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\begin{document}

\begin{frontmatter}

% Title, authors and addresses

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\title{The ALICE Silicon Drift Detectors: production and assembly}
%\title{Production and assembly of the ALICE Silicon Drift Detectors\thanksref{label1}}
%\thanks[label1]{}
\author[label1]{S.Beol\`e\corauthref{cor1}},
%\author[label2]{E.Crescio\thanksref{label2}}
%\author[label3]{P.Giubellino\thanksref{label3}}
\corauth[cor1]{Corresponding author}
%\author{Name\corauthref{cor1}\thanksref{label2}}
\ead{beole@to.infn.it}
\ead[url]{http://www.ph.unito.it/$\sim$beole/}
\author[tor]{B.Alessandro},
\author[bol]{S.Antinori},
\author[tor]{S.Coli},
\author[tor]{E.Crescio},
\author[bol]{D.Falchieri},
\author[rom]{R.Arteche Diaz\thanksref{rom1}},
\author[rom]{S.Di Liberto},
\author[bol]{A.Gabrielli},
\author[tor]{G.Giraudo},
\author[tor]{P.Giubellino},
\author[tor]{G.Masetti},
\author[tor]{G.Mazza},
\author[rom]{F.Meddi},
\author[tor]{F.Prino},
\author[tri]{A.Rashevsky},
\author[tor]{L.Riccati},
\author[tor]{A.Rivetti},
\author[label1]{S.Senyukov},
\author[tor]{L.Simonetti},
\author[tor]{L.Toscano},
\author[tor]{F.Tosello},
\author[rom]{G.M.Urciuoli},
\author[tri]{A.Vacchi},
\author[tor]{R.Wheadon}
\address[label1]{ Universit\`a di Torino/INFN, Torino, Italy}
\address[tor]{ INFN, Torino, Italy}
\address[tri]{INFN, Trieste, Italy}
\address[rom]{Universit\`a di Roma "La Sapienza"/INFN, Roma, Italy.}
\address[bol]{Universit\`a di Bologna/INFN, Bologna, Italy.}
\thanks[rom1]{on leave of absence from CEADEN, Havana, Cuba}
%\thanksref{rom1}{on leave of absence from CEADEN, Havana, Cuba}
%\author{Name\corauthref{cor1}\thanksref{label2}}
%\ead{beole@to.infn.it}
%\ead[url]{http://www.ph.unito.it/%7Ebeole/}
%\thanks[label2]{Universit\`a di Torino/INFN, Torino, Italy.}
%\address[label1]{ Universit\`a di Torino/INFN, Torino, Ita
%\address[label2]{ INFN, Torino, Italy}
%\address[label3]{INFN, Trieste, Italy}
%\address[label4]{Universit\`a di Roma/INFN, Roma, Italy.}
%\thanksref{label3}}
%\thanks[label3]{}
%\title{Pippo}
% use optional labels to link authors explicitly to addresses:
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%\author{}
%\address{}
\begin{abstract}
Silicon Drift Detectors (SDDs) have been selected to equip the two intermediate 
layers of the Inner Tracking System (ITS), of the ALICE experiment, 
since they couple a very good multi-track capability with 
d$E$/d$x$ information and excellent spatial resolution as described in references . 
In this paper we describe the different components of the SDD system as well as the 
procedures of the system assembly.
\end{abstract}
\begin{keyword}
% keywords here, in the form: keyword \sep keyword
Silicon Drift Detectors \sep Microcables \sep TAB bonding
% PACS codes here, in the form: \PACS code \sep code
\PACS 25.75.-q \sep 29.40.Gx \sep 29.40.Wk
\end{keyword}
\end{frontmatter}
%\cite{TDR}  \cite{gatti,dsi1,dsi2}
% main text
\section{Introduction}
The ALICE ITS, as described in detail in \cite{TDR,dsi1}, consists of 6 layers of silicon detectors.
Because of the high particle density,  up to 90~cm$^{-2}$, the four 
innermost layers ($r\leq 24$~cm) must be truly two-dimensional devices. 
For this task silicon pixel and silicon drift detectors were chosen. The 
outer two layers at $r\approx45$~cm, where the track densities are below
1~cm$^{-2}$, will be equipped with double-sided silicon microstrip 
detectors.  
The ALICE SDDs, $7.0 \times 7.53$~cm$^{2}$ active area each, are mounted on 
linear structures called ladders, each holding 
six detectors for layer 3, and eight detectors for layer 4. 
The layers are composed of 14 and 22~ladders and sit at the average radius of 14.9 and 23.8~cm 
respectively.
The front-end electronics is mounted on rigid heat-exchanging 
hybrids, clipped onto cooling pipes running 
along the ladder structure. 
The large number of channels in the layers of the ITS requires a 
large number of connections from the
front-end electronics to the detector and to the readout.
The requirement for a minimum of material within the acceptance does
not allow the use of conventional copper cables near the active
surfaces of the detection system.  Therefore TAB bonded aluminium multilayer
