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Laser-induced physical vapour deposition of diamond-like carbon films
Diamond and Related Materials, 2 (1992) 233-238
233
Laser-induced physical vapour deposition of diamond-like carbon films F. Mfiller and K. Mann Laser-Laboratorium G6ttingen eV, Im Hassel 21, W-MOOGdttingen (Germany)
Abstract Diamond-like carbon (DLC) films have been grown on various substrates at low temperatures and low pressure by ablation of carbon particles using KrF excimer laser pulses of 30 ns duration. It is shown that the film properties strongly depend on the energy density of the incident laser beam and the deposition temperature. At energy densities above 8 J c m - 2 and low substrate temperatures (below 200 °C), the coatings are transparent while, at lower energy densities or higher substrate temperatures, only opaque films are obtained. The thin films were characterized by optical spectroscopy, X-ray diffraction, Raman scattering and scanning electron microscopy. In addition to film growth and characterization, the kinetic energies and massesof laser-ablated carbon ions have been investigated by time-of-flightspectroscopy. We observe an almost linear relation between kinetic particle energy and laser energy density, with maximum values as high as 220 eV at 23 J c m - 2 , indicating a strong correlation between the laser energy density, the particle energies and the DLC film properties.
1. Introduction The laser-induced physical vapour deposition (LPVD) process has already become state of the art for the deposition of high T~ superconducting films [1]. Owing to the stoichiometric ablation of alloyed target materials and substantial particle energies within the lasergenerated plasma, the L P V D process offers great advantages also for the deposition of other novel materials such as hard coatings, e.g. TiN, SiC and a m o r p h o u s diamond-like carbon (DLC) films I-2-5]. The latter are of increasing interest as protective coatings for IR optical applications, in particular because of their high and broad-band optical transparency, high hardness and scratch resistance, thermal conductivity and chemical inertness against any solvent [6, 7]. The main advantage of the L P V D process with respect to D L C deposition is the possibility of film growth at low substrate temperatures and low pressures, in the absence of aggressive atomic hydrogen. Earlier studies have also revealed larger portions of diamond bonds than in the case of other deposition techniques 1-8]. However, there are still m a n y questions left open regarding the mechanism of diamond bond formation using a pulsed-laser-generated plasma; hence, fundamental investigations of this plasma will be of great value for an improved understanding as well as a possible upscaling of the process. In this work we have studied the correlation between the laser energy density and plasma emission characteris-
0925 9635/93/$6.00
tics on the one hand, and subsequent D L C film formation on the other hand. For these investigations a computer-controlled L P V D chamber has been constructed, the details of which are described in Section 2. The energy and mass distribution of excimer-laserablated carbon particles, as used for the D L C deposition on various substrates, have been determined by time-offlight (TOF) spectroscopy (Sections 3 and 4). The influence of these quantities on the properties and morphology of the deposited films gives insight into the mechanisms of diamond bond formation in the L P V D process, as discussed in Section 5.
2. Experimental set-up A schematic representation of the experimental setup is shown in Fig. 1. The deposition is performed in an ultrahigh vacuum chamber pumped by a wide-range turbomolecular p u m p (Balzers T P U 450). The vacuum system has a base pressure of less than 1 x 10-7 mbar; the residual gas composition is monitored with a quadrupole mass spectrometer (Leybold P G A 100). For thin film deposition, targets are ablated with an excimer laser (Lambda Physik E M G 202) of 248 nm wavelength and 30 ns duration at a repetition rate of 10 Hz. In order to obtain the necessary energy density and a homogeneous spatial intensity profile for ablation, the beam is imaged by a reducing spherical lens through an ultrahigh vacuum window (fused silica) onto the target,
© 1993 - - Elsevier Sequoia. All rights reserved
234
F. Miiller, K. Mann / Laser-induced PVD of DLC films
variable ttenuator
exci me r
spatial beam profiling
laser
pulse energy TOF analyser
E~.
L
su0stroto
n0ow ................................................................ uartz temporalprofile iii
..
quadrupole mass spec, 4 axis
stepper motor ] ~ [ ~ .
contro,
beam profiling system
.L_: x,y
•
..........
/
~0 o o
/tL
1, I,.................. r~
('o/
. . . . . . . . j . .................
1
\
Fig. 1. Computer-controlled experimental set-up for LPVD process.
using an angle of incidence of 45 °. Opposite to the target a heatable substrate is mounted on a manipulator at a distance of typically 8 cm. During deposition, both the target and the substrate to be coated can be rotated by computer-controlled stepper motors, allowing efficient use of the target material as well as a homogeneous film thickness on the substrate. In order to determine the energy density of the laser radiation in the target plane, the two-dimensional intensity profile is measured with a UV camera in combination with a personal-computer-based laserbeam-profiling system [9]. For this purpose the camera is brought into the chamber instead of the target manipulator. Corresponding measurements reveal a relatively smooth profile without any hot spots, with an irradiated target area of 6.4 x 10 -3 cm 2. The overall energy is measured with a pyroelectric energy monitor.
