rus
Issue #4/2022
A. A. Nikitin, K. O. Voropaev, A. A. Ershov, I. A. Ryabсev, A. V. Kondrashov, M. V. Parfenov, A. A. Semenov, A. V. Shamrai, E. I. Terukov, A. V. Petrov, A. B. Ustinov
Study of Silicon Nitride Film Deposition Technology for Application in the Photonic Integrated Circuits
Study of Silicon Nitride Film Deposition Technology for Application in the Photonic Integrated Circuits
DOI: 10.22184/1993-7296.FRos.2022.16.4.296.304
The article is devoted to the production technology of optical micro-waveguides made of silicon nitride. The silicon substrates with a silicon oxide sublayer were used to produce the waveguide structures. The silicon nitride films were deposited on the silicon oxide surface by plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition. The silicon nitride film thickness varied from 710 to 730 nm, depending on the chemical vapor deposition technology. Photolithography and reactive-ion etching were used to produce the waveguide structures. The waveguide structure width varied from 1 to 5 μm with a pitch of 500 nm. A cladding layer of silicon oxide was deposited on the structure surface. This paper describes the study of losses at a wavelength of 1.55 μm in the waveguide structures made by both chemical vapor deposition methods. A comparison of the deposition methods demonstrated that the developed method of plasma-enhanced chemical vapor deposition provided a significant reduction in the losses in structures compared to the low-pressure chemical vapor deposition.
The article is devoted to the production technology of optical micro-waveguides made of silicon nitride. The silicon substrates with a silicon oxide sublayer were used to produce the waveguide structures. The silicon nitride films were deposited on the silicon oxide surface by plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition. The silicon nitride film thickness varied from 710 to 730 nm, depending on the chemical vapor deposition technology. Photolithography and reactive-ion etching were used to produce the waveguide structures. The waveguide structure width varied from 1 to 5 μm with a pitch of 500 nm. A cladding layer of silicon oxide was deposited on the structure surface. This paper describes the study of losses at a wavelength of 1.55 μm in the waveguide structures made by both chemical vapor deposition methods. A comparison of the deposition methods demonstrated that the developed method of plasma-enhanced chemical vapor deposition provided a significant reduction in the losses in structures compared to the low-pressure chemical vapor deposition.
Теги: lpcvd microwave photonics pecvd photonic integrated circuits silicon nitride нитрид кремния радиофотоника фотонные интегральные схемы
Study of Silicon Nitride Film Deposition Technology for Application in the Photonic Integrated Circuits
A. A. Nikitin1, K. O. Voropaev3, A. A. Ershov1, I. A. Ryabсev1, A. V. Kondrashov1, M. V. Parfenov2, A. A. Semenov1, A. V. Shamrai2, E. I. Terukov2, A. V. Petrov3, A. B. Ustinov1
Saint Petersburg Electrotechnical University «LETI», Saint-Petersburg, Russia
Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Saint-Petersburg, Russia
OKB-Planeta JSC, Veliky Novgorod, Russia
The article is devoted to the production technology of optical micro-waveguides made of silicon nitride. The silicon substrates with a silicon oxide sublayer were used to produce the waveguide structures. The silicon nitride films were deposited on the silicon oxide surface by plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition. The silicon nitride film thickness varied from 710 to 730 nm, depending on the chemical vapor deposition technology. Photolithography and reactive-ion etching were used to produce the waveguide structures. The waveguide structure width varied from 1 to 5 μm with a pitch of 500 nm. A cladding layer of silicon oxide was deposited on the structure surface. This paper describes the study of losses at a wavelength of 1.55 μm in the waveguide structures made by both chemical vapor deposition methods. A comparison of the deposition methods demonstrated that the developed method of plasma-enhanced chemical vapor deposition provided a significant reduction in the losses in structures compared to the low-pressure chemical vapor deposition.
