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焦平面噪声包括探测器噪声、耦合噪声和读出电路噪声。探测器噪声包括探测器的暗电流产生的散粒噪声,探测器电阻产生的热噪声等;读出电路噪声包括由于来自于输入端放大器的复位噪声、采样电路的采样噪声,以及输出级噪声等;耦合噪声主要由于探测器与读出电路耦合过程中产生的。为抑制焦平面噪声,可采取以下减小噪声的措施:(1)在读出电路设计时,在符合满阱容量下优化积分电容设计;(2)优化读出电路单元内的运放参数;(3)减小探测器结电容和暗电流。
如图1所示,对于平面型In0.53Ga0.47As探测器,P+n结位于InGaAs吸收层中,可以认为结电容就是InGaAs的同质结。单边突变结的单位面积电容Cj为:
$$ {C_j} = {{A}}\dfrac{{{\varepsilon _0}{\varepsilon _s}}}{{{W_d}}} $$ (1) 式中:A为探测器的结面积;ε0为真空介电常数;εs为半导体的介电常数;Wd为耗尽区的宽度,耗尽区的宽度越宽,单位面积的结电容越小。耗尽层宽度Wd和外加电压V以及吸收层掺杂浓度Nd之间的关系为:
$$ {W_d} = {\left[ {\dfrac{{2{\varepsilon _0}{\varepsilon _s}}}{q}\dfrac{{{V_0} - V}}{{{N_d}}}} \right]^{1/2}} $$ (2) 式中:V0为P+n结的内建电势,将公式(2)代入公式(1),并重新整理可得:
$$ \frac{1}{{C_j^2}} = \dfrac{2}{{q{\varepsilon _0}{\varepsilon _s}{N_d}}}\left( {{V_0} - V} \right) $$ (3) 因此,降低材料吸收层的掺杂浓度可以有效减小探测器的结电容。采用不同吸收层浓度的材料制备的探测器的单位面积电容与电压的关系如图2所示,可见降低吸收层掺杂浓度,可有效抑制探测器电容。
图3为采用相同材料和工艺制备的不同直径的光敏元的暗电流密度,在室温−0.1 V偏压下,500 μm直径的单元器件暗电流密度为3.34 nA/cm2,而10 μm直径的单元器件暗电流密度为53.53 nA/cm2,暗电流密度增大了16倍之多。这说明针对高密度小像元探测器的暗电流,需要进行优化。
光敏芯片的暗电流由体电流和表面及侧面电流组成,体电流与光敏芯片的面积A成正比,而表面及侧面电流与光敏元的周长P成正比。探测器的光敏元暗电流密度可以表示为:
$$ J = \dfrac{I}{A} = {J_B} + {J_S}\dfrac{P}{A} $$ (4) 式中:JB为体内漏电流密度;JS为表面和侧面漏电流密度。计算不同尺寸的探测器暗电流密度和P/A的关系,可以分析出在总暗电流中体电流和表面及侧面漏电的贡献。从图4可以看出,随着P/A的增大,暗电流密度呈直线增大,表明与周长相关的侧面漏电和表面漏电作用凸显,需要优化器件表面钝化工艺,降低表面漏电。
采用低温ICPCVD方式镀SiNx钝化膜,调整镀膜过程中的气体总流量、N2/SiH4比例以及腔体压强,保持生长腔体温度和ICP功率不变,优化高密度小像元器件表面的钝化工艺,抑制表面漏电,最终有效降低了高密度小像元阵列探测器的暗电流。室温下−0.1 V偏压,15 μm中心距器件暗电流密度为9.43 nA/cm2,12 μm中心距器件暗电流密度为11.02 nA/cm2,10 μm中心距器件暗电流密度为11.62 nA/cm2,如图5所示。
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对研制的10 μm中心距2560×2048元InGaAs焦平面探测器组件进行光电性能测试,测试条件如表1所示(表中,1 in=2.54 cm)。
表 1 2560×2048元InGaAs焦平面探测器测试条件
Table 1. Test conditions of 2560×2048 InGaAs focal plane arrays
Item Value Item Value Black body temperature/K 900 Black body aperture/in 0.1 Distance/cm 60 Blackbody radiated power/W 1.65×10−11 Ambient temperature/℃ 22 Ambient humidity 23% Operating temperature/K 276 Circuit gain 0.8 Integral time/ms 100 Integral capacitance/fF 15 如图11所示为焦平面的响应光谱曲线,从图中可以看出,2560×2048元InGaAs焦平面光谱响应范围为0.80~1.71 µm。
图 11 2560×2048元InGaAs焦平面探测器组件光谱响应曲线
Figure 11. Response spectra curve of 2560×2048 InGaAs focal plane arrays
焦平面黑体响应信号如图12所示,测得黑体响应信号Vs=1.071 V,响应非均匀性为3.81%,盲元率0.26%。
图 12 焦平面响应信号测试结果。(a) 像元信号图;(b) 信号统计分布
Figure 12. Measured result of response signal of FPAs. (a) Pixel signal map; (b) Signal statistical distribution
焦平面噪声测试结果如图13所示,噪声电压为1.02 mV。
图 13 焦平面噪声测试结果。(a) 像元噪声图;(b) 噪声统计分布
Figure 13. Measured result of noise of FPAs. (a) Pixel noise map; (b) Noise statistical distribution
焦平面的响应率R、探测率D*和量子效率η分别为:
$$ {{{R}}_i} = \dfrac{{{V_s}{C_{int}}}}{{{T_{int}}{A_v}P}}G $$ (5) $$ {{{D}}^*} = G\sqrt {\dfrac{{{A_D}}}{{2{T_{int}}}}} \cdot \dfrac{{{V_S}}}{{P{V_N}}} $$ (6) $$ \eta = hc\dfrac{{{R_i}}}{{q{\lambda _p}}} $$ (7) 式中:Vs为平均信号值;VN为平均噪声值;Cint为积分电容;Tint为积分时间;Av为电路增益,取值0.8;G因子为光谱因子,值取78;h为普朗克常数;c为真空光速;
