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VISVESVARAYA TECHNOLOGICAL UNIVERSITY

BELGAUM, KARNATAKA

A SEMINAR REPORT

ON

FABRICATION OF ASPHERIC MICRO-LENS ARRAY BY EXCIMER LASER

MICRO MACHINING

A seminar report submitted in the partial fulfilment of the requirement of the

completion of the requirements for Bachelor of Engineering in

MECHANICAL ENGINEERING

SUBMITTED BY:

TANWEER KHAN

1GA06ME051

Under Guidance Of

Dr. M.S.Rajagopal BE, M.tech, PhD, FIE

DEPARTMENT OF MECHANICAL ENGINEERING

GLOBAL ACADEMY OF TECHNOLOGY

Raja Rajeshwari nagar, Ideal Home Township Bangalore-560098

YEAR-2012-2013


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DEPARTMENT OF MECHANICAL ENGINEERING

GLOBAL ACADEMY OF TECHNOLOGY

Ideal Homes Township, Raja rajeshwari nagar, Bangalore98

CERTIFICATE

This is to certify that the Seminar report entitled FABRICATON OF

ASPHERIC MICRO-LENS ARRAY BY EXCIMER LASER MICRO-MACHINING

submitted byTANWEER KHAN bearing 1GA06ME051, bonafide student of

GLOBAL ACADEMY OF TECHNOLOGY has successively completed the

Seminar work in partial fulfillment for the award of BACHELOR OF

ENGINEERING in mechanical Engineering prescribed by Visvesvaraya

Technological University, Belgaum during the year 20122013.

-----------------------

Guide

Name of the Guide

-----------------------

Head of the department

Dr. M.S.Rajagopal

-----------------------

Principal

Dr. Narendra Vishwanath

Date : Examiners :

1.

2.


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.

ACKNOWLEDGMENT

The satisfaction and euphoria that accompany that successful completion of any task

Would be incomplete without mentioning that the people who made it possible. So with

deep gratitude I acknowledge all those guidance and encouragement served as beacon

light and crowned my efforts with success.

With due respect, I sincerely thank to my guide and Head of the Department (HOD)

Dr. M.S. Rajagopal BE, M.tech, PhD,FIE, Department of Mechanical Engineering

Global Academy of Technology for his encouragement and support during this seminar

work.

I would express my sincere gratitude to Dr. Narendra Viswanath,principal of G.A.T.

Lastly, I express my sincere gratitude to all those who directly or indirectly helped

throughout this seminar work.

TANWEER KHAN

USN: 1GA06ME051


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ABSTRACT

This topic presents [1] a unique method for fabrication of aspheric micro-lens array

based on a KrF 248 nm excimer laser micromachining with precise surface profile

control. Based on a planetary contour scanning laser micro machining method along with

a shading metal mask and sample movable stage, an array of micro-lens with precisely

controlled surface profiles can be fabricated.

Each lens surface profile can be aspheric and pre-designed. In this experiments have

been carried out and the machining accuracy of each lens surface profile is examined.

Good surface roughness and profile accuracy are observed.

This paper [2] also demonstrates a newly developed laser scanning method is

introduced for machining refractive types of micro lenses, which have pre-designed

surface profiles aiming at minimizing the optical focal spot sizes. Optical testing on the

fabricated aspheric micro lenses shows significant improvement in focusing capability

and the focal spot sizes are approaching optical diffraction limits.

The proposed excimer laser micromachining method is flexible, versatile, and accurate,

hence can be very useful and powerful in machining 3D microstructures of complex

profiles and demanding profile accuracy.


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CONTENTS

1: Introduction [1]

2: Literature Survey

3: Technological aspects of laser micro machining

3.1: Definition

3.2: Construction

3.3: Operation

4: Components of typical industrial laser machining system

4.1: Process site

4.2: Laser light source

4.3: Beam delivery system

4.4: Part handling and motion control system

4.5: Control electronics

4.6: Laser support equipment

4.7: Structure and enclosure

5: Comparing with different types of laser [2]

6: Important Parameters

6.1: Absolute Energy

6.2: Laser Power

6.3: Intensity

6.4: Wave length

7: Configurations of UV excimer laser

7.1: UV excimer laser gases

7.2: UV excimer laser discharge

7.3: Extending laser gas life

8. Photo ablation process [2]


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9: Fabrication of Micro lenses [3]

