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Abstract
Compact spectrometers facilitate non-destructive and point-of-care spectral analysis. Here we report a single-pixel microspectrometer (SPM) for visible to near-infrared (VIS-NIR) spectroscopy using MEMS diffraction grating. The SPM consists of slits, electrothermally rotating diffraction grating, spherical mirror, and photodiode. The spherical mirror collimates an incident beam and focuses the beam on the exit slit. The photodiode detects spectral signals dispersed by electrothermally rotating diffraction grating. The SPM was fully packaged within 1.7 cm3 and provides a spectral response range of 405 nm to 810 nm with an average 2.2 nm spectral resolution. This optical module provides an opportunity for diverse mobile spectroscopic applications such as healthcare monitoring, product screening, or non-destructive inspection.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical spectroscopy allows rapid and non-destructive sample analysis using mid-infrared absorbance [1,2], diffuse reflectance [3,4], or Raman scattering [5,6]. Recently, compact spectrometers are combined with mobile or handheld devices for on-demand sample analysis. For instance, portable spectrometers provide industrial and agriculture product screening such as plastic classification [7,8] or food quality control [9,10]. Moreover, handheld spectroscopic probes have been applied for point-of-care (POC) healthcare and medical testing such as hemoglobin measurement [11] and intraoperative brain cancer detection [12,13]. However, the downsizing of spectrometers with high performance is still hampered by the trade-off between the physical volume, optical throughput, and spectral resolution [14].
Conventional dispersive spectrometers mainly comprise dispersive elements, collimating and focusing optical components, slits, and a photodetector. The spectral performance is mainly influenced by optical path length, angular dispersion, and slit width. For instance, high angular dispersion [15] and a long optical path length [16] substantially improve the spectral resolution. In addition, a narrow slit width improves spectral resolution even more while considerably decreasing optical throughput [17]. Conventional compact dispersive spectrometers achieve high spectral performance by employing highly dispersive elements [18] and narrow slits [19] in the limited optical path length. As a result, the optical throughput and resolution have a trade-off relation due to the narrow slit width. These technical limitations still hinder accurate spectral analysis from compact spectrometers.
Single-pixel spectrometers provide the technical solution by using a highly sensitive detector such as a photomultiplier tube, avalanche photodiode [20], or superconducting nanowire single-photon detector [21]. In addition, recently, MEMS (micro-electro-mechanical systems) techniques are applied to downsize the single-pixel spectrometer by employing MEMS mirrors [22,23], on-chip interferometers [24,25] or scanning diffraction gratings [26–28]. In particular, scanning diffraction gratings provide the integrated configurations of dispersive element and MEMS actuator, exceptionally favorable for the system miniaturization. For instance, on-chip electrostatic scanning diffraction grating integrates the slits, comb drive, and diffraction grating. [26] However, the electrostatic actuation requires high operation voltage [29], which limits its applicability. More recently, electromagnetic scanning gratings operate at a low driving voltage of less than 5 VPP [27,28], but suffer from a large footprint and small angular dispersion due to narrow scanning angle. Consequently, scanning diffraction grating still requires a large rotational angle at a low driving voltage while maintaining its compactness for on-demand precise spectral analysis.
Here we report a high-performance single-pixel microspectrometer (SPM) using electrothermal MEMS diffraction grating with a large rotational angle at low driving voltage. Figure 1(a) and (b) show the schematic illustration for electrothermally actuated MEMS grating and the working principle of SPM, respectively. Slits, reflective diffraction grating, and electrothermal actuator are fully integrated into a single-chip configuration of the MEMS grating. The SPM consists of the MEMS diffraction grating, a spherical mirror, and a single photodiode. An incident beam passes through the entrance slit and the spherical mirror collimates the beam. The collimated beam is dispersed by the diffraction grating and then focused on the exit slit. The photodiode detects target spectral signals by electrothermally rotating the diffraction grating. The collected spectral signals in unit time are converted into a unit wavelength depending on the rotational angle as shown in Fig. 1(b) (bottom). Rotation of diffraction grating permits tuning of spectral response, and electrothermal actuation provides a large rotational angle of 14° at only 7 VDC. The fully packaged SPM has a physical volume of 1.7 cm3 and an average spectral resolution of 2.2 nm in the spectral range of 405 nm to 810 nm.
