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Experimental and numerical study on pulsed-laser annealing process of diamond-like carbon thin films
Diamond and Related Materials 10 Ž2001. 905᎐909
Experimental and numerical study on pulsed-laser annealing process of diamond-like carbon thin films Toshiyuki Nakamiyaa,U , Shin-ichi Aoqui b, Kenji Ebihara c a
Department of Information Technology, Kyushu Tokai Uni¨ ersity, Toroku 9-1-1, Kumamoto 862-8652, Japan b Department of Electrical Engineering, Sojo Uni¨ ersity, Ikeda 4-22-1, Kumamoto 860-0082, Japan c Department of Electrical and Computer Engineering, and Graduate School of Science and Technology, Kumamoto Uni¨ ersity, Kurokami 2-39-1, Kumamoto 860-8555, Japan
Abstract The diamond-like carbon ŽDLC. films were deposited by KrF excimer laser Ž s 248 nm. ablation under H 2 atmosphere of 200 mtorr with 6 Jrcm2. The prepared DLC thin film was mounted on the holder and the film surface was irradiated with the third harmonic of a single pulsed of Nd:YAG laser Ž s 355 nm. or a pulsed KrF excimer laser. The experiments were performed in air and the substrate was held at room temperature. The threshold energy density for surface damage is measured by inspecting the surface after a single laser shot of increasing fluence to observe the onset of surface damage. The damage threshold energy density for DLC film flashed by a pulsed Nd:YAG laser is in the range 60᎐80 mJrcm2. The dynamics of pulsed nanosecond laser heating process is simulated by the solution of the one-dimensional heat conduction equation. The finite element method ŽFEM. is applied to solve the equation. At the laser fluence of 80 mJrcm2 with the Nd:YAG laser, the surface reaches the maximum temperature of 785⬚C at 48 ns. The experimental and calculated results show that the damage temperature of DLC film is smaller than the vaporization temperature of solid carbon. Moreover, the transmission spectra of DLC films before and after irradiation were measured. The graphitization occurred on the sample after irradiation over threshold energy for surface damage. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: DLC film; Pulsed-laser annealing; Damage temperature; FEM
1. Introduction DLC, an amorphous material which can have very interesting and useful properties, including high hardness, chemical stability, optical transparency, and dielectric properties, has been the subject of intense investigations w1᎐3x. Short pulsed laser annealing for DLC films has been interesting for the improvements
U
Corresponding author. Tel.: q81-96-386-2656; fax: q81-96-3862755. E-mail address: [email protected] ᎐ u.ac.jp ŽT. Nakamiya..
of film properties such as the adhesion to the substrate, the structural relaxation and the rearrangement of the dangling bonds. The heating process of pulsed laser irradiation on materials constitutes a three-dimensional heat flow problem. In nanosecond laser processing, the short thermal diffusion distances and the large dimensions of the laser beam compared to the melt depth limit the thermal gradients parallel to the interface to many orders of magnitude less than the gradients present perpendicular to the interface. Thus it essentially becomes a one-dimensional heat flow problem. The dynamics of pulsed nanosecond laser ablation is simu-
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 3 6 - 7
906
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909
lated by the solution of the one-dimensional heat flow equation. The heat flow equation is solved by the FEM w4,5x. In this paper, the heating process of DLC films Ž300 nm thickness . on quartz substrate flashed by a pulsed KrF excimer laser or a pulsed Nd:YAG laser is studied experimentally and numerically. The damage threshold energy is estimated from the optical micrographs of the DLC films. The transmittance of DLC film was measured either for the as-deposited films or for films given subsequent laser heating. The measurement results show that the transmittance decreases over threshold energy density for surface damage. From FEM calculated result, we can get the temporal temperature profile of DLC film and estimate the temperature of surface damage.
