Photodetector array and its fabrication method
By designing a parallel-structured photodetector array, the problems of insufficient responsivity and noise suppression capability of traditional photodetectors under low light conditions are solved, achieving high signal-to-noise ratio and high resolution pulse waveform detection, which is suitable for multi-dimensional health monitoring.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HEFEI UNIV OF TECH
- Filing Date
- 2026-01-21
- Publication Date
- 2026-06-02
Smart Images
Figure CN122138486A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of semiconductor optoelectronic device technology, and in particular to a photodetector array and its fabrication method. Background Technology
[0002] The clinical necessity of continuous blood pressure monitoring stems from its crucial role in early risk stratification, especially considering the intermittent nature of hypertensive events and diurnal blood pressure variability—characteristics often difficult to capture with traditional cuff-based blood pressure measurements. Given the challenges of blood pressure measurement, the reliability of photoplethysmography (PPG) signals is critical for accurate cardiovascular monitoring. As a non-invasive and cost-effective cardiovascular monitoring technology, PPG has gradually become the mainstream choice. This technology analyzes pulse waveforms to obtain key parameters such as heart rate (HR), oxygen saturation (SpO2), and arterial stiffness index.
[0003] The core of any pulse oximetry (PPG) system is a photodetector. This crucial component operates on the principle of "parallel photoelectric conversion," simultaneously converting the light signal on its surface into a corresponding electrical signal for data processing and analysis, thereby capturing or analyzing light information. However, in practical applications, traditional photodetectors suffer from insignificant signal waveform amplitude changes, particularly weak responsivity and noise suppression under low-light conditions, hindering subsequent pulse waveform analysis. Summary of the Invention
[0004] The main objective of this invention is to provide a photodetector array and its fabrication method, aiming to solve at least one of the aforementioned technical problems.
[0005] To achieve the above objectives, the present invention provides a photodetector array, comprising a P-type material substrate and a plurality of N-type material layers formed on the surface of the P-type material substrate at intervals, wherein each of the N-type material layers forms a PN junction with the P-type material substrate, and the plurality of N-type material layers are connected in parallel with each other.
[0006] Furthermore, the multiple N-type material layers are fan-shaped with consistent angles and distributed in a ring array.
[0007] Furthermore, the multiple N-type material layers are distributed in four quadrants, six quadrants, eight quadrants, or sixteen quadrants.
[0008] Furthermore, the diameter of the array ranges from 2mm to 10mm.
[0009] Furthermore, the spacing between two adjacent N-type material layers is 300 nm.
[0010] Furthermore, the thickness of the P-type material substrate is 100μm to 500μm.
[0011] Furthermore, the thickness of the N-type material layer is less than 2 μm.
[0012] Furthermore, the multiple N-type material layers are interconnected in parallel by wires with a width of 100 nm.
[0013] The present invention also provides a method for fabricating the above-mentioned photodetector array, comprising the following steps: S1: Cleaning process for P-type material substrate; S2: A silicon dioxide thin film is deposited on the cleaned P-type material substrate by thermal oxidation. S3: Photoresist is spin-coated onto the surface of a silicon dioxide thin film. The photoresist is exposed and developed through a mask. The cutout area of the mask matches multiple N-type material layers. Then, the photoresist corresponding to the cutout area of the mask is etched away. S4: N-type impurities are implanted into the area where the photoresist has been removed by ion implantation. The N-type impurities penetrate the silicon dioxide film and form multiple N-type material layers on the surface of the P-type material substrate. S5: Remove the remaining photoresist and silicon dioxide film in the corresponding areas of multiple N-type material layers by dry etching or wet etching; S6: Contact electrodes are fabricated on the surfaces of multiple N-type material layers and connected in parallel with wires to lead out the cathode and anode.
[0014] In the ion implantation process, the silicon dioxide film is very thin (usually in the tens to hundreds of nanometers range), and ion implantation uses a high-energy ion beam (energy generally in the range of keV to MeV) to bombard the material. Its penetrating power is sufficient to penetrate the thin oxide layer and directly enter the underlying P-type material substrate, so that impurities can be implanted without removing the oxide layer in advance.
[0015] The present invention also provides another method for fabricating the above-mentioned photodetector array, characterized by comprising the following steps: S1: Cleaning process for P-type material substrate; S2: Cover the cleaned P-type material substrate with a mask, and match the cutout area of the mask with multiple N-type material layers; S3: A solution of N-type material is dropped onto the cutout area of the mask, and after curing, multiple N-type material layers are formed; S4: Contact electrodes are fabricated on the surfaces of multiple N-type material layers and connected in parallel with wires to lead out the cathode and anode.
