Lecture #2 Nanoultrasonic imaging

Transcription

1 Lecture #2 Nanoultrasonic imaging Dr. Ari Salmi

2 Background Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

3 Ultrasonic imaging Ordinary imaging : contrast from light transmission / absorption / reflection Ultrasonic imaging: contrast from mechanical properties

4 Ultrasonic imaging Either through-transmission or pulse-echo g?version=1&modificationdate=

5 Ultrasonic imaging Applicable wave modes Longitudinal Shear Guided waves

6 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 )

7 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

8 Phonons Phonons exibit classical wavemotion like behavior Longitudinal and transverse phonons Also surface waves!

9 Brillouin scattering Photons ( light particles ) scatter from phonons ( sound particles

10 Nanoultrasonic pulseecho imaging Determining step size in nanoscale Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

11 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

12 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

13 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

14 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

15 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

16 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

17 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

18 Nanoacoustic imaging with OPTs Lin et al., APL 2006 Sample: patterned GaN structure coated with a SiO 2 Frequency: 500 GHz

19 Nanoacoustic imaging with OPTs Results: step clearly visible (compared to AFM)

20 Nanoacoustic imaging with OPTs Also the thickness of the SiO

21 Phonon spectroscopy Determining buried layer thicknesses in nanoscale Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

22 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

23 Case study: Detecting layer thickness Hettich et al., APL 2012 Excitation and pickup in a pump-probe scheme 50 fs nm Measure in reflection mode measure changes in the reflectivity

24 Case study: Detecting layer thickness Samples used: gold aminopropyltriethoxysilate (APTES) silicon sandwitches APTES layer was then damaged with a focused ion beam (Ga + ions)

25 Case study: Detecting layer thickness Reflectivity change measured after excitation Coherent acoustic phonons Damping of phonons related to film thickness Hettich et al., APL

26 Case study: Detecting layer thickness Results: Areas where molecules have been removed are clearly visible when a scan is made

27 Inspection of interfaces Determining roughness / specular reflection probability Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

28 Ultrasonic inspection of interfaces Interface roughness Determined by root-mean-square roughness σ Correlation length ξ Huang et al., Journal of Materials Science: Materials in Electronics

29 Ultrasonic inspection of interfaces Increasing roughness decreases obtained ultrasonic amplitude in pulse-echo mode Wavelength vs. roughness Increase in diffuse scattering when σ increases (3.5 MHz)

30 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

31 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

32 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!

33 Take-home Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

34 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)