Vjti Joshi Sir

Excimer Laser Micromachining

Dr. Suhas S. Joshi Associate Professor, Department of Mechanical Engineering Indian Institute of Technology, Bombay Email: [email protected] Phone: 022 2576 7527 (O) / 2576 8527 ® At Production Engineering Department V. J. T. I. Mumbai Date: 20.02.2009

Plan of Presentation • • • • • • •

Need for Lasers in Machining and Micro-machining Fundamentals of Excimer Lasers Excimer laser Micromachining Fabrication of Micro-arrayed structures using Excimer laser Micromachining Excimer Laser LIGA Pulsed Laser Deposition using Excimer Lasers Concluding Remarks

2

Laser Machining •

Need for laser processing of materials – Non-contact process involving no tool wear, no shock or mechanical loads on the work surfaces. – No chemicals or solvents are involved. – Selective material removal, where only surfaces of work material without affecting the surface below is possible. – Flexible processing in which beam acts as a soft tool.

3

Laser Machining Applications of Laser Machining [1]

4

Laser Micromachining • •

Types of lasers depend upon the lasing medium; these are: gas, solid-state, semiconductor, liquid-dye. Of these, only gas and solid-state lasers are practical for most of the industrial machining operations.

Typical Lasers for industrial machining [1] Gas lasers: i) Excimer – 193-351 nm ii) CO2 – 10 μm Solid state: i) Nd: YAG – 1.064 μm ii) Nd:YLF – 1.047 μm

5

Laser Micromachining Comparison between lasers and other manufacturing methods [1]

6

Fundamentals of Excimer Lasers • 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 an inert gas atom and a halide atom is produced that in the excited state. • These diatomic molecules have very short lifetimes (>5 ns) and dissociate releasing the excitation energy through UV photons. The stimulated emission must occur during this time.

7

Fundamentals of Excimer Lasers Formation of KrF [2] 1. Positive inert gas ion formation e + Kr => Kr+ + e + e 2. Inert gas in meta-stable condition e + Kr => Kr* + e 3. Negative halogen ion formation e + F2 => F- + F+ 4. KrF production Kr* + F- + M => KrF* + M 5. KrF production Kr* + F2 => KrF* + F 6. Stimulated emission KrF* + hv => Kr + F + 2hv (248 nm) Simplified diagram of molecular laser emission transition in excimer laser [1] The emission of UV photon occurs over relatively wide wavelength 0.4 nm. Typical gas mixture consists of Kr:F:Ne. The third element acts as a third body collision.

8

Fundamentals of Excimer Lasers •

The noble gases will not form compounds in the ground state but in the excited state they combine with certain elements.

•

The excimer lasers are available in six gas mixtures as indicated below. Types of Excimer lasers and their characteristics [1]

9

Fundamentals of Excimer Lasers • •

The laser medium in excimer lasers is pumped by a high-speed transverse electrical discharge. The high voltage pulse impressed across the preionization pins and electrodes cause ionization of the gas and pumps the excited atoms to the higher energy level.

Excimer laser resonant cavity [1]

10

Fundamentals of Excimer Lasers •

•

• •

•

The actual gas discharge process takes place in four stages: preionization, kinetic transfer, formation of excited dimers, and laser transition. An electron density of 107 – 108 electrons/cm3 is required to produce sufficient population inversion between lower and higher energy states. Typical excimer lasers employ spark ionization. Step 1: the discharge circuitry delivers an excitation pulse of 45kV to the electrodes and pre-ionization pins. Step 2: The electrons in the gas plasma are accelerated by the electric field between the electrodes and impart their kinetic energy to the surrounding atoms.

Preionization [1]

Gas discharge [1] 11

Fundamentals of Excimer Lasers •

•

• •

Step 3: Excited KrF* molecules are created by inelastic collisions with the electrons. They have very short life-time (>5 ns). They decay to spontaneously if not stimulated by an additional photon. Step 4: the laser transition is initiated by photons produced by spontaneous emission along the laser axis. These photons are reflected back and forth by the resonator optics to generate stimulated emission with other excited atoms. The laser emission occurs in about 20 ns pulse because the laser circuitry can not sustain a constant high voltage. After the pulse is completed, the gas constituents require a relaxation time of 120 ms before they can participate in the next discharge.

