Interdisciplinarity takes imagers to a higher level
01/01/2013
Els Parton, PIET DE MOOR, JONATHAN BORREMANS and ANDY LAMBRECHTS, imec, Leuven, Belgium
Specialty processing steps for different apps.
Belgian research institute imec shows the opportunities for imagers when teams of designers, software engineers, technologists, and system designers collaborate. A flexible fab is also a requirement for making innovative image systems. The recipe? Take a 0.13??m CMOS technology and add some back-side illumination technology, specialty processing steps and coatings, 3D stacking, embedded CCD, hyperspectral filters, ... and leverage system-on-chip and technology codesign as well as dedicated software development to open up unprecedented image sensor application fields. Some examples of projects that are the result of this multidisciplinarity are presented.
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| Figure 1. Only when technology and design challenges are met together can the image sensor target the application field. |
The market
The most widespread imagers today are the ones in our cameras and cell phones. These are standard imagers made in large volumes. However, there is also a market for specialty imagers. For these, no standard solution is available. Often, these imagers have special requirements depending on the application. For example, wafer and mask inspection tools for advanced semiconductor processing require imagers that are sensitive to extreme ultraviolet (EUV) wavelengths; and lab-on-chip solutions require miniaturized microscopes that can recognize cells at a high speed.
The market for specialty imagers is broad???ranging from high-end scientific, space, earth observation, medical imaging, high-end consumer, and machine vision and instrumentation. There is a growing demand from semiconductor equipment manufacturers for specialty imagers, and the medical imaging market segment is growing enormously.
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| Figure 2. The market for specialty imagers is varied. Applications typically require custom-made solutions with a combination of many technologies and expertises. |
New markets come with unprecedented noise, speed, and integration requirements, for example, for advanced high-end industrial tools. These unprecedented specifications necessitate innovative system-on-chip (SoC) design solutions in close collaboration with technology development and software post-processing development. Only when technology and design challenges are met together can the image sensor target the application field.
When analyzing the needs in the market for specialty imagers there is a clear trend towards imagers for non-visible wavelengths (e.g. UV, EUV). Also full SoC imager solutions are hot in the market today. Imec, as a research institute that closely collaborates with industry, has tackled some specific projects for industry. One by one, these projects are a clear illustration of the potential for imagers when bringing together different expertises. Below, we give a few examples of such realizations.
Microlens arrays for e-beam lithography
Reflective electron beam lithography (REBL) uses a beam of electrons to do lithography. The goal is to extend semiconductor manufacturing to the 16nm technology node and beyond. Electron beam lithography exists, but suffers from long writing times because this is essentially a serial technique. The advantage of the current development is that it enables writing of 1 million electron beams in parallel, leading to fast throughput.
A process for the fabrication of an electrostatic micro-lens (lenslet) array for the REBL tool has been developed. The lenslet device consists of an array of holes with a diameter of 1.4??m on a 1.6??m pitch. These holes are patterned through a stack with a total thickness of 4??m. This stack, consisting of electrode and insulating layers, acts as an electrostatic lens. By applying different voltages to the electrodes, electric fields are created that focus and either absorb or reflect an incoming electron beam.
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| Figure 3. Back-side-illuminated hybrid imager (1 Mpixel) connected to a CMOS readout circuit. Back-side-illuminated imagers show an improved light sensitivity as compared to conventional frontside-illuminated CMOS image sensors. |
The development of the lenslet structures together with the interconnects poses many design and processing challenges. A litho and etch process was developed to pattern the high aspect ratio devices with good overlay to underlying electrode layers. Bond pad and via design were done using some unconventional integration approaches in order to remain compatible with the lenslet processing steps.
EUV sensors
EUV detection is needed for EUV lithography tools and wafer and mask inspection equipment. The imagers for these applications require detection of light with an extremely short wavelength. Such light typically has a very limited penetration depth in silicon and dielectrics. Also, lithography equipment requires high doses, which cause reliability issues in the EUV sensors.
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| Figure 4. Wafer containing hyperspectral filter structures. These spectral filters are based on the principle of the Fabry???P??rot filter. Processed on a camera sensor, this structure can be used for hyperspectral imaging applications. |
Photodiodes were fabricated with a special structure. Key is a dedicated passivation that enables EUV penetration from the top to reach the sensitive silicon. These detectors can be used to sense the EUV dose in lithography tools. However, to check the uniformity, a 2D array is needed. For this reason, future work will focus on developing a complete imager. To do this, concepts like back-side illumination (BSI) become important.
