-
EQR SiPM的器件结构如图1所示,SiPM在反向偏置下工作,由少子触发雪崩事件,短波长光在硅中的吸收深度较浅,且因硅材料中电子的碰撞电离系数及迁移率均大于空穴,因此采用P on N的结构设计以增强器件对蓝紫光的探测灵敏度。器件表面重掺杂层P++与分区注入的N enrich区形成微单元PN结,结下方的N型外延层构成微单元淬灭电阻。由于外延层的掺杂浓度低于enrich区,微单元间Gap区的耗尽区深度大于微单元PN结的耗尽区深度,雪崩载流子将被限制在各自的微单元区域内,Gap区形成微单元间的电学隔离,最终信号通过公共电极输出。
图 1 P on N型EQR SiPM的结构示意图。(a)剖面结构;(b)平面结构
Figure 1. Structure diagram of P on N type EQR SiPM. (a) Profile structure; (b) Planer structure
EQR06与EQR15结构的不同之处在于:微单元尺寸分别为6 μm和15 μm。两类器件微单元尺寸不同,理论上特性的差别主要体现在直接串扰率(direct crosstalk probability, PDiCT)、PDE、增益。
EQR15-HE与EQR15-LE的唯一区别在于PN结的峰值电场强度。为了降低EQR15 SiPM的DCR,NDL优化器件设计和制作工艺,通过降低掺杂浓度降低PN结的峰值电场强度,有效抑制了场致隧穿引起的暗噪声成分,由EQR15- HE发展到EQR15-LE,从而降低了EQR15 SiPM的DCR,其他性能也获得优化。
-
SiPM主要表征I-V特性、DCR、PDiCT、PDE、单光子分辨谱、增益,以下特性无特殊说明外均为在20 ℃下的实验结果。
Keithley SMU对SiPM施加偏压,暗条件下测电流得 I-V曲线,确定器件击穿电压Vb及最大过偏压OVmax。暗条件下,Keithley SMU作为SiPM的偏置电源,其输出信号经过跨阻放大器(NDL AMP-40-1)输入示波器,Labview程序通过改变阈值测量计数率得到计数率随阈值的关系,由阈值为0.5 p.e.处的计数率得到DCR,由阈值为1.5 p.e.处的计数率与阈值为0.5 p.e.处的计数率之比得到PDiCT[10] ,其中1 p.e.代表SiPM中单个微单元发生雪崩时的输出信号幅度。
PDE是指一段时间内器件探测到的光子数与入射到器件表面的光子数之比。图2所示为PDE测试实验装置。图2(a)中采用函数发生器驱动LED产生脉冲光经窄带滤光片射入积分球,积分球出射光分别照射在已定标响应度的PIN和待测SiPM上, SiPM一端可通过加入衰减片以补偿PIN与SiPM的探测灵敏度差异,采集SiPM输出信号,根据泊松分布理论计算单次脉冲SiPM响应的平均光电子数μ[11],测量PIN的平均光电流IPIN,根据公式(1)得到单次脉冲入射至SiPM表面的光子数Nin:
$$ {{{N}}_{{\rm{in}}}} = \dfrac{{{{{I}}_{{\rm{PIN}}}} \cdot {\rm{\lambda }} \cdot {{{P}}_{{\rm{SiPM}} - {\rm{PIN}}}}}}{{{{R}} \cdot {{h}} \cdot {{c}} \cdot {{f}}}}$$ (1) 式中:
$ \lambda $ 为光波长;R为PIN的响应度;f为入射光频率;h为普朗克常量;c为光速;${{P}}_{\mathrm{S}\mathrm{i}\mathrm{P}\mathrm{M}-\mathrm{P}\mathrm{I}\mathrm{N}}\mathrm{反}\mathrm{映}$ SiPM与PIN位置处入射光功率的比例因子。μ与Nin的比值为器件在该波长下的绝对PDE。图2(b)中采用氙灯作光源经单色仪射入积分球,采集SiPM的平均光响应计数率NL、平均暗计数率ND及PIN的平均光电流IPIN,由此可得PDE随波长的相对变化关系为[12]:
$$ {\rm{PDE}} = \dfrac{{\left( {{{{N}}_{\rm{L}}} - {{{N}}_{\rm{D}}}} \right) \cdot {{R}} \cdot {{h}} \cdot {{c}}}}{{{{{I}}_{{\rm{PIN}}}} \cdot {{\lambda }} \cdot {{{P}}_{{\rm{SiPM}} - {\rm{PIN}}}}}}$$ (2) 将PDE随波长的相对变化关系归一化到基于泊松分布理论的某一波长下的绝对PDE,以此扣除关联噪声的影响,修正后得到PDE随波长的变化关系[11]。
采用脉宽为100 ps的激光器光源,用示波器统计SiPM响应信号,由此获得脉冲面积直方图即为单光子分辨谱,通过相邻光子峰的平均面积差求得增益。
-
从EQR15-HE~EQR15-LE,通过工艺和设计优化,器件特性取得一定改进:
(1)场致隧穿部分的DCR正比于器件内部电场强度,器件内部电场降低使得场致隧穿部分的DCR降低,因此,DCR随电场的斜率降低,即DCR随过偏压的斜率降低。
(2)电场强度的降低导致耗尽区宽度增大,使得SiPM微单元的等效电容减小,而增益正比于微单元等效电容及过偏压,因此与EQR15-HE相比,EQR15-LE增益随过偏压的斜率降低。两者温度系数的不同来源于耗尽区宽度的差异,耗尽区宽度越大温度系数越高[14],EQR15-LE具有更高的温度系数。每105个穿过高场区的载流子平均会产生3个串扰光子[15],这体现在对于单次雪崩事件而言,增益越高产生的串扰光子越多。因此,EQR15-HE与EQR15-LE增益及DCR随过偏压的变化趋势不同进而导致PDiCT的不同。
(3) PDE的大小由几何填充因子、量子效率、盖革效率决定。碰撞电离系数与电场呈指数关系且正相关[15],器件内部电场降低导致电子空穴的碰撞电离系数减小进而盖革效率降低,影响PDE,这体现在短波长处PDE的降低。与此同时,尽管盖革效率存在一定程度的降低,但PN结峰值电场降低使得耗尽区展宽,更深吸收长度的长波长光子量子效率反而提升,因此,长波长处PDE改变很小。
-
表1为NDL EQR15-LE与滨松同类型S14160-3015 PS[5]的主要参数,两类器件具有相同的有效面积及微单元尺寸。两器件的动态范围近似一样,EQR SiPM的器件结构使得其几何填充因子更高,因而器件在各自的推荐偏压下(Vop),EQR SiPM有更优PDE。EQR SiPM与MPPC的增益相近,只是其DCR及PDiCT不及MPPC。EQR SiPM的结电容远小于MPPC,意味其输出信号脉宽更窄,在高计数率应用中,脉冲堆叠现象的影响会比MPPC小。
