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Fluidic interconnects for modular assembly of chemical microsystems
Sensors and Actuators B 49 (1998) 40 – 45
Fluidic interconnects for modular assembly of chemical microsystems C. Gonza´lez, S.D. Collins, R.L. Smith * MicroInstrument and Systems Laboratory (MISL), Department of Electrical and Computer Engineering, Uni6ersity of California – Da6is, Da6is, CA 95616, USA Received 1 October 1997; received in revised form 30 December 1997; accepted 2 January 1998
Abstract A new assembly technology is presented which enables the modular interconnection, assembly and packaging of individual microfabricated components and/or modules (e.g., pumps, valves, reaction chambers, control circuitry, etc.). This technology can be used to make compression seal fittings utilizing self-aligning, interlocking, mechanical bonding structures and photo-patternable silicone O-ring seals. Two distinct types of fluidic interconnects for microfluidic system assembly are demonstrated using this technology. The first type demonstrates a means of interconnecting multiple, vertically stacked layers of micromachined channels. The second type demonstrates a means of connecting micromachined channels to tubing or other micromachined channels within the same plane. An important feature of these interconnects are that they are reversible, i.e., interconnections can be repeatedly made with the same structures allowing flexible system assembly. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Assembly; Interconnect; Microfluidics; Microsystems
1. Introduction Traditional analytical instruments are large, expensive and relegated to the laboratory environment. Hence, there is an increasing interest to replace these instruments with small and inexpensive handheld units. Palm-size, portable instruments and microlaboratories have become possible with recent advancements in materials science and microelectromechanical (MEM) devices, such as microvalves [1,2] and pumps [3,4]. Concomitant with the miniaturization of chemical analysis components is the need for miniature fluidic interconnects. One of the challenges is bridging the scale from the micron scale of microfabricated structures to the millimeter scale of components such as tubing, sample vials, and micropipettes, in order that prototype instruments can be interfaced with existing laboratory equipment. The structures and techniques presented in this paper establish a viable assembly and packaging technology for interconnecting MEMS components. The approach * Corresponding author. Tel.: +1 916 7524140; fax: + 1 916 7528428; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00035-5
is analogous to multi-chip-module (MCM) technology for integrated circuits. This general concept of a fluidic breadboard has been previously proposed [5] but a definitive, modular technique for aligning and attaching components was lacking. This paper describes a means for attachment that is versatile enough to provide alignment, accommodate either vertical or planar assembly, allow connections between components fabricated on different types of substrates (e.g., silicon, glass, plastic, alumina, gallium arsenide, metal) and be reversible, allowing each component to be unattached and either replaced or interchanged with another module. Assembly is based on stackable and plug-in, interlocking structures, similar in concept to those employed in the popular Lego® toys. One type of interlocking structures are micro-finger joints, made in two mating substrates, as shown in Fig. 1. By creating a set of periodic, vertical grooves into the surface of one substrate, a vacancy in the substrate is created, which can be filled with similar dimension grooves of opposite polarity. When two opposite sets of structures are pushed into each other, the fingers tend to separate by a small distance to accommodate the insertion of the fins of the opposite structure. The friction between the
C. Gonza´lez et al. / Sensors and Actuators B 49 (1998) 40–45
two side walls in contact, and the restoring force of the displaced fins, create a locking mechanism where both structures are held in place. The locking force is determined mainly by the friction of the two surfaces in contact, but the spring constant of the fins themselves also contribute to holding the structures together. The structures can be released by applying a force larger than the sum of the forces holding them in place. The very nature of these structures make them self-aligning and re-attachable, i.e., Lego®-like. These structures can readily be formed using a wafer saw to physically machine the substrates, making it applicable to many different types of substrates. Another type of interlocking structure is used to make connections to tubing and a quick connect for in plane modular connections. These structures are micromachined in silicon, but can be integrated with microchannels and the interlocking finger joints for interfacing with microsystems. This is demonstrated by the fabrication and assembly of a fluidic breadboard.