Fig. 1. Miniaturized single-pixel spectrometer using MEMS diffraction grating. Schematic illustrations for a) MEMS diffraction grating, b) the working principle of a miniaturized single-pixel spectrometer (SPM) (top) and wavelength calibration between applied voltage (VDC) and wavelength (λ) (bottom). Micro-slit, reflective diffraction grating, and electrothermal actuator are fully integrated into the MEMS diffraction grating. The SPM consists of spherical mirror, MEMS grating, and photodiode. The SPM detects dispersed spectral signals with a single photodiode by rotating the diffraction grating. The collected spectral signals in unit time are converted into a unit wavelength depending on the rotational angle.
2. Optomechanical design and microfabrication of electrothermal MEMS grating
The MEMS grating requires the optical and mechanical designs of diffraction grating, slit, and electrothermal actuator. Figure 2(a) shows the calculated spectral resolution and response range along with the period of the rectangular diffraction grating. The maximum rotating angle, which determines the maximum spectral response range, was set to 14.5°. The minimum spectral response range was determined as half of the maximum to avoid the overlap between the first and second-order diffractions. A short period improves the spectral resolution due to a large angular dispersion and reduces the spectral response range under a constant range of rotational angles. The grating period is set to 1.25 µm (800 lines/mm) by considering the spectral resolution and the photolithography resolution. Figure 2(b) shows the calculated diffraction efficiency at 800 nm along with the duty cycle of the diffraction grating by using the finite-difference time-domain method (FDTD, Lumerical FDTD solutions). The height of the diffraction grating was set to 200 nm, which enhances the diffraction efficiency at 800 nm by suppressing the zeroth order diffraction based on the destructive interference between reflection from the bottom and top surfaces [30]. Note that the duty cycle of 0.4 shows the maximum diffraction efficiency of 50% due to a substantial reduction of the zeroth order diffraction beam. The duty cycle was set to 0.4 by considering the high diffraction efficiency and the photolithography resolution. Figure 2(c) shows the calculated spectral resolution and the etendue depending on the width of the entrance slit. The etendue was calculated by multiplying the area of the entrance slit and the acceptable solid angle of the spectrometer. The high etendue improves the light collection of the system. The exit slit was set to 10 µm, i.e., half of the diffraction limit of the spherical mirror. Note that a large slit width improves the etendue, however, linearly deteriorates the spectral resolution due to the large focal spot size on the detection plane.
Fig. 2. MEMS diffraction grating. a) Calculated spectral resolution and response range at the rotating angle of less than 14.5° depending on the period of the rectangular diffraction grating. b) Calculated diffraction efficiency at 800 nm along with the duty cycle of the grating, which has a height of 200 nm. c) Calculated spectral resolution and etendue depend on the width of the micro-slit. d) Calculated initial angle and angle difference under different gravitational directions of the MEMS grating. e) Calculated horizontal and vertical curvature of the MEMS grating with various silicon rim structures after Al film deposition. f) Microfabrication procedure of the MEMS diffraction grating. The MEMS grating was fabricated on a 6-inch SOI wafer. g) Optical image of the microfabricated MEMS grating. SEM image of the microfabricated h) diffraction grating and i) top view of the bimorph.
