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Morphological and electrical modifications in silicon submitted to high-intensity laser irradiation
Materials Science and Engineering, A 168 (1993) 81-86
81
Morphological and electrical modifications in silicon submitted to high-intensity laser irradiation Olivier Muller* and Ren6 Joeckl6 Institut Franco-Allemand de Recherches de Saint-Louis (ISL), B P 34, F68301 Saint-Louis Cedex (France)
Abstract In an effort to study the nature of the damage processes which occur when detectors for visible light are irradiated with high-power laser beams, bulk (111) n-type (P doped) silicon samples were submitted to high intensity 580 nm dye laser pulses for microsecond duration and the induced morphological (cracks, melting) as well as electrical (sheet resistance) modifications were investigated. The irradiation was either in a variable diameter focused mode, or a uniform power density beam. The uniform illumination mode allows one to obtain more precise values for the morphological degradation occurring with increasing fluence: from the first appearance of a white coating (over 1.7 J cm-2) to the progressive formation of cracks (from 5.5 J cm -2 to melting) before melting occurs (at 10.7 J cm-2). A detailed study of the resistivity as a function of fluence is reported, using the latter optical regime, and it is shown that melting is accompanied by a strong increase in sheet resistance (from 50 to 180 kQ ).
1. Introduction During the past two decades, a great many works have been devoted to study of the interaction between intense laser beams and semiconducting materials. Most of these studies were stimulated by the fact that such an interaction paved the way for a new and rapidly growing technology called laser processing, with important applications in the field of microelectronics. Examples are the annealing of implantation-induced damage [1-3], or the processing of surfaces in terms of laser heating and melting of thin films on substrates [4] or crystallization of amorphous deposited layers [5-7]. Another important issue is study of the damage which is produced by the laser impact itself on semiconducting devices (e.g. flat-panels, C C D imaging sensors or photosensor arrays) and the possible resulting modifications of their properties. Zhang et al. observed laser-induced changes in silicon MOS structures [8] as well as laser-induced damage of silicon photosensor arrays [9]; laser-induced damage of silicon C C D imaging sensors was reported by Becker et al. [10]; laser-induced degradation and morphological damage have been observed in p - i - n photodiodes by Watkins et al. [11] and by Huffaker et al. [12].
Laser-induced damage was also studied in crystalline silicon, which is the basic material in detectors for visible light. Jhee et al. showed that first visible damage of silicon occurred at 0.45 J cm -2 (J. = 1.06 p m , l~p= 6 0 ps)[13], and it has been reported that the threshold for singlepulse damage of silicon is 1.6 J cm -2 (2 = 1.06 pro, rp = 5 ns)[14]. In an effort to investigate the damage processes which occur when detectors for visible light are submitted to a high-power laser beam, we report in this paper the results of experiments on laser-induced morphological and electrical changes in ( 111 ) silicon. The objective of this study was to contribute to our comprehension of the damage mechanism in silicon, going from the first visible surface modifications to the threshold of surface melting. The paper is organized as follows. In the next section, details are given of the laser source and the optical set-up used to generate uniform or focused illumination. In Section 3, the morphological modifications of the silicon surface are described as a function of the laser fluence. Section 4 reports on preliminary resistivity measurements obtained by the four-point probe method.
