This section presents a review of the emerging Si photonics platforms, which will be integrated into CORNERSTONE’s MPW service in 2021, and some research highlights from these platforms from the CORNERSTONE partners: University of Southampton and University of Glasgow.
3.1. Silicon Nitride
In recent years, silicon nitride has gained momentum as one of the preferred mid-index CMOS compatible materials for PICs. It is typically used in the form of hydrogenated amorphous films with thicknesses up to 1 μm. One of the most attractive characteristics of Si
3N
4 is its wider bandgap, which sets the lower limit of its transparency window to 450 nm in the visible spectrum and extends it all the way to the MIR [
83]. This much larger bandgap results in negligible two photon absorption (TPA) at 1550 nm, which makes Si
3N
4 a viable and more efficient material for non-linear processes, as it can support higher optical powers than Si with lower non-linear losses and a reasonable non-linear Kerr coefficient [
84]. In terms of its physical properties, Si
3N
4 has a thermo-optic coefficient (10
−5 °C), which is one order of magnitude lower than that of Si in the near-infrared regime [
85]. As a result, Si
3N
4 devices exhibit spectral shifts as small as 11 pm/°C under variable temperature environments [
86], which is a feature that makes them attractive for applications that require a high tolerance to temperature variations. When combined with SiO
2 cladding, the refractive index contrast that can be achieved with Si
3N
4 is lower than that of the SOI platform. However, it is enough to achieve a good optical confinement (60–80%) with the added advantage of providing a lower sensitivity to dimensional variations and a higher tolerance to surface roughness [
87,
88]. Hence, Si
3N
4 devices represent a compromise between the level of integration and a flexible dimensional control, despite their increased footprint.
The different properties discussed above have made Si
3N
4 an ideal candidate for the realization of a various linear and non-linear PICs spanning from visible to mid-infrared wavelengths. Si
3N
4 has been widely used to demonstrate passive linear devices for datacom and visible wavelengths [
89,
90], including low loss waveguides [
90,
91], multimode splitters [
83,
92] and Mach–Zehnder interferometers [
93]. Many research groups have worked on (de)multiplexers such as array waveguide gratings [
94], angled-multimode interferometers [
95,
96] and echelle gratings [
94,
97]. These devices exploit Si
3N
4 to achieve a high tolerance to temperature variations for wavelength (de)multiplexing in datacom circuits and optical spectrometers [
94,
98]. Si nitride has also become one of the key materials for nonlinear applications and many efforts have geared towards the demonstration of wavelength conversion and optical parametric amplification in the telecom band. Additionally, optical delay lines have been realized to enable digital filtering, pulse shaping and data storage for all optical signal processing [
99,
100,
101]. Si
3N
4 has also been attractive for frequency comb generation [
97,
99,
102] that can be used as an optical source for high-capacity data transmission [
99,
103], spectroscopy and optical metrology [
104]. Supercontinuum generation on Si
3N
4 has also been explored to obtain ultrabroadband spectra spanning from the visible to the mid-infrared [
105,
106]. As the transparency window of Si
3N
4 covers the spectrum between 730 and 920 nm, several research efforts have focused on creating highly sensitive biosensors [
107,
108,
109]. Other biomedical applications have also benefited from Si
3N
4 circuits that can provide optical sources and (de)multiplexers in a compact size [
110,
111], including super-resolution microscopy [
112], point-of-care diagnostics based on flow cytometry [
110,
113] and optical coherence tomography [
114,
115].
Many of the demonstrated devices use layers with a thickness of 300 nm and a stoichiometric composition (
n = 2.0) due to the readily availability of both low pressure chemical vapor deposition (LPCVD) layers with considerably low losses and low-temperature (<400 °C) plasma enhanced chemical vapor deposition (PECVD) films more suitable for multilayer integration. This 300 nm thick stoichiometric silicon nitride will become the standard CORNERSTONE thickness for MPW calls, with a 3 µm thick silicon dioxide under-cladding. Using 300 nm thick PECVD silicon nitride layers, we realized single-mode waveguides fabricated by e-beam lithography with propagation losses below 1 dB/cm in the O-band and close to 1.5 dB/cm in the C-band, as in
Table 4 [
116].
