Header for SPIE use Low-voltage operation of a CMOS image sensor based on pulse frequency modulation Jun Ohta * , Hirohiko Sakata, Takashi Tokuda, and Masahiro Nunoshita Graduate School of Materials Science Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara 630-0101 JAPAN ABSTRACT Inspired by biological information scheme, pulse frequency modulation (PFM) technique is robust for noise sources due to its digital encode of analog signals. In a viewpoint of image sensors, PFM is also useful for a wide dynamic range and has already been demonstrated over 60 dB. We have designed a pixel circuit of a CMOS image sensor using PFM for the next generation architecture of vision chips. The chip is fabricated using a standard 0.35 μm double poly, triple metal CMOS technology. The photodiode is a parasitic pn diode between p-well and n-diffusion with the size of 2 μm squares. The top of the photodiode is covered with third metal and 1 μm square hole is open for aperture. Feedback circuits consist of a Schmitt trigger and two inverters. We have demonstrated by introducing PFM the chip works well under the power supply voltage of 0.55V with 50 dB. Such a low voltage operation suggests deep sub-micron technologies, for example, 0.18 μm technologies could be applied to the sensor. The other important point in our chip is that the photodiode is very small in size of 2 μm x 2 μm with the aperture size of 1 μm x 1 μm. This enables us to realize an image sensor with a small fill factor, which is very useful for vision chips where functional circuits are integrated in each pixel. Keywords: CMOS image sensors, pulse frequency modulation, low voltage operation, vision chips 1. INTRODUCTION A vision chip is an image sensor that integrates a photo-sensor with signal processing circuits in each pixel [1]. Their advantages over conventional image processing systems based on CCDs (charge coupled devices) are in fast and versatile image pre-processing due to incorporating signal processing functions in each pixel. Many works on vision chips have been reported so far [2] and classified into an analog type and a digital type in a viewpoint of signal processing. The analog type is more natural because the input signal or light intensity is analog, although it is far from robust against noise and thus has less precision. On the contrary, the digital type is robust and more suitable to communication, while it requires analog-to- digital converter (ADC). In addition, digital multiplication is area consuming. In each type the integration density is critical to incorporating much function in a pixel and/or increasing resolution. The trend toward deep sub-micron fabrication process in LSI is suitable to vision chips, because such fine process enables to integrate more functions in a pixel. However, the operation voltage decreases accompanied with such a deep sub-micron process, and it may directly affect the signal quality and thus deteriorate a signal-to-noise-ratio (SNR) [3]. In addition, the area of a photodiode decreases as the process rule becomes fine, and consequently the signal capacity in the photodiode decreases, which also causes to degrade the SNR. Thus, the great issue is to develop architecture that can deal with lowering of the operation voltage and shrinking of the area of a photodiode. A pulse frequency modulation (PFM) technique uses pulse trains to represent analog signals in time domain [3]. Thus, in nature a PFM is robust in the signal transmission and well compatible with digital logic circuits. These features are very effective in vision chips where an image pre-processing function is integrated in each pixel. Neurobiological systems also use PFM in not only communication but also information processing as pulsed neural networks. This architecture is very suitable to deep sub-micron technologies in the following reasons. First, PFM is essentially digital circuits except for a photodiode, thus deep sub-micron technology is applicable to it with little modification. Second, the output is pulse trains or digital signals, which means the operation voltage hardly affects the performance of the signal quality. Third, the shrinkage * Correspondence: Email: ohta@ms.aist-nara.ac.jp; Telephone: +81-743-72-6051; Fax: +81-743-72-6059