Miniaturized infrared film thickness sensor based on on-chip spectrometer

Abstract

The application discloses a kind of miniaturization infrared film thickness sensor based on on-chip spectrometer.The application includes micro control unit, wide-spectrum infrared light source module, integrated optical fiber probe, grating coupler, on-chip spectrometer chip, signal conditioning circuit and spectral data processing module;The wide-spectrum infrared light source module is driven by micro control unit and emits wide-spectrum infrared light, which is transmitted to the surface of the sample to be measured by integrated optical fiber probe;The interference light signal carrying film thickness information reflected by the sample is collected again by integrated optical fiber probe, and enters the on-chip spectrometer chip after passing through grating coupler;The on-chip spectrometer chip has waveguide transmission module, light splitting module and detection array integrated therein.The application breaks through the architecture limitation of traditional infrared film thickness measurement system "probe+large desktop spectrometer", integrates using on-chip spectrometer chip, greatly reduces the size and weight of equipment, and reduces system cost.

Description

Technical Field

[0001] This invention belongs to the field of optical measurement technology, specifically relating to a miniaturized infrared film thickness sensor based on an on-chip spectrometer. Background Technology

[0002] Currently, in fields such as industrial inspection, semiconductor manufacturing, and materials analysis, infrared film thickness and micro-distance measurements often employ probe-type sensors based on spectral analysis. These systems typically consist of a separate fiber optic probe, a main controller incorporating a traditional grating spectrometer, and a data processing unit. Their stable operating principle and high measurement accuracy have led to their widespread application. However, the structure of traditional grating spectrometers results in a large main controller. This "probe + large controller" architecture makes it difficult to integrate the entire system into space-constrained online inspection platforms, portable devices, or embedded instruments, limiting its widespread application in field, real-time measurement scenarios.

[0003] Despite significant advancements in the miniaturization of spectroscopic devices, on-chip spectrometer technology still faces numerous challenges when applied to high-precision infrared film thickness measurement. Existing on-chip spectroscopic solutions often struggle to balance wide operating bandwidth and high spectral resolution, and tend to focus on single spectral acquisition functions, lacking deep integration with broadband light sources, interferometric optical paths, and back-end computational algorithms. In practical applications, the key bottlenecks preventing miniaturized film thickness sensors from transitioning from the laboratory to industrial applications lie in how to efficiently couple broadband interferometric signals into micro / nano waveguides, effectively suppress stray light interference and compensate for temperature drift at the chip level, and simultaneously implement complex thickness inversion algorithms with limited computing power. Therefore, a system-level solution capable of synergistically optimizing optical path integration, spectral quality, and data processing capabilities is urgently needed.

[0004] To address the challenges of miniaturization and integration in spectral measurement systems, on-chip spectrometer technology has emerged. This technology integrates optical components such as optical waveguides, arrayed waveguide gratings, tunable filters, or miniature detector arrays onto a single chip using micro- and nano-fabrication processes, achieving miniaturization of both spectral dispersion and detection functions. This provides a new technological path for sensor miniaturization. Therefore, this invention proposes a miniaturized infrared film thickness sensor based on an on-chip spectrometer. By highly integrating the light source, interference optical path, on-chip spectrometer, and data processing module, the system achieves comprehensive miniaturization and intelligence, effectively overcoming the size and weight limitations of traditional solutions. Summary of the Invention

[0005] The purpose of this invention is to provide a miniaturized infrared film thickness sensor based on an on-chip spectrometer, thereby realizing the design of a miniaturized infrared film thickness sensor.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A miniaturized infrared film thickness sensor based on an on-chip spectrometer includes: a microcontroller unit, a broadband infrared light source module, an integrated fiber optic probe, a grating coupler, an on-chip spectrometer chip, a signal conditioning circuit, and a spectral data processing module. The broadband infrared light source module emits broadband infrared light under the drive of the microcontroller unit, which is transmitted to the surface of the sample to be measured via the integrated fiber optic probe. The interference light signal carrying film thickness information reflected from the sample is collected again by the integrated fiber optic probe, passes through the grating coupler, and enters the on-chip spectrometer chip. The on-chip spectrometer chip integrates a waveguide transmission module, a beam splitting module, and a detector array. The beam splitting module is used to spatially demultiplex the coupled optical signals and guide optical signals of different wavelengths to the corresponding units of the detector array.

