Advanced metrology tools for assessing micro-optical lenses with structured surface
Y.Yu1#, D.J.Whitehouse2,1
The 5th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2013)
All accepted papers will be published in a journal of Key Engineering Materials (EI)
KEYWORDS : Structured surface, Fresnel lens, MOD lens, metrology, high aspect ratio, Interferometry
The increasing use of ultra-precision miniature structured surfaces in many applications such as automotive, optics, solar, biomedical, MEMS/NEMS and many other industries, has resulted in a critical need for highly refined inspection techniques for quality control. This has placed a great emphasis on the characterizing and subsequent measurement of such surfaces. This is not straightforward. The geometric structure on the surfaces are designed to fulfil a specific function which now requires extreme control because as the size of device reduces the structure on the surfaces, rather than the shape and size of the device, plays a more and more important role.
Some traditional measurement techniques such as stylus contact profilometry are no longer suitable for the measurement of structured surfaces having complicated factors such as high aspect ratio structures and soft surfaces because the components are small size and the contact pressure of the stylus is high. Advanced metrology tools are urgently needed to control the quality of the structured surface components. At present, it is difficult for metrologists to achieve such control. However, this paper describes some new instruments with which metrologists can now address such problems.
As examples, the assessments of the surface features of a miniaturized Fresnel lens and a MOD (Multi Order Diffractive) lens are presented. The micro-lenses were measured by a non-contact Coherence Correlation Interferometry (CCI). Also, the results for the MOD lens were compared with the results obtained from a stylus Phase Grating Interferometry (PGI). The measurement results show that the CCI can be used to characterize the key features (or parameters) such as the teeth angles, peak and valley radius and ring diameters, for the Fresnel lenses, which are not suited for contact measurement. It was also found that the CCI is a good metrology tool to study the micro-MOD lenses because it is a fast, accurate, non-contact and hence non-destructive measurement instrument. With on-going development of software and hardware the CCI will provide more details on the structured surfaces quickly and accurately.
The advanced contact Phase Grating Interferometry (PGI) instrument having sub-nanometre vertical resolution and sub-micron lateral resolution can provide detailed characterization of a wide range of components especially those with shallow and steep-sided optics but also including a comprehensive range of precision lenses and moulds. However, each technique has its limitations, which will be discussed in the paper. The correlation between the CCI and PGI will be presented and discussed in this paper.
Precision measurements of film THICKNESSES using scanning white light interferometry
Kevin Peng, Y Yu, M Conroy, D Mansfield
OMSEI 2011 Development Forum and Academic Conference Held in Chengdu, China
ABSTRACT
Accurate measurement of film thickness and its uniformity is very important to the performance of many surfaces and is critical for a large number of optical components. Effective inspection of the film thickness and uniformity is the key to high performance. Conventionally, film thicknesses are measured using a spectrophotometer / reflectometer, ellipsometer or a physical step measurement; however, these techniques all have limitations. Scanning white light interferometry (SWLI) is an established method to measure surface topography as this technique offers many advantages such as speed, ease of use and accuracy. A useful extension of SWLI is the ability to measure films thicker than 1.5 µm. Recent developments using a methodology known as a ‘helical complex field’ (HCF) function now allow film thicknesses to be measured down to ~25 nm. This new method, combined with Coherence Correlation Interferometry (CCI) offers film thickness measurements with sub-nanometre vertical resolution and ~1 µm lateral resolution. It is ideally suited for detailed analysis of coated optical surfaces. In this paper, the fundamentals of the techniques are described and recent results are presented.
Key Words:Film thickness, CCI, Thick film, HCF
Taylor Hobson Ltd, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
Characterization of optical components using contact and non-contact interferometry techniques-Advanced metrology for optical components
*Yang Yu, #Mike Conroy, #Richard Smith
Advanced Optical Manufacturing and Testing Technologies 2012 (SPIE)
ABSTRACT
Advanced metrology plays an important role in the research, production and quality control of optical components. With surface finish, form error and other parameter specifications becoming more stringent, precision measurements are increasingly demanded by optics manufacturers and users. The modern metrologist now has both contact and non-contact measurement solutions available and a combination of these techniques now provides a more detailed understanding of optical components. Phase Grating Interferometry (PGI) with sub-nanometre vertical resolution and sub-micron lateral resolution can provide detailed characterization of a wide range of components including shallow and steep-sided optics. PGI is ideal for precision form measurement of a comprehensive range of lenses, moulds and other spherical or aspheric products. Because of the complex nature of these components, especially precision aspheric and asphero-diffractive optics, control of the form is vital to ensure they perform correctly. Recent hardware and software developments now make it possible to gain a better understanding and control of the form and function of these optics. Another change is the use of high speed 3D non-contact measurement of optics which is becoming more popular. Often scanning interferometric techniques such as coherence correlation interferometry (CCI) can be used to study components not suited to 2D contact analysis, including fragile surfaces and structured surfaces. Scanning interferometry can also be used to measure film thickness and uniformity of any coating present. In this paper the use of both PGI and CCI to measure diffractive lenses and coatings is discussed.
