A High Resolution Vertex Tracker for the STAR Experiment using Active Pixel
Sensors
and
Recent work using APS Sensors
F. Bieser, R. Gareus, L. Greiner, J. King, J. Levesque, H.S. Matis, M.
Oldenburg, H.G. Ritter, F. Retiere, A. Rose, K. Schweda, A.
Shabetai, E. Sichtermann, J.H. Thomas, H. Wieman, Lawrence
Berkeley National Laboratory
S. Kleinfelder, S. Li, University of California, Irvine
H. Bichsel, University of Washington
Howard Matis
Pixel 2005
1
PART I - DETCTOR
physics motivation for a thin vertex detector
deconfinement
 Study initial properties of a
nuclear collision
 u, d, s quarks gain mass
Phase and Chiral transitions
become thermalized
 Final state effects
 Measures later/cooler
times of the collision
 d, b quarks produced at
early time
 Intrinsic mass
 Measure of early
collision


u-, d-quarks and
‘bound-states’
gain mass
Need to measure particles above 0.5 GeV/c
High collision density - more than 2000 tracks
 Measures secondary particles >100 µm from collision point
Howard Matis
Pixel 2005
2
detector requirements
 Study D0 measurement
 Multiple scattering in
beam pipe sets
fundamental limits
 “Dream” Detector
 Thickness 240 µm Si
equivalent
 Position resolution 8
µm
Howard Matis
Pixel 2005
3
star micro vertex detector
 Two layers
 1.5 cm radius
 4.5 cm radius
 24 ladders
 2 cm  20 cm each
 < 0.3% X0
 ~ 100 Mega Pixels
Howard Matis
Pixel 2005
4
close-up view
Howard Matis
Pixel 2005
5
sensor
•Sensor under development at IReS
•First prototype made using 0.25 µm process by TSMC
•Second version in production using 0.35 µm by AMS
Efficiency for min ionization 98%
Accidental rate
< 100 /cm2
Position resolution
< 10 m
Pixel dimension
30 m  30 m
Detector chip active area
19.2 mm  19.2 mm
Detector chip pixel array
640  640
Howard Matis
Pixel 2005
6
ladder
 10 thinned APS detectors
 Top of a matching row of
thinned readout chips
 Three-layer aluminum
Kapton cable
 Silicon cable structure is
bonded to a carbon
composite v, closing the
beam to make a rigid
structure
 Wire bonding to the cable
Howard Matis
Pixel 2005
7
ladder
2 carrier candidates – X0 =0.11 %
Top layer = 50 µm CFC
Outer shell = 100 µm CFC
(carbon fiber composite)
Middle layer = 3.2 mm RVC
Fill = RVC (reticulated
vitreous carbon foam)
Bottom layer = 50 µm CFC
Howard Matis
Pixel 2005
8
ladder prototypes

Mechanical Prototype with 4
MIMOSA-5 detectors glued to
the Kapton cable assembly.
Tested for
 Vibration
 Stiffness
•
A prototype cable (Cu) has been
designed, constructed and
tested.
•
Prototype ladder using thinned
50 µm MIMOSA-5 detectors.
Currently under test with DAQ
Howard Matis
Pixel 2005
9
heavy flavor tracker (hft) parameters
Total number of pixels
98  106
Number of pixels per chip
640 x 640
Pixel Readout rate
100 ns
Readout time per frame
4 ms
Dynamic range of the ADC
10 bits
Raw data from one sensor using a 10 bit ADC
1 Gb/s
Fixed pattern noise
2000 e
Noise after Correlated Double Sampling
10 e
Maximum signal
900 e
Dynamic range after Correlated Double Sampling
8 bits
Total power consumption
90 W
Howard Matis
Pixel 2005
10
mechanical requirements


Geometry

Maintain position resolution of ~ 10 µm

Low mass / radiation length (X0~ 0.3% / layer)

