- Email: [email protected]
Lateral and depth profiles of nitrogen in laser-nitrided iron
Nuclear Instruments and Methods in Physics Research B 122
( 1997)
420-422
lI?TlWB
Beam Interactions with Materials & Atoms
ELSEVIER
Lateral and depth profiles of nitrogen in laser-nitrided iron C. Illgner a, l? Schaaf ‘v*,K.-l? Lieb ‘, R. Queitsch b, K. Schutte b, H.-W. Bergmann b ’
Universitiit Giittingen, II. Physikulischrs Instirur, BunsenstraJQ 7/9, D-37073 GNtingen, Germany b Applikutions- und Technikzzentrum EVUS. RinosrraJe 1, D-92249 Vilseck. Germony
Abstract The effect of nitrogen take-up upon irradiation of iron or steel with excimer laser pulses in air or nitrogen atmosphere is well established. However, the influence of the laser parameters (energy density, time structure, wavelength, number of pulses) on the nitrogen profile is not yet understood. We used Resonant Nuclear Reaction Analysis to investigate the evolution of nitrogen profiles with the number of pulses. To improve the accuracy of the nitrogen profiling, the laser irradiation was carried out in a “N2 enriched atmosphere. This allowed to detect also very low nitrogen concentrations that made it possible to follow the development of the nitrogen depth profiles from 1 to 64 pulses and to resolve the lateral nitrogen profiles across the laser spot. The laser induced surface morphology was observed with optical microscopy and a surface protilometer. PACS: 61.80.Ba; 52.75-d;
81.60.Bn
1. Introduction
2. Experimental
It has recently been shown that excimer laser irradiation of Armco iron samples in air or in a nitrogen atmosphere can lead to a significant nitridation of the surface up to depths of more than 500 nm [ l-51 _Analysis of the nitrogen profiles showed characteristic nitrogen profiles with high nitrogen concentrations at the surface, decreasing with depth to a saturation value in the range of 5-10 at %. An analysis of the evolution of nitrogen profiles with the number of pulses was not possible, due to the low nitrogen concentrations at small numbers of pulses. In order to access this regime of laser parameters involving low nitrogen concentrations like small numbers of pulses or laser pulse energy densities below or near the threshold of nitridation, the iron samples were irradiated in an atmosphere enriched with the “RNRA sensitive” isotope “N. This enrichment enables the detection of very low nitrogen concentrations down to 50 ppm. Lateral scans of the nitrogen concentration in a given depth and nitrogen depth profiles in the central part of the irradiation spot of iron samples irradiated with a varying number of pulses will be described in this paper.
Samples of Armco iron with a diameter of 25 mm and a thickness of 2 mm were mechanically polished to 1 pm. They were mounted in a chamber, which was evacuated to lo-* Pa before it was filled with either nitrogen of the purity 4N8 or with nitrogen enriched with the isotope “N to 19.6( 17) at %. In comparison to the natural abundanceof 0.37 at % this means an enrichment by a factor of 53. The samples were irradiated with a Siemens XP2020 excimer laser (wavelength A = 308 nm, pulse length r = 55 ns). The pulse energy density was set to a mean value of 4 J/cm2 inside a laser spot size of 4 x 5 mm2. Targets were irradiated with numbers of pulses varying from 1 pulse to 64 pulses. For each number of pulses, a fresh sample was used. Nitrogen depth profiling was done by Resonant Nuclear Reaction Analysis using the 429.57(9) keV resonance of the reaction “N( p,ay) ‘*C having a resonance width of r = 124( 17) eV [ 61. The RNRA analysis measurement was done in Clottingen using the 530 keV implanter IONAS [ 71. During measurement the samples were cooled to 77 K in order to avoid changes due to diffusion induced by the analysis beam. One set of measurements was carried out at a fixed proton energy of 432.5 keV. The sample was moved relative to the analysing proton beam in steps of 0.5 mm with a step-to-step accuracy better than 0.1 mm. A second RNRA scan was done as a function of proton energy in the central part of the laser-irradiated spot. In both cases the diameter of the analysing proton beam was set to 2 mm by means of an aperture. The 4.43 MeV y radiation was taken by a 12 cm
* Corresponding author. Tel. +49 551 39 7672. fax +49 551 39 4493. e-mail [email protected].