3. Film deposition and characterization Carbon films have been deposited on polished and ultrasonically cleaned stainless steel and fused silica substrates of 2 0 m m diameter. Sintered graphite of purity 99.97% is used as the target material and irradiated with 2 x 104 laser pulses for each substrate. A new target site is exposed every 2 x 103 pulses; these parameters are kept constant for all the samples mentioned in this paper. Using a conical nozzle, an argon gas stream from the ultrahigh vacuum window onto the target is applied in order to avoid deposition of ablated particles on the window. The corresponding
pressure during deposition is about 1 x 10-3mbar. Carbon film deposition has been carried out at different substrate temperatures (from room temperature to 300°C) and different laser fluences--(2 15 J cm-2). Deposition rates have been found to increase with increasing laser fluence, reaching values of greater than 0.1 A pulse-1. Owing to the relatively high distance of 8 cm from the target the entire substrate area could be homogeneously coated at this rate. The films deposited at high substrate temperatures (above 200°C) and lower fluences are opaque and conducting, indicating a graphite bond character. The adherence to the substrate and scratch resistance are bad. On the contrary, transparent films with good substrate adhesion are obtained at low substrate temperatures and at high laser fluences (greater than 8 J cm-2). In these conditions the coatings are electrically insulating and have a scratch resistance higher than that of conventional optical coatings. From X-ray diffraction analysis we know that the films are amorphous with a very small crystalline portion, which can partially be attributed to microcrystalline diamond. A corresponding scanning electron micrograph is shown in Fig. 2, from which typical crystalline diamond structures such as cubes, pentadodecahedra, rhombododecahedra, and octahedra of micrometre size can be identified [10]. In addition to the far more numerous amorphous clusters, such microcrystals can lead to an increased surface roughness. On the contrary, when a substrate bias voltage is used, the films become homogeneous and smooth, with r.m.s, roughness values equal to the uncoated substrate (Fig. 3).
F. Miiller, K. Mann / Laser-induced PVD of DLC films
235
i Fig. 2. Scanning electron micrograph of diamond crystallites formed during pulsed-laser deposition of amorphous carbon films on a fused silica substrate (H= 12 J cm-2; T=25 °C).
The infuence of the laser energy density and substrate temperature on the transmission characteristics of the coatings, as measured with a UV-visible-near-IR spectrometer (Perkin-Elmer Lambda 19), is displayed in Fig. 4 in comparison with those of the uncoated fused silica substrate. The films grown at high laser fluences and low substrate temperatures show a high transmission particularly in the near-IR spectral range, in accordance with DLC coatings deposited with conventional chemical vapour deposition techniques [11]. Obviously the coatings deposited at low energy densities or high substrate temperatures show a much higher absorption over the entire spectral range, indicating a larger portion of sp 2 bonds. For further characterization of the deposited films, Raman spectra have been measured using the 514.5 nm line of an argon ion laser for excitation. Figure 5 shows the dependence of these spectra on the laser energy density. The Raman spectrum of the film deposited at low laser fluences (below 4 J cm -2) exhibits two broad peaks at 1340 cm 1 and 1580 cm- 1 (spectrum a), which are characteristic for sp2-dominated glassy carbon [12, 13]. The spectrum looked completely different for the smooth film deposited at higher laser energy densities and use of a bias voltage of 200 V, with only one broad peak centred around 1580 cm -1 (spectrum b). Because of its mechanical, electrical and optical properties the latter is DLC [14]. Note that the sharp peak of crystalline diamond at 1331 cm ~ is absent [15].
4. Plasma analysis
For an investigation of the energy and mass distributions of laser ablated ionic species, a T O F spectrometer has been installed perpendicular to the target surface
~/,i"Jf'l
u m
r
(a)
(b) Fig. 3. Scanning electron micrographs of DLC films deposited on fused silica (a) with a bias voltage of 200 V and (b) without bias voltage.
instead of the substrate manipulator. The corresponding set-up is shown in Fig. 6. After ablation the particles enter a field-free drift tube of length s = 60 cm; positive ions are detected at the end of this tube with an electron multiplier. After amplification and shaping, the ion pulses are displayed temporally resolved on a fast digital storage oscilloscope, which is triggered by the ablating
F. Miiller, K. Mann / Laser-induced PVD of DLC films
236
uncoated substrate
100
H=12J/cm 2 ,T=25 C
80 @
60
~ 40 20 506
1000
15l}O
2000
2500
3001
wavelength [nm] Fig. 4. Transmission spectra of carbon films deposited on fused silica substrates with KrF laser at different laser energy densities and different substrate temperatures in comparison to the uncoated substrate.
