Diffusion - limited photopolymerization in scanning micro - stereolithography

The trade-off between process speed and resolution in microstereolithog-raphy (µSL) roots on the diffusion-limited kinetics of photopolymerization. Using a numerical model, we have investigated the influence of diffusion dominant effect under high photon flux. Radical depletion turned out to limit the smallest feature achievable to the order of 10 µm under high process speed. A solution of pulsed laser curing is proposed in order to realize sub-micron resolution in high speed µSL process. 1 Introduction Microstereolithography (µSL) provides the designers with a simple approach to converting the computer aided designed (CAD) model into truly 3D microcomponents and devices with complex geometry [1–4]. Using a tight focused scanning laser beam to solidify photopolymer resin in a layer-by-layer manner, µSL forms micro-devices of arbitrary shape on stacked layers. In contrast to conventional silicon micro-machining process, µSL is free from constraints on the height of the components. In addition, µSL draws advantages from its precision, automatic process and capacity to incorporate a broad spectrum of functional materials. This promising prototyping technique will find potential applications in microfluidic systems [5], optical wave-guides [6], and 3D photonic band gap structures [7]. For the prototyping of 3D microde-vices, resolution and fabrication speed are two critical factors in µSL process. The former dominates the smallest feature size and the latter limits the process yield rate. We have recently reported an advanced microstereolithog-raphy system with 1–2 µm resolution using single photon absorption [2]. Figure 1 shows two examples of the fabricated polymeric photonic structures using our scanning µSL system. A common perception in microlithography is that photo-polymerization ratio is simply a function of the exposure dose, that is, the product of intensity and exposure time. This perception leads to an intuitive conclusion that an optimal process speed can be met by increas-FIGURE 1 The scanning electron micrograph (SEM) of a a 2D dot array and line patterns with 10 micron linewidth and 8-by-5 micron dot size and b a fabricated 16 layer 3D polymeric photonic crystal by scanning µSL. In b the dots are 3-micron in width with 7-micron spacing and 15-micron in depth. The scale bar in a and b is 20 and 50 microns, respectively ing light intensity while maintaining the exposure dose. However, the progression of photopolymerization under laser irradiation is an evolving process from liquid to solid which experiences an intermediate gelatinous state. The characteristic time and length scale of this evolving process will …