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Evaporation of solids by pulsed laser irradiation
150
ApplieASurfaceScience36 (1989) 150-156 North-Holland, Amsterdam
E V A P O R A T I O N O F S O L I D S BY P U L S E D LASER I R R A D I A T I O N H. STAFAST and M. VON P R Z Y C H O W S K I * Battelle.lnstitut e. V.. A m R6merhof 35, D-6000 Frankfurt/M 90, Fed. Rep. of Germany
Received 2 June 1988: accepted for publication 11 July 1988
The focused beam of a KrF laser (248 nm) h~ been applied to irradiate targets of AI203, SiC, graphite, Pb, Ni, Or, quartz, and NaCI at variable laser energy flux i. the range 0-13 J/cm2. The amount of target material ejected into the vacuum (background pressu,ozabout 8 x 10-4 Tort) was determined from the target weight before and after laser irradiation. The average number of particles (formula weight) evaporated per laser pulse and per unit of irradiated target area is non-linearly dependent on the laser energy flux. Tbe evaporation of AI203, SiC, and graphite is showing a well-definedflux threshold while the vaporization of Pb, Ni and Cr is rising smoothly with increasingflux. With both groups of materials laser evaporation is monotonicallyincreasing with the laser energy flux. NaCI and quartz, on the other hand, are showing an intermediate maximum in the laser vaporization efficiency.
1. Introduction Intense laser fight provides a convenient means to vaporize solid material by localized heating of a target surface [1,2]. With a laser beam of low intensity the evaporation process will be close to thermal equilibrium and characterized by a broad spatial distribution of the vaporized target material. Using (pulsed) laser radiation of high intensity, a plasma can be generated in front of the target surface and the vaporized material may be ejected in a directed jet-like gas (plasma) stream. Both processes can be used for laser sputtering, i.e. evaporation of target material and subsequent deposition onto near-by substrate surfaces. The overall process of laser sputtering may roughly be divided into the steps of evaporation, translation and relaxation in the gas (plasma) phase, and deposition onto the substrate surface. The present study is addressing the process of laser evaporation. Evaporation requires energy input from the laser beam into the target material. In the initial phase the energy input is dominated by the absorption behaviour of the solid. Absorption will be very efficient if the frequency of the incident laser light is higher than the plasma frequency of the illuminated * Graduate student at Frankfurt University,part of diploma thesis. 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
tl. Stafast. M. van Przycho'vski / Eoaporation of solids by p~.;sed laser irradiation
151
target. Many materials will meet this requirement if the short wavelength light of a KrF laser (248 nm) is applied. In addition, the Ifigh intensity of short excimer laser pulses is favourable with respect to the non-linearity of the entire evaporation process. In order to obtain some information about the main characteristics of the vaporization by pulsed laser irra,.~tiation a series of target materials with differei-..t (linear) optical properties has been examined. The solids selected are absorbing, reflecting ot transpare:al: to the applied laser light. The evaporation efficiency has been determined as a function of the incident laser energy flux (intensity) by measuring the weight loss of the target upon irradiation.
2. Expe~memal The experimental set-up is sketched in fig. 1. "Vr,e solid targets consisted of NaCl, quartz, graphite, A1203, SiC, Ni, Cr, and Pb. They were weighed on a balance (Mettler E5, +0.1 mg accuracy) before and after laser irradiation. Irradiation was performed in a vacuum vessel of - 15 d volume. The vessel was pumped by a diffusion (Leybold-Hcraeus, Leybodiff 30L) and a rotary pump (F,dwards, E D M 6). The background pressure was determined by a penning gauge (Leybold-Heraeus, PR 31) and aniounted to at least 5 x 10 -5 T o r t but increased t~ about 8 × 10 -a Torr when flushing the laser entrance window with N2, H2, or air to prevent window deposits. A tube of 4.5 cm inner diameter and 7 cm length with two baffles containing holes of 25 × 8 mm 2 and 10 × 5 mm 2 cross section towards the laser window and the target, respectively, provided additional protection against unwanted deposits on the window. The targets were tilted about 45 ° relative to the laser beam a.--d moved during irradiation in a direction vertical to the beam at a speed of 0.3 m m / m i n or 3 mr~/rnin (see below). A KrF laser (Lambda Physik, E M G 201 MSC) delivered pulse ¢n¢~:gies up to 300 mJ stabilized by a microprocessor .'2".Z.z,,~///////////////
inert
~J
.=8 x lO-4torr
vari.qble ~ positron ~
V/7~///~ pump ~////// Fig. 1. Experimental set-up for K r F laser evaporation of solid targets in vacuum, inert gas flushing of the laser entrance window provides a constant flow from the window towards the target to prevent deposits on the quartz plate (see text).
