Short-wave infrared light emission and fluorescence confocal microscopy system for integrated circuit detection

Abstract

The present application relates to a kind of microscope systems, in particular to a kind of short wave infrared waveband confocal microscopic system.The system includes bright / dark field microscopic subsystem, photonic emission microscopic subsystem, short wave infrared laser scanning confocal microscopic subsystem, fluorescent confocal microscopic subsystem, control module.The system presented in the present application can be through the electrical light emission and / or photoluminescence emission of semiconductor device in integrated circuit, obtain short wave infrared fluorescence lifetime curve, that is, the determination of semiconductor defect type can be made.In addition, the system uses the structure of multiple subsystem optical path multiplexing in structural design, while collecting the short wave infrared fluorescence generated by excitation defect or failure point, and has higher integration, only using the single system presented in the present application can realize the observation, positioning and failure analysis of defect or failure point.

Description

Technical Field

[0001] This invention relates to a microscope system, and more particularly to a confocal microscope system in the short-wave infrared band. Background Technology

[0002] During the production and use of integrated circuits, defects or failures in chips and their mechanisms require faster detection to identify the causes. Traditional testing methods determine the presence of faults by performing electrical measurements on semiconductor wafers or dies, providing basic data for subsequent failure analysis. However, to achieve fault tracing, traditional electrical testing alone is far from sufficient; precise location of defects and reconstruction of the failure scenario are essential for a deeper understanding of the failure mechanism. As chip integration increases, with more layers, more intricate circuits, and continuously decreasing operating voltages, extremely high demands are placed on the performance of electrical testing.

[0003] While increased integration brings enhanced chip performance, it also amplifies the risk of failure. Once a failure occurs, its causes are often complex, requiring a combination of sophisticated testing methods and functional modules, such as mechanical testing, electron beam testing (EBT), and X-ray imaging, for precise localization and analysis. This complicates the operational process, necessitating frequent movement of specific faulty samples during instrument switching, increasing the risk of sample contamination and damage.

[0004] Current chip failure detection technologies include destructive methods that require sample cutting, ranging from ordinary optical microscopy to transmission electron microscopy (TEM) and scanning electron microscopy (SEM). While these methods can effectively address a variety of defects, the irreversibility of sample cutting makes it difficult to reconstruct the failure mode, as a chip may exhibit different defects and failure problems after damage, and determining the true cause of failure is challenging. Therefore, a non-destructive, high-precision detection method is needed.

[0005] For integrated circuit manufacturers and research institutions, samples with defects or failures are extremely valuable due to economic benefits and production costs. They often aim to locate and identify problems using non-destructive methods and with minimal testing costs. A more comprehensive understanding, analysis, and assessment of the defects is crucial, followed by a classification and summary of their mechanisms, which are then fed back to the relevant design, production, or usage stages to prevent recurrence. However, conventional optical microscopy systems can only analyze morphology and cannot truly test the functionality of integrated circuit devices. Therefore, achieving non-destructive testing has become a major challenge for failure location analysis systems. Summary of the Invention

[0006] This invention innovatively proposes a highly integrated microscopic system that can obtain defect type feedback by collecting short-wave infrared fluorescence lifetime through the light emission and photoluminescence of semiconductors themselves. Utilizing the light transmission characteristics of silicon in the short-wave infrared band and the principle of stimulated emission, it uses a highly sensitive weak light detection method to detect the weak short-wave infrared light signal emitted by the fault point under electrical and optical excitation, and accurately locate the failed device.

[0007] The technical solution provided by this invention is: a short-wave infrared light emission and fluorescence confocal microscopy system for integrated circuit testing, comprising a bright / dark field microscopy subsystem, a photon emission microscopy subsystem, a short-wave infrared laser scanning confocal microscopy subsystem, a fluorescence confocal microscopy subsystem, and a control module; the bright / dark field microscopy subsystem observes the sample through vertical illumination from a bright field source or diffuse reflection from a dark field source; the photon emission microscopy subsystem drives the integrated circuit under test for testing; the short-wave infrared laser scanning confocal microscopy subsystem uses a short-wave infrared laser focused on the sample, and the reflected light is captured by a deep-cooled InGaAs imager after passing through a beam splitter and a focusing lens; the laser emitted by the fluorescence confocal subsystem enters the short-wave infrared microscope objective through a mirror and converges to the sample surface to detect the short-wave infrared fluorescence lifetime; the control module is used to control the data acquisition and working mode of each subsystem, and to process the acquired data to determine sample defects or failures.

