gas pressure and gas flow rate was experimentally determined. Optical emission spectroscopy measurements have shown that the sputtering process affects the gas composition at low values of the argon flow rate. This may explain the observed change of the electron density by the increase of the argon flow rate. The oxygen atoms resulting from sputtering of titanium oxide magnetron target form negative ions in the discharge plasma, which may affect the electron density. By the increase of the gas pressure, the electron density raised sharply at low gas pressure values ( < 2 mTorr) and very little at large pressure values. The effect of the gas pressure on the electron density was discussed on the basis of the effect of collisions on the high-energy electron movement in the trapping magnetic filed of the magnetron. Besides the increasing effect of the ionization probability in the electron trapping magnetic field of the magnetron, the increase of pressure causes also an increase in the escaping probability of the high-energy electrons through their momentum transfer collisions with neutral atoms. Therefore, at relatively large values of the gas pressure the two effects counterbalance each other and the electron density increases little by further increase of the gas pressure. The electron temperature computed by the mean kinetic energy of the electrons has shown a strong dependence on the gas pressure. Due to the low inelastic collision probability, large electron temperature values were measured at low gas pressure. By the increase of the gas pressure, the electron temperature decreased from approximately 5 eV at 1 mTorr to approximately 3 eV at 20 mTorr. It appeared that the electron temperature depended little on the discharge power and gas flow rate. The values for the electron temperature computed from the slope of the EEPF logarithm in the low energy region ( < 12 eV) were slightly larger than the effective values of the electron temperature computed by the mean electron kinetic energy. The differ-
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ence of the two temperature values is due to the departure of the EEDF from the Maxwellian distribution. At low electron kinetic energy values, the electron energy distributions resembled the Maxwell distribution, but at high electron kinetic energy values the distributions showed depletion. This depletion of electron energy distribution on high-energy electrons could be explained by the highenergy electron loss through inelastic collisions and recombination at the chamber wall of the magnetron.
References [1] F.H. Jones, Surf. Sci. Rep. 42 (2001) 75. [2] A. Fujishima, K. Hashimoto, T. Watanabe, TiO2 Photocatalysis Fundamentals and Applications, BKC Inc, Tokyo, 1999. [3] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, et al, Thin Solid Films 351 (1999) 260. [4] M. Takahashi, K. Tsukigi, T. Uchino, T. Yoko, Thin Solid Films 388 (2001) 231. [5] M. Nakamura, S. Kato, T. Aoki, L. Sirghi, Y. Hatanaka, J. Appl. Phys. 90 (2001) 3391. [6] L. Sirghi, Y. Hatanaka, Surf. Sci. 530 (2003) L323. [7] X. Wang, Y. Yu, X. Hu, L. Gao, Thin Solid Films 371 (2000) 148. [8] M. Nakamura, T. Aoki, Y. Hatanaka, D. Korzec, J. Engemann, J. Mater. Res. 16 (2001) 621. [9] L. Sirghi, T. Aoki, Y. Hatanaka, Thin Solid Films 422 (2002) 55. [10] K. Okimura, A. Shibata, N. Maeda, K. Tachibana, Y. Noguchi, K. Tsuchida, Jpn. J. Appl. Phys. 34 (1995) 4950. [11] K. Okimura, A. Shibata, Jpn. J. Appl. Phys. 36 (1997) 313. [12] M. Nakamura, T. Aoki, Y. Hatanaka, D. Korzec, J. Engemann, J. Mater. Res. 16 (2001) 621. [13] V.A. Godyak, R.B. Piejak, B.M. Alexandrovich, Plasma Sources Sci. Technol. 1 (1992) 36. [14] T.E. Sheridan, M.J. Goeckner, J. Goree, Jpn. J. Appl. Phys. 34 (1995) 4977. [15] M.J. Druyvesteyn, Z. Phys. 64 (1930) 781. [16] G.A. Martin, J.R. Fuhr, W.L. Wiese, J. Phys. Chem. Ref. Data 17 (Suppl) (1988) 3. [17] L. Bhale, P.R. Rao, Proc. Indian Acad. Sci. A 67 1968, pp. 350. [18] L. Sirghi, T. Kimura, K. Ohe, Phys. Plasmas 4 (1997) 1160. [19] R. Messier, A.P. Giri, R.A. Roy, J. Vac. Sci. Technol. A2 (1984) 500. [20] K. Tominaga, M. Chong, Y. Shintani, J. Vac. Sci. Technol. A 12 (1994) 1435.
