Abstract
Complementary metal oxide semiconductor (CMOS) image sensor is susceptible to proton single event effect when being applied to space environment. Proton irradiation experiments with different energy values are carried out on a commercial FSI and BSI CMOS image sensors. The proton single event effect is analyzed by on-line testing, the maximum proton energy is 200 MeV, and the total fluence is 10<sup>10 </sup>particle/cm<sup>2</sup>. The single-event transient bright spots of different shapes are observed in the pixel array. By extracting the deposition energy and size, the effects of different energy protons on transient bright spot characteristics are compared, and the transient bright spot characteristics between FSI and BSI are also compared. Finally, the energy deposition distribution of the transient bright spot generated by proton in CMOS image sensor pixel unit is predicted by comparing the simulation method with the experimental results. The simulation results verify that the decrease of PPD depletion region thickness and the thinning of epitaxial layer are the main factors leading the proton energy deposition distribution to shift leftward in BSI image sensor.
Highlights
图 2 质子辐照 FSI CIS 诱导单粒子瞬态亮斑 Fig 2 Single event transient bright spot of FSI CIS irradiated by proton
图 5 60MeV 质子辐照 FSI 和 BSI CIS 瞬态亮斑(a)沉积能量,(b)尺寸大小 Fig 5 (a) deposition energy and (b) size of FSI and BSI CIS transient bright spots irradiated by 60MeV proton
图 9 (a)100MeV (b)200MeV 质子辐照 FSI CIS 瞬态亮斑沉积能量分布仿真和实验结果比较图 Figure 9 Comparison of simulation and experimental results of FSI CIS bright spot deposition energy distribution irradiated by proton of (a)100MeV (b)200MeV
Summary
照外围电路时,注量率为 1×106 proton/(cm2·s)。辐照 CIS 像素阵列时,为避免注量率过大导 致瞬态亮斑互相重叠,选用的注量率为 1×105proton/(cm2·s)。试验中粒子垂直入射器件表面。 试验中器件已开盖,辐照在暗场下进行。. 之前的重离子实验研究表明,当重离子 LET 约 21 MeV/(mg/cm2)时,CMOS 图像传感器 出现单粒子闩锁效应(SEL)[11]。为了研究实验样品的质子单粒子效应敏感性,质子能量选 择最大值 200MeV, 注量率为 1×106 proton/(cm2·s),辐照总注量达 1×1010 proton/cm2,未观察 到单粒子闩锁现象,同时也未监测到外围电路发生单粒子翻转(SEU)和功能中断现象。所 以根据实验结果,对于本文所采用的样品的外围电路,当质子能量小于 200MeV 时,不会发 生单粒子闩锁和单粒子翻转现象。. 图 2 质子辐照 FSI CIS 诱导单粒子瞬态亮斑 Fig 2 Single event transient bright spot of FSI CIS irradiated by proton. 质子诱导半导体器件发生单粒子效应主要通过两种方式:一是通过直接电离,使核外电 子获得能量,从而被激发释放;二是通过核反应,即高能质子直接与原子核发生碰撞,使原 子发生位移,同时产生光子、轻粒子以及一些反冲原子[13]。依据瞬态亮斑的形状和总的灰度 值大小,可将质子辐照 CIS 像素阵列诱导的单粒子瞬态亮斑为三类。第一种如图 2(b) (c)中 红色标注所示,这种瞬态亮斑灰度值最低,占据像素单元个数最小,但是数量最多。因此可 以判断这种瞬态亮斑由质子直接电离导致。因为几乎所有的质子入射 CIS 进入敏感层,都会 和硅通过库伦相互作用激发释放硅核外电子,产生电子空穴对。第二种瞬态亮斑如图 2(c)中 较亮的光斑所示,和第一种亮斑形状相似,近似圆形,但亮度明显比第一种瞬态亮斑大,且 数量很少。这种亮斑是由高能质子与 CIS 像素单元发生核反应导致。第三种瞬态现象如图 3(b)中亮线所示,和前面两种亮斑最大的不同是,这种瞬态现象形状上呈一条亮线,灰度值 也比第一种亮斑大很多,这被称为单粒子瞬态亮线。和第二种亮斑一样,瞬态亮线数量很少。 因此可以判断瞬态亮线也是由于高能质子和 CIS 像素单元发生核反应导致。当核反应产生 的次级粒子和像素单元垂直面夹角较小,这种瞬态现象表现在图像上就是亮斑。而当产生次 级粒子出射角度大,次级粒子会经过多个像素,产生的电子空穴对会被粒子径迹周围像素收 集,表现在图像上就是单粒子瞬态亮线。. 为了进一步分析瞬态亮斑特征,我们提取瞬态亮斑两个重要参数,亮斑覆盖像素单元数 量和亮斑总的沉积能量。测试时,采集的图像信号值为灰度值,单位为 DN(Digital number), 表示由 AD 转换后直接得到的数字信号值。一个亮斑总的沉积能量 Ed 为: Ed. BSI CIS 中瞬态亮斑沉积能量和尺寸大小分布趋势和 FSI CIS 相同,区别是亮斑沉积能 量和尺寸大小分布范围发生明显左移。图 5 比较了 60MeV 质子辐照 FSI 和 BSI CIS 沉积能 量和尺寸大小分布。实验结果表明 BSI CIS 沉积能量分布整体明显左移,峰值能量由 0.0095MeV 减小到 0.0035MeV,最大沉积能量减小为 0.23MeV。和亮斑沉积能量变化一致, BSI CIS 亮斑尺寸大小分布也整体左移,即亮斑尺寸明显减小。
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