0168-583X/97/$17.00 Copyright @ 1997 Elsevier Science B.V. All rights reserved PIISO168-583X(96)00581-2
C. Illgner el al./Nucl.
421
Instr. and Meth. in Php. Res. B 122 (1997) 420-422
long Nal detector of 16 cm diameter. The surface profiles of the irradiated samples were taken by a Perthen Dektak3ST profilometer with a diamond tip of 2.5 ,um diameter.
3. Results 3. I. Surface topology Fig. 1 shows an optical micrograph of the central part of the sample irradiated with 4.0 J/cm2 and 64 pulses taken by an optical microscope. A characteristic pattern of the laser irradiated surface evolves, its periodicity being about 30 pm. A similar pattern was described by Gorodetsky et al. [8] for laser irradiated Si in terms of surface acoustic waves. This explanation seems reasonable in case of the samples described here, since the observed periodicity scale is much larger than the laser wavelength. Profilometer scans across the irradiated region show, as can for example be seen in Fig. 2 (top) for a sample irradiated with 64 pulses at 4 J/cm* that in the centre of the irradiated zone material has been removed up to a depth of 5 pm while material has been deposited at the border of the irradiated spot. Integrating the amount of material removed as well as the amount of material deposited in dependence of the number of pulses,
-2
-1
0
Position
I
2
x [mm]
-40. -500
100
200 Number
300
400
500
600
of pulses
Fig. 2. Top: Surface profileof iron irradiated with 64 pulses at 4 J/cm’ taken with a surface profiler DEKTAKjST (diamond tip of 2.5 firn diameter). Bottom: The plots named “Integral+” and ‘W.egral-” indicate an integral over the part of the surface profile, where material has been removed (dark shaded region) or deposited (light shaded region) with respect to the surface level before irradiation. which can be seen at x = -2.5 mm and .x = 2.5 mm. respectively.
depicted in Fig. 2 as “Integral+” and “Integral-“. shows that both processes proceed continuously up to 5 12 pulses. The material removal and deposition most probably are due to the “piston” mechanism [9]: a molten layer is pushed away from the central part of the irradiated spot by the recoil pressure of the evaporating material before plasma ignition. The material has gathered in a region where the laser intensity is not sufficient to establish enough pressure.
3.2. Lateral projiles
F’ig. 1. Optical micrograph of the surface of the iron sample irradiated with t,4 pulses at 4 J/cm2.
Fig. 3 shows the lateral profiles of the nitrogen concentration taken at a depth of about 20 nm. The nitrogen concentration is highest in the central part of the laser irradiated spot in case of single pulse irradiation, reaching saturation with the first pulse. With increasing numbers of pulses the nitrogen concentration in the outer regions of the spot further increases, forming two maxima. The maximum of nitrogen concentration moves outward, until, at 64 pulses, it is situated at the border of the irradiated zone. II. LASER
BEAMS
C. Il/pwr
422
rr ul./Nucl.
Insrr.
and Mrth.
in Ph,.s.
Res.
B 122
(1997)
420-422
4. Conclusions
4oo1E,=432.5 key F
(depth=20
“m)
_
,
300
3 3z
200
0 z s
100
0
-4
-2
0
2
Position Fig. 3. Lateral
nitrogen
Ep = 432.5 keV (z-20 with
4 J/cm’
profiles nm)
The error bars are smaller
6
across the laser spot at a proton energy of
measured by RNRA
and numbers
4
[mm]
of pulses ranging
for iron samples irradiated
I
from
pulse to 64 pulses.
than the symbols.
3.3. Depth projles Typical nitrogen profiles in the central part of the sample are shown in Fig. 4 in dependence of the number of pulses. Note that the y-ray yield at the surface, corresponding to the nitrogen concentration of the sample, is similar for all the samples independent of the number of pulses the samples were irradiated with. This holds even for low numbers of pulses. Towards deeper layers, the nitrogen concentration decreases to a constant value. Especially at low numbers of pulses this decrease resembles an exponential function. Extrapolating the known nitrogen diffusion coefficient [ IO] to the temperatures expected at the surface (about 4000 K for irradiation in vacuum, as was calculated by means of the model presented in Ref. [ I I ] ) and estimating the depth reached by the nitrogen via x = 2fi with r = 55 ns being the pulse width, yields x = 100 nm.