microcables are used. This 
type of cables are produced by the Scientific Research Technological 
Institute 
of Instrument Making Microelectronic Department, Ukraine, and are used both for
signal and power supply lines. A detailed description can be found in \cite{strip}. 
The same technology is used to connect the detector and its read-out hybrids to
the interface boards placed at the two ends of the ladder.
\section{The SDD module}
\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/modulo_1.jpg} &
\includegraphics[width=6cm]{figure/modulo_1_bw.jpg} 
%{\epsfig{figure=figure/schema_modulo.eps,width=6cm}} &
%\subfigure
%{\epsfig{figure=figure/det_HV_1.eps,width=6cm}}
%\includegraphics[width=6cm]{figure/det_HV_1.jpg}
%\includegraphics[width=6cm]{figure/det_HV_1_bw.jpg}
% \end{tabular}
%\caption{(a) SDD module scheme;(b)Detector equipped with the wrap-around (on the left) and transition microcable.}
\caption{(a) SDD module scheme.}
    \label{fig:det}
  \end{center}
\end{figure}
A SDD module (see figure \ref{fig:det}) consists of one silicon drift detector, 
two front-end hybrids each connected to the corresponding end-ladder LV
boards and a microcable specially designed to carry high voltage (up to 2.4kV) which
connects the detector to the HV end-ladder board. 
All the assembly steps are performed in the Technology Laboratory of INFN Torino.
The final
assembly is visible in figure \ref{fig:mod}.
\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
\includegraphics[width=6cm]{figure/HVcable_bw.jpg} 
\caption{Picture of a complete SDD module.}
\label{fig:mod}
\end{center}
\end{figure}
\subsection {Sensor}
The detectors are produced on 5-in. diameter neutron transmutation doped (NTD)
silicon wafers with resistivity of 3 K$\Omega$ cm and a thickness of 300 $\mu$m.
The detector has a bidirectional structure, where electrons drift from the
central $p^+$ cathode towards two linear arrays of 256 anodes (anode pitch 256
$\mu$m). The length of the sensitive area is 70.0 mm, and the sensitive-to
total area ratio is 83\%. 
The detailed description can be found in \cite{sasha01}.
The detector bias voltage is provided by specially  designed microcables. The
connections to the central bias cathode and to the injectors lines is provided
by microcable called {\it transition cable}, 
glued on the detector
 p-side. The bias lines are then bonded to the corresponding 
bonding pads. The high voltage is then brought to the n-side using the so
called {\it wrap-around cable}. The transition cable is then TAB bonded to the
long HV cable connected to the corresponding HV end-ladder board. 
Each sensor was carefully tested in INFN Trieste and after 
being accepted for modules production it was equipped with {\it transition } and {\it wrap-around
cables} and sent to Torino for the module assembly.
\subsection{The front-end electronics}
The front-end electronics, fully developed in the VLSI Laboratory of INFN Torino, 
consist of two integrated circuits 
both designed in the 0.25 micron CMOS technology. The 
first one, named PASCAL, performs the preamplification of the signals, 
their analog 
storage at a sampling frequency of about 40~MHz, 
and the analog-to-digital conversion. 
The second integrated circuit, AMBRA, is 
a digital four-event 
buffer which allows data derandomization and handles the communication protocol.
A detailed description of the SDD front-end system can be found in \cite{rivetti05}.
The production of 48
8-inch wafers holding about 150 potentially good pairs of PASCAL and AMBRA
finished in 2004. The validation tests started in the CL100 CleanRoom of INFN Rome, using a
Cascade Rel-6100 Probe station, in February 2005.
The yield for AMBRA was 74\%, while for PASCAL was 89 \%. 
At the end of November 2006, $\sim$3000 AMBRA and $\sim$3000 PASCAL were available for assembly. 
Each SDD is coupled to 2 hybrids each consisting of 4 couples of PASCAL and AMBRA chips.
Each couple of chips is TAB bonded to a circuit printed on a aluminium multilayer microcable 
called {\it chip-cable}. 
This circuit provides connections of data lines between AMBRA and PASCAl as well as the connections 
to the sub-hybrid circuit. The bondings on the chip-cables are protected by a thin layer
of glue, a thermally-conductive two-component alumina-filled epoxy (adhesive H70E-2),
