Inductively coupled plasma reactive ion etching is used to fabricate the monolithic beam splitter in silicon-on-insulator wafer. The near-field image shows that the symmetric 1×2 T-branch works well. The rms roughness of the corner mirror surfaces is measured by atomic force microscope, and the sidewall surface roughness of rib waveguide is evaluated by the corner mirror rms roughness. The scattering losses from the rough sidewall surfaces and the rough mirror surfaces are evaluated to be 0.5 dB/cm and 0.2 dB/mirror, respectively. And the fiber-waveguide insertion loss is measured approximately 5.0 dB.
©2004 Optical Society of America
In recent years, small radii of curvature in optical communication systems are highly desired to reduce the total chip size of integrated optical circuits. Corner mirrors offer a solution to achieve the reduction of device size. To induce abrupt changes of direction of the propagating light in semiconductor waveguides, corner mirrors that exploit total internal reflection at an interface to air are commonly employed. Such corner mirrors have been investigated by different groups in various waveguide systems [1, 2, 3, and 4]. Corner mirrors realize several kinds of integrated optical devices, such as integrated beam splitters, modulators, amplifier arrays and ring lasers.
Recently, much research work has focused upon the silicon-on-insulator (SOI) material system [5, 6, 7]. The top Si layer of the SOI substrate allows for the fabrication of high-index-contrast waveguide. Intel has fabricated an SOI-based Mach-Zehnder Interferometer (MZI) optical modulator with a modulation bandwidth of 2.5 GHz around 1.55 μm, which was based on a metal-oxide-semiconductor (MOS) capacitor . And it illustrated the concept of hybrid optical integration . In the art vision, the SOI material enables the monolithic integration of SOI-based optics and electronics on a single substrate.
A three-mask lithography process is used to fabricate the monolithic beam splitter in the SOI material system. Such a monolithic beam splitter realizes the integration of rib waveguide, corner mirrors and T-branches. Inductively coupled plasma reactive ion etching (ICPRIE) is used to form self-alignment U-groove, rib waveguide and corner mirrors. Atomic force microscope (AFM) is carried out to demonstrate the corner mirror rms roughness [10, 11]. The scattering losses induced from the corner mirror surfaces and the sidewall rough surfaces are evaluated by using the rms roughness data. We measure the fiber-waveguide insertion losses as the ratio between the output and input powers.
The monolithic beam splitters are realized in a separation by implanted oxygen (SIMOX) SOI wafer. The SIMOX process basically consists of two steps: first, a heavy dose of oxygen is implanted on a standard silicon substrate. Second, a thermal annealing is performed to obtain a buried SiO2 layer beneath a Si overlayer. Then the epitaxial growth is performed to attain single-crystal silicon with the desired height. The starting material in our work has a three-layer structure:_a typical 8.5 μm thickness single-crystal silicon layer on the top, a 375 nm thickness silicon dioxide layer in the middle, and a 520 μm thickness crystalline silicon substrate layer. The SOI wafer is cleaned and prepared for lithography using standard wafer preparation techniques. ICPRIE is a common processing technique for the fabrication of channel waveguides. A series of SOI waveguides with new fiber-waveguide endface are fabricated on a SOI wafer using a three-mask lithography process. The first lithography and timed ICPRIE steps define and form beam splitter structures. SOI rib waveguides are etched by using a SF6/O2/C4F8 gas mixture under the etching conditions of SF6/O2 130 sccm/13 sccm, the passivation condition of C4F8 85 sccm, 10 mTorr of working pressure, and 600 W inductive power. Upon a further wafer cleaning, a second lithography process and timed ICPRIE steps are performed to form corner mirrors. At the same time, the shallow U-groove channels for fiber alignment and the optical quality waveguide endface are achieved. Using the buffered oxide etch (BOE) cleaning, the buried silicon dioxide layer is removed. And a third lithography process and timed ICPRIE steps are used to form deep U-grooves. Large ribs also make possible low loss couplings to optical fibers as a result of the good fiber-waveguide fundamental modes overlapping. For our device, the rib width is 6 μm, the inner rib height is 8.5 μm, and the etching height is 3 μm, which conform to the single-mode condition .
3. Results and discussion
The micrograph in Fig. 1(a) shows a top view of the 1×3 monolithic beam splitter. The top-view of the symmetric T-branch is shown in Fig. 1(b). The dimensions of the T-branch are depicted directly on Fig. 1(b), and the branch length is approximately 125μm. The inset shows the near-field image of the single-mode 1×2 T-branch. It can be found that the T-branch works well though there exists distortion to some extent when the near-field image is transferred to computer from CCD monitor system. As shown in Fig. 1(c), a rectangle groove is etched to form the corner mirror.
The scattering losses from the rough sidewall surfaces and the rough mirror surfaces are the dominant scattering loss sources. The entire area of the corner mirror surface is approximately 8.5 μm×25 μm, and the area of rectangle is approximately 20 μm×35 μm. We saw the rectangle parallel to the corner mirror surface (as shown in Fig. 1(c)). The vertical mirror plane is about 90°±0.1°. Thus a special clamp fixes the corner mirror, so that the ultra sharp AFM tip can easily access the corner mirror surface of beam splitters perpendicular to the corner mirror surface. Under the same etch parameters, the sidewall surface roughness should be very similar to that of the corner mirror. And the sidewall surface roughness of SOI rib waveguide is evaluated by the corner mirror roughness.
