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Integrated Silicon Wavelength-Division Multiplexed Optical Transmitter using Low Loss Optical Waveguide

  • 이동호 (대학원 전기전자전파공학과 전자전기컴퓨터공학전공)

초록/요약

Materials used for optical element fabrication include lithium niobate (LiNbO3), gallium arsenide (GaAs), indium phosphide (InP), and silicon (Si). Most electronic devices are based on silicon, and optical interconnects using silicon photonics are very useful at short range. Recently, optical interconnects on chip to chip or board to board basis have been actively studied. This thesis describes a process technology for fabricating silicon based optical devices, arrayed waveguide grating (AWG) for separating wavelength, electro absorption modulator (EAM) for modulating electric signal into optical signal, a vertical interconnect technique that can be pulled out, and a wavelength-division-multiplexed optical transmitter that implements them on a single chip. It is very important to reduce the roughness caused by the etching process because the optical device transmits light by confining it in the waveguide. By controlling the SF6 and CF4 gas used for reactive ion etching (RIE) and using a hard mask of SiO2, the sidewall roughness of the device after the etching process is reduced, and the near-vertical etching is realized. The process conditions of the anisotropic vertical etching of 89 degre were established using RF power of 180 W and pressure of 40 mTorr using 10 sccm of SF6 and 10 sccm of CF4 gases mixture. When etching is performed using only photoresist (PR), the roughness of the photomask is transferred to the side wall of the device. Through this study, it was confirmed that propagation loss of less than 1 dB/cm could be obtained using SiO2 hard mask over 500 nm thick. An optical device is capable of transmitting and receiving light of different wavelengths in a waveguide. AWG is suitable for multi-channel devices because it can perform wavelength-division multiplexing (WDM). In this thesis, a silicon AWG is designed to be 8 channels. Superluminescent diode (SLD) with a wavelength in the range of 1500 ~ 1600 nm was used as the light source with the center wavelength of 1550 nm, which is mainly used for communication. The channel spacing in wavelength (Δλ) is 1.6 nm, and the shortest wavelength is 1544.4 nm and the longest wavelength is 1555.6 nm. The AWG was fabricated using silicon on insulator (SOI). The center wavelength was 1552.8 nm, Δλ was 1.33 nm, and the output was -47 dBm. In order to fabricate the silicon EAM, a rib type optical waveguide was designed to be capable of guiding light with a wavelength of 1550 nm using an effective refractive index method. The doping concentration of silicon was set in consideration of the carrier plasma dispersion effect and the bonding condition of metal-silicon. The optimized waveguide has a rib width of 4.8 μm, a height of 0.4 μm and an etch depth of 0.15 μm. The doping concentration of rib sides section forming the Schottky junction is 1015 cm-3, and the doping concentration for ohmic junction is 5 × 1019 cm-3. RF electrodes with coplanar waveguide (CPW) structure were designed and fabricated. The propagation loss after fabrication was measured to be 2.7 dB/cm and the RF electrode characteristics obtained a cut-off frequency characteristic of the input signal at 2 GHz. Using the optimized process technology for fabricating these optical devices, one integrated wavelength division multiplexed optical transmitter device based on each single device was designed and fabricated, and confirmed its operation characteristics. Its characteristics were similar to those of single devices. Preceding studies have been carried out on vertical optical interconnects for 3D highly integrated devices. the extraction of light vertically using SiON and SiN thin films with different refractive indices on a Si substrate was studied. The core of the optical waveguide is SiON with a width of 50 μm and a height of 2 μm, and a SiN layer capable of vertically extracting light is set to a thickness of 100 nm. The area of the SiN window was set to 48 × 48 μm2 for deposition on a waveguide having a width of 50 μm. lasers with wavelengths of 457 nm, 537 nm and 637 nm were used to measure the amount of extracted light from SiN windows. As a result, 30%, 45% and 55% of the light was extracted from the 12 windows.

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CONTENTS

ABSTRACT i
CONTENTS iv
LIST OF FIGURES vii
LIST OF TABLES xiii
LIST OF ACRONYMS xiv
LIST OF CHEMICAL SYMBOLS xvi

Chapter 1 INTRODUCTION 1
1.1 optical interconnect 2
1.2 Goal of the thesis 5
1.3 Outline of the thesis 8

Chapter 2 RESEARCH BACKGROUND 9
2.1 Plasma etching 10
2.1.1 Reactive ion etching (RIE) process 11
2.1.2 Fundamentals of plasma etching 13
2.2 Theory of optical waveguides 15
2.2.1 Maxwell equations 15
2.2.2 Modes of a waveguide 17
2.2.3 Basic three-layer planar waveguide 19
2.2.4 Normalized parameters 21
2.2.5 Losses in optical waveguides 23
2.3 Arrayed waveguide grating (AWG) 26
2.3.1 AWG overview 26
2.4 Schottky type silicon modulator 31
2.4.1 Schottky metal contact 31
2.4.2 Carrier depletion 33

Chapter 3 LOW-LOSS SILICON WAVEGUIDE 35
3.1 Silicon waveguide structure and fabrication 36
3.1.1 Silicon strip-loaded waveguide structure 36
3.1.2 Fabrication of the waveguide 37
3.2 Silicon waveguide etching mechanism 39
3.3 Properties of etched the silicon waveguide 41
3.3.1 Surface roughness 41
3.3.2 Sidewall roughness 45
3.3.3 Vertical profile of sidewall 47
3.3.4 SiO2 hard mask trim etching 51
3.4 Performance of the waveguide 53

Chapter 4 8 CHANNELS SILICON AWG 55
4.1 Silicon AWG design 56
4.1.1 Star coupler 57
4.1.2 Arrayed waveguides 60
4.2 Device fabrication of AWG 64
4.3 Performance of AWG 67

Chapter 5 SCHOTTKY TYPE SILICON MODULATOR 69
5.1 Schottky type modulator Design 70
5.1.1 Rib type waveguide 70
5.1.2 Electrode 74
5.2 Device fabrication of EAM 77
5.3 Characteristics of EAM 79
5.3.1 Characteristics of optical waveguide 79
5.3.2 Characteristics of RF electrodes 81
5.3.3 Characteristics of Schottky junction 82

Chapter 6 INTEGRATION OF OPTICAL TRANSMITER 83
6.1 Design of the optical transmitter 84
6.2 Fabrication of the optical transmitter 85
6.3 Characteristics of electro-optical transmitter 87
6.3.1 Vertical interconnect 87
6.3.2 AWG 89
6.3.3 EAM 90

Chapter 7 VERTICAL INTERCONNECT 92
7.1 Design of the device 93
7.2 Fabrication of the device 98
7.3 Optical characteristics 100

Chapter 8 CONCLUSION 106


REFERENCES 109

ABSTRACT IN KOREAN 115

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