vertex2002, kailua-kona, hi 11/07/02 b. quinn1 carbon fiber grounding issues in the dØ run iib...
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Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 1
Carbon Fiber Grounding Issues in the DØ Run IIb Silicon
Detector Design
Breese QuinnMarvin Johnson, Mike Matulik
FNAL
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 2
Carbon fiber (CF) has become ubiquitous in silicon detector design because of its high modulus and low mass.
Mechanical supportCooling tubes
However, the current pitch fabrication techniques that produce carbon fiber with ultra-high modulus (~1000 GPa) also yield very low resistivity (~100 Ω•cm)
Floating conductors present the potential for strong capacitive coupling to silicon sensorsCan result in a very troublesome source of noise, especially in close-packed L0 structures
Effective grounding of all carbon fiber elements is essential in producing a low-noise environment for silicon detectors
Carbon Fiber Issues
Before Ground Repair
L0 Support Structure,DØ Run IIb Silicon
After Ground Repair
Noise from Be support platecapacitor, DØ Run IIa ladder
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 3
To effectively ground a piece of carbon fiber, one must ensure excellent electrical coupling:
Tested coupling to carbon fiber (K13C) with copper tape
Measured the attenuation of a network analyzer input at the bandstop resonance
Saturation at ~10-15% coupling area
-29-28-27-26-25-24-23-22-21-20
0 10 20 30 40
Copper Tape Area (in^2)
Fre
qu
ency
Res
po
nse
M
inim
um
(d
B)
Coupling to Carbon Fiber
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 4
Power transfer functions measured with the network analyzer were complemented with manual, analog impedance measurements
Impedance Measurement
Carbon Fiber Capacitors
1
10
100
10000000 100000000
Freq. (Hz)
Mag
(Z
) (o
hm
s)
2 in^2
1 in^2
1/2 in 2̂
1/4 in 2̂
Minimum Impedance
1
1.11.2
1.3
1.4
1.51.6
1.7
1.8
0 0.5 1 1.5 2 2.5 3 3.5 4
Coupling Area (in^2)
Mag
(Z)
(oh
ms)
Inverse relationship between impedance (and thus power throughput) and coupling area was verified
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 5
L0/L1 Design
Silicon sensor/hybrid assemblies are mounted on crenellated carbon fiber support tubes
Sensors are sandwiched between carbon fiber/hybrid ground plane capacitors
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 6
The carbon fiber support underneath the silicon sensors must be shorted to small pads on the hybrid ground plane
Need low-mass coupling to the carbon fiberNeed small attachments to 2 mm ground pads on the hybridNeed to be effective over a frequency range from ~ 4.5 kHz – 10 MHz
Determined by integration time and 8-bit ADC dynamic range
Try aluminum foils embedded onto the carbon fiber surfaceNarrow aluminum strips folding up and over to the ground pads
Shorting the Carbon Fiber
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 7
L0/L1 Mock-up
K13CCarbon Fiber
Embedded Aluminum
Kapton Sensor (Al)Hybrid (Al)
NADrivingPower
FarDetector
~Carbon Fiber
Hybrid
Sensor
NA
GroundStrips
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 8
Studied transfer functions from the C fiber to the “sensor”Varied the amount of C fiber area covered by embedded aluminum
0.5 in2 4 in2 (2% 17%)
Quality of the electrical contact is crucialVarying the number and size of shorting strips had no significant effect
Far Detector Mock Up
-50.00
-40.00
-30.00
-20.00
-10.00
0.00
10.00
20.00
1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09
Frequency (Hz)
Fre
qu
ency
Res
po
nse
(d
B)
4 in 2̂
4 in 2̂
2 in 2̂
2 in 2̂
2 in 2̂
2 in 2̂
1 in 2̂
0.5 in 2̂
0.5 in 2̂
L0/L1 Mock-up
Data curves looka little suspicious –copper tape
contacts were not “fresh” for these measurements
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 9
Comparison of coupling to the simple CF capacitor with a somewhat subjective selection of L0 Mock-up coupling data
Estimate 25-30% coverage by embedded Al is needed for maximum attenuation
Carbon Fiber Coupling
L0 Mock-up Coupling Area
-35
-30
-25
-20
-15
-10
-5
0
0 1 2 3 4 5
in̂ 2
@ 100 kHz
Suspect data
CF Capacitor Coupling Area
-35
-30
-25
-20
-15
-10
-5
0
0 5 10 15 20 25 30
% Coupling Area
dB
L0 Mock-up Coupling Area
-35
-30
-25
-20
-15
-10
-5
0
0 5 10 15 20 25 30
% Coupling Aread
Bsaturation
point
?
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 10
Ongoing Studies
A significantly more complicated, though considerably more robust coupling concept…
(not even close to scale…)
Potentially less integrated mass than the embedded aluminum option
Less susceptible to oxidation, tears, silver epoxy
Carbon Fiber
FlexCircuits
Vias
Hybrid
Sensor
Copper Mesh
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 11
1
10
100
1000
1000000 10000000 100000000
Freq. (Hz)
Mag
(Z)
(oh
ms)
0.018" Cu
0.063" Al
0.014" SS
0.055" SS
0.005" CF
0.011" CF
0.015" CF
0.037" CF
Ongoing Studies
Why is the C fiber so conductive?Impedance measurements of capacitors made of different materials and thicknesses
Virtually indistinguishable from copper
Thin stainless steel is the only metal with significantly different impedanceThinnest plate (14 mils), and largest skin depth (~ 2 mils at 75 MHz)
C fiber has larger skin depth (~ 3 mils) and some plates were thinner ( 5 mils)
Other effect at work – graphene layer-plane characteristics, eddy currents?
Vertex2002, Kailua-Kona, HI 11/07/02
B. Quinn 12
Conclusions
Today’s ultra-high modulus types of carbon fiber are highly conductive, and demand careful attention to proper grounding when used in silicon vertex detectors.
Effective electrical coupling to the carbon fiber is the critical issue involved in designing a grounding scheme.
Thin aluminum foils embedded into the carbon fiber yield excellent ground connections
~ 25-30% coverage of the carbon fiber is optimal for the DØ Run IIb inner layer geometry
However, oxidation and fragility of thin Al foils are major concerns
Kapton/copper flex circuits together with an embedded copper mesh is a promising solution
Low-mass and robust
Open questions remain concerning the nature of carbon fiber conductivity
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