Atomic Force Microscopy. Presented by Psylla Christina MSc Applied and Engineering Physics Department of Physics TU München 1

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1 Atomic Force Microscopy Presented by Psylla Christina MSc Applied and Engineering Physics Department of Physics TU München 1

2 Outline Introduction Advantages of AFM Disadvantages of AFM How does AFM work? - How are the forces measured? Experimental setup of AFM - On which experimental parameters do the spatial resolutions depend? - How should a successful atomic force microsope be designed and constructed? - AFM tip - AFM cantilever Imaging methods -What types of forces are measured? -Modes of operation: Contact Mode, Non-Contact Mode, Tapping Mode -What are force curves? Examples of AFM images -Topography scanning -Elimination of extreme point -A better view - Thickness of a Thin Layer of Pd on Si Wafer -Surface roughness References 2

3 Introduction Atomic force microscopy is a high resolution type of scanning probe microscopy that allows us to see and measure surface structure in length scale 10nm-100μm with unprecedented resolution and accuracy (lateral resolution~30 nm, vertical resolution~0.1 nm) Unlike an imaging traditional microscope, AFM provides height information of the sample Almost any sample can be imaged, be it very hard (ceramic material) or very soft (human cells, individual molecules of DNA) We can generate images which look at the sample from any conceivable angle with simple analysis software Currently AFM is the most common form of scanning probe microscopy and is used in all fields of science as chemistry, biology, physics, materials science, nanotechnology, astronomy, medicine and more 3

4 Advantages of AFM minimal sample preparation it does not require a conductive sample provides a three-dimensional surface profile (ability to magnify in the X,Y,Z axes) works perfectly well in ambient air or even a liquid environment possible to study biological macromolecules and even living organisms it does not require expensive equipment due to its small size, it can also be combined with other microscopes or intruments 4

5 Disadvantages of AFM not practical to make measurements on areas greater than 100μm limited scanning speed, requiring several minutes for a typical scan images can be affected by nonlinearity, hysterisis and creep of the piezoelectric material possibility of image artifacts an AFM image does not reflect the true sample topography, but rather represents the interaction of the probe with the sample surface 5

6 How does AFM work? AFM provides a 3D profile of the surface on a nanoscale, by measuring forces (Van der Waals forces, dipole-dipole interactions, electrostatic forces) between a sharp probe (<10 nm) and surface at very short distance ( nm probe-sample separation) the probe is supported on a flexible cantilever the AFM tip gently touches the surface and records the small force between the probe and the surface forces between the tip and the sample lead to a deflection of the cantilever according to Hooke s law 6

7 How are the forces measured? the probe is placed on the end of a cantilever (which one can think of as a spring) the amount of force between the probe and sample is dependent on the spring constant (stiffness of the cantilever) and the distance between the probe and the sample this force can be described using Hooke s Law: F=-k x 7

8 Experimental setup of AFM 8

9 The cantilever is a bendable structure used to hold the tip. It is basically a spring with stiffness k The piezoelectric materials are used for controlling the motion of the probe as it is scanned across the sample surface A laser beam is reflected by the back side of a reflective cantilever onto the photodetector The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample The feedback loop includes all of the structural elements that are required to hold the probe at a fixed distance from the sample. 9

10 On which experimental parameters do the spatial resolutions depend? Τhe vertical resolution (~ Å) depends on: Laser intensity noise Photodiodes noise Thermal noise of cantilever Positionning noise of piezo-ceramic Z The lateral resolution (~ Å to few tens of nm) depends on: Positionning noise of piezoceramics X,Y Tip sharpness Long or short range interaction 10

11 How should a successful atomic force microsope be designed and constructed? A very sharp probe must be constructed for measurement of high-resolution images A feedback controller that permits rapid control so that the probe can follow the topography on the surface must be created An X-Y-Z piezoelectric scanner that has linear and calibrated motion must be created A stucture that is very rigid must be constructed, so that the probe does not vibrate relative to the surface 11

12 The tip of the AFM is used: AFM TIP for imaging for measuring forces (and mechanical properties) at the nanoscale as a nanoscale tool, i.e. for bending, cutting and extracting soft materials high-resolution image control In AFM all what is «seen», is seen by the tip, so everything depends on its shape 12

13 AFM TIP AFM tips with a polygon based pyramid shape positioned close to the free end of the AFM cantilever Macroscopic half-cone angle 20 to 25 viewed along the cantilever axis, 25 to 30 viewed from the side Half-cone angle smaller than 10 at the apex AFM tip height ~10-17 µm 13

14 AFM TIP Conical tips are more preferable than pyramidal tips 14

15 AFM artifact arising from a tip with a high radius of curvature with respect to the feature which is to be visualized 15

16 AFM CANTILEVER cantilevers are commonly fabricated from silicon, silicon nitride, or polymers the fabrication process typically involves undercutting the cantilever structure often with an anisotropic wet or dry etching technique without cantilever transducers, AFM would not be possible 16

17 Two equations are key to understanding the behavior of the cantilever: The first is Stoney's formula, which relates cantilever end deflection δ to applied stress σ: where ν is Poisson s ratio, E is Young s modulus, L is the beam length and t is the cantilever s thickness. The second is the formula relating the cantilever spring constant to the cantilever dimensions and material constants: where F is force and w is the cantilever width. The spring constant ω 0 is related to the cantilever resonance frequency by the usual harmonic oscillator formula: A change in the force applied to a cantilever can shift the resonance frequency. 17

