IOP PUBLISHING
JOURNAL OF MICROMECHANICS AND MICROENGINEERING
doi:10.1088/0960-1317/19/9/095010
J. Micromech. Microeng. 19 (2009) 095010 (10pp)
Dry etching of polydimethylsiloxane using microwave plasma Sung Jin Hwang1 , Dong Joon Oh1 , Phill Gu Jung1 , Sang Min Lee1 , Jeung Sang Go1 , Joon-Ho Kim2 , Kyu-Youn Hwang2 and Jong Soo Ko1,3 1 School of Mechanical Engineering, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea 2 Samsung Advanced Institute of Technology (SAIT), Nongseo-dong, Giheung-Gu, Yongin-si, Gyeonggi-do 446-712, Korea
E-mail:
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Received 25 March 2009, in final form 22 June 2009 Published 24 August 2009 Online at stacks.iop.org/JMM/19/095010 Abstract This paper presents a new polydimethylsiloxane (PDMS) dry-etching method that uses microwave plasma. The applicability of the method for fabricating microstructures and removing residual PDMS is also verified. The etch rate of PDMS was dominantly influenced by the gas flux ratio of CF4 /O2 and the microwave power. While the PDMS etch rate increased as the flux ratio of CF4 was increased, the etch rate decreased as the flux ratio of O2 was increased. The maximum etch rate of 4.31 μm min−1 was achieved when mixing oxygen (O2 ) and tetrafluoromethane (CF4 ) at a 1:2 ratio at 800 W power. The PDMS etch rate almost linearly increased with the microwave power. The ratio of the vertical etch rate to the lateral etch rate was in a range of 1.14–1.64 and varied with the gas fluxes. In consideration of potential applications of the proposed PDMS etching method, array-type PDMS microwells and network-type microprotrusion structures were fabricated. The contact angle was dramatically increased from 104◦ (non-etched PDMS surface) to 148◦ (etched PDMS surface) and the surface was thereby modified to be superhydrophobic. In addition, a thin PDMS skin that blocked holes and PDMS residues affixed in nickel microstructures was successively removed. (Some figures in this article are in colour only in the electronic version)
PDMS has unique characteristics such as excellent physical and chemical properties, good biocompatibility, low cost and simple fabrication with high replication fidelity [2–4]. PDMS is an elastomer with the following properties: Young’s modulus of ∼750 kPa [5], dielectric strength of 14 kV mm−1 [6], thermal conductivity of 0.16 W (m K)−1 [6], refractive index of 1.42 [7], optical transparency down to 300 nm wavelength [3] and high gas permeability (O2 , 79 × 10−7 (cm3 cm) (s cm2 kPa)−1 ); CO2 , 405 × 10−7 (cm3 cm) (s cm2 kPa)−1 ) [8]. It is chemically inert and non-flammable, and has a low surface free energy of 22 mJ m−2 [9], as a result of which it is hydrophobic in nature. PDMS can be formed by a simple casting process and has a very high replication fidelity of sub-0.1 μm [10]. Hence, it is cost effective and easy to use in high-precision manufacturing. In addition, it is non-toxic and biocompatible and, therefore, one of the materials that
1. Introduction Silicones are mixed inorganic–organic polymers that comprise atoms of silicon and oxygen that alternate in a chain, wherein some organic groups, including methyl, ethyl and phenyl, are bound to the silicon atoms. Silicones have been used in a very wide range of medical, industrial and household applications. From a biomedical point of view, silicones have been serving as useful biomaterials in the healthcare industry since the mid-1940s [1]. Among silicon-based organic polymers, polydimethylsiloxane (PDMS) is the most widely used. Recently, with increasing demand in nano/microscience and engineering fields, the importance and usefulness of PDMS have been greatly increased. 3
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J. Micromech. Microeng. 19 (2009) 095010
