- Email: [email protected]
Optical properties of alumina and zirconia thin films grown by pulsed laser deposition
Surface
and Coatings
Technology
100-101
(1998)
415-419
Optical properties of alumina and zirconia thin films grown by pulsed laser deposition J. Gottmann Lehrstuhl,fiir
Lasertechnik
*, A. Husmann, T. Klotzbiicher,
E.W. Kreutz
der Rheinisch- Westftilischen Technischen Hachschule Aachen. Steinbachstraj’e O-51074 Aachen, Germaq
15.
Abstract Sintered targets of ZrO, and AllO, are ablated by KrF excimer laser radiation. The processing gas atmosphere consists of 0, at typical pressures of 1O-3-1 mbar. Films with a thickness of 200-700 nm are deposited on a Pt/Ti/Si multilayer substrate. The analytical techniques used for the determination of structural characteristics of the films are X-ray diffraction and electron microscopy. The thickness and the complex refraction index are determined by ellipsometry by fitting a model for the film geometry to the measured data. The optical film thickness at different wavelengths is determined using interference reflection photometry. The investigations concentrate on the influence of the oxygen pressure, the target-to-substrate distance and the laser fluence on the refraction index of the films. which is correlated with the film density. The compaction of the films is achieved by particles impinging with kinetic energies above 30 eV on the growing surface. The kinetic energy of the particles depending on the processing parameters is modelled and related to the resulting film properties. 0 1998 Elsevier Science S.A. Keywords:
Alumina;
Optical properties;
Pulsed laser deposition;
1. Introduction
ablated particles is modelled and related to the resulting film structure.
Thin film coatings are widely used in a variety of applications either as structural overcoats or as functional coatings. The former case implies thin films on machine tools to decrease wear or protective overcoats to shield surfaces from an adverse environment. The latter case, for example, implies thin films on optical components for reflection modifications or gas proof films as ion conductors in high-temperature fuel cells. Normally, smooth surfaces of small corrugation and stoichiometric balanced films with a defined structure are desired, e.g. columnar structure for thermal heat protection or dense amorphous structure for corrosion resistance. These requirements can be met using pulsed laser deposition (PLD) [I]. The objective of the present work is to produce alumina and zirconia thin films with a desired structure using PLD. The influence of the processing parameters processing gas pressure, laser fluence and target-tosubstrate distance on the mean kinetic energies of the
* Corresponding author. Tel: +49 241 89 06 470; Fax: +49 241 89 06 121; e-mail: [email protected] 0257~8972/98/$19.00 0 1998 Elsevier PIf SO257-8972(97)00661-O
Zirconia
Science B.V. All rights reserved.
2. Experimental details 2.1. Set-up A KrF excimer laser (wavelength, 248 nm; pulse duration, 20 ns (FWHM); pulse energy, ~400 mJ) is used. The laser radiation is formed and focused by a telescopic lens system to a 1.1 x 1.7 mm2 rectangular spot on the target surface at an angle of incidence of 45 ‘. A laser fluence of up to 8 J cm -2 can be achieved on the target. The processing gas in the deposition chamber consists of oxygen at pressures of 10p3-1 mbar. The heatable substrate is positioned at a distance of 3 cm from the target at an angle of 45 n between target normal and substrate normal. The substrate surface is parallel to the laser beam, and therefore the substrate can be moved laterally without disturbance of to the laser beam. The substrate consists of a (111 )-Si wafer, coated with a 10 nm Ti adhesion layer and a 100 nm Pt bottom electrode using a d.c.-sputter coater.
