ultraprecision machining in optics manufacturing

© WZL/Fraunhofer IPT

Ultraprecision Machining in Optics Manufacturing

Dipl.-Ing. Martin Weinzierl

Fraunhofer IPT, Aachen, Germany

4M Workshop / CAS Conference, Sinaia

October 15th 2008

Seite 1© WZL/Fraunhofer IPT

Contents

Introduction

Diamond turning of spherical and aspherical optics

Diamond milling and fly-cutting

Manufacturing of high precision freeforms with fast tool servos

Summary

Source: Schneider GmbH


Contents

Introduction

Fundamentals




Summary



Applications - High Quality Optics in Daily Use

Automotive – Head-up displaySource: Siemens VDO

Medical technology –Intraocular lensSource: HumanOptics AG

LED FrontlightSource: Audi

Display technology for home entertainment Source: Philips

Color displays for mobile communicationSource: Casio


Process Chains for the Manufacture of Micro Structured Optics

PrototypingMastering

Galvanics

Recombination

Replication

Precision optics

Design

Process-iterations

Design based on reflective, refractive or diffractive principles

Prototyping or mastering of stamps or large area microstructures

Recombination of stamps to large area micro structured surfaces

Galvanic copying of microstructures into nickel

Replication into plastics

Design-iterations


Mastering and Mould Making

- Laser lithography

- Electron- and Ionbeamlithography

- Nano-Imprint-Lithography

- Screening

- Precision milling and turning

- Micro milling

- Grinding

- Ultraprecision machining

- Polishing

- (ECM, EDM)

- Step & Repeat

- Silicon moulding

- UV-curing

- Galvanic Nickel

- Electroless Nickel

Mechanical processes

Lithographic processes

GalvanicsRecombination


Ultraprecision Machining

Free form surface

Lens array

200 μm Processes

Turning (FTS), Milling, Fly-Cutting

CAD/CAM process chains for reduced geometrical complexity

Increasing machine-tool accuracy

Tools

Poly- and single crystalline diamond

Materials

Non-iron metals, Nickel-coatings, polymers

Precision

< 0.1 μm PV (depending on size and geometry)

Feature sizes down to 0.5 μm (V-grooves)

Surface quality

Optical, down to 2 nm Ra surface roughness

Micro pyramids


Ultraprecision Machining with Single Crystalline Diamonds

ρs : Cutting edge radius

Poly crystalline(fine grain)

w ~ 0,78 μm

0,5 μm

2,5 μm

5 μm ρs ≈ 2 μm

Cu

ttin

g e

dg

e

Single crystal

Wav

ines

s0,5 μm

2,5 μm

w < 0,05 μm

Highest surface qualities with Ra < 5 nm

5 μm ρs ≈ 50 nm

w ~ 2,06 μm

0,5 μm

2,5 μm

ρs » 4 μm 5 μm

Poly crystalline(coarse grain)

Poly crystalline(fine grain)


Base Materials for Diamond Milling and PlaningMetals

AluminiumBrassNickel silverCopperNickel coated steel samples

PlasticsPMMAPolycarbonate PCNylonAcetate

Brittle materialsGermaniumSilicon

Cu

8,96

129 pc

17

49-87 HV

1,69

Cu62Ni18

Zn20

8,72

120-135

16-17

75-190

29

ALZnMgCu0,5

2,76

72

23

170

18-22

Density [g/cm3]

Youngs modulus [GPa]

Thermal expansion [10-6 K-1]

Hardness [HB]

Elect. conductivity [μOhmcm]

Alum

iniu

m C

erta

l

Elec

trol

ess ni

ckel

Nickel

silv

er

OHFC

Bras

s MS

63

Cu63

Zn37

8,45

95-110

19-20

65-136

6,2-6,6

NI

8,9

199,5

13,3

100-190

6,9

Common materials for mould making


Ultraprecision Machining ProcessesFly-cutting

Diamond milling

Diamond turning

Fast-Tool turning


Contents

Introduction




Summary



Historical Development

1960

1970

1980

1990

USA– Continuous development in energy systems,

computers, military, metrology, sensors– Development of the laser

Requirements– Components with optical surfaces– Complex geometries and highest

form accuracy

Development of special machines with ultraprecision

First Applications– Laser mirrors– Hard drives– IR-Optics

Development of series machines with correlating process technology

Further applications– Aerostatic bearings– Scanner optics– Video heads– Copy machines

Systems and processes – for the micro structuring of surfaces– machining of smallest components


Precitech Ultra Precision Tuning Lathe

US-American machine tool builder since 1986Diamond tuning lathes since 1960 (Rank Pneumo)More than 1000 UP-Machines installed

