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Readout circuit gain measurement
Transfer function is obtainedsweeping VRST voltage andmeasuring the output DCvoltage.
= ( )( ) . ratio: = 1.08
IN
AVDD
VBP
OUT
AVSS
VBN
M1
M2
M3
M4M5
FB1
FB2
Cd
Cf
Cgd
VRST
Cp
IN
AVDD
VBP
OUT
AVSS
VBN
M1
M2
M3
M4M5
FB1
FB2
CdCp
Cf
Cgd
VRST
IN
AVDD
VBP
OUT
AVSS
VBN
M1
M2
M3
M4
M5
FB1
FB2
VRST
DC voltage Gain (bulk connected to source): =
Transistor M1: source follows the gate potential:is not charged, = + +
Decreasing with increasing = 0 ENC
ENC ENCe.g. shielding the input routing metal.
Ceff [fF]
CL = 6 fF:
Standard source follower (SF)
AVSS
Cf
AVDD
VBP
OUT
Cgd
IN
M2
M1
AVSSAVSS
Cd Cp
VRST
CL
Sensing node signal measurement, capacitance & ENC calculation
Source follower vs back bias voltage (VBB)
Source follower vs source-drain follower at -6 V VBB
Source-drain follower with/without shielding at -6 V VBB
VBB from -1 V to -6 V:• Signal amplitude increases
with increasing VBB.• Cluster signal is a little
wider than single signal.• Larger density moves to the
lower cluster multiplicity.• is reduced by ~ 38%.• ENC decreases by ~ 22%.
Measurement results with 5.9 keV X-ray from 55Fe source ~ 1640 electron/hole pairs generated in silicon.VRST = 1.3 V. Sensing node signal is calculated after gain calibration.Terms definition:• Cluster: group of contiguous pixels where >• Cluster multiplicity: number of pixels in a cluster (n)• Cluster signal: =• Seed signal: = max [ ] • Single signal: = with n=1
SF and SDF:• Signal amplitude is larger
for the SDF circuit.• Cluster signal is a little
wider than single signal.• The effect on the cluster
multiplicity is neglect.• decreases by ~ 9% for
the SDF circuit.• ENC decreases by ~ 25%
for the SDF circuit.
SDF with shielding:• Signal amplitude is larger.• Cluster signal is a little
wider than single signal.• The effect on the cluster
multiplicity is neglect.• decreases by ~18%.• ENC increases by ~ 34%.
A Novel Source-Drain Follower for MonolithicActive Pixel SensorsC. Gaob, G. Aglieria, H. Hillemannsa, A. Juniquea, M. Keila, D. Kimc , M. Kofaragoa, T. Kugathasana, M. Magera, C.A. Marin Tobona,P. Martinengoa, H. Mugnierd, L. Musaa, S. Leec, F. Reidta, P. Riedlera, J. Roussetd, K.M. Sielewicza, W. Snoeysa, J. W. van Hoornea, P. Yangba CERN, Geneva, Switzerland b Central China Normal University, Wuhan, China c Dongguk and Yonsei University, Seoul, Korea d Mind, Archamps, [email protected]
IntroductionMonolithic Active Pixel Sensors (MAPS) [1] for the ALICE Inner Tracking System (ITS)
upgrade [2]:MAPS advantages: lower production cost, easier assembly, good power - S/N ratio performance.Charge conversion gain (1/ ) for low power and low material budget:
( / ) for constant S/N ratio [3].[ : power consumption, : effective input capacitance, Q: input charge, S/N: signal-to-noise. ]Technology: TowerJazz 180 nm CMOS image sensor process.
• High resistive epitaxial layer ( > 1 ).• Deep pwell layer allows full CMOS circuitry in the pixel
Installation during the LS2 in 2019.A novel source-drain follower:
provides better charge conversion gain and power - S/N ratio performance.provides a way to estimate the capacitance of the sensor itself.
Three types of readout circuits from the sensing node to the output PAD: • SF with buffer: (a)• SDF with buffer: M0 in (a) circuit is substituted by (b) circuit.
with/without shielding the input routing metal.Deep nwell separates the periphery circuit ground from substrate. It allows the substrate back bias.
