High-pixel count optical distance sensors

The pixel area of optical distance sensors exploiting PN and PIN photodiodes has to be rather large, when a good distance measurement accuracy is required, to receive enough optical power for a sufficient signal to noise ratio. This results in a maximum of only a few thousand (1,000-4,000) dis-tance sensor pixels on one chip. Single- photon counting avalanche photodiodes (SPADs) require large guard-rings around to avoid edge-breakdown. Therefore SPADs lead to a low fill factor. Furthermore, their detection probability is low, due to a thin space- charge region. Other disadvantages of avalanche photodiodes are the need for a high voltage of several 10V, their rather high dark count rate (especially at higher temperatures) and their dead time. Phototransistors also possess a signal amplification like avalanche photodiodes. In conventional IC technologies, however, their base-collector space-charge region is also rather thin, which limits the signal amplification and responsivities of only a few (1-3)A/W result. The excellent knowledge of the group on PIN photodiode technologies, however, can be exploited as a significant innovation to investigate phototransistors in PIN technology. This innovative phototransistor has a thick base-collector space-charge region due to the thick low- doped (intrinsic) layer available in the PIN photodiode technology. Therefore, this new phototransistor possesses a large photon detection efficiency also in the red and near-infrared spectral range. According to estimations, the PIN phototransistor should achieve a responsivity of 70 to 90A/W, which is a factor of 100 to 200 higher than that of a PIN photodiode. The corresponding quantum efficiency of the PIN phototransistor should be in the range of 8000% to 16000%. This excellent signal amplification can be exploited to reduce the photodetector area. E.g. a light- sensitive area of 10*10m should enable the same photocurrent as a PIN photodiode with 100*100m. A noise model for the phototransistors will be created. Distance ranges from several meters to 15m will be examined. Another aim of the project, is to reduce the area of the signal processing circuits also, in order to increase the maximum pixel count up to 256*256, i.e. 65536, which is possible with a pixel area of 40*40m. New pixel circuits will be investigated to determine the minimum pixel area and the maximum pixel count. Electrical power consumption will be observed to avoid heat problems. The pixel circuits will also be investigated with respect to reduced electronic noise, power supply disturbance and substrate noise as well as high background light suppression and real-time capability. Such circuits also can be combined with smaller-size photodiodes. Test structures will be manufactured using Application Specific Integrated Circuit (ASIC) foundry services to verify the innovative approaches. A physical LED (light emitting diode) model to describe and compensate the temperature dependence of emitted optical power and of rise/fall times will be created and a light source including driver circuit will be constructed for characterisation of the high-pixel count distance measurement test chips with accuracies in the range of a few centimeters. In summary, the innovative phototransistors and novel small-footprint pixel circuits will be investigated to increase the pixel count possible on one chip by a factor in the range of 20 to 60.

Innovative phototransistors exploiting the PIN photodiode technology in 0.6m, 0.35m and 0.18m CMOS were investigated. Thereby the thick, low doped intrinsic layer of the PIN photodiode was used in order to widen the base-collector space-charge region of the phototransistors and to increase their photoresponsivity considerably. The values of 98A/W for the responsivity and of up to 125MHz for the bandwidth of these PNP PIN phototransistors achieved in the project exceed the goals written in the project proposal considerably and top the state of the art by more than an order of magnitude.Furthermore, innovative pixel circuits for optical distance measurement with a time-of-flight (ToF) principle more accurate: a phase correlation principle were examined. An ultra-small area pixel only 18.512.5m in size was realised, which enables 512512 pixels on one sensor test chip in 0.18m PIN photodiode CMOS. Single pixels and a 6448 multipixel sensor chip with PIN photodiodes designed and fabricated as ASICs achieved an outstanding measurement accuracy of 2-3mm with excellent suppression of the influence of background light. This showed that PIN photodiodes enable better optical distance sensors despite the high amplification of phototransistors. This is mainly due to larger dynamic range of PIN photodiodes. The development of a noise model confirmed these experimental results.In this project, in addition modulated powerful light sources, characterisation methods for phototransistors and optical multipixel distance sensors as well as FPGA programs and postprocessing software were developed and investigated. Furthermore, methods for the correction of errors due to the temperature dependence of LED light sources were investigated and developed. First, a reference pixel and a reference path with a known and constant fiber length was implemented. Second, the distortions of the measured signal served for the correction of the temperature dependence. In addition, an analytical correction function was derived and programmed within the FPGA in order to compensate the increasing systematic distance error when the number of phase steps is reduced.Summarising, innovative phototransistors and small-area, very precisely measuring pixel circuits were designed, fabricated as ASICs and characterised as well as verified experimentally.