[0008] Furthermore, the specific internal structure and connection relationships of the on-chip spectrometer chip are as follows:

[0009] A first coupling cone is provided between the grating coupler and the input waveguide in the waveguide transmission module; the width of the first coupling cone gradually narrows along the light propagation direction; the broadband optical signal coupled by the first coupling cone is divided into two parts, one part is directly output to the detector array as a reference waveguide, and the other part is used as an input waveguide to transmit the broadband optical signal to the free propagation region.

[0010] Furthermore, the beam splitter module consists of multiple sequentially arranged reflective tooth surfaces. The module uses silicon nitride as the waveguide core layer, with silicon dioxide cladding on both the top and bottom layers, and a silicon substrate (Si) as the bottom layer. A high-reflectivity metal film is deposited on the etched concave grating tooth surface. Diffused light waves are reflected after striking the concave grating. Due to the periodic or non-periodic arrangement of the grating teeth, light of different wavelengths is diffracted to different angles and simultaneously focused at different positions at the output end of the free propagation region. Multiple output waveguides are arranged along the focusing arc of the free propagation region. Each output waveguide corresponds to a specific diffraction angle, thereby receiving a monochromatic light signal of a specific center wavelength. The ends of the output waveguides are connected to the detector array via a second coupling cone. The detector array converts the received optical signal into an electrical signal for subsequent spectral analysis.

[0011] Furthermore, the broadband infrared light source module uses a superluminescent diode with a center wavelength located in the near-infrared band.

[0012] Furthermore, the integrated fiber optic probe adopts a "6+1" coaxial packaging structure, in which there is a receiving fiber at the center, the output end of which is connected to the grating coupler; and six transmitting fibers are distributed around the periphery, the input ends of which are connected to the broadband infrared light source module.

[0013] Furthermore, the detection end face of the integrated fiber optic probe is integrated with a miniature focusing lens, which is used to collimate and focus the diverging light emitted by the transmitting fiber onto the sample to be tested, and to efficiently converge the sample reflected light to the central receiving fiber.

[0014] Furthermore, the grating coupler adopts a linear chirped focusing structure, and its grating period and fill factor are linearly and gradually distributed along the light propagation direction.

[0015] Furthermore, the signal conditioning circuit mainly consists of a transimpedance amplifier, a low-pass filter, and an analog-to-digital converter; the transimpedance amplifier converts the weak photocurrent output by the detection array into a voltage signal; the low-pass filter filters out high-frequency noise; and the analog-to-digital converter converts the analog voltage signal into a digital signal and transmits it to the microcontroller unit.

[0016] Furthermore, the spectral data processing module is a thickness calculation algorithm. It converts the interference fringes in the frequency domain into amplitude signals in the thickness domain by using a fast Fourier transform on the collected reflectance spectral data, and calculates the physical thickness of the film under test by extracting the optical path difference corresponding to the main amplitude peak.

[0017] Furthermore, the present invention also provides an application of a miniaturized infrared film thickness sensor based on an on-chip spectrometer, which can be embedded in space-constrained production lines, robotic arms, or portable handheld devices.

[0018] Compared with the prior art, the technical solution provided by this invention has the following advantages:

[0019] 1. This invention breaks through the architectural limitations of traditional infrared film thickness measurement systems, which consist of a "probe + large benchtop spectrometer." It integrates the microcontroller unit, drive circuitry, and light source into a single housing using an on-chip spectrometer chip. This not only significantly reduces the size and weight of the device and lowers system costs, but also allows the sensor to be easily embedded in space-constrained production lines, robotic arms, or portable handheld devices, achieving a leap from "laboratory instrument" to "industrial sensor."

[0020] 2. To address the requirement for wide spectral bandwidth in infrared interferometry, this invention employs a linearly chirped focusing grating coupler. Compared to traditional uniform gratings, the linearly graded period and duty cycle design better matches the fiber mode field and the silicon nitride waveguide mode field, significantly broadening the effective coupling bandwidth and reducing back reflection loss. Combined with the efficient light-gathering design of the integrated "6-in-1" fiber optic probe, the signal-to-noise ratio of weak interferometric signals is greatly improved, thereby enhancing the measurement accuracy of weakly reflective surfaces or thin films.

[0021] 3. This invention uses a low-coherence superluminescent diode (SLD) as the light source, effectively suppressing parasitic interference noise from non-target surfaces in the optical path. Simultaneously, combined with the Fast Fourier Transform (FFT) algorithm of the host computer, the system can directly extract thickness information in the frequency domain, avoiding the complex phase unwrapping process. This results in fast computation speed, meeting the stringent real-time requirements of industrial online inspection. Attached Figure Description

[0022] Figure 1 This is a system integration principle block diagram of a miniaturized infrared film thickness sensor based on an on-chip spectrometer.