Keywords: microwave photonics, photonic integrated circuits, silicon nitride, LPCVD, PECVD
Article received: 16.05.2022
Article accepted: 03.06.2022
1. Introduction
One of the breakthrough aspects to develop in the modern microwave (MW) electronics is integrated microwave photonics [1, 2]. At present, the microwave photonics principles are actively used to develop various MW devices and integrated devices, for example, filters, delay lines, oscillators, synthesizers, and frequency standards [3–7]. Therefore, the topical issue is the development of production technology for the photonic integrated circuits (PICs). Silicon nitride is of particular interest [12–15] among various promising technological platforms for the PIC production, such as silicon-on-insulator [8–10], indium phosphide [11]. This material provides a sufficiently high contrast required for a high density of elements on the PICs; ultra-low losses, the value of which reaches 0.01 dB / cm at a bending radius of 80 microns in a band of the one octave order relative to the telecommunication wavelength of 1.55 microns; high Kerr-type nonlinearity (n2 = 2.4 · 10–19 m2 / W); low nonlinear attenuation, in particular, absence of two-photon absorption. In addition, silicon nitride is fully compatible with CMOS integrated circuit technology. Due to the above advantages, silicon nitride is promising not only for the development of passive linear microwave photonic devices, but also for the nonlinear applications such as the wavelength conversion, monochromatic signal generation, frequency combs, chaos, and supercontinuum [16–19].
At present, two production methods for the silicon nitride films are widely used. The first method is the chemical vapor deposition in the low-pressure flow reactors (in foreign references, this method is known as LPCVD – Low Pressure Chemical Vapor Deposition) [15, 20]. A decrease in pressure (1–1000 mT) during the LPCVD process leads to an increased diffusion rate of reagents in the gas phase, as a result of which the film is formed in the kinetic control mode that ensures high uniformity and quality of the growing film. The limiting factor of this method is the need to use a high substrate temperature (450–1000 °C) providing the occurrence of chemical reactions on the growing film surface that leads to a decreased growth rate (2–5 nm / min), as well as to the need for precise temperature control on the substrate. The second method is the plasma-enhanced chemical vapor deposition (in foreign references, this method is known as PECVD – Plasma Enhanced Chemical Vapor Deposition). PECVD, in contrast to the expensive LPCVD method, is a more accessible method for the silicon nitride film deposition with the released internal stresses up to 1 µm at a much higher deposition rate (25–350 nm / min) and a lower temperature of about 25–500 °C [14, 21]. Therefore, the objective of this paper was to produce the silicon nitride films on the silicon substrates with a silicon oxide sublayer by the PECVD and LPCVD methods, as well as to compare the obtained film quality by measuring the attenuation of optical signal propagating in the integral waveguides made from these films.
2. Production technology of microwave guides
The production process for Si3N4 optical micro-waveguides with a width of 1 µm to 5 µm is shown in Fig. 1. At the first stage (see Fig. 1.I), a silicon oxide film was grown on the three-inch silicon substrates by thermal oxidation in wet oxygen. The thickness and refractive index of the grown SiO2 layer were measured using ellipsometry at a wavelength of 1550 nm. These values were equal to 2.8 µm and 1.44, respectively. At the second stage (see Fig. 1.II), silicon nitride was deposited using the LPCVD or PECVD methods.
a) The PECVD deposition process was performed using a Sentec SI 500D setup. This process was performed with 5% mixture of silane SiH4 with nitrogen (200 cm3 / min) and pure nitrogen (5 cm3 / min). The substrate temperature during deposition was 250 °C, and the pressure was maintained at 4 Pa. The deposition process was carried out at a frequency of 13.56 MHz with a discharge power of 1000 W. The thickness and refractive index of the deposited film were measured using ellipsometry at a wavelength of 1550 nm. These values were 730 nm and 2.01, respectively.
b) In the LPCVD process, a low-pressure flow reactor and a diffusion furnace were used. The silicon nitride film was obtained by the following chemical reaction of silane and ammonia:
3SiH4 + 4NH3 → Si3N4 + 12H2.