Key Words: Metrology, Interferometry, Aspheric, Roughness, Coatings
*Taylor Hobson Ltd, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
#Taylor Hobson China, PartA 1st floor No.460 North Fute Road, Waigaoqiao, Free Trade Zone, 200131 Shanghai, China
Characterisation of laser scribes in thin film photovoltaics by Coherence Correlation Interferometry
B. Maniscalco1, P. Kaminski1, G. Claudio1, J.M. Walls1
Y. Yu2, D. Mansfield2
M.Crozier3, A.Brunton3
27th European Photovoltaic Solar Energy Conference (EU PVSEC) and Exhibition
ABSTRACT
In this paper we present results measuring the precise shape of laser scribes in thin film photovoltaics using Coherence Correlation Interferometry (CCI). Laser ablation is used in interconnect processes in all types of thin film devices, including those based on CdTe, CIGS and amorphous silicon. The work presented in this paper was focused on the use of a laser ablation process in the interconnect of thin film CdTe modules. This process is known as monolithic integration. The laser scribe measurements presented here were obtained on a nanometre scale using Coherence Correlation Interferometry. The Coherence Scanning Interferometry (CSI) technique is able to provide three and two dimensional topographical images of the sample surface, with an ultimate vertical resolution of 0.01nm. It provides two dimensional profiles of the laser ablated trenches, from which it is possible to extract quantitative information of the detailed shape of the scribe including precise measurements of both depth and width. In addition, the CCI is able to provide detailed analysis of surface roughness within the bottom of the trench which is important for efficient electrical contact.
Key words: CdTe, Laser Processing, Module Integration, Surface metrology
1Centre for Renewable Energy Systems Technology, (CREST), School of Electronic, Electrical and Systems Engineering, Loughborough University, Leicestershire, LE11 3TU, UK
2Taylor Hobson Ltd, AMETEK Ultra Precision Technologies, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
3 M-Solv Ltd, Oxonian Park, Langford Locks, Kidlington, OX5 1FP, UK
METROLOGY OF SILICON PHOTOVOLTAIC CELLS USING COHERENCE CORRELATION INTERFEROMETRY
B Maniscalco1, P M.Kaminski1, M Conroy2, D Mansfield2, Y Yu2
K Bass1, G Claudio1 and J M Walls1
THE 37th IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE
ABSTRACT
Surface metrology plays an important role in the development and manufacture of photovoltaic cells and modules. Coherence Correlation Interferometry (CCI) is a non-contacting surface metrology tool with potentially important applications in the characterization of photovoltaic devices. Its major advantages are that it is fast, non-destructive and it takes its data from a relatively large and hence representative area. A special mode, called “stitching x-y” can be used to provide information on a wider area, combining measurements taken in different zones of the sample. The technique is capable of providing surface roughness and step height measurements with sub-nanometre precision. It is also capable of measuring quantitatively surface texture and surface form in three dimensions and it now has a new capability to measure thin film and thick film thickness. CCI measurements are presented on a range of features on silicon photovoltaic cells including surface roughness, surface texture, the profile of laser grooves for buried contacts as well as the roughness and thickness of silicon nitride thin films. Complementary analysis using spectroscopic ellipsometry is also presented for verification. CCI is a sensitive, non-destructive metrology technique with potential use as an in-line quality assurance tool in the large scale production of photovoltaic modules.
