Ultraviolet-enhanced cmos image sensor based on fluorescence down-conversion and method of manufacturing the same

Abstract

This invention relates to a fluorescence down-conversion-based ultraviolet-enhanced CMOS image sensor and its fabrication method, belonging to the field of ultraviolet photoelectric detection technology. The image sensor comprises a CMOS image sensor and a down-conversion nanomaterial layer. The down-conversion nanomaterial layer is a benzene thin film deposited on the image-sensitive surface of the CMOS image sensor; the benzene thin film of the fluorescence down-conversion material layer has a thickness of 290 nm. This invention utilizes a low-cost fluorescence down-conversion material to achieve ultraviolet-to-visible light conversion. The emitted visible light can be well matched with the spectral response peak of the CMOS image sensor, improving the CMOS image sensor's response capability in the ultraviolet band. The down-conversion material in this invention possesses high light conversion efficiency, excellent thermal and light stability, and excellent transmittance in the visible and infrared regions. The thin film prepared by thermal evaporation exhibits better uniformity.

Description

Technical Field

[0001] This invention belongs to the field of ultraviolet photoelectric detection technology, and particularly relates to an ultraviolet-enhanced CMOS image sensor based on fluorescence downconversion materials and its fabrication method. Background Technology

[0002] Complementary Metal-Oxide-Semiconductor (CMOS) is a semiconductor-integrated optoelectronic device that integrates microlenses, amplifiers, analog-to-digital converters, memory, digital signal processing, and computer interface circuitry on a silicon substrate using standard CMOS processes. With the development of integrated circuit and semiconductor technologies, CMOS technology has made continuous breakthroughs in the semiconductor field since the mid-1980s, evolving from the initial tens of pixels to today's megapixel large-area CMOS sensors, with increasingly sophisticated performance. It is widely used in consumer applications such as smartphones, digital cameras, industrial vision systems, and security monitoring. At the same time, the performance of CMOS continues to be improved and optimized to adapt to the environmental needs of various application scenarios.

[0003] Silicon's suitable bandgap, high reliability, low cost, and compatibility with CMOS manufacturing processes make it the most suitable material for ultraviolet-visible light detection. However, because silicon semiconductors have an absorption coefficient greater than 10⁶ cm⁻¹ in the ultraviolet region, ultraviolet light penetration depth in silicon is less than 100 nm. Consequently, the optical signal cannot penetrate the silicon substrate to reach the photodiode, resulting in low response in the ultraviolet band for traditional silicon-based CMOS image sensors. Based on this, researchers in semiconductor process technology have conducted a series of studies to enhance the ultraviolet response of image sensors, including controlling ion implantation concentration, high-temperature annealing processes, doping techniques, back-thinning techniques, replacing polycrystalline silicon gates with ITO, and ultraviolet antireflection material fabrication processes. These efforts aim to further enhance the ultraviolet response capability of imaging devices, but they also significantly increase costs. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of low response and high cost of traditional silicon-based CMOS image sensors in the ultraviolet band, and to propose an ultraviolet CMOS image sensor based on fluorescence downconversion and its fabrication method.

[0005] The ultraviolet-enhanced CMOS image sensor based on fluorescent downconversion material is characterized in that the image sensor is composed of a CMOS image sensor and a downconversion nanomaterial layer, wherein the downconversion nanomaterial layer is a benzene thin film deposited on the image-sensitive surface of the CMOS image sensor.

[0006] The thickness of the fluorescent downconversion material layer is 290 nm.

[0007] The downconversion nanomaterial layer is deposited on the image sensor surface of the CMOS image sensor by thermal evaporation.

[0008] A method for fabricating an ultraviolet-enhanced CMOS image sensor based on fluorescence downconversion materials, characterized by the following steps:

[0009] S1, Remove the protective window of the CMOS image sensor to expose the image-sensitive surface of the CMOS image sensor.

[0010] S2, the CMOS image sensor with the protective window removed is fixed on the sample tray of the vacuum coating machine with the image sensor facing down, and the downconversion nanomaterial is placed in the quartz evaporation crucible;

[0011] S3 involves heating the downconversion nanomaterials inside the crucible to deposit a fluorescent downconversion film on the CMOS image sensor surface, thereby achieving coupling between the material and the device.

[0012] In step S3, the evaporation temperature is 120℃, the evaporation time is 60 minutes, and the vacuum degree is 5×10⁻⁶. -4 Below Pa.

