††thanks: These authors contributed equally to this work.††thanks: These
authors contributed equally to this work.

# Laser soliton microcombs on silicon

Chao Xiang  ECE Department, University of California Santa Barbara, Santa
Barbara, CA 93106, USA  Junqiu Liu  Institute of Physics, Swiss Federal
Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland  Joel
Guo  ECE Department, University of California Santa Barbara, Santa Barbara, CA
93106, USA  Lin Chang  ECE Department, University of California Santa Barbara,
Santa Barbara, CA 93106, USA  Rui Ning Wang  Institute of Physics, Swiss
Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland
Wenle Weng  Institute of Physics, Swiss Federal Institute of Technology
Lausanne (EPFL), CH-1015 Lausanne, Switzerland  Jonathan Peters  ECE
Department, University of California Santa Barbara, Santa Barbara, CA 93106,
USA  Weiqiang Xie  ECE Department, University of California Santa Barbara,
Santa Barbara, CA 93106, USA  Zeyu Zhang  ECE Department, University of
California Santa Barbara, Santa Barbara, CA 93106, USA  Johann Riemensberger
Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL),
CH-1015 Lausanne, Switzerland  Jennifer Selvidge  Materials Department,
University of California Santa Barbara, Santa Barbara, CA 93106, USA  Tobias
J. Kippenberg<EMAIL_ADDRESS>Institute of Physics, Swiss Federal
Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland  John
E. Bowers<EMAIL_ADDRESS>ECE Department, University of California Santa
Barbara, Santa Barbara, CA 93106, USA  Materials Department, University of
California Santa Barbara, Santa Barbara, CA 93106, USA

Silicon photonics enables wafer-scale integration of optical functionalities
on chip. A silicon-based laser frequency combs could significantly expand the
applications of silicon photonics, by providing integrated sources of mutually
coherent laser lines for terabit-per-second transceivers, parallel coherent
LiDAR, or photonics-assisted signal processing. Here, we report on
heterogeneously integrated laser soliton microcombs combining both InP/Si
semiconductor lasers and ultralow-loss Si3N4 microresonators on monolithic Si
substrate. Thousands of devices are produced from a single wafer using
standard CMOS techniques. Using on-chip electrical control of the microcomb-
laser relative optical phase, these devices can output single-soliton
microcombs with 100 GHz repetition rate. Our approach paves the way for large-
volume, low-cost manufacturing of chip-based frequency combs for next-
generation high-capacity transceivers, datacenters, space and mobile
platforms.

Figure 1: Device schematics, images and operation principle. A. Design layout
of a laser soliton microcomb device consisting DFB lasers, phase tuners and
high-$Q$ microresonators on a common substrate. A continuous-wave (CW) signal
(solid red line) emitted from the laser is partially back-scattered in the
microresonator. The back-scattered signal (dashed red line) is sent back to
the laser, triggering self-injection locking that assists the formation of
soliton pulse stream inside the microresonator. The optimization of laser
self-injection locking is realized by controlling the laser current
$I_{\text{laser}}$ and phase tuner current $I_{\text{phase}}$. B. Simplified
device cross-section schematics. The laser is based on InP/Si. The
microresonator is based on Si3N4. Silicon layer is used to deliver light from
the InP/Si layer to the Si3N4 layer. C. Photographs showing the completed
100-mm-diameter wafer, zoom-in of multiple laser soliton microcomb dies, and a
microscopic image showing a Si3N4 microring resonator with Si/ Si3N4
interface. D. Bird-view schematic illustration of the large-scale integrated
laser soliton microcomb devices enabled by multilayer heterogeneous
integration. The lasers, phase tuners and high-$Q$ microresonators are built
on InP/Si layer, Si layer and Si3N4 layer, respectively. E. Schematic
illustration of the microcomb generation with sweeping laser current
$I_{\text{laser}}$ and varying phase tuner current $I_{\text{phase}}$ (thus to
vary the optical phase difference between the forward and backward signals),
in the case of laser self-injection locking. This waterfall plot is based on a
full nonlinear simulation shown in the Supplementary Information.

