基底均匀和梯度掺杂下EBCMOS电荷收集效率的优化模拟
As a new type of low-light night vision imaging device technology, electron-bombarded complementary metal-oxide-semiconductor (EBCMOS) technology can realize photoelectric conversion, electric signal enhancement, digital processing, and target output below an illumination of 10-4 lx. It has the advantages of a small size, light weight, high gain, low noise, and fast response. Therefore, it has wide application prospects in military equipment, astronomical observation, remote-sensing mapping, and space detection. In the EBCMOS working process, the photoelectrons generated from the photocathode by the external photoelectric effect are accelerated by the negative high voltage between the photocathode and the surface of the electron-sensitive CMOS and bombard the P-type semiconductor substrate to obtain the gain from the secondary electrons in the multiplier layer. Because of the concentration difference of the minority carriers in the P-type substrate, the secondary electrons diffuse to the pixel region, are collected by the photodiode in the active pixel circuit, and are finally read out by the MOS transistor amplification circuit. Therefore, to improve the gain characteristics of EBCMOS devices, the design and optimization of the structural parameters of EBCMOS substrates and the building of corresponding theoretical models are important issues for researchers. In this study, secondary electron charge collection in EBCMOS substrates under different doping modes and structural parameters was investigated, laying a theoretical and technical foundation for the preparation of high-gain EBCMOS electron multiplier layers.
According to carrier transport theory and the Monte Carlo simulation algorithm, a theoretical model of the entire electronic trajectory of an EBCMOS substrate was established. Electron charge collection in the electron multiplier layer under uniform and gradient doping of the P-type substrate was simulated, and transport calculation models of photogenerated electrons and multiplier electrons in the proximity region of the EBCMOS were established. Various EBCMOS structural models were designed to simulate the electronic motions under the condition of different doping concentrations, substrate thicknesses, proximity distances, and gradient doping structures, and the influence of different structural parameters on the electron charge collection of the electronic multiplier layer was analyzed.
For a uniformly doped substrate, with an increase in doping concentration, the recombination rate of the carrier increases, the lifetime of minority carriers decreases, and the number of secondary electrons collected in the pixel region decreases, which causes the charge collection efficiency to decrease continuously (Fig. 4). When the substrate doping concentration reaches 1019 cm-3, the charge collection efficiency approaches 0—that is, the secondary electrons are completely recombined. As the thickness of the substrate increases, the diffusion range of the secondary electrons increases (Fig. 5), and the scattering radius of the secondary electrons collected in the pixel region increases, which is not conducive to improving the charge collection efficiency (Fig. 6). Therefore, a thinner P-type substrate treatment is necessary to obtain a higher charge collection efficiency. As the proximity distance between the cathode surface and the EBCMOS substrate increases, the initial energy obtained by the incident electrons decreases, and the number of secondary electrons that generate multiplication decreases, thereby reducing the number of electrons collected in the pixel area and reducing the charge collection efficiency (Fig. 8). When the substrate is divided into two sections for gradient doping, the range of secondary electron diffusion in the diffusion and depletion regions is obviously reduced, indicating that the electron focusing effect of gradient doping is better than that of uniform doping (Fig. 10). The built-in electric field distribution generated by gradient doping can provide an additional drift speed for secondary electrons in the direction of their movement, shortening the diffusion time of electrons in the diffusion region and obtaining a higher charge collection efficiency. The charge collection efficiency can reach a maximum of 86.28% when the width of the surface heavily doped region is 2 μm.
Based on the carrier transport mechanism in semiconductor physics and the Monte Carlo algorithm, the electronic trajectory of incident optoelectrons in EBCMOS is theoretically simulated. The electronic trajectory in the device is determined based on the simulation results, and the factors affecting the efficiency of charge collection are analyzed. The results show that the charge collection efficiency of the EBCMOS increases with the decreases in substrate doping concentration, substrate thickness, and proximity distance. The gradient doping of the substrate clearly improves the charge collection efficiency. The optimized gradient doping structure model achieves a charge collection efficiency of 86.28%. The results provide theoretical support for the fabrication of high-gain EBCMOS devices.