A comparison of the profilometer scan with the lateral distribution of the nitrogen concentration shows that the maxima of the concentration distribution are located near the position of the deposited material at the border of the irradiated spot. Considering the diameter of the analyzing proton beam of 2 mm, it cannot be decided if the maxima are due to nitrides formed by ablated material deposited from the plasma outside the laser irradiated spot [ 121 or from material which has been pushed aside from the centre of the spot. However, the “piston” mechanism, which has been shown to work in this case, moves the topmost layers containing the highest nitrogen concentrations to the boundary of the irradiated spot. Thus, the nitrogen concentration at the boundary should be expected to be higher. However, a conclusive distinction between plasma enhanced deposition and the effect of the “piston” mechanism on the concentration profile at the border can only be made after the lateral resolution of the analyzing proton beam has been reduced to 0.1-0.3 mm, requiring the techniques of microbeams. It has been shown that the use of nitrogen gas enriched with the RNRA isotope “N allows to access also regions of very low nitrogen contents. This should enable us to better understand the mechanisms of nitrogen take up and nitrogen diffusion by subsequent irradiations in natural/enriched nitrogen atmospheres. Acknowledgements The authors would like to thank D. Purschke for expertly running the IONAS accelerator. This work is supported by the Deutsche Forschungsgemeinschaft (DFG Scha 632/3I). References [ I 1 C. Illgner, Appl.
K.-P. Lieb, P. Schaaf. K. Mann, H. Ktister. and G. Marowsky.
Phys. A 62 ( 1996)
12 1 C. Illgner,
13] Depth 120
[nml
180
Fe irradieted -o-
240
1 pulse
-- *L- 2 pulses
---t---
Appl.
P. Schanf,
A. Emmel,
Bergmann,
Mater.
Schaaf,
C.
interactions
with 4 J/cm’ 32 pulses
---*--
64 pulses
Hyperfine
[6 I T.
Proton
energy
diated
with
4 J/cm*
measured
and numbers
Niederdrenk,
K.-P.
Hyperfine
and
H.W. 136
Lieb,
Bergman”,
and K.-P.
for iron samples irrafrom
1 pulse to 64
I I21
K. P. Lieb, and S. Btilssermann,
K. Pampus, F. Bergmeister, B 9 (1985)
Nucl.
Instr. and Meth.
J. Kanicki.
T. Kazyaka,
and R.L. Melcher,
J. Appl. Phys. 47 (1976)
H. Jehn. W. Hehn,
in Metals
Ferrous
(3):
Iron-Nitrogen
Appl. Phys.
5460.
H. Speck, and G. Hi%
(Thermodynamics,
Metals
D. Purschke, and K. I? Lieb.
234.
M. van Allmen,
R. Kelly, (1992)
Lieb.
1.
547.
Mathematik
[keV)
of pulses ranging
and H.W.
Kinetics, (Fe-N),
in: Gases and
and Properties), Physik
Date”.
H. Behrens and P. Luksch (FachinfommtionszentmmEnergie,
470
by RNRA
E. Schubert,
LI
232.
GmbH.
[ 11 [ S. FZhler and H.U. depth profiles
Lieb,
Lett. 46 (1985)
XV:
and H.W.
199. E. Schubert,
Instr. and Meth.
Carbon
Fig. 4, Nitrogen
M.
92 ( 1994)
Osipowicz,
I 101 E. Fromm,
460
K.-P.
A. Emmel,
I 8 1G.Gorodetsky,
450
C. Illgner,
R. Queitsch,
I.
Sci. Eng. A 197 ( 1995)
Illgner.
17 1 M. Uhrmacher,
[ 91
E. Schubert,
Interactions
B 18 (1987)
Nucl.
I.
Phys. A 61 (1995)
95 (1995)
151 P. Schaaf,
16 pulses
-x--
23
K.-P. Lieb,
Bergmann,
[41P.
300
P. Schaaf,
Karlsruhe,
2980.
eds.
Physik.
1982).
Krebs, Appl.
A. Miotello.
Part
B. Braren,
Surf. Sci. 2946 and C.E.