has been the most widely tested for biosafety [11]. Due to these unique characteristics, PDMS is utilized as a material for stamping tools in soft lithography [12, 13] and is one of the most common materials for microfluidics-based biochips [4, 5, 14]. PDMS nano-microstructures are typically produced by a single microcasting process [4, 15–18] but there are cases where PDMS etching must be performed in subsequent processes [4, 19, 20]. For example, during the casting process, a PDMS skin can remain at either the top or the bottom of microholes or channels, thus preventing the flow of fluid or air [4]. In addition, when PDMS casting is conducted using a mold with a high-aspect-ratio microstructure, PDMS residues can remain after demolding due to problems of jamming or adhesion. To remove these PDMS residues, a PDMS etching process is required. Although the areas of application of PDMS have rapidly expanded in the MEMS/NEMS fields, research on PDMS etching has thus far not been actively carried out. With regard to PDMS-etching methods, there are two main approaches: dry etching and wet etching. While reactive ion etch (RIE) [20, 21] and inductively coupled plasma (ICP) systems [22] have been used for dry etching, a mixture of tetrabutylammonium fluoride (TBAF) and n-methyl-2pyrrolidinone (NMP) has been used for wet etching [23]. However, dry-etching methods for PDMS that use RIE and ICP show very low etch rates on the order of hundreds of ˚ Angstroms per minute and cause serious surface damage due to the ion collision effect under high energy [24, 25]. In the case of wet etching, it is very difficult to acquire a smooth etched surface and maintain a uniform etch rate because the composition of the solution changes in the course of etching. In this study, we propose a new dry-etching technique for PDMS that uses a high-density microwave plasma system. In addition, to evaluate the usefulness of the proposed PDMS etching technique, we have fabricated PDMS microstructures with nickel shadow masks and removed PDMS residues by using the etching technique.
Figure 1. Schematic view of the microwave plasma system.
break carbon-based macromolecules. While etching can be done for a polymer that comprises carbon and hydrogen by using oxygen plasma, it is relatively hard to etch PDMS due to its strong Si–O bonds. Thus, another kind of etch chemistry is required [20]. Although an accurate etching mechanism for PDMS has not been identified thus far, the etching process can be understood through a dry-etching process that uses silicon (Si) and silicon dioxide (SiO2 ), since PDMS is a silicon-based polymer. Fluorine (F)-based chemistry, such as CF4 and SF6 , is mainly used for etching silicon and silicon dioxide, because fluorine can produce volatile compounds, including silicon. Also, for PDMS, a fluorine-based chemistry produces F atoms under a plasma condition and these F atoms react with the Si of PDMS to form a partial Si–Fx reaction layer. If the F atoms continue to penetrate into this partial reaction layer, etching is believed to occur as the F atoms change to SiF4 , which has strong volatilization [20, 22]. 2.2. Microwave plasma system The process technology that uses plasma has received attention as a new, fundamental technology in the manufacturing field. In particular, high-density plasma has a broad range of applications, including plasma etching [30], plasma-enhanced chemical vapor deposition (PECVD) [31, 32], surface treatments of polymers, glasses and metals [33, 34], plasma polymerization [35–37], etc. The density of microwave plasma, 1012 –1013 cm−3 [38], is at least 100 times higher than that of the radio frequency (RF) plasma, which is 108 – 1010 cm−3 [39]. It is believed that this high-plasma density will increase not only the ionization rate but also the degree of gas decomposition, thereby resulting in an increased etch rate of PDMS. Figure 1 shows a schematic view of the microwave plasma etching system (Tepla 300, PVA Tepla Co., Germany) used in this study. This microwave plasma etching system generates high-density plasma in the following manner. A microwave is produced by a magnetron, which can generate electromagnetic waves with a frequency of 2.45 GHz, and is transmitted along a waveguide. A strong electric field of electric waves, which resonate, is then transmitted through an antenna to a cylindertype quartz chamber (process chamber).