The film thickness and the refraction index are determined by ellipsometry, by fitting calculated Y-A values to lines-scan data across the inhomogeneous thickness distribution of the films, which is measured via singlewavelength ellipsometry [2]. The optical film thickness at different wavelengths is determined from interference figures measured by reflection spectroscopy. The analytical techniques used for the determination of structural characteristics of the films are X-ray diffraction (20 scan with grazing incidence) and scanning electron microscopy. The substrate temperature is measured by pyrometry of the rough SIC heater surface. 2.2. Deposition conditions The alumina thin films were deposited at room temperature using a fluence of 6 J crne2 by 3000-5000 pulses at a repetition rate of 30 Hz with a target-to-substrate distance (distance between laser spot on the target and point of intersection of target normal and substrate surface) of 3 cm. The pressure of the processing gas was varied in the range of 0.01-0.6 mbar. The zirconia thin films were deposited at room temperature at a target-to-substrate distance of 3 cm using a fluence of 3.6 or 7.2 J cme2 by 2000-5000 pulses at a repetition rate of 20 Hz and processing gas pressures of 0.01-0.5 mbar. Using a processing gas pressure of 0.15 mbar, zirconia thin films were deposited at a fluence of 3.6 J cmm2 and at substrate temperatures of 20-700 “C.
3. Modelling
after some impacts, all species have similar velocity distributions independent of their mass, e.g. a Maxwell distribution [7,8]. With these assumptions, the mean velocity, (tl), of the ablated particles and the dragged processing gas particles can be calculated from the laser pulse energy, E,, the number of ablated particles (in units of ablated ceramic molecules N,), the number of oxygen molecules No2 in the volume between a point on the target and a substrate area, A,, in units of the square of the lattice constant u2 of the ceramic, and the number of deposited monolayers per pulse, Nd, which is equal to the number of arriving ceramic molecules on the substrate area, A,. Half of the laser pulse energy is converted to kinetic energy of the ablated particles and the with-dragged oxygen molecules. The number of oxygen molecules dragged to the substrate area A, is No,/N, per ceramic molecule. Assuming the same velocity distribution for all species, the mean velocity is therefore
Assuming a Maxwell-distributed velocity, the mean velocity is 8% smaller than that calculated in Eq. (1). The number of ablated molecules, N, is determined experimentally by weighing the target, and the number of deposited monolayers per pulse, Nd, is calculated from thickness measurements with ellipsometry and complies to the number of molecules deposited on the substrate area, A, per pulse. For the mean kinetic energy of the particles depending on a specific particle mass, 17riholds therefore:
3.1. Kinetic energ?, of the particles (Ekin)i The compaction of growing alumina and zirconia thin films can be achieved by the transfer of momentum of the impinging particles to the film atoms [3]. In order to prevent sputtering of film atoms, which can result in oxygen deficient films due to preferential sputtering of oxygen [4], the kinetic energy of the particles has to be limited. Therefore, the mean kinetic energy of the impinging particles has to be controlled to allow the deposition of films with the desired stoichiometry and film structure. Assuming that approximately half of the laser pulse energy is converted into kinetic energy of the ablated particles [5], the energy balance can be considered. Because the number of processing gas particles between target and substrate is of the order of the number of ablated particles per pulse, and the mean free path is l-2 orders of magnitude smaller than the target-tosubstrate distance, the processing gas particles are dragged with the ablated particles [6]. This expansion process is not in a kinetic (thermal ) equilibrium, but
1 = -mi(u2)= 2
llli 1 EL -2 Na We, + Moz No1 INci ’
(2)
Because the number of oxygen molecules between target and substrate NoZ is proportional to the processing gas pressure and assuming that the number of ablated particles, Na, increases linearly with the laser pulse energy and that the number of deposited monolayers is Ndcx (eL/r.‘) where cL is the fluence on the target, the functional dependency of the mean kinetic energy of the impinging particles tni <&in > X
1 + cPOzr3/E,.
(3)
on particle mass, gas pressure, fluence and distance between target and a specific point on the substrate r can be assessed. For a species i with particle mass nri, the mean kinetic energy increases with the fluence E,. (Fig. 1) and decreases with processing gas pressure PO1 (Fig. 1) and distance r.
