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\begin{document}

\title{Chip Characterization}
\author{Giulia Casarosa}
\maketitle

The chip matrix has been  characterized in terms of noise, threshold dispersion and gain, the sensor response
has also been tested. 
A photograph of the front-end chip connected by bump-bonding to the high resistivity pixel sensor
matrix of $200\ \micro\meter$ thickness is reported in Fig.~\ref{fig:SPix0}. 
\begin{figure}
\centering
\includegraphics[width=3.0in]{./SuperPix0_bond.png} %
\caption{Photograph of the bump-bonded chip, the sensor matrix and the front-end chip are 
visible as well as the bondings to the carrier.}
\label{fig:SPix0}
\end{figure}

The first lab tests have shown a problem in the readout architecture that has been investigated with dedicated tests and
fully understood. 
A particular data acquisition configuration in the lab allowed us to overcome the problem but the measurements were limited to 
3\% of the pixels of the matrix for each run. Time constraints allowed us to characterize the front-end electronics 
 of around 10-20\% of the pixels in each matrix, depending on the chip.

The absolute gain calibration of the chip matrix has been performed using  an internal calibration circuit, implemented in the pixel, that allows to inject a charge from $0$ to $12\ \femto\coulomb$ in each channel preamplifier.
An average gain of 38\ \milli\volt/\femto\coulomb\ has been measured with a typical dispersion of about 6\% inside the examined
piece of  matrix. %, mainly due to XX.

Noise measurement and evaluation of threshold dispersion have been performed measuring the hit rate as a function of the discriminator threshold. With a fit to the turn-on curve we report a pixel average equivalent noise charge (ENC) of about 77 $e^-$ with 15\% dispersion inside the matrix and a threshold dispersion of about 500\ $e^-$. The latter is a little bit too high and therefore
 a threshold tuning circuit at pixel level is scheduled in the next submission.
In Table~\ref{tab:chip_charact} we report threshold dispersion, ENC and  gain values for each of the 5 chips 
characterized in lab.
 During test-beam the noise and the threshold dispersion have been re-evaluated for more than 98\% of the pixels 
showing compatible results with the lab characterization.

\begin{table}
\begin{center}
\begin{tabular}{c|ccc}
\hline
\hline
chip & thr. disp. ($e^-$) & ENC ($e^-$) & gain (\milli\volt/\femto\coulomb)\\
\hline %characterization in lab
12 & $460 \pm 30$ & $71 \pm 1$ &     37.3   \\
19 & $500 \pm 30$ & $85 \pm 1$ &     38.7   \\
53 & $520 \pm 30$ & $77 \pm 1$ &     38.6   \\
54 & $500 \pm 30$ & $77 \pm 1$ &     39.2   \\
55 & $580 \pm 30$ & $77 \pm 1$ &     36.9   \\
\hline
\hline
\end{tabular}
\caption{Lab characterization of the 5 chips tested during the test-beam.}
\label{tab:chip_charact}
\end{center}
\end{table}

A radioactive source of $^{90}$Sr has been used in order to test the sensor response and the
interconnection between the pixel electronics and the sensor. 
\begin{figure}[t!]
\centering
\includegraphics[width=3.2in]{./rate_Hz3.png}
\caption{Hit rate ($\hertz$) measured with chip 19 exposed to a $^{90}$Sr source.}
\label{fig:sr90}
\end{figure}
In Fig.~\ref{fig:sr90} we report the hit rate as seen from the sensor matrix. The illumination 
of the matrix is not uniform due to the collimation of the source. The two blank columns
are due to a known problem on the front-end chip. All five chips has shown a very good quality of the
 interconnection at $50\ \micro\meter$ pitch and a responding sensor.
Only $2\times 10^{-4}$ over more than 20\kilo\ channels have shown interconnection problems.



 
\end{document}