2. Material and microwave plasma etching system 2.1. Characteristics of PDMS PDMS comprises inorganic siloxane (Si–O–Si) backbones to which pendant organic methyl (CH3 ) groups are attached (more specifically, to the silicon in PDMS) [26]. The nonpolar methyl groups shield the polar siloxane backbone and form a hydrophobic sheath with very low intermolecular interactions [11]. Therefore, PDMS has low surface energy and low chemical reactivity, resulting in excellent biocompatibility [27]. Since siloxane bonds have nearly zero energy of rotation about the Si–O bond, silicon–oxygen chains are very flexible [11]. As a result, PDMS is also very flexible. The bond energy of Si–O (107 kcal) is about 20 kcal higher than that of C–C (83 kcal) and C–O (85 kcal) [28]. Characterized by the strong chemical bonds and polarity of Si–O, PDMS is not easily broken by hemolytic scission [29]. Therefore, more energy is required to break PDMS than to 2
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(b)
(a)
Figure 2. Nickel shadow mask with array-type circular holes (total size: 15 mm × 15 mm, hole diameter: 100 μm and gap between holes: 100 μm): (a) design and (b) SEM image of the fabricated nickel shadow mask.
3. Experimental details 3.1. Microwave plasma etching procedure for PDMS PDMS-etching experiments that use microwave plasma have been performed under two conditions: with and without a shadow mask. Hereafter, ‘PDMS without a shadow mask’ and ‘PDMS with a shadow mask’ are referred to as ‘open PDMS’ and ‘shadow-masked PDMS’, respectively. Mixtures of CF4 and O2 were used as etching gases. In order to identify the optimal etching conditions, we conducted etching experiments by varying the fluxes of the CF4 and O2 gases and the microwave power. Each sample was etched for 5 min under each condition and the etch rate was taken as the average of the data measured from five different samples. Changes in thickness before and after etching were measured by the α-step (KLA Tencor, USA). The surface roughness and the surface shape of the etched PDMS were measured using an atomic force microscope (AFM; XE-100, PSIA Corp., Korea). In addition, a scanning electron microscope (SEM) and a 3D confocal microscope (VK-9700, KEYENCE Corp., Japan) were used to acquire three-dimensional etching profiles.
(a)
(b)
(c)
(d )
Figure 3. The fabrication process of the nickel micromesh shadow mask: (a) deposition of the Cr seed layer, (b) photolithography, (c) nickel electroforming and (d) removal of the Si wafer and the photoresist.
of the photoresist layer was measured to be 15 μm. After that, nickel electroforming was performed with a current density of 5 mA cm−2 at 55 ◦ C (figure 3(c)). In order to avoid excessive electrodeposition, the height of the electroplated nickel had to be less than that of the photoresist. The thickness of the electrodeposited nickel layer was 11 μm. To obtain the nickel shadow mask, the silicon was then etched at 60 ◦ C using a 25 wt% potassium hydroxide (KOH) solution (figure 3(d)). Finally, the fabricated nickel shadow mask was cut into 15 mm × 15 mm squares. Figure 2(b) shows an SEM image of the fabricated nickel shadow mask. The microscopic inspection revealed that microholes of 100 μm diameter were formed within the entire 15 mm × 15 mm area.
