1
Electronics
P
P
+
+
Analysis and simulation of charge collection in Monolithic Active Pixel Sensors
(MAPS)

Enrico Giulio Villania*, R. Turchettaa, M. Tyndela
P Sensitive
volume
a
Rutherford Appleton Laboratory, CLRC, Chilton, Didcot OX11 0QX, UK (2 – 20 μm thick)


Monolithic Active Pixel Sensors (MAPS) are widely used in consumer imaging applications. They owe their
++ Substrate
success to the inherent features of the CMOS process they are fabricated P
with
i.e. low cost, simplicity, and low
power consumption, which allows the design of high resolution detectors (300
with integrated
processing and
– 500 μmsignal
thick)
data reduction. However, the signal collection time is dominated by diffusion and is relatively slow. The aim of
this paper is the analysis and simulation of a novel MAPS structure that can improve the performances in terms
of speed of charge collection and hence radiation tolerance.
1.
INTRODUCTION
The original proposal to use monolithic active
pixel sensors (MAPS) in imaging applications can
be traced back to the early 90's [1] and was followed
by proposals for their use as tracking detectors for
HEP experiments a few years later [2]. Spatial
resolution, of the order of a few μm has been
achieved with thin detectors which minimize the
multiple scattering of minimum ionizing particles
(MIPs). This taken together with all the other
advantages of the technology, like low power
consumption, low cost and the radiation tolerance of
deep sub micron technology make MAPS of
potential interest for a wide range of scientific
applications. MAPS sensors for both imaging and
HEP experiments rely on the integration of detector
and readout electronics onto a single CMOS chip but
there is an important difference in the requirements.
For imaging applications, the achievable fill-factor
(i.e. fraction of the pixel area sensitive to light) is
normally in the order of 30-40%. For particle
detection the fill-factor needs to be close to 100%
[3].


The typical time for charge collection is of the
order of tens of nanoseconds, which is not usually a
limitation in imaging applications. Conversely, in
HEP experiments there are many applications where
a much shorter charge collection time is necessary.
The radiation tolerance of these devices depends
both on the detecting part and the electronics. With
modern sub-micron processing, the electronics can
be designed to be very radiation tolerant. MAPS
detectors have been demonstrated to be sufficiently
radiation tolerant for use at a future Linear Collider
[4], but need to be improved for use at the LHC.
The paper is organized as follows. In the next
section the simulation results of a typical structure
are shown. In Section 3 a conceptually new structure
is introduced, and results of analysis and simulation
are presented. Section 4 cross-compares the results
of simulations of the two structures.
N+
cathode
*
E-mail address: g.villani@rl.ac.uk
2
Fig. 1 Standard MAPS sensitive section geometry
2.
SIMULATION
A typical structure of the sensitive section of a
MAPS can be seen in Fig.1. Owing to the doping
differences between the lightly doped P epitaxial
layer and P++ wells or P++ substrate, a potential
difference develops across these boundaries. The
ionisation generated by a charged particle remains
confined within this potential well and moves by
thermal diffusion, until it gets eventually collected at
the cathode. In order to establish the typical
electrical performances of such a structure, a
simulation was carried out using ISE TCAD [5],
with the parameters shown in Table 1:
Table 1
Standard cell characteristics
Cell size
Cathode size
Epitaxial thickness
Substrate thickness
20 μm
3 μm
5 μm
>10 μm
Cell parameters
NA (P-well)
NA (P-epitaxial)
NA (P-substrate)
Bias voltage
Temperature
1018 cm-3
1015 cm-3
1019 cm-3
2V
300 K
The parameters are representative of those typically
provided by foundries, but there are significant
variations. The thickness of the epitaxial layer varies
depending on the specific CMOS process, and the
defect densities in the various regions of silicon are
not well known. Nevertheless, using the
sophisticated modelling capabilities provided by ISE
TCAD [5] and some empirical models [6]
simulations can reproduce the results reported in the
literature reasonably well [7].
The simulation shows how the electric field
extends only weakly into the sensitive volume (Fig.
2). The consequence of this is clearly visible in Fig.
3, where the cathode voltage variation following a
hit by a Minimum Ionising Particle (MIP) is plotted
against time, with the parameter τF (fall-time)
defined as the time taken to cover 90% of full
voltage swing.
In the transient simulation, the device had the
substrate (anode) grounded and the cathode biased
with 2 V for the first 10 μs after which it is left
floating. A MIP hit is registered 1 ns after and the
behaviour of the device is analysed for the next
100ns.
The spread of charge among neighbouring cells is
a consequence of the dominance of diffusion over
drift in the sensitive section of the device.
It is clear that applying an electric field across the
full sensitive volume of the device improves the
charge collection speed and minimises the charge
spread.
3
Fig. 2 Internal Electric Field for standard structure
3.
NEW STRUCTURE
The underlying concept of the new structure relies
on the addition of a further N layer placed beneath
the P++ wells to generate an electric field across the
sensitive volume, and is shown in Fig. 4.
In order to investigate the achievable performances
of such approach, an analysis of the structure was
carried out. This in turn allowed the design of a
device to achieve the required charge collection
speed. Finally, the expected performance was
compared with results obtained by simulation
performed with the modelling package ISE TCAD
[5].
The cell, cathode size and temperature were kept
as described in Table 1. The thickness of the
epitaxial layer was also kept at the same value, so as
to compare the collection efficiency of the new
approach to the standard one. Initially a twodimension analysis and simulation was carried out.
The results and methods of this analysis will be
transferred to a full three-dimensional case.
3.1 Discussion
In order to analyse this structure, the Poisson’s
equation coupled with the continuity equation need
to be solved within the regions of interest.
Fig. 3 Cathode Voltage Variation following a MIP
hit at 10.001 μs
P+
N++ cathode
P++ substrate
N layer
P sensitive volume
Fig. 4 New MAPS with extended N-layer
n
 n F  D 2n  G  R
t
n
n
n
n
2
  

