Chapter 4 HOE-based AFM
4.1 Configuration of HOE-based AFM
Figure 4.1 introduces the block diagram of the developed HOE-based AFM system for both air and liquid environments. The whole system includes the scanning platform, the AFM probe holder, the holographic pickup head, and the signal acquisition system. All mechanical parts are made by stainless steel for increased rigidity. During the scanning process, the active vibration isolation table and acrylic acoustic enclosure are used to isolate vibrating noise.
Figure 4.1 Configuration of HOE-based AFM
Figure 4.2 displays the construction and the photograph of HOE-based AFM system.
The XYZ stage is a high resolution crossed roller guide stage. XYZ stage1 is used to carry the AFM probe holder to focus with the Holographic pickup head that is carried by XYZ stage2. Then the approaching stage, which is derived by a feed screw, is reached a suitable position for scanning with XYZ scanner. During scanning process, the signal acquisition system is captured the scanning data. The scanning data will be analyzed and imaged in the personal computer by the LabVIEW software. To avoid the vibration form the surroundings, the whole system is placed on an active vibration isolation table. A black acrylic shield is used to isolate the disturbance form the airflow and stray light.
Figure 4.2 HOE-based AFM
4.1.1 Scanning platform
The scanning platform contains two parts, which including a commercial precision piezoelectric scanning platform (P-363.3CD PicoCube, Physik Instrumente) and a home-made Z-axis approach stepper platform. The piezoelectric scanning platform has a scan range of 5 µm in XYZ-axes and a closed-loop resolution of 0.1 nm. The adjusting screw is used to provide screw pitch 0.5 mm when in manual approaching. Figure 4.3 illustrates the schematic diagram of scanning platform.
Figure 4.3 Construction and photograph of scanning platform
4.1.2 AFM probe holder
The AFM probe holder is designed for well functional in both air and liquid environments. The AFM probe (PPP-NCH) is clamped with the alignment chip. The piezoelectric plate is utilized to excite the AFM probe for tapping mode. For insolating the electrode and better stimulated efficiency, polyether ether ketone (PEEK) is placed between the piezoelectric plate and the tip alignment chip. Silicone is used for water proof. Figure 4.4 illustrates the construction and photograph of the AFM probe holder.
The glass is placed to isolate water and to provide a clear window when the cantilever is operated in water. The distance between the glass and the cantilever is designed as 0.5 mm to reduce the laser energy loss.
Figure 4.4 Construction and photograph of AFM probe holder.
For increasing the driving efficiency of the holder, the piezoelectric plate transmits energy to AFM cantilever directly. The AFM probe holder is suitable for small voltage driving. This design conceives to use a small voltage as 20 mV can drive the cantilever activated. Even in water, the driving voltage is not more than 2 V.
Another concept is conceived the behavior of the cantilever in liquid. For investigating the bio-molecules in liquid environment, the measurement in liquid is particularly
cantilever are influenced in liquid. When the cantilever is submerged under water, both resonant frequency and amplitude decrease significantly in liquid compared to in air. The Q-factor also decreases. In a piezoelectric excitation case, the amplitude and phase curves often show distortions due to an excitation of spurious resonance of the cantilever holder.
In order to avoid the spurious peak, a type of cantilever holder for spurious-free cantilever excitation in liquid was developed by Asakawa and Fukuma [48]. In this system, the piezoelectric excitation method with two pieces PEEK material as flexure drive mechanism is designed to reduce spurious peeks for the probe holder.
Besides, with the strength that the AFM probe holder can operate with a small voltage, the AFM probe holder also provide a clearer resonant frequency in air and water. Figure 4.5 shows the different resonant peaks form 20 to 200 mV when the cantilever is submerged under water. The spectrum of stimulated cantilever is clear and significant without any spurious peaks.
Figure 4.5 Spectra of stimulated cantilever in water
Utilizing the developed flexure mechanism, the excitation spectra in air and water are
demonstrated in Figure 4.6. The cantilever is excited by a sinusoidal signal with a voltage of 40 mV. From the spectrum in air, its amplitude, resonant frequency, and Q-factor are 7.19 V, 248 kHz, and 185, respectively. In comparison, the amplitude, resonant frequency, and Q-factor are 0.18 V, 124 kHz, and 22 in water. The spurious peaks are less than 4.3%
and 14.5% of the resonant peaks in air and water. Except for the resonant peak, no obvious spurious peak is detected. This result shows that the spurious peaks can be suppressed by the flexure mechanism effectively.
Figure 4.6 Spectra of stimulated cantilever in air and water
4.1.3 Holographic pickup head
The main core of the optical system is the holographic pickup head. The experimental allocation, the collimator lens and objective lens are equipped with the numerical aperture 0.18 and 0.4 (354560-B, CAY046, THORLABS). Holographic pickup head is used to transfer the cantilever deflection into the optoelectronic signal. The three-axis linear stages can carry the HOE tube and adjust the focus position on the micro-cantilever. The adapter block is designed with angle 10° so that the block can provide an angle to avoid
interference. Figure 4.7 illustrates the construction and photograph of holographic pickup head.
Figure 4.7 Construction and photograph of holographic pickup head
Figure 4.8 presents the exploded drawing of holographic pickup head. The dimension is 38 mm long × 13 mm wide × 13 mm thick. The total weight of holographic pickup head is about 10 g.
Figure 4.8 Exploded drawing of holographic pickup head
In order to obtain a stabile signals, a high frequency modulation (HFM) circuit is designed and equipped with HOE body. The original output signal of the focus error signal is distortional. After the modeling of the HFM circuit, the output signal is natural sine wave.
4.1.4 Signal acquisition system
The signal acquisition system includes a home-made signal amplifier (Bandwidth 1M Hz, theelectronic noise peak-to-peak is 10 mV), AD/DA, and a programmable embedded controller (sbRIO-9632, National Instrument), as shown in Figure 4.9. The Signal Board RIO is an embedded control and acquisition device which is integrated a real-time NI sbRIO-9632/9632XT embedded control and acquisition devices integrate a real-time processor, a user reconfigurable field programmable gate array (FPGA), and I/O on a single printed circuit board. The driving signal of the piezoelectric plate is generated from the digital signal to the analog output through a digital-to-analog converter (DC245A-A, Linear Technology). The reflective signal of the holographic pickup head is captured through an analog-to-digital converter (DC919A-A, Linear Technology).
The amplifier board is designed by using AD624, which is a high precision, low noise, high gain accuracy and high linearity instrumentation amplifier. For enlarging the output signals, the gain is set of 25 to have enough measurement range and for avoiding the output signal saturation. The offset of each channel is adjustable for absolute voltage level. Figure 4.10 shows the photograph of signal acquisition system.
In order to prevent the effect of low frequency signals, a high-pass filter is put between the amplifier and the electric board. A simple first-order electronic high-pass filter is implemented by placing a capacitor and a resistor. This filter can passes signals with a higher frequency and attenuates signals with lower frequency than the cutoff frequency.
In this system, the capacitor, the resistor and the corner frequency are 0.01 μF , 300 ohm and 53078.6 Hz, respectively.
Figure 4.10 Photograph of signal acquisition system