Deliverable 10.3

ML² – Multi Layer Micro Lab

Online film inspection system

1. Introduction ......................................................................................................................................... 2

1.1. Quality parameters ..................................................................................................................... 2

1.2. Process restrictions .................................................................................................................... 3

2. Inspection approaches ........................................................................................................................ 3

2.1. Imaging ellipsometry .................................................................................................................. 3

2.2. Coherence scanning interferometry ........................................................................................... 6

2.3. Vision inspection/ Line-scan camera ....................................................................................... 10

3. Summary........................................................................................................................................... 12

4. ANNEX .............................................................................................................................................. 13

4.1. Imaging ellipsometry ................................................................................................................ 13

4.2. Phase shifting interferometry ................................................................................................... 14

4.3. Vision inspection ...................................................................................................................... 15

1. Introduction

High throughput manufacturing approaches like continuous inkjet printing or UV and thermal embossing are robust and stable processes. Influences causing systematical defects and resulting in rejections are normally identified and avoided within the start-up phase of a production. Their impact on high throughput manufacturing is reduced and therefore comparably low. Randomly occurring changes in the production environment like dust, vibrations, thermal shifts or changes done by the human machine operator can be seen as the major cause for a lack of quality or even rejections. As their origin is not to be foreseen or measured by sensors, they can’t be controlled.

A strategy to control and measure quality, reliability and productivity of the continuous production process is to inspect the process outcome. Constant monitoring and evaluation provides the basis for a closed-loop controlled process. Here direct and indirect characteristic of the materials can be monitored. Figure 1 shows polymer substrates with microfluidic and optical functions processed in continuous UV imprint.

Microfluidic features Beam shaping effect of optical features

Figure 1: UV embossed functional layers

Regarding the microfluidics direct features like geometrical shapes of the channels, thickness of the coating or the absence of dust and other defects (e.g. air bubbles) should be monitored constantly to ensure the product quality. Besides the direct features the function of an optical film can be monitored as an indirect film feature as well. In this case not only the requirements, but also the requested technical function can be controlled in-line.

1.1. Quality parameters

Parameters or defects which should be monitored constantly are:

UV-/ Thermal embossing: Printed electronics:

Coating thickness

Coating homogeneity/ wetting behaviour

Impurities/ dirt

Geometrical shapes

Cracks/ Delaminations

Shrinkage cavities/ bubbles …

Coating thickness as a function of web size, coating geometry and material output is predictable and can be monitored and controlled by the dispensing or coating system.

Wetting behavior and coating homogeneity are strongly resulting from the chosen material combination. Once this composition works in an acceptable manner, the inspection of these parameters is more or less obsolete.

Therefore the parameters mentioned last are not necessarily needed to be monitored in-line. The parameters named only related to direct feature measurements. Depending on the function of the substrate different indirect quality parameters (e.g. optical behavior) can be measured individually.

1.2. Process restrictions

The continuity of the roll-to-roll process and the targeted precision of the produced structures define the requirements to the inspection system. Following restrictions can be derived:

100% inspection possible

300 to 500 mm web width

0,2 to 10 m/min web speed

Low/ No contrast between functional structures and substrate (highly transparent materials)

High tolerance towards material/ machine vibrations

5 – 8 µm geometrical resolution

Furthermore it is preferred to have the inspection system (e.g. signal emitter and respondence receiver) on the same side of the substrate.

2. Inspection approaches

To identify an inspection system suitable for the ML² project purpose a market and technology screening has been performed. Three competitive technologies have been defined to be tested in detail. Devices using imaging ellipsometry and phase shifting interferometry from suppliers will be tested by exemplary samples (optical, fludic). Vision inspection systems using area and line-scan cameras will be qualified Fraunhofer IPT. The systems and their detection principle are introduced and explained within the next sections.

2.1. Imaging ellipsometry

Ellipsometry involves measuring the change in polarisation state of light when it reflects off a surface, and constitutes a very accurate technique for analyzing ultrathin layers. It is used to probe monolayer and multilayer thicknesses from a few angstroms to several tens of micrometers. A broad range of properties of materials is also accessible using this technique, such as optical constants, the optical gap, chemical composition, crystallinity, and the depth and surface uniformity of films (roughness, porosity, interface, index gradient, anisotropy, and so on).