3.2. Preparation of the PDMS specimens A 10:1 (v/v) mixture of PDMS prepolymer with a curing agent was treated for 30 min in a vacuum oven to eliminate internal air bubbles. The PDMS mixture was then coated onto a 6 inch silicon wafer and subsequently cured at 20 ◦ C for 24 h. The thickness of the solidified PDMS samples was 100 μm. Finally, the wafers were cut into 15 mm × 15 mm size pieces. 3.3. Fabrication of Ni shadow masks The nickel shadow mask has circular shapes of diameter 100 μm arranged at 100 μm intervals, as shown in figure 2. Figure 3 depicts the fabrication process for the ˚ thick layer of Cr nickel shadow mask. First, a 1000 A was deposited on a 6 inch silicon wafer (figure 3(a)). Cr serves as a conducting seed layer for nickel electroforming. Photolithography was subsequently conducted using an AZ 9260 photoresist (Clariant, USA) (figure 3(b)). The thickness
4. Results and discussion 4.1. The etch characteristics of open PDMS The etch rate of PDMS is dominantly influenced by two crucial process parameters: the gas flux of CF4 and O2 and the 3
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Table 1. The etch rate of open PDMS as a function of the gas mixture and the electric power. No
O2 (sccm)
CF4 (sccm)
Power (W)
Pressure (mbar)
Vertical etch rate (μm min−1 )
˚ Ra (A)
1 2 3 4 5 6 7 8 9 10 11
0 100 100 100 100 150 150 200 100 100 200
200 200 200 200 150 150 200 200 100 0 0
800 800 600 400 800 800 800 800 800 800 800
0.528 0.862 0.839 0.815 0.785 0.848 0.935 0.998 0.712 0.357 0.617
0 4.31 2.60 0.77 2.73 2.41 3.44 2.29 1.81 0 0
10.78 13.06 13.35 14.46 7.21 9.56 14.4 10.12 12.83 35.26 36.26
6
O2 flow rate (sccm) 50
100
150
200
250
Vertical etch rate (mm/min)
0
Vertical etch rate (µm/min)
6 O2 100 sccm, 800W, Fixed CF4 200 sccm, 800W, Fixed
5
4
3
2
O2 100 sccm, CF4 200 sccm, Fixed 5 4 3 2 1
Open PDMS 1
0
Open PDMS
300
400
500
600
700
800
900
Microwave Power (W)
0 0
50
100
150
200
250
Figure 5. The vertical etch rate of open PDMS as a function of the microwave power.
CF4 flow rate (sccm) Figure 4. The vertical etch rate of open PDMS as a function of the CF4 /O2 flow rates at 800 W.
surface is larger than the etch rate that results from the reaction with F. However, if O2 is added to CF4 , O and/or O2 reacts with CFx and/or Cx Fy to form COF2 , CO and/or CO2 [40]. These reactions prevent Cx Fy types of polymer from being produced on the substrate, and the etch rate increases as the extracts of F that are required for PDMS etching increase. However, if a large amount of O2 is added to CF4 , etching is hindered as O2 or O causes the formation of oxides on the substrate surface [41]. If only O2 is injected, oxides solely in the form of SiOx are generated on the PDMS surface, and etching does not take place. As a result, to etch PDMS, both CF4 and O2 gases must be simultaneously injected, and we can observe that the ratio of CF4 :O2 is the significant factor that determines the PDMS etch rate. The maximum etching rate of open PDMS, as measured by this experiment, was 4.31 μm min−1 at 200 sccm for CF4 and 100 sccm for O2 at a fixed microwave power of 800 W. To examine the variation in the PDMS etch rate with the microwave power, an etching experiment was performed at 400 W, 600 W and 800 W microwave power and under fixed conditions of 100 sccm O2 and 200 sccm CF4 . Figure 5 displays the variation of the etch rate with the microwave power. The PDMS etch rate increased almost linearly from 0.77 μm min−1 to 2.60 μm min−1 and to 4.31 μm min−1 as
microwave power. Table 1 summarizes the etch results of open PDMS as a function of these two process parameters. The process pressures were determined by the fluxes of the CF4 and O2 gases. Figure 4 shows the variation with the gas fluxes of the measured etch rate of open PDMS at a fixed microwave power of 800 W. When only one type of gas was inserted, PDMS was not etched at all. At a fixed O2 flux of 100 sccm and a microwave power of 800 W, the etch rate of PDMS increased from 1.81 μm min−1 to 2.73 μm min−1 and finally to 4.31 μm min−1 as the CF4 flux was increased from 100 sccm to 150 sccm and to 200 sccm, respectively. On the other hand, at a fixed CF4 flux of 200 sccm and a microwave power of 800 W, the etch rate of PDMS decreased from 4.31 μm min−1 to 3.44 μm min−1 and to 2.29 μm min−1 , respectively, as the O2 flux increased from 100 sccm to 150 sccm and to 200 sccm, respectively. The measured PDMS etching rate of 4.31 μm min−1 is more than ten times faster than the PDMS etching rate under RIE [20]. It is believed that when only CF4 is injected into the processing chamber, etching is hindered because the production rate of polymers with a Cx Fy form on the PDMS 4
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(a)
(b)
(c)
(d )
Figure 6. AFM images of the etched open-PDMS surface for various O2 flow rates (sccm) at 800 W: (a) O2 :CF4 = 100:200, (b) O2 :CF4 = 150:200, (c) O2 :CF4 = 200:200 and (d) O2 = 200.