( 1996)
Otis, Appl.
61. Phys. Lett. 60
( 1997)
420-422
lI?TlWB
Beam Interactions with Materials & Atoms
ELSEVIER
Lateral and depth profiles of nitrogen in laser-nitrided iron C. Illgner a, l? Schaaf ‘v*,K.-l? Lieb ‘, R. Queitsch b, K. Schutte b, H.-W. Bergmann b ’
Universitiit Giittingen, II. Physikulischrs Instirur, BunsenstraJQ 7/9, D-37073 GNtingen, Germany b Applikutions- und Technikzzentrum EVUS. RinosrraJe 1, D-92249 Vilseck. Germony
Abstract The effect of nitrogen take-up upon irradiation of iron or steel with excimer laser pulses in air or nitrogen atmosphere is well established. However, the influence of the laser parameters (energy density, time structure, wavelength, number of pulses) on the nitrogen profile is not yet understood. We used Resonant Nuclear Reaction Analysis to investigate the evolution of nitrogen profiles with the number of pulses. To improve the accuracy of the nitrogen profiling, the laser irradiation was carried out in a “N2 enriched atmosphere. This allowed to detect also very low nitrogen concentrations that made it possible to follow the development of the nitrogen depth profiles from 1 to 64 pulses and to resolve the lateral nitrogen profiles across the laser spot. The laser induced surface morphology was observed with optical microscopy and a surface protilometer. PACS: 61.80.Ba; 52.75-d;
81.60.Bn
1. Introduction
2. Experimental
It has recently been shown that excimer laser irradiation of Armco iron samples in air or in a nitrogen atmosphere can lead to a significant nitridation of the surface up to depths of more than 500 nm [ l-51 _Analysis of the nitrogen profiles showed characteristic nitrogen profiles with high nitrogen concentrations at the surface, decreasing with depth to a saturation value in the range of 5-10 at %. An analysis of the evolution of nitrogen profiles with the number of pulses was not possible, due to the low nitrogen concentrations at small numbers of pulses. In order to access this regime of laser parameters involving low nitrogen concentrations like small numbers of pulses or laser pulse energy densities below or near the threshold of nitridation, the iron samples were irradiated in an atmosphere enriched with the “RNRA sensitive” isotope “N. This enrichment enables the detection of very low nitrogen concentrations down to 50 ppm. Lateral scans of the nitrogen concentration in a given depth and nitrogen depth profiles in the central part of the irradiation spot of iron samples irradiated with a varying number of pulses will be described in this paper.
Samples of Armco iron with a diameter of 25 mm and a thickness of 2 mm were mechanically polished to 1 pm. They were mounted in a chamber, which was evacuated to lo-* Pa before it was filled with either nitrogen of the purity 4N8 or with nitrogen enriched with the isotope “N to 19.6( 17) at %. In comparison to the natural abundanceof 0.37 at % this means an enrichment by a factor of 53. The samples were irradiated with a Siemens XP2020 excimer laser (wavelength A = 308 nm, pulse length r = 55 ns). The pulse energy density was set to a mean value of 4 J/cm2 inside a laser spot size of 4 x 5 mm2. Targets were irradiated with numbers of pulses varying from 1 pulse to 64 pulses. For each number of pulses, a fresh sample was used. Nitrogen depth profiling was done by Resonant Nuclear Reaction Analysis using the 429.57(9) keV resonance of the reaction “N( p,ay) ‘*C having a resonance width of r = 124( 17) eV [ 61. The RNRA analysis measurement was done in Clottingen using the 530 keV implanter IONAS [ 71. During measurement the samples were cooled to 77 K in order to avoid changes due to diffusion induced by the analysis beam. One set of measurements was carried out at a fixed proton energy of 432.5 keV. The sample was moved relative to the analysing proton beam in steps of 0.5 mm with a step-to-step accuracy better than 0.1 mm. A second RNRA scan was done as a function of proton energy in the central part of the laser-irradiated spot. In both cases the diameter of the analysing proton beam was set to 2 mm by means of an aperture. The 4.43 MeV y radiation was taken by a 12 cm
* Corresponding author. Tel. +49 551 39 7672. fax +49 551 39 4493. e-mail [email protected].
