Download Surface modification of polymers and ceramics induced by excimer

LASER ABLATION OF ELECTRONIC MATERIALS
Basic Mechanisms and Applications
E. Fogarassy and S. Lazare (Editors)
© 1992 Elsevier Science Publishers B.V. All rights reserved.
Surface modification of polymers and ceramics induced by
excimer laser radiation
D W Thomas, C Foulkes-Williams, P T Rumsby and M C Gower
Exitech Ltd, Hanborough Park, Long Hanborough, Oxford OX8 8LH, UK.
Abstract
Results on the modification of polymer and ceramic surfaces by exposure to excimer
laser light are given. Laser-induced changes in surface characteristics are presented and
potential applications of this treatment are discussed
1. INTRODUCTION
Excimer lasers are well known to be capable of micromachining polymeric and ceramic
materials with submicron precision without incurring thermal damage to the surrounding
unirradiated regions. With the short burst of high intensity uv light provided by the excimer
laser, ablative photodecomposition by direct chemical bond scission in the top layer of
material is generally believed to be the mechanism responsible for material removal. During
the past decade, uv excimer laser ablative cutting data as a function of laser energy fluence
and thresholds, laser wavelength and pulse duration have been extensively studied for a wide
range polymeric and ceramic materials. Photoablation by excimer laser irradiation is now
used on industrial production lines for operations such as the drilling of small feedthrough via
holes in polyimide insulating layers on printed circuit boards [1], for hole drilling ink jet
printer nozzles, for stripping the insulation from thin diameter wires and drilling small holes
through optical fibres that are used for biomedical probes [2].
Rather than the clean cutting properties of photoablative etching, in this paper we
describe results on the surface modification of various polymers and ceramics following
exposure to excimer laser light. While this field has received relatively little attention
compared to cutting and drilling studies, we believe laser-induced surface modification of
materials shows great promise for the future widespread industrial uses of excimer lasers.
D.W. Thomas et al.
2. POLYMERS
As an example of how excimer laser radiation can modify surfaces, we show in Fig 1(a) the
polymer PVC (polyvinyl chloride) which has been etched by ArF 193nm laser irradiation.
The clean surface obtained at an incident fluence of 1.06J/cm2 is typical of the excimer laser
ablative etching process. However, as shown in Fig 1(b) when working at an order of
magnitude lower
(a) Smooth etching at 1.06J/cm2
(b) Conical cones produced at 0.11J/cm2
Figure 1. PVC etched with an ArF 193nm laser.
fluence of 0.11J/cm2 near the threshold for ablation, interesting conical cone-like structures,
similar to those first observed when etching polyimide with 308nm radiation [3], begin to
appear. Such cones are thought to be caused by small particulate-type impurities, either
intrinsic or added, embedded in the polymer acting as masks that shadow the etching process
for material underneath. When working close to the threshold for ablation ejected material
may only be partially decomposed and have a low ejection velocity. Microparticles and
debris redeposited onto the surface may also shadow the etching during subsequent pulses.
Furthermore, in semicrystalline polymers such as polyethylene terphathalate (PET),
polyvinylidene fluoride (PVDF), and polyether-etherketone (PEEK) the small crystallites of a
few microns size may have different uv absorptive properties and etch at a slower rate than
the surrounding lower density amorphous material. When greatly exceeding the ablation
threshold fluence the particles or crystallites can also be ablated with the bulk material and
the etched region becomes smooth.
As shown in Fig 2, polyethylenimine (PEI) exhibits a much more densely packed conelike structure when etching with similar fluences at 193 and 248nm (and 308nm - not shown).
Because of the general trend in polymers to have lower threshold fluences for ablation at
shorter wavelengths, the sharpest cones with the smallest cone angles are obtained at 193nm.