152
tt. Stafast, M. yon Przychowski / Evaporation of solids by pulsed laser irradiation
control (Lambda Phys.~k, ILC). The laser control is based on a pulse energy measurement by a photo diode illuminated from a beam splitter in front of the laser outeoupling mirror. It was frequently recalibrated using a pyroelectric energy meter (Gentec, E D 500). This energy meter head also served to measure the laser pulse energy available on the target ( - 5 0 % of the original pulse energy). The laser beam was focused by a quartz lens (f= 20 era) and the beam cross section at the target surface was registered by the burn pattern on a photographic film. Its dimensions were measured using a magnifying glass with an engraved scale of 0.1 mm resolution. The estimated error limits are _+5% for large areas of 20 mm 2 and about _+15% for small areas of 1.3 mm 2. The irradiated target area varied with the distance d between the target and the focusing lens (20-25 cm) while keeping the laser pulse energy constant. The maximum laser energy flux achieved on the target amounted up to about 13 J / c m 2.
3. R e s ~
and discussion
The primary information obtained from the experiments is sketched in fig. 2 for the case of K r F laser irradiation of sintered aluminium oxide. The mass m of ablmed material (ordinate scale on the left hand side) is plotted against the distance d between the focusing lens and the irradiatex) target surface (lower abscissa scale). This plot is showing a maximum at d - - 23 cm corresponding to 11.5 nun 2 irradiated area and 1.45 J / e r a 2 laser energy flux. This maximum is reflecting two simultaneous but counterpropagating effects induced by the lens movement: ,'~ne irradiated area increases and the laser
13 10 5
3 2
~} (J/cm 2) 1.5 1
0.7
E ,o
o
2~
2'o
z~
b
z~
A
z's
o
d (crn) Fig. 2. Mass m of ablated target material (left ordinate scale) as a function of the distance d between the target surface and the lens (lower abscissa scale) focusing the KrF laser beam of stabilized pulse energy. The upper abscissa scale denotew the laser energy flux ~ and the ordinate scale on the left the (hypothetical) number N of AI203 particles ejected per laser pulse.
H. Stafast, M. yon Przychowski / Evaporation of solids by pulsed laser irradiation
153
energy flux decreases upon enlarging the distance between the lens and the target. For further considerations it appears convenient to express the amount of ablated material by the average number of particles (atoms, formula units) removed by a single laser pulse (ordinate scale at right hand side) to facihtate comparisons between different target materials. Furthermore the lens position is substituted by the related laser energy flux on the target (upper abscissa scale). Finally, the amount of ablated material is normalized with respect to the irradiated target area. Using this way of representation the experimental findings for K r F laser ablation of AI203, SiC, graphite, Pb, Ni, Cr, quartz, and NaCl are sketched in fig. 3.
,t /
~
s
0
5
~o (J/cm 2)
Fig. 3. Average amount of target material ablateQ per laser pulse and per umt of irradiated target
area given as the (hypothetical) numb~ N;, of formula weight particles determined as a function of the incident laser energy flux O. The functions have been evaluated for AI203, SiC, and graphite (C) (a), Pb, Ni, and Cr (b) as ~ell as quartz (SiO2) ~nd NaCl (c).
