bursts Machining of semiconductors and dielectrics with ultra-short pulses: Influence of the wavelength and pulse

Machining of semiconductors and dielectrics with ultra-short pulses:

Influence of the wavelength and pulse bursts

B. Neuenschwander, B. Jaeggi, S. Remund, E. Zavedeev¹, S. Pimenov¹

1 Prokhorov General Physics Institute, Moscow, Russia

https://doi.org/10.24451/arbor.9234 | downloaded: 14.2.2022

▶Introduction / Motivation

▶Experimental Setup

▶Experimental Results

▶Conclusions

Outline

Pulse Picker AmplifierBurst = sequence of pulsesn

t_B

Seed oscillator

t_L

▶Picking a sequence of n pulses instead of single pulses

▶Typically: t_B = 10 – 20 ns

▶Flex Burst^TM: Adjustable energy of the single pulses

Motivation: Pulse Bursts

Motivation: Steel AISI 304 Machined with Bursts

 = 532 nm:

0 0.5 1 1.5 2 2.5 3

0 0.5 1 1.5 2 2.5 3 3.5 4

dV/dE / µm3/µJ

f₀/ J/cm²

Spec. Removal Rates for AISI 304 with Bursts

Single Pulse 2 Pulse Burst 3 Pulse Burst 4 Puse Burst 5 Pulse Burst

 = 1064 nm:

0 0.5 1 1.5 2 2.5 3

0 0.5 1 1.5 2 2.5 3 3.5 4

dV/dE / µm3/µJ

f₀/ J/cm²

Spec. Removal Rates for AISI 304 with Bursts

Single Pulse 2 Pulse Burst 3 Pulse Burst 4 Puse Burst 6 Pulse Burst 8 Pulse Burst

▶Often:

▶Burst mode leads to reduced fluence of the single pulses

▶This fluence is nearer the optimum value -> increased specific removal rate

▶But maximum rate of single pulses is never reached

Motivation: Copper C12200 Machined with Bursts

 = 532 nm:  = 1064 nm:

0 1 2 3 4 5

0 2 4 6 8 10 12 14 16

dV/dE / µm3/µJ

f₀/ J/cm²

Spec. Removal Rates for Copper C12 200 with Bursts

Single Pulse 2 Pulse Burst 3 Pusle Burst

0 1 2 3 4 5

0 2 4 6 8 10 12 14 16

dV/dE / µm3/µJ

f₀/ J/cm²

Spec. Removal Rates for Copper C12 200 with Bursts

Single Pulse 2 Pulse Burst 3 Pulse Burst 4 Puse Burst 5 Pulse Burst

▶Sometimes:

▶ Strong shielding for 2 and 4 pulse bursts detected for copper

▶ Increased specific removal rate for 3 and 5 pulse bursts @ 1064nm -> Gain in efficiency

How do semiconductors and insulators behave?

▶Laser systems:

▶FUEGO

▶λ = 1064nm, 532nm

▶Pulse durations: 10 ps

▶Inter burst: 12ns

▶Flex Burst -> constant energy

▶Materials:

▶Silicon: 𝐸_𝑔 = 1.12𝑒𝑉 (1.108µm), t = 650µm, <111> cutted, p-doped (15.2 Ω ∙ 𝑐𝑚)

▶Germanium: 𝐸_𝑔 = 0.67𝑒𝑉 (1.852µm), t = 400µm, <111> cutted, undoped (30.0 Ω ∙ 𝑐𝑚)

▶GalliumPhosphide: 𝐸_𝑔 = 2.26𝑒𝑉 (0.549 µm), t = 420µm

▶Diamond-like nanocomposite (DLN) film (a-C:H,Si:O films) thickness 2.7µm, grown on Si

Experimental Setup

▶Satsuma HP2

▶λ = 515nm

▶Pulse durations: 320fs

▶Inter burst: 24ns

▶Decreasing energy

▶Galvo scanner: SCANLAB IntelliSCAN_se14

Experimental Setup

▶1064nm, 10ps

▶f_obj = 160mm

▶w₀ = 15.5µm

▶M² < 1.3

▶532 nm, 10ps

▶f_obj= 160mm

▶w₀ = 7.2µm

▶M² < 1.1

▶515 nm, 320fs

▶f_obj= 100mm

▶w₀ = 7.1µm

▶M² < 1.1

▶Machining squares:

▶t = 10 ps, f_r = 200kHz

▶Size: 1.5x1.5mm (1064nm), 1.0x1.0mm (532nm)

▶Synchronized Galvo scanner (raster mode) [1,2]

▶Pitch p_x: w₀/2, Line pitch p_y: w₀/2

▶Number of slices N_sl:

▶1064nm: 96 (1), 48 (2), ….

▶532nm: 48 (1), 24 (2), …

▶Measure its depth d [3] with smartWLI:

𝑑𝑉

𝑑𝐸 = ሶ𝑉

𝑃_𝑎𝑣 = 𝑑 ∙ 𝑝_𝑥 ∙ 𝑝_𝑦 𝑁_𝑆𝑙 ∙ 𝑓_𝑟

𝑃_𝑎𝑣

Experimental Setup: Semiconductors

[1]: Jaeggi B. et al.: "Ultra-high-precision surface structuring by synchronizing a galvo scanner with an ultra-short-pulsed laser system in MOPA arrangement," Proc. SPIE 8243, (2012)

[2]: Zimmermann M. et al.: "Improvements in ultra-high precision surface structuring using synchronized galvo or polygon scanner with a laser system in MOPA arrangement," Proc. SPIE 9350, (2015)

[3]: Neuenschwander B. et al.: "Burst mode with ps- and fs-pulses:

Influence on the removal rate, surface quality and heat accumulation," Proc. SPIE 9350, (2015)

▶Diamond-like nanocomposite (DLN) films:

▶Machine craters or grooves

▶Measure depths and/or cross section with AFM

▶In case of grooves:

𝑑𝑉

𝑑𝐸 = ሶ𝑉

𝑃_𝑎𝑣 = 𝐴 ∙ 𝑝_𝑥 𝑁_𝑟𝑒𝑝 ∙ 𝑓_𝑟

𝑃_𝑎𝑣

Experimental Setup: DNL Films

Silicon 1064nm

0 1 2 3 4 5 6

0 1 2 3 4 5 6 7 8

dV/dE / µm3/µJ

f₀/ J/cm²

Si: Specific Removal Rates with Bursts @ 1064 nm

Single Pulse 2 Pulse Burst 3 Pulse Burst 4 Pulse Burst 5 Pulse Burst 6 Pulse Burst 7 Pulse Burst 8 Pulse Burst

▶Maximum specific removal rate increases with the number of pulses in the burst

▶Its location is shifted towards lower fluences

Silicon 1064nm

▶Maximum specific removal rate increases with the number of pulses in the burst

▶Its location is shifted towards lower fluences

0 1 2 3 4 5 6

0 1 2 3 4 5 6

0 1 2 3 4 5 6 7 8

fopt/ J/cm2

dV/dE|max/ µm3/µJ

# Pulses in the Burst

Si: Specific Removal Rates with Bursts @ 1064 nm Max. Spec. Removal Rate Optimum Fluence

Silicon 1064nm

▶Maximum specific removal rate increases with the number of pulses in the burst

▶Its location is shifted towards lower fluences

▶Single pulses:

▶Low fluence: Black

▶High fluence: Shiny

▶Optimum fluences:

s_a = 450nm – 650nm

50 µm

50 µm 50 µm

1P 2P 3P 4P 5P 6P 7P 8P

f₀

Silicon 532nm

▶Maximum specific removal rate does only slightly increase with the number of pulses in the burst

▶It’s location is shifted towards higher values

0 0.5 1 1.5 2 2.5 3 3.5

0 5 10 15 20

dV/dE / µm3/µJ

f₀/ J/cm²

Si: Specific Removal Rates with Bursts @ 532 nm Single Pulse 2 Pulse Burst 3 Pulse Burst

