Rucha Natu and Rodrigo Martinez-Duarte
Multiscale Manufacturing Laboratory
Department of Mechanical Engineering
Photolithography refers to the use of light to pattern a photosensitive chemical or photoresist film. The patterning is enabled by the use of a photomask, a piece of material that only allows light to pass in selected areas. Photoresists that are commonly used include AZ, Shipley, polyimide (Metz et al., 2001), SU-8 and dry film resist such as Ordyl SY300/550. They are divided in positive and negative photoresists depending on the chemical phenomena induced in the volumes exposed to light of specific wavelength. A scission reaction is induced in a positive photoresist which renders such volumes more soluble to a developer solution. In contrast, light exposure of a negative photoresist induces cross-linking that makes the photoresist resistant to developing. In practical terms, the regions exposed to light are eliminated in positive photoresists while they are the ones that stay in the case of negative resists. Of special mention among negative photoresists is SU-8, since it has become a workhorse in microfluidics fabrication, either to make the channels themselves or to fabricate molds for soft lithography (Patrick Abgrall et al., 2005; Lin, Lee, Chang, & Chang, 2002; Martinez-duarte & Madou, 2011). Using photolithography to make the channels themselves leads to open channels that must then be closed by bonding a lid.
The main steps in photolithography of negative photoresists are substrate coating, soft bake, exposure, post exposure bake and development. Positive photoresists do not usually require a post exposure bake. Common substrates for microfluidics fabrication are glass and silicon, either in slides or wafers. Photolithography can also be accomplished on flexible substrates such as PET or polyimide films. To ensure industrially acceptable yields, photolithography processes should be carried out in a clean room, a specially designed area where the size and the number of airborne particulates are highly controlled, together with the temperature (±0.1°F), air pressure, humidity (from 0.5% to 5% RH), vibration, and lighting (yellow to avoid resist activation).
Given the importance of SU-8 in the field we overview a photolithography process using such resist. SU-8 photoresist as sold by different vendors (i.e. Microchem www.microchem.com and Gersteltec www.gersteltec.ch) and is a mix mainly composed by SU-8 resist, a casting solvent and a photoinitiator. Formulations meant for thin layers have low viscosity while those meant for thick layers have a consistency similar to thick honey. The first step in SU-8 fabrication is coating of the substrate, a step usually performed by spin coating. The spin speed, duration and acceleration are optimized to obtain a specific layer thickness based on the SU-8 formulation used. Once coated, the SU-8 layer is soft baked to evaporate the casting solvent and leave behind a pure SU-8 layer with photoinitiator. This layer is then exposed to light, with wavelength around 365 nm, through a mask. Since it is a negative photoresist, the areas meant to stay are exposed to initiate a crosslinking reaction. The non-exposed areas will remain un-crosslinked which will facilitate their dissolution in the development step. Once exposed, the layer is further baked to finalize the crosslinking reaction. The last step is developing, where the layer is immersed in a chemical bath to dissolve all non-exposed areas. More details on processing can be found elsewhere (Madou, 2011; Martinez-duarte & Madou, 2011). It is important to mention that the exposure step can be implemented with a grayscale mask instead of the more traditional binary mask. A grayscale mask allows for surface topography and can eliminate the need of multi-layer processes (Hung, Tseng, & Chou, 2005; Rammohan et al., 2011). Alternatively, the substrate can be inclined and/or rotated during exposure through a binary mask. This yields useful inclined structures that can be used as in-channel filters and scaffolding (Sato, Matsumura, Keino, & Shoji, 2006).
Microfluidics benefits from SU-8 photolithography in the batch fabrication of structures of high aspect ratio and/or large surface area, which can range in size from a few millimeters down to tens of nanometers. The good mechanical and excellent chemical properties of cross-linked SU-8 yield polymer microfluidics devices that can handle a variety of samples such as blood, urine, and milk as well as buffers and cleaning agents at a wide range of flow pressures and working temperatures. SU-8 has a high transparency for light at wavelengths more than 400 nm and allows for the use of fluorescence and other visualization and detection techniques that are common in microfluidics. The fabrication of thick structures of 1 mm and above, multilayered SU-8 structures, components like channels, check valves, micro pumps have been successfully demonstrated using this process (P Abgrall et al., 2006; Bohl et al., 2005; Lin et al., 2002).
SU-8 Photolithography is an efficient technique for batch production of chips, but it requires some practical experience for successful fabrication. Although the generic guidelines for the process are available, the process parameters need to be finely tweaked depending on equipment and facilities available to the user, the specific application, and most importantly, the geometry of the pattern. It is not the same process to fabricate high aspect ratio structures which are tens of micrometers apart than the one that must be used to fabricate dense arrays of the same structures (gaps <20 µm). The uniformity of the layer throughout the whole substrate must be monitored to minimize the risk of air pockets between SU-8 and mask during exposure which can lead to light diffraction and broadening of the features. Perhaps the most important variable to control in the manufacturing process of SU-8 structures of large footprint (>500 µm square and >50 µm high) is the internal stress. Thermal stresses can cause crack formation, bending, and debonding from the substrate and thermal management that includes the use of appropriate heating and cooling ramps during post exposure bake is needed (Martinez-duarte & Madou, 2011).
|Figure 1: SEM photomicrograph of the all SU-8 microfluidic chip with (a) Built in 3D micromeshes and (b) magnified images of micromesh structure. (Reprinted with the permission from IOPSCIENCE) (Sato et al., 2006)|