154
H. Stafast, M. yon Przychowski / Evaporation of solids by pulsed la~'er irradiatton
The data sketched in fig. 3 were typically obtained at a pulse repetition rate of 20 pps ~zad a target translation speed of 0.3 m m / m i n . In the cases of quartz and NaC! (part c), however, a lowered repetition rate of 2 pps and an inereased target speed of 3 m m / m i n turned out necessary to prevent the formation of deep holes in the target. Deep holes are found to diminish the amount of ejected target material. This reduction is attributed to less efficient evaporation and ejection processes. Evaporation is diminished because the laser beam is hitting the modified target surface at a larger angle (lower laser energy flux) than the original target surface. Matzrial ejection is hampered because the evaporated material first has to leave the laser engraved hole before it can freely expand into the vacuum. The latter process can be observed with the naked eye looking at the luminescent cloud of evaporated material: its volume decreases and its main direction turns from the normal of the or;.gina] target surface towards the direction of the incoming laser beam (axis of the laser engraved hole). It is pointed out that the absolute value of Np in fig. 3 is arbitrary. It is based on the assumption that the size of the particles ejected by the laser impact corresponds to one formula weight. But the actual chemical composition and size of the vaporized particles stil', have to be dete,=mined experimentally. In a preliminary way the Np(O) curves have been classified according to their shape. Fig. 3a comprises target materials absorbing the 248 x:~m radiation and showing a well defined laser energy flux threshold for vaporization. The threshold is lowest for AI20 3 and highest for SiC. Fig. 3b contains the data of three metals. Their laser evaporation is rising smoothly with increasing energy flux. The efficiently of laser vaporization is checked for correlations with the thermodynamic data given in table 1. The ease of vaporizing lead is consistent with its low melting point, bolting po2nt, and heat of atomization. The high bond strength of Pb-l~b furthermore indicates a tendency towards cluster formation which can redu:e the amount of laser energy required for evaporation. The vaporization of nickel and chromium is considerably less efficient than that of lead. Nickel is vaporized more efficiently than chromium in spite of its high boiling point and large heat
Table 1 Melting point mp (o C), boiling point bp (o C), heat of atom formation Ah r,29s (kJ/mol), and bond strengths of d/atomic molecules B (kJ/mol) for lead, nickel, and chromium [2,3] Lead Nickel Chromium
mp 328 1453 1857
bp 1740 2732 2672
AhL~ 8
B
195 430 398
339 262 155
H. Stafast. M. yon Przychowski / Evaporation of solids by pulsed laser irradiation
155
of atomization. This behaviour correlates, on the other hand, with the relatively low melting point of nickel and the N i - N i bond strength being higher than that of Cry. Fig. 3c shows the data recorded with polished quartz and NaCl plates as well as a quartz plate with a rough surface. Opposite to the above materials quartz and NaC! are transparent to 248 nm radiation. While the Np(~) curves in parts (a) and (b) increase monotonically with ~, the Np(~) curve of NaCI is approximately flat in the 3-6 J / c m 2 region. The data of quartz even provide a maximum at about 3-4 J / c m 2. Qualitatively the curves obtained with polished and rough quartz plates are similar. The polished plate is, however, showing a higher energy flux threshold than the rough plate. Furtherwore, Np(qb) increases more sharply at the threshold with the polished flat and its evaporation is about two times as efficient as that of rough quartz. The Np(~) curves of quartz with its maximum in the 3-4 J / c m 2 region might be rationalized in a preliminary way by two different mechanisms of laser light absorption. At low laser energy flux absorption of the essentially transparent material may dominantly be caused by impurities and structural imperfections. The laser penetration depth is large and much quartz can be vaporized a n d / o r ejected in tiny morcels. At increased flux non-linear absorption becomes likely and will reduce the penetration depth and material ejection, whereas the average energy con~znt of the ejected particles is rising. At even higher flux the Np(~) value increases again due to enhanced laser energy deposition in the solid. This preliminary rationalization is consistent with the different findings obtained with polished and rough quartz: relative to polished quartz the surface roughness (many imperfections) is lowering the threshold of particle ejection and reducing the laser penetration depth (amount of ejected material). Non-linear absorption is damped by light scattering at the rough surface (diffuse laser beam focus) resulting in diminished vaporization at high energy flux (fig. 3c). Summarizing, the amount of laser-evaporated material has been shown to have a maximum with respect to the position of the focusing lens relative to the solid target (fig. 2). The optimum lens position is different for individual target materials and of practical importance with respect to efficient laser sputtering. I. ookJng furthermore at the nor:-linearity of the Np(~) curves in fig. 3 it appears very likely that the average size and energy content of the particles ejected upon the laser impact are dependent on the irradiation parameters. The size and the energy content of the particles hitting the substrate surface, on the other hand, are important for the adherence and microscopic structure of the deposit. If appropriate parameters are selected laser sputtering may yield thin film properties which are hard or even impossible to achieve by other deposition techniques. To conclude with a typical example, laser sputtering has successfully been applied to deposit thin films of high T~ superconducting materials [4].