Germanium 1064nm

▶Shielding for 2 pulse burst

▶Strong increase for 3 pulse burst

▶Decrease for 4 pulse burst

▶Increase for 5 pulse burst

▶Small decrease for 6 pulse burst

▶“Copper-like” behavior

0 2 4 6 8 10 12 14

0 1 2 3 4 5 6

dV/dE / µm3/µJ

f₀/ J/cm²

Ge: Spec. Removal Rates with Bursts

Single Pulse 2 Pulse Burst 3 Pulse Burst 4 Pulse Burst 5 Pulse Burst 6 Pulse Burst

Germanium 1064nm

▶Shielding for 2 pulse burst

▶Strong increase for 3 pulse burst

▶Decrease for 4 pulse burst

▶Increase for 5 pulse burst

▶Small decrease for 6 pulse burst

▶“Copper-like” behavior

▶Optimum fluence:

▶Alternating

▶Tendency to smaller values

▶Roughness (Except single pulses):

▶Alternating

▶Tendency to higher values

s_a1 = 2.72µm s_a2 = 0.89µm s_a3 = 0.39µm s_a4 = 1.06µm s_a5 = 0.55µm s_a6 = 1.45µm 1P

2P 3P 4P 5P 6P

Germanium 1064nm: Influence of Roughness

▶Small fluences

0 2 4 6 8 10 12 14

0 1 2 3 4 5 6

dV/dE / µm3/µJ

f₀/ J/cm²

Ge: Spec. Removal Rates with Bursts

Single Pulse 2 Pulse Burst 3 Pulse Burst 4 Pulse Burst 5 Pulse Burst 6 Pulse Burst

Germanium 1064nm: Influence of Roughness

▶Small fluences

▶Abrupt changes in the spec. rate coincide with changes in the

roughness

0 0.5 1 1.5 2

0 1 2 3 4

0 0.2 0.4 0.6 0.8 1

sa/ µm dV/dE / µm3/µJ

f₀/ J/cm² Ge: 2 Pulse Burst

Spec. Rate Roughness

0 0.5 1 1.5

0 2.5 5 7.5 10 12.5 15

0 0.2 0.4 0.6 0.8 1

sa/ µm dV/dE / µm3/µJ

f₀/ J/cm² Ge: 3 Pulse Burst

Spec. Rate Roughness

0 0.2 0.4 0.6 0.8

0 2 4 6 8

0 0.2 0.4 0.6 0.8 1

sa/ µm dV/dE / µm3/µJ

f₀/ J/cm² Ge: 4 Pulse Burst

Spec. Rate Roughness

Germanium 1064nm: Influence of Roughness

▶Small fluences

▶Abrupt changes in the spec. rate coincide with changes in the

roughness

▶Hypotheses:

▶Spec. rate would drop

▶Absorption increases with roughness

▶Spec. rate first further increases

0 0.25 0.5 0.75 1 1.25

0 2 4 6 8 10

0 0.2 0.4 0.6 0.8 1

sa/ µm dV/dE / µm3/µJ

f₀/ J/cm² Ge: 5 Pulse Burst

Spec. Rate Roughness

Germanium 1064nm: Influence of Roughness

▶Small fluences

▶Abrupt changes in the spec. rate coincide with changes in the

roughness

▶Hypotheses:

▶Spec. rate would drop

▶Absorption increases with roughness

▶Spec. rate first further increases

0 0.2 0.4 0.6 0.8 1

0 2 4 6 8 10

0 0.2 0.4 0.6 0.8 1

sa/ µm dV/dE / µm3/µJ

f₀/ J/cm² Ge: 6 Pulse Burst

Spec. Rate Roughness

Diamond-Like-Nanocomposite: Ablation depth

▶3 main processes:

▶Graphitization

▶Spallation

▶Evaporation

▶Start: Single pulse E₀ = 0.5µJ

▶Green: N x depth

▶Black: single pulse 𝐸_𝑠𝑝 = 𝑁 ∙ 𝐸₀

▶Red: Burst 𝐸_{𝑏𝑢𝑟𝑠𝑡} = 𝑁 ∙ 𝐸₀

Diamond-Like-Nanocomposite: Temperature

▶The second pulse in a 2 pulse burst start on much higher temprature

▶Heat accumulation responsibel for higer rates?

