Seminar Presentation on BACK-ILLUMINATED SENSOR

Overview• A Back-illuminated sensor or BSI is a Digital

image sensor.• It is an advancement over Front-illuminated

senor.• A novel arrangement of sensor components

have been made to achieve this technology that produces brighter and sharper images.

Digital Image Sensor and Classification

• A Digital Image Sensor have mainly three components• 1)Lens 2) Active Matrix 3) Photo sites • Lens is the part where from light comes through inside the sensor.• Active matrix is a layer of transistor and capacitor circuitry which is the

wiring portion of the sensor is an addressing scheme which actively maintain the state of each pixel.

• Photo sites is the layer of tiny pixels which senses photons coming through lens part.

Digital Image Sensor and Classification

• Digital image sensor are classified broadly in two categories they are:• 1) Charge Coupled Device[CCD]. 2) Complimentary Metal Oxide Semiconductor[CMOS]

CCD sensor

CCD sensors are basically a shift register and the essence of design was the ability to transfer along the surface of semiconductor from one capacitor to the next.• These sensor are manufactured on a single wafer of silicon material.• In a CCD imager; the pixels are recorded on the chip and then one by one sent

through the analog to digital converter to build the data. This takes more energy than CMOS.

• One by one transfer of actuated charge makes this sensor extra load on power source and also increases data process time.

CCD Sensor

• charge coupled device-based imagers suffer from signal degradation during repetitive charge transfer through radiation-induced trap sites in silicon.

• Apart from sensor all the other components are mounted on PCB makes CCD sensor bulky in terms of size.

CCD sensor

• The charge collected on chip conveyed through one output node ,or only a few output nodes to external circuitry where charges are converted into voltage levels, buffered, amplified and sent out as analog signal and this signal is converted into a digital signal by A/D convertor outside the sensor as shown.

CMOS sensor and Classification

• CMOS stand for "Complementary Metal Oxide Silicon" . CMOS actually is only a technology to make transistor on silicon wafer, and have no further meaning.

• A CMOS sensor has circuitry at every photo sensor. So each pixel is read simultaneously and then transmitted as digital information at once. This set up leaves the chip relatively crowded with circuitry but is extremely efficient.

• CMOS sensor consumes lesser power than CCD around 25% to 50% power is conserved.

CMOS sensor and Classification

• CMOS sensor are classified in two categories:• 1) CMOS-FI 2)CMOS-BI• These sensor have similar components but in both they are arranged in

different architecture however in their principle of working have no such disparity.

CMOS-Back Illuminated sensor

• A Back-Illuminated sensor, also known as Backside illumination (BSI or BI) sensor, is a type of digital image sensor that uses a novel arrangement of the imaging elements to increase the amount of light captured and thereby improve low-light performance.

• The technique was used for some time in specialized roles like low-light security cameras and astronomy sensors, but was complex to build and required further refinement to become widely used.

• BSI sensor have photo sites over the wiring portion(Active matrix). This is done by Wafer Back Grinding technique ,in this technology wiring is taken behind the photocathode layer by flipping the silicon wafer during manufacturing and then thinning the reverse side so that light can strike the photocathode layer without passing through the wiring layer.

CMOS-Back Illuminated sensor

• This change improves chances of an input photon being captured from about 60% to 90%.

• The ability to collect more light meant that a similarly sized sensor array could offer higher resolution without the drop in low-light performance.

Back Illuminated sensor’s WORKING

• A CMOS-BSI sensor uses an array of millions of tiny light cavities or "photo sites" to record an image. When the exposure begins, each of these is uncovered to collect and store photons.

• All the collected photons makes a gray scale image cause image sensors are color blind and to make them colorful a Bayer array of alternating Red-Green and Green-Blue filter is placed over these photo sites.

Back Illuminated sensor’s WORKING

• There are two key concepts for using this RGB pattern• 1) Human eye perceive a much higher brightness resolution than we do

with color.• 2) Green light contributes roughly twice as much to our perception of

brightness than does the combined effect of red and blue.• So allocating more photo sites to green therefore produces a far better

looking image than if each color were allocated equally. Each primary color does not receive an equal fraction of the total area because the human eye is more sensitive to green light than both red and blue light.

Back Illuminated sensor’s WORKING

• Bayer “Demosaicing" is the process of translating this Bayer array of primary colors into a final image which contains full color information at each pixel.

• Demosaicing is an algorithm that makes true color by averaging the color sum of their nearby color areas from these primary color.

Back Illuminated sensor’s WORKING

• The photo sites however does not cover the entire area of sensor in order to accommodate other electronics.

• To gain these lost photons MICROLENS ARRAY is used to direct the photons into the cavity of photo sites.

Back Illuminated sensor’s WORKING

• These collected charges are now conveyed to the electronic circuitry to produce image.

Applications

• Low light security cameras. • Military services.• Microscope cameras. • Astronomy system.• Industrial sensors.• Compact cameras , camcorder and in smartphones.

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