The following technological parameters were used in the deposition process: substrate temperature – 850 °C; pressure in the reactor – 45 Pa; the ratio of SiH4 / NH3 flows – 1 / 3. As a result, a silicon nitride film with the thickness of 712 nm was obtained. To measure the thickness and refractive index of the grown film, we used the ellipsometry method at a wavelength of 1550 nm. The selected process parameters provided the same refractive index of 2.01 as in the previous case.
At the next stage (see Fig. 1.III), a layer of positive photoresist S1813 G2 SP‑15 with a thickness of 1.8 µm was deposited on the silicon nitride film surface by the centrifugation method. The photoresist was cured at a temperature of 90–110 °C. Then, the contact ultraviolet lithography was applied (see Fig. 1.IV) using a Suss MA6 / BA6 unit. The mask work used for lithography consisted of ten strips with a width of 1 µm to 5 µm and a pitch of 0.5 µm. The photoresist development (see Fig. 1.V) was performed in a 0.6% KOH solution. After development, the photoresist was dried at a temperature of 90–110 °C.
The next stage was the reactive-ion etching of silicon nitride on a photoresist mask (see Fig. 1.VI). Etching was performed at a pressure of 5 Pa in a mixture of CF4 and O2. The CF4 flow was equal to 80 cm3 / min and the O2 flow was equal to 16 cm3 / min. The plasma discharge power was 50 W. After etching, the masking photoresist was removed using SPR‑01F (see Fig. 1.VII). As a result of the etching process, the silicon nitride strips were obtained. The waveguide morphology was verified using the atomic force microscopy. As a result, it was found that the waveguides obtained have the sufficiently steep and even walls. The wall steepness of the micro-waveguides was at least 70°. The widths of the waveguides at the base were greater than the width of the waveguides on the mask work by 340 ± 140 nm. The width values of the waveguides on the mask work will be used to describe the results obtained in this text.
At the final stage (see Fig. 1.VIII), a silicon oxide cladding layer was deposited on the substrate surface. The deposition process consisted of two stages. At the first stage, a SiO2 layer with the thickness of 2 µm was deposited by the LPCVD method with the following parameters: temperature – 250 °C, time – 18 minutes, pressure – 4.5 Pa, silane and oxygen flows – 266 cm3 / min and 400 cm3 / min, respectively. This ensured high conformity of the micro-waveguide coating. According to the ellipsometry results at 1550 nm, the refractive index of this layer was equal to 1.42. At the second stage, an outer layer with the thickness of 1.13 μm was grown by the PECVD method. During the plasma-enhanced chemical deposition of silicon oxide, the following parameters were used: temperature – 250 °C, discharge power – 50 W, pressure – 4 Pa, silane and oxygen flows – 700 cm3 / min and 50 cm3 / min, respectively. According to the ellipsometry results at 1550 nm, the refractive index of the cladding layer was equal to 1.45.
The substrates with the waveguide structures were split in half after the production process. Half of the substrates were subjected to the high-temperature annealing at a temperature of 1100 °C for 90 minutes in an N2 atmosphere. A fragment of the produced waveguide array is shown in Fig. 2(a).
To measure the insertion loss, the plates with micro-waveguides were cut into the rectangular chips with a surface area of 5 × 5 mm2. In order to reduce the number of splits and scratches that occur when cutting the plate into chips, the samples were subsequently polished. Polishing was performed at an angle of 90 degrees using a KrellTech device.
3. Study of a transmission factor of the straight waveguides manufactured using the LPCVD and PECVD technology
The measuring equipment consisted of a laser with a wavelength of 1.55 μm, a polarization controller, input and output lensed fibers, differential translators, and a power meter. The input laser power applied to the fiber was 21.4 mW. The polarization controller was used to maintain the TE polarization of the input radiation, providing the minimal propagation losses. Each chip contained 4 identical series consisting of 9 micro-waveguides with a width of 1 to 5 μm. The radiation was introduced and received using the lensed fibers with a focal length of 12 μm and a focal spot size of 2 μm.
