laser micromilling · 2007-08-15 · turning/2d or 3d focused ion beam 200 nm/20 nm 100 nm 20-30...

October 04 Industrial Business Review

Martyn Knowles

Oxford Lasers Ltd.

Unit 8, Moorbrook Park

Didcot, Oxon OX11 7HP

Tel: +44-1235-814433

Laser Micromilling :

An Enabling Technology for MicroComponent Replication


Outline

• Introduction

• Process Technologies

• Laser Micro-milling

• Summary


Micro-manufacturing

• Enabling

Bridging the gap between Nano and Macro

• Disruptive

Change our thinking as to How, When & Where products

will be manufactured and used

• Strategic

Economic, Reduced Space and Energy Costs,

Portability, Productivity


The Main Challenges in Micro-Manufacture

• Feature sizes below 20 µm

• Tolerances in the range of 1 to

10% of the nominal dimensions

(precision engineering <0.01%)

• Surface roughness required in

the range of 10 to 50 nm that is

smaller than the grain size of

the materialNanotechnology


Relevant Toolmaking Expertise

Mechanical ProcessesMechanical Processes

•• Micro-Milling Micro-Milling

•• Micro-Turning Micro-Turning

Feature Sizes [Feature Sizes [µµmm]]

2020 0.10.1200200 100100

•• Laser Ablation Laser Ablation

Energy Assisted ProcessesEnergy Assisted Processes

•• Micro-EDM Micro-EDM

•• Ion Beam Machining Ion Beam Machining


Lasers in Micro-manufacturing

• Established Applications

Via drilling

Hard disk texturing

Lithography

etc

• Factors Enabling Growth

Small features

Wide variety of materials

Flexible

• Factors Limiting Growth

Too slow for mass production (in many but not all cases)


Serial Production / Parallel Processing / Replication

• Most laser processes employ serial production or

parallel processing

Process speed limited by laser or mechanics/handling

• Laser + Replication Process

Laser

- laser milling to create tool/mold

- process speed unimportant

Replication Process

- high speed, mass production

Enabling Technology

New Laser Applications


Why Laser Micro-milling?

Laser Micromilling

2.5D or 3D milling by laser ablation

Most materials can be machined

High resolution (very small spot sizes)

Flexible setup,

Easily interfaced with other processes

Easily automated with CAD/CAM software

Main Issues:

Laser choice: laser-material interaction expertise necessary.

Deleterious thermal effects (melt recast, debris deposit)

Post processing sometimes necessary (extra manufacturing step)

Example: 316 s.steel, 1064nm

Lady’s head16 x 10 x 2.5mm

Other micromilling methods:

MicroEDM (material dependent)

Focussed Ion Beam milling (accurate but v. slow)

Lithography (slow, expensive)

www.oxfordlasers.com


Micro-scale machining comparison

Any1,000 !m3/ sec100 nm100 nm/10 nmPROFIB/2D & 3D

Any>100,000 !m3/secsubmicron5 !m /

submicron

ps laser/2D or 3D

Conductive materials25 millions

!m3/sec

3 !m25 !m / 3 !mMicro-EDM/2D or

3D

Any13,000 !m3/secsubmicron2-4 !m/

submicron

fs laser/2D or 3D

Polymer, ceramics (&

metals)

40,000 !m3/secsubmicron5 !m/ submicronExcimer laser/2D or

3D

PMMA, aluminium,

brass, mild steel

10,400 !m3/sec3 !m25 !m/2 !mMicro-milling or -

turning/2D or 3D

Any20-30 !m3/sec100 nm200 nm/20 nmFocused Ion Beam

(FIB) / 2D & 3D

MaterialsMaterial removal

rate

Feature positional

tolerance

Min. feature

size/ feature

tolerance

Technology/Feature

& geometry

N.B. LIGA can also be used to fabricate parts in polymers, pressed powders, ceramics etc.

October 04 Industrial Business Reviewwww.oxfordlasers.com

Short-Pulse Laser Ablation

100ns 1ns 100ps 1ps

0%

20%

40%

60%

80%

100%

1 2 3 4

Laser

Irradiation

Light

Absorption

Vaporisation

Ablation

Material

Removal

Laser Ablation

- material removal by a combination of evaporation and melt expulsion.

Ejected material (vapour & melt)

Recast (melt) material

Proportion of

evaporation vs melt

At 1J/cm2 courtesy Dausinger et al.

Melt Vapour


Processing with Ultrashort Pulses

The penetration to which a laser pulse interacts with material is determined by

optical and thermal penetration

L = L op + L th

In dielectrics optical penetration dominates over thermal and for long pulses

strongly depends on wavelength.

In metals, optical penetration is very short (typically tenth of wavelength) and

thermal penetration dominates (function of thermal diffusivity and pulse duration t).

L th = 2 !(K.t) for t >10ps



Metals

Laser energy is initially absorbed in electrons

Electrons thermalize in about 100fs

Thermal equilibrium between the electrons and lattice occurs after a multiple of the

electron-phonon relaxation time.

Typical electron-phonon relaxation are 0.5 - 50ps.

Simple conclusion from this is that for laser pulses less than the electron-phonon

relaxation time then no heat transferred to lattice, therefore no melt, thermal

damage etc.

However, this in not observed in practice and a less simple model explains why.



Metals

As the lattice heats up evaporation starts and continues for several ns.

Material stays molten for tens of ns

So even for ultrashort pulses, the resulting thermal processes are still in the ns

regime and are almost independent of pulse duration for pulse durations <10ps

Hence the melt layer is never zero but does approach a minimum which is sub-

micron.

There are some effects which depend on pulse duration <10ps and also on pulse

frequency and fluence.



Dielectrics

Optical penetration dominates over thermal penetration.

