In-chip direct laser writing of a centimeter-scale acoustic micromixer

Jorick van ‘t Oever; Niels Spannenburg; Herman Offerhaus; Dirk van den Ende; Jennifer L. Herek; Frieder Mugele

doi:10.1117/1.JMM.14.2.023503

21 April 2015 In-chip direct laser writing of a centimeter-scale acoustic micromixer

Jorick van ‘t Oever, Niels Spannenburg, Herman Offerhaus, Dirk van den Ende, Jennifer L. Herek, Frieder Mugele

Journal of Micro/Nanolithography, MEMS, and MOEMS, Vol. 14, Issue 2, 023503 (April 2015). https://doi.org/10.1117/1.JMM.14.2.023503

Abstract

A centimeter-scale micromixer was fabricated by two-photon polymerization inside a closed microchannel using direct laser writing. The structure consists of a repeating pattern of 20 μm×20 μm×155 μm acrylate pillars and extends over 1.2 cm. Using external ultrasonic actuation, the micropillars locally induce streaming with flow speeds of 30 μm s⁻¹. The fabrication method allows for large flexibility and more complex designs

1. Introduction

Over the last decade, there has been a considerable development in the field of mixing in microfluidic devices.¹^–³ Mixing, passive or active, is part of many laboratory methods, therefore, adapting it to the microscale and nanoscale is important for lab-on-a-chip and microscale total analysis systems.⁴ Fluidics at this scale are typically dominated by viscous forces and turbulent mixing this is hard to achieve.

One class of active mixers is ultrasonic mixers, which use acoustic fields at ultrasonic frequencies. Typically, these mixers either excite a flow at the sidewalls of a microfluidic chamber⁵^–⁷ or excite a flow inside the chamber using a bubble.⁸^–¹⁰ In the case of homogeneous mixing, the chamber size is limited by the spatial extent of the induced flows, which subsequently limits the flow rate. Bubbles can be placed inside the volume and allow higher throughput compared with sidewall induced flows, enabling fast mixing.¹¹ However, bubble-based micromixers also show serious challenges⁷^,¹² with regard to stability. The excitation frequency must closely match the geometric resonance frequency,¹³ therefore, it is dependent on flow rate¹¹ and is influenced by contamination and dissolving over time.

The mixer shown in this paper operates by locally exciting flow around pillars embedded inside the microchannel, providing the main advantage of bubble-based mixing without the mentioned drawbacks. This method allows, in principle, for a larger channel cross section and effective mixing dispersed over that cross section, enabling high flow rates combined with improved stability compared with bubble mixers. To compensate for lower induced mixing flow speed, many micropillars in a row are used. In this paper, we explore the use of in-chip direct laser writing (DLW) as a fabrication technique for the mixing structure, which requires accurate and repeatable pillar placement and shape over an extensive distance.

DLW is a flexible fabrication method for small batches of arbitrarily shaped microstructures. Using two-photon polymerization, only the resist in the focal volume is cured. By displacing the focus with respect to the substrate arbitrarily shaped three-dimensional (3-D) structures can be created. The height of the freestanding structures is limited by their structural strength.

The maximum fabricated size in previously reported work is limited to hundreds of micrometers.¹⁴^,¹⁵ In this work, we explore a strategy for in-chip DLW fabrication of microstructures on a centimeter scale. An important aspect in this work is the choice of resist, enabling a faster and simpler workflow.¹⁵ The resist has a low viscosity, making it suitable for straightforward filling, development, and cleaning by flowing chemicals directly through the microsystem. The resist also does not require preexposure or postexposure baking. This, combined with the improved handling, enables a $10 \times$ faster preparation and development process (2.5 h versus 26 h) compared to earlier methods.¹⁶ Confocal laser scanning microscopy (CLSM) is used to nondestructively characterize the in-chip fabricated structures.¹⁷ As opposed to scanning electron microscopy, CLSM only requires optical access to the sample. Previously, DLW has been used to fabricate a wide variety of structures, such as nanochannels,¹⁸ “overpass”-structures for microfluidics,¹⁹ gold substrates for surface enhanced Raman scattering²⁰ (SERS), and nanoshells.²¹

Integration into larger structures, as those needed for microfluidics,²² is not straightforward. Due to the difference in scale, DLW cannot easily be used to fabricate both the structures and the channels since that would take an unacceptably long time. As such microstructures fabricated by DLW are usually fabricated separately from the channels, requiring fine control of structure size during fabrication and accurate alignment during assembly. The same is true for strategies where the structure is partially embedded into a channel which is subsequently covered with a glass or polydimethylsiloxane layer.²³ Examples of other in-chip fabrication techniques are 3-D patterning using laminar flows,²⁴ patterning of surfaces using microchannel networks,²⁵ and structured elastomeric membranes.¹⁷ However, these techniques do not offer the flexibility of arbitrary shapes as is the case with DLW.