2. Fabrication and assembly
2.1. Interlocking finger joints The interlocking finger joints used in the structures described in this paper are fabricated using a diamondtip, circular saw. However, grooves can also be fabricated using deep RIE etching or anisotropic wet chemical etching, e.g., 110-oriented silicon in KOH solutions. A set of periodic grooves are cut along the periphery of each interlocking die. When the groove period is twice the kerf of the blade, parts that are diced from the same substrate can fit into one another. However, one of the two interlocking dies requires a recess etch to accommodate the insertion of the complimentary structure, and its interlocking fingers need to be 180° out of phase with those on the opposite die. Mating dies can then be easily assembled as show in
Fig. 1. Diagram showing the interlocking fins concept.
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Fig. 2. Photograph of two mating dies with interlocking finger joints. Insert shows close-up of finger joints.
Fig. 2. Fig. 2 is a photograph of two assembled dies made in a single, (100) silicon substrate. The recess etch in this case was made by an anisotropic etch using KOH, which is evidenced by the characteristic 54° angle of the recess side-wall in the close-up (insert) of the finger joints. The groove width and spacings are 2509 5 mm. Using a dicing saw to form the finger joints has proven to be very effective, and sufficiently accurate for this application. Groove widths are accurate to 95 mm with accuracy primarily limited by chipping, which is somewhat dependent on the size of the diamond grit of the blade. Blades for cutting silicon can be obtained with grit size of 0.5 mm, resulting in chipping on order of 9 1 mm. Cutting glass requires larger grit sizes and consequently produces a higher degree of chipping and lower tolerances. However, widening of the tops of the groove widths due to chipping can be compensated with deeper grooves, since this will result in more flaring and larger sidewall surface contact area, increasing the holding force. Also, the bottom of the grooves is not affected by chipping. The interlocking finger joints have been used to fabricate several fluidic interconnecting structures. These connectors lend themselves nicely to fluidic applications because the locking mechanism facilitates interface sealing in addition to vertical assembly for multilevel channel interconnect. Placement of an O-ring or gasket at the interconnect interface can be held in compression via the clamping force of the interlocking fingers, thereby providing a viable sealing methodology (Fig. 3). The fluidic interconnect consists of two mating silicon parts, each containing fluid microchannels. An O-ring or gasket was fabricated on one of the parts using a UV curing, 10–100 mm thick polysiloxane film. With the appropriate choice of finger length, surface recess and O-ring thickness, the O-ring is held in compression when the two interlocking surfaces are pressed
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together, as shown in Fig. 3. Fluidic interconnects, bridges and a fluidic channel breadboard have been realized utilizing this technology. All were made with very similar fabrication steps, but only the fabrication of the fluidic interconnect is given here in detail. The fabrication process described here utilizes (100)oriented silicon wafers. Processing of the negative polarity structure begins with the deposition of a low-pressure chemical vapor deposition (LPCVD) sili˚ ), then patterning the con nitride masking layer (1500 A fluidic via and channel definition in the nitride using photolithography and RIE etching with CF4/4% O2. Then a second thin layer of LPCVD silicon nitride is ˚ ), and the fluidic via is repatterned and deposited (750 A etched open. The wafer is then placed in a 45 wt.% KOH solution and etched partially through the wafer. The thin nitride layer masking the channel is then stripped in a concentrated HF solution and the wafer is placed back in the KOH solution. In this way, the via hole can be etched completely through the silicon wafer while the channel depth is kept shallow. The etch is stopped when a 3–6 mm silicon membrane remains at the bottom of the via. We used a timed etch, but a p+ etch stop could be easily implemented. The etched wafer is then bonded to a second wafer of either glass
or silicon by either anodic or fusion bonding, respectively. The UV curable polysiloxane [6], which forms the O-ring or gasket for sealing, is then spin coated on the wafer. The resulting 10–20 mm films are exposed with UV radiation through a photomask to pattern the O-ring. The exposed regions cross-link and cure, while the unexposed regions are washed away in the xylene developer. Once cured and thoroughly dried, a wafer saw is used to cut the interlocking fins into the surface of the wafer. Finally, the thin silicon membrane is RIE etched, using SF6 and a shadow mask, opening the via hole. Alternatively, the O-rings have been fabricated separately and then applied to the interconnect before sealing. The positive polarity structure is fabricated in similar fashion, but includes a recess etched into the mating surface. The interlocking finger joints provide alignment, O-ring compression, and reversible connection. The finger joints can either be applied at the wafer level or at the die (module) level, making many component designs compatible with this technology. Space at the perimeter of each module for the finger joints can be readily accommodated in most cases. However, integrating the recessed etch into some module design is perceivably troublesome. The recess etch can be circumvented by making two positive structures and connecting them with micromachined splines.
