Fresenius J Anal Chem (2001) 371 : 270–275
DOI 10.1007/s002160101002

S P E C I A L I S S U E PA P E R

Ellis Meng · Shuyun Wu · Yu-Chong Tai

Silicon couplers for microfluidic applications

Received: 7 March 2001 / Revised: 8 June 2001 / Accepted: 19 June 2001 / Published online: 31 July 2001
© Springer-Verlag 2001

Abstract Several types of silicon fluidic coupler havebeen designed, fabricated, and tested to facilitate external
connections to MEMS (microelectromechanical systems)
fluidic devices. By using both bulk micromachining andDRIE (deep reactive ion-etching) techniques, couplers ofdifferent geometry have been produced for use with any
standard MEMS fluidic port. In addition, couplers are easily modified to accommodate any arbitrary fluidic port
geometry. For ease of use, these couplers interface withPEEK (polyetheretherketone) and fused-silica capillarytubing, both of which are commonly used in HPLC (highperformance liquid chromatography) systems and are supported by a wide range of plumbing products. Coupler
performance was evaluated and an operating range of atleast 0–8963 kPa (0–1300 psig) is attainable.

Introduction
Micromachined fluidic devices have great potential to
make an impact in the field of chemistry. The main advantages are small volume fluid flow, integrated subsystems, minimal power consumption, and small footprint.
One major problem that has hampered the development
and utilization of these MEMS (microelectromechanicalsystems) devices is the lack of a reliable and efficientmeans of accessing input/output ports to MEMS fluidicdevices. In the macro world, connections are easily made,because of the wealth of commercially available plumbing. Standard prefabricated tubing and fittings are easily
configured to achieve the desired result. On the MEMS
scale, which is of the order of millimeters or smaller, however, such a varied selection of suitable connections is not
currently available. Much research has been devoted toE. Meng (✉) · S. Wu · Y.-C. Tai
Caltech Micromachining Laboratory,
California Institute of Technology,
136-93, Pasadena, CA, 91126, USAe-mail: ellis@mems.caltech.edu

solving this problem, yet an adequate solution has not yet
been found [1, 2, 3, 4]. The most common method toachieve fluidic connections is to manually align and glue
tubing to ports. Numerous disadvantages plague thismethod including low yield, complex assembly, misalignment, large footprint, and difficulty of salvaging usedcomponents. In an effort to develop standardized MEMS
connections, device-independent prefabricated fluidic
couplers have been conceived [5].

DesignDirectly attaching tubing to fluidic ports has proven to be
an extremely difficult and unreliable way to achieve a fluidic connection. By introducing an intermediate piece that
accurately mates the tubing and port, this problem can beavoided. Micromachined fluidic couplers serve as a convenient intermediary between external fluidic plumbing
and the inlet/outlet ports of a fluidic device. They are discrete silicon structures consisting of alignment structuresand sleeves that enable geometrically matched fits to fluidic ports and tubing. A single fluidic connection to an array of fluidic ports using such a coupler is shown in Fig. 1,Fig. 1 Post coupler attached to a fluidic port

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Fig. 2 3D Exploded cross-sectional view of a bulk coupler assembly

