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Laser-initiated metal deposition on GaAs substrates
Volume 84A, number 4
PHYSICS LETTERS
27 July
1981
LASER-INITIATED METAL DEPOSITION ON GaAs SUBSTRATES Y. RYTZ-FROIDEVAUX, RP. SALATHE’ and H.H. GILGEN Institute of AppliedPhysics, University of Berne, Berne, Switzerland Received 12 November 1980
The local deposition of metal structures by thermal dissociation of trimethylaluminium, dimethylzinc and dimethylcadmium on GaAs surfaces heated by a cw krypton laser has been investigated. Piles of amorphous aluminium and zinc and crystalline structures of Cd have been deposited at temperatures between 200 and 1000°C. The smallest size of the deposits
was =4 tim.
The growth of semiconducting structures or the deposition of metal layers with microscopic features can be achieved by laser-induced chemical vapor deposition. So far thermal decomposition on laser heated surfaces [1,2] as well as photodecomposition by exciting electronic [3]or vibrational [1,4] transitions of molecules have been demonstrated. Generally the spatial resolution is better for the photodeposition, since both the laser spot size and the thermal conductivity of the substrate limit the resolution of the thermal method. However this latter process has the potential to interface with other semiconductor processes (e.g. diffusion, epitaxy, alloying, annealing). We report on experiments to deposit metallic structures by thermal decomposition of trimethylaluminium (Al(CH3)3, TMA1), dimethylzinc (Zn(CH3)2, DMZn) and dimethylcadmium (Cd(CH3)2, DMCd) on locally heated GaAs surfaces. These alkyls had been investigated because of their potential applications as doping or alloying materials in GaAs. A cw operated krypton laser at 520.8—568.2 nm wavelengths was used as heat source, resulting in a resolution improved by one order of magnitude compared to previously reported results obtained with a CO2 laser [1]. The experimental arrangement is shown in fig. 1. The laser beam is focused by means of a lens MO (f—~ 27 mm, NA = 0.25) ontocm3, a polished (100)-GaAs surface (Zn doped, 6 X 1016 0.047 ~ cm). The wafer is mounted on a copper heat sink in a chamber with the metal alkyl diluted in Pd-diffused H 2 at atmo216
Kr - Laser
S
MO
L
RM
R
BS I
I
B
1
Fig. 1. Expenmental arrangement. MO, L and B are lenses, BS is a beam splitter, S the sample, RM a swiveling mirror and R a Si-photodiode array.
spheric pressure. The chamber was mounted on a x—y translation stage with both axes perpendicular to the laser beam. The movement in x-direction (parallel to the optical bench plane) was performed by a motor. The focal position is controlled with the help of a lens L2 (f 500 mm) behind the beam splitter BS which focused the light reflected from the surface onto a photodiode array R (Reticon RL 256 Cl7). A swiveling mirror RM followed by binocular lenses allowed direct visual observations after irradiation. The diameter of the laser spot on the GaAs surface was measured with the photodiode array at low laser powers for2each exof the periment. Typical measured 1/e peak intensity werediameters on the order of 10 atim. The temperature on the GaAs surface is determined by the absorbed laser power, by the thermal conduc-
0 031—9163/81 /0000—0000/s 02.50 © North-Holland Publishing Company
Volume 84A, number 4
PHYSICS LETTERS
tivity of the semiconductor and the thermal resistances of the GaAs—copper interface and the copper mount. Uncertainties with estimates of these latter values have been eliminated by evaluating the temperature directly from the shift of the low-energy edge of the lumines cence emitted by the heated material. The long wavelength tail in the spectrum has been used in order to reduce errors due to averaging the luminescence signals from different depths. The thermal resistance was evaluated with the help of published data about the temperature dependence of the band gap [5]. For these measurements the photodiode array was replaced by a 1/8 m spectrometer with an optical multichannel analyser at the