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Overview of the experiment: the right ring is trimmed by electron beam compaction; the left one is kept original as a reference to exclude temperature or ambient variations.
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Silicon is becoming the preferable platform for future integrated components, mostly due to the mature and reliable fabrication capabilities of electronics industry. Nevertheless, even the most advanced fabrication technologies suffer from non-uniformity on wafer scale and on chip scale, causing variations in the critical dimensions of fabricated c...
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... is gaining interest as preferable material system for future ultra- compact integrated photonic components. The main advantages of this material system are firstly the high refractive index contrast between silicon (core) and oxide or air (cladding) enabling small bend radii and dense integration, and secondly mature fabrication facilities thanks to the electronics industry. In spite of the indirect band gap of silicon, both passive [1- 3] and active [4-7] devices have been demonstrated thanks to heterogeneous integration of III- V semiconductors. One of the applications aimed for by the telecom industry is optical filters for wavelength (de)multiplexing. SOI is the ideal platform to make these filters compact and low-cost. Several geometries, such as ring resonators [8] and photonic band gap materials [9] have good filtering characteristics. However, most of the demonstrated filters were fabricated with electron beam lithography, which is a serial fabrication technique and therefore unattractive for mass fabrication. In previous work we have demonstrated that these filters can also be fabricated in a parallel way, with 248 nm Deep-UV lithography (DUV) in a standard Complementary Metal Oxide Semiconductor (CMOS) facility [10]. However, variations of the critical dimensions of devices fabricated by optical lithography are inevitable. These variations can be caused by wafer non-uniformity, by e.g. varying layer thicknesses on wafer edges, or by non-uniformity within one chip, mainly caused by lithography imperfections near mask edges. A way to assess critical dimension variations in a photonic circuit is to evaluate the resonance wavelength shift of identically designed ring resonators, dispersed over a wafer. These resonators can be fabricated with Q-factors of about 10 4 [8], or a 3 dB bandwidth of 0.15 nm. In practice the resonance wavelength shifts exceed 1 nm, which is unacceptable for many applications. The most common solution for this is active thermal tuning [11, 12], however, when many resonators have to be integrated on a single chip, this would lead to high power consumption and important device complexity. Another approach to circumvent these process variations is trimming of the devices after fabrication. In this paper we present a technique to locally and independently trim the resonances of ring resonators on a silicon chip. This allows for complete compensation of resonance wavelength variations on silicon photonic integrated components. The resonance wavelength of a ring resonator is trimmed by changing the optical path length of the resonator, in our case by varying the effective index of the guided mode and not the length of the ring. An increase in effective index causes a red shift of the resonance wavelength. This was demonstrated in several (low to medium index contrast) material systems such as silica glass [13] and SiN/SiON [14, 15]. These core materials are sensitive to either UV or electron beam compaction (also other higher energy particles tend to compact these materials, but these are less commercially practicable). This means that the compacted material contains most of the optical mode and thus relatively large shifts in effective indices can be obtained. In SiN more than 10 nm of resonance wavelength shift was obtained for ring resonators operating at 1550 nm [14]. Electron beam [16-18] as well as UV [19-21] irradiation have been used to fabricate or alter waveguides in silica glass. In SOI, the preferable material system for future industrial deployment, the core material is silicon, which can not be compacted by either UV or electrons. Only the SiO 2 cladding is susceptible to compaction. Literature reports on SiO 2 volume compaction and refractive index changes upon irradiation with highly energetic particles. The particles include ions and gamma photons, by which the oxide compacts due to knock-on atom displacements, or electrons and UV photons, where the compaction is caused by ionization-induced relaxation of inherently strained Si-O bonds in the glass network [22-25]. Due to the imaging capabilities and the ease to precisely control the irradiation dose and energy we have used an electron beam (from an FEI Nova 600 scanning ion/electron microscope) to compact the SiO 2 cladding layer. The experiments presented here differ from previous reports in the fact that the optical core material – silicon – can not be compacted, thus strain induced by cladding compaction is used as trimming mechanism. The resonance frequency of silicon ring resonators is extremely sensitive to changes of temperature and of the surrounding medium; rings are therefore attractive as sensors [26, 27]. However, in this experiment we want to exclude all external factors and investigate the resonance shifts caused only by electron beam irradiation. Therefore we have fabricated