Coverage of -1 <  < 1
Function
•
Easy to calibrate
•
Easy to align
•
Easy to remove, repair and replace electronics (ladders
will need to have a local survey)
•
Fit easily into the existing detector and infrastructure at
STAR
Howard Matis
Pixel 2005
11
conceptual mechanical design
•Mounted to SVT cone
•Slides in and out on one end
Howard Matis
•Ladders moves as beam pipe
diameter increases
Pixel 2005
12
kinematic support structure
•Support bolts
unto STAR
•Green structure
provides stable
support for the
ladder
•Three point
kinematic mounts
assure accurate
positioning
•Can move
detector in and
out with
reproducibility
Howard Matis
Pixel 2005
13
PART II - APS RESEARCH
studies with scanning electron microscope
Access to 5 - 30 keV
scanning electron
microscope
Thought needed to
punch through 2-3
µm
Believed could
detect these
electrons
Howard Matis
Pixel 2005
12 µm
30 keV electrons
14
cross sectional view
(Tilt at 520)
Pt Layer
Top of IC
Artifact due
to charge
Top coating
Epi-layer
Howard Matis
Pixel 2005
15
element analysis
Pt
Al
Si
Ti
WHoward Matis
Ga
O
Pixel 2005
16
30 kev electrons do not penetrate to the epilayer
Howard Matis
Pixel 2005
17
can detect “electrons” with
reasonable accuracy
 Can see microscope
 Measuring
Bremsstrahlung
 Maximum intensity
~3000 /frame
 Evaluate charge
sharing of cell
 Evaluate position
resolution algorithms
 Best
Howard Matis
µm
x 3  x1
x1  x 2 /2  x 3
Pixel 2005
18
track efficiency is critical with noise level
 Monte Carlo study two
different algorithms with
MIMOSA 5
 Look for seed pixels
 Smooth data and then
look for seed pixels
 Real pedestal data with
imbedded electron spectrum
 Efficiency algorithm
dependent
 Algorithm choice dependent
on noise
Howard Matis
Pixel 2005
19
how much signal do you get out of an aps sensor?
 Calculations show that
energy loss in thin
materials much less
than thicker
 Bichsel & Saxon, Phys.
Rev. A 11, 1286 (1975).
Bichsel
& Saxon
Landau
Energy Deposited - eV
 Observed in aluminum
 Perez & Sevely, Phys.
Rev. A 16, 1061 (1977).
Howard Matis
Pixel 2005
0.76 µm Al
1 MeV e-
20
study at lbnl advanced light source
 Study 1.5 GeV/c
electrons
 Calculated expected
energy
 Use Bichsel formalism
 0.25 µm TSMC
 8 µm epitaxial layer
 Need to shift theory by
1.5 for good agreement
Howard Matis
Pixel 2005
21
p-well
MIP
n-well
some checks
 Epitaxial (epi) layer 8 µm
(error perhaps 1 µm)
 Use Bichsel formalism on
8.5 µm aluminum data
 1.66 keV scales to 1.43
keV silicon (most
probable)
 Bichsel predicts
 1.43 keV
 Total systematic error 10 20 %
 Cannot explain 50% excess
Howard Matis
Pixel 2005
P
epitaxial
layer
p++
substrate
epitaxial layer
22
a hypothesis
p-well
 Extra charge equivalent
to 4 µm
 Electrons could be
coming from upper pwell and p++ substrate
 Check with Mimosa-5
data (AMS 0.6 µm)
 Most Probable - 996 e Bichsel - 746 e Equivalent to extra 4.7
µm over nominal 14 µm
Howard Matis
Pixel 2005
MIP
n-well
P epitaxial
layer
p++
substrate
23
scaling of cell size
 UCI design a multi-spacing
chip
 5 µm, 10 µm, 20 µm and
30 µm
 All cell sizes on one chip
 Minimize systematic
errors
 Charge sharing very similar
 Can see small absorption of
charge in epitaxial layer
 Good scaling
Howard Matis
Pixel 2005
55
10
20
30
Log Scale
Linear
Scale
24
summary
 Proposal for a vertex detector with APS technology
 Awaiting funding
 Transmission scanning microscopes can be used
to probe sensors
 Software algorithms important to get high hit
reconstruction - choice very sensitive to absolute
noise
 Cell scales from 5 to 30 µm
 More charge then expected coming from APS
Howard Matis
Pixel 2005
25
A Heavy Flavor Tracker for STAR
Z. Xu
Brookhaven National Laboratory
Y. Chen, S. Kleinfelder, A. Koohi, S. Li
University of California, Irvine
H. Huang, A. Tai
University of California, Los Angeles
V. Kushpil, M. Sumbera
Nuclear Physics Institute AS CR
C. Colledani, W. Dulinski, A. Himmi, C. Hu, A. Shabetai, M. Szelezniak, I. Valin,
M. Winter
Institut de Recherches Subatomique, Strasbourg
M. Miller, B. Surrow, G. Van Nieuwenhuizen
Massachusetts Institute of Technology
F. Bieser, R. Gareus, L. Greiner, F. Lesser, H.S. Matis, M. Oldenburg, H.G. Ritter,
L. Pierpoint, F. Retiere, A. Rose, K. Schweda, E. Sichtermann, J.H. Thomas,
H. Wieman, E. Yamamoto
Lawrence Berkeley National Laboratory
I. Kotov
Ohio State University
Howard Matis
Pixel 2005
26
end
Howard Matis
Pixel 2005
27
backup slides
Howard Matis
Pixel 2005
28
precision tie points coupling the hft system to the star support
cone
Howard Matis
Pixel 2005
29
thin beam pipe
 Central beryllium region
 14.5 mm radius
 10   beam size
 500 µm thick walls
 Outer region
 30 mm radius aluminum
 Exoskeleton caries load
Howard Matis
Pixel 2005
30
end view showing the hft ladders between spokes of the inner
beam pipe support
Howard Matis
Pixel 2005
31
data flow and processing stages in the readout chip