tO)
._-.q r--
E r'd
I
1200
1
I
I
1400
I
1600
1800
wavenumber [cm ] Fig. 5. Raman spectra of carbon films deposited on fused silica at room temperature: spectrum a, glassy carbon deposited at H= 4 J cm-2; spectrum b, amorphous DLC deposited at H= 15 J cm-2.
photo diode i ',.~.v i amplifier
pulse shaper
.
excimer laser •
c ~ m ~
multiplier p
~rmm
/
- - ~ 1
stop ~ ~ digital storage oscilloscope
t
........................ ~"1.[-Lllh ["JJ'~ /~/~\ PC used as multichannelanalyser
Fig. 6. TOF arrangement for laser plasma analysis: PC, personal computer.
laser pulse. The trace obtained for a single pulse is transferred via an IEEE interface to a personal computer, which integrates the spectrum for a pre-selected number of pulses. For mass analysis the target is biased against a grid to a voltage U = + 2.5 kV. Mass separation is possible, since the corresponding energy of the accelerated ions of mass m and charge q is much higher than the particle energy Eab I obtained during the ablation process. The kinetic energy is given by
m(s:
Eki n = ' ~
= q U + Eab I
(1)
Having identified the masses of the ablated ions, this set-up can also be used for investigation of the initial particle energies gab I. For these measurements the target is grounded and the ions enter the drift tube with an energy given only by the laser ablation process. For a given mass number and measured flight time t the kinetic energy gab I c a n be calculated from eqn. (l). Plots of the count rate of laser-ablated carbon ions vs. T O F are presented in Fig. 7. In any case the spectra are divided into two temporal regions: one of fast atomic or small molecular ions and a broad distribution at higher flight times, which can be attributed to formation of large carbon clusters. As seen from Fig. 7, the data show a strong correlation between the laser energy density and the corresponding particle velocities. While the peaks of the small particles become sharper with increasing laser fluence, the broad cluster distributions are shifted towards higher flight times (lower velocities). Obviously the cluster size becomes larger with an increasing portion of absorbed energy. On the contrary, the peaked velocity distributions of the smaller ions observed above about 10 J cm -2 are shifted to the left for increasing fluence, indicating an enhancement of the kinetic energies of the ablated particles Czee below). As a further result, we observe a strongly increased portion of carbon clusters compared with smaller ions with increasing fluence. From the analysis of the smaller molecular masses corresponding to the peaks in the T O F spectra, it can be seen that, below 9 J cm-2, atomic carbon is formed in the plasma while, at higher fluences, only molecules of three or more atoms are observed as the smallest particles. Having identified the masses, we can calculate the kinetic ,energies Eab I from the flight times of the corresponding ions. In Fig. 8 these energies are compiled for C + and C~ as a function of the laser fluence, which has been varied from 0.5 to 23 J cm -2. We observe an almost linear relation, with maximum kinetic energies as high as 220 eV at 23 J cm -2. Obviously, the energy of the small molecular ions is independent of the mass and only a function of the laser fluence. Absolute values of the kinetic energy are in agreement with those in ref. 8.
F. Mailer, K. Mann / Laser-induced PVD of DLC Jilms
f]uence
H='~5.2',
C;
237
O;
I~
j,/cm 2
~~
t
.D
.
L"
X:5.23 J,.'cm~
,I
2
1
H=4.38 J/cm 2
Jo (a)
20
30
time of flight
40
250 (b)
[/,'s]
500
time of f l i g h t
750
1000
[~'s]
Fig. 7. T O F spectra of ablated carbon ions (without accelerating voltage): (a) fast atomic and small molecular particles; (b) distribution of slower particles (i.e. large clusters).
250
250
200
.
.
.
.
.
.
.
.
.
+ .
200
o
150
150
100
100
c
o c
50
0
50
~ 5
= 10 laser
~ 15 fluence
r 20
L 25
0 30
[J/cm 2]
Fig. 8. Kinetic energy of ablated carbon ions as a function of incident laser energy density: D, C+; + , C~.