a sample with two rings, with different resonance wavelengths, serially connected to the same waveguide, as depicted in Fig. 1. Only one of these rings is irradiated by imaging it with a scanning electron beam. The transmission spectrum features two superimposed ring spectra. By evaluating only relative peak shifts between the two ring spectra the external influences are excluded. The experiment was performed in situ, inside the vacuum chamber of a scanning electron microscope, by providing it with vacuum fiber feedthroughs. The optical input and output signals are transported by single mode fibers, glued (with UV-curable glue) in a near-vertical position above grating couplers. The optical circuit, with grating couplers, waveguides, tapering sections, and ring resonators, was fabricated by DUV lithography in a standard CMOS foundry [10]. Light was generated by a super luminescent LED with center wavelength at 1530 nm and detected by a spectrum analyzer with a resolution of 60 pm. The right ring resonator was trimmed in three subsequent steps, as depicted in Fig. 2. In a first step we have scanned a 0.21 nA beam across a 25.6 m x 22.1 m area for 360 s, and measured a resonance wavelength shift of 1.24 nm. In the next step the same beam was scanned over the same area for 300 s to reach an extra shift of 0.82 nm. In a final step a 0.84 nA beam was scanned over a 17.1 μ m x 14.8 μ m area for 400 s, leading to a subsequent shift of 2.77 nm. In total this leads to a 4.91 nm red shift of the resonance wavelength of the silicon ring. Figure 2 shows the evolution of the resonance peaks, extracted from the measured spectra (one spectrum was measured every 10 s). In the final step the resonance wavelength of the right ring was equated with that of the left ring. To evaluate the propagation losses we have extracted the Q-factors from the measured spectra, as depicted in Fig. 2. It is clear that the Q-factor slightly decreases in the third step of the experiment, which is also visible in the right part of Fig. 2. We argue that the increase during the second step is caused by a varying coupling constant, whereas the decrease in the third step is caused by increasing propagation losses. It is clear from the graph that in the beginning of the electron beam exposure there is a small blue shift of the resonance (between 100 pm and 200 pm blue shift in our experiments), and after the exposure there is settling of the peak towards longer wavelengths (we have measured 153 pm red shift, 45 minutes after the second step). We argue that this is partly caused by charging of the oxide and subsequent generation of carriers in the silicon, and partly by temperature variations due to electron bombardment. It was reported [24] that compacted silica tends to relax over a period of tens of hours after irradiation. However, we have measured a subsequent red shift of 0.15 nm, 5 days after the actual irradiation. This effect needs further investigation. Two distinct physical processes cause the effective index change of the mode in the silicon ring resonator, as depicted in Fig. 3. The first is a larger refractive index of the oxide cladding due to volume compaction; the second is the stress this oxide compaction induces in the silicon lattice. In our experiment we have worked with 2 keV electrons, which have a penetration depth of about 70 nm in Si and SiO 2 (this was calculated with Monte Carlo simulations, and confirmed by [24]). The silicon ring is 220 nm thick, on top of a 2 μ m oxide layer; therefore the electrons can not penetrate the silicon. The mode overlap with the compacted oxide is lower than 1.5%, as was calculated with a mode expansion tool. From [24, 28] we have estimated the maximum amount of refractive index change to be below 3% (i.e. for a compaction of about 10%), with a total irradiation dose of 2.8 x 10 23 keV/cm 3 (the total dose in our experiment). This can not lead to more than 0.5 nm shift in resonance wavelength. We can thus conclude that the largest fraction of the observed resonance wavelength shift is caused by strain in the silicon lattice. Finite elements simulations were used to evaluate this effect (with a variable mesh size, down to 10 nm where a large stress gradient is reached), as shown in Fig. 3. The overlay picture illustrates the deformed mesh (with a scale factor of 2) and the first principal strain in the case of a 10% compacted oxide layer with a thickness of 70 nm. It was reported [28] that the induced stress in the compacted oxide layer saturates upon high dose electron beam irradiation. However, we have not found reports of this effect for the energy and dose range considered in our experiments. Since a complete study was beyond the scope of this work, we have chosen to estimate the influence of compaction induced stress on the effective index of the supported modes without detailed simulations of the optical mode profile in the strained lattice. We have therefore calculated the average silicon strain in the dominant direction: perpendicular to the waveguide propagation direction and in the plane of the ...
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... in this experiment we want to exclude all external factors and investigate the resonance shifts caused only by electron beam irra- diation. Therefore we have fabricated a sample with two rings, with different resonance wavelengths, serially connected to the same waveguide, as depicted in Figure 1. Only one of these rings is irradiated by imaging it with a scanning electron beam. ...
