10000 pixels wide CMOS frame imager for earth observation from a HALE UAV

B. Delauréa, S. Livens a, J. Everaertsa , R. Kleihorsta, G. Schippersb, Yannick De Witb, John

Compietb, Bartosz Banachowiczb aFlemish Institute for Technological Research (VITO), Boeretang 200, B-2400 Mol, Belgium;

b Cypress Semiconductor Cooperation Belgium

ABSTRACT

MEDUSA is the lightweight high resolution camera, designed to be operated from a solar-powered Unmanned Aerial Vehicle (UAV) flying at stratospheric altitudes. The instrument is a technology demonstrator within the Pegasus program and targets applications such as crisis management and cartography. A special wide swath CMOS imager has been developed by Cypress Semiconductor Cooperation Belgium to meet the specific sensor requirements of MEDUSA.

The CMOS sensor has a stitched design comprising a panchromatic and color sensor on the same die. Each sensor consists of 10000*1200 square pixels (5.5µm size, novel 6T architecture) with micro-lenses. The exposure is performed by means of a high efficiency snapshot shutter. The sensor is able to operate at a rate of 30fps in full frame readout.

Due to a novel pixel design, the sensor has low dark leakage of the memory elements (PSNL) and low parasitic light sensitivity (PLS). Still it maintains a relative high QE (Quantum efficiency) and a FF (fill factor) of over 65%. It features an MTF (Modulation Transfer Function) higher than 60% at Nyquist frequency in both X and Y directions The measured optical/electrical crosstalk (expressed as MTF) of this 5.5um pixel is state-of-the art. These properties makes it possible to acquire sharp images also in low-light conditions.

Keywords: CMOS image sensor, frame imager, Unmanned Aerial Vehicle

1. INTRODUCTION

Traditionally earth observation is being conducted from airborne and satellite platforms, each having specific advantages regarding performance parameters such as spatial resolution, ground coverage, availability and flexibility. Depending on the application one platform is better suited than the other. Various technological developments over the last decade have lead to an new type of platform which bridges the gap between manned aircraft and satellites: a local geostationary system operated in the stratosphere. Various platform concepts, mostly airship or aircraft, are being developed aiming at persistent availability for not only earth observation but also for telecommunications [1,2]. Besides the research which is performed on the platform side, an additional challenge lies in the development of instruments which are adapted to the new specifications, environmental and operational conditions of those innovative platforms.

The Pegasus project [3] initiated in 2000 by VITO, the Flemish Institute for Technological research (Belgium), aims to demonstrate the feasibility of performing remote sensing from a high altitude unmanned aerial vehicle. Mercator 1 is an extremely light-weight platform of about 35 kg and a wingspan of 18m. It will be operated at an altitude of 18 km. This aircraft is powered by solar energy and high capacity batteries allowing it to fly for weeks or even months without landing. As such it can be used as local geostationary system, as it is able to remain above a region persistently. The platform is not bound to an orbit and moves at low speed in a region of the atmosphere where wind speeds are known to be limited. A similar airplane, QinetiQ’s Zephyr High Altitude Long Endurance (HALE) Unmanned Aerial Vehicle (UAV), has exceeded the official world record time for the longest duration unmanned flight with a 3 1/2 day flight in 2008 demonstrating [4] the potential of the solar cell-battery combination for persistent flights. MEDUSA is the first remote sensing instrument being developed by VITO for this type of platform and will act as a demonstration instrument in the framework of the Pegasus program.

2. MEDUSA INSTRUMENT DESCRIPTION

The high resolution camera system MEDUSA is being developed by an industrial consortium led by VITO [5,6]. Its first phase target applications are disaster monitoring and large-scale mapping. The top-level instrument requirements extracted from the target applications are summarized in Table 2-1.

Ground resolution 30 cm (@ 18 km ) or less

Wavelength range 400 – 650 nm (RGB)

Swath width 3000 m (>= 10 000 pixels)

SNR 100 @ 8:00 am equinox

Sensor type Frame

Shutter Electronic

Forward overlap 60%

RF downlink range 150 km from the ground station

Table 2-1 MEDUSA system requirements

The MEDUSA payload will operate under very strict and challenging physical and environmental constraints. As the UAV platform is extremely low-weight the total allowed weight for the payload is also restricted to 2-3kg depending on the flight window. The total power consumption is limited to 50 W during the day. No power is available at night, and all mechanical and electronics components have to survive severe thermal cycles with temperature ranges as wide as [-70°C, 60°C]. The barometric environment is 60 mbar. The available space of the payload is restricted to a more or less cylindrical volume of 1 meter length and an outer diameter of 12cm.