the microwave power was increased from 400 W to 600 W and to 800 W, respectively. These results indicate that an increase in the microwave power directly enhances the ionization of etching gases, excitations and the degree of dissociation of gas molecules. The surface roughness (Ra ) as a function of the etch condition is listed in table 1. The surface roughness of the ˚ After etching original PDMS was measured to be 1.8 A. under process conditions 1–9, the surface roughness increased ˚ However, when only CF4 was to a range of 7.21–14.46 A. injected (process conditions 10 and 11), the surface roughness ˚ Figure 6 shows the results increased greatly up to 36.26 A. of the AFM analysis of the etched PDMS surfaces when the level of O2 is increased. When only O2 was injected, as shown in figure 6(d), minute cracks seemed to appear on the PDMS
surfaces and, consequently, the surface roughness increased significantly. This was attributed to conversion of the PDMS surface to brittle SiOx layers as a result of O2 , which in turn gave rise to the observed minute cracks [42]. 4.2. The etch characteristics of shadow-masked PDMS After a nickel shadow mask was attached to the PDMS surface by applying strong pressure, the PDMS was etched. The etch conditions for shadow-masked PDMS were identical to those for open PDMS, as listed in table 1. Table 2 displays the measurement results for the vertical and lateral etch rates of shadow-masked PDMS in line with the etch conditions. Figures 7 and 8 are graphs of the vertical and lateral etch rates, respectively, as functions of the flux rates of CF4 and O2 gases at a fixed microwave power of 800 W. The vertical etch rates 5
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Table 2. The etch rate of shadow-masked PDMS as a function of the gas mixture and the electric power. No
O2 (sccm)
CF4 (sccm)
Power (W)
Pressure (mbar)
Vertical etch rate (μm min−1 )
Lateral etch rate (μm min−1 )
1 2 3 4 5 6 7 8 9 10 11
0 100 100 100 100 150 150 200 100 100 200
200 200 200 200 150 150 200 200 100 0 0
800 800 600 400 800 800 800 800 800 800 800
0.531 0.863 0.837 0.816 0.785 0.846 0.937 1.002 0.710 3.14 0.617
0 4.12 2.28 0.56 2.46 2.11 3.06 2.08 1.71 0 0
0 2.8 1.8 0.34 2.0 1.6 2.6 1.8 1.5 0 0
O2 flow rate (sccm) 0
50
100
150
200
250
4
5 4 3 2 1
50
O2 flow rate (sccm) 100
150
200
250
O2 100 sccm, 800W, Fixed
O2 100 sccm, 800W, Fixed CF4 200 sccm, 800W, Fixed
Lateral etch rate (µm/min)
Vertical etch rate (µm/min)
6
0
CF4 200 sccm, 800W, Fixed 3
2
1
Shadow-masked PDMS
Shadow-masked PDMS 0
0 0
50
100
150
200
0
250
50
100
150
200
250
CF4 f low rate (sccm)
CF4 flow rate (sccm) Figure 7. The vertical etch rate of shadow-masked PDMS as a function of the O2 /CF4 flow rates at 800 W.
