Back-illuminated sensor

Last updated
Comparison of simplified back-illuminated and front-illuminated pixel cross-sections Comparison backside illumination.svg
Comparison of simplified back-illuminated and front-illuminated pixel cross-sections

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.

Image sensor device that converts an optical image into an electronic signal

An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves into signals, small bursts of current that convey the information. The waves can be light or other electromagnetic radiation. Image sensors are used in electronic imaging devices of both analog and digital types, which include digital cameras, camera modules, medical imaging equipment, night vision equipment such as thermal imaging devices, radar, sonar, and others. As technology changes, digital imaging tends to replace analog imaging.


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. Sony was the first to reduce these problems and their costs sufficiently to introduce a 5-megapixel 1.75 µm BI CMOS sensor at general consumer prices in 2009. [1] [2] BI sensors from OmniVision Technologies have since been used in consumer electronics from other manufacturers as in the HTC EVO 4G [3] [4] Android smartphone, and as a major selling point for the camera in Apple's iPhone 4. [5] [6]

Sony Japanese multinational conglomerate corporation

Sony Corporation is a Japanese multinational conglomerate corporation headquartered in Kōnan, Minato, Tokyo. Its diversified business includes consumer and professional electronics, gaming, entertainment and financial services. The company owns the largest music entertainment business in the world, the largest video game console business and one of the largest video game publishing businesses, and is one of the leading manufacturers of electronic products for the consumer and professional markets, and a leading player in the film and television entertainment industry. Sony was ranked 97th on the 2018 Fortune Global 500 list.

Consumer electronics Electronic equipment intended for everyday home use

Consumer electronics or home electronics are electronic equipments intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications, and home-office activities. In British English, they are often called brown goods by producers and sellers, to distinguish them from "white goods" which are meant for housekeeping tasks, such as washing machines and refrigerators, although nowadays, these would be considered brown goods, some of these being connected to the Internet. In the 2010s, this distinction is not always present in large big box consumer electronics stores, such as Best Buy, which sell both entertainment, communication, and home office devices and kitchen appliances such as refrigerators.

OmniVision Technologies Inc. is a corporation that designs and develops advanced digital imaging technologies and products for use in mobile phones, notebooks, netbooks and webcams, security and surveillance cameras, entertainment, digital still and video cameras, automotive and medical imaging systems. Headquartered in Santa Clara, California, OmniVision Technologies has offices in the USA, Western Europe and Asia, including a design center and testing facility in Shanghai, China.


A traditional, front-illuminated digital camera is constructed in a fashion similar to the human eye, with a lens at the front and photodetectors at the back. This traditional orientation of the sensor places the active matrix of the digital camera image sensor—a matrix of individual picture elements—on its front surface and simplifies manufacturing. The matrix and its wiring, however, reflect some of the light, and thus the photocathode layer can only receive the remainder of the incoming light; the reflection reduces the signal that is available to be captured. [1]

Digital camera camera that captures photographs or video in digital format

A digital camera or digicam is a camera that captures photographs in digital memory. Most cameras produced today are digital, and while there are still dedicated digital cameras, many more are now incorporated into devices ranging from mobile devices to vehicles. However, high-end, high-definition dedicated cameras are still commonly used by professionals.

Human eye mammalian eye; part of the visual organ of the human body, and move using a system of six muscles

The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image, normally coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can differentiate between about 10 million colors and is possibly capable of detecting a single photon.

Lens (optics) optical device which transmits and refracts light

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis. Lenses are made from materials such as glass or plastic, and are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.

