Contact resistivity of ECA bonded joints · [5] Devoto et al., “Measuring the contact resistivity...

M. Ignacia Devoto1*, Tudor Timofte1, Andreas Halm1, Daniel Tune1

International Solar Energy Research Center (ISC) Konstanz

Corresponding author*: [email protected]

Contact resistivity of ECA bonded joints

M. Ignacia Devoto, Metallization & Interconnection Workshop 9th Edition, Online Conference, October 5th 2020

Agenda

2

• Introduction & motivation

• Transmission Line Method (or TLM)

•Contact End Resistance (or TLM-CER)

•Relevant issues

• Sample geometry/dimension

• Sample preparation

• Measurement procedure

• Method (TLM or TLM-CER)

•Results (ongoing)

•Conclusions


Introduction & motivation

3

• Soldering alternative

• Electrically Conductive Adhesives (ECAs)

• ECA broad formulation

• Conductive fillers (80-95 wt.%)

• Insulating polymer matrix (5-20 wt.%)

• ECA applications

• IBC or busbar-less cells (bowing)

• Shingling interconnection (no ribbon)

• Vehicle Integrated PV (vibrations)

• SHJ ribbon interconnection (low T°)

• Conductive backsheet (complexity)

• ECA-based joint advantages & disadvantages

• lead-free, low T°, mechanical flexibility, no flux, fine pitch

capabilities, curing during lamination

• use of silver, electrical/thermal conductivity, bleeding, storage,

long-term performance/reliability VIPV (Eolian Solar Car, Universidad de Chile)

Shingling PV Module (SunPower)ZEBRA IBC Solar Cell (ISC-Konstanz)


Introduction & motivation

4

• ECA-based joint

• Low cost interconnection solution

• Contact resistivity challenges

• Best Known Material/Method

• Long-term performance

• Measure/extrapolate contact resistivity

• Transmission Line Method (TLM)

• End Resistance Method (TLM-CER)

• TLM & ECA-based joints

• ECA transport mechanism (Percolation theory)

• ECA dispensing (Width & Thickness)

• Sample geometrical/dimension dependency4-point tester (GP Solar, ISC-Konstanz)

ECA-based joint (T. Geipel, 2018)

Interface

under study


Transmission Line Method

5

[1] H. H. Berger, “Models for contacts to planar devices,” Solid State Electron., vol. 15, no. 2, pp. 145–158, 1972.

[2] Y. Pan, “Versatile Circular Test Structure for Ohmic Contact Characterisation,”

2015.

𝐿𝑇

[1]

[1]

[1] [1]

The transfer length (𝑳𝑻) is defined as “the effective length over which the current must flow in the diffused layer before it transfers through

the alloy contacts into the metal layer”.[3]

[3] A. Goetzberger, R. Scarlett, R. Gereth, W. Schockley, V. Williams, and N. Zetterquist, “Research and investigation of inverse epitaxial UHF power

transistors,” 1964.

[2]



6



2015.

𝐿𝑇

[1]

[1]

[1] [1]

[2]



7



2015.

𝐿𝑇

[1]

[1]

[1] [1]

[2]



8



2015.

𝐿𝑇

[1]

[1]

[1] [1]

[2]


TLM with Contact End Resistance

9



10

[4] G. Reeves and H. Harrison, “Obtaining the Specific Contact Resistance from

Transmission Line Model Measurements,” IEEE Electron Device Lett., vol. EDL-3, no.

May, pp. 111–113, 1982.

𝑅𝑆𝐾 𝑅𝑆𝐻



11



May, pp. 111–113, 1982.


?

?

?

(𝑹𝑺𝑯 ≠ 𝑹𝑺𝑲)

For long contacts (𝑳 ≥ 𝟐𝑳𝑻):

[4]



12



May, pp. 111–113, 1982.


?

?

?



[4]



13



May, pp. 111–113, 1982.


?

?

?



[4]



14



May, pp. 111–113, 1982.


?

?

?



[4]



15



May, pp. 111–113, 1982.


?

?

?



[4]


Relevant issues

Sample geometry/dimension

16

• First proposal of metallization design:

• Results were presented in 37th EU PVSEC (Online Event 2020).

• Full proceeding publication is currently under revision to be published.