1 引言
电子轰击CMOS(EBCMOS)是在10-4 lx照度以下可以对目标实现光电转换、增强、处理的数字化微光成像器件[1-2]。其基本工作原理是:光电阴极产生的光电子经高压电场加速后轰击背部减薄CMOS芯片基底表面,电子轰击半导体基底产生二次倍增电子,形成电子增益,通过读出电路对电子增益进行放大,读出相应的电信号。因为不需要像增强电子耦合器件(ICCD)那样通过光学系统耦合将光信息转换为电信息,所以EBCMOS可以降低器件的重量,并易于实现高分辨率成像[3]。同时,由于外光电效应型器件光阴极的热噪声比内光电效应型器件的低,因而光电转换过程中的信噪比较高,能够在无需制冷的情况下实现单光子探测。目前商业化的EBCMOS主要应用在便携式和机载军用夜视设备上,这是数字化微光成像器件的一个重要发展方向[4],同时它在单光子探测、天文探测和生物成像等领域具有广阔的应用前景[5]。
电荷收集效率(CCE,在公式中记为CCE)是反映二次电子收集能力的一个重要指标,对器件增益有较大影响。电子倍增层的增益G的计算公式[6]为
式中:CCE为电荷收集效率;E0为入射电子能量;Ed为电子经过死层损失的能量;W为入射电子产生一个电子空穴对所消耗的能量,Si材料的W为3.6 eV。可以发现,增益与电荷收集效率成正比,采取适当措施提高电荷收集效率可以有效地提升器件的增益性能。2009年,Dominjon等[7]测量了EBCMOS电子倍增层的电荷收集效率和死层复合电子的数量(以X光作为入射光源),并通过曲线拟合的方式得到基底均匀掺杂下的电荷收集效率为32%,梯度掺杂下的电荷收集效率为60%。2011年,Barbier等[8]通过实验对比证实了EBCMOS在均匀掺杂下的电荷收集效率最高可达32%,在梯度掺杂下的电荷收集效率最高可达66%。2016年,Hirvonen等[9]研究了电子空穴对在器件内的产生过程以及加速电压与倍增电子数量之间的关系。2017年,本课题组[10]在国内率先研究了EBCMOS不同钝化层厚度对入射光电子能量损失的影响,同时对65 μm厚的电子倍增层进行了增益测试实验,实验结果显示:随着入射电子能量增大,电子倍增层的增益增大。2018年,刘虎林等[11]研制了一种紫外光响应的EBCMOS器件,并实现了40 mlx照度下的探测。2020年,Bai等[12]建立了局部的电子运动轨迹模型,并通过实验对模型进行了验证。同年,王巍等[1]对器件中光电阴极、BSB-CMOS及阳极电极的相对几何位置,以及阳极电极形状、BSB-CMOS背部表面处理等进行了理论模拟,设计了一种有利于EBCMOS电子聚焦的电子倍增层。然而,上述关于电荷收集效率的研究大多是通过实验测得或是对器件中的电子运动轨迹进行局部模拟获得的,整体的电子运动轨迹模型以及如何提高电荷收集效率目前还未见报道。
笔者建立了EBCMOS基底整体的电子运动轨迹理论模型,并对P型基底均匀掺杂和梯度掺杂两种情况下电子倍增层的电荷收集效率进行了模拟研究。根据载流子输运理论,采用蒙特卡罗模拟算法建立了EBCMOS近贴区内光生电子和电子倍增层内倍增电子输运过程的计算模型。同时,设计了EBCMOS的多种结构模型,模拟了不同掺杂浓度、基底厚度、近贴距离、梯度掺杂结构下的电子运动情况,分析了不同结构参数对电子倍增层电荷收集效率的影响。模拟结果可为具体的实验提供理论支撑。
2 理论模型
由于光生电子的运动是随机的,方向角
式中:v0为光生电子入射的初速度;d为光阴极与电子倍增层之间的近贴距离;m为电子质量;U为近贴电压;q为电荷量;t为时间。
背部减薄CMOS芯片表面有一层死层[14]。电子轰击电子倍增层时首先经过死层(结构如
式中:Z为原子序数;E为电子能量。利用蒙特卡罗法可以获得入射电子的散射角
散射步长表示电子经过一次弹性散射所走过的路程,假设其平均自由程λ与散射截面之间的关系为
式中:A为相对原子质量;NA为阿伏伽德罗常数;ρ为介质密度。因每次散射是随机过程,故而用蒙特卡罗方法得到散射步长的随机值,即
式中:Ji为电离能;Ci为原子浓度;k为修正系数。入射电子穿过死层后,会与硅原子发生碰撞,硅原子吸收入射电子的能量,产生电子空穴对,其中每产生一个电子空穴对大约消耗3.6 eV的能量[18-19]。然后,利用Bethe能量损失公式计算出电子在每一次散射步长内所损失的能量,就可以获得每一次散射的倍增电子数目。