A detailed comparison of silicon nitride technologies available at foundries and research groups around the world can be found in a publication by Muñoz et al. [
90]. Moreover, we demonstrated a library of devices working in the O and C telecom bands that are crucial for the fabrication of more complex PICs, including directional couplers, multimode splitters and Mach–Zehnder interferometers (see
Figure 14). In particular, we demonstrated 4-channel (de)multiplexers based on angled multimode interferometers with insertion losses <2.0 dB, crosstalk below 20 dB and a high tolerance to temperature variations with wavelength shifts below 10 pm/°C. Finally, we contributed to the realization of fiber-to-chip apodized grating couplers with a staircase teeth profile that exhibited a high directionality with coupling losses of only 1.5 dB and a 3 dB bandwidth of 60 nm in the C-band [
117].
3.2. Germanium-On-Silicon
MIR group IV photonic devices and systems are well suited to a broad variety of applications including environmental, biological, chemical and pharmaceutical sensing, industrial process control, toxin and contaminant detection, point-of-care diagnostics, communications and astrophysics. The absorption spectra in the fingerprint region (wavelength range 6–25 μm) are comprised of clearly defined peaks for many molecules [
118]. Therefore, this region is particularly appropriate for sensing because it can be used to identify molecular composition and quantify concentration, which is possible because the wavelength of each absorption peak corresponds to the vibrational frequency of a molecular bond and the intensity corresponds to concentration. MIR photonic devices can be used for non-destructive characterization of solid, liquid and gas phase substances with minimal sample preparation. The integration of microfluidics and MEMS systems for repeatable and efficient sample handling is straightforward due to the planar geometry of group IV photonic devices. When combined with the economies of scale associated with the fabrication techniques originally developed by the microelectronics industry, there is clear potential for miniaturized and mass producible MIR photonic sensors for many applications.
SOI is the most mature group IV photonics material platform. However, SiO
2 is highly absorbing at wavelengths greater than 4 μm so the SOI platform is inappropriate for longer wavelength operation [
119]. Several alternative material platforms have been investigated to extend the usable wavelength range to cover more of the fingerprint region and so enable a greater variety of applications. These include Si on sapphire (SOS) [
120,
121,
122], Si on porous Si [
123], Si on Si nitride [
124], germanium-on-silicon (GOS) in this section [
125,
126,
127,
128] and suspended Si [
129,
130,
131] (
Section 3.3).
The chief advantages of germanium-based material platforms for MIR photonics are the wide transparency range (2–15 μm) and, in comparison with Si, its higher carrier mobility and larger non-linear effects. The first reported germanium-based MIR photonic devices were Ge-core fibers [
132] and Ge slab waveguides on ZnS substrates [
133]. Chang et al. have demonstrated GOS waveguides with 2.5–3.0 dB/cm loss at wavelength λ = 5.8 μm [
126], and GOS multiplexers, Mach–Zehnder interferometers and thermo-optic modulators at λ = 5.3 μm [
125].
We used two different thicknesses of germanium for device fabrication: 2 μm and 3 μm [
134]. Initial investigations were performed using 2 μm Ge layers. The deposition of these layers required less time, which could result in lower cost devices. Latterly, a 3 μm Ge layer was adopted to allow larger waveguide structures; this platform will become the standard CORNERSTONE platform for Ge-on-Si MPW calls. The 3 μm thick Ge layer enabled single mode operation at longer wavelengths and allowed the mode to be more completely confined within the waveguide, thereby reducing its interaction with sidewall roughness and threading dislocations at the Ge-Si boundary, which occur due to the lattice mismatch between Si and Ge.
