**4.2 Probes**

152 Atomic Force Microscopy – Imaging, Measuring and Manipulating Surfaces at the Atomic Scale

The waviness height is also determined from the profile, which is just the height from the top of the peak to the bottom of the trough. It is usually at least three times the roughness

Roughness is often described as closely spaced irregularities or with terms such as 'uneven', 'irregular', 'coarse in texture', 'broken by prominences', and other similar ones (Thomas, 1999) (Figure 8). Similar to some surface properties such as hardness, the value of surface roughness depends on the scale of measurement. In addition, the concept of roughness has statistical implications as it takes into consideration factors such as sample size and sampling interval. It is quantified by the vertical spacing of a real surface from its ideal form. If these spacing are large, the surface is rough; if they are small the surface

Waviness Height

Roughness Height

Waviness Spacing

WAVINESS

Fig. 7. Waviness (adapted from B. C. MacDonald & Co., 2011).

Roughness Spacing

**4. Basic components of AFM** 

are not taken in opposite directions (

ROUGHNESS

Fig. 8. Roughness (adapted from B. C. MacDonald & Co., 2011).

This topic will present some basic ideas about basic components of AFM.

The movement of the tip or sample in the *x*, *y*, and *z*-directions is controlled by a piezoelectric tube scanner, similar to those used in STM. For typical AFM scanners, the maximum ranges are 80 µm x 80 µm in the *x*-*y* plane and 5 µm for the *z*-direction (Figure 9). The scanner moves across the first line of the scan, and back. It then steps in the perpendicular direction to the second scan line, moves across it and back, then to the third line, and so forth. The path differs from a traditional raster pattern in that the alternating lines of data

Odom, 2004).

average height.

is smooth.

**4.1 Scanner** 

The probe is a very important component of a SPM because different probes can measure different properties of the sample (Figure 10). In addition, the probe determines the force applied to the sample. Regarding AFM, the most common probes are the cantilevers that are highly suited to measure the topography of a sample. Different coatings on the cantilevers measure different properties of the sample.

The tip and the cantilever as an integrated component can be fabricated from silicon or silicon Nitride using photolithographic techniques. From a single silicon wafer it is possible to make more than 1000 probes. Regarding the physical properties, the cantilever ranges from 100 to 200 micrometers in length, 10 to 40 micrometers in width, and 0.3 to 2 micrometers in thickness.

Measurement of the Nanoscale Roughness by

**5. Measurement of surface profile** 

**4.4 How to select a probe** 

Atomic Force Microscopy: Basic Principles and Applications 155

The desirable properties for a probe are related to the imaging mode and the application. In contact mode, soft cantilevers are preferable because they deflect without deforming the surface of the sample, a silicon nitride microlever would be a good choice for most applications. In non-contact mode, stiff cantilevers with high resonant frequencies give optimal results. For applications other than topography, like MFM (Magnetic Force Microscopy), NSOM (Near-field scanning optical microscopy), SThM (Scanning thermal microscopy) etc., different types of probes have to be used. The probes are available mounted on AutoProbe mounts, mounted on TopoMetrix mounts or unmounted in pre-

The characterization of surface topography and its understanding is important in procedures involving friction, greasing and wear (Thomas, 1999). Surface measurement determines surface topography, which is essential for conforming a surface's suitability for a specific function. Surface measurement generally includes surface shape, surface finish and surface roughness. For example, engine parts may be exposed to lubricants to prevent potential wear, and these surfaces require precise engineering— at a microscopic level— to guarantee that the surface roughness holds enough of the lubricants between the parts under compression, so as not to make metal to metal contact. For manufacturing and design purposes, measurement is critical to ensure that the finished material meets the design specification. A profilometer is used to measure surface profile as the surface is moved relative to the contact profilometer's stylus, this notion is changing along with the

A diamond – sharp stylus is used for the measurement of the surface. The pen is placed on an irregular surface at a constant speed for the variation of surface height with horizontal displacement. According to international standards, a pen may have an angle of 60 and 90 degrees and a tip radius of curvature of 2 microns, 5 or 10. A truncated pyramid is one type with a 90 degree included angle between the opposite sides. It is likely that a profile containing many peaks and valleys with a radius of curvature of 10 microns or less and slopes greater