2.2. Tubing interconnects
Fig. 3. Cross-sectional view of a fluidic interconnect, with a compression seal formed by interlocking finger joints with interposed silicone O-ring.
The I/O devices described here demonstrate, for the first time, modular, microfabricated interfacing between the micro world (microfabricated channels), and the macro world (tubing). The interconnecting structures are microfabricated silicon tubes, which internally connect to microchannels, and externally connect to plastic tubing (Fig. 4). These structures can also be integrated with the fluidic channel interconnects described above for modular system assembly. The devices are fabricated in two (100) silicon layers by etching channels in each, and simultaneously etching from the opposite side of the channels to create a hex-shaped silicon tube. The module in Fig. 4 also has a via etched through one end of the channel in one substrate for interfacing to another layer of channels. The two silicon layers are aligned and fusion bonded together. The plastic tubing is selected to snugly fit over the outer diameter of the silicon tube. The starting silicon wafers were 350 mm thick, so the outer diameter of the bonded silicon tube is 700 mm. The closest fitting inner diameter tubing was 762 mm (30 mils). In some cases, after slipping the plastic tubing over the silicon tube, heat was applied to flow the plastic tubing over the silicon tube, sealing the connection. When the devices were diced apart, a portion of silicon was left on either side of the silicon tube to guide and secure the tubing in
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Fig. 4. Assembly diagram of silicon tubing and I/O interconnect to external tubing. Lower diagram shows integration of finger joints and vertical channel interconnect with tubing interconnect.
place, and lend structural support to the silicon tubing. The inner diameter of the silicon tube is 300 mm. The length varied, but a 5 mm long silicon tube provided enough structural support and sufficient overlap of the tubing to properly seal the connection. An I/O device with tubing attached is shown in Fig. 5. The addition of the finger joints allows this fluidic structure to become an integral part of a network of fluidic channels and connections, i.e., miniature fluidic systems, by providing the vital connection to the macro environment.
2.3. Fluidic quick-connects A fluidic ‘quick connect’ technique has also been developed to connect microchannels in the same plane on separate modules. Similar, complementary I/O structures provide a self-aligning and interlocking fluidic end-to-end, channel interconnect. The complementary structure is fabricated in the same method as illustrated in Fig. 4, but the external, releasing etch also produces silicon beams which match the voids in the I/O device. The two structures slide into each other (Fig. 6). The silicon tube in the I/O device pushes up against the opening of the micromachined channel in the matching structure. Sealing can be accomplished by sandwiching a gasket between the butted fluid channels. The structures are held together by flaring the outside base of the silicon tube on the I/O device, so when the silicon beams on the matching device reach the base they encounter a slightly larger diameter tube, which pushes them outward against the silicon guides on the I/O device, locking it in place. By applying a force which exceeds the clamping force of the structures, the devices are quickly released, i.e., quick-connects. Photographs of the opposing ends are show in Fig. 6 (top and center), and a connecting pair with finger joints and vias to the inner channels is shown at the bottom.
2.4. Fluidic breadboard
Fig. 5. Photograph of an I/O device, with tubing interconnect.
The interconnecting finger joints and I/O devices were combined to demonstrate a fluidic breadboard.
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board to seal the interfaces between the added bridges and input/output components. One can envision making a flexible analytical instrument using this scheme, where individual channels in the breadboard contain sensors, mixers and/or other elements. Bridges are used to connect the desired sequence of functional channels, in a serial or parallel fashion.
3. Testing
Fig. 6. Photographs of a silicon micromachined quick-connect. Top and center photos show opposing ends of the micromachined quickconnect. The inner silicon tubes transition into shallow microchannels. This type of connection has very small dead volume. The bottom photo shows pair of connects with the integrated vias and finger joints for attachment to other parts.
