FABRICATION AND CHARACTERIZATIONS OF SINGLE MODE OPTICAL WAVEGUIDE IN SILICON-ON-INSULATOR R.K. Navalakhe, N. Dasgupta and B.K. Das Department of Electrical Engineering, IIT Madras, Chennai - 600036 e-mail:
[email protected] Abstract: We have demonstrated the fabrication and characterization of single-mode optical rib waveguide (@ λ ~ 1550 nm) in silicon-on-insulator for the first time in India. The rib waveguide is defined photolithographically and fabricated by reactive ion etching. The single-mode guiding is verified by measuring the near-field output with an IR camera and the nearly polarization independent mode-size is found to be ~ 10 µm × 4.5 µm. We have also studied the polarization dependent waveguide losses by measuring the transmitted light outputs for various waveguide parameters. A typical waveguide loss has been measured to be < 1 dB/cm for single mode guiding. 1. INTRODUCTION There has been an increased interest in silicon for integrated optoelectronics, because of its transparency and low loss at the communication wavelengths (@ 1.3 µm and 1.55 µm) [1], and fabrication compatibility with CMOS technology on Silicon-On-Insulator (SOI) platform [2]. Many passive/active devices such as modulator [3], switch [4], filter [5], wavelength (de-)multiplexer [6], Raman laser/amplifier [7], detector [8] etc. have been demonstrated during recent years. The basic building block of these devices is the single mode optical waveguide. There are commonly two types of single mode waveguide structures in SOI: large crosssection rib waveguide (LCRW) [9] and smaller crosssection “photonic wire” waveguide [10]. The LCRW is preferred (over photonic wire) because it can be fabricated with conventional microelectronics technology. In addition to that light can be easily launched through a butt coupled fiber with a minimum optical loss as the mode-size of the guided light in LCRW is comparable to that of standard single-mode fiber. In this paper, we have studied design parameters and optimized the fabrication process of single-mode LCRW (@ λ ~ 1550 nm) using optical grade SOI substrate. The fabricated LCRWs have been analyzed by the measurements of guided mode profiles, waveguide losses and polarization dependencies for various waveguide parameters. 2. DESIGN OF SINGLE-MODE LCRW We have designed our single-mode LCRW based on the commercially procured optical grade SOI substrate (wafer size: 100 mm, device layer: 5 µm, BOX layer: 1 µm, substrate thickness: 500 µm). A conventional rib waveguide structure in SOI substrate has been shown schematically in Fig. 1. The 1-µm buried oxide (BOX) layer is used as lower cladding, so that substrate radiation can be avoided. The upper cladding can be either air or SiO2. The lateral confinement of light is possible because of the rib. By controlling the waveguide parameters of rib
width (W), rib height (H) and slab height (h), one can obtain single-mode guiding condition, optimum mode-size and required birefringence, etc. We have considered the following issues for the design of our LCRWs: (i) Single-mode guiding @ λ ~ 1550 nm. (ii) Optimum mode-size of the guided light to minimize the fiber to waveguide coupling loss (iii) Affordable sidewall roughness to minimize the scattering loss. Top Cladding Layer
W h
H
Mode Confinement
Device Layer
BOX: Bottom Cladding Layer (Thickness: ~ 1 µm)
Silicon Substrate (Thickness: 500 µm)
Fig. 1: Schematic diagram of an LCRW with symmetric slab heights in SOI substrate. Here W rib width, H - rib height, and h - slab height. 2.1 Single-Mode Condition: The rib waveguide structure of width W and height H with symmetric slab height h will support only the fundamental mode (@ λ ~ 1550 nm) if it satisfies the following expressions [9]:
r W ≤ 0.3 + H 1− r2 h r= ≥ 0.5 H
⎫ ⎪⎪ ⎬ ⎪ ⎪⎭
(1)
For the waveguide configuration shown in Fig. 1, single mode condition can be obtained with typical values of W ~ 4 µm and h ~ 4 µm for H = 5 µm. 2.2 Optimum Mode-Size: The mode-size/shape of guided light is an important parameter as it is directly