156
!t. Stafast, M. vorl Przychowski / Evaporation of solids by pulsed laser irradiation
References [1] M. v. Alimen, L'~ser-Beam Interactions with Materials, Vol. 2 of 'ipringer Series in Materials ,Science (Sp,cingcr, Berlin, 1987). [2] See, e.g., 3.E. Andrew, P.E. Dyer, R.D. Greenoagh and P.H. Key, Appl. Phys. Letlers 43 (1983) 1076; 3.T. Cheung, Mater. R,~,.. Soc. Syrup. Prec. 29 (1984) 301; F. Valerio, Spectrosc. Intern. J. 3 (1984) 427; R. Viswar.athan and I. Hussla, J. Opt. SOc. Am. B 3 (1986) 796; S. Metev, in: Proc. E-MRS Conf. 1986, Strasbourg (Editions de Physique, Paris, 1986) p. 71; H. Sanl,,ur, Appl Opt. 25 (1986) 1962. [3] R.C. Weast, Ed., Handbook of Chemistry and Physics, 59th ed. (CRC Press, Boca Raton, 1978--79). [4] See. e.g., D. Dijkkamp, T. Venkatesan. X.D. Wu, S.A. Shaheen, N. Jisraw;~ Y.H. Min-Lee, WL. McLean and M. Croft, Appl. Phys. Letters 51 (1987) 619; X.D. Wu, D. Dijkkamp, S.B. Ogale, A. inam, E.W. Chase, P.F. Miceli, C.C. Chang, J.M. Tarasoora and T. Venkatesan, Appl. Phys. Letters 51 (1987) 861; J. Narayan, N. Biumo, R. Singh, O.W. Holland and O. Aueiello, Appl. Phys. Letters 51 (1987) 1845: H. Koinuma, M. Kawasaki, M. Funabashl, T. Hasegawa, K. Kishio, K. Kitazawa, K Fueki and S. Nagata, J. Appl. Phys. 62 (1987) 1524; T. Venkatasan, Solid State Technol. (December 1987) 39; L. Lynds, B.R. Weinberger, G.G. Peterson and H.A. Krr~insky, Appl. Phys. Letters 52 (1988) 32O; X.D. Wu, A. Inam, T. Venkatesan, C.C. Chang, E.W. Chase, P. Barboux, J.M. Tarascon and B. Wilkens, Appl. Phys. Letters 52 (1988) 754.
ApplieASurfaceScience36 (1989) 150-156 North-Holland, Amsterdam
E V A P O R A T I O N O F S O L I D S BY P U L S E D LASER I R R A D I A T I O N H. STAFAST and M. VON P R Z Y C H O W S K I * Battelle.lnstitut e. V.. A m R6merhof 35, D-6000 Frankfurt/M 90, Fed. Rep. of Germany
Received 2 June 1988: accepted for publication 11 July 1988
The focused beam of a KrF laser (248 nm) h~ been applied to irradiate targets of AI203, SiC, graphite, Pb, Ni, Or, quartz, and NaCI at variable laser energy flux i. the range 0-13 J/cm2. The amount of target material ejected into the vacuum (background pressu,ozabout 8 x 10-4 Tort) was determined from the target weight before and after laser irradiation. The average number of particles (formula weight) evaporated per laser pulse and per unit of irradiated target area is non-linearly dependent on the laser energy flux. Tbe evaporation of AI203, SiC, and graphite is showing a well-definedflux threshold while the vaporization of Pb, Ni and Cr is rising smoothly with increasingflux. With both groups of materials laser evaporation is monotonicallyincreasing with the laser energy flux. NaCI and quartz, on the other hand, are showing an intermediate maximum in the laser vaporization efficiency.
1. Introduction Intense laser fight provides a convenient means to vaporize solid material by localized heating of a target surface [1,2]. With a laser beam of low intensity the evaporation process will be close to thermal equilibrium and characterized by a broad spatial distribution of the vaporized target material. Using (pulsed) laser radiation of high intensity, a plasma can be generated in front of the target surface and the vaporized material may be ejected in a directed jet-like gas (plasma) stream. Both processes can be used for laser sputtering, i.e. evaporation of target material and subsequent deposition onto near-by substrate surfaces. The overall process of laser sputtering may roughly be divided into the steps of evaporation, translation and relaxation in the gas (plasma) phase, and deposition onto the substrate surface. The present study is addressing the process of laser evaporation. Evaporation requires energy input from the laser beam into the target material. In the initial phase the energy input is dominated by the absorption behaviour of the solid. Absorption will be very efficient if the frequency of the incident laser light is higher than the plasma frequency of the illuminated * Graduate student at Frankfurt University,part of diploma thesis. 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
tl. Stafast. M. van Przycho'vski / Eoaporation of solids by p~.;sed laser irradiation
151
target. Many materials will meet this requirement if the short wavelength light of a KrF laser (248 nm) is applied. In addition, the Ifigh intensity of short excimer laser pulses is favourable with respect to the non-linearity of the entire evaporation process. In order to obtain some information about the main characteristics of the vaporization by pulsed laser irra,.~tiation a series of target materials with differei-..t (linear) optical properties has been examined. The solids selected are absorbing, reflecting ot transpare:al: to the applied laser light. The evaporation efficiency has been determined as a function of the incident laser energy flux (intensity) by measuring the weight loss of the target upon irradiation.
