chap02.tex

% !TeX spellcheck = <en-US>
%\chapter{Mask Aligner}

\section{Electron beam evaporation}
Electron beam evaporation, also known as \textbf{E}lectron-\textbf{b}eam \textbf{P}hysical \textbf{V}apor \textbf{D}eposition (EBPVD) is a \textbf{P}hysical \textbf{V}apor \textbf{D}eposition (PVD) technique that in \textbf{U}ltra \textbf{H}igh \textbf{V}acuum (UHV) deposits material onto a substrates surface.

\begin{figure}[H]
	\centering
	\includegraphics[width=0.5\linewidth]{img/EBeamDep.pdf}
	\caption{Schematic of a general electron beam evaporation chamber. The B-field is used to focus the beam onto the source. The shutter can interrupt the beam directed to the sample. The funnel is used to focus the vapor beam. }
	\label{fig:e-beam_evap}
\end{figure}

The setup of an electron beam evaporator is shown in Figure \ref{fig:e-beam_evap}. The source material is placed inside a tungsten crucible as pellets of ultrapure ($>99$ \%) material.
To heat the source material, it is bombarded with a high-voltage electron beam ($\mathcal{O}$($1$~kV)), which is emitted by either an electron gun or a filament. This beam usually is focused using magnetic fields to hit the source material. Energy transfer heats the hit atoms and eventually leads to the evaporation according to its vapor pressure.\\

%The crucible is also heated during the evaporation process, in order to prevent it from being damaged, a material with a high melting point is chosen. Tungsten with a melting point of 3695 K ~\cite{Tungsten_melt} is usually chosen. 
%Additionally, the crucible usually has to be water cooled to avoid outgassing during the evaporation process.
%The penetration depth of electron with ($<5$ kV) is less than 0.4 {\textmu}m (estimated using CASINO Monte Carlo software)~\cite{CASINO} so the heating occurs only very near to the source material's surface. This allows for less energy loss and more controlled evaporation as the crucible and the rest of the system is not heated by the electron beam directly, but only by the radiant heat emitted by the source material.\\

When the material's vapor pressure exceeds the surrounding environments pressure, a vapor forms. The sample is kept at a temperature much colder than the source material's temperature, due to this the material beam will condense on the substrate's surface forming a thin film. To regulate the deposition process, a shutter is employed, allowing for controlled release of the material. \\

In order to ensure the material beam reaches the sample in a direct path, the mean free path (MFP) of a traveling particle has to be larger than the distance to the sample's surface. For this reason, high vacuum (HV) (MFP of $10$ cm to $1$ km) or ultra-high vacuum conditions (UHV) (MFP of $1$ km to $10^5$ km) are needed.

The deposition rate of the evaporator can be measured using a molecular flux monitor. The deposition rate of a material is described by the Hertz-Knudsen equation:

\begin{equation}
	\frac{dN}{A dt} = \frac{\alpha (p_\text{e} - p)}{\sqrt{2 \pi m k_\text{B} T}}
	\label{eq:hertz_knudsen}
\end{equation}

Where $N$ is the number of gas molecules deposited, $A$ is the surface area, $t$ is time, $\alpha$ is the sticking coefficient, $p$ is the gas pressure of the impinging gas, $m$ is the mass of a single particle, $k_\text{B}$ is the Boltzmann constant, $p_\text{e}$ is the vapor pressure of the material at the sample temperature and $T$ is the temperature~\cite{knudsen}. The sticking parameter of a material can be looked up in literature. With this, the total deposition rate can then be estimated. In practice since the pressure of the impinging gas is difficult to determine this is difficult to estimate, since it requires precise knowledge on the temperature of the source, that is typically not measured. Instead, usually calibration evaporations are performed for different heating powers and different times to determine the deposition rate for a given setup.

Comparing e-beam evaporation with so-called sputtering of material onto a surface, it offers more controlled deposition~\cite{Vapor_depo_princ}. In sputtering, high energy particles are produced hitting the sample, which can lead to local roughening~\cite{sputter_damage}.
In contrast to thermal evaporation, where the source is typically heated by Joule heating from a resistive current, higher temperatures are available with e-beam evaporation. This is required, e.g.\ for \ce{Nb}~\cite{tungsten_evaporation}.

