Tuning Fork Sensor Construction

The tip holder locking system at the end of the scan tube incorporated crimp connectors and ∅1 mm copper wire. A circular MACOR block of 6 mm diameter with three crimp connectors mounted inside was glued into the end of the scan tube, as shown in Figure 3.7. Three copper wire prongs were secured in the (separate) MACOR tip holder to match the positions of the crimp connectors in the scan tube. A photograph of the Phase Two tip holder is displayed in Figure 3.10.

Figure 3.10: Photograph of the Phase Two tip holder through an optical microscope of the tip holder, viewing the AFM/STM tip ensemble from the

direction of the sample.

the tip tool. This tip tool could also fit into the yoke arm of the UHV system for storage (as shown in Figure 3.2).

Figure 3.11: Photographs of the Phase Two design tip tool, displaying the hole and slot to hold and lock the tip holder in place. The holes for the UHV arms to

manipulate the tip tool and the thread allowing the tip tool to screw into in the yoke arm are also labelled.

The TF itself was mounted with non-conductive epoxy (EPO-TEK H77 from Epoxy Technology Inc.) onto the tip holder (as shown in Figure 3.10) to secure one tine of the TF to the tip holder whilst the other TF tine was free to oscillate.

3.3.1 Tuning Fork Tip Attachment

The TF used in the construction of the AFM/STM tip ensemble was pur-chased from MICRO CRYSTAL, E158 as used in crystals DS26. These were the same TFs incorporated in the QPS system [85] and had a resonant frequency of 32.768 kHz.

The attachment of the tip to the free TF tine was critical, there was enough epoxy to hold the tip solidly whilst minimising the total amount of epoxy applied.

This was because the extra mass on the tuning fork reduced the cantilever resonant frequency, from 32 kHz to approximately 28 kHz, depending on the added mass

Chapter 3. Construction of a Combined AFM/STM UHV System 77

of epoxy. Thus the sensitivity of the AFM was reduced as the amount of epoxy applied to the TF was increased. The quality of the bond between the tip and the tuning fork was also crucial for the Q-factor of the QPS, if the tip was not attached rigidly the Q-factor would decrease.

In the Phase Two design of the system the TF tine was electrically isolated from the tip wire and a longer length of wire was used for the tip. The tip wire (with or without a tip already produced at one end) was attached to the free TF tine and the other end of the wire connected to a separate contact using conductive epoxy – EPO-TEK H20E-PFC from Epoxy Technology Inc.. The tip wire was manipulated into a right angle before attachment so that the tip wire could be attached with greater ease – this process is illustrated in Figure 3.12. The tip wire was either ∅25 µm Pt-Ir wire (90% to 10% ratio respectively) or ∅50 µm W wire (purity 99.95%), purchased from Goodfellow. Non-conductive epoxy was applied to the tip wire to attach it to the TF tine – as shown in Figure 3.12 – with special care taken that the tip wire was not in electrical contact with the TF electrode.

Figure 3.12: The different methods of attaching a tip wire to the top TF tine of the probe assembly.

The contacts on the tip holder were the same three ∅1 mm copper wires

The tip wire protruded perpendicular from the TF tine and bent 90^◦ to con-nect directly to a tip wire post, as shown in Figure 3.10. The tip wire post was situated as close to the end of the TF (and therefore the tip) as possible and was electrically connected to one of the copper wire contacts with conductive epoxy.

Thus the length of tip wire protruding from the tine was only ∽0.5 mm in length.

Connecting the tip wire directly to the ∅1 mm copper contact produced an ac-ceptable Q-factor for the TF in ambient conditions, but there was acoustic noise pick-up in UHV conditions due to the length and frailty of the tip wire. Hence the tip wire post was required to reduce any vibrational noise. This tip wire post structure is also illustrated in Figure 2a of Reference [86].

The tip wire post was initially constructed from ∅1 mm copper wire but this was too thick to allow any manipulation of the post. Therefore any lateral strain experienced by the top TF tine due to the tip wire could not be rectified after the epoxy had set. However, this method did produce some probe assemblies with an acceptable value of Q of ∼1000. The final design of the tip holder used

∅0.15 mm OFC wire as the tip wire post. This produced a similar Q-factor for the probe assembly as the thicker post design, but allowed the lower Q-factor probe assemblies to be improved upon by manipulating the tip wire post. The Q-factor of the probe assemblies varied from 500-1500, however only tip assemblies with a Q-factor ≥ 1000 were then used for experimentation.

Tip Apex Production

As discussed in Chapter 2, W tips were produced by chemically etching.

This was a more lengthy and difficult process than simply cleaving the wire at an angle, which produced acceptably shaped tips for Pt-Ir wires. If the tip wire was attached to the TF before etching, the TF could be damaged or destroyed during the etching process due to handling difficulties. If the tip wire was etched and then attached to the TF there was also a high risk of damaging the tip during the

Chapter 3. Construction of a Combined AFM/STM UHV System 79

attachment process shown in Figure 3.12. W tips were also harder to manipulate, there are physical limitations to how close to the delicate tip a right-angle bend can be created. Thus, although W tips may seem the obvious choice for the tip wire, W tips were much more difficult to implement.

A tip etching assembly was constructed to produce W tips more reliably and easily and is illustrated in Figure 3.13. This etching assembly allowed the tip holder to slowly and securely approach a thin meniscus of NaOH solution that bridged across a hole in a metal sheet. A voltage was applied (as described previ-ously in Section 2.2.3) to etch the tip wire. Other methods of aligning the tip wire to achieve a very short tip usually resulted in a high percentage of broken tips.

Figure 3.13: Diagrams of the AFM tip etching assembly, (a) the side view of the entire assembly including a tip holder with a TF and a W tip wire attached to

one tine, (b) the top view of the MACOR block that held a metal sheet with several holes for the NaOH solution.

The UHV system contained an argon sputterer that allowed sputtering of the surface of samples, as described in Section 2.2.3. The UHV sputterer could be used in the preparation of the AFM/STM tips as well. Techniques such as e-beam bombardment could not be performed on the AFM/STM probe assemblies as the heat produced caused the epoxy on the tip ensemble to decompose. This obviously

required mixing. The epoxy was then baked at approximately 150^◦C to produce a strong bond between surfaces. The ratio of the two materials mixed together to form the pre-baked epoxy affected the Q-factor of the probe assembly. It is hy-pothesised that this is due to variations in the rigidity of the resulting dry epoxy.

That is, the resonant frequency of the probe was damped when the epoxy was less rigid. The period of time the mixed pre-baked epoxy was exposed to ambient conditions also had an effect on the Q-factor of the TF secured onto the MACOR holder. It was found that the Q value of a TF (without a tip wire attached) would not exceed 1500 if the epoxy had been prepared more than 24 hours previously.

However the TF could achieve a Q-factor of ∼3000 with freshly mixed epoxy.