The MEMS grating consists of an electrothermal actuator with bimorph cantilever [31] and large area diffraction grating with a silicon rim structure. The bimorph cantilever design aims for a high initial angle and low angle variation during handheld use for wide spectral response and reliable spectral measurement, respectively. Figure 2(d) shows the calculated initial angle at the residual stress from 1 µm thick aluminum layers and the angle difference under different gravitational directions, depending on the silicon thickness. Thin silicon layer provides a high initial angle, however, suffers from high-angle variation due to a low stiffness. Therefore, the bimorph cantilever consists of 2.5 µm thick silicon and 1 µm thick aluminum layers, which provide both low angle variation and high initial angle over 10° for a wide spectral range including the near-infrared (NIR) region. In addition, the silicon rim structure protects the membrane deflection of the diffraction grating due to the residual stress of Al film [32]. Figure 2(e) shows the calculated horizontal and vertical curvature of the MEMS grating with various silicon rim structures. The closed silicon rim structure reduces the curvature of the substrate to less than 1/50 compared to the device with a free end or parallel rim structure.
The electrothermal MEMS diffraction grating was microfabricated on a 6-inch SOI wafer (silicon-on-insulator wafer, top Si: 2.5 µm, buried oxide layer: 1 µm, bottom Si: 430 µm) by combining stepper photolithography and contact aligner. Figure 2(f) shows the microfabrication procedure of the MEMS grating. First, a 200 nm thick low-stress silicon nitride layer was deposited by using low-pressure chemical vapor deposition (LPCVD). After stepper photolithography, the nitride film was defined by using reactive ion etching (RIE) for the insulation pattern and rectangular diffraction grating. The thickness of the silicon nitride layer determines the height of the diffraction grating. Next, two different aluminum layers are defined for reflective diffraction grating and bimorph cantilever. A 100 nm thick aluminum layer was defined for reflective diffraction grating by using thermal evaporation and wet etching. And then 1 µm thick aluminum film was lifted off for an electrothermal actuator with a silicon/aluminum bimorph structure, which contains a repeated line pattern of 1,000 µm in length, 14 µm in width, and 20 µm in a period. The top silicon layer was etched by using RIE for slit patterns. Deep RIE of the backside silicon was performed after frontside passivation followed by photoresist patterning of the silicon rim [32] and slit frame structure on the backside. The buried oxide layer worked as the etch stop layer during the deep RIE and was removed by buffered oxide etchant (BOE) after the etching process. The MEMS gratings show a fabrication yield of over 99% (191 successfully fabricated devices in 192 devices on a single wafer). Finally, a 120 nm thick aluminum layer was selectively evaporated on the backside of the micro-slit for the optical blocking layer by using a stencil mask. The optical blocking layer surrounding the slits reduces the transmittance of the silicon membrane to improve spectral performance. The microfabricated devices are tilted after release due to the residual stress from the aluminum line pattern on the bimorph. The silicon rim structures protect the deformation of the diffraction grating, resulting from the residual stress of the aluminum thin film. Figure 2 g and h show the optical image of the microfabricated MEMS grating and the scanning electron microscopy (SEM) image of the diffraction grating, respectively. Figure 2(i) shows the top view of the microfabricated bimorph cantilever.
3. Spectral calibration for rotating MEMS grating
The wavelength selection is achieved by the rotating angle of the MEMS grating. Figure 3(a) shows the rotating angle and current of MEMS grating depending on the applied DC voltage. The MEMS grating is initially tilted by 14.5° and rotated to 0.5° at 7 VDC, corresponding to nearly the in-plane of the MEMS grating chip. The measured rotational angles are well matched with the calculation results based on the finite element analysis (FEA, COMSOL Multi-physics ver. 5.5). The aluminum pattern on bimorph has a resistance of 350 Ω. The current in the aluminum patterns is gradually increased to 18 mA at 7 VDC. The electrothermal actuator requires an average electrical power of 80.9 mW. Figure 3(b) shows the step response of the MEMS grating. The device shows 90% rising and falling times of 1.9 sec and 1.8 sec, respectively, for a rotating angle of 22° at 10 VDC. In addition, the measured maximum temperature of the electrothermal actuator shows less than 350 K.
