Metrology challenges for highly parallel micro

Metrology challenges for highly parallel micro-manufacture Richard Leach1, Christopher Jones1, Ben Sherlock1, Adam Krysińscki1,2 1 2

Engineering Measurment Division, National Physical Laboratory, UK Centre for Microsystems and Photonics, University of Strathclyde, UK

Abstract A large range of high-value manufactured parts require structures to be produced over large areas (metres squared) to high resolution (micrometres and below). Examples include the structures for photo-voltaic cells and touch-screen plastic electronics, both of which are manufactured on large polymer substrates in a roll-to-roll process. Such parts present significant metrology challenges due to the high dynamic range of surface topography that needs to be measured. It is relatively simple to measure surface topography over large areas to low resolution (essentially form measurement), or over small areas to high resolution (texture measurement), but the combination leads to very long measurement times and large amounts of data. Also, the type of structures varies significantly, examples being repetitive structures such as micro-optical arrays, or randomly situated defects in large sheets of high-quality paper. To add to these challenges, many measurements need to be performed very quickly and on-line. These metrology challenges will be described along with some ideas of which directions to go to solve them. Keywords: roll-to-roll, high dynamic range, surface topography, defects 1.

Introduction

A clear trend in modern advanced manufacturing is to produce parts in a highly parallel fashion. Many structures are being produced on large area substrates (up to metres squared and larger) but with very high feature resolutions (down to micrometres and beyond). This form of “web-based” manufacturing (also called roll-to-roll manufacturing) throws up a whole range of metrology challenges. For example, the measurement of tiny defects during the manufacture of flat sheets of glass or the measurement of deviations from nominal in laser machined lines for plastic electronics. Further examples come from the trend to use deterministic surface structuring to produce functional surfaces. The following is a list of a small number of product examples: modern projectors, plastic PV sheets, highquality paper and packaging, touch-screen devices, back-lighters, vehicle HUDs, displays and HDI electronics. Figure 1 shows a drum diamond turning operation to produce a master stamp for micro-optics – the manufacture of this master takes several days and, because of a lack of metrology solutions to measure the structures, the masters are essentially “sold as seen”. Vision-based metrology solutions go some way to addressing the quality control issues with, what we shall call, “high dynamic range manufacturing” (see figure 2), but what is needed is the ability to measure three-dimensional (3D) structure over metres squared of area to very high spatial resolution. To make matters

worse, such metrology solutions need to be fast – webs may be travelling along the production line with speeds of several metres per second.

Fig. 1. A diamond turning application producing a master stamp for micro-optics sheets. The drum length is 1.5 m. Courtesy of Cranfield University.

Several advances have been made in the field of surface topography measurement over the last three decades, from the development of optical instrumentation, which is now a fully-fledged rival to contacting techniques, to the development of specification standards for areal topography. However, there are two distinct classes of instrument: 1. those

that measure over large areas (metres squared) with tens to hundreds of micrometres spatial resolution (for example, fringe projection, photogrammetry and Moiré interferometry) [1], and 2. those that measure over small areas (up to a few millimetres squared) with spatial resolutions of the order of a micrometre (for example, coherence scanning interferometry (CSI), confocal microscopy and focus variation microscopy) [2]. Essentially, the former class is camera-limited, and the latter is objective-limited. There have been several attempts to try and combine the two classes (see for example [3, 4] and discussion below), but more progress is required before such hybrids can be used in high dynamic range manufacturing. In this paper, we will attempt to describe some of the areas of research that require further attention if we are to satisfy the metrology requirements for high dynamic range manufacturing. Whilst some of the research being carried out at the National Physical Laboratory (NPL) will be briefly described, the paper will concentrate on generic issues that need to be solved by the community.

Fig. 2. Space representing the challenges of high dynamic range metrology.

2. Scanning technology The “brute force” approach to measuring over an area at high spatial resolution is to scan an optical spot over the area in a raster fashion, thus, either measuring height at given (x, y) co-ordinates or measuring a series of areal maps, which are subsequently stitched together. To build up an areal map usually requires a physical scan though focus at each (x, y) co-ordinate or for each areal map. In the point scanning case, the optical instrument acts analogously to a stylus instrument, but often requires a scan through focus to obtain the height data at each (x, y) co-ordinate, for example confocal and point autofocus instruments [2]. Triangulation instruments do not require an axial scan, but for many applications do not have the required axial resolution. Chromatic confocal instruments are potentially very fast, as they do not require a physical axial scan, but