[0008] Preferably, the bright / dark field microscopy subsystem includes a bright field light source, a dark field light source, a 200mm focal length lens and a 75mm focal length lens, a beam splitter, and an objective lens; the bright field light source illuminates the test sample perpendicularly after passing through the 75mm focal length lens and the beam splitter; the dark field light source is arranged diagonally above the sample stage.

[0009] Preferably, the short-wave infrared laser scanning confocal microscopy subsystem includes a 1310nm laser, a 250mm focal length lens and a 25.4mm focal length lens, a beam splitter, a fast-steering mirror, a scanning lens, a tube mirror, and a cryogenic InGaAs imager; the telecentric system formed by the scanning lens and the tube mirror ensures that the objective lens aperture is always fully illuminated when the fast-steering mirror moves.

[0010] Preferably, the fluorescence confocal subsystem includes a 660nm laser, a 200mm focal length lens and a 25.4mm focal length lens, a dichroic mirror, and part of the laser confocal optical path; the dichroic mirror is a long-pass type, transmitting wavelengths above 980nm; the fluorescence excited after the laser focuses the sample is transmitted through the optical path, and after passing through the dichroic mirror, beam splitter, and 20.1mm focal length focusing lens, the fluorescence lifetime is detected by a single-photon detector.

[0011] Preferably, the deep-cooled InGaAs imager includes a water-cooling module, a multi-stage thermoelectric cooler, and an InGaAs short-wave infrared imager; the cold end of the multi-stage thermoelectric cooler is connected to the water-cooling module, and the hot end is connected to the InGaAs short-wave infrared imager; the deep-cooled InGaAs imager acquires high-contrast tomographic images and three-dimensional structures of the failed device through the coordinated operation of an emission microscope, a short-wave infrared laser, and a three-dimensional displacement stage.

[0012] Preferably, the system has four operating modes: in bright / dark field mode, only the wide-field light source is used for macroscopic detection of the sample to locate the region of interest or failure site; in emission microscopy mode, the wide-field light source is turned off, and the emission image of the failed device is captured for precise location; in laser confocal scanning mode, the wide-field light source and short-wave infrared laser are used simultaneously, and the wide-field light source is turned off after determining the scanning range according to the size of the failed device, and the two-dimensional high-contrast image and three-dimensional stereo image of the key device of the sample are obtained by scanning with a displacement stage; in fluorescence confocal mode, the red laser is retained, and time-correlated single-photon counting technology is used to collect fluorescence lifetime data.

[0013] Compared to existing technologies, the system proposed in this invention can obtain short-wavelength infrared fluorescence lifetime curves through electroluminescence and / or photoluminescence emission of semiconductor devices within integrated circuits, thereby enabling the determination of semiconductor defect types. Furthermore, the system employs a multi-subsystem optical path multiplexing structure, facilitating observation while collecting short-wavelength infrared fluorescence generated by excitation defects or failure points. It also boasts high integration, allowing for the observation, location, and failure analysis of defects or failure points using only the single system proposed in this invention. Due to the generally small size of silicon-based chip devices, this invention utilizes a point scanning method to achieve more flexible control over the scanning size and direction. This invention employs a camera acquisition scheme, with all subsystems implementing the four modes using an optical multiplexing design to form a coaxial system, resulting in excellent compactness. Attached Figure Description

[0014] Figure 1This is a schematic diagram of the hardware structure of a short-wave infrared light emission and fluorescence confocal microscopy system for integrated circuit testing in an embodiment of the present invention; in the figure, 101. 660nm laser; 102. 1310nm laser; 103. First lens; 104. Second lens; 105. Third lens; 106. Fourth lens; 107. Two-dimensional scanning galvanometer; 108. Scanning lens; 109. Sleeve lens; 110. Dichroic mirror; 111. First beam splitter; 112. Seventh lens; 113. Single-photon detector; 114. Bright-field light source; 115. Dark-field light source; 116. Three-dimensional displacement platform; 117. Test sample; 118. Objective lens; 119. Piezoelectric ceramic sensor; 120. Second beam splitter; 121. Third beam splitter; 122. Fifth lens; 123. Sixth lens; 124. Deep-cooled InGaAs imager; Figure 2 This is a schematic diagram illustrating the working principle and detection process of the short-wave infrared light emission and fluorescence confocal microscopy system used for integrated circuit detection in an embodiment of the present invention. Detailed Implementation

