Signal processing circuit for infrared focal plane array

    Abstract: An overview of the development of infrared focal plane array signal processing circuits is introduced. It focuses on describing the basic structure, working methods and application fields of CCD multiplexer (CCD-MUX), time delay integration CCD (TDI-CCD), MOSFET, and CMOS multiplexer (CMOS-MUX). Finally, two application circuits of multiplex transmission devices are given.

    Keywords: multiplexer, infrared detector, infrared focal plane array, signal processing circuit

Signal detection in future infrared systems (such as thermal imaging systems, guidance systems, surveillance systems) will almost all be based on the focal plane array of infrared detectors. Opto-mechanical scanning can be simplified or eliminated to improve system performance using an infrared focal plane array (IRFPA). The infrared focal plane array device is a new structure that encapsulates devices that contain a large number of sensitive elements and require constant low temperature operation. In this new structure, the processing of the electrical signal output by the infrared detector (IRD) is done in the focal plane. People usually refer to the method of performing signal processing on the same substrate of the detector as the monolithic IRFPA infrared processing method, while the infrared detectors (IRD) are prepared on different substrates, and then flip-chip interconnection is performed. The connected method is called hybrid IRFPA. There are various types of infrared detectors (IRD), such as photovoltaic (PV) indium antimonide IRD, photoconductive (PC), HgCdTe, PbS, PbSe, PhSnTe IRD, metal silicide (Pd2Si, PtSi, IrSi). Special base barrier infrared detector (SBIRD), heterojunction barrier (SiGe/Si) internal photoemission IRD, superlattice quantum well (AlGaAs/GaAs, SiGe/Si) IRD, etc. At the same time, they also have various signal processing circuits, such as SCCD, BCCD, MCCD, SCD, TDI-CCD, MOSFET switches, CMOS static (or quasi-static) shift register multiplexer (CMOS-MUX), etc. This kind of signal processing circuit is currently monolithic (CMOS-MUX) and so on. These signal processing circuits are currently commonly used infrared signal processing devices for monolithic (or hybrid) infrared focal plane arrays. So far, the signal processing circuits used for infrared focal plane arrays have been made of silicon materials. Because there are still great technical difficulties in manufacturing high-performance signal processing circuits using materials other than silicon.

In order to develop an infrared system with excellent performance, in the development of infrared focal plane array (IRFPA), in addition to the mature infrared detector (IRD) preparation process, the design and development of signal processing circuits are the most critical factors to improve the performance of IRFPA. The main design requirements of the signal processing circuit are high charge storage capacity, high transfer efficiency, low noise, low power consumption, and functions such as background suppression and multiplexing. Moreover, after the infrared detector in the IRFPA converts the incident photons into charges, the generated signal charges must be able to be injected into the signal processing circuit for multiple outputs.

The following is a brief introduction to the basic structure, working methods and application fields of several signal processing circuits.

1 Signal processing circuit for infrared focal plane array

1.1 Silicon CCD multiplexer

The circuit principle of the silicon CCD multiplexer (SiCCD-MUX) is shown in Figure 1. In the figure, T1, T2..., TN are N taps of the input signal. The main function of SiCCD-MUX is to convert the parallel output signals of several (16, 32, 64, 128, 256, etc.) infrared detectors in a row into serial output signals in the infrared thermal imaging system. Its main advantages are low noise, high sensitivity and high operating frequency. Since all input signals are output through a low-noise, high-sensitivity charge detector, non-uniformity in the output signal is relatively easy to resolve.

SiCCD-MUX can not only match the photoconductive HgCdTe long-wave infrared detector, but also match the photovoltaic HgCdTe infrared detector (PVHgCdTe-IRD) in the advanced second-generation infrared detection system [1]. Due to this The matching is done by direct injection, thus eliminating the need for a low-temperature preamplifier in the photoconductive HgCdTe long-wave infrared detection system. This makes the entire system smaller and consumes less power, and at the same time greatly reduces the burden on the refrigerator, thus enabling long-wave infrared signal processing to enter the era of focal plane signal processing.

Direct injection SiCCD-MUX can be used in high-background applications such as tactical guidance and tracking systems that require strong background signal suppression capabilities. Therefore, the design of the direct injection SiCCD-MUX is more complicated than the design of the AC lily-type SiCCD-MUX in the light guide system. Its input structure should have the functions of injecting charge halo, segmentation, skimming and background elimination. The structural block diagram of direct injection SiCCD-MUX is shown in Figure 2.

