AFM/STM Microscopy Overcoming the challenges of the Micro and nano scale materials and devices characterization and measurement. Hassan Tanbakuchi Agilent Technologies Didier Pellerin ScienTec 1 Logo et nom de marque AFM/STM Microscopy ScienTec • Scientec: headquarters in Les Ulis (Paris) Metrology & Surface Analisis AFM / STM (Agilent Technologies) Profilers (KLA-Tencor) Interferometer (IDE-KLA / Sensofar) SNOM/AFM/Raman (Nanonics) Nano-Indenter ( MTS - Agilent) 2 Logo et nom de marque AFM/STM Microscopy Show Room Dark Room 3 Logo et nom de marque AFM/STM Microscopy Richard Feynman “There is Plenty of Room at the Bottom” 4 Logo et nom de marque AFM/STM Microscopy Outline • • • • • • STM Contact AFM Non Contact AFM Phase Measurements Vector Network Analyzer (VNA) VNA and AFM coupling – Technical Solutions – Results • Next generation 5 Logo et nom de marque AFM/STM Microscopy History • STM (Scanning Tunneling Microscope) • Developped in1981 by Gerd Binnig et Heinrich Rohrer (Nobel Price in 1986) – And then Contact AFM, SNOM, Non Contact AFM, MFM, EFM… 6 Logo et nom de marque AFM/STM Microscopy Scanning Probe Microscope Basic Principles Tip/Probe/Cantilever – Responds to a particular surface property as it is scanned over the surface. Feedback – Z position controlled to keep particular property constant. STM – Current AFM - Deflection AC Mode – Amplitude Applications limited mostly by experimenter’s imagination. 7 Logo et nom de marque AFM/STM Microscopy Basic Principles - STM Measures Tunneling Current Exponentially relation between separation and current. At low bias the image reflects the local electronic structure of the sample surface. Imaging Modes Constant Height Constant Current Spectroscopy I/V, I/S 8 Logo et nom de marque AFM/STM Microscopy Basic Principles - AFM Tip motion is monitored by the laser spot reflected into the photo detector. Topography signal derived from voltage applied to piezo to keep tip deflection constant times piezo sensitivity parameter. Contact mode force (≈0.1 to 1000nN) controlled by cantilever stiffness and deflection. 9 Logo et nom de marque AFM/STM Microscopy Schematic - Feedback Loop Photo-diode Z High Voltage ADC Topo Hybrid Servo A B D C Servo Gain Settings (Software) Deflection (AFM) Amplitude (AC Mode) + - ADC In DAC Out Setpoint 10 Logo et nom de marque AFM/STM Microscopy Contact Mode • Advantages – Easy set up – Fast imaging – Friction signal – Force vs. distance measurements • Disadvantages – Lower resolution – Possible tip and/or sample damage • Mode choice usually dictated by sample. 11 Logo et nom de marque AFM/STM Microscopy Current Sensing AFM (CSAFM™) Principle: A bias is applied to the sample; current flows through the conducting cantilever to preamp. Allows simultaneous probing of conductivity and topography. Preamplifiers with different sensitivities are available: - 10 nA/V (noise ≈ 30 pA rms) - 1.0 nA/V (noise ≈ 3 pA rms) 12 Logo et nom de marque AFM/STM Microscopy AC Methods - Acoustic Drive (AAC) A piezoelectric transducer shakes the cantilever holder at or near the resonant frequency of the cantilever (100 - 400 kHz typically). Interaction with the sample reduces the oscillation amplitude. This reduced amplitude is used as the feedback signal. 13 Logo et nom de marque AFM/STM Microscopy AC Methods – Magnetic AC (MAC) Mode The cantilever is coated on the top side with a proprietary magnetic film. A solenoid applies an oscillating magnetic field which is used to vibrate the cantilever. Since only the cantilever is oscillating, fewer resonances are excited. 