10: Application of Excimer Laser Machine

10.1: Laser Eye (or excimer laser) Surgery

10.2: Biological Matter and Organic Compounds10.3: Manufacturing of Micro Electronic Chip

10.4: Marking

10.5: Drilling

11: Advantages

11.1: Unique Properties of Excimer Laser Radiation

11.2: Resulting Benefits in Materials Processing

12: Disadvantages

12.1: Discharge Circuit

12.2: Toxic Laser Gas

12.3: Managing high-power UV

13: Future technology

14: Conclusion

15:Reference

LIST OF FIGURES

1: Fig.1 High intensity of laser beam

2: Fig.2 Operation of excimer laser micromachining

3: Fig.3 comparing different types of laser [2]

4: Fig.4 Typical excimer laser configuration [1]

5: Fig.5 Discharge of laser

6: Fig.6 The photo ablation process [2]

7: Fig.7 Fabricated micro lenses [4]

8: Fig.8 Shows measured 3D profile of lens [5]

9:Fig.9 Experimental setup [5]

10: Fig.10 Concept of eye surgery


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1. INTRODUCTION

Excimer laser micromachining technology is well known as a powerful tool for

fabrication of aspheric micro-lens array. It utilizes pulsed lasers of short wavelength suchas KrF(248nm), ArF (193 nm), XeCl (308 nm), and XeF (351 nm). The laser pulses

usually have high pulse energy and short pulse duration, around a few to a few tens of

nano-seconds.

In addition to the laser source, a typical excimer laser micromachining system

usually equipped with an optical project system, which modulates the laser beam pattern

with a photo-mask and then projects the pattern onto sample surface, and a servo

controlled mechanical scanning stage, which synchronizes the sample motion with the

laser pulse firing[1]. These additional capabilities allow an excimer laser

micromachining system to carry out fabricating aspheric micro-lens array with great

flexibility and machining accuracy.

Micro-lens and micro-lens array are important elements for many applications on

optical data storage, digital display, and optical communication. A number of fabrication

methods had been developed to fabricate these optical elements, especially for poly-

meric micro-lens and micro-lens arrays. Typical methods include photo resist thermal

reflow, photo thermal method, photo- polymer etching, micro jet method, laser ablation,

and micro-molding or hot embossing method. Although the above mentioned methods are

widely used, a common problem shared by all these methods is that the micro-lens

surface profile is not accurately controllable [2].

The mechanism of the material removal is based on laser-material interaction that

may induce thermal ablation or photo-ablation of the material. Photo-ablation is a coldmechanism for material removal and therefore results in better surface conditions on

polymer materials.

By using a excimerLaser eye surgery is a treatment to correct near sightedness,

far sightedness or astigmatism. The surgery may reduce or eliminate the need for contact

lenses or glasses. In a normal eye, the front of the eye (cornea), the lens of the eye and the

shape of the eye focus light to form an image on the back inside surface of the eye.


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2. LITERATURE REVIEW

After reading and understanding this paper demonstrates the unique and

exceptional capability of excimer laser micromachining in fabricating aspheric micro

lenses with precise surface profile control. A newly developed laser scanning method is

introduced for machining refractive types of micro lenses, which have pre-designed

surface profiles aiming at minimizing the optical focal spot sizes. The machining

accuracy and machined surface roughness are examined experimentally, and very good

results are obtained.

A new 3D micromachining method, called Hole Area Modulation (HAM), has

been introduced to enhance the current micromachining technology. In this method,

information on the machining depth is converted to the sizes of holes on the mask. The

machining is carried out with a simple 2D movement of the work piece or the mask. This

method can be applied for machining various kinds of micro cavities in various materials.

By mathematical model for excimer laser micromachining based on HAM anddetermination of the optimal laser ablation conditions (hole diameter, step size, mask

movement velocity, etc.) are described. The simulation and experiment of the

HAM-based laser ablation were carried out successfully to create micro lens.

This method has great importance in laser eye surgery. By using laser short

pulses of invisible ultraviolet light remove a small amount of tissue from the Cornea to

correct the curvature. The amount removed is typically less than the thickness of a human

hair. By correcting the curvature of the cornea, images are better focused on the retina

and the images are clearer.


