Lecture #2 Nanoultrasonic imaging Dr. Ari Salmi www.helsinki.fi/yliopisto 24.1.2014 1
Background Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 24.1.2014 2
Ultrasonic imaging Ordinary imaging : contrast from light transmission / absorption / reflection Ultrasonic imaging: contrast from mechanical properties 24.1.2014 3 http://www.soest.hawaii.edu/higp/faculty/zinin/images/sam/concrete_800.jpg
Ultrasonic imaging Either through-transmission or pulse-echo https://wiki.engr.illinois.edu/download/attachments/49742889/pulse+echo+transmit.pn g?version=1&modificationdate=1303393914000 24.1.2014 4
http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html Ultrasonic imaging Applicable wave modes Longitudinal Shear Guided waves 24.1.2014 5
Towards nanoscale What happens when the frequencies are high (100 GHz+)? Phonons are coherent lattice vibrations at the nanoscale Material vibration equivalent of photons Quantized sound! ( sound particles ) 24.1.2014 6
Phonons Two types of phonons Optical Atoms move out-of-phase In lattices made of atoms of different charge or mass Acoustic Atoms move in phase 24.1.2014 7 http://www.chembio.uoguelph.ca/educmat/chm729/phonons/optmovie.htm
Phonons Phonons exibit classical wavemotion like behavior Longitudinal and transverse phonons Also surface waves! 24.1.2014 8
Brillouin scattering Photons ( light particles ) scatter from phonons ( sound particles http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/brill.html#c2 24.1.2014 9
Nanoultrasonic pulseecho imaging Determining step size in nanoscale Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 24.1.2014 10
Nanoultrasonic pulse-echo imaging Pulse-echo imaging with nanoacoustic waves (NAWs) Terahertz acoustic waves, initiated optically from periodic piezoelectric nanostructures Optical piezoelectric transducers (OPTs) Based on multiple quantum well (MQW) technology http://www.oxfordplasma.de/images/qw01.jpg 24.1.2014 11
Optical piezoelectric transducers (OPTs) Lin et al., IEEE TUFFC 2005 Femtosecond laser pulses excite the material within the quantum wells Quantum well = material with lower bandgap surrounded by barriers 24.1.2014 12
Two effects: Piezoelectric effect Generation of photocarriers (electrons and holes) Deformation coupling Optical piezoelectric transducers (OPTs) Nonhomogenous distribution of photocarriers Distribution affected by the PZT strain 24.1.2014 13
Optical piezoelectric transducers (OPTs) Excited modes depend on direction of growth of the epitaxial layers of the MQW s The strain caused by the previously mentioned effects launches a NAW 24.1.2014 14
Detection of NAWs with OPTs Use the same OPT strain alters optical absorption (E-field changes because of PZT) Quantum-confined Franz-Keldysh (QCFK) effect 24.1.2014 15
Quantum-confined Franz- Keldysh Effect Change in optical absorption in a semiconductor when an E-field is applied Due to the fact that electron and hole wavefunctions become Airy functions under E-field They leak into the band gap! smaller energy required to absorb photon 24.1.2014 16
Detection of NAWs with OPTs Sun et al., PRL 2000 Transmit a probe pulse through the OPT E.g. 390 nm 250 fs pulse, 14 µm spot size 24.1.2014 17
Nanoacoustic imaging with OPTs Lin et al., APL 2006 Sample: patterned GaN structure coated with a SiO 2 Frequency: 500 GHz 24.1.2014 18
Nanoacoustic imaging with OPTs Results: step clearly visible (compared to AFM) 24.1.2014 19
Nanoacoustic imaging with OPTs Also the thickness of the SiO 2 24.1.2014 20
Phonon spectroscopy Determining buried layer thicknesses in nanoscale Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 24.1.2014 21
Acoustic phonon spectroscopy Principle: Absorption of ultrashort (~fs) light pulses causes a lattice vibration (coherent phonon burst) The phonons traveling in the medium cause a change in the refractive index This is detected with the probe beam Brillouin scattering 24.1.2014 22
Case study: Detecting layer thickness Hettich et al., APL 2012 Excitation and pickup in a pump-probe scheme 50 fs pulses @ 750-850 nm Measure in reflection mode measure changes in the reflectivity 24.1.2014 23
Case study: Detecting layer thickness Samples used: gold aminopropyltriethoxysilate (APTES) silicon sandwitches APTES layer was then damaged with a focused ion beam (Ga + ions) 24.1.2014 24
Case study: Detecting layer thickness Reflectivity change measured after excitation Coherent acoustic phonons Damping of phonons related to film thickness Hettich et al., APL 2011 24.1.2014 25
Case study: Detecting layer thickness Results: Areas where molecules have been removed are clearly visible when a scan is made 24.1.2014 26
Inspection of interfaces Determining roughness / specular reflection probability Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 24.1.2014 27
Ultrasonic inspection of interfaces Interface roughness Determined by root-mean-square roughness σ Correlation length ξ http://homepages.rpi.edu/~bloomm2/roughness.pdf Huang et al., Journal of Materials Science: Materials in Electronics 2010 24.1.2014 28
Ultrasonic inspection of interfaces Increasing roughness decreases obtained ultrasonic amplitude in pulse-echo mode Wavelength vs. roughness Increase in diffuse scattering when σ increases http://www.ndt.net/article/v09n04/abdelh1/abdelh1.htm (3.5 MHz) 24.1.2014 29
Nanoultrasonic inspection of interfaces Wen et al., PRL 2009 Three different wavelengths (frequencies) 117 GHz, 425 GHz and 890 GHz Different surface roughnesses Ranging from 2 Å to 1.2 nm 24.1.2014 30
Nanoultrasonic inspection of interfaces Study the reflected phonons from an interface From the reflection, determine the SSP Specular scattering probability Higher SSP more specular scattering 24.1.2014 31
Nanoultrasonic inspection of interfaces The results show that diffuse reflection increases as a function of roughness Also with reducing wavelenght! Inverse problem: determine roughness from reflected amplitude Cannot probe with light! 24.1.2014 32
Take-home Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 24.1.2014 33
Nanoultrasonic imaging take home Three different cases Pulse-echo imaging (OPT transducers) Layer thickness determination (phonon spectroscopy) Surface roughness determination (phonon reflection) Why use nanoultrasonic imaging? Probe structures you cannot with light (wavelengths are smaller than that of light) 24.1.2014 34