Optical frequency combs (OFC) Udem  _et al._ (2002); Cundiff and Ye (2003)
have revolutionized timing, spectroscopy and metrologyFortier and Baumann
(2019); Diddams  _et al._ (2020). Historically, OFCs have been made using
femtosecond mode-locked lasers with supercontinuum generation for octave-
spanning spectra, required for self-referencing. Discovered a decade ago
Del’Haye  _et al._ (2007), OFCs can also be generated in driven Kerr-nonlinear
optical microresonators. These types of OFCs are commonly referred to as
"microcombs". Microcombs can be operated in regimes where they form coherent,
temporal, dissipative structures Lugiato and Lefever (1987); Akhmediev and
Karlsson (1995), i.e. bright dissipative Kerr solitons (DKS) Herr  _et al._
(2013). DKS have unlocked the full potential of microcombs by providing
coherent, broadband OFCs with repetition rates in the terahertz to microwave
domains, and have been successfully employed in many system-level applications
such as RF photonicsTorres-Company and Weiner (2014); Wu  _et al._ (2018),
coherent communication Marin-Palomo  _et al._ (2017); Corcoran  _et al._
(2020), astronomical spectrometer calibration Suh  _et al._ (2019); Obrzud
_et al._ (2019), massively parallel coherent LiDAR Riemensberger  _et al._
(2020), optical frequency synthesizers Spencer  _et al._ (2018) and photonic
neromorphic computing Feldmann  _et al._ (2021); Xu  _et al._ (2021). On a
fundamental level, DKS microcombs have allowed explorations of a plethora of
novel nonlinear dynamics and phenomena Xue  _et al._ (2015); Yang  _et al._
(2017).

In parallel, there has been substantial progress in photonic integrated
platforms Kovach  _et al._ (2020) for microcomb generation, which to date
include Si3N4Moss  _et al._ (2013); Xuan  _et al._ (2016); Ji  _et al._
(2017); Liu  _et al._ (2020a); Ye  _et al._ (2019), AlNJung  _et al._ (2014);
Liu  _et al._ (2020b), LiNbO3He  _et al._ (2019a); Wang  _et al._ (2019); Gong
_et al._ (2020), AlGaAsPu  _et al._ (2016); Chang  _et al._ (2020); Moille
_et al._ (2020), GaPWilson  _et al._ (2020), and SiCLukin  _et al._ (2020).
Among them, the leading platform is Si3N4, which has already been used widely
in CMOS microelectronics as diffusion barriers and etch masks. In addition to
its native properties such as the absence of two-photon absorption, high Kerr
nonlinearity, and weak Raman and Brillouin gains Gyger  _et al._ (2020),
recent advances in fabrication of nonlinear Si3N4 photonic integrated circuits
(PIC) have enabled optical propagation losses below 1 dB/m Ji  _et al._
(2017); Liu  _et al._ (2020a). These ultralow losses have significantly
reduced the soliton formation threshold power to the levels that integrated
lasers can provide Huang  _et al._ (2019a); McKinzie  _et al._ (2021), and
have yielded soliton repetition rates in the microwave X-band Liu  _et al._
(2020c). In combination with negligible thermal effects and almost purely
Kerr-dominated frequency-dependent response Liu  _et al._ (2020a), reliable
soliton generation can be attained without complex or fast laser tuning.
Additionally, laser self-injection locking Liang  _et al._ (2015a); Pavlov
_et al._ (2018) and hybrid integration of soliton microcombs with RSOAs Stern
_et al._ (2018) or DFB lasers Shen  _et al._ (2020) allow for current-
initiated and electrically controlled modules with low electrical power Stern
_et al._ (2018); Raja  _et al._ (2019); Shen  _et al._ (2020); Jin  _et al._
(2021).