• Second proposal of metallization design:

[5] Devoto et al., “Measuring the contact resistivity of ECA-based joints,” EU PSEC 37th, 2020.


Relevant issues

Sample preparation

17


Relevant issues

Sample preparation

18

Hot plate

Sample


Relevant issues

Measurement procedure

19


Relevant issues

Method (TLM or TLM-CER)

20

•Results are only valid when the following

requirements are fulfilled:

• TLM plot slope is positive (𝑅𝑆𝐻 > 0)

• TLM plot ordinate axis intersection is positive

(𝐿𝑇 , 𝑅𝑆𝐾 > 0)

• The effective contact area is smaller than the

physical contact area (𝑳 > 𝟐𝑳𝑻)


Results

Tested contact areas and valid results

21

Finger pad

width [mm]

ECA line width

[mm]

Contact area

[mm²]Valid results

0.04

0.16

0.64

1.00

1.44

2.251.5

0.2

0.4

0.8

1.00

1.2

1.5

35 µm width finger with square pad at the end

0.2

0.4

0.8

1.00

1.2

Finger width

[mm]

ECA line width

[mm]

Contact area

[mm²]Valid results

0.80 0.16

1.00 0.20

1.20 0.24

1.50 0.30

0.80 0.32

1.00 0.40

1.20 0.48

1.50 0.60

0.80 0.64

1.00 0.80

1.20 0.96

1.50 1.20

0.80 0.80

1.00 1.00

1.20 1.20

1.50 1.50

0.80 0.96

1.00 1.20

1.20 1.44

1.50 1.80

0.80 1.20

1.00 1.50

1.20 1.80

1.50 2.25

1.5

Wider fingers

0.2

0.4

0.8

1.00

1.2


Results

Tested contact areas and valid results

22

Finger pad

width [mm]

ECA line width

[mm]

Contact area

[mm²]Valid results

0.04

0.16

0.64

1.00

1.44

2.251.5

0.2

0.4

0.8

1.00

1.2

1.5

35 µm width finger with square pad at the end

0.2

0.4

0.8

1.00

1.2

Finger width

[mm]

ECA line width

[mm]

Contact area

[mm²]Valid results

0.80 0.16

1.00 0.20

1.20 0.24

1.50 0.30

0.80 0.32

1.00 0.40

1.20 0.48

1.50 0.60

0.80 0.64

1.00 0.80

1.20 0.96

1.50 1.20

0.80 0.80

1.00 1.00

1.20 1.20

1.50 1.50

0.80 0.96

1.00 1.20

1.20 1.44

1.50 1.80

0.80 1.20

1.00 1.50

1.20 1.80

1.50 2.25

1.5

Wider fingers

0.2

0.4

0.8

1.00

1.2

Contact areas under

study

0.04

0.16

0.20

0.24

0.30

0.32

0.40

0.48

0.60

0.64

0.80

0.96

1.00

1.20

1.44

1.50

1.80

2.25


Results

New designs differences

23


Results


24


Results


25


Results

𝝆𝒄 versus contact area

26

M: 8.8e-5

N: 21

M: 1.1e-4

N: 108

M: 1.6e-3

N: 4

M: 2.0e-4

N: 84

M: 4.0e-4

N: 40

M: 9.0e-6

N: 3

M: 5.0e-4

N: 1

M: Median

N: Number of points TLM valid but

TLMCER invalid

M: 2.9e-5

N: 91

Take away:

• TLM is valid for

bigger contact

areas.


Results


27

TLMCER valid

but TLM invalid

M: 8.0e-3

N: 256

M: 5.2e-3

N: 240

M: 2.7e-3

N: 120

M: 2.1e-3

N: 9

M: 1.9e-3

N: 22

M: 1.2e-3

N: 12

M: 1.2e-3

N: 28

M: 3.1e-4

N: 93M: 2.5e-4

N: 30

M: 1.0e-4

N: 62

M: Median

N: Number of points

Take away:

• TLMCER is valid

for smaller contact

areas.

• 𝜌𝑐 values may be

one order of

magnitude bigger

compare to TLM.

• TLMCER is valid

for a wider range

of contact areas.

(2.9e-5)

(1.1e-4)

(1.6e-3)

(2.0e-4)

(4.0e-4)

(5.0e-4)


Results


28

TLMCER and TLM simultaneously valid Take away:

• When both

methods are

valid, results are

in the same order

of magnitude.


