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Nuclear Instruments and Methods in Physics Research B 12 1 (I 997) 363-366
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ELSMER
Beam Interadams with Materials&Atoms
Department of
Materials Science and Engineering, University of Florida, Gainesoille, FL 32611, USA
Abstract
Laser induced micro-rough structures (LIMS) are a by-product of laser ablation process and are created during multiple pulse irradiation on the surface of the material. Although LIMS have been found to be deleterious for the thin film deposition process, these surfaces have wide variety of applications in synthesis of adherent coatings in thermal expansion mismatched systems. Earlier models, based on interference effects of the laser beam, to explain the evolution of LIMS, are not consistent with the experimental results. Experiments were conducted on a wide variety of materials (e.g. Sic, alumina,
YBaCuO superconductor, etc.) to understand the mechanisms for generation of the micro-rough structures. A novel model was developed to explain the characteristics of LIMS such as (i> feature orientation (ii) evolution of surface structures as a function of pulses, (iii) formation of LIMS within a energy window near ablation threshold and (iv) periodicity which is independent of the laser wavelength and incident angle.
1. Introduction
Laser induced micro-rough structures (LIMS) are a common by-product of pulsed laser deposition of thin films [l-4]. During laser ablation, the surface of the target material is transformed from a planar morphology to undu- lating, high surface area, micro-rough morphology. LIMS surfaces evolve as the number of pulses are increased and assume different shapes such as cones, grooves, ridges, etc with varying periodicities [5]. These structures have been attributed with deleterious effects on the thin film deposi- tion process by laser ablation. For example, the deposition rate of the films have been found to decrease substantially once these structures evolve. The composition of these structures have been found to be different the nominal composition of the bulk material, thus creating non-uni- form composition in the deposited thin film. Some meth- ods to suppress LIMS involve rotation of the target, chang- ing the angle of incident of the laser beams, surface polishing, etc. [5].
Recently, LIMS, created under atmospheric conditions, has been utilized to synthesize a new class of materials called surface composites in which two materials near the surface possess linearly graded chemical composition [6].
This novel engineering materials is significantly different from other engineered materials like bulk composites and functionally graded materials. Surface composites are very different from existing engineered materials such as “bulk composites” and “functionally graded materials” (FGM).
The fabrication of surface composites is discussed in detail elsewhere [6,7].
Although LIMS have been observed for a wide variety of laser-based processes, the mechanisms for generation of these structures have not been clearly understood. A consistent quantitative model to explain the characteristic of these structures has not yet been developed 151. This is mainly because of a wide variety of micro-rough features found in different materials. These micro-rough structures are much different from other morphological surface modi- fication phenomena such as laser induced periodic struc- tures (LIPS) which has been ascribed to interference be- tween the incident beam and radiation scattered parallel to the mirror surface by dust and other particles [8,9]. To describe the two different ripple periods, depending upon whether the scattered wave and components traveling with or against the traveling wave vector, the period can be approximately expressed as d=-
A
(1) where, A is the wavelength and 0 is the incident angle of the laser beam. The ripple spacing formula was further refined to account for the polarization phase velocity [IO].
It should be noted that the propagating surface waves need not originate from dust on scratches, they can be generated as a result of roughness and inhomogeneity characteristic of any surface. During melting, the induced temperature variations cause gradients in the evaporation flux above the molten pool or in the surface tension which expels the melt from the irradiation zone. The associated surface deformation causes permanent freezing of the patterns following laser irradiation. Patterns caused by this interfer-
0168-583X/97/$17.00 Copyright 0 1997 Published by Elsevier Science B.V. All rights reserved
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364 R.K. Singh. J.M. Fitz-Gerald/A&l. Insrr. and Merh. in Phys. Rex B 121 (1997) 363-366 ence are characterized by a constant lateral spacing related to the wavelength and their k-vector orientation being determined by the polarization of their impinging radiation
[IO]. However, the interference related structures mecha- nisms do not correlated well with the characteristics of
LIMS. Firstly, the micro-rough structures are not periodic but have a wide ranging periodicity varying from 2 pm to
100 km. In contrast to predictions from Eq. (l), the spacing of these structures appears to be independent of the wavelength and the angle of incidence. In addition these structures are formed with laser beams which possess very low spatial coherence like excimer lasers. Typically interference related effects are not observed when low coherence laser is used in materials processing. Thus, these observations suggest that other mechanisms must play a significant role in formation of LIMS.