By seeding with small graphite particles and measuring the cone angle, the ablation threshold
for the polymer and wavelength can be determined [4]. Since the steep wall angle of the cone
reduces the local fluence below the etching threshold, once a cone structure has started to
form it tends to become stable during further etching even if the opaque obstruction is
removed. Using many laser pulses structures with depths of several hundred microns can be
produced.
Excimer Laser Radiation
(a) 193nm at 0.24J/cm2
(b) 248nm at 0.29J/cm2
Figure 2. PEI etched with ArF and KrF lasers showing conical cone-like structures.
We have found that even more unusual structures can be produced such as the wavelike forms in polycarbonate (PC) shown in Fig 3. The material was a thin-film form of
Makrofol (a trade name of Mobay Corp). As can be seen, the ripples were coarser when
etching at 248nm than 193nm and the structure was retained at all fluences. Since for all
other forms of polycarbonate that we studied (eg Lexan, trade name of GE Plastics Corp.) we
only ever observed smooth excimer laser etching, we assume that this structure was initiated
by stretching or other orientational stress in the manufacturing process. Partially disrupted
polymer bonds will have a different etch rate to pristine bonded material. Such laser
roughened material has scale sizes ranging from sub to many microns and is often in the form
of channels or troughs aligned with or perpendicular to the direction of the stretching of the
film.
(a) 248nm at 1.36J/cm2
(b) 193nm at 0.14J/cm2
Figure 3. Polycarbonate etched with ArF and KrF lasers showing wave-like structure.
Some materials only produce structures when etching with a particular wavelength. At
193nm polyvinylidene chloride (PVDC) is smoothly etched with only a few cone-like
structures near threshold. However, irradiation of the surface at 248nm produces the strangely
beautiful 'ice-cream cone' like structures shown in Fig 4. Their density decreases at higher
laser fluences. Fluffy-like structures like that shown in Fig 5 are produced when etching
polystyrene (PS) with an ArF laser over a wide range of fluences. In this case it seems the
material jetted up from the surface redeposits, cools and solidifies after each laser pulse.
D.W. Thomas et al.
Figure 4. PVDC etched at 248nm and 1.4J/cm2 showing ice-cream cone-like structure.
Such surfaces contain upstanding surfaces that resist further etching due to the steep angle of
incidence of the incoming beam. After several pulses such surfaces take on the appearance of
fibrous material.
(a) Near threshold at 0.064J/cm2
(b) At 0.15J/cm2
Figure 5. Polystyrene etched with an ArF laser at 193nm showing fluffy-like structure.
Etching the polymer can greatly increase the exposed surface area. With closely packed
identical cones this increase is approximately given by the ratio of the cone slant height to
base radius. We have produced densely packed cones with aspect ratios of >10:1.
3. CERAMICS
Ceramic materials are characterised by their hardness, strength, chemical inertness,
high temperature and insulation properties which make them difficult to bond and
mechanically work. Using incident fluences typically an order of magnitude higher than those
required to process polymers, it is well known that excimer lasers can also cleanly
micromachine structures in ceramic materials. However as shown in Fig 6 for alumina
(Al203) and silicon nitride (Si3N4) just above the ablation threshold their surfaces can be
modified in a manner similar to that described above for polymer surfaces. Cone and lavalike features can be created that are
Excimer La ser Rad ia tio n
substantially rougher than the original surface. Smooth surfaces of silicon carbide and
zirconia (Zr02) can also be roughened by excimer laser treatment.
200µm
(a) Alumina. KrF at 248nm, 3.9J/cm2
40µm
(b) Silicon Nitride. XeCl at 308nm, 2.9J/cm2.
Figure 6. Cone and lava-like structures on ceramics produced by excimer laser treatment.
4. EXCIMER LASER SURFACE MODIFICATION: Potential Applications
It is well known that metal and semiconductor surfaces can be substantially modified
by excimer laser treatment to remove oxide layers, clean, anneal and smooth them and
improve their wear and corrosive properties. Below we give some results and examples of
potential applications of excimer laser treated polymer and ceramic surfaces.
