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Abstract
Silicon-based image sensors are attractive for applications in the near-infrared (NIR) range owing to their low-cost and high availability. However, novel approaches are required to enhance their light absorption, hindered by the silicon band gap. In this study, we proposed a light trapping strategy in a silicon absorption layer by plasmonic diffraction and reflection within a pixel to improve the sensitivity at a specific NIR wavelength for complementary metal-oxide semiconductor image sensors. The plasmonic grating diffracted light under the quasi-resonant condition of the surface plasmon polaritons. We simulated the silicon absorption efficiency for plasmonic diffraction combined with metal-filled trenches and a pre-metal dielectric (PMD) layer. Backward propagation light in silicon by a total internal reflection at the bottom decoupled with plasmonic grating. A single SiO2 protrusion was added at the silicon bottom to prevent decoupling by scattering the light in the silicon and trapping it within the pixel. In addition, the light transmitted to the PMD layer is reflected by the wiring layer used as a mirror. The photon confinement in silicon by these constructions improved the absorption by approximately 8.2 times at an NIR wavelength of 940 nm with 3-µm-thick. It is useful for NIR imaging system with active laser illumination.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Near-infrared (NIR) image sensors are sensitive to light in the 800 nm to 1100 nm wavelength range, which is invisible to the human eye. Thus, these sensors have been found interesting for a large variety of applications including time-of-flight (ToF) [1–6], medical inspection [7–10], and surveillance [11–14]. Further, applications in light detection and ranging (LiDAR) sensors have been developed for automatic driving and advanced driver assistance systems [15–17]. Owing to their low-cost complementary metal-oxide semiconductor (CMOS) compatibility, silicon-based image sensors have been attractive for light detection in the NIR wavelength range. However, these sensors suffer from a low photon detection efficiency due the low absorption coefficient of silicon in the NIR wavelength range inferred by its band gap near 1100 nm. Therefore, improving the NIR sensitivity of silicon-based image sensors has become an important technological requirement. Several approaches have been investigated so far in the literature for this goal.
On the one approach, plasmonic enhancement has been proposed to extend the effective sensitivity of silicon image sensors, where silicon metal Schottky junction-type photodetectors have been already reported [18–31]. Hot carriers generated by the enhanced field, owing to surface plasmon resonance, contribute to an increase in the photocurrent response by exceeding the Schottky barrier. This approach is a promising photo detection technique to improve the sensitivity not only the NIR wavelength range but also the short-wavelength infrared range between 1100 nm and 1700 nm, which is mainly applied in telecommunications.
On the other approach, extending the effective absorption length in silicon has been suggested to improve the sensitivity of silicon image sensors in the NIR wavelength range (i.e. 800 - 1100 nm). It has been reported that diffractive silicon pyramid arrays and SiO2 deep trench isolation (DTI) improve the NIR sensitivity of back-illuminated (BI) CMOS image sensors [32,33]. In these pixels structure, the incident light was refracted by the silicon pyramid arrays on the top surface and reflected by the SiO2 trench, resulting in the trapping of light within the pixels. DTI generally avoids crosstalk between pixels [34–37] and is applied to photon trapping. Recently, Cobo et al. proposed a polysilicon nano-grating of a CMOS image sensor to diffract the incident light and a SiO2 trench for light confinement [38]. In our previous work [39], we have proposed plasmonic diffraction under quasi-resonant conditions of surface plasmon polaritons as a route to improve the NIR sensitivity of BI silicon image sensors. The proposed image sensor was constructed using metal gratings and metal-filled DTI for pixel boundaries. The metal grating diffracted the incident light to the silicon absorption layer at a large diffraction angle of approximately 90°, and the diffracted light was reflected by the metal DTI. Increasing the effective propagation length improved the absorption efficiency of silicon.
In the present study, we extend the possibilities for improving the NIR absorption of the BI silicon image sensors by adding a light confinement structure composed of a SiO2 protrusion in the pre-metal-dielectric (PMD) layer at the silicon bottom and a mirror in the wiring layer (Fig. 1). The light propagating between the metal DTIs in silicon is reflected by the PMD layer. It is then scattered by the added protrusion at the middle of the silicon/PMD interface. This confines the light within the pixel by mismatching the decoupled angle between the silicon and metal grating, which will be explained in more detail later. Furthermore, the light transmitted to the PMD layer returned to the silicon by the reflection mirror utilizing the first metal layer and contributed to silicon absorption. The optical confinement structures increased the propagation length in the silicon, which dramatically improved the NIR absorption efficiency.
Fig. 1. Schematic diagram of the light confinement structure with plasmonic grating in an image sensor pixel. NIR incident light is diffracted by the metal grating at a diffraction angle close to 90°. The diffracted light is trapped in the silicon absorption layer by reflecting at the metal trenches and PMD layer. The protrusion scatters light and prevents leakage from the top surface of the sensor. The mirror contributed to photon harvesting by reflecting transmitted light back into the silicon layer.