Fig. 3. Spectral wavelength depending on the rotational angle of the MEMS grating. a) Measured rotating angles and current of MEMS grating depending on the applied DC voltage. b) Step response of MEMS grating. c) Schematic illustrations of calibration methods and d) measured laser spectra depending on the applied DC voltage. e) Linear relationship between target wavelengths and squared voltage differences for calibration. f) Normalized photo response of the MEMS grating, which is measured by multiplying diffraction efficiency and photosensitivity of the photodiode. g) Comparison of the calibration precision before and after calibration. The Inset image illustrates the experimental setup to perform pseudo-handheld conditions.
The applied DC voltage determines the rotating angle and the target wavelength. The spectral calibration was then conducted via the relation between the applied DC voltage and the target wavelength. Figure 3(c) illustrates the calibration method and each color represents different trials for single spectrum measurement. The thin bimorph cantilever of the MEMS grating may cause some spectral variation under different gravitational conditions before calibration. For this experiment, the consistent relationship (ΔVDC) between the first and the zeroth-order diffraction beams was utilized for the calibration. Figure 3(d) shows the spectral measurements of 5 different laser spectra. After the first-order diffraction beam for each laser spectrum is arranged with respect to the zeroth-order diffraction beam, all the wavelengths are clearly distinguished depending on ΔVDC. Figure 3(e) shows a linear relationship between target wavelengths and squared voltage differences (ΔVDC2), which is calibrated for each device depending on initial angle variation. The target wavelengths show a roughly linear relation to the rotating angles in the operating range of the device. The measured values indicate the average of 30 operations and show high repeatability with a standard deviation of less than 0.6 nm. Finally, the spectral response was calibrated by considering the sensitivity of the photodiode and the diffraction efficiency. Figure 3(f) shows the normalized photo response of the MEMS grating, measured by multiplying the diffraction efficiency and sensitivity of the photodiode. Note that each single measurement was performed in 5 sec with 10 sec cooling time for the full spectral range.
The calibration was further verified under pseudo-handheld conditions, i.e., different gravitational directions. Figure 3 g shows the spectral variation before and after the calibration. The inset image illustrates the experimental setup of a pseudo-handheld condition. The rotational movement of the spectrometer (θs) may slightly affect the angle of the diffraction grating (θg) due to gravity. In this experiment, spectral variations were measured under 30 different random conditions among three different rotational movements of 0, 90, and 180 degrees for θs. The spectral variation is initially more than 10 nm but substantially reduced by less than 2 nm after the calibration. As a result, the MEMS grating provides a spectral response range of 405 nm to 810 nm at an applied voltage of 7 VDC with a spectral variation of less than 2 nm under the handheld condition.
4. Fully packaged single-pixel spectrometer and fluorescence detection
The SPM was fully packaged within 1.7 cm3. Figure 4(a) illustrates the micro-packaging of SPM. The MEMS grating and a bare chip photodiode (Hamamatsu Inc., S5972, diameter of photosensitive area: 800 µm) were mounted on a printed circuit board (PCB) and vertically stacked with a single spherical mirror (Edmund optics, 43-462) using a 3d printed spacer. The MEMS grating is mounted at a vertical distance of 300 µm from the photodiode due to the wiring. The distance between the spherical mirror and the MEMS grating is half of the radius of curvature of the spherical mirror. The distance between the MEMS grating and the spherical mirror was set to 6 mm for maintaining the spectral resolution in such a compact volume. In addition, the minus first-order diffraction beams are directed into the sidewall of the spacer to prevent the measurement of unwanted signals. The spacer is 3D-printed by using black ABS (acrylonitrile butadiene styrene) with a black ink-based dull finish to minimize the reflection. The module package can be further simplified by integrating photodetector and MEMS grating on a single wafer. Figure 4(b) and 4(c) show the optical images of the PCB-mounted MEMS grating with a bare chip photodiode and a fully packaged SPM, respectively. The photodiode was connected to the home-built current-to-voltage converter on a breadboard for spectral measurement. The measured system noise is 2 mVrms for an RC circuit with 500 MΩ and 10 pF. The sensitivity of the fully packaged SPM is 13.3 nW at 400 nm and 3.4 nW at 800 nm with a dynamic range of 37 dB. Figure 4(d) shows the diffraction colors reflected from the MEMS grating with different rotational angles (top) and the measured spectral resolutions (bottom), depending on the applied DC voltage. The spectral resolution is 2.2 nm on average, which is measured with five different lasers of 405 nm (magenta), 488 nm (blue), 532 nm (green), 638 nm (red), and 785 nm (black). The 3 µm core fiber was coupled with the fully packaged SPM and the incident angle was determined by the numerical aperture of the fiber (NA = 0.1). The measured diffraction efficiency ranges from 15% in the short visible region to 50% in the NIR region. Note that the fully packaged SPM has a maximum stray light level of -15 dB in the spectral range of 450 nm to 480 nm under light illumination with a wavelength of 405 nm.
