Error analysis and fabrication of low-stepped mirrors
doi: 10.3788/CO.20191204.0791
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摘要: 本文提出了一种基于多级微反射镜和栅格分束器的静态轻型傅立叶红外变换光谱仪，通过两个多级微反射镜实现光程差的空间离散和干涉图的静态二维采样，通过引入栅格分束器有效降低了系统的体积和重量。作为该光谱仪的核心光学器件，多级微反射镜的阶梯高度一致性、面型平整度和结构精度是决定采样间隔、分辨率和噪声等仪器指标的主要因素。本文提出了基于MOEMS技术的厚度依次减半多层膜法，制作了台阶高度为0.625 μm，阶梯数为32的低阶梯多级微反射镜。测得实际阶梯高度平均值为626.9 nm，表面粗糙度均方根值为1.72 nm。分析了阶梯高度误差对光谱复原的影响，提出了两种阶梯高度误差校正方法，分别为通过修正因子来减小膜厚监控误差，和利用最小二乘余弦多项式算法对复原光谱进行校正。校正后的复原光谱误差(SCE)降低为2.34%，满足系统对光谱复原的要求。最后，将该低阶梯多级微反射镜置入光谱仪中，得到乙腈样品的干涉图和复原光谱图。
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关键词:
- 傅立叶变换红外光谱仪 /
- 低阶梯多级微反射镜 /
- 高度误差分析
Abstract: In this study, a static and light Fourier transform infrared spectrometer based on stepped mirrors and a grid beam splitter was proposed. By introducing two stepped mirrors into the interference system, the optical path difference is discretized and the 2-dimensional sampling of the interferogram is obtained. Furthermore, by introducing the grid beam splitter into the interference system, the volume and weight are decreased. Stepped mirrors as the core optical devices of such a spectrometer, its step height consistency, face flatness and the structure's precision determine the spectral sampling interval, resolution and noise of the system. We propose a method based on MOEMS technology involving multiple depositions accompanied by a 50% reduction in thickness at every iteration to fabricate a low-stepped mirror with 32 steps and 0.625 μm in step height. The test results show that the root-mean-square of roughness is 1.72 nm and that the average height of the real steps is 626.9 nm. The effect of the height error on the recovered spectrum is analyzed. In order to reduce the influence of this error, two methods are proposed:one is through using tooling factor to reduce the monitoring error of the film thickness, thus reducing the height error; the other is through using the least-squares approximation cosine polynomial algorithm to correct the recovered spectrum. The spectrum-constructing error(SCE) is reduced to 2.34%, which meets the requirements of spectral restoration. Finally, the experiment was carried out using low stepped mirrors and the interferograms were obtained before and after the addition of the sample. The absorption spectrum of the sample acetonitrile can be obtained using a Fourier transform. -
图 3 光谱评价函数SCE随阶梯高度标准差σ变化曲线
Figure 3. SCE varies with height standard deviation σ of the sub-mirror
图 4 低阶梯多级微反射镜制备流程图(以8台阶为例)
Figure 4. Flow diagram of low-stepped multi-level micromirrors(with 8 steps as an example)
图 7 低阶梯多级微反射镜粗糙度测试图
Figure 7. Roughness test chart of low-step multi-level micro-mirrors
图 8 32级低阶梯多级微反射镜台阶高度测试图
Figure 8. Step height test results of low stepped micromirror with 32 steps
图 9 台阶高度测试值与理论值差值图
Figure 9. Step height difference between the measured values and their theoretical values
图 10 使用FFT算法时高度误差影响下的实际光谱和理想复原光谱对比图
Figure 10. Comparison of the ideal spectrum and the actual spectrum using the FFT algorithm with height error
图 11 使用LSC算法时高度误差影响下的实际光谱和理想复原光谱对比图
Figure 11. Comparison of the ideal spectrum and the actual spectrum using the LSC algorithm with height error
图 12 静态空间调制FTIR原理样机.1光源; 2准直系统; 3样品池; 4栅格分束板和栅格补偿板; 5高阶梯多级微反射镜; 6低阶梯多级微反射镜; 7缩束系统; 8红外探测器
Figure 12. Static spatial modulation FTIR prototype. 1 light source; 2 collimating system; 3 sample chamber; 4 beam splitter and compensation plate; 5 high-stepped micromirror; 6 low-stepped micromirror; 7.constricting system; 8 infrared detector array
图 13 干涉图(a)加入样品前; (b)加入样品后
Figure 13. Interferogram (a)without the sample; (b) with the sample
表 1 Statistical test results of step height
Table 1. Statistical test results of step height
Step Height Value Maximum value/ Highest positive deviation 640/15 Minimum value/Lowest negative value 618/-7 Average value/Average deviation 626.9/1.9 Standard deviation 7.4 
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