2. Simulation results and discussion
2.1 Simulation model for plasmonic diffraction
The silicon absorption efficiency by plasmonic diffraction was simulated using commercial software DiffractMOD and FullWAVE by RSOFT. The dispersion relation of the plasmonic diffraction grating was calculated using a rigorous coupled-wave analysis (RCWA) algorithm. The finite-difference time-domain (FDTD) algorithm was applied to calculate the electric-field intensity and absorption distributions. The metal grating was modeled as a two-dimensional rectangle to represent stripe patterns. To maintain accuracy at the edges of the material boundaries, we used a non-uniform spatial grid, which has been gradually changed from 1 nm to 10 nm on both the x- and z-axes. The boundary conditions of the x- and z-axis were perfectly matched layer (PML) absorption boundaries.
The wavelength of the incident light was selected at 940 nm, which was used for NIR sensing applications because of its high atmospheric transmittance and low sunlight disturbance. The incident light was defined as a continuous wave at normal incidence with p-polarized light, which excited surface plasmons along the direction orthogonal to the metal grating period. Silver, gold, copper, and aluminum, whose surface plasmons are excited at the NIR wavelength, are applicable to the grating. We selected silver as the metal grating owing to its lowest absorption loss at the 940 nm wavelength. The silver grating was defined by the period p, width w, height h, and SiO2 protection layer thickness t. The structural parameters of the silver grating and SiO2 protection layer were chosen to maximize the silicon absorption at a depth of 1 µm: p = 265 nm, w = 230 nm, h = 85 nm, and t = 205 nm [39]. A natural oxide film with a thickness of 2 nm remained on the Si surface. Silver (Ag) was also applied to the metal trench because of its high reflectance at 940 nm. The width of the silver trench was set to 180 nm to ensure sufficient reflectance. The refractive indices of the complexes at 940 nm were 3.595 + i0.001 for silicon [40], 1.471 for SiO2 [41], 0.080 + i6.784 for silver [42], and 0.244 + i6.029 for copper [43]. The pixel size of the image sensor is 6.5 µm and the PMD layer was placed at the bottom of silicon.
2.2 High efficiency diffraction under quasi-resonance of surface plasmon polaritons
Figure 2(a) shows the dispersion relation calculated using the incident angle dependence of the reflectance spectra for the silver grating. The reflection dip indicates that the surface plasmon was excited by the silver grating. The dispersion curve around ω = 4.00 × 1015 rad/s was derived from the surface plasmon resonant mode at the top side of the silver grating. This dispersion was obtained from the dielectric constant of Ag and the average dielectric constants of SiO2 and air. The surface plasmon resonant mode at the bottom of the silver grating appeared at ω = 2.01 × 1015 rad/s owing to the large dielectric constant of silicon. The free space wavelength of 940 nm corresponding to ω = 1.99 × 1015 rad/s is slightly shorter than the surface plasmon resonance according to the quasi-resonance concept.
Fig. 2. Simulation results of the plasmonic diffraction under the quasi-resonance condition of surface plasmons. (a) Dispersion relation of reflectance in plasmonic diffraction model. (b) Electric-field vector distribution at an incident wavelength of 940 nm. The incident light is diffracted to the silicon due to the electron quadrupole oscillations in silver grating.
The incident light at a wavelength of 940 nm, which is close to the surface plasmon resonance, diffracted toward the silicon side with a large diffraction angle (Fig. 2(b)). Under the surface plasmon resonance, the incident light diffracts horizontally and propagates on the metal surface. However, it diffracted nearly 90° from the silicon side under the quasi-resonant condition. The electric field vector distribution indicates that the quadrupole oscillation is excited in the silver grating, and the upper and lower poles are not orthogonal, but are slightly tilted. This tilt causes the incident light to diffract at a large diffraction angle of 80.6° [39]. The photon energy is efficiently coupled with the electric vibrational energy owing to quasi-resonance. The diffracted light radiation associated with electric dipole oscillations is emitted in a specific direction by the silver grating pitch. Therefore, the enhancement of the electric field contributes to a high-efficiency diffraction, which exhibits an anomalous 1st-order diffraction angle and diffraction efficiency and differs from the ordinary Bragg diffraction theory with a dielectric grating [44]. A large diffraction angle with high diffraction efficiency was obtained under the quasi-resonance of surface plasmon polaritons.