Formation of excimer molecules [1]

Laser transition [1] 12

Fundamentals of Excimer Lasers • •

•

The relaxation time imposes a constraint on the pulse rate of the excimer laser. Normally, this constraint is overcome by recirculating the laser gas so that the volume of gas is completely refreshed and exchanged several times between the pulses. The gas is cooled and filtered during the recirculation process such that repetition rates up to 400 pulses per second are achievable. Excimer laser head [1]

13

Components of Excimer Laser System •

A typical excimer laser system consists of following element: – Laser source – Attenuator – Homogenizer – Lenses – Masks

Schematic of a typical Excimer laser system 14

Components of Excimer Laser System Attenuator: Any excimer laser application requires precise control of energy levels. An attenuator helps in setting a accurate setting of energy required. The beam hits the dielectrically coated attenuator plate. Some portion of light is transmitted and some is reflected. The ratio between transmitted and reflected depends upon the angle of incidence. The beam then hits a compensator plate which has antireflection coating. Both attenuator and compensator plates rotate simultaneously to avoid shifting of beam axis. Any light beam reflected by the reflector is absorbed within the attenuator housing.

Photograph of attenuator [2]

15

Components of Excimer Laser System Typical transmission curve of an attenuator [2].

16

Components: Excimer Laser System Homogenizer: The point to point fluctuation in the energy density of excimer beam could be +/- 10%. This is very high to carry out precision micromachining. The homogenizer optics cuts the beam into pieces and mixes the parts of the beam such that the product is a complete homogenous distribution. It consists of an array of lenses (lenslets) in which the laser beam segments are superimposed. A condenser lens overlays all the beamlets at a focal plane of the condenser lens and creates homogenous profile .

Typical structure of homogenizer [2]

17

Components: Excimer Laser System •

A typical profile of excimer laser beam before and after homogenization [2]

Typical effect of homogenizer [2]

18

Components: Excimer Laser System

Metal and dielectric coated mirrors: In metal (Al) mirrors are highly reflective hence are more suitable. But they are soft and form aluminium oxide over a period of time which absorbs UV radiation. So Al mirrors with thin coating of MgF2 has better reflectivity and durability (80% reflection at 193 nm). If coating thickness is λ/4 then the two reflecting waves, Ir1 and Ir2 will have a phase shift of λ/2. If n2 = (n1 +n3)1/2, the Ir1 and Ir2 are equal and cancel each other completely as a result of interference. n1, n2 and n3 are refractive indexes of the media shown. Thus, we get an anti-reflection coating for wavelength λ. The dielectric coatings include coating of Al2O3, SiO2, etc. 99.8% reflectivity can be obtained.

Effect of dielectric coating on lens [2]

19

Components: Excimer Laser System • • • • • •

Typical mask imaging optics for micromachining applications using excimer lasers. The feature to be ablated is first made in a mask which contains an enlarged pattern. This helps especially if small features are to be made. In the mask, the dimensions are 5 to 10 times larges, which simplifies the manufacturing. Using the 10:1 image reduction, the structure can be projected on to the sample. The mask must be made of a material that can resist the energy densities during illumination. Usually, fused silica plates or stainless steel foils are used for making the mask. Mask projection system of 20 excimer laser

CCD camera

Laser Source

Beam Delivery System

XYZ Stage

Exceimer laser System at Suman Mashruwala Laboratory, I.I.T., Bombay

21

Components: Excimer Laser System Technical Specifications of Excimer laser system Excimer Laser generator Beam Delivery System Wavelength Energy

Make: Lambda Physik Make : Micro-lass 248 nm (KrF) 650 mJ

Pulse Duration

20 ns

Repetition Rate

1-50 Hz

Homogenizer XYZ stage

7 x 7 lenslets array 1 µm accuracy

Projection lens

4x, 8x & 20x Demagnification

Beam Size

15 mm x 15mm

Modes Machining resolution

HV, Energy Constant ~3 μm

22

Excimer Laser Micromachining • Interaction of excimer laser with solids involves absorption of short wave length and high-energy photons in a sub-micrometer thick surface layer. The two different ablation mechanisms that prevail are: 1. Photochemical 2. Photo-thermal • In the photochemical the also called as ‘cold ablation’, the processed material is nearly free of debris and thermal effects, which are limited to the edges of the generated structure. • In the photo-thermal reaction, the conduction and generation of melt are the decisive mechanisms.