2. Experiments The DLC films were deposited by KrF excimer laser ŽLambda Physik LPX305icc, pulse width 21 ns. ablation of a rotating graphite target Ž⭋, 30 mm= 5 mm.. The targets used were high purity graphite Ž99.999%. with a density of 1.83 grcm3 and the substrates were quartz and used at normal temperature. The ambient pressure was 200 mtorr. The pulsed laser was irradiated to the target with the laser energy density of 6 Jrcm2 . The deposition time was 20 min with 10 Hz repetition rate. The prepared DLC film Ž300 nm thickness . was mounted on the holder and the film surface was flashed with a single pulse of KrF excimer laser or the third harmonic of a pulsed Nd:YAG laser ŽInfinity 40᎐100, pulse width 3 ns.. The laser energy density for DLC film flashed by a pulsed KrF excimer laser was varied in the range of 16᎐220 mJrcm2 and that for a pulsed Nd:YAG laser was varied in 18᎐110 mJrcm2 . The transmittance measurements of DLC films before and after irradiation were undertaken on a visible light spectrometer ŽSHIMAZU UV-160..
3. Computer model for surface heating 3.1. Go¨ erning equation The present analysis is restricted to the heat conduction model. The absorbed laser energy is instantaneously converted into the local heat, which can diffuse by thermal conduction. If the laser radiation is spatially uniform, the differential equation for heat conduction can be written in terms of the temperature distribution T Ž X,t . at the depth X and time t as C P
⭸T Ž X ,t . ⭸ ⭸T Ž X ,t . s q ␣ I Ž X ,t . ⭸t ⭸X ⭸X
ž
/
where is the mass density, C P is the specific heat, ␣ is the absorption coefficient, and is the thermal conductivity. The laser power density I Ž X,t . is determined by the interaction of the laser radiation with the DLC film and the subsequent transfer of the energy to the lattice. The laser power density will be I Ž X ,t . s Ž 1 y R . I0 Ž t . exp Ž y␣ X .
Specific heat w7x
3.2. Material properties The thermal and optical properties of DLC film have been studied by many researchers. The thermal and optical properties of DLC film used in these calculations are listed in Table 1. The temperature dependence of the thermal conductivity and specific heat are considered. The thermal conductivity of DLC film is extrapolated into the high temperature regime. The
: 9.68163y 0.00533T q 2.48844= 10y6 T2 y 5.82665= 10y10 T3 q4.7823= 10y1 4 T4 wWrcm ⬚Cx Cp : ŽT F 727⬚C. w16.299q 5.262= 10y3 ŽT q 273. y ŽT ) 727⬚C.
8.602= 103 xr12.011 wJrg ⬚Cx Ž T q 273. 2
w24.943q 4.892= 10y4 ŽT q 273. y Density w8x Reflectivity w8x Absorption coefficient w8x
Ž2.
where R is the reflectivity and I0 Ž t . is the temporal distribution of the laser power. The FEM is applied to compute the temperature profiles as a function of depth X and time t in the sample w4x. The sample is divided into a set of line element which are not of equal length. The time interval is divided into equal time steps.
Table 1 The thermal and optical properties of DLC film Thermal conductivity w6x
Ž1.
: 1.35 wgrcm3 x R: 0.08 Ž s 355 nm and 248 nm. ␣: 4.212= 104 wcmy1 x Ž s 355 nm. 1.378= 105 wcmy1 x Ž s 248 nm.
