椭圆偏振光谱是一种无损无接触的光学测量技术，基于测量线偏振光经过薄膜样品反射后偏振状态发生的改变，通过模型拟合后得到薄膜、界面和表面粗糙层的厚度以及光学性质等等，可测厚度范围为几埃至几十微米。而且，椭圆偏振光谱既能够实现在线监测又能进行原位测试，可根据各类不同的应用需要提供静态和动态测量模式，应用非常广泛。
Fig.1: Optical setup of the UVISEL serie.
椭圆偏振光谱仪可以测量椭偏角ψ 和 Δ 这两个可以描述线偏振光经过薄膜反射后的椭圆偏振光的状态变化的参数。ψ 和Δ 与菲涅尔反射系数相关，满足方程ρ=tanψeiΔ= rp/rs 。
测量得到ψ 和Δ 后，必须建立一个薄膜的模型来确定厚度或者光学常数。
通常，椭圆偏振光谱仪不会直接测量ψ 和 Δ，而是测试ψ 和 Δ 的函数。在相位调制椭圆偏振光谱仪中，例如 UVISEL 和 UVISEL 2，可以测试Is, Ic 和 Ic’, 它们是ψ 和 Δ 的函数，即Is = sin2ψ sin Δ, Ic = sin 2ψ cos Δ, 和 Ic’ = cos 2ψ。通过Is 和Ic 可以精确测量0° ~360°范围的Δ，而通过Is 和 Ic’可以精确测量0° ~90°范围ψ。
Ex-situ spectroscopic ellipsometry.
It is important to note that spectroscopic ellipsometry is an indirect technique which does not measure thin film thickness and/or optical properties directly. In order to determine thin film thickness and/or optical properties, a model-based approach must be used. Ex-situ spectroscopic ellipsometry allows for the characterization of a range of thin film properties including layer thickness, surface roughness thickness, interface thickness, optical constants, composition, band gap, composition, crystallinity, grading, anisotropy, and uniformity by depth and area. It can also be used to calculate the depolarization factor and the Mueller Matrix coefficients.
In-situ spectroscopic ellipsometry can be used to determine nucleation and growth parameters, precise optical properties without significant surface roughness or oxides, and film growth profiles.
It is important to note that spectroscopic ellipsometry is an indirect technique which does not measure thin film thickness and/or optical properties directly. In order to determine thin film thickness and/or optical properties, a model-based approach must be used. Besides thin film thickness and optical properties, in-situ spectroscopic ellipsometry can also be used to determine nucleation and growth parameters, precise optical properties without significant surface roughness or oxides, and film growth profiles.
Since ellipsometry is a model-based approach, it is helpful to know something about your sample (number of layers, materials, etc.). However, if little to nothing is known about the sample, it can still be studied with ellipsometry, as long as it is a simple single layer on a known substrate. Ellipsometry can also be used to determine the optical properties of the substrate if its material is unknown.
Polarization can be described as a superposition of two orthogonal waves.
Polarization is defined by the orientation and phase of the electric field vector. We can describe polarization as a superposition of two orthogonal waves. The most general state of polarization, known as elliptical, allows for an arbitrary phase difference and arbitrary relative amplitudes of the two orthogonal waves.
Circular polarization is a special case of elliptical polarization. To obtain circular polarization, the two orthogonal waves must be 90° out of phase and have equal amplitudes.
Two special cases of elliptical polarization are known as circular and linear polarization. To obtain circular polarization, the two orthogonal waves must be 90° out of phase and have equal amplitudes.
Linear polarization is a special case of elliptical polarization. To obtain linear polarization, the two orthogonal waves must be in phase but they may have arbitrary amplitudes.
To obtain linear polarization, the two orthogonal waves must be in phase but they may have arbitrary amplitudes.
Images taken from Fujiwara, H. “Spectroscopic Ellipsometry Principles and Applications,” John Wiley and Sons, 2007.
Optical properties are comprised of two components: the refractive index, and the extinction coefficient. The refractive index, denoted by n, is the ratio of the speed of light in a vacuum to the speed of light in the material. The extinction coefficient, denoted by k, is related to the absorption loss in the material. Together, these two components make up the complex refractive index, given by N = n-ik, which describes electromagnetic radiation’s interaction with materials (speed change and absorption loss).
