Laser Micro Projection for Micro Mechanical Device Fabre Cation

8/6/2019 Laser Micro Projection for Micro Mechanical Device Fabre Cation

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Laser Microprojection for Micromechanical Device Fabrication

P. T. Rumsby, E. C. Harvey and D. W. Thomas

Exitech Ltd, Hanborough Park, Long Hanborough, Oxford, OX8 8LH

ABSTRACT

Using advanced laser mask projection methods it is possible to manufacture miniature complex 3D structures in polymers

and ceramics for sensor and micromechanical device prototyping or mass manufacture. Subtle movements of workpiece

and mask (separately or together) during processing allow complex devices to be realized. Multilevel sensor structures can

be constructed using lamination techniques.

The full capabilities of laser micromachining techniques are explained in detail and a examples of novel 3D structures made

by laser machining are shown.

Keywords: Excimer laser, microfabrication, LIGA, 3D micro structures, prototyping, lamination.

1. INTRODUCTION

Development of complex 3D products often requires the rapid building of prototypes to test design ideas. Many

machines are currently available for fabrication of 3D objects layer by layer in metals and polymers using laser

polymerization or sintering or droplet delivery systems. Such machines address the issue of prototyping of macroscopic

objects with dimensions up to a metre in size 1.

Sensors, biomedical products and other micromechanical devices however have sub-mm overall dimensions andcannot be developed using such macroscopic laminate building methods. Requirements thus exists for a microscopic rapid

prototyping technology able to construct objects with few micro resolution.

Some miniature fluid flow devices and micromechanical structures have been prototyped successfully using deep resist

based x-ray LIGA techniques but such methods are limited to vertical high aspect ratio structures (2 ½ D) and require

complex masks and exposure stations so that development cycle times are long 2.

684 SPIE Vol. 2921 • 0277-786X 97 10.00



Newly developed excimer laser polymer micromachining methods can be used to solve both of these problems3. Using

excimer laser mask projection methods with complex motion of mask and workpiece. 3D structures can be machined directly

into deep polymer layers with resolutions of a few microns over areas of many cm 2. Such structures can be used directly for

devices or replicated by electroforming and molding methods. Complex 3D microstructures can be created by the

combination of these processes with lamination techniques so that prototyping of miniature devices is flexible and rapid.

This paper describes the fundamentals of excimer laser micromachining methods and shows how a wide variety of

microscopic 3D structures can be fabricated either for direct use or for subsequent replication.

2. FUNDAMENTALS OF EXCIMER LASER MICROMACHINING

Excimer lasers emit short (20ns) bursts of incoherent radiation in the UV (193, 248nm or 308nm) at repetition rates up

to several hundred Hz. When this radiation is delivered on to the surface of a polymer the incident energy is absorbed in athin layer (eg 0.1µm) which is rapidly decomposed, heated and ablated. Each incident laser pulse removes a well defined thin

layer of material so that depth control is very exact. If the laser pulse energy is low then the beam must be focused onto the

material to achieve the necessary energy density. In this case processing can only be carried out in a serial mode (as with a

focused Yag, C02 or copper vapour laser). One of the main advantages of excimer lasers however is that they can be used in a

mask projection mode to transfer a complex pattern onto the workpiece.

Figure 2a

Fig.2b High aspect ratio structures

with vertical side walls machined in

a ol imide and b alumina.

Figure 1. Optical components in an excimer laser micromachining tool.



Figure 1 shows the fundamental components needed in the optical system of an excimer laser mask projection

micromachining tool. The beam from the laser is reshaped and homogenized using special optics. The function of the

homogenizer is two fold. First it creates a highly uniform (eg ±5% RMS) beam at the plane of the mask which contains the

pattern to be transferred. Secondly it introduces a range of angles (eg ±20m rad) into the beam which are significantly

greater than the inherent laser divergence (approx ±2m rad) so that wall cutting angle can be controlled to give vertical or

tapered side walls. The high resolution high numerical aperture (NA) projection lens transfers the mask pattern into the

workpiece. For cheapness and speed of manufacturing masks are standard chrome on quartz reticules as used for semi

conductor device fabrication. Such masks require to be operated at low (eg 100mJ/cm2 ) pulsed laser energy density whereas

efficient ablation of most polymers occurs at substantially higher levels of 1-2J/cm2. Hence the projection lens usually has a

demagnification factor in the range 3x to 10x, increasing the range of angles incident on the substrate substantially (up to ±

0.2 rads).

Maximum pulse energies emitted by excimer lasers are typically in the range of a few 100mJ so that the area that can beprocessed in a single laser shot is limited to approx 20 to 30mm2 for polymer materials. The wavelength of choice for

polymer micromachining is 248nm since at this wavelength most materials have sufficiently high absorption coefficient, laser

operation is efficient and optics lifetimes are long. Some polymers (eg PMMA and some fluorinated polymers) require

193nm but at this wavelength processing techniques became more complex and operating costs increase.