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3. TECHNOLOGICAL ASPECTS OF LASER MICRO

MACHINING

3.1 DEFINITION

An excimer laser (sometimes more correctly called an exciplex laser) is a form

ofultraviolet laserwhich is commonly used in the production ofmicroelectronic devices

(semiconductorintegrated circuits or chips), eye surgery, and micromachining.

Fig.1 High intensity laser beam

3.2 CONSTRUCTION

An excimer lasertypically uses a combination of a noble gas (argon, krypton,

orxenon) and a reactive gas (fluorine orchlorine). Under the appropriate conditions of

electrical stimulation and high pressure, a pseudo-molecule called an excimer(or in the

case of noble gas halides, exciplex) is created, which can only exist in an energized state

and can give rise to laserlight in the ultra violetrange.
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3.3 OPERATION

Fig2 Operation of excimer laser micromachining

Laser action in an excimer molecule occurs because it has a bound

(associative) excited state, but a repulsive (dissociative) ground state. This is because

noble gases such as xenon and krypton are highly inert and do not usually form chemical

compounds. However, when in an excited state (induced by an electrical discharge or

high-energy electron beams, which produce high energy pulses), they can form

temporarily bound molecules with themselves (dimers) or with halogens (complexes)

such as fluorine and chlorine. The excited compound can give up its excess energy by

undergoing spontaneous or stimulated emission, resulting in a strongly repulsive ground

state molecule which very quickly (on the order of a picosecond) dissociates back into

two unbound atoms. This forms a population inversion.

4. COMPONENTS OF TYPICAL INDUSTRIAL LASER

MACHINING SYSTEM

Although laser machining systems can differ in their layout and their application,

they generally share a typical component set.

Laser machining system components include:

4.1 Process site
http://en.wikipedia.org/wiki/Excited_statehttp://en.wikipedia.org/wiki/Repulsive_statehttp://en.wikipedia.org/wiki/Ground_statehttp://en.wikipedia.org/wiki/Kryptonhttp://en.wikipedia.org/wiki/Inerthttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Complex_(chemistry)http://en.wikipedia.org/wiki/Fluorinehttp://en.wikipedia.org/wiki/Chlorinehttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Picosecondhttp://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Picosecondhttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Chlorinehttp://en.wikipedia.org/wiki/Fluorinehttp://en.wikipedia.org/wiki/Complex_(chemistry)http://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Inerthttp://en.wikipedia.org/wiki/Kryptonhttp://en.wikipedia.org/wiki/Ground_statehttp://en.wikipedia.org/wiki/Repulsive_statehttp://en.wikipedia.org/wiki/Excited_state


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This is the enclosed area in which the material is processed.

4.2 Laser light source

The laser light source (laser head) provides photon energy with which to process

materials. Selecting the appropriate laser type for your application is essential to

achieving the desired results. Different types of lasers have distinct applications.

4.3 Beam delivery system

The beam delivery system (BDS) directs the laser energy to the material under

process. The BDS determines laser power density on target and the size and shape of the

laser beam on the target.

4.4 Part handling and motion control system

The part handling system is typically an X,Y table that positions parts beneath the

laser beam. Z-axis motion is used to focus the beam at the delivery site. Rotary stages

provide lathe-style operations. You can incorporate part loading systems with a part

handling system to integrate robotics or conveyors.

The motion control system controls the part handling system. The motion control

system is sometimes used for articulated beam positioning relative to the process

material.

4.5 Control electronics

The control electronics include a system CPU, the motion control electronics and the

laser operator controls. The electronics control the hardware and software that allow you

to automate many system functions, providing laser beam delivery and part handling

system integration.

4.6 Laser support equipment

Laser support equipment includes:

The electrical distribution system supply; a multi-kilowatt supply required for

high-powered lasers.


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Cooling systems.

Gas delivery systems to provide laser gas (in certain lasers) and purge gases to the

process site.

4.7 Structure and enclosure

The laser machining system structure must mechanically integrate all of the system

components. It must be designed for rigidity and thermal stability to ensure stable beam

pointing. Systems range from tabletop size to footprints of many square meters. Safety

enclosures and interlock systems prevent operator exposure to laser radiation.