The breadboard consists of an anisotropically etched, 4 inch (100) silicon wafer and a 4 inch 7740 Pyrex® wafer. The fluid channels were etched into one side of the silicon wafer, while the vias were etched through the wafer, and the finger joints were machined into the top side of the wafer. The etched wafer was then anodically bonded to the Pyrex® wafer, such that the glass substrate forms the bottom of the channels. The fluid bridges are fabricated in a similar fashion on a 4 inch wafer but have a recess etch to allow them to plug into the breadboard, and are diced apart after bonding to a Pyrex® wafer. On a 4 inch wafer, channels of various lengths, with widths of 1.2 mm and depths of 200 mm, were fabricated to demonstrate the flexibility of the breadboarding concept. A photograph of two fabricated breadboards is shown in Fig. 7. One is pictured with bridges of various lengths attached, which couple fluid from one channel to another, and input/output devices which attach to the breadboard in the same fashion as the bridges. The other breadboard is flipped over so the channels can be seen through the Pyrex® wafer. The insert in Fig. 7 illustrates the assembly concept with fluid flow from one bridge through a breadboard channel and into the other bridge. The full diameter of the wafer is used as the breadboard. A polysiloxane O-ring layer is patterned on the bread-
In order to test the integrity of the fluidic interconnects, two mating finger joint modules were each attached to two separate Plexiglas® substrates. The Plexiglas® substrates each contained a channel drilled from one side up to the surface where the fluidic interconnect structures were mounted over the hole and sealed using a two part epoxy. The side entry was threaded to accommodate a standard tubing fitting. A 30 mm thick, free standing polysiloxane gasket was placed between the two mating parts and then they were pressed together to form the seal. Water was then pumped from one Plexiglas substrate through the interconnect junction and out the other Plexiglas structure, using a recirculating pump on a water bath. No leaks were visually observed. To test the maximum seal pressure, one of the interconnecting modules was capped with a solid mating structure to block flow through the channel. A polysiloxane O-ring was sandwiched between the two parts. The via opening of the capped fluidic interconnect was 1× 1 mm2. Water was pumped into the Plexiglas® substrate until the capped interconnect failed. Pressures of up to 20 psi were applied with no noticeable leaks. Above 20 psi, the glued interface between the Plexiglas® and the silicon fluidic interconnect failed, such that the seals could not be tested above this pressure.
4. Conclusions The fluidic interconnect methodology and devices presented here is a low-cost, effective means of creating modular fluidic microsystems. This technique can be readily applied to materials other than silicon, such as GaAs, glass, and high-density PVC. It is therefore a generic interconnecting technique that provides a high degree of flexibility in the assembly of microsystems and a completely modular approach, here-to-fore not feasible. For example, it enables the aligned, direct coupling of individual components, built with different technologies and potentially even in different materials, with or without a ‘mother board’ or common substrate. Components or modules can be assembled horizontally or vertically to form truly three-dimensional microsys-
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Fig. 7. Photograph of two silicon–glass breadboards. The upper breadboard has bridges and I/O devices plugged into it. The bottom breadboard is turned over to reveal the microchannels, which are visible through the glass. The insert illustrates the fluidic breadboard concept. Fluid flows from an upper channel in a bridge, down to lower channel in the breadboard, then back up to another bridge. Connections are reversible. Bridging connectors are plugged into the breadboard, directing the flow of fluid through selected channels.
tems. We liken this assembly concept to the well known Lego® building blocks. Although fluidic components are the focus of this presentation, we have also fabricated electrical and optical interconnects utilizing this technology.
