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. 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. BI sensors from OmniVision Technologies have since been used in consumer electronics from other manufacturers as in the HTC EVO 4G Android smartphone, and as a major selling point for the camera in Apple's iPhone 4.
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.
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. This change can improve the chance of an input photon being captured from about 60% to over 90%, (i.e. a 1/2 stop faster) with the greatest difference realised when pixel size is small, 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. BSI-CMOS sensors are most advantageous in partial sun and other low light conditions. 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 crosstalk, 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.
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.
A major step in the adoption of BI sensors was made when OmniVision Technologies sampled their first sensors using the technique in 2007. These sensors, however, did not see widespread use due to their high costs. The first widely used BI sensor was the OmniVision OV8810, which was announced on 23 September 2008, and contained 8 megapixels which were 1.4 µm in size. The OV8810 was used in the HTC Droid Incredible and HTC EVO 4G, which were released in April and June 2009, respectively. In June 2009, OmniVision announced the 5MP OV5650, which had the best low-light sensitivity at 1300 mV/lux-sec and the lowest stack height at 6 mm in the industry. Apple selected the OV5650 to use in the back camera of the iPhone 4, which garnered good reviews for its low-light photos.
Sony's work on new photo diode materials and processes allowed them to introduce their first consumer back-illuminated sensor as their CMOS-based "Exmor R" in August 2009. 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. The iPhone 4s employed an image sensor manufactured by Sony. In 2011, Sony implemented their Exmor R sensor in their flagship smartphone Sony Ericsson Xperia Arc.
In January 2012, Sony developed the back-side illuminated sensor further with Stacked CMOS, where the supporting circuitry is moved below the active pixel section, giving another 30% improvement to light capturing capability. This was commercialized by Sony in August 2012 as Exmor RS with resolutions of 13 and 8 effective megapixels.
In September 2014 Samsung announced the world's first APS-C sensor to adopt BSI pixel technology. 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 May 2021, Sony announced a new back-illuminated, stacked sensor for the Micro Four Thirds format.
- Sony, 2009
- US patent 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
- Zimmerman, Steven (12 October 2016). "Sony IMX378: Comprehensive Breakdown of the Google Pixel's Sensor and its Features". XDA Developers. Retrieved 17 October 2016.
- "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.
- 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.
- Apple, 2010
- US patent 4266334, Edwards, Thomas W. & Pennypacker, Ronald S., "Manufacture of thinned substrate imagers", issued 1981-05-12, assigned to RCA Corporation
- Swain and Cheskis, 2008
- Yoshua Goldman. "Why the iPhone 4 takes good low-light photos: BSI CMOS sensors explained!". Retrieved 29 September 2014.
- Yoshida 2007
- "OmniVision premieres world's first 1/3-inch, 8 megapixel CameraChip™ sensor with 1.4 micron OmniBSI™ technology". EDN. 23 September 2008.
- Brian Klug (20 July 2010). "Motorola Droid X: Thoroughly Reviewed". Anandtech.
- "OmniVision delivers DSC-quality imaging to high performance mobile phone market" (PDF). OmniVision. 22 June 2009.
- "DSC-Quality Imaging for High-Performance Mobile Phones: OV5650 5 megapixel product brief" (PDF). OmniVision. January 2010.
- Philip Berne (24 June 2010). "Review: iPhone 4". PhoneScoop.
- Vlad Savov. "Sony Ericsson Xperia Arc review". Engadget. AOL. Retrieved 16 August 2015.
- "Sony's Stacked CMOS Image Sensor Solves All Existing Problems in One Stroke" (PDF). Sony. 12 June 2012. Archived from the original (PDF) on 12 June 2012.
- "Sony Global - News Releases - Sony Develops "Exmor RS," the World's First*1 Stacked CMOS Image Sensor". Retrieved 16 August 2015.
- "Samsung Semiconductors Global Site". Retrieved 16 August 2015.
- "Fujifilm announces the new X-T3, a mirrorless digital camera evolving X Series into fourth generation". Fujifilm. Retrieved 27 September 2018.
- "Canon announces development of the EOS R3 full-frame mirrorless camera that delivers high speed, high sensitivity and high reliability to expand users' range of photographic possibilities". Canon. Retrieved 17 April 2021.
- "Sony announced a new 20MP stacked BS1 Micro Four Thirds sensor. Is this for the future Olympus OMD camera?". Retrieved 30 May 2021.
- (Sony), "Sony unveils 'Exmor R' back-illuminated CMOS technology", Sony press release, 6 August 2009
- P.K. Swain and David Cheskis, "Back-Illuminated Image Sensors Come to the Forefront", Photonics, August 2008
- Junko Yoshida, "OmniVision adopts backside illumination technology for CMOS imager", EE Times, 27 May 2007
- (Apple), "iPhone 4: Camera", Apple Inc., 7 June 2010