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