71.156 3.649= 106 xr12.011 wJrg ⬚Cx y Ž T q 273. Ž T q 273. 2
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909 Table 2 The thermal properties of quartz Thermal conductivity w9x Thermal capacity w9x
: 0.0147 wWrcm ⬚Cx Cp : 1.806 wJrcm3 ⬚Cx
specific heat of DLC film is assumed to be the same as the graphite. The thermal properties of quartz are listed in Table 2. 4. Results and discussion In Fig. 1a, the optical micrograph of a sample irradiated with a single pulsed Nd:YAG laser of 60 mJrcm2 , the irradiated region is not visible. Fig. 1b shows the surface of sample after a pulsed Nd:YAG laser irradiation of 80 mJrcm2 and the boundary of the irradiated region is clearly visible. Therefore, the damage threshold energy density of DLC film with a pulsed Nd:YAG laser can be estimated to be in the range 60᎐80 mJrcm2 . Fig. 2 shows the surface morphology of DLC film after a single pulsed KrF excimer laser irradiation. The laser energy density is changed from 60 mJrcm2 for the case shown in Fig. 2a to 70 mJrcm2 for that of Fig. 2b. It is clearly seen from the photograph that we can
907
find out the trace of damage at the laser power of 70 mJrcm2 ŽFig. 2b.. This fact suggests that the surface damage threshold energy density for a pulsed KrF excimer laser ranges from 60 to 70 mJrcm2 . The typical temporal temperature profiles are shown in Fig. 3. The DLC film is flashed with a pulsed KrF excimer laser of 80 mJrcm2 . The sample temperature at the bottom Ž X s 1.0 mm. is kept at 20⬚C. It is noticed that the temperature at the surface rises rapidly with increase of laser power. A maximum surface temperature of 785⬚C is reached at 48 ns. A depth of 300 nm Žboundary of DLC film and quartz substrate . the temperature shows 783⬚C at 48 ns and it is almost same with surface temperature because of high thermal conductivity of film. The temperature profile at depth 500 nm shows slow increase with time. The temporal temperature profiles for several depths are shown in Fig. 4. The DLC film is irradiated with a pulsed Nd:YAG laser of 80 mJrcm2 . The temperature at the surface rapidly increases with time. A maximum surface temperature of 706⬚C is reached at 7 ns. The results show that the damage temperature of DLC film flashed by a pulsed Nd:YAG laser or KrF excimer laser is smaller than the vaporization temperature Ž3327⬚C. of solid carbon w10x. The transmission spectrum of three DLC films over
Fig. 1. Surface morphology of DLC film irradiated with a Nd:YAG laser pulse at different energy densities: Ža. 60; and Žb. 80 mJrcm2 .
Fig. 2. Surface morphology of DLC film irradiated with a KrF excimer laser pulse at different energy densities: Ža. 60; and Žb. 70 mJrcm2 .
908
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909
Fig. 3. The temperature profiles at the surface, 300 nm and 500 nm depths for the DLC film flashed with a KrF excimer laser fluence of 80 mJrcm2 .
the range 200᎐800 nm is shown in Fig. 5. The as-deposited film and two DLC films irradiated by a pulsed laser were used in the transmission measurement. One sample was irradiated by a single shot of 80 mJrcm2 with a pulsed Nd:YAG laser. The other sample was used the film flashed with a single pulse of 80 mJrcm2 from a pulsed KrF excimer laser. The maximum temperature of samples surface after irradiation reach 706⬚C ŽNd:YAG laser. at 7 ns and 785⬚C ŽKrF excimer laser. at 48 ns with our calculations, respectively. At the wavelength of 400 nm, the transmittance before irradiation is 58%, the transmittance after irradiation is 55% ŽNd:YAG laser. and 47% ŽKrF excimer laser., respectively. At the wavelength of 800 nm, the transmittance before irradiation and after irradiation of Nd:YAG laser are the same values. However, the transmittance after irradiation of KrF excimer laser is 88%. The transmission of the film damaged with a pulsed Nd:YAG laser is so little changed. It seems that the temperature of the sample surface irradiated by a pulsed Nd:YAG laser of 80 mJrcm2 is smaller than
Fig. 4. The temperature profiles at the surface, 300 nm and 500 nm depths for the DLC film flashed with a Nd:YAG laser fluence of 80 mJrcm2 .
Fig. 5. Transmittance vs. wavelength for DLC film as-deposited and after irradiation with a pulsed KrF excimer laser or third harmonic Nd:YAG laser with the same energy density of 80 mJrcm2 , respectively.
that of KrF excimer laser. The figure shows that the graphitization occurs on the sample after irradiation over threshold energy density for surface damage. Smith has reported that the hydrogenated amorphous carbon Ža-c:H. films are annealed in a flow-through oven under an atmosphere of Ar at 50⬚C intervals from 300 to 850⬚C and the transmittance of the films show a considerable variation with annealing up to 750⬚C w8x. The growth of amorphous and graphitic component upon annealing is explained by the reduced transparency. Ageev et al. have reported that the graphitization threshold energy of XeCl Ž s 308 nm. excimer laser for the diamond film is in the range of 20᎐80 mJrcm2 by measuring the transmission w11x.
