Daniel Haefliger

Processing of thin SU-8 films

This paper summarizes the results of the process optimization for SU-8 films with thicknesses ≤5 µm. The influence of soft-bake conditions, exposure dose and post-exposure-bake parameters on residual film stress, structural stability and lithographic resolution was investigated. Conventionally, the SU-8 is soft-baked after spin coating to remove the solvent. After the exposure, a post-exposure bake at a high temperature TPEB ≥ 90 °C is required to cross-link the resist. However, for thin SU-8 films this often results in cracking or delamination due to residual film stress. The approach of the process optimization is to keep a considerable amount of the solvent in the SU-8 before exposure to facilitate photo-acid diffusion and to increase the mobility of the monomers. The experiments demonstrate that a replacement of the soft-bake by a short solvent evaporation time at ambient temperature allows cross-linking of the thin SU-8 films even at a low TPEB = 50 °C. Fourier-transform infrared spectroscopy is used to confirm the increased cross-linking density. The low thermal stress due to the reduced TPEB and the improved structural stability result in crack-free structures and solve the issue of delamination. The knowledge of the influence of different processing parameters on the responses allows the design of optimized processes for thin SU-8 films depending on the specific application.

Optimized plasma-deposited fluorocarbon coating for dry release and passivation of thin SU-8 cantilevers

Plasma-deposited fluorocarbon coatings are introduced as a convenient method for the dry release of polymer structures. In this method, the passivation process in a deep reactive ion etch reactor was used to deposit hydrophobic fluorocarbon films. Standard photolithography with the negative epoxy-based photoresist SU-8 was used to fabricate polymer structures such as cantilevers and membranes on top of the nonadhesive release layer. The authors identify the plasma density as the main parameter determining the surface properties of the deposited fluorocarbon films. They show that by modifying the pressure during fluorocarbon deposition, the surface free energy of the coating can be tuned to allow for uniform wetting during spin coating of arbitrary thin SU-8 films. Further, they define an optimal pressure regime for the release of thin polymer structures at high yield. They demonstrate the successful release of SU-8 cantilevers and membranes with thicknesses down to 2.3 and 1.7μm respectively, which is a c...

Fabrication of thin SU-8 cantilevers: initial bending, release and time stability

SU-8 cantilevers with a thickness of 2 ?m were fabricated using a dry release method and two steps of SU-8 photolithography. The processing of the thin SU-8 film defining the cantilevers was experimentally optimized to achieve low initial bending due to residual stress gradients. In parallel, the rotational deformation at the clamping point allowed a qualitative assessment of the device release from the fluorocarbon-coated substrate. The change of these parameters during several months of storage at ambient temperature was investigated in detail. The introduction of a long hard bake in an oven after development of the thin SU-8 film resulted in reduced cantilever bending due to removal of residual stress gradients. Further, improved time-stability of the devices was achieved due to the enhanced cross-linking of the polymer. A post-exposure bake at a temperature TPEB = 50 ?C followed by a hard bake at THB = 90 ?C proved to be optimal to ensure low cantilever bending and low rotational deformation due to excellent device release and low change of these properties with time. With the optimized process, the reproducible fabrication of arrays with 2 ?m thick cantilevers with a length of 500 ?m and an initial bending of less than 20 ?m was possible. The theoretical spring constant of these cantilevers is k = 4.8 ? 2.5 mN m?1, which is comparable to the value for Si cantilevers with identical dimensions and a thickness of 500 nm.

Three-dimensional microfabrication in negative resist using printed masks

We present a process based on contact printed shadow masks for three-dimensional microfabrication of soft and sensitive overhanging membranes in SU-8. A metal mask composed of a 50 nm thick gold and 20 nm thick chromium film is transferred onto unexposed SU-8 from an elastomer stamp made of polydimethylsiloxane. The mask transfer is mediated by the strong bonding of the reactive chromium surface to the SU-8 and the low adhesion of the gold surface to the rubber stamp. The printed mask is embedded into the negative resist by layering an additional SU-8 film on the top. The mask protects buried resist from subsequent UV exposure enabling monolithic fabrication of cavities in SU-8. Unlike direct evaporation–deposition of a mask onto the SU-8, printing avoids high stress and radiation, thus preventing resist wrinkling and prepolymerization. The effective fabrication of soft, 4 µm thick and 100 µm long cantilevers integrated into a microfluidic system is demonstrated. The process yields very flat and well-defined membrane surfaces, paving the way for three-dimensional fabrication of lab-on-a-chip devices with integrated fine filters and highly sensitive cantilevered probes, valves and pressure sensors useful in research and diagnostics.

Self-actuated polymeric valve for autonomous sensing and mixing

We present an autonomously operated microvalve array for chemical sensing and mixing, which gains the actuation energy from a chemical reaction on the valve structure. An 8-/spl mu/m-thick flapper valve made in SU-8 is coated with stress-loaded Al on one side and Ti on the other side. The metal films keep the flapper in a flat, stress-balanced closed position. Upon contact with an analyte composed of a NaOH solution the Al film is etched from the valve surface unbalancing the surface stress and bending the flapper. A deflection of up to 45 /spl mu/m is observed allowing for effective release of a green marker from a reservoir. Calculations reveal that valve operation with stress originating from biochemical processes will require considerable enhancement of the actuation efficiency.

Autonomous valve for detection of biopolymer degradation

We present a polymer microvalve that allows the detection of biopolymer degradation without the need of external energy. The valve is based on a polymer container filled with a colored marker solution and closed by a thin lid. This structure is covered by a film of poly(L-lactide) and degradation of the biopolymer triggers the release of the color which is detected visually. The autonomous valve has potential for the fast testing of biopolymer degradation under various environmental conditions or by specific enzymes.