In-chip DLW enables the direct integration of various functional structures into existing microchannels. Resist is flowed into the existing channels and structures are directly written and anchored into place. This ensures a good fit and reduces the amount of chemicals required. In-chip DLW was first used for enzymatic 3-D microreactors by Iosin et al.²⁶ Amato et al.¹⁶ fabricated a porous filter into an existing microchannel and Serra et al.¹⁴ fabricated nematic liquid crystal cells. Wu et al.²⁷ demonstrated fabrication of both channels and microstructures by combined DLW and direct femtosecond-laser writing.

DLW allows rapid prototyping of 3-D microstructures without the need for masks for each variation. The work presented in this paper provides the basis for more complicated structures, including structures which require full 3-D capability fabricated with DLW with an extensive length. At the same time, in-chip DLW provides good bonding of structures which is important for the robustness of the micromixer and mitigates the problem of alignment during assembly. In the next section, we will first explain the in-chip DLW fabrication process, after which we characterize the structure in terms of geometry and strength. We show the limits of this fabrication method for long microstructures and finally we show acoustic mixing around the micropillars.

2. Experimental Methods

2.1.

Fabrication

The DLW system (Photonic Professional, Nanoscribe) uses a femtosecond pulsed laser at a 780-nm wavelength and an 80 MHz repetition rate. The system contains a Zeiss Axiovert inverted microscope with an autofocus/interface finding system and charge-coupled device (CCD) camera. The sample is moved using a positioning system consisting of a motorized stage and piezo stage. The motorized stage is used for coarse movements and has a lateral range of 100 mm in both directions and a position accuracy better than $1 μ m$ . The piezo stage is used for the laser writing and has a range of $300 μ m \times 300 μ m \times 300 μ m$ .

An objective with a higher numerical aperture (NA) produces a smaller focal spot and higher resolution but also produces a smaller polymerized volume. For writing large structures, a lower NA objective and increased laser power reduce writing time at the expense of resolution. However, the aspect ratio of the focal volume scales inversely with the NA, so that a lower NA translates into a more elongated focal volume. For in-chip laser writing the objective working distance is also important, as this limits the maximum axial size of the structures.

The objective we used is a Zeiss LD Plan-Neofluar $63 \times$ with an NA of 0.75 and a working distance of 1.7 mm, resulting in lateral and axial DLW resolutions of 560 nm and $5.5 μ m$ , respectively. The resist is a negative tone acrylate-based resist (IP-L, Nanoscribe). IP-L does not require prebaking or postbaking and has a low viscosity (0.8 Pa s) such that direct injection into microchannels is possible.

The microchannel used for in-chip DLW was fabricated using photolithography and deep reactive-ion etching on a $〈 100 〉$ silicon wafer. Access holes of 1-mm diameter were made from the backside in the same way as the channels. The channel wafer is anodically bonded to a $500 - μ m$ thick borofloat glass wafer and then diced into glass-silicon microchannel chips measuring $6 \times 1.5 cm$ (see Fig. 1). The length $L_{c}$ , width $w_{c}$ , and depth $h$ of the microchannel are 4 cm, $373 μ m$ , and $155 μ m$ , respectively.

Fig. 1

(a) Photograph and (b) cross-sectional sketch of the mixing device (not to scale). The chip consists of a microchannel etched into silicon (gray), sealed with borofloat glass (blue). Access holes are etched into the backside of the chip on which tubing is connected (orange). The mixing structure fabricated using direct laser writing (green) consists of two parallel rows of square pillars, both rows extending over a region of 1.2 cm. The acoustic mixing is excited using a piezo-element which is placed on top of the chip (illustrated in purple in both photograph and sketch). See Fig. 2 for a cross-sectional sketch in the $x z$ -direction.

The microchannel is filled with resist by connecting a tube to one of the access holes and manually filling the channel using a syringe. The tube is then removed and the chip is placed glass-down into the sample holder in the DLW system.

Sample tilt and rotation correction are performed using the interface finder and CCD. The writing of structures is performed from the silicon toward the glass (see Fig. 2) to prevent an aberrated structure due to diffraction from previously polymerized resist.

Fig. 2

A cross-sectional sketch of the microchannel with the designed structure. The chip consists of a microchannel with width $w_{c} = 373 μ m$ and height $h = 155 μ m$ . The structure (green) consists of two rows of square pillars, each extending over the full height of the channel and having a width of $w_{p} = 20 μ m$ . The rows are placed at a quarter and three quarters of the channel width. Each pillar is written using the focused laser beam (red) with the starting position of the focus designed to be at an anchoring depth $d_{a} = 7.5 μ m$ inside the silicon. The progression of writing is from the silicon toward the glass as indicated by the arrow.

