Nanoscale Material Characterization with Differential Interferometric Atomic Force Microscopy

Transcription

1 Nanoscale Material Characterization with Differential Interferometric Atomic Force Microscopy F. Sarioglu, M. Liu, K. Vijayraghavan, A. Gellineau, O. Solgaard E. L. Ginzton Laboratory University Tip-sample Interaction in TM-AFM Repulsive Regime Force 0 Attractive Regime Tip-sample distance Dynamic tip-sample interaction Attractive Regime van der Waals Forces, Magnetic Forces, Electrical Forces Repulsive Regime Elasticity 1

2 High-bandwidth Force Probes Coupled Torsional Cantilevers (Sahin et. al 2007) Use of torsional mode to increase the bandwidth Can capture a set of interaction force harmonics with improved SNR HarmoniX Nanoscale Material Property Mapping Full-spectrum harmonic image processing. High Resolution, Realtime Quanitative results Increased bandwidth require better signal to avoid lowering the SNR High-Frequency Force-Sensing AFM Probes An AFM probe that can measure tip-sample interaction forces during tapping-mode AFM (TM - AFM) imaging High-bandwidth force sensor provides increased temporal resolution Soft cantilever provides gentle resonant interaction with the surface 2

3 Displacement sensor Interferometric relative displacement sensor for tip motion Phase-sensitive diffraction grating λ/8 offset is required for maximum sensitivity and linearity 0 th order 1 st order z Displacement sensor Light intensity of even modes is maximized when the displacement is an even integer multiple of λ/4 Light intensity of odd modes are maximized when the displacement is an odd integer multiple of λ/4 n*λ/2 displacement (2n-1)*λ/8 displacement (2n-1)*λ/4 displacement Light intensity (a.u.) Light intensity (a.u.) Light intensity (a.u.) Position (a.u.) Position (a.u.) Position (a.u.) 3

4 Mechanical Bandwidth A conventional AFM cantilever can not fully capture the high frequency tip-sample interaction in tapping-mode AFM Cantilever Fundamental Resonance Cantilever Drive Frequency Sensor Fundamental Resonance Cantilever Drive Frequency Probe with an integrated sensor An integrated high bandwidth force sensor on the cantilever k cantilever Cantilever k tip Tip F tip-sample 4

5 Cantilever flexural resonances Mechanical design Sensor Reference Cantilever oscillations couple to the differential grating signal Coupling can be minimized by making the outer moving fingers shorter Sensor transfer function can be approximated as a simple harmonic oscillator Start with an SOI (silicon on insulator) wafer Fabrication 5

6 Fabrication Grow thermal oxide Deposit LPCVD silicon nitride Fabrication Pattern silicon nitride layer with plasma etch Grow thermal oxide 6

7 Fabrication Etch silicon nitride in H 3 PO 4 Etch oxide in HF Fabrication Bond an oxidized DSP (double-sidepolished) wafer on patterned SOI device layer Anneal at 1050C 7

8 Fabrication Etch SOI handle wafer using TMAH (tetra methyl ammonium hydroxide) Fabrication Deposit LPCVD silicon nitride Pattern silicon nitride layer using plasma etch Pattern oxide layer using HF 8

9 Form tip and support regions using isotropic dry etch Fabrication Remove oxide layer using HF Fabrication 9

10 Oxidize at 950C to sharpen the tip Fabrication Pattern oxide layer Etch device layer to form the cantilever outline Fabrication 10

11 Fabrication Deposit TEOS (tetraethyl orthosilicate) Deposit LPCVD silicon nitride Pattern backside silicon nitride layer Etch handle wafer using KOH Fabrication Etch silicon nitride in a plasma etch Etch oxide in HF to release the cantilever 11

12 Fabricated Cantilevers Height offset Set of cantilevers optimized for different materials Ratio of Sensor resonance to cantilever resonance varies from quadrant photodiode Probe Operation Lateral = relative tip displacement Vertical = cantilever oscillation Individual tapping events can be identified The ringing is due to the force sensor resonance The sensor is ~18x faster than the cantilever body 12

13 Imaging Setup Cantilever Drive Signal Phase Detector Phase Image Quad Cell Photodiode Cantilever Oscillation Feedback Control Electronics Topography Laser diode Piezo tube Force Signal Lock-in Amplifier Harmonic Image Computer f Material Mapping Ion beam assisted deposition Platinum, Tetraethyl orthosilicate (TEOS) TEOS Pt Topography image 10 th harmonic image SEM image Harmonic image reveals checkerboard pattern due to difference in the tip-sample interaction Increased spatial resolution for the samples with large topographical differences 13

14 Topography image C 18 SAM on Gold Phase Image 18 th harmonic image Vertical and horizontal states have different mechanical properties Tip-sample interaction measurements Oscilloscope trace 0-10 Frequency spectrum Signal Power (dbm) Frequency (MHz) Cantilever is driven on resonance The sensor is ~30x faster than the cantilever body Spectrum contains harmonics of the drive signal with high SNR Cantilever drive frequency 14

15 Measurement of ω s and Q Tip Displacement Cantilever Displacement By observing a single tip-sample rupture event, the resonance frequency and the quality factor of the force sensor are measured. Spring Constant Calibration Thermal spectrum is measured at the room temperature Cantilever fundamental resonance peak Displacement (m) Equipartition theorem mω 0 q = k T 2 2 B Frequency (Hz) 15

16 Calibration Calibration is performed to convert photodetector output to force values Displacement (nm) Piezotube Input Time (s) Laser diode Piezotube Detector Output (V) 0 th mode Time (s) Dual cell photodiode Hard sample - + Detector Output (V) 1 st mode Time (s) 0 th mode 1 st mode Detector Output (V) Time (s) Signal Processing k, ω s and Q Calibration Parameters Inverse Filter Force signal Average FFT IFFT Peak Repulsive Force Peak Attractive Force Max Min + - Labview program Calculations are done in 2-4 ms sin(ω 0 t) sin(2ω 0 t) sin(nω 0 t) 16

17 Force Imaging Setup Quad Cell Photodiode Laser diode Cantilever Oscillation Feedback Control Electronics Computer Piezo tube Force Signal DAQ Card Force Image F repulsive F attractive Adhesive carbon tape 6 um Topography image Amplitude Phase image 13 th harmonic (amp) 13 th harmonic (phase) Peak repulsive force Peak adhesive force The adhesive force map identifies the particles that are responsible for adhesion and show a range of different adhesion forces 17

18 SBS triblock copolymer Polystyrene-polybutadiene-polystyrene - Periodicity ~ 40nm Bulk Young s Modulus (room temp): Polystyrene ~ 2-4 GPa, Polybutadiene ~ 1MPa Topography 8.1 nm Phase 17.1 o Reduced Young s Modulus 1.54 GPa -5 nm 0.6 o 0 Harmonic Imaging in Liquids Topography image Phase Image 10 th harmonic image In liquid, the effect of dissipative processes due to adhesion on the surface disappears The phase image can not differentiate between the two surfaces The elasticity difference between materials creates the contrast in the harmonic image 18

19 Summary Designed and fabricated cantilevers that can measure high-frequency tip-sample forces with high force sensitivity and high temporal resolution Demonstrated high contrast compositional mapping by utilizing harmonics of the tip displacement Imaging in air and in water Demonstrated quantitative nanoscale material characterization by measuring reduced Young s modulus of the tip-sample contact Acknowledgements: Dr. Sergei Magonov - SBS polymer sample Agilent Technologies Funding and Equipment support NSF, CPN, DARPA, Center for Integrated Systems 19