J. Gottmann
Zr parbcles
et al. / Surfhce
ablation with laser
of ZrO,
and Coatings
Technology
0.01
0.1 processing
gas
pressure
05
thickness
T2-2T+2TR
cos @
1+R2-2RcosQ T= tI t,
and
R=r,r,,
with (4)
where 5 is the transmission coefficient and r is the reflection coefficient of the air-film interface (index 1), respectively, of the film-substrate interface (index 2). FFar thephasq. @ hoIds
Fig. 2. Refraction thin films (right)
0 0 0 1 0.2 0.3 0.4 0 5 0 6 0 7 oxygen pressure PO, [mbar]
with the wavenumber of the light k and film thickness A. The. &&cSaRct 9, is m&ii&s& *>a \& ;ohase Q ‘>u varying the wavelength with its extremes at multiples of 7K. -2kw5ul‘e. ~WYhU\~~ 5 4--- ‘il-;e il&cxrs iI6 ‘he &wxns versus l/A yields a curve with slope s = 4dn(A), which is proportional to the optical film thickness depending on the wavelength. If the change of the refraction index n(R) of film and substrate between two extremes of the reflectanz is -smaS axiqard to n,
4. Results and discussion
4.1. Zirconiafilm The ellipsometrically determined refraction indices the zirconia ttin fi’ms increase with decreasing pxcx5ssing gas pressure duirng ckposition 1Fig. 2). Al processing gas pressures be.10~ 0.1 &oar {Auence of 3.6 J cmP2) and 0.14 mbar (7.2 J cm-‘), respectively, a refraction index of 2.18 & 0.02 at a wavelength of 633 nm
index, N, of zirconia thin films (left) versus processing gas pressure Po,.
and alumina
is achieved, which correlates with the bulk density of 5.2 g cmm3 [9]. With increasing pressure, the refraction index is decreases to 1.3-1.6, correlating with a density of 30-50% of the bulk value. The refraction index of these films is highest in the film centre and decreases with increasing target-to-substrate distance, Y, in qualitative agreement with Eq. (3). In comparison with the dependence of the mean kinetic energy on the processing gas pressure (Fig. 1 ), it can be estimated that particles with energies above 40-50 eV induce the compaction of the growing film [lo]. The decreasing porosity of the films with decreasing processing gas pressure is visible using scanning electron microscopy (Fig. 3). The absorqtion coefficient of the films is kcO.01. ,if L~~e-pruceSSrir~~~-p~SSu~-ls-~Ind;rt-ul-m~~I;
ad
0.01 mbar (at eL=3.6 J cmP2), the absorption coeffi&xv ,i%cTz&-s & ,&=d?&~~~d?i&? a+ ,i%*tn -0ms>\\> .YSSbEp .3%>. ‘h canbe e~&m~&& that Zr particles with mean kinetic energies of more y&g) &! ctxcsz pri+.~~~~~~~ ‘ilna h&~ 5 $??eaxTR~ u+ oxygen atoms from the growing film surface (Fig. 1). The refraction indices of Zr02 thin films (deposited with PO, = 0.15 mbar, eL = 3.6 J cm-‘) increase with substrate temperature during deposition Tsub from n = 1.6 (T,,,=2~ “C) to n=2.2Gf_G.C2 ~Ts,b=40WW “C 3 in agreement with the zone model [ L!]. Films deposited at elevated substrate temperatures ( T,,,=433-?3?l % $ &we z c.h-k cFy%2&~~L~~~~~~~~: 3&q diffraction data show a strong (111) peak at 0=30 ’ and smaller peaks at 6’ = 35 ’ (200), 50 o (220) and 60 ’ (311). Films deposited at room temperature are amorphous, independent on the processing gas pressure, showing a broad short-range order peak at 0 = 30-33 @. Interference reflection spectroscopy indicates that the refraction index n =2.20 f0.05 of ZrO, films is nearly constant in the range of h=450-\QQQnm. At a skorter wave~m~h, tne refracliun inbex inmease3 fn =2.% _+%I at x =380nm and ?P=3.% _+0.2 at 3,=25onmj. Hence, interference spectroscopy is useful for thickness determination of thick films (l-5 pm) using wavelengths of at
05
0 05 0.10 0 15 0.20 0.25 oxygen pressure PO, [mbar]
PO, [mbar]
The reflectance, Rx, of a film on a reflecting substrate can be calculated using the Fresnel equations to R,ccl+
411
415-419
target
Fig. 1, Mean kinetic energy of Zr (Al ) particles during the deposition of ZrO, ( Al,O,) films depending on processing gas pressure and laser fluence with a target-to-substrate distance of 3 cm according to Eq. (2).
