Fabrication of topologically-complex 3D microstructures by femtosecond laser machining and polymer molding

We demonstrate the demolding of topologically complex three-dimensional elastomeric microstructures from a femtosecond laser micromachined glass substrate. Demolding success rates of >90% are achieved, which are qualitatively supported by a simple mechanical model.


Introduction
The creation of three-dimensional microscaled features on surfaces has many applications, including the control of surface properties such as adhesion [1] and wettability, as well as for fabricating microfluidic, optofluidic, or micromechanical devices. Mask-based photolithography processes offer limited inherent three-dimensionality, and other methods for creating more complex three-dimensional shapes on the microscale, such as 2-photon polymerization and electron-beam lithography, are generally serial (and thus time-consuming) in nature.
As an alternative fabrication technique, we use a two-step process combining femtosecond laser patterning with a chemical etch. The nonlinear multiphoton absorption process locally modifies (but does not ablate) the material in a micron-scaled laser-affected volume; in this volume, the local etching rate in hydrofluoric acid is increased compared to the pristine material. We use this process to fabricate monolithic glass (fused silica) micromolds with complex three-dimensional surface topology, and demonstrate the replication of the negative structures with polydimethylsiloxane (PDMS). The molding process allows many copies of one machined surface to be produced at a low cost, overcoming the limitations of the serial mold fabrication process. after the application of a monolayer to decrease mold adhesion (3), PDMS is poured over the glass mold, cured, and removed (5).

Fabrication
An overview of the fabrication process is shown in Fig. 1. Following laser exposure (wavelength 1030 nm; pulse duration 400 fs; repetition rate 400 kHz; average energy 220 mW; pulse energy of 550 nJ/pulse), the fused silica sample was etched (2.5% hydrofluoric acid, room temperature, 9.5 hours) and then coated with a monolayer to decrease the adhesion between the glass and the molding polymer, as described previously [2]. PDMS (Sylgard 184) is mixed in a 1:10 ratio of hardener:base, poured on the mold, cured at 65⁰C for 2 hours, and allowed to return to room temperature over several hours. The glass mold and polymer replica are separated by hand.

Mechanical modeling
To predict the success of the demolding process, we choose the structure shown in Fig 2a as a representative example. We model the stress at the point of highest deformation, when the narrow-diameter section of the PDMS (section e to f, Fig. 3) is stretched around the wider-diameter section of the glass (section a to b, Fig. 3). The loading is a combination of the hoop and friction forces resisting the demolding, for which we evaluate the equivalent von Mises stress and compare that to the ultimate tensile strength of PDMS. We take the elastic modulus of PDMS at 1.  with >90% success rate, and the few broken samples were at the extreme of d/D < 0.5. While the model is only an approximation, it qualitatively aligns with the high demolding success rates we observed.

Conclusion
The successful demolding of topologically-complex three-dimensional microstructures has been demonstrated. This technique is made possible by non-linear laser-material interactions, which allows the formation of arbitrary threedimensional patterns in glass, creating a monolithic, mechanically robust mold. Structures which require significant stretching of the molding material around the glass are possible with a sufficiently elastic polymer; with PDMS, demolding success rates of over 90% are achieved, and a simple mechanical model is able to qualitatively predict the demolding success of such structures. This method enables the creation of shapes which are difficult or impossible to achieve with other fabrication techniques, and can produce repeatable, complex molded microstructures with a high yield over an arbitrarily large surface area.