Low-Pressure Plasma-Etching of Bulk Polymer Materials Using Gas Mixture of
CF4 and O2.
Hirofumi Nabesawa,1,2 Takaharu Hiruma,1 Takeshi Hitobo,3 Suguru Wakabayashi,4
Toyohisa Asaji,3,5 Takashi Abe,6 and Minoru Seki1*
1
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering,
Chiba University, Chiba, 263-8522, Japan
2
Central Research Institute, Toyama Industrial Technology Center, Takaoka, 933-0981, Japan
3
Tateyama Machine Co. Ltd., Toyama, 930-1305, Japan
4
Tateyama Kagaku Industry Co. Ltd., Toyama, 930-1305, Japan
5
Electronic-Mechanical Engineering Department, Oshima National College of Maritime
Technology, Suo-Oshima, 742-2193, Japan
6
Department of Mechanical and Production Engineering, Graduate School of Science and
Technology, Niigata University, Niigata, 950-2181, Japan
* Corresponding author
Supplemental Information
Polymer Plate Pretreatment
PMMA plates with 2 mm thickness (softening temperature: 373K) and PFA plates with 1 mm
thickness (melting point: 583K) were cut to 25 mm2 and used as etching samples. The
arithmetic mean surface roughnesses (Ra) of PMMA and PFA were below 10 nm and 70 nm,
respectively. These were ultrasonically cleaned with deionized water and absolute ethanol
before the etching process.
Etching Systems
The magnetically enhanced RIE system used herein was originally developed for polymer
plate etching (see Supplementary Material (Figure S917). A high plasma density under a low
pressure condition, which supports high ion-kinetic-energy and high directivity of incoming
ions at the same time, was generated by six permanent magnets on the quartz cover located in
a circular disposition with successive opposite polarities. High ion-kinetic-energy contributes
to immediate micromask removal to form a smooth etched surface. As a result, the plasma
generated by this system was capable of etching polymer substrates with smooth etched
surfaces in an anisotropic manner. A coolant channel is embedded in the sample stage 10
mm under its surface to prevent the temperature increase of the thermoplastic samples.
Etching Conditions
All the etching procedures were carried out under the conditions of a chamber pressure of 0.1
Pa, a radio frequency power of 50 W, and a stage temperature of 273K. A gas mixture of 75
mol% O2 and 25 mol% CF4, and pure oxygen gas (total gas flow rate of 10 cm3 /min) were
adopted as an etching gas. Although there was no difference between the etch rates of the two
gases against the polymers, the surface roughness of the etched sidewall using the former gas
mixture was slightly superior to that using the latter.
Therefore, a 75 mol% O2-CF4 gas
mixture was used for investigating the characteristics of shape change for PMMA and PFA.
Because the CF4-containing gas mixture might form fluorocarbon groups on the etched
sidewall, pure oxygen gas was used for measuring the wettability of PMMA and PFA, as well
as for fabricating a high-aspect-ratio pillar array or stacked spheres on the pillar array
structure. The etch rates for the PMMA plate, PFA plate, PS sphere, and silica sphere using a
75 mol%O2-CF4 gas mixture were 0.42 m/min, 1.04 m/min, 0.18 m/min, and 30 nm/min,
respectively. In a previous report, it was suggested that the reason why the etch rate of PFA
plates is higher than that of PMMA plates, is the increase in the atomic fluorine concentration
due to the generation of fluorine as an etching product of PFA or the fragmentation of
fluorinated products in the plasma.18 On the other hand, the lower etch rate of PS spheres is
caused by the chemical structure of PS, which includes a thermochemically stable aromatic
rings.19, 20 The uniformity of all the etch rates was within 10%.
Figure S1. Schematic flow chart for different etching procedures to fabricate various array
structures with high aspect ratio and narrow gap.
Figure S2. SEM photographs of the array structures of pillar, frustum, and cone formed on
PFA plates by use of various diameters of PS spheres as etching mask. The etching durations
are shown on the upper left corner of each picture.
(a)-(c); 0.5 m spheres.
spheres.
(d)-(f); 1.0 m spheres.
(g)-(i); 2.1 m spheres.
(j)-(l); 3.0 m
Figure S3. Time courses of variation of PMMA micropillar height during the etching process
using PS spheres with different diameters as etching mask.
Figure S4. Lower base diameter of the etched PMMA micropillar with respect to etching
duration.
(a)
(b)
Figure S5. Applications of PMMA frustum array structure.
(a): Optical waveguide array: Visible light radiation from top surface of frustum arrays,
whose lateral faces were sputter-coated with gold.
(b): Fluorescence frustum array: Fluorescence by red and green fluorescent microspheres,
which were placed on the top of the frustum arrays as masks.
Figure S6. Time courses of variation of PFA micropillar height during the etching process
using PS spheres with different diameters as etching mask.
Figure S7. Lower base diameter of the etched PMMA micropillar with respect to etching
duration.
(b)
(a)
3 m
(c)
10 m
3 m
Figure S8. SEM photographs of stacking PS-coated silica spheres on the etched PMMA
micropillar array structure; a) array structure. b) two-spheres on the pillar, and c)
three-spheres on the pillar.
Figure S9. Schematic illustration of the magnetically enhanced RIE system.