Test Chip VRST
SUB
AVDD
AVSS
Pixel
PixSel
PixSel
AVDD
ColSel
OUT
PeripheryI0
I1I3
I2
M0M2
M3
M4IN
D
S
AVSS AVSS AVSS
CED spacing
Pitch
8 x 8 mini matrix surrounded by dummy pixels. AVSS
VBN
IN
D
M5
M6
M7
M8
S(b)
(a)
Pixel layout:Collection Electrode Diameter (CED): 3 um.Spacing: 3 um.Pitch: 20 um
= + = + = + + = +
IN
+gm1 vsg1 ro1
ro2
OUT
vOUT-vsg1
gm4 vsg4 gmb4vsb4
FB1
ro4
gm3 vsg3
FB2
ro5
ro3
+
-vsg3
+ -vsg4
a b c
+
-vi
+
-
+
-vFB2
+
-
vFB1
Novel source-drain follower (SDF)
+ + + = 0 + + = 0 + + + = 0
= (1 + )1 + (1 + )= 1 + (1 + )
= ( + )1 + (1 + ) = ( + ) +
Transistor M1: source and drain follow the gate potential.
DC voltage Gain
and arenot charged:= +
Shielding the input routing metal:
, and are not charged, = .
KVL & KCL:
Source-drain follower:• DC voltage gain is closer to unity• Lower • Lower bandwidth.• Lower ENC• Larger area.
VBB [V] -1 -6 Ceff [fF] 4.94 3.04 ENC [e-] 65 51
Table 3. source follower vs VBB
VBB = -6 V SF SDF Ceff [fF] 3.04 2.76 ENC [e-] 51 38
Table 4. SF and SDF at -6 V VBB
VBB = -6 V /W /O Ceff [fF] 2.27 2.76 ENC [e-] 51 38
Table 5. SDF with and without shielding at -6 V VBB
-3 dB bandwidth [MHz]
SF SDF 208 125
Table 1. SF and SDF: bandwidth from simulation
ConclusionsMAPS achieves very low sensor capacitance. Charge conversion gain ( ) is critical for ENC and power consumption.
The novel source-drain follower circuit reduces the hence increases voltage swing of sensing node, reduces ENC.The DC voltage gain of the source-drain follower is closer to unity than the one of the standard source follower.The bandwidth of the source–drain follower is smaller than the one of the standard source follower.Measurement results with 55Fe radiation source:Standard source follower decreases by 38% and ENC decreases by 22% changing VBB from -1 V to -6 V.Source-drain follower reduces by 9% and ENC by 25% compared to the standard source follower at VBB = -6 V.Source-drain follower with shielding achieves 18% reduction of , however ENC increases by 34% at VBB = -6 V.
10th International "Hiroshima" Symposium on the Development and Application of Semiconductor Tracking Detectors, Xi'an, China
References[1] H. Hillemanns et al., Radiation hardness and detector performance of new 180 nm CMOS MAPS prototype test structures developed for the upgrade of the ALICE Inner racking System, Nuclear Science Symposium and Medical Imaging Conference, Oct.27–Nov.2, 2013 Seoul.[2] ALICE Collaboration, The ALICE experiment at the CERN LHC, 2008 JINST 3 S08002.[3] W. Snoeys, ”Monolithic pixel detectors for high energy physics ”, Nucl. Instr. and Meth. A, 731 (2013), p. 125, doi:10.1016/j.nima.2013.05.073.
Equivalent Noise Charge (ENC):
ENC =
mean rms
SF * Buffer 0.722 0.019 SDF * Buffer 0.778 0.014
Table 2. voltage gain for 8 x 8 pixels
https://indico.cern.ch/event/340417/contribution/94 /
e.g. a cluster of 4 pixels
pwellDeep Pwell
Collection electrode
Deep Pwellpwell pwell pwell
Deep PwellNwell
Collection electrode
Epitaxial Layer p--
P-Substrate
ENC:• ENC • ENC
