accounting for copper surface roughness for close correlation between simulation and measurement in...

Accounting for Copper Surface Roughness for Close Correlation between Simulation

and Measurement in a 10Gbps

Package Channel

Jacov Brener, PHY EM Design Engineer Intel Corporation, Datacenter and Connected Systems Group, Communication & Storage Silicon Engineering

• Motivation

• Measurement setup and results

• Initial correlation results

• Surface roughness Hall-Huray model

• Surface roughness measurement

• Final correlation results

• Summary

• Q&A

Agenda

2 J. Brener, “Accounting for Copper Surface Roughness for Close Correlation between Simulation and Measurement in a 10Gbps Package Channel”, November 26th 2013

Motivation

• TP1 is on the DUT balls [1]

• The only high speed channel exits in the test is package channel

• Need to account for package impact on waveform jitter, rise time, amplitude etc…


TP1

Silicon

Package

Board

PTH

BGA

C4 bumps Trace

Motivation cont.

• A typical 10mm package has BW of at least 20GHz

• Package effects mostly on waveform on the balls so we’ll focus on ILdiff magnitude

100GHz 3D full wave typical package channel model


Measurement Setup and Results

• 4 port 20GHz equipment • SOLT calibration • Dual side probing station


VNA

Package

Probes

Measurement Setup and Results cont.

• Low measurement noise • Low package to package variation • Good measurement repeatability


Initial Correlation Results

• Small correlation error in DC due to inaccuracy of the field solver • Nearly linear increase in the correlation error – looks like a loss mechanism • ~12.5% error @20GHz


Initial Correlation Results cont.

• Cross section of package traces were made to debug the correlation error

• Copper surface roughness was discovered to be on the order of skin depth in GHz range

• Both on the traces an the planes found to be equally rough

• Same phenomena occurs on boards and was discussed extensively in DesignCon [2]


Surface Roughness Hall-Huray Model

• Traditional Hammerstad model describes surface roughness as a repeating series of peaks and valleys [3]

• Models the surface as a 1 dimensional effective cross section [3]

9

RMSh0

2

21 arctan 1.

2

4Hammerstad

smoothr

rough

skin

smooth

RMSr

skin

f

hf

J. Brener, “Accounting for Copper Surface Roughness for Close Correlation between Simulation and Measurement in a 10Gbps Package Channel”, November 26th 2013

2

2

2

0

4

31

2

2

2

1Hall Huary

smoothr

rough

skin

smooth

r

rr

skin skin

f

a Ns

A

sf

a a

Surface Roughness Hall-Huray Model cont.

• Hall-Hurray model models a 2 dimensional surface [4]

• Describes the surface as an effective matrix of half-spheres [4]

10

a


N half-spheres per area A

Surface Roughness Measurement

• Copper surface was exposed from the dielectrics [5]

• SEM (Scanning Electro Microscopy) pictures were taken to visualize the phenomena [5]

• AFM (Atomic Force Microscopy) 3D profile map was taken for analysis [5] 11


Surface Roughness Measurement cont.

Results obtained from AFM [5]:

• RMS: RMS height

• SAD: Surface Area Difference

12

N

ZRMS

i

2)(

( _ _ )1

( _ )

i

j

real surface areaSAD

scan area


Final Correlation Results

Rough trace loss estimation [6]

13

1

20log

smoothrough

r

rough

l

f

Rwt

R j L G j C

j

Attenuation dB e


• Hall-Huray model usage decreases the error by a factor of 10 up to 13GHz • Hall-Huray model usage decreases the error at 20GHz to less than 5% • Hammerstad model is still away from reality


Final Correlation Results cont.

Summary

1. Package can and will become important

2. Surface roughness adds significant loss at GHz range and beyond for any PCB trace

3. Surface roughness can be accurately measured by AFM

4. Hall-Huray model proven to be effective

5. Accounting for surface roughness decrease correlation error by x2.5


Acknowledgments & References

• Acknowledgments: – Manukovsky Alex, Intel Corp.

– Valentina Korchnoy, Intel Corp.

• References

1. IEEE 802.3-2012, Clause 72

2. E. Bogatin, D. DeGroot, P.G. Huray, Y.Shlepnev, “Which one is better? Comparing options to describe frequency dependent losses”, DesignCon 2013

3. E. Hammerstad , O. Jensen, “Accurate models for microstrip computer aided design”, IEEE MTT-S Int. Microw. Symp. Dig., May 1980, pp.407–409.

4. S. Hall, S. Pytel, P. Huray, D. Hua, A. Moonshiram, G. Brist, and E. Sijercic, “Multigigahertz Causal Transmission Line Modeling Methodology Using a 3-D Hemispherical Surface Roughness Approach”, IEEE trans. Microwave Theory and Tech., vol.55, no.12, Dec.2007, pp.2614-2624

5. V. Korchnoy, J. Brener, “A Practical Method for Trace Exposure and Roughness Measurements and Implementation in High Speed Package Design”, ISTFA 2012, November 2012

6. W.R. Eisenstadt, Y. Eo“S-Parameter-Based IC Interconnect Transmission Line Characterization”, IEEE trans. Components Hybrids and Manufacturing Tech., vol. 15, no. 4, August 1992, pp483-490


Thank you!

Q&A

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