2. Experiments *Also with Centre de Recherches Nucldaires, IN2P3-CNRS/ ULP, Groupe de Recherches Physiques et Matdriaux, BP20, F67037 Strasbourg Cedex 2, France. 0921-5093/93/$6.00
During these experiments, n-type, single-crystal silicon material was examined, with a nominal electri© 1993 - Elsevier Sequoia. All rights reserved
O. Muller, R. Joeckl~ / Laserirradiation of silicon
82
cal resistivity value of 900 ~2 cm. (111) oriented samples were cut into rectangular (5 × 10 mm 2 or 10 × 10 mm 2) pieces with thicknesses of 200 /~m. They were laser irradiated without any specific surface treatment before illumination. The laser source was a home-designed dye laser (R6G) delivering at 580 nm single pulses (2 /~s (FWHM) in duration) of tunable energy up to 0.8 J per pulse. Compared with irradiation used elsewhere [13, 14], it should be noted that the pulse duration is much longer than that of the Q-switched YAG lasers, whereas the shorter wavelength is located inside the sensitivity range of the silicon visible light detector. The beam divergence was about 3 mrad. The spatial profile of the laser beam was measured using a CCD video camera and a laser-beam profiler, for the different types of irradiation used. The irradiations were performed using two different modes: the uniform mode in which the optical beam is homogenized as much as possible using a suitable optical set-up, and the focused mode which simulates a more realistic situation. In principle, the uniform regime of illumination allows well defined uniformly irradiated areas to be obtained on the semiconducting target, on which further measurements using for example electrical or optical probes can be taken appropriately. The experimental set-up used to obtain such a uniform regime is shown in Fig. 1. It consists of a BK7 glass rod with a square section (40 × 3 × 3 mm 3) onto which the laser beam is focused, and an output projection lens. The sample environment is arranged in such a manner that an energy meter or a video CCD camera can be used to control both the total energy of the pulse and its spatial profile. Fine optical adjustments are realized with the help of an auxiliary H e - N e laser beam. The position of
the output projection lens F2 was chosen to obtain on the target square impact areas with edge 1.27 mm. Given the optical losses through the whole system (i.e. 65%) this set-up can provide energy fluences of up to about 12 J cm- 2 on the target plane. In the focused regime the optical set-up simply consists of a lens with a suitable focal length to achieve melted zones with diameter in the range 300-800/~m. In the tightly focused regime (i.e. for melted zone diameters of a few tens of micrometres), the main difficulty occurs when the position of the spot as well as its profile have to be controlled with a high accuracy. As shown in Fig. 2, this control was realized by lighting the target with an auxiliary incoherent source which gives, under specific conditions, the image of the incident laser spot on the CCD camera. Detailed morphological examinations were subsequently performed using optical (50 × 200 Nomarski) and scanning electron (SEM) microscopes. Electrical characterizations were exclusively performed on samples irradiated in the uniform regime. We used the standard four-point probe (f.p.p.) method in the in-line equidistant configuration [15].
3. Morphological modifications Figure 3 shows SEM (a) and Nomarski (b) images of quasi-uniform laser irradiation on silicon. These types of damage are typical of two distinct modifications which appear under increasing energy fluences. The SEM view was observed after irradiation at 8.2 J cm-2 and reveals a density of cracks, whereas the optical image (Fig. 3(b)) shows a well defined area where the molten material (10.7 J cm-2) has again solidified. As a function of the laser fluence, a complete chronology of the illumination-induced damage on
CCDcamera Hc-Ne
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Fig. 1. Schematic diagram of the optical set-up used for uniform illumination of the sample (F1, F2 lenses; H homogenizer; M semi-transparent mirror (RF/T=8/92); El, E2 energy meters).
El
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;
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Fig. 2. Schematic diagram of the optical set-up used in the focused regime showing the auxiliary equipment allowing direct visualization of the irradiated area on the silicon sample surface.
O. Muller, R. Joeckl~
/
83
Laser irradiation of silicon
TABLE 1. Chronologyof illumination-induceddamageon silicon
Fig. 3. (a) SEM and (b) Nomarski images showingquasi-uniform laser irradiations realized on n-Si: (a) high density of cracks at 8.2 J cm- 2;(b) uniform meltingand resolidifyingat 10.7 J cm 2.
