# Electron/Ion Microscopes

Like for optical microscopes there exist different types of electron/ion microscopes and we have access to several types of them, too. Here we will shortly discuss why they are necessary and then go through the microscopes that are important for fabricating and imaging of our electro-optical nano-antennas.

## Intro

Optical microscopes* are limited in resolution to $$\lambda/{2\,\mathrm{NA}}$$ with $$\lambda$$ being the wavelength and $$\mathrm{NA}$$ the numerical aperture of the device. Assuming a minimum visible wavelength of 400 nm and a maximum $$\mathrm{NA}$$ of 1.49, we obtain a resolution limit of ~135 nm.

Fortunately, due to the wave-particle duality we are not bound to photons but can use electrons or ions instead. The de Broglie wavelengths of these particles is given by: $$\lambda_{\text{de Broglie}} =\frac {h}{\sqrt{2\cdot m \cdot q\cdot V_{\text a}}}$$ with $$\text h$$ being the Planck constant, $$m$$ the mass and $$q$$ the charge of the electrons/ions, respectively, and $$V_{\text a}$$ the acceleration voltage. This means the higher we turn the voltage the shorter the wavelength of the particle gets, and we can easily overcome the 135-nm barrier. The only drawback is that we need a vacuum environment and conductive substrates to perform the imaging.

In the case of electrons there are two types of microscopes: Transmission Electron Microscopes (TEMs) and Scanning Electron Microscopes (SEMs). TEMs work in principle quite similarly to optical transmission microscopes with the exception that electrons interact much more strongly with light, i.e., one must prepare very thin lamellas (<100 nm) first. As this is a very complex task by itself and would mean to destroy our structures, the TEM will be skip in this discussion.

*that do not exploit near-field or non-linear effects

## SEM

SEMs might be best compared with a laser scanning microscope (a subtype of confocal optical microscope): We have an electron beam which is focused and scanned over a sample and the amount of (inelastically) scattered electrons is recorded, for example with a Secondary Electron (SE2) or InLens detector. As different materials (atoms) scatter differently, one can reconstruct an image by evaluating the signal vs. position.

We own a Zeiss GeminiSEM 450 for acquiring high-resolution images of our samples. A notable feature of the device is a nozzle which constantly injects oxygen close to the electron beam and therefore prevents the deposition of carbon on the sample. Carbon would electrically shortcut our antennas. Furthermore, we have micromanipulators built in which can be used to directly perform electrical measurements within the vacuum environment.

## FIB

A Focused-Ion Beam (FIB) device is a microscope which uses ions instead of electrons. Usually, gallium ions are used which are much heavier and more destructive than electrons and, therefore, can also be utilized to mill material away. This process can be envisioned like sandblasting on the nanoscale.

Furthermore, also additive nano-manufacturing can be done by injecting gases that are reduced within the focus to their elementary constituents. Platinum for example is often used for “welding” TEM lamellas to a mount.

Fortunately, we have access to a FEI Helios NanoLab 650 which is a dual-beam FIB. Dual beam means that besides the gallium the device also features an electron beam which impinges the sample under an offset angle of 52°. This is used for monitoring the ion-milling process and crucial for our antenna fabrication process.

## HIM

The last device in our list is a Helium-ion microscope (HIM). Helium ions are much lighter than gallium ions and, hence, less destructive and more precise. So, HIMs are much more suitable for imaging than FIBs and under certain circumstance can even excel against SEMs as it is easier to neutralize charges with them. Furthermore, the precise beam is ideal for fine structuring nano-structures.

We purchased a Zeiss Orion Nanofab which combines a gallium with a helium beam. The gallium is used for coarse structuring while the helium does the monitoring and an later on the fine structuring. In our case this allowed to decrease the feature sizes of our structures from ~20 nm down to ~5 nm.