Chapter 6 Micro Reflective System
6.2 Fabrication
The device was fabricated on a silicon-on-insulator (SOI) wafer with a 5 μm thick silicon device layer and a 2 μm thick buried silicon dioxide layer. The fabrication
Fig. 6-5. Process flow for the planar micro-reflective device.
process is illustrated in Figure 6-5. A 0.5 μm thick thermal oxide layer on the front-side and a 4-μm thick PECVD oxide layer on the backside were grown as the hard mask for silicon etching. After patterning the waveguides, parabolic mirrors, prisms, and thermal
based chemistry (step a). After stripping the photoresist, the front-side silicon device layer was dry etched using HBr based chemistry to the buried oxide. The wafer was then dipped in BOE for 1 minute to remove the oxide-like byproduct from the HBr dry etching process. After patterning the backside using thick photoresist, the backside silicon dioxide layer was etched (step b). The backside silicon was etched to the buried oxide layer by a deep reactive ion etching (DRIE) using Bosh process, which consists of cycled etching and passivation steps (step c). The wafer was then released using HF vapor at 40 °C. A shadow mask was used before evaporating the anti-reflection layer, ZrO2, twice by e-beam under 30° tilting to coat the sidewalls of the Si-slab and the prism, respectively (step d). A composite metal film, 50 Å Ti/ 800 Å Pt, was finally coated as the heat source in the actuator area using another shadow mask. The SEM of the fabricated planar micro-reflective system is shown in Fig. 6-6.
(a) (b)
Fig.6-6. SEM image of the fabricated PRS (a) The entire PRS including the waveguide to guide the incident light. (b) The enlarged view of the prism and the parabolic reflector.
6.3 Experimental Results and Discussion
TE mode) was used. A polarizer and a quarter wave plate were adjusted to obtain the required polarization states. The experimental setup is shown in Fig. 6-7. The beam was coupled into a rectangular waveguide, 5 μm wide and 5 μm high, from a lensed fiber tilted by 45 degrees as shown in Fig. 6-8(a). At the end of the rectangular waveguide was a curved mirror and a plane mirror, which were used to collimate and direct the
Fig. 6-7. Experimental setup for the optical measurement of the PRS.
beam to the PRS, respectively. The prism of the PRS was actuated by applying a DC bias across the pads of the two V-shape electro-thermal beams to change the optical path of the beam. After outputting from the PRS, the beam was reflected by an air-gap based mirror and redirected to the facet of the chip. Infrared (IR) images of the optical beam were observed at the facet of the chip as shown in Fig. 6-8(b), which verified the dynamic switching capability of the PRS. The reflection angle as a function of displacement of the prism in the PRS is shown in Fig. 6-9. The calculated value is based on the Gaussian beam approximation and geometry of the parabolic mirror and prism. The maximum equivalent scan angle is 38° at a prism displacement of 45 μm.
The discrepancy from the calculated value is partly due to the linewidth deviation during the fabrication process and the instability of the electro-thermal actuator under
high temperature.
(a) (b)
Fig. 6-8. The infrared near field images of the optical beams at (a) the front end of the rectangular waveguide, which is the input spot; and (b) the facet of the chip, which is the output spot.
0 5 10 15 20 25 30 35 40 45
0 10 20 30 40 50 60
Prism Displacement (micron)
Reflection Angle (degree)
Calculated Measurement
Fig. 6-9. The reflection angle as a function of prism displacement.
The maximum equivalent angle could be increased in principle by designing an actuator with larger displacement. As compared with the previous design proposed by Chi[58], the equivalent scan angle of our device is three times larger. Besides, when
applied in a waveguide system, there are no these drawbacks: (a) the position of the reflection point varies with rotation angle, (b) the rotation angle is reduced when the light beam re-enters the silicon slab, and (c) the larger air gap between the slab and the mirror results in high diffraction loss. The illustration of these drawbacks is shown in Fig. 6-10. The effective rotation angle inside the Si PRS is reduced by n, the refractive index of silicon, according to the Snell’s law:
n n
air si
θ θ θ = − sin )≅
( sin 1
The position of the reflection point varies from R1 to R2 as the mirror rotates. The air gap between the Si slab and the mirror also increases with scan angle, which induces high diffraction loss.
Fig. 6-10. Schematic of the light reentering the silicon slab.
6.4. Summary
Using a parabolic mirror and a prism, the micro reflective system, which reflects the incident beam to large angle under constant air gap and fixed reflection point, has been demonstrated. The maximum equivalent scan angle is 38°, which is almost three times larger as compared with that of conventional micro rotary mirror. The optical
performance of the micro-device shows its potential application in planar waveguide optics. The concept can be also applied in a surface micro-machined pickup to realize a CD/DVD/HD-DVD compatible system. As illustrated in Fig. 6-11, the prism and the parabolic reflector can be replaced with a set of mirrors and a vertical parabolic reflector, respectively.