GOS devices that were 2 μm thick were based on rib waveguides using an etch depth of 1.2 μm and core width of 2.25 μm, which were optimized for single mode operation, with a measured loss of 3 dB/cm in the region λ = 2–3.8 μm. The non-linear response of germanium due to TPA was also investigated using GOS waveguides. The TPA non-linearity
βTPA was measured to be greater than 1 × 10
3 cm/GW in the wavelength range λ = 1.9–2.3 μm, which is approximately 1000 times greater than that of Si, and corresponds closely to
βTPA measured for bulk Ge [
135,
136]. The magnitude and ultrafast nature of the TPA nonlinearity mean this behavior can be used to implement a variety of all-optical functions including modulators, switches, logic gates and pulse shapers.
All-optical modulation was demonstrated using GOS waveguides at λ = 2 μm, where the high
βTPA value enabled high speed cross-absorption modulation (XAM). This experiment used a pump-probe setup where a high-power pump induced an absorption dip on a low-power probe that was too weak to induce TPA itself. Increasing pump power was shown to increase modulation depth, albeit with some roll-off due to pump saturation. A maximum extinction ratio (ER) of 8.1 dB was recorded for a coupled input peak power of 10 W, which is the highest published value for any group IV waveguide device [
137,
138]. Alternatively, due to the fact that free carrier effects are significantly stronger in Ge than Si [
139], electrically induced modulation based on carrier injection into 1 mm long PIN devices has been demonstrated [
140]. A modulation depth of >35 dB with a 7 V forward bias at λ = 3.8 μm was demonstrated when operating as an electroabsorption device, and when operating as an electrorefraction device integrated into an MZI, a
Vπ L of 0.47 V cm, driven by a 2.5
Vpp RF 60 MHz signal, was demonstrated.
3 μm thick GOS devices were based on rib waveguides using an etch depth of 1.7 μm and core width of 2.7 μm, which were optimized for single mode operation. A scanning electron microscope (SEM) image of a ductile machined end facet of a GOS waveguide is shown in
Figure 15. Light was coupled into and out of the devices using surface grating couplers. The etch depth of the grating was kept the same as the waveguide etch depth of 1.7 μm to reduce the number of required fabrication steps. The period (2 μm) and duty cycle (0.5) were optimized for TE mode operation using Lumerical. Subsequently, coupling efficiency has been increased and reflections reduced by using inverse taper excitation. Propagation loss was measured as 0.58 ± 0.12 dB/cm at λ = 3.8 μm, which is significantly less than the data published for other GOS devices [
125,
126,
127].
A wide variety of devices for building PICs have been demonstrated using the 3 μm GOS platform (see
Table 5), including MMI splitters [
141], angled MMIs [
142] and cascaded ring and racetrack resonators [
143]. Low loss 1 × 2 and 2 × 2 MMIs are discussed here as an example.
The optimized 1 × 2 MMI widths (W), lengths (L) and center-to-center output port separation (S) were W
MMI = 10 μm, L
MMI = 58.2 μm, W
taper = 4.75 μm, L
taper = 30 μm and S = 4.75 μm. The optimized 2 × 2 MMI dimensions were identical except for the length of the multimode region, which was L
MMI = 114 μm. MMI loss was calculated from the difference in transmission between different numbers of successively linked MMIs. Loss was measured as 0.21 ± 0.02 dB/MMI for 1 × 2 MMIs and 0.37 ± 0.07 dB/MMI for 2 × 2 MMIs [
141].
GOS long wavelength operation was demonstrated in the wavelength range λ = 7.5–8.5 μm, with a minimum experimentally measured propagation loss of 2.5 dB/cm at λ = 7.575 μm [
144]. An input facet for end-fire coupling was fabricated using ductile machining instead of cleaving, allowing optical-quality end facets with rapid wafer-scale fabrication.