Ellipsometry has been part of the industrial scene for a decade or so now and is exploited in various stages of production, from optimisation and validation of processes to quality control on the production line. Used in areas as varied as electronics, telecommunications, flat screens, and optical films, it is a powerful and versatile tool for applications in bio- and nanotechnologies as well.

As a non-destructive technique, its sensitivity is unequalled, with spot sizes as small as 10 µm. It is a modular technique, working in-situ and adapting easily to any type of reactor, where it can monitor growth or etching of films and carry out fast kinetic measurements.

Possibilities of this technique

In contrast to most optical methods, ellipsometry measures two quantities at the same time, viz., the modulus ||ρ|| = ||rp/rs|| = tanΨ and the phase Δ = δp − δs. This 2D feature is a key point, making this a highly sensitive technique. Ellipsometry provides a very accurate way of characterising thin films, surfaces, and interfaces. It delivers three types of information:

Thicknesses between a few angstroms and a few tens of micrometers. The sample can be a simple monolayer deposited on a substrate or a stack of complex multilayers. This highly sensitive technique can easily detect a native layer, an interface, or surface asperities. In the latter case, many papers refer to the very good correlation between AFM and ellipsometric measurements.

Optical constants:

– refractive index n,

– extinction coefficient k.

Properties of the material such as:

– The composition of type III-V or type II-VI semiconductor alloys.

– Microstructure, e.g., density or porosity of the layer, with the possibility of estimating the empty volume within a porous layer.

– Crystallinity, with the most representative example being silicon, whose three types (crystalline, semi-crystalline, or amorphous) possess widely different optical constants.

– The optical gap Eg.

– The homogeneity of surface or buried layers. Films produced by deposition, synthesis, annealing, etc., are often inhomogeneous. Ellipsometry can characterise inhomogeneities via index gradients, anisotropy, or depolarisation phenomena due for example to excessive roughness.

Figure 2: Optical setup in an ellipsometer

The main advantages of ellipsometry are:

it is non-invasive,

it is highly sensitive, down to a few atomic monolayers,

it can be used to control growth or etching in real time.

Ellipsometer Configurations

Three main ellipsometer technologies are available on the market. All three are based on an optical arrangement like the one shown in Figure 2 with a light source, polariser, sample, analyser, and detector.

The idea of ellipsometry is to send a light beam of known polarization (after passing through a polariser) at oblique incidence onto the sample, and then to analyse the polarisation of the reflected beam (by passing it through an analyser). Recall that the change in polarisation is due to the interaction with the sample, and it is through this that one can reconstitute its properties. Different elements can be adjoined to the setup, e.g., modulator, compensator, and this greatly contributes to the sensitivity of ellipsometric analysis. Apart from these differences in the general configuration, there are two main families of ellipsometer:

Laser Ellipsometer. This measures the parameters Ψ and Δ at a single wavelength, generally that of the helium–neon laser at 632.8 nm. For two measured parameters, two quantities can be determined, such as the thickness and the refractive index of a transparent monolayer deposited on a known substrate. These ellipsometers have the advantage of being highly accurate, but they are limited to very simple applications.

Spectroscopic Ellipsometer. This uses a white light source, whence it is possible to cover a broad spectral range, from the far ultraviolet (FUV) to the near infrared (NIR), i.e., typically from 190 nm to 2,000 nm. The ellipsometric angles Ψ and Δ are measured at each wavelength, so complex structures can be characterised, e.g., stacks of multilayers, inhomogeneities in materials, etc.

Spectroscopic ellipsometers include a wavelength selection system which can be one of two types, either a monochromator carrying out sequential acquisition, wavelength by wavelength, or a CCD carrying out simultaneous acquisition.

1. Nulling Ellipsometer

These ellipsometers exploit extinction of the signal to determine the ellipsometric angles Ψ and Δ. They use a polariser followed by a compensator (usually a quarter-wave plate), which transform the linear polarisation into an elliptical polarisation. The role of the compensator is to cancel the delay introduced by reflection from the sample, in such a way as to make the polarization linear once again. The compensator thus plays a symmetrical role with respect to the sample. Successive adjustment of the polariser and the analyser leads to extinction of the signal.

This classic technique is very accurate, but measurement is slow and difficult to automate. For this reason, these ellipsometers have been supplanted by modulation ellipsometers, which extract information from the changing intensity at the detector.