2. Expe~memal The experimental set-up is sketched in fig. 1. "Vr,e solid targets consisted of NaCl, quartz, graphite, A1203, SiC, Ni, Cr, and Pb. They were weighed on a balance (Mettler E5, +0.1 mg accuracy) before and after laser irradiation. Irradiation was performed in a vacuum vessel of - 15 d volume. The vessel was pumped by a diffusion (Leybold-Hcraeus, Leybodiff 30L) and a rotary pump (F,dwards, E D M 6). The background pressure was determined by a penning gauge (Leybold-Heraeus, PR 31) and aniounted to at least 5 x 10 -5 T o r t but increased t~ about 8 × 10 -a Torr when flushing the laser entrance window with N2, H2, or air to prevent window deposits. A tube of 4.5 cm inner diameter and 7 cm length with two baffles containing holes of 25 × 8 mm 2 and 10 × 5 mm 2 cross section towards the laser window and the target, respectively, provided additional protection against unwanted deposits on the window. The targets were tilted about 45 ° relative to the laser beam a.--d moved during irradiation in a direction vertical to the beam at a speed of 0.3 m m / m i n or 3 mr~/rnin (see below). A KrF laser (Lambda Physik, E M G 201 MSC) delivered pulse ¢n¢~:gies up to 300 mJ stabilized by a microprocessor .'2".Z.z,,~///////////////
inert
~J
.=8 x lO-4torr
vari.qble ~ positron ~
V/7~///~ pump ~////// Fig. 1. Experimental set-up for K r F laser evaporation of solid targets in vacuum, inert gas flushing of the laser entrance window provides a constant flow from the window towards the target to prevent deposits on the quartz plate (see text).
152
tt. Stafast, M. yon Przychowski / Evaporation of solids by pulsed laser irradiation
control (Lambda Phys.~k, ILC). The laser control is based on a pulse energy measurement by a photo diode illuminated from a beam splitter in front of the laser outeoupling mirror. It was frequently recalibrated using a pyroelectric energy meter (Gentec, E D 500). This energy meter head also served to measure the laser pulse energy available on the target ( - 5 0 % of the original pulse energy). The laser beam was focused by a quartz lens (f= 20 era) and the beam cross section at the target surface was registered by the burn pattern on a photographic film. Its dimensions were measured using a magnifying glass with an engraved scale of 0.1 mm resolution. The estimated error limits are _+5% for large areas of 20 mm 2 and about _+15% for small areas of 1.3 mm 2. The irradiated target area varied with the distance d between the target and the focusing lens (20-25 cm) while keeping the laser pulse energy constant. The maximum laser energy flux achieved on the target amounted up to about 13 J / c m 2.
3. R e s ~
and discussion
The primary information obtained from the experiments is sketched in fig. 2 for the case of K r F laser irradiation of sintered aluminium oxide. The mass m of ablmed material (ordinate scale on the left hand side) is plotted against the distance d between the focusing lens and the irradiatex) target surface (lower abscissa scale). This plot is showing a maximum at d - - 23 cm corresponding to 11.5 nun 2 irradiated area and 1.45 J / e r a 2 laser energy flux. This maximum is reflecting two simultaneous but counterpropagating effects induced by the lens movement: ,'~ne irradiated area increases and the laser
13 10 5
3 2
~} (J/cm 2) 1.5 1
0.7
E ,o
o
2~
2'o
z~
b
z~
A
z's
o
d (crn) Fig. 2. Mass m of ablated target material (left ordinate scale) as a function of the distance d between the target surface and the lens (lower abscissa scale) focusing the KrF laser beam of stabilized pulse energy. The upper abscissa scale denotew the laser energy flux ~ and the ordinate scale on the left the (hypothetical) number N of AI203 particles ejected per laser pulse.
