A new material platform of Si photonics for implementing architecture of dense wavelength division multiplexing on Si bulk wafer

Figures & data

Figure 1. Information capacity vs. year. Signal multiplexing technologies, such as wavelength division multiplexing (WDM) and electrical time division multiplexing (ETDM), assume the most vital role to meet the considerable growth of communication capacity. Reproduced with permission from The Electrochemical Society Interface, 9–2, 20, (2000).

Figure 2. Local heating of central processing unit (CPU). This figure is from [9], which is in black and white. Thanks to Y. Vlasov for the color version.[10]

Figure 3. Minimum channel spacing. Thermal shift and crosstalk should determine the minimum channel spacing.

Table 1. The relation of minimum channel spacing with Q factor at 1550 NM.

Quality factor (Q)	Minimum channel spacing (GHz)
	Crosstalk of −30 dB	Crosstalk of −20 dB
10^4	107	60
3 × 10^4	36	20
10^5	10.7	6

Note:

Here, no thermal shift is assumed.

Figure 4. Materials chart for Si photonics platform. MiDex and HiDex are shown. Horizontal axis shows the optical index difference, Δn = n_core – n_clad at 1.55 μm, where the cladding is SiO₂. (a) Design figure for minimum bending radius and scattering loss with experimental results shown by squares. (b) Design figure for thermo-optic coefficient and bandgap of materials group. Symbols show literature data. Calculation of minimum bending radius is regarding that bending loss less than 0.1 dB/turn as acceptable.

Figure 5. Typical SEM images of fabricated SiN_x. (a) A cross sectional SEM image, (b) a plan view image of SiN_x ring and coupled waveguides used in the present work.

Figure 6. Transmission characteristics of PVD- and CVD- SiN_x waveguides on SiO₂. (a) Transmission spectra. The PVD-SiN_x waveguide is shown in red, the CVD-SiN_x is plotted in black and in blue (annealed at 900 °Cfor 3 h). The transmission loss of the CVD-SiN_x after 1150 °Cannealing for 3 h (not shown here) is identical to that of the PVD-SiN_x (red). (b) The transmission loss of the PVD-SiN_x waveguide. The loss ~5.6 dB cm^–1 is obtained using the waveguide with air upper cladding. Simulation indicates that the loss is reduced to ~1 dB cm^–1 with the SiO₂ upper cladding.

Figure 7. Transmission characteristics of the PVD- and CVD-SiN_x ring resonators at temperatures from room temperature (24 °C) to 76 °C. (a) The spectra of the drop port show Q of 31,600. (b) The resonance peak shifts of CVD-SiN_x rings are reproduced by the reported thermo-optic coefficient, 2.4 × 10⁻⁵ 1/K, obtained for CVD stoichiometric Si₃N₄.[15] The shifts are slightly larger for PVD than CVD rings. The thermo-optic coefficient of the PVD-SiN_x is 2.8 × 10⁻⁵ 1/K.

Figure 8. The peak wavelength shift of the PVD-SiN_x ring resonators with athermalization using a ‘silicone’ polymer as the upper cladding (red). The shifts were measured between 24 and 70 °C. The black and red lines are simulations. An athermalized SiN_x (MiDex) ring shows a smaller thermal shift than reported for an athermalized Si rings (black).[36] The athermalized SiN_x ring covers S-, L- and U-bands within the 50 GHz channel spacing at this temperature range, while the reported, athermalized Si rings cover C band.

Figure 9. Design figure of thermo optic coefficient (blue circles) and bandgap (red squares) vs. Δn. The PVD-SiN_x and CVD-SiN_x are within the green circle of Si₃N₄. The following four materials, (1) Al₂O₃, (4) HfO₂, (7) Ta₂O₅ and (12) Ga₂O₃, should be excellent candidates because of their low TO coefficients < 10⁻⁵ 1/K and the wide bandgaps >4 eV.

Table 2. The MiDex materials characteristics.

Number in Figure 9	Material	Refractive index	Thermo-optic coefficient × 10^−5 (1/K)	Bandgap (eV) at 1.55 μm
1	Al2O3	1.76	0.8	7
2	ZrSiO4	1.9	6	6
3	HfSiO4	1.9	19.7	6
4	HfO2	2	0.58	6
5	Si3N4, SiNx (*)	2.0, 1.96 (*)	2.4, 2.8(*)	4.9
6	Y2O3	2.1	2.17	6
7	Ta2O5	2.2	0.23	4.4
8	AlN	2.19	3.0	6.3
9	LiNbO3	2.21	4	3.7
10	ZrO2	2.19	10	5.8
11	GaN	2.3	7	3.4
12	Ga2O3	1.94	0.7	5
13	SiC	2.58	5	3
14	GaAs	3.38	15	1.43
15	c-Si	3.5	20	1.12

^* The present PVD work.

The refractive indices were reported at near infrared around 1.55 μm for 13, 14, 15, and others are visible range at around 633 nm.

Figure 10. Schematics of a monolithic electronic and photonic integrated circuits on a Si bulk wafer (cross section). WG stands for waveguide.

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