Ultra Short Pulse Lasers for Micromachining

Question 1

What kind of process is the mechanism of laser micro machining? 2

Answer 1

Short pulse Photo chemical process or short pulse photo thermal.

Question 2

What is photo chemical process for micro machining?3

Answer 2

Where there is absorption at very specific wavelengths.
Clean scission(cutting) of the material
UVC processing of polymers

Question 3

What is photo thermal process for micro machining?3

Answer 3

Laser is absorbed at the optical penetration depth
Heat is conducted through the material to the thermal penetration depth
Material is ejected ie melted material is blown out, vaporising ejects material, evaporation and plasma or shockwaves from these effects

Question 4

What are the values from these prefixes: milli, micro, nano, pico and femto.

Answer 4

milli 10-3, micro 10-6, nano 10-9, pico 10-12 and femto10-15.

Question 5

What are ultra fast laser characteristics? 3

Answer 5

1. Pulse Width
   Laser pulse width ~ 10 ps to 5 fs
   (1×10^(−11)s to 5×10^(−15)s);
2. Pulse Energy
   millijoules mJ to picojoulse pJ;
   (too low for some applications)
   Average powers = milliwatts to
   watts;
   3.Peak Power !!!
   Ep = 5 mJ in tp= 100 femtoseconds, the peak power of such a laser would be 50
   gigawatts (GW)
   Irradiance ranges from 1015−1022
   W/cm2
   Laser repetition rates = MHz or
   even GHz.

Question 6

What are the laser material interaction times for electron heating, electron energy relaxation time and electron energy transfer to lattice.

Answer 6

Electron heating (into excited states) by laser: 10 fs.
* Electron energy relaxation time: 1 ps.
* Electron energy transfer to lattice (thermal): 10 ps.
* During fs laser pulse, lattice (bulk) is not heated.

Question 7

What are the 4 mechanisms of ultra short pulse?

Answer 7

1.Single photon Absorption
2.Multi Photon Absorption
3.Femtosecond ablation
4.Pico Second ablation

Question 8

What is Single photon absorption?

Answer 8

• How our photothermal and photochemical interactions have been taking place.
• Laser ablates a material by exciting electrons and through avalanche ionization; eventually excite electrons resulting in material breakdown which is the case with single photon absorption.

Question 9

What is multi photon absorption?

Answer 9

• The greater peak power of ultra-short pulses means it is possible for an electron to absorb several photons in multi-photon absorption
  mechanisms.
• This absorption mechanism dependent on the
  intensity of the laser pulses so this effect is more pronounced with ultrafast pulses.

Question 10

What is femtosecond ablation?

Answer 10

There is no transfer of energy to the lattice during this process. All the energy is stored in a thin surface layer.
* This energy will be more than the specific heat of evaporation and there will be vigorous evaporation after the incidence of the pulse.
* The ablation process is a direct solid-vapour transition.
* The result is a precise and pure laser ablation of materials.

Question 11

What is Picosecond ablation?

Answer 11

• Here the pulse length is of the same order as that of the transfer of energy
  from electrons to the lattice.
• The heat flow by the free electrons to the lattice is higher.
• It results in the formation of a solid-vapor or solid-plasma transition front but deeper in the material liquid phase is present.
• Compared with femtosecond lasers, picosecond laser sources are often more economical and in applications such as micromachining, they offer similar performance.

Question 12

What is the two temperature model?

Answer 12

Hot electrons in the ejected material
and cold ions (phonons) in the bulk
material
* Telectrons – Tions ≈ 6,000 to10,000K)
* This causes significant ejection of the
material.

Question 13

What are the two regimes of ablation rate?

Answer 13

Coulomb explosion
Phase explosion

Question 14

What is Coulomb Explosion (gentle)?

Answer 14

• Occurs at relatively low fluence.
• The energy transfer during laser–material interaction occurs only within the area characterized by the optical absorption depth.
• Positively charged ions repel each other causing removal of materials – cold (non-thermal ablation).
• Typical ablation rate: 1-30 nm/pulse.
• Smooth surface after material removal and no chemical changes or no debris.

Question 15

What is Phase Explosion(strong)?

Answer 15

• At high laser fluence the ablation rate is dominated by the thermal diffusion
  length.
• Vapour/thermal plasma heating dominates (up to few nano second).
• Strong thermal ablation and melt ejection.
• Rougher surface.
• High ablation rate > 100 nm/pulse.
• Material removal is proportional to incident energy.

Question 16

What can the high peak intensity of a fs laser cause?

Answer 16

Induce nonlinear absorption of materials

Question 17

What are the best types of laser to used for ms, ns, ps and fs processing?

Answer 17

ms Nd:YAG, ns(DPSS Escimer), ps(Nd:YAG or Nd:YVO4 Fibre) and fs(Ti:sapphire)

Question 18

Difference between ns to ps to fs in laser material defect result?

Answer 18

ns is the worst as there will be more debris, less clearer edges and recast.

Question 19

What is the benefit of laser patterning of thin Mo on glass?

Answer 19

Burr Free, melt free, no delamination and isolated channel

Question 20

What are the limitations in ultra fast processing?

Answer 20

• claim to remove substrate melting resulting in pure ablation as the pulse time is significantly shorter than thermal heat diffusion by conduction.
  -Plasma effects in fs lasers can exist for several micro seconds providing enough energy and time for melting. Either by low fluence(energy per unit area) = linear process related to radiation based solution or high fluence = break down threshold for air and water, the create plasma here is responsible for primary effect on process

Question 21

Compare the pulse length in regard to thermal melting/evap or explosion

Answer 21

ms- Thermal Melting/heat conduction is
dominant
ns - Thermal Evaporation and resolidification
ps- Phase explosion and thermal build-up
fs- Phase explosion

Question 22

What is the relationship of processing rate and energy efficieny?

Answer 22

As the energy increases the ablation also increases however this is more efficient for fs laser than ns for example.