MEMS: Fabrication

Lecture 12: Micromachining with

Laser

Prasanna S. GandhiAssistant Professor,Department of Mechanical Engineering,Indian Institute of Technology, Bombay,

Recap

Laser fundamentalsStimulated emission Laser beam characteristics

Laser demo: Excimer laser

Today

Practical laser design: Excimer laser exampleLaser micromachining: Excimer laser example

Population Inversion

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This probability increase in spontaneous emission to power 3 of inverse wavelength makes it difficult to design lasers with short wavelength

Principle Components of Laser System

Active mediumSource of pumping energy

Discharge excitation (electrical current thro medium)Electrical or ion beam excitation (pulses of e/ions deposited in medium from accelerator)Microwave excitationChemical excitation (exothermic reaction)Nuclear excitation (nuclear reaction)

Resonating cavity

L

plane-parallel resonator (marginal stability)

r1 hemiconfocal resonator (stable)

r1confocal resonator r2

r1 r2concentric resonator

Light oscillator (laser cavity, laser resonator) r1= L

Laser medium r1= r2= L

mirror (R=100%)

Output mirror (R<100%)

221Lrr ==

Lrr=

+2

21

confocal unstable resonatorr1

r2

Principle set-up of various mirror configurations for a laser resonator

Source of Pumping

Electron or ion beam excitation (pulses of e/ions deposited in medium from accelerator)

High pulse energy, Low pulse frequencyShort tube life and higher cost

Microwave excitationLow pulse energy (100µJ),Good pulse freq (upto 8kHz)

Chemical excitation (exothermic reaction)Ok but not feasible

Discharge excitation: After 10-30ns discharge in excimer spark discharge: pulse limit

Excimer Laser• Created by IBM, Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton or xenon. When electrically stimulated, a diatomic pseudo molecule (dimer) usually of aninert gas atom and a halide atom is produced that in the excited state. These diatomic molecules have very short lifetimes and dissociate releasing the excitation energy through UV photons.

Consider collision between hydrogen and chlorine atoms. At very small energies, the two atoms come close to each other and the repulsive forces are converted to attraction forces primarily due to interaction of outer shells. This is called ‘molecular bonding’. Not all elements can form such bonds.

In Xe and Ar atoms, the forces remain repulsive. Under normal circumstances no bonding is possible. But in the excited stage chemical reaction takes place and a bond is formed between them.

Excimer Laser: KrF

Pumpinge + Kr Kr+ + e + e - positive inert gas ion formatione + Kr Kr* + e - inert gas in metastable conditione + F2 F- + F+ - negative halogen ion formationKr* + F- + M KrF* + M -KrF productionKr* + F2 KrF* + F - KrFproductionStimulated emissionKrF* + hv Kr + F + 2hv (248 nm) - laser emission.

Excited dimerExcited Complex : Exciplex

Excimer LaserExcitation: the method of excitation can be either by electron beams (highest pulse energy, high cost), micorwaves (with two electrodes outside the a laser gas filled capillary tube) low pulse energy) or gas discharge.

In gas discharge a high pressure gas vessel is used.

0.1 to 0.5 % - Halogen component

5 to 10 % of inert gas component

Buffer gas

•The preionization pins produce a spark discharge which produce UV radiation that is sufficient to preionize the laser gas between the electrodes.

•Discharge circuits::

•10 ns Limit on pulse duration because of arcing

Excimer Laser

The excimer lasers operating at about 2% conversion efficiency between electrical power input and optical power output, the excess energy is to be removed as heat.

A heat exchanger with water as a cooling medium is used.

Optics for Laser Micromachining

Beam conditioning opticsHomogenizer (array of cylindrical lenses) Beam collimator

Optics for reduction and focussing

Laser Micro-machining - Definitions

Ablation: The use of a laser to remove any material by vaporization.Absorption: The loss of light as it passes through a material, generally due to its conversion to other energy forms (typicallyheat).Avalanche Ionization: Free electrons colliding with a surrounding atoms, and breaking off more free electrons, create additional free electrons at an exponential rate.Conductivity: A material property that is the inverse to its resistance to the flow of electricity.Features: While we have yet to create features this small to date in materials, the principle has been demonstrated. Free Electrons: Electrons in the outer orbit around the nucleousof an atom, they can be moved out of orbit comparatively easily.