Diagnostics of the radio frequency magnetron discharge plasma used for TiO2 thin film sputtering deposition Lucel Sirghi a,*, Toru Aoki b, Yoshinori Hatanaka c a
Faculty of Physics, ‘Al. I. Cuza’ University, blvd. Carol I, no. 11, Iasi 700506, Romania b Research Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan c Aichi University of Technology, 50-2 Manori, Nishihazama, Gamagori 443-0047, Japan Received 27 June 2003; accepted in revised form 30 January 2004 Available online 20 April 2004
Abstract The density, temperature and energy distribution function of the electrons of the plasma nearby the deposition substrate of a radio frequency magnetron discharge used for sputtering deposition of TiO2 thin films were measured by a cylindrical Langmuir probe. In the lowenergy domain (0 – 15 eV), the electron energy distributions resembled a Maxwell distribution with a temperature value that decreased from 5 to 3 eV by the increase of the argon pressure from 1 to 20 mTorr. In the high-energy domain the electron energy distribution showed a depletion. At low values (approx. 0.5 sccm) the flow rate of argon affected the plasma density. This effect was attributed to the gas composition change induced by the atoms sputtered from the cathode titanium dioxide target, change that was proved by optical emission spectroscopy measurements. D 2004 Elsevier B.V. All rights reserved. Keywords: Radio frequency magnetron discharge; Titanium oxide; Langmuir probe; Electron density; Optical emission spectroscopy
1. Introduction Recently, titanium dioxide (TiO2) has received much attention as a coating material because of its good bio compatible [1], photocatalytic and hydrophilic [2] properties. The UV light induced hydrophilicity of this material is related to its photocatalytic property, which is due to the strong oxidizing power of UV light generated holes in the valence band. Studies of the photoinduced hydrophilicity of different crystal faces revealed that this property depends on the surface structure, particularly on the density of oxygen bridging sites [3]. Recent studies revealed the importance that the charge carriers transport phenomena, which depend on the thickness [4] and microscopic structure [5] of the TiO2 films, have for the UV induced hydrofilicity [6] or photocatalytic activity. Therefore, a strong dependence of surface properties on the preparation method for TiO2 coatings is expected. A variety of deposition techniques have been developed to produce TiO2 films with good surface proper-
* Corresponding author. Tel.: +40-232-201186; fax: +40-232-201150. E-mail address: [email protected] (L. Sirghi). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.035
ties [7,8]. Among these methods, the deposition by r.f. magnetron sputtering of a pure ceramic TiO2 target is known to produce films with good hardness and adhesion to the substrate, but with relatively low photocatalytic activity [8]. The low photocatalytic activity of these TiO2 thin films has been attributed to their heterogeneous, crystalline and amorphous, mesostructure [9]. The crystalline structure of TiO2 thin films obtained by r.f. magnetron sputtering of a pure titanium target in argon – oxygen mixture gas has been reported to depend strongly on the gas pressure [10]. This discharge parameter affects the plasma density, electron temperature and the flux and energy of positive ions bombarding the growing film [11]. The crystalline structure of films deposited on heated substrates by r.f. magnetron sputtering of a sintered TiO2 target in argon– oxygen mixture gas has been also found to depend on gas pressure, amorphous films being deposited at relatively large pressure values [9,12]. Mass spectroscopy measurements showed that the development of the rutile phase in the TiO2 films deposited by reactive r.f. magnetron sputtering is favored by the bombardment of the growing film by energetic Ar+ ions [11]. Generally, because of the large mobility of the electrons, the surface of a growing film is biased negatively
L. Sirghi et al. / Surface & Coatings Technology 187 (2004) 358–363
with respect to the nearby plasma, which results in the occurrence of an ionic sheath that accelerates the positive ions towards the film surface. The temperature and density of the plasma electrons nearby the ionic sheath determine the positive ion flux towards the film surface. Therefore, the temperature and density of the electrons of the plasma in the vicinity of the deposition substrate are relevant plasma parameters in determining the structure and surface properties of the deposited film. This paper reports results concerning the effect of the gas pressure, gas flow rate and r.f. discharge power, on the temperature, density and energy distribution function of the plasma electrons nearby the deposition substrate of a TiO2 deposition system that used the r.f. magnetron sputtering of a sintered TiO2 target in an argon plasma. The results and discussion presented here are based on measurements of the Langmuir probe current – voltage characteristics and optical emission spectra of the plasma nearby the deposition substrate. The results concerning the dependence of the deposited film properties on the deposition parameters has been published elsewhere [9].