A photonic method was used to measure loss instead of the cutback method. This used a splitting tree of 1 × 2 MMIs to divide power equally between eight waveguides, which terminated in output grating couplers. This allowed power to be quantified using a long wavelength infrared camera above the device instead of a conventional detector, thereby avoiding large coupling losses associated with output end-fire coupling. This improved reproducibility by avoiding both the need for consistent end facet preparation for all eight waveguides and the need to align to each waveguide separately. This also improved the tolerance to fabrication errors due to the symmetry of the MMI devices, which in theory have perfectly even splitting ratios.
The future contains a large variety of potential applications that would benefit from the wide transparency and low loss operation of GOS devices. One obvious next step is to extend the wavelength range of the all-optical GOS modulator to demonstrate modulation at higher wavelengths, in order to take full advantage of the TPA window of Ge.
3.3. Suspended Silicon
In photonics Si is most commonly encountered in the SOI platform for optical communications at the telecoms wavelengths of 1.31 μm or 1.55 μm. Si is a viable mid-infrared material given that its usable transparency window extends up to wavelengths of 8 µm; the factor limiting the wavelength range of SOI devices is the absorption of the buried silica layer above λ = 4 μm.
Air-clad Si structures offer an alternative to germanium-based material platforms (see
Section 3.2) for extending the usable wavelength range beyond that of SOI by elevating a Si waveguide so the propagating mode does not interact with the substrate material. Such structures are implemented in one of two ways: pedestal-type structures [
145], which are supported by a thin rib of Si, or suspended structures, where the region under the waveguide is completely removed and the waveguide is supported by lateral bars or membranes [
146,
147,
148]. Such air-clad platforms avoid using a new material platform and thereby allow the full range of established SOI fabrication techniques to be used.
The major advantage of laterally suspended Si structures is that the suspending structure can be fabricated using lithographically patterned etching. Material underneath the waveguide must be removed using an isotropic etch, whether this is partial (as for pedestal structures) or complete (as for suspended Si devices). This limits the degree of longitudinal patterning that can be achieved underneath the waveguide, for example in the pedestal. Conversely, the lateral supports of a suspended Si can be periodically structured to form a grating [
148]. This enables the mode confinement in the waveguide to be optimized so that the sample–light interaction and therefore sensitivity can be adjusted for a particular application.
Three techniques have been used to suspend Si waveguides. First, Si membrane waveguides have been fabricated using an array of holes far from a rib waveguide core to expose the buried oxide (BOX) to hydrofluoric (HF) acid and undercut the Si rib [
146]. Second, suspended Si membrane rib waveguides were fusion-bonded to a Si substrate, which had prepatterned air trenches, to provide more support to the waveguide and improve stability [
147]. Third, we have demonstrated a method where a subwavelength grating is used to suspend a waveguide core [
148]. The grating is etched through the entire upper Si layer to expose the BOX for removal with HF acid. A schematic of this structure is shown in
Figure 16.
The subwavelength grating (SWG) supporting structure performs several functions, each with its own constraints. The SWG cladding must be strong and stiff enough to physically support the central waveguide structure without sagging or breaking. The periodicity must be such that it forms a subwavelength structure at mid-infrared wavelengths to suppress any propagation outside of the waveguide. The lateral refractive index can be controlled to optimize mode confinement, which is achieved by varying the longitudinal proportion of Si to air gap (i.e.,
LSi:
Lhole in
Figure 16b). A higher fraction of Si provides a lower index contrast with the waveguide. Finally, the gaps between the supporting bars allow the circulation of HF acid to selectively etch SiO
2 under the Si to form the undercut.
The suspended Si platform shares the potential for the same variety of mid-infrared applications as the GOS platform, discussed in detail in
Section 3.2. The choice of which platform is more suitable is application specific. GOS provides a wider transparency region so is better suited to sensing applications that require access to the longer wavelength part of the fingerprint region. The two platforms have comparable loss although GOS is generally slightly lower. We demonstrated straight suspended Si waveguides with loss of 0.82 dB/cm at λ = 3.8 μm and 3.1 dB/cm at λ = 7.7 μm compared with GOS, which exhibited loss 0.58 ± 0.12 dB/cm at λ = 3.8 μm and 2.5 dB/cm at λ = 7.575 μm.
