Atomic Force Microscopy

Frontmatter

Chapter 1. Introduction

Abstract

In many areas of science and technology there is a trend toward the nanoscale or even the atomic level. For instance, electronics is already undergoing a transition from microelectronics to nanoelectronics. As transistors with critical dimensions in the the single digit nanometer range are now in production, consumer electronics products contain now real nanoelectronic devices. Also in many other areas the progress toward the nanoscale is under way.

Bert Voigtländer

Chapter 2. Harmonic Oscillator

Abstract

In atomic force microscopy, vibrations play a central role in several areas: in vibration isolation and in dynamic atomic force microscopy. Therefore, In this chapter we will study the mechanical harmonic oscillator.

Bert Voigtländer

Chapter 3. Technical Aspects of Atomic Force Microscopy

Abstract

In order to perform nanoscale motions in AFM (e.g. during scanning) very precise actuators are required. Piezoelectric actuators achieve the required precision. We describe the principles of operation of these actuators and present examples of specific actuators. In the following principles of vibration isolation are considered, because the amplitude of floor vibrations is much larger than the desired amplitude of the tip-sample vibrations.

Bert Voigtländer

Chapter 4. Atomic Force Microscopy Designs

Abstract

The design of an AFM has to enable two different tasks: First it has to allow for a xyz-motion during scanning (fine motion, or scan motion), for the acquisition of the surface topography. As the range of the piezo actuators performing this motion is limited to usually \({<}100\,\mathrm {\upmu m}\), the second task of an AFM design is to bring the cantilever tip and the sample initially so close together, that their distance is within the range of the z-fine motion. This task is called the coarse approach. Both of these tasks have to be satisfied while simultaneously maintaining a stiff mechanical structure with high resonance frequencies allowing for good vibration isolation and small (thermal) drift of the tip relative to the sample. In this chapter we discuss several types of coarse positioners as well as scanners for the fine motion and introduce the principles of some particular AFM designs.

Bert Voigtländer

Chapter 5. Electronics and Control for Atomic Force Microscopy

Abstract

We introduce the time domain and the frequency domain approaches to electronic signals. Then we discuss some basic electronic components, such as voltage divider, low-pass filter, and operational amplifier. We continue to discuss topics more closely related to atomic force microscopy such as the feedback electronics, which in AFM serves to stabilize the tip-sample distance. We close this chapter on electronics by discussing how digital-to-analog converters and analog-to-digital converters work in principle.

Bert Voigtländer

Chapter 6. Lock-in Technique

Abstract

A lock-in amplifier measures a signal amplitude hidden in a noisy environment. An AC modulation is used to measure the signal in a very narrow frequency range. Using the lock-in technique the noise can be even much larger than the signal which can nevertheless be measured precisely. In dynamic atomic force microscopy is used for instance to detect the oscillation amplitude.

Bert Voigtländer

Chapter 7. Data Representation and Image Processing

Abstract

Scanning probe microscopy data usually have the form of a matrix where the topography (height) or some other signal such as the phase in dynamic AFM is measured as a function of the lateral xy-position on the surface. Data representation is the task to map the heights (i.e. the output of the z-controller) to gray levels in an image in an optimal way. Image processing is used in order to enhance the image representation further, i.e. by removing image artifacts such as high-frequency noise, noise pixels or noise lines (Klapetek, Quantitative data processing in scanning probe microscopy, 2nd edn. Elsevier, Amsterdam, 2018, [1]; Eaton, Batziou, Artifacts and practical issues in atomic force microscopy. In: Santos N, Carvalho F (eds) Atomic force microscopy. Methods in molecular biology, vol. 1886. Humana Press, New York, 2019, [2]).

Bert Voigtländer

Chapter 8. Artifacts in AFM

Abstract

The ideal AFM tip is (from the point of view of surface imaging) a sharp needle which can image even surface features with high aspect ratio. If the tip has a broader shape, artifacts occur due to a convolution of the tip shape with the surface features. Nearby micro tips can lead to a doubling of surface features in the acquired AFM image. Other kinds of artifacts in atomic force microscopy (Klapetek, Quantitative data processing in scanning probe microscopy, 2nd edn. Elsevier, Amsterdam, 2018, [1]; Golek, Mazur, Ryszka, Zuber, Appl Surf Sci 304, 11–19, 2014, [2]; Eaton, Batziou, Atomic force microscopy, vol. 1886. Humana Press, New York, 2019, [3]) include thermal drift, feedback overshoot, piezo creep, and electrical noise.

