ABSTRACT
The
world might have been reaching new heights in technology, man might have made
artificial residential environment in outer space. But all this will end up to
one primary objective. The aim of every human research is how to make life more
comfortable and decrease the mortality of human beings.
For
this reason a crucial visual study of animal tissue and environment is highly
critical. The number of CMOS technology used as light sensors in different
imaging system has been on rise in recent decade. The use of appropriate CMOS
photo-detector structures and imagers can bring a revolution in field of
medicine, agriculture and bio-defence. Some of the pronounced fields where
photo-detectors are regularly used are protein detection, gene expression, cell
migration and evaluation of animal models of human cancer.
Therefore,
to develop we need to make the available technology cheap and miniaturise it,
so that it can be made available through hand held devices, and can be brought
to masses as a low cost technology implementation.
INTRODUCTION
Imaging
has been very attractive sphere of interest for many individuals. Some refer it
to be hobby, some refer it to quest, some refer it to research, and some refer
it to be revelation. The desire of man to represent and record the 3-D world
around him and to be capable of retrieving the data at any desired moment has
led to development of many new technologies which wouldn’t have existed 2-3
decades ago.
Medicine
industry has seen many transitions in technology, because it needs state of art
artefacts to detect diseases and different infections. The technology currently
used is Fluorescence. Next to it is Photo Multiplier Tubes. These devices are
capable of rendering high gain to currents, and require high operating
voltages.
The
technology which is creeping into use is CMOS based technology. These include pixels with photo current
integration, high dynamic range imaging, and avalanche photodiode system. These
systems are briefly discussed in this paper and their results have been
compared.
Fluorescence
Cells contain molecules, which become fluorescent when excited by
UV/Vis radiation of suitable wavelength. This fluorescence emission, arising
from endogenous fluoro-phores, is an intrinsic property of cells and is called
auto-fluorescence to be distinguished from fluorescent signals obtained by
adding exogenous markers. The majority of cell auto-fluorescence originates
from mitochondria and lysosomes. Together with aromatic amino acids and
lipo-pigments, the most important endogenous fluorophores are pyridinic (NADPH)
and flavin coenzymes. In tissues, the extracellular matrix often contributes to
the auto-fluorescence emission more than the cellular component, because
collagen and elastin have, among the endogenous fluorophores, a relatively high
quantum yield. Changes occurring in the cell and tissue state during
physiological and/or pathological processes result in modifications of the
amount and distribution of endogenous fluoro-phores and chemical-physical
properties of their microenvironment. Therefore, analytical techniques based on
auto-fluorescence monitoring can be utilized in order to obtain information
about morphological and physiological state of cells and tissues. Moreover,
auto-fluorescence analysis can be performed in real time because it does not
require any treatment of fixing or staining of the specimens. In the past few
years spectroscopic and imaging techniques have been developed for many
different applications both in basic research and diagnostics.
Photo Multiplier Tubes
The current state of art
light sensitive device is PMT (Photo Multiplier Tube). They have very high
gain, approaching 1 million. And for the very same reason they are very bulky
and also very costly. They require operating voltages of around hundreds to
thousands of volts. So they are out of question to be used in hand held device.
Another disadvantage of PMT is that the efficiency of photon detection is near
about ~4% which is very low and in-efficient. Their bulky size renders them
incapable to be used in dense arrays.
Charge-Coupled Devices
Another alternative is
CCD. These are solid state replacement for PMTs. CCDs require special Silicon
processing for manufacturing, and hence have higher integration and
manufacturing costs. Their integration with other electronics and CMOS
technologies is very difficult to implement. Other disadvantages of CCDs
compared to CMOS photo detectors are its higher power consumption, higher
system costs, lower efficiency, lower speed of operation, i.e. higher dead
time.
DESIGN,
RESULTS & DISCUSSIONS
Pixels with
Photo Current Integration
Passive Pixel Structure
This is one of the
simplest pixel structures available in Photo current integration mode. In the
given figure 3, we can identify a transistor and a photodiode integrated to
select and output lines. They combine and operate to capture light intensity.