responsible for the coupling efficiency of laser light from a standard single-mode fiber to the LCRW. Coupling loss occurs because of mode mismatch between optical fiber and rib waveguide. Therefore, coupling loss is estimated by the numerical calculation of overlap integral [11]: +∞ +∞
Γ=
∫∫
E f ( x, y ) Ew ( x, y ) dxdy
(2)
−∞ −∞ 1/2
+∞ ⎡ +∞ 2 ⎤ E x , y dxdy Ew2 ( x, y ) dxdy ⎥ ) ⎢∫ f ( ∫ −∞ ⎣ −∞ ⎦
70 65 60 r = 0.5 55
r = 0.6 r = 0.7
50 2
4
6
8
10
Waveguide Width [µm]
Fig. 2: Coupling efficiency as a function of waveguide width and the r as parameter. It is clear from Fig. 2 that as waveguide width increases coupling efficiency decreases first and then increases. This is because at smaller waveguide width evanescent field is large and hence the coupling efficiency is increased. However, as the width increases, mode confinement increases and thus increases the coupling efficiency. Again as r is increased, the evanescent field enhances laterally resulting into higher coupling efficiencies. Therefore, by a suitable choice of r value, we can limit our waveguide width in the range of 2.5 -5.5 µm to achieve single mode guiding and maximum coupling efficiency as well as tight lateral confinement which is important for compact planar circuits. 2.3 Effect of Side-Wall Roughness: The side-wall roughness is very much influential for the waveguide losses, as they serve basically as the scattering points. The scattering loss can be expressed analytically by
⎞ ⎟g f e ⎟ ⎠
(3)
Where σ is the rms roughness, w is the waveguide width, k0 is free space wave vector, neff is the effective refractive index of guided mode, g is a function estimated by waveguide parameters and fe is a special function derived from the analytical assumption of roughness distribution [12]. The plot of scattering loss as a function of rms roughness is shown in Fig. 3, taking waveguide width as a parameter. It is clear from the plot that the scattering loss can be reduced by increasing the waveguide width. Because as the waveguide width increases, interaction of mode with sidewall decreases this in turn reduces the scattering loss. IT is evident from Fig. 3 that scattering loss can be limited to < 1 dB/cm if the rms roughness is < 10 nm. 15 w = 2 µm w = 4 µm 10
w = 6 µm w = 8 µm
5
0 0
r = 0.8 45 0
⎛ σ2 αs = ⎜ ⎜ 2 k w4 n 0 eff ⎝
Scatering Loss [dB/cm]
% Power Coupling Efficiency
Where Ef and Ew are the electric field profiles of the guided modes for fiber and waveguide, respectively. We have calculated the coupling efficiency between fiber and LCRW with a standard Gaussian mode-field distribution of fiber, where mode-field diameter is assumed to be 10 µm. Ew has been calculated by effective index method [11]. The calculated power coupling efficiencies as a function of waveguide width W, for different values of r have been shown in Fig. 2.
the equation [12]:
w = 10 µm
10
20
30
40
50
RMS Roughness [nm]
Fig. 3: Scattering loss as a function of rms roughness and waveguide width as a parameter. 3. FABRICATION We have fabricated LCRWs in optical grade SOI substrate with device layer thickness of 5 µm and BOX thickness of 1 µm. The waveguide widths (2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 5.5 µm) are defined photolithographically using Cr-coated mask plate which was also fabricated in our laboratory using Heidelberg Laser Writing System (DWL-66). The rib structure is fabricated by selective Si removal by a reactive ion etching (RIE) system. Since side-wall roughness plays an important role in waveguide losses, we have optimized the RIE process first so that a smooth rib walls can be achieved. 3.1 Optimization of RIE Process: We have studied the surface roughness created during RIE of silicon with SF6/Ar gas recipe for different gas flow rate and pressures, keeping the RF power and temperature constant at 150 watt and 20oC, respectively. Basically, four different samples have been etched with different etching parameters as shown in Table1.
