Bench-Top MEMS

Background  | Supplies Needed | Formation of Microfluidic Channels  |  Use as Etch Mask

Background

It is possible to use photo-curable adhesives to make MEMS structures. They have lower resolution than negative photoresists such as SU8, but they do have a number of other attractive features:

• rapid, inexpensive fabrication
• a range of mechanical properties, from stretchable to rigid
• can be spin-coated to thin films or cast to thick films
• layers up to 1 cm thick, with relatively high aspect ratios
• curing times of less than a minute when exposed to UV light
• resolution of hundreds of µm if exposed to UV light and developed
• resolution of tens of µm if micro-molded
• little sensitivity to room light
• irreversible bonding, like PDMS, after oxygen plasma exposure
• like SU8, can be used for molding PDMS
• conductors can be mixed into the material, rendering the composites conducting
• magnetic particles can be mixed into these material, rendering the composites magnetic
• can be peeled off of the substrate (may require a non-stick layer)
• can be used as a mask for etching
• is biocompatible with at least some cell cultures
• can be used in conjunction with sacrificial layers to produce multi-level structures.

Examples of free-standing, flexible structures made using Loctite 3108 are shown here.

Figures are from R. Delille, M. Urdaneta, S. Moseley, and E. Smela, "Benchtop polymer MEMS," J. Microelectromech. Syst., 15 (5), 1108-1120 (2006).

An example of the features you can obtain in Loctite 3340 by photolithography are shown here.

Figures are from R. Delille, M. Urdaneta, S. Moseley, and E. Smela, "Benchtop polymer MEMS," J. Microelectromech. Syst., 15 (5), 1108-1120 (2006).

One can also use the 3340 for short-term packaging (a week or two) of CMOS chips for immersion in cell medium, as discussed in the cell-based sensing pages and as shown in this figure. The high aspect ratio allows the bond wires to be covered, but the surface of the chip to be exposed, with nearly vertical sidewalls.

For further information, see:

R. Delille, M. Urdaneta, S. Moseley, and E. Smela, "Benchtop polymer MEMS," J. Microelectromech. Syst., 15 (5), 1108-1120 (2006).

Further information and detailed instructions for fabricating various structures can be downloaded from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1707770. On the left side of the page, click “Multimedia”. The instructions below on this page are examples from that document.

M. Urdaneta, R. Delille, and E. Smela, "Benchtop polymer MEMS as a low-cost educational tool," Solid State Sensors, Actuators, and Microsystems Workshop, Hilton Head, SC, Educational Poster Digest (eds. Spangler, Kenny, and Schmidt), p. 14-15 (June 4-8, 2006).

Supplies Needed for Benchtop MEMS

This work led to the development of MEMS education kits, for school and college students. This is possible because the photopatternable elastomer used in this technique does not require special protection equipment or chemical hoods, and does not need spinner, aligner, or other special machinery. A typical set of supplies needed for benchtop MEMS is shown here.

Figure by M. Urdaneta.

Formation of Microfluidic Channels, Instructions

In 2000, Beebe et al. (D. J. Beebe, J. S. Moore, Q. Yu, R. H. Liu, M. L. Kraft, B.-H. Jo, and C. Devadoss, "Microfluidic tectonics: A comprehensive construction platform for microfluidic systems," PNAS, 97 (25), 13488–13493 (2000).) demonstrated a method to fabricate a complete microfluidic channel system based on liquid-phase photopolymerization and lithography of hydgrogels, which they called microfluidic tectonics (micro-FT). In-situ creation of the channels is attractive because the structures are fabricated all at once without the need for later alignment and bonding. Such structures can also be made using these commercially available adhesives.

The following instructions were originally prepared by S. Moseley and M. Urdaneta. For further information, refer to R. Delille, M. Urdaneta, S. Moseley, and E. Smela, "Benchtop polymer MEMS," J. Microelectromech. Syst., 15 (5), 1108-1120 (2006). For detailed instructions on fabricating various other structures, download the supplementary materials (pdf).

Step 1. Drill 2 holes into a piece of Plexiglas; these will serve as the substrate with an inlet and outlet for the finished channel.

Step 2. Apply a bead of 3108 to the surface of the substrate. Place a transparency mask directly on the surface of the 3108.

Step 3. Expose this sandwich to 365 nm light for 40 seconds.

Connect commercially available nylon fittings (such as 1/16” McMaster Carr 5117k41) and hoses (Tygon Super-Soft High Purity Tubing, McMaster Carr 9449k11, OD 3/16 in, ID 1/16 in, wall 1/16 in) via the holes drilled in the substrate. Place one hose in beaker of acetone and connect the other hoses to a syringe (e.g. 10 ml disposable). Create suction with the syringe and draw acetone through the channel. This clears the undeveloped 3108 out of the channel.

Step 4. The acetone can be cleaned out of the channel by drawing water through the channel. The figure on the left shows a close-up of a developed reservoir and channel (without a cover). To demonstrate channel operation, food dye and water can be drawn through it.

To avoid the use of plastic wrap to prevent the photopatternable elastomer from sticking to other surfaces, since plastic wrap can create “wiggles” of nonuniform height on the surface, Sylgard 184 polydimethylsiloxane (PDMS) base precursor (Dow Corning) can be wiped onto any solid surface to leave a thin coating. The Loctite 3108 will not stick to this treated surface. This can be done to e.g. the mask, allowing it to be brought into direct contact with the photopatternable elastomer and then pealed off after exposure.

Instructions for Using 3108 as an Etch Mask

These instructions were originally prepared by S. Moseley and Mario Urdaneta.

Step 1. Start with a sheet of metalized plastic film; the example here is with gold (Au). This will serve as a substrate. Apply a small quantity of 3108 to the surface.

Step 2. Cover the surface of the 3108 with a piece of plastic wrap, such as Saran or Sealview; the cured polymer will not stick to these plastics. Place a mask face-down over the film and cover with a piece of glass. The mask shown is an inexpensive transparency made on a high-resolution (3000 dpi) printer from a commercial print shop. It has areas with black ink that block light, including UV wavelenghts. Steps 1 and 2 can be done in normal room light, even if the room has windows.

Step 3. Expose the Loctite 3108 to a 365 nm source light from a UV lamp for approximately 40 seconds. The 3108 that is exposed to the UV light crosslinks.

Step 4. Remove glass, mask, and Saran wrap. Rinse the sample with acetone to remove the unexposed 3108. You now have a mask that can be used to etch the Au.

Step 5. Place the sample in gold each for 35 to 40 seconds. (Gold etch is an aqueous solution of I2 and KI that can be either mixed or purchased commercially). After the gold has been etched, rinse the sample in water.

Step 6. Remove the substrate from the water and peel the 3108 from the surface. Rinse.

Background  | Supplies Needed | Formation of Microfluidic Channels  |  Use as Etch Mask

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