LASER STENCILING FOR POLYMER MICROFLUIDIC DEVICES Whitney Longsine1 and Arum Han1,2*

1Department of Electrical and Computer Engineering and 2Department of Biomedical Engineering, Texas A&M University, College Station, TX, USA

ABSTRACT This paper presents a novel laser micromachining method that utilizes a microfabricated silicon stencil and a CO2

laser for low-cost 10 µm-resolution machining of polymer substrates. With this stenciling method, arrays of microchannels and microholes were patterned in poly(dimethylsiloxane) (PDMS) and Kapton® films, with minimum recorded feature sizes of 8.4 µm and 12.6 µm, respectively. An example multi-layer microfluidic device, comprised of PDMS layers with stenciled features, is presented to demonstrate the prototyping capability of this technique. KEYWORDS: Laser micromachining, Microstenciling, Polymer microfluidic devices INTRODUCTION

Polymer substrates are often the material of choice for the production of microfluidic devices, due to low cost and relatively basic fabrication techniques. Polymer microdevices can be fabricated using methods such as photolithography, hot embossing, injection or replica molding, or reactive ion etching (RIE), though the selection of processing methods is strongly material-dependent. Conversely, with laser micromachining almost all polymer materials can be directly ablated without the requirement of a cleanroom facility [1, 2]. The use of a CO2 laser allows rapid micromachining at low cost; however, the minimum feature size achievable (50 – 350 µm) pales in comparison to the sub-micron resolution of expensive excimer lasers [2, 3].

THEORY

In order to significantly improve the resolution of a low-cost CO2 laser micromachining tool, we have developed a laser stenciling method where a laser beam (typical spot size = 100 µm) is raster scanned across a silicon stencil with openings smaller than the spot size, to selectively pattern an underlying sample (Figure 1(I)). This results in ablated patterns on the polymer substrate with dimensions corresponding to the sizes of the stencil openings, overcoming the resolution limitation of a conventional CO2 laser micromachining tool. Using a single stencil, several polymer substrates can be patterned; thus, we can achieve small volume production at a low cost. Similar stencils have been used as masks for selective metal or dielectric deposition, ion implantation and dry etching [4]. Like our method, these configurations allow direct pattern transfer to a sample by means of a microfabricated stencil.

Figure 1: (I) Schematic of the laser stenciling method. (II) Fabrication steps for the Si stencil: (A) Si3N4 deposited Si wafer, (B) Photoresist patterning and Si3N4 removal using RIE, (C) Si wafer with topside Si3N4 etched, (D) KOH etching through the Si wafer, (E) Removal of backside Si3N4 layer, (F) Metal coating on topside of the Si stencil. EXPERIMENTAL

The microstencil was fabricated by potassium hydroxide (KOH) etching through a silicon wafer (7.62 mm diameter, 380 µm thick) with LPCVD-deposited Si3N4 on both sides (t = 150 nm) to serve as an etch mask. A metal layer was deposited on the surface of the stencil to block the infrared laser (λ ≈ 10.6 µm) from transmitting through the un-etched areas of the stencil. The complete stencil fabrication process is presented in Figure 1(II).

In the stenciling application, the laser was operated at a power of 6 W or below, with values of 2.4 – 3.6 W commonly employed and the standard speed used was approximately 54 mm/s. When using multiple passes, these selected parameters achieved successful sample patterning without causing damage to the stencil and allowed design completion in an acceptable period of time.

(I) (II)

978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS 1196 14th International Conference onMiniaturized Systems for Chemistry and Life Sciences

3 - 7 October 2010, Groningen, The Netherlands

The microfabricated stencil is placed on top of the polymer sample to be patterned. The laser beam is then raster scanned across a selected area of the stencil and the underlying sample is patterned to match the exposed stencil geometries. For increased depth penetration of the pattern, multiple passes at a relatively low power were used in lieu of increasing the power of the laser to minimize feature sizes and reduce stencil damage. This method was used to pattern arrays of microchannels and microholes in Kapton® and poly(dimethylsiloxane) (PDMS) films (Figure 2).

Figure 2: Laser stencil machined holes using Si stencils with 25 µm openings in (A) 50 µm thick Kapton® films and (B) 50 µm thick PDMS films: (I) Frontside view of hole array, (II) Backside view of hole array [scale bars = 500 µm], (III) SEM of individual hole from frontside. RESULTS AND DISCUSSION

Using this stenciling method, the minimum recorded feature sizes were 8.4 µm in PDMS and 12.6 µm in Kapton® films when using a 25 µm opening. To achieve through-holes, while keeping power low to minimize feature size, multiple raster scanning passes were required. Figures 3 and 4 show the relationship between number of raster scanning passes and the front and backside opening widths of the resulting patterns in Kapton® and PDMS, respectively. The frontside openings of holes in PDMS films were on average 135% the size of the stencil openings, while the backside hole openings were 58%. The frontside openings of holes in Kapton® films were on average 179% the size of the stencil openings, while the backside hole openings were 95%. This data indicates that the resulting openings have a conical, or Gaussian, profile. Standard deviation was typically within 10%, showing that this method can reproducibly generate features with variations of only a few microns.

Figure 3: Number of raster scanning passes vs. patterned width (front side and back side) at different Kapton® film thicknesses, using Si stencil having 25 µm openings [(A), (B)] and 55 µm openings [(C), (D)]. N = 9 for all samples.

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Figure 4: Number of raster scanning passes vs. patterned width (front side and back side) at different PDMS film thicknesses, using Si stencil having 25 µm openings [(A), (B)] and 55 µm openings [(C), (D)]. N = 9 for all samples.

These results establish the ability of the laser stenciling method to produce features with 10 µm-resolution that can be incorporated into multi-layer microfluidic devices. Figure 5 presents an example of a PDMS multi-layer, 4-channel microfluidic device, where all channels were patterned simultaneously and the individual layers were assembled by means of O2 plasma bonding. The resulting channels were shown to be leak-free and demonstrate the capability to rapidly prototype microfluidic devices with laser stenciled channels.

Figure 5: Multilayer microfluidic devices. (A) Schematic of the three device layers [inlet/outlet, microfluidic channel, substrate layers] and assembly, (B) PDMS 4-channel device filled with blue color dye for visualization, (C) Enlarged view of the dye-filled PDMS channel (10X magnification). CONCLUSION

In this work, we have introduced stencil-based laser micromachining and demonstrated the capability of this method. This technique effectively reduces the minimum feature size that can be patterned on polymer substrates using a CO2 laser system. Multiple raster passes are performed to achieve through-holes in Kapton® and PDMS films. When machining hole arrays, the ablated patterns on the front surface are on average 157% the size of the stencil opening and the backside opening is 79%. Stenciled channels have been successfully incorporated into multi-layer microfluidic chips, proving the usefulness of this method in device prototyping. REFERENCES [1] “Polymer microfluidic devices,” H. Becker and L.E. Locascio, Talanta, 56, 267 (2002). [2] “Laser processing for bio-microfluidics applications (part II),” C.G. Khan Malek, Anal Bioanal Chem, 385,

1362 (2006). [3] “Microstructure fabrication with a CO2 laser system,” D. Snakenborg, H. Klank, and J.P. Kutter, J. Micromech.

Microeng., 14, 182 (2004). [4] “Direct etching of high aspect ratio structures through a stencil,” G. Villanueva, O. Vazquez-Mina, C. Hibert, and J.

Brugger, IEEE MEMS 2009, 144 (2009). CONTACT *Arum Han, Tel: 1-979-845-9686, Fax: 1-979-845-6259, E-mail: arum.han@ece.tamu.edu.

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