Due to the mass constraints it is not possible to incorporate a high grade IMU (Inertial Measurement Unit) that would allow georeferencing with sub-pixel spatial accuracy from an altitude of 18 km. Therefore direct georeferencing will only be used for a rough positioning of the images at the earth surface. Improved positional accuracy down to sub-pixel level will be realized by means of aerial triangulation. Bundle block adjustment typically includes strips of 2D images with approximately 60% overlap in the flight direction and 20% - 30% overlap of the neighbored flight strips.

3. MEDUSA WIDE SWATH CMOS SENSOR

3.1 Top level sensor specifications

In order to combine the swath requirement of 3 km with the GSD of 30 cm the frame sensor needs to be 10000 pixels wide. In order to allow block bundle adjustment to be applied for accurate georeferencing true 2D information is required. Therefore a frame sensor rather than a line scanning imager is used. Additionally the shuttering mechanism on the frame sensor should be global (snapshot rather then rolling) to guarantee acquisition of true 2D data at a well-defined instance of exposure. A pixel size of maximum 5.5 µm was required to keep the focal length and the optical system fit within the restricted volume of the instrument.

Taking into account the fact that a forward overlap of at least 60% is recommended for block bundle adjustment and taking into account the platform behavior of the UAV platform flying at operational altitude a frame rate of 0.7 fps is required. In order to reduce the parasitic light sensitivity (PLS) the sensor has been designed in such a way that it can be read-out in 33ms (approx 30 fps). Apart from this the actual parasitic light sensitivity of the pixel design itself needs to be minimized.

System performance simulations performed at an early design stage have indicated that a COTS Bayer coded color sensor would not allow to meet the SNR requirement under low light conditions (8:00 am at equinox above Flanders) together with minimizing the motion blur (short integration time) to obtain the required MTF. For this reason it has been decided to implement a double sensor system (panchromatic and color) on 1 chip allowing to do pan-sharpening [7]. The gray-scale image is recorded with short integration time minimizing the motion blur in the image and preserving the geometric information. The color sensor is exposed with a longer integration time having impact on the spatial resolution through motion blurring. On ground panchromatic and color images are fused to deliver a high resolution color image as end product. More details on those trade-offs can be found in [8].

Table 2: key specifications of the MEDUSA CMOS sensor.

Technology 0.18µm CMOS process Die size 58mm*9.92mm Array size 10000 (H) x 1200 (V) Frame rate 33 fps at full resolution Shutter type Pipelined snapshot shutter Power consumption <0.5W Operating temperature -40ºC to 60ºC Color Panchromatic and Color

(same die) Pixel type 6T variant Pixel size 5.5 µm x 5.5 µm

3.2 Sensor layout and pixel design

This 12Mpixel snapshot shutter sensor is primary designed to realize high shutter efficiency. The pixel array contains 10000*1200, 5.5um square pixels with microlenses and color. It is able to operate at 33fps in full frame readout with 8 analog outputs each operating at 50 Msamples/s bringing the total sensor output data rate to 0.4 GSamples/sec. A simplified block diagram of the sensor is presented in Figure 2. Both the monochrome and color sensor are processed on the same die. This has the advantage that both the monochrome and color sensor are surface-aligned. The possibility to only dice one sensor (color or mono) is available due to the addition of scribe lines between the two sensors. A picture of the sensor is shown in Figure 1.

Figure 1: MEDUSA wide swath CMOS sensor with color and panchromatic sensor on 1 die.

Figure 2: Layout of the MEDUSA CMOS sensor.

Figure 3: low PLS/PSNL, high fill factor 6T pixel architecture.

