Semiconductor Image Sensing | EPFL Graph Search

Silicon is an excellent detector material for electromagnetic radiation in the wavelength range of 0.1 to 1000 nm. In the visible spectral range (400-700 nm), external quantum efficiencies approaching 100% are obtained. When combined with the amazing miniaturization capabilities of the semiconductor industry, this fact explains why silicon is the material of choice for very efficient, highly integrated, cost-effective image sensors: In 2007 about one billion image sensors were produced and employed in camera systems. Profiting from the unrelenting progress of semiconductor technology, silicon-based image sensors with astounding performance have been demonstrated, in terms of resolution, pixel size, data rate, sensitivity, time resolution and functionality: 111 million pixels on a single CCD chip were produced, pixels with a period of 1.2 m were fabricated, sustainable image acquisition and readout rates of 4 billion pixels per second were realized, single-photon sensitivity at room temperature and at video rates was achieved, timing resolution of the pixels in lock-in image sensors below 5 ps was obtained, and the processing complexity of ―smart pixels‖ was raised to several ten transistor functions per pixel. The future of semiconductor image sensing lies in the extension of the accessible wavelength range to the infrared spectrum (1.5-10 m), the development of affordable, high-performance X-ray image sensors in particular for the medical energy range (20-120 keV), the realization of sensitive and cost-effective sensors for Terahertz imaging (100-500 m), as well as the integration of an increasing amount of analog and digital functionality on single-chip custom camera systems. The Holy Grail is the ―seeing chip‖, capable of analyzing the contents of a scene and of recognizing individual objects of interest.

Ultra-Low-Temperature Silicon and Germanium-on-Silicon Avalanche Photodiodes

Mahdi Aminian

In this thesis we propose the use of photodiodes fabricated in planar technologies to address the detection problem in these applications. A number of solutions exist, optimized for these wavelengths, based on Germanium (Ge) and other III-V materials. In this thesis we focused on Ge thanks to its versatility and ease to use in the clean room. The main advantage of this material is in fact a good compatibility with Silicon and standard CMOS processes. Note that the proposed technology is not based on Silicon/Germanium (SiGe), whereby Ge is used to strain Si to achieve higher bandwidth in Si, not higher sensitivity. In our pure Ge approach, Ge is grafted onto Si (Ge-on-Si), achieving high responsivity at wavelengths of 900nm and higher. The proposed devices can operate in avalanche mode (avalanche photodiodes - APDs), and in Geiger mode (Geiger mode APDs (GAPDs) or single-photon avalanche diodes (SPADs)). To combine the advantages of Ge with single-photon sensitivity and excellent timing resolution of Si-based SPADs, this thesis proposes a new generation of SPADs, achieved in collaboration with Prof. Nanver at TUDelft, aimed at near-infrared range. The fabrication process of the Ge-on-Si SPAD approach, which we are investigating together with the TUDelft group, consists of a standard CMOS process combined with post-processing steps to grow Ge on top of a Si/SiO2 layer. In our study we have investigated the potential for a new generation of massively parallel, Ge-on-Si sensors fabricated in fully CMOS compatible technology. The objective was to address the next challenges of super-parallel pixel arrays, while exploiting the advantages of Ge substrate. The key technology developed in the thesis is a selective chemical-vapor deposition (CVD) epitaxial growth. A novel processing procedure was developed for the p+ Ge surface doping by a sequence of pure-Ga and pure-B depositions (PureGaB). The resulting p+n diodes have exceptionally good I-V characteristics with ideality factor of ~1.1 and low saturation currents. They can be operated both in proportional and in Geiger mode, and exhibit relatively low dark counts. We also looked at techniques to improve red and infrared sensitivity in conventional deep-submicron CMOS processes, by careful selection of standard layers at high depths in the Si substrate. Using the proposed approach, 12 µm-diameter SPADs were fabricated in 0.18µm CMOS technology showing low dark count rates (363 cps) at room temperature and considerably lower rates at cryogenic temperatures (77 K), while the FWHM timing jitter is as low as 76 ps. That of cryogenic SPADs is a novel research direction and in this thesis it was advocated as a significant trend for the future of optical sensing, especially in mid-infrared wavelengths. Low temperature characterizations reported in this thesis exposed how the relevant properties of fabrication materials, such as strength, thermal conductivity, ductility, and electrical resistance are changing. One of the most important properties is superconductivity in materials cooled to extreme temperatures: this is an important trend that will be pursued in the future activities of our group.

EPFL2014

A new versatile hybrid MEMS technology for high sensitivity, fluid proof sensing applications.