[0013] This invention employs a low-cost fluorescent down-conversion material to achieve ultraviolet-to-visible light conversion. The emitted visible light can be well matched with the spectral response peak of a CMOS image sensor, improving the CMOS image sensor's response capability in the ultraviolet band. The down-conversion material in this invention exhibits high light conversion efficiency, excellent thermal and light stability, and excellent transmittance in both the visible and infrared regions. The thin film prepared by thermal evaporation also demonstrates better uniformity. Attached Figure Description

[0014] Figure 1 This is a schematic diagram illustrating the downconversion ultraviolet enhancement principle of Example 1.

[0015] Figure 2 The images show the photoexcitation and emission spectra of the fluorescent downconversion material thin film in Example 1.

[0016] Figure 3 The figure shows the thermal stability test results of the fluorescent downconversion thin film prepared in Example 1.

[0017] Figure 4 The image shows the photostability test results of the fluorescent downconversion film prepared in Example 1.

[0018] Figure 5 This is a physical image of a CMOS device after the fluorescent downconversion film was deposited under visible light irradiation, as shown in Example 1.

[0019] Figure 6This is a CMOS image of Example 1 after the fluorescent downconversion film was deposited under ultraviolet light (365nm) irradiation. Figure 7 This is an ultraviolet (365nm) image of a CMOS monochrome camera without a conversion film in Example 1.

[0020] Figure 8 This is an ultraviolet (365nm) image of a CMOS monochrome camera after thinning in Example 1.

[0021] Figure 9 These are ultraviolet images of a CMOS monochrome camera before and after coating.

[0022] Figure 10 The images are ultraviolet images of a CMOS monochrome camera before and after coating in Example 1.

[0023] Figure 11 This is an image of the reflected light from the target object when the thickness of the downconversion film is 120 nm, as shown in Example 2.

[0024] Figure 12 This is an image of the reflected light from the target object when the thickness of the downconversion film is 500 nm, as shown in Example 3. Detailed Implementation

[0025] The present invention will be further described in detail below through specific embodiments, but it should not be construed as limiting the scope of the invention to the following examples. Various substitutions or modifications made based on ordinary technical knowledge and conventional methods in the art without departing from the above-described methodological spirit of the invention should be included within the scope of the invention.

[0026] Example 1: A UV-enhanced CMOS image sensor based on fluorescence downconversion, consisting of a CMOS image sensor and a downconversion nanomaterial layer. The downconversion nanomaterial layer is a benzene thin film deposited on the image-sensitive surface of the CMOS image sensor.

[0027] A method for fabricating an ultraviolet-enhanced CMOS image sensor based on fluorescence downconversion includes the following steps:

[0028] S1, Remove the protective window of the CMOS image sensor to expose the image-sensitive surface of the CMOS image sensor.

[0029] S2, the CMOS image sensor with the protective window removed is fixed on the sample tray of the vacuum coating machine with the image sensor facing down, and the downconversion nanomaterial halobenzene is placed in the quartz evaporation crucible;

[0030] S3 employs the principle of resistive evaporation to heat downconversion nanomaterials in a crucible, causing them to break free from intermolecular binding forces under vacuum, thereby depositing a fluorescent downconversion thin film on the image-sensitive surface of a CMOS image sensor, achieving coupling between materials and devices.

[0031] In step S3, the evaporation temperature is 120℃, the evaporation time is 60 minutes, and the vacuum degree is maintained at 5×10⁻⁶. -4 Below Pa, a benzene film with a thickness of 290 nm can be obtained.

[0032] After the fluorescent downconversion material layer absorbs ultraviolet light, electrons (mainly π electrons and f and d electrons) transition from the ground state (valence band) to a higher-energy excited state (conduction band), leaving valence band holes. This energy level change causes radiative decay, releasing photons and generating fluorescence, thus achieving the conversion from ultraviolet to visible light. The principle of downconversion ultraviolet enhancement is as follows: Figure 1 As shown.

[0033] The peak excitation wavelength of the downconversion nanomaterial is at 290 nm, and the peak emission wavelength is at 500 nm, which can be well matched with the peak spectral response of the CMOS image sensor. Figure 2 As shown.

[0034] The downconversion nanomaterial film exhibits excellent thermal stability. Within a temperature range of 25-250℃, the fluorescent downconversion film was annealed in air using a tubular annealing furnace at different temperature gradients. The photoluminescence spectrum of the benzene film under 290nm ultraviolet light excitation was recorded. The relationship between the fluorescence intensity of the emission spectrum peak and the annealing temperature is as follows: Figure 3 As shown, the photoluminescence intensity of the thin film remains essentially unchanged within a temperature range of 25-175℃. At 200℃, the peak intensity of the emission spectrum is 95.7% of the initial value, indicating that the downconversion nanomaterial has excellent thermal stability.