表 1 NDL与滨松SiPM的主要特性参数对比
Table 1. Main characteristic parameters comparison between NDL and HAMAMATSU SiPM
Research institute HAMAMATSU NDL Series S14160-3015 PS EQR15 11-3030 D Active area/mm2 3.0×3.0 3.0×3.0 Microcell size/μm 15 15 Microcell number 39984 40000 Breakdown voltage (Vb)/V 38±3 28±0.2 Recommended operating voltage (Vop)/V Vb+4 Vb+7 Photon detection efficiency (PDE) @Vop 32% @460 nm 46% @410 nm Gain @Vop 3.6×105 3.5×105 Dark count rate (DCR) @Vop Typical: 700 kHz Typical: 2 000 kHz Crosstalk probability @Vop <1% 11% Terminal capacitance/pF 530 48 FBK于2018年报道出微单元尺寸为5 μm、微单元密度高达46190个/mm2的超高密度UHD SiPM[6],在接近6 V的工作条件下,增益约为1.8×105,在545 nm处,PDE约为12%,但DCR高于800 kHz/mm2。与其相比,EQR06 SiPM微单元密度低于UHD SiPM,在推荐过偏压处,增益为7×104,峰值波长处PDE为28%,DCR约为240 kHz/mm2,整体特性优于FBK 微单元尺寸为5 μm的UHD SiPM。
Recent research progress of silicon photomultiplier with epitaxial quenching resistor
-
摘要: 北京师范大学新器件实验室(NDL)一直致力于研制结构紧凑、工艺相对简单的外延电阻淬灭型硅光电倍增器(silicon photomultiplier with epitaxial quenching resistor, EQR SiPM)。近期为了满足硅光电倍增器(silicon photomultiplier, SiPM)在核医学成像方面的需要,NDL通过优化器件设计和制作工艺,成功研制出微单元尺寸为15 μm、有效面积为9 mm2的EQR SiPM。相较以往同类型器件,实现了器件暗计数率(dark count rate, DCR)的进一步降低同时保持了较高的光子探测效率(photon detection efficiency, PDE),在环境温度为20 ℃、过偏压为7 V时,DCR的典型值为226 kHz/mm2、峰值PDE为46%。另外,为了进一步提升EQR SiPM的动态范围,NDL还研制出微单元尺寸为6 μm、有效面积为9 mm2、微单元数目为244720的EQR SiPM,在环境温度为20 ℃、过偏压为7 V时,DCR的典型值为240 kHz/mm2、峰值PDE为28%,其较大的动态范围特别适合高能宇宙射线的测量、强子量能器等应用。Abstract: The Novel Device Laboratory (NDL) of Beijing Normal University has been developing a silicon photomultiplier with an epitaxial quenching resistor (EQR SiPM), which has a compact structure and a relatively simple fabrication process. Recently, to meet the requirements of nuclear medicine imaging, NDL has successfully developed an EQR SiPM with a microcell size of 15 μm and an active area of 9 mm2 by optimizing the device structure and fabrication technology. Compared to previous devices of the same type, the dark count rate (DCR) of the EQR SiPM is further reduced while still maintaining high photon detection efficiency (PDE). At an ambient temperature of 20 ℃ and an operating overvoltage of 7 V, the typical DCR is 226 kHz/mm2, and the peak PDE is 46%. In addition, to further increase the dynamic range of the EQR SiPM, NDL has developed an EQR SiPM with a microcell size of 6 μm, an active area of 9 mm2 and a microcell number of 244720. At an ambient temperature of 20 ℃ and an operating overvoltage of 7 V, the typical DCR is 240 kHz/mm2, and the peak PDE is 28%. It has large dynamic range that is very suitable for the measurement of high-energy cosmic rays and other applications in hadron calorimeters.
