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\journal{Nuclear Instruments and Methods A }
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\begin{document}
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\begin{frontmatter}
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\title{Characterization of "n-in-p" pixel sensors for high radiation environments}
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\author{I.~Tsurin}
\ead{ilya.tsurin@liverpool.ac.uk}
\author{A.~Affolder, P.~P.~Allport, G.~Casse, V.~Chmill, T.~Huse, M.~Wormald}
\address{Oliver Lodge Laboratory, University of Liverpool, United Kingdom }
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\begin{abstract}
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This work presents the first held at Liverpool University measurements of
pixel sensors with n-type readout implant in the p-type bulk before and after
irradiation of samples by 24GeV protons to doses 7$\cdot$10$^{15}$ protons/cm$^2$
and 1.5$\cdot$10$^{16}$ protons/cm$^2$. A~comparison is given for two measurement
techniques; one based on the FE-I3 readout chip designed for the ATLAS and the
other using the Beetle chip developed for the LHCb experiments at CERN.
\end{abstract}
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\begin{keyword}
Pixel detector, ATLAS upgrade
%% MSC codes here, in the form: \MSC code \sep code
%% or \MSC[2008] code \sep code (2000 is the default)
\end{keyword}
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\end{frontmatter}
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\linenumbers
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\section{Introduction}
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%--------1---------2---------3---------4---------5---------6---------7---------8
In the future luminosity upgrade of the Large Hadron Collider at CERN
pixel sensors are anticipated to occupy a larger volume to satisfy the need
of increased granularity imposed by the much higher track density. The total
surface covered by pixel sensors would go from present $\sim$1.8~m$^2$ to
10~m$^2$ in the upgraded ATLAS detector. This significant increase in area
can justify the use of cost-effective p-type silicon bulk instead of present
n-type for the n-type readout pixels needed for enhanced radiation tolerance.
Savings up to 50\% come from avoiding double-sided photolithography required
for the backplane of n-type silicon to implant a diode structure there.

The much larger area short strip detectors have already adopted such an
approach as a default with 10x10~cm$^2$ detectors now being delivered. It
has been proven that "n-in-p" (n-type readout implant in the p-type bulk)
segmented silicon sensors offer radiation tolerance similar to the "n-in-n"
ones, fully satisfying the requirements for all pixel layers of the upgraded
experiments. On the other hand, the bias voltage required to operate
heavily-irradiated sensors has to be remarkably high (up to 1000V for the
innermost layers). It must be demonstrated that "n-in-p" silicon pixel
detectors can be biased to this level without electrical breakdown or
discharges through the overlapping readout chip.

Novel technologies resulting from research and development programmes (R\&D)
on radiation-tolerant detectors need planning for being transferred to large
scale applications for optimisation of performance and expenses. Pixellated
detector assemblies (where the electronics is hybridised by bump-bonding to
the sensors themselves) are particularly valuable due to high production costs.
At the system level, engineering of the front-end services, i.e. power feed,
cooling, etc. should provide reliable operation of sensors throughout their
lifetime. Having long traditions in design and construction of the silicon
tracking detectors \cite{VELO, ECAP}, the Oliver Lodge Laboratory of the
Liverpool University can contribute substantially to the ATLAS upgrade.

%--------1---------2---------3---------4---------5---------6---------7---------8
The "n-in-p" planar technology is being evaluated at \mbox{Liverpool}
University as a candidate for the future strip and pixel detectors. It has
already been shown \cite{REF} that after expected dose of 2$\cdot$10$^{16}$
neq/cm$^2$ they still produce signals compatible with 99\% efficient tracking.
Variety of vendors, low manufacturing costs (the DC-coupled pixel sensors
require 3 or 4 mask levels only) and high production yield are evident
advantages of the "n-in-p" process in the entire class of semiconductor
particle detectors. The paper describes the development programme of planar
pixel sensors at Liverpool University in collaboration with the Micron
Semiconductor~Ltd.\footnote{Micron Semiconductor Ltd. 1~Royal Buildings,
Marlborough Road, \mbox{\hspace{5mm}Lancing} Business Park, Lancing
Sussex, BN15 8SJ, United Kingdom}.
%
%
\section{Pixel sensor design}
%
Studies of pixel detectors at Liverpool University have started with the
preparation of a new wafer containing devices reproducing the geometry of
standard ATLAS pixel sensors~\cite{APIX} for a single chip assembly (SCA)
based on the FE-I3 chip~\cite{FEI3}. The SCA detector with its 2800 pixels
has been redesigned to be read out by only two Beetle chips \cite{BEETLE}.
The new sensor has eight interleaved pixels per column connected to one wire
bond pad at the die boundary, Fig.~\ref{fig:APC}. Such a geometry allows for
the cluster size measurements up to 7 pixels with 50~$\mu$m pitch. In the
second version of that detector every other pixel (400 $\mu$m long) in the
row is connected to the same wire bond pad. The readout pixel matrix consists
of 128$\times$16 implants grouped for the column-parallel (APC) or row-parallel
(APR) readout. The second metal was needed for the pixel interconnect.