[0035] Photostability (lifetime) refers to the change in efficiency over time and under light exposure; a short lifespan will lead to frequent sensor replacements. The fluorescence down-conversion film was tested for its emission spectrum at a 280 nm ultraviolet excitation wavelength to study its photodegradation performance. Figure 4 The normalized fluorescence downconversion film's maximum fluorescence intensity as a function of the number of irradiations is shown. After approximately 60 minutes of irradiation, the fluorescence intensity decays exponentially to 64% of its initial value, a decrease of 36%. Compared to general organic materials, the downconversion nanomaterial exhibits better photostability.

[0036] The actual image of the CMOS after coating is shown below. Figure 5As shown, in order to detect the coupling between the downconversion nanomaterial and the CMOS, the coated CMOS was placed under ultraviolet light (365nm). Strong fluorescence was observed on the CMOS image sensor surface, indicating that the coupling between the downconversion nanofilm and the CMOS can be well achieved by thermal evaporation.

[0037] Figure 7 and Figure 8 By comparing the imaging of target objects in the visible light region by CMOS cameras before and after coating, it can be clearly seen that the sharpness and contrast of the objects before and after coating are not significantly different when the sensitivity remains unchanged. This indicates that the thin film at this thickness does not affect the transmittance of the device in the visible light region.

[0038] To verify the enhancement effect of the UV-enhancing film, a CMOS image sensor was mounted on a monochrome camera. The target object was illuminated with a UV lamp with a center wavelength of 365nm, and the reflected light from the target object was captured by the CMOS monochrome camera. This allows for a qualitative analysis of the UV enhancement effect of the downconversion nanofilm. Figure 9 and Figure 10 These are ultraviolet images of a CMOS monochrome camera before and after coating. Compared to before coating, Figure 10 The objects in the image are more detailed than those captured by an uncoated CMOS monochrome camera. Figure 9 It is brighter because the photons actually detected by the UV-enhanced camera are green visible photons after being down-converted by the down-conversion nanofilm, and the CMOS camera has the highest spectral response in this band. Therefore, the CMOS camera coated with the down-conversion film is more sensitive to ultraviolet light.

[0039] Example 2: Compared to Example 1, the preparation steps were exactly the same, except the thickness of the downconversion nanofilm was reduced to 120 nm. Under the same conditions, the reflected light from the target object was imaged using a CMOS monochrome camera. Figure 11 As shown, its UV enhancement effect is not as good as that described in Example 1.

[0040] Example 3: The preparation steps were exactly the same as in Example 1, except the evaporation time was extended to 100 min and the downconversion film thickness was 500 nm. At this time, it was observed that the imaged object and its surrounding environment in the visible light region of the CMOS camera became darker. Figure 12 As shown. This is due to Fresnel reflection between the thin film interface and the air. When incident photons are scattered inside the thin film, some photons no longer propagate along their original direction, resulting in a reduction in the number of photons collected by the photodiode, thus making the photographed target object appear darker. Compared to Examples 2 and 3, the thickness of the down-conversion thin film in Embodiment 1 can improve the device's response to ultraviolet light without affecting the device's transmittance in the visible light region.

Claims

1. An ultraviolet-enhanced CMOS image sensor based on fluorescence downconversion materials, characterized in that... The image sensor consists of a CMOS image sensor and a downconversion nanomaterial layer, which is a benzene thin film deposited on the image-sensitive surface of the CMOS image sensor. The thickness of the fluorescent downconversion material layer is 290 nm.

2. A method for fabricating an ultraviolet-enhanced CMOS image sensor based on fluorescence down-conversion materials, characterized in that... The method includes the following steps: S1, Remove the protective window of the CMOS image sensor to expose the image-sensitive surface of the CMOS image sensor. S2, the CMOS image sensor with the protective window removed is fixed on the sample tray of the vacuum coating machine with the image sensor facing down, and the downconversion nanomaterial is placed in the quartz evaporation crucible; S3, heating the downconversion nanomaterials in the crucible to deposit a fluorescent downconversion film on the CMOS image sensor surface, thereby achieving coupling between the material and the device; In step S3, the evaporation temperature is 120℃, the evaporation time is 60 min, and the vacuum degree is 5×10⁻⁶. -4 Below Pa.

Images