3.2. OpticalJilrn
(1998)
fluence
0.00
0.005
100-101
418
Fig. 3. Scanning electron micrograph (SEM film deposited at a processing gas pressure
) of cross-sections of zirconia thin films deposited at a fluence of 0.3 mbar; right. dense film deposited at 0.1 mbar.
45OGlOOO nm. where the variation of n with two extremes of the reflectance is negligible.
i. between
4.2. Alun~inaJilnu The refraction index of alumina thin films increases with decreasing processing gas pressure P,, (Fig. 2). At pressures of up to 0.1 mbar, a refraction index of II= 1.72 F0.02 (bulk value 1.766) is achieved, which corresponds to a density of 94+ 3% of the bulk density. Al particles with mean kinetic energies above about 40 eV are necessary for compaction of the growing film (Fig. 1). At processing gas pressures above 0.2 mbar, the refractive index decreases to 1.3- 1.4, which corresponds to a density of 40-50% of the bulk value. The decreasing porosity of the columnar AlzO, films with decreasing processing gas pressure is visible using scanning electron microscopy (Fig. 4). At a processing gas pressure of 0.01 mbar, dense, smooth and homogenous films can be deposited at room temperature (Fig. 4). X-ray diffraction data of the films deposited at 7’,,,=20 C show only a broad short-range order peak around 28=25 “C besides the substrate peaks, indicating, that the films are amorphous independent of their density.
Fig. 4. Scanning electron microgrdph (SEM ) of cross-sections film deposited at a processing gas pressure of 0.3 mbar: right,
of Ed = 3.6 J cm -I. Left, porous
columnar
5. Conclusions Dense alumina and zirconia thin films are deposited using PLD without any further ion-bombardment or heat treatment. Amorphous zirconia thin films with a bulk density and alumina thin films with a density of up to 94% of the bulk density are achieved using low processing gas pressures (0.01-o. 1 mbar). Processing gas pressures of 0.1-0.5 mbar lead to porous, columnar and amorphous films with a density of 30-50% of the bulk density. Zirconia thin films deposited on heated substrates (400-700 “C) have a bulk density and cubic crystal structure. For compaction of the growing zirconia films at room temperature, the mean kinetic energy of the Zr particles has to be above 40-50 eV, and to prevent preferential sputtering of oxygen, the mean kinetic energy has to be below about 100 eV. The compaction of the alumina films is achieved by Al particles with a mean energy of more than 40 eV. The relation between the mean kinetic energy of the impinging film-forming particles and the processing parameters processing gas pressure, fluence and target-to-substrate distance presented here allows the processing parameters to be determined for pulsed laser deposition of films with the desired structure for certain applications.
of alumina thin films deposited at a fluence of E,. = 6 J cm -l. Left. dense uniform film deposited at 0.01 mbar.
porous
columnar
J. Gotttnunn
et ul. / Surj&e
and Coatings
Acknowledgement We are grateful to C. and S. Hoffmann (IWE, RWTH Aachen) for assistance in XRD-measurements, scanning electron microscopy and for stimulating discussions. Part of the work is financially supported by the BMBF (Bundesministerium ftir Bildung und Forschung), reference-no. : 13 N 6647/3.