5. Discussion
In this work we have investigated the deposition of carbon films by the LPVD technique. In agreement with the results of other workers I-8, 16] we find that the formation of DLC films is already possible at moderate laser power densities (greater than 3 x 108W cm 2), indicating the advantage of the higher photon energies in comparison with the use of visible or IR lasers [17]. X-ray diffraction analysis reveals that, independent of the laser energy density, all deposited coatings are amorphous; nevertheless, scanning electron micrographs indicate that a very small amount of micrometre-sized crystalline diamond particles can be formed. In order to obtain a better understanding of the
mechanisms responsible for DLC growth, the two separated steps of the LPVD process, i.e. target ablation and subsequent deposition of carbon particles on a substrate, have to be investigated in detail. For this purpose the energy and mass distributions of the positive ions in the ablation plume and their correlation with the thin film microstructure and film properties have been studied using TOF spectroscopy. Although the experimental results cannot directly be extended to the (probably much stronger) emission of neutral particles, which are not detectable with the given set-up, some important dependences could be obtained. First of all, we observe a linear relationship between the kinetic energy of ablated carbon ions and the laser energy density, with a slope of approximately 10eV j - l c m 2 (see Fig. 8). A pronounced DLC character of the deposited films, as demonstrated particularly by the corresponding Raman spectra (see Fig. 5), is observed above a threshold of roughly 8 J cm- 2 (i.e. 2.7 x 108 W cm 2 for 30 ns pulses). This is equivalent to a relatively high kinetic energy of about 80 eV, at which an efficient preferential sputtering of spZ-bonded carbon is already very likely [18]. Hence, the data support the corresponding theoretical model for an enrichment of sp3-bonded sites described by Miyazawa et al. [19]. The predominant role of the kinetic particle energy with respect to DLC growth may also be deduced from the interesting observation that the small ionic particles are assuming almost discrete kinetic energy values above a threshold laser fluence, which coincides with the onset for DLC formation. On the contrary, this threshold also 'coincides with an increased occurrence of C [ and C ; ions (see Fig. 7), indicating the additional influence of
238
F. Mfiller, K. Mann / Laser-induced PVD of DLC films
the mass distribution of the emitted particles in the plasma plume on the film growth. It is conceivable that these particles are already in a metastable sp3-bonded configuration, which is frozen after deposition on the substrate surface. This may be justified by the observed substrate temperature dependence of the coating properties, i.e. DLC character only at lower temperatures. A similar quenching process has been discussed elsewhere
[8]. A major disadvantage of the higher laser fluences necessary for DLC film deposition is the increased portion of large clusters observed both in the TOF spectra and in the scanning electron micrographs of the films, which leads to absorption and scatter losses not tolerable for coatings with optical quality [12, 20]. However, this effect could be reduced by the use of a bias voltage. The influence of this bias on the plasma plume may be explained by additional acceleration of charged particles, which leads to decomposition of the clusters during impact on the substrate. Further improvements with respect to the suppression of cluster formation can be expected for the near future from laser beam homogenizing optics, which will guarantee a flat-topped laser intensity profile and thus an extremely uniform target ablation. Moreover, the use of a short-pulse excimer laser (500 fs at 248 nm) is planned for DLC growth. The first TOF measurements of ablated graphite using this laser have already shown that the formation of particle clusters can be totally avoided. Acknowledgments The authors would like to thank T. R6dle of the IV. Physical Institute of the University of G6ttingen for carrying out the Raman investigations.