A long-standing goal is to monolithically integrate lasers and high-$Q$
nonlinear microresonators onto a common silicon wafer. Heterogeneously
integrated silicon photonics Komljenovic  _et al._ (2016); Liang and Bowers ;
Zhang  _et al._ (2019a) offers a compelling solution using low-cost, industry-
standard silicon substrates, aided by mature CMOS-compatible fabrication
facilities. High-performance, large-scale heterogeneous PICs with complete
functionalities, including lasers Fang  _et al._ (2006); Liang and Bowers
(2010); Xiang  _et al._ (2020), modulators Xu  _et al._ (2005); Reed  _et al._
(2010); Hiraki  _et al._ (2017), and photodetectors Xie  _et al._ (2016);
Michel  _et al._ (2010), are disrupting optical interconnect technology and
other applications Roelkens  _et al._ (2010); Jones  _et al._ (2019); Sun  _et
al._ (2013). The material-by-design nature enabled by heterogeneous
integration offers best-in-class performance. However, heterogeneous
integration of high-power, narrow-linewidth, semiconductor lasers with
high-$Q$ Si3N4 microresonators has not been demonstrated yet, because multiple
material platforms (Si, Si3N4, and III-V) with significantly different optical
properties have to be deployed and combined. Here we overcome these challenges
and present the first demonstration of heterogeneously integrated laser
soliton microcombs. This is achieved using ultralow-loss Si3N4 PICs based on
the photonic Damascene fabrication process in conjunction with direct SOI
wafer bonding and heterogeneous III-V integration. The result is a wafer-scale
fabrication process that produces thousands of devices from one silicon wafer.
Each device allows electrical initiation and control of soliton microcombs.
Our technology demonstrates the viability of large-volume, low-cost
manufacturing of chip-based OFCs, and allows incorporation of soliton
microcombs as new building block into existing complex silicon photonics
systems, and in applications requiring reliable performance, small footprint,
low cost and low power consumption, such as space or mobile platforms.

Figure 2: Simplified wafer-scale fabrication process flow. A. Multilayer
heterogeneous integration process. Left panel shows that a pre-patterned Si3N4
Damascene substrate is planarized and subsequently bonded with an SOI wafer
and an InP MQW epi wafer. The wafer undergoes 1.) Si3N4 photonic Damascene
process, 2.) Si processing and 3.) InP processing. Right panel shows selected
key steps in the wafer fabrication with 1.) Si3N4 photonic Damascene process
including LPCVD Si3N4 deposition on the patterned SiO2 substrate (top), excess
Si3N4 removal and planarization (middle), and SiO2 spacer deposition,
densification and polishing (bottom); 2.) Si processing including Si substrate
removal (top), Si waveguide etch for laser and thermal tuner (middle), and for
Si grating and Si-Si3N4 mode conversion taper (bottom); 3.) InP processing
including InP substrate removal (top), InP mesa etch (middle), and excess Si
removal, laser passivation, contact formation, vias open, heaters, and probe
metal formation (bottom). B. Left: Atomic-force microscopy (AFM) shows 0.27 nm
RMS roughness of the Si3N4 wafer surface, after the second CMP on the SiO2
spacer, before bonding with the SOI wafer. Right: SEM image of Si grating for
laser before bonded with InP MQW wafer. C. False-colored SEM image showing the
cross-section of the complete device at laser area with multilayer device
structure.

Device schematic. As shown in Fig. 1A, a chip-scale laser frequency comb
consists of three main parts: a distributed feedback (DFB) laser, a thermo-
optic phase tuner, and a high-$Q$ nonlinear microresonator, by leveraging
multilayer heterogeneous integrationXiang  _et al._ (2020) (Fig. 1B). Figure
1C shows the photographs of the complete wafer (before dicing into chips),
multiple dies, single-die-level devices, and an optical microscope image of a
Si3N4 microring resonator interfaced with Si layer. As illustrated in Fig. 1D,
the DFB lasers, phase tuners, and nonlinear microresonators are built on
indium phosphide/silicon (InP/Si), silicon (Si), and Si3N4 layers,
respectively. This vertical, multilayer structure is realized through
sequential wafer bonding of a silicon-on-insulator (SOI) wafer and an InP
multiple-quantum-well (InP MQW) epi wafer to a pre-patterned Si3N4 substrate
that is fabricated using the photonic Damascene process Liu  _et al._ (2020a);
Xiang  _et al._ (2020). Deep ultra-violet (DUV) stepper lithography is used to
ensure pattern alignment on each individual layer with an accuracy better than
100 nm. Here we directly apply heterogeneous integration on 100-mm-diameter Si
substrates and process the entire substrate on the wafer scale. Our process
could be further scaled up with larger wafer substrates such that it is
compatible with standard CMOS foundry production lines Jones  _et al._ (2019).