1.2 Time delay integration CCD

The structural block diagram of time delay integration CCD (TDI-CCD) is shown in Figure 3. Its main function is to apply different time (4Tc, 8Tc,..., 32Tc) delays to the input signals of each tap (such as the signal output by the HgCdTe infrared detector), and perform synchronous superposition (integration) at the output end to improve the signal. Noise ratio purpose. Because the output signal of the HgCdTe infrared detector matching the TDI-CCD represents the intensity of infrared radiation from the same target. Therefore, its output signal will grow by a factor M after linear superposition (M represents the number of input taps). The noise of the HgCdTe infrared detector is related to the noise of each channel corresponding to the TDI-CCD. Therefore, its output will increase by a factor M 1/2. In this way, the increase in the output signal-to-noise ratio should be M/(M)1/2=M 1/2 times.

In addition to detecting weak infrared signals in infrared thermal imaging systems, TDI-CCD can also be used as a CCD correlator in spread spectrum systems to detect signals with a signal-to-noise ratio less than 1.

TDI-CCD is a functional circuit that, in addition to being used alone, can also be used as a signal processing device for a hybrid infrared focal plane array. 1.3 Time delay integration CCD unit

This circuit integrates time delay integration CCD (TDI-CCD) and silicon CCD multiplexer (SiCCD-MUX) on one chip, so its structure and function will be more complex. When used as a signal processing circuit for a hybrid HgCdTe infrared focal plane array [2,3], the indium pillar can be directly welded to the 4N series photovoltaic HgCdTe infrared detector to form a so-called infrared focal plane array signal processing circuit.

The arrangement of the 4N series HgCdTe infrared detector array is shown in Figure 4. The Y direction is the time delay integration direction, and the X direction is the output direction of the CCD multiple transmitters. The structure of the matching TDI-CCD chip is shown in Figure 5.

Table 1 shows the number of pixels and operating wavelength of hybrid HgCdTe infrared focal plane arrays with TDI-CCD readout mode from each company.

Table 1 4N series hybrid HgCdTe-IRFPA[3]

1.4 MOSFET switch multiplexer

The basic structure of a MOSFET switch multiplexer is shown in Figure 6. Each of these photodiodes is associated with a memory circuit in the cell. A digital horizontal scan shift register (usually fabricated on a chip) is located in the upper part of the figure and a multiplexer is located on the left side of the figure. Scanning the shift register selects one column of diodes and storage capacitors at a time. At the same time, the vertical scan shift register selects a row of bus lines. In this manner, each pixel is individually addressed sequentially and the pixel signal charge is transferred to a multiplexer for readout.

MOSFET switching multiplexers have many advantages as signal processing devices for infrared focal plane arrays. First, the circuit density can be made very high, so that there is more space for storing electric tea, and the noise performance is good, so the dynamic range is very high. Second, the silicon processing technology used for MOSFET design is highly standardized, so it can be produced in batches to reduce costs. The third point is that the signal readout device for the infrared focal plane array has simple interface requirements, because some clock and switching circuits can be placed next to the chip,This reduces the number of clock waveforms that need to be fed through the Dewar wall. In the development of refrigerated infrared focal plane arrays, indium antimonide line arrays with MOSFET switch multiplexer readout structures of 64 yuan and 128 yuan and area arrays of 128×128 yuan and 256×256 yuan infrared have been developed. Focal plane array. In the development of uncooled infrared focal plane arrays, the British company Plessey Research caswell has developed 16-yuan, 40-yuan and 64-yuan pyroelectric wire arrays with MOSFET switch multiplexer readout structures. Focal plane array. In order to further improve the performance of the infrared focal plane array, a MOS structure field-effect transistor (MOSJFET) switch multiplexer has now been developed based on the development of MOSFET switch multiplexers.

1.5 CMOS multiplexer

In the development of infrared focal plane array (IRFPA), people always hope to develop infrared signal processing devices with lower cost, better producibility and larger dynamic range, which makes people turn to CMOS multiplexed infrared signal processing devices Research. Now, the emergence of advanced CMOS technology makes it possible to design high-density, multi-functional CMOS multiplexers. This multiplexer is capable of performing signal portioning, transmission, processing and scanning of dense linear and area infrared focal plane arrays [4] (monolithic or hybrid). Therefore, using the infrared focal plane array (IRFPA) of the CMOS-MUX readout device can design an infrared system that is smaller in size, lighter in weight, lower in power consumption, and has better performance.

