In this paper, we present a new concept and fabrication of micro optical switch of which application is transmissive
display devices. The micro optical switch consists of two parallel plate electrodes and is driven by electrostatic force.
The first electrode is patterned on the glass substrate and the second electrode is disposed spaced apart from the first
electrode. Each electrode has holes and the holes in each electrode do not overlap with one another. Light passes through
the holes in the second electrode via the holes in the first electrode. All dimensions and fabrication process of the micro
optical switch were designed to be compatible with Liquid Crystal Display (LCD) fabrication process. The size of the
fabricated micro optical switch was 254μm × 254 μm. The micro optical switch was fabricated by surface
micromachining. Aluminum was used as electrodes and patterned by plasma etching process. Photoresist was used as a
sacrificial layer, which defined the gap between the two electrodes. Plasma ahsing was used to remove the sacrificial
layer. Finally, anti-stiction layer was coated by Molecular Vapor Deposition (MVD) process. When voltage was applied,
the second electrode moved down and contacted the first electrode. When voltage applying stopped, the second electrode
returned to its original position. The voltage required to pull in the second electrode was below than 15 V. The sum of
transition time, from on to off-state and from off to on-state, was below than 100 ㎲ and operating frequency was more
than the 10 kHz.
This paper presents a micro electromechanical system (MEMS) based micro optical switch for display devices. The size
of the micro optical switch was 254 μm × 254 μm. The micro optical switch moves vertically by electrostatic force and
consists of two parallel plate electrodes which have rectangular aperture array. The lower electrode was directly
patterned on a glass substrate and the upper electrode was suspended by cantilever spring which was supported by
anchors. The electrodes were made of thin aluminum film and the gap between two electrodes was 3.3 μm. The thickness of the electrodes was 0.3 μm respectively and the lower electrode was covered with 0.3 μm silicon dioxide. The width of the aperture and the metal were 8 μm and 5 μm respectively. The upper aperture array was aligned with the lower aperture array with 6.5 μm offset horizontally and the overlapping width of the two electrodes was 1.5 μm. The micro optical switch is in open state when no voltage is applied and the light passes through the two electrodes by the leakage or the reflection between the two electrodes. The micro optical switch is in close state when voltage is applied and the light is reflected to the backlight unit. The required voltages for pull in and pull out were 16.0 V and 11.4 V respectively. The switching time was 30 μs from the open state to the closed state and 50 μs from the closed state to the open state.
Single layer of aluminum film was sputter deposited on to (100) oriented 4 inch silicon wafer to study effect of film thickness, D.C. power and sputtering gas pressure on the film stress. The as-deposited stress appeared to be increasing as film thickness increases and argon pressure decreases. Thermal stress originated from difference in CTE and temperature variation during and after sputtering seems to be a main factor in room temperature sputter deposited aluminum films. From observation of temperature-stress behavior, it was found that the pure aluminum film has an elastic modulus of 56GPA and compressive yield strength of -100MPA. The yield strength was improved to about -175MPA by alloying with 3wt.%Ti. Titanium alloying also proved to be useful in extending linear elastic region before plastic deformation occurs. However, it was hard to determine the stress level with buckling phenomena of ring/beam microstructures because of imperfections such as stress gradient and thermal deformation. In stead, those diagnostic microstructures could be applied to give an information on whether a plastic deformation was introduced or not in a structure of specific dimension.