Ultrafine str u c t u r i n g
of flexible
c i rc u i t s
The continuing trend
towards miniaturisation
requires the development
of new cost-effective and
environmentally friendly
materials and processes
for the electronics
industry. The gap between
chip production, now
producing sub-0.2 ?m
lines and spaces, and PCB
and packaging, must
inevitably shrink.
Dr. Dieter J. Meier,
LPKF Laser & Electronics AG, Germany
Fig. 1 – A possible
application for UV
lasers – laser
direct patterning
(LDP) of circuit
structures
Fig. 2 – LPD polymer film on
Si-wafer. A UV laser was used
to produce
³30 ?m features
2
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ANUFACTURING
ablation possible at a rate of 0.13?m
/shot using a wavelength of 248nm
(KrF laser). The resultant structured
films can be used as alkaline as well as
acid resists and can be etched (subtractive)
or additively built-up (semiadditive).
Another way to laser structure thin
polymer films for lithographic applications
in electronics manufacture
an electrochemically synthesised
100nm thick polythiophene film that
can be ablated by a He-Cd laser at
325nm. Polythiophene has proved to
be chemically stable against all acids
and dilute alkalis, and it seems that it
may be used as a self-developed UV
resist. Using the process, it is possible
to produce 1.9-2.7?m grooves at a
pitch of 20?m.
Pd-doped polymer films laid onto
polyimide substrate may also be laser
patterned
by using an electroless metallising process
with copper as shown in fig. 3.
This produces
electroless metallisation process with
Cu, Ni and Au allows further processing
for the electronics industry.
Thin metal films laid down on a polymer
may also be patterned by a UV
laser. Here, the UV radiation penetrates
the metal film and cracks the chemical
bonds at the metal-polymer interface.
The resulting plasma plume lifts off the
metal layer in a mini explosion. Due to
the high photonic energy of the UV
laser it is assumed that this process
also includes thermal ablation
A UV laser was used to structure thin
Al/Zn-layers on polypropylene foils
with an XeCl-laser (wavelength
308nm)
150?m structures using a fluence
of 200mJ/cm
spaces.
[8] uses[9] and circuit tracks built up³20?m lines, and the[12] .[5], making it possible to produce2 and ³20 ?m lines and
Laser direct patterning
(LDP)
Fig. 4 shows the general idea for producing
flexible circuits in just three
steps. Given the foregoing results, it
should be possible to produce flexible
circuitry using LDP, and this article
looks at the results of research into this
potential use of laser technology
[ 1 1 ]
using a KrF UV laser source that “ima-
Fig. 3 – UV-patterned Pd-doped
polymer film
Fig. 4 – The production of
flexible circuits in a 3-step
LDP process
Additive build up
•
electroless Cu
•
electroless Ni
•
Film with layer Laser structuring
Mask
electroless Au
Fig. 5 – Laser ablation of flexible circuitry
through a chromium mask
Fig. 6 – Detail of a microcoil
patterned with optimised
power density Fig. 7 – 15 ?m (0.6 mil) ablated
tracks compared with human
hair
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ST QUARTER 2001ST QUARTER 2001 3
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ANUFACTURING
ges” the film through a chromium
mask (see fig. 5). This process uses an
adhesiveless flex polyimide substrate
on which is a 15nm Cr tiecoat and a
50nm Cu seed layer that has been processed
in a proprietary vacuum metallisation
process by an experienced
base materials manufacturer
[10].
Results and discussion
Optical evaluation of the first laser
ablated Cr/Cu metallisation indicated
the need for extensive tests to determi -
ne an optimum imaging power density,
that varies between 150-350mJ/cm
Figs. 6 and 7 show the results of
using the optimised technology
to produce circuits with 15
micron lines and spaces, and
table 1 shows some pro c e s s
parameters.
2.
After ablation
Although commercial quality
baths are used for the electroless
metallisation of the laser- s t ru c t u re d
substrates, special care is necessary
for the activation of the first ultra-thin
layers. Following the careful ultrasonic
removal of the laser debris, cleaning
agents should be used at low-concentrations
to avoid damaging the layers.
Electroless copper metallisation using
an appropriate bath then selectively
builds up at approx. 2?m/hour.
T
M
M
S
M
OTAL PROCESSING RANGE (INCHES) 8 X 8AX. LAYOUT DIMENSIONS (INCHES) 5,5 X 5,5IN. LINE AND SPACE WIDTH 15?m (0.6MIL)YSTEM RESOLUTION 2?m (0.08MIL)AX. THROUGHPUT 1.55 SQ. INCH/ SEC
M
W
AX. LASER POWER (W) 50AVELENGTH (nm) 248
T
ABLE 1 – PROCESS PARAMETERS FOR LDP PROCESS
2655.63nm
1327.81nm
0 nm
56 ?m
56 ?m 28 ?m
28 ?m
0 ?m
0 ?m
Fig. 8 – Additive build up with
electroless copper
(19 ?m line width)
R
EFERENCES
[1] C. Dunsky et al:. ”High Quality Microvia Formation with Imaged UV YAG Lasers”, Proceedings of the Techn.