Fluence (J cm- 2)
Damage behaviour and comments
10.7 8.7-10.4 7.4-8.4 5.5-6.4 1.7-4.1 < 1.5
Uniform melting Local melting High densityof cracks Low densityof cracks Change in surface colour No visibledamage
silicon was established for the uniform regime, where the fluences are more accurately determined. This chronology is reported in Table 1. As shown, it includes meaningful values, for example the threshold of no visible damage or the melting threshold. Figure 4 shows similar results after illumination of a silicon sample in the focused mode, i.e. for melted zone diameters ranging from 300 to 800 ktm. In this regime, the diameter of the optical impact at the FWHM of the laser pulse was found to vary with the total energy of the pulse: from about 400 /~m at the lowest energy value to about 1600/~m for the highest value. Figure 4(a) shows an optical micrograph of a superficial crack that appeared at 8 J cm- 2; the angle between the linear traces is 120 °. This characterizes the behaviour of (111) silicon surfaces and suggests that linear damage develops preferentially along dislocations, as reported by Birnbaum [16]. These cracks occur well below the melting threshold; moreover, the irradiation geometry is unidimensional, meaning that the temperature gradients and the related stresses are only important in the thickness direction. In that case, compressive stresses develop at the surface but are unable to produce cracks because the compressive yield strength of silicon is fairly high. The observed increase in density of cracks suggests that these cracks are initiated on particular sites, corresponding either to defects or dislocations at the surface, where small melted pits occur for low fluences, or to local non-uniformities of the laser beam, yielding local overheated zones, where melting can occur. During the cooling phase, strong tensile stresses develop around these sites and the observed cracks can develop. The SEM image of Fig. 4(b) shows details of a crater obtained on the sme material after a more energetic ( 16 J cm -2) irradiation. At the periphery of the melted zone, a ripple pattern is clearly visible. Such periodic ripples have also been observed by other authors [13, 14, 17-21], who explain that they are due to a constructive interference effect occurring between surface waves and the incident laser beam; thus the distance between two adjacent ripples should mirror the wave-
84
O. Muller, R. Joeckl~
/
Laser irradiation of silicon
length of the laser beam, i.e. 0.58 pm. Measurements were taken from different SEM images, giving for this distance a value slightly lower than 1/tm, which could be considered in good agreement with theory [20]. In the tightly focused regime, i.e. for melted zone diameters of the order of 20 pm, experiments were performed at low deposited energies limited typically to 1 or 2 mJ per pulse. The SEM images shown in Fig. 5 are representative of what is observed for a single pulse of 0.65 mJ. As shown, the figure looks quite symmetric with a mean diameter of 20 /am and a characteristic cone-shaped protuberance in its centre. In this case the deposited energy fluence was of the order of 210 J cm -2. Protuberances located in the centre of the laser beam impact have also been observed by other authors [22, 23] and their origin is to be found in the positive difference between the density of silicon in its liquid and solid phases [22]. The particular shape observed in Fig. 5 is believed to occur as a consequence of the very small dimension of the actual laser spot combined with the high deposited fluence.
4. Electrical characterization 4.1. Electrical m e a s u r e m e n t s
As mentioned above, the four-point probe method was applied to samples processed under the uniform irradiation regime. Furthermore, it was assumed that the film being characterized reduces to the thin processed layer, the thickness of which is not known. Given these assumptions, the sheet resistance of the film is given by the following relation [15] (in-line equidistant configuration),
R:~ = (~lln 2)Ull where U is the open-voltage measured between the inner points, for a current I flowing through the outer points. Our results are reported in Fig. 6 as a function of the laser fluence. In the investigated range of fluence, the sheet resistance of the sample has a rather constant value of the order of 50 k ~ at low fluence values, and shows a sudden increase for irradiation fluences higher than 11 J cm -2 which, according to Table 1, corresponds to the melting threshold of this material. 4.2. Discussion
Fig. 4. (a) Nomarski and (b) SEM images of single-pulse damage observed with increasing energy fluence on n-Si targets: (a) superficial crack occurring at 8 J cm-2; (b) ripple pattern at the periphery of the melted zone at 16 J cm- 2.