5. COMPARING WITH DIFFERENT TYPES OF LASER

Consider the following two examples of excimer laser machining compared to

CO2 and Nd:YAG. As you can see, there is a marked difference in the resolution of the

cuts. The following comparison shows 300m-diameter holes drilled in 75m thick

polyimide.

CO2 Excimer

Nd:YAG (IR) CO2 Excimer

Fig.3comparining different types of laser


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In both examples it is clear that the excimer laser is better suited to high-precision

micromachining. CO2 and Nd:YAG lasers produce round spots suited to drilling, welding

and profile cutting.

While the high average power and lower feature resolution make CO2 and Nd:YAG

laser systems well-suited to heavy industrial applications, excimer lasers are often the

only efficient, practical and cost-effective manufacturing method available for high-

precision micromachining applications.

While CO2 and Nd:YAG lasers generate thermal effects that can affect surrounding

materials, excimer laser beams do not generate heat. Instead, they turn materials directly

into gas by breaking chemical bonds, a process calledphoto-chemical ablation, that

provides cleaner, more precise cuts.

6. IMPORTANT PARAMETERS

6.1 ABSOLUTE ENERGY

Absolute Energy is the total energy in a laser pulse or system, unit is Joule (J), a

typical value for a single laser pulse is 100 mJ.

6.2 LASER POWER

The power of a laser is the output optical power of the laser, we need to know the

normal working power and its maximum allowable power. Lasers operate in either

continuous wave state or pulsed state. Both operation states have lots of applications. For

pulsed laser, an important parameter is the peak power. In general, CO2 lasers have

relatively high continuous wave power, while Nd:YAG lasers can provide relatively high

peak power for pulsed operation. Output power is closely related with processing time

and operation expense. If the selected laser power is lower, the processing time will be

increased, if the selected laser has too high power than necessary, the operation expense

will be higher than necessary. So the proper choosing of laser power is very important.

6.3 INTENSITY

Energy Intensity is the area average of laser power, unit is W/cm2 in laser

processing. When the interaction between energy field and target is not continuous, we


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know that energy intensity is usually the deciding factor. Intensity is closely related with

laser focus spot size and pulse lasting time.

6.4 WAVELENGTH

The Wavelength of a laser is decided by the related stimulated energy transition.

Laser light is very pure, this means their wavelength vary in very small ranges when

compared with normal light. Different wavelengths may have different effects when

interacting with matters. The shorter the wavelength, the higher the energy of the photon.

Laser can also be divided into infrared, visible, ultraviolet lasers according to their output

light wavelength. Wavelength also affects the maximum resolution and focalization, the

shorter the wavelength, the higher the resolution, the better the focal property.

7. CONFIGURATIONS OF UV EXCIMER LASER

While excimer lasers are available in a variety of packages, all commercial excimer

lasers employ the modules shown here.

Laser light is generated in the laser cabinet. The electrical energy required by the

laser to form laser pulses is generated by the high voltage supply. A gas supply and a

vacuum pump are required to fill the laser with the appropriate laser gas mixture. The

control computer is usually linked to the laser cabinet and high-voltage supply by a fiber

optic network. The computer provides laser function user control.

Fig.4 Typical excimer laser configuration


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7.1 UV EXCIMER LASER GASES

Excimer comes from the term excited dimer, which refers to a diatomic molecule,

usually of an inert gas atom and a halide atom, which are bound in excited states only.

The gases that JPSA excimer laser systems use include helium, neon, krypton and

fluorine (for 157nm, 193nm an 248nm systems) and xenon (for 308nm and 351nm

systems). These gases must be ultra high-purity grade (UHP). A reactive gas, such as

fluorine, is mixed with inert gases such as helium or krypton. When electrically

stimulated, a dimer molecule is produced that, when lased, produces light in the

ultraviolet range.

7.2 UV EXCIMER LASER DISCHARGE

In an excimer laser, the laser medium is excited by means of a high-speed transverse

electrical discharge. DC high voltage is supplied to a pulse-forming network that consists

of a high-performance thyratron switch, a magnetic pulse compression system and banks

of storage capacitors. These components generate a high-speed current pulse across the

electrodes. As the current pulse traverses the electrode gap, the lasant gases are ionized

and form the excimer molecule. The following graphic depicts the laser discharge

process.