dispensed automatically with a Champion 3700 glue dispensing machine.
The production yield of chip cables at the beginning of the production phase was very high both before 
and after the encapsulation process ($\sim$90\%). Unfortunately in spring 2006 
some problems arised  and
since then the production yield went down to $\sim$70\%. 
The reason of the high number of bad chip-cables (maily due to Pascal 
problems) is still not well understood, and it can be found in the TAB-bonding phase as
well as in the thinning and
dicing of the wafers. 
At the end of November 2006 we had bonded more than 3000 chip-cables instead of the foreseen 2400. 
After the encapsulation process the chip-cables are glued on a sub-hybrid circuit. 
The relative alignment of the 4 chip-cables needed
for each hybrid is obtained using the SDD anodes array as a reference. 
The sub-hybrid is glued onto a rigid carbon-fiber heat-dissipator called {\it heat-bridge}. 
This will be clipped to the 
cooling tubes running along both sides of a ladder.
Due to the large number of bonds on the hybrids it is very important to
avoid that the inevitable occasional weak bonds lead to failure of the
hybrid after only a few operational cycles. For this purpose, and
following the experience of the ALICE SSD development \cite{strip}, a thermal
cycling test has been introduced in the production sequence. Sample
hybrids have been demonstrated to withstand 200 thermal cycles from 20
to 65 degrees C (the likely extremes that the hybrids can be expected to
experience), and survive 10 cycles up to 100 degrees C despite
the mismatch in the thermal expansion coefficients between the different
layers of the hybrid. The hybrids are therefore exposed to ten thermal
cycles from 20 to 65 degrees C. 
\subsection{Modules production and tests}
At the end of November 2006 300 modules have been assembled and tested. 
The assembly procedure is composed by more than 20 steps.
Each TAB bonding phase is followed by encapsulation of bonds 
by means of different kinds of glue. After each bonding step and before glue protection
an electrical test is performed to ensure the validity of electrical connection
and exclude any damaging of the components. 
The complexity of the assembly and test procedures leads to an average time of 3 days 
to have the complete module ready for final test with laser.
This test was developed to get a complete map for doping fluctuations for each detector. 
The non uniformity of doping concentration leads to non uniformity of drift field i.e. 
to non-linear 
trajectory of the electron cloud as well as variations of drift speed as explained in \cite{dsi1}.
This problem has been studied carefully in the beam tests and the results published in 
\cite{crescio}. 
\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
\includegraphics[width=6cm]{figure/mappa_2_bw.jpg} 
\caption{Maps of residuals in a detector with doping fluctuations: please note the
typical shape of dopant distribution}
\label{fig:betta}
\end{center}
\end{figure}
The test set up uses a laser source to inject charge in a known position on the detector and
extract the map of residuals both along the drift and the anode directions.
Only 4 modules turned out to have residual values up to 400 microns, similar to those observed in the past, 
in the beam-test data; in picture \ref{fig:betta} the typical map of residual for this kind of detector is shown.
The laser test set-up constituted also
a complete functioning test of the modules. It was used to detect any kind
of problem both in the front-end electronics and in the sensor,
as well as to detect noisy and dead channels.
\section{Assembly of ladders}
Each SDD module is handled using a special box equipped with vacuum holders.
In fact the different elements of each module (sensor, hybrids, end ladder boards) 
are connected one to the other by microcables, resulting in a completely flexible structure. 
Thus each element needs
to be firmely held while handling to avoid mechanical stress on TAB bonding connections.
The potential weakness of this connections, even if protected by glue deposition,
 is one of the critical points of the SDD system.
A special mechanical tool has been developed to position and align SDD modules, 
as visible in figure \ref{fig:silvia1}. 