Fig. 3 Comparison of a conventional fluidic interconnection andour method

Both fused-silica capillary and PEEK (polyetheretherketone) tubing can be used with couplers. These werechosen for their ease of use and wide range of commercially available plumbing accessories. Adhesive can beused to connect couplers to tubing, however, when the dimensions of the mating receptacle are appropriately calibrated, cryogenic insertion is also an option (Fig. 4). Bymeans of this technique the tubing size can be reduced bycooling it in liquid nitrogen or other similar agent for easeof insertion into a coupler. Previously, couplers were individually attached to fluidic ports by means of adhesive[7]. This approach is time-consuming, inconsistent, andextremely inconvenient. To mitigate these issues, a batchprocess was developed (Fig. 5). By using Crystalbond, athermoplastic adhesive, wafer level spray coating to thousands of couplers at once is achieved. The thermoplastic
nature of Crystalbond enables easy attachment and removal of parts by thermal cycling. Alternatively, couplers
can also be molded with a thermoplastic material and subsequently re-flowed to form a connection. It is also possible to use other adhesive joining methods, and solder andeutectic bonding at the coupler-to-I/O port interface.
Modified couplers with fitting structures at both ends ofthe capillary can also be used to concatenate microfluidicdevices.
Fluidic ports are typically created by anisotropic etching, drilling, or DRIE (deep reactive ion-etching). The
geometries associated with these fabrication methods dictate coupler design. Various coupler designs stem fromtwo common geometries encountered in MEMS fluidic
devices (Fig. 6). The first incorporates the truncated pyramidal geometry associated with anisotropic etching. Although it is possible to glue tubing directly to pyramidalpits, leakage paths because of mismatch (non-interlockinghalves) result in clogging when less viscous adhesives areused. In addition, each of these structures can be designedwith a shoulder to constrain the usable height of the coupler. The second scheme can be used with fluidic ports ofcircular cross-section that are formed by drilling or DRIE.
Furthermore, by using DRIE, virtually any cross-sectional
port or tubing geometry can be matched. Each of these designs incorporates alignment structures and has a low
dead volume, and only a minimum amount of adhesive is
exposed to fluid flow.

Fabrication

Fig. 4 Process flow for the cryogenic insertion techniqueand the interlocking nature of the couplers is depicted inFig. 2. The spatial advantages gained by using couplers,compared with traditional manual coupling techniques, isdemonstrated on thermopneumatic valves from Ref. [6] inFig. 3. In addition to alignment structures, stepped sleevesthat serve as a tubing insertion depth guide can also be included in the couplers.

Three types of coupler using different sizes of capillaryand PEEK tubing have been fabricated. Process flows
for fabricating the various types of couplers are shown in
Figs 7, 8, 9.
Bulk coupler
Bulk couplers are formed by taking advantage of thestructures formed when anisotropically etching silicon.
The name “bulk couplers” arises from the use of bulk mi-272

Fig. 5 Process flow for the thermoplastic spray-coating techniqueFig. 6 (a) Bulk, (b) molded, and (c) post couplers with capillary tubing

approximately 270 µm high are formed. These islands
form the basic bulk coupler structure. Press-fit pits fortubing are then etched into the center of these islands byDRIE. Depending on the application, a stepped pit can beformed by leaving approximately 50 µm silicon then etching a smaller diameter hole through the remaining silicon.
This additional shoulder forms a tubing stop and is shownin Figs 10 and 11.
As is evident from the desired geometry, corner compensation is required to fabricate bulk couplers. Twoschemes were investigated and etching results are shownin Fig. 12. The merits of each method are described inRefs [8] and [9], respectively. Additional features inFig. 12a, b are artifacts attributed to the DRIE of masking
gaps, because of poor photoresist step coverage. Althougha certain amount of undercut is observed for both convex
corners, slight modifications to the etch mask and carefulattention to etching progress should eliminate these features.
Molded bulk coupler
Fig. 7 Bulk coupler process flow

cromachining techniques to fabricate these structures(fabricating structures by etching into the substrate material). First, oxidized silicon wafers (~1.5 µm) are patternedand etched in KOH (potassium hydroxide) until islands

Molded bulk couplers are a variation on the standard bulk
coupler and are fabricated from two bonded wafers (Figs. 8and 13). One is an oxidized wafer etched with KOH tocreate through-holes; the other wafer is DRIEd to form anarray of circular pegs. These pegs form alignment structures for tubing. By bonding these two structures together

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Fig. 8 Molded coupler process flow

Fig. 10 SEMs of bulk couplers

Fig. 9 Post coupler process flow

and coating with a PTFE (polytetrafluoroethylene)-based
release layer, a mold is formed. The release layer facilitates removal of the molded coupler from the mold structure and prevents adhesion of the molded coupler to themold. It is also possible to substitute other types of moldrelease layers. Plasma-deposited Teflon and Parylene C
are also being investigated as candidates for mold-releaselayers. Moldings are realized by melting raw material
such as a hot-melt polyolefin around fused silica tubing
fitted on mold posts. When the polyolefin material has
cooled, couplers can be release simply by pulling themFig. 11 Cross-sectional SEM view of an assembled bulk coupler

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Fig. 12 (a) and (b) Corner compensation structures with respective mask layouts (c) and (d)can be used. An advantage of using these couplers is thereduction in fabrication costs afforded by a reusable moldand polymer structural material.
Post coupler
The nature of DRIE processing enables flexibility in creating two-dimensional geometries across a wafer surface.
A two -dimensional feature corresponding to the desired
coupler geometry can be defined and etched in silicon.
Thus, post couplers (Figs 9 and 14) can be used with virtually any fluidic port and tubing. Here, a generic process
flow is described and the process flow for a cylindricalcoupler is depicted.
First, an opening is defined in an oxidized wafer to forma tubing insertion depth guide. Then alignment structures
and tube receptacles are etched. Tubes are then press-fitted into the receptacles and joined to a fluidic port.