[0015] To facilitate understanding of the present invention, it will be described in more detail below with reference to the accompanying drawings and specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described in this specification. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0016] This invention proposes a highly integrated nondestructive testing system that combines a bright / dark field microscopy subsystem, a light emission microscopy subsystem, a laser scanning confocal microscopy subsystem, and a fluorescence confocal microscopy subsystem. It utilizes the transparency of silicon in the short-wave infrared band and the principle of stimulated emission. The system mainly includes a wide-field illumination source, a 1310 nm short-wave infrared laser, a 660 nm visible light laser, a deep-cooled single-photon detector, a deep-cooled short-wave infrared imager, a three-dimensional displacement stage, and optical lenses. It employs an optical multiplexing structure to achieve a high degree of system integration.

[0017] like Figure 1As shown, the bright / dark field microscopy subsystem comprises a bright field module and a dark field module. The bright field module includes a bright field light source 114, a fifth lens 122 with a focal length of 200mm and a sixth lens 123 with a focal length of 75mm, a third beam splitter 121, an objective lens 118, a piezoelectric ceramic sensor 119, and a deep-cooled InGaAs imager 124. The test sample 117 is observed through vertical illumination from the bright field light source 114. The dark field module includes a dark field light source 115, a fifth lens 122, a third beam splitter 121, an objective lens 118, a piezoelectric ceramic sensor 119, and a deep-cooled InGaAs imager 124. The dark field light source 115 is arranged diagonally above the sample stage, allowing observation of the sample through diffuse reflection. The bright / dark field microscopy subsystem utilizes the excellent penetrability of short-wave infrared light to silicon, combining two illumination methods to achieve non-contact, non-destructive testing of unmetallized wafers or back-packaged chips, depending on requirements. This subsystem's wide-field imaging of macroscopic regions also provides a working basis for subsequent precise defect localization.

[0018] Photonic emission microscopy (PEMMS) exhibits unique advantages in integrated circuit defect localization, particularly in detecting minute electrical performance anomalies and internal structural defects, providing a more precise diagnostic tool. The core of the PEMMS is an InGaAs short-wave infrared imager, which not only performs macroscopic observation of the failure area but also monitors weak light emission points in the field of view in real time, enabling precise localization of failed devices.

[0019] When observing integrated circuits or semiconductor devices using a bright / dark field microscopy system, appropriate objectives must be selected for observation depending on the integrated circuit's manufacturing process. As the magnification of the objective increases, its aperture decreases accordingly, necessitating a corresponding increase in the imager's gain and integration time to ensure image quality. This also means effectively suppressing the impact of thermal noise and readout noise on image quality. In applications, due to light intensity loss and the low effective numerical aperture (NA) of macro objectives, the relative signal intensity collected by each objective may differ from the calculated value. InGaAs detectors exhibit good overlap between the short-wave infrared light signals they collect and the spectra of silicon-penetrating devices, and can observe issues involving chemical impurities, physical defects, and recombination radiation, making them a superior choice for light emission microscopy systems. This invention develops a high-efficiency, deep-cooled InGaAs short-wave infrared imager for this system, serving as the primary detector for short-wave infrared wide-field microscopy, light emission microscopy, and laser confocal microscopy systems. The deep-cooled InGaAs short-wave infrared imager comprises a water-cooling module, a multi-stage thermoelectric cooler, and an InGaAs short-wave infrared imager. The cold end of the multi-stage thermoelectric cooler is connected to the water-cooling module, and the hot end is connected to the InGaAs short-wave infrared imager. The water-cooling module and the multi-stage thermoelectric cooler (TEC) work together to achieve -50°C cooling, effectively reducing dark current and thermal noise, and minimizing the generation of "hot" pixels and dead pixels. These methods significantly improve the overall imaging quality and signal-to-noise ratio in low-light detection.