Such an increase in the sheet resistance value could result from a decrease in the residual free-carrier concentration and/or in their mobility in the processed layer. This was the hypothesis of Kamins [24] and Seto [25] for the analysis of their Hall-effect measurements on laser-irradiated silicon. These authors assumed that
O. Muller, R. Joeckl(
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Laser irradiation of silicon
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Fig. 6. Sheet resistance vs. incident laser energyfluencefor n-Si.
under irradiation the free-carriers are trapped by voids or dangling bonds, thus giving rise to a barrier potential which reduces strongly both their population and their mobility. However, taking into account that in the present case the irradiation was performed in ambient atmosphere, it is also possible that the increase in resistivity observed on n-Si for fluences higher than the melting threshold, is due to an oxidation process which occurs during resolidification of the molten area. Further experiments, such as resistivity measurements of silicon samples preliminarily etched and illuminated in a neutral atmosphere, and other measurements using more doped silicon will be undertaken to understand the reason for such an increase in sheet resistance value.
5. Summary
Fig. 5. SEM views of single-pulsedamage on the surface of n-Si in the tightly focused case: (a) cone-shaped protuberance rising at 210 J cm 2;(b) top view of the same impact.
Bulk (weakly) n-type silicon samples were submitted to high-power visible (580 nm) laser pulses of 2 #s in duration with a view to describing the induced morphological and, possibly electrical, modifications. By working in the uniform illumination regime, in which the deposited energy fluence is determined with good accuracy, a typical test-list of the observed damage was established as a function of the irradiation fluence. The test-list includes the following: a change in surface colour, the appearance of an increasing density of cracks, local and finally uniform melting with subsequent resolidification. In these experiments, the sheet resistance was also measured as a function of the laser fluence. No noticeable variation was observed, excepted for fluences higher than the melting threshold where an oxide layer is probably grown at the surface of the resolidified volume.
O. Muller, R. JoecklO /
86
Acknowledgment We w o u l d like to a c k n o w l e d g e B. P r e v o t for helpful discussion.
References 1 J. C. Muller, A. Grob, J. J. Grob, R. Stuck and P. Siffert, Appl. Phys. Lett., 33(1978)287. 2 A. Gat, J. E Gibbons, T. J. Magee, J. Peng, V. R. Deline, P. Williams and C. A. Evans, Jr., AppL Phys. Lett., 32 (1978) 276. 3 W. A. Porter, D. L. Parker, T. Wm. Richardson and E. J. Swenson, Appl. Phys. Lett., 33 (1978) 886. 4 R.A. Ghez and R. A. Laff, J. Appl. Phys., 46 (1975) 2103. 5 J. C. Bean, H. J. Leamy, J. M. Poate, G. A. Rozgonyi, T. T. Sheng, J. S. Williams and G. K. Celler, Appl. Phys. Lett., 33 (1978) 227. 6 J. Feinleib, J. de Neufville, S. C. Moss and S. R. Ovshinsky, Appl. Phys. Lett., 18(1971)254. 7 E. Fujii, K. Senda, E Emoto, A. Yamamoto, A. Nakamura, Y. Uemoto and G. Kano, IEEE Trans. Electron. Devices, 37 (1990) 121. 8 C. Zhang, S. E. Watkins, R. M. Walser and M. F. Becker, Laser-induced changes in the electrical performance of silicon MOS device structures, Proc. 20th Annu. Symp. on
Optical Materials for Higher Power Lasers, NIST (NBS) Special Publication 775, NIST, Boulder, CO, 1989, p. 105. 9 C. Zhang, T. Benchetrit, S. E. Watkins, R. M. Walser and M. E Becker, Laser-induced damage to silicon photosensor arrays, Proc. 20th Annu. Symp. on Optical Materials for
Higher Power Lasers, NIST (NBS) Special Publication 775, NIST, Boulder, CO, 1989.
Laser irradiation of sificon 10 M. F. Becker, C. Zhang, S. E. Watkins and R. M. Walser, SPtE, 1105 (1989) 68. 11 S. E. Watkins, C. Zhang, R. M. Walser and M. F. Becker, Appl. Opt., 29(1990)827. 12 D. L. Huffaker, R. M. Walser and M. E Becker, Correlation of surface topography and coating damage with changes in the responsivity of silicon PIN photodiodes, Proc. 21stAnnu.