References 1 G. Endres, B. Roas, K. J. Schmatjko and L. Schultz, Opt. Elektron. Mag., 6 (1990) 66. 2 E. D'Anna, G. Leggieri, A. Luches, M. Martino, A. V. Drigo, I. N. Mihailescu and S. Ganatsios, J. Appl. Phys. 69 (3) (1991) 1687. 3 M. Balooch, R. J. Tench, W. J. Siekhaus, M. J. Allen, A. L. Connor and D. R. 0lander, Appl. Phys. Lett., 57 (15) (1990) 1540. 4 S. S. Wagal, E. M. Juengerman and C. B. Collins, Appl. Phys. Lett., 53 (3) (1988) 187. 5 F. Davanloo, T. J. Lee, D. R. Jander, H. Park, J. H. You and C. B. Collins, J. Appl. Phys., 71 (3) (1992) 1446. 6 A. Bubenzer, B. Dischler, G. Brandt and P. Koidl, Opt. Eng., 23 (2) (1984) 153. 7 P. K. Bachmann, D. Leers and D. U. Wiechert, Ber. Bunsenges. Phys. Chem., 11 (1991) 95. 8 J. J. Cuomo, D. L. Pappas, J. Bruley, J. P. Doyle and K. L. Saenger, J. Appl. Phys., 70 (3) (1991) 1706. 9 K. Mann, A. Hopfm/iller, H. Gerhardt, P. Gorzellik, R. Schild, W. St6ffler, H. Wagner and G. Wolbold, Laser Optoelektron., 24 (l) (1992) 42. 10 H. J. R6sler, Lehrbueh der Mineralogie, VEB Verlag f/ir Grundstoffindustrie, Leipzig, 1981, p. 290. 11 W. M/iller-Sebert, Ch. Wilde, P. Koidl, N. Herres, J. Wagner and T. Eckermann, Mater. Sci. Eng., Bll (1992) 173. 12 L. Soukup, I. Gregora, L. Jastrabik and A. Konakova, Mater Sci. Eng., B l l (1992) 355. 13 P. K. Bachmann and D. U. Wiechert, Diamond and Diamond-like Films and Coatings, Plenum, New York, 1991, p. 677. 14 M. Kamo and Y. Sato, Mater. Sci. Eng., B l l (1992) 191. 15 P. V. Huong, Mater. Sci. Eng., B l l (1992) 235. 16 T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, Appl. Phys. A, 45 (1988) 355. 17 D. L. Pappas, K. L. Saenger, J. Bruley, W. Krakow, J. J. Cuomo, T. Gu and R. W. Collins, J. Appl. Phys., 71 (11) (1992) 5675. 18 J. A. Thornton, in R. F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Park Ridge, NJ, 1982, Chap. 2, p. 30. 19 T. Miyazawa, S. Misawa, S. Yoshida and S. Gonda, J. Appl. Phys., 55 (1984) 188. 20 K. Mann and H. Gerhardt, Proe. Soc. Photo-Opt. Instrum. Eng., 1503 (1991) 176.
233
Laser-induced physical vapour deposition of diamond-like carbon films F. Mfiller and K. Mann Laser-Laboratorium G6ttingen eV, Im Hassel 21, W-MOOGdttingen (Germany)
Abstract Diamond-like carbon (DLC) films have been grown on various substrates at low temperatures and low pressure by ablation of carbon particles using KrF excimer laser pulses of 30 ns duration. It is shown that the film properties strongly depend on the energy density of the incident laser beam and the deposition temperature. At energy densities above 8 J c m - 2 and low substrate temperatures (below 200 °C), the coatings are transparent while, at lower energy densities or higher substrate temperatures, only opaque films are obtained. The thin films were characterized by optical spectroscopy, X-ray diffraction, Raman scattering and scanning electron microscopy. In addition to film growth and characterization, the kinetic energies and massesof laser-ablated carbon ions have been investigated by time-of-flightspectroscopy. We observe an almost linear relation between kinetic particle energy and laser energy density, with maximum values as high as 220 eV at 23 J c m - 2 , indicating a strong correlation between the laser energy density, the particle energies and the DLC film properties.
1. Introduction The laser-induced physical vapour deposition (LPVD) process has already become state of the art for the deposition of high T~ superconducting films [1]. Owing to the stoichiometric ablation of alloyed target materials and substantial particle energies within the lasergenerated plasma, the L P V D process offers great advantages also for the deposition of other novel materials such as hard coatings, e.g. TiN, SiC and a m o r p h o u s diamond-like carbon (DLC) films I-2-5]. The latter are of increasing interest as protective coatings for IR optical applications, in particular because of their high and broad-band optical transparency, high hardness and scratch resistance, thermal conductivity and chemical inertness against any solvent [6, 7]. The main advantage of the L P V D process with respect to D L C deposition is the possibility of film growth at low substrate temperatures and low pressures, in the absence of aggressive atomic hydrogen. Earlier studies have also revealed larger portions of diamond bonds than in the case of other deposition techniques 1-8]. However, there are still m a n y questions left open regarding the mechanism of diamond bond formation using a pulsed-laser-generated plasma; hence, fundamental investigations of this plasma will be of great value for an improved understanding as well as a possible upscaling of the process. In this work we have studied the correlation between the laser energy density and plasma emission characteris-
0925 9635/93/$6.00
tics on the one hand, and subsequent D L C film formation on the other hand. For these investigations a computer-controlled L P V D chamber has been constructed, the details of which are described in Section 2. The energy and mass distribution of excimer-laserablated carbon particles, as used for the D L C deposition on various substrates, have been determined by time-offlight (TOF) spectroscopy (Sections 3 and 4). The influence of these quantities on the properties and morphology of the deposited films gives insight into the mechanisms of diamond bond formation in the L P V D process, as discussed in Section 5.