%--------1---------2---------3---------4---------5---------6---------7---------8
A possibility of wire bonding at room temperature is one major advantage
of these detectors for irradiation and annealing studies. Re-use of their
bonding pads makes it possible to irradiate them without the readout chip.
Furthermore these devices facilitate measurements of the punch-through
voltage, inter-strip resistance and capacitance before and after
\mbox{irradiation.}

APC and APR devices were manufactured in 2009 by Micron Semiconductor Ltd.
on 300~$\mu$m thin 6-inch wafer in the double-metal "n-in-p" process on
float-zone silicon with 13~kOhm$\cdot$cm bulk resistivity and the p-spray
isolation \cite{SPRAY}. The wafer includes also strip detectors with bias
options and pads for testing the high voltage performance of guard structures
and dicing schemes. In addition the RD50
collaboration\footnote{http://rd50.web.cern.ch} has provided prototypes of the
ATLAS pixel sensors (produced by Micron Semiconductor Ltd. on float-zone p-type
silicon with 10~kOhm$\cdot$cm bulk resistivity) which were used for populating
the SCAs.
%
\begin{figure}[ht] 
  \begin{center}
  \resizebox{7.5cm}{!}{\includegraphics{./apc.eps}}
  \caption{Sensor with interleaved readout pixels. Second metal layer
           provides their interconnect and routing to the wire bond pad.
	   Shuffling of readout channels minimises their cross-talk.}
  \label{fig:APC}
  \end{center}
\vspace{-7mm}
\end{figure}
%
\section{Characterisation of sensor layout components}
%--------1---------2---------3---------4---------5---------6---------7---------8
A detailed analysis was needed to characterise the new detector structures
including the matrix of readout implants, high voltage termination (multiple
floating guard rings) and dicing options. Their influence on the readout
electronics (before and after irradiation), their operating limits and
tolerance to layout variations were investigated.

Prior to measurements all sensors underwent several HV cycles to the breakdown
point with the current limitation~at~1~$\mu$A. Most of detectors could stand the
maximum voltage of \mbox{Keithley} 2410 source-measure unit of 1100V. For some
other devices the HV training (burn-in) has helped to improve the breakdown voltage
by almost 30\% up to 900V after which the IV and CV characteristics of pixel sensors
were obtained to check that the reverse bias voltage could be applied safely for
the full bulk depletion. However the HV performance of SCA detectors had degraded
(probably due to excessive thermal and mechanical stress) after their bump bonding
at the Fraunhofer Institute\footnote{Fraunhofer IZM Gustav-Meyer-Allee~25, D-13355
Berlin, Germany}.