For fluidic sensing, the sample will occupy the evanescent field both above and below a suspended Si waveguide compared with above but not below a GOS waveguide. This gives an increased light-sample interaction for the suspended Si platform so it could potentially achieve higher sensitivity and lower limits of detection.
Suspended Si is more suitable for microfluidic integration than GOS and can implemented, for example, by bonding polydimethylsiloxane flow cells to SiO2 via surface functionalization with oxygen plasma. The bond requires a surface monolayer of SiO2 so can be formed using native oxide. Conversely GOS cannot be covalently bonded to common microfluidic materials so would require an additional cladding or a compression-sealed flow cell instead.
Suspended Si devices are inherently less robust than their fully supported SOI or GOS counterparts so have a lower limit for high pressure fluidic applications. However, if the SWG structure is not used to provide any flow functionality, careful design of the fluidic circuit should mitigate this disadvantage.
The SWG structure limits the size of suspended particles than could be safely transported within liquid samples. A nanoparticle suspension would likely not be influenced by the SWG, surface effects notwithstanding. However, the microparticulate content of biological samples such as cells, blebs and other microvesicles are of a comparable size to the holes in the SWG cladding and would be likely to become trapped, causing flow problems and interfering with the index contrast between the waveguide and the SWG.
Suspended Si has been used to implement components and devices for operation at both λ = 3.8 μm and 7.7 μm including 90° bends, S-bends, MMI couplers and grating couplers. Typical device performance metrics are listed in
Table 6 for TE polarization. Devices for λ = 3.8 µm operation were fabricated using an SOI wafer, which is formed of a 500 nm layer of Si on 3 µm BOX on a Si substrate, which will become the standard CORNERSTONE platform for suspended Si MPW calls. Longer wavelength operation required a thicker 1.4 μm Si layer on top of the buried oxide, which was epitaxially grown on top of standard 500 nm SOI, but this modified thickness will not form part of the standard CORNERSTONE MPW calls.
The future outlook for MIR sensing using the suspended Si platform is broadly similar to that of Ge-on-Si discussed above, albeit with the additional benefits of offering simpler integration with Si MEMS and polydimethylsiloxane (PDMS) microfluidics. One exemplar application would be exploiting the bioanalytical data we have obtained using Fourier-transform infrared spectroscopy (FTIR), including a detailed study of the MIR properties of blood [
149]. When combined with comprehensive knowledge of the MIR performance of the GOS and suspended Si platforms, sensitivity can be maximized for blood-based medical diagnostics.
3.5. Pick-and-Place of Light Sources onto SOI
Although there are a variety of different technologies to integrate III–V active devices on SOI chips, these can be broadly classed into three main groups, namely heteroepitaxial growth, wafer or die bonding and flip-chip integration [
152]. Heteroepitaxial growth of III–V semiconductor layers on a Si substrate would provide the ideal route for large scale industrial manufacturing of PICs; however, current technology in this area lacks the required reliability and performance. In fact the large lattice constant mismatch between silicon and III–V materials such as GaAs and InP, and the formation of antiphase domains at the III–V/silicon interface pose major technological challenges that affect the quality of the epitaxial layers [
153].