A very important thing to consider is that the variation of the radius of stylus tip may affect the shape of the profiled surface because the radius of the tip of the pen draws a single envelope of the actual profile. The resolution depends on the real contact between the pen and the actual profile. As the radius stylus is increased contact is made with fewer points on the surface, and therefore the profile is modified. Increasing the radius stylus tends to reduce the measured amplitude of the parameter like Ra (Roughness average). However, the relative effect on roughness is not as great as the peak to valley, and other parameters

The stylus instruments can be used with two different attachments. The first one has a fixed reference, which limits the movement of the pick-up to a horizontal and the transducer gives the height difference between the instantaneous movement of the pen and the whole pick-up. This is the ideal way to measure the surface profile. Unfortunately, this method requires a setup procedure for leveling by a skilled operator. In order to reduce the setup process, a skid

than 45 degrees would be misunderstood by such a stylus (Vorburguer & Raja, 1990).

separated quantities or as a half wafer (Howland & Benatar, 2000).

emergence of numerous non-contact profilometery techniques.

that are best suited for analysis of sensitive surface structures.

Fig. 10. Probe (1nm radius of curvature HI'RES-W probe – MikroMasch, 2011).

#### **4.3 Properties of cantilever**

The spring constant of a cantilever (Figure 11) has a critical importance in atomic force microscopy and it is lower than the spring constant between atoms in a solid, which are on the order of 10 N/m. The spring constant of the cantilever depends on its shape, its dimension and the material from which it's made of. Shorter and thicker cantilevers tend to be stiffer and consequently have higher resonant frequencies. Commercial available cantilevers range over four orders of magnitude, from thousands of a Newton per meter to tens of Newton's per meter. Resonant frequencies range from a few Kilohertz to hundreds of Kilohertz allowing high-speed response for non-contact operation.

Fig. 11. Microcantilever with microfabricated tip for a contact-mode AFM. This silicon nitride cantilever was manufactured by Park Scientific Instruments, Mountain View, California. (Photograph by Greg Kelderman – Rugar & Hansma, 1990).

#### **4.4 How to select a probe**

154 Atomic Force Microscopy – Imaging, Measuring and Manipulating Surfaces at the Atomic Scale

Fig. 10. Probe (1nm radius of curvature HI'RES-W probe – MikroMasch, 2011).

Kilohertz allowing high-speed response for non-contact operation.

The spring constant of a cantilever (Figure 11) has a critical importance in atomic force microscopy and it is lower than the spring constant between atoms in a solid, which are on the order of 10 N/m. The spring constant of the cantilever depends on its shape, its dimension and the material from which it's made of. Shorter and thicker cantilevers tend to be stiffer and consequently have higher resonant frequencies. Commercial available cantilevers range over four orders of magnitude, from thousands of a Newton per meter to tens of Newton's per meter. Resonant frequencies range from a few Kilohertz to hundreds of

Fig. 11. Microcantilever with microfabricated tip for a contact-mode AFM. This silicon nitride cantilever was manufactured by Park Scientific Instruments, Mountain View,

California. (Photograph by Greg Kelderman – Rugar & Hansma, 1990).

**4.3 Properties of cantilever** 

The desirable properties for a probe are related to the imaging mode and the application. In contact mode, soft cantilevers are preferable because they deflect without deforming the surface of the sample, a silicon nitride microlever would be a good choice for most applications. In non-contact mode, stiff cantilevers with high resonant frequencies give optimal results. For applications other than topography, like MFM (Magnetic Force Microscopy), NSOM (Near-field scanning optical microscopy), SThM (Scanning thermal microscopy) etc., different types of probes have to be used. The probes are available mounted on AutoProbe mounts, mounted on TopoMetrix mounts or unmounted in preseparated quantities or as a half wafer (Howland & Benatar, 2000).