current systems are limited to scanning frequencies of around 70 kHz and their performance suffers as their scan speed increases [5]. All current point scanning instruments are far too slow for many high dynamic range applications that require fast, large area scanning. Line-scanning technology is being developed (see for example [6, 7] and it is feasible to use banks of parallel sensors, but the cost of such systems soon becomes prohibitive. A further alternative is the use of optical scanning, i.e. moving the beam across the surface without the need for physical xy scanning, but speeds are still limited (for example, [8]). In the case of areal sensors, for example CSI, digital holographic microscopy (DHM) and focus variation microscopy [2], there is a trade-off required between the beam coverage area and the numerical aperture (NA) of the objective system. For example, to push the size of the coverage area to a few millimetres squared, NAs of less than 0.3 are used, and the lateral resolution and maximum measureable slope angle tolerance suffer. DHM systems have been shown to scan at several hundred kilohertz [1, 9], but they require multiple wavelength sources to cover the axial ranges needed for many high dynamic range applications. Wavelength scanning techniques are being developed [8] to circumnavigate the need for axial scanning through focus, but the sampling speeds are still not high enough for many high dynamic range applications. Ptychography [10], or lens-less imaging, is a potential candidate technique for high dynamic range sensing, but it has yet to be demonstrated for such high-speed applications. The simple conclusion of this section is that brute force optical scanning technology is not appropriate for many high dynamic range applications and more research is needed to demonstrate the limits of such technologies – essentially to push them as far as they can go.

3. Full-field technology There are a number of full-field techniques that can measure surface topography (principally form) over large areas, for example, fringe projection and photogrammetry [1]. However, such techniques do not have the required resolution (axial or lateral) or accuracy for many high dynamic range applications. It is not clear how such techniques could be adapted to increase resolution and accuracy without severely limiting the field of view. Techniques such as digital holography and speckle interferometry have been used to measure topography over large areas to high resolution, but they often have limitations in terms of height range, dynamic range and the need for very stable environments [11]. One potential solution to the high dynamic range challenge is the use of hybrid instrumentation, i.e. to detect areas of interest with a relatively low resolution sensor (for example, camera-based sensors), then to “home-in” on the areas of interest using a high resolution sensor (see for example, [4, 12]). Data

fusion techniques can then be used to combine the data from the different sensors [13]. In some scenarios the low resolution sensor could use approaches that detect scattering from points of interest, such as defects or scratches, therefore, allowing high resolution detection without the need for imaging.

although it has not been demonstrated on high dynamic range applications yet (to the knowledge of the authors). Lastly, methods need to be developed for handling the potentially very large datasets that will be produced when measuring to high resolution over large areas.

4. Resolution

6. Traceability and standards

The high dynamic range metrology challenge is essentially a challenge of maintaining high resolution whilst extending range, or bandwidth. Potential solutions may involve the use of a priori information and iterative approaches to extend resolution of farfield optical imaging systems, i.e. super-resolution [11]. Often super-resolution is discussed in the context of imaging systems that are already operating close to their natural resolution limits, i.e. high NA systems. However, it is possible to extend the resolution of low NA systems using a priori information, therefore, extending the dynamic range, i.e. also maintaining a large range. Many advanced products require surface features to be measured below the micrometre scale (see for example, [14, 15]. Currently, scanning probe microscopes (SPMs) can be used. The lateral resolution of SPMs is usually limited by the geometry and size of the proximal tip, and varies from hundreds of nanometres to less than a nanometre. However, with the exception of a few dedicated instruments in research laboratories (see for example, [16]), SPMs are limited in their field of view to a few hundred micrometres squared, often much less at higher spatial resolutions. A further limitation of SPMs is their relatively slow measurement speed, which is a result of the need to keep the probe in intimate contact (or near contact) with the surface being measured and the need to raster scan to build up a 3D topography map. SPMs are, therefore, not suitable for quality control over large areas (up to metres squared) unless employed in a highly parallel fashion, i.e. banks of SPMs – this is a very expensive solution to implement. Far-field optical techniques with enhanced resolution could offer a significant breakthrough in this field, provided a number of technical challenges can be overcome, for example, fast iterative algorithms and intelligent use of a priori information [17].

Specification and standards for calibrating areal surface topography measuring instruments are only just becoming available [20, 21]. Whether such calibration techniques can be applied for high dynamic range applications remains to be seen. The traditional use of transfer artefacts may need to be re-assessed when such large areas and high speeds are required.

5. Background mathematics

Acknowledgements

Development of techniques to address the high dynamic range metrology challenge requires advances in a number of mathematical areas. Where two or more sensors are used with different lateral resolutions, data fusion techniques are required to combine the data and to match the co-ordinate systems of the sensors [15]. To reduce scanning times and make full use of all the available information, intelligent sampling techniques will need to be utilised [18]. One relatively promising newcomer to this field is compressed sensing [19],

This paper was funded by the UK National Measurement System Engineering & Flow Metrology Programme and the FP7 project NANOMend.

6. Current projects at NPL NPL is currently taking two approaches to tackle the high dynamic range metrology challenge. 1. Enhancing and extending existing technologies to push them to their limits. 2. Developing novel approaches. In approach 1, we have reviewed all the existing sensing technology for surface topography measurement or defect detection, and have developed a platform comprising a simple two-axis roller-bearing slideway and a bridge to support the probe under test. The dynamic performance of a number of sensors will be investigated. Also, we are working with a number of industrial partners to develop demonstrators for in-line sensing. For approach 2, we are developing full-field and super-resolution approaches that will be reported in future publications. 7. Summary As is often the case with new techniques, the metrology for high dynamic range manufacturing is lagging behind the manufacturing capability. We have discussed some of the areas of metrology that need to be developed to address this lag. NPL is currently engaged in projects to develop solutions for high dynamic range metrology.

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