Orit Skorka and Dileepan Joseph University of Alberta, Canada Reducing Crosstalk in Vertically- Integrated CMOS Image Sensors

2 Introduction

Vertically-integrated image sensors 3 Vertical-integration is a new trend in IC design. Each die can be fabricated in a process optimized for the devices it contains. Image sensors contain photodetectors, analog circuits, and digital circuits. More degrees of freedom in the design of the photodetector; no longer limited to a certain CMOS process.

Vertically-integrated image sensors 4 Vertical stacking allows more circuits to be placed within the same pixel area. Enables reasonable pixel dimensions while having: More pixel-level electronics to improve signal-to-noise ratio (SNR) and dynamic range (DR); High fill-factor.

5 Background

6 Device structure VI-CMOS image sensor made by flip-chip bonding

Reduction of crosstalk 7 Crosstalk – A situation where signals reach destinations other than their original ones. Flow of lateral currents in the light-sensitive semiconductor can cause crosstalk in the VI- CMOS image sensor shown. Lateral currents are caused by drift and diffusion of charge carriers.

Reduction of crosstalk 8 One way to prevent flow of lateral currents is by device patterning. However: Edges of patterned devices introduce imperfections and defect states; The additional lithography steps increase the overall manufacturing costs. In standard CMOS image sensors, the photodetector must have well-defined borders. Therefore, the problem is unique to VI-CMOS image sensors.

Reduction of crosstalk 9 The method used here is based on maintenance of a uniform vertical electric field across the unpatterned photodetector array.

Primary circuit requirements 10 Although this idea has been used by Schneider et al., they do not address important design considerations such as stability and compensation. In standard CMOS APS image sensors, the read-out is based on the voltage across the photodetector. Since the photodetector voltage must be kept constant, its current should be used as the input signal. For an efficient design, one needs to know the expected range of the photodetector current, I pd.

Primary circuit requirements 11 Hydrogenated amorphous silicon photodetector:

Proposed solution 12 With a proper design of the read-out circuit, a high DR image sensor can be made with a-Si:H photodetectors. Logarithmic response is preferred because it supports a higher DR than a linear one. To conclude circuit requirements: Maintain a constant voltage on the photodetector; Use I pd as the input signal; Have a logarithmic response to illuminance.

13 Method

Feedback circuit overview 14 A feedback element varies I pd to keep node a at a constant potential, V ref. The feedback element should generate I pd that is related to V out logarithmically. The resistance, R ph, decreases with illuminance.

Optional circuit topologies 15 There are three main feedback topologies that achieve a logarithmic response:

Bias point analysis 16 The common-drain is the only topology in which the feedback loop does not draw current from the op-amp. If I pd is drawn from the op-amp, I b » max(I pd ), which significantly increases the power consumption.

Optional circuit topologies 17 The NMOS transistor can be replaced with a PMOS one if an opposite polarity is required.

Small-signal analysis 18 In spite of having a negative feedback, the pixel circuit might oscillate because phase changes may result in positive feedback. The frequency response of the simplified small-signal model shows that the loop gain has two poles and a finite zero.

19 Results

DC response 20 Two op-amps have been designed: In the common-drain topology, I b equals 1µA; In the others, I b equals 38µA.

Compensation capacitor 21 The operating point of the circuit changes with illuminance, and so does the frequency response. The phase margin (PM) decreases with illuminance. To ensure PM ≥ 60 0 at all bias points, the value of a compensation capacitor, C C, was calculated for the highest expected I pd. Two equations (with two unknowns) need to be solved: |ßA OL ( f 0dB, C C )| = 1; ßA OL ( f 0dB, C C ) = 60°. The solution gives f 0dB = 29MHz and C C = 60fF.

Loop gain Simplified modelCadence simulation 22

Transient response 23 The transient response is checked to ensure that the circuit does not oscillate. Transition times are asymmetric, typical of logarithmic circuits.

24 Conclusion

25 This work introduced circuits to reduce crosstalk in VI-CMOS image sensors with an unpatterned photodetector array. Flow of lateral currents can be reduced by applying a constant electric potential at all pixels of the unpatterned photodetector array. The read-out circuit is required to: Maintain a constant potential at the photodetector; Use the photodetector current as the input signal; Generate a logarithmic response.

Conclusion 26 The circuit is implemented using a logarithmic amplifier with feedback control. The common-drain configuration is found to be the most power efficient. A simplified small-signal model of the circuit has been developed to test the system for stability and determine the value of the compensation capacitor. Transient simulations confirm that the circuit does not oscillate in bright light when C C is added.

Acknowledgments 27 The authors gratefully acknowledge the support of: Alberta Ingenuity; The Natural Sciences and Engineering Research Council (NSERC) of Canada; Micralyne Inc; CMC Microsystems.

28 Appendix