应力变化对多层薄膜窄带滤光片透射光谱的影响

Influences of stress changes on transmission characteristics of multilayer thin-film narrow bandpass filters

利用薄膜应力公式和弹性力学的小挠度弯曲理论分析了在应力变化情况下,多层薄膜应变与膜厚变化的关系,并建立其数学模型,提出多层薄膜各层厚度变化的不均匀性理论。利用这个模型在应力减小200MPa条件下对138层4腔的100GHz滤光片光谱进行了模拟,与设计值比较发现中心波长增加了0.562nm;0.5dB处的带宽比设计值减小0.124nm;25dB处的带宽比设计值减小0.01nm;谱线矩形化变差;插损也有明显的增加。说明了应力变化引起的光学薄膜厚度变化的不均匀性是引起这种窄带干涉滤光片光谱退化的主要原因之一。实验结果为100GHz的窄带滤光片热处理后中心波长增加0.325nm;0.5dB处带宽减少0.01nm;20dB处带宽增加0.014nm;插损增加,谱线通带变形严重,纹波增大,光谱退化了。实验结果与理论分析一致。

On the base of film stress formula and micro-bending principle of elasticity, the relation between multilayer film strain and film thickness was analyzed. A mathematic model was established, and a new theory that thickness of multilayer film changes disuniform was brought forward. By using this model, the spectrum of 100 GHz filter with 138 layers and 4 cavities was simulated when the stress decreased by 200 MPa. Compared with the designed value, it was found that central wavelength increased by 0.562 nm, bandwidth decreased by 0.124 nm at 0.5 dB and 0.01 nm at 25 dB, rectangular spectrum deformed, and insert loss increased apparently. These demonstrated that uneven film thickness caused by stress change was the main reason that caused spectrum of narrow-band interference filter to degrade. Experimental results were that central wavelength of 100 GHz narrow-band filter increased by 0.325 nm after heat treatment, bandwidth decreased by 0.01 nm at 0.5 dB and 0.014 nm at 20 dB, insert loss increased, spectrum of pass band deformed badly, and ripple enlarged. The experimental result conforms to the theory analysis.