Large structures are written in sections using the piezo stage, then the motorized stage is used to move the sample to the beginning of the next section.

After writing, the sample is removed from the sample holder and tube connectors are connected to the access holes. Using a syringe pump (PHD 4400, Harvard Apparatus), first 20 mL propylene glycol monomethyl ether acetate (RER600, Fujifilm) developer is flowed through the channel at a flow rate of $20 mL h^{- 1}$ , followed by 20 mL isopropanol at $20 mL h^{- 1}$ . A gentle flow of air is applied over the access holes until all isopropanol has evaporated, which take less than 30 s.

2.2.

Characterization

2.2.1.

Structural analysis

For the 3-D analysis of the structures, we use CLSM. The channel is filled with index matching oil (Nikon immersion oil, $n = 1.5$ ) to limit image deterioration by diffraction. We use a home-built CLSM based on a Nikon TE300 inverted microscope, with a 25 mW, 488-nm laser (iFlex-Mustang 488, QiOptiq), spinning disk-based confocal scanner (UltraView ERS, Perkin Elmer), and camera (C4742-95-12ERG, Hamamatsu). The axial $z$ -position of the objective (S Plan Fluor ELWD 60X NA 0.7, Nikon) was scanned using an objective piezo scanner (PIFOC P-720, Physik Instrumente) with a range of $100 μ m$ . Only the emitted autofluorescence from the structure was transmitted to the camera using a low-pass emission filter with a cut-off wavelength of 525 nm. For the overview image in Fig. 3, a regular fluorescence microscope (TE2000, Nikon) was used with a lower magnification objective (S Plan Fluor ELWD 45X NA 0.45, Nikon). An autofluorescence excitation-emission map of the cured polymer can be found in the Appendix.

Fig. 3

Fluorescence image of part of the double row structure with $Λ = 30 μ m$ . Visible are four consecutively written pillar sections, with small jumps between sections by the motorized stage visible. The lines (red) indicate the microchannel walls.

2.2.2.

Acoustic mixing

To visualize the acoustic mixing, we use the same CLSM. The channel is filled with glycerol water mixture (80:20 ratio by weight), in which an 18% volume fraction of fluorescent silica tracer particles is suspended. The mixture is index matched to the silica particles. The spheres have an outer diameter of $2 r = 1 μ m$ and a core of $0.5 - μ m$ diameter filled with fluorescein isothiocyanate fluorophore. The fundamental acoustic resonance of the channel at $f = 1.92 MHz$ , corresponding to a standing wave along the channel width, is excited using a piezo plate (PZ 26-302, Ferroperm) connected to an amplifier and function generator. The piezo plate is attached on top of the chip using a drop of paraffin oil as the coupling liquid.

Assuming a plane wave resonance, the magnitudes of the acoustic radiation force and the Stokes drag are²⁸

Eq. (1)

F_{rad} = 4 π Φ k r^{3} E_{ac} \sin (2 k x),

Eq. (2)

Φ = \frac{1}{3} [\frac{5 \frac{ρ_{p}}{ρ_{m}} - 2}{2 \frac{ρ_{p}}{ρ_{m}} + 1} - \frac{k_{p}}{k_{m}}],

Eq. (3)

k = \frac{2 π f}{c_{m}} = 2 π f \sqrt{ρ_{m} k_{m}},

Eq. (4)

F_{drag} = 6 π η r v_{p},

where

Φ

is the acoustic contrast factor,

E_{ac}

is the acoustic energy density,

k

is the wavenumber of the sound wave,

x

is the position along the width of the channel,

c_{m}

is the speed of sound in the mixture,

ρ_{p} = 2648 kg m^{- 3}

and

ρ_{m} = 1208 kg m^{- 3}

are the densities of the particles and the mixture,

k_{p} = 280 {TPa}^{- 1}

and

k_{m} = 255 {TPa}^{- 1}

are the respective compressibilities,

η = 60 mPa s

is the viscosity of the mixture, and

v_{p}

is the particle speed (values from Refs. 29 and 30). Using

E_{ac} = 100 J / m^{3}

, a particle speed of

30 μ m s^{- 1}

, and evaluating the radiation force at its peak position

x = w_{c} / 4

gives a ratio of forces

F_{drag} / F_{rad} \approx 85

. Therefore, the silica particles are small enough to be used as tracers since the acoustic radiation force is negligible compared with the Stokes drag.

3. Results and Discussion