Atomic force microscopy is the principal analysis tool utilized in modified surface research. This tool allows accurate and immediate detailing of the surface topology with analysis of the surface roughness in conjunction with 2D and 3D imaging. AFM was developed (1985 Binnig, Quate and Gerber) (299) to overcome a fundamental flaw with surface tunnelling microscopes (STM), the lack of ability to image non-conductive surfaces (300) (301) (302). With AFM analysis, practically any surface type may be is imaged including ceramics, glass, composites, ceramics and biological samples (298). Surface characterisation is taken at the microscopic level with a resolving accuracy from approximately 1µm to sub-nanometer (298). AFM operates by the utilization of incredibly fine sharp tips that trace the sample surface either in contact or near proximity (0.2-10nm) (303). Tips are of the order of several microns long and less than 100Å in diameter located on the free end of a flexible cantilever of approximately 100-200 µm long (302). Interactive forces between the tip and the surface either attract of repel the tip, with the resulting deflections being recorded and processed as a computer image (297) (298).
Tip (probe) is placed on the cantilever, where the amount of force administered between the probe and sample is dependent on the spring constant of the cantilever and the tip to sample separation. This force is described using Hooke's Law in equation (5. 3)(301);
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(5. 1)
- Force - Spring Constant - Cantilever Deflection
The spring constant for a cantilever is usually between 0.1-1 N/m (300). If this spring constant is less than equivalent spring between the atoms of the sample, the cantilever will bend with the resultant deflection (open air deflection 10-6 to 10-9) being monitored electronically (300). Modern AFM design exploits a laser beam deflection system for sensitive measurement of the probing tip as shown in Figure 34 (298). The laser emits a beam upon the back surface of the travelling probe, with a position sensitive detector tracking its movements as it scans across the material surface. This particular configuration proves useful in terms of resolution, as the minute deflection in the tip is registered by contrast as significantly large deflections in the lasers path length (301). Advancements in tip composition and design further aid the resolution attained by the AFM with standard cantilevers being microfabricated from Si or Si3N4 with a typical tip radius <10nm (300) (303).
Page | 105 Probe motion is monitored by a feedback loop system with piezoelectric scanners to maintain probe height or contact against the surface (297). Van der Waals interactions dominate the small probe to surface force interaction. During contact with the sample the probe experiences repulsive Van der Waals forces deflecting the tip, this is known as contact mode (301) (303)
. As the tip loses contact from the surface it experiences attractive Van der Waals forces, known as non-contact mode (303). Figure 35 depicts the AFM mode with respects to probe to sample separation and force (300).
Figure 35 - Force as a function probe-sample separation (300)
There are three primary operation modes for AFM imaging, contact, tapping and non-contact (300)
.
1. Contact mode mainly functions using repulsive Van der Waal forces. As the spring constant is less that the atoms on the sample surface the cantilever bends with a repulsive force resulting on the tip. Constant cantilever deflection is maintained by a series of feedback loops and the forces required maintaining said deflection, results in an image being obtained. This mode is good at scanning rough surfaces at fast rates and provides good analysis for frictional properties, but the contact may result in soft sample damage.
Page | 106 2. Tapping (Intermittent) mode is similar to contact operation; however the tip is
oscillated at its resonant frequency. Due to the movement in the probe, the tip taps the surface during scanning. Maintenance of stable oscillation amplitude permits a constant tip-sample interaction, allowing imaging of the surface. This mode gives high resolution of samples that may be easily damaged or soft surface adhesion (particularly cellular biological samples). Due to the nature of the intermittent tapping of the surface, slower scan rates are required.
3. Non-contact mode utilises the attractive Van der Waals forces experienced between tip and sample. Samples have a designated liquid absorbed on the surface for scanning or maybe conducted under Ultra High Vacuum conditions. The probe oscillates above the fluid layer during scanning and using a feedback loop monitors amplitude changes due to the attractive forces. The ultimate change in forces results in the imaging of the surface topology. With this method there is a very low force exerted on the sample, a consequence of this is low resolution and contamination of the surface. Generally this technique requires vacuum for best imaging due to the surface interference with oscillation and contaminants.
Resolution of AFM is limited significantly by the aspect of the tip (297) (298). It's difficult to obtain true surface topology on the atomic scale using the AFM methods as there will always be a degree of error due to the tip width (297). We generally encounter the absolute resolution determination as the interaction between the probe and surface, this is described as tip convolution (297) (303). The shape of the tip does not affect the resultant image feature height but alters the lateral resolution. Figure 36 shows an example of how the perceived feature differs based on probe aspect ratio.
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Figure 36 - High and Low aspect ratio tips resolution variation (300).
Roughness data is a useful measurement of the sample surface, enabling characterization of films. Specifically for TCO applications rough films are required with smooth features to prevent defects in final solar cell construction. Therefore the desired films should exhibit large smooth features which reflect a high roughness value. Typically as the film thickness is reduced the roughness and therefore sharpness of the features is reduced. Texture of the film may be visually assessed with roughness being calculated from the AFM probe data. Multiple methods can be applied to determine AFM roughness depending on how the data is handled. Roughness average ( ) is the most commonly applied measurement for AFM. Root mean square ( ) is primarily applied in this investigation as this is commonly used within the field of TCO assessment.
Roughness average ( ) is the arithmetic mean of the absolute values of the height of the measured sample surface. It is calculated from equation (5. 2)(304).
∫ | ( )| (5. 2)
Z(x) is the function that describes the surface profile analysed in terms of height (Z) and position (x) of the sample over the evaluation length “L”.
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Figure 37 – AFM Measurements and Roughness Determination (304)
This value is just the mean absolute profile which makes no distinction between the valleys and peaks of the sample. Therefore samples of various different structures/textures can exhibit identical roughness values. determination is a statistical measurement, taking the square of the measurements. The function is defined in equation (5. 3)(299) (305).
√ ∫ | ( )| (5. 3)
measurements are therefore more sensitive to the valley and peak information compared to the roughness average due to the amplitude squaring during calculation. This type of assessment is therefore more suited for TCO films for comparative assessment.