Scalable integration of nano-, and microfluidics with hybrid two-photon lithography

Abstract

Nanofluidic devices have great potential for applications in areas ranging from renewable energy to human health. A crucial requirement for the successful operation of nanofluidic devices is the ability to interface them in a scalable manner with the outside world. Here, we demonstrate a hybrid two photon nanolithography approach interfaced with conventional mask whole-wafer UV-photolithography to generate master wafers for the fabrication of integrated micro and nanofluidic devices. Using this approach we demonstrate the fabrication of molds from SU-8 photoresist with nanofluidic features down to 230 nm lateral width and channel heights from micron to sub-100 nm. Scanning electron microscopy and atomic force microscopy were used to characterize the printing capabilities of the system and show the integration of nanofluidic channels into an existing microfluidic chip design. The functionality of the devices was demonstrated through super-resolution microscopy, allowing the observation of features below the diffraction limit of light produced using our approach. Single molecule localization of diffusing dye molecules verified the successful imprint of nanochannels and the spatial confinement of molecules to 200 nm across the nanochannel molded from the master wafer. This approach integrates readily with current microfluidic fabrication methods and allows the combination of microfluidic devices with locally two-photon-written nano-sized functionalities, enabling rapid nanofluidic device fabrication and enhancement of existing microfluidic device architectures with nanofluidic features.

Keywords: Nanofabrication and nanopatterning; Nanofluidics.

Fig. 1. Process outline for nanofluidic device fabrication via combination of two-photon lithography and mask UV lithography.

UV lithography; (a) mask-based UV lithography is used to project an arbitrary microfluidic chip design onto a SU-8 coated silicon wafer; b nanofluidic channels are added via two-photon lithography in areas of interest. The jablonsky diagram illustrates that two spatially correlated IR-photons can interact with the photoinitiator like one single UV photon with half the wavelength if they are absorbed by the molecule within the lifetime of a virtual state excited by a single-IR photon; c the wafer is developed and a microfluidic master wafer with integrated nanofluidics obtained; d the wafer is covered with PDMS for soft lithography; e after curing of the PDMS, devices are peeled off the surface and inlets added; f the final PDMS-chip is plasma bonded onto a glass coverslip and filled with, e.g., fluorescent dye

Fig. 2. Correlated SEM and AFM analysis of 2-photon nano structures.

a SEM micrograph of nanofluidic PDMS imprint fabricated by the combination of UV mask lithography and 2-photon writing; b three nanofluidic areas with nanochannels of 75 micron length joining the two microchannels; c higher magnification shows 420 nm wide nanochannels imprinted in PDMS

Fig. 3. Correlated SEM and AFM analysis of 2-photon nano structures.

a SEM image of power offset parameter test pattern at a writing speed of 400 µm/s. From bottom to top the power was varied from 50 to 120 mW. Horizontally, from the left to right the 2P-voxel was deepened into the wafer from zero offset—relating to the focal spot being on the wafer surface—to −3 µm, which relates to the focal spot being 3 microns inside the silicon from the wafer surface. One can see that at constant power just by varying the height, the lateral line width can be pushed to the nanoscale; b SEM-image of the position where the polymerization inducing voxel is disappearing into the wafer; (B*) Correlated AFM image of position B; (B**) AFM line plot of height profile measurement along red line as indicated in (B*) verifying a size down to 52 nm; c detailed SEM imaging verifies soft lithography compatible structures down to 280 nm in width and illustrate how channel size can be controlled at constant offset by variation of power; (C*) AFM height profiles along colored lines as indicated in (c)

Fig. 4. Super-resolution imaging of nanofluidic devices.

a Super-resolved dSTORM image of a nanofluidic PDMS chip with Rhodamine 6G diffusing through 2-photon written nanochannels. b Zoomed in region, verifying the successful imprinting of fluidically connected nanochannels in PDMS; the channel size was measured by averaging the vertical line profiles reaching from position 1 to position 2; c plot of averaged line profiles as indicated in (b)—verifying the intended channel width of 420 nm on chip and a FWHM of 200 nm. Reconstruction was computed with the ThunderSTORM software plugin for ImageJ.