CMOS Active Pixel at Lfoundry: Brief description - Agenda

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CMOS Active Pixel at Lfoundry:
Brief description, main quality factors and criticalities from
the device concept to the production phase.
VI Scuola Nazionale:
"Rivelatori ed Elettronica per Fisica delle Alte Energie, Astrofisica,
Applicazioni Spaziali e Fisica Medica“
INFN Laboratori Nazionali di Legnaro (PD)
23 - 27 marzo 2015
Presented by Giovanni Margutti
[email protected]
Principal Process Integration Eng
R/D department - Lfoundry
confidential
© 2013 LFoundry. All rights reserved.
Course Outline
 Introduction to Photo detectors
 Principle of operation
 CCD and CIS
 PPS vs APS
 Image Sensors
 Photodiode 4T
 Main merit functions and their optimization
 QE, Dark I/hot pixels, Dynamic range
 Further image quality improvement: Rolling vs GS, 5T
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Photodetectors
 Devices that convert light into electrical signal (charge, current, voltage)





Photo conductors
Photo transistors
Photo diodes
Photogates
Etc..
 The basic principle of operation is the same for all the devices and it will briefly
discussed in the next pages
 The definition of the main merit functions is similar for all the photo detectors
and will be also discussed
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Photodetectors - Principle of operation
 Photons impinging the semiconductor can have enough energy to create electron/hole
pairs;
 Electrons and/or holes are then separated and collected;
 An output signal due to the collected charges is generated.
In case of “ high energy” photons w e may have the follow ing mechanisms of interactions:
-Photoelectric effect
- Compton scattering
-Pair production
.. but the main mechanism of photon absorption in silicon, in case of low energy photons (Emin< E< 10 eV)
is the photovoltaic effect
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Principle of operation: photo generation
~1.1 eV
(Si)
Silicon band gap betw een valence band and conduction band is 1.12 eV (at 300 K). This is an indirect
bandgap, so Incident photons w ith energy> 1.12eV (λ= 1100 nm) may be absorbed, causing electrons
to jump from valence to the conduction band,. but the transition must be assisted by phonons.
A very useful parameter to represent the chance a photon on a given energy is adsorbed into the silicon
is the absorption coefficient
I(x)= I(0)exp(-
α, usually
expressed in cm-1
αx);
w here I(x) is the intensity of the light penetrating the silicon at a depth X, X= 0 is the silicon surface;
thus at a depth of X= 3/ α 95% of the light is absorbed.
To give an idea; Blue light is adsorbed in 0.2 µm; Green in about 1 µm; Red in 3 µm;
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Principle of operation: charge collection
Once electrons and holes are generated, they must be separated and collected. This is
performed by an electric field, causing drift of the charges.
It’s important at this stage taking into account for the recombination mechanism, whereby an
electron and a hole recombine and annihilate each other to reach the equilibrium state:
np= ni2
Where n, p are the concentration of free electrons and holes, and ni is the
intrinsic concentration of carrier in silicon, at a given temperature
Recombination may be due to the following mechanisms:
•recombination trough recombination centers (traps, impurities etc..) known as SRH
(Shockley–Read–Hall) mechanism
•direct recombination (unlikely in silicon)
•Auger effect (more likely in heavily doped substrate)
So we need to use “good” silicon, with low trap density, ( few silicon defects/low metal
contaminations) to avoid recombination . This is key for CIS fabrication. Metal contamination as
well as silicon damage must be kept as low as possible as they may be responsible for both
recombination or generation (causing dark current, hot pixels.. this will be briefly shown later on)
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Photodiode - Photogate
 The Photodiode (PD) basically consists of a N/P junction reverse biased; electrons
generated into the depletion region (proportional to the flow of the impinging light) are
stored in the PD capacitor. Changes in the PD voltage are then amplified and read
out. Photodiodes are the basic element of CMOS image sensors
 The Photo gate it’s a MOS capacitor with polysilicon as the top terminal. It’s biased in
order to have a wide depletion region where electron/holes are generated. The charge
generated and collected into the photogate must be transferred to a read out unit to be
converted in signal. Photogates are the basic elements of CCD
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Imager sensors: CCD vs CIS
Imaging is the process of using arrays of photo detectors to create and store images. The
most important devices used in the image sensor market are CCDs (charge coupled
devices) and CMOS Image Sensors (CIS). In both cases the a charge proportional to the
incident light is generated and collected, for each pixel.