Laser Micro-machining - Definitions

Heat-Affected Zone (HAZ): The process has a threshold. Below that threshold energy from the laser pulse may be absorbed into the material and converted to heat that will dissipate into the surrounding material. Since the beam profile typically does not have sharp edges, some energy in the beam may be below the threshold for ablation. How much gets into the surrounding material depends on the exact beam shape, its relation to the threshold for ablation and the repetition rate of the laser. Typically, however, some set of conditions can be chosen to minimize these effects. The price for this minimization may be thruput; i.e. how fast material can be removed from the target. In some cases this may be unacceptably slow.Heat-diffusion time: All tool bits deposit mechanical energy into the material that is being machined, a portion of which is converted to heat energy.

Laser Micro-machining - DefinitionsUltrafast lasers can produce this state of matter because they pack so many particles of light called photons into so small a time interval that when they interact with the atoms in the surface of the material, they strip as many as 15 electrons off the atom. Physicists call this process multiphoton ionization.Peak Power: The maximum power supplied by a laser pulse.Picosecond: A fraction of a second (10-¹²). Abbrieviated as p.Power Density: In laser beam welding or heat treating, the instantaneous laser beam power per unit area. This parameter is key in determining the fusion zone profile (area of base metal melted) on a workpiece.Recast Layer: Molten metal which forms a layer of debris on the surface of the material during picosecond machining.Slag: The unwanted material that is removed from metal when it is heated to a liquid state.Terawatt: A unit or power equal to one trillion watts.

Laser Micro-machining - Definitions

Ultrafast: As it relates to micromachining, a laser capable of generating light pulses that last only a few femtoseconds. This can be achieved by nonlinear filtering to increase bandwidth andcompress the pulse or by passive modelocking or synchronous pumping in conjunction with pulse-shaping techniques.

Laser Micro-machiningThe laser beam is characterized by the half divergence angle (θ = D/2f) and radius of beam waist, w

For an ideal beam θw = λ/π (an invariant over the whole beam trajectory). A beam quality parameter is M2 number = [(θw) / (λ/π)]

The minimum spot diameter of a beam δ is given by –

δ = (4/π) M2 λ (f/D)

For micromachining smallest spot size is obtained when M2 ~1; with a short wavelength and short focal length lens.

Laser Micro-machining

Laser Micro-machiningThe primary requirement for laser micro-machining is availability of laser energies in a very small pulse durations.

Ultrafast pulses of light interact with materials in micromachining process is one on a different time scale.

Ultrafast pulses of light interact with materials therefore micromachining process is one on a different time scale.

Miliseconds 1x10-3 secondMicroseconds 1x10-6 secondNanoseconds 1x10-9 secondPicoseconds 1x10-12 secondFemtoseconds 1x10-15 second

In other words, a femtosecond is a million times shorter than a nanosecond. Femtosecond pulses are the fastest man-made “objects”. Nothing, absolutely nothing, is faster!

Mechanics of Micromachining

Simple modelI(X) = (1-R)I0 e(-αx)

α: absorption coefficientR: reflectance

h = 00

1 εεεα ⎟⎟⎠

⎞⎜⎜⎝

⎛− In : threshold fluence

Mechanics of Micromachining

Simple modelTemperature variation

T(x,t) = T0 erf ⎟⎟⎠

⎞⎜⎜⎝

⎛kt

x2

T0: surface temperature

κ: coefficient of temperature diffusion

erf: error function

http://mathworld.wolfram.com/

Mechanics of Micromachining

Two important parameters:Optical penetration depth Thermal penetration depth

Mechanics of Micromachining

To get proper machining with excimer:AbsorptionOptical length vs thermal diffusion length

MicromachiningTable

Material α-1[µm] L30=2 Ratio Material Characteristics

(τ=30 ns) kE=α-1/L30

Aluminum 0.007 3.3 0.002 Metallic behaviorkE<0.01

Silicon nitride 0.06 1.0 0.06 Strong absorberAlumina 0.08 0.8 0.1 0.01<kE < 10PI 0.07 0.16 0.4PET 0.1 0.14 0.7PC 0.2 0.13 1.5

PE 6 0.16 38 Weak absorberPMMA 16 0.11 145 10<kE < 1000

Quartz glass >108 0.32 >108 Transparent region1000 < kE

Thank You

Summary:

Laser Micro-machining – By long pulsesAbsorption: depends upon the w/p material, power density and wavelength. CO2 lasers – 20% absorption whereas Nd: YAG and exciemer 40-80% is absorbed.

The optical penetration depth is a depth for which power density is reduced to 1/e of the initial density. With CO2 lasers this depth is 15 nm and with Nd: YAG it is 5 nm.

