OVERVIEW: The design of the microscope’s core is based on the setup used in the research group of D. Rugar at IBM Research Division, Almaden Research Center. It has been re-built and further evolved in the research group of M. Poggio at Universität Basel. There exist other similar setups as for example used in the Group of C. Degen at ETHZ, which is mainly just horizontally mirrored compared to the present setup [18].
3.7 Motion Detection 55
An other configuration, where the axis of the cantilever is normal to both the external magnetic field and the rf micro-wire source, was demonstrated by Xue et al. [101].
(A)
(B)
FIGURE3.8: THEMICROSCOPE’SCORE
(A) In the upper back: the assembled force sensor with the L -block (i) mounted on the support-stage (ii); in the lower front: the piezoelectric positioner stack (iii) with an (arbitrary) chip (iv) on top. The three beryllium-copper (v) and Teflon (vi) springs isolate the microscope’s core from environmental vibrations. Through the three copper leads (vii) the core is thermally connected to the cryostat. The semi-rigid coaxial RF lines are fed from the right side (spiral) underneath the positioner stack to its left side (straight), where they are fixed and connected to the wires of the micro-wire chip. (B) A zoom-in showing the cantilever chip (i) on the general holder (ii) and actuation lever (iii), the lens (iv) at the end of the sheath (v), the actuation piezo (vi), and an MRFM chip (micro-wire & nano-magnet) (vii) with the flexible wires (viii) to the coax-lines’ connectors (ix). Photographs from the Poggiolab archive.
The core of the apparatus consists of two parts: (1) the force sensor composed of cantilever, fibre, lens and actuation piezo; and (2) the chip positioner, a stack of piezoelectric positioners on top of which a chip can be brought into vicinity of (1). For MRFM, this is the micro-wire chip with the nano-magnet. Though in general any other item close to an edge of a flat surface27can be measured by the force sensor [108,109,
153]. Or it is used solely, without any other object nearby [106,110,151].
The core is placed on a custom designed support-stage which is suspended on springs from a corresponding ceiling-stage. The two stages are thermally connected by ∼ 7 mm
27The microscope can only operate within ∼ 100 µm to the edge of a chip. Further away, the laser beam
(A) (
B)
FIGURE3.9: THEREARSIDE OF THEMICROSCOPE’SCORE&THESHEATH
(A) Rear side of the support stage (i) showing the sheath’s back end (ii), the set-screws for the fine alignment (iii) (section3.7.1) and the L -block’s fixation screws (iv) with the mentioned big washers. The hole in the middle is for accessing the set-screw which pushes on the actuation piezo. (B) Front end of the sheath with the integrated lens. With the tweezers the fibre (not visible) is held in place inside the sheath while waiting for the glue to be cured. Photographs from the Poggiolab archive.
thick flexible Cu leads. Twisted cable pairs and semi-rigid coaxial lines are fixed with interconnectors to the ceiling-stage, from where the leads go directly or via an additional fixation on the supporting-stage (coaxial lines) to the individual elements.
If not specified differently, the elements of the microscope’s core, including all screws, are made of titanium. It provides minimal thermal contraction and is first and foremost paramagnetic compared to steel. The second material of choice is copper (Cu, diamagnetic) exhibiting the best available thermal properties
FORCE SENSOR: The ∼ 100 µm long cantilever sits at the edge of the cantilever chip (see fig.3.1, ∼ 2 × 5 × 1 mm3).28 To simplify the handling the chip is clamped into a general-holder(∼ 4 × 10 × 3 mm3), which in turn is mounted on the actuation lever
as shown in fig.3.8b. In order for the cantilever to eventually be precisely vertical, all elements are carefully aligned orthogonal, respectively parallel.
This assembly and the course alignment below have to be made anew for every experiment. The rest of the microscope’s core is either semi-permanently or irreversibly
3.7 Motion Detection 57
assembled.
The actuation lever (∼ 1 × 4 × 0.2 cm3) is rigidly clamped to the L -block(fig.3.8a,
∼ 2 × 3 × 1.5 cm3).29 The actuation piezo, a stack of two piezoelectric discs, is glued
to the back of the lever and faces the L -block. The hot pin is thereby in the middle and the outsides go to ground. By a set-screw, pushing from the L -block on the actuation piezo, the actuation lever it is bended and brought under tension. An oscillating voltage on the actuation piezo thereby drives the actuation lever and finally the cantilever.
On the other side of the cavity is the sheath (fig.3.9bholding lens and fibre, which has been introduced previously. As mentioned, it is held by the fibre-holder enabling the fine alignment. The fibre-holder itself is screwed to a relatively big (∼ 3 × 5 × 1.5 cm3), massive support block. On one side, it serves as support for the L -block and has the according openings (holes) to access the L -block from behind. On the other side it fixes the whole force sensor to the support-stage.
COURSE ALIGNMENT: The above assembling is made on the lab table and to some extent under a microscope. The course alignment is subsequently made on the system itself. It consist of attaching the assembled L -block(with cantilever and sample) to the support block, so that the laser beam impacts somewhere on the cantilever’s paddle.
For the fixation, the support block does not exhibit threaded holes, but rather much wider openings than the diameter of the attachment screws which are fed trough these openings. Thereby the assembled L -block can be moved to some extend in the y-z-plane, but not in x. The fixation is finally made by clamping the two pieces together with accordingly big washers covering the openings.
For the course alignment, the assembled L -block is pressed against the support block. Unspecific light is coupled into the fibre from behind which makes the fibre’s core (the lens’ focal spot) visible in the frontal view. Looking with a microscope on the cantilever and the fibre’s core, the assembled L -block is moved in the y-z-plane until the two overlap. While one person holds by hand the L -block in this position an other person tightens the screws (fig.3.9a) without distorting the alignment.
In x-direction the distance is set by means of the fine alignment, if necessary in an iterative way.
CHIPPOSITIONER: The stack of the piezoelectric positioners (attocube) consists of a vertical, linear positioner (ANPz51), two horizontal linear positioners (ANPx51) and a horizontal scanner (ANSxy50) screwed on top of each other. With the linear positioners a 3 mm × 3 mm × 2.5 mm region can be covered whilst the scanner enables finer positioning than with the linear positioners (subnm precision on 15 × 15 µm2).
On top of the stack, a holder (Cu) designed to receive the micro-wire chip is mounted.
29The term refers to the side view, which has the shape of an L that has been turned upside down: L . In
Positioning with the positioner stack is straight-forward and simple. But the stack exhibits the disadvantage of noticeable oscillations. By coupling trough non-contact friction to the cantilever’s oscillation they can increase the measurement noise floor.