[0020] This deep-cooled InGaAs short-wave infrared imager operates on a 5V power supply, consuming only 2.5W in uncooled mode. The thermoelectric cooler (TEC) at the cooled end requires a separate power supply, consuming approximately 20W. Furthermore, the operating frequency and integration time of the area-array staring short-wave infrared imager are flexibly adjustable: the operating frequency can be adjusted from 100 kHz to 20 MHz, and the integration time can be adjusted from 100 ns to 100 s, achieving a maximum imaging frame rate of 244 fps. Using a 100× objective lens with the smallest aperture, images of the same area of ​​a wafer sample were acquired under both uncooled and -50℃ conditions, with a uniform integration time (5 ms). The results show that the uncooled imager, due to heat accumulation during operation, has an actual operating temperature much higher than room temperature, resulting in significantly reduced image quality due to thermal noise. In contrast, cooling to -50℃ effectively suppresses thermal noise, significantly improving image clarity and signal-to-noise ratio.

[0021] This light emission microscopy subsystem and the bright / dark field microscopy subsystem use an optical path multiplexing structure. By combining the light emission characteristics of the device under bias conditions with bright / dark field microscopic images, it can achieve efficient and rapid localization of failure points or defective devices in integrated circuits.

[0022] The laser scanning confocal microscopy subsystem comprises a 15mW 1310nm laser 102, a first lens 103 with a focal length of 250mm, a second lens 104 with a focal length of 25.4mm, a first beam splitter 111, a two-dimensional scanning galvanometer 107, a scanning lens 108, a sleeve lens 109, and a deep-cooled InGaAs short-wave infrared imager 124. The telecentric system formed by the scanning lens 108 and the sleeve lens 109 ensures that the objective aperture is always fully illuminated when the two-dimensional scanning galvanometer 107 moves. After the short-wave infrared laser is focused onto the test sample 117, the reflected light is focused by the third beam splitter 121 and the fifth lens 122 before being captured by the deep-cooled InGaAs imager 124.

[0023] The fluorescence confocal subsystem comprises a 120mW 660nm laser 101, a third lens 105 with a focal length of 200mm, a fourth lens 106 with a focal length of 25.4mm, a dichroic mirror 110, and part of the laser confocal optical path (first beam splitter) and a seventh lens 112. The dichroic mirror 110 is a long-pass type, designed to transmit wavelengths above 980nm. The subsystem's workflow is as follows: The high-power 660nm pulsed laser passes through a beam expander and collimator system, collimating into a parallel beam. This beam is then filtered by a spatial filter to obtain a spatially uniform Gaussian beam, which illuminates the high-damage-threshold dichroic mirror. After transmission, it reaches a high-precision two-dimensional scanning galvanometer. The laser beam passes through a relay telecentric system composed of a scanning lens and a sleeve lens. The beam exiting the telecentric system becomes parallel again, passes through a mirror, and enters a short-wave infrared microscope objective, converging onto the surface of the test sample. The fluorescence excited after laser focusing on the test sample is transmitted through the optical path, passing through a dichroic mirror 110, a first beam splitter 111, and a seventh lens 112 with a focal length of 20.1 mm, before being detected by a single-photon detector (SPD) 113 for fluorescence lifetime. Data from each subsystem is integrated and analyzed to ultimately achieve rapid determination of this type of failure problem.

[0024] Another core component of the fluorescence confocal subsystem is the single-photon detector 113. To improve the detection efficiency of fluorescence excited from the test sample, this invention employs a low-noise short-wavelength infrared single-photon detector in free-running mode based on a deep-cooled InGaAs negative feedback avalanche diode (NFAD). It uses spatial optical coupling and has a photosensitive surface diameter of up to 50 μm, ensuring high light collection efficiency and reducing the difficulty of optical path adjustment. The detection wavelength range is 950 nm to 1650 nm, with a peak detection efficiency of up to 25%, and a dark count rate of only about 3 kHz at a detection efficiency of 15%.

[0025] In applications, a single chip may experience various defects and failures due to different reasons. To address this, this system focuses on integrated circuit testing, utilizing the short-wave infrared band. Its structural design employs an optical path multiplexing structure, meeting the high integration requirements of the system while facilitating the collection of short-wave infrared fluorescence generated by excitation defects or failure points. Ultimately, through the coordinated operation of the entire system, the failure mechanism can be determined more quickly by comparing the short-wave infrared fluorescence lifetime curves of defects or failure points.