Silicon at different f

: Spec. Removal Rate

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

0 1 2 3 4 5 6 7 8

dV/dE / µm3/µJ

f₀/ J/cm²

Si @ 1064nm: Spec. Removal Rates for different f_r

200 kHz 300 kHz 400 kHz 600 kHz 1000 kHz 1600 kHz

▶Surface temperature should increase with the repetition rate

▶Silicon shows 2 regimes with changeover between 1.0J/cm² and 1.5 J/cm²

▶In low fluence regime higer repetition rates leads to lower spec. removal rates

▶In the high fluence regime the differences almost vanishes

Silicon at different f

: Changeover

▶Changeover: black to shiny

▶Changeover is only slightly shifted to smaller fluences

▶Dependency on temperature seems not to be dominant

200 Hz 300 Hz 400 Hz 600 Hz 1000 Hz 1600 Hz

Silicon at different f

: Changeover

▶Changeover: black to shiny

▶Changeover is only slightly shifted to smaller fluences

▶Dependency on temperature seems not to be dominant

▶Changeover goes with a

significant drop in the surface roughness

▶Absorptivity(s_a) = ???

0 0.5 1 1.5 2 2.5

0 0.5 1 1.5 2 2.5 3 3.5 4

sa/ µm

f₀/ J/cm²

Si @ 1064nm: Surface Roughness for different f_r

200 kHz 300 kHz 400 kHz 600 kHz 1000 kHz 1600 kHz

Silicon at f

= 600kHz: Calorimetry

▶Partially transparent for 1064nm

▶During ablation small penetration depth -> residual heat is

measured

▶Low fluence on machined surface -> (1-R) is measured

▶Can be assumed as absorptivity during ablation process

Silicon at f

= 600kHz: Calorimetry

▶Partially transparent for 1064nm

▶During ablation small penetration depth -> residual heat is

measured

▶Low fluence on machined surface -> (1-R) is measured

▶Can be assumed as absorptivity during ablation process

▶Changeover:

Drop in the Absorptivity to almost its initial value

0 0.2 0.4 0.6 0.8 1

0 2 4 6 8 10

hHeat, 1-R

f₀/ J/cm²

Si: Energy Deposition on Machined Surface

Heat 1-R polished

Estimation of Surface Temperature

𝜙₀ ≈ 1 𝐽

𝑐𝑚² 𝜙₀ ≈ 1.6 𝐽 𝑐𝑚²

Estimation of Surface Temperature

f₀ = 1 J/cm² ; h_heat = 0.91

0 100 200 300 400 500 600 700 800 900 1000

-30 -20 -10 0 10 20 30

T / °C

x / µm

Heat Accumulation Below Changeover

200kHz 300kHz 400kHz 600kHz 800kHz 1000kHz 1600kHz

f₀ = 1.6 J/cm² ; h_heat = 0.7

0 100 200 300 400 500 600 700 800 900 1000

-30 -20 -10 0 10 20 30

T / °C

x / µm

Heat Accumulation Above Changeover

200kHz 300kHz 400kHz 600kHz 800kHz 1000kHz 1600kHz

▶ Heat accumulation is not the dominant effect for the changeover

▶ The changeover is expected to be caused by the fluence

▶ But heat accumulation could explain the drop in the spec. removal rate for higher rfepetition rates below the changeover

▶Semiconductors (Si, Ge, GaP) and DLN-films show an increase in ablation efficiency for the burst mode

▶More pronounced for IR radiation

▶Factor of 3 between single pulses and 8 pulse burst for Si and 1064nm

▶Surface quality rests high

▶Si single pulses: changeover from rough to shiny surfaces

▶Surface roughness influences the absorptivity and therefore the specific removal rate

▶Heat accumulation is expected to be another driving factor but its influence has to be clarified

▶Further experiments needed to better quantify the influence of roughness and heat accumulation

Conclusions

Thank you for your attention