References [I] E.W. Kreutz, M. Alunovic, A. Voss, W. Alleging, H. Sung. D.A. Wesner, Surf. Coat. Technol 6869 (1994) 238. [2] H.G. Tompkins, A Users Guide to Ellipsometry, Academic Press, San Diego. CA, 1993. [3] F.A. Smidt, Int. Mater. Rev. 35 (1990) 61.
Technology
100-101
(1998)
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419
[4] J. Gonzalo. C.N. Alfonso. J. Perriere, Appl. Phys. Lett. 67 (1995) 1335. [5] M. Mertin, PhD thesis. RWTH-Aachen, Shaker, Aachen, 1996. [6] D.B. Geohegan, A.A. Puretzky. Appl. Phys. Lett. 67 (1995) 197. [7] D.B. Chrisey, G.K. Hubler. Pulsed Laser Deposition of Thin Films, Wiley, New York. 1994. [8] M. Mertin. D. OtYenberg, C.W. An, D.A. Wesner. T. Klotzbiicher. E.W. Kreutz. Appl. Surf. Sci. 969798 (1996) 842. [9] P.J. Martin. R.P. Netterfield. W.G. Sainty. J. Appl. Phys. 55 ( 1984) 235. [lo] K.-H. Miiller. R.P. Netterfield, P.J. Martin, Phys. Rev. B 35 ( 1986) 2934. [ 1 l] G.A. Smith. L.-C. Chen, M.-C. Chuang. Photons and low energy particles in surface processing. Mater. Res. Sot. Symp. Proc. 236 (1992) 429. [l?] B.A. Movchan, A.V. Demchishin. Phys. Met. Metallogr. 28 (1969) 83.
and Coatings
Technology
100-101
(1998)
415-419
Optical properties of alumina and zirconia thin films grown by pulsed laser deposition J. Gottmann Lehrstuhl,fiir
Lasertechnik
*, A. Husmann, T. Klotzbiicher,
E.W. Kreutz
der Rheinisch- Westftilischen Technischen Hachschule Aachen. Steinbachstraj’e O-51074 Aachen, Germaq
15.
Abstract Sintered targets of ZrO, and AllO, are ablated by KrF excimer laser radiation. The processing gas atmosphere consists of 0, at typical pressures of 1O-3-1 mbar. Films with a thickness of 200-700 nm are deposited on a Pt/Ti/Si multilayer substrate. The analytical techniques used for the determination of structural characteristics of the films are X-ray diffraction and electron microscopy. The thickness and the complex refraction index are determined by ellipsometry by fitting a model for the film geometry to the measured data. The optical film thickness at different wavelengths is determined using interference reflection photometry. The investigations concentrate on the influence of the oxygen pressure, the target-to-substrate distance and the laser fluence on the refraction index of the films. which is correlated with the film density. The compaction of the films is achieved by particles impinging with kinetic energies above 30 eV on the growing surface. The kinetic energy of the particles depending on the processing parameters is modelled and related to the resulting film properties. 0 1998 Elsevier Science S.A. Keywords:
Alumina;
Optical properties;
Pulsed laser deposition;
1. Introduction
ablated particles is modelled and related to the resulting film structure.
Thin film coatings are widely used in a variety of applications either as structural overcoats or as functional coatings. The former case implies thin films on machine tools to decrease wear or protective overcoats to shield surfaces from an adverse environment. The latter case, for example, implies thin films on optical components for reflection modifications or gas proof films as ion conductors in high-temperature fuel cells. Normally, smooth surfaces of small corrugation and stoichiometric balanced films with a defined structure are desired, e.g. columnar structure for thermal heat protection or dense amorphous structure for corrosion resistance. These requirements can be met using pulsed laser deposition (PLD) [I]. The objective of the present work is to produce alumina and zirconia thin films with a desired structure using PLD. The influence of the processing parameters processing gas pressure, laser fluence and target-tosubstrate distance on the mean kinetic energies of the
* Corresponding author. Tel: +49 241 89 06 470; Fax: +49 241 89 06 121; e-mail: [email protected] 0257~8972/98/$19.00 0 1998 Elsevier PIf SO257-8972(97)00661-O
Zirconia
Science B.V. All rights reserved.
