Charge Coupled Device

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Charge Coupled Device

Retrieved From Circuits Today



Inception

The CCD, or charge couple device, was developed in 1969 by Williard Boyle and George Smith at AT&T Bells Lab in New Jersey. It was originally developed as a memory storage device but has gained widespread notion for its function in digital imaging. CCDs are commonly used in most camera or video camera devices. Therefore, they are a crucial component to colonoscopes and many other medical imaging devices.

When Smith and Boyle developed the CCD their idea was to focus light on the surface of the CCD so that the light values could be stored electrically. These values, stored in the form of charge, could then be passed from capacitor to adjacent capacitor until they were read at the end of the line. This first CCD contained 8 pixels so the imaging was rather crude. During the initial stages of the project Smith and Boyle noticed the device was extremely sensitive to alpha particles of incoming light. This made it difficult to function as a memory device, but also made it ideal for imaging.

In 1974 Fairchild Electronics had improved the CCD enough to produce them commercially. Their unit was comprised of a 100 X 100 array of pixels, which compared to today's standards isn't significant, but at the time was unheard of. Their CCD, however, did not gain attention until the following year when it was adopted for use in video cameras for television networks. By 1980 a CCD with a 320 X 512 pixel array had been incorporated into a telescope at the Kitt National Observatory in Arizona. NASA soon adopted this practice and began using CCDs in all their telescopes. Thus began the widespread use of the charge coupled device.
Retrieved from Latent Imager

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Function

The CCD we're currently investigating can be summarized in 2 layers, the first being an organized layer of silicon which acts as the photoactive layer, and the second being the transmission region. When an image is projected through the lens it becomes incident on the capacitor array. This allows the capacitors to develop a charge across them that is proportional to the light intensity at that point. Next, the charge in one capacitor is transferred to its neighboring capacitor. This continues down the line of capacitors until the end is reached. At the final capacitor in the array, where the overall charge has accumulated, the net charge is emptied into a charge amplifier which converts this charge into a voltage. This process is repeated until all of the charge present across the capacitors is converted into a corresponding voltage. This voltage can then be digitized and stored in memory or fed continuously into an analog device for transmission or recording.

As mentioned before the surface of the CCD is composed of a microscopic layer of silicon. This superficial layer of silicon is usually doped with Boron yielding a positive increase in the number of free charge carriers. Underneath this layer is a substrate layer with twice the number of free charger carriers. When light strikes the superficial layer of silicon atoms a free electron will be produced. This creates a temporary "hole" in the silicon crystalline lattice. The free electron will then be collected in a "well" known as the depletion layer which is located deep within the silicon. At the same time, the "hole" is moved away from the well, where the electron is currently present, until it is displaced elsewhere within the silicon. Movement of this "hole" is how the charge is carried through the crystalline lattice.
Retrieved From Molecular Expressions

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Creation Process

Step Number Description Image
1 A CCD begins as a layer of p-type silicon. P doping allows for the carriers to be positively charged electron holes
Click Image to Enlarge
2 The second step is to create channel stops by diffusing boron ions around the edges of the block of silicon
3 Phosphorous ions are then implanted every 2E-7 to 3E-7 meters between the silicon
4 The entire block is then coated with a thin layer of silicon dioxide
5 Polysilicon(red layer) is then added on top of the silicon dioxide and doped with phosphorous to increase conductivity
Click Image to Enlarge
6 On top of the polysilicon is a layer of ultraviolet light sensitive photoresist
7 A mask is then placed over the block to allow to ultraviolet light to hit certain areas of the photoresist. These exposed areas are hardened while whose beneath the mask stay soft
8 The soft photoresist is then washed with a solvent and removed
9 The hardened photoresist is then removed with chemical solvent leaving only two polysilicon areas and the p doped silicon surface
10 Another layer of silicon dixoide is then added over the entire surface
11 More polysilicon is then added on top of the silicon dioxide followed by a layer of photoresist blue
12 Another mask (black) is laid across the layer of photoresist creating a channel through the middle of the block. The mask is then exposed to ultraviolet light
Click Image to Enlarge
13 Washing then removes both the soft and the hardened photoresist
14 A third layer of silicon dioxide is then added over the surface covering everything
15 Again photoresist is added, a mask applied this time creating small individual squares on one side of the block
16 Washing with a chemical solvent removes the photoresist blue
17 The entire plate is then covered with a layer of aluminum which creates a linkage between adjacent photodiodes
18 A layer of photoresist is again added, followed by a mask which will be subjected to ultraviolet light and washed off by chemical solvent
19 A planarization layer and a color filter are then added to restrict certain wavelengths
20 Finally the CCD is ready for use. The floating red circles are light hitting the photodiode array. The charge they create(red circles at bottom of picture) is then captured in the charge amplifier where it will eventually be changed to a corresponding voltage.
Click Image to Enlarge