High intensity “rips” electrons out of the lattice.

Resulting ions repel each other and cause a “Coulomb” explosion.

Coulomb explosion is a non-thermal ablation mechanism.

Tests have shown that in dielectrics material is partly removed by Coulomb

explosion and partly thermally.

In metals there is no evidence for Coulomb explosion.

Ultra-high intensity is desirable to enhance the Coulomb explosion in dielectrics

crystals or glass so fs is preferable to ps for processing crystals and glass.


Summary of Ultrafast Laser Ablation

Metals

- Can be processed with fs and ps sources

- No advantage for fs, certain disadvantages for fs

- 10ps is optimum pulse duration for processing metals.

- Lower fluence improves “quality”

- Higher pulse frequency improves speed and there is no quality degradation when

the fluences are low and/or velocity high.

- Material removal rate for ps laser is > order of magnitude higher than fs because

of intensity and pulse frequency considerations.

Dielectrics - plastics, polymers (inc. teflon)

- High quality results can be achieved with ps and fs sources.

- Shorter wavelengths (eg 355nm) enhance results.

Dielectrics - crystals and glass

- Can be processed with with ps and fs sources.

- Shorter wavelengths (eg 355nm) for the ps source will enhance results.

- Best results probably achieved with fs source

ns laser pulses can also be

used but greater wavelength

dependence and increased

thermal input


Lasers Used in this Study

cNd:YAG aMP250 aDP100-355 bML Nd:YVO4 bML Nd:YVO4

Wavelength(nm) 1064 511 355 1064 355

Av. Power (W) 100 45 10 10 2

Pulse Freq. (kHz) 4 - 50 5 - 20 0 - 100 0 - 500 0 - 500

Max Energy (mJ) 10 4.5 1 0.3 0.02

Pulse Duration 10µs 17ns 30ns 15ps 12ps

a Oxford Lasers Ltd

b Lumera Laser GmbH

c DMG


Micro-milling Metals : Stainless Steel

20µm 20µm 20µm

Thick recast

Rough

Many voidsRa=0.86 - 2.20µm

Thin recast

Cracking of recast

layerRa=0.79 - 2.18µm

Recast too thin to

measure

Ra =0.29 - 0.56µm

1064nm, 10 µs 511nm, 17ns 1064nm, 10ps


Nanosecond Pulse Ablation : Micro-Milling

Polyimide

355nm

Alumina

511nmTungsten

511nm

Diamond

511nm

Examples of optimized processes with nanosecond laser sources


Nanosecond Laser Drilling :

Special Case of Micro-milling


Alumina 650µm thick

50µm Ø, 60µm pitch

44

45

46

47

48

49

50

1 51 101 151 201 251

Hole Number

Exit

Dia

mete

r [_

m]

Hole Size Repeatability

50µm square holes

entrance

exit

Si3N4 500µm thick, 511nm

90µmØ


Ceramics – ps Nd:Vanadate Laser

1mm2 square milled with 6 µm raster steps

Scan speed: 300 mm/s

50 kHz, spot size 17µm

Floor surface roughness Ra~0.90 µm

Unirradiated surface roughness Ra~0.73 µm

Surface roughness increases with raster step size

Typical excimer laser removal rate: 105 µm3/s1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0.1 1 10 100

Volume Removal Rate [x107 _m3/sec]

Su

rfa

ce

Ro

ug

hn

es

s,

Ra

[_

m]

Alumina (Al2O3) – 1064nm

100µm 50µm 10µm



Dielectrics – ps Nd:Vanadate Laser

1mm2 square milled out with 2 µm raster steps

Floor surface roughness Ra~0.99 µm

0.5W, 50 kHz

Scan speed 5-200 mm/s

Milled depth 30-200 µm

100µm100µm500µm

Fused silica - 1064 nm

Some edge chipping present

No evidence for laser induced microcracking



Dielectrics – ps Nd:Vanadate Laser

2 µm raster steps

Floor surf.roughness

Ra~0.434 µm

0.4

0.6

0.8

1

1.2

1.4

1.6

0 50 100 150

Laser Milled Depth [_m]

Su

rfa

ce

Ro

ug

hn

es

s,

Ra

[_

m]

5 µm

Fused silica - 355 nm



Workstation


General Manufacturing workflow

3D CAD modelling: Tool Design

Structuring of tool topography by FIB, Laser ablation,

mechanical and chemical processes

“hard tooling”

in polymers in metal, quartz or ceramics

moulds/inserts

for µIM

stamps

for HE

templates

for NIL

Replication of polymer, ceramic or glass components

“soft” tooling (pattering of small

patchs of the tool surface including

critical features)

Quality control (SEM, WLI, etc)

CAD model


Possible Process Chains involving FIB,

Laser ablation, NIL and electroforming

Nano- and micro- structuring of quartz templates

for the S-FIL process

3D CAD modelling: design of quartz template

Micro or nano- pattering of large areas (up to 4”)

in polymers

Transfer the polymer topography in Ni

Ni inserts and tool

integration

FIB,

Laser ablation

NIL

via S-FIL process

Serial production of

polymer components

Hot embossing,

µInjection Molding

Ni plating and

eletroforming

Serial production of Ni

components


Summary

• Micro-machining (laser & non-laser) processes have not yetreached the level of precision (%) achieved in macro-machining

• Laser micro-machining can be an enabling technology but islimited in many cases by throughput.

• Combining laser with other processes e.g. replication, canovercome the throughput problem.

• Ultrafast lasers are an enabling technology for lasermicromilling, especially “difficult materials.”

• Picosecond lasers are a good choice for combination of rangeof materials, machining quality and etch rate.

laser micromilling · 2007-08-15 · turning/2d or 3d focused ion beam 200 nm/20 nm 100 nm 20-30...

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