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\title{Sensors for the Run IIb CDF Silicon Detector}

\author{N. Bacchetta\address[FNAL]{Fermilab, Batavia, IL 60510, USA},
        G. Bolla\address[Purdue]
            {Purdue University, West Lafayette, IN 47907-2036, USA},
        D. Bortoletto\addressmark[Purdue],
        A. Canepa\addressmark[Purdue],
        B. Flaugher\addressmark[FNAL],
        I. Gorelov\address[UNM]{University of New Mexico, Albuquerque, 
             NM 87131, USA},
        K. Hara\address[Tsukuba]{University of Tsukuba, Tsukuba, 
             Ibaraki 305-8571, Japa}\thanks{Supported by Grant in Aid 
             for Scientific Research (KAKENHI) on Priority Areas, 13047101},
        M. Hoeferkamp\addressmark[UNM],
        S. Kim\addressmark[Tsukuba],
        D. Pellett\address[Davis]{University of California, Davis, 
                 CA 95616, USA},
        S. Seidel\addressmark[UNM],
        Y. Takei\addressmark[Tsukuba] and
        P. Watje\addressmark[UNM]}

\begin{abstract}

We describe the characteristics of Hamamatsu silicon sensors prototyped 
for CDF Run IIb silicon detector. In the prototype program, 116 
micro-strip sensors have been fabricated.  Five sensors
were irradiated with neutrons up to 1.4 $\times$ 10$^{14}$ cm$^{-2}$. 
The sensors have shown superior electrical 
characteristics in all the aspects, fulfilling our specifications.
\vspace{1pc}
\end{abstract}

%\begin{keyword}
%silicon microstrip; radiation damage; CDF; Tevatron Run IIb
%\end{keyword}
%\end{frontmatter}
\maketitle

\section{Introduction}

The CDF collaboration is building a new silicon tracker system for 
Tevatron Collider Run IIb, replacing the present L00 and SVXII detectors. 
The new silicon system will be a six-layer device located between 2.1 cm 
and 16 cm radius, consisting of ~2300 single sided silicon microstrip
sensors\cite{Run2b}.    

The main specifications are; (1) Sensors should be operational up to 500V,
(2) Full depletion voltage should be in the range from 120 to 250V;
(3) Dead channel fraction should be less than 1\%;
(4) Sensors should be uniform in coupling capacitance ($>$12pF/cm); 
bias resistance ($1.5\pm0.5 M\Omega$), interstrip resistance
($<1.2$pF/cm), and other electrical properties.
We have employed single sided $p^+$-on-$n$ sensors with dimensions of
96.4 mm length and 40.6 mm width for axial and 41.1 mm width for
1.2$^\circ$ stereo sensors, so that two sensors can be taken from a 6''
wafer. The readout pitch is 75 (80) $\mu$m for axial (stereo) sensors.
Single intermediate strip is implemented to improve the position
resolution.

Prototypes of 63 axial and 53 stereo types were fabricated by 
Hamamatsu Photonics (HPK). 
We have performed detailed electrical characterization of 18 sensors for
each type along with other more general tests such as I-V and C-V. 
The innermost sensor
will receive radiation corresponding to 1.4 $\times$ 10$^{14}$ 1-MeV eq
neutrons/cm$^{2}$ in 30 fb$^{-1}$ of data taking period. We have
irradiated five sensors up to this fluence.
The performance results of irradiated sensors are also reported.

\section{Initial electrical characteristics}

The I-V measurement results are summarized in Fig.\ref{IV}. The leakage
currents are normalized to 20$^\circ$C. Most of the sensors do not exceed
0.2 $\mu A$ even at 950~V. Only two sensors out of 116 showed noticeable
micro-discharge.

Scanning the individual strip current for the two leaky sensors, 
we found only one strip for each sensor is
contributing to the large leakage.
The micro-discharge onset
voltage showed to shift higher as we repeat the I-V measurements.

%%The depletion voltages estimated from C-V curves distributed in a narrow
%%region of between 115 to 142~V. Three sensors were slightly below the
%%lower limit specification of 120~V.

HPK reported in total 21 defective strips for 14 sensors out of
116. Our full characterization for
36 sensors, including these 14 sensors, recognized all the defects
reported by HPK. In addition, we found 5 implant opens and a pair of low bias
resistors. 
The new 5 implant opens are due to that HPK testing is not sensitive to
detect them.
The low bias resistor pair is obviously due to a discharge caused
probably at probing. 
%Actually we found 13 new oxide punch-throughs for axial sensors. It 
%turned out that HPK tested punch-throughs at a lower voltage of 100~V
%than anticipated 120~V; for stereo sensors, we found no punch-throughs 
%since HPK testing was performed as anticipated.
The estimated dead channel fraction is 0.08\% 
including these defects altogether.

\vspace{-5mm}
\begin{figure}[h]
\begin{center}
\includegraphics[width=65mm]{IVcurve_mono.eps}
\vspace{-3mm}
\caption{Leakage current distributions of 116 prototype sensors at 500
 and 950V. The overflows are histogrammed on the extreme right. The
 inset is the I-V curves of all the sensors.}
\label{IV}
\end{center}
\end{figure}
\vspace{-5mm}
\section{Electrical characteristics of irradiated sensors}
The irradiation took place at MNRC Irradiation Facility at UC Davis.
Three sensors were irradiated to 1.4$\times$ 10$^{14}$ /cm$^2$ and two to
0.7$\times$ 10$^{14}$ /cm$^2$. 

The damage constant evaluated from five sensors is 
(2--3)$\times$10$^{-14}$ A/cm, being consistent with previously known
value\cite{Rose}. 
The I-V curves are good to 1000~V without showing significant
micro-discharge. 

The full depletion voltage was evaluated when the
initial beneficial annealing is almost completed. 
It resulted to 100~V (40--50~V) for the sensors
irradiated to 1.4 (0.7)$\times$ 10$^{14}$ /cm$^2$, whereas 90~V (40~V)
is an expectation from Rose parameterization\cite{Rose} . 

The capacitance between two neighboring readout strips is plotted in
Fig.\ref{cint}. The data is for the two sensors irradiated to 1.4
$\times$ 10$^{14}$ /cm$^2$. The curves show shoulders around 130~V
corresponding to the full depletion voltages (note: these were measured
at annealing conditions different to the above), then decrease
gradually with the bias voltage. Though the asymptotic value is
consistent with non-irradiated sensors, an excess bias of 200--250~V is 
required to reach the minimum.   
The interstrip resistance of $>$1 G$\Omega$ is achieved for a bias of 300~V.


\vspace{-5mm}
\begin{figure}[h]
\begin{center}
\includegraphics[width=65mm]{cint_mono.eps}
\vspace{-3mm}
\caption{Interstrip capacitance of two irradiated sensors: one pair and
 two pairs for each.}
\label{cint}
\end{center}
\end{figure}

\vspace{-5mm}
\section{Summary}

We have evaluated the performance of prototype sensors for CDF
Run IIb silicon detector. The electrical characteristics are superior in
every aspects. 
The irradiated sensors also performed as expected. 
An excess bias of 200--250~V is required to recover the electrical
performance.

\begin{thebibliography}{9}
\bibitem{Run2b}B. Flaugher (CDF Run IIb Silicon Group), These proceedings.
\bibitem{Rose} G. Lindstrom \it{et. al.} (Rose Collaboration),
	Nucl. Instrum. and Methods A466 (2001) 308.
\end{thebibliography}

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