23

Excimer Laser Micromachining •

Typical interaction of excimer lasers with various materials [2]

Comparison between laser machining and excimer laser ablation Laser machining

Excimer laser ablaiton

24

Work materials and their energy requirements for excimer laser ablation [2]

25

Methods of Excimer Laser Micromachining 1. Ablation using Mask Projection Method

Schematics of mask projection system 26

Masks for Excimer Laser Micromachining •

Contact mask technique: For low fleunces (<0.8 J/cm2), the mask is directly placed on the workpiece. This method few optical elements, metallic masks are used. The top layer of the material is structured to give desired shape.

•

Proximity mask technique: in which there is a small gap between the mask and the workpiece. Sharpness of the beam depends upon the gap and the divergence of the beam. Resolution can be around 100 μm.

•

Masking using imaging optics: A carefully designed optics system can provide a resolution of about 2 μm. The masks are produced precisely by laser cutting or etching or other lithographic techniques.

•

Chrome on quartz mask consists of a dielectric films coated with metal to improve its reflectivity.

•

Dielectric masks have surfaces with differing transmission characteristics. The surfaces are made such way that they absorb some of the wave lengths at some locations and all complete transmission at some other locations. 27

Masks for Excimer Laser Micromachining Techniques for mask making [2]

Photo-chemical machining

<2

Moderate

Mechanical micromachining methods

< 20

Low but have problems 28

of burrs

Methods of Excimer Laser Micromachining Step and Repeat Ablation using Mask Projection Method ¾Beam size larger than cutout feature on mask ¾ Similar feature can be ablated at different location ¾ Different microstructures on same workpiece by changing the mask ¾ Features such as blind holes (reservoirs in microfluidics) ablated by this technique

Step and Repeat

29

Methods of Excimer Laser Micromachining Workpiece Dragging Ablation using Mask Projection Method ¾Mask is stationary and workpiece is moving while laser is firing ¾ The shape of mask determines cross section of feature ¾ Long and curved microchannels can be fabricated ¾ Traverse speed, shot overlap and material decides smoothness of microchannels

Curved micro-channels by workpiece dragging 30

Methods of Excimer Laser Micromachining Mask Dragging Ablation using Mask Projection Method ¾Translating opaque mask into laser beam and keeping work piece stationary ¾ Linearly varying change in number of shots at the sample ¾ Ramps can be fabricated using this technique

Ramps fabricated by mask dragging

31

Methods of Excimer Laser Micromachining Synchronized Mask Scanning Ablation using Mask Projection Method ¾Both mask and workpiece are moving in synchronization while laser is firing ¾ Cylinder objects can be micromachined ¾ Devices such as biomedical catheters, micro-motors

Synchronized mask scanning 32

Methods of Excimer Laser Micromachining Hole Area Modulation technique ¾New 3D micromachining technique to ablate the cavities with depth information ¾ The principle is, if laser power density is fixed, ablated depth is proportional to exposure time of laser ¾ Array of holes of different diameters on mask

HAM Technique

¾ Mask is moved within the area of array pitch Micro-lens by HAM

33

Fabrication of Micro-channels ¾ In this experiment, micro-channels of gradually reducing size (100μm - 25μm) were fabricated to identify the effect of size variation on error in width of microchannel and ablation depth. ¾ Polycarbonate (PC) of 1.5 mm thickness selected as work material. ¾ E/Eo, repetition rate and number of pulses: Independent variables. ¾ Error in width of micro-channel and ablation depth: Dependent variables. ¾ 40 experimental runs were carried out with a variety of combinations of processing parameters mentioned in the table below using an experimental design ‘Response Surface Method’. ¾ Dimensions of micro-channels including depth were measured by Form Talysurf. Experimental parameters and their levels

Parameters

Level 1

Level 2

Level 3

Level 4

Level 5

E/Eo

2.636

4

6

8

9.363

Repetition rate(Hz)

1

2

4

6

8

Number of shots

2

5

10

15

19

34

Mask Design and Fabrication Microchannel Mask Design 2

3

1

4

Steps in Photochemical machining Mask for Excimer Laser

Microchannel Mask Prepare PhotoTool

Flattened Copper Sheet

Spin Coat Photo resist

Photoresist Stripping

Chemical Etching

UV exposure (Photolithography) 35

Micro-channel Fabrication

5.85μm

Depth Measurement on 100 μm wide channel Error in width = 0.25-0.5μm Ablated microchannel at 4 Hz, E/Eo = 6 and number of shots = 10 36

Micro-channel Fabrication Depth Measurement Location

13.3μm

Depth Measurement on 100 μm wide channel Error in width = 25-30 μm

Ablated microchannel at 6 Hz, E/Eo =8 and number of shots = 15 37

Excimer Laser Micromachining: Applications Micro-drilling of polymers

SEM image of laser micro-drilling of micro-pores on spin-cast thin PCL film at a laser fluence of 25 J/cm2, repetition rate of 10,000 Hz with an exposure time of 500 ms for 5000 pulses [2].

