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氧化钒薄膜的电阻温度系数
$\alpha $ (TCR)为电阻随温度的变化与电阻倒数的乘积:利用探针和温控热板搭建了电阻温度系数测试系统,并对两种在不同条件下沉积的薄膜制备的探测器的电阻-温度关系进行了测试,在各自合适的电流密度下,把制备氧化钒薄膜时两种不同功率条件分别称为条件一(以下简称T1)和条件二(以下简称T2)。两个样品薄膜方块电阻相近,均为500 kΩ/□左右,采取先将热板加热至80 ℃后再缓慢降温,最后降至26 ℃,每隔2 ℃测一次电阻。根据公式(2)计算出薄膜电阻温度系数曲线如图1所示。测试温度范围为26~80 ℃,取常温段26~40 ℃范围内TCR的均值为最终TCR的值。
如图2所示,在T1和T2条件下沉积的氧化钒薄膜在常温段26~40 ℃下的TCR分别为−2.27% K−1和−2.69% K−1,且在68 ℃附近不具有相变特性,薄膜电阻温度系数与测试温度在20~80 ℃范围内呈线性关系。由此可知,T2条件有利于热敏材料氧化钒TCR的提升。
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电子能谱分析是通过分析样品在入射粒子作用下发射出来的电子所带的信息(例如能量、强度、角分布等),从而了解样品的组成及原子和分子电子结构的一种方法[9]。XPS定量分析的基本依据是谱峰的强度(峰面积或峰高)与元素的含量有关。通常,光电子峰面积的大小主要取决于样品中所测元素的含量(或相对浓度)。因此,通过测量光电子峰的强度就可进行定量分析[10]。
使用VG Multilab 2000 X射线光电子能谱仪分析在T1和T2条件下制备的氧化钒样品中钒的价态,X射线源:AIK (hv=1486.6 eV),电压12.5 kV,电流密度250 W,工作压强为0.8~3×10−6 Pa,分辨率为通能采用25 eV时,FWHM Ag 3d5/2=1.45 eV (for metal),XPS结果如图2所示。
如图3(a)和表1所示,氧化钒生长为T1条件时,5+价的钒峰面积为134749.37,4+价的钒峰面积为75060.59,3+价的钒峰面积为17687.09,通过计算5+,4+和3+价钒占比分别为59.24%,32.99%和7.77%。如图3(b)和表1所示,氧化钒生长为T2条件时,5+价的钒峰面积为151143.03,4+价的钒峰面积为35430.81,3+价的钒峰面积为42235.11,5+,4+和3+价钒占比分别为66.07%,15.48%和18.45%。对比发现,在合适的电流密度下,调整沉积功率可以改变氧化钒薄膜中不同价态钒的占比,其中V5+含量增加,TCR提升。
Condition Valence of
vanadiumPeak area of
vanadium/PPercentage of
totalTCR T1 V5+ 134749.37 59.24% 2.27% V4+ 75060.59 32.99% V3+ 17687.09 7.77% T2 V5+ 151143.03 66.07% 2.69% V4+ 35430.81 15.48% V3+ 42235.11 18.45% Table 1. XPS results of vanadium oxide films under different conditions
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为直观了解薄膜表面状况,将在T1和T2条件下制备的氧化钒薄膜通过原子力显微镜来进行材料表面形貌分析,通过表面粗糙度等表征来进一步深入理解氧化钒薄膜的特性。两种不同条件下薄膜的原子力显微镜扫描(AFM)照片如图4所示。
在T1和T2条件制备薄膜的8 in晶圆上,取其上下左右中相同位置各制5个样品测粗糙度,试验结果如表2所示,其中Ra和Rq分别代表算术平均粗糙度和均方根粗糙度。对比发现,T2相比于T1条件制备的薄膜致密性更好,粗糙度Ra和Rq有明显降低。
Sample T1 Ra/nm T2 Ra/nm T1 Rq/nm T2 Rq/nm C 0.825 0.083 1.05 0.106 T 0.823 0.079 1.06 0.101 B 0.817 0.080 1.05 0.104 L 0.795 0.084 1.03 0.111 R 0.818 0.082 1.06 0.107 Mean 0.816 0.082 1.05 0.106 Table 2. Properties comparision of vanadium oxide films under T1 and T2 conditions
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对T1和T2两种条件生长的氧化钒薄膜在高温下进行热稳定性试验,将高温处理前后电阻分别进行对比,数据如表3所示。
Resistance change after
high temperatureSample one Sample two Sample three T1 sample −6.4% −5.1% −7.5% T2 sample −0.6% −0.1% −0.2% Table 3. Thermal stability comparision of vanadium oxide films under T1 and T2 conditions
试验发现俩种不同条件下生长的氧化钒,其中T2条件下的氧化钒高温下电阻降幅较小,说明T2条件下生长的氧化钒热稳定性更好。
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非制冷红外焦平面探测器是利用外界红外光的辐射引起探测器像元温度变化,致使像元内热敏材料的电阻随温度发生变化。这种变化经由探测器内部的读出电路转换为电信号输出,经过探测器外部的信号采集和数据处理电路实现了对外热成像。表征微测辐射热计性能的参数主要包括响应率ℜ、噪声f和噪声等效温差NETD,他们之间的关系为NETD=f/ℜ。