2. Experimental details 2.1. Set-up A KrF excimer laser (wavelength, 248 nm; pulse duration, 20 ns (FWHM); pulse energy, ~400 mJ) is used. The laser radiation is formed and focused by a telescopic lens system to a 1.1 x 1.7 mm2 rectangular spot on the target surface at an angle of incidence of 45 ‘. A laser fluence of up to 8 J cm -2 can be achieved on the target. The processing gas in the deposition chamber consists of oxygen at pressures of 10p3-1 mbar. The heatable substrate is positioned at a distance of 3 cm from the target at an angle of 45 n between target normal and substrate normal. The substrate surface is parallel to the laser beam, and therefore the substrate can be moved laterally without disturbance of to the laser beam. The substrate consists of a (111 )-Si wafer, coated with a 10 nm Ti adhesion layer and a 100 nm Pt bottom electrode using a d.c.-sputter coater.
The film thickness and the refraction index are determined by ellipsometry, by fitting calculated Y-A values to lines-scan data across the inhomogeneous thickness distribution of the films, which is measured via singlewavelength ellipsometry [2]. The optical film thickness at different wavelengths is determined from interference figures measured by reflection spectroscopy. The analytical techniques used for the determination of structural characteristics of the films are X-ray diffraction (20 scan with grazing incidence) and scanning electron microscopy. The substrate temperature is measured by pyrometry of the rough SIC heater surface. 2.2. Deposition conditions The alumina thin films were deposited at room temperature using a fluence of 6 J crne2 by 3000-5000 pulses at a repetition rate of 30 Hz with a target-to-substrate distance (distance between laser spot on the target and point of intersection of target normal and substrate surface) of 3 cm. The pressure of the processing gas was varied in the range of 0.01-0.6 mbar. The zirconia thin films were deposited at room temperature at a target-to-substrate distance of 3 cm using a fluence of 3.6 or 7.2 J cme2 by 2000-5000 pulses at a repetition rate of 20 Hz and processing gas pressures of 0.01-0.5 mbar. Using a processing gas pressure of 0.15 mbar, zirconia thin films were deposited at a fluence of 3.6 J cmm2 and at substrate temperatures of 20-700 “C.
3. Modelling
after some impacts, all species have similar velocity distributions independent of their mass, e.g. a Maxwell distribution [7,8]. With these assumptions, the mean velocity, (tl), of the ablated particles and the dragged processing gas particles can be calculated from the laser pulse energy, E,, the number of ablated particles (in units of ablated ceramic molecules N,), the number of oxygen molecules No2 in the volume between a point on the target and a substrate area, A,, in units of the square of the lattice constant u2 of the ceramic, and the number of deposited monolayers per pulse, Nd, which is equal to the number of arriving ceramic molecules on the substrate area, A,. Half of the laser pulse energy is converted to kinetic energy of the ablated particles and the with-dragged oxygen molecules. The number of oxygen molecules dragged to the substrate area A, is No,/N, per ceramic molecule. Assuming the same velocity distribution for all species, the mean velocity is therefore