2.3 Absorption enhancement by plasmonic diffraction combined with metal trench and PMD layer
In the following simulation results, we adapted a silver trench and a PMD layer consisting of SiO2 on the silicon bottom side to image sensor pixels to improve their NIR absorption. The light diffracted by the plasmonic grating extended the propagation length in silicon by the silver trench and the PMD layer. Figure 3(a) shows the electric field intensity distribution of the plasmonic image sensor pixel with the silver trench and the PMD layer. The diffracted light at 80.6° was reflected by the silver trench and propagated in the silicon by repeating the trench reflection and going through a total internal reflection at the Si/SiO2 layer interface. In the silicon absorption layer of Fig. 3(a), both standing waves due to the transverse propagation between the silver trenches and longitudinal standing waves due to the total reflection at the bottom of the silicon were observed. The diffraction angle was calculated from the number of standing waves and size of the silicon layer. The number of standing waves in x- and z-axis were mx = 48 and mz = 4, respectively. The sizes of the silicon layer in x- and z-axis were lx = 6.32 µm and lz = 3 µm. The diffraction angle θ is given by:
From Eq. (1), the diffraction angle was calculated at 80°, which is in good agreement with the diffraction angle of 80.6° obtained from the theoretical equation [39]. A clear standing wave indicates that 1st-order diffraction is dominant and 0th-order transmission is negligible. This high diffraction efficiency is a feature of plasmonic diffraction and differs from ordinary diffraction grating. Figure 3(b) shows the distribution of the light-consumption ratio. The reflectance and transmittance monitors were set to 3 µm above the silicon top surface and silicon/PMD layer interface, respectively, with the same width as the pixel size of 6.5 µm. The backscatter monitors had a length of 3 µm along the z-axis from the top of each trench. The simulated reflectance, transmittance, back scattering, silicon absorption, silver grating absorption, and silver trench absorption were 44.0%, 9.6%, 7.5%, 31.3%, 1.9%, and 5.7%, respectively. With an increase in the effective propagation length in silicon owing to repetition reflection, the silicon absorption for a thickness of 3 µm increased to 31.3%. However, the reflectance reached 44.0% of the light-consumption ratio. This high reflectance was attributed to the reverse excitation of surface plasmon resonance in the silver grating, similar to the reverse Kretschmann configuration [45]. The total reflected light at the silicon/PMD layer interface propagated with the opposite optical pass to the plasmonic diffraction and was satisfied with the reverse decoupling. Figures 3(c) and 3(d) show the electric field intensity distributions to support reverse decoupling. Two symmetrical incident angles of the plane wave were irradiated from the silicon side, assuming the return of the diffracted light. In Fig. 3(c), the incident angle is set to 80°, which is the same as the plasmonic diffraction angle. Enhanced electric field due to surface plasmon resonance on the silver grating and decoupling to the air side with a normal angle were clearly observed. This reverse excitation of the surface plasmon resonance indicated the cause of the high reflectance value of 44.0% in Fig. 3(b). On the other hand, in Fig. 3(d), no decoupling light is observed under one of the off-resonant conditions with an incident angle of 70°. Therefore, managing the light angle mismatch with reverse excitation is required to increase the absorption efficiency of the silicon layer.
Fig. 3. Simulation results of the absorption enhancement combining the silver trench and the PMD layer. (a) Electric-field intensity distribution of plasmonic image sensor at a wavelength of 940 nm. Incident light diffracted by the silver gratings was reflected between the silver trenches and total reflection at the silicon/PMD layer interface. (b) Light consumption ratio in (a), (c) electric-field intensity distribution of reverse excitation at a wavelength of 940 nm incident from silicon side with the incident angle of 80°, (d) electric-field intensity distribution of off-resonant excitation at the incident angle of 70°.
2.4 Light scattering in the image sensor pixel by protrusion
For further optical confinement in the image sensor pixel, we investigated three types of additional structures at the bottom of the silicon to manage the light angle. These structures included a scattering type with a single protrusion at the middle of the silicon bottom, a diffraction grating type with rectangular or triangular gratings, and a direction converter type with a tapered silicon bottom surface to change the reflection angle. In particular, we selected a rectangular scattering-type structure to improve absorption efficiency and practical fabrication. A single SiO2 protrusion was applied to the middle of the silicon bottom to scatter the propagating light in the silicon absorption layer, as illustrated in Fig. 1. The width and height of the SiO2 protrusion were investigated in the range of 100 nm to 500 nm, and the values 300 nm and 310 nm, respectively, were selected to improve silicon absorption. This protrusion will be fabricated using the same process as that used for the shallow trench isolation. The standing waves were disturbed because the diffracted light in the silicon was scattered by the SiO2 protrusion (Fig. 4(a)). The propagating light was scattered by the SiO2 protrusion, drastically decreasing the decoupling efficiency. The reflectance dropped to 5.5% by mismatching with the reverse excitation, resulting in the improvement of the integrated absorption of the 3-µm-thick silicon to 48.0% (Fig. 4(b)). The transmittance, backscattering, silver grating absorption, and silver trench absorption ratios were 19.2%, 8.1%, 10.5%, and 8.7%, respectively.
