Nick Waltham

CCD and CMOS sensors

The charge-coupled device (CCD) has been developed primarily as a compact image sensor for consumer and industrial markets, but is now also the preeminent visible and ultraviolet wavelength image sensor in many fields of scientific research including space-science and both Earth and planetary remote sensing. Today’s scientific or science-grade CCD will typically maximise pixel count, focal plane coverage, photon detection efficiency over the broadest spectral range and signal dynamic range whilst maintaining the lowest possible readout noise. The relatively recent re-emergence of complementary metal oxide semiconductor (CMOS) image sensor technology is arguably the most important development in solid-state imaging since the invention of the CCD. Current sub-micron CMOS technology now enables the integration on a single silicon chip of a large array of photodiode pixels alongside all of the ancillary electronics needed to address the array and digitise the resulting analogue video signal. Compared to the CCD, CMOS promises a more compact, lower mass, lower power and potentially more radiation tolerant camera.

A large-area CMOS monolithic active pixel sensor for extreme ultraviolet spectroscopy and imaging

We describe our programme to develop science-grade CMOS active pixel sensors for future space science missions, and in particular an extreme ultra-violet spectrograph for solar physics studies on the ESA Solar Orbiter. Our goal is the development of a large format 4k x 4k pixel CMOS sensor with useful sensitivity in the extreme ultra-violet (EUV) for solar physics spectroscopy and imaging. Our route to EUV sensitivity relies primarily in adapting the back-thinning and rear-illumination techniques first developed for CCD sensors; however we are also exploring the alternative approach of using a front-etch to expose the CMOS photodiodes. We have successfully back-thinned several 525 x 525 prototype CMOS sensors and proved that the devices survived the process both structurally and functionally. We have also been successful in removing the oxide from the front side of a small array of pixels, using focused ion beam etching. Preliminary results from these pixels show they are sensitive in the Ultra Violet. We have also designed a working large format 4k x 3k prototype on a 0.25 micron CMOS imager process.

Design of a 3 μm pixel linear CMOS sensor for earth observation

A visible wavelength linear photosensor featuring a pixel size of 3µm has been designed for fabrication using commercial 0.25µm CMOS technology. For the photo-sensing element, the design uses a special deep N-well in P-epi diode offered by the foundry for imaging devices. Pixel reset is via an adjacent p-FET, thus allowing high reset voltages for a wide pixel voltage swing. The pixel voltage is buffered using a voltage-follower op-amp and a sampling scheme is used to allow correlated double sampling (CDS) for removal of reset noise. Reset and signal levels are buffered through a 16:1 multiplexer to a switched capacitor amplifier which performs the CDS function. Incorporated in the CDS circuit is a programmable gain of 1-8 for increased signal-to-noise ratio at low signal levels. Data output is via 4 analogue output drivers for off-chip conversion. Each driver supplies a differential output voltage with a ± 1V swing for 6.25kHz. This gives a peak data rate at each output driver of 10M sample/s. The device will operate on a 3.3V supply and will dissipate approximately 950mW. Simulations indicate an equivalent noise charge at the pixel of 66.3e - for a full well capacity of 255,000e - , giving a dynamic range of 71.7dB.

Magnetically guided Cesium interferometer for inertial sensing

In this paper, we demonstrate a Talbot-Lau interferometer with magnetically guided Cesium atoms for inertial sensing using both three-pulse and prolonged four-pulse interferometer schemes. The recoil frequency of the Cesium atoms and the acceleration along the waveguide symmetry axis are measured. An acceleration measurement uncertainty of 7 mGal is achieved. We also realize an enclosed area of 0.018u2009mm2 for rotation sensing. The system limitations and its advantages are also discussed.

A multi-channel CCD clock driver ASIC for space-based applications

A high voltage mixed signal ASIC is described that provides multiple fully programmable clock outputs capable of driving large format CCD capacitive electrodes. The COMET ASIC provides 6 independent clock buffering channels each with individually programmable rising/falling current drive and high/low voltage levels. Output voltage levels are controlled with integrated fast response regulators that operate over a 16.368V range without the need for external decoupling capacitors. Clock drive currents can be adjusted for the load capacitance and voltage swing required over a 409.6mA range, with edge speeds <15ns achievable for small loads. Setup and control of the ASIC is via a simple SPI interface with safety features to ensure correct sequencing of channel operation and prevent driver supply reverse biasing. The ASIC also features an under-voltage lock out circuit to safeguard the chip in the event of power loss. All necessary biases are generated internally and only supply decoupling, a single filtering capacitor, and a resistive divider are required to operate the device. The circuit is manufactured in a commercial 0.35um HV CMOS technology and uses established layout techniques to harden against both Total Ionising Dose (TID) and Single Event Latchup (SEL) radiation effects. The device has been manufactured and test results are shown.

DCDS weighted averaging theory and development for improved noise filtering in scientific CCD applications

Readout noise is a key factor in the performance of optical systems based on charge coupled devices (CCDs). Recent developments have shown that digital correlated double sampling (DCDS) using weighted averaging may provide a further reduction in the system readout noise. This paper describes recent advances in noise filtering using DCDS. Particular emphasis is placed on optimising weighted averaging filters to reduce 1/f noise and the characterisation of system performance when using the unsettled samples within the pixel period. Experimental results are presented and compared with theoretical predictions based on the extracted noise spectrum. The analysis provides a detailed study of the relationship between the 1/f corner frequency, the pixel frequency and weighted averaging technique in comparison with the theory of matched filters. Furthermore, the results include a comparison of the noise profile with measured and simulated noise patterns. Key system metrics, including linearity and gain stability, have been characterised and are presented to confirm the suitability of this technique for high-performance scientific applications.

