Acoustic Attenuation in Silica in the 100-250 GHz Range Using Colored Picosecond Ultrasonics Patrick EMERY1, Arnaud DEVOS2, Simon AYRINHAC3, Marie FORET3, Benoît RUFFLE3 1 ST
Microelectronics, 850 rue Jean Monnet, 38920 CROLLES
2 Institut 3
d’Electronique, de Microélectronique et de Nanotechnologies, Cité Scientifique - Avenue Poincaré, BP 60069 59652 Villeneuve d'Ascq Cedex
Laboratoire des Colloïdes, Verres et Nanomatériaux - UMR 5587 CNRS-UM2 MONTPELLIER Attenuation in the hypersonic range
Abstract We report on new measurements of the attenuation of longitudinal acoustic waves in vitreous silica using picosecond ultrasonics. We present a new way of using this ultrafast technique which overcomes the difficulties encountered in the pioneering work of H. J. Maris. [P. Emery, A. Devos, Applied Physics Letters 89, 191904 (2006)]
Picosecond Ultrasonics
Ultrafast technique based on a pump-probe scheme that enables non destructive mechanical measurements on µm to nm scale stacks Acoustic pulse
probe
Any materials[4] :
Transparent materials[5] :
Comparison between the Fourier transform of the successive ehoes
Analysis of the damping of the Brillouin oscillation
• A high impedance contrast is needed to produce several echoes,
• Needs thick sample to reveal the low frequency oscillation and the attenuation effect
• Reflections at the free surface can affect the result
pump
Substrate ∆ R/R (u.a.)
Thin film Ex: W e=163nm
The given example illustrates the classical Picosecond ultrasonic technique to measure acoustic attenuation 4 C. Thomsen, H. T. Grahn, H. J. Maris, J. Tauc, Phys. Rev. B 34, 6, p4129 (1986) Time (ps)
5 H. –N. Lin, R. J. Stoner, H. J. Maris, and J. Tauc, JAP 69, 3816 (1991)
Protocol description and Experimental results Definition : The protocol enables attenuation measurements in thin films. We measure α, defined by: Along the distance z, the acoustic energy is attenuated by a factor e-αz at a given frequency[6]
Al
Samples configuration Si
Si
Dielectric (SiO2)
In some cases attenuation has to be measured in thin films deposited on a substrate with a low impedance contrast :
The probed frequency depends on the material characteristics[7]
0.03
300nm 600nm 900nm 1200nm
0.02
Al/SiO2/Si ∆R/R (unités arb.)
No impedance mismatch • 1 weak echo !!
DR/R (a.u.) .
In thin films, high frequencies are needed to be sensitive to the attenuation effect on small distances ~ a few 100 nm
Classical technique :
fSiO2 = 40 GHz (negligible)
λsonde=804 nm
0 0
50
100
150
200
-0.02
Wavelength protocol :
Time delay (ps) -0.03
2 frequencies come out 600nm 900nm
50
100
150
u.a.
• 240 GHz (travelling in Silicon) 0
Superimposition of experimental results obtained on 4 thicknesses of SiO2 (300, 600, 900, 1200nm) probed at 400nm
300nm
• 40 GHz (travelling in Silica)
λsonde=402 nm
Results :
200
TimeTemps delay (ps) 0
v Probe interferences
0.2 f (THz)
0.4
0.6
0.8
1
measured energy attenuation at 236 GHz in v–SiO2 (LPCVD sample) α = 5,1 ± 0,9 .10-3 nm-1
6 P. Emery, A. Devos, Appl. Phys. Lett. 89, 191904 (2006) 7 A. Devos, R. Côte, G. Caruyer and A. Lefèvre, Appl. Phys. Lett. 86, 211903 (2005)
Strain pulse Fourier components after different propagation distances assuming a f² dependent attenuation coefficient
Oscillations
(corresponding to β=1.16 ± 0,2.10-3 nm-1.THz-2 strain field)
Wide band results : Using different probe wavelengths and substrates, the protocol can provide attenuation measurements in the hypersonic frequency range
2nv cos θ
f =
Results :
Use of different wavelengths Si Brillouin at different probe wavelengths
Fréquence sondée en fonction du substrat utilisé 3.5
SiO2 (1000nm) sur GaP SIO2 (900nm) sur Si
λ=430 nm
2.5 2
f = 192 GHz
1.5
178 GHz
DR/R (u.a.) .
3 .
frequency dependence of LA attenuation in v–SiO2 at 300 K
λ
Use of different substrates
DR/R (u.a.)
250
-0.01
fSi = 240 GHz (measurable)
Echo
0.01
193 GHz
218 GHz
231 GHz
390 nm 400 nm 410 nm 430 nm 450 nm
1 250 GHz
0.5
f = 108 GHz
0 0
50
100
t (ps)
150
200
250
100
110
300
130
140
150
t (ps)
760 to 900 nm Achievable frequency range :
120
380 to 450 nm
GaP
GaP
Si GHz
0
50
100
150
200
250
Conclusion • We measured a LA attenuation for v–SiO2 two times lower than that found in the pioneering work of H.J.Maris [Zhu et al PRB 44 4281 (1991)]. • Our results seems to be in line with a quadratic frequency dependence of attenuation up to 250 GHz.
