Development of new ultra-high sensitivity Super-HARP camera tube

    Abstract: The new ultra-high-sensitivity Super-HARP camera tube is a new solid-state photoelectric camera device made using the principle of gun power multiplication. This article gives a comprehensive introduction to the working principle and structural characteristics of the device. The technical characteristics and technical parameters of some devices are given, and the future development direction of ultra-high-sensitivity camera tubes is initially discussed.

    Keywords: HARP, high-sensitivity camera tube, micro-indium column, collapse multiplication film

1 Introduction

With the continuous development of semiconductor technology, microelectronics technology and optoelectronics technology, in recent years, the development of imaging devices has been able to meet the requirements of high-quality images. However, if imaging and observation are performed under dark conditions, higher sensitivity of the imaging device is required. At present, the avalanche multiplication film method is generally used to improve sensitivity. This method has a simple structure, convenient manufacturing, and low cost. It is a hybrid structure. The light-receiving surface using this structure is an avalanche multiplication film formed on the glass sheet. Then use micro-indium pillars to connect the multiplication film to the solid readout circuit (such as MOS or CMOS readout circuit). The micro-indium pillar mainly serves as a mechanical and electrical connection.

The high sensitivity and ultra-high sensitivity of the camera tube marks the birth of ultra-high-sensitivity cameras, which are not only suitable for high-resolution television (HDTV) camera systems, but also greatly improve the quality of standard TV camera systems. Its use will definitely bring new changes to TV program production and studio lighting technology.

2 The development process of high-sensitivity camera tubes

Research work on camera tubes has currently made progress in improving sensitivity and resolution, reducing inertia and noise effects. Japan has successively launched HARP and Super-HARP camera tubes. These camera tubes are considered to be representatives of today's new generation of camera tubes due to their extremely high sensitivity.

In the past, when shooting with Station or Plumbicon three-tube cameras and CCD cameras, as long as the illumination was lower than 20Lux, the image quality was difficult to guarantee. Although silicon target intensified (SIT) camera tubes and image intensified (IT) camera tubes have higher sensitivity, they are difficult to form ideal images due to their strong inertia, large noise, and low resolution. Image. In 1987, Japan's NHK Research Institute discovered that amorphous selenium has the same avalanche multiplication effect as crystalline semiconductors. On this basis, it invented a new photoconductor and target structure that can produce good television images under low illumination conditions, namely high-gain avalanche. High gain Avalanvhe Rushing amorphous Photoconductor, referred to as HARP target. The camera tube developed using this structure is 10 times more sensitive than ordinary camera tubes and can produce the same high-quality images as HDTV camera tubes. And this camera tube is not only suitable for standard TV systems, but also very suitable for HDTV systems. From 1988 to 1993, Japan's NHK successfully developed a variety of HARP target camera tubes with optical responses in the 20nm to 1.9 μm and even x-bands, and were named "Harpicon" and "Super-Harpicon". In 1991, NHK introduced a higher-sensitivity Super-HARP camera tube. Its sensitivity is 100 times higher than ordinary camera tubes and CCD camera devices, and it can form high-quality images under a wide range of lighting conditions from sunlight to moonlight, thus becoming a new generation of television camera tubes.

3 Principles of HARP and Super-HARP camera tubes

When the electric field in the space charge region in the semiconductor increases, the energy obtained by the electrons and holes passing through the space charge region under the action of the electric field also increases. In this way, the electrons and holes moving in the crystal will constantly collide with crystal atoms. When the energy of electrons and holes is large enough, through collision, the electrons in their valence bonds can be excited to form free electron-hole pairs. This phenomenon is called impact ionization. The newly generated electron-hole pairs, like the original electrons and holes, also move in the opposite direction under the action of the electric field and regain energy. New electron-hole pairs will be generated through collision, which is the multiplication effect of carriers. Since this avalanche multiplication phenomenon also exists in amorphous selenium, HAPR and Super-HARP camera tubes can use the avalanche multiplication phenomenon in the target layer to obtain high sensitivity. Figure 1 is a comparison of the working principles of ordinary camera tubes and HARP camera tubes.