exit. A pinhole of 24 pm diameter mounted at the entrance slit discriminated against light from less heated regions. The spatial resolution of the detection system on the sample surface was 1.1 pm. A temperature rise of 3.3°Cper mW of incident power was measured. This is a factor of two higher than would be expected, if the radiation is absorbed in a semi-infinite GaAs wafer [6]. Experiments with TMA1 have been performed at a partial pressure of 9.5 Torr and a H 2 flow rate of 1 2/h. The onset of deposition was characterized by a decrease in reflectivity by two orders of magnitude within a fraction of a second. A variable speckle pattern mdicating turbulences could be observed at the same time. This mode of deposition occurred above 200°C.A typical structure deposited from TMAI at 400°Cas observed with a SEM is shown in fig. 2. The macroscopic structure of the spikes also indicates growth conditions in a turbulent atmosphere. The spikes were grown collinear to the laser beam. The diameters of the spikes ranged from 20 to 90 pm and were much larger than the laser beam waist. However, the nucleation areas had diameters of about 10 pm in agreement with the focal spot diameter. The length of the spikes was dependent on the laser power. The maximum length obtamed under optimum conditions was 500 pm. This limit was given by the shear stress at the interface to the substrate introduced by gravitation. Changes in the gas flow rate from 1 to 4.4 2/h did not affect the experimental results. This indicates that the growth was not terminated by depletion of the organometallic cornpound. The spikes consisted of amorphous Al and crystame Al3 C4. This was confirmed by X-ray spectroscopy and X-ray powder diffraction investigations, By comparing the length and the growth time of
27 July 1981
--~ -_____
________
1
Fig. 2. SEM photograph ofAl deposited at 400°C.
the spikes, a mean growth velocity of a few pm per
-
-
second was evaluated. A rough idea of the dependence of the growth velocity on growth time was obtained by moving the translation stage horizontally at constant speed during growth. In this case the spikes showed a curvature of decreasing radius with increasing time. A translation speed (v I pm/s) below the growth velocity had been selected in order to ensure that the growth conditions do not differ too much from the case v = 0. Under this assumption the time-dependent growth velocity can be evaluated from the slopes of the spike curvatures. Fig. 3 shows that the growth yelocity is decreasing with time. This corresponds to the concept that a minimum temperature is required to sustain the growth. The temperature on top of the growing spike is determined by the- irradiation density and the thermal conductivity. A decrease of temperature and correspondingly a decrease in growth velocity would be expected if, on a logarithmic scale, the irradiation density on top of the spike decreases more rapidly than the thermal conductivity. According to these model assumptions, the beginning of the growth would depend on the position ofthe laser focus. lixperimentafly best conditions were achieved for a focal 217
Volume 84A, number 4
PHYSICS LETTERS I
I
I
I
I
I
I
+
-
5
-
-
1
-
‘S.-,
+ I
0
I
I
I
‘-
I
100 (sec)
Fig. 3. Growth velocity v~parallel to the laser beam versus irradiation time t. plane located at 30 pm in front of the surface. This indicates that the growth velocity has to increase with time for the beginning of the deposition. The growth velocities parallel and perpendicular to the surface are comparable in this case. Thus the diameter of the spikes increases from 10 pm, the spot size, to typically 80 pm. Zn was deposited from DMZn at a pressure of 20 Torr and a H2 flow rate of 1.5 2/h. Deposition was obtained at laser powers of 65—190 mW correspondir~g to temperatures of 215—635°C.The lower temperature necessary for the onset of deposition in this case is explained by the smaller metal binding energy in the molecule [7]. Zn piles with typical lengths of. 70 pm and diameters of 20 pm have been deposited in the -
.
