【摘要】 微光图像传感器需通过光电倍增以增强光生电荷信号，目前以单光子雪崩二极管（SPAD）为代表的微光图像传感器的光电倍增机制是雪崩倍增，但较高的工作电压限制了像素与读出电路的单芯片集成。根据重掺杂硅的Auger激发效应和传统正照式（FSI）CMOS图像传感器（CIS）像素中光电二极管（PD）结构，用相同厚度的重掺杂杂质补偿区替换其PN结，设计出一种具有重掺杂杂质补偿结构的新型PD(PD_C-HD)。根据Auger激发与价带电子跃迁的关系，提出了一种基于Auger激发诱导的光电倍增机制。研究结果表明，PD_C-HD能在3.3 V的反向偏置电压下实现低压光电倍增。与传统PD相比，PD_C-HD的峰值量子效率能从90%提升至140%，且光响应波长范围从可见光扩展到近红外区。TCAD仿真研究表明，具有重掺杂杂质补偿结构PD_C-HD的FSI-CIS新型像素设计在工艺上是可行的。更多还原

【Abstract】 Objective Weak-light image sensors have a wide range of applications in astronomical observation, medical and military fields. Because the photogenerated charge signal of an image sensor is weak at weak-light condition, photomultiplier is the key to achieve high-contrast imaging and improve image quality. At present, the photomultiplier mechanism of the single photon avalanche diode（SPAD） as the representative of weak-light image sensors is avalanche multiplication. However, the high operating voltage in avalanche multiplication limits the single-chip integration between pixels and readout circuits. Therefore, how to reduce the operating voltage of the weak-light image sensors is the difficulty and hotspot of research. To address the above problem, a CMOS image sensor（CIS） with low-voltage photomultiplier based on the induction mechanism of Auger excitation is proposed in this paper, which can realize photomultiplier at a reverse bias voltage of 3.3 V. This basic study may be helpful for the research of the CIS with low-voltage photomultiplier.Methods According to the pixel structural characteristics of a front-side illuminated（FSI） CIS, the heavily doped（HD） impurity compensation region（ICR） is designed under the N well of photodiode（PD）, thus a novel PD(expressed as PD_C-HD) is formed. First, a full impurity compensation model is established to investigate the Auger excitation effect happened at a low voltage, which is related to photogenerated electrons or holes generated by short-wavelength absorption in N well and long-wavelength absorption in HD-ICR. Second, the effects of the photomultiplier based on the Auger excitation effect on the performance of the PD_C-HD is studied by the quantum efficiency（QE） of the PD_C-HD over wave length. Finally, the TCAD（technology computer-aided design） simulations of the novel pixel with PD_C-HD are performed to verify the process feasibility by the combination of ion implantation and fast CVD epitaxial process. Ion implantation process is used to form the HD-ICR with boron（B） and phosphorus（P） dopants. The N well of the PD_C-HD consists of three gradientally-doped layers, and the n-type impurity concentration of each layer is decreased from the top to bottom in the order of PDN1, PDN2, and PDN3（Fig. 2）.Results and Discussions In the wavelength range of 400-1100 nm, the QE of PD_C-HD is more than that of PD. At the wavelength of 1100 nm, the QE of PD decreases to 0, whereas the QE of the PD_C-HD is still as high as 22%. The PD_C-HD still has light response in the near-infrared region of 1100-2200 nm and its QE is not zero（Fig. 4）. The PD_C-HD achieves its QE-peak value of approximately 140% at 576 nm, whereas PD obtains its QE-peak value of approximately 90% at 500 nm. The high QE-peak value of the PD_C-HD at long wavelength is conducive to improving the utilization of long-wavelength light. When the HD-ICR thickness is changed, the QE of the PD_C-HD changes accordingly（Fig. 5） as follows: the QE of the PD_C-HD is improved with the increase of the HD-ICR thickness at the wavelength of more than 400 nm. The QE-peak value of the PD_C-HD is improved with the increase of the HD-ICR thickness, and its corresponding peak position drifts toward long wavelength. In the TCAD simulation, the concentration of B and P dopants in the HDICR is more than 1×10¹⁸ cm^-3, and a high degree of impurity compensation（higher than 98%） can be achieved in HD-ICR, which is close to full impurity compensation（Fig. 7）. In addition, the PD_C-HD can be fully depleted after reset, and the electric potential difference between the two ends of the HD-ICR is 1.142 V, which is basically the same as the theoretical value（approximately 1.126 V）. After TG turning on, there are no potential barriers or wells in the transfer path, which is conducive to the electron transfer in PD_C-HD（Fig. 8）.Conclusions A novel PD_C-HD with HD impurity compensated structure is designed in this paper, in which its PN junction is replaced by HD-ICR with same thickness. According to the relationship between Auger excitation and the electron transition of valence band, a photomultiplier mechanism induced by Auger excitation effect is proposed. The photomultiplier can stop on its own, so quenching circuits are not needed like the case of SPAD-CIS. Comparative study indicates that the PD_C-HD can achieve its QE-peak value of 140% over 576 nm at 3.3 V, whereas the conventional PD obtains its QE-peak value of 90% at 500 nm. Furthermore, the spectral response range of PD_C-HD is broadened from visible light to near-infrared region（400-2200 nm）, which should be related to the light absorption of impurity compensated silicon. The simulation results indicate that the novel pixel with the PD_C-HD designed for an FSICIS is feasible in fabrication process. In summary, the low-voltage photomultiplier of a CIS can be achieved by the Auger excitation effect happened in a HD impurity compensated structure. The high QE of the CIS with PD_C-HD over a wide spectral range is conducive to weak-light imaging. This paper may provide an idea for the study of weak-light CIS with silicon as photosensitive material.更多还原