[0016] The above two preparation methods: Method 1 offers advantages in ultra-high precision, high reliability, and superior performance. Through photolithography and ion implantation, precisely sized PN junctions with controllable doping concentrations can be formed, forming the foundation for high-performance photodetectors. This method achieves high consistency and stability of device units and allows for easy monolithic integration of photodetector arrays and silicon-based readout circuits onto a single chip, enabling high-density, high-speed imaging systems. However, this method involves complex processes, significant equipment investment, and stringent requirements for the production environment (cleanroom).
[0017] The advantage of method two is that the process is simple, the cost is low and it does not require harsh conditions such as high temperature and vacuum. However, the limitation of this method is that the crystal quality and electrical properties of the film formed by solution curing are usually not as perfect as those of the crystal formed by high temperature process, which will limit the upper limit of device performance and uniformity.
[0018] The beneficial effects of this invention are reflected in: 1. The photodetector array of this invention is connected in parallel in the circuit, which linearly increases the total photosensitive area available for carrier generation, thereby increasing the photocurrent without proportionally increasing the dark current. This achieves synergistic optimization of the signal-to-noise ratio and enables the observation of extremely low dark current (on the order of nanoamps) at low bias voltages. Such a small dark current is a key property for suppressing intrinsic noise, thus enabling the accurate detection of weak photocurrent signals generated under weak ambient light, overcoming the limitations of traditional photodetectors. The photodetector array of this invention can be used in the field of multi-dimensional health monitoring based on photoplethysmography (PPG).
[0019] 2. Compared with traditional commercial photodiodes, the multi-quadrant photodetector array of this invention has a larger effective active area, which promotes the collection of more photogenerated carriers while minimizing inherent noise. In actual pulse detection, a clear and stable PPG waveform was successfully generated, with distinct contraction peak, double-peak groove, and diastolic peak, indicating a significant improvement in signal-to-noise ratio.
[0020] 3. From single-quadrant to N-quadrant devices, the photocurrent has been increased by nearly N times. This significant improvement stems from its core design: the N-quadrants are connected in parallel, which allows the total photosensitive area available for carrier generation to increase linearly.
[0021] 4. The photodetector array of the present invention can achieve high integration while having high resolution, and can be used for high-quality, high-resolution imaging.
[0022] 5. While the AC signal generated by a single-quadrant device is detectable, its amplitude is extremely low, making accurate positioning of the reference point difficult and prone to errors. Although the AC amplitude of the output of a dual-quadrant device is higher, there is significant baseline (DC component) drift. Baseline drift manifests as low-frequency fluctuations caused by motion, thermal effects, or circuit artifacts, which distort the pulse waveform and obscure key reference points such as the contraction peak and bifurcation depression, resulting in inaccurate extraction of parameters such as heart rate, respiratory rate, and blood pressure in the time and frequency domains. In contrast, the multi-quadrant device (three or more) of this invention provides a pure, high-fidelity pulse wave signal with excellent signal-to-noise ratio and significant baseline stability. This characteristic is crucial for achieving robust ambient light pulse wave acquisition over a wide range of illumination intensities. Attached Figure Description
[0023] Figure 1 This is a schematic diagram of the structure of the eight-quadrant distributed photodetector array prepared in Example 1.
[0024] Figure 2 This is a schematic diagram of the structure of the six-quadrant distributed photodetector array prepared in Example 2.
[0025] Figure 3 This is a schematic diagram of the mask used in Example 2.
[0026] Figure 4 This is a schematic diagram showing the distribution and parallel connection of the photodetector arrays in quadrants four, six, eight, and sixteen of the present invention.
[0027] Figure 5 The current-voltage (IV) characteristic curves of single-quadrant, six-quadrant, and eight-quadrant devices under dark visible light are shown.
[0028] Figure 6 To achieve this under 635nm illumination (1mW / cm) 2 Photocurrent images of single-quadrant, six-quadrant, and eight-quadrant devices.
[0029] Figure 7 For cloudy conditions (190 μW / cm) 2 PPG signals acquired by single-quadrant, six-quadrant, and eight-quadrant devices.
[0030] Figure 8 For sunny conditions (2mW / cm) 2 PPG signals acquired by single-quadrant, six-quadrant, and eight-quadrant devices.