\noindent
The silicon sensors, the SCAs and some test structures were irradiated by protons
at IRRAD-1 facility \cite{IRRAD} at CERN to doses 7$\cdot$10$^{15}$ protons/cm$^2$
and 1.5$\cdot$10$^{16}$ protons/cm$^2$.
%
\subsection{Measurements of readout implants}
%--------1---------2---------3---------4---------5---------6---------7---------8
Each pixel features a punch-through biasing circuit that connects the readout
implant to the bias grid through a narrow gap of an accumulation layer. The
build-up voltage between the grid and readout implants, called "punch-through
voltage" U$_{pt}$, was measured using a Keithley 6517A electrometer for gap
lengths ranging from L = 3~$\mu$m to 50~$\mu$m. The voltage across the gap for
unirradiated sensor follows one-to-one the reverse bias voltage applied either
to pixels or to the grid. For the APC irradiated to 7$\cdot$10$^{15}$
protons/cm$^2$ this dependence is about 50 times weaker. After reaching the
value U$_{pt} \approx$~1V/$\mu$m~$\times$~L~[$\mu$m] the punch-through voltage
becomes independent of further increase of the bias voltage for both measurement
samples. The readout electronics connected to the pixel sensor has to cope with
this voltage. To comply with absolute maximum ratings of deep-submicron CMOS
process the potential of the bias grid bump bonded to the FE-I3 chip on the SCA
was controlled to be equal to an average potential of pixels. For measurements
with the Beetle chip the bias grid of the APC was left floating.

Isolation of pixels from the bias grid (gap resistance) was parameterised as an
inverse slope of the IV characteristics with the voltage differential (smaller
than U$_{pt}$) applied between the readout implants and the grid. The number of
joined pixels was increased to eliminate the systematic error and for all pixels
in parallel their gap resistance at room temperature and before irradiation was in
the TOhm range, independent of the bias voltage above 50V. For the APC irradiated
to 7$\cdot$10$^{15}$ protons/cm$^2$ the gap resistance of all pixels in parallel
at -25$^{\circ}$C was 100~kOhm, independent of the bias voltage above 1kV. Similarly,
the inter-strip resistance was measured between two groups of implants (even and odd
columns of the APC). It equals to 200~GOhm/cm for the non-irradiated sensor at room
temperature with the bias voltage above 50V and 10~MOhm/cm at~-25$^{\circ}$C and 1kV
bias for the APC irradiated to 7$\cdot$10$^{15}$ protons/cm$^2$.

The IV and CV scans were made for the entire pixel matrix through its bias grid.
The full depletion voltage amounts to 80V for unirradiated APC detectors as found
from the 1/C$^2$ plot. However this technique cannot be used for irradiated sensors
and therefore one relies on the charge collection measurements. The reverse leakage
current density at 600V amounts to 25~nA/cm$^2$ for the unirradiated APC detector at
room temperature and 60~$\mu$A/cm$^2$ and 100~$\mu$A/cm$^2$ for doses 7$\cdot$10$^{15}$
and 1.5$\cdot$10$^{16}$~protons/cm$^2$ \mbox{at~-25$^{\circ}$C}. The breakdown voltage
of all APC samples exceeds 1100V.
%
\subsection{Studies of the guard structures}
%--------1---------2---------3---------4---------5---------6---------7---------8
The breakdown voltage of a bare implant with 600~$\mu$m distance to the cut
edge is about 150V before irradiation that is well above the full depletion
voltage of 80V. However the ratio between the full depletion and breakdown
voltages becomes worse for irradiated implant in this geometry. The optimum
HV range could be significantly extended by using guard rings. Several diodes
with different guard structures: varying gaps between implants, varying metal
width, reverse orientation of the field plates have been evaluated as shown in
Fig.~\ref{fig:GUARD}.\par
%
\begin{figure}[ht] 
  \begin{center}
  \resizebox{7.5cm}{!}{\includegraphics{./guard.eps}}
  \caption{Four types of evaluated guard structures. Each has 4 dicing options.
           Type 1 is the standard RD50 layout, types 2 and 3 are used for the
	   CMS and \mbox{ATLAS} pixel sensors and type 4 with reverse orientation
	   of metal plates and steps between implants was needed to prove the
	   tolerance of the first three designs. Central implant is 600~$\mu$m
	   away from the cut edge on the left and top.}
  \label{fig:GUARD}
  \end{center}
\end{figure}

