PDMS - Dry Etching

PDMS - Dry Etching

Material Name: PDMS
Recipe No.: 10352
Primary Chemical Element in Material: No data
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: The PDMS used throughout the study was Sylgard 184 from Dow Corning Corp, Midland, MI. The elastomer was mixed in a 10:1 ratio with its curing agent and then placed in a desiccator for at least 45 min to remove the bubbles created during mixing. Samples were then made by either spinning the polymer on precleaned glass slides and curing at 90 °C, or by pouring it into plastic weighing dishes and curing at low heat.

The wet etch experiments were performed using a solution of tetrabutylammonium fluoride (C16H36FN) in n-methyl-2-pyrrolidinone (C5H9NO), 3:1 (v/v) NMP:TBAF, both supplied by Sigma-Aldrich. The surface of the PDMS was first coated with copper or aluminum either by sputtering or electron-beam evaporation. Rectangular windows were then etched in the metal masking layer using standard photolithographic processing and wet etching. The PDMS was then immersed in the solution and gently agitated during the etch. Agitation was required due to the relatively slow etch rate, and the need to replace etchant at the surface, as is commonly done in the laminar flow etch process described by Takayama et al.3 The substrates were periodically removed from the etchant, rinsed with isopropanol and water, dried with nitrogen, and examined under the microscope in order to monitor the etch progress. Results of this approach are shown in Fig. 3 and discussed in the following section.

Dry etching experiments were performed in an Oxford Plasma Technology mP 80 parallel plate reactor equipped for reactive ion etching. Oxygen (O2) and tetrafluoromethane (CF4) gases were supplied by Messer, MG Industries, Malvern, PA. All samples used for etch rate determination were etched for 60 min while the CF4/O2 ratio, total gas pressure, and RF power were varied (see Table 1). The surface of each sample was partially masked with Kapton tape prior to etching in order to create a distinct step feature that could be measured using a Dektak 3030 stylus profilometer. The step height was also measured using a Leitz Ergolux optical microscope with a calibrated reticle. Each sample was cross sectioned and laid flat on a microscope slide to obtain a side view of the etch profile. Several cross sections were taken for each sample and the average step-height value was used.
Procedure (Condition): No data
Note: A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 mm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches.
Reference: J. Garra, et al., Dry etching of polydimethylsiloxane for microfluidic systems, J. Vac. Sci. Technol. A 20(3), May.Jun 2002, pp. 975-982.

Figure 1: (a) Thick PDMS layer cast in SU-8 mold. The SU-8 features are patterned in the surface of the PDMS. (b) The PDMS cast in the SU-8 mold has the same thickness as the SU-8 itself. The mold features extend through the membrane.

Table 1: Dry etch data for PDMS at varying pressures and gas ratios. All etches were performed for 60 min.

Figure 2: Cross sections of two different structures, (a) and (b), etched in PDMS as seen under the optical microscope at 203 magnification. The twin structures in (a) are roughly 140 µm wide and 60 µm high. The rectangular etch cavity in (b) is 350 mm wide and 60 mm deep. The measurement reticle is not visible in the pictures.

Figure 3: Two different patterns were etched into the PDMS samples through lithographically defined aluminum masking layers to show how dry etching can be used to bulk micromachine structures in the surface of PDMS. The first structure, shown in (a) and at increased magnification in (b) consists of a series of parallel channels 350 µm wide and 70 µm deep. The second structure, shown in (c) and again in (d) is an array of octagons (300 µm wide) and rhombuses (45 µm wide) raised 70 µm from the bottom surface. The micrographs show the roughness characteristic of the etched PDMS surface.