A back-illuminated sensor contains the same elements, but arranges the wiring behind the photocathode layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light can strike the photocathode layer without passing through the wiring layer. [7] This change can improve the chance of an input photon being captured from about 60% to over 90%, [8] (i.e. a 1/2 stop faster) with the greatest difference realised when pixel size is small,[ citation needed ] as the light capture area gained in moving the wiring from the top (light incident) to bottom surface (paraphrasing the BSI design) is proportionately smaller for a larger pixel.[ citation needed ] BSI-CMOS sensors are most advantageous in partial sun and other low light conditions. [9] Placing the wiring behind the light sensors is similar to the difference between a cephalopod eye and a vertebrate eye. Orienting the active matrix transistors behind the photocathode layer can lead to a host of problems, such as cross-talk, which causes noise, dark current, and color mixing between adjacent pixels. Thinning also makes the silicon wafer more fragile. These problems could be solved through improved manufacturing processes, but only at the cost of lower yields, and consequently higher prices. Despite these issues, early BI sensors found uses in niche roles where their better low-light performance was important. Early uses included industrial sensors, security cameras, microscope cameras and astronomy systems. [8]

The photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force. The photon has zero rest mass and always moves at the speed of light within a vacuum.

Cephalopod eye

Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye which consists of an iris, a circular lens, vitreous cavity, pigment cells, and photoreceptor cells that translate light from the light-sensitive retina into nerve signals which travel along the optic nerve to the brain. For the past 140 years, the camera-type cephalopod eye has been compared with the vertebrate eye as an example of convergent evolution, where both types of organisms have independently evolved the camera-eye trait and both share similar functionality. Contention exists on whether this is truly convergent evolution or parallel evolution. Unlike the vertebrate camera eye, the cephalopods' form as invaginations of the body surface, and consequently they lack a cornea. Unlike the vertebrate eye, a cephalopod eye is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does. The eye is approximately spherical, as is the lens, which is fully internal.

Other advantages of a BSI sensor include wider angular response (giving more flexibility for lens design) and possibly faster readout rates. Disadvantages include worse response uniformity.

Industry observers[ who? ] noted that a back-illuminated sensor could theoretically cost less than a similar front-illuminated version. The ability to collect more light meant that a similarly sized sensor array could offer higher resolution without the drop in low-light performance otherwise associated with the megapixel (MP) race. Alternatively, the same resolution and low-light capability could be offered on a smaller chip, lowering costs. Key to attaining these advantages would be an improved process that addressed the yield problems, largely through improving the uniformity of an active layer on the front of the detectors. [8]

A major step in the adoption of BI sensors was made when OmniVision Technologies sampled their first sensors using the technique in 2007. [10] These sensors, however, did not see widespread use due to their high costs. Sony's work on new photo diode materials and processes allowed them to introduce the first consumer back-illuminated sensor as their CMOS-based "Exmor R" in August 2009. [1] According to Sony, the new material offered +8 dB signaling and −2 dB noise. When combined with the new back-illuminated layout, the sensor improved low-light performance by as much as two times. [1]

Active pixel sensor an image sensor consisting of an integrated circuit

An active-pixel sensor (APS) is an image sensor where each picture element ("pixel") has a photodetector and an active amplifier. There are many types of integrated circuit active pixel sensors including the complementary metal–oxide–semiconductor (CMOS) APS used most commonly in cell phone cameras, web cameras, most digital pocket cameras since 2010, in most digital single-lens reflex cameras (DSLRs) and Mirrorless interchangeable-lens cameras (MILCs). Such an image sensor is produced using CMOS technology, and has emerged as an alternative to charge-coupled device (CCD) image sensors.

Competitors followed suit, and by the end of the year most companies were offering a version in their high-end products. OmniVision has continued to push the technology down their product lines. By contrast, the iPhone 4s employs a sensor manufactured by Sony. Another example is the HTC EVO 4G [4] [3] which has an 8 MP, 1.4 µm pixel BSI sensor from OmniVision. In 2011, Sony implemented their Exmor R sensor in their flagship smartphone Sony Ericsson Xperia Arc. [11]

In January 2012 Sony developed the back-side illuminated sensor further with Stacked CMOS, [3] where the supporting circuitry is moved below the active pixel section, giving another 30% improvement to light capturing capability. [12] This was commercialized by Sony in August 2012 as Exmor RS with resolutions of 13 and 8 effective megapixels. [13]

In September 2014 Samsung announced the world's first APS-C sensor to adopt BSI pixel technology. [14] [3] This 28 MP sensor (S5KVB2) was adopted by their new compact system camera, the NX1, and was showcased along with the camera at Photokina 2014.