Bert Voigtländer

Chapter 9. Work Function, Contact Potential, and Kelvin Probe AFM

Abstract

We already used the term work function when we introduced the tunneling barrier height in STM. The work function can be considered as the energy difference between the vacuum level and the Fermi level of a metal. Here we will see that also a surface term contributes to the work function. The work function is a measurable quantity and the operative definition of the work function is that it is the energy required to remove an electron from the bulk Fermi level of a metal to a certain distance from the solid.

Bert Voigtländer

Chapter 10. Forces Between Tip and Sample

Abstract

The idea behind the atomic force microscope (AFM) is to measure the force between the surface and the scanning tip in order to track the surface topography. Before we describe the atomic force microscopy technique in detail, we consider the forces acting between tip and sample as well as the tip-sample contact mechanics. We consider also the snap-to-contact phenomenon, which can occur due to attractive tip-sample forces.

Bert Voigtländer

Chapter 11. Cantilevers and Detection Methods in Atomic Force Microscopy

Abstract

We consider basic requirements for force sensors and introduce a fabrication process for cantilevers. Subsequently, the most common detection method for measuring the cantilever deflection, the beam deflection method, is discussed in detail.

Bert Voigtländer

Chapter 12. Static Atomic Force Microscopy

Abstract

In static atomic force microscopy the force between the tip and the sample leads to a deflection of the cantilever according to Hooke’s law. This cantilever bending is measured, for instance, by the beam deflection method. The name static comes from the fact that the cantilever is not excited to oscillate, as in the dynamic modes of AFM. In the following, we will discuss the static mode, while the dynamic variants are considered in the subsequent chapters. At the end of this chapter, we discuss how force-distance curves can be used to identify the tip-sample interaction regime in which subsequent imaging is performed.

Bert Voigtländer

Chapter 13. Amplitude Modulation (AM) Mode in Dynamic Atomic Force Microscopy

Abstract

In dynamic atomic force microscopy the cantilever is excited at a driving frequency which is close to the resonance frequency of the free cantilever. Due to the interaction between tip and the surface, the resonance frequency of the cantilever changes. As shown in this chapter, an attractive force between tip and sample leads to a lower resonance frequency of the cantilever, while for repulsive tip-sample forces the resonance frequency increases.

Bert Voigtländer

Chapter 14. Intermittent Contact Mode/Tapping Mode

Abstract

Here we introduce the intermittent contact mode (or tapping mode) which is the mode that is used most frequently at ambient conditions.

Bert Voigtländer

Chapter 15. Mapping of Mechanical Properties Using Force-Distance Curves

Abstract

The concept behind mapping of mechanical properties by force-distance curves is to acquire a force-distance curve at each image point and to extract images of elasticity, adhesion and other mechanical properties. Besides access to the mechanical properties, this mode also allows high-resolution imaging, it is a tapping mode under additional force control.

Bert Voigtländer

Chapter 16. Frequency Modulation (FM) Mode in Dynamic Atomic Force Microscopy—Non-contact Atomic Force Microscopy

Abstract

In the FM detection scheme of AFM the cantilever does not oscillate at a fixed driving frequency (as in the tapping mode), but always oscillates at resonance. If the resonance frequency shifts due to a tip-sample interaction, the cantilever oscillation frequency follows this shift.

Bert Voigtländer

Chapter 17. Noise in Atomic Force Microscopy

Abstract

In topographic images, the noise in the vertical position of the tip (i.e. the noise in the tip-sample distance) should be considerably smaller than the topography signal on the sample to be measured. If atomic steps are imaged, the noise should have an amplitude much smaller than 1 Å. In the following we do not consider noise due to floor vibrations or sound, but more fundamental limits of noise due to thermal excitation of the cantilever, or due to the detection limit of the preamplifier detecting the signal.

Bert Voigtländer

Chapter 18. Quartz Sensors in Atomic Force Microscopy

Abstract

As an alternative to the most frequently used silicon cantilevers, quartz oscillators can be used as sensors in AFM. It is possible to obtain atomic resolution in FM atomic force microscopy using quartz sensors. These quartz sensors are characterized by a large spring constant (>1,000 N/m). Both quartz tuning forks, which are used in wristwatches, as well as quartz needle oscillators can be used as sensors in AFM. An advantage of using quartz sensors is that the detection of the oscillation signal can be performed completely electrically, without any optical elements, like a laser diode, a lens, a fiber, or a photodiode being needed. This simplifies the experimental setup.

Bert Voigtländer

Backmatter