Unexposed SU-8 Research Articles

3D-microfabrication by two-photon polymerization of an integrated sacrificial stencil mask

This work aims at developing a new and unconventional Sacrificial Stencil Mask (SSM) technology by exploiting Two-Photon Polymerization (2PP) in an IP-L/SU-8 double layer resist system. The process consists of the sequential deposition of two different resists, such as SU-8 and IPL, onto the same glass substrate, followed by 2PP lithography and distinct development processes. The 2PP writing process was used to polymerize structures inside the top and bottom resist layers to form, in one single exposure process, both SSM and a permanent polymeric structure, in our case a plain pedestal. The top IPL resist was developed using Isopropyl Alcohol (IPA), which does not affect either exposed or un-exposed SU-8 regions. In this way, structures written into the bottom layer remained latent, while exposed areas of the top IPL resist, including the stencil mask, were developed. The realization of 3D stencil masks, designed to be anchored inside the un-exposed bottom layer, was combined with metal evaporation to demonstrate the deposition of a plain metal line through the stencil mask. The final development of the bottom layer led to the lift off of the sacrificial stencil mask, uncovering the underlying, permanent polymer-metal structure. The combination of sacrificial polymer structures with permanent ones opens new possibilities in 3D MEMS design, enabling the integration of distributed electronic transducers in flexible polymeric structures.

Self-Aligned Electrodes on SU-8 Negative Photoresist Pedestals

We report a novel technique by which self-aligned thin-film electrodes are fabricated on SU-8 negative photoresist pedestals. A bilayer aluminum and titanium structure is used to align the thin-film electrode and to serve as the optical mask for the UV exposure of the SU-8. The SU-8 developer is used to remove both the unexposed SU-8 and the bilayer structure of aluminum and titanium. The result is a thin-film electrode aligned on an SU-8 pedestal with a minimal undercut beneath the thin-film electrode.

Fabrication of size controllable SU‐8 nanochannels using nanoimprint lithography and low‐pressure thermal bonding methods

A new and simple method is demonstrated to achieve size controllable nanochannels by using nanoimprint lithography and low-pressure thermal bonding methods. The flexible free-standing SU-8 allows the bonding of the nanochannels' pattern features at such a low pressure primarily because of ‘sequential’ bonding made possible by the bonding layer flexibility and the conformal contact made between the bonding layer and the structure layer in a large area. The simple geometrical argument shows that the height of the enclosed nanochannel can be determined by the depth of the cross-linked SU-8 trenches as well as by the initial thickness of the thin unexposed SU-8 layer. The height of the nanochannels can also be controlled by adjusting the ratio of the ridge width to the trench width on the SU-8 trenches. These simple relationships can serve as a guideline in choosing the parameters for SU-8 nanochannel fabrication.

A novel hybrid patterning technique for micro and nanochannel fabrication by integrating hot embossing and inverse UV photolithography

Nanofluidic devices with micro and nanostructures are becoming increasingly important for biological and chemical applications. However, the majority of the present fabrication methods suffer from a low pattern transfer quality during the simultaneous embossing of the microscale and nanoscale patterns into a thermoplastic polymer due to insufficient polymer flow. In this work, a novel hybrid patterning technique, integrating hot embossing and inverse ultraviolet (UV) photolithography, is developed to fabricate micro and nanochannels with a high replication precision of the SU-8 layer. The influence of embossing temperature and time on the replication precision was investigated. The effect of UV lithography parameters on the micro and nanochannel pattern was analyzed. To improve the SU-8 bonding strength, the influence of the O2 plasma treatment parameters on the water contact angles of the exposed and unexposed SU-8 layer were studied. A complete SU-8 nanofluidic chip with 130 nm wide and 150 nm deep nanochannels was successfully fabricated with a replication precision of 99.5%. Compared with most of the current processing methods, this fabrication technique has great potential due to its low cost and high pattern transfer quality of the SU-8 micro and nanochannels.