Figure 2(b) demonstrates the averaged measurement results for 4 series of micro-waveguides produced using the LPCVD technology without any high-temperature annealing (red lines and symbols), LPCVD with annealing (black lines and symbols), PECVD without annealing (green lines and symbols), PECVD with annealing (blue lines and symbols). Based on the measurement results, the following conclusions can be drawn:
The waveguide structures made using the LPCVD technology demonstrate rather high losses, the level of which is decreased almost linearly with the increased width of the waveguide structures.
The high-temperature annealing has almost no effect on the level of losses. One of the most probable reasons for such behavior is the high level of defects developed as a result of mechanical stresses generated during the silicon nitride film growth with a thickness of 712 μm.
It is known that during the Si3N4 growth using the LPCVD method, the tensile stresses occur that make it difficult to obtain the films with a thickness of more than 400 μm. To reduce stresses, the grooves can be made in the Si3N4 film [22–24], and the two-stage deposition with intermediate high-temperature annealing at a temperature of 1200 °C for 3 hours in an argon atmosphere can also be used [15]. The multilayer deposition with periodical annealing and grooving relate to the additional processing stages that greatly slow down the production process and increase the production costs. As can be seen from the results presented in Fig. 2(b), the selected annealing mode did not provide the stress relaxation.
The PECVD technology provided the waveguide structures with a significantly lower level of insertion attenuation. The provided dependence can be conditionally divided into two sections. In the first section up to 3 μm, the losses are rapidly decreased with the increased waveguide width. The losses in this area are related to the use of classical photolithography and can be reduced by fine-tuning of the photolithography and etching processes. In the second section, beginning from a width of 3 μm, the losses weakly depend on the waveguide width. It is known that the factor limiting the use of the PECVD method for the PIC production on silicon nitride is a significantly higher level of propagation losses. The reason for this phenomenon is the absorption on Si-H and N-H complexes remaining during the film growth process [25]. In this case, the main method for reducing insertion loss is thermal annealing that ensures the bond disruption in Si-H, N-H complexes and free hydrogen removal from the film [26, 27]. As can be seen from Fig. 2(b), the structure annealing provides a 1 dB loss reduction that is most likely due to the decomposition of the Si-H and N-H complexes and free hydrogen release.
4. Conclusion
The paper compares the optical properties of silicon nitride microwave guides produced by the plasma-enhanced chemical vapor deposition and chemical vapor deposition in the low-pressure flow reactors. Moreover, special attention is paid to the possible transfer of the results obtained to the mass production of photonic integrated circuits using the classical lithography. It is shown that the developed method of plasma-enhanced chemical deposition using photolithography provides the losses in structures with a cross section of 0.7 × 3 μm2 at a level of 12 dB, including the losses for the optical signal input and output. Moreover, the developed production methods for the waveguide structures can be used for mass production of the photonic integrated circuits. To further reduce insertion losses, it is necessary to study and refine the annealing methods, as well as to refine the photolithographic methods in order to improve the waveguide quality that will make it possible to reduce the waveguide width to a level of 1.5 μm.
Acknowledgement
The work was performed as a part of the state task from the Ministry of Science and Higher Education of the Russian Federation (grant No. FSEE‑2020-0005).