Hence for effective polymer micromachining for miniature device fabrication the conditions at the workpiece are

typically wavelength 248nm, pulsed energy density 1-2J/cm2, illumination NA 0.1 to 0.3.

3. MASK PROJECTION TECHNIQUES

When mask and workpiece are stationary machining is limited to the fabrication of high resolution, high aspect ratio 2 ½

D patterns over areas of a few 10mm2 where depth can be controlled to a few 10s of nm by laser shot control. Figure 2

shows SEM photographs of high aspect ratio structures with vertical side walls cut in polyimide and alumina by such

methods. For machining ceramic materials significantly higher energy densities (eg 10J/cm2 ) are needed than for polymers.

In this mode of operation the processing is similar to that achieved using X ray resist exposure techniques with the

exception that for laser machining depth can be accurately controlled.

Another major difference between X ray and laser exposure is that wall angles can be controlled over the approximate

range of ±20° to the vertical by adjustment of fluence (J/cm2 ) and illumination (homogenizer) NA. This is demonstrated in

fig 3 which shows an SEM picture of the rear side of an inversely tapering hole cut in a polyimide inkjet printer nozzle plate

of thickness 50µm using an optical system with an NA of 0.4. In this case the ink exit (laser input) hole is circular with a

diameter of 35µm whereas the ink input (laser exit) hole is transformed to a square of 60x60µm by the highly divergent

beamlets generated by the high NA optical system.

To increase device complexity and size it is necessary to introduce some sort of motion to the mask and/or the workpiece



during the cutting process. There are various methods of doing this as detailed below:

3.1 Workpiece step and repeat mode:

In this mode a single mask pattern is projected onto the workpiece to define a single structure of limited area. This pattern

is repeated by stepping the workpiece as often as required to fabricate large numbers of identical devices.

Figure 3. Rear surface of polyimide

nozzle plate drilled with DANA

optics to create an inversely tapering

hole varying from circular to square

in cross section.

Figure 4. Stepped nozzle and reservoir

structure machined in polyimide using

mask stepping techniques.

Figure 5. Element of Moiré fringe

based security code plate machined

into PVC by excimer lasermask and workpiece scanning.

3.2 Mask stepping mode:

In this mode complex 3D structures are formed at the substrate by substituting different masks as laser processing

proceeds. Figure 4 shows a structured nozzle drilled in polyimide for an ink jet printer device manufactured by this process.

All the sequential mask patterns can be contained on a single mask plate which is positioned in the beam on high speed X Y

mask stages. Alternatively for structures that are based on square or rectangular patterns a dynamical aperture with

programmed adjustable blades can be used whereas for circular structures a time varying circular iris is suitable as a mask.

Such nozzle structures can be repeated to drill an array by stepping the workpiece or a complete linear array of nozzles can

be drilled simultaneously by configuring the beam in a line and using a projection lens of appropriate (large) field.

3.3 Mask and workpiece stepping mode:

In this mode both mask and workpiece are stepped sequentially to generate structures over relatively large (eg few

cm2 ) areas4. Initially the mask or aperture is set to the largest size possible and the workpiece programmed to move with the

laser firing at programmed times linked to the motion of the stages. In this way the over lap and position of ablation spots

are controlled so that specific areas receive more laser shots than others to create a pattern of controlled and varying depth.

After this rough initial cutting the mask or aperture is changed to one of smaller size and the operation repeated. The

sequence continues with the aperture being reduced in size and changed in shape sequentially until the smallest required

features are cut. This mode sometimes termed “dynamic path dissolved material removal” allows highly complex 3D

structure to be cut to unlimited depth over large areas but has the main disadvantage that the process time is extremely long.



3.4 Mask and workpiece scanning mode:

In this mode both mask and workpiece are simultaneously moved in exact synchronism through a stationary laser beam

to transform a large area complex pattern from the mask to the substrate. The travel distances and the velocities of the mask

are at all times related exactly to the workpiece travel and velocity by the lens demagnification factor. For a typical mask to

workpiece demagnification of 4x and a 6" chrome on quartz mask, areas up to 30x30mm can be patterned at the workpiece

with the resolution set by the accuracy, resolution and repeatability of the work -piece stages (and a lesser extent the mask

stages) rather than the projection lens optical resolution. Fig 5 shows an SEM picture of a small area of a 30x30mm pattern

cut into polycarbonate using such synchronized mask and workpiece scanning techniques. The object shown is a mold for

mastering a Moiré fringe based security device. When used with a similarly patterned “reader” plate the device creates a Moiré

image of an encoded pattern. The merit of mask and workpiece scanning techniques is that devices of high accuracy and

resolution can be made with areas set only by the mask size and stage limits and at high speed. A device such as that shown in

fig 5 with an area of 30x30mm and depth of 200µm can be machined with a 50watt laser source in a few minutes.

