Multilayer Soft Lithography

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Multilayer soft lithography (MSL) is a fabrication process in which microscopic chambers, channels, valves and vias are molded within bonded layers of elastomer.

1. Description

2. Why

3. How

4. Future Trends

5. Related Links

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Description

Soft lithography—a set of techniques for nonphotolithographic pattern transfer based on contact printing and polymer molding—is now established as an alternative to photolithography in microfabrication. While it has been demonstrated that soft lithography has obvious applications in optical5–7 and microanalytical systems,8 its potential for application to microelectronic devices remains to be established.Fabrication of such devices requires high reproducibility in pattern transfer, accurate registration between layers, low distortion of the elastomeric molds, and little surface contamination or damage during processing.  

Why

Soft lithography is an alternative to silicon-based micromachining that uses replica molding of nontraditional elastomeric materials to fabricate stamps and microfluidic channels. We describe here an extension to the soft lithography paradigm, multilayer soft lithography, with which devices consisting of multiple layers may be fabricated from soft materials.This technique is used to build active microfluidic systems containing on-off valves, switching valves, and pumps entirely out of elastomer. The softness of these materials allows the device areas to be reduced by more than two orders of magnitude compared with silicon-based devices. The other advantages of soft lithography, such as rapid prototyping, ease of fabrication, and biocompatibility, are retained. 

How 

The fabrication of field effect transistors ~FETs! on a GaAs/AlGaAs heterostructure using a representative soft lithographic technique: micromolding in capillaries ~MIMIC!.2 The fabrication process required three molding steps and two registration steps. The FETs have an overall size of 1 mm30.55 mm with gate length (L) and gate width (Z) both ranging from about 20 to 50 mm.

  • The FET was made on a molecular beam epitaxially ~MBE!-grown GaAs/AlGaAs heterostructure composed of GaAs, Al0.3Ga0.7As, and a Si d-doped layer n53.831012 cm22 400 Å from the interface.
  • The two-dimensional electron gas is located 1500 Å below the surface. At room temperature, the measured 2DEG density and mobility were 3.9310 cmand 4.0310cm2/V s.
  • In the source and drain are AuNiGe ohmic contacts, the channel is defined by a mesaetch, and the gate is a Cr/Au Schottky contact.

  • In MIMIC, an elastomeric mold with interconnected recessed regions is put in conformal contact with the substrate.

  • Continuous channels are formed by the recessed regions on the mold and the substrate.

  • A liquid prepolymer is applied to the open ends of the channels and fills the channels automatically by capillarity.

  • The prepolymer is cured either thermally or by long wavelength ultraviolet ~UV! light, and the mold is then removed. 

The elastomeric molds ~or stamps! were made by casting polydimethylsiloxane ~PDMS, Sylgard 184, Dow-Corning, A:B51:15) on masters generated by a rapid prototyping technique based on patterns printed using a high resolution image-setting system. The thickness of the PDMS molds used in FET fabrication was typically ;1 mm while the relief structures had 10 mm steps. The recessed regions on the molds were interconnected and had open ends.PDMS is a good choice for the mold material because it wets the sample surface and because it is transparent to the UV light used to cure the prepolymer. The transmission coefficient of a 1-mm-thick PDMS block was 92% at l5365 nm. Samples were cleaned in trichloroethylene, acetone, and methanol before each application of the PDMS mold.Their characteristics are similar to those of FETs fabricated using conventional photolithographic techniques. These dimensions are not lower limits; we have made simple test structures with dimensions down to 30 nm by soft lithography. 

Future Trends

These FETs establish that soft lithography is compatible with multilayer fabrication of functional microelectronic devices, and set a benchmark against which to measure further development in this area.It will be interesting to evaluate the precision of the softlithography mixing device and compare it with programmable EHD droplet systems. To increase the space of potential applications, additional sensors and agitators will be integrated on the device.Scheduling algorithms will be developed to effectively utilize parallel hardware and increase the throughput of experiments. There are rich opportunities for future work in programmable microfluidics.Using high-level programs, scientists will be able to orchestrate large, adaptive, and reusable procedures that are beyond the grasp of today’s hardware-oriented user interface. As many of the computational sciences have increasingly found overlap with aspects of biology, we are excited that programmable microfluidics might offer a new avenue for programming language and computer architecture researchers to join in the discussion. 

Keywords

Soft Lithography, Micro-aspiration assisted lithography,  microelectronic devices.

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