Spatial-temporally resolved high-frequency surface acoustic waves on silicon investigated by femtosecond spectroscopy

Martin Schubert, Martin Grossmann, Oliver Ristow, Mike Hettich, Axel Bruchhausen, Elaine C. S. Barretto, Elke Scheer, Vitalyi Gusev, Thomas Dekorsy
2012 Applied Physics Letters  
Various types of surface acoustic waves are generated by femtosecond pulses on bulk silicon with aluminium stripe transducers. Rayleigh and leaky longitudinal surface acoustic wave modes are detected in the time domain for various propagation distances. The modes are identified by measuring on various pitches and comparing the spectra with finite element calculations. The lifetimes of the modes are determined quantitatively by spatially separating pump and probe beam, showing a significant
more » ... a significant difference in the lifetimes of both modes. We were able to excite and measure Rayleigh modes with frequencies of up to 90 GHz using a 100 nm period grating. V C 2012 American Institute of Physics. [http://dx.Surface acoustic waves (SAWs) have been of great interest for the last 50 years since the invention of interdigital transducers. 1 While low frequency SAWs in the ultrasonic range have made their success path in communication and signal processing technologies, 2 higher frequency SAWs have found applications in several areas, such as material characterization, 3,4 photonic modulation, 5 phononic crystals, 6 optomechanics, 7 and transport by phonons of other excitations in solids. 8, 9 The ability to excite higher acoustic frequencies and identify their correspondent modes is therefore the key to enable further advances in these and other areas. While there are a number of techniques to create SAWs, the highest frequencies achieved so far are by photoacoustic excitation using thin stripe metal transducers on transparent and semi-transparent substrates. The waves are excited by the thermoelastic expansion of the fingers, creating a stress in the material underneath. 10, 11 SAWs with frequencies of up to 50 GHz using nickel stripes on sapphire have been recently obtained by Nelson et al. 12 Slightly lower frequencies with aluminum stripes on silicon (Si) were generated by Sadhu et al. to investigate the effect of changing the ratio between the period of the grating and finger width. 13 In both cases the waves were excited by pumping a large area of the grating. In our measurements we are locally exciting and probing the SAWs by using small pump and probe spots, and thereby directly measuring propagation distances and the decay of the excited waves up to 90 GHz frequencies. Instead of measuring surface displacement, we detect the change in optical reflectivity due to the strain created by the waves. While other methods have been used to completely map the acoustic band structure of SAWs such as Rayleigh-like waves (RSAW) and leaky longitudinal SAWs (LLSAW), 14 no measurements at such high frequencies have been performed so far that were able to spatially and temporally map these modes. The samples were fabricated using standard techniques on (001) Si wafers. The process was started by cleaning the samples and spinning a thin layer of PMMA on top. Grating patterns were defined by electron beam lithography and subsequent development. A short reactive ion etching was used to etch a few nanometers ($3 nm) into the material to improve the adhesion of the aluminium (Al) fingers and to generate an initial stress that better matches the profile of the SAWs. Afterwards the Al was evaporated on top and the process was finished by a lift-off to remove the photoresist as well as the Al on top of it, leaving the fingers of the transducers on the surface. The fingers were aligned perpendicularly to the [110] direction. Samples with five different pitches were fabricated, ranging from a period of 600 nm down to 100 nm in steps of 100 nm. There were no samples with a pitch of 500 nm. Finger widths varied and were typically half the period except in the case of the 600 nm period where it was a third of the period. The fingers have a length of $80 microns and the entire grating covers a 80 lm by 35 lm area. The samples were excited and measured using the highspeed asynchronous optical sampling (ASOPS) technique. 15 High-speed ASOPS is a pump probe technique that uses two Ti-Sapphire femtosecond lasers with a repetition rate of 800 MHz. The lasers have an offset in repetition rate of 5 kHz and are locked to each other. Due to the difference in repetition rate, the pulses have a varying time delay without the need of a mechanical delay line. This offers improved stability and, due to the high repetition rate, an improved signal-to-noise ratio compared to conventional pump-probe techniques that use mechanical delay stages. The pump laser was set to a wavelength of 790 nm and the probe laser to 820 nm. Both lasers have a pulse length of $50 fs. They are focused onto the sample using a microscope objective with a numerical aperture of 0.55. The probe light polarization is parallel to the grating while the one of the pump is perpendicular. The spot size is resolution limited a)
doi:10.1063/1.4729891 fatcat:fuezidsabbe77of7m3it773wda