High Yield Polymer MEMS Process for CMOS/MEMS Integration

ABSTRACTMEMS community is increasingly using SU-8 as a structural material because it is self-patternable, compliant and needs a low thermal budget. While the exposed layers act as the structural layers, the unexposed SU-8 layers can act as the sacrificial layers, thus making it similar to a surface micromachining process. A sequence of exposed and unexposed SU-8 layers should lead to the development of a SU-8 based MEMS chip integrated with a pre-processed CMOS wafer. A process consisting of optical lithography to obtain SU-8 structures on a CMOS wafer is described in this paper.

Fabrication of aspherical SU-8 microlens array utilizing novel stamping process and electro-static pulling method

A simple and novel method is proposed for the fabrication of aspherical SU-8 microlens arrays with a wide range of tunable focal lengths utilizing a soft SU-8 stamping process and an electro-static pulling method. In the proposed approach, an SU-8 stamp incorporating a micro-nozzle array and a reservoir containing unexposed SU-8 is fabricated on a glass substrate using a dose-controlled exposure process. Microlens arrays with diameters ranging from 20 to 500 μm and various radii of curvature are successfully fabricated using the proposed method. The low surface roughness (Ra = 3.84 nm) and high dimensional uniformity of the SU-8 microlens arrays (variation < 5% designed diameter) confirm both the optical quality of the individual microlenses and the general feasibility of the fabrication method. The innovative fabrication method proposed in this study provides a simple and efficient means of producing high quality aspherical microlens arrays with tunable focal lengths.

The fabrication of non-spherical microlens arrays utilizing a novel SU-8 stamping method

SU-8 photoresist (PR) is an excellent material for optical applications due to its high refractive index and high transmittance at visible and near-IR wavelengths. This paper reports a simple and novel method of fabricating non-spherical microlens arrays by soft stamping unexposed SU-8 PR. An SU-8 based stamp composed of micro-nozzle arrays and a reservoir structure are first fabricated on a glass substrate using a process of dosage control exposure. The unexposed SU-8 encapsulated in the crosslinked SU-8 shell is used as the ‘ink’ for the stamping process. The present SU-8 microlens array is then formed by stamping the formed SU-8 structure on a bare glass substrate at a temperature higher than the glass transition temperature (Tg) of the unexposed SU-8 PR. A non-spherical lens with various radii of curvature structure can be formed by controlling the working temperature during the stamping process. Microlens arrays with diameters ranging from 50 to 500 µm are successfully fabricated using this novel process, with the height of the fabricated microlens reaching 200 µm. Experimental investigation results indicate that the formed microlenses have a good surface profile, good uniformity and good optical properties. The measured numerical apertures (NA) of the fabricated microlens range from 0.22 to 0.58. The relationships between the focal length of the fabricated microlens and the lens dimension as well as the process temperature are systematically investigated. The method presented in this study will substantially impact the process for microlens array fabrication.

A Simple Imprint Method for Multi‐Tiered Polymer Nanopatterning on Large Flexible Substrates Employing a Flexible Mold and Hemispherical PDMS Elastomer

AbstractThis work reports a facile method to fabricate multi‐tiered polymer nanopatterns on SU‐8 by the combination of imprint‐ and photo‐lithography. First, SU‐8 is imprint patterned using a polymeric flexible mold with an anti‐adhesion coating that is deposited on a transparent and flexible substrate, at room temperature under low pressure. Next, the resulting SU‐8 nanopatterns are exposed to UV light through a chromium mask by a photolithographic process. Removal of the unexposed SU‐8 leaves behind multi‐tiered structures. The use of a hemispherical poly(dimethylsiloxane) pad facilitates the evacuation of trapped air during the imprinting process. Line/space patterns of 500 nm with the smallest line width of 200 nm were homogeneously imprint‐patterned on SU‐8 on a large flexible substrate, and three‐tiered structures, ranging in thickness from 300 nm to 2 µm, were successfully formed.magnified image

