1.
Introduction
Polydimethylsioxane (PDMS) belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones, which is the most widely used silicon based organic polymer[1]. After being introduced into the micro fabrication process, PDMS has come to be the most widely used material in polymer based micro fabrication and nanofabrication such as microfluidics, micro-optics, BioMEMS, and other chemical or biological sensors. Its obvious advantages are a simple fabrication process, low cost, optically clear, nontoxic, inert and nonflammable. What is more, PDMS is biocompatible, which can be used to fabricate devices for biology and medical applications[2–5].
Conventionally, the device manufacturing process is to cast PDMS over the SU8 mold in a single process. However, the single level of PDMS over the SU8 mold cannot achieve complex microstructures because it is difficult to peel off the PDMS device from the SU8 master mold after casting. Especially, the single level process cannot get high aspect ratio microstructures due to the pillars being broken when peeling off high aspect ratio ones from the SU8 mold surface.
Using a PDMS mold to create PDMS structures was recognized as having potential as a technique for high aspect ratio complex microstructures. Some researchers have studied several methods to realize high aspect ratio microstructures. Lee et al.[6] has developed a special process for high aspect ratio PDMS microstructure, which needs a complicated and expensive device manufacturing process. Sun et al.[7] and Gitlin et al.[8] have used PDMS as casting materials to replicate nano-patterns from a PDMS negative mold with a non-adhesive layer evaporated on the PDMS mold surface. Although both reports presented good fabrication results for their applications, only simple low aspect ratio structures were investigated. Sun et al.[5] presented a high aspect ratio structure with 20; however, they adopt an expensive and time consuming chemical process to form a non-adhesive layer on the PMDS mold surface and the high aspect ratio structure having a supporting flank. Natarajan et al.[9] have achieved the high aspect ratio with 17; however, they have changed the soft lithography process and the structure having large area which would support the high aspect ratio structure with stress. Kan et al.[12] explored PDMS casting on Ni masters as an alternative way to replicate high aspect ratio microstructures. Ni masters with grooves spaced 2.5 apart and 13 μm deep were successfully replicated in PDMS. However, these PDMS structures have an aspect ratio of only a little more than 5. Metin Sitti[13] gave three ways to achieve a high aspect ratio using nanoembossing, nanomolding and directed self-assembly, which are all complex and expensive. Dinesh Chandra et al.[14] fabricated three kinds of high aspect ratio structures, which were up to 12 using poly hydroxyethyl methacrylate, poly hydroxyethyl methacrylate-co-N-isopropylacrylamide, and poly ethylene glycol dimethacrylate. Hung et al.[15] also presented a high aspect ratio with 20 in their paper, in which the large size micro chambers can support these high aspect ratio structures. Kung et al.[16] fabricated a 3D high aspect ratio in a PDMS chip, however they adopted a complex hybrid stamp and its aspect ratio was not large enough.
From what we discussed above, although many papers presented large high aspect ratio microstructures, some methods need expensive chemicals, or procedures that are complex and time consuming, or the devices need a large size structure for support.
In this paper, a simple two steps PDMS transferring process has given rise to high aspect ratio structures. During the process, after achieving an SU8 mold with a template mask, a negative PDMS mold was peeled off the SU8 mold.Then a non-adhesive layer was covered on the PDMS mold to help peel off the PDMS device. At last, PDMS devices will be produced using the PDMS mold. The experimental results showed the method is effective and promising.
2.
Experiments
The paper presented a simpler two-step PDMS device fabrication process than the methods reported for high aspect ratio microstructures. During the process, a two times transferring process has happened, and only simple surface solution immersion was adopted to achieve a PDMS layer peeling off a PDMS mold. The whole process is described in Fig. 1, and the details are given in the following sections.
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Figure1.
(Color online) The simple two-step PDMS transferring process.
2.1
Materials preparation
The classic choice of PDMS kits for microstructure in microfluidic or lab on chip is Dow Corning’s Sylgard 184 with mix ratio of cross linker and curing agent to siloxane being 1 to 10. SU8 2050 negative tone photoresist and its developer were from MicroChem Corporation. Brij@52 was purchased from Sigma Aldrich Corporation (Canada).
2.2
SU8 mold fabrication
Because there are many reports available about high aspect ratio SU8 mold fabrication[17–19], only a brief process description is given in this section. For the two-step template transferring process, the mask should be negative. The SU8 2050 photoresist was spin coated on a clean silicon wafer at 500 rpm for 10 s with acceleration of 100 rpm/second and then 2000 rpm for 30 s with acceleration of 300 rpm/second. Then the SU8 layer was soft baked on a hot plate for 5 min at 65 °C and 10 min at 95 °C. After that, the resulting SU8 layer was 85 μm thick with low internal stress. The baked SU8 layer was then clamped under the negative mask and exposed with the 215-240 MJ/CM2 UV at 10 s. Moreover, the SU8 layer was post exposure baked with 5 min at 65 °C and 10 min at 95 °C. When the post exposure bake was finished, the SU8 layer was immersed in the SU8 developer and shaken for 5 min. A clean IPA bath was used to check the completion of the development process. Finally, the SU8 mold was reached after drying.
2.3
PDMS mold fabrication
The SU8 mold was ready with using the negative mask. Then we used the SU8 mold to achieve a PDMS negative mold. The PDMS pre-polymer was prepared by mixing the base and curing agent at the ratio of 10 : 1. The PDMS kits mixture was then evacuated in a vacuum desiccator for 15 min to remove the air bubbles. Then, the PDMS prepolymer was poured onto the SU8 mold and evacuated again in the vacuum desiccator. After that, the SU8 mold with PDMS prepolymer was cured at 90 °C for 2 h. Then they were kept at room temperature to cool down, the PDMS layer was peeled off from the SU8 mold and cut into the desired feature according to the PDMS negative mold.
