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多层复合膜设计的原位椭偏检测

In situ ellipsometric monitoring of complex multilayer designs 

Current developments in optical multilayer design and computation make it possible to calculate filters that satisfy the most demanding optical specifications. Some of the designs are highly sensitive to manufacturing errors and require accurate monitoring and control during thin film deposition. Ellipsometric monitoring enables the accurate deposition of any thickness, including very thin layers, and in situ measurement of both refractive index and thickness of the layers during deposition, which facilitate the subsequent real-time design reoptimisation. In this letter, a number of complex multilayer designs with the aid of ellipsometric monitoring are presented, including a laser notch plus band-blocker filter, dichroic filter, beamsplitter, and a wide-range broadband multiplayer antireflection coating.

  OCIS codes: 240.2130, 310.1860, 120.2130.

  doi: 10.3788/COL201008Sl.0044. 

In recent years, significant progress has been made in the field of multilayer optical coating design[1?5]. Nearly any optical filter specification can be designed theoretically, and a number of solutions have become available for very complex and challenging structures. Unfortunately, the required accuracy and reproducibility of the properties of a material for such structures are sometimes so high that mass production may be impossible, even in state-of-theart deposition systems. Hence, the accuracy of in situ monitoring and effective feedback control of the deposition process remain significant issues to be addressed.

   A number of well-established optical and non-optical techniques allow the monitoring and control of the layer deposition process[6]. The applicability of a particular method is dependent on the deposition technique and the design requirements. For a deposition process being able to maintain a constant rate and stability of the optical constants of the material, satisfactory results can be achieved by using a simple time-termination process. Deposition for a specific period is often adequate for the sputtering techniques when the required accuracy of film thickness is within a few nanometres[7?9]. However, in many coating methods, including evaporation, the material refractive index, and deposition rate resulting from simply timing the deposition are not sufficiently stable to produce precise optical filter coatings. Quartz crystal monitoring, another frequently used non-optical method for thickness monitoring, indirectly measures the deposition rate and film thickness. It is simple, easy to install, and relatively cheap. However, the random thickness errors of the crystal monitoring systems used in production are in the order of 4%[10], which makes it inadequate for the production of certain complex optical structures. Optical in situ monitoring can produce much better results since the monitoring is conducted using the optical parameters of the structure, which are more directly related to the end performance. Optical monitoring techniques can be subdivided into two major categories: photometric methods (e.g., reflection, transmission, and others) and polarisation-dependent methods (e.g., ellipsometry, polarimetry, and others). Many optical designs can be successfully produced using single or multi-wavelength photometric monitoring. Single wavelength optical monitoring is the most widely used production technique. Turning point[11] and level monitoring[12,13] are the popular single wavelength monitoring methods suitable for designs based on periodic quarterwave optical thickness (QWOT).

  Broadband transmittance and/or reflectance have been widely used for deposition monitoring of non-quarterwave designs over the last three decades[14]. The enhancement of the computational power of modern computers has boosted the practical application of photometric methods and has led to the increased capability in data processing and deposition control. For example, sub-nanometre accuracy in thickness control has been reported for single layer deposition[15]. Real-time reoptimisation based on broadband reflectance and transmittance data has been implemented for the manufacture of high-performance non-quarterwave designs[16]. Moreover, advances in broadband photometric monitoring have allowed nanometre-level thickness control in multilayer production.

   Ellipsometry is one of the most widely used polarisation-dependent methods. In situ ellipsometry has been successfully applied to a diverse range of applications in both research and industry. It is relatively simple, reliable, and provides highly accurate real-time measurements of both the thickness and refractive index of growing layers[17?20]. It also provides robust information for the reoptimisation of the design according to the measured properties of the deposited layer. In many cases, this can be accomplished without interrupting of the deposition process. Owing to its advantages, ellipsometry has grown in popularity. It has been extensively used in the study of optical thin film material properties and in the monitoring of multilayers with demanding specifications[21?26] . 

  Ellipsometry is a century-old technique, and its theory and applications are covered at length[17?20]. Progress in the automation of ellipsometric instruments started in the early 1960s and has accelerated significantly during the last decade. The introduction of fast and inexpensive computers has allowed the development of broadband spectroscopic ellipsometers with the latest commercial instruments, enabling their expansion into both the vacuum ultraviolet and mid-infrared ranges[27]. Currently, high-accuracy data can be acquired over broad spectral ranges within seconds[28]. Improvements in ellipsometric instrumentation have advanced research and industrial capabilities in traditional areas such as multilayer optical coatings, and have triggered the development of a new range of applications[19] .

   Ellipsometry has a number of advantages compared with conventional photometric monitoring. Unlike photometric monitoring, where only one value is obtained per measurement, ellipsometers measure two ellipsometric values, Ψ and ?. As a result, two parameters of a film can be simultaneously determined for each measurement point: refractive index n, and thickness d. In the case of a bare substrate, the optical constants n and extinction coefficient