2. Rotating Element Ellipsometer

The polarisation can be modulated in three different ways: rotating the polariser, the analyser, or the compensator. Ellipsometers with rotating polarisers and analysers have been around for at least fifteen years now. They are well-suited to spectroscopic studies, since the response of all the elements, apart from the sample surface, is independent of the wavelength. Furthermore, these systems are sensitive to the residual polarisation of the source or the detector, but also to inhomogeneities in the polariser/analyser due to rotation of the beam. These imperfections can be reduced to some extent by calibrating the system, but they can give rise to significant errors. The rotation frequency is of the order of a few hundred hertz.

By calculating the ellipsometric angles for these two types of system, one obtains tan Ψ and cosΔ. There remains an indeterminacy with regard to the sign of Δ, which is only known up to addition of 180°. The accuracy in Δ is therefore poor in regions where Δ is equal to 0° or 180°,

which correspond to applications with a transparent substrate, e.g., glass or plastics. Finally, Δ is the most sensitive parameter to small variations, such as ultrathin films or films with small index contrast. These two technologies are therefore inaccurate for this type of advanced application.

A rotating compensator ellipsometer comprises the same elements as the nulling ellipsometer just described, except that the quarter-wave plate is motorised, generally rotating at a few hundred hertz. This type of system overcomes all the polarisation constraints in the source and detector, but requires spectral calibration of the compensator, a source of systematic error in the measurement. The signal equation of these ellipsometers delivers tanΔ and tan Ψ, so the ellipsometric angles are unambiguously defined.

3. Phase Modulation Ellipsometer

The optical setup includes a photoelastic modulator placed before the analyser. All elements are fixed, and the polarisation is modulated by a birefringent modulator. This type of ellipsometer does not therefore require any special characteristics with regard to the polarisation for the source or the detector. Measurements can be made over a broad spectral range and at a high acquisition rate. The use of a photoelastic modulator does not involve very accurate alignment (no rotating elements). However, this technology does need high-performance electronics, able to carry out acquisition and analysis of harmonics of order 0, 1, and 2 in a signal modulated at a frequency of 50 kHz. Moreover, the modulator is a chromatic element that must be calibrated with respect to the wavelength.

This ellipsometer measures tanΔ and sin 2Ψ. By using two measurement arrangements (rotating the modulator), the ellipsometric angles are unambiguously defined, since the combination of two measurements yields tanΔ and tan Ψ.

The ML² solution

For ML² project a phase modulation ellipsometer is chosen. It’s an monochromatic in-line imaging system without any moving parts. It has been qualified for a line-length (web width) of 250 mm, but is adaptable to higher length. It’s developed by the Vienna University of Technology in cooperation with the German company ACCURION. A conference poster/ datasheet of the system can be found in the annex (see 4.1).

References:

Altschuh, D.; Ricard-Blum, S.; Ball, V.; Gaillet, M.; Schaaf, P.; Senger, B.; Desbat, B.; Lavalle, P.; Legrand, J.-F.: Surface Methods. In: Nanoscience. Boisseau, P.; et al. (eds.). 2010 Springer, DOI: 10.1007/978-3-540-88633-4_9, pp. 477 - 594

Bammer, F.; Huemer, F.; Jamalieh, M.: Ellipsometry-Report. Vienna University of Technology. 2015 (product presentation)

2.2. Coherence scanning interferometry

In the field of surface inspection by interferometry there are two complementary or even competitive approaches: phase shifting interferometry and coherence scanning interferometry.

Phase shifting interferometry (PSI) is a well-established technique for areal surface characterisation that relies on digitisation of interference data acquired during a controlled phase shift, most often introduced by controlled mechanical oscillation of an interference objective. The technique provides full 3D images with typical height measurement repeatability of less than 1 nm independent of field size. Microscopes for interferometry employ a range of specialised interference objectives for roughness and microscopic form measurement.

Height-dependent variations in fringe visibility related to optical coherence in an interference microscope provide a powerful, non-contact sensing mechanism for 3D measurement and surface characterisation. Coherence scanning interferometry (CSI) extends interferometric

techniques to surfaces that are complex in terms of roughness, steps, discontinuities, and structure such as transparent films. Additional benefits include the equivalent of an autofocus at every point in the field of view and suppression of spurious interference from scattered light.

