Laser machining of silicon with bursts of ultra-short laser pulses: Factors influencing the process efficiency and surface quality

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Laser machining of silicon with bursts of ultra-short laser pulses:

Factors influencing the process efficiency and surface quality

Beat Neuenschwander, Stefan M. Remund, Thorsten Kramer

source: https://doi.org/10.24451/arbor.9235 | downloaded: 14.2.2022

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▶ Specific Removal Rate

▶ Experimental procedure

▶ Results for ps and fs

▶ Surface Roughness

▶ Calorimetry (ps vs fs)

▶ Transmission experiments (ps)

▶ Conclusion

Outline

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Experimental Procedure

▶ Standard galvo-scanner set-up

▶ Machine squares in Si

with side length s = 1.6 mm

▶ select a fixed number of pulses per area i.e. N_Sl depends on the pitch p and

number of pulses per burst

▶ Increase 𝑃_𝑎𝑣 i.e. the fluence from the threshold up to several J/cm²

▶ Measure the depth 𝑑 of the squares with a white-light interferometric microscope

▶ Specific removal rate g:

𝜸 = 𝑑𝑉 ൗ

𝑑𝑡 𝑃_𝑎𝑣 = 𝑑𝑉

𝑑𝐸 = 𝑠² ∙ 𝑑

𝑑𝑡 ∙ 𝑃_𝑎𝑣 = 𝑑 ∙ 𝑝_𝑥 ∙ 𝑝_𝑦 ∙ 𝑓_𝑟 𝑁_𝑆𝑙 ∙ 𝑃_𝑎𝑣

▶ p-type, 𝜌 = 1 − 100 Ω ∙ 𝑐𝑚, (100),

one side polished, thickness = 650 µm

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Experimental Procedure

Picoblade2, Dt = 10 ps, 1064 nm

▶ Identical sub pulses, Dt_B = 12 ns

▶ Refer to sub pulse peak fluence

▶ 1, 2, ….. 6 Pulse burst

▶ w₀ = 16.8 µm

▶ M² = 1.35

▶ 8, 10 ad 14 pulse burst

▶ w₀ = 13.9 µm

▶ M² = 1.77

Dt

_B

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Experimental Procedure

SATSUMA HPII, Dt = 350 fs, 1030 nm

-25 0 25 50 75 100

t / ns

fs Pulse Bursts

100% 88%

100%

93% 82%

93% 87%

77%

▶ Varying sub pulses, Dt_B = 25 ns

▶ Refer to 1st sub pulse peak fluence

▶ 1, 2, 3 and 4 Pulse burst

▶ w₀ = 16.5 µm

▶ M² = 1.53

▶ Due to limited P_av @ f_min = 505kHz, only up to 4 PB

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Comparison 10 ps vs 350 fs: Specific Removal Rate

0 1 2 3 4 5 6 7 8

0 2 4 6 8

g/ µm3 /µJ

Sub Pulse Peak Fluence f₀/ J/cm² Dt = 10 ps, Dt_B= 12 ns

1 Pulse 2 Pulse Burst 3 Pulse Burst 4 Pulse Burst

▶ Dt = 10 ps

▶ Except SP,

g increases with # of sub pulses

▶ Dt = 350 fs

▶ similar rates for

▶ SP and 2PB

▶ 3PB and 4PB

▶ Higher rates

compared to ps

0 1 2 3 4 5 6 7 8

0 2 4 6 8

g/ µm3 /µJ

First Sub Pulse Peak Fluence f₀ / J/cm²

Dt = 350 fs, Dt_B= 25 ns

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Dt = 10 ps, up to 14 Pulses per Burst

[1] Kerse et al., ”Ablation-cooled material

removal with ultrafast bursts of pulses”, Nature, 2016 537 (7618):84-88

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6 7

g/ µm3 /µJ

Sub Pulse Peak Fluence f₀ / J/cm² Dt = 10 ps, Dt_B= 12 ns

1 Pulse 2 Pulse Burst 3 Pulse Burst 4 Pulse Burst 5 Pulse Burst 6 Pulse Burst 8 Pulse Burst 10 Pulse Burst 14 Pulse Burst

▶ Maximum specific removal rate

increases with the number of pulses in the burst

▶ Gain up to a factor of 5

▶ Optimum point is shifted towards lower fluences

▶ 𝛾_𝑚𝑎𝑥 ≈ 8 𝜇𝑚³/𝜇𝐽

▶ Near the maximum value of 11 𝜇𝑚³/𝜇𝐽 obtained in [1] with 800pulse burst and ∆𝑡_𝑏 = 290 𝑝𝑠

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Comparison 10 ps vs 350 fs: Surface Roughness