Fig. 6-11. Schematic of a CD/DVD/HD-DVD compatible micro-optical pickup, which uses a surface micro-machined linear actuated mirror and a vertical parabolic reflector to switch between each light source.
Chapter 7
Conclusions and Future Works
7.1 Conclusions
The combination of optics and micro-fabrication technologies has emerged as a new branch of science during the past 10-20 years and is gradually making its way towards commercialization in a number of fields. Because of its capabilities of miniaturization and design flexibility, micro-optical devices have become the key technology for building compact optoelectronic systems.
Micro-optical pickup, a kind of optoelectronic system, has the desired features of compact size, light weight, low power consumption and batch-fabrication. All these properties of micro-optical pickups can fulfill the requirements of the next generation optical storage applications, which are revolutionizing our society.
In this thesis, we demonstrated three novel micromachining devices to be applied in the next generation micro-optical pickups. The first is a micromachined free-space switchable grating, which can be used to realize a multi-beam optical pickup. The second is a stacked micro polarization beam splitter, which can be used with a quarter wave plate to achieve high extinction ratios and realize a high efficiency micro-optical pickup. The third is a planar reflective system, which is consisted of a prism and a parabolic reflector. The concept has a potential to realize a CD/DVD/HD-DVD compatible surface micro-machined optical pickup. Based on these experiences, we propose a new type micro-optical pickup to fully take advantages of the above surface micro-machinined devices in the future work.
7.1.1 Switchable grating in a micro optical pickup
In some conventional optical pickups, using multiple beams in parallel is a straightforward solution to improve the data rate. However, weight, large volume, adding more components and intensive assembly are the main issues of conventional solutions.
In order to improve these drawbacks, we demonstrated a silicon-based micro switchable grating, which can be used to switch between single-beam recording and multi-beam reading. This device was composed of a binary phase grating and a stress-induced actuator. Low stress silicon nitride was used as the optical material of the binary phase grating due to its high transparency in the visible spectrum and its superior chemical and mechanical properties. The switching can be easily controlled using an electrostatically driven stress-induced curved actuator. Furthermore, the actuator and the grating were batch-fabricated using a two-layer poly-silicon and one-layer silicon nitride micro-machining process. The measurement showed that three diffracted beams with nearly equal intensity from the grating were generated when a voltage was applied to the actuator to switch its position. The single-beam and multi-beam configurations can be used for writing and reading optical disc, respectively. By using these well-developed fabrication processes and smart designs, the switchable grating integrated with other micro-optical components on a silicon substrate can be produced economically and reproducibly in large volume.
7.1.2 High-extinction-ratio Polarization Beam Splitter
In some optical pickups, polarizing beam splitters (PBSs) are used with a quarter wave plate to simultaneously produce high TM-mode transmitted light and high TE-mode reflected light. The PBS is usually realized using a larger-size birefringence
crystal or a high-cost films-coated glass. However, in a miniature optical system, it is difficult to realize such a device at low cost and compatible process.
Our proposed high-extinction-ratio PBS possesses the similar function of a conventional bulky PBS. This PBS used low-absorption silicon nitride layers for blue wavelength applications and consisted of novel stack of two silicon nitride layers separated by an air gap. An optimized PBS model was developed to achieve high extinction-ratio for polarized light with an adequate surface micromachining margin.
The polarization extinction ratios of 25 dB for the reflected light and 15 dB for the transmitted light were experimentally achieved at λ=405 nm. Furthermore, the fabrication of the PBS is compatible with other diffractive elements and can be used to build up a micro optical bench for short wavelength optical storage applications.
7.1.3 Micro Reflective System
In the development of optical pickups, the current trend is towards being CD/DVD/ HD-DVD compatible. To realize this function in a micro-optical pickup, one of the solutions is to use a rotary actuated mirror to assist switching between each light source. However, the maximum rotation angle of conventional surface micromachined rotary actuator is below 2.5°. Increase the distances between the laser diodes and the rotary actuated mirror is a straightforward way to solve the issue of low rotation angle of the actuator, but the side effect is the chip size would triple.
Using a parabolic mirror and a prism, the micro reflective system, which reflects the incident beam to large angle under constant air gap and fixed reflection point, has been demonstrated. The maximum equivalent scan angle is 38°, which is almost three times larger as compared with that of conventional micro rotary mirror. The concept can be applied in a surface micro-machined pickup to realize a CD/DVD/HD-DVD
compatible system by replacing the prism and the parabolic reflector with a set of mirrors and a vertical parabolic reflector, respectively.
7.2 Future works
The proposed microoptical components, the switchable grating, the polarized beam splitter, and the micro reflective system, successfully demonstrated to enhance the function of micro-optical pickups. Their applications may not only be limited to the micro-optical pickup systems. For example, the switchable grating can be applied in the optical communication as a dynamic beam steering device. Polarized beam splitters can combine with actuators as an array of filters for optical signal processing.
