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Influence of the substrate on the temperature induced by pulsed laser irradiation of thin films
Applied Surface North-Holland
Influence of the substrate irradiation of thin films A. Jadin, I.V. Filiouguine Department Received
375
Science 46 (1990) 375-377
on the temperature
*, M. Wautelet
induced
by pulsed laser
and L.D. Laude
of Materruls, University of Mom, B-7000 Mans, Belgium
30 May 1990: accepted
for publication
23 July 1990
When the thickness of thin films compares with the light absorption length and the diffusion length of heat during pulsed irradiation, the thermal properties of the substrate influence the temperature in the film. In order to understand these phenomena, numerical simulation of temperature of thin film-substrate systems are performed. influence of the nature of the substrate and of the thickness of the film is studied.
1. Introduction Thin film processing by intense pulsed laser irradiation may lead to various effects: crystallization, ablation, chemical reactions, etc. [l]. It is often believed that pulsed laser irradiation gives mainly rise to thermal effects, although, under some circumstances, there is evidence that socalled photonic effects are necessary ingredients to a complete understanding of the processes [2]. However, even in these cases, thermal effects have to be taken into account. This is evidenced experimentally by the fact that these photonic effects are often temperature dependent. For film thicknesses ranging from 20 to 500 nm, irradiation by laser pulses in the 20 ns duration range (like most excimer laser pulses), gives a heat diffusion length comparable to the film thickness, d. It is also of the order of the optical absorption depth of visible and ultra-violet light. In this regime, it is expected that both the substrate and the film play roles in the temperature distribution in the system. It is the aim of the present work to calculate numerically this temperature, as a function of d and of the thermal
* Permanent address: Moscow. USSR. 0169-4332/90/$03.50
Moscow
Physical-Technical
Institute,
0 1990 - Elsevier Science Publishers
laser The
properties of the substrate, in a region where d compares with the heat diffusion length. Heat diffusion equations are solved, for the case of a selenium film on insulating substrates. Contrary to silicon for example, the optical properties of Se do not change much from the solid to the liquid phase, and then interference effects between both phases are not considered. For this system, it has been demonstrated experimentally that excimer laser irradiation gives rise to a complete ablation of the film, at threshold fluences that depend on d and on the nature of the substrate [3]. It is shown that this is compatible with a model in which ablation takes place at a fixed temperature.
2. Numerical simulation For a numerical simulation, we have to solve classical heat diffusion equations. One assumes that the laser beam travels along the z-axis perpendicularly to the surface of the film, and is uniform in the x-y plane, perpendicular to z. The target is also supposed to be uniform in the x-y plane, structural changes occurring only along the z-axis. This target is composed of a film deposited on a semi-infinite substrate. A one-dimensional heat-flow formalism is used, the laser beam cross
B.V. (North-Holland)
section being much larger than the heated sample thickness. According to the previous assumptions. the heat equation [4] may be written:
Table 1 Parametera
ar -=
Se
at +J’ ,,
z,t)+LA pcP
aZ i
Kg
( i
(1)
where T is the temperature. t is the time, p, Cp, K and (Y are respectively, the density, specific heat, thermal conductivity and absorption coefficient of the film or the substrate, depending on Z. The laser pulse intensity is given by:
used in numerical (Y(cm
‘)
6X to
calculation
D (cm’/s) 1.46x10
’
(J/g.K)
P &/cm’)
C;,
4.26
0.3x1
5.3x10
i
2.5
0.665
Quartz
5.07
8.3X10
3
2.2
0.755
Sapphire
5.07
4.0x
2
7.2
0.755
114.5
Glass
10
f ( Id5 J Cm-2 K-’ )
5-
I( -_, t) = [ At2 exp( -ut’)](l
- R) exp( -a?), L-
(2)
3-
where A and u are constants. The term between brackets in the right-hand side of eq. (2) fits the temporal shape of the excimer laser pulse by taking u = 0.25 X lOI spz. In this first approach, where mainly the shape of temperature variations is considered, optical and thermal parameters are assumed to be independent of temperature, which is partly justified by the fact that the reflectivity of Se is similar in the solid and liquid phases [6]. The temperature is then proportional to the laser fluence. R is the reflectivity. Eq. (1) was solved by the finite-difference method [4], with the following boundary and initial conditions: - optical and thermal parameters are different in film and substrate; * the temperature distribution in the target at time t = 0 is assumed to be homogeneous and the initial temperature equal to room temperature.