18 Imaging methods Contact mode tip is in contact with the substrate high resolution can damage fragile surfaces Non-contact mode (NCM) tip is oscillating and not touching the sample Tapping mode tip is oscillating and taps the surface Lateral force microscopy (LFM) tip is scanned sideways used to measure friction forces on the nanoscale Force Modulation Microscopy rapidly moving the tip up and down while pressing it into the sample. possible to measure the hardness of the surface and characterize it mechanically 18

19 Imaging methods Electrical force microscopy If there are varying amount of charges present on the surface, the cantilever will deflect as it is attracted and repelled Kelvin probe microscopy By applying an oscillating voltage to an oscillating cantilever in non-contact mode and measuring the charge induced oscillations, a map can be made of the surface charge distribution Magnetic Force Microscopy If the cantilever has been magnetized it will deflect depending on the magnetization of the sample. Liquid sample AFM Immersing the cantilever in a liquid Image wet samples Electrochemical AFM 19

20 What types of forces are measured? The dominant interactions at short probe-sample distances in the AFM are Van der Waals (VdW) interactions During contact with the sample, the probe predominately experiences repulsive Van der Waals forces (contact mode) As the tip moves further away from the surface attractive Van der Waals forces are dominant (noncontact mode) 20

21 There are 3 primary imaging modes in AFM: Modes of operation Contact AFM (< 0.5 nm probesurface separation) Intermittent contact (tapping mode AFM) (0.5-2 nm probe-surface separation) Non-contact AFM ( nm probe-surface separation) 21

22 Contact mode (repulsive VdW) When the spring constant of cantilever is less than surface, the cantilever bends The force on the tip is repulsive By maintaining a constant cantilever deflection (using the feedback loops) the force between the probe and the sample remains constant and an image of the surface is obtained. 22

23 23

24 Contact mode Advantages Disadvantages High resolution High contact pressure (GPa) Fastest of all the topographic modes Lateral forces are experienced by both probe and sample No problem with surface pollution (Imaging in air and liquid is possible) Measurement of physical parameters like electrical and thermal properties Tip sharpness is limited and so does lateral resolution It can modify/destroy the observed surfaces, so no soft materials can be used 24

25 Non-Contact mode (attractive VdW) The probe does not contact the sample surface The cantilever is oscillated near its resonant frequency (about 100 to 400 khz) with an amplitude of a few nanometers (<10 nm) As the tip comes near the sample surface, the system detects variations in the resonant frequency or vibration amplitude The frequency deviation is used to make an image of the sample 25

26 A virus coated on a multilayered polymer surface was imaged. The delicate surface of the virus can be clearly seen from the phase information acquired by Non-Contact mode. 26

27 Non-contact mode Advantages Disadvantages soft materials (very low force exerted on the sample(10-12 N) low lateral resolution because of the long range forces no limitation in tip s sharpness surface pollution extended probe lifetime usually needs ultra high vacuum (UHV) for better imaging 27

28 Tapping mode The imaging is similar to contact The cantilever is driven to oscillate up and down at near its resonance frequency by a small piezoelectric element mounted in the AFM tip holder The probe lightly taps on the sample surface during scanning, contacting the surface at the bottom of its swing The amplitude is used for the feedback and the vertical adjustments of the piezoscanner are recorded as a height image 28

29 a: the oscillation amplitude of the cantilever is constant, representing the free space situation where there is no interaction between the tip and the surface b: the amplitude decreases when the tip approaches close enough to the sample surface so that it feels attractive and/or repulsive forces c: The cantilever stops oscillating when the tip is brought in to mechanically contact the surface 29

30 Tapping mode Advantages Disadvantages improved lateral resolution (~ 5 nm) compared to contact mode and noncontact mode (short range repulsive forces dominate) reduced forces applied on surface compared to contact mode (so we can observe soft materials) 5 to 10 times slower than contact mode (We need 5-20 minutes to obtain an image) tip is damaged after several scans no friction forces, so sharper tips can be used pollution layer is not a problem (works in air, liquids) 30

31 Epitaxial Silicon (1x1μm 2 ) 31

32 32

33 What are force curves? Force curve analyses can be used to determine chemical and mechanical properties such as adhesion, elasticity, hardness and rupture bond lengths Force curves measure the amount of force felt by the cantilever as the probe tip is brought close to - and even indented into - a sample surface and then pulled away In a force curve analysis the probe is repeatedly brought towards the surface and then retracted 33

34 The slope of the deflection (C) provides information on the hardness of a sample The adhesion (D) provides information on the interaction between the probe and sample surface as the probe is trying to break free 34

35 Examples of AFM images Topography Scanning Example of generated image upon scanning Pd thermally evaporated on Si 35

36 Elimination of Extreme Points This targets the highest points of the sample and eliminates them It then manipulates the image to create a smaller dynamic depth 36

37 A better view Now: Removed extreme points Digitally decreased the height of analysis Less than 1/3 as high as initial scan 37

38 Thickness of a Thin Layer of Pd on Si Wafer Systematic error 38

39 Surface roughness 39

40 References Atomic Force Microscopy by Peter Eaton and Paul West Atomic Force Microscopy & CT-AFM by K.Bouzehouane Automatic drift elimination in probe microscope images based on techniques of counter-scanning and topography feature recognition Yurov, A. N. Klimov (1994) Scanning tunneling microscope calibratiion and reconstruction of real image: Drift and slope elimination G. Schitter, M. J. Rost (2008) Introduction to Scanning Probe Microscopy (SPM) Basic Theory Atomic Force Microscopy (AFM) Robert A. Wilson and Heather A. Bullen,* Department of Chemistry, Northern Kentucky University, Highland Heights, KY

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