14 Logo et nom de marque AFM/STM Microscopy Advantages of AC modes • • • • Soft surfaces stiffened by viscoelastic response Impact predominantly vertical Pull-off from sticky (contaminated) surfaces Instrumental advantage (1/f noise) 15 Logo et nom de marque AFM/STM Microscopy Higher Resolution • Gentler contact preserves asperities same tip gives better resolution • Functionalized tips survive • More control of interactions Plasmid DNA in water. (a) MAC (b) Contact - same tip 16 Logo et nom de marque AFM/STM Microscopy Polymers Phase imaging • Topography / Phase Topography Phase Measurement 17 Logo et nom de marque Magnetic Fields AFM/STM Microscopy Magnetic tip 1 st scan (Topography) 2 nd scan (Magnetic field Measurement) 30 µm scan – MFM measurement - Hard disk 18 Logo et nom de marque Electric Fields AFM/STM Microscopy conductive tip 1 st scan (Topography) 2 nd scan ( Electric field Measurement) 2 µm scan – EFM measurement -Squares made by elctric charge deposition on SiO2, only revealed by EFM 19 Logo et nom de marque AFM/STM Microscopy Electric Field Measurements (EFM) Topography EFM MFM EFM measurement on active circuit 20 Logo et nom de marque AFM/STM Microscopy Nanoscale Electronic Devices Characterization All MW measurement system have 50 Ohms characteristic impedance. Classical nano devices impedances can conveniently be expressed as multiples of the resistance quantum, R0 ≡ h/2e^2 = 12.96kΩ. 21 Logo et nom de marque AFM/STM Microscopy Traditional SCM VCO Detector • Scanning only • qualitative • poor sensitivity • limited 1015-1020 Atoms/cm3 • No Conductors/Insulators dV dC 22 Logo et nom de marque AFM/STM Microscopy Network Analyzer Basics Oct 2008 Logo et nom de marque Agilent private/key customers Page 23 AFM/STM Microscopy What is a Vector Network Analyzer? S21 Transmission • Vector network analyzers (VNAs)… DUT – Are stimulus-response test systems Reflection S11 S22 S12 – Characterize forward and reverse reflection and transmission responses (Sparameters) of RF and microwave components – Quantify linear magnitude and phase Magnitude – Are very fast for swept measurements – Provide the highest level of measurement accuracy RF Source Phase LO R1 A Test port 1 R2 B Test port 2 24 Logo et nom de marque AFM/STM Microscopy Lightwave Analogy to RF Energy Incident Reflected Transmitted Lightwave DUT RF 25 Logo et nom de marque AFM/STM Microscopy Transmission Line Terminated with Zo Zs = Zo Zo = characteristic impedance of transmission line Zo V inc Vrefl = 0! (all the incident power is absorbed in the load) For reflection, a transmission line terminated in Zo behaves like an infinitely long transmission line 26 Logo et nom de marque AFM/STM Microscopy Transmission Line Terminated with Short, Open Zs = Zo V inc o Vrefl In-phase (0 ) for open, o out-of-phase (180 ) for short For reflection, a transmission line terminated in a short or open reflects all power back to source 27 Logo et nom de marque AFM/STM Microscopy Transmission Line Terminated with 25 W Zs = Zo ZL = 25 W V inc Vrefl Standing wave pattern does not go to zero as with short or open 28 Logo et nom de marque AFM/STM Microscopy High-Frequency Device Characterization Incident Transmitted R B Reflected A TRANSMISSION REFLECTION Reflected Incident SWR S-Parameters S11, S22 Transmitted A = R Reflection Coefficient G, r Incident Return Loss Impedance, Admittance R+jX, G+jB B = R Group Delay Gain / Loss S-Parameters S21, S12 Transmission Coefficient T,t Insertion Phase 29 Logo et nom de marque AFM/STM Microscopy Standard Vector Network Analyzer as a reflectometer Very small capacitor High SNR Low Resolution kW Source A LO A/D S11 Z L Z0 Z L Z0 B LO A/D Probe Highly resistive load High SNR Low Resolution Load close to 50 Ohms Low SNR High Resolution Figure 1: reflection coefficient vs.. impedance Low resistive load High SNR Low Resolution 30 Logo et nom de marque AFM/STM Microscopy Fully Automated Proposal • • • System drift correction via ECAL Phase shifting and Attenuation are done through DSP Low IF frequency, and