The DFB laser is a high-power, single longitudinal-mode pump source with an
excellent side mode suppression ratio (SMSR) such that the laser wavelength
can be tuned to a microring resonanceColdren  _et al._ (2012). The laser has a
1.8-mm-long InP/Si gain section, and the grating is etched on both sides of
the shallow-etched Si waveguide rib with a 170 nm gap separation to the Si
waveguide core. The fully etched grating provides the optical resonant
feedback for the pump laser and determines the lasing wavelength
$\lambda_{\text{pump}}$ by its pitch $\Lambda$
($\lambda_{\text{pump}}=2n_{\text{eff}}\Lambda$). Here $\Lambda=238$ nm and
$\lambda_{\text{pump}}\sim 1550$ nm. A quarter wavelength shift section is
included at the grating length center to supply non-degenerate phase
conditions favoring single longitudinal mode lasing.

The single-wavelength laser output passes through a thermo-optic resistive
heater (for optical phase control), and couples into a high-$Q$ Si3N4
microring resonator where nonlinear four-wave mixing processes generate Kerr
microcombsKippenberg  _et al._ (2004); Del’Haye  _et al._ (2007). The Si3N4
microresonator exhibits anomalous group velocity dispersion (GVD) in the
telecommunication C band and have a free-spectral range (FSR) of 100 GHz. The
laser directly pumps the microresonator without an intermediate optical
isolator, and the entire device is operated via laser control and phase
control. Laser self-injection locking Kondratiev  _et al._ (2017); Liang  _et
al._ (2015b, a); Raja  _et al._ (2019); Shen  _et al._ (2020); Jin  _et al._
(2021) leverages the narrow-band optical feedback at desired phase relations
from a high-$Q$ microresonator to stabilize the pump laser and pulls the laser
frequency towards the microring resonance. In this scenario, soliton
microcombs can form when optimum laser-resonator frequency detuning is
reached. Only the laser and phase tuner currents are electronically
controlled, and no sophisticated external electronics feedback controls are
required. The DFB laser wavelength increases with increasing laser current, as
the grating index increases due to injected electrical power heating. As a
result, certain gain currents triggers comb generation if the laser wavelength
coincides with a microresonator resonance. The comb generation region resides
where the laser is red-detuned to the resonance and the phase of the back-
scattered light from the microresonator to the laser fulfills certain phase
relations (Fig. 1E, see Supplementary Information for details). Note that the
back-scattered light originates from Rayleigh scattering inside the
microresonator due to surface roughness and bulk inhomogeneity.

Fabrication process flow. Figure 2A presents the wafer-scale fabrication
process flow. It starts with the photonic Damascene process Liu  _et al._
(2020a) to fabricate the Si3N4 PIC on a Si substrate with 4-µm-thick thermal
wet SiO2. The Si3N4 PIC is exposed with DUV stepper lithography and
transferred to the SiO2 substrate to form the waveguide preform.
Stoichiometric Si3N4 is deposited on the patterned SiO2 preform using low-
pressure chemical vapor deposition (LPCVD), filling the trenches and forming
waveguide cores. Chemical-mechanical polishing (CMP) is used to remove excess
Si3N4 and planarize the wafer front surface, followed by spacer SiO2
deposition of 300 nm thickness on the Si3N4 substrate. The entire substrate is
further annealed at 1200∘C to drive out the residual hydrogen content in the
Si3N4 and SiO2 films, and to densify the spacer SiO2.

