The preceding sections described all the components needed to do NV spin coherence and relaxation measurements and magnetometry on fixed-position samples, for example external spins placed on the diamond surface. The AFM was added to perform 3D
nanoscale magnetic imaging in a sample-on-tip configuration. The Bruker BioScope
Catalyst AFM contains a sample-scanning x, y stage on which we scan a bulk diamond sample. The AFM head controls the coarse and fine z positioning of the AFM tip for approach and scanning. Figure 2.17(a) shows a photograph of the combined inverted CFM and AFM sample stage. During AFM imaging this tower section of the setup was enclosed in a box of lead-lined panels with acoustic-damping foam (Fig. 2.17(b)) to further minimize stray light, temperature drift, and air currents. A close-up photo of the AFM head and sample region is shown in Fig. 2.17(c). An infrared laser in the head is directed by a small mirror to the AFM cantilever from which it then reflects into a quadrant photodiode (QPD) to read out the dc deflection and on-resonance lock-
in amplitude. The diamond sample sits on a shorting path of a Ti/Au CPW on a
glass coverslip. A flexible microwave cable is soldered to the CPW to deliver the pulsed microwaves. This PCB-SMP connector was required to be small and well positioned so as not to block the IR beam path into the QPD, as inferred from Fig. 2.17(c).
Figure 2.17(d) is a top-down micrograph of the system when a magnetic AFM can- tilever and its tip are aligned to an optically addressed NV residing in a clear window of the CPW. The procedure for alignment the 532-nm laser spot, NV, and AFM tip to a given x, y position is illustrated in Chapter 4. We summarize here the degrees of freedom: the objective x, y position is mechanically fixed, but the laser spot can scan three-dimensionally over the diamond. The entire AFM baseplate is mounted on a bread- board on top of a x, y translation stage (Newport 406). This manually moves the AFM
magnetic probe AFM head (Z)
AFM baseplate (X,Y) Inverted high-NA objective (hidden) Top-down optics, for probe alignment external magnet rf cable to CPW magnetic tip aligned to diamond IR detector Au CPW laser
(a)
(b)
(c)
(d)
diamond on CPW Optics pathFigure 2.17: Photographs of Magnetometer A setup on an optical table for NV scan- ning magnetometry. (a) AFM, sample area, and optical imaging region of the confocal microscope. The diamond sample is scanned in x and y while the AFM head controls z positioning of the magnetic tip. (b) Fully assembled isolation box around structure shown in (a) for acoustic damping, blocking air currents, and mitigating temperature drifts. (c) Up-close AFM head region showing AFM infrared detection optics and rf waveguide circuit for NV spin rotations. (d) Micrograph using top-down optics showing magnetic probe cantilever aligned to a metal-clear gap on the gold waveg- uide-on-glass where the confocal microscope laser is focused on the diamond surface. Microscope oil is between the waveguide and diamond.
baseplate (i.e., diamond sample) and AFM head (i.e., tip) together relative to the ob- jective. The AFM baseplate coarse x, y motor positioners translate the diamond sample relative to both the objective and tip.
Because the diamond-on-CPW is scanned relative to the objective and tip, then during a magnetometry image acquisition the laser must scan to “follow” the changing
position of the NV to keep excitation rate and PL constant. AFM scan sizes were
typically larger than the lateral resolution of the CFM. NV following requires quantitative calibration of the AFM scanner’s linear translation and scan angle to the CFM laser scanning. In summary, calibration involves fine-tuning the CFM’s galvo µm/V in x and y to match the scanner displacement as well as transforming all AFM scan coordinates
by the offset angle θscan before executing a scan. In other words, the AFM scan distance
was used as the linear ruler and the two CFM scan mirrors defined the x and y directions.
The Catalyst AFM was interfaced with the Matlab confocal microscope software via a Distributed Component Object Model (DCOM) connection on the local network (see Appendix B). The “Nanoman” nanolithography functions from Bruker were used to control the tip z position, sample x, y, feedback settings, and stage position of the AFM through DCOM. Figure 2.18 is a basic schematic of the AFM electronics and network connection to the CFM.
The combined CFM and AFM was sufficiently stable against mechanical vibrations to resolve sub-nanometer height features. This is demonstrated for example by Fig. 2.19, which shows a contact-mode topographic image of a diamond film surface acquired with the system loaded with a standard silicon nitride probe (Bruker SNL). The atomic-scale steps of the step-flow diamond growth mode are clearly visible and no comparable noise amplitude is present in the image. For the NV magnetometry measurements in Chapter 4
AFM PC DSP card Confocal PC AFM head Nanoscope controller E-‐box LAN ethernet Hi gh v ol tag e A/ D li nes
Serial COM AFM scanning baseplate
x,y stage motor (coarse posi?oning) x,y stage
scanner z scan and IR op?cs control NI-‐DAQ card Pulseblaster card ethernet analog signal monitor outputs (scan voltages, ?p amplitude, …)
Figure 2.18: Schematic of optical-based AFM scanning electronics for Magnetome- ter A. The AFM PC, E-box, Nanoscope controller, and AFM baseplate and head are part of the Bruker Catalyst system. For synchronization with ODMR for NV magnetometry the AFM positioning commands are controlled from the custom confo- cal microscope Matlab software on “Confocal PC” through a lab network connection using Microsoft Distributed Component Object Model (DCOM).
2 µm
Figure 2.19: Topographic AFM image of a CVD-grown [001] diamond surface using contact mode with a sharp SiN AFM tip on Magnetometer A. The scan reveals a atomic-scale steps on a flat surface and demonstrates the stability of the combined AFM-CFM setup. The lower panel shows a line cut at the dashed purple line. The growth-formed steps proceed in the [110] crystal direction as discussed in Chapter 3.
scan measurements.