Multiple-Height Microstructure Fabricated by Deep Reactive Ion Etching and Selective Ashing of Resist Layer Combined with Ultraviolet Curing
UV-cured photoresist is applied to the delay masking process to realize a multiple-height microstructure for the first time. Although the UV-cured photoresist is a soft mask, its material property becomes stable against resist thinner and UV exposure. A layered resist pattern can be realized by stac...
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| Published in | Japanese Journal of Applied Physics Vol. 51; no. 1; pp. 01AB04 - 01AB04-6 |
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| Main Authors | , , , , , |
| Format | Journal Article |
| Language | English |
| Published |
The Japan Society of Applied Physics
01.01.2012
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| Online Access | Get full text |
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| Summary: | UV-cured photoresist is applied to the delay masking process to realize a multiple-height microstructure for the first time. Although the UV-cured photoresist is a soft mask, its material property becomes stable against resist thinner and UV exposure. A layered resist pattern can be realized by stacking normal photoresist on the UV-cured photoresist. The UV curing increases the glass transition temperature from 120 to 160 °C. By controlling the temperature, the normal photoresist can be ashed twice as fast as the UV-cured photoresist. This selectivity can be combined with the deep reactive ion etching of Si, and a microstructure having three different etching depths is realized, thus forming a micromirror. The multiple-height structure is advantageous for reducing the inertia of the mirror plate. For the same rotational spring constant, the micromirror has a higher resonant frequency, which is required for realizing higher-resolution laser displays. |
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| Bibliography: | Process sequence for adding one layer of photoresist. (a) Underlying layer is normal resist after postbaking. (b) Underlying layer is UV-cured resist. (a) Micrograph of mesh pattern consisting of normal resist on UV-cured resist. (b) Micrograph when the sample is heated at 120 °C. (c) Micrograph when the sample is heated at 160 °C. The color change is attributed to the resist reflow. Perspective profiles of normal resist after (a) patterning, (b) heating at 120 °C, and (c) heating at 160 °C for 30 min. (d) Cross section indicated by dot-dash line AB in (a). Perspective profiles of (e) UV-cured resist and (f) after heating at 160 °C for 30 min. (g) Cross section indicated by dot-dash line CD in (e). Schematic drawing of the ashing equipment used to actively control the sample temperature. Ashing rate of normal and UV-cured photoresists. The resist used is AZ1500 (38 cp). The vertical error bar shows the standard deviation of the data measured at 5 points on the wafer. The pressure is 10 Torr. The gases used are 10 and 10 sccm of O 2 and Ar, respectively. The 13.56 MHz RF power is 450 W. Sample image showing remaining UV-cured resist after selective ashing of normal resist. Schematic drawing of drum-type micromirror consisting of rigid c-Si ring and thin membrane for decreasing the moment of inertia. H 1 , H 2 , and H 3 are the etching depths. Calculated moment of inertia of drum-type micromirror. The rotation center is assumed to be at the middle height of the torsion bar as illustrated in Fig. . Fabrication sequence of drum-type micromirror. The process is illustrated at the cross section labeled A--O--B in Fig. . Sample micrographs (a) before and (b) after selective ashing after process step 10 in Fig. . The thickness of the remaining resist in (b) is about 2 μm. (a) Top and (b) bottom views of elliptical micromirror. (a) Profile of circular micromirror. (b) Typical surface profile of mirror. The peak-to-valley height difference is about 52 nm. The poly-Si edge has a small protrusion. The faint traces of four small circles in the membrane are the holes shown in Fig. . They were originally designed as openings for the air flow, which were eventually not opened. This is because the holes in the membrane decrease the yield ratio significantly. (a) Schematic drawing of the experimental setup for inducing mirror rotation. (b) Mirror rotation angle as a function of driving frequency. The filled and open circles show the results for solid cylinder-type and drum-type micromirrors, respectively, shown as insets. The Q factor is about 72 for both micromirrors. |
| ISSN: | 0021-4922 1347-4065 |
| DOI: | 10.1143/JJAP.51.01AB04 |