Results

𝝆𝒄 versus current injection

29

M: 1.9e-4

SD: 3.9e-4

N: 69

M: 1.0e-4

SD: 3.5e-4

N: 99

M: 9.3e-5

SD: 3.2e-4

N: 93

M: 9.3e-5

SD: 4.9e-4

N: 91

M: Median

SD: standard deviation

N: Number of points

TLM valid but

TLMCER invalid

Take away:

• Standard deviation

is in the range of

the values.

Accuracy must be

improved.

• 𝜌𝑐 is in the same

order of magnitude

for the showed

current injection

range.

• Smaller current

injections do not

deliver valid results.


Results


30

M: 5.7e-3

SD: 2.8e-3

N: 206

M: 3.1e-3

SD:3.2e-3

N: 237

M: 4.0e-3

SD:3.3e-3

N: 259 M: 4.0e-3

SD:3.4e-3

N: 259

M: Median

SD: standard deviation

N: Number of points

TLMCER valid

but TLM invalid

Take away:


is in the range of

the values.

Accuracy must be

improved.

• 𝜌𝑐 is in the same

order of magnitude

for the showed

current injection

range.

• Smaller current

injections do not

deliver valid results.


Results


31

TLMCER and TLM simultaneously valid Take away:

• When both

methods are

valid, results are

in the same order

of magnitude.

• At current

injection 10-4 A,

data tend to

spread more for

both methods.


Results

Rsk versus Rsh

32

Take away:

• The sheet

resistance under

the contact (𝑅𝑆𝐾) is

significantly higher,

by several orders

of magnitude, in

comparison to the

contact sheet

resistance outside

the contact.


Results

Transfer length versus contact area

33

M: 0.1979

SD:0.0789

N: 21

M: 0.1538

SD:0.1125

N: 91

M: 0.3536

SD:0.1133

N: 108

M: 0.2607

SD:0.0222

N: 4

M: 0.3496

SD:0.1421

N: 84

M: 0.2687

SD: 0.1467

N: 40

M: 0.1109

Sd:0.0242

N: 3

M: 0.2073

N: 1

M: Median

Sd: standard deviation

N: Number of points

TLM valid but

TLMCER invalid

Take away:


of the transfer

length tend to be

high for all contact

areas.


Results

Transfer length versus contact area

34

M: 0.042

SD:0.0302

N: 255

M: 0.042

SD: 0

N: 240

M: 0.042

SD: 0

N: 120

M: 0.042

Sd: 0

N: 90

M: 0.3242

SD: 0.1291

N: 9

M: 0.3213

Sd: 0.0710

N: 62

M: 0.2717

SD: 0.1100

N: 22

M: 0.2665

SD: 0.0415

N: 12

M: 0.74

SD: 0.0868

N: 93

M: 0.5027

SD:0.0436

N: 30

M: Median

N: Number of points

TLMCER valid

but TLM invalid

Take away:


of the transfer

length is

significantly smaller

at small contact

areas.


Conclusions

35

•TLM and TLMCER methods can be used on ECA-based joints.

•Fingers with square pads at the end demonstrate current crowding.

Wide fingers generate the best datasets.

• It was not possible to study the effect of the non-probed contacts in

between the probed contacts.

• We will manufacture samples without non-probed fingers using wide fingers.


Conclusions

36

•During the current sweep, only injections in the range 10−4 − 10−1 A

generate reliable and consistent datasets for TLM plot.

•Accuracy of the method can be further improve by improving the

sample manufacture as follows:

• Instead of dispensing by pressure, ECA will be jet dispensed.

• Instead of manual resistance measurements, resistance will be measured by an

automated machine.


Acknowledgement

37

• I would like to thank to the following funding projects:

• This work was supported by the German Federal Ministry for Economic Affairs and Energy

(BMWi) as part of the Zquadrat project with funding reference number 03EE1005A.

• This work was supported by the German Federal Ministry for Economic Affairs and Energy

(BMWi) as part of the HoSSa project with funding reference number 03EE1014D.

• Finally, I would like to thank to Niklas Grab for helping me with the manual

resistance measurements at ISC Konstanz.

Thank you for your attention!

Contact resistivity of ECA bonded joints · [5] Devoto et al., “Measuring the contact resistivity...

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