Fig. 4. Fully packaged single-pixel microspectrometer. a) Schematic illustration for micro-packaging of SPM. Optical images of b) PCB-mounted MEMS grating with a bare chip photodiode and c) fully packaged SPM. d) Diffraction color reflected from the MEMS grating with different rotational angles (top) and the measured spectral resolution (bottom). The MEMS grating rotates with applied DC voltage, resulting in different diffraction colors. The spectral resolution is 2.2 nm on average, measured with five different lasers of 405 nm (magenta), 488 nm (blue), 532 nm (green), 638 nm (red), and 785 nm (black).
The fluorescence spectra are further measured by using the fully packaged SPM. Figure 5(a) shows the schematic illustration of the experimental setup for fluorescence measurement. A 532 nm laser with 1.8 mW power was used as an excitation beam and the long pass dichroic mirror (Semrock, FF555-Di03) decouples the laser beam and fluorescence signals. Figure 5(b) shows the measured fluorescence emission spectra of 300 µM Rhodamine 6 G (R6G) and 1 mM Rhodamine B (RB) solutions using the SPM and a commercial spectrometer (Hamamatsu Inc., C10082CAH, spectral resolution: 1 nm). The peaks of measured R6G and RB spectra are 561 nm and 571 nm and the spectra are detected with cross-correlations over 0.97, compared to the commercial spectrometer. The measured fluorescence emission in the R6G solution is higher than in the RB solution due to the high fluorescence quantum yield of R6G [33,34]. The fully packaged SPM clearly distinguishes the fluorescence emission signals of R6G and RB.
Fig. 5. Comparative fluorescence detection of Rhodamine 6 G (R6G) and B (RB) using SPM and commercial spectrometer. a) Schematic illustration of the experimental setup for fluorescence measurement. b) Measured fluorescence emission spectra of each 300 µM R6G and 1 mM RB solution using the fully packaged SPM (blue line) and a commercial spectrometer (red line).
5. Conclusion
In summary, we have successfully demonstrated the VIS-NIR SPM using electrothermal MEMS diffraction grating. The MEMS grating consists of the micro-slit, the reflective diffraction grating, and the electrothermal actuator. The precise rotation of diffraction grating facilitates spectral measurement via a single photodiode, and electrothermal actuation provides a large rotational angle of 14° at only 7 VDC. The SPM was fully packaged within 1.7 cm3, providing a measurable spectral response range of 405 nm to 810 nm and an average 2.2 nm spectral resolution. This compact spectrometer can provide diverse applications for advanced mobile spectral analysis in healthcare monitoring, product screening, or non-destructive inspection.
Funding
Korea Medical Device Development Fund (KDMF_PR_20200901_0074); Ministry of Science and ICT, South Korea (2021R1A2B5B03002428, 2022M3H4A4085645).
Acknowledgments
This work was supported by the Korea Medical Device Development Fund (KDMF_PR_20200901_0074) and the National Research Foundation of Korea (NRF) funded by the Ministry of Science ICT & Future Planning (2021R1A2B5B03002428, 2022M3H4A4085645).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon request.
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