\section{Molecular beam evaporation chamber}
\begin{figure}[H]
    \centering
    \includegraphics[width=0.9\linewidth]{img/MaskAlignerChamber.pdf}
    \caption{Circuit diagram of the mask aligner and its associated vacuum
system. It consists of the mask aligner (MA) chamber, the main chamber, the
Pb evaporator and the \ce{Au} evaporator. The \ce{Au} evaporator is attached to the same vacuum system, but is unrelated to the Mask Aligner. The configuration depicted is used for
evaporation. The \textcolor{tab_green}{green} line shows the sample/mask extraction
and insertion path with the wobble stick. The black arrow shows the molecular beam
path from the \ce{Pb} evaporator. BA stands for Bayard-Alpert pressure gauge. This diagram is accurate for the setup on 01.08.24.}
    \label{fig:mask_aligner_chamber}
\end{figure}

The Mask Aligner vacuum system (Figure \ref{fig:mask_aligner_chamber}) consists of two areas that can be separated with vacuum gate valves~\cite{Mask_Aligner}. The \textbf{M}ain \textbf{C}hamber (MC) with the Mask Aligner (MA) chamber, and the \ce{Pb} evaporator. The second one is the \textbf{L}oad \textbf{L}ock (LL), a vacuum suitcase that is used to insert new samples and masks into the system. The system is pumped to UHV pressures by a turbomolecular pump and a prepump (rotary vane). 
Between prepump and turbo molecular pump is a pressure sensor to determine if the prepump is providing suitable backing pressure. A valve to a nitrogen bottle allows the system to be vented with an inert gas to avoid contamination.\\
The main chamber is equipped with an Ion Getter Pump, such that the Load Lock can be separated from the turbomolecular pump, without loss of UHV conditions. The pumping system is separated via $2$ \textbf{A}ll \textbf{M}etal \textbf{C}orner \textbf{V}alves (AMCV) (Fig. \ref{fig:mask_aligner_chamber}). Additionally, the Load Lock and the main chamber are separated by a Gate Valve. In order to detect leaks or contaminants in the vacuum system, a mass spectrometer is attached to the main chamber. \\
The Load Lock is equipped with a small ion getter pump, that runs on its own, allowing it to keep UHV conditions, even while separated from the main pump loop. A garage with spaces for up to $14$ samples ($10$: $12\times12$ cm$^2$, $4$: Omicron size) is part of the Load lock. Masks occupy $2$ sample slots due to additional height. For insertion and removal of masks and sample into the Mask Aligner, a wobble stick is attached to the Load lock chamber. The path of the wobble stick to the Mask Aligner is marked \textcolor{tab_green}{green} in Figure \ref{fig:mask_aligner_chamber}. \\
Another device, unrelated to this thesis, a gold evaporator, is connected to the vacuum system. It is not further discussed in this thesis. \\

\subsection{Lead evaporator}
The electron beam evaporator used for the lead evaporation in the mask aligner chamber was built by Florian Forster in $2009$~\cite{florian_forster}. It is shown schematically in Figure \ref{fig:ma_evap}. The evaporator uses a filament placed near the crucible to bombard the crucible with electrons. To accomplish this, a high voltage (up to $1$ kV) is applied between filament and crucible to accelerate electrons to the crucible. In addition, the system is heated by radiative heat from the filament current. The resulting heating power is linearly dependent on the voltage applied and quadratic in the current. This heat is used to degas the evaporator and to prevent contaminants from settling on the filament, when no evaporation is taking place. The filament and crucible are surrounded by a copper cylinder, that functions as a heat sink. The heat sink is water cooled to prevent outgassing of the surrounding due to heating by the filament or crucible. To control the temperature of the \ce{Cu} cylinder a thermal sensor is placed on the copper cylinder. \\

\begin{figure}[H]
    \centering
    \includegraphics[width=0.8\linewidth]{img/MA/Evaporator.pdf}
    \caption{Solidworks diagram of the evaporator used on the Mask Aligner.}
    \label{fig:ma_evap}
\end{figure}

In order to control the molecular flux, one can change the current applied to the filament or the voltage accelerating the electrons. Additionally, the crucible can be shifted on the z-axis closer to or further away from the filament. This method of temperature control is the least reliable and was not used in this thesis. In order to determine the flux current of $\text{Pb}^+$ ions leaving the crucible, it is measured by a flux monitor positioned at the top of the evaporator. Above the flux monitor is a shutter which can be used to open the molecular flux to the MA chamber. \\

\begin{figure}[H]
    \centering
	\begin{subfigure}{0.42\textwidth}