CMOS-MUX can usually be used as a signal processing device for platinum silicide Schott barrier infrared detector arrays. In the development of infrared detector arrays such as HgCdTe, InSb, AlGaAs/GaAs, InAsP/InP, PbS, PbTe, InGaAs, etc., the design Investors often hope to use CMOS-MUX as an infrared signal processing device. The functional diagram of the CMOS multiplexer is shown in Figure 7.

2 Application circuit

2.1 Signal readout circuit of multiplexer

Infrared detector arrays, especially infrared detectors not made of silicon materials, generally require specially designed and prepared multiplexers (MUX) as the infrared signal processing device of the infrared detector array. At present, in the design and development of multiplexers, mature silicon materials have been used to develop silicon multiplexers with different types of structures (such as Si-CCD, Si-MOSFET, Si-CMSO, Si-JFET, etc.) , in the development of monolithic (or hybrid) IRFPA, in addition to mature infrared detector preparation technology, the key factors to improve the performance of IRFPA are the development of low pull-out, low noise, high charge storage capacity and easy integration with infrared detectors. Detectors integrated or coupled to silicon multiplexers. Table 2 gives the charge storage capacity of silicon multiplexers.

Table 1 Charge storage capacity of silicon multiplexers

An application circuit for performing line array interleaved readout using two multiplexers is shown in Figure 8. In the figure, the multiplexers are located on either side of the detector array in an interleaved readout of the standard and mirror images so that the two multiplexers are mirror images of each other. The even pixels of the detector array are connected to solder pins on one side of the array, while the odd pixels are connected to the other side of the array. These solder pins of the array are then connected to the corresponding multiplexers. and read from two multiplexers. Finally they are recombined off-chip to return the video signal to a single data stream. This approach doubles the length of the imaging array being used and enables closer detector pixel-to-pixel spacing, while still using off-the-shelf components.

The timing mechanism will time one of the solder legs of the multiplexer connected to the switch on the array, and then time all the multiplexers connected to the video. This process will continue until all rows have been read. until completed.

2.2 Buffer multiplexer application circuit

EG & G Reticon offers buffered multiplexers with 64, 128 or 256 channel lengths with solder pins located one on a row and spaced 100 μm apart . The device's input bias current is as low as 100fA. The preamplifier conversion gain is 1 μV per 100 electrons . Figure 9 is a schematic diagram of an information target display of an enable and block detector array including two sample and hold gates. To integrate the multiplexer with the detector array, the device connects the photoreceptor to solder pins and associates each solder pin with a channel that uses a 15pF feedback integration capacitor to convert the input charge into The output voltage. A switched sampling circuit immediately following the capacitor provides correlated double sampling to reduce noise and offset.

3 Conclusion

Any kind of IRFPA, whether monolithic or hybrid, is composed of infrared photoelectric conversion and signal processing. Almost all the signal processing parts of the infrared focal plane array (IRFPA) are implemented using silicon materials and silicon signal processing circuits. This is because silicon is the most mature material today, and the design and process technology of signal processing circuits made of silicon can already meet the needs of IRFPA.

The silicon signal processing circuit in IRFPA has functions such as signal readout, integration, background suppression, preamplification, sampling and holding, and multiplexing. In principle, current microelectronics technology can fully integrate these functions on a silicon chip. However, for IRFPA, especially area arrays, the area where these circuits can be placed is often very limited. Therefore, very simple and effective input stage circuits must be considered, usually using silicon CCD multiplexers (surface trench CCD, buried trench CCD CCD, curved groove CCD, buried groove type curved groove CCD), time delay integration CCD, MOSFET, charge scanning device (CSD) and CMOS multiplexer and other circuits to realize the signal processing of IRFPA. However, due to the small charge storage capacity of the potential well of the silicon CCD multiplexer, it requires high impedance of the infrared detector, and has shortcomings such as transfer loss and complex process. Therefore, in the development of IRFPA in recent years, whether it is refrigeration The signal processing circuitry of both type and uncooled IRFPA increasingly uses highly developed CMOS multiplexers.