[0031] Figure 9 FFT analysis of PPG signals from commercial photodiodes under cloudy conditions.
[0032] Figure 10 FFT analysis of the PPG signal of the eight-quadrant device in Example 3 under cloudy conditions.
[0033] Figure 11 The graph shows a comparison of the PPG signals measured by the 4×2 row / column device and the eight-quadrant device of Example 3 under the same area and conditions.
[0034] Figure 12 This is the reference drift diagram for a dual-quadrant device.
[0035] Explanation of reference numerals in the attached figures: 1. P-type material substrate; 2. N-type material layer; 3. Contact electrode; 4. Wire; 5. Cathode; 6. Anode. Detailed Implementation
[0036] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.
[0037] Unless otherwise specified, the raw materials, reagents, or devices used in the following examples can be obtained from conventional commercial sources or by existing known methods; unless otherwise specified, the methods used in the embodiments of the present invention are methods mastered by those skilled in the art. Among them, the MXene (Ti3C2Tx) colloidal aqueous solution, monolayer, concentration 15 mg / ml, was purchased from Suzhou Beike Nanotechnology Co., Ltd.
[0038] Example 1 Eight-quadrant photodetector array fabricated by ion implantation A schematic diagram of the photodetector array prepared in this embodiment is shown below. Figure 1 As shown, the substrate includes a P-type substrate 1 (using a P-type single-crystal silicon wafer) and eight N-type material layers 2 (made of arsenic) formed on the P-type substrate 1, spaced apart from each other. Each N-type material layer 2 forms a PN junction with the P-type substrate 1, and the eight N-type material layers 2 are connected in parallel. The eight N-type material layers 2 are fan-shaped with consistent angles and arranged in a ring array. The spacing between two adjacent N-type material layers 2 is 300 nm. The fabrication method is as follows: S1: Perform standard RCA cleaning on a P-type monocrystalline silicon wafer (12mm in length and width, 400μm in thickness) to obtain a clean surface; S2: A thin film of silicon dioxide (300nm thick) is deposited on the surface of a cleaned P-type single crystal silicon wafer by thermal oxidation. S3: Spin-coat photoresist onto the surface of a silicon dioxide thin film. Use a photomask (the cutout area of the mask is the eight-quadrant pattern that needs to form eight N-type material layers) to expose the photoresist. After development, etch away the photoresist corresponding to the cutout area of the mask. S4: By ion implantation, AsH3 gas containing N-type impurities As is injected into the area where the photoresist is removed. The N-type impurities penetrate the silicon dioxide film and form multiple N-type material layers on the P-type material substrate. S5: Remove the remaining photoresist and silicon dioxide film in the corresponding areas of multiple N-type material layers by dry etching (plasma) or wet etching (hydrofluoric acid solution); S6: A thin film of aluminum (or other metals such as gold) is deposited on the surface of each of the eight N-type material layers as contact electrodes 3, and silver wires are used as conductors 4 to connect them in parallel, leading out the cathode 5 and anode 6.
[0039] Example 2 Fabrication of a six-quadrant photodetector array using the mask method A schematic diagram of the photodetector array prepared in this embodiment is shown below. Figure 2 As shown, the substrate includes a P-type substrate 1 (using a P-type single-crystal silicon wafer) and six N-type material layers 2 (using MXene thin films) formed on the P-type substrate 1 at intervals. Each N-type material layer 2 forms a PN junction with the P-type substrate 1, and the six N-type material layers 2 are connected in parallel. The six N-type material layers 2 are fan-shaped with consistent angles and arranged in a ring array. The spacing between two adjacent N-type material layers 2 is 300 nm. The fabrication method is as follows: S1: Perform standard RCA cleaning on a P-type monocrystalline silicon wafer (12mm in length and width, 400μm in thickness) to obtain a clean surface; S2: A customized laser-cut stainless steel mask is precisely aligned and applied to the cleaned surface of a P-type monocrystalline silicon wafer. The structure of the mask is as follows: Figure 3 As shown, it includes two parts: a pattern plate and a substrate plate. During operation, the P-type monocrystalline silicon wafer is first placed between the two, and then the pattern plate and the substrate plate are fixed with bolts through the threaded holes around the perimeter to clamp the P-type monocrystalline silicon wafer. Figure 3 The mask design shown features three six-sector ring arrays with diameters of 6mm, 8mm, and 10mm on the pattern plate, allowing for the simultaneous fabrication of three devices of different sizes. S3: MXene (Ti3C2Tx) colloidal aqueous solution is dropped onto all the cut-out areas of the patterned mask until it covers the silicon wafer surface. Then, it is annealed at 60°C for 5 minutes, naturally cooled to room temperature and waited at room temperature until the colloidal aqueous solution is completely dry to form six continuous, transparent MXene films with a thickness of about 650nm, thus defining six identical N-type material layers. S4: Deposit a layer of aluminum film (or other metals such as gold) on the surface of each of the six MXene films as contact electrodes 3, and connect them in parallel using silver wires as conductors 4 to lead out the cathode 5 and anode 6.