2. Experimental set-up A schematic representation of the experimental setup is shown in Fig. 1. The deposition is performed in an ultrahigh vacuum chamber pumped by a wide-range turbomolecular p u m p (Balzers T P U 450). The vacuum system has a base pressure of less than 1 x 10-7 mbar; the residual gas composition is monitored with a quadrupole mass spectrometer (Leybold P G A 100). For thin film deposition, targets are ablated with an excimer laser (Lambda Physik E M G 202) of 248 nm wavelength and 30 ns duration at a repetition rate of 10 Hz. In order to obtain the necessary energy density and a homogeneous spatial intensity profile for ablation, the beam is imaged by a reducing spherical lens through an ultrahigh vacuum window (fused silica) onto the target,
© 1993 - - Elsevier Sequoia. All rights reserved
234
F. Miiller, K. Mann / Laser-induced PVD of DLC films
variable ttenuator
exci me r
spatial beam profiling
laser
pulse energy TOF analyser
E~.
L
su0stroto
n0ow ................................................................ uartz temporalprofile iii
..
quadrupole mass spec, 4 axis
stepper motor ] ~ [ ~ .
contro,
beam profiling system
.L_: x,y
•
..........
/
~0 o o
/tL
1, I,.................. r~
('o/
. . . . . . . . j . .................
1
\
Fig. 1. Computer-controlled experimental set-up for LPVD process.
using an angle of incidence of 45 °. Opposite to the target a heatable substrate is mounted on a manipulator at a distance of typically 8 cm. During deposition, both the target and the substrate to be coated can be rotated by computer-controlled stepper motors, allowing efficient use of the target material as well as a homogeneous film thickness on the substrate. In order to determine the energy density of the laser radiation in the target plane, the two-dimensional intensity profile is measured with a UV camera in combination with a personal-computer-based laserbeam-profiling system [9]. For this purpose the camera is brought into the chamber instead of the target manipulator. Corresponding measurements reveal a relatively smooth profile without any hot spots, with an irradiated target area of 6.4 x 10 -3 cm 2. The overall energy is measured with a pyroelectric energy monitor.
3. Film deposition and characterization Carbon films have been deposited on polished and ultrasonically cleaned stainless steel and fused silica substrates of 2 0 m m diameter. Sintered graphite of purity 99.97% is used as the target material and irradiated with 2 x 104 laser pulses for each substrate. A new target site is exposed every 2 x 103 pulses; these parameters are kept constant for all the samples mentioned in this paper. Using a conical nozzle, an argon gas stream from the ultrahigh vacuum window onto the target is applied in order to avoid deposition of ablated particles on the window. The corresponding
pressure during deposition is about 1 x 10-3mbar. Carbon film deposition has been carried out at different substrate temperatures (from room temperature to 300°C) and different laser fluences--(2 15 J cm-2). Deposition rates have been found to increase with increasing laser fluence, reaching values of greater than 0.1 A pulse-1. Owing to the relatively high distance of 8 cm from the target the entire substrate area could be homogeneously coated at this rate. The films deposited at high substrate temperatures (above 200°C) and lower fluences are opaque and conducting, indicating a graphite bond character. The adherence to the substrate and scratch resistance are bad. On the contrary, transparent films with good substrate adhesion are obtained at low substrate temperatures and at high laser fluences (greater than 8 J cm-2). In these conditions the coatings are electrically insulating and have a scratch resistance higher than that of conventional optical coatings. From X-ray diffraction analysis we know that the films are amorphous with a very small crystalline portion, which can partially be attributed to microcrystalline diamond. A corresponding scanning electron micrograph is shown in Fig. 2, from which typical crystalline diamond structures such as cubes, pentadodecahedra, rhombododecahedra, and octahedra of micrometre size can be identified [10]. In addition to the far more numerous amorphous clusters, such microcrystals can lead to an increased surface roughness. On the contrary, when a substrate bias voltage is used, the films become homogeneous and smooth, with r.m.s, roughness values equal to the uncoated substrate (Fig. 3).
F. Miiller, K. Mann / Laser-induced PVD of DLC films
235
i Fig. 2. Scanning electron micrograph of diamond crystallites formed during pulsed-laser deposition of amorphous carbon films on a fused silica substrate (H= 12 J cm-2; T=25 °C).