2.4
PDMS device fabrication
After getting the PDMS mold, we cannot use it directly to produce a PDMS device because it would be difficult to try to peel off devices from the mold without surface treatment. Therefore, preparing a PDMS surface solution is needed, which was used to help peel off the PDMS device from the PDMS mold. At first, we mixed 2 mg Brij@52 and 2 mg acetone, and then kept them in a seal glass cup for half an hour. After all Brij@52 resolved in the acetone, 16 mg de-ionized water was added into the glass cup and shaken. Also, we can use the ratio with 1 : 1 : 8 for Brij@52 : acetone : de-ionized water to reach a different volume.
When the surface solution was available, we put the PDMS mold into the surface solution for 15 min, and then rinsed the PDMS mold using de-ionized water and dried it with airflow. In the following, the steps were the same as in the normal PDMS device fabrication, which are: pour the PDMS solution into the PMDS mold, cure it at 90 °C for one and half hours, and peel off the final chip from the PDMS mold after it has cooled down.
3.
Results and discussion
3.1
Experimental results
The resulting PDMS chip is shown in Fig. 2. For the PDMS chip, a channel has been designed with 200 μm in width and 100 μm in height. At the middle of channels, there were cavities 1.2 mm in diameter and 100 μm in height. Also, there were lots of pillars in the cavity to form capturing microstructures. Each pillar had a width of 10 to 15 μm, and the distance between pillars in a capturing microstructure was 15 μm. The designed height of pillars was 70 μm.
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Figure2.
The final PDMS chip microstructure.
The measurement results of the pillars were given in Fig. 3. Two representative heights have been chosen to demonstrate the method is effective. According to the measurement, the height of pillars was 67 μm. So the high aspect ratio can be around 7 or more for only one pillar without any supporting structure. Also, the whole process only required Brij@52 chemical material that is cheap and easy to use.
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Figure3.
(Color online) The measurement results of the pillar sizes in PDMS device.
3.2
Discussion
The designed height of pillars was 70 μm; however, the experimental result was 67 μm because the thickness of SU8 mold was fluctuated during SU8 mold fabrication. We can get a different thickness of SU8 layer by changing the spin speed and time. Such as when we decreased the speed by 10%, the thickness of SU8 changed to 52.7 μm, shown in Fig. 4.
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Figure4.
(Color online) The results for test thickness of SU8 mold.
Also, after using the surface solution that covered the PDMS mold for 15 min, it was necessary to dry the PDMS mold. Otherwise, there were many bubbles in the PDMS chips that exited when curing, which is shown in Fig. 5. Fig. 5(a) shows a chip using a dry mold but Fig. 5(b) shows the results for a wet mold. Moreover, at the final step, the PDMS solution would pour on the PDMS mold and cure. The temperature value should be less than 90 °C because the acetone will evaporate at high temperature.
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Figure5.
(Color online) (a) The test chip based on a dry mold. (b) The test chip based on a wet mold.
3.3
Experimental development
When we reviewed papers about manufacturing high aspect ratio microstructure, the results showed the high aspect ratio value could be achieved to 17, although these studies need expensive chemical materials, complex and time consuming processes, or supporting structures for high aspect ratio microstructure. Thus, we continue the experiments using different high aspect ratio and different width values. The experimental results are given in Table 1.
| Parameter | Experimental results | |||||
| Width (μm) | 5 | 5 | 10 | 10 | 15 | 20 |
| Length (μm) | 5 | 10 | 15 | 20 | 15 | 20 |
| Aspect ratio | 6 | 7 | 12 | 15 | 18 | 25 |
Table1.
Experimental results with aspect ratio.
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| Parameter | Experimental results | |||||
| Width (μm) | 5 | 5 | 10 | 10 | 15 | 20 |
| Length (μm) | 5 | 10 | 15 | 20 | 15 | 20 |
| Aspect ratio | 6 | 7 | 12 | 15 | 18 | 25 |
During the experiments, we changed the size of pillar with different width and length, and then we tried several aspect ratios and achieved the typical results. According to Table 1, the high aspect ratio value can be up to 25 when using the simple two steps PDMS transferring process. Also if we increase the width and length of pillar, the aspect ratio can be greater than 25. The aspect ratio can be increased when enlarging the size of pillar. Although the aspect ratio value is not large with small width and length, the manufacturing procedure is much simpler than the others and is also cheaper for only using Brij@52 for surface treatment. When compared with others, we find that the method in this paper is promising and economic.
4.
Conclusion
High aspect ratio units are important and indispensable parts of complex micro/nano structures in microfluidic devices. Some of the available methods to achieve a high aspect ratio require expensive materials or complex chemical processes; other methods are difficult to use to reach simple high aspect ratio structures, which need supporting structures.
The paper presented a two-step PDMS transferring process for high aspect ratio microstructures. The method is simpler and cheaper than others, which has achieved a single pillar with 6 to 25 in aspect ratio or more. Compared with the method given in Ref. [5], the aspect ratio is not lower than that of the other methods. Moreover, the process only requires Brij@52 as the surface solution for the two PDMS layers to peel off. Also compared with Ref. [12], we reached a higher aspect ratio with single pillar structures, which is a necessary part of complex microstructure. Finally, the experimental results demonstrated the method is effective and efficient.
Acknowledgements
The SU8 mold fabrication happened at the Centre of Microfluidic Systems in University of Toronto, Canada. The chip manufacture and rest works were worked at the ACμTE lab in York University, Canada. The experiment development happened in Northwestern Polytechnical University, China. Thanks go to them for their support.