Fig. 13 SEMs of coupler molds

Testing
To determine the failure point and the operating range,
blocked couplers were pressurized using filtered (0.1 µm)
nitrogen gas from a cylinder (Fig. 15). The testing equipment was constructed from stainless-steel Swagelok plumbing to withstand high pressures of the order of 1000 psig.
PEEK and Valco stainless-steel high-performance liquid
chromatography (HPLC) fittings were used to connect thesmall-diameter PEEK and fused-silica capillary tubing

Fig. 14 SEMs of post couplers

away from the mold. Released structures are then attached
to fluidic ports simply by re-heating the polyolefin, enabling it to reflow and adhere to the silicon port. In applications where heating is unacceptable, adhesive joining

Fig. 15 Schematic diagram of testing equipment

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Table 1 Comparison of different methods of fluidic coupling
Description of Coupler

Operation Range

Refs

Silicon finger microjoint with silicone gasket and Tygon tubing
Silicon/plastic coupler with silicone gasket and capillarytubing
Silicone gasket sealed silicon coupler with capillary tubing
Silicon sleeve coupler with capillary tubing
Polymer coupler with fused capillary tubing
Silicon coupler with cryogenically inserted PEEK tubing
Silicon coupler with fused capillary or PEEK tubing>30 psig (210 kPa)
~60 psig (415 kPa)
~80 psig (550 kPa)
~500 psig (3550 kPa)
~900 psig (6200 kPa)
>1300 psig (8960 kPa)
>1300 psig (8960 kPa)

[1]
[2]
[4]
[2]
[5]
This work
This work and [5]from the coupler to the rest of the pressure-testing equipment. Pressure was measured by use of an Omega PX120–
2KGV pressure transducer in conjunction with an Omega
DP25-S strain gage panel meter.
Bulk and post couplers with capillary tubing were able
to withstand at least 8963 kPa (1300 psig). This corresponds to the maximum measurable pressure of the experimental apparatus. Post couplers with cryogenically inserted PEEK tubing were also tested and are identical inperformance with couplers with glued tubing. Molded
couplers, however, failed at approximately 6200 kPa(900 psig) because of poor bonding between the molding
material and the capillary. These results are summarized
and compared with other work in Table 1.

Conclusion
To create standardized microfluidic connections to MEMS
fluidic devices/systems, micromachined fluidic couplers
have been designed, fabricated, and tested. These couplers
are compatible with PEEK and fused silica capillary tubing and are capable of withstanding pressures up to 8963 kPa(1300 psig). These couplers can also be used with typical
fluidic ports and can, if necessary, be customized for use
with special geometries.
Acknowledgements The authors would like to thank Mr Tuan
Hoang for help with proofreading and Mr Trevor Roper and Mr
Hung Bui for assistance with fabrication.

ReferencesDiscussion
Determination of the failure load of the adhesive joint between the coupler and the fluidic port is a difficult problem because of complex joint geometry and the inability
to ascertain the exact failure mechanism(s). General observations concerning the design and resulting mechanicalbehavior of the joints can be made, however. To maximize
strain capability and toughness of adhesives, joints are
ideally designed such that compression and shear are
more dominant than tension, cleavage, or peel stresses.
Thus, post couplers are expected to have better performance, because compression and shear have the largest
stress contributions under loading. In addition, the surface
roughness accrued during the DRIE process promotes mechanical interlocking and increases the surface area available for bonding. Bulk couplers, however, experience a
combination of tension and shear stresses. By using liquidadhesives, the excess adhesive squeezed from the joint, orspew fillet, can provide the added benefit of reducing thestress concentration at the edges of joints, further increasing overall strength [10].

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