Retrieved From Molecular Expressions

An in depth view of a Charge Coupled Device (CCD) developed by Molecular Expressions.
Retrieved from Molecular Expressions

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CCD Usage in Colonoscopies

Charge Coupled Devices have been a standard in colonoscopes since their development. These CCDs provide the means for actual viewing of the inside of the colon without using a physical camera. As explained above, the CCD acts like a camera and saves the image from the lens as a compilation of voltages emitted by an array of photodiodes. This image is then transported back to the base unit where the voltage is converted back into a form viewable by us. As you can see, the CCD is an essential component to the operation of the colonoscope. Below is the datasheet from the type of CCD that would be found in a colonoscope. Retrieved from Texas Instruments

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Advancements in CCD

The CCD is a function of the number of pixels it contains. A pixel is the smallest element viewable on a screen. Therefore, a CCD with a higher number of pixels will produce a more detailed image. This is accomplished by decreasing the size of each individual photodiode so that more of them can be placed in the same area. Producing a more detailed image can benefit the physician in a number of ways. When speaking to Dr. Silas he specifically mentioned that only recently had the quality of the images from inside the colonoscope become high definition. Dr. Silas Interview He explained that Pentax, the company that designs the scopes he works with, made a point to mention the HD capability of their scopes. Ideally, this ability should translate to an increase in polyp detection but hasn't been proven yet.

CCDs are rather expensive to make. Each one must be covered in silicon, photoresist, and special masks multiple times during the course of the its creation. They must then be individually inspected by a technician and each pixel checked for proper function. These costs begin to add up. Moreover, the CCDs used in colonoscopes are of the highest pixel count and thus quality so their cost is even higher. A typical CCD can run between 15 and a couple hundred dollars. Thus mass production and consumption isn't cheap. To combat this rising cost the idea of using CMOS, complementary metal oxide semiconductors, instead of using CCDs has been discussed. CMOS devices are much cheaper to manufacture and can be produced faster than CCDs. This would make them ideal for "one and done" devices like the old disposable cameras. However, the tradeoff lies in the quality of the images. CMOS devices tend to have a lower pixel count and since physicians don't like to sacrifice quality for convenience, many have chosen to shy away from scopes with CMOS devices. More information on the difference between CCD and CMOS can be found at the Dalsa Wesbite.

Another area for CCD advancement is in the use of filters. Dr. Silas Interview Dr. Silas mentioned that scopes are currently equipped with filters capable of processing the image in 15 to 20 ways. These include improving the ability to see flat polyps which are thought to be more cancerous, using color processing to differentiate between discolored areas and normal tissue, removing blood so the intestinal wall can be seen more clearly, and filtering that makes blood vessels stand out so they can be studied individually. Some of these ideas are still in the developmental stage but others are well on their way to becoming a staple in colonoscopes across the globe. All however, can be related to the charge coupled device and its function in the scope.

Below is a Photodiode similar to one that would be used in a CCD. Retrieved from Micropac Industries

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The images below are from the colonoscope display we were able to secure courtesy of Pentax. The caption on the board reads, CCD Unit Assembly.


Click Image to Enlarge

Click Image to Enlarge

Sources

(1) Davidson, Michael W. Anatomy of a Charge Coupled Device Molecular Expressions: Images from the Microscope. Florida State University. [Online]. Available <http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdanatomy.html>.

(2) Williamson, Mark. The Latent Imager. Engineering and Technology. [Online]. Available <http://web.ebscohost.com.proxy2.library.uiuc.edu/ehost/pdfviewer/pdfviewer?hid=11&sid=a2f52d2e-4b36-4aa6-b4a2-e01669ddaad1%40sessionmgr4&vid=3>.

(3) (2010). CCD vs. CMOS. DALSA Corporation. [Online]. Available <http://www.dalsa.com/corp/markets/ccd_vs_cmos.aspx>.

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