As can be seen from Figure 1, in an ordinary camera tube, one incident photon only produces one photogenerated electron-hole pair. In the HARP camera tube, the strong electric field added to the target layer will accelerate the holes in the direction of the electron beam scanning movement. The accelerated holes will collide with the atoms in the target and ionize them to generate new electron holes. right. Due to avalanche multiplication, a large number of carriers will be read out for each incident photon in the HARP camera tube. The avalanche multiplication procedure is determined by the target thickness pressure value. The target thickness of the HARP camera tube was previously 2 μm and has now been increased to 4 μm , while the thickness of the Super-HARP camera tube has been significantly increased to 25 μm . Figure 2 is the relationship curve between target and signal current when blue light is incident. It can be seen that near the target pressure of 420V, the sensitivity of Super-HARP is the same as that of ordinary camera tubes. At this time, there is no avalanche multiplication phenomenon, and the blue light quantum efficiency is 70%. . When the target pressure exceeds 420V, the signal current increases rapidly. At a target pressure of 676V, the sensitivity can reach 100 times that of ordinary camera tubes. Since the Super-HARP camera tube uses a 6 μm target thickness and a target pressure of 676V, the avalanche multiplication phenomenon in the target is very strong, so extremely high sensitivity can be obtained. Super-HARP camera tubes have the same appearance as regular MSSATICON (2/3-inch) camera tubes. In 1996, Japan's NHK Institute of Science and Technology developed a new ultra-high-sensitivity Super-HARP camera. The core device used in this camera is a new Super-HARP target structure (shown in Figure 3). In order to integrate with the old HARP target and The current-voltage characteristics of the original Super-HARP target are compared. Figure 4 shows the relationship between the signal and dark current of the new Super-HARP and the target voltage.

4 Technical characteristics of HARP and Super-HARP camera tubes

4.1 Main features

HARP and Super-HARP camera tubes are highly sensitive and ultra-sensitive new structural photoelectric multiplication camera devices respectively. Its main features are as follows:

●High sensitivity, small dark current

As can be seen from Figure 2, after the target voltage exceeds 420V, the signal current of the device increases rapidly. At a target voltage of 676V, the sensitivity can reach 100 times that of an ordinary camera tube, but there is only less than 2 μA of dark current at this point. In addition, it can be seen from Figure 4 that for the new Super-HARP target, when the target voltage exceeds 1500V, the signal current increases rapidly. When the target voltage reaches 2500V, the sensitivity can reach 600 times that of the ordinary SATICON TV camera tube, and at this point Dark current is only 2nA.

●Low inertia

The inertness of the HARP tube is almost purely capacitive, so its inertness can be improved by using a low-temperature electron gun, increasing the target thickness, or biasing the light speed. When the Super-HARP camera tube using a diode electron gun has the bias light turned off, the target voltage is 676V, the signal current is 200nA, and the electron beam current is 600nA, after turning off the incident light, the inertia of the third field is 1.6%, which is less than that of the Super-HARP camera tube using a diode electron gun. Inertness of HARP camera tube for the same electron gun (4.6%). If bias light is used, only 10nA of signal current is generated, and its inertness is negligible at this time.

●High resolution

Since there is no light diffusion in the amorphous selenium target layer and it has high resistance, the HARP camera tube has high resolution. The current horizontal limit resolution exceeds 800 lines. The main factor affecting its resolution is the cross-sectional area of ​​the scanning electron beam. Therefore, if an HDTV electron optical system is used, its resolution can be further improved.

●Low noise

In the Super-HARP camera tube, the additional noise caused by avalanche times is very small, fixed noise is not observed at all, and the measured noise includes photon noise. Theoretically, photon noise is unavoidable under very low lighting conditions. However, due to the high light utilization efficiency of the Super-HARP camera tube, such low photon noise will have almost no impact on image quality.

