back-illuminated sensor

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, block some of the light, and thus the photocathode layer can only receive the remainder of the incoming light; the blockages 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.{{cite patent

| country = US

| number = 4266334

| status = patent

| title = Manufacture of thinned substrate imagers

| pubdate =

| gdate = 1981-05-12

| fdate = 1979-11-29

| pridate =

| inventor = Edwards, Thomas W.

| invent1 =

| invent2 = Pennypacker, Ronald S.

| assign1 = RCA Corporation

| assign2 =

| class =

}} This change can improve the chance of an input photon being captured from about 60% to over 90%,Swain and Cheskis, 2008 (i.e. a 1/2 stop faster) with the greatest difference realised when pixel size is small,{{citation needed|date=September 2014}} as the light capture area gained in moving the wiring from the top (light incident) to bottom surface (paraphrasing the BSI design) is proportionately larger for a smaller pixel.{{citation needed|date=September 2014}} BSI-CMOS sensors are most advantageous in partial sun and other low light conditions.{{cite web |url=http://www.cnet.com/news/why-the-iphone-4-takes-good-low-light-photos-bsi-cmos-sensors-explained/ |title=Why the iPhone 4 takes good low-light photos: BSI CMOS sensors explained! |author=Yoshua Goldman |access-date=29 September 2014}} 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 image 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|date=August 2011}} noted (in 2008) 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.Yoshida 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.{{cite web |url=https://www.edn.com/omnivision-premieres-worlds-first-1-3-inch-8-megapixel-camerachip-sensor-with-1-4-micron-omnibsi-technology/ |title=OmniVision premieres world's first 1/3-inch, 8 megapixel CameraChip sensor with 1.4 micron OmniBSI technology |date=23 September 2008 |publisher=EDN }} The OV8810 was used in the HTC Droid Incredible{{cite web |url=https://www.anandtech.com/show/3826/motorola-droid-x-thoroughly-reviewed/15 |title=Motorola Droid X: Thoroughly Reviewed |author=Brian Klug |publisher=Anandtech |date=20 July 2010 }} and HTC EVO 4G, which were released in April and June 2009, respectively. In June 2009, OmniVision announced the 5MP OV5650,{{cite web |url=https://www.ovt.com/download/datasets/2766/OV5650_OV2665_FINAL.pdf | title=OmniVision delivers DSC-quality imaging to high performance mobile phone market | publisher=OmniVision | date=22 June 2009}} which had the best low-light sensitivity at 1300 mV/lux-sec and the lowest stack height at 6 mm in the industry.{{cite web |url=https://docs.rs-online.com/3cdc/0900766b80e999cc.pdf | title=DSC-Quality Imaging for High-Performance Mobile Phones: OV5650 5 megapixel product brief | publisher=OmniVision | date=January 2010}} Apple selected the OV5650 to use in the back camera of the iPhone 4, which garnered good reviews for its low-light photos.{{cite web | url=https://www.phonescoop.com/articles/article.php?a=387&p=2810 | title=Review: iPhone 4 | publisher=PhoneScoop | date=24 June 2010 | author=Philip Berne}}

Sony's work on new photodiode 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.{{cite web|url=https://www.engadget.com/2011/03/30/sony-ericsson-xperia-arc-review/|title=Sony Ericsson Xperia Arc review|author=Vlad Savov|publisher=AOL|work=Engadget|access-date=16 August 2015}}

In October 2012, GoPro used a Sony IMX117 sensor as the first BSI sensor in their action cameras, in the Hero3 Black.{{Cite web |date=2013-07-18 |title=GoPro HERO3 Black Edition: Super Hero... |url=https://www.dxomark.com/gopro-hero3-black-edition-super-hero/ |access-date=2022-09-06 |website=DXOMARK |language=en-US}}

In September 2014 Samsung announced the world's first APS-C sensor to adopt BSI pixel technology.{{cite web|url=http://www.samsung.com/global/business/semiconductor/news-events/press-releases/detail?newsId=13683|title=Samsung Semiconductors Global Site|access-date=16 August 2015}} 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.

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.{{cite web|url=http://www.fujifilmusa.com/press/news/display_news?newsID=881484|title=Fujifilm announces the new X-T3, a mirrorless digital camera evolving X Series into fourth generation|publisher=Fujifilm|access-date=27 September 2018}}

In April 2021, Ricoh released the Pentax K-3 III featuring a BSI 26 megapixel APS-C sensor from Sony and a PRIME V image processor.