Due to a new pixel design, this sensor has low dark leakage of the memory elements (PSNL, pixel storage node leakage), low parasitic light sensitivity (PLS) while still maintaining a relative high FF(fill factor) and QE (Quantum efficiency) as well as a high MTF. The pixel design schema is presented in Figure 3. As can be seen in the schema, the memory capacitor exists of a transistor where the signal is sampled on the poly-gate of the memory transistor which is properly biased to form a stable capacitor. Another improvement compared to traditional 6T pixels is a better way of precharging this memory capacitor. By applying these new design improvements, the stored value inside the pixel will be much less sensitive to light, dark junction leakage and drain source leakage of the precharge transistor, improving PLS and PSNL of the pixel while maintaining a high fill factor. Also a novel column structure and bus driving circuit has been

implemented. Due to this new bus driving topology, the internal bus speed has increased by a significant decrease in bus settling time. As a consequence of the increased bus speed, single pixel kernels are possible avoiding the need to do after-bus multiplexing. The new column amplifier design also results in an improvement for the column fixed pattern noise of the sensor (column FPN). Both the old traditional column structure, as well as the new column structure, have been included in this sensor to evaluate both structures against each other.

4. CHARACTERISATION RESULTS

The sensor has passed a complete test program to verify compliance against all requirements. The most important results are listed in Table 3. More details on a number of measurements are described in the next sections.

Table 3: most important test results of the sensor characterization campaign

Peak QE 48% 550nm, with micro lenses Conversion gain 35 µV/e¯ Temporal dark noise 28e¯ Pixel saturation charge >35000 e¯ Dynamic range 62dB SNR(shot noise limited) >48dB Dark current 6mV/s at 20ºC PRNU 1.6 % @ half saturation FPN 30mV Column FPN <3mV PLS <1/30000 PSNL <2mV/s @ 20ºC MTF @ 630nm @ Nyquist freq.

>60% in both X and Y directions

4.1 Spectral response and quantum efficiency

The measurement of the spectral response is done on an on chip test pixel array. This array contains a number of pixels (identical to the imager-pixel) of which the photodiodes are connected together in order to measure the photocurrent. The measurement consists of three phases: Using a monochromator a parallel beam is produced with a wavelength between 400nm and 1000nm variable in 10nm steps. The band pass is approximately 5nm. Before the actual measurement, the illumination power is measured at each wavelength of interest using a calibrated radiometer. After calibration of the set-up, the devices are positioned such that the test-pixel array is at exactly the same location as the center of the reference detector. Next the output-current of the test-structure is measured for every wavelength of the calibrated light source.

Where spectral response λSR and Quantum efficiency λQE have been defined as:

q

chSRQE

AP

ISR

arrayill

array

⋅⋅⋅

=

⋅=

λλ

λ

λ

with arrayI the photocurrent of the test structure [A], illP the illumination power [W/cm²], arrayA surface of the test

structure [cm²], h Planck constant [Js], c speed of light [m/s], λ wavelength [m] and q the charge of the electron [C].

Figure 4: spectral response curve for Red (R), Green (G), Blue (B), panchromatic (mono) pixels and full well.

Figure 5: quantum efficiency of 3 different MEDUSA devices for panchromatic and full-well diode.

4.2 Column FPN

As seen in the raw image shown in Figure 6, column FPN is not noticeable by naked eye when the new column structure is operational. Figure 7 presents the image when the traditional column structure is used. Comparing these figures proves

that the image quality has improved significantly, the column FPN has decreased by a factor of 3.8 with the new developed column structure.

Figure 6: Raw image with new column structure: no column FPN observed with naked eye.

Figure 7: raw image with old column structure: column FPN is introduced as well as a significant lower dynamic range.

5. CONCLUSION

In order to meet the specific requirements imposed by the high resolution MEDUSA camera system a 10000 pixels wide CMOS frame imager has been developed. This has resulted in a sensor with a unique geometry (10000x1200 pixels, two sensors on single die) and pixel design with novel global shutter. Key parameters for this imager are high shutter efficiency (>86dB), low crosstalk (MTF>60%) and a dynamic range of 62dB (linear) to 90dB (multiple slope operation). The sensor is designed in a standard 0.18µm CMOS process and consumes less than 0.5W in full operating mode at 33fps. Apart from earth observation for which the sensor has been designed, the sensor has interesting characteristics for other application fields like industrial inspection.

ACKNOWLEDGEMENTS

The MEDUSA phase C/D project is funded by the European Space Agency (ESA) under the PRODEX program.

REFERENCES

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[7] Zhang, Y., Understanding Image fusion, Photogrammetric engineering and remote sensing, June 2004, pp. 658-661. [8] Van Achteren, T., Delauré, B., Everaerts, J., 2006. “Instrument Design for the Pegasus HALE UAV Payload.”

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