\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/tool_ladder.jpg} &
\includegraphics[width=6cm]{figure/tool_ladder_bw.jpg} 
%\subfigure
% \includegraphics[width=6cm]{figure/tool_det.jpg}
%\includegraphics[width=6cm]{figure/tool_det_bw.jpg}
%\end{tabular}
\caption{The ladder assembly jig: the 8 support for sensors and the hybrid folding tool}
\label{fig:silvia1}
\end{center}
\end{figure}
It consists of 8 supports for the sensors that can be moved in the z coordinate to allow the positioning 
of overlapping detectors.
The SDDs are positioned so that the
electrons drift orthogonally to the beam axis. The anodes  therefore are
aligned along the ladder length.  The modules are mounted at different
distances from the ladder structure both in the $R\varphi$ and in the $Rz$
planes. This is in order to allow the overlap of the guard regions and a small overlap of the active areas. 
Each module is lifted by a tool equipped with vacuum cups which can be moved along the structure and 
places it on the corresponding base, visible in figure \ref{fig:silvia2}.
n the mean time the end-ladder boards are placed on dedicated supports. 
\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/tool_ladder.jpg} &
%\includegraphics[width=6cm]{figure/tool_ladder_bw.jpg} &
%\subfigure
% \includegraphics[width=6cm]{figure/tool_det.jpg}
\includegraphics[width=6cm]{figure/tool_det_bw.jpg}
%\end{tabular}
\caption{The tool for module positioning}
\label{fig:silvia2}
\end{center}
\end{figure}
After being placed on its base each SDD must be aligned. 
The alignement is performed using the Mitutoyo measuring machine in 
INFN Torino Technological Laboratory. In figure \ref{fig:camera} the video-camera used in the alignement procedure is
shown. It is used to measure the position of one of the reference crosses on the detector with respect to
the main reference point, a ruby sphere placed on the ladder foot.
\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_01.jpg}&
\includegraphics[width=3.5cm]{figure/allinea_zoom_bw.jpg}
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_04.jpg} 
%\includegraphics[width=6cm]{figure/ladder3_bw.jpg} 
%\end{tabular}
\caption{The Mytutoyo video camera while measuring reference crosses on a SDD}
\label{fig:camera}
\end{center}
\end{figure}