Fig.5 discharge of laser

Due to the excimer constituent relaxation times, thermal effects influence discharge

and particulate formation. Laser discharge can only occur at repetition rates of less than

1Hz unless the gas between the electrodes is constantly replaced with fresh gas. The gas


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reservoir function is to provide a large volume of lasing gas so that gas refreshment can

occur. A high-speed blower fan recirculates the laser gas at speeds of 1 to 5 meters per

second so that the volume of the gap is completely refreshed between pulses at repetition

rates of up to 400Hz. After discharge, the spent gas mixture flows across heat exchangers,

which remove the heat that was generated during the pulse. Most lasers employ someform of particle filtration to clean the gas mixture prior to its passing again across the

electrodes. As the gas flows around in the recirculation loop, excited gas molecules return

to normal sizes.

7.3 EXTENDING LASER GAS LIFE

The following methods can help to extend the life of excimer laser gases:

Choose the appropriate laser for the job

Find the right combination of power and rep rate.

Lasers that operate at lower pulse energy and higher rep rate deliver longer gas

lifetimes than high pulse energy lasers operating at the same average power.

Operating the laser at less than the maximum rated output can decrease gas

consumption.

Select a laser with superior laser head interior components

Laser head construction:

Material compatibility with corrosive gas mixtures, such as mechanical

assemblies and o-rings.

Material compatibility with high voltage environment, e.g. ceramics.

Effective particle filtering of laser gas.

Effective window flushing system to retard optics contamination.

High quality optics selection and manufacture.

Alternate pre ionization schemes:

UV spark ionization is most common.

Corona pre ionization.

Microwave pre ionization; developmental.


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Gas processing

Extends gas lifetime by removing impurities from gas.

Effective with ArF, KrF, XeCl, and KrCl mixtures.

Ineffective with XeF mixture.

8. PHOTO ABLATION PROCESS

When matter is exposed to focus excimer light pulses, the pulse energy is absorbed

in a thin layer of materials, typically less than 0.1m thick, due to the short wavelength of

deep UV light. The high peak power of an excimer light pulse, when absorbed into this

tiny volume, results in strong electronic bond breaking in the material. The resultant

molecular fragments expand in a plasma plume that carries any thermal energy away

from the work piece. As a result, there is little or no damage to the material surrounding

the produced feature.

Fig.6 The photo ablation process


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9. FABRICATION OF MICROLENSES

Fig.7 fabricated micro lenses [2]

In this section, the fabricated micro lenses will be characterized in terms of machined

surface profile accuracy, surface roughness, and focusing capability. First of all, the

surface profiles of the fabricated micro lenses are measured by a non-contact 3D confocal

surface measurement system (Nano Focus mSurf C, Nano Focus AG, Oberhausen,

Germany).

Fig.8 Shows measured 3D profile of lens[3]

Very good surface profile accuracy is achieved in the central region of a radius

approximately 75 mm. To closely examine the profile accuracy, the deviations between

the machined surface profiles and the designed ones are displayed in Fig. It is observed

that the profile accuracy is much compensate this error by fine-tuning the mask

probability function and the photo-mask window-opening pattern. For optical

components, surface roughness is of great importance and hence needs to be


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characterized. The surface roughness of the six machined micro lenses is characterized

with an Atomic Force Microscope (AFM) system. Finally, the focusing capability of the

fabricated micro lenses is tested experimentally.

Fig.9 experimental setup [3]

The experimental setup is shown in Fig. above, in which a 20mW HeNe laser (1101,

JDS Uniphase Co., San Jose, CA) of wavelength 632.8nm is used as the light source.

After adjusting its intensity by the polarizer, the laser beam is passed through a spatial

filter to eliminate higher-order components. An iris allows a laser beam of diameter 200

mm and uniform intensity distribution to passes through the micro lens under evaluation.

A CCD camera with a 100 x objective lens . For example, within the range of a radius of

75 mm the maximum profile error is mostly within 71 mm. On the other hand, in the

areas close to the lens aperture edge, greater surface profile deviation is observed. The

reason could be that for increasing machined depth, the machining surface is moving far

away from the focal plane of the projection lens system, and hence the machining rate is

down. This explains why the final machined depth is always less than what is room of

improvement in approaching the diffraction limits if the surface profile accuracy can be

further improved over the whole lens aperture.