The transistor used is called row select transistor (S). The internal
capacitance of photo diode integrates the current running through it (photo
current). When the pixel is addressed the select line turns on the transistor,
and the output of pixel is recorded onto the output line. The charges from
pixel are read in parallel. As can be seen from the figure the circuit has only
one photo diode, and only one transistor. Hence it has very high fill factor.
(Fill Factor is referred as the ration of area covered by the photo detector to
the area covered by the total pixel structure.) But this system is prone to
noise as no noise reduction system is present. Noise results in very harmful
effects in this system.
Active
Pixel Structure
In the figure 4 we can see
there are 3 transistors and a photo diode. R is reset transistor, B is buffer
transistor, and S is the source follower which isolates the read out column
from sense node.
The APS has better signal
to noise ration. The B present buffers the output from the photo diode; as a
result, a standard noise won’t be able to break through the system. The
individuality in every circuit results in non uniformity of output for surface
illuminated with same intensity. This is termed a fixed pattern noise (FPN).
The large part of FPN is due to the variation on sense node voltage just after
reset. One of the ways to prevent FPN is Correlated Double Sampling (CDS). In
this procedure we take two samples, one before the read out, and second just
after the reset. And the output will be the difference between the two output
levels.
Digital
Pixel Sensor
In this pixel structure an
ADC is already present in the structure, so the output is Digital in nature,
which is easy to be incorporated into any arrayed operation or data bus. The
figure 5 shows a reset transistor and an ADC connected to the sense node. It is
very good for integrated solutions and high speed digital imaging. However the
Fill factor is very low, so high density arrayed structure is not possible with
DPS.
Avalanche Photodiode System
This is the most advanced
technology available in the field of Image sensors using CMOS technology. The
circuit consists of two transistors, Quenching transistor, and reset
transistor. It also consists of two control blocks namely Quench activation and
Reset control. An avalanche
photodiode (APD) is
a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity.
APDs can be thought of as photo detectors that provide a built-in first
stage of gain through avalanche
multiplication. From a functional standpoint, they can be regarded as the
semiconductor analogous to photomultipliers. By applying a high reverse bias
voltage (typically 100-200 V in silicon),
APDs show an internal current gain effect (around 100) due to impact ionization (avalanche effect). However, some
silicon APDs employ alternative doping and bevelling techniques compared to
traditional APDs that allow greater voltage to be applied (> 1500 V) before
breakdown is reached and hence a greater operating gain (> 1000). In
general, the higher the reverse voltage, the higher is the gain.
Operation:
When no light is falling
on APD, the potential across it is
Vs
= (Vop + Vdd)
When light falls on APD,
even of very low intensity, Impact ionization occurs, and current starts to
built up in the APD. Hence the potential at sense node starts falling. The
Quench control detecting this fall turns on the quench transistor. The quench transistor
takes the sense node voltage down to zero. As soon as the sense node reaches
zero, quench transistor is turned off and reset control turns the reset
transistor on which takes the sense node to Vdd voltage. After ‘Reset’, the
transistor is turned off; the sense node returns back to the potential of
(Vdd+Vop).
To explain the operation
of Avalanche photo diode system we approximate the Geiger mode i.e. single
photon detection. And hence the device is named Single Photon Avalanche
Photo-Diode (SPAD). The APD operating in Geiger mode is capable of both high
speed and high sensitivity operation. We can also say that APD system is the
nearest replacement for photo multiplier tubes in CMOS technology.
In the previous Photo
Current integration section the pixel structures had one problem which couldn’t
be removed. The problem was that if a light pulse had very high illumination
but stayed for only a short duration, the photo diodes would have been unable
to register it. Similarly, if the intensity of light was very small, but
appeared for longer interval of time, its integration is also not possible. But
in APDs this problem was overcome, because it can operate in Geiger mode.
CMOS
IMAGERS
High Dynamic Range Imaging
Dynamic
range and signal to noise ratio of a single collected image is strongly
correlated with the shot noise and the so called read out noise of charge
transfer and output amplification. Common CCDs offer a dynamic range of near
about 50 db-80 db. Most of the integrating CMOS imagers use diffused photo
diodes in combination with in-pixel amplification. Their blooming resistance
and non destructive read out allow a number of individually timed read outs
with same integrating period. For dynamic range extension this read out is
typically combined with a piece wise linear compressed output signal.