Figure 8. The lateral etch rate of shadow-masked PDMS as a function of the O2 /CF4 flow rates at 800 W.
of shadow-masked PDMS were slightly lower than those of open PDMS. The vertical etch rate of PDMS increased from 1.71 μm min−1 to 2.46 μm min−1 and to 4.12 μm min−1 as the CF4 flux was increased from 100 sccm to 150 sccm and to 200 sccm at a fixed O2 flux of 100 sccm and a microwave power of 800 W. On the other hand, at a fixed CF4 flux of 200 sccm and a microwave power of 800 W, the vertical etch rate of PDMS decreased from 4.12 μm min−1 to 3.06 μm min−1 and to 2.08 μm min−1 , respectively, as the O2 flux was increased from 100 sccm to 150 sccm and to 200 sccm. The variation of the lateral etch rate with the etch conditions (figure 8) was similar to that of the vertical etch rate (figure 7). The highest lateral etch rate was 2.8 μm min−1 under process conditions of an O2 flux of 100 sccm, a CF4 flux of 200 sccm and a microwave power of 800 W. The ratio of the vertical etch rate to the lateral etch rate was in a range of 1.14–1.64 and varied with the gas fluxes. Figure 9 graphically presents the vertical and lateral etch rates of shadow-masked PDMS as functions of the microwave power at a fixed gas flux of (200 sccm)/(100 sccm) for CF4 /O2 . As the microwave power was increased from 400 W to 600 W and to 800 W, the vertical etch rate increased from
0.56 μm min−1 to 2.28 μm min−1 and to 4.12 μm min−1 , and the lateral etch rate increased from 0.34 μm min−1 to 1.8 μm min−1 and to 2.8 μm min−1 . While the vertical etch rate linearly increased with the microwave power, the lateral etch rate did not vary linearly. This nonlinear variation of the lateral etch rate is attributed to insufficient in-draft of fresh gases into the laterally etched cavity and insufficient efflux of volatile etching products from the laterally etched cavity. This phenomenon is heightened as the microwave power is increased. Figure 10 shows SEM and three-dimensional confocal microscopic images of a PDMS sample that was etched for 10 min at 150 sccm O2 , 150 sccm CF4 and 800 W.
5. Application 5.1. Fabrication of PDMS microstructures Figure 11 shows an SEM image of array-type PDMS microwells. To obtain the PDMS microwells, etching was conducted for 10 min using a nickel shadow mask under process conditions of 150 sccm O2 , 150 sccm CF4 and 6
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500 µ m
Figure 9. The etch rate of shadow-masked PDMS as a function of the microwave power.
Figure 11. An SEM micrograph of plasma-etched PDMS microwells using a nickel shadow mask.
for 20 min at 100 sccm O2 , 200 sccm CF4 and 800 W. Network-type microstructures as well as standalone-type microstructures can be fabricated through this etching procedure. Microprotrusions reduce the contact area between water and the surface, and thus the surface energy is decreased, resulting in an increase of hydrophobicity [43]. The contact angle was dramatically increased from 104◦ (non-etched PDMS surface, figure 12(b)) to 148◦ (etched PDMS surface, figure 12(a)) such that the surface was modified to be superhydrophobic. The water used for measurement of the contact angles was deionized (DI) water and the volume of a water droplet was 4 μl. The measured contact angle was taken as the average obtained from five measurements.
