Back-illuminated sensor
Encyclopedia
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 enough to introduce a 5 Mpx 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 manufactures such as HTC's EVO 4G
, and as a major selling point for the camera in Apple's iPhone 4
.
consists of a matrix of individual picture elements. Each element is constructed in a fashion similar to the human eye
, with a lens
at the front, wiring in the middle, and photodetectors at the back. This orientation of the sensor places the active matrix
on its front surface and simplifies manufacturing. The matrix and its wiring, however, reflect some of the light and the photocathode layer then respond to 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 orients 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%. 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.
Industry observers 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 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. 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. 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.
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. In 2010, back-illumination came to the low-end of the market when Apple included a 5 Mpx OmniVision detector in the last revision of the mobile phone, the iPhone 4
.. By contrast, the recently introduced iPhone 4s employs a sensor manufactured by Sony. Another example is the HTC EVO 4G
which equipped with an 8 Mpx, 1.4 µm pixel BSI sensor from OmniVision. In 2011, Sony implemented their Exmor R sensor in their flagship smartphone Sony Ericsson Xperia Arc
.
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
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 enough to introduce a 5 Mpx 1.75 µm BI CMOS sensor at general consumer
Consumer electronics
Consumer electronics are electronic equipment intended for everyday use, most often in entertainment, communications and office productivity. Radio broadcasting in the early 20th century brought the first major consumer product, the broadcast receiver...
prices in 2009. BI sensors from OmniVision Technologies
OmniVision Technologies
OmniVision Technologies Inc. is a corporation that designs and develops digital imaging products. 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. The company...
have since been used in consumer electronics from other manufactures such as HTC's EVO 4G
HTC Evo 4G
The HTC Evo 4G is a smartphone developed by HTC Corporation and marketed as Sprint's flagship Android smartphone, running on its WiMAX network...
, and as a major selling point for the camera in Apple's iPhone 4
IPhone 4
The iPhone 4 is a touchscreen slate smartphone developed by Apple Inc. It is the fourth generation iPhone, and successor to the iPhone 3GS. It is particularly marketed for video calling , consumption of media such as books and periodicals, movies, music, and games, and for general web and e-mail...
.
Description
A traditional, front-illuminated digital camera sensorDigital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
consists of a matrix of individual picture elements. Each element is constructed in a fashion similar to the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
, with a lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
at the front, wiring in the middle, and photodetectors at the back. This orientation of the sensor places the active matrix
Active matrix
Active matrix is a type of addressing scheme used in flat panel displays. The term describes a method of switching individual elements of a flat panel display, using a CdSe or Silicon-based thin-film transistor for each pixel...
on its front surface and simplifies manufacturing. The matrix and its wiring, however, reflect some of the light and the photocathode layer then respond to 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 orients 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
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
being captured from about 60% to over 90%. Placing the wiring behind the light sensors is similar to the difference between a Cephalopod eye
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 a lens projecting an image onto a retina. Unlike the vertebrate camera eye, the cephalopods' form as invaginations of the body surface , and...
and a vertebrate eye. Orienting the active matrix
Active matrix
Active matrix is a type of addressing scheme used in flat panel displays. The term describes a method of switching individual elements of a flat panel display, using a CdSe or Silicon-based thin-film transistor for each pixel...
transistors behind the photocathode layer can lead to a host of problems, such as cross-talk, which causes noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
, dark current
Dark current
Dark current is the constant response exhibited by a receptor of radiation during periods when it is not actively being exposed to light. It may refer to:...
, 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.
Industry observers 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 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
OmniVision Technologies
OmniVision Technologies Inc. is a corporation that designs and develops digital imaging products. 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. The company...
sampled their first sensors using the technique in 2007. 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
Active pixel sensor
An active-pixel sensor is an image sensor consisting of an integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier. There are many types of active pixel sensors including the CMOS APS used most commonly in cell phone cameras, web...
-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.
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. In 2010, back-illumination came to the low-end of the market when Apple included a 5 Mpx OmniVision detector in the last revision of the mobile phone, the iPhone 4
IPhone
The iPhone is a line of Internet and multimedia-enabled smartphones marketed by Apple Inc. The first iPhone was unveiled by Steve Jobs, then CEO of Apple, on January 9, 2007, and released on June 29, 2007...
.. By contrast, the recently introduced iPhone 4s employs a sensor manufactured by Sony. Another example is the HTC EVO 4G
HTC Evo 4G
The HTC Evo 4G is a smartphone developed by HTC Corporation and marketed as Sprint's flagship Android smartphone, running on its WiMAX network...
which equipped with an 8 Mpx, 1.4 µm pixel BSI sensor from OmniVision. In 2011, Sony implemented their Exmor R sensor in their flagship smartphone Sony Ericsson Xperia Arc
Sony Ericsson Xperia Arc
The Sony Ericsson Xperia arc is an Android smartphone from Sony Ericsson. It was launched on April 1st, 2011 in Europe. The Xperia arc has a touch-screen, the mobile BRAVIA engine which optimises the picture and runs at a resolution of 854×480 pixels, a 1 GHz Qualcomm Snapdragon processor,...
.