Portfolio:

Diamond turning

Fast-, Slow-Tool

Fly-Cutting, Grooving

Milling

Focus:

Multi axes machines

Diamond tuning

Fly-Cutting, Grooving Source: Precitec


Geometry:

Radius rε = 1 mm

Angle of rake γ = 0°

Clearance α =12°conic

Opening angle ε = 100°

Typical products:

– Turning of contact lenses

– Ruling or lenticularlenses

Single Crystalline Diamond Tools - Standards

Diamond: natural/synthetic

Radii: defined,5 μm - 10 mm

Negative and positive rake angles

Controlled waviness Standard optional < 0,25 μm < 0,05 μm

Source: Contour Fine Tooling


Diamond Turning – Process Characteristics

Kinematics:

Transverse tool movement

Rotation of the work piece

Longitudinal turning and facing

Continuous cut

Rotational symmetric shapes

Advantages:

High geometrical variety

Direct machining of optical surfaces (Ra < 10 nm)

Realization of highest form accuracies (P-V < 250 nm)

Machining of metals, crystalline materials and polymers

Longitudinal turning Facing


Off-Axis Diamond Turning

Process characteristics:Work piece is mounted at an offset of the spindle centreDiscontinuous cutSymmetric mounting of work piece (at least two) due to rotor balance

Advantages:Machining of non-symmetric work piece surfacesNo centre artefacts due to tool misalignment Turning of non-symmetric work piece surfaces without using fast- or slow-tools servos

Disadvantages:Discontinuous cutHigh centripetal forces may cause deviations in the work piece surface Difficult alignment of the work pieces on the spindle


Challenges in Diamond Turning: Machining of Continuous Diffractive Structures

vf

Blazed diffractive structure

– Step hight down to 0.5 μm

– Contrast:Sharp corner geometrySmooth optical surface

15 μm

Diffractive structures:

LED technology

High performance optics

Structure: 0.5 – 5 μm

Blazed geometry

Aspheric base geometry


Contents

Introduction




Summary



Max. working area 1000 x 1000 x 200 mm³

Rotary table (C-axis)

Hydrostatic bearings for the main feed axes (not realised in vertical direction)

Two portal slides for either mass compensation or usage of two tools

Linear direct drives and hydrostatic lead screws

Equipped with standard NC controller

Ultraprecision Machine for Large Area Micro Structuring

Source: Fraunhofer IPT


Basic Machine Design

Vertical slides(z1- and z2-axis)

Rotary table(c-axis)

Table(y-axis)

Hydrostatic leadscrew

Portal slides(x1- and x2-axis)

Portal bridges(cast iron)

Bridge support(granite)

Base(granite)


Diamond Milling – Process Characteristics

Diamond ball-end milling tools

– Available tool radii:0.2 mm - 1.5 mm

Vertical alignment of the tool

Tilted alignment of the tool

Source: ALMT, Fraunhofer IPT


Diamond Milling – Applications

Source: ALMT, Kaleido, Fraunhofer IPT

Channel Structures

Spherical cavities

Aspherical lens arrays


Fly-Cutting – Process Characteristics

Rotating diamond toolAir bearing spindleLinear feed movement of the spindle over the work piece surfaceRotor revolutions between 2000- 3000 rpmInfeed up to 2 mmFeed rates between 5 – 100 mm/min Main process parameters:

– Spindle rotor revolution– Feed rate– Infeed

Applications:– Linear micro structuring – Planing of surfaces

Accuracy:– 0.1 μm / 100 mm– Ra < 20 nm

Rotational axis of the spindle rotor

aP = InfeedΔx= Structure pitch

Δx

aP

Feed direction

Single crystaldiamond tool

Micro structure 1 mm


Single Crystal Diamond Tools for Surface Structuring

200 μm

Facet tool

20 μm

V-shape tool

50 μm

Radius blade

Single crystal diamond

Posalux tool shaft

a

b

g

a : Clearance angle: Wedge angleg

Tool geometry

Blade radius

Cutting edge radius

< 50 nm

Waviness < 250 nm


Fly-Cutting – Applications

2 mm 50 mm

100 mm

10 mm

100 µm10 mm

0,5 mm 0,5 mm

Master of an illumination panel

Retroreflector element

Hot embossing tool

Master structure of a large surface reflector


Planing – Process Characteristics

Rotating tool

Linear movement of the spindle across the work piece surface

Rotating speeds between 1000 - 3000 rpm

Infeed up to 1 mm

Feed rates between 5 – 100 mm/min

Process parameter:– Spindle revolution– Feed rate– Infeed

Applications:– Planing of optical

surfaces– Machining of

reference surfaces

Infeed

Feed direction

Schematic process description

z

xy

Feed direction

Diamond tool Rotational direction of the spindle

Raw work piece surface

Optical surface

In machine quality controlof the surface (Fizeauinterferometer)


