摘要
在红外系统中,红外探测器是最关键的元件之一,是红外装置的心脏,利用红外探测原理将红外辐射信号转换为电信号输出。热电堆红外探测器是非制冷型探测器,具有以下特点：1)可以在室温下工作,而且制造成本低；2)对各个波长的红外辐射均有响应；3)接收到红外辐射后,是通过声子对晶格温度的影响,进而影响该探测器的电学性能；4)检测恒定的辐射量,在恒定的红外辐射量下就会有响应输出。热电堆红外探测器在军事和民用上应用也极其广泛。本论文以热电堆红外探测器为研究对象,从理论与工艺两方面进行了探索,在此基础上,对该探测器进行了性能测试。
理论上,针对本文设计的热电堆红外探测器结构,建立了探测器一维热稳态热传导模型,从五个方面(吸收区尺寸、热偶条尺寸、介质支撑膜尺寸、背腔腐蚀窗口尺寸、测试条件)表述了对热电堆红外探测器性能的影响。此外,对探测器进行了热稳态有限元仿真和瞬态有限元仿真,得到了探测器的温度分布云图、热量梯度分布云图、热通量分布云图、温度分布路径分析图和响应时间仿真图,对于探测器接受红外辐射之后的热响应有了直观的表象。
热电堆红外探测器的整个制备过程是在中国科学院微电子研究所加工完成的,为了能够顺利地完成整个流片过程,我们首先进行了一系列的关键工艺的单项实验：热电堆红外探测器介质支撑膜的制备单项实验；热电堆红外探测器多晶硅热偶条方阻测试单项实验；热电堆红外探测器多晶硅、氮化硅刻蚀条件单项实验；热电堆红外探测器钝化层制备的单项实验；热电堆红外探测器反射层制备的单项实验；热电堆红外探测器背腔腐蚀硅方案验证实验。在此基础上,设计了完整的工艺流片过程,成功地制备出基于单层低应力氮化硅薄膜的热电堆红外探测器。
在实验设计方面,本文对热电堆红外探测器进行了以下几方面的测试：热电堆红外探测器吸收区红外透射谱测试；热电堆红外探测器不同结构尺寸性能测试对比实验；热电堆红外探测器频率响应特性实验；热电堆红外探测器黑体炉温度响应测试实验；热电堆红外探测器吸收层红外吸收特性对比实验；热电堆红外探测器反射层覆盖性能对比实验。以上测试结果表明探测器受结构尺寸变化(吸收区尺寸、热偶条长度、背腔腐蚀窗口尺寸)而表现出不同的响应；在不同调制频率作用下,探测器的响应率和探测率会随着频率的增大而减小,这与探测器本身的热容变化密不可分；在不同黑体炉温度下(范围在403K-823K),本文所设计的探测器会由于不同的等效红外吸收率而在温度较低区域显示更好的性能；在性能改进实验中,添加碳黑和金属反射层对探测器的性能有显著提高。
本文研究创新点主要体现在：
a)突破了传统的SiO2-Si3N4三明治式介质支撑膜的制备方法,采用单层低应力氮化硅薄膜作用为探测器的介质支撑膜,实现了厚度可调的探测器介质支撑膜的简单工艺制备方法。
b)结合APIE封蜡和CRY封蜡保护探测器正面图形的方法,方法简单易于实现,大大提高了探测器的成品率。
c)本文研究了热电堆红外探测器吸收区红外透射光谱,得到了不同厚度的红外吸收薄膜的红外透射谱和添加碳黑前后的吸收区薄膜的红外透射谱,采用分段拟合的方法更为准确地定量描述了探测器实际吸收的红外光谱。
d)在探测器的吸收区添加碳黑,在热偶条冷端覆盖钛金反射层,通过实验证明了二者有助于探测器性能的提高。
In the infrared system, infrared detectors are one of key elements, which can transfer infrared radiation into electrical output signal based on the infrared detecting theory. Thermopile infrared detectors are one of non-cooled detectors, with the following characteristics:1) they can work under room temperature with low cost of fabrication; 2) they are sensitive to infrared radiation with various wavelengths; 3) when they receive infrared radiation, they generate phonon to make the temperature increase of crystal lattice which will influence the electrical performance of the detectors; 4) they can detect the radiation with stable amount and have electrical response. Due to their advantages, they are broadly applied in the military and commercial fields. In this thesis, thermopile infrared detectors are studied from the point of view of theory and fabrication process, and then the performance of thermopile infrared detectors is measured.
First, for the thermopile infrared detector mentioned above, one-dimensional Fourier's stationary heat model is built to analyze the temperature gradient between hot junctions and cold junctions of thermo couples. The influences of absorbing area, thermocouple's size, supporting membrane's size, back-etching window's size and measurement condition on the performance of the thermopile infrared detector are established. Moreover, heat static state finite simulation and transient state finite simulation are conducted. Thus, the figures of temperature distribution, heat gradient, heat flux, temperature path analysis and heat response time are obtained to make direct impression of heat response of the detector, after the detector receives infrared radiation.
The whole fabrication process of the thermopile infrared detector is conducted at Institute of microelectronics of Chinese academy of Sciences (IMECAS). In order to finish the whole fabrication process, firstly, a series of single experiments for key fabrication process are conducted, including the fabrication of a single layer of low-stress SiNx membrane, square resistance testing of poly-Si, dry etching of poly-Si and SiNx, fabrication of the passivation layer, fabrication of the reflective coating, and wet etching of Si substrate. Based on these single key experiments, the whole fabrication process of the detector is designed, and the thermopile infrared detected based on a single layer of low-stress SiNx membrane is fabricated successfully.
In this thesis, a series of experiments are conducted to quantify the performance of the thermopile infrared detector, such as measurement of infrared transmission spectra of the detector, performance comparison of the detector under different structural size, frequency response of the detector, blackbody temperature response of the detector, performance enhancement experiments of the detector. All these testing results show that the performance of the detector is distinct under different absorbing area, the length of thermo-couples and back-etching window's size; due to heat capacity of the detector itself, the performance of the detector decreases with the adding of the frequency of the chopper; during the blackbody temperature range of 403K-823K, the detector in this thesis shows higher performance in low temperature region because of its equivalent absorbing ratio; and the application of carbon-black in the absorbing area and metal reflective-coating in cold junctions of the detector enhances the performance of the detector distinctly.
This thesis brings forward the main innovation spots as follows:
a) Breaking through the traditional the fabrication method of the SiO2-Si3N4 sandwich-type supporting membrane, a single layer of low-stress SiNx membrane is used as the supporting membrane of the detector to achieve easy fabrication method of the low-stress supporting membrane with adjustable depth.
b) The anti-etch protection of the thermo-couples is easy to be achieved combining APIE wax with CRY wax, and the productivity of the detector is enhanced distinctly.
c) In this thesis, infrared transmission spectra of the absorbing layer of the thermopile infrared detector are taken into consideration. The infrared transmission spectra of the absorbing layer with different depth and that of the absorbing layer with carbon-black and without carbon-black are obtained and fitted to quantify the actual amount of infrared radiation by the detector more accurately.
d) Carbon-black, applied to the absorbing area of the detector, and Titanium/Gold coating, applied to the cold junctions of the detector, enhance the performance of the detector.
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