CIS
CCD
• Charge to voltage conversion is
made w ithin each pixel of the array
• Voltage signal is pre-amplified by a
source follow er transistor on each
pixel
• Charge is carried across each columns
by means of coupled devices
• Charge to voltage conversion is made
out of the array
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CIS vs CCD at the beginning…
…The gap in terms of image quality is not true anymore: highend cameras are now equipped with CIS
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CIS: Core and SOC
CIS allow s the
integration in
the same chip!
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PIXELS - PPS
Core of the imager sensors is the Pixel; the active element of the pixel is the photodiode
The easiest PD is a diode w ith a w ide depletion region (inverse polarization); electrons/holes are generated
inside the silicon; the ones generated in the depleted region are driven by the electric field tow ards the
ground potential (holes) or tow ards the anode (positive potential) changing the potential difference of the
PD;
Charges generated outside of the depleted region can diffuse up to hit the depletion region. Of course in this
case electrons have higher chance to recombine w ith holes (in the case depicted in figure).
Changes in the PD potential (proportional to the flow of the impinging light) can be now sampled and read.
A possible simplified scheme is the follow ing one:
The PD is inversed biased .
During the integration time the row is disabled and the
PD is left floating for a time Dt;
• During this time the photo-generated charges are
collected in the PD capacitance
• When the row is enabled the current flow s into the
column (charging a capacitor until all the generated
charge is transferred).
• Once the charge is completely transferred the PD bias is
restored to its initial value
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Pixels array: PPS and APS
PPS (one X-tor per pixel)
APS (at least three X-tors per pixel)
• High fill factor (only one TX):
• High noise (charge to signal conversions and signal
amplification is performed at the end of each column
and all the components of the noise coming from the
signal path are amplified)
• Slow read out-reset sequence
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• Low fill factor (no more true w ith decreasing Xtors size)
• Low noise (charge to signal conversions and
amplification is performed for each PD) w ith
respect PPS but higher than CCD at the
beginning..
• fast read out sequence
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4-T cell (and pinned Photodiode)
 The 4T arxchitecture allows to dramaticaly reduce the noise and makes the CIS
competitive to CCD
The main feature are:
 The introduction of the pinned photodiode, that eliminates two important
sources of temporal noise:
 the componenet of the dark I coming from the silicon surface;
 the photodiode reset noise;
 The introduction of the DCS ( double correlated sampling) that eliminates
 the FD reset noise;
 the spatial noise coming from the Reset and Source follow ers Vt pixel to
pixel variability.
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Pinned photodiode and 4-T cell
4-T cell architecture
4-T cell architecture
TX
Positive value potential
Reset TX
(FD floating diffusion)
Source Follow er
Row Select
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Pinned photodiode and 4-T cell
Conventional photodiode (p-n
junction)
Pinned photodiode
(p-n-p structure)
TX
Silicon
surface
the N- region
is completely
When
depleted the maximum
potential accross the PD is hit; increasing the reset voltage the
potential doesn’t change. So if the reset voltage Vrst is high
enough the PD potential after reset is unique and determined.
This is not true for the stardar NP photodiode, for w hich the
potential value and depleted region after the reset are affected by
the RST noise
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Double Correlated Samplig
The charge collected in the PD is transferred to the FD and converted to voltage, whose value is Q/Cfd. The
capacitance of the FD is usually small, so that we can have a good conversion gain and responsivity. If some
noise is generated in the FD area, this is amplified and sent to the read out circuitery. The two main contributors to
the FD noise are the reset noise and the charges generated and collected into the FD during the integration time
(due to thermal generation, unfiltered light, etc..). The charges are eliminated by resetting the FD right before
charge from the PD is transferred.