The infuence of the laser energy density and substrate temperature on the transmission characteristics of the coatings, as measured with a UV-visible-near-IR spectrometer (Perkin-Elmer Lambda 19), is displayed in Fig. 4 in comparison with those of the uncoated fused silica substrate. The films grown at high laser fluences and low substrate temperatures show a high transmission particularly in the near-IR spectral range, in accordance with DLC coatings deposited with conventional chemical vapour deposition techniques [11]. Obviously the coatings deposited at low energy densities or high substrate temperatures show a much higher absorption over the entire spectral range, indicating a larger portion of sp 2 bonds. For further characterization of the deposited films, Raman spectra have been measured using the 514.5 nm line of an argon ion laser for excitation. Figure 5 shows the dependence of these spectra on the laser energy density. The Raman spectrum of the film deposited at low laser fluences (below 4 J cm -2) exhibits two broad peaks at 1340 cm 1 and 1580 cm- 1 (spectrum a), which are characteristic for sp2-dominated glassy carbon [12, 13]. The spectrum looked completely different for the smooth film deposited at higher laser energy densities and use of a bias voltage of 200 V, with only one broad peak centred around 1580 cm -1 (spectrum b). Because of its mechanical, electrical and optical properties the latter is DLC [14]. Note that the sharp peak of crystalline diamond at 1331 cm ~ is absent [15].
4. Plasma analysis
For an investigation of the energy and mass distributions of laser ablated ionic species, a T O F spectrometer has been installed perpendicular to the target surface
~/,i"Jf'l
u m
r
(a)
(b) Fig. 3. Scanning electron micrographs of DLC films deposited on fused silica (a) with a bias voltage of 200 V and (b) without bias voltage.
instead of the substrate manipulator. The corresponding set-up is shown in Fig. 6. After ablation the particles enter a field-free drift tube of length s = 60 cm; positive ions are detected at the end of this tube with an electron multiplier. After amplification and shaping, the ion pulses are displayed temporally resolved on a fast digital storage oscilloscope, which is triggered by the ablating
F. Miiller, K. Mann / Laser-induced PVD of DLC films
236
uncoated substrate
100
H=12J/cm 2 ,T=25 C
80 @
60
~ 40 20 506
1000
15l}O
2000
2500
3001
wavelength [nm] Fig. 4. Transmission spectra of carbon films deposited on fused silica substrates with KrF laser at different laser energy densities and different substrate temperatures in comparison to the uncoated substrate.
tO)
._-.q r--
E r'd
I
1200
1
I
I
1400
I
1600
1800
wavenumber [cm ] Fig. 5. Raman spectra of carbon films deposited on fused silica at room temperature: spectrum a, glassy carbon deposited at H= 4 J cm-2; spectrum b, amorphous DLC deposited at H= 15 J cm-2.
photo diode i ',.~.v i amplifier
pulse shaper
.
excimer laser •
c ~ m ~
multiplier p
~rmm
/
- - ~ 1
stop ~ ~ digital storage oscilloscope
t
........................ ~"1.[-Lllh ["JJ'~ /~/~\ PC used as multichannelanalyser
Fig. 6. TOF arrangement for laser plasma analysis: PC, personal computer.
laser pulse. The trace obtained for a single pulse is transferred via an IEEE interface to a personal computer, which integrates the spectrum for a pre-selected number of pulses. For mass analysis the target is biased against a grid to a voltage U = + 2.5 kV. Mass separation is possible, since the corresponding energy of the accelerated ions of mass m and charge q is much higher than the particle energy Eab I obtained during the ablation process. The kinetic energy is given by
m(s:
Eki n = ' ~
= q U + Eab I
(1)
Having identified the masses of the ablated ions, this set-up can also be used for investigation of the initial particle energies gab I. For these measurements the target is grounded and the ions enter the drift tube with an energy given only by the laser ablation process. For a given mass number and measured flight time t the kinetic energy gab I c a n be calculated from eqn. (l). Plots of the count rate of laser-ablated carbon ions vs. T O F are presented in Fig. 7. In any case the spectra are divided into two temporal regions: one of fast atomic or small molecular ions and a broad distribution at higher flight times, which can be attributed to formation of large carbon clusters. As seen from Fig. 7, the data show a strong correlation between the laser energy density and the corresponding particle velocities. While the peaks of the small particles become sharper with increasing laser fluence, the broad cluster distributions are shifted towards higher flight times (lower velocities). Obviously the cluster size becomes larger with an increasing portion of absorbed energy. On the contrary, the peaked velocity distributions of the smaller ions observed above about 10 J cm -2 are shifted to the left for increasing fluence, indicating an enhancement of the kinetic energies of the ablated particles Czee below). As a further result, we observe a strongly increased portion of carbon clusters compared with smaller ions with increasing fluence. From the analysis of the smaller molecular masses corresponding to the peaks in the T O F spectra, it can be seen that, below 9 J cm-2, atomic carbon is formed in the plasma while, at higher fluences, only molecules of three or more atoms are observed as the smallest particles. Having identified the masses, we can calculate the kinetic ,energies Eab I from the flight times of the corresponding ions. In Fig. 8 these energies are compiled for C + and C~ as a function of the laser fluence, which has been varied from 0.5 to 23 J cm -2. We observe an almost linear relation, with maximum kinetic energies as high as 220 eV at 23 J cm -2. Obviously, the energy of the small molecular ions is independent of the mass and only a function of the laser fluence. Absolute values of the kinetic energy are in agreement with those in ref. 8.
