Two High-Precision Proximity Capacitance CMOS Image Sensors with Large Format and High Resolution

Abstract

This paper presents newly developed two high-precision CMOS proximity capacitance image sensors: Chip A with 12 μm pitch pixels with a large detection area of 1.68 cm²; Chip B with 2.8 μm pitch 1.8 M pixels for a higher resolution. Both fabricated chips achieved a capacitance detection precision of less than 100 zF (10^-19 F) at an input voltage of 20 V and less than 10 zF (10^-20 F) at 300 V due to the noise cancelling technique. Furthermore, by using multiple input pulse amplitudes, a capacitance detection dynamic range of up to 123 dB was achieved. The spatial resolution improvement was confirmed by the experimentally obtained modulation transfer function for Chip B with various line and space pattens. The examples of capacitance imaging using the fabricated chips were also demonstrated.

Keywords: CMOS; high precision; high resolution; image sensor; large format; proximity capacitance.

Figure 1

The 3D models of the sensor and the target, and the captured images for each target: (a) conductive target; (b) dielectric target; (c) particulate targets.

Figure 2

The schematics of the two chips: (a) the circuit block diagram; (b) the circuit schematic of the pixels.

Figure 3

Principle of operation of proximity capacitance image sensor: (a) simplified circuit and noise cancelling principle; (b) timing diagram of normal mode; (c) timing diagram of HDR mode.

Figure 4

The pixel layout diagrams of the two chips: (a) up to the 1st metal layer of Chip A; (b) up to the 1st metal layer of Chip B; (c) the top metal layer.

Figure 5

The micrograph of the fabricated CMOS proximity capacitance image sensors: (a) Chip A; (b) Chip B.

Figure 6

The cross-sectional pixel TEM image of the two chips: (a) Chip A; (b) Chip B.

Figure 7

The measurement system for characterizing the performance of fabricated sensor chips: (a) picture; (b) block diagram.

Figure 8

Various assembly methods of the sensor chips: (a) ceramic package with bonding wires; (b) resin potting; (c) polyimide film with flexible wires.

Figure 9

Captured images without and with noise cancelling and their histograms in the absence of targets: (a) Chip A; (b) Chip B.

Figure 10

Measurement to determine the capacitance value of C_C: (a) sensor chip irradiated with light; (b) pulse timing; (c) histogram of measured C_C of Chip B and C_C values of the two chips.

Figure 11

Measured transfer characteristics of the two chips: (a) measurement methods; (b) Chip A; (c) Chip B.

Figure 12

Calculation of capacitance and distance detection precision: (a) cross-sectional schematic diagram of a chip surface and a conductor target with small irregularities; (b) calculation results of the relation between the measurement capacitance C_S and the capacitance detection precision ΔC_Smin at each input voltage V_IN; (c) relation between the reference distance x between the chip surface and the target and the distance detection precision Δx_min.

Figure 13

Resolution measurement of Chip B: (a) cross-sectional structure of the chip with metal wiring created on the surface; (b) captured images of the wiring for each pitch; (c) output signal for each pixel when 20 µm pitch wiring captured by Chip B; (d) MTF calculated from the measurement results.

Figure 14

Captured image of a fingerprint by the large format Chip A (blurred for privacy).

Figure 15

Captured images of a general-purpose logic IC (TC74HC02): (a) captured by optical image sensor using a microscope; (b) captured by Chip A; (c) captured by Chip B.

Figure 16

Comparison of images captured of a general-purpose IC chip with the sensor operating in normal mode and HDR mode: (a) Normal mode; (b) HDR mode.

Figure 17

Captured images of a drop of saline solution on the sensor surface drying out as time advances captured by Chip B: (a) sample images; (b) closed-up images of red rectangle in (a).

Figure 18

Captured images of printed circuit board wiring by optical image sensor and Chip A: (a) no defects; (b) open defect; (c) short defect. φC was applied only to the center wire indicated by the red arrows.

Figure 19

Captured image of flat panel display with a line defect by Chip A.

Figure 20

Benchmark comparison with other sensors [6,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,26]: (a) comparison of detection precision and pixel area; (b) comparison of detection precision and pixel pitch and examples of target.