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1、Paper for ANTEC 2002 Annual Technical Conference May 05-09,2002,San Francisco,CA SESSION T49:Joining of Plastics and Composites IR and Laser Session 2(Tuesday Afternoon,May 7)J.-W.Chen Page 1 16.10.2003 LASER ASSEMBLY TECHNOLOGY FOR PLANAR MICROFLUIDIC DEVICES Jie-Wei Chen,Leister Process Technologi
2、es,Riedstrasse CH-6060,Switzerland and Jerry Zybko,Leister Technologies,LLC,Schaumburg,IL 60173/U.S.A.Abstract The assembly of plastic micro fluidic devices,requiring high positioning and welding accuracy in the micrometer range,was successfully achieved using a new technology based on laser transmi
3、ssion welding combined with the mask technique.This paper reviews a laser assembly system for the joining of micro fluidic plastic parts with its main related process characteristics and its potential for low-cost and high volume manufacturing.The system consists of a of di-ode laser with a mask and
4、 automated alignment function to generate micro welding seams with freely definable geome-try.A fully automated mask alignment system with a resolu-tion of 2 m and a precise,non-contact energy input allows a fast welding of micro structured plastic parts with high re-producibility and excellent weld
5、ing quality.Background The research and development of micro fluidic device,so-called lab-on-a-chip technology,is one of the fastest grow-ing areas of medical and biological diagnostics for a variety of applications including DNA analysis,drug discovery and clinical diagnostics 1,2.Up to now,the pre
6、ferred materials have been silicon,glass or quartz,mainly because micro fabrication methods for these materials have been exten-sively developed in the microelectronics industry.However,for many applications,these materials,and associated fab-rication processes,are not cost-effective for commercial
7、production and typically for single-use devices.Plastics are playing an important and ever-increasing role in micro technology-especially in low-cost,mass-produced applications.It is relatively easy to produce micro structure on the plastic substrates with complex patterns of 50-100m-sized channels
8、using state-of-the-art replication techniques such as injection molding,hot embossing 3 and UV molding.These substrates are typically used to produce devices on which reactions and high-efficiency electrophoretic separations of biomolecules have been achieved in timescales of seconds to minutes.Plas
9、tics like polymethylmethacrylate(PMMA),polycarbon-ate(PC)and Cyclopolyolefinpolymere(COC)have been in-tensively investigated because of their resistance to certain chemicals and biocompatibility.The complete fabrication approach of such a micro fluidic device involves two pri-mary steps:(1)formation
10、 of micro-channels in a plastic base wafer or layer,and(2)bonding of the base layer with a cover sheet to form closed channels.There are several joining procedures for plastic parts including glue cured by heat or UV light,ultrasonic welding and hot gas welding.However,most of these methods cannot,o
11、r can only with great difficulty,be adapted to micro structured plastic parts due to the dispensing problems,the use of additional mate-rial with different chemical and surface properties,and the lower precision of energy deposition.The goal of this research was to develop an assembly tech-nology fo
12、r micro structured parts made from plastics with the goal of producing single-use fluidic devices at an acceptable cost level.The assembly process and the re-lated equipment presented are based on the laser through transmission welding(TTIr)principle.This report describes a device fabrication proces
13、s and packaging results.Mask assisted laser transmission welding Laser welding of plastics was developed in the 1960s,but not until recently has it become a cost effective technology which offers a wide range of advantages because of the non-contact and localized energy input by laser beam 4.Laser r
14、epresents a source of clean,controllable,and con-centrated thermal and photochemical energy.Due to the recent progress in semiconductor laser technology,com-pact,economical and reliable laser sources in the NIR range(700 to 1500 nm)are now available which promote the direct use for the material proc
15、essing with desired power and beam quality.Fig.1.Principle of laser transmission welding.The principle of the TTIr welding process is indicated in Figure 1.The two plastic parts to be joined must have dif-ferent optical transmission characteristics at the laser wave-Paper for ANTEC 2002 Annual Techn
16、ical Conference May 05-09,2002,San Francisco,CA SESSION T49:Joining of Plastics and Composites IR and Laser Session 2(Tuesday Afternoon,May 7)J.-W.Chen Page 2 16.10.2003 length,one must be transparent or translucent and the other absorbent.The energy of the laser light is transmitted through the tra
17、nsparent part with minimal loss and con-verted into heat in the absorbing part.By applying a clamp-ing pressure,physical contact between the two parts is en-sured and the transparent part is heated by thermal con-duction.Surface melting under the illumination of laser beam results in the excitation
18、of convective fluxes within the liquid layer.These liquid layers from both welding parts permit the physical mixture within a cavity at the contact in-terface and the welding is initiated.Fig.2.Mask welding.A line focused laser beam is scanned over a mask,which discriminates the energy to the desire