Concept and Overview

Coherence scanning interferometry (CSI), which in addition to, or as a substitute for the detection of phase (PSI), evaluates changes in interference signal strength related to optical coherence. In the simplest conceptualisation, surface heights are inferred by noting where the interference effect is strongest. Thus one defining feature of CSI instruments, with respect to PSI instruments, is that by design, interference fringes are only strongly observed over a narrow surface height range. Figure 3 (left) shows how the appearance of the interference fringes varies as an interference objective is scanned vertically in the figure along its optical axis. A visual interpretation leads to the conclusion that the outside edge of the sample must be lower than the centre, as is evident from the scan position ζ being lower for high contrast fringes at the edge as compared to those at the centre. Although modern instruments employ a variety of optical configurations and data processing methods to extract surface data, a height dependent variation in fringe contrast is a common behaviour of all CSI instruments. CSI instruments are automated systems with electronic data acquisition that provide a signal for each image pixel as a function of scan position. The resulting signals appear as in Figure 3 (right), which shows the signal itself and an overall modulation envelope with a peak position that at least conceptually provides a non-contact optical measurement of surface height.

Images of interference fringes on a curved surface with low coherence illumination

CSI signal for a single pixel showing the modulation envelope

Figure 3: CSI principle

CSI has evolved to become the dominant technique in interference microscopy and development continues. When compared to phase shifting methods, CSI has a superior tolerance for variations in surface texture and greater capability for measuring surface features and structures such as step heights. An additional benefit of CSI is that data for every image pixel are gathered at the accurate point of best focus for that pixel. CSI maintains the basic interferometry advantage of subnanometre vertical resolution regardless of the numerical aperture or field of view of the microscope. The technique continues to develop in capability, performance and flexibility, including advanced methods for transparent film and other surface structure analyses.

Typical Configurations of CSI

Figure 4 illustrates a generic CSI instrument. The object has height features h that vary over the object surface. A mechanical scanner provides a smooth, continuous scan of the interference objective in the z direction. During the scan, a computer records intensity data I for each image point or pixel in successive camera frames. Light sources for CSI are incoherent with a broadband spectrum (white light), spatial extent, or both. A classic example is an incandescent lamp such as a tungsten halogen bulb, while currently the most common source is a white-light LED. The Köhler illumination optics images the light source into the pupil of an interference objective. The aperture stop controls the numerical aperture of the illumination, while the field stop controls the illuminated area of the object surface. Most often, the illumination fills the pupil to minimise spatial coherence and maximise lateral resolution. For dynamically moving objects, the light source may be flashed stroboscopically to freeze the object motion (Nakano 1995, Novak and Schurig 2004).

Components of a microscope suitable for CSI

Path of a single ray bundle through a CSI instrument

Figure 4: Geometry of an interference microscope suitable for CSI

CSI instruments, just as PSI microscopes, are most often configured in a manner similar to a conventional microscope with the normal objective replaced by a twobeam interference objective; although a few systems have a Twyman-Green geometry (Dresel et al. 1992, de Groot et al. 2002a). Interference objectives are most commonly of the Michelson, Mirau, or Linnik type.

The interference objective should be compatible with a low coherence light source. The two interferometer paths should be balanced for dispersion of refractive index with wavelength, and the position of best focus should be coincident with the position of zero optical path difference. For some applications, a non-flat reference surface may be used that better matches the sample shape (Biegen 1990). A scanner moves the interference objective or the objective turret; in other cases an actuator moves the object moves. Although less common, it is feasible to move the reference mirror, beam-splitter or some combination of optical elements within the objective in place of translating the entire interference objective (Colonna de Lega 2004). Generally, the scan motion is along the optical axis of the objective perpendicular to the sample surface, i.e., in the direction, shown in Figure 4. The scan length is typically between 10 μm and 200 μm for piezo-electric scanners, and several millimetres for motorized scanners.

Interferometer Design

Modern interference microscopes have evolved to instrument platforms recognizable as refined versions of a conventional microscope, configured for digital data acquisition and low vibration, combined with one or more removable interference objectives that take the place of the

conventional imaging microscope objective. This basic design provides flexibility and convenience, allowing the use of a turret of interference objectives of varying magnification and interferometer design depending on the needs of the application.