[4]
References
[5]
[1] P.W. Barth, Silicon microvalves for gas flow control, Tech. Digest, Transducers ’95, Int. Conf. Solid State Sensors and Actuators, June 1995, paper no. 301-B7. [2] M.J. Zdeblick, R. Anderson, J. Jankowski, et al., Thermopneumatically actuated microvalves and integrated electro-fluidic cir-
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cuits, Solid State Sensors and Actuator Workshop, Hilton Head, SC, USA, June 13 – 16, 1994, pp. 251 – 255. V. Gass, B.H. Van Der Schoot, S. Jeanneret, N.F. De Rooij, Integrated flow-regulated silicon micropump, Sensors and Actuators A 43 (1/2) (1994) 335 – 338. R. Zengerle, W. Geiger, M. Richter, et al., Transient measurements on miniaturized diaphragm pumps in microfluid systems, Sensors and Actuators A 47 (1/3) (1995) 557 – 561. T.S.J. Lammerink, V.L. Spiering, M. Elwenspock, et al., Modular concept for fluid handling systems: a demonstrator micro analysis system, Proc. MEMS ’96, San Diego, CA, USA, 1996, pp. 389 – 384. A. Van Den Berg, A. Grisel, E. Verneynorberg, An ISFET-based calcium sensor using a photopolymerized polysiloxane membrane, Sensors and Actuators B 4 (3/4) (1991) 235 – 238.
Fluidic interconnects for modular assembly of chemical microsystems C. Gonza´lez, S.D. Collins, R.L. Smith * MicroInstrument and Systems Laboratory (MISL), Department of Electrical and Computer Engineering, Uni6ersity of California – Da6is, Da6is, CA 95616, USA Received 1 October 1997; received in revised form 30 December 1997; accepted 2 January 1998
Abstract A new assembly technology is presented which enables the modular interconnection, assembly and packaging of individual microfabricated components and/or modules (e.g., pumps, valves, reaction chambers, control circuitry, etc.). This technology can be used to make compression seal fittings utilizing self-aligning, interlocking, mechanical bonding structures and photo-patternable silicone O-ring seals. Two distinct types of fluidic interconnects for microfluidic system assembly are demonstrated using this technology. The first type demonstrates a means of interconnecting multiple, vertically stacked layers of micromachined channels. The second type demonstrates a means of connecting micromachined channels to tubing or other micromachined channels within the same plane. An important feature of these interconnects are that they are reversible, i.e., interconnections can be repeatedly made with the same structures allowing flexible system assembly. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Assembly; Interconnect; Microfluidics; Microsystems
1. Introduction Traditional analytical instruments are large, expensive and relegated to the laboratory environment. Hence, there is an increasing interest to replace these instruments with small and inexpensive handheld units. Palm-size, portable instruments and microlaboratories have become possible with recent advancements in materials science and microelectromechanical (MEM) devices, such as microvalves [1,2] and pumps [3,4]. Concomitant with the miniaturization of chemical analysis components is the need for miniature fluidic interconnects. One of the challenges is bridging the scale from the micron scale of microfabricated structures to the millimeter scale of components such as tubing, sample vials, and micropipettes, in order that prototype instruments can be interfaced with existing laboratory equipment. The structures and techniques presented in this paper establish a viable assembly and packaging technology for interconnecting MEMS components. The approach * Corresponding author. Tel.: +1 916 7524140; fax: + 1 916 7528428; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00035-5
is analogous to multi-chip-module (MCM) technology for integrated circuits. This general concept of a fluidic breadboard has been previously proposed [5] but a definitive, modular technique for aligning and attaching components was lacking. This paper describes a means for attachment that is versatile enough to provide alignment, accommodate either vertical or planar assembly, allow connections between components fabricated on different types of substrates (e.g., silicon, glass, plastic, alumina, gallium arsenide, metal) and be reversible, allowing each component to be unattached and either replaced or interchanged with another module. Assembly is based on stackable and plug-in, interlocking structures, similar in concept to those employed in the popular Lego® toys. One type of interlocking structures are micro-finger joints, made in two mating substrates, as shown in Fig. 1. By creating a set of periodic, vertical grooves into the surface of one substrate, a vacancy in the substrate is created, which can be filled with similar dimension grooves of opposite polarity. When two opposite sets of structures are pushed into each other, the fingers tend to separate by a small distance to accommodate the insertion of the fins of the opposite structure. The friction between the
C. Gonza´lez et al. / Sensors and Actuators B 49 (1998) 40–45
two side walls in contact, and the restoring force of the displaced fins, create a locking mechanism where both structures are held in place. The locking force is determined mainly by the friction of the two surfaces in contact, but the spring constant of the fins themselves also contribute to holding the structures together. The structures can be released by applying a force larger than the sum of the forces holding them in place. The very nature of these structures make them self-aligning and re-attachable, i.e., Lego®-like. These structures can readily be formed using a wafer saw to physically machine the substrates, making it applicable to many different types of substrates. Another type of interlocking structure is used to make connections to tubing and a quick connect for in plane modular connections. These structures are micromachined in silicon, but can be integrated with microchannels and the interlocking finger joints for interfacing with microsystems. This is demonstrated by the fabrication and assembly of a fluidic breadboard.