使用T1和T2条件制备的氧化钒薄膜在阵列规模为640×512, 像元尺寸为12 μm的非制冷红外焦平面探测器上进行验证,两种条件下制备的氧化钒红外芯片封装成探测器之后,依据国标GB/T 13584—2011和GB/T 17444—1998对探测器相关参数进行测试和统计。该两种条件的探测器在相同且最佳的偏置条件下,得到所有像元的响应率、噪声和NETD的分布,并进行了一定样本量的统计,结果如表4所示。
VOx deposition
conditionsNoise/mV Response rate/mV·K−1 NETD/mK Sample size T1 0.632 16.6 38 200 T2 0.456 17.2 26.5 50 Performance change of
T2 relative to T1−27.8% 3.6% −30.3% Table 4. Performance comparision of detectors made of vanadium oxide under T1 and T2 conditions
由表4可知,在各自合适的电流密度下,随着氧化钒沉积功率由T1到T2,探测器NETD降低了30%,响应率提高了3.6%,噪声降低了约28%,与理论关系相符。
探测器噪声包括电路噪声和材料噪声,文中该两种不同条件沉积的氧化钒薄膜均是在相同阵列规模和像元尺寸的读出电路上进行验证,其T1和T2两种条件电路噪声一致,为0.2~0.3 mV,所以测试结果对比显示T2条件薄膜制备的探测器噪声降低了28%,主要是薄膜材料噪声降低影响。这是因为T2条件通过改变氧化钒的沉积功率,改变了钒原子接触到基片表面时的沉积速率,使薄膜结晶状态改变,结晶度增加,减少颗粒间晶界和缺陷,使热敏薄膜的噪声降低。
图5是T2条件下氧化钒制备工艺生长的氧化钒薄膜制成的探测器对物体的成像图,画面比较细腻,建筑物和卡车等细节都清晰可见,边缘锐利,对比度高,整体成像效果很好。
Preparation and application of a high-performance vanadium oxide thermosensitive film
doi: 10.3788/IRLA20200349
- Received Date: 2020-09-08
- Rev Recd Date: 2020-12-03
- Available Online: 2021-02-07
- Publish Date: 2021-02-07
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Key words:
- vanadium oxide film /
- reactive magnetron sputtering /
- TCR /
- uncooled infrared focal plane detector
Abstract: A method for preparing high performance vanadium oxide thermosensitive thin films and its application were reported. Using reactive magnetron sputtering film deposition technology, the preparation process of vanadium oxide thin films was optimized by changing the sputtering power during the deposition of vanadium oxide thermosensitive thin films, the deposition rate of vanadium atoms was adjusted when they touched the surface of the substrate after being sputtered. At the same time, the equipment was modified and upgraded, that is, a control power supply outside the vanadium sputtering chamber was added to accurately control the sputtering voltage and oxygen partial pressure and other parameters to accurately control the current density in the reaction process. A vanadium oxide film with a sheet resistance of 500 kΩ/□ and a temperature coefficient of resistance (TCR) of −2.7% K−1 was prepared. The experimental results show that the noise equivalent temperature difference (NETD) performance of uncooled infrared focal plane detector made of high-performance vanadium oxide thermal sensitive film is reduced by 30% and the noise is reduced by 28%. The overall performance of the uncooled focal plane detector has been improved significantly.