(a) Improved Adhesive Bonding
The fabrication of nearly all products, consumer or otherwise, crucially relies on the
successful adhesive bonding of materials together. Obtaining a strong adhesive bond usually
requires a good mechanical key, the surface to be free from grease and water and often have a
high oxygen content. To provide a good mechanical key, surface roughening by mechanical
(peel ply or grit blasting), chemical or plasma etching are standard techniques often used
prior to adhesive bonding of polymers and ceramics. All of these techniques have drawbacks
- from introducing additional contamination to the surface, to being very slow and expensive.
Also when roughening fibre composite materials prior to bonding, the fibres themselves are
often damaged which leads to a weakening of the joint.
We have seen that the effect of excimer laser radiation on a polymer or ceramic surface can
simply be to clean, roughen and greatly increase the exposed surface area and frictional
properties of the material. It can also chemically activate the surface (see (b) below) which
may also improve the strength of adhesively bonded materials. In Fig 7 we show a KrF laser
treated surface of a woven carbon fibre/epoxy composite that has had the top surface layer of
epoxy removed without damaging the carbon fibres. Lap shear testing of adhesively bonded
l0mm overlapped joints of detergent degreased surfaces of this material showed the failure
load and stress of the joint increased from 0.48kN and 2.40MPa respectively, to 3.74kN and
18.41 Mpa
D.W. Thomas et al.
when KrF laser pretreated surfaces were used (a factor of x7.7 improvement in joint strength).
We have also found that KrF laser treated surfaces also enhanced adhesively bonded joints of
PEEK/carbon fibre and polypropylene/glass fibre composite materials.
40um
Figure 7. Woven carbon fibre/PEEK composite surface treated with 150 pulses of 248nm
radiation at 2.1J/cm2
(b) Surface Wettability
Depending on the material, laser wavelength and fluence, excimer laser radiation can clean the
top surface layer of hydrocarbon contaminants and leave behind a chemically
activated/deactivated surface of the polymer or ceramic [5]. By using excimer laser irradiation
the surface wettability can be changed between being hydrophobic and hydrophilic. The
surface may be roughened or a surface charge induced so water and other fluids can be made to
better adhere to or repelled from it. Treating surfaces of polycarbonate (Lexan) for example,
which is normally extremely hydrophobic, with ~30 pulses of unconcentrated 248nm laser
radiation at a fluence of ~0.3J/cm2 produces an near-zero contact angle for water droplets and a
highly wettable surface.
LASER FLUENCE. J/cm 2
Figure 8. Static contact angle of a water droplet on KrF laser treated surfaces. 600 pulses.
Excimer Laser Radiation
The measured change in the static surface contact angle for water droplets on three types of
fibre composite surfaces is shown in Fig 8 as a function of laser fluence. We see that the
surfaces of carbon fibre/epoxy and PEEK composites can be made hydrophilic by treating them
with 2.1J/cm2 of KrF laser radiation. Also, treating the PEEK/carbon fibre material at a lower
fluence induces an increase in its hydrophobicity. The ability to manipulate the wettability of
surfaces by excimer laser treatment could find wide applicability to the printing and adhesive
industries. Because irradiation of surfaces by large patterned excimer laser beams can be
carried out by contact or projection replication printing of a mask, it is easy to treat only certain
predefined regions. For example, the adhesion of paint or dye to only the treated areas would be
useful to a variety of industries.
(c) Particle filtration
By constraining a fluid so it passes over the surface, the large roughened area created can
be used to filter any particles that might be present in the fluid in the size range of several to
fractions of a micron. A suitable filter device could consist of a laminated stack of polymer
films with treated surfaces each with laser-machined hole arrays offset to permit passage of the
fluid through from the film above to. the one below. For example, such a filter might be used to
separate out red cells from blood plasma or bacteria from fluids. As well as physically
constraining particles smaller than the mesh size range of the filter, an electrically charge
activated surface can also restrain any particles or molecules that have a permanent charge of
the opposite sense. By passing a variety of fluids over the surface of such charged areas,
separation, filtration and trapping of selected molecules and particles may be possible.