19、d weld area.When very small and high accuracy welding seams in the form of lines or areas are required,exact local discrimina-tion of the deposited laser energy must be ensured.With the mask technique the shape of welding seams can be controlled with a high resolution 5.The principle is indi-cated i
20、n Figure 2.A reflective or absorbing mask is placed between the welding part and the laser source,which gen-erates the lateral energy distribution at the welding surface.The mask is illuminated by a diode bar focused to a line,which scans over the mask.The precision of the welding process depends bo
21、th on the quality of the mask and beam quality of the laser.The mask can be exchanged quickly,al-lowing for great flexibility in production.Process equipment All assembly procedures of micro fluidic devices described in this paper are performed by the mask welding system il-lustrated in Figure 3.The
22、 basic process equipment consists of a holder for welding parts,a laser source with optics,a mask,and a vision system for mask alignment.High power diode laser with an emission wavelength in the NIR range is used,typically between 808 nm and 980 nm.For a rapid heating and cooling process of plastic,
23、a power in the range of 80-120 W is required.Fig.3.Mask welding system for assembly of plastic micro fluidic device.a)laser source,b)micro alignment,c)vision system and d)clamping pressure system.In the mask welding process,the welding parts are placed underneath a transparent glass sheet with clamp
24、ing pres-sure to assure contact of the mating parts.The mask is fixed on a 3-axis stage(X-Y-)allowing for precise align-ment of the mask relative to the parts.The vision system moving between the mask and holder of welding parts,measures the locations of mask and welding part by using a two point me
25、asuring principle and well defined position marks selected directly from the micro structures on the welding part.The locating of defined position marks is car-ried out by using a pattern matching procedure correlating information about the location and orientation of a known object.According to the
26、 measured deviation of the positions the mask will be adjusted with an accuracy of 2 m to the micro structured welding parts.After the adjustment proc-ess the distance between mask and welding part will be minimized.The laser welding process is carried out by scanning of laser source over the mask.T
27、his work investi-gated various levels of system integration that may improve the performance of the system for the mass-fabrication of micro fluidic device.Plastic micro fluidic device There are number of diagnostic devices that require micro channels.In determining the manufacturing method of the c
28、hannel structure and the packaging of the device the di-mensional tolerances,material specifications,and costs should be taken into consideration.For example,a device for the injection and pumping of a precise volume of liquid is achieved through a combination of hydrophobic and hydro-philic regions
29、 defined inside the micro-channels.Solutions laser source laser beam mask welding parts abc d Paper for ANTEC 2002 Annual Technical Conference May 05-09,2002,San Francisco,CA SESSION T49:Joining of Plastics and Composites IR and Laser Session 2(Tuesday Afternoon,May 7)J.-W.Chen Page 3 16.10.2003 pla
30、ced at the inlet port are drawn in by capillary action.Once the liquid is positioned at the desired point,definite volume drops of approximately 1020 nl can be injected us-ing air pressure from the side-channel.A device for such a task usually consists of a solid plastic base on top of which a thin
31、plastic film is bonded.In the top surface of the plastic base are numerous channels through which the fluids will flow during the handling and analysis process.The plastic film forms the top boundary of these channels.Figure 4 shows some typical macro and micro channel structures of polycarbonate(PC
32、)micro fluidic parts which are 14 mm 10 mm in size and 2.5 mm in thickness(Fig.4a).This sam-ple has an injection channel and reservoir at the ends of the channel.It consists of a base plate and two tow cover sheets on both sides of the base plate.The cover sheets are 1 mm in thickness.The base plate
33、 was an injection-molded part.The fine structure of the channel has a dimen-sion of only 100 m.Fig.4.The key component for a microfluidic handling system with macro and mi-cro channels.Common requirements for assembly of the above men-tioned micro fluidic device are joining area,gas and liquid tight
34、 joints,and resistance to a few bars.Welding the film to the base must not block any channels,and there can be no delaminations between the film and base,which would permit leakage between adjacent channels.The overall cross-sections of the channels must not be changed during the welding process bec
35、ause of their influence on the effect of capillary action and the transport behavior of the fluids.An exact generation of the welding seams is needed.Results and discussion-Controlling of melt spreading The concept of mask welding in micrometer range has im-plications relating to the rapid heating a
36、nd cooling of plastic during the welding process.The key factors in establishing the desired welding accuracy are not only the quality of mask and beam shape of the laser,which mainly determine the precise heat transfer,but also the speed of heat treat-ment and response of material due to the melt f
37、low.Fig.5.Test welding seams containing various micro structures with different di-mensions.a)and b):Welding on non-structured plastic parts,the black areas are welded.c):Micro channels with melt spreading due to the miss-matching of mask.d):Micro channel welded using the excessive coverage of mask.