2. Experimental apparatus Fig. 1 shows a sketch of the experimental apparatus used in the experiments. The magnetron chamber consisted of a stainless steel cylinder with the diameter of 39 cm and height of 19 cm, which was connected to a vacuum system (minimum pressure of 10 6 Torr). The working gas (high purity argon) was fed into the chamber through a mass flow controller (MFC). The gas pressure was controlled by the conductance of the chamber connection to the vacuum pumps. The magnetron target, which consisted of a sintered TiO2 (99.99% purity) disk of 10 cm in diameter, was installed at the bottom of the chamber and connected to the r.f. power supply through a matching box. The deposition substrate holder was mounted at a distance of 10 cm from the magnetron target. A Langmuir probe consisting of a tungsten wire with a length of 5 mm and a diameter of 0.15 mm was installed at 1 cm from the substrate holder and 4 cm from the chamber axis. Because of the relatively large distance between the target and the Langmuir probe (approx. 10 cm), the probe measurements were not affected by the magnetic filed of the target. The I(V) characteristic of the probe was digitally acquired by a data acquisition system comprised of a digital analog converter (DAC) that generated a rump biasing voltage ranged from 30 V to + 30 V, a differential operational amplifier (OPA) that collected the current signal across a resistor R installed in the probe biasing circuit, an analog digital converter (ADC) that converted the probe current signal into digital data, and a personal computer (PC). Three parallel LC circuits tuned on the fundamental and second harmonic components of the r.f. discharge current (13.56 MHz), respectively, were installed in series in the probe circuit in order to reject the r.f.
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Fig. 1. Sketch of the experimental setup.
component of the probe current [13] (for simplicity, only one LC filter is represented in Fig. 1). Before each measurement, the probe was biased to a high positive potential (200 – 300 V) and heated to incandescence by the electronic current drown from the plasma in order to prevent the probe surface contamination by the deposition of titanium oxide. Each probe characteristic, Ip(Vp), was digitally acquired as a sequence of 1200 probe current data (16-bit resolution) by increasing of the probe bias potential from 30 V to + 30 V in steps of 50 mV. The probe current was smoothed using a smoothing technique based on a convolution of the data with a Gaussian error distribution function with a half-width of 600 mV. To determine the electron energy distribution function (EEDF), plasma potential (Vs), electron temperature (Te) and electron density (ne), the second derivative of the probe current was computed by a method using the least square fitting of a sequence of 40 probe current data (which corresponded to 2 V biasing voltage interval) with a thirdorder polynomial function. Assuming an isotropic plasma [14], the EEDF, f(u)u1/2, was determined using the Druyvesteyn [15] formula: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 ! 2me Vs Vp d Ip 2 f ðuÞu1=2 ju¼eðVs Vp Þ ¼ 2 ; ð1Þ e Ap e dVp2 where me and e are the electron mass and electric charge, respectively, u is the electron kinetic energy, Vp, the probe biasing potential and Ap is the probe surface area. The plasma potential Vs was identified as the probe voltage at which the second derivative of probe current crossed through zero. Taking into account the level of noise in the Langmuir probe current data, a noise level approximately 10 4 in the normalized EEDF data has been estimated, so that the results at energies larger than 20 eV were discarded. The electron density was computed by the EEDF integral over the whole resolved energy spectrum: where umax = 20 eV. ne ¼ mu0max f ðuÞu1=2 du;
ð2Þ
To compare the electron energy distribution function obtained in various experimental conditions, the electron
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Fig. 2. Typical optical emission spectrum of the negative glow plasma (discharge power was 100 W and the argon flow rate was 6 sccm). The light intensity signal in the range 300 – 500 nm is represented multiplied by ten on the plot.
