Digital Instruments (DI) Multymode Nanoscope IIIa

Transcription

1 Lecture 6 From Microsystems to the Nanoworld Freiburger Materialforschungszentrum Institut für Mikrosystemtechnik

2 Basics of AFM AFM provides very hogh resolution images of various sample properties PSD Laser Tip Cantilever Sample 50 nm Piezo Three basic components: Piezoelectric scanner Cantilever with a sharp tip Digital Instruments (DI) Multymode Nanoscope IIIa Position sensitive detector (PSD) coupled with a feed-back system

3 Historical steps of development 1981-invention of STM 1985-invention of AFM Christoph Gerber 1986-Nobel Price IBM s Zurich Research Center in Rüschlikon I t 0 2kz ( z ) = I e, I t 1 k = 2mΦ / h ~ 1Α o V t Restriction: conductive samples

4 The first achievement Si{111} 7-per-7 18 per 8 nm 2 z=0.04 nm SPMs opened a new window into the nanoworld and have been a major force driving the current development of nanoscience and engineering

5 The man who almost saw atoms In the late 1950s Erwin Müller (Pennsylvania State University) atom-resolving device - field ion microsocpe In the late 1960s Russell Young (former student) (NIST) topographiner Technical and Bureaucratic difficulties Vibration, perturbation Lost the job The Nobel committee acknowledged Young s close approach

6 General components and their functions

7 Position sensitive detector (PSD) coupled with a feed-back system Optical Lever 5mW Peak output at 670nm AFM signal: (A+B)-(C+D) The simplest method for constructing an optical readout

8 Other detection ti systems for cantilever deflections Tunneling sensor (Binnig, i Rohrer) Optical interferometer detection system STM tip I t V t Piezoresistive detection special design of cantilever changing of resistivity with the applied stress Wheatstone bridge

9 Piezoelectric scanner

10 Piezoelectric scanner SPM scanners are made from a piezoelectric material that expands and contracts proportionally to an applied voltage Whether they expand or contract depends upon the polarity of the applied voltage. Digital it Instruments t scanners have AC voltage ranges of +220 to 220V 0 V -V + V No applied voltage Extended Contracted In some versions, the piezo tube moves the sample relative to the tip. In other models, the sample is stationary while the scanner moves the tip PZT Cantilever Solenoid

11 Introduction Wristwatch: quartz Mobile phone: surface tuning fork acoustic wave filters Ink-jet printer Vabration damping slalom skis: Piezoelectric i Fibers piezoelectrics Tennis rackets Gas grill lighter Auto focusing camera

12 Piezoelectric Effect Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric field in response to applied mechanical stress. If the material is not short-circuited, the induced charge generates a voltage across the material. The piezoelectric effect is reversible, that is, the piezo-materials exhibit: the direct piezoelectric effect the production of electricity when stress is applied, the inverse piezoelectric effect the production of stress and/or strain when an electric field is applied.

13 Simple molecular model for explaining the piezoelectric effect unperturbed molecule molecule subjected to an external force polarizing effect on the material surfaces Cane sugar Quartz 20 crystal symmetry y classes can exhibit piezoelectricity Rochelle salt Topaz Tourmaline- group minerals

14 Man-made ceramics: PZT (lead zirconate titanate) tetragonal crystal structure BaTiO 3 : octahedra of O 2 - ions, which is the Perovskite family. General formula is ABX 3. negatively charged oxygen ions and positively charged Ti 4+ ion are slightly displaced from their symmetrical positions (upward displacement) resulting in a permanent ionic dipole moment along the c-axis b h i b i C i l f i bi by heating above its Curie temperature, tetragonal structure transforms into cubic (symmetric), thus no spontaneous dipole moment, becoming dielectric material

15 Ferroelectric ceramics A large number of such dipoles line up in clusters (called domain) Domains are randomly oriented giving no net electric dipol moment They can be aligned by an external electric field - poling With applied external field, domains are oriented parallel to electric field When the electric field is removed, most of the dipoles are locked into a configuration of near alignment The element has now a permanent polarization, the remanent polarization and is permanently elongated The sample takes on its macroscopic piezoelectric properties

16 Fabrication of piezoelectric ceramics Fine powders of the component metal oxides are mixed in specific proportions Heated to form a uniform powder The powder is mixed with an organic binder and is formed into structural elements The elements are fired according to a specific time and temperature program The powder particles sinter and the material attains a dense crystalline structure The elements are cooled, then shaped or trimmed to specifications Electrodes are applied to the appropriate surfaces

17 Motor Actions of a Piezoelectric Element If a voltage of the same polarity as the poling voltage is applied to a ceramic element, the element will lengthen and its diameter will become smaller If the opposite direction, the element will become shorter and broader If an alternating voltage is applied, the element will lengthen and shorten cyclically, at the frequency of the applied voltage This is motor action -- electrical energy is converted into mechanical energy The principle is adapted to piezoelectric ic motors, sound or ultrasound generating devices, and many other products.