5. Conclusions Laser-irradiated DLC film is studied experimentally by measuring the transmittance beforerafter irradiation and numerically solving the one-dimensional heat conduction equation by means of the finite element method. The following results are obtained. 1. The damage threshold energy density for DLC film flashed by a pulsed KrF excimer laser is in the range 60᎐70 mJrcm2 . 2. The damage threshold energy density for DLC film flashed by a pulsed Nd:YAG laser is estimated to be in the range 60᎐80 mJrcm2 . 3. The damage temperature of samples surface after irradiation reaches 706⬚C ŽNd:YAG laser. at 7 ns and 785⬚C ŽKrF excimer laser. at 48 ns, respectively. These temperatures are smaller than the vaporization temperature Ž3327⬚C. of solid carbon. 4. The transmission spectrum of DLC films show that the graphitization occurs on the sample after irra-
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909
diation over threshold energy density of surface damage.
Acknowledgements The authors would like to thank Tamiko Ohshima and Naotomo Miyata, Graduate School of Science and Technology of Kumamoto University, for their technical help for this work. References w1x A. Grill, B.S. Meyerson, V.V. Patel, IBM J. Res. Develop 34 Ž1990. 846.
909
w2x B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 Ž1983. 636. w3x A. Grill, Thin Solid Films 355r356 Ž1999. 189. w4x T. Nakamiya, K. Ebihara, IEEE Jpn. A 108 Ž1988. 443. w5x T. Nakamiya, T. Ikegami, K. Ebihara, Comput. Mater. Sci. 17 Ž2000. 409. w6x K. Belay, Z. Etzel, D.G. Onn, T.R. Anthony, J. Appl. Phys. 79 Ž1996. 8336. w7x B.T. Kelly, Physics of Graphite, Applied Science, London, 1981. w8x F.W. Smith, J. Appl. Phys. 55 Ž1984. 764. w9x K. Kobayashi, Thermophysical Properties Handbook, Yohkendo, Tokyo, 1990. w10x C.A. Klein, M.J. Berry, P.A. Miles, J. Appl. Phys. 65 Ž1989. 3425. w11x V.P. Ageev, L.L. Builov, V.I. Konov et al., Sov. Phys. Dokl. 33 Ž1988. 840.
Experimental and numerical study on pulsed-laser annealing process of diamond-like carbon thin films Toshiyuki Nakamiyaa,U , Shin-ichi Aoqui b, Kenji Ebihara c a
Department of Information Technology, Kyushu Tokai Uni¨ ersity, Toroku 9-1-1, Kumamoto 862-8652, Japan b Department of Electrical Engineering, Sojo Uni¨ ersity, Ikeda 4-22-1, Kumamoto 860-0082, Japan c Department of Electrical and Computer Engineering, and Graduate School of Science and Technology, Kumamoto Uni¨ ersity, Kurokami 2-39-1, Kumamoto 860-8555, Japan
Abstract The diamond-like carbon ŽDLC. films were deposited by KrF excimer laser Ž s 248 nm. ablation under H 2 atmosphere of 200 mtorr with 6 Jrcm2. The prepared DLC thin film was mounted on the holder and the film surface was irradiated with the third harmonic of a single pulsed of Nd:YAG laser Ž s 355 nm. or a pulsed KrF excimer laser. The experiments were performed in air and the substrate was held at room temperature. The threshold energy density for surface damage is measured by inspecting the surface after a single laser shot of increasing fluence to observe the onset of surface damage. The damage threshold energy density for DLC film flashed by a pulsed Nd:YAG laser is in the range 60᎐80 mJrcm2. The dynamics of pulsed nanosecond laser heating process is simulated by the solution of the one-dimensional heat conduction equation. The finite element method ŽFEM. is applied to solve the equation. At the laser fluence of 80 mJrcm2 with the Nd:YAG laser, the surface reaches the maximum temperature of 785⬚C at 48 ns. The experimental and calculated results show that the damage temperature of DLC film is smaller than the vaporization temperature of solid carbon. Moreover, the transmission spectra of DLC films before and after irradiation were measured. The graphitization occurred on the sample after irradiation over threshold energy for surface damage. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: DLC film; Pulsed-laser annealing; Damage temperature; FEM
1. Introduction DLC, an amorphous material which can have very interesting and useful properties, including high hardness, chemical stability, optical transparency, and dielectric properties, has been the subject of intense investigations w1᎐3x. Short pulsed laser annealing for DLC films has been interesting for the improvements
U
Corresponding author. Tel.: q81-96-386-2656; fax: q81-96-3862755. E-mail address: [email protected] ᎐ u.ac.jp ŽT. Nakamiya..