(d) Surface catalysis
The large surface areas provided by the laser-induced cone-like structures and
microroughened surfaces on polymers and ceramics can provide an effective substrate for
enhanced catalysis of chemical reactions. When coated with appropriate catalytic elements or
compounds, structured films could be fabricated into laminated structures similar to the fluid
filter device described in (c) to make devices for the promotion of catalytic conversion
reactions.
(e) Antireflection treatment
The deep cone-like structures produced by irradiating polymers - particularily after coating
with a thin reflective metallic layer, can be highly effective light absorbers or traps. Incident
radiation passes into the cone-like structure and is reflected, scattered and absorbed at the sides
of the cones down to the material at the base. There being little specular reflection of light from
the surface, it can be made to appear black in much the same manner as the pincushion like
structure of a moth's eye. The cone angles and their density can be adjusted so that only certain
wavelengths of interest are trapped. Also, hypersonic sound waves with wavelengths similar to
the cone separation and depth scale size may be similarily damped by such a surface. Laser
treated surfaces that have low levels of backscattered radiation may be of potential industrial
interest in areas such as identification marking and defence countermeasures.
D.W. Thomas et al.
(f) Field-emission cathode arrays
Some of the pointed structures produced by excimer laser irradiation of polymer
surfaces can be used as the basis for producing large area array field-emission cold cathode
devices. In their low voltage form these devices are of interest as electron sources in cold,
low power vacuum microelectronic circuits, while at higher voltage can provide high voltage
electron beams. In this case the substrate on which the surface is fabricated needs to be
electrically conducting and can be made by applying a thin layer of polymer to a metallic
substrate. Under appropriate conditions this polymer is then laser etched to produce a high
density of cones. Laser etching through to the metal backing followed by subsequent
metallisation of the roughened surface provides a suitable structure for a field-emitting
cathode array.
(g) Friction improvement
The laser roughened surface produced on some polymers and ceramics increases the
surface friction and may be useful for improving handling of the material. Since it can give
similar results to conventional mechanical metal knurling to increase friction and improve
grip, such a process might be called microknurling. For example, a laser roughened or
microknurled strip down each edge of a polymer film could improve its winding stability and
control.
5. SUMMARY
Excimer laser radiation can be used to substantially modify the surfaces of polymer and
ceramic materials. As well as cleaning the surface of contaminants, these lasers can be used
to create highly irregular microscopic structures on the surface that may be useful for
improving adhesive bonding, surface friction and in fabricating filtration, catalytic, fieldemission cathode and light-trap devices for example. Exposure to excimer laser light can also
change the charge state of a surface that dramatically modifies how it interacts with fluids.
We are grateful for partial funding of this work by the UK Department of Trade and
Industry as part of the EUREKA EU205 High Average Power Excimer Laser EUROLASER
programme. We are also grateful to Dr J Wingfield of the Joining Technology Research
Centre, Oxford Polytechnic for assisting with the adhesive bond strength and droplet
measurements.
6. REFERENCES
1) F Bachmann, Chemtronics, 4 (1989) 149.
2) M C Gower. Excimer Lasers: Their Current and Future Applications to Industry and
Medicine. Chapter in High Power Laser Applications. Ed R C Crafer. Chapman and
Hall. 1992. To be published
3) P E Dyer, S D Jenkins, and J Sidhu, Appl. Phys Letts, 49 (1986), 453
4) P E Dyer, S D Jenkins and J Sidhu, Appl. Phys. Letts, 52 (1988), 1880
5) P T Rumsby and M C Gower. UK Patent filed. No GB 2233334A, 29 June 1989. Surface
treatment of polymer materials by the action of pulses of uv radiation.