Conference, San Diego 2000, S 15-5-1
[2] C. Vaucher: “Direct Imaging: will it fly or not?”, PCB-Fab, June 2000, 28ff
[3] R. Rhodes: “Laser Direct Imaging”, CircuiTree June 2000, 62ff
[4] E. Tadic: “Haaresbreite Feinststrukturen für zukünftige Produktgestaltungen”, SMT Ausgabe 1-2/2000, 12ff
[5] W. Ziegler et al.: “Ein Excimerlaser strukturiert metallbedampfte Folien”, F&M 101 (1993), 189ff
[6] R. Srinivasan et al.: “Self-developing photoetching of poly(ethylene terephthalate) films by far-ultraviolet
excimer laser radiation”, Appl. Phys. Lett. 41, 576,1982
[7] K. Suzuki et al.: “Polymer resist materials for excimer ablation lithography”, Applied Surface Science 127-129
(1998) 905-910
[8] T.K.S. Wong et al.: “Patterning of poly(3-alkylthiophene) thin films by direct-write ultraviolet laser
lithography”, Mat. Science and Eng. B55 (1998) 71-78
[9] J. Kickelhain: “Untersuchungen zur additiven Herstellung flexibler Feinstleiterstrukturen durch
Excimerlaserablation festhaftender, metallorganisch aktivierter Schichten auf Polyimidfilmen”, Dissertation,
Rostock 1999
[10] T. Bergstresser et al.: “The Effect of Moisture on Peel Strength of Adhesiveless Polyimide Laminates”,
Proceedings of the 5th Annual National Conference on Flexible Circuits, Denver, 1999
[11] D. J. Meier et al.: “Laser Structuring of Fine Lines”, Proceedings of the 5th Annual National Conference on
Flexible Circuits, Denver, 1999
[12] J. Koo et al. : “Removal of thin films from substrates by laser induced explosion”, US-Pat. 4,081,653
4
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Summary
Copper layers applied to polyimide
films by PVD processes to a maximum
thickness of 50nm can be ablated and
patterned with a UV laser machine. A
system has been developed that is suitable
for industrial use that allows the
production of
and spaces. Structures for flexible circuit
applications can be produced by
the additive build-up of functional
layers.
The advantages that this new technology
brings to the production of fine
line structures for HDI applications are
as follows:
³15?m (0.6mil) lines
•
etching
precision lines and spaces without
•
³15?m geometries
•
no photo imaging
•
fewer process steps => cost reduction
•
reduced chemicals and waste
•
desired
Now, as well as being used for microvia
drilling and laser direct imaging, the
UV laser can be used for laser patterning.
The author believes that this will
reduce the gap between thin film techniques
and PCB production.
tracks can be built up to 6?m as3
LPKF Laser and Electronics AG
Osteriede 7
30827 Garbsen – Germany
Tel.: +49 (0)5131 7095 0
Fax: +49 (0)5131 7095 90
E-mail: lpkf@lpkf.de
Website: www.lpkf.de
A
CKNOWLEDGEMENTS
The author would like to thank Mr. T.
Bergstresser (GOULD Electronics), Mr. A.
Boenke (LPKF Laser & Electronics AG),
Mr. C. Böker (Zeiss), Mr. L. Bruderreck
(Technolab Berlin), Mr. T. Kohlmeier
(Universität Hannover) and the company
Enthone-OMI.
Applications for the LDP
process
Figs. 9 to 12 illustrate some applications
for the LDP process.
Fig. 9 shows a detail of a microcoil
with 15 micron lines and spaces,
produced using LDP.
Fig. 10 shows a printer head application
with 12 micron lines and spaces.
Fig. 11 shows a flexible interposer.
This micropackaging (Chip Size
packaging) application has 15
micron lines and spaces.
Fig. 12 shows an LDP interposer.
This was completed as a CSP demonstrator
and has been tested successfully
according to the general reliability
test for electronic components.
Fig. 10 – Applications for the
LDP process.
High density interconnect
(12 ?m lines and spaces)
Fig. 11 – Applications for the
LDP process.
Flex interposer for CSPs
(15 ?m lines and spaces)
Fig. 9 – Microcoil
(15 ?m lines and spaces)
Fig. 12 – Laser-structured CSP
flexible interposer
application. This has been
tested successfully