Symp. on Optical Materials for Higher Power Lasers, NIST (NBS) Special Publication, NIST, Boulder, CO, 1990. 13 Y. K. Jhee, M. F. Becker and R. M. Walser, J. Opt. Soc. Am. B, 2(1985) 1626. 14 M. F. Becker, F. E. Domann, A. F. Stewart and A. H. Guenther, Charge emission and related precursor events associated with laser damage, 15th Annu. Symp. on Optical
Materials for High Power Lasers, NIST (NBS) Special Publication, 668, NIST, Boulder, CO, 1984. 15 L.B. Valdes, Proc. IRE, 42 (1954) 420. 16 M. Birnbaum, J. Appl. Phys., 36 ( 1965) 3688. 17 Y. Jee, M. F. Becker and R. M. Walser, J. Opt. Soc. Am. B, 5 (1988)648. 18 A. L. Huang, M. F. Becker and R. M. Walser, Appl. Opt., 25 (1986) 3864. 19 D. C. Emmony, S. E. Clark, N. C. Kerr and B. A. Omar, SPIE, 1397(1990) 651. 20 J. F. Young, J. S. Preston, H. M. van Driel and J. E. Sipe, Phys. Rev. B, 27(1983) 1155. 21 H. J. Leamy, G. A. Rozgonyi and T. T. Sheng, Appl. Phys. Lett., 32 (1978) 535. 22 D. C. Emmony, N. J. Phillips, J. H. Toyer and L. J. Willis, J. Phys. D, 8(1975) 1472. 23 M. Bertolotti, F. de Pasquale, P. Marietti, D. Sette and G. Vitali, J. Appl. Phys., 38(1967) 4088. 24 T.I. Kamins, J. Appl. Phys., 42 (1971 ) 4357. 25 J.Y.W. Seto, J. Appl. Phys., 46 (1975) 5247.
81
Morphological and electrical modifications in silicon submitted to high-intensity laser irradiation Olivier Muller* and Ren6 Joeckl6 Institut Franco-Allemand de Recherches de Saint-Louis (ISL), B P 34, F68301 Saint-Louis Cedex (France)
Abstract In an effort to study the nature of the damage processes which occur when detectors for visible light are irradiated with high-power laser beams, bulk (111) n-type (P doped) silicon samples were submitted to high intensity 580 nm dye laser pulses for microsecond duration and the induced morphological (cracks, melting) as well as electrical (sheet resistance) modifications were investigated. The irradiation was either in a variable diameter focused mode, or a uniform power density beam. The uniform illumination mode allows one to obtain more precise values for the morphological degradation occurring with increasing fluence: from the first appearance of a white coating (over 1.7 J cm-2) to the progressive formation of cracks (from 5.5 J cm -2 to melting) before melting occurs (at 10.7 J cm-2). A detailed study of the resistivity as a function of fluence is reported, using the latter optical regime, and it is shown that melting is accompanied by a strong increase in sheet resistance (from 50 to 180 kQ ).
1. Introduction During the past two decades, a great many works have been devoted to study of the interaction between intense laser beams and semiconducting materials. Most of these studies were stimulated by the fact that such an interaction paved the way for a new and rapidly growing technology called laser processing, with important applications in the field of microelectronics. Examples are the annealing of implantation-induced damage [1-3], or the processing of surfaces in terms of laser heating and melting of thin films on substrates [4] or crystallization of amorphous deposited layers [5-7]. Another important issue is study of the damage which is produced by the laser impact itself on semiconducting devices (e.g. flat-panels, C C D imaging sensors or photosensor arrays) and the possible resulting modifications of their properties. Zhang et al. observed laser-induced changes in silicon MOS structures [8] as well as laser-induced damage of silicon photosensor arrays [9]; laser-induced damage of silicon C C D imaging sensors was reported by Becker et al. [10]; laser-induced degradation and morphological damage have been observed in p - i - n photodiodes by Watkins et al. [11] and by Huffaker et al. [12].