3. Results and discussion The method has been applied to KrF excimer laser irradiation of selenium films deposited on various substrates, including glass and quartz, for thicknesses ranging from 20 to 230 nm and a mean power density of 2~10~ W cmP2. The parameters used in the calculation are shown in table 1. The maximum temperature attained in the film is proportional to the absorbed laser fluence and the results of the calculation give, for the particu-
@Jo 2-
0
D = 4 x 10-2 cm2,’
I
D=
lo3
cm?<’
n D=53x103cm2s-’
l-
.
D~15xlCT3cm2,_’
0’ 001
Fig.
2~1O-~crn~s-’
b D = 8.3x
0.02
0.03
005
004
dj(nm-‘)
I. Calculated variation of temperature a$ a function film thickness d for various substrates.
lar pulse shape considered
of
here and laser fluence
F.
1 - = T
f/(&‘).
where f(d ‘) is displayed in fig. 1 for different values of the thermal diffusivity D of the substrate, d being the film thickness. It can be seen that the temperature is independent of the film thickness when d is large, above the heat diffusion length in Se (1;‘201r = 94 nm in Se, with a pulse duration
t, = 30 ns). Below a threshold
s(cm.
I
value of d.
k’)
0.7. 05.
-3-F
0.11
,
I Fig. 2.
3 of the diffusivity
4
(1O-2 cm-2
in fig. of
as a substrates.
of the
A. Jadin et al. / Temperature
induced by pulsed laser irradiation of thin films
377
4. Conclusions
d-'(nni') 0 0
OOL 005 0.03 002 Fig. 3. Experimental evolution of the fluence threshold for plasma formation F, in Se films as a function of film thickness d. 0.01
T-’ is proportional to d-i, with a slope depending on D, as shown in fig. 2. No saturation occurs in the curves when d decreases down to 20 nm, which is close to the light penetration depth in Se, (Y-’ = 17 nm, where only half of the incident power is absorbed by the film. In parallel, it was observed experimentally [5] that selenium films irradiated by excimer laser emit a plasma above a certain fluence F,. The evolution of F2 versus the film thickness d was measured and is shown in fig. 3. Below d = 100 nm, F2 is found to be proportional to dP’. The ratios of slopes between glass and quartz in figs. 2 and 3 are in agreement. From these indications and according to eq. (3) F, might be related to a given temperature.
The present numerical simulation shows that, under pulsed laser irradiation, the nature of the substrate plays no important role, in the temperature evolution during the pulse, when the film thickness is larger than the heat diffusion length. When the thickness decreases, for the same incident fluence, the temperature is a function of both the film thickness and the thermal properties of the substrate. The same reasoning may be extended to multilayer stacks or multilayer films on substrates, as encountered, for instance, during laser-assisted compound synthesis or processing of silicon multilayers. In a further approach, the effects of the temperature on the optical and thermal properties will be taken into account.
References [l] D. Bluerle, Chemical Processing with Lasers (Springer. Berlin, 1986). [2] M. Wautelet. P. Quenon and A. Jadin, Semicond. Sci. Technol. 3 (1988) 54. [3] A. Jadin, M. Wautelet and L.D. Laude. Semicond. Sci. Technol. 3 (1988) 499. [4] E. Bimini, in: Cohesive Properties of Semiconductors under Laser Irradiation, Ed. L.D. Laude (Nijhoff, The Hague, 1983) p. 71. [5] A. Jadin, I.V. Filiouguine, M. Wautelet and L.D. Laude, SPIE Proc., The Hague 1990, in press. [6] F. Greuter, J. Phys. C (Solid State Phys.) 18 (1985) 2527.