2. Fabrication and assembly
2.1. Interlocking finger joints The interlocking finger joints used in the structures described in this paper are fabricated using a diamondtip, circular saw. However, grooves can also be fabricated using deep RIE etching or anisotropic wet chemical etching, e.g., 110-oriented silicon in KOH solutions. A set of periodic grooves are cut along the periphery of each interlocking die. When the groove period is twice the kerf of the blade, parts that are diced from the same substrate can fit into one another. However, one of the two interlocking dies requires a recess etch to accommodate the insertion of the complimentary structure, and its interlocking fingers need to be 180° out of phase with those on the opposite die. Mating dies can then be easily assembled as show in
Fig. 1. Diagram showing the interlocking fins concept.
41
Fig. 2. Photograph of two mating dies with interlocking finger joints. Insert shows close-up of finger joints.
Fig. 2. Fig. 2 is a photograph of two assembled dies made in a single, (100) silicon substrate. The recess etch in this case was made by an anisotropic etch using KOH, which is evidenced by the characteristic 54° angle of the recess side-wall in the close-up (insert) of the finger joints. The groove width and spacings are 2509 5 mm. Using a dicing saw to form the finger joints has proven to be very effective, and sufficiently accurate for this application. Groove widths are accurate to 95 mm with accuracy primarily limited by chipping, which is somewhat dependent on the size of the diamond grit of the blade. Blades for cutting silicon can be obtained with grit size of 0.5 mm, resulting in chipping on order of 9 1 mm. Cutting glass requires larger grit sizes and consequently produces a higher degree of chipping and lower tolerances. However, widening of the tops of the groove widths due to chipping can be compensated with deeper grooves, since this will result in more flaring and larger sidewall surface contact area, increasing the holding force. Also, the bottom of the grooves is not affected by chipping. The interlocking finger joints have been used to fabricate several fluidic interconnecting structures. These connectors lend themselves nicely to fluidic applications because the locking mechanism facilitates interface sealing in addition to vertical assembly for multilevel channel interconnect. Placement of an O-ring or gasket at the interconnect interface can be held in compression via the clamping force of the interlocking fingers, thereby providing a viable sealing methodology (Fig. 3). The fluidic interconnect consists of two mating silicon parts, each containing fluid microchannels. An O-ring or gasket was fabricated on one of the parts using a UV curing, 10–100 mm thick polysiloxane film. With the appropriate choice of finger length, surface recess and O-ring thickness, the O-ring is held in compression when the two interlocking surfaces are pressed
42
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together, as shown in Fig. 3. Fluidic interconnects, bridges and a fluidic channel breadboard have been realized utilizing this technology. All were made with very similar fabrication steps, but only the fabrication of the fluidic interconnect is given here in detail. The fabrication process described here utilizes (100)oriented silicon wafers. Processing of the negative polarity structure begins with the deposition of a low-pressure chemical vapor deposition (LPCVD) sili˚ ), then patterning the con nitride masking layer (1500 A fluidic via and channel definition in the nitride using photolithography and RIE etching with CF4/4% O2. Then a second thin layer of LPCVD silicon nitride is ˚ ), and the fluidic via is repatterned and deposited (750 A etched open. The wafer is then placed in a 45 wt.% KOH solution and etched partially through the wafer. The thin nitride layer masking the channel is then stripped in a concentrated HF solution and the wafer is placed back in the KOH solution. In this way, the via hole can be etched completely through the silicon wafer while the channel depth is kept shallow. The etch is stopped when a 3–6 mm silicon membrane remains at the bottom of the via. We used a timed etch, but a p+ etch stop could be easily implemented. The etched wafer is then bonded to a second wafer of either glass
or silicon by either anodic or fusion bonding, respectively. The UV curable polysiloxane [6], which forms the O-ring or gasket for sealing, is then spin coated on the wafer. The resulting 10–20 mm films are exposed with UV radiation through a photomask to pattern the O-ring. The exposed regions cross-link and cure, while the unexposed regions are washed away in the xylene developer. Once cured and thoroughly dried, a wafer saw is used to cut the interlocking fins into the surface of the wafer. Finally, the thin silicon membrane is RIE etched, using SF6 and a shadow mask, opening the via hole. Alternatively, the O-rings have been fabricated separately and then applied to the interconnect before sealing. The positive polarity structure is fabricated in similar fashion, but includes a recess etched into the mating surface. The interlocking finger joints provide alignment, O-ring compression, and reversible connection. The finger joints can either be applied at the wafer level or at the die (module) level, making many component designs compatible with this technology. Space at the perimeter of each module for the finger joints can be readily accommodated in most cases. However, integrating the recessed etch into some module design is perceivably troublesome. The recess etch can be circumvented by making two positive structures and connecting them with micromachined splines.