Assuming a Maxwell-distributed velocity, the mean velocity is 8% smaller than that calculated in Eq. (1). The number of ablated molecules, N, is determined experimentally by weighing the target, and the number of deposited monolayers per pulse, Nd, is calculated from thickness measurements with ellipsometry and complies to the number of molecules deposited on the substrate area, A, per pulse. For the mean kinetic energy of the particles depending on a specific particle mass, 17riholds therefore:
3.1. Kinetic energ?, of the particles (Ekin)i The compaction of growing alumina and zirconia thin films can be achieved by the transfer of momentum of the impinging particles to the film atoms [3]. In order to prevent sputtering of film atoms, which can result in oxygen deficient films due to preferential sputtering of oxygen [4], the kinetic energy of the particles has to be limited. Therefore, the mean kinetic energy of the impinging particles has to be controlled to allow the deposition of films with the desired stoichiometry and film structure. Assuming that approximately half of the laser pulse energy is converted into kinetic energy of the ablated particles [5], the energy balance can be considered. Because the number of processing gas particles between target and substrate is of the order of the number of ablated particles per pulse, and the mean free path is l-2 orders of magnitude smaller than the target-tosubstrate distance, the processing gas particles are dragged with the ablated particles [6]. This expansion process is not in a kinetic (thermal ) equilibrium, but
1 = -mi(u2)= 2
llli 1 EL -2 Na We, + Moz No1 INci ’
(2)
Because the number of oxygen molecules between target and substrate NoZ is proportional to the processing gas pressure and assuming that the number of ablated particles, Na, increases linearly with the laser pulse energy and that the number of deposited monolayers is Ndcx (eL/r.‘) where cL is the fluence on the target, the functional dependency of the mean kinetic energy of the impinging particles tni <&in > X
1 + cPOzr3/E,.
(3)
on particle mass, gas pressure, fluence and distance between target and a specific point on the substrate r can be assessed. For a species i with particle mass nri, the mean kinetic energy increases with the fluence E,. (Fig. 1) and decreases with processing gas pressure PO1 (Fig. 1) and distance r.
J. Gottmann
Zr parbcles
et al. / Surfhce
ablation with laser
of ZrO,
and Coatings
Technology
0.01
0.1 processing
gas
pressure
05
thickness
T2-2T+2TR
cos @
1+R2-2RcosQ T= tI t,
and
R=r,r,,
with (4)
where 5 is the transmission coefficient and r is the reflection coefficient of the air-film interface (index 1), respectively, of the film-substrate interface (index 2). FFar thephasq. @ hoIds
Fig. 2. Refraction thin films (right)
0 0 0 1 0.2 0.3 0.4 0 5 0 6 0 7 oxygen pressure PO, [mbar]
with the wavenumber of the light k and film thickness A. The. &&cSaRct 9, is m&ii&s& *>a \& ;ohase Q ‘>u varying the wavelength with its extremes at multiples of 7K. -2kw5ul‘e. ~WYhU\~~ 5 4--- ‘il-;e il&cxrs iI6 ‘he &wxns versus l/A yields a curve with slope s = 4dn(A), which is proportional to the optical film thickness depending on the wavelength. If the change of the refraction index n(R) of film and substrate between two extremes of the reflectanz is -smaS axiqard to n,
4. Results and discussion
4.1. Zirconiafilm The ellipsometrically determined refraction indices the zirconia ttin fi’ms increase with decreasing pxcx5ssing gas pressure duirng ckposition 1Fig. 2). Al processing gas pressures be.10~ 0.1 &oar {Auence of 3.6 J cmP2) and 0.14 mbar (7.2 J cm-‘), respectively, a refraction index of 2.18 & 0.02 at a wavelength of 633 nm
index, N, of zirconia thin films (left) versus processing gas pressure Po,.