Influence of the substrate irradiation of thin films A. Jadin, I.V. Filiouguine Department Received
375
Science 46 (1990) 375-377
on the temperature
*, M. Wautelet
induced
by pulsed laser
and L.D. Laude
of Materruls, University of Mom, B-7000 Mans, Belgium
30 May 1990: accepted
for publication
23 July 1990
When the thickness of thin films compares with the light absorption length and the diffusion length of heat during pulsed irradiation, the thermal properties of the substrate influence the temperature in the film. In order to understand these phenomena, numerical simulation of temperature of thin film-substrate systems are performed. influence of the nature of the substrate and of the thickness of the film is studied.
1. Introduction Thin film processing by intense pulsed laser irradiation may lead to various effects: crystallization, ablation, chemical reactions, etc. [l]. It is often believed that pulsed laser irradiation gives mainly rise to thermal effects, although, under some circumstances, there is evidence that socalled photonic effects are necessary ingredients to a complete understanding of the processes [2]. However, even in these cases, thermal effects have to be taken into account. This is evidenced experimentally by the fact that these photonic effects are often temperature dependent. For film thicknesses ranging from 20 to 500 nm, irradiation by laser pulses in the 20 ns duration range (like most excimer laser pulses), gives a heat diffusion length comparable to the film thickness, d. It is also of the order of the optical absorption depth of visible and ultra-violet light. In this regime, it is expected that both the substrate and the film play roles in the temperature distribution in the system. It is the aim of the present work to calculate numerically this temperature, as a function of d and of the thermal
* Permanent address: Moscow. USSR. 0169-4332/90/$03.50
Moscow
Physical-Technical
Institute,
0 1990 - Elsevier Science Publishers
laser The
properties of the substrate, in a region where d compares with the heat diffusion length. Heat diffusion equations are solved, for the case of a selenium film on insulating substrates. Contrary to silicon for example, the optical properties of Se do not change much from the solid to the liquid phase, and then interference effects between both phases are not considered. For this system, it has been demonstrated experimentally that excimer laser irradiation gives rise to a complete ablation of the film, at threshold fluences that depend on d and on the nature of the substrate [3]. It is shown that this is compatible with a model in which ablation takes place at a fixed temperature.
2. Numerical simulation For a numerical simulation, we have to solve classical heat diffusion equations. One assumes that the laser beam travels along the z-axis perpendicularly to the surface of the film, and is uniform in the x-y plane, perpendicular to z. The target is also supposed to be uniform in the x-y plane, structural changes occurring only along the z-axis. This target is composed of a film deposited on a semi-infinite substrate. A one-dimensional heat-flow formalism is used, the laser beam cross
B.V. (North-Holland)
section being much larger than the heated sample thickness. According to the previous assumptions. the heat equation [4] may be written:
Table 1 Parametera
ar -=
Se
at +J’ ,,
z,t)+LA pcP
aZ i
Kg
( i
(1)
where T is the temperature. t is the time, p, Cp, K and (Y are respectively, the density, specific heat, thermal conductivity and absorption coefficient of the film or the substrate, depending on Z. The laser pulse intensity is given by:
used in numerical (Y(cm
‘)
6X to
calculation
D (cm’/s) 1.46x10
’
(J/g.K)
P &/cm’)
C;,
4.26
0.3x1
5.3x10
i
2.5
0.665
Quartz
5.07
8.3X10
3
2.2
0.755
Sapphire
5.07
4.0x
2
7.2
0.755
114.5
Glass
10
f ( Id5 J Cm-2 K-’ )
5-
I( -_, t) = [ At2 exp( -ut’)](l
- R) exp( -a?), L-
(2)
3-
where A and u are constants. The term between brackets in the right-hand side of eq. (2) fits the temporal shape of the excimer laser pulse by taking u = 0.25 X lOI spz. In this first approach, where mainly the shape of temperature variations is considered, optical and thermal parameters are assumed to be independent of temperature, which is partly justified by the fact that the reflectivity of Se is similar in the solid and liquid phases [6]. The temperature is then proportional to the laser fluence. R is the reflectivity. Eq. (1) was solved by the finite-difference method [4], with the following boundary and initial conditions: - optical and thermal parameters are different in film and substrate; * the temperature distribution in the target at time t = 0 is assumed to be homogeneous and the initial temperature equal to room temperature.
