Integration of Photo Sensors and Signal Processing

Fraunhofer Institute for Integrated Circuits

Integration of Photo Sensors and Signal Processing

Standard CMOS processing normally provides several pn-junctions which can be used as photo sensors. Together with the signal processing an integration offers a high level of failure immunity. The amplified analog sensor signals/photoelectric currents are transformed into a digital signal which is transferred outside.

Characteristics

  • Wavelength range for optical sensors: 400 nm – 1100 nm
  • Cost-effective standard CMOS processing
  • High dynamic range: 107
  • On-chip integration of analog and digital signal processing
  • Availability of standard cell libraries for several applications

Application spectra

These embedded photo sensors offer a wide range of applications:

Light intensity measurement with digital interface

Many applications require an easy to use interface transmitting the sensor signal to the connecting unit. We therefore developed a sensor front end in which a capacitor is discharged by the photo current of a photodiode and is recharged by a Schmitt-Trigger. The output frequency of the Schmitt-Trigger is consequently an adequate value for the light intensity. Additional on-chip counter circuits allow to determine a digital value and to pass it to an external micro processor through a serial interface.

Sensors with high sensitivity and dynamics

To achieve a low detection limit we use dark current compensation by placing a photo diode with metal cover close to the actual detector and subtract their currents. A sensitivity of less than 1 pW of optical output power and a dynamic range of more than 107 could be achieved in a special application. It was implemented in 0.6 µm CMOS technology.

On-chip signal processing

The devices are manufactured in commercially available standard CMOS processes. This ensures cost-effective production without any additional process steps and a high reproducibility of the integrated sensors. Furthermore, the on-chip integration of analog and digital post-processing of the recorded signal is possible. Integrable signal processing blocks are e. g. preamplifier, filters, analog-digital converters and the provision of serial and parallel interfaces. For various applications, designs can be derived from standard cell libraries at Fraunhofer IIS.