In June 2015 Sony announced the first camera employing a back-side illuminated full frame sensor, the α7R II. [3]

In August 2017 Nikon announced that its forthcoming Nikon D850, a full-frame digital SLR camera, would have a back-illuminated sensor on its new 45.7 MP sensor.

In September 2018 Fujifilm announced the availability of the X-T3, a mirrorless interchangeable-lens camera, with a 26.1MP APS-C Fujifilm X-Trans sensor back-illuminated sensor. [15]

See also

Related Research Articles

LBCAST is a type of photo sensor which the manufacturer claims is simpler and thus smaller and faster than CMOS sensors. It was developed over ten years by Nikon, in parallel with other manufacturer's development of CMOS, and resulted in shipping product in 2003.

Super CCD camera parts, features and technologies

Super CCD is a proprietary charge-coupled device that has been developed by Fujifilm since 1999. The Super CCD uses octagonal, rather than rectangular, pixels. This allows a higher horizontal and vertical resolution to be achieved than a traditional sensor of an equivalent pixel count.

Sony produces professional, consumer, and prosumer camcorders.

Image sensor format camera parts, features and technologies

Note: If you came here to get a quick understanding of numbers like 1/2.3, skip ahead to table of sensor formats and sizes. For a simplified discussion of image sensors see image sensor.

Exmor digital camera

Exmor is the name of a technology Sony implemented on some of their CMOS image sensors. It performs on-chip analog/digital signal conversion and two-step noise reduction in parallel on each column of the CMOS sensor.

Sony Alpha 550 digital camera model

The Sony Alpha a550 (DSLR-A550) is a midrange-level digital single-lens reflex camera (DSLR) marketed by Sony and aimed at enthusiasts, it was released in August 2009. The camera features a 14.2 megapixel APS-C Type CMOS Exmor Sensor and features Sony's patented SteadyShot INSIDE stabilisation system which works with any attached lens. The Sony Alpha a550's main selling point is its dual Live View mode's, Sony's normal secondary; smaller sensor based Live View mode and another which uses the main sensor with no autofocus. The a550 also features a maximum of 7frame/s continuous shooting speed when operating in speed-priority mode and a maximum ISO of 1600 when in auto mode and 12800 ISO when in manual mode.

Sony Xperia S smartphone model

The Sony Xperia S is an Android smartphone from Sony launched at the 2012 Consumer Electronics Show. It is the first Sony-only branded smartphone after Sony acquired Ericsson's stake in Sony Ericsson in January 2012. The Xperia S has a 4.3 in (110 mm) touch-screen with the mobile BRAVIA engine which optimizes the picture, a 1.5 GHz dual core processor, a 12.0-megapixel rear camera, HDMI-out, 1 GB of RAM, and 32 GB of internal storage.

Fujifilm X-Pro1 digital camera made by Fujifilm

The Fujifilm X-Pro1 is a mirrorless interchangeable-lens digital camera announced in January 2012 and launched in March 2012. It is part of Fujifilm's X-Series of cameras. In October 2012 Fujifilm has released a very similar, yet smaller, camera named the X-E1. In January 2016 Fujifilm announced its successor the X-Pro2.

Sony Cyber-shot DSC-RX100 digital camera model

The Sony Cyber-shot DSC-RX100 is a 20.2 megapixel compact camera, part of the Cyber-shot line of digital cameras made by Sony and announced on 6 June 2012. It was named as the "European Advanced Compact Camera for 2012–2013" by the European Imaging and Sound Association (EISA) and one of the 'Best Inventions of 2012' by TIME.

