Hot-embossing process

Datta, P., and Goettert, J. (2007). Method for polymer hot embossing process development. Microsystem Technol. 13, 265—270. 

Polymer to emboss FIGURE 20.11 Hot embossing process flow. 

Fig. 5 Graphical representation of the temperature and pressure program in a typical hot embossing process...

Mechanical properties of PDMS affect the hot embossing process. PDMS masters are not rigid like the nickel or silicon tools discussed earlier and have a much lower Young s modulus [16]. Thus, embossing pressure is very important. PDMS has a higher thermal expansion and will expand and deform considerably more than conventional tools. Thus optimizing temperature is equally important in order to reduce discrepancies in replication. Overall, the use of PDMS masters can substantially reduce time, complexity, and costs involved in fabricating prototype microfluidic chips, but limits the master lifetime to approximately 20 cycles. 

In previous chapters, many emerging microlenses based on various mechanisms covering non-tunable and tunable types were presented. However, these microlenses have their optical axes perpendicular to the substrates, thus requiring optical alignment of the different layers. This causes complicated structures for applications such as labs on chips. In this chapter, we discuss horizontal microlenses integrated in microfluidics. Their optical axes are parallel to the substrates of the microfluidic networks. These horizontal microlenses include those formed by a hot embossing process hydrodynamically tuned cylindrical microlenses and tunable and movable liquid droplets as microlenses. 

Hsiung et al. did not use a PDMS layer. They utilized a hot embossing process to form microlenses [10]. The hot embossing technique is relatively simple and reliable [11]. The manufacturing process involved three major steps (1) fabrication of the glass-based master mold, (2) fabrication and bonding of the polymethylmethacrylate (PMMA) chips, and (3) insertion of the optic fibers. 

In the hot-embossing process a heated die carrying the negative of the conductor layout presses a specially coated copper film on to a thermoplastic substrate, applying thermal loading and mechanical pressure. The die cuts out the film, forming a positive bond to the locally melted plastic close to the surface of the blank. Figure 3.16 shows the process chain in simplified, schematic form. 

Rates On the order of 5-7 minutes per part [1]. 2 minute prototype cycle time [13] Faster than hot embossing process of individual chips [10]. On the order of 1-3 minutes per part [1]. Several hours curing time [1]. Curing - 1/10 second [2]. Chemical preparation may require more time. High volumes and potential roll-to-roll process [8]. 

The preliminary results obtained in our experiments demonstrate the feasibility of punching throu -holes in a single-step hot embossing process. Our holes diameters range from 0.5 to 2 mm, and we believe these sizes should be useful either as interface ports or as interlayer connects in a multi-layer microfluidic device. Future woik concerning varied process parameters and yielding mechanics are planned. 

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