High speed ADC are chosen to minimize the computational round off error in DSP. Source DUT ECAL A B LO1 LO1 10 KHz IF 10 KHz IF LO2 LO2 A/D1 - + U93 DIF1 - DAC DSP1 DSP2 DAC + A/D A/D High resolution amplitude tweaker 31 Logo et nom de marque AFM/STM Microscopy How do we measure small things? -SMM is a near field system. The resolution is determined by the Electric field interaction area with the sample. This is on the order of 5-10 nm -SMM uses a network analyzer to measure the vector reflection coefficient caused by the tip-sample interaction; this gives information about the material properties (dielectric properties) -While an AFM needs “contact” to make a measurement the SMM can measure without contact. You can be 1-10 nm away from the sample and still have good sensitivity 5400 AFM PNA network analyzer 5 nm Tip/sample interaction area Nosecone assembly Applying cantilever to substrate holder 32 Logo et nom de marque AFM/STM Microscopy Electromechanical coupling Balanced Pendulum: How Does It Work • Laser tracking spot remains fixed relative to Z-piezo & AFM cantilever • Z-piezo does not bend Y scan Tube Design Pendulum Design 33 Logo et nom de marque AFM/STM Microscopy Early Design Suffers from Abrupt Localized Bend of Coax Connecting TIP to Diplexer Load Diplexer RF to PNA Coax from the Tip to diplexer Scanner head With Conductive Tip 34 Logo et nom de marque AFM/STM Microscopy Distribution of Electromechanical Coupling Through Coaxial Loop Extraction tool Conductive Tip Conductive Tip Diplexer 50 Ohm CKT Looped cable Looped cable • Half Wavelength Cable Diffusion of electrical/mechanical coupling with integration of enhanced VNA and Precision Machining 35 Logo et nom de marque AFM/STM Microscopy How to Interpret the Measurement Tip and cantilever impedance Tip ( Shank L=100 um ,Dia=20 nm) Cantilever (L=300 um ,W=60 um) C2 Oxide capacitance cantilever capacitance C3 dopant capacitance C1 Oxide (Thickness ~2 nm) Substrate Dopant C (measured Cap)= C1 II (C2 in series C3) C1 nominal=3.18 fF with100 nm change gives 3.18 af change in capacitance. C2 nominal=5.3 aF and tip capacitance of 2 nm dielectric over a conductor. The tip over a silicon substrate has .53 aF effective capacitance 36 Logo et nom de marque AFM/STM Microscopy How to Interpret the Measurement Tip and Cantilever 37 Logo et nom de marque AFM/STM Microscopy DPMM Dopant Profile Measurement Module DPMM approach: Use the Flatband transfer function that is function of dopant density ( variable capacitor) that can be used as an AM mixer to modulate the reflected MW signal at the rate of Flatband drive frequency (<100 KHz). The said AM modulation index is function of the dopant density. 38 Logo et nom de marque AFM/STM Microscopy IMEC p-type after CO2 snowjetcleaning RH~23.5% T~71.4 F Beautiful SMM channels 39 Logo et nom de marque AFM/STM Microscopy Life Science Examples AFM Cell SMM AFM Virus SMM 40 Logo et nom de marque AFM/STM Microscopy 41 Logo et nom de marque AFM/STM Microscopy SRAM Image 42 Logo et nom de marque AFM/STM Microscopy Nanoscale Materials Electronic Measurements Available SPM-based techniques to probe materials electric properties: Scanning near-field microwave microscopy (SNMM) Scanning capacitance microscopy (SCM) Scanning spreading resistance microscopy (SSRM) Electrostatic force microscopy (EFM) Current-sensing (or conductive) AFM (CSAFM) Kelvin force microscopy (KFM) More … Scanning Probe Microscopy, edited by S. Kalinin and A. Gruverman, Springer, New York, 2007. 43 Logo et nom de marque AFM/STM Microscopy Nanoscale Materials Electronic Measurements Vector network analyzer + AFM Scanning Microwave Microscopy (SMM) Absolute measurements for: impedance capacitance dopant density more … 44 Logo et nom de marque AFM/STM Microscopy Merci 45 Logo et nom de marque