STM

3.2.1 SPM controller and current preamplifier

SPM controller is a brain of STM and NSMM. Our RHK R9 SPM controller (Fig.

3-1) (Fig. 3-2) plays multiple roles as power supply, high voltage amplifier, PI feedback control, piezoelectric stage driving, current preamplifier, data acquisition and data post-processing. We take numbers of advantages from its high flexibility of hardware configuration ADC, DAC I/Os, LabView VI support and ultra-low noise performance of electronics.

Fig. 3-1 RHK R9 controller (rear)

Fig. 3-2 RHK R9 controller (front)

31

Tunneling current in STM is extremely tiny, typically range from 0.01 to 50 nA.

Current preamplifier is thus an essential component to convert tunneling current into voltage and feed back to R9 SPM controller. Our IVP-200 and IVP-R9 (Fig. 3-3) are two-stage high performance current preamplifiers from the RHK technology. The first stage, IVP-200 has a gain of 10⁹ (10nA/V) with 30 kHz (50 kHz) bandwidth at 0 pF (100 pF) input capacitance. IVP-R9, as the second stage, provides a selectable gain and bandwidth, and it is also responsible for outputting bias voltage to the sample.

In practice, the distance between tip and input port of the first-stage preamplifier should be as short as possible. The shorter traveling distance, the less equivalent stray capacitance, the smaller electrically coupling noise.

Fig. 3-3 Current preamplifier. IVP-200 [right]. IVP-R9 [left].

32

3.2.2 Piezoelectric stage and controller

To handle nanometer-scale movement including fine approach and scaning task, we exploited n.point NPXY100Z25-102 three dimensional piezoelectric stage (Fig. 3-5) controlled by n.point C.300 series DSP controller (Fig. 3-4). In x, y, and z, the piezoelectric stage features a maximum translation range of 100 μm, 100 μm and 25 μm and position noises are respectively 0.3 nm, 0.3 nm and 0.1 nm. PID mode and AFM scan mode are both supported.

Fig. 3-4 n.point C.300 DSP controller.

Fig. 3-5 n.point NPXY100Z25-102 three dimensional piezoelectric stage .

33

3.2.3 Stepper motor and motor drive

SGSP40-5ZF stepper motor stage and SHOT-102 stepper motor drive (Fig. 3-6) are used to practice coarse approach. This is a five-phase stepper motor stage featuring 0.5 μm per step resolution, 5 mm maximum travel distance and 2 kg maximum loading weight.

Fig. 3-6 SIGMA KOKI SHOT-102 motor drive [right] and SIGMA KOKI SGSP40-5ZF five phase stepper motor stage [left].

34

3.2.4 Sample stage

Our acrylic-made sample stage (Fig. 3-7) was then revolutionized into an aluminum-made one (Fig. 3-8 Aluminum-made sample stage. The remodeled aluminum-made one has better sample-attaching design, more secure bias volatge attachment as well as larger and more flexible scanning area. The height of the sample stage was deliberately modeled for LFMR to work under an in-plane magnetic field in the future.

Fig. 3-7 Acrylic-made sample stage

Fig. 3-8 Aluminum-made sample stage

35

3.2.5 Tip holder

Tip holder is responsible for firmly holding the tip and transmitting both tunneling current and microwave. SMA connector is typically and widely used for coaxial transmission line with signal frequency available from DC to 18 GHz microwave. Thus, we tailored a SMA connector by drilling a 0.4-mm-diameter and 3-mm-depth hole on SMA Jack, turning the jack into a tube to hold a 0.25-mm-diameter tip (Fig. 3-9). Also, this tip holder features 50Ω impedance and excellent electrical shielding to prevent from coupling with electrical noise.

Fig. 3-9 Drawing of tip holder.

36

3.2.6 Tip and tip-fabricating equipment

The importance of tip treatment is recognized by Binning and Rohrer in the very beginning of the STM’s birth. The tip is suggested as sharp as possible to achieve better spatial resolution. In addition, for NSMM, tip shape is of the most primary factor for spatial resolution and sensitivity [26]. (Fig. 3-10) illustrates that image quality affected by different shape of the tip. Sharp tip (a) performs high spatial resolution but low sensitivity. Blunt tip (b) performs low spatial resolution but high sensitivity. Hybrid tip (c) with decreasing tapered angle gives the image with both high spatial resolution and high sensitivity. Therefore, tip fabrication is a critical and imperative technique for a high-quality image, and apparently the hybrid tip is the most wanted.

In our experiment, the tip was fabricated by electrochemically etching a 0.25-mm-diameter tungsten wire in 4M KOH solution. First, we sandpapered the oxide layer off the tungsten wire surface, cut the tungsten wire into roughly 0.7-cm-length clips before a 5-minute ultrasonic cleaning in methyl alcohol and acetone respectively. Second, we had a tungsten clip clamped as anode and submerged into KOH with 2-3 mm out the surface, and a copper wire was connected as cathode. Once the set-up is ready, turn on the DC power supply and increase the voltage slowly from 2 V to 8 V. To avoid the asymmetry of etching caused by absorbing foam on the tungsten wire, the voltage can be adjusted in above range to control the etching speed, and the two electrodes should be separated as far as possible to prevent bubbles of hydrogen gas from interfering the observation. The tungsten clip under KOH surface would be considerably sharpened.

Not until the bottom portion is about to drop off, decrease the voltage down to 2 V and Fig. 3-10 Tapered tip shape and corresponding image [26].

37

give intermittent pulses. Cut-off timing is significantly critical for the sharpness of tip.

The quicker the electrochemical reaction can be stopped as soon as the wire breaks, the sharper tip apex will be shaped [29]. Chemical formulas of electrochemical reaction for tip etching are shown as below,

Anode: W + 8OH⁻ → 𝑊𝑂₄⁻²+ 4𝐻₂𝑂 + 6𝑒⁻ (3.1) Cathode: 6𝐻₂𝑂 + 6𝑒⁻ → 3𝐻₂+ 6𝑂𝐻⁻ (3.2)

Our tip-etching equipment consists of a TOE 8704 DC power supply, a momentary switch, wires, a vertical displacement stage, a tip-fixing stage, a beaker, a LED light, a clip of copper wire as the anthode, and a long-len microscope (Fig. 3-11).

Fig. 3-11 Tip-fabricating equipment.

38