Optimization of Direct-Write 3D Two Photon Photolithography in Poly (methyl methacrylate)
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Date
2015-05
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The Ohio State University
Abstract
Direct-write multiphoton photolithography (DWMP) is a technique which exploits the localization of multi-photon processes which occur at a tightly focused femtosecond laser to write 3D patterns in a photosensitive polymer. In conventional photolithography devices are fabricated by using masks to tailor light exposure onto photo sensitive material, developing the photoresist, and this process is driven by a single photon. DWMP differs in that the energy of at least two photons is required to reach criticality for exposure chemistry. This means that whereas traditional photolithography will polymerize a material throughout the volume of the beam, DWMP will only polymerize a material where the probability of two or more photons interacting with a molecule simultaneously is incredibly high, i.e. at the focus. This is the essential idea behind DWMP which allows arbitrary 3D shapes to be created, in contrast to traditional photolithography where devices are produced in layers with strict limitations on complexity.
DWMP also allows for the creation of very small, high resolution shapes. This is possible because of the tight laser focus which produces “voxels” (volume-pixels) of polymerized material. To zeroth order the dimensions of a voxel can be estimated by a Gaussian laser’s diffraction limit. However in the DWMP case, because two or more photons must interact at the beam waist to induce polymerization the effective volume is reduced in proportion to the number of photons required for the interaction. This reduces the effective volume further as a function of the cross-sectional intensity, allowing for voxels smaller than the diffraction limit.
The majority of DWMP work to date has used negative photoresists, in which exposed material is made less soluble. This results in solidified material where the focus was scanned and is useful for creating high resolution freeform structures. Here we explore and attempt to optimize DWMP with regards to the positive resist Poly (methyl methacrylate) (PMMA) using short wavelength (~387 nm) light. Not only is PMMA a widely used and durable material in the biological community, but because it is a positive resist it is the exposed rather than unexposed material which is removed upon development. This property combined with the complexity allowed by DWMP should make it possible to make wells and intricate channels imbedded on all sides within a block of PMMA. Such a technique would prove useful in the creation of arbitrary microfluidic devices, as are often needed for biological research. We find that the technique is indeed viable while outlining a general method and defining future work to optimize the resolution of the process.
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Keywords
PMMA, Poly(methyl methacrylate), photolithography, lithography, multiphoton, two photon