In this paper, based on the experimentations done with a wide variety of materials, we have developed a model to explain the formation of LIMS. In contrast to diffraction related morphological effects observed in various materi- als, the mechanisms for formation of the LIMS structures involve non-uniform spatial etching/ablation of the sur- face. Under specific fluence conditions, slight changes in the spatial energy density can result in large changes in the ablation rates. However, at higher energy densities, the ablation rate is nearly independent of energy density. Thus, an energy window is created for the formation of these structures. Additionally at energy densities near the abla- tion threshold, the ablation selectivity at the grain- boundaries is much higher than that of the bulk, thus resulting in non-uniform spatially dependent rates.
2. Experimental
To understand the mechanisms for formation of the
LIMS structures, studies were conducted on several mate- rials such as alumina, silicon carbide, silicon nitride, hy- droxyapatite and YBaCuO superconductor. The studies were confined to ceramics because the formation of freely flowing liquid phase in metals and related shock waves effects further complicates the experimental observations.
In our experiments a KrF excimer laser (A = 248 nm) with a pulse duration of approximately 25 ns was used. Gener- ally the number of pulses applied in a single areas was varied from 50 to 2000. The surface morphology of the laser-irradiated material was monitored by scanning elec- tron microscopy (SEM) and atomic force microscopy
(AFM).
3. Results
To understand the evolution of the LIMS structures, the surface morphology was monitored as a function of energy density and number of pulses. Fig. l(i-iii) shows the evolution of LIMS on silicon carbide substrates with in- creasing number of laser pulses. The silicon carbide sub- strate was irradiated with a fluence of 2.9 J/cm’. As the surface features were perpendicular to the surface (in the direction of the laser beam), the scanning electron micro- graphs were obtained at a tilt of 30”. The micrographs show that the surface was initially smooth with an average roughness of 0.2 p.m. After 20 pulses, “holes” are ob-
Fig. 1. Surface morphology of silicon carbide: (i) without laser irradiation (ii) irradiation with 20 pulses at 2.90 J/cm’,
800 pulses at 2.90 J/cm’, (iv) irradiation with 800 pulses at
3.90
J/cm’.
(iii) irradiation with

R.K. Singh, J.M. Fitz-Gerald/Nucl. Insrr. and Meth. in Phys. Res. B 121 (1997) 363-366 served at the grain boundaries of Sic. These holes corre- spond to regions of higher ablation rates during the irradia- tion process. As the number of pulses are increased at the same fluence conditions, a surface structure with semi- periodic features begins to evolve. After 800 pulses the features mature into truncated cone-like structures which possesses an average feature depth of 16 pm, and average periodicity of _ 10 pm.
The fluence of the laser beam plays a critical role in generation of these structures. Fig. l(iv) shows that the morphology of the Sic surface after excimer laser irradia- tion with 800 pulses at an energy density of 3.90 J/cm*.
In contrast to earlier results, no cone-like or micro-rough surface morphology was observed. The surface shows features characteristic of removal of material from the surface. Our experiments showed that LIMS evolved within a specific fluence window from 2.6 + 0.2 J/cm* to 3.5 +
0.2 J/cm*. Detailed experimental results on the change in surface morphology as a function of energy density will be presented elsewhere.
365
The periodicity of the micro-rough structures in ce- ramic materials was found to be related to the creation of the “hole-like” structure shown in Fig. I(ii). The periodic- ity of the cone-like structures was geometrically evaluated and compared with the “holes” which are formed at the grain boundaries of the ceramic material. Fig. 2 shows the distribution of periodicities of the (i> grain boundary
“holes” and (ii> truncated cone-like structures formed during multiple excimer laser radiation of silicon carbide ceramic substrate. Although the cone-like features visually appear to be very periodic in Fig. l(iii), the periodicity of these structures vary from 8 pm to 22 p.m with the most probable and mean periodicity of approximately 10 pm.
