弹光调制测椭偏参量的数字锁相数据处理

为了实现椭偏参量ψ和Δ的高速、高灵敏测量,建立了一种测量成本低、重复性好和便于工业化集成的椭偏测量方案。本文对弹光调制型椭偏参量测量系统进行了原理分析,针对弹光调制器的工作模式及调制光信号特点,设计了基于现场可编程门阵列的数字锁相数据处理方案。现场可编程门阵列提供弹光调制器工作的信号源,并控制AD采样;同时产生正弦和余弦参考序列,并完成直流项、一倍频项和二倍频项的同相分量和正交分量的提取,进而求解出椭偏参量。利用搭建的试验系统对SiO2薄膜厚度为3.753nm的硅片样品进行了实验分析。实验结果表明,采样时间为20ms时,椭偏参量ψ和Δ的平均值分别为9.622°和168.692°,标准偏差分别为0.005°和0.008°;采样时间设置为200ms时,椭偏参量测量平均值与20ms的非常接近,标准偏差减小,并且都在0.001°量级,揭示了本系统具有较高的灵敏度和较好的重复性。

Digital phase-locked data processing for ellipsometric parameter measurements based on photoelastic modulation

A system based on photoelastic modulation was analyzed in principle to realize high-speed, high-sensitivity measurements of ellipsometric parameters (ψ and △), thereby establishing a low-cost ellipsometric measurement scheme characterized by good repeatability and easy integration. On the basis of the Photoelastic Modulator (PEM) working mode and characteristics of modulated optical signals, a digital phase-locked data-processing scheme, based on Field-programmable Gate Array (FPGA) was designed. FPGA provides a signal source to drive PEM and controls Analog to Digital (AD) sampling. Meanwhile, sine and cosine reference sequences were generated, and the DC term as well as in-phase and quadrature components of the first and second harmonics were extracted. Subsequently, values of ellipsometric parameters are solved. The proposed system was used to analyze an SiO₂ film sample measuring 3.753 nm thick placed on a silicon wafer. Experimental results yielded average values of the ellipsometric parameters ψ and △ to be 9.622 and 168.692, respectively, with corresponding standard deviations of 0.005° and 0.008°, respectively. The sampling time is set to 20 ms. When the sampling time is increased to 200 ms, corresponding average values of ellipsometric parameters did not demonstrate much change from those observed in the 20 ms case; corresponding standard deviations were observed to be much smaller assuming values of the order of 0.001° for both parameters. These results confirmed high sensitivity and good repeatability of the proposed system.