${\lambda _p} $ 为峰值波长,为1.6 μm。根据测得的光谱响应、信号、噪声,计算得到焦平面的响应率为0.95 A/W,量子效率为73.7%,峰值探测率为1.11×1013 cm·Hz1/2/W。采用了高动态范围(High-Dynamic Range, HDR)技术进行试验,通过两次或多次采样(Multiple Capture),当目标物的光强时,采用短积分时间Tint1;当目标物的光弱时,采用长积分时间Tint2,综合得到整个目标内的光强分布,获得大动态范围。采用短波红外激光器(MIL-1342 nm,200 mW)照到墙壁上,形成圆斑区域强光,读出电路饱和信号Vsat约为1.85 V。采用HDR技术,读出电路试验片Tint1=0.06 ms、Tint2=100 ms,成像图像如图14所示:等效的饱和信号约为2.604 V,动态范围增大到128.1 dB。
如图15所示,采用2560×2048元InGaAs焦平面探测器组件进行成像验证。如图16所示,短波红外成像可以清晰地辨别液位、穿透硅片对背后的火焰成像,以及具有分辨不同物质的能力。
2560×2048 short-wave infrared InGaAs focal plane detector (Invited)
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摘要: 高性能大规模小像元短波红外InGaAs焦平面探测器是新一代航天遥感仪器向高空间分辨率、高能量分辨率、高时间分辨率发展需要的核心器件。文中报道了高密度InGaAs探测器阵列设计与制备,并与匹配的Si-CMOS读出电路倒焊互连形成焦平面的最新研究进展,重点突破了高密度小像素探测器的暗电流和噪声抑制、百万像素焦平面倒焊互连等关键技术,解决了高平整度芯片面形控制、In柱凸点形貌和高度一致性控制、高密度倒焊偏移控制等倒焊新工艺,研制了像元中心距10 μm的2560×2048元焦平面探测器,其峰值探测率优于1.0×1013 cm·Hz1/2/W,响应不均匀性优于3%,有效像元率优于99.7%,动态范围优于120 dB。采用该焦平面进行了实验室演示成像,图片清晰。Abstract: The new generation of aerospace remote sensing instruments are developing towards high spatial resolution, high energy resolution, and high time resolution. Its core components are high-performance large-scale small-pixel short-wave infrared InGaAs focal plane detectors. The latest research progress in the design and fabrication of high-density InGaAs detector arrays was reported, and hybrided with matching Si-CMOS readout circuits to form a focal plane. The breakthroughs in dark current and noise suppression of high-density small-pixel detectors , megapixel focal plane flip chip interconnection and other key technologies were focused. The new flip chip interconnection technologies such as high flatness chip surface shape control, indium bumps convex morphology and high consistency control, and high-density flip chip interconnection control were solved. Developed 10 μm pitch 2560×2 048 focal plane detectors, which D* was better than 1.0×1013 cm·Hz1/2/W, the response non-uniformity was better than 3%, the effective pixel rate was better than 99.7%, and the dynamic range was better than 120 dB. This focal plane was used for laboratory demonstration imaging, and the picture was clear.
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Key words:
- InGaAs /
- focal plane /
- short-wave infrared /
- detectivity /
- dark current
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图 9 采用优化工艺制备的铟柱回融前后的形貌 。(a) 15 μm中心距铟柱回融前SEM图; (b) 15 μm中心距铟柱回融后SEM图;(c) 10 μm中心距铟柱回融前 SEM图;(d) 10 μm中心距铟柱回融后SEM图; (e) 15 μm中心距铟柱直径统计分布 ;(f) 10 μm中心距铟柱直径统计分布
Figure 9. Indium bump arrays fabricated via the modified SiNx recipe: SEM images for 15 μm pitch (a) before and (b) after reflow, and 10 μm pitch (c) before and (d) after reflow; (e) and (f) Statistical diameter distribution of the ball arrays for 15 μm pitch and 10 μm pitch respectively
表 1 2560×2048元InGaAs焦平面探测器测试条件
Table 1. Test conditions of 2560×2048 InGaAs focal plane arrays
Item Value Item Value Black body temperature/K 900 Black body aperture/in 0.1 Distance/cm 60 Blackbody radiated power/W 1.65×10−11 Ambient temperature/℃ 22 Ambient humidity 23% Operating temperature/K 276 Circuit gain 0.8 Integral time/ms 100 Integral capacitance/fF 15 -
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