倍增后的电子存在一定的浓度梯度,而由于电子的无规则热运动,电子从浓度高的地方向浓度低的地方扩散,也就是从P型基底向N阱一侧运动,并最终被N阱收集。电子的扩散方向是各向同性的,所以扩散的方向角满足
式中:Ln为扩散长度,
若在EBCMOS的基底区域进行梯度掺杂,则掺杂区域内的电场[21-22]满足
式中:N0为初始掺杂浓度;Ns为表面掺杂浓度;D为掺杂深度;kB为玻尔兹曼常数;T为热力学温度。梯度掺杂区域的掺杂浓度[23]满足
耗尽区内P区和N区之间的电场满足
式中:Na为受主杂质浓度;ε为介电常数;xp为P区的宽度。在耗尽区,电子既会受到扩散速度的影响,又会受到漂移速度的影响。通过计算可知,扩散速度是漂移速度的百分之一甚至是千分之一。所以,在耗尽区,电子的扩散速度可以忽略不计。电子经过耗尽区后,最终会落在像素区内,这里假设5×5像素区为电子有效收集区域。
由于电子在扩散过程中会发生复合,所以在收集到的电子中需要除去复合后的电子,所以实际收集到的电子数[19]为
式中:τ为少子寿命;t为电子在扩散过程中所经历的时间。那么,电荷收集效率可以用像素区内收集到的电子数与倍增后所有电子数的比值[24]来表示,即
3 模拟结果与分析
3.1 基底均匀掺杂下电荷收集效率的影响因素
图 4. 不同掺杂浓度下的电荷收集效率
Fig. 4. Charge collection efficiency for different doping concentrations
图 5. 不同基底厚度下电子在像素区内的散点图。(a)5 μm;(b)15 μm;(c)25 μm;(d)35 μm
Fig. 5. Scatter diagram of electronics in pixel area for different substrate thicknesses. (a) 5 μm; (b) 15 μm; (c) 25 μm; (d) 35 μm
图 6. 不同基底厚度下的电荷收集效率
Fig. 6. Charge collection efficiency for different substrate thicknesses
图 7. 不同近贴距离下电子在像素区内的散点图。(a)5000 μm;(b)3000 μm;(c)1000 μm;(d)500 μm
Fig. 7. Scatter diagram of electronics in pixel area for different proximity distances. (a) 5000 μm; (b) 3000 μm; (c) 1000 μm; (d) 500 μm
图 8. 不同近贴距离下的电荷收集效率
Fig. 8. Charge collection efficiency for different proximity distances
3.2 基底梯度掺杂下电荷收集效率的影响因素
为了模拟分析基底梯度掺杂结构变化对电荷收集效率的影响,笔者设计了梯度掺杂下电子倍增层结构模型,如
模拟分析采用的参数如下:近贴距离为1000 μm,近贴电压为2700 V,基底厚度为10 μm,死层厚度为100 nm。
图 10. 梯度掺杂下的电子运动轨迹图
Fig. 10. Diagram of electron motion trajectories under gradient doping
图 11. 不同结构模型下的电荷收集效率
Fig. 11. Charge collection efficiency under different structural models
4 结论
笔者依据半导体理论中的载流子输运机理并结合蒙特卡罗算法对入射光电子在EBCMOS中的电子运动轨迹进行了理论模拟,并根据模拟结果确定了器件中的电子运动轨迹,同时分析了电荷收集效率的影响因素。研究结果表明:EBCMOS的电荷收集效率随着基底掺杂浓度的降低、基底厚度的减少、近贴距离的缩短而提高。基底梯度掺杂能够明显提高电荷收集效率。优化后的梯度掺杂结构模型可使电荷收集效率达到86.28%。本文研究结果可以为高增益EBCMOS器件的制备提供理论支撑。
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