of film properties such as the adhesion to the substrate, the structural relaxation and the rearrangement of the dangling bonds. The heating process of pulsed laser irradiation on materials constitutes a three-dimensional heat flow problem. In nanosecond laser processing, the short thermal diffusion distances and the large dimensions of the laser beam compared to the melt depth limit the thermal gradients parallel to the interface to many orders of magnitude less than the gradients present perpendicular to the interface. Thus it essentially becomes a one-dimensional heat flow problem. The dynamics of pulsed nanosecond laser ablation is simu-
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 5 3 6 - 7
906
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909
lated by the solution of the one-dimensional heat flow equation. The heat flow equation is solved by the FEM w4,5x. In this paper, the heating process of DLC films Ž300 nm thickness . on quartz substrate flashed by a pulsed KrF excimer laser or a pulsed Nd:YAG laser is studied experimentally and numerically. The damage threshold energy is estimated from the optical micrographs of the DLC films. The transmittance of DLC film was measured either for the as-deposited films or for films given subsequent laser heating. The measurement results show that the transmittance decreases over threshold energy density for surface damage. From FEM calculated result, we can get the temporal temperature profile of DLC film and estimate the temperature of surface damage.
2. Experiments The DLC films were deposited by KrF excimer laser ŽLambda Physik LPX305icc, pulse width 21 ns. ablation of a rotating graphite target Ž⭋, 30 mm= 5 mm.. The targets used were high purity graphite Ž99.999%. with a density of 1.83 grcm3 and the substrates were quartz and used at normal temperature. The ambient pressure was 200 mtorr. The pulsed laser was irradiated to the target with the laser energy density of 6 Jrcm2 . The deposition time was 20 min with 10 Hz repetition rate. The prepared DLC film Ž300 nm thickness . was mounted on the holder and the film surface was flashed with a single pulse of KrF excimer laser or the third harmonic of a pulsed Nd:YAG laser ŽInfinity 40᎐100, pulse width 3 ns.. The laser energy density for DLC film flashed by a pulsed KrF excimer laser was varied in the range of 16᎐220 mJrcm2 and that for a pulsed Nd:YAG laser was varied in 18᎐110 mJrcm2 . The transmittance measurements of DLC films before and after irradiation were undertaken on a visible light spectrometer ŽSHIMAZU UV-160..
3. Computer model for surface heating 3.1. Go¨ erning equation The present analysis is restricted to the heat conduction model. The absorbed laser energy is instantaneously converted into the local heat, which can diffuse by thermal conduction. If the laser radiation is spatially uniform, the differential equation for heat conduction can be written in terms of the temperature distribution T Ž X,t . at the depth X and time t as C P
⭸T Ž X ,t . ⭸ ⭸T Ž X ,t . s q ␣ I Ž X ,t . ⭸t ⭸X ⭸X
ž
/
where is the mass density, C P is the specific heat, ␣ is the absorption coefficient, and is the thermal conductivity. The laser power density I Ž X,t . is determined by the interaction of the laser radiation with the DLC film and the subsequent transfer of the energy to the lattice. The laser power density will be I Ž X ,t . s Ž 1 y R . I0 Ž t . exp Ž y␣ X .
Specific heat w7x
3.2. Material properties The thermal and optical properties of DLC film have been studied by many researchers. The thermal and optical properties of DLC film used in these calculations are listed in Table 1. The temperature dependence of the thermal conductivity and specific heat are considered. The thermal conductivity of DLC film is extrapolated into the high temperature regime. The
: 9.68163y 0.00533T q 2.48844= 10y6 T2 y 5.82665= 10y10 T3 q4.7823= 10y1 4 T4 wWrcm ⬚Cx Cp : ŽT F 727⬚C. w16.299q 5.262= 10y3 ŽT q 273. y ŽT ) 727⬚C.