0168-583X/97/$17.00 Copyright @ 1997 Elsevier Science B.V. All rights reserved PIISO168-583X(96)00581-2
C. Illgner el al./Nucl.
421
Instr. and Meth. in Php. Res. B 122 (1997) 420-422
long Nal detector of 16 cm diameter. The surface profiles of the irradiated samples were taken by a Perthen Dektak3ST profilometer with a diamond tip of 2.5 ,um diameter.
3. Results 3. I. Surface topology Fig. 1 shows an optical micrograph of the central part of the sample irradiated with 4.0 J/cm2 and 64 pulses taken by an optical microscope. A characteristic pattern of the laser irradiated surface evolves, its periodicity being about 30 pm. A similar pattern was described by Gorodetsky et al. [8] for laser irradiated Si in terms of surface acoustic waves. This explanation seems reasonable in case of the samples described here, since the observed periodicity scale is much larger than the laser wavelength. Profilometer scans across the irradiated region show, as can for example be seen in Fig. 2 (top) for a sample irradiated with 64 pulses at 4 J/cm* that in the centre of the irradiated zone material has been removed up to a depth of 5 pm while material has been deposited at the border of the irradiated spot. Integrating the amount of material removed as well as the amount of material deposited in dependence of the number of pulses,
-2
-1
0
Position
I
2
x [mm]
-40. -500
100
200 Number
300
400
500
600
of pulses
Fig. 2. Top: Surface profileof iron irradiated with 64 pulses at 4 J/cm’ taken with a surface profiler DEKTAKjST (diamond tip of 2.5 firn diameter). Bottom: The plots named “Integral+” and ‘W.egral-” indicate an integral over the part of the surface profile, where material has been removed (dark shaded region) or deposited (light shaded region) with respect to the surface level before irradiation. which can be seen at x = -2.5 mm and .x = 2.5 mm. respectively.
depicted in Fig. 2 as “Integral+” and “Integral-“. shows that both processes proceed continuously up to 5 12 pulses. The material removal and deposition most probably are due to the “piston” mechanism [9]: a molten layer is pushed away from the central part of the irradiated spot by the recoil pressure of the evaporating material before plasma ignition. The material has gathered in a region where the laser intensity is not sufficient to establish enough pressure.
3.2. Lateral projiles
F’ig. 1. Optical micrograph of the surface of the iron sample irradiated with t,4 pulses at 4 J/cm2.
Fig. 3 shows the lateral profiles of the nitrogen concentration taken at a depth of about 20 nm. The nitrogen concentration is highest in the central part of the laser irradiated spot in case of single pulse irradiation, reaching saturation with the first pulse. With increasing numbers of pulses the nitrogen concentration in the outer regions of the spot further increases, forming two maxima. The maximum of nitrogen concentration moves outward, until, at 64 pulses, it is situated at the border of the irradiated zone. II. LASER
BEAMS
C. Il/pwr
422
rr ul./Nucl.
Insrr.
and Mrth.
in Ph,.s.
Res.
B 122
(1997)
420-422
4. Conclusions
4oo1E,=432.5 key F
(depth=20
“m)
_
,
300
3 3z
200
0 z s
100
0
-4
-2
0
2
Position Fig. 3. Lateral
nitrogen
Ep = 432.5 keV (z-20 with
4 J/cm’
profiles nm)
The error bars are smaller
6
across the laser spot at a proton energy of
measured by RNRA
and numbers
4
[mm]
of pulses ranging
for iron samples irradiated
I
from
pulse to 64 pulses.
than the symbols.
3.3. Depth projles Typical nitrogen profiles in the central part of the sample are shown in Fig. 4 in dependence of the number of pulses. Note that the y-ray yield at the surface, corresponding to the nitrogen concentration of the sample, is similar for all the samples independent of the number of pulses the samples were irradiated with. This holds even for low numbers of pulses. Towards deeper layers, the nitrogen concentration decreases to a constant value. Especially at low numbers of pulses this decrease resembles an exponential function. Extrapolating the known nitrogen diffusion coefficient [ IO] to the temperatures expected at the surface (about 4000 K for irradiation in vacuum, as was calculated by means of the model presented in Ref. [ I I ] ) and estimating the depth reached by the nitrogen via x = 2fi with r = 55 ns being the pulse width, yields x = 100 nm.