 o Si
(1)
(2)
4
The continuity equation (1) describes how the
charge travels through the active volume and the
neutral regions. Poisson’s equation (2) describes
how the potential varies within the structure.
Once the appropriate boundary conditions are set
and the solutions are obtained, it is possible for
example to relate internal electric field and charge
movement to doping, geometry and bias of the
structure. However, the full solutions of this system
of non-linear equations is not a trivial task, even in
case of a simplified structure like that one depicted
above. Furthermore, some parameters are not well
known.
Fortunately, some simplifications are possible. For
example, it was found that a conservative
approximation for the extension W of the epitaxial
layer interested by an electric field is given by:
W 
2 Si o kT

q2N 
A
2 Si  o kT
qN A

(Vbi V )
Table 2
Designed cell parameters and expected
characteristics
Cell parameters
Cell size
Cathode size
Epitaxial thickness
N well thickness
Substrate thickness
NA (P-well)
ND (N-well)
NA (P-epitaxial)
NA (P-substrate)
Collection time
Capacitance
Collected charge
20 μm
3 μm
5 μm
1.1 μm
>10 μm
≈5*1018cm-3
≈5*1017 cm-3
2*1014 cm-3
≈1019 cm-3
<5 ns
8.5 fF
800 e-
Bias voltage
Temperature
2V
300 K
(3)
4.
where NA+ is the doping concentration of the
epitaxial layer. This is assumed to be some orders of
magnitude lower than that of P substrate and N-well.
Vbi is the built-in voltage between N-well and Pepitaxial layer and V is the voltage bias. In eq. (3)
the first contribution comes from the depleted area
around the P-epi/P-substrate boundary while the
second term comes from the Nwell/P-epi junction.
Using equation (3), it is possible to simply relate
doping profile and bias voltage, both obviously
subjected to specific constraints stemming from
technology, to the condition of non zero electric
field extending all over the sensitive area.
With an added drift component in the active volume,
it was found that the collection charge process has
its upper limit dominated by the diffusion of charge
taking place in the substrate. This process was
analysed using the continuity equation (1), in which
the drift component in the neutral regions was
assumed to be negligible.
With these ‘simplified’ solutions, the behaviour of
the device under different configurations (i.e. with
different combinations of doping, bias and
geometry) can be predicted.
As an example, in Table 2, some expected
characteristics (in bold) for a device of defined
geometry, peak doping and bias are shown.
SIMULATION RESULTS
Using ISE TCAD [5], simulations were performed
on the new designed structure, in order to crosscheck the solutions of the systems of equations. The
cell was covered by approximately 4500 nodes with
finer mesh close to the substrate- epitaxial layer and
N-well –epitaxial layer junctions with a minimum
resolution of 20 nm.
In Fig. 5 the internal electric field is plotted and
compared to the standard device, depicted in Fig. 2.
It is evident that the sensitive region has a drift
component added. The superimposed cathode
voltage variations following a hit by a MIP under the
same aforementioned conditions are plotted in Fig. 6
both for the standard structure and for the new one.
Because of the increased capacitance, the voltage
variation is smaller than in the former case, being
around 19 mV. This would still allow an adequate
signal-to-noise ratio. The fall time (defined as
before) is around 2 ns, approximately 8.5 times
smaller than in the previous case and in agreement
with what expected.
5
Vcath ≈ 1.981
τF ≈ 1.9ns
Vcath ≈ 1.883
τF ≈ 17ns
Fig. 5 New MAPS internal field configuration
5.
Fig. 6 Cathode Voltage Variations comparison
CONCLUSIONS
The aim of this paper was to describe a new concept
to the structure of standard MAPS and to perform
analysis and simulations of this new device.
Simulations performed on a standard structure match
well with what is reported in the literature giving
some confidence in the results obtained for the new
device.
In order to take this concept into a prototyping
phase we have identified an existing process, which
would allow the structure to be fabricated. The next
step will be to carry out a full 3D simulation and
analysis of the structure and then to proceed with the
fabrication and test of the device.
At the same time, several different architectures of
readout electronics will be explored, so as to fully
exploit the potentialities of a new faster device.
Technology, in Nucl. Instr. And Meth A 458
(2001) 677-689
3.
G.Deptuch et al., Design and Testing Of
Monolithic Active Pixel Sensors for charged
Particle Tracking, in IEEE Trans. on Nucl.
Sciences, vol. 49, n. 2, April 2002, 601-610
4.
R.Turchetta et al., Monolithic Active Pixel
Sensors (MAPS) in a VLSI CMOS Technology,
presented at Vertex 2001, to be published on
Nucl. Instrum. and Methods A
5.
ISE TCAD 8.0, Integrates Systems Engineering
Zurich Switzerland
6.
S.M.Sze, Physics of Semiconductor Devices,
Wiley, New York, 1981
7.
G.Deptuch et al., Simulation and Measurements
of Charge Collection in Monolithic Active Pixel
Sensors, in Nucl. Instr. And Meth A 465 (2001)
92-100
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2.
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Electronic Camera-On-A-Chip", in IEEE Trans.
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1997, 1689-1698
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