3.4.1 The fiber
The fiber end is used for detecting the deflection of the cantilever and is very sensitive to dirt and scratches. Scratches on the fiber can occur when for instance
Table 3.1: Overview of the maximum voltages applied to the piezos and the corresponding ranges the piezos have at different temperatures.
Piezo Max V RT Max V 77/4 K Range RT Range 77 K Range 4 K
x-scan 60 V 150 V 42 µm 47 µm 29 µm
y-scan 60 V 150 V 45 µm 45 µm 20 µm
z-scan 60 V 150 V 20 µm 24 µm 13 µm
x-pos 30 V 40 V 4 mm 4 mm 4 mm
y-pos 30 V 40 V 4 mm 4 mm 4 mm
z-pos 30 V 40 V 5 mm 5 mm 5 mm
Table 3.2: Overview of the capacities of the piezos at different temperatures.
Piezo Capacitance RT (nF) Capacitance 77 K (nF) Capacitance 4 K (nF)
x-scan 1863 1125 645
y-scan 2021 1203 577
z-scan 845 288 157
x-pos 723 269 141
y-pos 638 243 127
z-pos 811 297 157
dither 338 119 64
(a)
(b)
( )c
Figure 3.4: The movement of the piezo and the main body in three steps of the movement cycle. m1is the main body and m2 is a mass attached to the piezo.
(a) The starting position of the positioner. (b) The piezo extends slowly and mass m1 follows the motion. (c) The piezo retracts so fast that m1 does not follow the movement.
the cantilever touches the fiber end. A lens tissue and some ethanol can be used to clean the fiber. If it is still dirty/scratched afterwards the fiber end has to be replaced. The laser spot of a dirty or scratched fiber differs from a laser spot from a good fiber end. A good fiber end produces a perfectly round spot. To make a new clean end of the fiber, the fiber is detached from the fiber holder where it is glued onto. The fiber is cleaved and the new fiber end should be stripped from its cladding for approximately 12 mm using a fiber stripper. The fiber is then clamped with a fiber adaptor that holds the fiber without damaging it. The fiber can then be polished with help of very fine grains. The polishing is done in an ”eight”-motion to ensure the fiber end is not tilted. While polishing the reflection signal will change a lot and the polishing is done right when the signal is maximized and the spot is round with no light leaving the fiber end sideways. The unstripped part of the fiber should be just as large as the fiber holder. The fiber end should extend a bit at the cantilever end. At the other side the cladding should be present at the position where the fiber holder stops in order to resist the forces on the fiber there. The bare fiber can be attached to the fiber holder with two-components glue and placed inside the scanhead again.
Replacing the fiber end shortens the fiber approximately 2 cm so damaging the fiber should be avoided if possible.
3.4.2 The cantilever
The cantilever is a sensitive and fragile measuring tool and can be damaged easily. If the cantilever is damaged, broken, or if you want to switch from AFM to MFM, it has to be replaced by a new one. The first step in doing this is removing the dither piezo from the rest of the setup. This can be done by
Fiber holder
Piezostack holder x Positioner
y Positioner z Positioner z Scanner x/y Scanner Sample holder
Dither/Cantilever
Dither Fiber end Cantilever Chip
Fiber holder
(a)
(b)
Figure 3.5: (a) A picture of the piezostack and the piezostack holder with the fiber and cantilever holders. Normally the piezostack is mounted in the holder.
(b) A zoomed in picture of the fiber and cantilever.
detaching the scan head from the stick, turning the fiber/dither holder upside down and unscrewing the dither piezo with the screw seen in figure 3.6. With a tweezer the old cantilever can be detached from the dither. The dither then has to be cleaned with a surgical knife to remove all the glue that is left behind.
After that the new wafer part of the cantilever is glued onto the dither with two-component glue of the brand UHU sofortfest. The wafer has to be placed with the back end on the dither as is shown in figure 3.7.
After the cantilever is glued onto the dither, the dither can be attached to the scan head again. The fiber should be retracted first to avoid contact between the fiber and the cantilever. The fiber can move in one direction only, and that is the direction perpendicular to the cantilever. A good tip fiber alignment is important because the interference signal depends on the distance between the cantilever and the fiber and on the relative position of the cantilever and
Dither Fiber end Cantilever Chip
Fiber holder
Screw
Figure 3.6: Picture of the cantilever on top of the dither piezo. The fiber ends just under the cantilever.
dither
wafer cantilever
laser spot
mounting screw cantilever
(a) (b)
Figure 3.7: (a) A schematic view of the cantilever on top of the dither. The cantilever should be placed approximately half way on the dither. On the left of the dither the mounting screw can be seen to attach the ditherholder to the scan head. (b) The cantilever with the laser spot striking it. The laser spot should be at the end of the cantilever.
the fiber. With the fiber at a safe distance, the cantilever has to be placed exactly on top of the fiber as is shown in figure 3.7. By holding a piece of paper on the other end of the cantilever the spot can be seen on the paper. If the cantilever is positioned right, the fiber is moved closer to the cantilever. This can be done by adjusting a screw that pushes the fiber holder closer to the cantilever. An interference signal as was calculated in section 2.1 is observed.
The right distance between cantilever and fiber is where the visibility is largest, and thus the difference between a maximum and minimum is largest. However by cooling down the system the fiber-cantilever distance decreases so for low temperature measurements the starting distance should be a bit larger, around 50 µm. Because adjusting a screw with a screwdriver while maintaining a close look on the interference signal can be difficult the signal can also be checked by applying a voltage sweep on the dither piezo. Now the dither and thus the wafer and the cantilever move and the distance between the cantilever and fiber changes. This gives rise to an interference signal as can be seen in figure 3.8. At low voltages there is a small nonlinearity in the dither extension which results in a distorted sine shape.
0 10 20 30 40 50 60 0.12
0.13 0.14 0.15 0.16 0.17 0.18 0.19
Interferencesignal(V)
Dither Voltage (V)
Figure 3.8: Interference signal of a changing voltage on the cantilever.