\vspace{-3mm}
\noindent
%--------1---------2---------3---------4---------5---------6---------7---------8
All devices had a good HV performance with an average breakdown voltage of 900V.
It is not as good as for the APC and APR detectors due to locating wire bond pads
on the guard rings for test purposes. The new pixel sensors for the ATLAS upgrade
\cite{IBL} have their alignment marks on the guard rings, Fig.~\ref{fig:MARK}. It
has been shown that the HV properties of such detectors do not degrade if the
guard structure remains uniform throughout.
%
\begin{figure}[hb] 
  \begin{center}
  \resizebox{8.8cm}{!}{\includegraphics{./mark.eps}}
  \caption{Probe pads (design 2) and reference marks (designs A, B) on the
           guard structure. Layouts 1 and A are preferable over 2 and B.}
  \label{fig:MARK}
  \end{center}
\end{figure}

The surface current has been measured for all diodes before irradiation by
connecting an amperemeter between any pairs of adjacent guard rings. This
current was the same \mbox{everywhere:} close to the central implant and near
the cut edge and it contributed up to 80\% to the total leakage current before
breakdown. It has been shown \cite{GRPN} that after irradiation the ratio
between the surface and the bulk currents changes and it depends on the
bombarding particle type. In case of protons the surface current can still
be sizeable.
%
\subsection{Evaluation of the dicing schemes}
%--------1---------2---------3---------4---------5---------6---------7---------8
One major role of guard rings in the planar technology is to make the surface
current isotropic and therefore the guard structure has to be on the depleted,
i.e. on the readout side. This reduces the active detector area. To find the
minimum number of guard rings, the pad detectors with all guard types were
stretched to have the cut line going through the 8th, 4th and the 2nd innermost
guard rings. The IV curves were scanned after their dicing before and after
irradiation. The breakdown voltage versus distance from the readout implant to
the cut edge for unirradiated samples has the same slope of 1~V/$\mu$m as for
the punch-through voltage measurements meaning that the breakdown occures when
the bulk depletion in lateral direction reaches the cut edge. Diodes with one
remaining guard ring show a \mbox{10-fold} increase in the breakdown voltage
after doses 7$\cdot$10$^{15}$ and 1.5$\cdot$10$^{16}$~protons/cm$^2$. Sensors
with 3 and 7 rings do not break down under 1100V.
%
%
\section{Charge collection measurements}
%--------1---------2---------3---------4---------5---------6---------7---------8
The charge collection in pixel detectors was measured using a $^{90}$Sr
radioactive source. The SCA with its FE-I3 chip was read out by the ATLAS pixel
"Turbo-DAQ" system\footnote{http://physik2.uni-goettingen.de/~jgrosse/TurboDAQ/}.
The FE-I3 chip utilises the time over threshold (ToT) technique to digitise
the input charge \cite{GROS}.

The cross-calibration of charge collection for the pixel detectors becomes
possible with APC and APR devices using a newly developed ALiBaVa system
\cite{ALI}. The signal source is either $^{90}$Sr or movable infra-red
laser (980 or 1060 nm wavelength) with a variable \mbox{focus} distance,
light intensity and exposure time. The analogue \mbox{ALiBaVa} readout
allows for accurate charge cluster analysis taking into account small
amplitudes that are not available in the ToT technique due to its readout
threshold.

The pixel efficiency and spatial resolution measurements require track
information from the particle telescope. Irradiated samples have been
tested with 24 GeV protons at CERN in October 2010. The SCAs were readout
by the "Turbo-DAQ" integrated into EUDET tracking system~\cite{EUDET}.
The beamtest data analysis is ongoing.
%
\subsection{Measurement system}
%--------1---------2---------3---------4---------5---------6---------7---------8
Two dedicated test cards were designed for the SCA and APC detectors. The cards
feature low-dropout voltage regulators to protect against the noise the FE-I3
and Beetle analogue supplies fed through the long mixed-signal cables.
A collimated scintillation trigger mounted underneath each test card provided
small variation in the incidence angles and reasonably high energies of beta
particles crossing the silicon.

To avoid thermal runaway of heavily irradiated sensors at high bias voltages
and moderate cooling temperatures, each sensor is being cooled from its
backplane by a heat sink with a soft thermal interface. The heat sink and the
test card have holes to let through the particles from the radioactive source
to avoid energy loss in front of the SCA and APC sensors.