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Citations
... Photonic resonators can be tuned by a variety of mechanisms, including static methods such as post-fabrication modification of the resonator [13][14][15][16] or cladding [17][18][19][20][21][22], or active methods via temperature, electro-optic effect, or mechanical strain [1,2,[5][6][7][8]12,[23][24][25]. Such methods have been shown to enable double or even triple resonance provided the tuning mechanism has a different effect on each mode [1,8,23,25]. ...
Resonant enhancement of nonlinear photonic processes is critical for the scalability of applications such as long-distance entanglement generation. To implement nonlinear resonant enhancement, multiple resonator modes must be individually tuned onto a precise set of process wavelengths, which requires multiple linearly-independent tuning methods. Using coupled auxiliary resonators to indirectly tune modes in a multi-resonant nonlinear cavity is particularly attractive because it allows the extension of a single physical tuning mechanism, such as thermal tuning, to provide the required independent controls. Here we model and simulate the performance and tradeoffs of a coupled-resonator tuning scheme which uses auxiliary resonators to tune specific modes of a multi-resonant nonlinear process. Our analysis determines the tuning bandwidth for steady-state mode field intensity can significantly exceed the inter-cavity coupling rate g if the total quality factor of the auxiliary resonator is higher than the multi-mode main resonator. Consequently, over-coupling a nonlinear resonator mode to improve the maximum efficiency of a frequency conversion process will simultaneously expand the auxiliary resonator tuning bandwidth for that mode, indicating a natural compatibility with this tuning scheme. We apply the model to an existing small-diameter triply-resonant ring resonator design and find that a tuning bandwidth of 136 GHz ≈ 1.1 nm can be attained for a mode in the telecom band while limiting excess scattering losses to a quality factor of 10 ⁶ . Such range would span the distribution of inhomogeneously broadened quantum emitter ensembles as well as resonator fabrication variations, indicating the potential for the auxiliary resonators to enable not only low-loss telecom conversion but also the generation of indistinguishable photons in a quantum network.
... Previous work based on electron beam modification of various cladding materials like silicon dioxide (SiO 2 ), hydrogen silsesquioxane, and polymethyl methacrylate (PMMA) has shown promise in this regard. [30][31][32] However, a major challenge with these approaches is that the wavelength shifts obtained are not stable with time. An alternative approach to tuning the optical properties is by amorphizing crystalline silicon waveguides with germanium (Ge) ion implantation. ...
Photonic Integrated Circuits (PICs) are revolutionizing the realm of information technology, promising unprecedented speeds and efficiency in data processing and optical communication. However, the nanoscale precision required to fabricate these circuits at scale presents significant challenges, due to the need to maintain consistency across wavelength‐selective components, which necessitates individualized adjustments after fabrication. Harnessing spectral alignment by automated silicon ion implantation, in this work scalable and non‐volatile photonic computational memories are demonstrated in high quality resonant devices. Precise spectral trimming of large‐scale photonic ensembles from few picometers to several nanometres is achieved with long‐term stability and marginal loss penalty. Based on this approach spectrally aligned photonic memory and computing systems for general matrix multiplication are demonstrated, enabling wavelength multiplexed integrated architectures at large scales.
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... . This increases the process complexity and complicates limiting the deposition area. 5. Use electron beam-induced compaction upon silica cladding to introduce strains into the waveguide core [37]. This method is costly and difficult to control and repeat since the strains relax over time. ...
Fabrication errors inevitably occur in device manufacturing owing to the limited processing accuracy of commercial silicon photonic processes. For silicon photonic devices, which are mostly processing-sensitive, their performances usually deteriorate significantly. This remains an unsolved issue for mass production, particularly for passive devices, because they cannot be adjusted once fixed in processes. This study presents a post-processing trimming method to compensate for fabrication errors by changing the cladding equivalent refractive indices of devices with femtosecond lasers. The experimental results show that the resonant wavelengths of micro-ring resonators can be regularly shifted within their free spectral range via tuning the illuminating area, focusing position, emitting power, and scanning speed of the trimming femtosecond laser with an acceptable loss increase. These experiments, as well as the trimming experiments in improving the phase balance of Mach-Zehnder interferometer switches, indicate that the femtosecond laser trimming method is an effective and fast method for silicon photonic devices.
... An additional cover layer is modified in trimming of the waveguide cladding. This approach does not deteriorate the waveguide core, it is more universal and can be used for many material platforms [20][21][22][23][24]. Oxide materials are a good choice for an additional cover layer. ...