0 1 2 3 4 5 6 7 8

0 2 4 6 8

sa/ µm

0 1 2 3 4 5 6 7 8

0 2 4 6

sa/ µm

First Sub Pulse Peak Fluence f₀ / J/cm² Dt = 350 fs, Dt_B= 25 ns

▶ Dt = 10 ps

▶ SP: s_a increases and then decreases to low values

▶ s_a increases with

#pulses per burst

▶ Dt = 350 fs

▶ SP: s_a increases with f₀

▶ s_a lower for bursts

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Comparison 10 ps vs 350 fs: Surface Roughness

0 0.5 1 1.5 2

0 2 4 6 8

sa/ µm

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Comparison 10 ps vs 350 fs: Surface Roughness

0 1 2 3 4 5 6 7 8

0 2 4 6

sa/ µm

First Sub Pulse Peak Fluence f₀/ J/cm² Dt = 350 fs, Dt_B= 25 ns

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Dt = 10 ps, up to 14 Pulses per Burst

0 0.5 1 1.5 2 2.5 3 3.5 4

0 1 2 3 4 5 6 7

sa/ µm

Sub Pulse Peak Fluence f₀ / J/cm² Dt = 10 ps, Dt_B = 12 ns

▶ s_a increases with increasing

▶ Peak fluence f₀

▶ # pulses per burst

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Dt = 10 ps, up to 14 Pulses per Burst

▶ s_a increases with increasing

▶ Peak fluence f₀

▶ # pulses per burst

▶ At optimum point (max g) s_a shows a tendency to increase with the #pulses per burst

▶ Could the higher s_a explain the gain in the specific removal rate?

0 0.2 0.4 0.6 0.8 1 1.2

0 2 4 6 8 10 12 14

sa,opt/ µm

#Sub Pulses per Burst

Roughness @ Optimum for Dt = 10 ps, Dt_B= 12 ns

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T / °C

t / s Sensor SIgnal

Calorimetry

[2]: F. Bauer, A. Michalowski, Th. Kiedrowski, S. Nolte, Opt.

Expr. 23, 1035 – 1043, (2015)

▶ From the incoming energy a part is always converted to heat

▶ Sample is heated up and cooled after irradiation

▶ T measured with a PT1000

▶ From this curve the residual energy in the sample can be calculated [2]

▶ 𝐸_{𝐻𝑒𝑎𝑡} respectively 𝜂_{𝐻𝑒𝑎𝑡} = 𝐸_{𝐻𝑒𝑎𝑡}/𝐸_𝑖𝑛 is measured

Silicon

Copper

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Calorimetry

▶ With ablation:

▶ 𝜂_{𝐻𝑒𝑎𝑡} corresponds to the part of the incoming energy finally remaining in the sample

▶ 𝜂_𝑟𝑒𝑠 = 𝜂_{𝐻𝑒𝑎𝑡} = 𝐸_𝑟𝑒𝑠/𝐸_𝑖𝑛

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Calorimetry

▶ With ablation:

▶ 𝜂_{𝐻𝑒𝑎𝑡} corresponds to the part of the incoming energy finally remaining in the sample

▶ 𝜂_𝑟𝑒𝑠 = 𝜂_{𝐻𝑒𝑎𝑡} = 𝐸_𝑟𝑒𝑠/𝐸_𝑖𝑛

▶ No ablation

▶ 𝜂_{𝐻𝑒𝑎𝑡} = 𝐸_{𝐻𝑒𝑎𝑡}/𝐸_𝑖𝑛 corresponds to the absorbed part of the energy (nontransparent materials)

▶ Absorptance 𝜂_𝑎𝑏𝑠 = 𝜂_{𝐻𝑒𝑎𝑡} = (1 − 𝑅)

▶ For non-transparent materials the part of the absorbed energy effectively remaining in the sample reads:

𝜂_{𝑟𝑒𝑠,𝑒𝑓𝑓} = 𝜂_𝑟𝑒𝑠/𝜂_𝑎𝑏𝑠

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Comparison 10 ps vs 350 fs: Calorimetry

0 0.2 0.4 0.6 0.8 1 1.2

0 1 2 3 4 5 6 7

hres/habs

Sub Pulse Peak Fluence f₀/ J/cm² Dt= 10 ps, Dt_B= 12 ns

0 0.2 0.4 0.6 0.8 1 1.2

0 1 2 3 4 5 6

hres/habs

First Sub Pulse Peak Fluence f₀/ J/cm² Dt= 350 fs, Dt_B= 25 ns

▶ Except SP similar behavior of h_res/h_abs

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▶ For all burst situations 𝜂_𝑟𝑒𝑠 is almost identical

▶ Almost constant values at optimum point

▶ The Surface Roughness cannot

explain gain in the specific removal rate

0 0.2 0.4 0.6 0.8 1

0 2 4 6 8 10 12 14

h

# Sub Pulses per Burst h_resand h_abs @ Optimum Point

res abs res/abs

Dt = 10 ps, up to 14 Pulses per Burst

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Results from 1d Two-Temperature Model

▶ Strong non linear absorption in Silicon

▶ Situation changes during the pulse

Intensity Carrier density

t / ps z / µm

0

1.5

-30 30

t / ps z / µm

0

1.5

-30 30

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Results from 1d Two-Temperature Model