While
HRDC (High dynamic range CMOS) uses exponentially sub threshold characteristics
of a MOS transistor. This transistor is directly connected to the photo diode
as a permanent working shunt for photo currents.
HDRI
is the result of years of research done to optimize the dynamic range within
which a photo detector is capable to work. HDRC is based on n+ -p
junction in a low doped p-substrate. This arrangement provides an optimum
combination of low photo diode leakage current and spectral response.
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The CMOS image sensor as the heart of an imager system on
the one hand defines the optical properties of the imager system, such as
sensitivity, spectral response and dynamic range. On the other hand its layout and
design determines the resolution, readout speed, random access of regions of
the viewed scene and the kind of sampling in time of the image information,
which are all related to the circuit design. Basically there are two different
sampling mechanisms, i.e. line synchronous and frame synchronous sampling, also
described as rolling shutter and global shutter.
Rolling shutter (also known as line
scan)
is a method of image acquisition in which each frame is recorded not from a
snapshot of a single point in time, but rather by scanning across the frame
either vertically or horizontally. In other words, not all parts of the image
are recorded at exactly the same time, even though the whole frame is displayed
at the same time during playback. This produces predictable distortions of fast
moving objects or when the sensor captures rapid flashes of light.
A global shutter, unlike the rolling
shutter, exposes all pixels at the same time.
Uses:
Today HRDC
technology is in high demand. The line synchronous and frame synchronous
techniques of sampling the image finds wide spread application in different
fields. Fig. Adjoining shows a typical scene captured with a HDRC®
sensor developed for a night vision driver assistance system. In this
automotive application the challenges for the sensor are the wide spectral
sensitivity from visible (VIS) to near infrared (NIR), to recognize the lane
and obstacles far away on the dark road without getting blinded by the high
beam of the approaching car.
In
the field of medicine the growing use of image sensors has opened new avenues
for photo diode development. The integration of cameras in endoscopes and
inspection elements for minimal invasive surgery, led to miniaturization of
sensors and optimization of image qualities.
Fig. adjoining shows an example of a CMOS imager in a standard
CMOS 0.18μm technology. The array is based on 256 APS pixels of 20 μm × 30 μm
size with a 60% FF. Only the photosensitive area of the photodiodes is exposed
in the array, and everything else is covered with the top metal layer. Here,
row and column scanners were used instead of decoder circuits in order to
reduce the control lines coming into the chip. Also, only one input clock is
needed to control the row and column circuitry.
The
signals received from individual pixel structures are fed to multiplexers. The
output of these multiplexers is then connected to Op-amp for getting analog
signal equivalent of the image. This in turn is given as input to Sample and
Hold circuit. S/H circuit is embedded with capacitor of high value to prevent
any charge injection effects. This is followed by Analog to digital converter,
then state and machine control, followed by output latch, which gives parallel
port output. If serial output is desired, a parallel to serial port convertor
is used after output latch. The analog equivalent output can be extracted from
Op-amp.
CONCLUSION
We
have briefly reviewed different CMOS photo detectors used till dates and their
implications, the new technologies which are available to us. We also discussed
the prospects of imaging technologies. A few years ago, Passive pixel sensor
was used to capture images and different noise reduction algorithms were
developed to remove the noise from it. But as complexity of different
requirements raised, a demand for high definition image with lower Dead time
and lower Noise increased. And hence Active pixel sensor came into being. But
still the dead time was high due to usage of 3 transistors and Correlated
Double Sampling. So to improve its performance, Digital pixel sensor was
introduced, which was designed particularly to have high speed image acquiring
capability. HRDC implemented on real world has set standards for other imagers.
Finally
Avalanche Photo diode system was introduced. Combining all the features which
we require from a Photo detector, and providing alternative technology which is
cheap to implement and suitable for mass production, we get APD. APD has all
the features we expect a cheap alternative of PMT to have. Its integration with
different Imagers is quite easy; soon we will be able to see APD, in different
fields.
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