Since 2010, there has been growth beyond expectations in the adoption of mobile devices, such as smart phones and tablets, which has called for larger volumes of CMOS image sensor chips to be produced. The resolution and miniaturization races are ongoing, and performance metrics are also becoming more stringent. In addition to the conventional pixel shrinkage, a “more than Moore” trend is increasingly evident. Resolutions of over 20 Mpixels are commercially available for mobile devices employing enhanced small-size pixels. Thanks to the innovative readout and ADC architectures embedded at the column and chip levels, data rates approaching 50Gb/s and a noise floor below single electron have been demonstrated. In addition to the conventional applications, ultra-low-power vision sensors, 3D, high-speed, and multispectral imaging are the front-running emerging technologies.
Back-side Illumination (BSI) is now the mainstream technology for high-volume, high-performance mobile applications, 1.12μm BSI pixels are available, and the industry is potentially moving towards 0.9μm pixel pitch and below. Additional innovative technologies outside of the traditional scaling include advanced 3D stacking of a specialized image sensor layer on top of deep-submicron digital CMOS (65nm 1P7M) using through silicon vias (TSVs) and micro-bumps. The importance of digital-signal-processing technology in cameras continues to grow in order to mitigate sensor imperfections and noise, and to compensate for optical limitations. The level of sensor computation is increasing to thousands of operations-per pixel, requiring high-performance and low-power digital-signal-processing solutions. In parallel with these efforts is a trend throughout the image sensor industry toward higher levels of integration to reduce system costs.
Ultra-low-power vision sensors are being reported in which more programmability and computation is performed at the pixel level in order to extract scene information such as object features and motion.
Lightfield/plenoptic commercial cameras, which have been available since 2010, are now gaining popularity and are being marketed for 3D imaging and/or all-in-focus 2D imaging. On-chip stereoscopic vision has been demonstrated through digital micro lenses (DML), paving the way to next-generation passive 3D imaging for mobile and entertainment applications, e.g. through gesture control user interfaces.
Significant R&D effort is being spent on active 3D imaging time-of-flight (TOF) applications to support requirements from autonomous driving, gaming, and industrial applications, addressing open challenges like background light immunity, higher spatial resolution, and longer distance range. Deep-submicron CMOS single-photon avalanche diodes (SPADs) have been developed by several groups using different technology nodes. They are now capable of meeting the requirements for high resolution, high timing accuracy by employing highly parallel time-to-digital-converters (TDCs) and small pixel pitch with better fill factor.
Ultra-high-speed image sensors for scientific imaging applications with up to 20Mfps acquisition speed have been demonstrated.
Multispectral imaging is gaining a lot of interest from the image sensor community: several research groups have demonstrated fully CMOS room-temperature THz image sensors, and a hybrid sensor capable of simultaneous visible, IR, and THz detection has been reported.
The share of CCDs continues to shrink in machine vision, compact DSC and security applications. Only for high-end digital cameras for astronomy and medical imaging do CCDs still maintain a significant market share.
Sensors & MEMS
|A 4x4 array of sensing cells, developed by Dr. Peng Peng of Seagate Technology, from Flexible Microtactile Sensor for Normal and Shear Elasticity (IEEE Transactions on Industrial Electronics)
MEMS inertial sensors are finding widespread use in consumer applications to provide enhanced user interfaces, localization, and image stabilization. Accelerometers and gyroscopes are being combined with 3D magnetic-field sensors to form nine-degree-of-freedom devices, and pressure sensors will eventually add a 10th degree. The power consumption of such devices is becoming sufficiently low for the sensor to be on all the time, enhancing indoor navigation. There have been further advances in heterogeneous integration of MEMS with interface circuits in supporting increased performance, larger sensor arrays, reduced noise sensitivity, reduced size, and lower costs.
To address the stringent requirements of automotive, industrial, mobile, and scientific application, MEMS inertial sensors, pressure sensors and microphones are becoming more robust against electromagnetic interference (EMI), packaging parasitics, process voltage temperature (PVT) variations, humidity, and vibration.