Sensor system development for the WSO-UV (World Space Observatory–Ultraviolet) space-based astronomical telescope

As part of a strategy to provide increasingly complex systems to customers, e2v is currently developing the sensor solution for focal plane array for the WSO-UV (World Space Observatory – Ultraviolet) programme, a Russian led 170 cm space astronomical telescope. This is a fully integrated sensor system for the detection of UV light across 3 channels: 2 high resolution spectrometers covering wavelengths of 115 – 176 nm and 174 – 310 nm and a Long-Slit Spectrometer covering 115 nm – 310 nm. This paper will describe the systematic approach and technical solution that has been developed based on e2v’s long heritage, CCD experience and expertise. It will show how this approach is consistent with the key performance requirements and the overall environment requirements that the delivered system will experience through ground test, integration, storage and flight.

EUV and Soft X-Ray Quantum Efficiency Measurements of a Thinned Back-Illuminated CMOS Active Pixel Sensor

We report the first absolute effective quantum efficiency (e-h pairs collected/predicted) measurements of a monolithic thinned back-illuminated CMOS active pixel sensor (APS) in the extreme ultraviolet and soft X-ray region (13-600 Å). The sensor was designed and fabricated under a joint Rutherford Appleton Laboratory/e2v research program and characterized in the Lockheed Martin Solar and Astrophysics calibration facility. We compare our QE results to the data and models developed for thinned CCDs with similar back surface passivation. Our results demonstrate that CMOS APS arrays show significant promise for use in space-based solar physics and astrophysics missions.

A pair of custom ASICs for bias generation and clock buffering in space-based CCD camera systems

A pair of radiation hardened high-voltage mixed signal Application Specific Integrated Circuits (ASICs) are described that provide the biasing and clocking functions required to drive large format CCDs used for space-borne cameras and focal planes. The use of these ASICs allows the CCD drive electronics to be realised in a compact and energy efficient manner saving volume, mass, and power when compared with traditional space-qualified discrete implementations. The STAR ASIC provides 24 independent voltage outputs with a 32.736V range at 10 bit resolution and with <100μV noise. Each voltage output provides a drive current of up to +/-20mA and is stable for capacitive loads of up to 10μF. An on-board telemetry system featuring a 12-bit ADC and programmable gain buffer allows internal monitoring of the output voltages plus up to 32 single ended and 4 differential external voltages, such as from PRT bridge circuits for temperature monitoring. A simple SPI serial interface provides control and telemetry read back, while all required voltages and currents are generated from internal bandgap circuits. The COMET ASIC provides 6 fully independent clock buffering channels each with individually programmable rising/falling current drive and high/low voltage levels. Output voltage levels are controlled with integrated fast response regulators that operate over a 16.368V range without the need for external decoupling capacitors. Clock drive currents can be adjusted for the load capacitance and output slew rate required over a 409.6mA range, with edge speeds <15ns achievable for small loads. Setup and control of the ASIC is also via an SPI interface with integrated safety features to ensure correct sequencing of channel operation and to prevent reverse biasing of the driver programmable voltage supplies. The COMET ASIC also features an under-voltage lock out circuit to safeguard the chip in the event of unexpected power loss. All necessary biases are generated internally and only supply decoupling, a single filtering capacitor, and a resistive divider are required to operate the device. Both devices have been designed in a commercial 0.35μm 50V tolerant HV CMOS technology using Triple Module Redundancy (TMR) and established layout techniques to harden against Total Ionising Dose (TID), Single Event Upset (SEU), and Single Event Latch-up (SEL) radiation effects. The latch-up detection circuits often needed for space electronics are therefore not required for either ASIC. Details of the architectures and circuit implementations of both ASICs will be presented. Test results from manufactured devices will be shown under representative load conditions.

Management of the camera electronics programme for the World Space Observatory ultraviolet WUVS instrument

World Space Observatory Ultraviolet (WSO-UV) is a major international collaboration led by Russia and will study the universe at ultraviolet wavelengths between 115 nm and 320 nm. The WSO Ultraviolet Spectrograph (WUVS) subsystem is led by a consortium of Russian institutes and consists of three spectrographs. RAL Space is contracted by e2v technologies Ltd to provide the CCD readout electronics for each of the three WUVS channels. The programme involves the design, manufacturing, assembly and testing of each Camera Electronics Box (CEB), its associated Interconnection Module (ICM), Electrical Ground Support Equipment (EGSE) and harness. An overview of the programme will be presented, from the initial design phase culminating in the development of an Engineering Model (EM) through qualification whereby an Engineering Qualification Model (EQM) will undergo environmental testing to characterize the performance of the CEB against the space environment, to the delivery of the Flight Models (FMs). The paper will discuss the challenges faced managing a large, dynamic project. This includes managing significant changes in fundamental requirements mid-programme as a result of external political issues which forced a complete re-design of an existing CEB with extensive space heritage but containing many ITAR controlled electronic components to a new, more efficient solution, free of ITAR controlled parts. The methodology and processes used to ensure the demanding schedule is maintained through each stage of the project will be presented including an insight into planning, decision-making, communication, risk management, and resource management; all essential to the continued success of the programme.