Surface modification of SU8 photoresist for shrinkage improvement in a monolithic MEMS microstructure

The effect of O2 plasma treatment on the surface property of exposed and unexposed SU8 photoresist has been investigated for the fabrication of a monolithic MEMS microstructure. It can solve the non-uniformity problem of second resist coating on the SU8 with high intrinsic shrinkage after exposure and post-exposure baking (PEB) in the fabrication of the stacked polymer–metal or polymer–polymer structure, which was used in the application of microfluid, bio and chemistry. The thickness difference of untreated SU8 before PEB between the exposed and unexposed SU8 was about 0.3% while that after PEB increased to about 6%. It could result in large non-uniformity of about 18 µm thickness difference for the following second resist coating on the hydrophobic surface without plasma treatment. The surface property of SU8 in terms of the contact angle and surface energy can be adjusted by O2 plasma treatment for enhancing the coating uniformity of the following resist. The measured contact angles of the exposed and unexposed SU8 decrease with O2 plasma time, corresponding to the increased surface energy determined by the Lifshitz–van der Waals/Lewis acid–base approach. It displayed that the similar hydrophilic surface property can minimize the thickness difference of second resist coating on the first shrunken SU8. A monolithic nozzle plate with a physical resolution of 600 dpi in a single column was demonstrated for an inkjet application based on the improved uniformity.

Fabrication of the monolithic polymer–metal microstructure by the backside exposure and electroforming technology

Monolithic polymer–metal microstructures can be fabricated on the silicon or glass substrate using two kinds of photoresists and electroforming technologies for the inkjet and microfluidic application. However, it suffers from the high shrinkage problem of first SU8 resist after exposure and post exposure baking. This paper reports a novel approach to solve the shrinkage problem by introducing backside exposure of first SU8 resist for the fabrication of the monolithic polymer–metal microstructure. In combination with the light absorption layer coating on the unexposed SU8 resist, metal seed layer deposition, frontside exposure for second JSR resist on the seed layer and the nickel (Ni) electroforming together with release process, we have demonstrated a high physical resolution of 1,200 dpi monolithic Ni nozzle plate with negligible shrinkage. It also has the advantages of low cost and high resolution for the improvement of the traditional bonding of polymer and metal nozzle plate, which is generally in need of a complex alignment to stick the metal nozzle plate and dry film polymer on the heating chip together.

Photothermally enabled lithography for refractive-index modulation in SU-8 photoresist

We present a new lithographic technique based on a hybrid photothermal process to modulate the refractive index in commercial SU-8 photoresist. Owing to a difference in cross-linking, the refractive index of unexposed SU-8 cross-linked by thermally induced polymerization is 0.0072 higher than that of SU-8 cross-linked by UV exposure and postbaking. Making use of this property, we fabricated two thick, flat-topped index-modulated diffractive optical elements (DOEs) that contain different phase distributions and measured their wavefront reconstruction. The good experimental reconstructions of the index DOEs demonstrate the potential to extend the refractive-index modulation technique for the fabrication of three-dimensional optical elements without needing a development step.

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.

Thermal ablation of PMMA for water release using a microheater

A new approach was developed in this work to ablate a micropore through a polymethylmethacrylate (PMMA) layer using a microheater for the on-demand release of water stored in a microreservoir. The size of the ablated micropore in the PMMA capping layer is mainly governed by the heater temperature profile and ablation time. Furthermore, the molten PMMA resulting from the heating energy tends to retract away from the micropore location, taking away the gold heater lines from the pore area. This prevents the possibility of any gold traces blocking the flow of water released from within the microreservoir. Simulation was conducted to find temperature profiles on the surface of the microheater, and the results were used to interpret the phenomena observed in the ablation process. Simulation was also conducted for the case when the microreservoir was filled with water. In addition, two alternative ablation materials, unexposed SU-8 and polydimethylsiloxane (PDMS), were examined as possible microreservoir capping layers. The approach developed has potential applications in microfluidic systems to release encapsulated fluids in a controllable manner.