АВТОРЫ
Никитин А. А., к. ф.‑ м. н., кафедра физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0002-4226-4341
Ершов А. А., аспирант, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0003-3600-4946
Рябцев И. А., аспирант, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID 0000-0001-8158-8827
Кондрашов А. В., к. ф.‑ м. н., кафедра физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0002-7337-8907
Семенов А. А., д.т.н., доцент, зав. кафедрой физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0003-2348-3773
Устинов А. Б., д. ф.‑ м. н., руководитель лаборатории магноники и радиофотоники им. Б. А. Калиникоса, профессор, кафедра физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0002-7382-9210
Воропаев К. О. начальник группы, отдел 1. разработка ИЭТ,
АО «ОКБ-Планета», Великий Новгород, Россия.
ORCID: 0000-0002-6159-8902
Петров А. В., к.т.н., генеральный директор, АО «ОКБ-Планета», Великий Новгород, Россия.
Парфенов М. В., м.н.с., лаб. квантовой электроники, ФТИ им. А. Ф. Иоффе РАН, С-Пб, Россия.
ORCID: 0000-0003-3867-9007
Шамрай А. В., д. ф.‑ м. н., зав. лаб. квантовой электроники, ФТИ им. А. Ф. Иоффе, С-Пб, Россия.
ORCID: 0000 0003 0292 8673
Теруков Е. И., д.т.н., зав. лаб. физико-химических свойств полупроводников, ФТИ им. А. Ф. Иоффе РАН, С-Пб, Россия.
ORCID: 0000-0002-4818-4924
A. A. Nikitin1, K. O. Voropaev3, A. A. Ershov1, I. A. Ryabсev1, A. V. Kondrashov1, M. V. Parfenov2, A. A. Semenov1, A. V. Shamrai2, E. I. Terukov2, A. V. Petrov3, A. B. Ustinov1
Saint Petersburg Electrotechnical University «LETI», Saint-Petersburg, Russia
Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Saint-Petersburg, Russia
OKB-Planeta JSC, Veliky Novgorod, Russia
The article is devoted to the production technology of optical micro-waveguides made of silicon nitride. The silicon substrates with a silicon oxide sublayer were used to produce the waveguide structures. The silicon nitride films were deposited on the silicon oxide surface by plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition. The silicon nitride film thickness varied from 710 to 730 nm, depending on the chemical vapor deposition technology. Photolithography and reactive-ion etching were used to produce the waveguide structures. The waveguide structure width varied from 1 to 5 μm with a pitch of 500 nm. A cladding layer of silicon oxide was deposited on the structure surface. This paper describes the study of losses at a wavelength of 1.55 μm in the waveguide structures made by both chemical vapor deposition methods. A comparison of the deposition methods demonstrated that the developed method of plasma-enhanced chemical vapor deposition provided a significant reduction in the losses in structures compared to the low-pressure chemical vapor deposition.
Keywords: microwave photonics, photonic integrated circuits, silicon nitride, LPCVD, PECVD
Article received: 16.05.2022
Article accepted: 03.06.2022
1. Introduction
One of the breakthrough aspects to develop in the modern microwave (MW) electronics is integrated microwave photonics [1, 2]. At present, the microwave photonics principles are actively used to develop various MW devices and integrated devices, for example, filters, delay lines, oscillators, synthesizers, and frequency standards [3–7]. Therefore, the topical issue is the development of production technology for the photonic integrated circuits (PICs). Silicon nitride is of particular interest [12–15] among various promising technological platforms for the PIC production, such as silicon-on-insulator [8–10], indium phosphide [11]. This material provides a sufficiently high contrast required for a high density of elements on the PICs; ultra-low losses, the value of which reaches 0.01 dB / cm at a bending radius of 80 microns in a band of the one octave order relative to the telecommunication wavelength of 1.55 microns; high Kerr-type nonlinearity (n2 = 2.4 · 10–19 m2 / W); low nonlinear attenuation, in particular, absence of two-photon absorption. In addition, silicon nitride is fully compatible with CMOS integrated circuit technology. Due to the above advantages, silicon nitride is promising not only for the development of passive linear microwave photonic devices, but also for the nonlinear applications such as the wavelength conversion, monochromatic signal generation, frequency combs, chaos, and supercontinuum [16–19].
