Laser Induced Nanofabrication on Titanium Thin Films
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microscope (LSM) [2], a modified optical microscope which focuses the 514 nm beam of an argon-ion laser to a minimum spot diameter of 500 nm (1/e for 1st peak of Airy beam-profile). The focused light is then rastered across a sample in a manner similar to a scanning electron microscope, providing high resolution optical images. A schematic of this technique, as applied to this experiment, is shown in Figure 1. 514 nrn Ar VV ion laser
Optical microscope lens
Experimental arrangement.
M=50x,
f
NA=0.85, r=250 nm
Theoretical Assumptions: 1) Gaussian beam
Beam intensity profile
Surface
Figure 1:
\
temperature
profile
Hfeated zone M w-->O Titanium thin film Float glass substrate
2) Stationary beam 3) Infinitely thin, absorbing film 4) Transparent, thermally insulating, and infinitely thick substrate
625 Mat. Res. Soc. Symp. Proc. Vol. 397 ©1996 Materials Research Society
The samples are laser ablated titanium thin films (3 to 240 nm thick) on float glass substrates. When illuminated, feature fabrication occurs according to the following extensively-studied model [3-7]. The optical properties of the metal film cause laser power absorption, inducing a temperature gradient within which oxidation occurs. The lateral extent of this heightened temperature region is controlled by the thermal properties of the substrate. Due to the non-linearity of the oxidation reaction with temperature, the reaction zone can be laterally confined to regions narrower than the diffraction limit of optical resolution. In this case the oxide forms a surface protrusion, because the molar volume of TiOx is less than that of Ti. The dimensions of these features were measured using a atomic force microscope (AFM) [81 as well as a scanning tunneling microscope (STM) [9]. In order to establish the feasibility of this study, a preliminary experiment was first conducted using a higher power laser than the LSM utilizes. Measurements by translation indicate its non-focused spot intensity is Gaussian (Eq. 1). Using this data, the surface temperature as a function of lateral position may be predicted by Eq. 2 [71. The spatial extent of the reaction of Ti to TiOx (assumed TiO 2 throughout this experiment) is then estimated assuming a parabolic growth rate, Eq. 3 [6). When compared to the intensity profile, the local oxidation can be smaller than the illuminating beam radius as illustrated in Figure 2. The variables for these calculations were as follows for a 30 nm Ti film: Ro= measured beam radius at 1/e= 756 gtm; P= measured laser power= 4.07 W; A= measured absorption coefficient= 51%; k= thermal conductivity of glass= 0.0146 W/(cm*K) [10]; Io=modified Bessel function of order zero, T(o)= calculated maximum temperature= 824 Kelvin; wo= parabolic rate constant= 330 m 2 /sec [4]; t= reaction time =10 sec; Q/R= oxidation activation energy term= 33000 K [4].
0(r) = exp -[ T(r,t
=
(1)
o]
OO)Gaussian
thickness(parabolic)
2* k * Ro *
=
2*
* *e 3
r
_x
2
*T
\
Gaussian beam profile
2-08
Normalized laser beam, temperature, and oxide t
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