27 July 1981
temperature range of 450—630°C.In contrast to Al, Zn was also deposited outside the focal region with a thickness according to the surface temperature profile. Circular patterns of up to 1.3 mm radius have been observed in the microscope. At lower temperature deposition outside the focal region was eliminated and deposits of ~10 pm diameter with thicknesses in the pm range have been obtained. DMCd has the lowest binding energy of the investigated metal-alkyl molecules [7]. The experiments were performed at a partial pressure of 0.14 Torr and a H2 flow rate of 1 2/h. Deposition was observed at laser powers of 70—300 mW corresponding to temperatures of 235—1000°C.Significantly reduced growth velocities of typically 20 nm/s were obtained at this low partial pressure. The deposited structures consisted of .
several small crystalhtes within the spot diameter (flg.4). Typical dimensions of these crystaffites ranged from 3—10 pm in diameter and 2 pm in thickness. They show the typical pyramidal structure observed for the growth of single-crystalline Cd [8]. The deposits obtained at higher temperatures (5 50—1000°C)showed the same circular pattern as was observed with the Zn deposits. In conclusion, metal deposition by thermal dissociation of metal alkyls on laser-heated GaAs surfaces has been investigated. Depending on the partial pressure and the irradiation parameters, growth of amorphous and crystalline metal structures with dimensions in the pm range have been obtained. The authors are grateful to Professor H.P. Weber for the opporturnty to perform this work, to Dr. P. Engel for the X-ray investigations, to R. Flick for SEM micrographs and to the Swiss foundation for microtechnical research for financial support. [1] C.P. Christensen and K.M. Lakin, Appl. Ploys. Lett. 32 (1978) 254. [2] S.D. Allen, in: Digest of technical papers of the 1979 IEEE/OSA Coni. on Laser engineering and applications (IEEE, New York) p. 43. [3] T.F. Deutsch, D.J. Ehrlich and R.M. Osgood Jr., Appl. Phys. Lett. 35 (1979) 175. [4] Hanabusa and Akira Namiki, App!. Ploys. Lett. 35 (1979) 626. [5] H.C. Casey Jr. and M.B. Panish, Heterostructure lasers, part B (Academic Press, New York, 1978) p. 9. [6] M. Lax, AppI. Phys. Lett. 33 (1978) 786.
J.K. Kochi, Organometallic mechanisms and catalysis (Academic Press, New York, 1978) p. 238. [8] M. Straumanis, Z. Ploys. Chem. B26 (1934) 246. [7]
Fig. 4. SEM photograph of Cd structure grown at about 400°C. 218
PHYSICS LETTERS
27 July
1981
LASER-INITIATED METAL DEPOSITION ON GaAs SUBSTRATES Y. RYTZ-FROIDEVAUX, RP. SALATHE’ and H.H. GILGEN Institute of AppliedPhysics, University of Berne, Berne, Switzerland Received 12 November 1980
The local deposition of metal structures by thermal dissociation of trimethylaluminium, dimethylzinc and dimethylcadmium on GaAs surfaces heated by a cw krypton laser has been investigated. Piles of amorphous aluminium and zinc and crystalline structures of Cd have been deposited at temperatures between 200 and 1000°C. The smallest size of the deposits
was =4 tim.
The growth of semiconducting structures or the deposition of metal layers with microscopic features can be achieved by laser-induced chemical vapor deposition. So far thermal decomposition on laser heated surfaces [1,2] as well as photodecomposition by exciting electronic [3]or vibrational [1,4] transitions of molecules have been demonstrated. Generally the spatial resolution is better for the photodeposition, since both the laser spot size and the thermal conductivity of the substrate limit the resolution of the thermal method. However this latter process has the potential to interface with other semiconductor processes (e.g. diffusion, epitaxy, alloying, annealing). We report on experiments to deposit metallic structures by thermal decomposition of trimethylaluminium (Al(CH3)3, TMA1), dimethylzinc (Zn(CH3)2, DMZn) and dimethylcadmium (Cd(CH3)2, DMCd) on locally heated GaAs surfaces. These alkyls had been investigated because of their potential applications as doping or alloying materials in GaAs. A cw operated krypton laser at 520.8—568.2 nm wavelengths was used as heat source, resulting in a resolution improved by one order of magnitude compared to previously reported results obtained with a CO2 laser [1]. The experimental arrangement is shown in fig. 1. The laser beam is focused by means of a lens MO (f—~ 27 mm, NA = 0.25) ontocm3, a polished (100)-GaAs surface (Zn doped, 6 X 1016 0.047 ~ cm). The wafer is mounted on a copper heat sink in a chamber with the metal alkyl diluted in Pd-diffused H 2 at atmo216
Kr - Laser
S
MO
L
RM
R
BS I
I
B
1
Fig. 1. Expenmental arrangement. MO, L and B are lenses, BS is a beam splitter, S the sample, RM a swiveling mirror and R a Si-photodiode array.