Local laser oxidation of a thin titanium film is considered as a means of a precise adjustment of losses and effective refractive index of dielectric optical waveguides. A fine phase control of an operating point and extinction ratio enhancement up to 57 dB were demonstrated using an integrated optical Ti:LiNbO 3 Mach-Zehnder modulator. This technique only slightly affects the dielectric waveguide material and is very promising for a high precision permanent trimming of photonic devices based on dielectric waveguides of different material platforms and fabrication technologies.
... Many of these benefits exhibited by ring resonators are due to their resonance characteristics which, unfortunately, also make them extremely susceptible to fabrication variations. Even typical nm-scale fabrication variations in the ring's geometry can significantly modify the roundtrip phase of the light inside the ring, causing its resonance wavelength to vary by a few nanometers (i.e., hundreds of gigahertz near 1310 nm wavelength) [5][6][7][8][9]. Therefore, the exact resonance wavelengths of ring resonatorbased devices cannot be accurately designed and fabricated [10] and, hence, these devices require post-fabrication adjustments of their resonance wavelengths. ...
... While real-time tuning may still be required to account for any resonance drifts due to ambient temperature changes, post-fabrication trimming largely eliminates the power needed to correct for initial fabrication variations. Previous methods to implement changes to the refractive index include exposing fabricated devices to highpower laser beams [11][12][13][14][15][16][17][18][19], electron beams [6,20,21], or visible/UV light [14,22]. These methods are difficult to implement at wafer-scale with high throughput and at low cost. ...
Silicon ring resonator-based devices, such as modulators, detectors, filters, and switches, play important roles in integrated photonic circuits for optical communication, high-performance computing, and sensing applications. However, the high sensitivity to fabrication variations has limited their volume manufacturability and commercial adoption. Here, we report a low-cost post-fabrication trimming method to tune the resonance wavelength of a silicon ring resonator and correct for fabrication variations at wafer-scale. We use a Ge implant to create an index trimmable section in the ring resonator and an on-chip heater to apply a precise and localized thermal annealing to tune and set its resonance to a desired wavelength. We demonstrate resonance wavelength trimming of ring resonators fabricated across a 300 mm silicon-on-insulator (SOI) wafer to within +/-32 pm of a target wavelength of 1310 nm, providing a viable path to high-volume manufacturing and opening up new practical applications for these devices.
... temperature [16,17], electric field [18,19]) or trimmed via processes that permanently change device parameters (e.g. etching [20][21], introducing strain [22,23], modification of cladding material [24][25][26][27]). Resonant wavelength modification of ring resonators for frequency conversion processes presents a particularly complex challenge due to the simultaneous involvement of multiple wavelengths. ...
We demonstrate post-fabrication target-wavelength trimming with a gallium phosphide on a silicon nitride integrated photonic platform using controlled electron-beam exposure of hydrogen silsesquioxane cladding. A linear relationship between the electron-beam exposure dose and resonant wavelength red-shift enables deterministic, individual trimming of multiple devices on the same chip to within 30 pm of a single target wavelength. Second harmonic generation from telecom to near infrared at a target wavelength is shown in multiple devices with quality factors on the order of 104. Post-fabrication tuning is an essential tool for targeted wavelength applications including quantum frequency conversion.
... Electron beam and laser irradiation are two techniques that have been used for trimming, with each utilising a different process to realise a refractive index change [27]. Inducing strain and compaction into an oxide cladding by e-beam can change the effective index of the optical mode in the devices [28,29]. The polymethyl methacrylate cladding allows the resonant wavelength of ring resonators to be trimmed after irradiation by ultraviolet light [30]. ...
Germanium (Ge) ion implantation into silicon waveguides will induce lattice defects in the silicon, which can eventually change the crystal silicon into amorphous silicon and increase the refractive index from 3.48 to 3.96. A subsequent annealing process, either by using an external laser or integrated thermal heaters can partially or completely remove those lattice defects and gradually change the amorphous silicon back into the crystalline form and, therefore, reduce the material’s refractive index. Utilising this change in optical properties, we successfully demonstrated various erasable photonic devices. Those devices can be used to implement a flexible and commercially viable wafer-scale testing method for a silicon photonics fabrication line, which is a key technology to reduce the cost and increase the yield in production. In addition, Ge ion implantation and annealing are also demonstrated to enable post-fabrication trimming of ring resonators and Mach–Zehnder interferometers and to implement nonvolatile programmable photonic circuits.