and alumina
is achieved, which correlates with the bulk density of 5.2 g cmm3 [9]. With increasing pressure, the refraction index is decreases to 1.3-1.6, correlating with a density of 30-50% of the bulk value. The refraction index of these films is highest in the film centre and decreases with increasing target-to-substrate distance, Y, in qualitative agreement with Eq. (3). In comparison with the dependence of the mean kinetic energy on the processing gas pressure (Fig. 1 ), it can be estimated that particles with energies above 40-50 eV induce the compaction of the growing film [lo]. The decreasing porosity of the films with decreasing processing gas pressure is visible using scanning electron microscopy (Fig. 3). The absorqtion coefficient of the films is kcO.01. ,if L~~e-pruceSSrir~~~-p~SSu~-ls-~Ind;rt-ul-m~~I;
ad
0.01 mbar (at eL=3.6 J cmP2), the absorption coeffi&xv ,i%cTz&-s & ,&=d?&~~~d?i&? a+ ,i%
05
0 05 0.10 0 15 0.20 0.25 oxygen pressure PO, [mbar]
PO, [mbar]
The reflectance, Rx, of a film on a reflecting substrate can be calculated using the Fresnel equations to R,ccl+
411
415-419
target
Fig. 1, Mean kinetic energy of Zr (Al ) particles during the deposition of ZrO, ( Al,O,) films depending on processing gas pressure and laser fluence with a target-to-substrate distance of 3 cm according to Eq. (2).
3.2. OpticalJilrn
(1998)
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0.005
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Fig. 3. Scanning electron micrograph (SEM film deposited at a processing gas pressure
) of cross-sections of zirconia thin films deposited at a fluence of 0.3 mbar; right. dense film deposited at 0.1 mbar.
45OGlOOO nm. where the variation of n with two extremes of the reflectance is negligible.
i. between
4.2. Alun~inaJilnu The refraction index of alumina thin films increases with decreasing processing gas pressure P,, (Fig. 2). At pressures of up to 0.1 mbar, a refraction index of II= 1.72 F0.02 (bulk value 1.766) is achieved, which corresponds to a density of 94+ 3% of the bulk density. Al particles with mean kinetic energies above about 40 eV are necessary for compaction of the growing film (Fig. 1). At processing gas pressures above 0.2 mbar, the refractive index decreases to 1.3- 1.4, which corresponds to a density of 40-50% of the bulk value. The decreasing porosity of the columnar AlzO, films with decreasing processing gas pressure is visible using scanning electron microscopy (Fig. 4). At a processing gas pressure of 0.01 mbar, dense, smooth and homogenous films can be deposited at room temperature (Fig. 4). X-ray diffraction data of the films deposited at 7’,,,=20 C show only a broad short-range order peak around 28=25 “C besides the substrate peaks, indicating, that the films are amorphous independent of their density.
Fig. 4. Scanning electron microgrdph (SEM ) of cross-sections film deposited at a processing gas pressure of 0.3 mbar: right,
of Ed = 3.6 J cm -I. Left, porous
columnar
5. Conclusions Dense alumina and zirconia thin films are deposited using PLD without any further ion-bombardment or heat treatment. Amorphous zirconia thin films with a bulk density and alumina thin films with a density of up to 94% of the bulk density are achieved using low processing gas pressures (0.01-o. 1 mbar). Processing gas pressures of 0.1-0.5 mbar lead to porous, columnar and amorphous films with a density of 30-50% of the bulk density. Zirconia thin films deposited on heated substrates (400-700 “C) have a bulk density and cubic crystal structure. For compaction of the growing zirconia films at room temperature, the mean kinetic energy of the Zr particles has to be above 40-50 eV, and to prevent preferential sputtering of oxygen, the mean kinetic energy has to be below about 100 eV. The compaction of the alumina films is achieved by Al particles with a mean energy of more than 40 eV. The relation between the mean kinetic energy of the impinging film-forming particles and the processing parameters processing gas pressure, fluence and target-to-substrate distance presented here allows the processing parameters to be determined for pulsed laser deposition of films with the desired structure for certain applications.
of alumina thin films deposited at a fluence of E,. = 6 J cm -l. Left. dense uniform film deposited at 0.01 mbar.
porous
columnar
J. Gotttnunn
et ul. / Surj&e
and Coatings
Acknowledgement We are grateful to C. and S. Hoffmann (IWE, RWTH Aachen) for assistance in XRD-measurements, scanning electron microscopy and for stimulating discussions. Part of the work is financially supported by the BMBF (Bundesministerium ftir Bildung und Forschung), reference-no. : 13 N 6647/3.
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