Each test card is placed inside the electromagnetic shielding box in a freezer
whose temperature is controlled down to -25$^{\circ}$C with 2$^{\circ}$C accuracy
using a K-type thermocouple attached to a dummy heat sink. Two blowers provide
cold air flow through each shielding box. ALiBaVa and "Turbo-DAQ" log temperature
of the Beetle and FE-I3 chip which differ from the temperature of
sensor on the heat sink by approximately~3$^{\circ}$C.
%
\subsection{Measurements of single chip assemblies}
%--------1---------2---------3---------4---------5---------6---------7---------8
Prior to their irradiation the SCAs were temporarily mounted on test cards
for the chip tuning and calibration as described in~\cite{GROS}. It has been
found that the current consumption of the FE-I3 pixel chips with conventional
1.6V analogue supply voltage has dropped from 110 mA down to 40~mA after
irradiation and the minimum analogue voltage of 1.8V was required for both
SCAs to operate. Similarly the minimum bias voltage of 300V was required for
DC-coupled pixel sensors after irradiation to get any response from the pixel
chip. The high voltage becomes unstable above 700V for both irradiated sensors:
their bias currents drop and the trigger circuit starts to register electromagnetic
spikes whose frequency correlates with HV with approximately 10 Hz/V slope. For a
safe long-term operation the bias voltage of 600V was chosen at which the leakage
currents without annealing of 60~$\mu$A/cm$^2$ and 100~$\mu$A/cm$^2$ for doses
7$\cdot$10$^{15}$ and 1.5$\cdot$10$^{16}$~protons/cm$^2$ were measured at
-25$^{\circ}$C (same as for APC at 600~V) that agrees with expectations for
the given detector volume, accumulated dose, the bias voltage and annealing
time~\cite{LEAK}.

The sub-threshold signals could not be read out in the ToT method which
introduces an offset into the formula for measured charge:
%
\begin{equation}
\label{CCOL}
\hspace{-6mm}
Charge[ToT] = Gain \left [\frac{ToT}{e}\right ]\cdot(Charge - Threshold)[e]
\end{equation}
%
The gain is related to the chip tuning parameters: 60~ToT counts should match
by convention 20000 electrons, reduced by 3200 electrons threshold. An inverse
function to Eq.~\ref{CCOL} describes the physical charge produced by beta
particles and collected in sensors that gives roughly 7200 and 6400 electrons
at 600V for corresponding doses of 7$\cdot$10$^{15}$ and 1.5$\cdot$10$^{16}$
protons/cm$^2$. These results originate from the "Turbo-DAQ" fitting algorithm
based on the $\chi ^2$ minimisation and, if understood as mean values of the
Landau distribution and recalculated to most probable values (MPV) \cite{PDG},
are consistent with data presented in~\cite{REF}. The maximum estimate of the
systematic uncertainty due to non-linearities in the FE-I3 calibration,
amplification and threshold circuits is 10\%.
%
\subsection{Measurements of APC with analogue readout}
%--------1---------2---------3---------4---------5---------6---------7---------8
All pixels of the APC sensor have a DC-coupling to the \mbox{Beetle} chip.
The leakage current of detector diodes is drained by the input amplifiers.
The test with laser pulses has shown that each input of the Beetle chip can
tolerate, without degradation of gain, up to 1~$\mu$A leakage current using
current generated by an additional constant infra-red light source.

Measurements with the $^{90}$Sr source of the charge collection
efficiency (CCE) for the irradiated to 7$\cdot$10$^{15}$~protons/cm$^2$ APC
device as a function of its bias voltage are shown in Fig.~\ref{fig:CHAR}.
The two lines differ in the event selection: the upper shows amplitudes for
single hits only (one pixel at a time has a signal above \mbox{threshold})
the lower contains all events including multi-pixel clusters for which the
cluster seed (central pixel) amplitude is presented. All values are
normalized to non-irradiated detector at 250V whose full depletion voltage
of 80V is taken from its 1/C$^2$ characteristics. The most probable value
(MPV) of 6000 electrons at 600V and 8000 electrons at 900V agrees with data
in~\cite{REF}. The systematic uncertainty has been eliminated by calibrating
the Beetle chip with a pulse generator and by measuring the charge collection
with 140~$\mu$m and 300~$\mu$m thin unirradiated sensors. The statistical errors
on MPV are negligibly small for data samples of 100k events. Results are being
published as preliminary since more detectors need to be analysed.
%
\begin{figure}[ht] 
  \begin{center}
  \resizebox{7.5cm}{!}{\includegraphics{./cce.eps}}
  \caption{Charge collection efficiency for the 300$\mu$m thin APC detector
           irradiated by 24~GeV protons to dose $7\cdot 10^{15}$~particles/cm$^2$.}
  \label{fig:CHAR}
  \end{center}
  \vspace{-5mm}
\end{figure}
%
%
\section{Summary}
%--------1---------2---------3---------4---------5---------6---------7---------8
This manuscript describes briefly the properties of planar silicon pixel
detectors designed at Liverpool University and methods for their evaluation.
Two measurement techniques: the time-over-threshold of the FE-I3 pixel chip
and analogue pipeline of the Beetle chip were compared for the charge
collection from heavily-irradiated planar pixel sensors.