spheric pressure. The chamber was mounted on a x—y translation stage with both axes perpendicular to the laser beam. The movement in x-direction (parallel to the optical bench plane) was performed by a motor. The focal position is controlled with the help of a lens L2 (f 500 mm) behind the beam splitter BS which focused the light reflected from the surface onto a photodiode array R (Reticon RL 256 Cl7). A swiveling mirror RM followed by binocular lenses allowed direct visual observations after irradiation. The diameter of the laser spot on the GaAs surface was measured with the photodiode array at low laser powers for2each exof the periment. Typical measured 1/e peak intensity werediameters on the order of 10 atim. The temperature on the GaAs surface is determined by the absorbed laser power, by the thermal conduc-
0 031—9163/81 /0000—0000/s 02.50 © North-Holland Publishing Company
Volume 84A, number 4
PHYSICS LETTERS
tivity of the semiconductor and the thermal resistances of the GaAs—copper interface and the copper mount. Uncertainties with estimates of these latter values have been eliminated by evaluating the temperature directly from the shift of the low-energy edge of the lumines cence emitted by the heated material. The long wavelength tail in the spectrum has been used in order to reduce errors due to averaging the luminescence signals from different depths. The thermal resistance was evaluated with the help of published data about the temperature dependence of the band gap [5]. For these measurements the photodiode array was replaced by a 1/8 m spectrometer with an optical multichannel analyser at the exit. A pinhole of 24 pm diameter mounted at the entrance slit discriminated against light from less heated regions. The spatial resolution of the detection system on the sample surface was 1.1 pm. A temperature rise of 3.3°Cper mW of incident power was measured. This is a factor of two higher than would be expected, if the radiation is absorbed in a semi-infinite GaAs wafer [6]. Experiments with TMA1 have been performed at a partial pressure of 9.5 Torr and a H 2 flow rate of 1 2/h. The onset of deposition was characterized by a decrease in reflectivity by two orders of magnitude within a fraction of a second. A variable speckle pattern mdicating turbulences could be observed at the same time. This mode of deposition occurred above 200°C.A typical structure deposited from TMAI at 400°Cas observed with a SEM is shown in fig. 2. The macroscopic structure of the spikes also indicates growth conditions in a turbulent atmosphere. The spikes were grown collinear to the laser beam. The diameters of the spikes ranged from 20 to 90 pm and were much larger than the laser beam waist. However, the nucleation areas had diameters of about 10 pm in agreement with the focal spot diameter. The length of the spikes was dependent on the laser power. The maximum length obtamed under optimum conditions was 500 pm. This limit was given by the shear stress at the interface to the substrate introduced by gravitation. Changes in the gas flow rate from 1 to 4.4 2/h did not affect the experimental results. This indicates that the growth was not terminated by depletion of the organometallic cornpound. The spikes consisted of amorphous Al and crystame Al3 C4. This was confirmed by X-ray spectroscopy and X-ray powder diffraction investigations, By comparing the length and the growth time of
27 July 1981
--~ -_____
________
1
Fig. 2. SEM photograph ofAl deposited at 400°C.
