YONSEI UNIVERSITY

YONSEI UNIVERSITY

25Gb/s 1V-driving CMOS ring modulator with

integrated thermal tuning

Kim, Sungjin

YONSEI UNIVERSITY

Contents

2

Introduction

Device design and fabrication

Device DC performance

Small-signal RF tests

High-speed data modulation

Conclusion

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Introduction

3

Carrier-depletion Si ring modulators with thermal tuning

Advantages

- High speed modulation

- consuming low energy for modulation

Disadvantages

- Additional power consumptions occur in thermal tuning

→ Advanced manufacturing techniques and higher modulation speed

can reduce this penalty

Interconnect power and density issues are the

main obstacles for supercomputing

Silicon photonics is candidates incoming

systems

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Waveguide structure

rib waveguide structure for applying electrical modulation

Tradeoff in modulator design

Minimize resistance and capacitance by thicker Si slab and smaller waveguide height

↔ Increase optical loss and reduce optical confinement

Minimize optical bending loss with wider waveguide width

↔ Difficult to maintain single-mode operation

Device design and fabrication

4

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Ring waveguide dimensions

- 7.6μm ring radius to achieve 12.8nm of free spectral range for 8-wavelength-channel WDM link

- 380nm wide ring waveguide with 220nm etch depth and 80nm thick Si slab

- 300nm wide bus waveguide

PN junction doping

- symmetric lateral PN junction

- vertically uniform doping

- laterally increasing density from junction

- to achieve small capacitance, resistance, optical loss

- caution with impurities diffusion occurred during implantations

and multiple CMOS thermal cycles

Device design and fabrication

5

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Doping profile

The impurity densities decrease linearly from 𝑁𝑑 to zero at the center of the junction

PN junction doping determines the depletion width

→ impact to the modulation depth and optical loss

→ determines the quality factor and modulation bandwidth

Impact of the junction doping to the modulation depth

∆𝜆𝑠𝑠𝑠𝑠𝑠∆𝜆𝐹𝐹𝐹𝐹

∝ 𝑁𝑑1 3⁄

𝛼𝑑𝑑𝑑+𝛼𝑑𝑠𝑠𝑜𝑜 , 𝑤𝑤𝑤𝑤𝑤 𝛼𝑑𝑑𝑑 ∝ 𝑁𝑑

∆𝜆𝑠𝑠𝑠𝑠𝑠 ∶ 𝑡𝑤𝑤 𝑤𝑤𝑟𝑟𝑟𝑟𝑟𝑟𝑤 𝑤𝑟𝑤𝑤𝑤𝑤𝑟𝑤𝑡𝑤 𝑟𝑤𝑠𝑠𝑡 𝑢𝑟𝑢𝑤𝑤 𝑤𝑟𝑤𝑡𝑟𝑤𝑤 𝑟𝑤𝑠𝑟𝑤

Δ𝜆𝐹𝐹𝐹𝐹 ∶ 𝑠𝑢𝑤𝑤 𝑤𝑠𝑢𝑡𝑤 𝑟𝑡 𝑤𝑟𝑤𝑠 𝑚𝑟𝑚𝑠𝑚𝑢𝑚 𝑟𝑠 𝑡𝑤𝑤 𝑤𝑤𝑟𝑟𝑟𝑟𝑟𝑟𝑤 𝑤𝑠𝑢𝑡𝑤

𝛼𝑑𝑑𝑑 ∶ 𝑤𝑟𝑟𝑟 𝑟𝑟𝑤𝑠𝑠𝑠𝑟𝑠𝑤𝑟𝑡 𝑢𝑢𝑤 𝑡𝑟 𝑗𝑢𝑟𝑟𝑡𝑠𝑟𝑟 𝑢𝑟𝑑𝑠𝑟𝑤,𝛼𝑑𝑠𝑠𝑜𝑜 ∶ 𝑤𝑟𝑟𝑟 𝑟𝑟𝑤𝑠𝑠𝑠𝑟𝑠𝑤𝑟𝑡 𝑢𝑢𝑤 𝑡𝑟 𝑟𝑡𝑤𝑤𝑤 𝑚𝑤𝑟𝑤𝑟𝑟𝑠𝑟𝑚𝑟

- to maximize modulation depth, 𝛼𝑑𝑑𝑑 = 0.5𝛼𝑑𝑠𝑠𝑜𝑜

- typical 𝛼𝑑𝑠𝑠𝑜𝑜 is 6dB/cm, which requires junction doping of 3 × 1017𝑟𝑚−3

Device design and fabrication

6

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Modulation bandwidth

Bandwidth is proportional to the waveguide loss

𝑠𝑑 = 𝑐𝛼 𝜋𝑛𝑔⁄

- maximized modulation depth results in small bandwidth of 5GHz

- to achieve higher modulation bandwidth, increase the junction doping density

and sacrifice the modulation depth

- to have 𝑠𝑑 > 25𝐺𝐺𝐺

𝑄 < 8000

𝜆𝑠𝑠𝑠𝑠𝑠 > 40𝑑𝑚

𝑚𝑟𝑢𝑢𝑤𝑟𝑡𝑟𝑤 𝑑𝑤𝑟𝑟𝑤𝑡𝑝 > 9𝑢𝑑

Device design and fabrication

7

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Device layout

67% of the ring : PN diode for high-speed modulation with junction doping of 3 × 1018𝑟𝑚−3

Upper-right 25% : N-type doped as a Si resistor for thermal tuning

Between the PN diode and the thermal resistor section : undoped waveguide

2𝜇𝑚 wide isolation gaps

Device design and fabrication

8

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DC performance

The resonance wavelength shift ∝ 𝑉𝑏 − 𝑉 2 3⁄

Positive bias beyond 0.5V will make the diode close to turn-on condition

26pm resonance shift is achieved with voltage swing from -0.5V to 0.5V

Device DC performance

9

< Resonant spectrum with different voltages applied >

< Resonance wavelength shift at different bias voltages >

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DC performance

750Ω of Si resistor

Tuning efficiency of 0.19nm/mW

66mW tuning power is needed to tune the 12.6nm FSR

Device DC performance

10

< Resonant spectrum with different tuning power applied >

< Resonance wavelength versus tuning power >

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Circuit model

High-speed behavior of the ring modulator can be approximated by circuit model

𝐶𝑑 : capacitance between the electrodes through top dielectrics

𝑅𝑆 and 𝐶𝐽 : model the current path through the reverse-biased PN junction

𝑅𝑆𝑠 and 𝐶𝑂𝑂 : model the current path through the BOX and the Si handle

Based on the circuit model, modulation energy is 7fJ/bit with 25Gb/s pseudo-random data

Small-signal RF tests

11

< fitted circuit model > < curve-fitting using the circuit model>

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Experimental results

High-speed data modulation

12

Eye-diagram with 25Gb/s modulation 1Vpp voltage swing

>5dB extinction ratio

Laser wavelength is tuned over the

5.3nm

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Demonstrated an error-free 25Gb/s tunable carrier-depletion ring

modulator with 7.5μm radius

Small junction capacitance leads to 7fJ/bit modulation energy with 1V

driving at 25Gb/s

The carrier-depletion Si ring modulators can enable ultrahigh-speed

and ultralow-power WDM photonic links with compact footprint

Conclusion

13

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