The heat flow in the bulk of the material is a approx. one dimensional phenomenon and the temperature at the surface is given by –

T = (Ia/λ) (4at/π)1/2

Where, Ia – power density (W/cm2) ; a - thermal diffusivity = λ/ρc; t - time.

For Ia = 109 W/cm2 on steel, the melting point is reached in 300 ns. If power density is increased 10 times, time to melt is reduced to 3 ns.

Laser Micro-machining – By long pulsesThe high vaporization rate causes a shock wave and a high vapor pressure at the liquid surface considerably increases the boiling temperature. Finally, the material is removed by the expulsion of melt and explosive like boiling of the superheated liquid at the end of a laser pulse. Machining of metals generate a rim of resolidified material.

In plastics, the material is removed by breaking of chemical bonds of macro-molecules, and is dispersed as gas or small particles so no melting is found.

Laser Micro-machining – By long pulses

Machining with long pulse lasers – Long pulse machining

Example of a 25 micron (1 mil) channel machined in 1 mm (40 mils) thick INVAR with a nanosecond laser. INVAR, an alloy formed of Nickel and Iron, has an extremely small coefficient of thermal expansion at room temperature. INVAR is often called for in the design of machinery that must be extremely stable. This sample was machined using a “long” pulse laser. The laser pulse parameters are: pulse duration 8 ns, energy 0.5 mJ. The machining was not assisted by an air jet.

Laser Micro-machining – By long pulsesPlume or cloud development in laser micromachining takes a time. If a detectable plume is assumed to form at a time tp when the surface reaches a temperature of Tv (vaporization temperature) then

tp = π/4 (E/Ia2) where E- erosion resistance of the material given by λρcTv2.

It is a significant process variable as it is a measure of absorbed power density. It in collaboration with the focus distance, determines the plume initiation time.

In the focal area the process is mainly drilling, smooth machining is possible at a distance zopt.

Laser Micro-machining – By long pulsesMicroscopic view of Ablation by (Evaporation) long pulses: It involves –formation of a plasma and formation of a dielectric transparent layer on the work surface and a melt front.

There can not be any heat absorption in the super heated layer. Material above the layer is removed by evaporation. At side walls, the material is forced away by plasma pressure. At the end of pulse, pressure drops suddenly, the material is removed by boiling of superheated liquid and get redeposited around the processing area.

Laser Micro-machining – By long pulsesIn a numerical model of the process, a drop in absorptivity at the critical temperature causes decrease in the specific heat of evaporation with temperature.

Lv = Lv0 [1 - (T/Tc)2 ]1/2

At the critical temperature (about 1.4 times the boiling temperature) no extra heat is required for evaporation. Thus, above a certain fluence(energy per unit area), the critical temperature remains constant.

Ablation still occurs at the end of a pulse. For a given fluence, the ablation depth is maximum at a given pulse length.

Effect of ratio of free to bound electrons is a parameter. If the laser field is intense enough, a free electron colliding with a surrounding atom will knock off an additional electron. They in turn can knock two more electrons off atoms in the surrounding material and so on. This type of multiplication effect is called an avalanche effect, and because it createselectrons by ionizing atoms, it is called 'avalanche ionization.‘

More free electrons Less electrons Lesser free electrons Comparison

Laser Micro-machining – By Short pulsesThe laser-material interaction consists of a set of physical steps each characterized by its typical time constant.

The laser energy is transformed to the electrons first especially in the case of metals. The electrons will transfer the energy to lattice and finally within the lattice heat is distributed further by lattice collisions.

The Absorption of a photon by an electron requires about 10-15 s (1 femtosecond).

The relaxation time, the time required to transfer the energy to the lattice is 10-12 s (1 picosecond).

The time to diffuse heat in the lattice by conduction over a distance of optical penetration depth is 10-12 s (1 picosecond).

The time constants of these processes change the material removal process laser micromachining.

Laser Micro-machining – By Short pulsesFemtosecond ablation: 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 depth per pulse is given by –

Za ~ α-1 ln(Fa/Fth)

Where, Fa is the absorbed fluence and Fth is the threshold fluence = energy required to evaporate the irradiated volume of material; α -penetration depth (absorption). For α-1 = 10nm, Fth = 0.1 J/cm2.

For the material removal to occur, fluence should be about 3 times that of the threshold fluence.

The ablation process is a direct solid-vapor transition.The energy is transferred to the lattice from electrons after the pulse in a picosecond.

The result is a precise and pure laser ablation of materials.

Laser Micro-machining – By Short pulses

Laser machining by ultra-short pulses – Femtosecond pulse

Picosecond Ablation: Here the pulse length is of the same order as that of the transfer of energy from electrons to the lattice.