The FD reset noise is eliminated by the DCS (double correlated sampling), instead.
DCS sequence
PD and FD are reset.
Photo electrons are generated in the PD during the integration time.
FD is reset and signal stored in the SHR. The signal is Vaapix-Vt Reset-Vt Row Select-Vt Source Follower
+Reset noise.
Then the charge Q is transferred to the FD, added to the previous signal, sent and stored into the the SHS.
The output from SHS and SRH is then sent to operation amplifiers who return a signal proportional to SHSSRH=Q/Cfd. So the reset noise is eliminated. The contributions coming from the Vt of the arrah devices (RST TX,
RS TS, SF) are also eliminated, and this improves the spatial noise as we are no more sensivite to pixel to pixel
differences.
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4-T cell readout - DCS
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CMOS image sensors
Section II
Image Sensors
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Outline
 Main quality factors: definition and improvement
 Quantum Efficiency
 QE increase – stack optimization, LG, deep PD
 Back Side Illumination (BSI)
 Dark I
 Hot Pixels
 Further improvement of image quality -Rolling vs GS;
Dynamic range – 5T
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QE (quantum efficiency)
 Quantum efficiency (QE) is the measure of the efficiency with which incident photons are
detected. The quantum efficiency is the ratio of the number of detected electrons divided by
the number of incident photons.
It’s usually expressed in % and detailed for wave lenght (see picture)
Of course higher QE results in higher signal and lower SNR, so it’s preferred
Color filters are realized with colored photo resists;
Bayer pattern
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Optical microscope image
How to increase external QE
1) Micro lenses.. In the APS some «active space» is consumed to built in pixel read out
electronics. The dead space can be recovered by added lenses (micro-lenses) focusing the
light into the PD;
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Microlenses to enhance fill factor
w /o
microlens
Without
ulens
Optical
simulation
Light is
focused
into the PD
area
After coat,
expose and
develop
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Bleach
and reflow
light into the PD;
2) Reduce reflected/adsorbed light
A certain portion of impinging light is reflected at the interface between adjacent layers forming
the optical stack;
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light into the PD;
the optical stack;
this can be reduced by eliminating the interfaces (when possible);
Low k, high
n (1.6/1.7)
polymer
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light into the PD;
the optical stack;
Or reducing the refractive index differences between adjacent layers;
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light into the PD;
the optical stack;
Or reducing the refractive index differences between adjacent layers;
• CFA
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Step index polymeric light guides
Microlens and planarization layer
Color filter array
Nitride passivation
M2
M1
Oxide
layers
High refractive index
Polymer (n = 1.6-1.7)
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QE (how to increase internal quantum efficiency)
We need to increase the generation and collection of fotogenerated electrons;
Because of the low recombination probability, we can assume all electrons generated inside the depleted regions
are collected in to the PD node;
To hit 100% of internal QE all the light impinging the silicon must be adsorbed into the PD depletion region
So a possibility is increasing the PD depth(indeed in the 4T APS pixel the PD is empty after the being reset). The
photo litography mask be so thick to block implanted ions and the dimensions to be printed (CDs) can be ~1/15
time the mask thickness: this may be very challenging, particularly for small pixels
Light absorption in
silicon
Dee PD
isolation
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BSI - CIS
 Backside illumination (BSI) flips the image sensor upside down so it absorbs light from
the backside
 As an alternative to the more common front-side illumination (FSI) technique, it offers the
most direct path for light to strike the pixel
 In principle, BSI provides a better fill factor, higher QE, lower cross talk, and metal routing
flexibility
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FSI vs BSI pixel
Microlenses
Bayer filters
Metal
runners
Antireflective
coating
Dielelectric
stack
Access
transistors
Photodiodes
BSI
FSI
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Pixel Shrinking - BSI
Bonding loop
Carrier w afer &
Device w afer
preparation
Device w afer
edge trim
Carrier w afer &
Device w afer
direct bond
CFA/uLens
Thinning loop
Antireflective layer
Device w afer
grinding
Device w afer final
thinning (< 5um)
Thinned silicon
Additional modules
Device w afer additional
optical/electrical modules
Critical items
Front Side structures
 Bonding voids;
 Device w afer distortion;
 Silicon surface damage.