1 CREST, Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK
2 Taylor Hobson, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
METROLOGY MEASUREMENTS OF THIN FILMS USED IN PHOTOVOLTAIC DEVICES BY COHERENCE CORRELATION INTERFEROMETRY
Y Yu1, D Mansfield1, M Conroy1, B Maniscalco2, P M Kaminski2, K Bass2, G Claudio2and J M Walls2
PHOTOVOLTAIC TECHNICAL CONFERENCE- THIN FILM & ADVANCED SOLUTIONS 2011
Conclusions and perspectives
The CCI Sunstar is a new metrology tool with potentially important applications in photovoltaic device research and quality assurance in solar module manufacturing. Metrology measurements are used to optimize device efficiency. The CCI technique provides a rapid and accurate three-dimensional analytical capability. The vertical resolution at 0.01nm is at the atomic scale. The lateral resolution is governed by the wavelength of light (~0.5μm) and by the NA of the objective lens and this will limit its suitability for some applications. A wide range of important metrology parameters can be measured using relatively simple analyses. These parameters include surface roughness, step height, surface form and surface texture. Its extension to measure accurately thin film thickness is important since it now allows the technique to address the range of thin films used in silicon, thin film and third generation photovoltaic devices.
1 Taylor Hobson Ltd, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
2 CREST, Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK
Precision measurements of PHOTORESIST film THICKNESSES using scanning white light interferometry
Y Yu, M Conroy, D Mansfield, R Smith
Taylor Hobson Ltd, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
ABSTRACT
Accurate measurement of photoresist film thickness and its uniformity is very important to the performance of all photoresist devices. Effective inspection of the film thickness and uniformity is the key to high performance of the devices. Conventionally, the film thicknesses are measured using a spectrophotometer/ reflectometer, an ellipsometer or a physical step measurement. Scanning white light interferometry (SWLI) is an established technique to measure the surface topography. This technique offers many advantages in measurement such as speed, ease of use and accuracy. A useful extension of SWLI is the ability to measure films larger than about 1.5 µm. The recently introduced ‘helical complex field’ (HCF) function now allows film thicknesses to be measured down to ~25 nm (index dependent). This new method combines novel Coherence Correlation Interferometry (CCI) with sub-nanometre vertical resolution and ~1 µm lateral resolution and can therefore be used for precision measurement of thin films. In this paper, the fundamentals of the techniques are described and some results from SiO2 and photoresist on Si substrate are presented.
Key words:Photoresist, Film thickness, CCI, Thick film, HCF
The distorted helix: thin film extraction from scanning white light interferometry
Daniel Mansfield
Proc. SPIE 6186, MEMS, MOEMS, and Micromachining II, 61860O (April 24, 2006); doi:10.1117/12.664036
Scanning white light interferometry (SWLI) is now an established technique for the measurement of surface topography. It has the capability of combining sub-nanometre interferometric resolution with a range limited only by the z-traverse, typically at least 100μm. A very useful extension to its capability is the ability to measure thin films on a local scale. For films with thicknesses in excess of ~2μm (depending on refractive index), the SWLI interaction with the film leads simply the formation of two localised fringe bunches, each corresponding to a surface interface. It is evidently relatively trivial to locate the positions of these two envelope maxima and therefore determine the film thickness, assuming the refractive index is known. For thin films (with thicknesses ~20nm to ~2μm, again depending on the index), the SWLI interaction leads to the formation of a single interference maxima. In this context, it is appropriate to describe the thin film structure in terms of optical admittances; it is this regime that is addressed through the introduction of a new function, the 'helical conjugate field' (HCF) function. This function may be considered as providing a 'signature' of the multilayer measured so that through optimization, the thin film multilayer may be determined on a local scale. Following the derivation of the HCF function, examples of extracted multilayer structures are presented. This is followed by a discussion of the limits of the approach.
Application of linear systems theory to characterise coherence scanning interferometry
Rahul Mandal1 Kanik Palodhi 2 Jeremy Coupland 1 Richard Leach2 Daniel Mansfield3
Proc. SPIE 8430, Optical Micro- and Nanometrology IV, 84300T (April 26, 2012); doi:10.1117/12.922435
This paper considers coherence scanning interferometry as a linear filtering operation that is characterised by a point spread function in the space domain or equivalently a transfer function in the frequency domain. The applicability of the theory is discussed and the effects of these functions on the measured interferograms, and their influence on the resulting surface measurements, are described. The practical characterisation of coherence scanning interferometers using a spherical reference artefact is then considered and a new method to compensate measurement errors, based on a modified inverse filter, is demonstrated.