▶ Even when ablation takes place first part of the pulse

▶ penetrates deep into the material

▶ can ev. be transmitted

▶ Calculation is

▶ time consuming for simulating a burst sequence

▶ sensitive to unknown initial values

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Transmission through Si-wafer

PD

▶ The focused laser beam is guided

▶ via a scattering surface (white paper)

▶ onto a fast photodiode

▶ monitored with oscilloscope

▶ A thin silicon wafer is moved trough the focal position

(p-type, 𝜌 = 0.1 − 10 Ω ∙ 𝑐𝑚, (100)

both sides polished, thickness = 220 µm)

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Transmission through Si-wafer

▶ The transmission significantly drops when the wafer reaches the focal

position

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

26 27 28 29 30

T / %

z / mm

Transmission Silicon 220 µm

Ep = 1.0 µJ Ep = 2.5 µJ Ep = 7.5 µJ

Focal position

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Transmission through Si-wafer

▶ The transmission significantly drops when the wafer reaches the focal

position

▶ Transmission as a function of the peak fluence (assumed Gaussian

beam) depends on the pulse energy?

▶ Beam not Gaussian (M²=1.77)

▶ Nonlinear effects like self focusing?

0 0.1 0.2 0.3 0.4

0 0.5 1 1.5 2 2.5

T / %

f₀/ J/cm²

Transmission Silicon 220 µm

Ep = 1.0 µJ Ep = 2.5 µJ Ep = 7.5 µJ

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Burst Transmission through Si-wafer

▶ Wafer in the focal plane

▶ For 14 single pulses with ∆𝑡 ≈ 1𝑠 (blue)

Transmission slightly drops by about 15%

0 50 100 150 200 250 300

-20 30 80 130 180

A / mV

t / ns

Focal Plane, f₀ = 2.47 J/cm²

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Burst Transmission through Si-wafer

▶ Wafer in the focal plane

▶ For 14 single pulses with ∆𝑡 ≈ 1𝑠 (blue)

Transmission slightly drops by about 15%

▶ Significant drop of about 75% for the 14 pulse burst (orange)

0 50 100 150 200 250 300

-20 0 20 40 60 80 100 120 140 160 180

A / mV

t / ns

Focal Plane, f₀= 2.47 J/cm²

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Burst Transmission through Si-wafer

▶ The drop for the burst depends on the peak

fluence of the sub-pulses

▶ Energy part responsible for ablation is dominated by the change in the

transmission

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

-25 0 25 50 75 100 125 150 175

PD Signal / V

t / ns

200µm Wafer: Transmitted 14 Pulse Burst @ Different Peak Fluences

1.7 mJ/cm2 10 mJ/cm2 16 mJ/cm2 28 mJ/cm2 62 mJ/cm2 0.23 J/cm2 0.72 J/cm2 2.47 J/cm2

∆𝑈 ∝ ∆𝐸

_{𝑛𝑜𝑛,𝑙𝑖𝑛}

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Burst Transmission through Si-wafer

▶ The drop for the burst depends on the peak

fluence of the sub-pulses

▶ Energy part responsible for ablation is dominated by the change in the

transmission

▶ For Bursts this part changes with f₀

▶ Effect might partially be responsible for the

observed higher specific removal rate with bursts

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

-25 0 25 50 75 100 125 150 175

PD Signal / V

t / ns

200µm Wafer: Transmitted 14 Pulse Burst @ Different Peak Fluences

1.7 mJ/cm2 10 mJ/cm2 16 mJ/cm2 28 mJ/cm2 62 mJ/cm2 0.23 J/cm2 0.72 J/cm2 2.47 J/cm2

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6 7

g/ µm3/µJ

Sub Pulse Peak Fluence f₀/ J/cm² Dt= 10 ps, Dt_B= 12 ns

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▶ Silicon shows a higher maximum specific removal rate when it is machined with bursts

▶ Gain of a factor of 5 for a 14 pulse burst compared to single pulses for 10 ps

▶ fs pulses (1030nm) are more efficient than ps pulses (1064nm)

▶ low surface roughness achievable with single pulses of 10ps (melting effect) or bursts of fs pulses

▶ Calorimetry for ps and fs bursts

▶ Similar behavior of effective residual heat 𝜂_𝑟𝑒𝑠/𝜂_𝑎𝑏𝑠

▶ Aborptance can not explain the gain in the specific removal rate

▶ Transmission experiments clearly show the nonlinear absorption of silicon at 1064nm

▶ Nonlinearities might be responsible for the achieved significant gain in the specific removal rate

Conclusion

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▶ We thank

▶ Josef Zürcher for all the SEM pictures

▶ Patrick Neuenschwander the calorimetry measurements

Acknowledgements

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Laser machining of silicon with bursts of ultra-short laser pulses: Factors influencing the process efficiency and surface quality