A series of AFM measurements were carried out to demonstrate the rms roughness of the mirror surface. The rms roughness value was calculated according to the corresponding three-dimensional surface profile. The values for all the measurements are the average numbers and these results are reproducibility. A typical three-dimensional image of the mirror surface profile obtained by AFM is presented in Fig. 2(a). The entire scanned area is 2 μm×2 μm. Vertical striations on the corner mirror surface are clearly visible because of the characteristic of ICPRIE. Figure 2(b) shows the corner mirror profiles in the etching direction and X direction, respectively. X direction is perpendicular to the etching direction. The rms roughness in the etching direction is higher than that of the X direction. The rms roughness of the corner mirror surface is approximately 12.4±0.5 nm. The surface roughness can be improved with some processing techniques to reduce the scattering loss. Kevin K. Lee et al. have reduced the rms roughness of the sidewall surface by oxidation smoothing and anisotropic etching . The two methods would change the configuration of the rib waveguide. In order not to change the shape of the rib waveguide, we have adopted hydrogen annealing to smooth the sidewall surface obtained by three-mask lithography. After hydrogen annealing, there is a significant drop in the rms roughness of the sidewall surface . And the value of surface roughness can be reduced to 8 nm by the multiple-step HARSE process using an ICP system .
Roughness of the mirror surface increases the scattering losses, and the scattering losses are theoretically analyzed by different groups [1, 16, 17, 18]. Akira Himeno et al. analyzed the scattering loss using the plane-wave scattering loss theory . And Shing Man Lee et al. have modeled the rough-surface effects in turning mirror using the finite-difference time-domain method . In their model, the simulation region can be shrunk to a small area containing only the details of the rough mirror surface, and then they calculated the forward-reflected and back-reflected powers of a guided mode from a rough turning mirror. Their results are in good agreement with that of reference . We estimate the scattering loss induced from corner mirror rms roughness using the analysis results of reference . The scattering loss is approximately 0.2 dB/mirror for a corner mirror with the rms roughness of 12.4 nm.
Sidewall surface induced scattering is proportional to σ2 where σ is the rms roughness [19, 20]. Tien has derived a convenient closed-form expression for scattering loss due to sidewall surface roughness, based on the Rayleigh criterion . We can take the rib as a symmetric waveguide. And the sidewall surfaces are the bottom Si/air interface and the top Si/air interface of the symmetric waveguide, respectively. Thus, the scattering loss induced by sidewall surface roughness can be evaluated using Tien’s theory. The scattering loss is evaluated to be approximately 0.5dB/cm with the rms roughness of 12.4 nm.
Figure 3 shows the monolithic fiber-wavguide endface etched by ICPRIE. The U-groove is used to locate the single-mode fiber in front of the rib waveguide. And the inset shows the cross-sectional morphology of anti-reflection coating on the rib waveguide endface. A special clamp fixes the SOI rib waveguide, and the waveguide endfaces are perpendicular to the deposition direction so that the anti-reflection coating can be easily deposited onto the waveguide endfaces.
We measured the fiber-waveguide insertion losses as the ratio between the output and input powers using Agilent 8164A lightwave measurement system. A laser beam from a Fabry-Perot laser with wavelength 1.55 μm is launched into the device without polarization, and the polarization sensitivity of the device is not measured in the present work. It is free-space coupling, and we use an Agilent 81624B InGaAs optical head to collect all the light at the output. The results at λ=1.55 μm are 5.0±0.5 dB and 5.2±0.5 dB, respectively, in the two output waveguides of the 1×2 single-mode T-branch The split ratio is nearly 52:48. In the measured 5.0 dB fiber-waveguide-fiber loss, 3 dB is due to the fact that the symmetric T-branch is designed to be 3dB at λ=1.55 μm, and the reflection loss is below 0.2 dB for both endfaces after the AR-coatings are deposited onto the both waveguide endfaces. The scattering losses induced from sidewall and corner mirror rms roughness are estimated to be 0.9 dB from the above analysis. The remaining 0.9 dB loss is the injection loss on the both endfaces. The resulting fiber-waveguide coupling efficiency is approximately η=0.91, which is close to the theoretical value .
4. Summary and conclusions
In conclusion, monolithic beam splitters are fabricated in the SOI wafer by ICPRIE. We focus on the symmetric 1×2 T-branch, and then the near-field image shows that the single-mode 1×2 T-branch works well. The rms roughness of the corner mirror surface is measured by AFM, and the sidewall surface roughness of the SOI rib waveguide is evaluated by the corner mirror roughness. The scattering losses from the rough sidewall surfaces and the rough mirror surfaces are 0.5 dB/cm and 0.2 dB/mirror, respectively. And the insertion loss for the 1×2 T-branch is measured approximately 5.0 dB as the ratio between the output and input powers.
This work was financially supported by the grant from the 863-programe of the Ministry of Science and Technology of China (No. 2001AA312070). The authors would like to thank Ms. Yuan Jiang for her help and enlightening discussion.
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