A second CMP is performed to create a flat and smooth wafer surface. As shown
in Fig. 2B left, the measured RMS roughness of the wafer surface using atomic-
force microscopy (AFM) is 0.27 nm, enabling direct substrate bonding with an
SOI wafer. After removing the Si substrate of the bonded SOI wafer, the Si
device layer is processed to create waveguide structures with different etch
depths, including shallow-etched Si waveguides for the lasers and phase
tuners, fully-etched hole structures for gratings (cf. Fig. 2B right), and
thin-thickness Si tapers for mode conversion between the Si waveguide and
underlying Si3N4 waveguideXiang  _et al._ (2020). InP MQW epi is then bonded
to the patterned Si device surface at the active regions. The InP process
starts with InP substrate removal. InP mesa etches including P-type InP,
AlInGaAs MQW, and N-type InP etches are performed using selective dry etching
and wet etching. The P- and N-type contact metals are deposited on the
P-InGaAs layer and N-InP layer respectively. The excess Si on top of the Si3N4
microresonators is removed before laser passivation using hydrogen-free
deuterated SiO2 depositionJin  _et al._ (2020). Vias are then etched through
it for laser electrical contact. Proton implantation is performed on the laser
mesa structure to reduce electrical current leakage. Heater and probe metals
are deposited at the end of the full process. Finally, the entire wafer is
diced into dozens of dies/chips to facilitate testing. Each chip contains tens
of devices for soliton generation. Figure 2C shows the scanning electron
microscopic (SEM) images of the device cross-section, which is false-colored
to illustrate the multilayer structure.

Figure 3: Device characterization and experimental generation of soliton
spectra. A. Schematics of the experimental setup for laser and soliton
characterization. $I_{\text{laser}}$ and $I_{\text{phase}}$ are the current
sources to drive the laser and the phase tuner. POW: power meter. WAV:
wavelength meter. OSA: optical spectrum analyzer. PNA: phase noise analyzer.
OSC: oscilloscope. ESA: electrical spectrum analyzer. ISO: isolator. BPF:
band-pass filter. PD: photodetector. B. Light-current sweep measurement with
stepped laser current and fixed phase tuner current. Grey color shows the
corresponding laser center wavelength as a function of the laser current. Red
circles indicate the laser wavelength coinciding with microresonator
resonances. Soliton microcombs are generated with laser currents and
wavelengths at resonance #1 and resonance #2. C. Single-mode DFB laser spectra
at the wavelengths of resonance #1 and resonance #2. D. Optical spectra of
soliton microcombs. Inset shows the relative position of multi-solitons
circulating inside the microring resonator for 4-, 3-, and 2-FSR soliton
crystal states and low-frequency RF spectrum of the single soliton state.
Figure 4: Laser frequency noise spectrum and comb generation with self-
injection locking. A. Frequency noise spectra of the self-injection-locked
pump line (green), comb lines with $\pm 100$ GHz frequency offset to the pump
(blue and red) in the single soliton state, free-running single-mode DFB laser
output without self-injection locking (dark grey), and comb line around 1550
nm for the 2-FSR soliton crystal state using optical phase retrieval method
(see Supplementary Information). B. Comb power evolution with sweeping laser
current (10 milliseconds sweep) under varying electrical power on the phase
tuner.

Soliton generation. The experimental setup to characterize the final chip
devices is shown in Fig. 3A. We first characterize the laser performance.
Figure 3B shows the measured light-current curves (solid blue curve), i.e. the
laser output power versus stepped laser injection current of 0.5 mA, fixed
phase tuner current of 7 mA, and 20∘C stage temperature. The measured laser
threshold current is 64 mA. The laser power is out-coupled from the Si3N4
waveguide inverse-taper to a lensed fiber. The maximum power in the output
fiber is measured as 8 mW, and the corresponding on-chip power in the Si3N4
waveguide is approximately 20 mW. The measured laser center wavelength using a
wavelength meter is shown in Fig. 3B (dashed grey curve). Several dips on the
optical power are observed at laser currents of 133, 231, 329.5, and 418 mA,
where the laser wavelength coincides with microresonator resonances. These
dips also verify the microresonator FSR of 100 GHz. Note that a high-
reflection (HR) coating is applied on the other side of the DFB laser to boost
the laser output power. Future devices can avoid using this coating to yield
mode-hop-free DFB lasers and linearized laser wavelength dependence on the
laser current. The DFB laser has high SMSR at resonance #1 and #2, where high
output powers are also obtained that are advantageous for comb generation.
Figure 3C shows the single-longitudinal-mode laser spectra at the two
resonance wavelengths, with 60 and 57 dB SMSR, respectively.