3. Results and discussion The method has been applied to KrF excimer laser irradiation of selenium films deposited on various substrates, including glass and quartz, for thicknesses ranging from 20 to 230 nm and a mean power density of 2~10~ W cmP2. The parameters used in the calculation are shown in table 1. The maximum temperature attained in the film is proportional to the absorbed laser fluence and the results of the calculation give, for the particu-
@Jo 2-
0
D = 4 x 10-2 cm2,’
I
D=
lo3
cm?<’
n D=53x103cm2s-’
l-
.
D~15xlCT3cm2,_’
0’ 001
Fig.
2~1O-~crn~s-’
b D = 8.3x
0.02
0.03
005
004
dj(nm-‘)
I. Calculated variation of temperature a$ a function film thickness d for various substrates.
lar pulse shape considered
of
here and laser fluence
F.
1 - = T
f/(&‘).
where f(d ‘) is displayed in fig. 1 for different values of the thermal diffusivity D of the substrate, d being the film thickness. It can be seen that the temperature is independent of the film thickness when d is large, above the heat diffusion length in Se (1;‘201r = 94 nm in Se, with a pulse duration
t, = 30 ns). Below a threshold
s(cm.
I
value of d.
k’)
0.7. 05.
-3-F
0.11
,
I Fig. 2.
3 of the diffusivity
4
(1O-2 cm-2
in fig. of
as a substrates.
of the
A. Jadin et al. / Temperature
induced by pulsed laser irradiation of thin films
377
4. Conclusions
d-'(nni') 0 0
OOL 005 0.03 002 Fig. 3. Experimental evolution of the fluence threshold for plasma formation F, in Se films as a function of film thickness d. 0.01
T-’ is proportional to d-i, with a slope depending on D, as shown in fig. 2. No saturation occurs in the curves when d decreases down to 20 nm, which is close to the light penetration depth in Se, (Y-’ = 17 nm, where only half of the incident power is absorbed by the film. In parallel, it was observed experimentally [5] that selenium films irradiated by excimer laser emit a plasma above a certain fluence F,. The evolution of F2 versus the film thickness d was measured and is shown in fig. 3. Below d = 100 nm, F2 is found to be proportional to dP’. The ratios of slopes between glass and quartz in figs. 2 and 3 are in agreement. From these indications and according to eq. (3) F, might be related to a given temperature.
The present numerical simulation shows that, under pulsed laser irradiation, the nature of the substrate plays no important role, in the temperature evolution during the pulse, when the film thickness is larger than the heat diffusion length. When the thickness decreases, for the same incident fluence, the temperature is a function of both the film thickness and the thermal properties of the substrate. The same reasoning may be extended to multilayer stacks or multilayer films on substrates, as encountered, for instance, during laser-assisted compound synthesis or processing of silicon multilayers. In a further approach, the effects of the temperature on the optical and thermal properties will be taken into account.
References [l] D. Bluerle, Chemical Processing with Lasers (Springer. Berlin, 1986). [2] M. Wautelet. P. Quenon and A. Jadin, Semicond. Sci. Technol. 3 (1988) 54. [3] A. Jadin, M. Wautelet and L.D. Laude. Semicond. Sci. Technol. 3 (1988) 499. [4] E. Bimini, in: Cohesive Properties of Semiconductors under Laser Irradiation, Ed. L.D. Laude (Nijhoff, The Hague, 1983) p. 71. [5] A. Jadin, I.V. Filiouguine, M. Wautelet and L.D. Laude, SPIE Proc., The Hague 1990, in press. [6] F. Greuter, J. Phys. C (Solid State Phys.) 18 (1985) 2527.