At present, two production methods for the silicon nitride films are widely used. The first method is the chemical vapor deposition in the low-pressure flow reactors (in foreign references, this method is known as LPCVD – Low Pressure Chemical Vapor Deposition) [15, 20]. A decrease in pressure (1–1000 mT) during the LPCVD process leads to an increased diffusion rate of reagents in the gas phase, as a result of which the film is formed in the kinetic control mode that ensures high uniformity and quality of the growing film. The limiting factor of this method is the need to use a high substrate temperature (450–1000 °C) providing the occurrence of chemical reactions on the growing film surface that leads to a decreased growth rate (2–5 nm / min), as well as to the need for precise temperature control on the substrate. The second method is the plasma-enhanced chemical vapor deposition (in foreign references, this method is known as PECVD – Plasma Enhanced Chemical Vapor Deposition). PECVD, in contrast to the expensive LPCVD method, is a more accessible method for the silicon nitride film deposition with the released internal stresses up to 1 µm at a much higher deposition rate (25–350 nm / min) and a lower temperature of about 25–500 °C [14, 21]. Therefore, the objective of this paper was to produce the silicon nitride films on the silicon substrates with a silicon oxide sublayer by the PECVD and LPCVD methods, as well as to compare the obtained film quality by measuring the attenuation of optical signal propagating in the integral waveguides made from these films.
2. Production technology of microwave guides
The production process for Si3N4 optical micro-waveguides with a width of 1 µm to 5 µm is shown in Fig. 1. At the first stage (see Fig. 1.I), a silicon oxide film was grown on the three-inch silicon substrates by thermal oxidation in wet oxygen. The thickness and refractive index of the grown SiO2 layer were measured using ellipsometry at a wavelength of 1550 nm. These values were equal to 2.8 µm and 1.44, respectively. At the second stage (see Fig. 1.II), silicon nitride was deposited using the LPCVD or PECVD methods.
a) The PECVD deposition process was performed using a Sentec SI 500D setup. This process was performed with 5% mixture of silane SiH4 with nitrogen (200 cm3 / min) and pure nitrogen (5 cm3 / min). The substrate temperature during deposition was 250 °C, and the pressure was maintained at 4 Pa. The deposition process was carried out at a frequency of 13.56 MHz with a discharge power of 1000 W. The thickness and refractive index of the deposited film were measured using ellipsometry at a wavelength of 1550 nm. These values were 730 nm and 2.01, respectively.
b) In the LPCVD process, a low-pressure flow reactor and a diffusion furnace were used. The silicon nitride film was obtained by the following chemical reaction of silane and ammonia:
3SiH4 + 4NH3 → Si3N4 + 12H2.
The following technological parameters were used in the deposition process: substrate temperature – 850 °C; pressure in the reactor – 45 Pa; the ratio of SiH4 / NH3 flows – 1 / 3. As a result, a silicon nitride film with the thickness of 712 nm was obtained. To measure the thickness and refractive index of the grown film, we used the ellipsometry method at a wavelength of 1550 nm. The selected process parameters provided the same refractive index of 2.01 as in the previous case.
At the next stage (see Fig. 1.III), a layer of positive photoresist S1813 G2 SP‑15 with a thickness of 1.8 µm was deposited on the silicon nitride film surface by the centrifugation method. The photoresist was cured at a temperature of 90–110 °C. Then, the contact ultraviolet lithography was applied (see Fig. 1.IV) using a Suss MA6 / BA6 unit. The mask work used for lithography consisted of ten strips with a width of 1 µm to 5 µm and a pitch of 0.5 µm. The photoresist development (see Fig. 1.V) was performed in a 0.6% KOH solution. After development, the photoresist was dried at a temperature of 90–110 °C.