Table -1: Parameters used for RIE optimization process Sample #. 1 2 3 4
SF6/Ar [sccm:sccm] 30:10 30:10 20:20 20:20
Pressure [mTorr] 100 200 100 200
Etch Rate [µm/min] 0.21 0.38 0.19 0.35
After each of the etching processes, we have studied the surface morphologies with an interferometric microscope as well as scanning electron microscope (SEM).
(a)
(c)
(b)
sample #1 followed by oxidation at 1000oC for 1.5 hour. 3.2 Waveguide Fabrication: For the optimization, we used two-step RIE process for fabrication of waveguide. One step was used for making the rib structure and the other for end face preparation. Two different approaches were followed: (a) End Face First Method: In this method end face was prepared first by etching 5 µm top Si layer, 1 µm BOX and then again 5 µm silicon in succession. Then rib structure was created. In this case waveguide was found to be tapered at the end and was not developed completely near end faces. This is because of edge build up created during second lithography. (b) Rib First Method: To overcome the problem associated with end face first method, we prepared rib first and then end faces. SEM image of end face fabricated by this method (see Fig. 5) is very smooth and light could be coupled. However, maximum light coupling was achieved when the end-facets were mechanically polished perpendicular to the waveguide axis.
(d)
Fig. 5: The SEM picture of waveguide end-facet prepared by RIE process.
(e)
(f)
Fig. 4: SEM pictures of different samples showing side-walls prepared by RIE process with different recipes tabulated in Table-1: (a) sample #1, (b) sample #2, (c) sample #3, (d) sample #4, (e) sample #1 after oxidation smoothening, and (d) rib waveguide fabricated with optimized recipe (see text for details). SEM images of the sidewalls for above four RIE processed samples have been shown in Fig. 4a - 4d. It is observed that the surface roughness can be reduced by decreasing the flow rate ration of SF6 and Ar. Analyzing the SEM pictures, sample #4 has been found to be best (vertical sidewalls with less roughness). We have also studied the effect of oxidation on sidewall roughness and we found that it can be reduced significantly if RIE is followed by oxidation. Fig. 4e is the SEM image of recipe of
4. WAVEGUIDE CHARACTERIZATIONS The fabricated waveguides were characterized with experimental setup shown in Fig. 6, in terms of guided mode profiles, insertion loss and polarization dependencies using a DFB laser source with an emission wavelength of 1550 nm. As expected, all the waveguides are single-moded (see inset of Fig. 6). A typical and nearly polarization independent mode-size is measured as ~ 10 µm × 4.5 µm for a 3µm wide waveguide (H = 5 µm, h = 4 µm), which is nearly matching our simulation results. We carried out the insertion loss measurement of fabricated waveguides (length ~ 1 cm) using the end fire coupling method (see Fig. 6). The corresponding results are given in Fig. 7 as a function of waveguide parameters.. It is evident from the figure that the total optical loss is > 9 dB. If we deduct the Fresnel losses at the end-facets of the waveguide (~ 3.5 dB), lens losses (~ 1.5 dB) and polarizer losses (~1 dB) and finally the mode mismatch losses (~ 2.5 dB), the waveguide losses can be approximated to < 1 dB/cm. It is also clear from Fig. 7 that the total loss decreases as the waveguide width or slab height
increases. It is also evident that lower slab heights (lower r) introduce some polarization dependent losses, which could be due to differential change in boundary conditions. PD / IR Camera L
P
L
DUT
L
P
FC
PC Computer
DFB Laser