10. APPLICATIONS OF EXCIMER LASER MACHINE

10.1 LASER EYE (OR EXCIMER LASER) SURGERY


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Laser eye (excimer laser) surgery is a treatment to correct near sightedness, far

sightedness or astigmatism. The surgery may reduce or eliminate the need for contact

lenses or glasses. In a normal eye, the front of the eye (cornea), the lens of the eye and the

shape of the eye focus light to form an image on the back inside surface of the eye

(retina).

Fig.10 concept of eye surgery

Using excimer laser short pulses of invisible ultraviolet light remove a small amount of

tissue from the cornea to correct the curvature. The amount removed is typically less than

the thickness of a human hair. By correcting the curvature of the cornea, images are better

focused on the retina and the images are clear.

10.2 BIOLOGICAL MATTER AND ORGANIC COMPOUNDS

The ultraviolet light from an excimer laser is well absorbed by biological

matterand organic compounds. Rather than burning or cutting material, the excimer laser

adds enough energy to disrupt the molecular bonds of the surface tissue, which

effectively disintegrates into the air in a tightly controlled manner through ablation rather

than burning. Thus excimer lasers have the useful property that they can remove

exceptionally fine layers of surface material with almost no heating or change to the

remainder of the material which is left intact. These properties make excimer lasers well
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suited to precision micromachining organic material (including certain polymers and

plastics).

10.3 MANUFATURING OF MICRO ELECTRONIC CHIP

Excimer lasers are widely used in high-resolution photolithography machines, one

of the critical technologies required for manufacturing of microelectronic chip. Current

state-of-the-art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF

excimer lasers with wavelengths of 248 and 193 nano meters (the dominant lithography

technology today is thus also called excimer laser lithography, which has enabled

transistor feature sizes to shrink below 45 nano meters. Excimer laser lithography has

thus played a critical role in the continued advance of the so-called Moores lawfor the

last 20 years. The most widespread industrial application of excimer lasers has been in

deep-ultraviolet photolithography, a critical technology used in the manufacturing

ofmicroelectronic devices.

10.4 MARKING

Fig.11 marking on ceramic chip

Marking has been a standard application for both the CO2 and excimer lasers for

more than 30 years. The CO2 laser's powerful beam can instantly mark a wide variety of

non-metals. The UV energy from the excimer laser can produce dramatic colour changes

in plastics and ceramic materials. These marks are permanent and can be used for simple

identification or traceability or anti-conterfeit.

Fig.12 marking on nail polish cap
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The foil on this nail polish bottle cap has been removed to reveal the base plastic. The

Laser Mark laser provides very sharp clear lettering required for this type of cosmetics

application.

10.5 DRILLING

Fig.13 drill holes on 75 micron thick plastic

The drilling of small holes is one of the most common applications of the excimer lasers.

Lasers can be focussed to very small spots, the very small holes in ink jet nozzles or the

micro-vias in smart phone printed circuit boards. The clean walls created in the this fast

'cold' process have no melting or charring.

11. ADVANTAGES

In this work present a modified machining method based on the previous planetary

scanning machining method and the goal is to remove the undesired outside skirt

machining problem and therefore to achieve micro-lens array fabrication. Basically, we

add one shading mask and one translation mechanism so that we can partially block the

laser beam from undesired laser machining and in the mean time acquire array type of

micro-lens fabrication. Excimer lasers bring the following advantages to micromachining

applications:

11.1 Unique Properties of Excimer Laser Radiation

Short wavelength: 193nm to 351nm.

High optical resolution: less than 1m.

Shallow absorption depth: 0.1 to 0.5m.

Small interaction volume.

Energy highly absorbed by materials.

Large area multi-mode beam.


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Uniform power density over relatively large area.

High peak power: approximately 107 watts.

Sufficient power to ablate materials at low beam demagnifications.

11.2 Resulting Benefits in Materials Processing

High peak power and small interaction volume results in high-energy material

ablation with little heat transfer to surrounding material.

Shallow absorption depth allows tight control of feature depth by controlling

number of pulses to which the material is exposed.

Short optical wavelength provides high resolution generation (approximately 1m

features in process materials).