A comparison of the profilometer scan with the lateral distribution of the nitrogen concentration shows that the maxima of the concentration distribution are located near the position of the deposited material at the border of the irradiated spot. Considering the diameter of the analyzing proton beam of 2 mm, it cannot be decided if the maxima are due to nitrides formed by ablated material deposited from the plasma outside the laser irradiated spot [ 121 or from material which has been pushed aside from the centre of the spot. However, the “piston” mechanism, which has been shown to work in this case, moves the topmost layers containing the highest nitrogen concentrations to the boundary of the irradiated spot. Thus, the nitrogen concentration at the boundary should be expected to be higher. However, a conclusive distinction between plasma enhanced deposition and the effect of the “piston” mechanism on the concentration profile at the border can only be made after the lateral resolution of the analyzing proton beam has been reduced to 0.1-0.3 mm, requiring the techniques of microbeams. It has been shown that the use of nitrogen gas enriched with the RNRA isotope “N allows to access also regions of very low nitrogen contents. This should enable us to better understand the mechanisms of nitrogen take up and nitrogen diffusion by subsequent irradiations in natural/enriched nitrogen atmospheres. Acknowledgements The authors would like to thank D. Purschke for expertly running the IONAS accelerator. This work is supported by the Deutsche Forschungsgemeinschaft (DFG Scha 632/3I). References [ I 1 C. Illgner, Appl.
K.-P. Lieb, P. Schaaf. K. Mann, H. Ktister. and G. Marowsky.
Phys. A 62 ( 1996)
12 1 C. Illgner,
13] Depth 120
[nml
180
Fe irradieted -o-
240
1 pulse
-- *L- 2 pulses
---t---
Appl.
P. Schanf,
A. Emmel,
Bergmann,
Mater.
Schaaf,
C.
interactions
with 4 J/cm’ 32 pulses
---*--
64 pulses
Hyperfine
[6 I T.
Proton
energy
diated
with
4 J/cm*
measured
and numbers
Niederdrenk,
K.-P.
Hyperfine
and
H.W. 136
Lieb,
Bergman”,
and K.-P.
for iron samples irrafrom
1 pulse to 64
I I21
K. P. Lieb, and S. Btilssermann,
K. Pampus, F. Bergmeister, B 9 (1985)
Nucl.
Instr. and Meth.
J. Kanicki.
T. Kazyaka,
and R.L. Melcher,
J. Appl. Phys. 47 (1976)
H. Jehn. W. Hehn,
in Metals
Ferrous
(3):
Iron-Nitrogen
Appl. Phys.
5460.
H. Speck, and G. Hi%
(Thermodynamics,
Metals
D. Purschke, and K. I? Lieb.
234.
M. van Allmen,
R. Kelly, (1992)
Lieb.
1.
547.
Mathematik
[keV)
of pulses ranging
and H.W.
Kinetics, (Fe-N),
in: Gases and
and Properties), Physik
Date”.
H. Behrens and P. Luksch (FachinfommtionszentmmEnergie,
470
by RNRA
E. Schubert,
LI
232.
GmbH.
[ 11 [ S. FZhler and H.U. depth profiles
Lieb,
Lett. 46 (1985)
XV:
and H.W.
199. E. Schubert,
Instr. and Meth.
Carbon
Fig. 4, Nitrogen
M.
92 ( 1994)
Osipowicz,
I 101 E. Fromm,
460
K.-P.
A. Emmel,
I 8 1G.Gorodetsky,
450
C. Illgner,
R. Queitsch,
I.
Sci. Eng. A 197 ( 1995)
Illgner.
17 1 M. Uhrmacher,
[ 91
E. Schubert,
Interactions
B 18 (1987)
Nucl.
I.
Phys. A 61 (1995)
95 (1995)
151 P. Schaaf,
16 pulses
-x--
23
K.-P. Lieb,
Bergmann,
[41P.
300
P. Schaaf,
Karlsruhe,
2980.
eds.
Physik.
1982).
Krebs, Appl.
A. Miotello.
Part
B. Braren,
Surf. Sci. 2946 and C.E.
( 1996)
Otis, Appl.
61. Phys. Lett. 60