Soliton microcomb is generated by tuning the laser frequency to the
microresonator resonance via biasing the laser current, together with tuning
the current of the phase tuner to control the relative forward/backward phase
relations. As no optical isolator is used between the laser and the Si3N4
microresonator, laser self-injection lockingKondratiev  _et al._ (2017); Liang
_et al._ (2015b); Shen  _et al._ (2020) occurs when the laser frequency
coincides with a microresonator resonance. More information of the phase
dependence on comb generation is revealed by simulating the nonlinear self-
injection locking processVoloshin  _et al._ (2021) and is discussed in the
Supplementary Information. The calculated Kerr parametric oscillation
threshold Kippenberg  _et al._ (2004) for Si3N4 microresonators of $Q_{0}\sim
7\times 10^{6}$ (see Supplementary information for $Q$ measurement) and 100
GHz FSR, is estimated to be around 3 mW. With around 329 mA laser current, the
estimated on-chip laser power to pump the Si3N4 microresonator exceeds 16 mW.
As shown in Fig. 3D, perfect soliton crystal statesKarpov  _et al._ (2019) are
observed by fine-tuning the laser frequency. We also observe soliton crystal
states with decreasing crystallization orders when increasing the laser
current (red-detuning). Further increasing the laser power to around 418 mA
current, a two-soliton crystal state with 200 GHz repetition rate and a
single-soliton state with 100 GHz repetition rate is generated. The coherent
soliton nature is revealed by photodetection of the amplitude noise of the
comb lines beat signal. Once generated, the soliton states can be stable for
hours in standard lab environments without any external feedback control. This
stability benefits from the monolithic nature of the chip device and the
laser-microresonator coupling through laser self-injection locking.

Self-injection locking and phase noise reduction. To further characterize the
laser self-injection locking, we measured the frequency noise spectra of the
pump line and comb line in the single soliton state and laser free-running
state, as shown in Fig. 4A. Leveraging a high-$Q$ external cavity, self-
injection locking exhibits frequency noise reduction, i.e. laser linewidth
narrowing Liang  _et al._ (2015b). The fundamental linewidth of the free-
running DFB laser is around 60 kHz, and is reduced to about 25 Hz in the self-
injection-locked single soliton state. A 10-dB noise reduction is observed at
1 kHz Fourier offset frequency and is further increased to more than 30 dB
above 300 kHz offset frequency. It has been revealed recently that the
frequency noise of a self-injection-locked laser to a high-$Q$ Si3N4
microresonator can be dominantly limited by the thermo-refractive noise of the
microresonator Huang  _et al._ (2019b); Jin  _et al._ (2021). Therefore,
further noise reduction can be realized by using microresonators of larger
sizes (i.e. smaller FSR) or engineered material properties with lower thermo-
optic transduction. Though the free-running linewidth of the DFB laser is
broad, due to the linewidth narrowing provided by self-injection locking, the
laser is still able to directly generate soliton states, surpassing its
intrinsic limitation of spectral impurity. Additionally, the coherence of the
injection-locked pump line is transferred to other comb lines. For example,
the first neighboring comb lines, 100 GHz apart from the pump, have a
fundamental linewidth around 200 to 300 Hz, and their frequency noise below 10
MHz offset frequency directly inherits that of the self-injection-locked pump.
The self-injection locking scheme thus enables multi-wavelength, narrow-
linewidth laser sources. Our device represents the first demonstration of a
self-injection-locked, narrow-linewidth laser on Si.