38

Excimer Laser Micromachining: Applications Micro-porous tubes made from Segmented Polyurethane (SPU)

Organic tubular micro-filters made of Segmented Polyurethane (SPU) [4]. The hole size is close to 100 μm 39

Excimer Laser Micromachining: Applications The promise and potential for microfluidics technology for the biomedical and life sciences is great. Microfluidic devices allow scientists to manipulate nanoliter volumes of fluids (such as drugs), permitting automation of complex chemical and biochemical processes, providing useful analytical and diagnostic procedures. A variety of fabrication techniques have been used for microfluidic devices, and excimer laser ablation is a promising one among them.

Nozzle structure centered on 10µm exit hole in ceramic [5]

40

Excimer Laser Micromachining: Applications Optically induced refractive index changes have many applications; one of them is ‘all optical switching in waveguide devices’. High quality gratings of extended area are necessary to demonstrate the switching effects and guarantee reliable performance of the device.

Grating structures generated by UV-photo-ablation. Applied wavelength: (a) 248 nm, (b) 193 nm, (c) 355 nm. Pulse duration: (a) 500 fs, (b) 20 ns, (c) 40 ps. Fluence: (a) 11.2mJ/cm2, (b) 350 mJ/cm2, (c) 7 mJ/cm2. Number of pulses (a) 50, (b) 2, (c) 100. Grating period: (a) 386 nm, (b) 1.0 micron, (c) 720 nm [6]. 41

Excimer Laser Micromachined Components

A cantilever of 50 μm on Kapton

A gear of 100 μm diameter on PMMA

A gear die 100 μm diameter on PMMA

42

Excimer Laser Micromachining: Applications Spinnerets are used as tiny nozzles for chemical fibre production [7]. The ceramic spinneret is a part with a high demand of precision. An excimer laser (Type Lambda LPX315i) with wavelength 248 nm is used as a laser source for micro-structuring ceramic spinnerets. The high processing precision achieved has been confirmed by measured nozzle geometries and also by several tests of laser-manufactured nozzles using a special computer-controlled flow measuring system. Typical ablation rates of less than 0.1 μm per pulse are seen (depending on the ceramic and the applied energy density). The processing time for the structuring of a 200 μm strong ceramic plate is less than 20 seconds at a repetition rate of 100 Hz.

43

Excimer Laser Micromachining: Applications • Surface oxidized BN/Cu coating films on stainless steel substrates have been synthesized by magnetron co-sputtering combined with KrF excimer laser irradiation. The irradiated films had very low frictional coefficients (µ = 0.075) in Ultra High Vacuum conditions. The low frictional property was maintained even after oxidization and prolonged UV irradiation. This coating film is therefore suitable for several aerospace applications [8]. • The use of a KrF excimer laser, with a 100-micron diameter spot size and 248 nm wavelength has been investigated, to define stripe patterns on magnetically soft multilayer thin films by ablation [9]. • Aerospace tribology and composite coatings for aerospace applications: Nanocomposite coatings made of carbide, diamond-like carbon (DLC) and transition-metal dichalcogenide phases are used in aerospace applications for reducing friction and wear. The preparation of such coatings within the W–C–S material system using a hybrid of magnetron sputtering and pulsed laser deposition has been done. These adaptive mechanisms achieve low friction coefficients of 0.02–0.05 and an endurance above two million cycles in space simulation tests [10]. 44

Excimer Laser Micromachining

Individual sets of ablation

Stage movement between ablations

Typical gear structure inside a square pocket by excimer laser micromachining

Micro-filter: pore size - 14.4µm. In-set 10X magnified image of icrofilter #1. Processing conditions: Energy: 300 mJ, Rep. rate: 3 Hz, No. of pulses: 300.

Ref: Ishan Saxena, Amit Agrawal and Suhas S. Joshi, Journal of Micromechanics and Microengineering, 2009. Vishal Barde and Suhas S. Joshi, Journal of Micromechanics and Microengineering, 2009 (Under 45 Review)

Generation of 3D Microstructures (ISRO 2009-2011)

Desired 3D structure on the work surfaces to achieve hydrophobic properties

Microstructures made on titanium surface by excimer laser micromachining at IIT Bombay

46

X – Ray LIGA •

• •

LIGA involves deep etching based on lithography, electroplating and molding. In its original version uses X-rays from synchrotron to expose thick layer of photo resist to a depth of 1000 μm and lateral resolution of 1 μm at very high speed. Excimer laser LIGA forms one of the cheaper alternatives. The methodology involves – – Excimer laser micromachining – Electroplating – Machining and separation of polymer – Injection molding – Demolding to get 3D structures

1500µm gear fabricated from SU-8 [Ref.]