the spikes, a mean growth velocity of a few pm per
-
-
second was evaluated. A rough idea of the dependence of the growth velocity on growth time was obtained by moving the translation stage horizontally at constant speed during growth. In this case the spikes showed a curvature of decreasing radius with increasing time. A translation speed (v I pm/s) below the growth velocity had been selected in order to ensure that the growth conditions do not differ too much from the case v = 0. Under this assumption the time-dependent growth velocity can be evaluated from the slopes of the spike curvatures. Fig. 3 shows that the growth yelocity is decreasing with time. This corresponds to the concept that a minimum temperature is required to sustain the growth. The temperature on top of the growing spike is determined by the- irradiation density and the thermal conductivity. A decrease of temperature and correspondingly a decrease in growth velocity would be expected if, on a logarithmic scale, the irradiation density on top of the spike decreases more rapidly than the thermal conductivity. According to these model assumptions, the beginning of the growth would depend on the position ofthe laser focus. lixperimentafly best conditions were achieved for a focal 217
Volume 84A, number 4
PHYSICS LETTERS I
I
I
I
I
I
I
+
-
5
-
-
1
-
‘S.-,
+ I
0
I
I
I
‘-
I
100 (sec)
Fig. 3. Growth velocity v~parallel to the laser beam versus irradiation time t. plane located at 30 pm in front of the surface. This indicates that the growth velocity has to increase with time for the beginning of the deposition. The growth velocities parallel and perpendicular to the surface are comparable in this case. Thus the diameter of the spikes increases from 10 pm, the spot size, to typically 80 pm. Zn was deposited from DMZn at a pressure of 20 Torr and a H2 flow rate of 1.5 2/h. Deposition was obtained at laser powers of 65—190 mW correspondir~g to temperatures of 215—635°C.The lower temperature necessary for the onset of deposition in this case is explained by the smaller metal binding energy in the molecule [7]. Zn piles with typical lengths of. 70 pm and diameters of 20 pm have been deposited in the -
.
27 July 1981
temperature range of 450—630°C.In contrast to Al, Zn was also deposited outside the focal region with a thickness according to the surface temperature profile. Circular patterns of up to 1.3 mm radius have been observed in the microscope. At lower temperature deposition outside the focal region was eliminated and deposits of ~10 pm diameter with thicknesses in the pm range have been obtained. DMCd has the lowest binding energy of the investigated metal-alkyl molecules [7]. The experiments were performed at a partial pressure of 0.14 Torr and a H2 flow rate of 1 2/h. Deposition was observed at laser powers of 70—300 mW corresponding to temperatures of 235—1000°C.Significantly reduced growth velocities of typically 20 nm/s were obtained at this low partial pressure. The deposited structures consisted of .
several small crystalhtes within the spot diameter (flg.4). Typical dimensions of these crystaffites ranged from 3—10 pm in diameter and 2 pm in thickness. They show the typical pyramidal structure observed for the growth of single-crystalline Cd [8]. The deposits obtained at higher temperatures (5 50—1000°C)showed the same circular pattern as was observed with the Zn deposits. In conclusion, metal deposition by thermal dissociation of metal alkyls on laser-heated GaAs surfaces has been investigated. Depending on the partial pressure and the irradiation parameters, growth of amorphous and crystalline metal structures with dimensions in the pm range have been obtained. The authors are grateful to Professor H.P. Weber for the opporturnty to perform this work, to Dr. P. Engel for the X-ray investigations, to R. Flick for SEM micrographs and to the Swiss foundation for microtechnical research for financial support. [1] C.P. Christensen and K.M. Lakin, Appl. Ploys. Lett. 32 (1978) 254. [2] S.D. Allen, in: Digest of technical papers of the 1979 IEEE/OSA Coni. on Laser engineering and applications (IEEE, New York) p. 43. [3] T.F. Deutsch, D.J. Ehrlich and R.M. Osgood Jr., Appl. Phys. Lett. 35 (1979) 175. [4] Hanabusa and Akira Namiki, App!. Ploys. Lett. 35 (1979) 626. [5] H.C. Casey Jr. and M.B. Panish, Heterostructure lasers, part B (Academic Press, New York, 1978) p. 9. [6] M. Lax, AppI. Phys. Lett. 33 (1978) 786.
J.K. Kochi, Organometallic mechanisms and catalysis (Academic Press, New York, 1978) p. 238. [8] M. Straumanis, Z. Ploys. Chem. B26 (1934) 246. [7]
Fig. 4. SEM photograph of Cd structure grown at about 400°C. 218