2.2. Tubing interconnects
Fig. 3. Cross-sectional view of a fluidic interconnect, with a compression seal formed by interlocking finger joints with interposed silicone O-ring.
The I/O devices described here demonstrate, for the first time, modular, microfabricated interfacing between the micro world (microfabricated channels), and the macro world (tubing). The interconnecting structures are microfabricated silicon tubes, which internally connect to microchannels, and externally connect to plastic tubing (Fig. 4). These structures can also be integrated with the fluidic channel interconnects described above for modular system assembly. The devices are fabricated in two (100) silicon layers by etching channels in each, and simultaneously etching from the opposite side of the channels to create a hex-shaped silicon tube. The module in Fig. 4 also has a via etched through one end of the channel in one substrate for interfacing to another layer of channels. The two silicon layers are aligned and fusion bonded together. The plastic tubing is selected to snugly fit over the outer diameter of the silicon tube. The starting silicon wafers were 350 mm thick, so the outer diameter of the bonded silicon tube is 700 mm. The closest fitting inner diameter tubing was 762 mm (30 mils). In some cases, after slipping the plastic tubing over the silicon tube, heat was applied to flow the plastic tubing over the silicon tube, sealing the connection. When the devices were diced apart, a portion of silicon was left on either side of the silicon tube to guide and secure the tubing in
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Fig. 4. Assembly diagram of silicon tubing and I/O interconnect to external tubing. Lower diagram shows integration of finger joints and vertical channel interconnect with tubing interconnect.
place, and lend structural support to the silicon tubing. The inner diameter of the silicon tube is 300 mm. The length varied, but a 5 mm long silicon tube provided enough structural support and sufficient overlap of the tubing to properly seal the connection. An I/O device with tubing attached is shown in Fig. 5. The addition of the finger joints allows this fluidic structure to become an integral part of a network of fluidic channels and connections, i.e., miniature fluidic systems, by providing the vital connection to the macro environment.
2.3. Fluidic quick-connects A fluidic ‘quick connect’ technique has also been developed to connect microchannels in the same plane on separate modules. Similar, complementary I/O structures provide a self-aligning and interlocking fluidic end-to-end, channel interconnect. The complementary structure is fabricated in the same method as illustrated in Fig. 4, but the external, releasing etch also produces silicon beams which match the voids in the I/O device. The two structures slide into each other (Fig. 6). The silicon tube in the I/O device pushes up against the opening of the micromachined channel in the matching structure. Sealing can be accomplished by sandwiching a gasket between the butted fluid channels. The structures are held together by flaring the outside base of the silicon tube on the I/O device, so when the silicon beams on the matching device reach the base they encounter a slightly larger diameter tube, which pushes them outward against the silicon guides on the I/O device, locking it in place. By applying a force which exceeds the clamping force of the structures, the devices are quickly released, i.e., quick-connects. Photographs of the opposing ends are show in Fig. 6 (top and center), and a connecting pair with finger joints and vias to the inner channels is shown at the bottom.
2.4. Fluidic breadboard
Fig. 5. Photograph of an I/O device, with tubing interconnect.