8.602= 103 xr12.011 wJrg ⬚Cx Ž T q 273. 2
w24.943q 4.892= 10y4 ŽT q 273. y Density w8x Reflectivity w8x Absorption coefficient w8x
Ž2.
where R is the reflectivity and I0 Ž t . is the temporal distribution of the laser power. The FEM is applied to compute the temperature profiles as a function of depth X and time t in the sample w4x. The sample is divided into a set of line element which are not of equal length. The time interval is divided into equal time steps.
Table 1 The thermal and optical properties of DLC film Thermal conductivity w6x
Ž1.
: 1.35 wgrcm3 x R: 0.08 Ž s 355 nm and 248 nm. ␣: 4.212= 104 wcmy1 x Ž s 355 nm. 1.378= 105 wcmy1 x Ž s 248 nm.
71.156 3.649= 106 xr12.011 wJrg ⬚Cx y Ž T q 273. Ž T q 273. 2
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909 Table 2 The thermal properties of quartz Thermal conductivity w9x Thermal capacity w9x
: 0.0147 wWrcm ⬚Cx Cp : 1.806 wJrcm3 ⬚Cx
specific heat of DLC film is assumed to be the same as the graphite. The thermal properties of quartz are listed in Table 2. 4. Results and discussion In Fig. 1a, the optical micrograph of a sample irradiated with a single pulsed Nd:YAG laser of 60 mJrcm2 , the irradiated region is not visible. Fig. 1b shows the surface of sample after a pulsed Nd:YAG laser irradiation of 80 mJrcm2 and the boundary of the irradiated region is clearly visible. Therefore, the damage threshold energy density of DLC film with a pulsed Nd:YAG laser can be estimated to be in the range 60᎐80 mJrcm2 . Fig. 2 shows the surface morphology of DLC film after a single pulsed KrF excimer laser irradiation. The laser energy density is changed from 60 mJrcm2 for the case shown in Fig. 2a to 70 mJrcm2 for that of Fig. 2b. It is clearly seen from the photograph that we can
907
find out the trace of damage at the laser power of 70 mJrcm2 ŽFig. 2b.. This fact suggests that the surface damage threshold energy density for a pulsed KrF excimer laser ranges from 60 to 70 mJrcm2 . The typical temporal temperature profiles are shown in Fig. 3. The DLC film is flashed with a pulsed KrF excimer laser of 80 mJrcm2 . The sample temperature at the bottom Ž X s 1.0 mm. is kept at 20⬚C. It is noticed that the temperature at the surface rises rapidly with increase of laser power. A maximum surface temperature of 785⬚C is reached at 48 ns. A depth of 300 nm Žboundary of DLC film and quartz substrate . the temperature shows 783⬚C at 48 ns and it is almost same with surface temperature because of high thermal conductivity of film. The temperature profile at depth 500 nm shows slow increase with time. The temporal temperature profiles for several depths are shown in Fig. 4. The DLC film is irradiated with a pulsed Nd:YAG laser of 80 mJrcm2 . The temperature at the surface rapidly increases with time. A maximum surface temperature of 706⬚C is reached at 7 ns. The results show that the damage temperature of DLC film flashed by a pulsed Nd:YAG laser or KrF excimer laser is smaller than the vaporization temperature Ž3327⬚C. of solid carbon w10x. The transmission spectrum of three DLC films over
Fig. 1. Surface morphology of DLC film irradiated with a Nd:YAG laser pulse at different energy densities: Ža. 60; and Žb. 80 mJrcm2 .
Fig. 2. Surface morphology of DLC film irradiated with a KrF excimer laser pulse at different energy densities: Ža. 60; and Žb. 70 mJrcm2 .