[0040] Example 3 Fabrication of an eight-quadrant photodetector array using a mask method The photodetector array structure and fabrication method in this embodiment are basically the same as in Embodiment 2. The only difference is that the number of N-type material layers in this embodiment is eight, and the pattern of the mask used in the corresponding fabrication process is adjusted accordingly.
[0041] Of course, devices containing different numbers of N-type material layers can also be fabricated using the ion implantation method or mask method described above. For example, schematic diagrams of the distribution and parallel connection of photodetector arrays with four, six, eight, and sixteen quadrants are shown below. Figure 4 As shown.
[0042] Comparative Example 1 Fabrication of single-quadrant photodetectors The photodetector structure and fabrication method of this comparative example are basically the same as those of Example 2. The only difference is that this comparative example uses a circular N-type material layer, and the pattern of the mask used in the corresponding fabrication process is adjusted accordingly.
[0043] Comparative Example 2 Fabrication of a dual-quadrant photodetector array The photodetector array structure and fabrication method of this comparative example are basically the same as those of Example 2. The only difference is that this comparative example uses two nearly semi-circular N-type material layers, and the pattern of the mask used in the corresponding fabrication process is adjusted accordingly.
[0044] Experimental Example 1 Performance testing of photodetectors I. The photodetectors prepared in Examples 2 and 3, and Comparative Examples 1 and 2, were subjected to tests such as current-voltage (IV) characteristic curves under the same conditions. All devices used in the tests had an array diameter of 8 mm. The test results are as follows: Figure 5 The current-voltage (IV) characteristics of single-quadrant (single), six-quadrant (hexa), and eight-quadrant (octa) devices under dark-visible light reveal their superior rectification behavior, achieving a ratio of 6 × 10^4 over a ±2V bias range. This high rectification ratio directly indicates the formation of a high-quality Schottky junction and is the fundamental reason for the extremely low dark current (on the order of nanoamps) observed in all device configurations (single-quadrant, six-quadrant, and eight-quadrant devices) under low bias voltages. Such a small dark current is a key property for suppressing intrinsic noise, enabling the precise detection of weak photocurrent signals generated under weak ambient light.
[0045] Figure 6 Under 635nm illumination (1mW / cm) 2 Photocurrent images of devices from single-quadrant to six-quadrant and eight-quadrant configurations. To quantify the performance improvement brought by the eight-quadrant structure, we measured the photocurrent of single-quadrant and eight-quadrant configurations under standardized 635 nm illumination (1 mW / cm²) conditions. Figure 6 The results show that the photocurrent is increased nearly eightfold from a single-quadrant to an eight-quadrant device. This significant improvement stems from its core design: the eight quadrants are connected in parallel, resulting in a linear increase in the total photosensitive area available for carrier generation.
[0046] Figure 7 For cloudy conditions (190 μW / cm) 2 The PPG signals obtained were compared with single-quadrant, six-quadrant and eight-quadrant configurations to demonstrate the significant signal-to-noise ratio advantage of the six-quadrant and eight-quadrant designs in low-light environments.
[0047] Figure 8 For sunny conditions (2mW / cm) 2 The PPG signals obtained show a performance comparison of different quadrant configurations. The eight-quadrant device maintains excellent baseline stability and significant amplitude variation.
[0048] Figure 12 The baseline drift plot of the dual-quadrant device prepared for Comparative Example 2 shows that although the AC amplitude of the dual-quadrant device output is higher, there is a significant baseline (DC component) drift. The baseline drift manifests as low-frequency fluctuations caused by motion, thermal effects, or circuit artifacts, which distort the pulse waveform and obscure key reference points such as the contraction peak and bifurcation depression, thus leading to inaccurate extraction of parameters such as heart rate, respiratory rate, and blood pressure in the time-frequency domain.