Sony Xperia ZR Android smartphone

The Sony Xperia ZR is a touchscreen-enabled, HD Android flagship smartphone designed, developed, and marketed by Sony Mobile.

Sony α7R II Mirrorless interchangeable-lens camera

The Sony α7R II is a full-frame mirrorless interchangeable-lens camera. It was announced by Sony on 10 June 2015. At the time of its release, it had the largest backside illuminated CMOS sensor of any camera in the market, the previous largest being used in the Samsung NX1 released only months earlier. The camera was released 5 August 2015. Some of its most notable features are the 42 megapixels and the 399 on-phase detection points.

The HTC One M9+ is an Android smartphone manufactured and marketed by HTC which was announced on April 8, 2015. Initially on launch, the device was only sold in China. In July 2015, the device was released in Europe excluding the United Kingdom.

Fujifilm X-Trans sensor

The Fujifilm X-Trans is a CMOS sensor developed by Fujifilm and used in its Fujifilm X-series cameras. Unlike most CMOS sensors featuring a conventional Bayer filter array, X-Trans sensors have a unique 6 by 6 pattern of photosites. Fujifilm claims that this layout can minimise moiré effects, and increase resolution by eliminating the need for a low-pass filter.

Sony Xperia XZs Android smartphone

The Sony Xperia XZs is an Android smartphone manufactured and marketed by Sony. Part of the Xperia X series, the device was announced to the public along with the Xperia XZ Premium at the annual Mobile World Congress last February 2017.

The Sony Xperia XZ1 is an Android smartphone manufactured and marketed by Sony. Part of the Xperia X series, the device was announced to the public along with the Xperia XZ1 Compact at the annual IFA 2017 on August 31, 2017. It is the direct successor to the Sony Xperia XZ according to Sony, and is the latest flagship after the Xperia XZ Premium.


  1. 1 2 3 4 Sony, 2009
  2. USpatent 7521335,Yamanaka, Hideo,"Method and apparatus for producing ultra-thin semiconductor chip and method and apparatus for producing ultra-thin back-illuminated solid-state image pickup device",issued 2009-04-21, assigned to Sony Corporation
  3. 1 2 3 4 5 Zimmerman, Steven (12 October 2016). "Sony IMX378: Comprehensive Breakdown of the Google Pixel's Sensor and its Features". XDA Developers . Retrieved 17 October 2016.
  4. 1 2 "Inside the HTC EVO 4G Smart Phone with a Teardown to the Silicon". chipworks. 4 June 2010. Archived from the original on 22 July 2011. Retrieved 3 August 2011.
  5. Tufegdzic, Pamela (3 September 2010). "iPhone 4 Drives Adoption of BSI Image Sensors in Smart Phones". iSuppli. Archived from the original on 19 July 2011. Retrieved 3 August 2011.
  6. Apple, 2010
  7. USpatent 4266334,Edwards, Thomas W.&Pennypacker, Ronald S.,"Manufacture of thinned substrate imagers",issued 1981-05-12, assigned to RCA Corporation
  8. 1 2 3 Swain and Cheskis, 2008
  9. Yoshua Goldman. "Why the iPhone 4 takes good low-light photos: BSI CMOS sensors explained!" . Retrieved 29 September 2014.
  10. Yoshida 2007
  11. Vlad Savov. "Sony Ericsson Xperia Arc review". Engadget. AOL. Retrieved 16 August 2015.
  12. "Wayback Machine" (PDF). 12 June 2012.
  13. "Sony Global - News Releases - Sony Develops "Exmor RS," the World's First*1 Stacked CMOS Image Sensor" . Retrieved 16 August 2015.
  14. "Samsung Semiconductors Global Site" . Retrieved 16 August 2015.
  15. "Fujifilm announces the new X-T3, a mirrorless digital camera evolving X Series into fourth generation". Fujifilm. Retrieved 27 September 2018.