47

Micro-electro-deposition Set-up 1 4

2 3

5 6

Micro-electro-deposition Set-up Identifiers: 1-continuous bath recirculation and filtration system; 2-constant current DC power supply; 3digital pH meter; 4-nickel sulphamate bath; 5-anode; 6-magnetic stirrer with temperature controlled hot plate 48

Micro-hot Embossing Set-up

Micro-Hot embossing set-up

49

Micro-pin hole array

Micro-slot 40 µm wide

Peripheral micromilling cutter

Fig. 1 Nano-delivery pin micro-wire-EDM

Micro-channels 400µm wide

a.

60 µm deep

c.

b.

Fig. 2 a-c Peripheral micro-milling of micro-channels 400µm wide x 60µm deep on 100µm foil

Fig. 3 Micro-pin hole matrix, 70 holes in seven rows, of φ60 µm by microEDM on 100µm thick foil / W electrode φ60 µm from φ3 mm by micro-turning 10 mm

Micro-gripper

Fig. 4 Spiral grooves on a spherical punch of φ 5 mm, groove depth at pole 100 µm and equator 60 µm using EDM

Actuator

10 mm

400 μm thin foil object

Fig. 5 Micro-gripper for pick-place Ref: Micro-component Development for BARC Mumbai (2004-2007).

Fig. 6 3D Microstructures 20µmx20µm on Titanium using excimer lasers 50

Initial Results: Reverse Micro-EDM 400µm sq.

Ф 200µm

800µm

Typical geometry of electrodes for reverse micro-EDM

Typical micro-rods/pins fabricated at IIT Bombay. φ 200 μm cylindrical, 400 μm sq. rod, . φ 60 μm micro-pin

Typical surface plots on reverse micro-EDMed surfaces

51

References [1] Micro-machining seminar by Resontics, Single Stop Solutions, www.resonetics.com [2] Dirk Basting (Ed), Excimer Laser Technology: Laser sources, Optics and Systems and Applications, March 2001, Lambda Physik. [3] K. S. Tiawa, M.H. Honga, S.H. Teoh, ‘Precision laser micro-processing of polymers’, Journal of Alloys and Compounds, Vol. 449, 1-2, p 228-231, Jan. 2008. [4] Kiyoshi Doi; Yasuhide Nakayama; Takehisa Matsuda; ‘Novel compliant and tissuepermeable microporous polyurethane vascular prosthesis fabricated using an excimer laser ablation technique’, Journal of Biomedical Materials Research, Vol. 31, 27-33, 1996. [5] Dr. Lamar E. Bullock, Advanced Technology & Manufacturing Center, University of Massachusetts Dartmouth, ‘Developing Microfluidic Devices Using UV Excimer Laser Materials Processing’. [6] M. A. Bader, S. Soria and G. Marowsky, Laser-Laboratorium Göttingen e.V., Hans-AdolfKrebs-Weg 1, 37077 Göttingen, Germany, ‘Excimer Laser Produced Polymeric Grating Waveguide Structures for All-Optical Switching’. [7] Klaus Dickmann, Laserzentrum FH Münster (LFM), Muenster, Germany, ‘Excimer Laser for High Precision Processing of Ceramic Spinnerets’. [8] Masahiro Goto, Akira Kasahara, Masahiro Tosa, Jonathan Hobley, Maki Kishimoto, Kazuhiro Yoshihara, Hiroshi Fukumura, ‘Low frictional coating by cosputtering in combination with excimer laser irradiation for aerospace applications’, J. Vac. Sci. Technol., A 20.4., Jul-Aug 2002, p 1458-1461. [9] Craig A. Grimes, Department of Electrical Engineering, The University of Kentucky, Lexington, KY 40506, ‘Fabrication of Magnetic Thin Film Structures for Control of Electromagnetic Interference’. [10] A.A. Voevodin, J.P. O’Neill, J.S. Zabinski, ‘Nanocomposite tribological coatings for 52 aerospace applications’, Surface and Coatings Technology, 116–119 (1999), p 36–45.