908
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909
Fig. 3. The temperature profiles at the surface, 300 nm and 500 nm depths for the DLC film flashed with a KrF excimer laser fluence of 80 mJrcm2 .
the range 200᎐800 nm is shown in Fig. 5. The as-deposited film and two DLC films irradiated by a pulsed laser were used in the transmission measurement. One sample was irradiated by a single shot of 80 mJrcm2 with a pulsed Nd:YAG laser. The other sample was used the film flashed with a single pulse of 80 mJrcm2 from a pulsed KrF excimer laser. The maximum temperature of samples surface after irradiation reach 706⬚C ŽNd:YAG laser. at 7 ns and 785⬚C ŽKrF excimer laser. at 48 ns with our calculations, respectively. At the wavelength of 400 nm, the transmittance before irradiation is 58%, the transmittance after irradiation is 55% ŽNd:YAG laser. and 47% ŽKrF excimer laser., respectively. At the wavelength of 800 nm, the transmittance before irradiation and after irradiation of Nd:YAG laser are the same values. However, the transmittance after irradiation of KrF excimer laser is 88%. The transmission of the film damaged with a pulsed Nd:YAG laser is so little changed. It seems that the temperature of the sample surface irradiated by a pulsed Nd:YAG laser of 80 mJrcm2 is smaller than
Fig. 4. The temperature profiles at the surface, 300 nm and 500 nm depths for the DLC film flashed with a Nd:YAG laser fluence of 80 mJrcm2 .
Fig. 5. Transmittance vs. wavelength for DLC film as-deposited and after irradiation with a pulsed KrF excimer laser or third harmonic Nd:YAG laser with the same energy density of 80 mJrcm2 , respectively.
that of KrF excimer laser. The figure shows that the graphitization occurs on the sample after irradiation over threshold energy density for surface damage. Smith has reported that the hydrogenated amorphous carbon Ža-c:H. films are annealed in a flow-through oven under an atmosphere of Ar at 50⬚C intervals from 300 to 850⬚C and the transmittance of the films show a considerable variation with annealing up to 750⬚C w8x. The growth of amorphous and graphitic component upon annealing is explained by the reduced transparency. Ageev et al. have reported that the graphitization threshold energy of XeCl Ž s 308 nm. excimer laser for the diamond film is in the range of 20᎐80 mJrcm2 by measuring the transmission w11x.
5. Conclusions Laser-irradiated DLC film is studied experimentally by measuring the transmittance beforerafter irradiation and numerically solving the one-dimensional heat conduction equation by means of the finite element method. The following results are obtained. 1. The damage threshold energy density for DLC film flashed by a pulsed KrF excimer laser is in the range 60᎐70 mJrcm2 . 2. The damage threshold energy density for DLC film flashed by a pulsed Nd:YAG laser is estimated to be in the range 60᎐80 mJrcm2 . 3. The damage temperature of samples surface after irradiation reaches 706⬚C ŽNd:YAG laser. at 7 ns and 785⬚C ŽKrF excimer laser. at 48 ns, respectively. These temperatures are smaller than the vaporization temperature Ž3327⬚C. of solid carbon. 4. The transmission spectrum of DLC films show that the graphitization occurs on the sample after irra-
T. Nakamiya et al. r Diamond and Related Materials 10 (2001) 905᎐909
diation over threshold energy density of surface damage.
Acknowledgements The authors would like to thank Tamiko Ohshima and Naotomo Miyata, Graduate School of Science and Technology of Kumamoto University, for their technical help for this work. References w1x A. Grill, B.S. Meyerson, V.V. Patel, IBM J. Res. Develop 34 Ž1990. 846.
909
w2x B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 Ž1983. 636. w3x A. Grill, Thin Solid Films 355r356 Ž1999. 189. w4x T. Nakamiya, K. Ebihara, IEEE Jpn. A 108 Ž1988. 443. w5x T. Nakamiya, T. Ikegami, K. Ebihara, Comput. Mater. Sci. 17 Ž2000. 409. w6x K. Belay, Z. Etzel, D.G. Onn, T.R. Anthony, J. Appl. Phys. 79 Ž1996. 8336. w7x B.T. Kelly, Physics of Graphite, Applied Science, London, 1981. w8x F.W. Smith, J. Appl. Phys. 55 Ž1984. 764. w9x K. Kobayashi, Thermophysical Properties Handbook, Yohkendo, Tokyo, 1990. w10x C.A. Klein, M.J. Berry, P.A. Miles, J. Appl. Phys. 65 Ž1989. 3425. w11x V.P. Ageev, L.L. Builov, V.I. Konov et al., Sov. Phys. Dokl. 33 Ž1988. 840.