●Spectral response characteristics

For amorphous selenium materials, the cut-off wavelength of red light sensitivity is 620nm, which will affect the sensitivity of the red channel camera tube of TV cameras. However, red-enhanced Super-HARP camera tubes can be manufactured by incorporating a certain amount of tellurium (Te) into amorphous selenium materials to solve the problem of low red light sensitivity.

4.2 Technical parameters

In 1996, Japan's NHK developed a new type of ultra-high-sensitivity Super-HARP camera. Its performance parameters are listed in Table 1 and Table 2. The data calculation conditions in Table 1 are: target capacitance 800pF (4 μm) , 400pF (8 μm) , 128pF (25 μm) , beam temperature 3000°C, beam current 0.48 μADC , 2nADC.

Table 1 Output current and amplifier gain of three camera tubes

Table 2 Performance parameters of new Super-HARP camera

In 1999, the Japan Broadcasting Association successfully developed an ultra-high-sensitivity solid-state imaging device, which coupled an avalanche multiplication high-gain amorphous photoconductor (HARP) and a high-voltage MOS transistor switch array through indium pillar flip-chip interconnections. Its performance parameters are listed in Table 3.

Table 3 HARP-MOS tube switch array performance parameters

5 The development potential of Super-HARP

Solid-state imaging devices generally consist of a HARP film and a low-noise readout circuit (such as MOS and CMOS). Its typical structure is shown in Figure 5. Figure 6 shows the structure of the HARP film MOS image sensor and the composition of a single pixel. The electron-hole pairs generated by the HARP film after being exposed to light will produce an avalanche effect under the action of a sufficient electric field to form avalanche amplification. The amplified signal charges (holes) flow to the drain of the MOS tube and increase the drain potential. Electrons from the source flow to the drain and combine with holes. After the signal current from the source is amplified by the amplifier, the noise is eliminated by the correlated double sampling (CDS) circuit and read out, so that the output signal has a high signal noise ratio. Since the multiplication effect of strong light incident on the HARP film is very strong, the stored signal voltage is too high, which requires the MOS tube circuit to be able to withstand higher voltages. Secondary photolithography can also be used to form indium micro-bumps to solve the signal voltage problem. Too high problem. Therefore, there are two structures of HARP film-type high-sensitivity imaging devices. One is a MOS tube directly combined with a HARP film, and the other is a convex indium plate type. In addition, in 1998, Japan's NHK Research Institute also developed a HARP film CMOS image sensor.

In the coming period, camera devices will mainly develop in the direction of high sensitivity, high resolution, low power consumption, low cost and miniaturization. To realize the above functions, the use of CMOS technology is extremely critical. It can also be said that the main trend in the current development of camera devices is CMOS.

6 Applications of Super-HARP cameras

Japan's Hitachi Manufacturing Co., Ltd. and NHK Research Institute have developed high-sensitivity ultra-high-sensitivity HARP target UV and It can be used in astronomy, medicine, biological observation and research, fingerprint identification, calligraphy and painting identification, and securities identification; the latter is mainly used in fields such as observation of material crystallization, large-scale integrated circuit inspection and analysis, and quality monitoring.

7 Conclusion

The current development of vacuum camera tubes has advantages in both high definition and high sensitivity, and has more advantages in HDTV applications; while solid-state camera devices have not yet reached this point. The development direction of vacuum camera tubes and solid-state imaging devices will mainly be characterized by high sensitivity, high resolution, low power consumption, low cost and miniaturization. To meet these requirements, the use of CMOS technology is extremely critical. The most likely development direction of the Super-HARP camera tube is to connect the HARP target and the MOS readout circuit or CMOS readout circuit with micro-indium pillars. At present, the new ultra-high-sensitivity Super-HARP camera developed using this technology has Can be used as an HDTV camera system.