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.
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.
Contact-less optical distance measurement systems are necessary to obtain 3D-information of an entire scene. To be able to determine depth information of the scene by a sensor without moving parts like e.g. scanner, it is necessary to measure the distance from the camera to an object in every single pixel. A new pixel for such a 3D-camera is presented. The operating principle is based on the time-of-flight (TOF) of laser light from 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 fast, efficient PIN-photodiode having a 3dB bandwidth of about 1.35 GHz, a single-stage transimpedance amplifier and an electronic mixer on a single silicon chip. By correlating the received optical signal and the original electronic modulation signal, the phase-shift between sent and received signal can be determined. By performing correlation with a delayed modulation signal it is possible to eliminate the influence of object reflectivity and background illumination. The measurement time for a single distance measurement is 500μs for a range up to 3.7m. The standard deviation at 2.5m is better than 3cm for a transmitted optical power of 1.44mW at a wavelength of 650nm. The OEIC was fabricated in a slightly modified BiCMOS 0.6μm process. The diameter of the photosensitive area of the integrated PIN-photodiode is 100μm. The effective pixel size is about 220x400μm2. Therefore a fill factor of ~9% is reached.
Integrated optical distance measurement systems based on the Time-of-Flight (TOF) principle open up 3D vision for various applications like e.g. inspection systems. The introduced single pixel consists of both, a PIN photodiode and a signal-processing circuit on chip. Due to eye-safety reasons, the optical illumination power is limited (Popt<2mW). For diffuse reflecting objects in distances up to several meters, signal attenuation of about -50dB occurs with 1-inch optics. Therefore high responsivity of the photodiode is required: R=0.36A/W at 660nm. Resolutions of centimeters matter TOF far below 1ns, i.e. the photodiode has to feature high bandwidth (f3dB=1.35GHz). Distance information is gained by correlation between the modulated transmission signal and the run- ime delayed, attenuated received signal. The readout circuit consists of three stages: the first stage is a broadband current amplifier, realised with current mirrors. The correlation is performed in the second stage by a switching mixer. Amplification and smoothing is performed in the third, active integrator stage. The distance information is derived from the output signal by external sampling and simple data processing. A standard deviation of better than 1% (2%) for distances up to 2m (3.7m) is achieved for measurement durations of 10ms. The primary linearity error of less than 6cm is educed by error correction. The pixel has a fill factor of ~10%, including the overall pixel area of ~460µm×170µm and the photodiode with a diameter of 100µm. The chip was realised in a 0.6µm BiCMOS ASIC process.