[0023] Figure 2 This is a schematic diagram of the mechanical structure of a miniaturized infrared film thickness sensor based on an on-chip spectrometer.

[0024] Figure 3 This is a schematic diagram of the optical structure of a two-in-one integrated fiber optic probe.

[0025] Figure 4 This is a schematic diagram of the cross-sectional distribution of the fiber bundle.

[0026] Figure 5 This is a schematic cross-sectional view of a linear chirped focusing grating coupler.

[0027] Figure 6 This is a schematic diagram of the optical path structure of an on-chip integrated spectrometer.

[0028] Figure 7 This is a flowchart of the spectral signal acquisition and processing process.

[0029] Figure 8 This is a schematic diagram illustrating the principle and data processing of thin film thickness measurement based on the interference principle. Detailed Implementation

[0030] To further understand the content of this invention, the invention will be described in detail with reference to the embodiments. The following embodiments are used to illustrate the invention, but are not intended to limit the scope of the invention.

[0031] See attached document Figure 1 and Figure 2 As shown, this invention provides a miniaturized infrared film thickness sensor based on an on-chip spectrometer. This invention employs a highly integrated design, integrating core components such as a microcontroller unit (MCU) 6, a broadband infrared light source module 5, an integrated fiber optic probe, an on-chip spectrometer chip 4, a grating coupler 3, a signal conditioning circuit, and a spectral data processing module into a single circuit board or compact housing, overcoming the bulky nature of traditional external spectrometers.

[0032] Furthermore, the microcontroller unit (MCU) 6 acts as the control center, responsible for coordinating the overall operation. Specific functions include: sending data to the host computer via the communication interface 8; controlling the light source drive module and adjusting the luminous intensity of the infrared light source via pulse width modulation signals or analog signals; generating timing signals to drive the on-chip spectrometer chip for data acquisition; performing preprocessing such as buffering and noise reduction on the acquired raw spectral data; and finally uploading the data to the host computer via the data transmission interface. The aforementioned light source drive module is an existing module that integrates a constant current source circuit to ensure the long-term stability of the output optical power and center wavelength.

[0033] Furthermore, the broadband infrared light source module 5 preferably adopts an infrared superluminescent diode (SLD). The SLD combines the high brightness of a laser diode with the broadband characteristics of a light-emitting diode. Its low coherence can effectively suppress long-distance parasitic interference noise, making it particularly suitable for the measurement of thin film thickness in the micrometer to millimeter range.

[0034] like Figure 3 and Figure 4 As shown, this invention employs a "two-in-one" integrated fiber optic probe, achieving compact integration of the transmitting and receiving optical paths. Figure 4 As shown, the fiber bundle adopts a "6+1" distribution structure. Section 1-1 shows that the outer six fibers are transmitting fibers, with their other ends connected to the light source to ensure uniform brightness and sufficient energy of the light spot illuminating the sample. Section 1-2 shows that the central fiber is the receiving fiber, with its other end connected to the grating coupling port of the on-chip spectrometer to collect the reflected interference signal. A miniature focusing lens 2 (e.g., ...) is integrated at the end of the fiber optic probe. Figure 3 (As shown). The diverging light emitted from the transmitting fiber is collimated and focused by lens 2 and then illuminates the sample 10 under test; the light signal reflected from the sample is again focused by lens 2 and efficiently coupled into the receiving fiber in the center. This common optical path design effectively improves the signal collection efficiency and enhances the resistance to vibration interference.

[0035] The optical signal enters the on-chip spectrometer chip through the grating coupler. For example... Figure 5 As shown, the grating coupler adopts a linear chirped focusing structure, consisting of a silicon substrate, a silicon dioxide (SiO2) cladding, and a silicon waveguide core layer from bottom to top. To improve the mode matching efficiency between the optical fiber and the waveguide and reduce back reflection, the grating coupler in this embodiment is divided into N variable-period grating regions (chirped regions) and M uniform grating regions along the light propagation direction. The structural parameters of the variable-period grating regions satisfy the following relationship:

[0036] Periodic changes:

[0037] Changes in fill factor (duty cycle):

[0038] in, Let i be the period of the i-th unit in the variable periodicity region. For the periodicity of the uniform region, The step size is a periodic linear variation. Let i be the fill factor of the i-th unit. For the uniform region fill factor, Where is the fill factor variation step size, and N is the number of chirped gratings. Through the above linear gradient design, the grating coupler can efficiently convert the Gaussian mode field of the optical fiber into the waveguide mode field.