F. Mailer, K. Mann / Laser-induced PVD of DLC Jilms
f]uence
H='~5.2',
C;
237
O;
I~
j,/cm 2
~~
t
.D
.
L"
X:5.23 J,.'cm~
,I
2
1
H=4.38 J/cm 2
Jo (a)
20
30
time of flight
40
250 (b)
[/,'s]
500
time of f l i g h t
750
1000
[~'s]
Fig. 7. T O F spectra of ablated carbon ions (without accelerating voltage): (a) fast atomic and small molecular particles; (b) distribution of slower particles (i.e. large clusters).
250
250
200
.
.
.
.
.
.
.
.
.
+ .
200
o
150
150
100
100
c
o c
50
0
50
~ 5
= 10 laser
~ 15 fluence
r 20
L 25
0 30
[J/cm 2]
Fig. 8. Kinetic energy of ablated carbon ions as a function of incident laser energy density: D, C+; + , C~.
5. Discussion
In this work we have investigated the deposition of carbon films by the LPVD technique. In agreement with the results of other workers I-8, 16] we find that the formation of DLC films is already possible at moderate laser power densities (greater than 3 x 108W cm 2), indicating the advantage of the higher photon energies in comparison with the use of visible or IR lasers [17]. X-ray diffraction analysis reveals that, independent of the laser energy density, all deposited coatings are amorphous; nevertheless, scanning electron micrographs indicate that a very small amount of micrometre-sized crystalline diamond particles can be formed. In order to obtain a better understanding of the
mechanisms responsible for DLC growth, the two separated steps of the LPVD process, i.e. target ablation and subsequent deposition of carbon particles on a substrate, have to be investigated in detail. For this purpose the energy and mass distributions of the positive ions in the ablation plume and their correlation with the thin film microstructure and film properties have been studied using TOF spectroscopy. Although the experimental results cannot directly be extended to the (probably much stronger) emission of neutral particles, which are not detectable with the given set-up, some important dependences could be obtained. First of all, we observe a linear relationship between the kinetic energy of ablated carbon ions and the laser energy density, with a slope of approximately 10eV j - l c m 2 (see Fig. 8). A pronounced DLC character of the deposited films, as demonstrated particularly by the corresponding Raman spectra (see Fig. 5), is observed above a threshold of roughly 8 J cm- 2 (i.e. 2.7 x 108 W cm 2 for 30 ns pulses). This is equivalent to a relatively high kinetic energy of about 80 eV, at which an efficient preferential sputtering of spZ-bonded carbon is already very likely [18]. Hence, the data support the corresponding theoretical model for an enrichment of sp3-bonded sites described by Miyazawa et al. [19]. The predominant role of the kinetic particle energy with respect to DLC growth may also be deduced from the interesting observation that the small ionic particles are assuming almost discrete kinetic energy values above a threshold laser fluence, which coincides with the onset for DLC formation. On the contrary, this threshold also 'coincides with an increased occurrence of C [ and C ; ions (see Fig. 7), indicating the additional influence of
238
F. Mfiller, K. Mann / Laser-induced PVD of DLC films
the mass distribution of the emitted particles in the plasma plume on the film growth. It is conceivable that these particles are already in a metastable sp3-bonded configuration, which is frozen after deposition on the substrate surface. This may be justified by the observed substrate temperature dependence of the coating properties, i.e. DLC character only at lower temperatures. A similar quenching process has been discussed elsewhere
[8]. A major disadvantage of the higher laser fluences necessary for DLC film deposition is the increased portion of large clusters observed both in the TOF spectra and in the scanning electron micrographs of the films, which leads to absorption and scatter losses not tolerable for coatings with optical quality [12, 20]. However, this effect could be reduced by the use of a bias voltage. The influence of this bias on the plasma plume may be explained by additional acceleration of charged particles, which leads to decomposition of the clusters during impact on the substrate. Further improvements with respect to the suppression of cluster formation can be expected for the near future from laser beam homogenizing optics, which will guarantee a flat-topped laser intensity profile and thus an extremely uniform target ablation. Moreover, the use of a short-pulse excimer laser (500 fs at 248 nm) is planned for DLC growth. The first TOF measurements of ablated graphite using this laser have already shown that the formation of particle clusters can be totally avoided. Acknowledgments The authors would like to thank T. R6dle of the IV. Physical Institute of the University of G6ttingen for carrying out the Raman investigations.
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