A CMOS Photodiode Array with Linearized Spectral Response and Spatially Distributed Light Intensity Detection for the Use in Optical Storage Systems Ingo Hehemann, Erol Özkan, Armin Kemna, Werner Brockherde, and Bedrich J. Hosticka Fraunhofer Institute of Microelectronic Circuits and Systems, Finkenstr. 61, 47057 Duisburg, Germany ingo.hehemann@ims.fhg.de Holger Hofmann Thomson Multimedia, Optical Disk Recording Lab, 78003 Villingen-Schwenningen, Germany Abstract This paper presents a special photodiode array layout to be used in optical pickup units in combination with optical storage systems. Apart from the features that a standard array offers, this design enables higher resolution of the data recovery diodes. Furthermore, additional diodes for the determination of the spatial light intensity are included. The special layout of the array ensures crosstalk isolation and linearizes the spectral response of the data recovery diodes enabling less circuitry needed for the readout of the photocurrent. The structure was fabricated in standard 0.6μm twin well CMOS technology. 1. Introduction A fundamental component of every optical storage system is the optical pickup unit that reads and retrieves data from the storage media. Apart from a laser diode, optical elements, and an actuator for tracking and focussing, the optical pickup unit contains a photodetector array with readout circuitry. The classical detector arrangement is a 2x2 photodiode matrix as sketched in Figure 1(a). Summing the signals of quadrant A to D in different manners delivers the electrical signals for data recovery and servo signals for focus and tracking control [1]. Standard detectors have different spectral responsitivities at different wavelengths so that a readout circuit with variable gain is necessary [2]. Furthermore, all approaches to generate the servo signals suffer from sensitivity to a nonideal positioning of the detector caused by misalignment or thermal drift of the laser beam position. In addition to this, optical offsets caused by substrate thickness variations and tilt of the storage media as well as radial objective lens movement are leading to a degradation of the readout signals resulting in a bit-error rate increase. The proposed new arrangement has a higher resolution of photodiodes for data recovery (PDDR) which allows a more detailed analysis of the laser beam reflected from the storage media. Furthermore, it offers the possibility not only of sensing the position of the light spot's center of gravity but also of measuring its intensity distribution with 25 spatially distributed photodiodes (PDID). This information can be processed to generate an electrical compensation of signals affected by alignment errors and optical offsets. The newly introduced 25 photodiodes have been used to linearize the spectral responsivity of the PDDRs employing a special layout. Their shape is used to trap diffusing charges which improves the transient behaviour and works as crosstalk isolation between the PDDRs. Figure 1. (a) Standard array, (b) structure of the proposed array. 2. Structure and Layout Figure 1(b) shows a schematic representation of the proposed arrangement. The standard 2x2 detector of Figure 1(a) was splitted into a 4x4 array of PDDRs. The more detailed information achieved by the spatial refinement is usable in future generation of storage systems [3,4] to improve the storage capacity. Illumination of two tracks on the storage media due to A B C D X Y Y X (a) (b) PDDR PDID