The silicon wafer with analogue readout pixels was designed at Liverpool
University and manufactured by Micron~Semiconductor~Ltd. Layout rules
for improved HV performance of the "n-in-p" planar detectors were finalised
during the the workflow. This knowledge was implemented in the second 6-inch
wafer produced by Micron Semiconductor~Ltd. for the IBL \mbox{ATLAS} pixel
project~\cite{IBL}.

The beam telescope based on the ALiBaVa readout with an integrated "Turbo-DAQ"
is currently under construction in collaboration between Valencia, Liverpool
and Barcelona Universities. The plan is to extend laboratory measurements
of charge collection with different beam types and to complement them by
characterisation of spatial efficiency and coordinate resolution of pixels.
%
\section{Acknowledgments}
%
This work was supported by the UK Research Council grant "Development of
Radiation-Hard Single-Sided Planar P-type Silicon Pixel Detectors using
Planar p-type \mbox{Technology"}.

The authors would like to express their gratitude to Micron Semiconductor
Ltd company for their cooperation and excellent product quality. A special
thank to Prof. Dr. Norbert Wermes for help with establishing the pixel
development programme at Liverpool University. It is very pleasant to
mention Mr.~Phil R.~Turner whose contribution to this and many other projects
is truly appreciated. The last but not least: "gracias a Ricardo Marco" for
his comprehensive technical support!
%
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%
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\bibitem{ECAP}  A.~Abdesselam et al., Nucl.\ Instrum.\ Meth.\  A {\bf 575} (2007) 353
\bibitem{REF}   A.~Affolder, P.~Allport, G.~Casse, Nucl.\ Instrum.\ Meth.\  A {\bf 623} (2010) 177
\bibitem{APIX}  M.S.~Alam et al., Nucl.\ Instrum.\ Meth.\  A {\bf 456} (2001) 217
\bibitem{FEI3}  I.~Peri\'{c} et al., Nucl.\ Instrum.\ Meth.\  A {\bf 565} (2006) 178
\bibitem{BEETLE} M.~Agari et al, Nucl.\ Instrum.\ Meth.\  A {\bf 518} (2004) 468
\bibitem{SPRAY} R.H.~Richter et al., Nucl.\ Instrum.\ Meth.\  A {\bf 377} (1996) 412
\bibitem{IRRAD} M.~Glaser et al., IEEE Trans. on Nucl. Sc. 53(4) (2006) 2016
\bibitem{IBL}   ATLAS Technical Design Report 019 CERN/LHCC 2010-013
\bibitem{GRPN}  A.~Beck et al., Nucl.\ Instrum.\ Meth.\  A {\bf 396} (1997) 214
\bibitem{ALI}   R.~Marco-Hern\'{a}ndez, et al., IEEE Trans. on Nucl. Sc. 56(3) (2009) 1642
\bibitem{EUDET} Philipp Roloff, Nucl.\ Instrum.\ Meth.\  A {\bf 604} (2009) 265
\bibitem{GROS}  J\"{o}rn Gro\ss e-Knette  Hab. Thesis BONN-IR-2008-04, ISSN-0172-8741 95
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\bibitem{PDG}   K. Nakamura et al. (PDG), J. Phys. G 37, 075021 (2010) 290
%
\end{thebibliography}
\end{document}