Figure 5 illustrates the optical configuration of a microscope for interchangeable interference objectives, using a common interferometer design derived from the modified Michelson geometry.

Figure 5: Inteferometer for areal surface measurement employing a Michelson-type interference objective

Within the objective, a beam-splitter placed between the imaging optics and the sample establishes the reference and measurement paths. The beam-splitter is most often a prism. Variants of the basic design include those with fixed or adjustable dispersion compensation for use with samples having a transparent glass cover (Hedley 2001, Han 2006).

The system magnification is determined by the ratio of the tube lens focal length to the focal length of the objective. The objective magnification is defined in terms of a nominal unity-magnification tube lens focal length, which varies between 160 mm and 200 mm, depending on the manufacturer. Thus a 10x objective for a defined 200 mm tube lens has a focal length of 20 mm.

The working distance from the objective housing to the object is a function of the design of the objective, and relates to factors such as interferometer geometry, focal length, mechanical structure and lens design. A large working distance is generally preferred and can be a deciding factor in the choice of interferometer geometry.

At higher magnifications, usually for 20x and above, there is insufficient working distance to accommodate a large beam-splitter between the objective lens and the sample and the Michelson geometry is no longer practical. Figure 6 (left) shows a Mirau objective with a parallel beam-splitter and reference surface, both aligned with the optical axis of the imaging lenses (Mirau 1952). The reference is a small reflective disk, somewhat larger in diameter than the field of view of the objective, and typically comprised of a coating on an otherwise transparent supporting plate of identical thickness to the beam-splitter. The design inherently relies on a sufficiently large numerical aperture to allow at least a portion of the illuminating and imaging rays to pass around the reflecting reference disk, often referred to as the central obscuration. The Mirau is by far the most common high magnification interference objective in use today.

Figure 6: Mirau interference objective (left) and Linnik interference objective

For very high magnifications of 100x and above, the presence of even the thin beam-splitter plate in the Mirau becomes awkward enough to require the movement of the beam-splitter to above the lenses, as shown in the objective of Figure 6 (right) (Linnik 1938). For the Linnik objective the imaging optics are included directly in the measurement and reference paths, similarly to Figure 6 (right). This makes the Linnik objective more challenging to design and to manufacture because the separate imaging objectives ideally must perfectly match in terms of aberrations and dispersion characteristics, which is much less an issue with the Michelson and Mirau designs. This complication is balanced by superior working distance and greater achievable numerical aperture at high magnification. In practice, the Michelson is the preferred geometry for low magnifications of 10x and lower. The Mirau is common for 20x to 100x, while the Linnik is sometimes used at 50x and 100x.

The ML² solution

To measure the printed and embossed structures produced in ML² consortium a system of Swiss company Heliotis is tested. The heliProfiler

TM M3-XL (see 4.2) makes use of a patented

smart pixel sensor for parallel optical coherence tomography with in-pixel signal processing. The architecture of the installed interferometer is of Mirau type. The system is capable of processing up to 1 million 2D-slices per second. An additional 2D-camera enables live-view mode for navigation on sample.

References:

De Groot, P.: Coherence Scanning Interferometry. In: Optical Measurement of Surface Topography. Leach, R. (ed.). 2011 Springer, DOI: 10.1007/978-3-642-12012-1, pp. 187 – 208

Heliotis. Advancing 3D metrology: heliProfiler M3/ M3-XL – Efficient 3D Measurement. 2015 (URL: http://www. heliotis.ch/html/m3xl.htm; date: 30.10.2015)

2.3. Vision inspection/ Line-scan camera

In scenarios where moving, continuous material needs to be inspected for faults, line-scan cameras will generally provide a better solution than traditional area scan cameras. Line-scan cameras offer an inexpensive method of generating high-resolution images and making them available to the software for evaluation on standard PC platforms whose performance is increasing all the time. Figure 7 depicts the basic working principle of vision inspection of large area surfaces with line-scan and area cameras. The major advantage of line-scanning is covering a wider field of interest on a moving substrate. Stitching algorithms synchronized with the web speed provide 2D images as commonly known for the conventional area cameras.

Working principle line-scan vs. area camera Schematic structure of a line-scan camera application with potential

error locations indicated.