The ruby sphere is also used for ladder positioning on the ITS support structure.
The positioning and alignement operation must be repeated 6 times for each ladder for ITS layer 3, and 8 times
for each ladder for ITS layer 4.
In figure \ref{fig:ladder_al} we show six aligned modules leaning on the corresponding bases and the 
ladder already placed above them.

\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_01.jpg}&
%\includegraphics[width=3.5cm]{figure/allinea_zoom_bw.jpg}&
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_04.jpg} 
\includegraphics[width=6cm]{figure/ladder3_bw.jpg} 
%\end{tabular}
\caption{The SDD ladder placed on the 6 aligned module (ITS layer 3)}
\label{fig:ladder_al}
\end{center}
\end{figure}

Once the ladder is placed on the modules 4 small pillars are glued to each SDD. 
In the last phase we fold the hybrids and the LV micro-cables, then clip of the heat-bridges to the cooling tubes 
and the place the end-ladder boards, 1 HV and 2 LV for each module, in their final position (see figure \ref{fig:end_ladder}
).
\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_01.jpg}&
\includegraphics[width=6cm]{figure/end_ladder.jpg}
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_04.jpg} 
%\includegraphics[width=6cm]{figure/ladder_cones.jpg} 
%\end{tabular}
\caption{ End-ladder boards}
\label{fig:end_ladder}
\end{center}
\end{figure}

The ladder production rate reached a maximum of 1 ladder per day, with an average value of 3-4 ladders per week.
Due to different problems (one module broken during alignement, 
some modules with TAB-bonds on the LV end-ladder boards broken during folding of microcables) 
we had to repair 6 ladders (4 of layer 3 and 2 of layer 4) replacing damaged modules with good ones. 
At the end of November 2006 all the 14 ladders of layer 3 and 18 of layer 4 were ready 
for being mounted on the cones.
In figure \ref{fig:ladder_cones} you can see five ladders of layer 3 mounted on the cones (for a brief description of the
carbon fiber support structures see \cite{beole05}) and ready for tests.


\begin{figure}[htbp]
\begin{center}
\vglue0.2cm
%\begin{tabular}{cc}
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_01.jpg}&
%\includegraphics[width=6cm]{figure/end_ladder.jpg}&
%\subfigure
%\includegraphics[width=6cm]{figure/4mod_04.jpg} 
\includegraphics[width=6cm]{figure/ladder_cones.jpg} 
%\end{tabular}
\caption{Five SDD ladders of layer 3 mounted on the carbon-fiber support structure}
\label{fig:ladder_cones}
\end{center}
\end{figure}


%\label{}
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% appendix sections are then done as normal sections
% \appendix

% \section{}
% \label{}
\begin{thebibliography}{00}
% \bibitem{label}
% Text of bibliographic item
\bibitem{TDR} ALICE Collaboration, Technical Design Report, CERN/LHCC 99-12.
%\\ http://alice.web.cern.ch/Alice/documents.html.
%\bibitem{gatti} E. Gatti and P. Rehak, Nucl. Instr. and Meth. A225 (1984) 608.
\bibitem{dsi1} S. Beol\'e et al., Nucl. Instr. and Meth. A377 (1996) 393.
%\bibitem{dsi2} S. Beol\'e et al., Il Nuovo Cimento Vol. 109A, N. 9 (1996).
\bibitem{strip}
A.P.de Haas et al., 
%Aluminium microcable technology for the Alice silicon strip detector : a status report, 
Proc. of 8th Workshop on Electronics
for LHC Experiments, Colmar, September 2002
\bibitem{sasha01}
A. Rashevsky et al., 
%Characteristics of the ALICE Silicon Drift Detector, 
Nucl. Instr. and Meth. A461 (2001) 133
\bibitem{rivetti05}
A.Rivetti et al. 
%The front-end system of the silicon drift detectors of ALICE, 
Nucl. Instr. and Meth. A 541 (2005) 267-273.
\bibitem{crescio}
E.Crescio et al., 
%Results from beam tests of large area silicon drift detectors.
Nucl. Instr. and Meth. A 539 (2005) 250-261.
\bibitem{beole05}
S.Beol\'e et al, 
%Production and assembly of the ALICE Silicon Drift Detectors, 
Nucl. Instr. and Meth. A 570 (2007) 236-240.
% notes:
% \bibitem{label} \note

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\end{document}