Sensor interfaces achieve increasingly high resolution and dynamic range while maintaining or improving power or energy efficiency. This is achieved through techniques such as zooming, non-uniform quantization, and compensation for baseline values.
New calibration approaches, such as voltage calibration, are being adopted for BJT-based temperature sensors to reduce cost. In addition to thermal management applications (prevention of overheating in microprocessors and SoCs), temperature sensors are also increasingly co-integrated with other sensors (e.g. humidity, pressure, and current sensors) and MEMS resonators for cross-sensitivity compensation. Alternative temperature-sensing concepts find their way into applications with specific requirements not easily addressed by BJTs: thermal diffusivity-based sensing for high-temperature applications; thermistor-based and Q-based concepts for in-situ temperature sensing of MEMS devices and for ultra-low voltage operation.
MEMS oscillators continue to improve; phase noise is now low enough for demanding RF applications, 12kHz-to-20MHz integrated jitter is now below 0.5ps, and frequency accuracy is now better than 0.5ppm. Consumer applications are adopting new low-power and low-cost oscillators.
There have been continuous achievements in the area of ICs for neural and biopotential interfacing technologies. Spatial resolution of neural monitoring devices is being reduced utilizing the benefits of CMOS technology. IC providers are increasing their component offerings towards miniaturization of portable medical devices.
Telemedicine and remote-monitoring applications are expanding with support from IC manufacturing companies. The applications of such systems are not limited to services targeted for elderly or chronically ill patients; for example there are several technologies developed to enhance the way clinical trials are conducted by monitoring patient adherence and by improving data collection. Low power WiFi, and Bluetooth-low-energy is emerging as a standard wireless connection between portable communication services and wearable technology.
Smart biomolecular sensing is another major trend that marries solid-state and biochemical worlds together with the ultimate goal of enabling a more predictive and preventative medicine. With the help of the accuracy and parallelism enabled by CMOS technology, time, cost, and error rate of DNA sequencing may be significantly improved. Direct electronic readout may relax the need for complex biochemical assays. Similar trends are becoming increasingly evident in the space of proteomics and sample preparation.
Even for medical imaging, there is a trend from hospital imaging toward point-of-care and portable devices. A key example is in the space of portable high-resolution ultrasounds in which larger scientific imaging setups are being integrated onto the sensor by process technology (e.g. integrated spectral filters, CMUT). Another example is in the space of molecular imaging. The advent of silicon photomultipliers (SiPM) providing a solid-state alternative to PMTs enable the realization of PET scanners compatible with MRI, opening the way to new frontiers in the field of cancer diagnostics. More recently, SiPMs realized within deep-submicron CMOS technologies have allowed the integration at pixel- and chip-level of extra features, e.g. multiple timestamp extraction, allowing in perspective a dramatic reduction of the system cost.
The desire to put much higher-resolution and higher-definition displays into mobile applications is one of the display technology trends, and it is now opening a Full HD smartphone era. 440ppi high-definition displays are expected, even for 5-inch display sizes. Low-temperature polysilicon (LTPS) technology seems to have more merits over a-Si TFT technology. But a-Si TFT and oxide TFT technologies supported by compensating driver systems are being prepared to compete with it. Very-large-size LCD TVs over 84 inches, and UD (3840×2160) resolution are now the leading entertainment systems. 55-inch AMOLED TVs with Full HD resolution are also opening new opportunities in consumer applications.
As touch-screen displays for mobile devices become increasingly thin, capacitive touch sensors move closer to the display. The resulting in-cell touch displays come with reduced signal levels due to increased parasitics, and increased interference from the display and switched-mode chargers. Noise immunity is improved by adopting noise filtering and new signal modulation approaches.
This and other related topics will be discussed at length at ISSCC 2013, the foremost global forum for new developments in the integrated-circuit industry. ISSCC, the International Solid-State Circuits Conference, will be held on February 17-21, 2013, at the San Francisco Marriott Marquis Hotel.