energy probability function (EEPF) was computed as f(u) of which integral Eq. (2) was normalized to unity. For the energy domain where the EEPF resembled the Maxwellian distribution, the electron temperature was computed from the slope of EEPF logarithm: TEEPF ¼
dln f ðuÞ 1 : du
ð3Þ
The optical spectroscopy measurements were performed with a resolution of 3 nm by an optical spectrometer (Hamamatsu Plasma Processing Monitor C6670-04) with an optical fiber probe that collected the light emitted by the negative glow plasma. Typical emission spectrum of the light emitted by the negative glow plasma showing the emission lines corresponding to Ar, O2+, Ti and Ti+ is presented in Fig. 2. Because of the relatively low resolution of the optical spectrometer, the emission lines corresponding to Ti and Ti+ (337.1, 337.3 and 336.1 nm) [16] are merged on the spectrum. Also, the lines of the second negative emission system of O2+ (312.3, 314.1, 321 and 323.1 nm) [17] are merged into a broad peak. From the strong emission lines of Ar spectrum corresponding to 4p ! 4s transitions in the 650 –950 nm range we have chosen the line at 750.4 nm.
Fig. 3. Dependence of the plasma density on the Ar flow rate at four values of the discharge power (Ar pressure was 4 mTorr).
flow rate, the electron density increased at small values (approx. 0.5 sccm) and remained constant at values larger than 1 sccm. However, this pattern changed for the case of a low discharge power (50 W), when the electron density remained constant for the whole range of the argon flow rate values. This dependence may be explained by the effect of the sputtered O and Ti atoms on the discharge plasma. At low argon flow rate and relatively high discharge power the gas composition in the discharge chamber is modified by the sputtering process and a large concentration of Ti and O atoms is expected. Because of their high electron affinity, the O atoms capture low energy electrons from plasma and form negative ions with the effects of decreasing of density and increasing of temperature of the plasma electrons. This phenomenon is negligible at low discharge power values ( < 50 W), when the sputtering rate is low. To prove the effect of sputtering on the gas composition, we performed optical emission spectroscopy measurements of the light emitted by the negative glow plasma. Fig. 4 presents the dependence on the argon flow rate of the emission intensities of some lines from Ar (line 750.4 nm), Ti and Ti+ (lines 337.1, 337.3 and 336.1 nm) and O2+ (lines 312.3, 314.1, 321 and 323.1 nm) emission spectra. The discharge power was kept at 100 W and gas pressure at 4 mTorr in these measurements. By the increase of the argon flow rate, the intensity of the lines corresponding to Ar
3. Results and discussion We studied the effect of gas flow rate, discharge power and gas pressure on the electron density, temperature and energy distribution function. Optical emission spectroscopy measurements were performed to support the explanation of plasma parameter dependence on the gas flow rate. 3.1. Dependence of plasma parameters on the r.f. discharge power and gas flow rate Fig. 3 presents the dependence of the plasma density near the deposition substrate on the flow rate of argon at four values of the discharge power. By the increase of the argon
Fig. 4. Intensity of lines corresponding to Ar, O+2,Ti+ and Ti emission in negative glow plasma as they changed by the increase of the argon flow rate (the discharge power was 100 W and the gas pressure was 4 mTorr).
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emission spectrum increased, while the intensity of the lines corresponding to Ti, Ti+ and O2+ emission spectra decreased. The line intensities changed largely at low values of the argon flow rate and very little at large values of the argon flow rate. Since the Langmuir probe measurements have not showed significant changes of the electron temperature by the change of the gas flow rate, the change in the light emission intensity is attributed mainly to the changes of plasma density and gas composition. The optical emission spectroscopy measurements show that the important changes in the gas composition occur mainly at low values of the argon flow rate, when the yield of the sputtered atoms is comparable to the contribution of the argon atoms fed into the discharge. The change in discharge power and Ar flow rate did not significantly affect the electron temperature and the EEDF. At relatively low energy values ( < 12 eV) the EEPF resembled the Maxwellian distribution with a temperature value approximately 4.2 eV, but at large energy values the distribution showed a much steeper decrease that depended on the argon flow rate. The depletion of the EEPF in its high-energy tail could be the effect of the high-energy electron loss at the magnetron chamber wall [18]. The electrons with kinetic energy exceeding the retarding potential drop of the ion sheath at the magnetron chamber wall can reach the wall and recombine with positive ions. The height of the potential barrier for the low-energy electrons is determined by the voltage of the ionic sheath at the plasma – wall interface, which is given by the difference between plasma potential and wall potential. Since the magnetron chamber wall was grounded, the d.c. value of the plasma potential set a d.c. value of the height of the wall sheath potential barrier to a value less than 10 eV. Thus, the electrons with kinetic energy values larger than 10 eV may escape and recombine at the magnetron chamber wall. The inelastic collisions may also enhance the loss of high-energy electrons. Electrons with kinetic energy values