Each stage can be bypassed to allow raw or partially unprocessed data to be
routed to the DAQ
The first stage is a CDS preprocessor which is followed by pedestal
subtraction and a pixel masking filter
Further processing allows us to sum up the value of 1, 4 or 9 pixels before a
threshold cut is applied.
The last stage includes zero suppression and transcoding to hit positions.
Howard Matis
Pixel 2005
32
readout layout


Howard Matis
Pixel 2005
sketch of the
readout-topology
on a detector
ladder
one of ten APS and
the corresponding
readout chip
layout.
33
rdo asic
•
ADC – 10 bit ADC for signals
from sensor chip
•
CDS – Chip will perform
correlated double sampling
•
High speed LVDS output
•
Configuration, control, clock,
synch functions
• Both chips thinned to 50 µm
thickness.
• X0 = 0.053 % each
Howard Matis
Pixel 2005
34
daq
ladders can be
combined to one
optical link.
Figure5:
Howard Matis
Pixel 2005
35
hit loading
Au+Au Luminosity
1  1027 cm-2s-1
dN/d
170
(min bias)
Min bias cross section
10 barns
Interaction diamond size,
σ
30 cm
Outer Layer
Inner Layer
Radius
5 cm
1.5 cm
Hit Flux
4.3 kHz/cm2
18 kHz/cm2
17/cm2
72/cm2
Projected Tracking Window Area
0.6 mm2
0.15 mm2
Probability of Tracking Window
Pileup
10 %
10 %
0.001 mm2
0.001 mm2
0.14%
0.58%
Hit Density 4 ms Integration
HFT Hit Resolving Area
Probability of HFT Pileup
Howard Matis
Pixel 2005
36
Comparison with mimosa-5
Parameter
Detector
MIMOSA-5
98% @30 – 40 C
Detection efficiency
~ 99% ≤ 20
resolution
< 10 µm
~ 2 µm
pixel pitch)
30 µm
17 µm
Read-out time
4 – 10 ms
24 ms (
Ionizing radiation
tolerance
2.6 kRad/yr
Fluence tolerance
2
Power dissipation
Chip size
Chip thickness
Howard Matis
1010 neq/cm2
100 mW/cm2
~2  2 cm2
50
m
20 ms possible)
100 kRad
≤ 1012 neq/cm2
~ 10 mW/cm2
1.9  1.7 cm2
120
Pixel 2005
C
m
37
material budget
Material
Beryllium beam pipe
Material Thickness
(µm of Si)
% X0
500 µm of Be
0.1417
MIMOSA detector
50
0.0534
Adhesive
13
0.0143
RDO chip
50
0.0534
Adhesive
13
0.0143
Cable assembly
84
0.0896
Adhesive
13
0.0143
Carbon fiber / RVC beam
103
0.1100
Total for the ladder
components
327
0.349
Howard Matis
Pixel 2005
38
using an aps as a camera
Howard Matis
Pixel 2005
39