[0039] like Figure 6 The diagram shows the structure of the on-chip spectrometer chip of this invention, mainly including a waveguide transmission module, a beam splitting module, and a detector array. The specific structure and connections are as follows: A first coupling cone is provided between the grating coupler and the input waveguide in the waveguide transmission module. The width of the first coupling cone gradually narrows (or widens, depending on the specific design) along the light propagation direction, used to achieve adiabatic mode conversion and reduce mode mismatch loss when the optical signal enters the narrow waveguide region from the wide grating region. The broadband optical signal coupled by the first coupling cone is divided into two parts: one part is directly output to the detector array as a reference waveguide, and the other part is used as the input waveguide. The input waveguide transmits the broadband optical signal to the free propagation region. The free propagation region (FPR) is a plate-shaped waveguide where the beam is no longer restricted in the horizontal direction, thus undergoing free diffraction and diffusion. It is located at the arc-shaped boundary at the far end of the free propagation region. Figure 6 The lower left inset shows the beam splitting module, which consists of multiple sequentially arranged reflective tooth surfaces. In this embodiment, the beam splitting module preferably uses silicon nitride (Si3N4) as the waveguide core layer, covered with silicon dioxide (SiO2) cladding layers on the top and bottom, with a silicon substrate (Si) as the bottom layer. A high-reflectivity metal film (such as Ag or aluminum (Al) is deposited on the etched concave grating tooth surface. The function of this metal layer is to enhance the reflectivity of the grating interface, reduce the loss caused by the light signal penetrating the grating, and thus improve the signal-to-noise ratio of the system. Diffused light waves are reflected after impacting the concave grating. Due to the periodic or non-periodic arrangement of the grating teeth, light of different wavelengths is diffracted to different angles and simultaneously focused at different positions at the output end of the free propagation region. Multiple output waveguides are arranged on the focusing arc line of the free propagation region. The position of each output waveguide corresponds to a specific diffraction angle, thereby receiving a monochromatic light signal of a specific center wavelength. The end of the output waveguide is connected to the detector array through a second coupling cone. The detector array converts the received light signal into an electrical signal for subsequent spectral analysis.

[0040] Figure 7For the spectral detection system and its signal processing flow, the light signal reflected from the sample under test is first transmitted to a grating coupler, which efficiently couples the light signal in the optical fiber into the waveguide transmission module of the on-chip spectrometer chip. The light signal output from the waveguide transmission module is then processed by a beam splitter (such as...). Figure 6 The planar concave grating shown in the lower left corner performs wavelength demultiplexing to separate light of different wavelengths in space. Each monochromatic beam after being split by the beam splitter passes through an output waveguide and then connects to the detector array via a second coupling cone. The detector array utilizes the photoelectric effect to convert the light intensity signal into a weak analog photocurrent. Since the photocurrent output by the detector array is typically extremely weak (nanoamperes to microamperes), it cannot be processed directly. Therefore, the photocurrent output by the detector array is conditioned by a signal conditioning circuit, which includes a transimpedance amplifier, a filter circuit, and an analog-to-digital converter. The photocurrent first amplifies the signal through the transimpedance amplifier; then it passes through the filter circuit to remove high-frequency noise, power frequency interference, and thermal noise from the circuit itself. The analog voltage signal output by the filter circuit is sent to the analog-to-digital converter to obtain a conditioned digital signal; the MCU reads the quantized digital signal and transmits it to the host computer.

[0041] Figure 8 This is a method for thin film thickness measurement and data processing based on the principle of interference. Light emitted from a broadband light source illuminates the thin film sample, such as... Figure 8 The Gaussian spectrum is shown in the diagram. The light beam is reflected at both the upper and lower surfaces of the thin film. Due to the film thickness *d*, an optical path difference exists between the two reflected beams, resulting in periodic interference fringes in the superimposed spectrum. After the detection array acquires this reflected spectral signal, the generated signal is processed by a host computer. The host computer uses algorithms such as Fast Fourier Transform (FFT) to transform the spectral data from the wavelength domain to the thickness domain. Figure 8 As shown in the interference principle model section, the film structure in this example is an air-SiO2-Si substrate, and the refractive indices of the three materials are n0, n1+ik1, and n2+ik2, respectively. In the frequency domain analysis, the frequency characteristics of the interference fringes are transformed into significant amplitude spikes. The horizontal coordinate of these spikes directly corresponds to the optical thickness of the film, and the physical thickness of the film can be accurately calculated by combining the refractive indices of the materials.