In this dissertation, although the novel functions of the devices were demonstrated, the realization of a commercial micro-optical pickup is still limited by the following challenges:
(a) Efficiency: The optical efficiency of each element is limited to the available quantized levels and minimum line-width. The total efficiency of the pickup is also significantly reduced as the number of elements increases. To enhance efficiency, a finite conjugate system with fewer optical elements should be adopted.
(b) Material: The optical performance suffers from high curvature of the polysilicon frame and the optical film itself due to the residual stress. This issue can be solved by using a thicker and stress-less single crystal silicon film to replace the poly-silicon film.
(c) Release: HF solution is used to remove the silicon-dioxide sacrificial layer.
However, it also damages the silicon nitride optical layer. The solution of this process issue depends on a more smart design of the pickup by reducing the
contact time of the optical film with HF solution during the release step.
(d) Objective lens: To fabricate a SiN diffractive element with NA> 0.5 requires a minimum line-width of 0.3 μm. This is beyond the capability of current etching tool. Besides the optical performance of aspherical surfaces in both sides of the objective lens can’t be fully realized using a single diffractive element, which also induces optical aberration.
To cover these challenges, a finite conjugate system based micro optical pickup is proposed. It combines the advantages of the simple fabrication of the stacked type pickup and the design flexibility of the surface micro-machined type pickup. The structure is consisted of one glass, two general wafers, and one silicon on insulator wafer (SOI). The optical devices include a laser diode, a photo diode, a 45° upward beam splitter, a hologram, and an refractive objective lens, as shown in Fig. 7-1(a).
In the forward optical path, the beam splitter can be used to direct the incident light to the objective lens and the disc. In the backward optical path, the beam splitter is tranmissive. The HOE is used to produce asymetrical spot on the photo diode, which monitors the focusing status of the disc. As compared with a conventional optical pickup, the required optical elements are significantly reduced. Besides, the HOE is realized on a flat wafer with its back side being etched using the bulk etching, so no HF release procedure is required. The beam splitter is composed of a silicon nitride film mounted on a single-crystal silicon frame to reduce the curvature. The specification obtained from ZEMAX is shown in Fig. 7-1(b). The simulation value shows the dimension of the pickup is feasible using the current fabrication tools. The tolerance tilt of the 45° upward beam splitter is 0.08°, which is achievable using stacked bonding technology.
(a) (b)
Fig 7-1. A surface-micromachined optical pickup based on the finite conjugate design (a) and its specification according to ZEMAX (b).
The features of the new design include:
(a) 45° upward beam splitter, which is used to replace the beam splitter and the 45°
upward mirror in the previous design. In this way, the optical elements can be reduced.
(b) 5-μm silicon frame, which is supplied by a SOI wafer. Since most optical elements are attached within the thick silicon frame, the curvature can be reduced.
FEM simulation shows the deformation of the beam splitter with a 500-μm diameter is below 5.6e-3, as shown in Fig. 7-2.
(c) No laborious assembly. Since only the 45° upward beam splitter requires to be assembled, the yield of the new design can be enhanced as compared with the previous design.
(d) Smart mask design. There are no etch holes and dimples in the optical pattern, which induces no optical noise. To help release, a back side bulk etching is designed. All etch holes are defined in the side latches as shown in Fig. 7-3.
Fig 7-2. FEM shows the deformation of the beam splitter is below 5.6e-3.
Fig 7-3. Photograph of the smart mask design. The etch holes concentrate on the side latches.
(e) Astigmatic HOE. A hologram element (HOE) is used to monitor the working distance between the disc and the objective lens. It is based on a 4-phase levels design, as shown in Fig. 7-4.
(f) Reasonable S-curve. ZEMAX simulation in Fig. 7-5 shows the linear range of the focus error signal (S-curve) is 11 μm, which is comparable with that of the
Fig 7-4. Photograph of the 4-phase levels HOE.
Fig 7-5. Simulation result of focus error signal.
For future works, in the short term the HOE and BS in SOI wafer and the laser diode bonding on wafer#2 can be integrated together to demonstrate its optical performance. In the long term, this work can be further incorporated with the previous developed devices to realize a multifunctional commercial micro-pickup as shown in Fig. 7-6. It is a finite-conjugate optical reading/writing system including a semiconductor laser diode (LD), a photodetector, a switchable grating, a 3 layered stacked-film beam splitter, a hlographic optical element (HOE), and an objective lens.
state.
Disc Objective lens
Beam splitter
HOE
Photo diode Laser
diode
Wafer # 1 Wafer # 2 Glass # 3
Switchable grating
Fig 7-6. A surface-micromachined optical pickup based on the finite conjugate design.
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