1 Scanning Probe Microscopy Dr. Benjamin Dwir Laboratory of Physics of Nanostructures (LPN) Benjamin.dwir@epfl.ch PH.D3.344
Outline: Introduction: What is SPM, history STM AFM Image treatment Advanced SPM techniques Applications in semiconductor research and industry
3 What is SPM? Scanning Probe Microscopy : The characterization of a sample by scanning its surface with a probe, at a small distance Usually, only surface properties are observable
4 How does SPM compare with other microscopy techniques? Microscope: Optical Confocal SEM TEM STM AFM SNOM XY resolution 400 nm 150 nm 1 nm 0.1 nm 0.1 nm (0.1) 1-10 nm <50 nm Z resolution - 100 nm - - 0.01 nm 0.01 nm (0.01nm) Ambience Sample preparation air (liquid) Air (liquid) none none none / coating vacuum vacuum polishing, ion milling Vacuum (air) Air (liquid) Air None / UHV cleaving None None Damage to sample none none Contamination Contamination, heating None None (scratches) None Price (kfr) 5-30 50-00 00-500 500-000 70-300 70-300 70-300
3D imaging High spatial and vertical resolutions No sample preparation Simple to operate Low-cost Advantages of SPM Main disadvantage : slow (5-0 min/image) 5
History of SPM : An old principle Tip Scanner (rotating cylinder) Stabilizer Tip+reading Surface features (grooves) Surface features (grooves) Scanner (rotating+ advancing cylinder) Source: Wikipedia 6
The Stylus Profilometer ( Alpha-step ) Source: pc-optimize.com Source: CMI-EPFL Scans a line profile of the surface with a tip Z-resolution: 5-10 nm X-resolution: 1-10 mm Scan length: up to 10-100 mm 7
8 How to get nm resolution in X,Y,Z?
9 First attempt: The Topografiner (197) R. Young, J. ward, F. Scirer, Rev. Sci. Inst. 43, 999 (197) A noncontacting instrument for measuring the microtopography of metallic surfaces Field-emission tip to generate narrow electron beam (in UHV) Sample current is measured Feedback keeps tip distance from sample by keeping constant current Source: CMI-EPFL Scans the surface of the sample by a piezo scanner (scan length: up to 8 mm)
10 System construction: The Topografiner (197) Source: CMI-EPFL
11 The Topografiner (197) Results: scan of a Pt optical grating replica Z-resolution: 3 nm XY-resolution: 400 nm Source: CMI-EPFL
1 Noise: The Topografiner (197) Thermal drift Source: CMI-EPFL
13 Resolution: The Topografiner (197) Comparison with other microscopes (197): Source: CMI-EPFL
14 The main problem: How to get nm resolution? Potential problems: Solutions: 1. 1. Tip Tip size 1. Short-range interactions.. High-resolution XY XY scanning. Piezo scanner 3. 3. Non-destructive 3. Non-contact 4. 4. Keep distance from sample 4. Height feedback 5. 5. Vibrations 6. 6. Thermal stability 5. Rigid structure, isolation The first solutions: G. Binnig, H. Rohrer, 1986 6. Compensation
F [nn] Let s look at the solutions: 1. Short-range interactions: Do you know any? Nuclear (strong) forces But range is too short! Quantum-mechanical electron tunneling Van-der-Waals forces 1.5 1 0.5 0-0.5 0.5 Z [nm] 1 1.5-1 15
16 Outline: Introduction: What is SPM, history STM AFM Image treatment Advanced SPM techniques Applications in semiconductor research and industry
STM: uses quantum tunneling STM = Scanning Tunneling Microscope The principle of quantum-mechanical tunneling (198): Electron wavefunctions "leak" into vacuum ("tail") At short distances, current can flow: Wavefunction decay length is very short: ~ 1 Å! mf F = potential difference from vacuum level to Fermi level I e d / Electron wavefunctions 17
18 Tunneling as surface probe: We approach the sample with a sharp metallic tip, biased to a small potential (1-1000 mv) At a very close distance, tunneling current will start to flow between the tip's atoms and the samples' surface atoms This current is measurable (na) at tip-sample distance of 1Å
19 Details of quantum Tunneling When two metals are close enough, electron wavefunctions y 1, y can overlap and tunneling current flows: E 1, E are the electron levels, M 1, is the matrix element: V = applied voltage ) ( ) ( ) 1 ( 1 1,, 1 1 E E M ev E f E f e I E E 1 1 1, y y y y m M
0 Tunneling between metals At low temperatures, between planar identical metals, we get an approximate formula: 3 e kd I V y1 y e m 1 mf Where: k ~ 1 Å -1 Since 1/k~1Å, tunneling is significant only at very short distances STS = Scanning Tunneling Spectroscopy: Between metals, the I/V curve looks like : (no gap)
1 Tunneling between plane and tip At low temperatures, between plane and spherical tip of identical metals, we get the tunneling current: I 3 3 e 4 Kd VF DT ( EF ) R K e y s ( r ) 0 ( E s s E F ) Where D = density of states at tip, R = tip radius There is still exponential dependence on distance Tip radius plays a secondary role DOS can be measured as well
Metal-semiconductor tunneling Tunneling between the metallic STM tip and a semiconductor shows the energy gap in the I/V curve (STS) In many cases the derivative di/dv is plotted vs. V to show more clearly the DOS, states in the gap etc. Surface states (oxidation) can pin the Fermi level UHV is needed