[0049] II. To compare the performance differences between the present invention and existing devices, the following tests were performed on a commercial photodiode (Thorlabs FDS1010) and the eight-quadrant distributed photodetector array prepared in Example 3: Figure 9 For FFT analysis of PPG signals from a commercial photodiode (Thorlabs FDS1010) under cloudy conditions, due to poor signal quality and predominantly noise, the FFT analysis could not produce usable RR and HR estimates.
[0050] Figure 10 FFT analysis of the PPG signal of the eight-quadrant device under cloudy conditions showed an RR of 14 times / minute and an HR of 73 times / minute, which is very close to the reference values of commercial wearable devices.
[0051] In summary, the performance of the eight-quadrant photodetector was directly compared with that of a commercially available silicon photodiode (Thorlabs FDS1010) of the same specifications under the same lighting and physiological conditions. While the commercial photodiode performed reasonably well under strong light conditions, it failed to generate a usable PPG signal in cloudy conditions, with its output signal primarily dominated by noise. It is noteworthy that the self-powered photodetector system of this invention can reliably acquire PPG waveforms even when the commercial reference fails. The obtained waveforms are of excellent quality, exhibiting distinct morphological characteristics, and under the same constraint conditions, their amplitude is significantly higher than that of similar commercial products.
[0052] III. To compare the performance differences between the array distribution method of this invention and the ordinary row-column distribution method, a device with eight square MXene thin films arranged in a 4×2 row-column configuration (total area is the same as that in Example 3, with an arrangement spacing of 300 nm) was fabricated and its PPG signal was measured under the same conditions as the eight-quadrant photodetector device in Example 3. The results are as follows: Figure 11 As shown, the pulse waveform measured by the eight-quadrant array arrangement of the present invention has excellent quality, and the generated PPG waveform is clear and stable, with a distinct systolic peak, biphasic dip and diastolic peak.
[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A photodetector array, characterized in that, It includes a P-type material substrate and multiple N-type material layers formed on the P-type material substrate at intervals, each of the N-type material layers forming a PN junction with the P-type material substrate, and the multiple N-type material layers are connected in parallel with each other.
2. The photodetector array as described in claim 1, characterized in that, The multiple N-type material layers are fan-shaped with consistent angles and distributed in a ring array.
3. The photodetector array as described in claim 2, characterized in that, The multiple N-type material layers are distributed in four quadrants, six quadrants, eight quadrants, or sixteen quadrants.
4. The photodetector array as described in claim 2 or 3, characterized in that, The diameter of the array ranges from 2mm to 10mm.
5. The photodetector array as described in claim 1, 2, or 3, characterized in that, The spacing between two adjacent N-type material layers is 300 nm.
6. The photodetector array as described in claim 1, 2, or 3, characterized in that, The thickness of the P-type material substrate is 100μm to 500μm.
7. The photodetector array as described in claim 1, 2, or 3, characterized in that, The thickness of the N-type material layer is less than 2 μm.
8. The photodetector array as described in claim 1, 2, or 3, characterized in that, Multiple N-type material layers are interconnected in parallel by wires, the width of which is 100 nm.
9. The method for fabricating a photodetector array as described in any one of claims 1 to 8, characterized in that, Includes the following steps: S1: Cleaning process for P-type material substrate; S2: A silicon dioxide thin film is deposited on the cleaned P-type material substrate by thermal oxidation. S3: Photoresist is spin-coated onto the surface of a silicon dioxide thin film. The photoresist is exposed and developed through a mask. The cutout area of the mask matches multiple N-type material layers. Then, the photoresist corresponding to the cutout area of the mask is etched away. S4: N-type impurities are implanted into the area where the photoresist has been removed by ion implantation. The N-type impurities penetrate the silicon dioxide film and form multiple N-type material layers on the surface of the P-type material substrate. S5: Remove the remaining photoresist and silicon dioxide film in the corresponding areas of multiple N-type material layers by dry etching or wet etching; S6: Contact electrodes are fabricated on the surfaces of multiple N-type material layers and connected in parallel with wires to lead out the cathode and anode.
10. The method for fabricating a photodetector array as described in any one of claims 1 to 8, characterized in that, Includes the following steps: S1: Cleaning process for P-type material substrate; S2: Cover the cleaned P-type material substrate with a mask, and match the cutout area of the mask with multiple N-type material layers; S3: A solution of N-type material is dropped onto the cutout area of the mask, and after curing, multiple N-type material layers are formed; S4: Contact electrodes are fabricated on the surfaces of multiple N-type material layers and connected in parallel with wires to lead out the cathode and anode.