TECHNOLOGY - ACTIVE PIXEL SENSORS
The CMOS active pixel sensor (APS), first developed in the 1990s, has progressively replaced other forms of solid state imaging technology such as the charge-coupled device (CCD) and thin film transistor (TFT). While these technologies are still used for certain applications, CMOS is now the mainstream technology in almost every imaging device today, ranging from low-cost mobile phone cameras to advanced 3-d sub-atomic particle detection systems.
The reasons behind the explosion of CMOS imaging devices are cost and performance. CMOS devices can be mass produced with low cost, while the advance of chip technology has ensured that CMOS performance can excel in terms of low noise, stability and high speed.
The original APS was built around a simple circuit topology of a photodiode and 3 or 4 transistors. As Moore’s Law enabled ever greater complexity in a smaller scale, pixels can now have complexity, and can incorporate photon counting and digital logic.
ISDI’s patented pixel design combines advanced low noise design with radiation-hard properties, making it ideal for x-ray applications where the total dose may be up to 100kGy.
Our wide experience in research projects and non-standard sensor design provides capabilities to design for a wider spectral sensitivity, ranging from infrared to ultraviolet. Our knowledge of scintillators and conversion techniques allows us to design detectors with photon detection from low kV to MV.
DynAMITe – a dual resolution radiation hard sensor
DynAMITe was commissioned for a EPSRCC Grand Challenge project for radiotherapy imaging. Designed in 2010, the 13 x 13cm imager was the largest wafer-scale radiation-hard detector ever created.
A unique and original feature of DynAMITe was the combination of two pixel resolutions, providing a 100µm and 50um pixel array covering the same area, and allowing simultaneous low-resolution and high-resolution readout combined with multiple ROIs (regions of interest). In a typical MV imaging application, this allows fiducial markers implanted in a prostate tumour to be tracked at high speed while tissue images are read out at higher resolution and lower speed.
- 13.1 x 13.4cm active area
- Dual spatial resolution – 2,560 x 2,624 pixels at 50 µm pitch, and 1,280 x 1,312 pixels at 100 µm pitch
- Four independent cameras in one physical array with random addressing
- Radiation-hard array – considered “best-in-class”
- Two-side butting
- Frame rates up to 15 fps for 50 um pixels; 90 fps for 100 um pixels
- Fully programmable multiple regions of interest
- Multiple readout options including non-destructive and full CDS
- Built-in redundancy to enhance yield
- Power consumption 1W
DUOS – multiple ROIs with programmable exposure times
The DUOS sensor was designed to acquire images with extreme dynamic range. Used mainly in scientific and measurement applications such as X-ray crystallography and spectral analysis, the sensor can define ROIs, each with its own frame rate.
Using this technique, the sensor can combine sub-millisecond (ROI) exposure times with several seconds of background exposure, on the same image. The example below shows a single video image capture of a room, capturing both the shadow detail in the room and the filament of the light bulb. Simultaneous frame rates, with a ratio as high as 10,000:1 can be achieved.
Programmable regions of interest (ROIs) with different integration times/frame rates
- Position and size of multiple non-overlapping ROIs are dynamically programmable
- 1K x 1K pixel array at 20 µm pitch
- Leakage current = 400 e–/s
- Noise floor = 30 e– rms
- Minimum ROI = 4 x 1 pixels
- Full-frame rate up to 80 Hz
- Example ROI frame rate : 48 x 48 pixels ~ 10,000 fps
- Transmission electron microscope (TEM) detector built for Pi-Phi project (http://eeepro.shef.ac.uk/piphi/index.html), and installed on the Joel R005,one of the world's highest resolution microscopes -.
- TEM used at the University of Oxford (http://www-em.materials.ox.ac.uk/)
- TEM for the German SALVE I-II project (http://www.salve-project.de).
- The x-ray version of DUOS has been installed on the Diamond Light Source - the UK's Synchrotron Facility (http://www.diamond.ac.uk).