6. Integration and Packaging
6.1 Integration of the PDMS and silicon substrate
After a wafer is removed from TRL, it is diced with the ICL diesaw. This is a straightforward process that results in the release of twelve 2cm x 2cm devices per wafer. After dicing, the devices are taken to EML, where the protective photoresist coating is stripped from the dies individually with an acetone dip. They are then cleaned with methanol, isopropanol, and rinsed in water in preparation for plasma cleaning and bonding.
Likewise, the PDMS fluidic channels are diced, leaving a centimeter or so of overhang on either end for mounting to the acrylic mask adapter used for alignment (that process is discussed in detail in the next section). Dicing is done by hand with a razor blade. Next, fluidic contact holes are punched in the PDMS with a 16 gauge (approx. 1mm inner diameter) syringe tip. The holes are punched slowly so that the polymer does not split or crack. The PDMS pieces are then cleaned with isopropanol and rinsed with water.
The next step is to activate the surfaces of the PDMS and the silicon substrate in an oxygen plasma so that they will permanently bond. Each surface is oxidized in the plasma, and covalent siloxane (Si – O – Si) bonds form when the two surfaces are brought into contact [13]. The activation is done with a bench-top RF plasma cleaner/sterilizer unit which was brought into EML from the Media Lab. The silicon and PDMS are placed on a glass microscope slide, which is then loaded into the glass chamber of the cleaner. After a two to three minute vacuum pumpdown of the chamber, the RF is turned on high power, establishing a pale blue plasma. The inlet valve on the chamber is then slowly opened to let in a small stream of air, turning the plasma pink. We found that a 45 second exposure to the oxygen plasma (after it has turned pink) consistently yielded strong bonds. After 45 seconds, the RF power is turned off and the chamber is vented, and the pieces are be removed.
Within two to three minutes of surface activation, the silicon substrate and PDMS pieces must be brought into contact for permanent bonds to form [12]. During this short window of time, they must be precisely aligned before they are brought into contact. To accomplish this, we have developed the following method for alignment using the high resolution mask aligner in EML. After the silicon and PDMS surfaces have been plasma activated, these pieces are loaded into the mask aligner using simple adapters which were laser cut out of acrylic (see schematic in Fig. 6.1, and appendix __). One adapter holds the optically transparent PDMS in the aligner, where a photomask would ordinarily go. The other adapter holds the silicon substrate below the PDMS in the aligner, where a wafer would ordinarily go. The height of the substrate is slowly raised with a lever while the x, y, and theta micrometers are used to align it with the PDMS. The substrate comes into clear focus as it approaches the height of the PDMS. When the substrate and PDMS are well aligned, they are brought into conformal contact, and the surfaces quickly "wet" and form permanent bonds. This method is capable of aligning and bonding the PDMS and substrate with a precision of approximately 5 microns. Figs. 6.2 and 6.3 show top views of a die integrated with PDMS fluidic channels. A full SOP for the alignment procedure is included in appendix __.
6.2 Packaging the integrated PDMS/silicon unit
A laser-cut acrylic package (Fig. 6.4) was designed to hold the integrated PDMS/silicon module and four ceramic wirebonding chips. First, the integrated unit and the wirebonding chips are epoxied to the acrylic package. After the epoxy has dried, the bond pads on the die are wirebonded to the ceramic chips using the gold wire ball/wedge bonder in EML. The wirebonds are coated with RTV (silicone rubber) sealant for protection. After the RTV has cured, polyethylene tubing is inserted into the PDMS fluidic contact holes and is expoxied in place. Tubing with an inner diameter of .015" and and outer diameter of .043" was used. Syringe tips (26 gauge) are inserted into the free ends of the tubing and are epoxied in place. Finally, stranded hookup wire is soldered to the wirebonding chips. A fully packaged device, ready for testing, is shown in Fig 6.5.