larger than the first excitation energy threshold (11.54 eV) or ionization energy threshold (15.66 eV) are lost also by inelastic collisions with the gas atoms. Therefore, the electron loss rate should be larger in the high-energy part than in the low-energy part of electron kinetic energy spectrum. Because of the depletion of the EEPF in high-energy electrons, the effective values of the electron temperature (two thirds of the electron average energy) were smaller than the electron temperature computed by the slope of the EEPF logarithm. Thus at a discharge power of 100 W in Ar at 4 mTorr, the EEPF logarithm showed a slope corresponding to a temperature value approximately 4.2 eV, while the effective electron temperature had values approximately 3.7 eV (see Fig. 6). The plasma potential showed a very weak dependence on the discharge power and, by the increase of the argon flow rate, it decreased slightly from values approximately 8 V at 0.5 sccm to values approximately 6 V at 4 sccm. Since the d.c. component of the discharge current should be zero (the
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discharge is coupled to the r.f. power supply through a d.c. current blocking capacitor in the matching box), the voltage across the ionic sheath, which forms at the magnetron chamber wall should be close to the plasma floating potential. Since the chamber of the magnetron was grounded, the plasma nearby the wall and deposition substrate has positive values. The richer is plasma in highenergy electrons, the higher the plasma potential is. Therefore, the little higher values of the plasma potential at low argon flow rate indicate the presence of more high-energy electrons in the plasma nearby the wall of the magnetron chamber. 3.2. Dependence of the plasma parameters on the gas pressure It is well known that the gas pressure has a major effect on the discharge plasma parameters. Usually, the electronatom inelastic collision rates are increased by the increase of the gas pressure with a large effect on the plasma density, electron temperature and EEDF. From the point of view of plasma as a fluid, the gas pressure has a major impact on the plasma transport coefficients, which determine the particle and power balance of the plasma. We performed probe measurements at gas pressure values ranged between 1 and 20 mTorr and constant discharge power and argon flow rate. Fig. 5 shows the dependence of plasma density on argon pressure for three values of the discharge power. By increase of the gas pressure, the plasma density increased steeply at low gas pressure ( < 2 mTorr) while it increased little at higher gas pressure. The dependence of the plasma density on the gas pressure can be understood if the escape mechanism of the high-energy electron from the strong magnetic field near the cathode target is analyzed. The high-energy electrons, which are emitted by the cathode target and accelerated into the cathode sheath, are trapped in the transversal magnetic field of the target and produce the negative glow plasma through ionization collisions. They can escape from the region of high transversal magnetic filed through either
Fig. 5. Dependence of the plasma density on the argon pressure at three values of the discharge power (the argon flow rate was 4 sccm).
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collisions with neutral atoms or drift towards regions of low magnetic field intensity. At very low pressure ( < 2 mTorr) the momentum transfer collisions between the high-energy electrons and neutral atoms are rare and they do not contribute to the electron escape from the trapping magnetic field. In this case, the ionization probability in the negative glow plasma is proportional to the gas pressure and the plasma density allover the magnetron chamber increases steeply by the increase of the gas pressure. As the gas pressure increases towards higher values, the increase of the ionization probability is counteracted by the increase of the escape probability of the highenergy electrons from the trapping magnetic field of magnetron through momentum transfer collisions with neutral atoms. Therefore, the plasma density increases little by the increase of the argon pressure over 2 mTorr. By the increase of the gas pressure, the electron mean energy (effective temperature) decreases (see the Fig. 6). At low gas pressure, the electrons lose little energy in inelastic collisions with the atoms and their mean value of the kinetic energy (electron temperature) is high. As the pressure increases, the electron kinetic energy loss through electron inelastic collisions with neutral atoms increases and the electron temperature decreases. A difference between the electron temperature computed by the slope of the EEPF logarithm in the low-energy region and the effective temperature computed by the electron mean energy was observed for whole range of the gas pressure values and this difference was larger at low pressure values. This means that the departure of EEPF from the Maxwellian distribution is larger at low pressure. Semilogarithmic plots of EEPF measured at four values of the gas pressure and a discharge power of 100 W are presented in Fig. 7. The slope of EEPF logarithm in the low-energy region showed a strong dependence on the gas pressure. A depletion of electron energy distribution in the high-energy electrons was noticed for the whole range of gas pressure values. As it was discussed above, this depletion of EEDF in the high-energy region is due to the inelastic (excitation and ionization) electron-atom collisions and high-energy electron loss through recombination at the magnetron chamber wall.