1 University of Loughborough
2 NPL
3 Taylor Hobson Ltd
Extraction of film interface surfaces from scanning white light interferometry
Daniel Mansfield
Proc. SPIE 7101, Advances in Optical Thin Films III, 71010U (September 25, 2008); doi:10.1117/12.797978
In the field of scanning white light interferometry (SWLI), it is well known that films of optical thickness in excess of the coherence length may be measured by simply taking advantage of the fact that such films exhibit interference maxima corresponding to each interface. In fact for the majority of such 'thick' films the determined thickness has a DC error arising from the spectral phase-change on reflection at the two interfaces. For thinner films, the interference maxima coalesce and it was for this regime that the HCF (helical complex field) was previously introduced to allow thin film extraction. This work has now been significantly extended with a demonstrated capability to extract film interfaces with a lateral (XY) resolution of 1.25μm and a (Z) surface rms noise of ~0.75A (angstrom). It is also capable of covering both the thin and thick film regimes (from ~50nm to several microns, both limits being material dependant). Results are presented showing the performance of this approach, these include 'micro-scratches' that are apparent in the thickness of the deposited layers as well as substrate/film and film/air interfaces. These are compared to the original 'surface' as determined by SWLI and by AFM surface measurements. Additionally a brief comparison is made between film thickness determination using this approach, spectrophotometry, ellipsometry and stylus profilometry.
Thin film extraction from scanning white light interferometry
Daniel I. Mansfield
Scanning white light interferometry (SWLI) is now an established technique for the measurement of surface topography. It has the capability of combining sub-nanometre interferometric resolution with a range limited only by the z-traverse, typically at least 100µm. A very useful extension to its capability is the ability to measure thin films on a local scale. For films with thicknesses in excess of ~2µm (depending on refractive index), the SWLI interaction with the film leads simply the formation of two localised fringe bunches, each corresponding to a surface interface. It is evidently relatively trivial to locate the positions of these two envelope maxima and therefore determine the film thickness, assuming the refractive index is known. For thin films (with thicknesses ~20nm to ~2µm, again depending on the index), the SWLI interaction leads to the formation of a single interference maxima. In this context, it is appropriate to describe the thin film structure in terms of optical admittances; it is this regime that is addressed through the introduction of a new function, the ‘helical conjugate field’ (HCF) function. This function may be considered as providing a ‘signature’ of the multilayer measured so that through optimization, the thin film multilayer may be determined on a local scale. Following the definition of the HCF function, examples of extracted multilayer structures are presented. This is followed by a discussion of the minimum film thickness limit of the approach.
Phase grating interferometer gauge
D. Mansfield and I. K. Buehring (Taylor Hobson, Ltd.)
Surface Metrology - Novel Measuring Instruments/Oral/Abstract/1997 Spring
Taylor Hobson has recently launched the Phase Grating interferometer (PGI) gauge into the market. This metrological gauge, currently with a respective range and resolution of 10mm and 13nm, represents a profound change in gauging technology. The modus operandi of the PGI gauge is discussed together with a brief review of its performance.
用扫描白光干涉法精确测量薄膜厚度
彭传红, Y Yu, M Conroy, D Mansfield
摘要
精确测量薄膜厚度及其均匀性对于许多工程表面的性能以及光学元件的性能是非常重要的。对薄膜的厚度和均匀性进行有效的检测，同样也是被测元件能否取得高性能的关键。按照常规，薄膜厚度一般使用分光光度计/反射仪，椭偏仪或触针式计量仪器进行测量，然而，所有这些技术都有其局限性。扫描白光干涉仪（SWLI）是测量表面形貌的一个有效方法，这种技术拥有许多优点，如速度快、易于使用和测量准确性高。 SWLI还可以用来扩展到测量大于1.5微米厚的薄膜。随着技术的发展，通过使用一种称为‘螺旋复数领域’ (HCF)的函数, SWLI可以用来测量小至只有25 nm的薄膜厚度。这种新方法与本就具有亚纳米级垂直分辨率和亚微米级横向分辨率的相干相关干涉法(CCI)相结合，因而非常适合于镀膜光学表面的详细分析。在本文中，我们对技术的基本原理进行了阐述，并列出了最新的研究测试结果。
关键词:薄膜厚度，CCI，厚膜，HCF
Taylor Hobson Ltd, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK
Note: This paper was presented in Chinese.