[0026] Since the overall optical path of the system is coaxial and the short-wave infrared imager is the key imaging component, a 1310nm laser can be turned on as an indicator laser. At the imaging end, a 660nm laser is adjusted to a suitable focal plane on the test sample to excite the integrated circuit unit under test. Due to the reversibility of the optical path, the excited short-wave infrared fluorescence becomes parallel light after passing through the objective lens. After reflection by a semi-transparent mirror, it converges and passes through a sleeve lens and a scanning lens, becoming parallel light again to illuminate the two-dimensional scanning galvanometer. The fluorescence passes through a dichroic mirror and a filter, and is then focused onto the detection target surface by an aspherical lens. Using a single-photon detector and time-correlated single-photon counting technology, the lifetime of the short-wave infrared fluorescence generated by the failed device after laser excitation is measured. By comparing the fluorescence lifetime curves of corresponding defects and combining effective information such as the characteristics, manufacturing process, and operating environment of the device under test, the defect type is analyzed. After integrating and analyzing the data from each subsystem, rapid identification of this type of failure problem is ultimately achieved.

[0027] The system provided by this invention can operate in four modes: in bright / dark field mode, only a wide-field light source is used for macroscopic chip inspection to locate regions of interest or failure sites; in emission microscopy mode, the wide-field light source is turned off, and the emission image of the failed device is captured for precise location; in laser scanning confocal scanning mode, both a wide-field light source and a short-wave infrared laser are used simultaneously, and after determining the scanning range based on the size of the failed device, the wide-field light source is turned off, and a displacement stage is used to scan and acquire two-dimensional high-contrast images and three-dimensional stereo images of key chip components; in fluorescence confocal mode, the red laser is retained, and time-correlated single-photon counting (TCSPC) technology is used to collect fluorescence lifetime data, which, combined with the failure mechanism, provides data support for the rapid detection of similar failures. The working principle and process of the system are as follows: Figure 2 As shown.

[0028] To enable the detection of various materials and device properties, a multiplexed optical path structure allows for the switching of light sources and detection components in the visible / shortwave infrared bands, providing comprehensive monitoring information. The subsystem employs a multiplexed optical path design; the laser confocal microscopy subsystem and the fluorescence lifetime subsystem share coaxial optical paths; the telecentric system and multiple point scanning modes are also applicable to the fluorescence lifetime microscopy subsystem, better integrating multiple detection methods. For automated control and data processing, the host computer of the entire system integrates the control of the fluorescence lifetime microscopy subsystem and the acquisition and processing of photoluminescence signals.

[0029] To improve overall system usability and reduce user complexity, the functions and moving parts of the bright / dark field microscopy subsystem, light emission microscopy subsystem, laser scanning confocal microscopy subsystem, and fluorescence lifetime microscopy subsystem were integrated. Host computer software was developed and tightly integrated with each subsystem. This software not only enables real-time data acquisition, processing, and display but also possesses powerful analytical capabilities, helping researchers accurately detect and locate failed samples.

[0030] The control module ensures image integrity and display quality through efficient image data reception and processing algorithms. The image data from the shortwave infrared imager is uncompressed and transmitted in Bulk mode using the USB 3.0 SuperSpeed ​​protocol, guaranteeing the integrity of high-speed image information. Since the imager's imaging device may contain blind pixels such as overheated pixels, and there are issues with pixel background noise and non-uniform photoresponse, the control module incorporates a blind pixel compensation algorithm based on selective median filtering and a two-point non-uniformity correction algorithm with temperature compensation, thereby ensuring the image's visual quality. Simultaneously, the module supports flexible adjustment of camera parameters such as exposure time, cooling temperature, background subtraction, and non-uniformity correction coefficients to meet the imaging requirements of different modes.

[0031] One advantage of this system is its ability to simultaneously observe the laser spot and bright-field image. In addition to laser illumination, an extended light source can be used to illuminate the sample, making it easy to locate the region of interest. To improve the positioning accuracy of the visible laser in the fluorescence confocal subsystem, the short-wave infrared laser spot can be used as an indicator, thanks to the coaxial optical path designed for both short-wave infrared and visible lasers within the system.