(a)
5.2. Removal of PDMS residue Figure 13(a) is a residual PDMS skin with a thickness of 4 μm that is caused by imperfect contact between the top and bottom molds [18]. This kind of residual PDMS skin can be easily removed through microwave plasma etching. The PDMS with residual skin shown in figure 13(a) was etched for 2 min under process conditions of 150 sccm O2 , 150 sccm CF4 and 800 W. As a result, the residual PDMS skin was completely removed, resulting in opening of the hole, as shown in figure 13(b). When PDMS casting is conducted using a mold with high-density and high-aspect-ratio microstructures, PDMS can adhere to the narrow and deep gaps between neighboring microstructures. Figure 14(a) shows PDMS residues that were stuck in the nickel micromold following the PDMS casting process. The dimensions of the micropillars formed on the mold were 40 μm (width) × 40 μm (length) × 50 μm (height) and the gap between the neighboring pillars was 10 μm. To remove the PDMS residues shown in figure 14(a), PDMS etching was conducted for 30 min at 150 sccm O2 , 150 sccm CF4 and 800 W. As a result, the PDMS residues were completely removed and the nickel micromold was restored to its original shape, as shown in figure 14(b).
(b)
Figure 10. An SEM and a 3D confocal micrograph of etched PDMS using a nickel shadow mask (diameter: 100 μm, pattern gap: 100 μm): (a) SEM image and (b) 3D confocal microscope image.
800 W. Figure 12(a) shows network-type microprotrusion structures. These structures were created by etching PDMS 7
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(a)
(b)
Figure 12. SEM images of the surfaces of PDMS and measured contact angles: (a) after etching through the nickel shadow mask and (b) before etching. Residual PDMS skin 4µm
100 µ m
100 µ m
Figure 14. SEM images of a nickel mold with dimensions of 40 μm (width) × 40 μm (length) × 50 μm (height) and a gap of 10 μm: (a) PDMS residue stuck in the nickel microstructures and (b) PDMS residue eliminated through microwave plasma etching.
Figure 13. (a) An SEM image of a micro-hole that is clogged with a residual PDMS skin and (b) an SEM image of a micro-hole after removal of the PDMS skin with microwave plasma etching.
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6. Conclusion We introduced a new dry-etching method for PDMS using microwave plasma and verified its applicability for the fabrication of microstructures and removal of residual PDMS. PDMS etching has been performed with and without a shadow mask. The etch rate of PDMS was dominantly influenced by the gas-flux ratio of CF4 /O2 and the microwave power. From the point of view of the gas-flux ratio, while the PDMS etch rate increased as the flux ratio of CF4 was increased, the etch rate decreased as the flux ratio of O2 was increased. The maximum etch rate of PDMS without a shadow mask was 4.31 μm min−1 under process conditions of 200 sccm CF4 , 100 sccm O2 and 800 W microwave power. This high PDMS etch rate is more than ten times faster than the etch rate under RIE. When only one gas was injected into the processing chamber, PDMS was not etched at all. From the point of view of microwave power, the PDMS etch rate almost linearly increased with the microwave power. PDMS etching has also been performed with a nickel shadow mask comprised of circular shapes with a diameter of 100 μm arranged at 100 μm intervals. The vertical etch rates of PDMS with the shadow mask were slightly lower than those of PDMS without a shadow mask. The ratio of the vertical etch rate to the lateral etch rate was in a range of 1.14–1.64 and depended on the gas fluxes. Executing PDMS etching with a shadow mask, arraytype PDMS microwells and network-type microprotrusion structures were fabricated. The contact angle was dramatically increased from 104◦ (non-etched PDMS surface) to 148◦ (etched PDMS surface) such that the surface was modified to be superhydrophobic. In addition, the proposed PDMS etching was utilized to remove PDMS residues. A residual PDMS skin with a thickness of 4 μm was successfully removed by using microwave plasma etching, which resulted in opening of the hole. PDMS residues that had adhered to high-density and high-aspect-ratio nickel microstructures were completely removed and the nickel micromold was restored to its original shape. We verified that the proposed PDMS etching method using microwave plasma can serve as a very useful way of creating various PDMS nano-microstructures and removing PDMS residues.
Acknowledgment This work was supported by the Samsung Advanced Institute of Technology (SAIT) grant funded by the Ministry of Commerce, Industry and Energy Republic of Korea (no. 20060150000).
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