Laser-induced damage was also studied in crystalline silicon, which is the basic material in detectors for visible light. Jhee et al. showed that first visible damage of silicon occurred at 0.45 J cm -2 (J. = 1.06 p m , l~p= 6 0 ps)[13], and it has been reported that the threshold for singlepulse damage of silicon is 1.6 J cm -2 (2 = 1.06 pro, rp = 5 ns)[14]. In an effort to investigate the damage processes which occur when detectors for visible light are submitted to a high-power laser beam, we report in this paper the results of experiments on laser-induced morphological and electrical changes in ( 111 ) silicon. The objective of this study was to contribute to our comprehension of the damage mechanism in silicon, going from the first visible surface modifications to the threshold of surface melting. The paper is organized as follows. In the next section, details are given of the laser source and the optical set-up used to generate uniform or focused illumination. In Section 3, the morphological modifications of the silicon surface are described as a function of the laser fluence. Section 4 reports on preliminary resistivity measurements obtained by the four-point probe method.
2. Experiments *Also with Centre de Recherches Nucldaires, IN2P3-CNRS/ ULP, Groupe de Recherches Physiques et Matdriaux, BP20, F67037 Strasbourg Cedex 2, France. 0921-5093/93/$6.00
During these experiments, n-type, single-crystal silicon material was examined, with a nominal electri© 1993 - Elsevier Sequoia. All rights reserved
O. Muller, R. Joeckl~ / Laserirradiation of silicon
82
cal resistivity value of 900 ~2 cm. (111) oriented samples were cut into rectangular (5 × 10 mm 2 or 10 × 10 mm 2) pieces with thicknesses of 200 /~m. They were laser irradiated without any specific surface treatment before illumination. The laser source was a home-designed dye laser (R6G) delivering at 580 nm single pulses (2 /~s (FWHM) in duration) of tunable energy up to 0.8 J per pulse. Compared with irradiation used elsewhere [13, 14], it should be noted that the pulse duration is much longer than that of the Q-switched YAG lasers, whereas the shorter wavelength is located inside the sensitivity range of the silicon visible light detector. The beam divergence was about 3 mrad. The spatial profile of the laser beam was measured using a CCD video camera and a laser-beam profiler, for the different types of irradiation used. The irradiations were performed using two different modes: the uniform mode in which the optical beam is homogenized as much as possible using a suitable optical set-up, and the focused mode which simulates a more realistic situation. In principle, the uniform regime of illumination allows well defined uniformly irradiated areas to be obtained on the semiconducting target, on which further measurements using for example electrical or optical probes can be taken appropriately. The experimental set-up used to obtain such a uniform regime is shown in Fig. 1. It consists of a BK7 glass rod with a square section (40 × 3 × 3 mm 3) onto which the laser beam is focused, and an output projection lens. The sample environment is arranged in such a manner that an energy meter or a video CCD camera can be used to control both the total energy of the pulse and its spatial profile. Fine optical adjustments are realized with the help of an auxiliary H e - N e laser beam. The position of
the output projection lens F2 was chosen to obtain on the target square impact areas with edge 1.27 mm. Given the optical losses through the whole system (i.e. 65%) this set-up can provide energy fluences of up to about 12 J cm- 2 on the target plane. In the focused regime the optical set-up simply consists of a lens with a suitable focal length to achieve melted zones with diameter in the range 300-800/~m. In the tightly focused regime (i.e. for melted zone diameters of a few tens of micrometres), the main difficulty occurs when the position of the spot as well as its profile have to be controlled with a high accuracy. As shown in Fig. 2, this control was realized by lighting the target with an auxiliary incoherent source which gives, under specific conditions, the image of the incident laser spot on the CCD camera. Detailed morphological examinations were subsequently performed using optical (50 × 200 Nomarski) and scanning electron (SEM) microscopes. Electrical characterizations were exclusively performed on samples irradiated in the uniform regime. We used the standard four-point probe (f.p.p.) method in the in-line equidistant configuration [15].
