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Ultrashort Pulse Laser Micromachining of Silicon: Effect of Repetition Rate and Assessment of Surface Integrity of Microchannels

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Abstract

Achieving high surface integrity of micro features on the silicon wafers is highly challenging, owing to the inherent material properties like higher reflectivity, brittleness, and low toughness. In the present work, ultrashort pulse laser micromachining is used to create high-depth microchannels in silicon. Utilization of high average power ultrashort pulse laser for producing high-depth microchannels in silicon causes adverse effects on the feature quality. To investigate this, repetition rates from 10 to 500 kHz produce average power from 0.18 to 9 W at a minimum constant pulse energy of 18 μJ and peak fluence of 0.28 J/cm2. Surface integrity is evaluated in terms of surface roughness, melting thickness, and crack formation. The ablation depth of 18.24 μm is obtained in silicon at high average power of 9 W. The influence of heat accumulation is dominant at high repetition rates. The temporal separation between pulses decreases with an increase in repetition rates. This induces residual stress at the ablated region, eventually leading to the formation of a crack. Higher melting thickness, surface roughness, and presence of crack were significantly detected at high repetition rates from 333 to 500 kHz. The optimum surface integrity was obtained at a repetition rate of 200 kHz with a pulse energy of 18 μJ. The outcomes from the present research work are highly significant for manufacturing MEMS components. The fabrication of higher-quality micro-scale features on silicon wafers is still considered the most important area of research due to its wide range of industrial applicability.

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Abbreviations

c p :

Heat capacity of material

CW:

Continuous wavelength

d:

Spot diameter

P ed :

Laser energy density

EDAX:

Energy Dispersive X-ray Analysis

E p :

Pulse energy

fs:

Femtosecond

Fp :

Peak fluence

Fth :

Ablation threshold fluence (J/cm2)

FESEM:

Field-Emission Scanning Electron Microscope

h :

Convective heat transfer coefficient

HAZ:

Heat affected zone

ms:

Millisecond

ns:

Nanosecond

MT:

Melting thickness

N:

Number of pulses per spot

ps:

Picosecond

Q:

Heat source

Ta :

Initial temperature

T:

Surface temperature

x r :

Beam movement in x axis

x d :

Standard deviation of beam

Ra :

Surface Roughness (nm)

Si:

n-type doped silicon

Si3N4 :

Silicon nitride

β :

Trains of pulses

f :

Repetition rate (kHz)

k :

Thermal conductivity (W/mK)

v :

Scanning speed (mm/s)

ρ :

Density of material (g/cm3)

σ:

Residual stress (Pa)

Δω :

Difference in peak (cm−1)

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Acknowledgments

The present work is performed using the Ultrashort pulse laser facility at the Indian Institute of Technology, Madras, developed by Aeronautics Research and Development Board (ARDB).

Funding

Aeronautics Research and Development Board (ARDB). Project Number: ARDB/01/2031768/M/I.

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Authors and Affiliations

  1. Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India

    Shalini Singh & G. L. Samuel

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  1. Shalini Singh
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Contributions

Shalini Singh and G L Samuel investigated the concept, prepared the materials, gathered the data, and carried out the analysis. The methodology and reviewing are done by Shalini Singh and G L Samuel. The manuscript was reviewed by all authors.

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Correspondence to G. L. Samuel.

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Singh, S., Samuel, G.L. Ultrashort Pulse Laser Micromachining of Silicon: Effect of Repetition Rate and Assessment of Surface Integrity of Microchannels. Silicon 15, 4229–4246 (2023). https://doi.org/10.1007/s12633-023-02325-6

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