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Vision Art by Electrons––The Imaging Technology


By
Muwei Zhang

October 25, 2016 - Posted in Discussion
​Thousands of years, people always hold great interests in recording the marvelous scenes in their lives. However, our eyes are just like a high-resolution cache video cameras without a sharing button of Twitter or Facebook. Back to the ancient time, the date when people still live in a cave, our creative ancestors have already learned to use rocks like talcum painting on the walls to cherish their hunting events. As time goes by, painting tools and skills have developed significantly. At Renaissance period, with focus on the reality and human being, paintings even mixed the false with the genuine. Nowadays, hyperrealism as a branch of painting art is still trying to close in upon the reality only using brushes.

 

In 1825, the first permanent photographic image was created by an old French inventor, Nicéphore Niépce, and that picture is a contacted-exposed copy of an engraving. Just after 2 years later, in a comfortable morning with plenty of sunshine, the old man used his new heliography (which literally means “sun drawing” in Greek) machine, implementing a photograph that reproduced the view from the window of his house, which becomes the first photograph under the meaning of modern concept.

Improved by Nicéphore Niépce’s coworker, the new process so called daguerreotype has become the pioneer invention or idea that has influenced the photography technology over centuries. The fundamental of this process is, using specific photosensitive materials to record the light intensity, then etching or using other chemical development processes to get positive picture. Later film imaging technology based on silver halide has shirked the camera size and lowered the cost, and ruled the whole photography industries until late 20th century.

 

Nowadays, an advanced brush has taken the place of photosensitive materials. That is the arrays of electronic photodetector, or imaging sensors. Digital cameras based on imaging sensors own a lot of advantages such as lower recurring cost, higher light sensitivity and more convenient storages. Comparing to the high quality and low cost of digital cameras, you can barely see the traditional film based cameras in the market.

Generally, imaging sensor is a circuit chip integrated with arrays of photodetectors, and every detector represents a unit at the coordinate (N, M) in this array, which is so called “pixel”. There are two major solid state device technologies of imaging sensors: CCD (Charge-Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor).

CCD is an integrated chip that stores and transmits signals by electrons, in a relatively small size lined with numerous pixels (2.5 cm × 2.5 cm for 1024 × 1024 pixels). The basic component of a pixel is a transparent MOS (Metal-Oxide-Semiconductor) or MIS (Metal-Insulator-Semiconductor) device. This structure is often comprised by p-type silicon, oxide layer and metal electrodes like the figure below. The depletion region in the p-type semiconductor can generate electron-hole pairs whether receiving the light from topside or backside, and the substrate is very thin to let the light penetrates through.
 
When voltage at gate VG = +V, the photo-generated electrons in the depletion region will accumulate near the surface of the device. Those electrons are confined in the potential well formed by the gate voltage. The volume of electrons is proportional to the light exposure, and those electrons contributed the useful signal, so the image is actually “decomposed and stored” in every pixel with certain amount of electrons. In 2009, Willard S. Boyle, George Elwood Smith and Kuen Kao shared the Nobel Prize in Physics for the great contribution to the CCD technology.

CMOS imaging sensor basically is an Active Matrix Array (AMA), and every pixel is comprised by one optoelectronic diode and one or more CMOS transistors. Unlike CCD, CMOS is mainly based on switch property of transistors, the current only passes when the transistor is on, so the power consumption remains low. Due to the mature silicon fabrication, this kind of imaging sensors are also cheaper.

 
Recently, structure of CMOS imaging sensors improved in order to get better picture quality. Other MOSFET units like photogate and cascode has been tried to improve the light sensitivity. What appeals to people mostly, is the size of this kind of sensors can be much smaller than CCD, which allows it can be applied into smaller devices, like our cellphone. All the cameras at our smartphones are based on CMOS imaging sensors. In 2013, researchers from SONY incorporation designed an 8Mpixel stacked structure of CMOS, claiming that this structure can allow device to get better quality image from various scenes and different environments. 

 
 When voltage at gate VG = +V, the photo-generated electrons in the depletion region will accumulate near the surface of the device. Those electrons are confined in the potential well formed by the gate voltage. The volume of electrons is proportional to the light exposure, and those electrons contributed the useful signal, so the image is actually “decomposed and stored” in every pixel with certain amount of electrons. In 2009, Willard S. Boyle, George Elwood Smith and Kuen Kao shared the Nobel Prize in Physics for the great contribution to the CCD technology.

CMOS imaging sensor basically is an Active Matrix Array (AMA), and every pixel is comprised by one optoelectronic diode and one or more CMOS transistors. Unlike CCD, CMOS is mainly based on switch property of transistors, the current only passes when the transistor is on, so the power consumption remains low. Due to the mature silicon fabrication, this kind of imaging sensors are also cheaper.

Recently, structure of CMOS imaging sensors improved in order to get better picture quality. Other MOSFET units like photogate and cascode has been tried to improve the light sensitivity. What appeals to people mostly, is the size of this kind of sensors can be much smaller than CCD, which allows it can be applied into smaller devices, like our cellphone. All the cameras at our smartphones are based on CMOS imaging sensors. In 2013, researchers from SONY incorporation designed an 8Mpixel stacked structure of CMOS, claiming that this structure can allow device to get better quality image from various scenes and different environments.

 
 Because of the stable and wide range light sensitive properties, CCD now is still applied in professional areas like astronomy and infrared detection. But for the commercial, CMOS sensors almost took over the whole market, just like CCD replaced the film imaging in late 1990s. As to the customers or photographers, we always want a camera with better performance, and that’s the reason drives technology keep updating.

Our ancestors in the cave, they had no ideas about what’s silver halide and film; our first photographer Nicéphore Niépce, he didn’t have a clue about the existence of electrons and the relationship between electrons and light. But travelling thousands of years, their pictures convey an indescribable feeling of the eternity. Can you imagine what eternity is? Well, every time when we take photos using our cameras, we are making eternity by electrons.

 
 
 Reference:
[1] https://en.wikipedia.org/wiki/Nic%C3%A9phore_Ni%C3%A9pce
[2] Safa, Kasap. Optoelectronics and Photonics: Principles and Practices. Pearson Education India, 2009.
[3] http://www.ysctech.com/digital-microscope-CCD-camera-info.html] Sukegawa, Shunichi, et al. "A 1/4-inch 8Mpixel back-illuminated stacked CMOS image sensor." 2013 IEEE International Solid-State Circuits Conference Digest of Technical Papers. IEEE, 2013.
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