Figure 7: Vision inspection principles

Concept and overview

The traditional fields of application for line-scan cameras include situations where continuous materials need to be analysed (see Figure 7).

The reason why line-scan cameras are more suitable than area scan cameras for such continuous processes is explained by the fundamentally different design of the two technologies. Depending on the camera type, area scan cameras deliver a fixed (synchronous) or variable (asynchronous) sequence of images of a moving object. In practice, uninterrupted capture of continuous materials is achieved by capturing overlapping images. When this has been done, software is required to painstakingly crop the individual images, eliminate distortion and assemble the images in the correct sequence.

Line-scan cameras, on the other hand, only have a single row of light- sensitive pixels, which constantly scan moving objects at a high line frequency. The resolution of line cameras varies between 512 and 12,888 pixels. Sensors with pixel edge lengths of 7 µm, 10 µm and 14 µm are commonly available on the market. In the case of very high resolutions, smaller pixels are used in the design of the sensor in order to take the optics into consideration, since a sensor with 8000 pixels and an edge length of 10 µm, for instance, could only be “exposed" without distortion by a lens with an image diameter of at least 8 cm.

In operation, the charges of the individual pixels are read into a horizontal register arranged in parallel and converted pixel by pixel to digital values. These can then be stored and processed by the PC. To illustrate this point, if a line-scan camera were to observe a motionless object and was operated at a constant line rate of 1 kHz, 1,000 lines of a PC monitor would be filled with lines of identical greyscale information in 1 second.

This example makes it clear that a 2D image of a flat object can only be generated with a line-scan camera if the object moves under the camera at a sensible speed. Of course, it is also possible to move the camera across the object., The line rate of the camera must be synchronised to the speed of a moving object if the same resolution in the direction of travel (Y) is to be achieved, as the resolution across the object width. If this is not the case, and the line rate is fixed while the object speed varies, the object image on the monitor or in image memory is elongated or compressed. However, the speed of conveyors or the positioning equipment is often subject to load changes and acceleration or braking, which means that they rarely move at exactly the same speed. This in turn means that it is not generally possible to work with a fixed line rate. The hardware must provide a method of adjusting the line rate to match the current

speed of the material under inspection, as only then can a precise, meaningful 2D analysis of the image be made, using software algorithms.

In practice, this is usually implemented using an incremental encoder coupled to the drive unit. Of course, this feedback should be generated at a position where the minimum of slippage is expected with respect to the object. This issue needs to be considered on a case-by-case basis. The pulses generated by the encoder are then passed to the frame grabber and conditioned for the required resolution in the direction of travel using adjustable frequency scalers, before being passed to the line-scan camera in order to trigger the illumination.

The use of appropriate timing signals allows the integration time of the sensor to be kept constant. The setting is made on the frame grabber and is geared to the maximum expected line frequency.

The ML² solution

Fraunhofer IPT will use both approaches, area and line-scan camera, with the ML² production equipment. Mentioned in the first section, direct and indirect properties of the printed and embossed structures can be inspected to control the process quality. Therefore the area camera approach will be used to monitor optical functions of the film, in particular the beam shaping properties. The line-scan camera approach is used to inspect the geometrical features and the replication quality directly. The setup of the systems is depicted in Figure 8.

Area camera Line-scan camera

Figure 8: Setups for machine integration

Area and line-scan camera have been selected from the product portfolio of Stemmer Imaging (see 4.3). This also includes illumination units, PC hardware (framegrabber) and lenses suitable for the inspection process requirements.

References:

Demant, C.; Streicher-Abel, B.; Springhoff, A.: Industrielle Bildverarbeitung. Wie optische Bildverarbeitung wirklich funktioniert. 3. Akt. Auflage. Springer, Heidelberg. 2015 (978-3-642-13096-0)

Stemmer Imaging: The Imaging & Vision Handbook. 2015 (URL: http://www.stemmer-imaging.co.uk/en/the-imaging-vision-handbook/; date: 30.10.2015)

3. Summary

The tasks and requirements of online film inspection within ML² project have been introduced. Three different measuring technologies/ approaches have been presented. Their technologies have been explained and a selection of market available devices has been given. Results will be shown in deliverable 11.7 “Demonstration of inspection system”.

4. ANNEX

4.1. Imaging ellipsometry

4.2. Phase shifting interferometry

4.3. Vision inspection