[0032] Another advantage of the system proposed in this invention is the use of optical multiplexing technology. Confocal detection can be achieved by adding point illumination and a scanning platform to a wide-field microscope. Furthermore, the use of optical multiplexing eliminates the need for pinholes and photomultiplier tubes (PMTs), which are replaced by low-noise short-wave infrared imagers. In the proposed system, the centroid of the laser spot in the image can be automatically located in software, and individual pixels are treated as point detectors, such as PMTs. Since the pixel diameter is as small as 25 μm, it is equivalent to a 25 μm pinhole. Currently, the imager of this invention can scan up to 20 points per second and utilizes region of interest (ROI) adjustments to improve scanning speed and quality. However, the development of short-wave infrared confocal microscopes for chip inspection in recent years has remained relatively limited, using traditional APDs as detectors, largely due to the faster readout speed of APDs compared to InGaAs imagers.

Claims

1. A short-wave infrared light emission and fluorescence confocal microscopy system for integrated circuit inspection, characterized in that: It includes a bright / dark field microscopy system, a photon emission microscopy system, a short-wave infrared laser scanning confocal microscopy system, a fluorescence confocal microscopy system, and a control module; the bright / dark field microscopy system observes samples through vertical illumination from a bright field light source or diffuse reflection from a dark field light source; The photon emission microscopy subsystem performs testing by driving the integrated circuit under test; the short-wave infrared laser scanning confocal microscopy subsystem uses a short-wave infrared laser focused on the sample, and the reflected light is captured by a deep-cooled InGaAs imager after passing through a beam splitter and focusing lens; the laser emitted by the fluorescence confocal microscopy subsystem enters the short-wave infrared microscope objective through a mirror and converges to the sample surface to detect the short-wave infrared fluorescence lifetime; the control module is used to control the data acquisition and switching of the working modes of each subsystem, and to process the acquired data. The data is processed to determine sample defects or failures; the bright / dark field microscopy subsystem includes a bright field light source, a dark field light source, a 200mm focal length lens and a 75mm focal length lens, a beam splitter, and an objective lens; the bright field light source illuminates the test sample perpendicularly after passing through the 75mm focal length lens and the beam splitter; the dark field light source is arranged diagonally above the sample stage; the short-wave infrared laser scanning confocal microscopy subsystem includes a 1310nm laser, a 250mm focal length lens and a 25.4mm focal length lens, a beam splitter, and... The system comprises a fast-turning mirror, scanning lens, tube mirror, and a deep-cooled InGaAs imager. A telecentric system consisting of the scanning lens and tube mirror ensures that the objective aperture is always fully illuminated as the fast-turning mirror moves. The fluorescence confocal microscopy subsystem includes a 660nm laser, a 200mm focal length lens, a 25.4mm focal length lens, a dichroic mirror, and part of the laser confocal optical path. The dichroic mirror is a long-pass type, transmitting wavelengths above 980nm. The fluorescence excited after the laser focuses the sample is transmitted through the optical path, passing through the dichroic mirror and beam splitter. After the device and a focusing lens with a focal length of 20.1 mm are used, fluorescence lifetime is detected by a single-photon detector; the deep-cooled InGaAs imager includes a water-cooling module, a multi-stage thermoelectric cooler, and an InGaAs short-wave infrared imager; the cold end of the multi-stage thermoelectric cooler is connected to the water-cooling module, and the hot end is connected to the InGaAs short-wave infrared imager; the deep-cooled InGaAs imager acquires high-contrast tomographic images and three-dimensional structures of the failed device through the coordinated operation of an emission microscope, a short-wave infrared laser, and a three-dimensional displacement stage.

2. The short-wave infrared light emission and fluorescence confocal microscopy system for integrated circuit detection according to claim 1, characterized in that, The system has four operating modes: in bright / dark field mode, only the wide-field light source is used for macroscopic detection of the sample to locate the region of interest or failure site; in emission microscopy mode, the wide-field light source is turned off, and the emission image of the failed device is captured for precise location; in laser confocal scanning mode, both the wide-field light source and short-wave infrared laser are used simultaneously, and the wide-field light source is turned off after determining the scanning range according to the size of the failed device, and the two-dimensional high-contrast image and three-dimensional stereo image of the key device of the sample are obtained by scanning with a displacement stage; in fluorescence confocal mode, the red laser is retained, and time-correlated single-photon counting technology is used to collect fluorescence lifetime data.

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