H. Stafast, M. yon Przychowski / Evaporation of solids by pulsed laser irradiation
153
energy flux decreases upon enlarging the distance between the lens and the target. For further considerations it appears convenient to express the amount of ablated material by the average number of particles (atoms, formula units) removed by a single laser pulse (ordinate scale at right hand side) to facihtate comparisons between different target materials. Furthermore the lens position is substituted by the related laser energy flux on the target (upper abscissa scale). Finally, the amount of ablated material is normalized with respect to the irradiated target area. Using this way of representation the experimental findings for K r F laser ablation of AI203, SiC, graphite, Pb, Ni, Cr, quartz, and NaCl are sketched in fig. 3.
,t /
~
s
0
5
~o (J/cm 2)
Fig. 3. Average amount of target material ablateQ per laser pulse and per umt of irradiated target
area given as the (hypothetical) numb~ N;, of formula weight particles determined as a function of the incident laser energy flux O. The functions have been evaluated for AI203, SiC, and graphite (C) (a), Pb, Ni, and Cr (b) as ~ell as quartz (SiO2) ~nd NaCl (c).
154
H. Stafast, M. yon Przychowski / Evaporation of solids by pulsed la~'er irradiatton
The data sketched in fig. 3 were typically obtained at a pulse repetition rate of 20 pps ~zad a target translation speed of 0.3 m m / m i n . In the cases of quartz and NaC! (part c), however, a lowered repetition rate of 2 pps and an inereased target speed of 3 m m / m i n turned out necessary to prevent the formation of deep holes in the target. Deep holes are found to diminish the amount of ejected target material. This reduction is attributed to less efficient evaporation and ejection processes. Evaporation is diminished because the laser beam is hitting the modified target surface at a larger angle (lower laser energy flux) than the original target surface. Matzrial ejection is hampered because the evaporated material first has to leave the laser engraved hole before it can freely expand into the vacuum. The latter process can be observed with the naked eye looking at the luminescent cloud of evaporated material: its volume decreases and its main direction turns from the normal of the or;.gina] target surface towards the direction of the incoming laser beam (axis of the laser engraved hole). It is pointed out that the absolute value of Np in fig. 3 is arbitrary. It is based on the assumption that the size of the particles ejected by the laser impact corresponds to one formula weight. But the actual chemical composition and size of the vaporized particles stil', have to be dete,=mined experimentally. In a preliminary way the Np(O) curves have been classified according to their shape. Fig. 3a comprises target materials absorbing the 248 x:~m radiation and showing a well defined laser energy flux threshold for vaporization. The threshold is lowest for AI20 3 and highest for SiC. Fig. 3b contains the data of three metals. Their laser evaporation is rising smoothly with increasing energy flux. The efficiently of laser vaporization is checked for correlations with the thermodynamic data given in table 1. The ease of vaporizing lead is consistent with its low melting point, bolting po2nt, and heat of atomization. The high bond strength of Pb-l~b furthermore indicates a tendency towards cluster formation which can redu:e the amount of laser energy required for evaporation. The vaporization of nickel and chromium is considerably less efficient than that of lead. Nickel is vaporized more efficiently than chromium in spite of its high boiling point and large heat
Table 1 Melting point mp (o C), boiling point bp (o C), heat of atom formation Ah r,29s (kJ/mol), and bond strengths of d/atomic molecules B (kJ/mol) for lead, nickel, and chromium [2,3] Lead Nickel Chromium
mp 328 1453 1857
bp 1740 2732 2672
AhL~ 8
B
195 430 398
339 262 155
H. Stafast. M. yon Przychowski / Evaporation of solids by pulsed laser irradiation
155