Claims

1. A miniaturized infrared film thickness sensor based on an on-chip spectrometer, characterized in that, include: Microcontroller unit, broadband infrared light source module, integrated fiber optic probe, grating coupler, on-chip spectrometer chip, signal conditioning circuit and spectral data processing module; Driven by the microcontroller unit, the broadband infrared light source module emits broadband infrared light, which is transmitted to the surface of the sample under test via an integrated fiber optic probe. The interference light signal carrying thin film thickness information reflected by the sample is collected again by the integrated fiber optic probe, passes through a grating coupler, and enters the on-chip spectrometer chip. The on-chip spectrometer chip integrates a waveguide transmission module, a beam splitting module, and a detector array. The beam splitting module is used to spatially demultiplex the coupled light signal and guide light signals of different wavelengths to the corresponding units of the detector array. A first coupling cone is provided between the grating coupler and the input waveguide in the waveguide transmission module; the width of the first coupling cone gradually narrows along the light propagation direction; the broadband optical signal coupled by the first coupling cone is divided into two parts, one part is directly output to the detector array as a reference waveguide, and the other part is used as an input waveguide to transmit the broadband optical signal to the free propagation region; The beam splitter module consists of multiple sequentially arranged reflective tooth surfaces. It uses silicon nitride as the waveguide core layer, covered with silicon dioxide cladding layers on both the top and bottom, with a silicon substrate (Si) at the bottom. A highly reflective metal film is deposited on the etched concave grating tooth surfaces. Diffused light waves are reflected after striking the concave grating. Due to the periodic or non-periodic arrangement of the grating teeth, light of different wavelengths is diffracted to different angles and simultaneously focused at different positions at the output end of the free propagation region. Multiple output waveguides are arranged along the focusing arc of the free propagation region. Each output waveguide corresponds to a specific diffraction angle, thereby receiving monochromatic light signals of a specific center wavelength. The end of the output waveguide is connected to the detector array via a second coupling cone; the detector array converts the received optical signal into an electrical signal for subsequent spectral analysis.

2. The miniaturized infrared film thickness sensor based on an on-chip spectrometer according to claim 1, characterized in that: The broadband infrared light source module uses a superluminescent diode with a center wavelength in the near-infrared band.

3. The miniaturized infrared film thickness sensor based on an on-chip spectrometer according to claim 1, characterized in that: The integrated fiber optic probe adopts a "6+1" coaxial packaging structure, in which there is a receiving fiber at the center, the output end of which is connected to the grating coupler; and six transmitting fibers are distributed around the periphery, the input ends of which are connected to the broadband infrared light source module.

4. The miniaturized infrared film thickness sensor based on an on-chip spectrometer according to claim 3, characterized in that: The integrated fiber optic probe has a miniature focusing lens integrated on its detection end face, which is used to collimate and focus the diverging light emitted by the transmitting fiber onto the sample to be tested, and to efficiently converge the sample reflected light to the central receiving fiber.

5. The miniaturized infrared film thickness sensor based on an on-chip spectrometer according to claim 1, characterized in that: The grating coupler adopts a linear chirped focusing structure, and its grating period and fill factor are linearly and gradually distributed along the light propagation direction.

6. The miniaturized infrared film thickness sensor based on an on-chip spectrometer according to claim 1, characterized in that: The signal conditioning circuit mainly consists of a transimpedance amplifier, a low-pass filter, and an analog-to-digital converter. The transimpedance amplifier converts the weak photocurrent output by the detection array into a voltage signal; the low-pass filter filters out high-frequency noise; and the analog-to-digital converter converts the analog voltage signal into a digital signal and transmits it to the microcontroller unit.

7. The miniaturized infrared film thickness sensor based on an on-chip spectrometer according to claim 1, characterized in that: The spectral data processing module is a thickness calculation algorithm. It converts the interference fringes in the frequency domain into amplitude signals in the thickness domain by using a fast Fourier transform on the collected reflection spectral data, and calculates the physical thickness of the film under test by extracting the optical path difference corresponding to the main amplitude peak.

8. An application of a miniaturized infrared film thickness sensor based on an on-chip spectrometer, characterized in that, The sensor as described in any one of claims 1-7 can be embedded in space-constrained production lines, robotic arms, or portable handheld devices.

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