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Dark current and its
impact on image quality
Undesired signal integrated in dark
conditions
Sources of dark current and hot pixels
1. STI around the photodiode
2. Photodiode surface
3. Substrate
4. Overlap of PD with transfer gate
5. Contamination & defects
Medium light
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Transfer gate
p+ implantation
STI
TX
n- implantation
epi substrate
Low light
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Hot pixels
• Causes – eletrons generated in the depletion region trough SRH
mechanims
• Mitigation – reduce generation centers by:
• Using “ good silicon”
• Optimize processes and thermal budget
• Reduce metal contamination (gettering, environment,
maintenance, etc..)
Mid gap state for various impurity, in silicon –
after S. M. SZE KWOK K.
NGE - Physic of Semiconductor device
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Further improvement of image quality
Distortion induced by the Electronic Rolling Shutter
(ERS)
Blooming
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Dynamic range limiting the view of details in darker and brighter
areas
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High Dynamic Range, 5T
Aptina DR-pix technology, aimed to increase dynamic
Dynamic range= ratio betw een maximum and minimum signal can be detected in linear regime, usually expressed in db.
range
The maximum signal is determined by the full w ell, the minimum one by the noise (read noise floor)
DCG on
Saturation
Signal
DCG off
Variable conversion gain
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Electronic rolling shutter
Reset
(from aptina.com
w ebsite)
Readout
Row 1
Row 2
…
Row n
t
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The integration time of
different row s starts
and ends in a different
time, causing the
artifacts © 2013 LFoundry. All rights reserved.
Global shutter
Introducing the “in pixel memory” all the rows start
and end the integration time at the same time…
In pixel
memory
… While the in-pixel
memories can be
read row by row.
No distortion!
But the “In pixel memory”
needs an effective shield
from light!
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(from aptina.com
w ebsite)
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Conclusion
Technological development has enabled us to improve the performances of CMOS
imager and make them competitive with CCDs, keeping the advantages of low cost
and low power consumption. This is why their diffusion has increased in the last year in
several market segments. Nowadays they are also used for high performances digital
cameras.
Current and future trends
•Die stacking (Sony, already in mass production)
•Organic photodiode (announced by Fujifilm)
•Black Silicon (SiOnyx)
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Biblio
Some reference:
Text books from general guideline to deep understanding of imager sensors architectures
J. Nakamura Image Sensors and Digital Processing for Digital Still Camera . CRC Press 2005
O. Yadid-Pecht, R. Etienne-Cummings - Cmos imagers from Phototransduction to Image
Processing - Kluw er Academic Publisher
Introduction to semiconductors devices (including detailed description of CMOS)
S. M. SZE KWOK K. NGE - Physic of Semiconductor device– Wiley and sons
Articles on CIS at the beginning of their history
E. R. Fossum (1993), " Active Pixel Sensors: Are CCD' s Dinosaurs?" Proc. SPIE Vol. 1900, p.
2–14, Charge-Coupled Devices and Solid State Optical Sensors III, Morley M. Blouke; Ed
And some time later
E.R. Fossum, " CMOS Image Sensors—Electronic Camera on a Chip” , IEEE Micro, vol. 18(3),
pp 8-15, May/June (1998).
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Thank you
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CMOS Active Pixel at Lfoundry: Brief description - Agenda

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