of atomization. This behaviour correlates, on the other hand, with the relatively low melting point of nickel and the N i - N i bond strength being higher than that of Cry. Fig. 3c shows the data recorded with polished quartz and NaCl plates as well as a quartz plate with a rough surface. Opposite to the above materials quartz and NaC! are transparent to 248 nm radiation. While the Np(~) curves in parts (a) and (b) increase monotonically with ~, the Np(~) curve of NaCI is approximately flat in the 3-6 J / c m 2 region. The data of quartz even provide a maximum at about 3-4 J / c m 2. Qualitatively the curves obtained with polished and rough quartz plates are similar. The polished plate is, however, showing a higher energy flux threshold than the rough plate. Furtherwore, Np(qb) increases more sharply at the threshold with the polished flat and its evaporation is about two times as efficient as that of rough quartz. The Np(~) curves of quartz with its maximum in the 3-4 J / c m 2 region might be rationalized in a preliminary way by two different mechanisms of laser light absorption. At low laser energy flux absorption of the essentially transparent material may dominantly be caused by impurities and structural imperfections. The laser penetration depth is large and much quartz can be vaporized a n d / o r ejected in tiny morcels. At increased flux non-linear absorption becomes likely and will reduce the penetration depth and material ejection, whereas the average energy con~znt of the ejected particles is rising. At even higher flux the Np(~) value increases again due to enhanced laser energy deposition in the solid. This preliminary rationalization is consistent with the different findings obtained with polished and rough quartz: relative to polished quartz the surface roughness (many imperfections) is lowering the threshold of particle ejection and reducing the laser penetration depth (amount of ejected material). Non-linear absorption is damped by light scattering at the rough surface (diffuse laser beam focus) resulting in diminished vaporization at high energy flux (fig. 3c). Summarizing, the amount of laser-evaporated material has been shown to have a maximum with respect to the position of the focusing lens relative to the solid target (fig. 2). The optimum lens position is different for individual target materials and of practical importance with respect to efficient laser sputtering. I. ookJng furthermore at the nor:-linearity of the Np(~) curves in fig. 3 it appears very likely that the average size and energy content of the particles ejected upon the laser impact are dependent on the irradiation parameters. The size and the energy content of the particles hitting the substrate surface, on the other hand, are important for the adherence and microscopic structure of the deposit. If appropriate parameters are selected laser sputtering may yield thin film properties which are hard or even impossible to achieve by other deposition techniques. To conclude with a typical example, laser sputtering has successfully been applied to deposit thin films of high T~ superconducting materials [4].
156
!t. Stafast, M. vorl Przychowski / Evaporation of solids by pulsed laser irradiation
References [1] M. v. Alimen, L'~ser-Beam Interactions with Materials, Vol. 2 of 'ipringer Series in Materials ,Science (Sp,cingcr, Berlin, 1987). [2] See, e.g., 3.E. Andrew, P.E. Dyer, R.D. Greenoagh and P.H. Key, Appl. Phys. Letlers 43 (1983) 1076; 3.T. Cheung, Mater. R,~,.. Soc. Syrup. Prec. 29 (1984) 301; F. Valerio, Spectrosc. Intern. J. 3 (1984) 427; R. Viswar.athan and I. Hussla, J. Opt. SOc. Am. B 3 (1986) 796; S. Metev, in: Proc. E-MRS Conf. 1986, Strasbourg (Editions de Physique, Paris, 1986) p. 71; H. Sanl,,ur, Appl Opt. 25 (1986) 1962. [3] R.C. Weast, Ed., Handbook of Chemistry and Physics, 59th ed. (CRC Press, Boca Raton, 1978--79). [4] See. e.g., D. Dijkkamp, T. Venkatesan. X.D. Wu, S.A. Shaheen, N. Jisraw;~ Y.H. Min-Lee, WL. McLean and M. Croft, Appl. Phys. Letters 51 (1987) 619; X.D. Wu, D. Dijkkamp, S.B. Ogale, A. inam, E.W. Chase, P.F. Miceli, C.C. Chang, J.M. Tarasoora and T. Venkatesan, Appl. Phys. Letters 51 (1987) 861; J. Narayan, N. Biumo, R. Singh, O.W. Holland and O. Aueiello, Appl. Phys. Letters 51 (1987) 1845: H. Koinuma, M. Kawasaki, M. Funabashl, T. Hasegawa, K. Kishio, K. Kitazawa, K Fueki and S. Nagata, J. Appl. Phys. 62 (1987) 1524; T. Venkatasan, Solid State Technol. (December 1987) 39; L. Lynds, B.R. Weinberger, G.G. Peterson and H.A. Krr~insky, Appl. Phys. Letters 52 (1988) 32O; X.D. Wu, A. Inam, T. Venkatesan, C.C. Chang, E.W. Chase, P. Barboux, J.M. Tarascon and B. Wilkens, Appl. Phys. Letters 52 (1988) 754.