Large beam size and high peak power allow simultaneous large area

12. DISADVANTAGES

One drawback of the planetary contour scanning method is that beside the

machined micro-lens which one needs, the outside skirt region was also machined by the

laser. This problem is particularly important in the laser machining of micro-lens array

because it will limit the filling ratio of lenses.

Like other technologies, excimer lasers are not without their disadvantages.

12.1 Discharge Circuit

High performance discharge circuit is required to generate laser light.

High speed switches and pulse compression electronics are required.

High performance electronics require frequent maintenance.

12.2Toxic Laser Gas

Laser gas mixture is toxic and corrosive. Reactivity of lasant mixtures result in impurities formation during laser operation.

The laser must be refilled with fresh gas regularly.

A computer control system is required to maintain stable laser light output.

12.3 Managing high-power UV

High power ultraviolet beams are difficult to handle optically.

Advanced optical materials are required to efficiently transmit the beam.

Optic transmissivity degrades over long-term exposure to high-power UV beams.


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13. FUTURE TECHNOLOGY

In future the thermal side effects of carbon dioxide and neodymium: yttrium

aluminium-garnet lasers limit their clinical applications. These high-powered, infrared

lasers result in zones of charring and carbonization even in soft tissues and the bone. In

contrast, the pulsed, ultraviolet radiation emitted by excimer lasers causes limited

thermal, denaturative damage to surrounding tissues. Therefore, treatment of dental

tissues with the non thermal process of photo ablation with excimer lasers may present

alternatives to traditional dental practice. Possible future applications of the excimer laser

include selective caries removal, the conditioning of tooth surfaces, and cleaning of root

surfaces; the zones of necrosis are small, so that there is no residual debris.

In future work, the proposed laser machining method can be in conjunction with

several low-cost micro-manufacturing methods including micro- electroforming and

micro-injection molding. Such a LIGA-like micro-fabrication process is completed and

the laser fabricated micro-lens array can be replicated in a fast and inexpensive way

In future the new Wave front technology can measure and correct the unique

imperfections of each individuals vision and most often this provides them with the

potential to experience better vision than is possible with glasses or contact lenses. This

technology was originally developed for use in high powered telescopes to reduce

distortions when viewing distant objects in space. Now, surgeons can identify, measure

and correct imperfections in an individuals eyes 25 times more precisely t han with

conventional methods used for glasses and contact lenses.


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14. CONCLUSIONS

In this paper , I demonstrate the fabrication of an array of aspheric micro-lens based

on an improved excimer laser machining method. According to this newly developed

planetary scanning method it can fabricate circular symmetrical 3D microstructures with

an arbitrary profile based on the sample rotation and revolution approaches. The planetary

contour scanning method combined the metal mask screen and an xy movable stage

solve the problem of outskirt laser machining and allow complete micro-lens array

fabrication.

A 4x4 aspheric micro-lens array with a high-quality controllable surface profile had

been experimentally demonstrated. This method brings a new chance to expand more

future optical applications in using the planetary contour scanning method. The method

proposed in this work is far superior to other currently existing micro machining or micro

fabricating methods in terms of profile flexibility, versatility, and profile accuracy.

In future work, the proposed laser machining method can be in conjunction with

several low-cost micro-manufacturing methods including micro- electro forming and

micro-injection molding .Such as LIGA-like micro-fabrication process is completed and

the laser fabricated micro-lens array can be replicated in a fast and in expensive way.


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REFERENCES

1. Chi-Cheng Chiu, Fabricating of aspheric micro-lens array by excimer laser micro

machining ,optics and Lasers in Engineering 49 (2011) 1232

1237.

2. Yung-Chun Lee, Excimer laser micromachining of aspheric micro lenses with precise

surface profile control and optimal focusing capability ,Optics and Lasers in Engineering 45

(2007) 116125.

3. Kyung Hyun Choi , Johan Meijer, Excimer laser micromachining for 3D

microstructure, Journal of Materials Processing Technology 149 (2004) 561566.

4. Chun-Ming Chena, A new excimer laser micromachining method for axiallysymmetric 3D microstructures with continuous surface profiles, Sensors and

Actuators A 117 (2005) 349355.

5. Jean-Philippe Desbiens,ArF excimer laser micromachining of Pyrex, SiC and PZT

for rapid prototyping of MEMS components, Sensors and Actuators A 136 (2007)

554563.

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