Different from the conventional pumping scheme with an optical isolator, the
comb generation in the self-injection locking scheme relies critically on the
phase relation between the forward signal (i.e. the laser emission to the
microresonator) and backward signal (i.e. the back-scattered light from the
microresonator to the laser). In this case, comb generation is only allowed
when the optical phase difference is within a certain optimized range. In
previous works using hybrid integrated devices, controlling the phase
difference is realized by varying the gap distance between the laser chip and
Si3N4 chip, which however also causes power coupling efficiency fluctuation
between these two chips. In our monolithic device, this optical phase
difference can be directly controlled by varying the current to the phase
tuner. We experimentally studied the comb formation with laser current sweep
under different currents on the phase tuner. In order to exclude the
interference from the mode-hop phenomenon when monitoring the power of the new
frequency components, we sweep the laser current across the resonance #1 shown
in Fig. 3B. Results shown in Fig. 4B unveils that the comb generation is only
permitted with certain phase conditions. More study shown in the Supplementary
Information indicates that the back-scattered signal needs to be in phase with
the forward signal. Additionally, the phase altering effect is periodic and
deterministic with the applied electrical power on the phase tuner. Thus the
allowed comb generation area depends mainly on the pump power and the
intensity of the back-scattered light (see Supplementary Information).

In summary, we have presented the first heterogeneously integrated laser
soliton microcomb on silicon. Thousands of devices are manufactured from a
full wafer-scale fabrication process using standard CMOS techniques such as
DUV stepper lithography, CMP, wafer bonding, etc. We have successfully
demonstrated single soliton formation with 100 GHz repetition rate, and
characterized laser performance with self-injection locking. More functions
could be added based on our current technology. For example, active, high-
speed tuning of the Si3N4 microresonators (thus the injection-locked laser
frequency) can be realized using piezoelectric actuators Jin  _et al._ (2018);
Liu  _et al._ (2020d). Meanwhile, full integration with other nonlinear
photonics platforms such as (Al)GaAs Chang  _et al._ (2020, 2019) and LiNbO3
Zhang  _et al._ (2019b); Boes  _et al._ (2018); He  _et al._ (2019b) could
provide $\chi(2)$ nonlinearity that can be used for electro-optic modulation
and second harmonic generation. The laser gain material can also be modified
to utilize quantum-dots Liu  _et al._ (2015) or extended to different
wavelength ranges from the visible to mid-infrared, significantly
complementing existing integrated nonlinear photonic systems. Though
demonstrated here on 100-mm-diameter Si substrates, our process could be
upgraded to 200- or 300-mm-diameter substrates using modified CMOS foundry
pilot lines. Our work paves the way for large-volume, low-cost manufacturing
of electrically controlled, chip-based frequency comb modules for future
applications in data-centers, space and mobile platforms.

Acknowledgments: We thank Mario Dumont and Jijun He for their help in the
device characterization. This work is supported by the Defense Advanced
Research Projects Agency (DARPA) under DODOS (HR0011-15-C-055) programmes of
the Microsystems Technology Office (MTO).

Author contribution: C.X., L.C., J.L., T.J.K. and J.E.B. conceived the idea
and initiated the collaboration. C.X., J.L. and L.C. designed the wafer layout
and process flow. J.L. and R.N.W. fabricated the Si3N4 substrate. C.X. and
J.P. fabricated the heterogeneous InP/Si lasers and Si circuits with the
assistance from W.X., and Z.Z. C.X., J.G., and J.L. tested the final chips.
C.X. and J.G. performed the laser soliton experiments with the assistance from
J.R.. W.W. performed the numerical simulations of self-injection locking. J.S.
took the FIB-SEM image. All authors discussed the data. C.X., J.L. and W.W.
wrote the manuscript, with input from others. T.J.K. and J.E.B supervised the
project.

Data Availability Statement: All data generated or analysed during this study
are available within the paper and its Supplementary Information. Further
source data will be made available on reasonable request.

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