A further development is the stacked CMOS sensor, which layers the circuitry and image signal processor (ISP) behind the pixels, allowing the active pixel to occupy even more area, further increasing the chance of light capture. Sony, which announced the first stacked sensor in January 2012, claims a 30% increase in light captured.{{cite web|url=http://www.sony.net/Products/SC-HP/cx_news/vol68/pdf/sideview_vol68.pdf| title=Sony's Stacked CMOS Image Sensor Solves All Existing Problems in One Stroke |publisher=Sony |date=12 June 2012|archive-url=https://web.archive.org/web/20120612000427/http://www.sony.net/Products/SC-HP/cx_news/vol68/pdf/sideview_vol68.pdf|archive-date=12 June 2012}} Stacked CMOS also allows more complex processing circuitry to be used simply by increasing the layer count, enabling faster frame rates and readout speeds.{{cite web |author1=Possibility Editorial |title=The Future of CMOS is Stacked |url=https://possibility.teledyneimaging.com/the-future-of-cmos-is-stacked/ |website=Possibility {{!}} Teledyne Imaging |date=August 30, 2023}}

In August 2012, Sony commercialized their stacked sensor technology as Exmor RS with resolutions of 13 and 8 effective megapixels.{{cite web|url=http://www.sony.net/SonyInfo/News/Press/201208/12-107E/index.html|title=Sony Global – News Releases – Sony Develops "Exmor RS," the World's First*1 Stacked CMOS Image Sensor|access-date=16 August 2015}}

In April 2021, Canon announced their new EOS R3 would feature a 35mm full-frame, back illuminated, stacked CMOS sensor and a DIGIC X image processor.{{cite web|url=https://global.canon/en/news/2021/20210414.html|title=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|publisher=Canon|access-date=17 April 2021}}

In May 2021, Sony announced a new back-illuminated, stacked sensor for the Micro Four Thirds format, IMX472-AAJK.{{cite web|url=https://www.43rumors.com/sony-announced-a-new-20mp-stacked-bs1-micro-four-thirds-sensor-is-this-the-new-panasonic-gh6-sensor/|title=Sony announced a new 20MP stacked BS1 Micro Four Thirds sensor. Is this for the future Olympus OMD camera?|access-date=30 May 2021}}

In May 2022, Fujifilm commercialized its first stacked sensor, the X-Trans 5 HS, used in the Fujifilm X-H2S.{{cite web |title=X-Trans CMOS |url=https://fujifilm-x.com/global/products/x-trans-cmos/ |website=fujifilm-x.com |access-date=3 July 2022}}

• Wafer backgrinding

{{Reflist|refs =

{{cite web

|first=Steven

|last=Zimmerman

|url=http://www.xda-developers.com/sony-imx378-comprehensive-breakdown-of-the-google-pixels-sensor-and-its-features/

|title=Sony IMX378: Comprehensive Breakdown of the Google Pixel's Sensor and its Features

|publisher=XDA Developers

|date=12 October 2016

|access-date=17 October 2016}}

}}

{{refbegin}}

• (Sony), "[http://www.sony.net/SonyInfo/News/Press/200806/08-069E/index.html Sony unveils 'Exmor R' back-illuminated CMOS technology]", Sony press release, 6 August 2009
• P.K. Swain and David Cheskis, "[http://www.photonics.com/Article.aspx?AID=34685 Back-Illuminated Image Sensors Come to the Forefront]", Photonics, August 2008
• Junko Yoshida, "[http://www.eetimes.com/news/semi/showArticle.jhtml?articleID=208200587 OmniVision adopts backside illumination technology for CMOS imager]", EE Times, 27 May 2007
• (Apple), "[https://www.apple.com/iphone/features/camera.html iPhone 4: Camera]", Apple Inc., 7 June 2010

{{refend}}

{{refbegin}}

• [https://web.archive.org/web/20100725153737/http://sales.hamamatsu.com/en/products/solid-state-division/image-sensors/ccd.php List of Hamamatsu Backthinned CCD sensors]
• [http://www.azooptics.com/details.asp?newsID=6961 Back thinned CCD used in Security cameras]
• [http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/quantumefficiency.html Microscopy Primer on Quantum Efficiency]
• [https://www.allaboutcircuits.com/technical-articles/back-illuminated-ccd-improving-light-sensitivity/ The Back-Illuminated CCD: Improving Light Sensitivity]

{{refend}}

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Category:Digital cameras

Category:Image sensors