The next stage was the reactive-ion etching of silicon nitride on a photoresist mask (see Fig. 1.VI). Etching was performed at a pressure of 5 Pa in a mixture of CF4 and O2. The CF4 flow was equal to 80 cm3 / min and the O2 flow was equal to 16 cm3 / min. The plasma discharge power was 50 W. After etching, the masking photoresist was removed using SPR‑01F (see Fig. 1.VII). As a result of the etching process, the silicon nitride strips were obtained. The waveguide morphology was verified using the atomic force microscopy. As a result, it was found that the waveguides obtained have the sufficiently steep and even walls. The wall steepness of the micro-waveguides was at least 70°. The widths of the waveguides at the base were greater than the width of the waveguides on the mask work by 340 ± 140 nm. The width values of the waveguides on the mask work will be used to describe the results obtained in this text.
At the final stage (see Fig. 1.VIII), a silicon oxide cladding layer was deposited on the substrate surface. The deposition process consisted of two stages. At the first stage, a SiO2 layer with the thickness of 2 µm was deposited by the LPCVD method with the following parameters: temperature – 250 °C, time – 18 minutes, pressure – 4.5 Pa, silane and oxygen flows – 266 cm3 / min and 400 cm3 / min, respectively. This ensured high conformity of the micro-waveguide coating. According to the ellipsometry results at 1550 nm, the refractive index of this layer was equal to 1.42. At the second stage, an outer layer with the thickness of 1.13 μm was grown by the PECVD method. During the plasma-enhanced chemical deposition of silicon oxide, the following parameters were used: temperature – 250 °C, discharge power – 50 W, pressure – 4 Pa, silane and oxygen flows – 700 cm3 / min and 50 cm3 / min, respectively. According to the ellipsometry results at 1550 nm, the refractive index of the cladding layer was equal to 1.45.
The substrates with the waveguide structures were split in half after the production process. Half of the substrates were subjected to the high-temperature annealing at a temperature of 1100 °C for 90 minutes in an N2 atmosphere. A fragment of the produced waveguide array is shown in Fig. 2(a).
To measure the insertion loss, the plates with micro-waveguides were cut into the rectangular chips with a surface area of 5 × 5 mm2. In order to reduce the number of splits and scratches that occur when cutting the plate into chips, the samples were subsequently polished. Polishing was performed at an angle of 90 degrees using a KrellTech device.
3. Study of a transmission factor of the straight waveguides manufactured using the LPCVD and PECVD technology
The measuring equipment consisted of a laser with a wavelength of 1.55 μm, a polarization controller, input and output lensed fibers, differential translators, and a power meter. The input laser power applied to the fiber was 21.4 mW. The polarization controller was used to maintain the TE polarization of the input radiation, providing the minimal propagation losses. Each chip contained 4 identical series consisting of 9 micro-waveguides with a width of 1 to 5 μm. The radiation was introduced and received using the lensed fibers with a focal length of 12 μm and a focal spot size of 2 μm.
Figure 2(b) demonstrates the averaged measurement results for 4 series of micro-waveguides produced using the LPCVD technology without any high-temperature annealing (red lines and symbols), LPCVD with annealing (black lines and symbols), PECVD without annealing (green lines and symbols), PECVD with annealing (blue lines and symbols). Based on the measurement results, the following conclusions can be drawn:
The waveguide structures made using the LPCVD technology demonstrate rather high losses, the level of which is decreased almost linearly with the increased width of the waveguide structures.
The high-temperature annealing has almost no effect on the level of losses. One of the most probable reasons for such behavior is the high level of defects developed as a result of mechanical stresses generated during the silicon nitride film growth with a thickness of 712 μm.
It is known that during the Si3N4 growth using the LPCVD method, the tensile stresses occur that make it difficult to obtain the films with a thickness of more than 400 μm. To reduce stresses, the grooves can be made in the Si3N4 film [22–24], and the two-stage deposition with intermediate high-temperature annealing at a temperature of 1200 °C for 3 hours in an argon atmosphere can also be used [15]. The multilayer deposition with periodical annealing and grooving relate to the additional processing stages that greatly slow down the production process and increase the production costs. As can be seen from the results presented in Fig. 2(b), the selected annealing mode did not provide the stress relaxation.