38、e):Complete welded sample.In order to achieve an optimum weld quality for small struc-tures,the melt flow was investigated using both of non-structured(flat)and structured(with micro channels)plastic parts.Figure 5 shows selected test welding seams.Welding structures with a minimum size of 100 m hav
39、e been achieved.The behavior of melt flow depends strongly on the welding velocity and viscosity of plastic materials with a characteristic speed of extension that depends additionally on the laser-light intensity and illumination time.The control of this flow was achieved via process optimizing(Fig
40、.6).The non-structured plastic welding parts were scanned by a laser source with a constant power of 80 W,equipped with a 150 m slit mask.The width of the non-welded line was measured as a function of scan velocity.A nearly linear behavior is shown within the narrow range.With scan ve-locities less
41、than 30 mm/s no well definable structure could be measured due to excessive melt beyond the mask area.For scan velocities greater than 50 mm/s no melt fluidic phase was produced by the laser illumination.It was ob-served that the speed of melt flow is about 23 mm/s.In order to achieve an accuracy of
42、 welding seams within 510 m,a maximum illumination time of 3.5 ms is required.A 500 m 400 m 100 m 100 ma)b)c)d)e)200 m a)b)c)Paper for ANTEC 2002 Annual Technical Conference May 05-09,2002,San Francisco,CA SESSION T49:Joining of Plastics and Composites IR and Laser Session 2(Tuesday Afternoon,May 7)
43、J.-W.Chen Page 4 16.10.2003 scanning procedure with a line focused laser beam and a high-energy intensity allows for a short illumination time.Fig.6.Melt spreading vs.process time The use of a laser beam to excite the melting fluxes within the liquid layer plays an important and decisive role for th
44、e heat transport,which ultimately determines the quality of the weld.Most plastic materials absorb the laser beam in NIR range only on the top surface with a depth of about a few micrometers.The deep penetration of laser energy can be reached by thermal interaction through liquid plastic flu-idics.T
45、he quick heating and cooling process is of benefit to the restriction of melt spreading but it may result in a very thin effective convection zone,which reduces the welding strength.The compromise must be taken by the excessive coverage of mask to reserve enough places for the expan-sion of plastic
46、melt fluxes because the melt spreading can-not be completely ruled out by process optimization.The typical dimension of the over coverage is about of 10 to 20 m,which is dependent on the material to be welded and should be integrated during the mask design phase.In ad-dition,high viscosity of liquid
47、 plastic is often of benefit to the restriction of melt spreading due to the slower melt flow.Figure.7 shows a microstructure assisted mechanism,which makes it possible to control or stop the spreading of the liquid melt.This structure forms a thin air slit next to the illumination area.As soon as t
48、he joining area is illuminated with the laser beam,the liquid melt is guided in a controlled manner into this air slit space.The form of this assisted structuring determines the shape of the edge of melt spreading.Fig.7.Controlling of melt spreading using assisted micro structures.Surface deformatio
49、n and material compatibility Because the mask welding process is based on a discrimi-native heating procedure,the difference of temperature on the welding parts will result in residual stresses6.Figure 8 shows the welding seam achieved with a slit mask.A wave-form structure at the boundary of weldin
50、g zone is visible.The reason for that is the temperature gradient near the boundary zone,where the plastic is not efficiently heated to melting point due to the cool surrounding.A growth at the absorbing part with height of 250 nm and a sinking of the surface at the same place on the transparent par