3. Morphological modifications Figure 3 shows SEM (a) and Nomarski (b) images of quasi-uniform laser irradiation on silicon. These types of damage are typical of two distinct modifications which appear under increasing energy fluences. The SEM view was observed after irradiation at 8.2 J cm-2 and reveals a density of cracks, whereas the optical image (Fig. 3(b)) shows a well defined area where the molten material (10.7 J cm-2) has again solidified. As a function of the laser fluence, a complete chronology of the illumination-induced damage on
CCDcamera Hc-Ne
Dye
Laser
Laser
M
F1
H
F2
I
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video
I
,
I
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Centronlcs
interface
I 5VDC computer
R/Jr. ~ ~ g e
t
!12v video
DC
input
|j video monitor
Fig. 1. Schematic diagram of the optical set-up used for uniform illumination of the sample (F1, F2 lenses; H homogenizer; M semi-transparent mirror (RF/T=8/92); El, E2 energy meters).
El
En• ]Pgy--Mete~
;
"'"-
i1 noo]ltePen t ighCentn9
Fig. 2. Schematic diagram of the optical set-up used in the focused regime showing the auxiliary equipment allowing direct visualization of the irradiated area on the silicon sample surface.
O. Muller, R. Joeckl~
/
83
Laser irradiation of silicon
TABLE 1. Chronologyof illumination-induceddamageon silicon
Fig. 3. (a) SEM and (b) Nomarski images showingquasi-uniform laser irradiations realized on n-Si: (a) high density of cracks at 8.2 J cm- 2;(b) uniform meltingand resolidifyingat 10.7 J cm 2.
Fluence (J cm- 2)
Damage behaviour and comments
10.7 8.7-10.4 7.4-8.4 5.5-6.4 1.7-4.1 < 1.5
Uniform melting Local melting High densityof cracks Low densityof cracks Change in surface colour No visibledamage
silicon was established for the uniform regime, where the fluences are more accurately determined. This chronology is reported in Table 1. As shown, it includes meaningful values, for example the threshold of no visible damage or the melting threshold. Figure 4 shows similar results after illumination of a silicon sample in the focused mode, i.e. for melted zone diameters ranging from 300 to 800 ktm. In this regime, the diameter of the optical impact at the FWHM of the laser pulse was found to vary with the total energy of the pulse: from about 400 /~m at the lowest energy value to about 1600/~m for the highest value. Figure 4(a) shows an optical micrograph of a superficial crack that appeared at 8 J cm- 2; the angle between the linear traces is 120 °. This characterizes the behaviour of (111) silicon surfaces and suggests that linear damage develops preferentially along dislocations, as reported by Birnbaum [16]. These cracks occur well below the melting threshold; moreover, the irradiation geometry is unidimensional, meaning that the temperature gradients and the related stresses are only important in the thickness direction. In that case, compressive stresses develop at the surface but are unable to produce cracks because the compressive yield strength of silicon is fairly high. The observed increase in density of cracks suggests that these cracks are initiated on particular sites, corresponding either to defects or dislocations at the surface, where small melted pits occur for low fluences, or to local non-uniformities of the laser beam, yielding local overheated zones, where melting can occur. During the cooling phase, strong tensile stresses develop around these sites and the observed cracks can develop. The SEM image of Fig. 4(b) shows details of a crater obtained on the sme material after a more energetic ( 16 J cm -2) irradiation. At the periphery of the melted zone, a ripple pattern is clearly visible. Such periodic ripples have also been observed by other authors [13, 14, 17-21], who explain that they are due to a constructive interference effect occurring between surface waves and the incident laser beam; thus the distance between two adjacent ripples should mirror the wave-
84
O. Muller, R. Joeckl~
/
Laser irradiation of silicon
length of the laser beam, i.e. 0.58 pm. Measurements were taken from different SEM images, giving for this distance a value slightly lower than 1/tm, which could be considered in good agreement with theory [20]. In the tightly focused regime, i.e. for melted zone diameters of the order of 20 pm, experiments were performed at low deposited energies limited typically to 1 or 2 mJ per pulse. The SEM images shown in Fig. 5 are representative of what is observed for a single pulse of 0.65 mJ. As shown, the figure looks quite symmetric with a mean diameter of 20 /am and a characteristic cone-shaped protuberance in its centre. In this case the deposited energy fluence was of the order of 210 J cm -2. Protuberances located in the centre of the laser beam impact have also been observed by other authors [22, 23] and their origin is to be found in the positive difference between the density of silicon in its liquid and solid phases [22]. The particular shape observed in Fig. 5 is believed to occur as a consequence of the very small dimension of the actual laser spot combined with the high deposited fluence.