Fig. 6: Waveguide characterization setup: PC – polarization controller, FC – fiber connector, L – focusing/collimating lens, DUT – device under test, PD – photodetector. Inset: measured mode profile of the guided light. r = 0.8 (TE) r = 0.8 (TM) r = 0.7 (TE) r = 0.7 (TM)
Total Loss (dB/cm)
14 13 12 11 10 9 2.5
3.0
3.5
4.0
4.5
5.0
5.5
Waveguide Width [µm]
Fig. 7: Polarization dependent total loss as a function of waveguide width and r as parameter. CONCLUSION We have designed, fabricated and characterized large cross section rib waveguide structure in SOI platform. The waveguides are characterized as single moded @ λ ~ 1550 nm and nearly polarization independent. Typical mode size and waveguide losses have been measured to be ~ 10 µm × 4.5 µm and 1 dB/cm, respectively. The waveguide losses can be improved further by oxidation smoothening which is under progress in our laboratory. ACKNOWLEDGEMENT This research work has been carried out with the financial supports of DIT, Govt. of India and DST, Govt of India. The Authors acknowledge gratefully the Photonics Research Group, Ghent University, Belgium for SEM facilities, IRDE Dehradun / SAMEER Mumbai for sample polishing. REFERENCES [1] A. G. Rickman, G. T. Reed and Ferydoon Namavar, “Silicon-On-Insulator Optical Rib
Waveguide Loss and Mode Characteristics”, Journal of Lightwave Technology, 12, no. 10, pp. 1771-1776, Oct. 1994. [2] Richard A. Soref, “Silicon Based Optoelectronics”, Proceedings of the IEEE, 81, no. 12, pp. 1687-1706, Dec. 1993. [3] C. Angulo Barrios, V. R. Almeida, R. Panepucci and M. Lipson, ‘Electrooptic Modulation of Silicon-on-Insulator Submicrometer-size waveguide Devices”, Journal of Lightwave Technology, 21, no. 10, pp. 2332-2339, Oct. 2003. [4] Y. L. Liu, E. K. Liu, S.L. Zhang, G. Z. Li and J. S. Luo, “Silicon 1×2 digital optical switch using plasma dispersion”, Electronics Letters, 30, no. 2, pp. 130-131, Jan. 1994. [5] B. Liu, Y. H. Zuo, B. W. Cheng, R. W. Mao, L. Zhao, W. H. Shi, L. P. Luo, J. Z. Yu and Q. M. Wang, “Thermally tunable optical filter with crystalline silicon cavity”, Optics Communication, 244, pp. 167-170, Jan. 2005. [6] Kermino Jia, Weohui, Yanzhe Tang, Yiron Yang, Jianyi Yang, Xiaoquing Jiangm, Yaming Wu, Minghua Wang and Yuelin Wang, “Silicon-onInsulator Optical Demultiplexer Employing Turning-Mirror-Integrated Arryed-Waveguide Grating”, IEEE Photonics Technology Letters, 17, no. 2, pp. 378-380, Feb. 2005. [7] Ozdal Boyraz and Bahran Jalai, “Demonstration of a silicon Raman laser”, Optics Express, 12, no. 21, pp. 5269-5273, Oct. 2007. [8] Tao Yin, Rami Cohin, Mike M. Morse, Gadi sarid, Yoel Cherit, Doron Rubin and Mario J. Panniccia, “31 GHz Ge n-i-p wveguide photodetectors on Silicon-on-Insulator substrate”, Optics Express, 15, no. 21, pp. 13965-13971, Oct. 2007. [9] Souren P. Poggosian, Lili Vescan and Adrian Vonsovici, “The single mode condition for semiconductor rib waveguide with large cross section”, Journal of Lightwave Technology, 16, no. 10, pp. 1851-1853, Oct. 1998. [10] Pieter Dumon, Wim Bogaerts, Vincent Wiaux, Johan Wouters, Stephen Beckx, Joris Van Campenhout, dirk Taillaert, Bert Luyssaert, Peter Bienstman, Dries Van Thourhout and Roel Baets, “Low-Loss SOI Photonics Wires and Ring Resonators Fabricated With Deep UV Lithography”, IEEE Photonics Technology Letters, 16, no. 5, pp. 1328-1330, May 2004. [11] G. T. Reed, A. P. Knights, “Silicon Photonics-An Introduction”, John Wiley & Sons, Ltd, 2004. [12] F. P. Payne, J. P. R. Lacey, “A theoretical analysis of scattering loss from planar optical waveguides”, Optical and Quantum electronics, 26, pp. 977-986, 1994.
