A new scanning force microscope for temperature-controlled static and dynamic friction measurements, using an extended normal and lateral working range with high spatial resolution

Abstract

In this work, a new and improved SFM is presented. This SFM can measure many variables such as the individual dynamic and active temperature regulation of the sample and the cantilever, miscellaneous friction modes, x-, y-, z- linearization and an increased scan speed, simultaneous with high spatial resolution. The scan width is also laterally extended to 1 millimetre by 1 millimetre. This extension of the lateral scan range requires a larger tilt compensation. Therefore, the z-scanner has a range of more than 300 µm. Summary In this thesis, a novel scanning force microscope was presented. Other than the large scan range of approximately 1000 µm by 1000 µm, the thermal guidance of the cantilever and sample is the most labour-intensive component of the microscope. Detailed construction of the cantilever holder and sample holder, light beam guidance for the normal and lateral forces, the positioning and scan unit, and ECU, control software and communication software have been discussed. The entire microscope was constructed as symmetrically as possible using mechanical construction to avoid thermal drift. For thermal measurements and thermally assisted friction measurements, a concept for separately heating the cantilever holder and the sample holder was discussed. A simulation on the cantilever holder showed that a temperature difference of 150°K could be reached after 40 s and that the cool down cycle took only 38 s, without any disturbance of the nearest piezo. The same analysis was performed for the sample holder. Only the junction temperature for unknown samples and the temperature behaviour of these samples was calculated. After 15 s of heating, the junction reached 150 °K above the ambient temperature. The thermal path to the next piezo element is more critical than the one for the cantilever holder setup, therefore, the thermal management is more complex. However, the analysis has shown a maximum rise of 32 K above the ambient temperature at the piezo element for the heating and cooling scenario described above. For the beam deflection setup, a single-mode glass fibre supply of the laser light beam was used. A special grinding technique was used to form fibre tips, which can be short-duration fused to form a small lens for pre-focusing the light beam (two patents have been submitted). A sample setup showed that the self-assembled laser beam has a diameter of approximately 60 µm. With 10 µW of light on the 4Q-PD using a standard cantilever, the electronics produced a displacement equivalent to the noise of approximately 0.1 nm with a 10 kHz low-pass filter. With the beam deflection system introduced here, even the first and second resonances, in addition to the basic one, were observable. The torsional basic mode and its first harmonic were also observed. The Z-drive was separated into low, mid-range and high-frequency components. This separation aids with the thermal management of the microscope to avoid and/or remove unwanted thermal heating in areas of the system where mechanical drift disturbs the scanning force measurements. The hydraulically driven low frequency Z-component fulfils this requirement. A European patent has been granted for the hydraulic cylinder design. While linearization in x-, y-, and z-direction is always desirable, it is a necessity for the scan range of the microscope introduced here. Each scan range could be assembled with a specially adapted linearization unit (fibre bundle and mirror) as the linear behaviour of each linearization unit is proportional to the numerical aperture of the guiding fibres. Finally, the ECU and the control, communication and visualisation software were presented. The centrepiece of the ECU is a Texas Instruments 6711 floating point DSP. This unit features the real-time capability that is necessary to drive such a microscope. The DSP communicates via a serial, insulated interface (FireWire®) with the PC in burst mode, while collecting data from the sensor channels in strict, fixed time-slices. Again, the data channels have to be precise and very fast due to the large scan range, to keep the scan times in a manageable regime. Six channels have 16 bit resolution at 1 MSPS, four channels have 16 bit resolution at 500 kSPS, and eight channels have 16 bit resolution at 250 kSPS. One time-slice was also used up to 128 times for multiplexing pre-selection for one arbitrary channel, which can be used to detect slowly changing signals. At this sample rate, a meander-shaped scan line could be achieved. Different types of deceleration and acceleration behaviour were calculated to obtain the largest possible linear scan range, resulting in cubical behaviour during deceleration and acceleration for even the smallest disturbance in the linear scan regime. The mixer electronics were designed with offset and zoom capabilities. This results in linear DAC sample steps within the chosen region, independent of the user-chosen region of interest (angle and size).