-
表 1 NDL与滨松SiPM的主要特性参数对比
Table 1. Main characteristic parameters comparison between NDL and HAMAMATSU SiPM
Research institute HAMAMATSU NDL Series S14160-3015 PS EQR15 11-3030 D Active area/mm2 3.0×3.0 3.0×3.0 Microcell size/μm 15 15 Microcell number 39984 40000 Breakdown voltage (Vb)/V 38±3 28±0.2 Recommended operating voltage (Vop)/V Vb+4 Vb+7 Photon detection efficiency (PDE) @Vop 32% @460 nm 46% @410 nm Gain @Vop 3.6×105 3.5×105 Dark count rate (DCR) @Vop Typical: 700 kHz Typical: 2 000 kHz Crosstalk probability @Vop <1% 11% Terminal capacitance/pF 530 48 -
[1] Nagai A, Alispach C, Volpe D D, et al. SiPM behaviour under continuous light [J]. Journal of Instrumentation, 2019, 14(12): P12016. doi: 10.1088/1748-0221/14/12/P12016 [2] Gundacker S, Heering A. The silicon photomultiplier: fundamentals and applications of a modern solid-state photon detector [J]. Physics in Medicine & Biology, 2020, 65(17): 17TR01. [3] Gola A, Acerbi F, Capasso M, et al. NUV-sensitive silicon photomultiplier technologies developed at fondazione bruno kessler [J]. Sensors, 2019, 19(2): 308. doi: 10.3390/s19020308 [4] Simon F. Silicon photomultipliers in particle and nuclear physics [J]. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019, 926: 85-100. [5] Hamamatsu. S14160-3015PS: Low breakdown voltage, wide dynamic range type MPPC with small pixels[EB/OL]. [2021-09-12]. https://www.hamamatsu.com/jp/en/product/optical-sensors-/mppc/mppc_mppc-array/S14160-3015PS.html [6] Acerbi F, Gola A, Regazzoni V, et al. High efficiency, ultra-high-density silicon photomultipliers [J]. IEEE Journal of Selected Topics in Quantum Electronics, 2018, 24(2): 1-8. [7] Acerbi F, Paternoster G, Capasso M, et al. Silicon photomultipliers: technology optimizations for ultraviolet, visible and near-infrared range [J]. Instruments, 2019, 3(1): 15. doi: 10.3390/instruments3010015 [8] Zhang G Q, Hu X B, Hu C Z, et al. Demonstration of a silicon photomultiplier with bulk integrated quenching resistors on epitaxial silicon [J]. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010, 621(1-3): 116-120. doi: 10.1016/j.nima.2010.04.040 [9] 刘红敏, 龙金燕, 代雷, 等. 大动态范围外延电阻淬灭型硅光电倍增器[J]. 光学精密工程, 2020, 28(3): 535-541. doi: 10.3788/OPE.20202803.0535 Liu Hongmin, Long Jinyan, Dai Lei, et al. Research progress of large dynamic range silicon photomultiplier with epitaxial quenching resistor [J]. Optics and Precision Engineering, 2020, 28(3): 535-541. (in Chinese) doi: 10.3788/OPE.20202803.0535 [10] Nagai A, Alispach C, Barbano A, et al. Characterization of a large area silicon photomultiplier [J]. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019, 948: 162796. doi: 10.1016/j.nima.2019.162796 [11] Eckert P, Schultz-Coulon H C, Shen W, et al. Characterisation studies of silicon photomultipliers [J]. Nuclear Inst & Methods in Physics Research A, 2010, 620(2): 217-226. [12] Zappalà G, Acerbi F, Ferri A, et al. Set-up and methods for SiPM Photo-Detection Efficiency measurements [J]. Journal of Instrumentation, 2016, 11(8): P08014. doi: 10.1088/1748-0221/11/08/P08014 [13] Serra N, Ferri A, Gola A, et al. Characterization of new FBK SiPM technology for visible light detection [J]. Journal of Instrumentation, 2013, 8(3): P03019. doi: 10.1088/1748-0221/8/03/P03019 [14] Piemonte C, Gola A. Overview on the main parameters and technology of modern Silicon Photomultipliers [J]. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019, 926: 2-15. doi: 10.1016/j.nima.2018.11.119 [15] Fa A, Sgb C. Understanding and simulating SiPMs [J]. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019, 926: 16-35.