Matthias Neuenschwander

The field of micro electromechanical systems (MEMS) evolved from the microelectronic industry and the technologies developed to fabricate integrated circuits. As a result, MEMS are commonly fabricated on silicon wafers. The development of MEMS has been driven by three main merits: miniaturisation, microelectronics integration, and parallel fabrication with high precision. Many operational properties scale well with smaller sizes. In addition, integrated electronic circuitry allows embedding MEMS with computing or networking capabilities, while parallel manufacturing enables the fabrication of many identical devices on a single wafer, reducing the unit cost. The main MEMS application is transducers which transform signals from one form of energy to another, and can be used for perception (sensors) or to produce actions (actuators). Silicon and other microelectronic materials like metals have enabled very high-performance MEMS transducers because these materials can be used for a range of very effective actuating and sensing principles.More recently, polymers have started to be used in MEMS because of their unique electrical, physical and chemical properties which include biocompatibility, viscoelasticity and mechanical shock tolerance. They are also much softer than silicon or metals, and they can be processed using many techniques that allow unique low-cost, batch-style fabrication and packaging. However, polymers have relatively low glass-transition and melting temperatures. As a result, the advantages of polymers cannot easily be combined with high-performance actuating and sensing elements as these are based on silicon technology and require high-temperature fabrication steps. Here, a hybrid-MEMS fabrication process is used to address this issue. The developed devices are based on a trilayer structure, where a thick polymer core is sandwiched between two hard thin films, while electronic layers are embedded within in a fluid-compatible way.The objective of this thesis is to turn hybrid-MEMS into a benchmark MEMS technology. This involves many different aspects. First, sensing and actuation elements are integrated into trilayer devices, and their performance is analysed. Then, some more unique possibilities offered by hybrid-MEMS are exploited. A way to fabricate electronics on multiple layers is developed, which allows different electronic features to be integrated in parallel. In addition, three-dimensional bulk features fabricated at several levels are demonstrated. This is possible because the hybrid-MEMS process is based on the bonding of multiple wafers.All these new fabrication features are demonstrated in atomic force microscopy (AFM) applications. Self-actuated trilayer AFM cantilevers with sharp silicon tips are used to boost imaging speeds in the off-resonance tapping (ORT) mode, thanks to an order of magnitude faster surface probing rates. Furthermore, fully integrated ORT imaging is demonstrated with self-actuated piezoresistive cantilevers. Next, trilayer cantilevers with shielded conductive tips are introduced for electrical AFM modes. Finally, ongoing research projects are touched upon, including approaches to fabricate hybrid-MEMS with single crystal silicon piezoresistors for improved sensitivity, and an analysis of reproducibility of fabricated trilayer cantilevers.

EPFL2022

Application of CMOS image sensors in optical sectioning and polarization microscopy

Jelena Mitic

In this thesis new imaging approaches for optical microscopy are proposed and studied. They are based on the use of dynamic structured illumination in combination with a demodulation detection concept employing CMOS image detectors. Two particular implementations are suggested: real-time optical sectioning microscopy and whole field quantitative polarization microscopy. Optical sectioning, i.e depth resolved imaging, allows observation of thin sections in volumetric samples. In its classical configuration the wide-field microscope does not allow optical sectioning of the investigated objects. In this thesis the optical sectioning in a wide-field microscope is achieved by illuminating the sample with a continuously moving single spatial frequency pattern, which temporarily modulates the signal obtained on the photodetector. Only the object parts that are in the focus have a good contrast and consequently generate stronger signal on the detector. The optically sectioned images are obtained employing a CMOS image detector, which demodulates the time-dependent component of the image. Two different systems for real-time acquisition of optically sectioned images are proposed. The first one is based on a specially designed smart-pixel-detector-array (SPDA), which allows performing the signal processing by an electronic circuit integrated to each pixel of the sensor. The second system utilizes a commercial CMOS image sensor. Here, the images are treated by a digital signal processor (DSP) integrated into the camera (iMVS-155). Both approaches provide real-time acquisition of optically sectioned images. The proof of principle for the new method is demonstrated by imaging artificial three-dimensional reflective samples. The optical sectioning imaging of biological samples in bright-field mode and in fluorescence mode is also demonstrated. The optical sectioning performances of the studied systems are similar to that of the confocal microscope. However the new method has some advantages over the classical confocal system: e.g. the difficulties with the alignment and the post-processing inherent to the confocal system are avoided. The theoretical framework presented in the thesis describes the image formation in structured illumination for both reflective and fluorescent samples. The influence of longitudinal chromatic aberration and spherical aberration caused by specimen induced index mismatch on depth discrimination property are studied. In extension to the work related to optically sectioned microscopy, the quantitative polarization microscopy imaging method is proposed as a new application for CMOS detectors. The polarization state of the illuminating light is dynamically changed and the demodulation detection approach is employed to extract the retardance and azimuth of the birefringent sample in real-time; for a whole field of view. Examples of polarization sensitive measurements for different samples are presented. The theoretical analysis is performed using the Jones formalism. Finally, the potentials and limitations of the CMOS detectors for applications in optical microscopy are discussed. Thanks to constant advancements in CMOS technology, the acquisition speed, resolution and sensitivity of new CMOS detectors generations have been improved. This will allow adapting the methods proposed in this thesis for investigations of fast dynamic process where high temporal and spatial resolution is required.

EPFL2004