... Trimming of waveguide cladding has been demonstrated via electron beam induced compaction and strain of silicon dioxide (SiO2) [18], electron bleaching of chromophore-doped polymer [19], photodarkening of chalcogenide (A2S3) glass [20] and corrective etching of nitride films [21]. The additional cover layer is modified in this approach without affecting the waveguide core. ...
Local laser oxidation of a thin titanium film is considered as a means for achieving a precise adjustment of absorption coefficient and effective refractive index of dielectric optical waveguides. A fine phase tuning of an operating point and extinction ratio enhancement up to 57 dB were demonstrated on an integrated optical Ti:LiNbO3 Mach-Zehnder modulator. The technique affects only slightly the dielectric waveguide material and is very promising for a high accuracy permanent trimming of photonic devices based on dielectric waveguides of different material platforms and fabrication technologies.
... temperature [16,17], electric field [18,19]) or trimmed via processes that permanently change device parameters (e.g. etching [20][21][22], introducing strain [23,24], modification of cladding material [25][26][27][28]). Resonant wavelength modification of ring resonators for frequency conversion processes presents a particularly complex challenge due to the simultaneous involvement of multiple wavelengths. ...
We demonstrate post-fabrication target-wavelength trimming with a gallium phosphide on silicon nitride integrated photonic platform using controlled electron-beam exposure of hydrogen silsesquioxane cladding. A linear relationship between the electron-beam exposure dose and resonant wavelength red-shift enables deterministic, individual trimming of multiple devices on the same chip to within 30 pm of a single target wavelength. Second harmonic generation from telecom to near infrared at a target wavelength is shown in multiple devices with quality factors on the order of $10^4$. Post-fabrication tuning is an essential tool for targeted wavelength applications including quantum frequency conversion
... An alternative approach is to permanently trim the device to a resonance at a desired wavelength, usually achieved by changing the effective index of the optical waveguide, often the cladding which is accessible post-fabrication. Prominent examples of modification of an SiO 2 cladding include the work of Spector et al. [21] who employed integrated micro-heaters formed within the silicon waveguide slab region of a silicon photonic circuit to anneal low-temperature deposited SiO 2 ; Schrauwen et al. [22] who used electron beam irradiation to induce SiO 2 compaction, straining a silicon waveguide core causing a shift of 4.91 nm in an MRR; Biryukova et al. [23] who showed a shift of 1.1 nm in a silicon MRR clad in hydrogen silsesquioxane (HSQ) using visible irradiation solicited through a commercial micro-Raman spectrometer; and Hagan et al. [24] in which a 4-channel silicon filter was trimmed via ion implantation and integrated annealing of lattice defects. Other trimming methods have also been reported, including changing the index of a photosensitive cladding polymer by UV irradiation [25,26], visible irradiation of a chalcogenide cladding on a silicon waveguide [27]; local oxidation of Si waveguides [28]; deposition of thin films followed by partial etching [29]; UV laser annealing [30]; application of an electric field to a polymerizable liquid crystal [31]; and direct UV irradiation of the core of a Si 3 N 4 waveguide [32]. ...
... In [21] trimmed Si MRR cladded with PECVD oxide underwent more than 500 pm shift after 1 day of storage in open air (essentially total relaxation to their original resonances). Reference [22] reported a Si MRRs trimmed with electron beam induced compaction of SiO 2 cladding to a total of 4.91 nm experience a further red shift of 0.15 nm along the trimming direction 5 days after the irradiation. Photo-induced trimming of chalcogenide-cladded Si MRRs in [27] observed a 0.16 nm drift of a device trimmed to a total of 6.7 nm. ...
We report a resonance trimming technique, applicable to waveguides employing an SiO 2 cladding. The SiO 2 is deposited by a room temperature sputtering process. Resonance shifts of micro-ring resonators of 4.4 nm were achieved with furnace annealing, whereas a resonance shift of 1.4 nm was achieved using integrated micro-heaters. For our device layout, with 30 μ m ring separation, the thermal cross-talk is negligible, and isolated trimming of each micro-ring is achieved. Three, single-channel ring filters on the same substrate were aligned to the same wavelength within a 20 pm precision. The stability of trimmed micro-rings was assessed following extended storage in atmospheric ambient. For a ring shifted by 4.4 nm using furnace annealing, relaxation of 540 pm is observed, while for a ring shifted by 1.4 nm using integrated heaters, the relaxation is 270 pm.