The PECVD technology provided the waveguide structures with a significantly lower level of insertion attenuation. The provided dependence can be conditionally divided into two sections. In the first section up to 3 μm, the losses are rapidly decreased with the increased waveguide width. The losses in this area are related to the use of classical photolithography and can be reduced by fine-tuning of the photolithography and etching processes. In the second section, beginning from a width of 3 μm, the losses weakly depend on the waveguide width. It is known that the factor limiting the use of the PECVD method for the PIC production on silicon nitride is a significantly higher level of propagation losses. The reason for this phenomenon is the absorption on Si-H and N-H complexes remaining during the film growth process [25]. In this case, the main method for reducing insertion loss is thermal annealing that ensures the bond disruption in Si-H, N-H complexes and free hydrogen removal from the film [26, 27]. As can be seen from Fig. 2(b), the structure annealing provides a 1 dB loss reduction that is most likely due to the decomposition of the Si-H and N-H complexes and free hydrogen release.
4. Conclusion
The paper compares the optical properties of silicon nitride microwave guides produced by the plasma-enhanced chemical vapor deposition and chemical vapor deposition in the low-pressure flow reactors. Moreover, special attention is paid to the possible transfer of the results obtained to the mass production of photonic integrated circuits using the classical lithography. It is shown that the developed method of plasma-enhanced chemical deposition using photolithography provides the losses in structures with a cross section of 0.7 × 3 μm2 at a level of 12 dB, including the losses for the optical signal input and output. Moreover, the developed production methods for the waveguide structures can be used for mass production of the photonic integrated circuits. To further reduce insertion losses, it is necessary to study and refine the annealing methods, as well as to refine the photolithographic methods in order to improve the waveguide quality that will make it possible to reduce the waveguide width to a level of 1.5 μm.
Acknowledgement
The work was performed as a part of the state task from the Ministry of Science and Higher Education of the Russian Federation (grant No. FSEE‑2020-0005).
АВТОРЫ
Никитин А. А., к. ф.‑ м. н., кафедра физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0002-4226-4341
Ершов А. А., аспирант, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0003-3600-4946
Рябцев И. А., аспирант, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID 0000-0001-8158-8827
Кондрашов А. В., к. ф.‑ м. н., кафедра физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0002-7337-8907
Семенов А. А., д.т.н., доцент, зав. кафедрой физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0003-2348-3773
Устинов А. Б., д. ф.‑ м. н., руководитель лаборатории магноники и радиофотоники им. Б. А. Калиникоса, профессор, кафедра физической электроники и технологии, ЛЭТИ им. В. И. Ульянова (Ленина), С-Пб, Россия.
ORCID: 0000-0002-7382-9210
Воропаев К. О. начальник группы, отдел 1. разработка ИЭТ,
АО «ОКБ-Планета», Великий Новгород, Россия.
ORCID: 0000-0002-6159-8902
Петров А. В., к.т.н., генеральный директор, АО «ОКБ-Планета», Великий Новгород, Россия.
Парфенов М. В., м.н.с., лаб. квантовой электроники, ФТИ им. А. Ф. Иоффе РАН, С-Пб, Россия.
ORCID: 0000-0003-3867-9007
Шамрай А. В., д. ф.‑ м. н., зав. лаб. квантовой электроники, ФТИ им. А. Ф. Иоффе, С-Пб, Россия.
ORCID: 0000 0003 0292 8673
Теруков Е. И., д.т.н., зав. лаб. физико-химических свойств полупроводников, ФТИ им. А. Ф. Иоффе РАН, С-Пб, Россия.
ORCID: 0000-0002-4818-4924
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