Planing - ApplicationsMould master with optical reference surfaces

Injection moulding toolwith optical mould inserts

Detail: Mould insert with a plane optical surface

Referencesurfaces

Functional surface


Challenges in Diamond Milling: Large Area Micro Structuring

Workpiece:Structured area: 640 x 640 mm2

Structure: 4-sided pyramids

Process:Tool: single crystalline diamondSpindle rotation: 3000 rpmTooth engagement frequency: 50 HzFeed: 40 mm/minMachining time: 15 daysDistance of cut: 546 mInvestigation on tool wearActive compensation geometrical offsets 10 mm


Tool Wear and Characteristic Damages During Fly-Cutting

Cutting edge break out:

Excessive load on the cutting edge :

– Mechanical reasons: e.g. planing at high infeeds

– Chemical reasons: Material (iron-contents!)

Effect: crucial to structure quality

Crater wear:

Long term non-stop machining in combination with high infeeds

Effect: drawbacks in surface quality

Cutting edge Break out:

Crater wear:


Tool Referencing and Tool Wear Investigation

Quasi-continuous investigation of tool wear

Optical inspection of the cutting edge

High resolution CCD-camera (pixel size approx. 4 μm)

Hardware resolution of 0.4 μm by use of 10-fold telecentric objective

Interpolated resolution < 0.1 μm by measurement software

Automated tool exchange

In-process offset compensation via NC-control

Point 1Name

0.00000X

0.000018Y

0.000000Z

Tool tip

50 μm


Contents

Introduction




Summary



Fast Tool Servo

Design– Aerostatic– CFR-Light-weight– Voice-Coil– Invar housing

Specification– Stroke: 10 mm– Max. Force: 500 N– Resolution: 2 nm

(after interpolation)– Stiffnes/pad: 90 N/μm– Max. Force: 100 N

each direction– Accelaration:

amax=500 m/s2

– Control loop: 16 kHz, 32 kHz

Fast Tool ServoSource: FHG IPT


Fast-Tool-Servo Machining – Composition of Free Form Surfaces

rotationally-symmetric optical element

non-rotationally-symmetricoptical element

Splitting of function-based surface encoding

z* = f (r, ϕ) = + f (r, ϕ)nrsf (r)rs


Limitations of Fast-Tool-Servo MachiningNon-rotationally symmetric structure

micro lens array

Main limitation:

Tool geometry– Clearance angle limits

the infeed angle

Examples:

Linear groove– Clearance angle: 20°– Length: 20 μm– Depth: 5 μm– Usable length: 5 μm

Lens array– Clearance angle: 20°– Radius: 2,5 mm– Max. depth: 130 μm

angle of structurevc

R =

2,5

mm

D = 1,6 mm

20 μm

FTS-dynamic

vc

tool

5 μm

workpiece

Usable groove length

angle of structure


Fast-Tool-Servo Machining – Applications

Examples:

Phase modulation mirror to reduce the transversal coherency of lasers

Facet mirror for beam shaping and beam guiding

Optical free form surfaces

– E.g. mould inserts for glass moulding

Materials:

Copper (OFHC)

Aluminium (6061, Certal)

Source: OEC AG, Fraunhofer IPT

00 μmMould insert Glass lens Application

Phase modulation mirror Facet mirror


Challenges in Production

Design– Calculation of optical surface– Analysis of machinability

Data conformity– Interface compatibility– Fitting and referencing

FTS control system design– Online set point calculation (conformity)– Controller of system

Metrology– Characterisation of surface without symmetry– Data analysis

Shrinkage compensation (replication)– No symmetry for simplification– Volumetric shrinkage approach


Contents

Introduction




Summary

Source : Schneider GmbH


Summary

Optical mould and die making with diamond machining processes offers:

– Structure sizes of only a few microns with submicron resolution

– Optical surface quality – Great variety of machinable materials

Freeform Optics are the latest development in optics manufacturing and of high interest for beam shaping:

– Advertisement, illumination, compact optic design– Enabling element for LED illumination

Replication of optics requires an iterative optimization:– Data conformity– Machine & control optimization– Deterministic characterisation– New means for compensation



Thank you for your attention !

For more information visit:

www.opticsmanufacturing.net

ultraprecision machining in optics manufacturing

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