In the wafer or die bonding approach, most commonly called heterogeneous integration, pieces of III–V material are bonded onto a patterned Si wafer followed by substrate removal and then processed using standard lithography tools [
154,
155]. The two most exploited bonding techniques are covalent molecular direct bonding and adhesive bonding using polymers, each with its own advantages and limitations [
156]. Wafer bonding allows for very high accuracy lithographic alignment of the III–V devices as it employs the same registration markers used for the SOI chip fabrication; however, this requires running fully processed silicon PIC wafers through an III–V foundry. An alternative approach that circumvents this limitation is transfer printing, in which fully fabricated III–V membrane devices are directly transferred and bonded onto the SOI chip [
157]. Transfer printing retains the advantages of a full post-fabrication process at the expense, however, of a more complicated III–V fabrication processing that require the release of the III/V membranes from the native substrate, usually achieved through selective wet etching of a sacrificial layer [
158]. Additionally, transfer printing does not provide a good heat flow when only Van der Waals forces are used for the bonding. In fact, efficient heat dissipation is a prime requirement of any heterogeneous integration technique as it directly impacts the reliability, lifetime and performance of the III–V active devices. This issue can be mitigated by using a metal, e.g., gold, as a bonding layer [
159] or by bonding the membrane directly on the Si substrate after removal of the SiO
2 BOX layer [
160,
161]. The use of thin III–V membranes in the wafer bonding and transfer printing approaches is ideal to implement vertical mode coupling via efficient adiabatic mode coupling between III/V and SOI waveguides or to design hybrid coupled modes [
162,
163,
164,
165].
Although wafer bonding technology has substantially progressed in the last decade, hybrid integration by flip-chip bonding III–V active devices onto SOI chips is still the industry standard for the manufacturing of PICs in large markets such as telecoms and data centers [
166,
167,
168]. In this approach, prefabricated III–V active devices are picked and placed onto silicon photonic chips usually with microgrippers [
169,
170] and attached to the host substrate via metal-to-metal flip-chip bonding. Crucially, because integration by flip-chip bonding is a post-fabrication technique it allows to independently optimize and prescreen devices fabricated in different foundries, which adds substantial design flexibility, reduces packaging cost and improves device yield. One major requirement of flip-chip integration is an accurate and robust alignment process to minimize the insertion losses that arise by any misalignment between the III/V active devices and the SOI waveguides. From a mechanical point of view, the accuracy of the flip-chip technique can be enhanced via the use of patterned registration markers [
171], computer vision registration techniques and active alignment [
172]. The design of both the III–V and SOI chips in the coupling region can also be optimized to improve the tolerance to misalignment and minimize the coupling losses [
173,
174,
175]. Due to these approaches III/V devices flip-chip bonded to SOI have insertion loss as low as 1.1–1.5 dB [
176,
177,
178].
As detailed in
Section 3.1, CORNERSTONE will also offer PIC development on a silicon nitride platform because of the advantages offered by its wide transparency down to the visible spectral range and beyond. The SiN transparency range is well-covered by a variety of III–V compound semiconductors such as aluminium indium gallium nitride (AlInGaN) [
179,
180], gallium indium phosphide (GaInP), aluminium gallium indium phosphide (AlGaInP) and aluminium gallium arsenide (AlGaAs) [
181]. The integration of these III-V active devices on a SiN material platform could be of great impact to several applications in healthcare, imaging and quantum technologies [
182]. However, SiN presents additional integration challenges due to its lower refractive index, i.e.,
n = 2.0 for the stoichiometric composition, compared to the III/V materials, in the range of
n = 3–3.6. This is often mitigated using a mode-matching waveguide between the III/V and the SiN waveguides made by a material with an intermediate refractive index waveguide, such as silicon or polymer. Examples of integration of III/V devices on a SiN platform are reported in the literature for both flip-chip bonding [
183,
184,
185] and transfer printing [
186,
187,
188] with an insertion loss as low as 2.1 dB.
CORNERSTONE will offer flip-chip bonding by metal-to-metal thermocompression with submicron alignment. Due to the large demand for telecoms applications, the initial focus will be on InP-based distributed feedback (DFB) lasers and InGaAs photodetectors integrated on an SOI platform operating at a wavelength of 1550 nm. This activity will be supported by the design of suitable waveguide geometries to mitigate the impact of misalignment tolerances and by the establishment of design rules that will ultimately be part of the CORNERSTONE PDK. A key feature of this model is that, provided compliance to design and fabrication rules is followed, users will be able to use the CORNERSTONE capability to integrate any III-V device, which adds a unique level of flexibility to support both established and emerging applications at non-telecom wavelengths.