At CERN, a prototype triple GEM detector (Large GEM, abbreviated LG) was built\footnote{Design by S. D. Pinto, CERN, on behalf of RD51 group.} with an active area of about $2000cm^2$ using $66 \cdot 66cm^2$ kapton copper-clad foils \cite{pinto-lg}. Two innovative techniques were used to manufacture this detector: \textit{single-mask} etching and \textit{GEM foils splicing}.\\

GEM foils are manufactured through the same photolitographic processes used to produce common printed circuit boards. Tipically, small GEMs are etched on both sides to obtain symmetric holes. However, it can very uncomfortable to align masks when the GEM foil dimensions grow. Due to the very large area of this prototype, it was necessary to renew a single-mask etching technique. As shown in \figurename~\vref{LGmasks}, after the top copper layer is etched, a basic mixture containing potassium hydroxide (KOH, which etches isotropically) and ethylene diamine (C\subscript{2}H\subscript{4}(NH\subscript{2})\subscript{2}, which etches anisotropically) was used to make the holes in the polymide material (Kapton). An anisotropic etcher was needed to keep the holes aspect ratio large, defined as $\dfrac{depth}{width}$.\\

\begin{figure}%[!htbp]
	\centering
	\includegraphics[width=13.5cm]{Images/masks-BW}
	\caption{Comparison of the standard \textit{double-mask} and the new \textit{single-mask} GEM etching procedures \cite{pinto-lg}}
	\label{LGmasks}
\end{figure}
The bottom copper layer was then etched on both sides, using the holes in the kapton as masks, dipping the whole foil in an acidic etchant mixture in order to finalize the holes and to reduce the thickness of the electrodes.\\

When the detector was built, suppliers of the GEM foils base material could only offer $50 \div 70cm$ wide rolls. A $66 x 66cm^2$ foil was cut crosswise, and the two halves were spliced together by covering the junction with a $25\mu m$ Kapton adhesive substrate. The glue polymerized by baking. \\

Rate capability tests demonstrated that the chamber performance remains unaffected except for the $2mm$ wide seam zone \cite{pinto-cmsw}. In addition, a $3.5mm$ wide plastic spacer was placed, in the triple GEM detector we tested, exactly over the foils junction areas, covering them totally. This made it impossible to recognize the loss of efficiency due to the seam.\\
\begin{figure}%[htbp]
	\centering
	\subfigure[View of the prototype]
	{\includegraphics[width=6.6cm]{Images/lglayout}\label{layout1}}
	\hspace{2mm}
	\subfigure[Layout of the GEM foils sectors]
	{\includegraphics[width=6.5cm]{Images/lglayout2}\label{layout2}}
	\label{lg-layout}
	\caption{A photo and a scheme of the large area triple GEM prototype}
\end{figure}
\begin{figure}%[htbp]
	\centering
%	\vfill
%	\subfigure[View of the prototype]
%	{\includegraphics[width=6.6cm]{Images/lglayout}\label{layout1}}
%	\hspace{2mm}
%	\subfigure[Layout of the GEM foils sectors]
%	{\includegraphics[width=6.5cm]{Images/lglayout2}\label{layout2}}
	\subfigure[Divider board]
	{\includegraphics[width=6.6cm]{Images/divider_board}\label{lgdiv}}
	\hspace{2mm}
	\subfigure[Readout plane, made of pads]
	{\includegraphics[width=6.6cm]{Images/LG-Pads}\label{lgpads}}
	\label{divpads}
	\caption{A view of the compact divider board, and a sketch of the pad-based readout showing the beam-tested regions}
\end{figure}

Foreseeing the possibility of discharges, the cathode electrodes are segmented and connected to the power supply via $10M\Omega$ resistors in order to reduce their energy storage. A compact divider board supplies high voltage, in order to make it easier to debug the circuit if needed. Since the board was not designed to handle such high voltages, groups of pins were connected together and alternated with groups of floating strips (see \figurename~\vref{lgdiv}).\\

The readout configuration consists of 1024 pads, with a surface of $0.25cm^2$ (in the narrowest part of the detector) to $6cm^2$. \figurename~\vref{lgpads} shows the different pad dimensions; the image also evidences the zones of the chamber where we directed the beam during the tests.\\

Small hybrid printed circuit boards bound to the chips themselves connect the readout VFAT2s to the detector, on its outer round edge.