4. Electrical characterization 4.1. Electrical m e a s u r e m e n t s
As mentioned above, the four-point probe method was applied to samples processed under the uniform irradiation regime. Furthermore, it was assumed that the film being characterized reduces to the thin processed layer, the thickness of which is not known. Given these assumptions, the sheet resistance of the film is given by the following relation [15] (in-line equidistant configuration),
R:~ = (~lln 2)Ull where U is the open-voltage measured between the inner points, for a current I flowing through the outer points. Our results are reported in Fig. 6 as a function of the laser fluence. In the investigated range of fluence, the sheet resistance of the sample has a rather constant value of the order of 50 k ~ at low fluence values, and shows a sudden increase for irradiation fluences higher than 11 J cm -2 which, according to Table 1, corresponds to the melting threshold of this material. 4.2. Discussion
Fig. 4. (a) Nomarski and (b) SEM images of single-pulse damage observed with increasing energy fluence on n-Si targets: (a) superficial crack occurring at 8 J cm-2; (b) ripple pattern at the periphery of the melted zone at 16 J cm- 2.
Such an increase in the sheet resistance value could result from a decrease in the residual free-carrier concentration and/or in their mobility in the processed layer. This was the hypothesis of Kamins [24] and Seto [25] for the analysis of their Hall-effect measurements on laser-irradiated silicon. These authors assumed that
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Laser irradiation of silicon
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160 140 120
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60 o
o
o
o
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o
-
-
cc
40 o
20 0 2
4
6
8
10
12
F l u e n c e [Jlcrn ~)
Fig. 6. Sheet resistance vs. incident laser energyfluencefor n-Si.
under irradiation the free-carriers are trapped by voids or dangling bonds, thus giving rise to a barrier potential which reduces strongly both their population and their mobility. However, taking into account that in the present case the irradiation was performed in ambient atmosphere, it is also possible that the increase in resistivity observed on n-Si for fluences higher than the melting threshold, is due to an oxidation process which occurs during resolidification of the molten area. Further experiments, such as resistivity measurements of silicon samples preliminarily etched and illuminated in a neutral atmosphere, and other measurements using more doped silicon will be undertaken to understand the reason for such an increase in sheet resistance value.
5. Summary
Fig. 5. SEM views of single-pulsedamage on the surface of n-Si in the tightly focused case: (a) cone-shaped protuberance rising at 210 J cm 2;(b) top view of the same impact.
Bulk (weakly) n-type silicon samples were submitted to high-power visible (580 nm) laser pulses of 2 #s in duration with a view to describing the induced morphological and, possibly electrical, modifications. By working in the uniform illumination regime, in which the deposited energy fluence is determined with good accuracy, a typical test-list of the observed damage was established as a function of the irradiation fluence. The test-list includes the following: a change in surface colour, the appearance of an increasing density of cracks, local and finally uniform melting with subsequent resolidification. In these experiments, the sheet resistance was also measured as a function of the laser fluence. No noticeable variation was observed, excepted for fluences higher than the melting threshold where an oxide layer is probably grown at the surface of the resolidified volume.
O. Muller, R. JoecklO /
86
Acknowledgment We w o u l d like to a c k n o w l e d g e B. P r e v o t for helpful discussion.
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