18 Displacement of Piezo Actuator + - U L 0 L 0 + ΔL ΔL = S L 0 E dij L 0 D 0 +ΔD S is the strain (relative length change ΔL/L, /, D 0 dimensionless) L 0 is the ceramic length (m) E is the electric field strength (V/m) d ij is the piezoelectric coefficient of the material (m/v)

19 Applications AFM Scanner Design of Simple Lever Amplifier PZT Active Optics (Piezoelectric Tilting Platforms) Fiber Positioning System Micropositioning Systems Unaffected by energy efficiency losses that limit the miniaturization of electromagnetic motors, and have been constructed to sizes of less than 1 cm 3 A potentially important additional advantage to piezoelectric motors is the absence of electromagnetic noise

20 Geometry of PZT scanner Tb Tube scanner The tripod Not stable The outer electrode is segmented in four equal sectors of 90 degrees The inner electrode is driven by the z signal Bipolar configuration Δ x = KΔV, K ~ 3nm / V Model Scan Size Vertical Range A 0.4 μm 0.4 μm 0.4 μm E 10 μm 10 μm 2.5 μm J 125 μm 125 μm 5 μm

21 Triangular pattern Fast scan speed v f = 2lv Hz Slow scan speed v s = l v N Hz Fast scan direction

22 Feedback loop

23 AFM Probe Construction Low spring constant (k to 10 2 N/m) Sharp protruding tip (r=5-50 nm) High resonance frequency enc ω = 1 2ππ k m Three common types of AFM tip normal supertip ultralever

24 Common types of cantilevers Si 3 N 4 Si Diamond

25 Fabrication of cantilevers

26 Calibration of cantilever Theoretical method Static method Dynamic method Measuring of thermal response of the cantilever Measuring of the change of resonance frequency caused by the addition of known masses ( Z ) ' Zt ks = Ztkc t

27 How do I measure k? first, measure the resonance response of the cantilever using an AFM (if you are used to working in an AC mode such as tapping, you already know how to do this.) second, attach a spherical particle to the cantilever. Make sure you know what the particle is made of, since you want to be able to calculate l its mass. third, take an SEM image of the whole cantilever top down to measure the dimensions. In the same session take an image of a calibration grid so you really know. Take an image of the sphere and measure its diameter. Take a side view and measure the thickness of the cantilever (This is critical as k is proportional to the cube of the thickness!) fourth, remeasure the resonance response of the cantilever. Now you have heaps of data with which to determine k.

28 Superposition of two geometries

29 Reconvolution of the tip shape I II r d D=d real d D 2 D = 4 r

30 Calibration of the tip shape 2r L L 2 2 L L R = R = 4r 2 h h

31 Useful formulas 2 d Δz = 8R d 2. 8nm Δz = d > 3nm d 2 ( 4d + 8R ) d ( ) = 2 R Δ z + Δz + Δh

32 Oxide-Sharpened Tips increasing g aspect ratio reducing tip radius SiO 2 HF etching Aspect ration- 10:1 Radius r~1nm

33 Electron beam deposition (EBD) High-aspect-ratio tips L=(1-5)μm R=(20-40)nm Carbon materials are deposed by the dissociation of Carbon materials are deposed by the dissociation of background gases in the SEM vacuum chamber

34 Carbon Nanotube Tips Single-walled carbon nanotubes (SWNT), d=(0.7-3)nm Multiwalled carbon nanotubes (MWNT) (nested, concentrically arranged SWNT, d=(3-50)nm High-aspect-ration AFM probes Very stiff, E=10 12 Pa (the stiffest known materials) Buckled nanotubes Labor intensive Not t amenable to mass production

35 Pick-up Tips d=0.9nm d=2.8nm

36 Chemical Vapor deposition (CVD) Direct grow nanotubes onto AFM tip Heating of nanocatalyst particle (r~3.5 nm) Presipitates carbon nucleates a grow of nanotube

37 Direct grow of nanotubes Alumina/iron/molybdenumpowdered catalyst 2 nm in diameter 2μm in length