Fig. 4. Simulation results of plasmonic image sensor applying a protrusion. (a) Electric-field intensity distribution of image sensor pixel with a protrusion. The propagation light at a diffraction angle of 80.6° was scattered by the protrusion and confined to the silicon layer. (b) light consumption ratio in (a).
2.5 Transmitted light harvesting using reflection mirror
To harvest the 19.2% of the transmitted light to silicon absorption, we set a metal reflection mirror by applying a wiring layer. Copper is commonly used as a wiring layer in image sensors, which was selected as the metal mirror owing to its high reflectivity at the 940 nm wavelength, similar to silver. The thickness of this copper mirror was set to 600 nm, which was placed 800 nm below the bottom of silicon, assuming the first metal wiring layer. The light transmitted from the silicon to the PMD layer is reflected by the copper mirror and returned to the image sensor pixel. Figures 5(a) and 5(b) show the intensity and absorption distribution of the plasmonic image sensor with a copper mirror, where the light was confined to the silicon layer. From the light consumption ratio, the silicon absorption increased to over 50%, and the transmittance of the Si/SiO2 boundary was reduced to 6.9% by the mirror (Fig. 5(c)). A part of the transmitted light (3.2%) was absorbed by the copper reflection mirror, and the remaining light was dissipated to the side in the PMD layer. Figure 5(d) shows the integrated silicon absorption with respect to the silicon thickness for flat (Fig. 3(a)), protrusion (Fig. 4(a)), and protrusion with mirror (Fig. 5(a)). The silicon-integrated absorption increases with the silicon thickness. The maximum absorption was 53.3% for silicon with a thickness of 3 µm. This constitutes an 8.2 enhancement factor over the integrated absorption of 6.5% for bare silicon with the PMD layer, indicating a significant increase in sensitivity at a wavelength of 940 nm. The fluctuation in this graph indicates the coherence of the interference between trenches in the lateral direction and the interference by finite silicon thickness in the vertical direction. The increased absorption contributes to a short integration time of CMOS image sensors, which leads to an increase in the frame rate.
Fig. 5. Simulation results of light confinement in an image sensor pixel using a reflection mirror. (a) Electric-field intensity distribution and (b) absorption distribution of the image sensor pixel with the added copper mirror. The transmitted light from the silicon to the PMD layer was reflected by the copper mirror, which improved the silicon absorption. (c) Light consumption ratio in (a), (d) silicon integrated absorption dependence on the thickness.
3. Conclusions
In this study, we proposed a new architecture of a silicon image sensor pixel to improve its NIR absorption at a wavelength of 940 nm. The new design with plasmonic diffraction and optical confinement structures incorporated a metal trench, PMD layer, SiO2 protrusion at the silicon bottom, and a reflection mirror in the wiring layer. The incident light diffracted efficiently to the silicon side at a large diffraction angle of nearly 90° because of plasmonic diffraction under the quasi-resonance condition of surface plasmon polaritons. The propagation distance of the diffracted light in silicon increased owing to repeated reflection between the metal trenches and total internal reflection at the silicon/PMD layer interface, resulting in improved photon detection efficiency. From the viewpoint of photon confinement in silicon, we set the SiO2 protrusion at the middle of the silicon bottom to scatter light in the silicon. In addition, the silicon absorption efficiency was harvested by the reflection of the light transmitted to the PMD layer by a mirror applying a part of the wiring layer. Silicon integrated absorption for a thickness of 3 µm was 53.3%, which induced an 8.2-fold improvement compared to the 6.5% absorption of bare silicon including the PMD layer. The plasmonic diffraction combined with optical confinement led to a significant improvement in the NIR sensitivity of CMOS image sensors over half the silicon absorption efficiency, which is comparable to the visible light of red wavelength absorption. The SiO2 protrusion and mirror in the wiring layer increased the absorption per unit depth of silicon and contributed to a higher sensitivity in the NIR range. A photon confinement image sensor can be fabricated using advanced CMOS technology and a full trench isolation technique buried with metal [46,47]. In front side process, SiO2 protrusion is fabricated by the same technique with shallow trench isolation. First Cu wiring layer is applied for the metal mirror. In back side process, silver for deep trench isolation is embedded by atomic layer deposition. Silver grating is patterned by liquid immersion ArF lithography with lift-off process or with etching process of silver film. In this paper, we have simulated in pixel sizes of 6.5 µm in width and 3.0 µm in thickness. Similar conclusions will be obtained even for different pixel sizes by re-designing the grating pitch and other structural parameters. Compact NIR image sensors based on silicon are cost-effective and have the potential to be applied to a wide variety of consumable devices such as automobiles, ToFs, optical communications, and biological sensors.
Funding
National Institute of Information and Communications Technology (03601); Japan Society for the Promotion of Science (JP22K18984).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
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