This correlates with the “holes” distribution which also average periodicity of approximately IO pm. Thus these measurements validate that the non-uniform preferential ablation at the grain boundaries of the ceramic material is responsible for the formation of LIMS.
Fig. 2. The periodicity of (i) holes after irradiation with 20 pulses at 2.90 J/cm*, and (ii) cone-like stntctures after irradiation with
800 pulses at 2.90 J/cm*.
4. Discussion
From the above experimental results we can infer the following characteristics of LIMS: (i) existence of the specific energy window for formation, (ii) formation in the direction of the laser beam, (iii) periodicity of the struc- tures is dependent on the periodicity of the surface
“holes”, and (iv) evolution based on the number of laser pulses at a particular area.
To understand the effect of energy density and pulse repetition on the fabrication of the micro-rough structures, detailed simulation experiments were conducted [l I]. Fig.
3 shows the calculated and experimentally obtained abla- tion depths per pulse for 248 nm excimer laser irradiation on YBaCuO target. YBaCuO also exhibits similar charac- teristics of SIC. Thus this material was chosen for these calculations, because the etching rates for these fluence rates have been experimentally determined. The calcula- tions were conducted to two different input values to simulate the ablation characteristics in the bulk materials and grain boundaries. Standard values of YBaCuO (ther- mal conductivity 0.025 Wcm- ’ K-‘, volume heat capac- ity 2.4 J/cm3, absorption coefficient 2.5 X lo5 cm-‘, vaporization temperature = 1900 K) were chosen to simu- late the ablation depth, AQ,, of bulk YBaCuO [ 121. A plasma absorption coefficient of 4.0 X IO5 cm -’ was used to account for absorption of the incident laser beam by the plasma 1121. The figure also shows the experimentally obtained ablation depths as a function of energy density obtained by Savva et al. [ 131. Good agreement between the experimental and calculated values was obtained by choos- ing and appropriate values of the plasma absorption coeffi- cient. Fig. 3 also shows that at low fluence values, the slope of the curve is much higher near ablation threshold
(&,,I than at high energy densities. Thus very small changes of energy density, E, is expected to bring large changes in
V. LASER PROCESS/APPLICATIONS

366 R.K. Singh, J.M. Fit:-Geruld/Nucl. Instr. and Meth. in Phys. Res. B 121 (1997) 363-366
5. Conclusions
In conclusion, we have made preliminary investigations into the mechanisms for the creation of laser-induced micro-rough surfaces. The effects of pulses and energy density have been analyzed for SIC ceramic materials.
Based on these observations, a new model has been devel- oped to explain the evolution of these structures.
0
’
I
1
I
2
Energy Density
J/cm’
1
3
I
4
Fig. 3. The calculated and experimentally obtained (see Ref. [IO]) ablation depth of YBaCuO as a function of KrF laser energy density. Calculations were done to two sets of input parameters to simulate the ablation in the bulk material (A rat,,) and at the grain boundaries (Ax,,). the ablation rates at energy densities near the ablation threshold. In contrast, dA x,,,/d E is nearly constant when the energy density values is much higher than the ablation threshold. This can be mathematically expressed as
Thus, spatial variations in laser fluence play a significant role at energy densities near the ablation threshold, while at high energy densities they are relatively insignificant.
Spatial variations in energy density are expected to occur due to the following reasons: (i) beam inhomogeneity, (ii) shadow effects and (iii) multiple reflection on rough sur- faces (as the rough surfaces evolve). Thus, this phenomena explains the evolution of LIMS at fluences near the abla- tion threshold, and the existence of the upper energy window for creation of the micro-rough structures.
Acknowledgements
The authors would like to acknowledge the financial support of the University of Florida, National Science
Foundation, The Engineering Research Center for Particle
Science and Technology at the University of Florida, NSF grant # EEC 94-02989.
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