The interconnecting finger joints and I/O devices were combined to demonstrate a fluidic breadboard.
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board to seal the interfaces between the added bridges and input/output components. One can envision making a flexible analytical instrument using this scheme, where individual channels in the breadboard contain sensors, mixers and/or other elements. Bridges are used to connect the desired sequence of functional channels, in a serial or parallel fashion.
3. Testing
Fig. 6. Photographs of a silicon micromachined quick-connect. Top and center photos show opposing ends of the micromachined quickconnect. The inner silicon tubes transition into shallow microchannels. This type of connection has very small dead volume. The bottom photo shows pair of connects with the integrated vias and finger joints for attachment to other parts.
The breadboard consists of an anisotropically etched, 4 inch (100) silicon wafer and a 4 inch 7740 Pyrex® wafer. The fluid channels were etched into one side of the silicon wafer, while the vias were etched through the wafer, and the finger joints were machined into the top side of the wafer. The etched wafer was then anodically bonded to the Pyrex® wafer, such that the glass substrate forms the bottom of the channels. The fluid bridges are fabricated in a similar fashion on a 4 inch wafer but have a recess etch to allow them to plug into the breadboard, and are diced apart after bonding to a Pyrex® wafer. On a 4 inch wafer, channels of various lengths, with widths of 1.2 mm and depths of 200 mm, were fabricated to demonstrate the flexibility of the breadboarding concept. A photograph of two fabricated breadboards is shown in Fig. 7. One is pictured with bridges of various lengths attached, which couple fluid from one channel to another, and input/output devices which attach to the breadboard in the same fashion as the bridges. The other breadboard is flipped over so the channels can be seen through the Pyrex® wafer. The insert in Fig. 7 illustrates the assembly concept with fluid flow from one bridge through a breadboard channel and into the other bridge. The full diameter of the wafer is used as the breadboard. A polysiloxane O-ring layer is patterned on the bread-
In order to test the integrity of the fluidic interconnects, two mating finger joint modules were each attached to two separate Plexiglas® substrates. The Plexiglas® substrates each contained a channel drilled from one side up to the surface where the fluidic interconnect structures were mounted over the hole and sealed using a two part epoxy. The side entry was threaded to accommodate a standard tubing fitting. A 30 mm thick, free standing polysiloxane gasket was placed between the two mating parts and then they were pressed together to form the seal. Water was then pumped from one Plexiglas substrate through the interconnect junction and out the other Plexiglas structure, using a recirculating pump on a water bath. No leaks were visually observed. To test the maximum seal pressure, one of the interconnecting modules was capped with a solid mating structure to block flow through the channel. A polysiloxane O-ring was sandwiched between the two parts. The via opening of the capped fluidic interconnect was 1× 1 mm2. Water was pumped into the Plexiglas® substrate until the capped interconnect failed. Pressures of up to 20 psi were applied with no noticeable leaks. Above 20 psi, the glued interface between the Plexiglas® and the silicon fluidic interconnect failed, such that the seals could not be tested above this pressure.
4. Conclusions The fluidic interconnect methodology and devices presented here is a low-cost, effective means of creating modular fluidic microsystems. This technique can be readily applied to materials other than silicon, such as GaAs, glass, and high-density PVC. It is therefore a generic interconnecting technique that provides a high degree of flexibility in the assembly of microsystems and a completely modular approach, here-to-fore not feasible. For example, it enables the aligned, direct coupling of individual components, built with different technologies and potentially even in different materials, with or without a ‘mother board’ or common substrate. Components or modules can be assembled horizontally or vertically to form truly three-dimensional microsys-
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Fig. 7. Photograph of two silicon–glass breadboards. The upper breadboard has bridges and I/O devices plugged into it. The bottom breadboard is turned over to reveal the microchannels, which are visible through the glass. The insert illustrates the fluidic breadboard concept. Fluid flows from an upper channel in a bridge, down to lower channel in the breadboard, then back up to another bridge. Connections are reversible. Bridging connectors are plugged into the breadboard, directing the flow of fluid through selected channels.
tems. We liken this assembly concept to the well known Lego® building blocks. Although fluidic components are the focus of this presentation, we have also fabricated electrical and optical interconnects utilizing this technology.
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