3.5 Step and scan mode:

This mode is generally used when a large number of identical complex structures are required for direct device

fabrication. An array of identical large area high resolution patterns are built up by stepping the workpiece to a new location

between mask and workpiece scan cycles as described in 3.4 above.

3.6 Pattern scanning mode:

In this mode a complex mask pattern is projected onto the surface of the workpiece which is moved at constant velocity

to form a linear structure of constant profile (or accelerated or decelerated to form curved structures). By rotating the

workpiece and repeating the process crossing of the linear patterns gives rise to a periodic regular 3D structure. Fig 6 shows

an example of a structured pattern on a Caesium Iodide far IR lens to form a graded index anti-reflection coating. In this case

a mask with adjacent diamond apertures was projected on to the moving lens to cut linear groves. Fig 7 shows the pattern cut

in polymer by this mask when the workpiece is stationary. Moving the workpiece and then repeating the operation after

rotation by 90° leads to the type of structure shown in Fig 8. Grooves or pyramid patterns with pitches down to a few µm can

be produced directly by this method of processing.

Figure 6. Antireflection pyramidal

structure machined into the surface

of a CsI far IR lens at 193nm using

pattern scanning techniques.

Figure 7. Diamond pattern statically

ablated into polycarbonate using

'diamond' mask.

Figure 8. Linear grooves and

pyramids ablated in the surface

of polyimide by scanning a

'diamond' mask pattern over



3.7 Mask or workpiece oscillation mode:

This mode of operation is a variation on mask and workpiece stepping (3.3) but in this case the mask pattern is held

constant and the workpiece is moved in a programmed oscillation manner in order to control the edge profile of the

structure cut and create concave or convex slopes on nozzles as required. This type of process requirement is particularly

useful for ink jet printer nozzles where there is a requirement to generate nozzles with half taper angles of over 20°. a value

that is too large to achieve by direct ablation at low fluence. Fig 9a shows an SEM picture of a section of an ink jet nozzle

plate drilled using asymmetric workpiece ‘wobbling’ to achieve a low taper angle input structure. Rather than moving the

workpiece stages the mask can also be programmed to oscillate to achieve the desired profile. This is also achieved in

practice by spinning a tiltable plate placed just down stream of the mask which causes the effective mask pattern to rotate

with time variable amplitude. The structure can be axisymmetric or otherwise as required. Fig 9b shows an array of nozzles

with high entry angle drilled using this spinning plate technique.

Ink jet nozzle in polyimide with a) asymmetric input profiles created by programmed workpiece osollation and

b) axisymmetric high taper angle created by spinning plate mask movement techniques.

3.8 Mask rotation:

This mode of operation is used purely for the generation of symmetric nozzle structures as the mask is rotated on an

axis centred on the beam giving rise in all cases to axisymmetric ablation structures. If the beam at the mask plane is

uniformly distributed in energy density then the ablation profile at the workpiece created by a rotating mask pattern depends

on the total laser dose (J/cm2 ) each area receives which is a function of the width and diameter of the mask structure. For

example a narrow slit mask rotated about its centre point gives rise to a convex hole profile of the type shown in fig 10 since

the total dose depends roughly quadratically of the inverse of the radius. By varying the mask shape a wide variety of hole

profiles ranging from convex to concave can be generated. For the generation of individual axisymmetric nozzles with

complex profile this method is much faster than osollation of the mask or workpiece. By modifying the rotating slit mask into

an S shape the sub micron deep beam overlap structures formed on the surface can be made to spiral as shown in fig 11. This

shows a nozzle with super imposed surface structure ‘rifling’ used to impart rotation to the ejected droplet. An array of such

nozzles can be formed by step and repeat methods and then replicating using metal forming and moulding techniques.



Figure 10. Convex arisymmetric nozzle

drilled through a glass plate using a

rotating slit mask.

4. 3D STRUCTURES BY LAMINATION METHODS

Excimer laser micromachining in single layers of material is limited to structures that progressively decrease in open

area with depth or in special cases to cutting nozzles that have inverse tapers of up to 20°. Construction of sealed cavities

or closed channels in single layers is not possible. To achieve full 3D structures with enclosed sealed cavities or buried

pathways requires the use of lamination techniques as are used in conventional macro stereo lithography. The critical

difference however is that macro stereo lithography is an additive process where material is added layer by layer to form a

closed structure, whereas 3D prototyping by ablative excimer laser machining is a subtractive process with material being

removed from each added layer.

For deep (eg 1mm) 3D devices lamination involves application of polymer layers of 50-100 microns thickness. In this

case each layer has a UV curing adhesive layer backing applied to it and is laminated on to the previously patterned layer.