Due to the heat flow by the free electrons is also significantlyhigher.

Ii results in the formation of a solid-vapor or solid-plasma transition front but deeper in deeper in the material liquid phase is present.

Nanosecond Ablation: In this process, the heat of laser is used for melting and evaporation of the surface. The main energy loss is by conduction of heat into the work surface. Therefore, in the above equation thermal diffusion depth (at)1/2 replaces α-1.

For 20ns the thermal diffusion depth is 0.5 µm, threshold fluence is 4J/cm2.

Laser Micro-machining – Organic Polymers

In plastics, the mechanism of material removal is based on the photochemical reactions with photons.

The typical bonding energy for many macromolucules is 3-15 eV. This corresponds to the photon energy in the ultra violet range.

UV photons are absorbed in the top layer of thickness 0.2 microns.

Long chin of molecules is broken into parts. Molecules are removed from the processing area in the form of vapor.

Laser Micro-machining - ApplicationsLaser micro-drilling: by direct focusing. The focal length should chosen so that the focal diameter corresponds to the required hole diameter.

CO2 lasers for large number of holes in thin materials

Nd: YAG lasers for drilling of precision holes in harder materials.

Diamond is one of the difficult to machine materials, is optically transparent over a wide range of wavelengths. Under high power densities, the diamond is transformed into graphite which absorbs the laser power and is removed by ablation.

Laser Micro-welding: Of wires of 0.1 mm diameter.

Laser micro-adjustment: generation of thermal-mechanical stresses in metal structures.

Laser cleaning: for particles below 1 µm size get strongly adherent to the surfaces. They can be ablated by 124 nm eximer laser pulses.

Laser Micro-machining – Femtosecondlaser

All lasers produce light over some natural bandwidth or range of frequencies which is determined primarily by the gain / laser medium that the laser is constructed from.

For example, a typical helium-neon (HeNe) gas laser has a gain bandwidth of approximately 1.5 GHz (around 0.002 nm wavelength range), whereas a titanium-doped sapphire (Ti:Sapphire) solid-state laser has a bandwidth of about 128 THz (a 300 nm wavelength range).

The second factor which determines a laser's emission frequencies is the optical cavity or resonant cavity of the laser. For a simple plane-mirror cavity, the allowed modes are those for which the separation distance of the mirrors L is an exact multiple of half the wavelength of the light λ, such that L = q λ/2, when q is an integer known as the mode order.

In practice, the separation distance of the mirrors L is usually much greater than the wavelength of light λ, so the relevant values of q are large (around 105 to 106).

Laser Micro-machining – Femtosecondlaser

Of more interest is the frequency separation between any two adjacent modes q and q+1; this is given by ∆ν:

∆ν = c/2L c: speed of light

Using the above equation, a small laser with a mirror separation of 30 cm has a frequency separation between longitudinal modes of 0.5 GHz. Thus for the two lasers referenced above, with a 30 cm cavity the 1.5 GHz bandwidth of the HeNe laser would support up to 3 longitudinal modes, whereas the 128 THz bandwidth of the Ti:sapphire laser could support approximately 250000 modes.

Modelocking theory:In a simple laser, each of these modes will oscillate independently, with no fixed relationship between each other, inessence like a set of independent lasers all emitting light at slightly different frequencies. If instead of oscillating independently, each mode operates with a fixed phase between it and the other modes, the laser output behaves quite differently. Instead of a random or constant output intensity, the modes of the laser will periodically all constructively interfere with one another, producing an intense burst or pulse of light.

Laser Micro-machining – Femtosecondlaser

Such a laser is said to be mode-locked or phase-locked. These pulses occur separated in time by τ = 2L/c, which is the time taken for the light to make exactly one round trip of the laser cavity. This time corresponds to a frequency exactly equal to the mode-spacing of the laser, ∆ν = 1/τ.

If there are N modes locked with a frequency separation ∆ν, the overall modelocked bandwidth is N∆ν, and the wider this bandwidth, the shorter the pulse duration from the laser.

For example, for a laser producing pulses with a Gaussian temporal shape, the minimum possible pulse duration ∆t is given by: ∆t = 0.44/N∆ν

The value 0.44 is known as the time-bandwidth product of the pulse, and varies depending on the pulse shape. Using this equation, we can calculate the minimum pulse duration which can be produced by a laser. For the HeNe laser with a 1.5 GHz bandwidth, the shortest Gaussian pulse which can be produced would be around 300 picoseconds; for the 128 THz bandwidth Ti:sapphire laser, this duration would be only 3.4 femtoseconds.