Most imagers today use front-side illumination. The light has to go through the back-end-of-line with the metals and dielectrics. These materials reflect the light and even absorb part of the light. When you go to light beyond the visible spectrum, this becomes a problem. For this reason, it is better to use in this case the back-side illumination concept for the imager. By applying the BSI concept, EUV imagers become possible for applications such as monitoring the exposure dose, and calibrate, align, and focus the lithography tool's lens systems.
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| Figure 5. Compact hyperspectral camera and hyperspectral image sensor based on the integration of dedicated filters on top of image sensors. |
Hyperspectral filters on top of an imager
Hyperspectral imaging exists today as large, expensive tools typically used in research environments. However, innovative integration of filters and image sensors can turn this around and can enable high-speed, low-cost, and compact hyperspectral cameras. Such cameras could be used for industrial inspection, anti-counterfeiting, food quality control, and medical applications such as screening of skin cancer.
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| Figure 6. Hyperspectral filter structure (stepwise wedge consisting of Fabry???P??rot interferometers) that is directly post-processed on top of the image sensor. |
A hyperspectral imager was developed by integrating a group of 100 spectral filters, arranged in the shape of a wedge, on top of a commercial CMOS imager. To enable the low-cost processing of such a microscopic wedge filter, imec introduced a design that is able to compensate for process variability. The result is a compact and fast hyperspectral camera made with mass-producible and fully CMOS-compatible process technology.
The integrated spectral filters are narrow banded Fabry???P??rot interference filters. The Fabry???P??rot filter is typically made of a transparent layer (called cavity) with a mirror at each side of that layer. The length of the cavity defines the central wavelength of the optical filter and the reflectivity of the mirrors defines the full width half maximum (FWHM) of the filter. Using these filters, different hyperspectral imager designs can be realized. As an example, a line scan hyperspectral imager can record a full 3D cube (i.e. an image in all the different wavelengths) for a linear moving object.
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| Figure 7. Concept of a 3D-stacked imaging system with different active layers for the different functionalities of a smart imaging system |
The hyperspectral filters can be processed in principle on any image sensor to match different application specs. Similarly, the spectral range can be tuned, and currently an extended spectral range of 400???1000nm is under development.
System-on-chip imagers
High-end imagers require a lot of intelligence to be integrated in the imager: just think about the complex and fast read-out circuitry needed. The solution is a CMOS-based SoC approach for the imagers. The CCD approach often used for specialty imagers cannot handle this need for integrated intelligence.
For example, analog-to-digital conversion for imagers pushes the boundaries of frame rate and resolution to meet new performance requirements from the application side. A prototype was developed with fast and low-power ADCs for each column on the imager. For high-performance SoC imagers, the co-design of technology, design, and system is essential, as well as a flexible CMOS platform with add-ons such as back-side illumination, embedded CCD, or hyperspectral filters.
3D stacking in imagers
In addition to system-on-chip technology, 3D stacking technology can also be used to make imagers smarter. During the last few years, a lot of development effort has been spent on through-Si vias (TSVs), enabling 3D stacking of active Si dies. The main driver for this technology is memory stacking and memory on logic stacking. This technology is now becoming mature (at the R&D level), and the implementation in industry is expected to happen in the future.
Also for imagers, 3D stacking creates opportunities. A first advantage is the decoupling of functions of different layers in an imager: sensing layer, analog ROIC, ADC, and digital system. Each of the different layers can be optimized separately in the most adequate technology and subsequently stacked. A second advantage is the enhancement of the read-out structure using a vertical interconnect scheme in terms of speed (massive parallel processing), and performance (complex image processing). Thirdly, 3D application mapping allows us to distribute the functionality of a specific sensor in an optimized way over the heterogeneous layers to obtain a cost-efficient realization.
Els Parton is editor-in-chief of imec's Dutch magazine InterConnect, which highlights trends, technologies, and collaborations with Flemish SMEs. Piet De Moor focuses his current research on advanced CMOS imagers such as back-side illuminated and hybrid imagers suitable for high-end imaging applications. Jonathan Borremans leads the Imager Design Group at imec. Andy Lambrechts leads the Integrated Imaging team and is working on hyperspectral imaging, lens-free microscopy, and other activities.