A double-cathode photodetector (DCP) featuring a buried-finger structure to achieve improved separation efficiency is
presented. The interleaving comb-shaped cathodes are realized with n-buried implants and they are located in the
p-epitaxial layer roughly 1μm below the surface. Based on MEDICI device simulations several layout variations have
been realized in a slightly modified BiCMOS process. Best results are achieved with a finger distance of 12μm and a
finger width of 1μm: separation efficiencies of 50, 67, and 54% and responsivities of 0.23, 0.47, and 0.38A/W were
measured for the optical wavelengths 410nm, 660nm, and 850nm, respectively. All test structures occupy optical active
areas of around 100×100μm2. A maximum 3dB-modulation bandwidth of almost 300MHz was measured, while dark
currents in the picoampere range are typical for these detectors up to a bias voltage of 5V at room temperature. In the
application of a time-of-flight (TOF) distance measurement sensor, the DCP serves as optical detector and correlating
device at the same time. Distance measurements up to 6.2m were performed with a 650nm laser source that emitted an
average optical power of 1mW using rectangular modulation signals at 10MHz. The standard deviation is better than
1cm up to 3.4m for a total measurement time of 20ms per acquired distance point.
Within this work a 28 pixels line sensor for distance measurement applications is presented based on the time-of-flight
principle and the double-correlator circuit concept. An on-chip oscillator block generates 15-phase steps of the
fundamental 10 MHz clock with σ = 1.28 %, which is necessary for calculating the triangular correlation function out of
which the distance information is obtained. Measurement results in a range up to 3 m with a standard deviation ≥ 3.5 cm
are achieved. The pixel autonomous background light suppression is capable of managing background illumination
> 100 kLux. A smart bus concept reduces the number of control signals to the pixels and guarantees 80 dB attenuation
from the oscillator signals to the analog differential outputs of the chip. The line sensor was realized in a single-chip
solution embodying the silicon PIN photodiode detectors.
Within this work a new correlating photodetector concept using current carrying photogates for Time-Of-Flight (TOF)
based optical distance measurements is presented. The integrated photodetector consists of a PIN setup with a P+ doped
anode, two N+ doped cathode fingers and a wide low doped intrinsic region in between. Furthermore a resistive
polysilicon photogate is located in between of the readout cathode fingers on top of field oxide. Applying an electrical
modulation signal to this photogate causes a linear potential drop along the resistor as a result of the control current.
Therefore constant electric field is achieved in the photodetector regions below thus effecting photogenerated electrons
to be directed to one or the other cathode, depending on the sign of the field. While positive charges are collected by the
anode below, the modulation signal controls whether photocurrent of incident light is led to readout cathode 1 or 2. Due
to this setup, applied modulation signals cause an optimal potential distribution for efficient correlation of ηsep=80% with
incident optical signals. A responsivity of 0.23A/W {0.21A/W}, a rise time of 19.3ns {18.3ns} and a bandwidth of
f-3dB=22.7MHz {29.6MHz} is measured at 660nm {850nm} together with low dark current of Idark<0.5pA. The capability
of this photodetector is demonstrated at an integrated rangefinder chip in a range of 1.5m-3.5m achieving a standard
deviation of σ<5cm at a white paper target and an optical power of Popt=1.5mW. A comparison of three realized
photodetectors with different shapes of the photogate is done, each with an active area of 100μm×100μm and processed
in 0.6μm BiCMOS.
For various industrial applications contact-less optical 3D distance measurement systems with active illumination are suitable. A new approach for a pixel of such a 3D-camera chip for applications in displacement and 3D-shape measurement is presented here. The distance information is gained by measuring the Time-of-Flight (TOF) of photons transmitted by a modulated light source to a diffuse reflecting object and back to the receiver IC. The receiver is implemented as an opto-electronic integrated circuit (OEIC). It consists of a double-cathode photodetector performing an opto-electronic correlation, a decoupling network and an output low-pass filter on a single silicon chip. The correlation of the received optical signal and the electronic modulation signal enables the determination of the phase-shift between them. The phase-shift is directly proportional to the distance of the object. The measurement time for a single distance measurement is 20 ms for a range up to 6.2 m. The standard deviation up to 3.4 m is better than 1cm for a transmitted optical power of 1.2 mW at a wavelength of 650 nm. The OEIC was fabricated in a slightly modified 0.6 &mgr;m BiCMOS technology with a PIN-photodetector. The photosensitive area of the integrated PIN-photodetector is 120x115 &mgr;m2. A fill factor of ~67% is reached.
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