Mask projection using either mask and workpiece scanning or step and repeat processing depending on device size is then

used to pattern each sequential layer. If all level mask designs are on one mask plate then switching between patterns is

rapid.

Fig 12 shows a picture of a prototype of a 12 level gas filled diffractive optical filter made by this method. The filter is

part of a low cost methane gas sensor device and consists of a 1mm thick grating type structure with 25 micron wide

channels filled alternately with methane or air. The active filter area is 5mm wide, 20mm long and 1mm thick with CaF2

windows at top and bottom. Appropriate gas manifolds, reinforcing structures and inlet and outlet ports are incorporated

into the structure as laser fabrication proceeds layer by layer.

Using lamination techniques it is also possible to incorporate active layers into devices. Fig 13 shows an SEM picture

of a miniature intelligent gas valve used to control gas flow. In this case step and repeat methods are used to cut a window

pattern through a 2 layer film of polyethylene and polyester. Such films have very different coefficients of thermal

expansion so that the valve opens and closes as temperature changes. Such valve devices can be either powered by film

resistive heaters and incorporated into miniature fluid flow devices or used in the ambient environment to control gas

flow in and out of food containers.

Figure 11. ‘Rifled’ nozzle drilled in PEEK

using a rotating S mask.



Figure 12. Prototy pe diffractive

optical filter for a methane gas

sensor made by mask and workpiece

scanning sequential patterning of 12

layers of 80µm polyester.

Figure 13. Temperature actuated valve

made by laser machining window

structure into PE/PET laminate.

Figure 14. Dielectrophoretic

bioparticle manipulator made by

direct laser ablation (using mask

and workpiece scanning) of 3 layers

of gold, polyimide and gold.

For multi layer devices made with thin (eg sub micron) films sequential layers are applied by spinning or dipping

techniques for polymers (or resists) and by PVD or sputtering techniques for metal layers. Each additional layer must be laser

patterned on top of the previously patterned layer. Consequently care has to be taken to select appropriate laser processing

parameters where damage to the lower layer does not occur. Multi sequential layer laser patterning of thin films is an ideally

way to build prototype and production complex 3D interconnected structures that can act as sensors, detectors or other

electronic or fluid flow devices. Fig 14 shows an example of such a device. It shows part of a complex miniature 3 layer

dielectrophoretic bio particle manipulator made entirely by direct laser patterning of all layers (gold, polymide, gold) using

both mask and workpiece scanning and step and repeat techniques.The total depth of the device is only 2µm with anelectrode pitch of 20µm. This laser prototyped device is one of key components needed for the development of ‘bio factory

on a chip’ technology. Excimer laser direct patterning of films can be used to develop such devices rapidly and also

manufacture in quantity at low cost.

5. CONCLUSIONS

Excimer laser are ideal tools to fabricate complex 3D structures in polymers. There is almost no limit to the range of

structures that can be built using appropriate mask and workpiece movements. Such structures can be used directly as sensor

or nozzle devices or can be replicated by electroforming and molding methods For microstructures manufacturing time isshort allowing rapid prototyping of devices. Using lamination techniques full 3D microscopic devices with built in channels,

valves or interconnects can be built at high speed.

6. ACKNOWLEDGMENT

The authors would like to acknowledge the assistance of all the staff at Exitech who were involved in the developmentof



the techniques outlined above and in the production of the samples. The bio-manipulator device shown in fig 14 is

reproduced by kind permission of Prof Ron Pethig.

7. REFERENCES

1) P F Jacobs (Ed) ‘Rapid Prototyping and Manufacturing’ Soc of Mfg Engineers (SME) (1992).

2) W Ehrfeld and D Munchmeyer ‘3 dimensional micro fabrication using synchrotron radiation’ Nue Inst and

Methods in phy Res, pp 523-531, A303 (1991).

3) E C Harvey, P T Rumsby, M C Gower and J L Remnont. ‘Microstructuring by excimer laser’ SPIE Vol 2639,

Micromachining and Microfabrication Technology pp 266-277 (1995).

4) H K Tonshoff, D Hesse, H Kappel and J Mommsen, Laser Zentrum Hannover, ‘Excimer Laser Systems’

Laser Assisted Net shape Engineering, Proc. of the LANE 94 Vol 11 ed M Geiger and F Vollertsen, pp 627-650

(1994).

5) M T Duignan, ‘Micromachining of Diffractive Optics with Excimer Laser’ in Diffractive Optics, Vol 11, 1994

OSA Technical Digest Series, Optical Society of America, Washington, D.C. pp 129-132 (1994).

6) M S Talary, J P H Burt, J A Tame and R Pethig, ‘Electromanipulation and separation of cells using travelling

electric fields’ J Phys D, Appl Phys, 29, pp 2148-2203 (1996).

Laser Micro Projection for Micro Mechanical Device Fabre Cation

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