Fig. 7. EEPF at four values of Ar pressure (the argon flow rate was 4 sccm and the r.f. discharge power was 200 W).
The microstructure and properties of the deposited films has been noticed to depend on the gas pressure [9]. Thus, the films deposited at pressure values lower than 4 mTorr presented a heterogeneous amorphous and crystalline (nano crystals of rutile) structure, while films deposited at pressure values larger than 10 mTorr were amorphous. The films and their substrates (silicon wafers) were grounded during the deposition, which fixed the voltage drop across the ionic sheath at the film-plasma interface to the d.c. value of the plasma potential. The ion sheath at the film-plasma interface generated a flux of positive ions of Ar+, Ti+, O+ and O2 + that impinged the film surface during the deposition. The positive ion bombardment of a film surface during deposition increases the mobility of the adatoms and leads to a more compact film microstructure [19]. However, it was observed that crystalline films were obtained at low values of the gas pressure, at which the flux density of low-energy ions coming from the nearby plasma was weak (due to the low values of the plasma density). This discrepancy was explained by taking into account that the kinetic energy of the positive ions coming from the nearby plasma is too small ( < 10 eV) to induce a mobility of the adatoms over a scale long enough to lead to the formation of a crystalline microstructure [9]. Growing of crystalline films is favored by the high-energy ion impingement more than the low-energy ion impingement. The change of the gas pressure resulted also in a change of the film surface bombardment by the energetic particles coming directly from the magnetron target [20] (sputtered O ions accelerated in the cathode sheath nearby the target, energetic backscattered Ar+, and electrons escaped from the magnetic trap of the magnetron cathode). Because of the scattering collisions, the increase of the gas pressure weakened the flux of high-energy particles going from the cathode region of the magnetron discharge towards the growing film, which may explain the observed dependence of the deposited film structure on the gas pressure.
4. Conclusion Fig. 6. Dependence of the electron temperature on the argon pressure at the argon flow rate of 4 sccm and two values of the discharge power.
A cylindrical Langmuir probe was used to measure the density, temperature and energy distribution function of the
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electrons of plasma nearby the deposition substrate of a radio frequency magnetron discharge used in deposition of TiO2 thin films. The dependence of these plasma parameters on the discharge power, gas pressure and gas flow rate was experimentally determined. Optical emission spectroscopy measurements have shown that the sputtering process affects the gas composition at low values of the argon flow rate. This may explain the observed change of the electron density by the increase of the argon flow rate. The oxygen atoms resulting from sputtering of titanium oxide magnetron target form negative ions in the discharge plasma, which may affect the electron density. By the increase of the gas pressure, the electron density raised sharply at low gas pressure values ( < 2 mTorr) and very little at large pressure values. The effect of the gas pressure on the electron density was discussed on the basis of the effect of collisions on the high-energy electron movement in the trapping magnetic filed of the magnetron. Besides the increasing effect of the ionization probability in the electron trapping magnetic field of the magnetron, the increase of pressure causes also an increase in the escaping probability of the high-energy electrons through their momentum transfer collisions with neutral atoms. Therefore, at relatively large values of the gas pressure the two effects counterbalance each other and the electron density increases little by further increase of the gas pressure. The electron temperature computed by the mean kinetic energy of the electrons has shown a strong dependence on the gas pressure. Due to the low inelastic collision probability, large electron temperature values were measured at low gas pressure. By the increase of the gas pressure, the electron temperature decreased from approximately 5 eV at 1 mTorr to approximately 3 eV at 20 mTorr. It appeared that the electron temperature depended little on the discharge power and gas flow rate. The values for the electron temperature computed from the slope of the EEPF logarithm in the low energy region ( < 12 eV) were slightly larger than the effective values of the electron temperature computed by the mean electron kinetic energy. The differ-
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ence of the two temperature values is due to the departure of the EEDF from the Maxwellian distribution. At low electron kinetic energy values, the electron energy distributions resembled the Maxwell distribution, but at high electron kinetic energy values the distributions showed depletion. This depletion of electron energy distribution on high-energy electrons could be explained by the highenergy electron loss through inelastic collisions and recombination at the chamber wall of the magnetron.
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