CIS Output Signal Characterization

imagesensorex05.in : CIS Output Signal Characterization

Required Athena/Monte Carlo Implant/Luminous/Deckbuild
Minimum Version Athena 5.22.3.R, Atlas 5.28.1.R

This examples shows the creation and calibration of a 3.3 Volt CMOS Image Sensor (CIS), using a standard 10 ohm-cm epitaxy process. The Image Sensor is created using Athena process simulation and monte-carlo implantation.

The pass transistors have a high threshold voltage of approximately 2.5 volts to minimize leakage currents in their off state. Since the signal output contact of the image sensor is floating, the threshold voltage of the two pass transistors is determined by using the probe statement, whereby the electron concentration under their gates, tg_conc and rst_conc, are used to determine the relative conductivity of the channel, rather than the usual drain current plot.

The first transient simulates what happens when the image sensor is depleted of electrons, and is simply left to integrate thermally generated charge (leakage current) in the dark for a period of 10 seconds. The transient_dark.log plot shows that the concentration of electrons in the depleted image sensor well, starts to rise after approximately 0.1 seconds, so this effectively sets a maximum integration time limit on the image sensor to obtain a normal contrast image.

The second set of transients, numbering 10 in all, calibrate the image sensor output for a typical light intensity range that would be expected in a normal out door setting. The simulated intensity range was one order of magnitude, between 0.02 and 0.2 times the intensity of a standard solar AM-1.5 spectrum. It was found that the optimum integration time for such a photograph with this intensity range, was approximately 7.5e-04 seconds for this particular image sensor. This integration time indicates the image sensor can be used in a range from bright sunshine images down to an average intensity of approximately four orders of magnitude less than this.

The 10 simulations for the different intensities for written using the loop capability in deckbuild. The loops are bounded by the deckbuild syntax loop steps=16 and l.end The values of intensity are increased inside each loop for each simulation using deckbuild's set statements, which assign a value to the light intensity variable defined in the set statement.

The output signal voltage of the image sensor is taken from the common source / drain contact of the two series pass transistors. In order to save the complication of creating a separate contact at this location, and then defining this contact as a high resistance voltage sense contact, the probe statement was used once again, to probe the potential at this location.

The figure of image sensor output voltage (fd_potential) versus light integration time up to the chosen integration time of 7.5e-5 seconds, shows the effect of light intensity variation on the output voltage. The output voltage at the final time of 7.5e-5 seconds is the final outptut signal of the image sensor and is captured by an extract statement. The values of the extract statement were then collated in the final plot and show the output signal characteristics of the image sensor versus light intensity reflected from an image.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.