Fabrication of multi-level 3-dimension microstructures by phase inversion process

One process based on phase inversion of fillers in microstructures for the fabrication of multi-level three-dimensional (3-D) microstructures is described using SU-8, a kind of epoxy photoresist, as the model constructing materials. This process is depicted by use of the routine photolithography technique to construct the top layer of 3-D microstructures on the bottom layer of 3-D microstructures layer by layer. This process makes it possible to fabricate multi-level 3-D microstructures with connectors at desired locations, and to seal long span microstructures (e.g. very shallow channels with depth less than 50 μm and width more than 300 μm) without blockage. In addition, this process can provide a sealing layer by the solidification of a liquid polymer layer, which can be as strong as the bulk constructing materials for microstructures due to a complete contact and cross-linking between the sealing layer and the patterned layers. The hydrodynamic testing indicates that this kind of sealing and interconnection can endure a static pressure of more than 10 MPa overnight and a hydrodynamic pressure drop of about 5.3 MPa for more than 8 hours by pumping the tetrahydrofuran solution through a 60 μm wide micro-channels.

Micro and nano fluidic devices are useful not only because they allow manipulation with fast response times, handle small fluid volumes, sense and control flows and pattern substrates on small lengths scale, but also promise selectively address the cellular cell [1,2]. As a result of the uniform reactor conditions in mass and heat transfer similar as in microbial cells obtained, a high degree of reaction control is observed [3,4]. Modern developments in the design and utilization of micro fluidic devices for fluid transport have found many applications, ranging from the life science industries for pharmaceuticals and biomedicine (drug discovery, drug delivery and detection, diagnostic devices) [4,5] to industrial applications of combinatorial synthesis (such as, stereoselective synthesis [3], nanoparticle synthesis [6][7][8], rapid in-situ chemical analyses and high throughput screening [9,10]). Hence, there is an urgent need for developing rapid and economically viable prototyping processes for manufacturing micro fluidic devices with suitable materials compatible with the application environment [11,12].
In order to realize lab-on-a-chip in practical sense, micro-temperature controller (heat-exchanger and T-sensor), micro pressure controller, micro flow rate controller, micro separator, micro detector and other micro devices are necessarily integrated in one chip, most probably realized in multi-level 3-D microstructures with connectors at any locations. Building microstructures in 3-D still presents significant challenges when working with the inherently planar geometries that are accessible through projection photolithography [11][12][13][14][15]. Alternatives to photolithography for fabrication schemes that "write" patterns serially in metals and polymers either carve in a wise manner from a solid object or cause localized deposition of material in a series manner [15,16]. However, these methods are often limited in the connectivity and dimensionalities of structures they can  D patterns to 3-D   microstructures through pseudo-3-D patterns in a cylindrically   symmetrical substrate remains a challenge to form multi-level   3-D microstructures with connectors and lose the convenience to   build complicated microstructures with more than one layer of   3-D structures on the normal chip [16].
Compared with silicon, glass or stainless steel based micro fluidic devices, polymer based micro devices are more promising as microchips because of the availability of different types of polymers, low cost and ease of fabrication using LIGA, embossing, casting, injection molding and imprinting process [17,18]. Careful selection of polymers and fabrication process can lead to commercially viable fabrication processes and different application environment for micro fluidic devices [12].
SU-8, an epoxy-based negative photo resist, has been gaining much attention as a material of choice for the fabrication of microstructures and microchannels due to its superior chemical and mechanical properties besides its ease of fabrication using photolithography process [12,16,[18][19][20]. However, when using SU-8 for the fabrication of multi-level 3-D microstructures, there are still some issues that need to be addressed. The multi-level 3-D microstructures fabricated with connectors at desired locations, a strong binding interface for complete sealing of these microstructures and the sealing for long span microchannels with a shallow depth (<50µm) without any blockage are still not trivial problems although many processes have been developed for these problems [11,12,16]. The developed process by thermal binding two pre-patterned solid SU-8 layers to form multi-level 3-D microstructures needs to overcome the micro gas-gaps between the two solid layers or operate under vacuum and then to form a complete inter-connecting between the two layers, which is usually not a trivial matter [11,12,21]. Furthermore, it is difficult to get connecting parts at some desired locations between the two layers or needs tedious alignment [11,21]. In addition, once the substrate is used to support the SU-8 layer, additional work is required to release the substrate from the SU-8 layer for the construction of other levels. Otherwise, it will lose the opportunity to construct additional SU-8 layer on the pre-formed SU-8 layers [11].
This report demonstrates a flexible process based on the phase inversion of fillers in the pre-patterned microstructures for the design, the fabrication and the sealing of multi-level 3-D microstructures. This process favors to get free-selected connectors between two contacting layers and the strong sealing of the long span microstructures with depth less than 50 μm using the routine photolithography process. Figure 1 shows the schematic diagram for the fabrication of multi-level 3-D microstructures with freely designed connectors.
The first layer with embedded structures is formed according to our previous process (see Fig. 1(i)) [12]. Then a kind of filler (i.e.
wax, polymers, etc.) will be heated above its melting point and filled into the pre-formed microstructures (see Fig. 1(ii)) by a syringe at a large opening orifice. The second layer of liquid photo resist (e.g. SU-8) is coated on the filled first level of microstructures (see Fig. 1(iii)) after the liquid filler changes to solid. Using the routine photolithography process described in reference 12, the second level of microstructures can be constructed in the second layer of photo resist on the first level of microstructures (see Fig. 1(iv)). The second level of microstructures is then filled again by the fillers like step 2 (see   between layers can be obtained that will result in a strong binding. Using this phase inversion process, many complex multi-layer 3-D microstructures can be constructed although some of them cannot be built up using multi-layer fabrication process mentioned by some reports [11,21]. In the following, some typical multi-level 3-D microstructures will be demonstrated using SU-8 as constructing materials and mondang wax or PE wax as fillers. Figure 2 shows the typical complex 2-level 3-D microchannels with flexible connectors using the phase inversion process. In Fig. 2(a), the transparent optical photograph clearly indicates the first layer channels is covered by the second layer channels, which can have lots of opened connecting parts in the first layer channels to the second layer channels for exchanging information, which may be used as mixers for two-layer liquid-liquid mixing or liquid-gas mixing.
Their cross-section optical image (see Fig. 2(b)) suggests that the bottom surface of the second layer channels is connected with the top surface of the first layer channels with many connecting parts. Figure 2(b) shows that the sealing surface is uniformly flat without any blockage and deformation, which can be further evidenced by their Scanning Electron Microscope (SEM) images for some local places (see Fig. 2(c) and 2(d)).  In the microfabrication, another important field is to sealing very shallow channels with long span, which is also difficult since the blockage occurs often. Using the developed phase inversion process, it will become easy for channels with depth of less than 50 μm and span more than 300 μm. Figure  Unlike other solid bonding process (e.g. hot-embossing), the interlayer for the sealing between the two layers cannot be envisioned since a uniform and complete contact interface is formed during sealing process using the liquid photo resist (e.g. SU-8) (see Fig. 2, Fig. 3 and Fig. 4), which will lead to ultra-strong bonding between two contacting layers. In order to These results indicate that the sealing process preserves the potential to reach the binding as strong as the bulk photoresist (i.e. SU-8 100) [22].
In summary, one process based on the phase inversion of