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Anatomy of a Digital Camera: Image Sensors

CCD Architecture

At its most basic, an image sensor needs to achieve five key tasks: absorb photons, generate a charge from the photons, collect the charge, transfer the charge, and convert it to a voltage. Both CCD and CMOS sensors perform all five tasks. The first three tasks are performed similarly but they diverge in their methods of charge transfer and voltage conversion.

Simple Elegance
CCDs perform fewer functions on-chip than CMOS sensors (see CMOS Architecture), but the simple elegance of the CCD results in superior image quality. Of course, simply because a digital camera has a CCD doesn't mean that the camera itself will produce a superb image. The image quality produced by a digital camera is the result of the entire camera system including the optics, analog to digital conversion, image processing, image sensor, and all the other camera components and processes. Further, the way these components work together is an important factor in determining final image quality.

CCDs are so named for the way they transfer charges between pixel wells, and ultimately out of the sensor. The charges are shifted from one horizontal row of pixels to the next horizontal row from top to bottom of the array. This is a parallel (or vertical) shift register architecture, with multiple vertical shift registers used to transport charges vertically down the rows. The charges are "coupled" to each other (thus the term charge-coupled device) so that as one row of charge is moved vertically, the next row of charge (which is coupled to it) shifts into the pixels thus vacated.

With the charges shifted down the parallel array row by row, you might wonder what happens to the charges in the last row of the sensor device. Using a serial shift register architecture, the last row is actually a horizontal shift register. Charges in that row serially transferred out of the sensor using the charge-coupling technique, making room for the next row to be shifted out, and the next, and so on. This serial transfer of charge out of the CCD is often described as a "bucket brigade," referring to its similarity to the old-fashioned fire department's bucket brigade.

Before being transferred out of the CCD serially, each pixel's charge is amplified resulting in an analog output signal of varying voltage. This signal is sent to a separate off-chip analog to digital converter (ADC) and the resultant digital data is converted into the bytes that comprise the raw representation of the image as captured by the sensor, prior to any post-processing. Unlike computer RAM that represents a 1 or 0 by either storing a charge or not, the charge on a CCD remains in analog form until the ADC stage late in the process.

Because the CCD transfers a pure electric charge over the entire sensor via the charge-coupling process with little resistance or interference from other electronic components, it tends to produce a cleaner, less noisy signal than CMOS sensors (which have much more circuitry than CCDs). The transfer, however, is never 100 percent efficient; some electrons will inevitably be lost somewhere between the pixel well and the sensor readout. A sensor's charge transfer efficiency (CTE) is a defining specification provided by manufacturers.

The Gatekeepers
Electrodes act as gatekeepers to the entire process. Electrodes are conductors that permit current to flow in or out of an electronic device and can act as electronic gates. They are also called by other names in CCDs, according to their function in the sensor design (i.e. transfer gates, exposure control gates, and overflow gates). In the case of transfer gates, the electrodes receive clock pulses of varying voltage that enable the transfer of charge from one pixel well to the next. This includes transfer of pixel charges from row to row down the array, and the final serial readout of the last row. The electronic shutter on a sensor involves using voltage controls and electrodes to limit the integration time (exactly how long a pixel will accept photons and generate electrons), performing an exposure control function. And overflow gates are used to keep electrons from spilling and contaminating adjacent pixel charges.

The most common electrodes are made of polysilicon, though Kodak has introduced another type of electrode made from indium tin oxide (ITO). This can improve the process of capturing electrons in the pixel wells, because ITO is optically more transparent than polysilicon. An unfortunate side effect of polysilicon electrodes is that they can reflect or absorb incoming photons at certain wavelengths.

CMOS electrodes function differently than those on CCDs because of the inherent differences in the way the two kinds of sensors transfer the charge. In other words, CMOS doesn't use the CCD's charge-coupled transfer process. Therefore, CMOS doesn't use electrodes the way CCD does for that process. However, electrodes are used on CMOS to reduce noise and for transfer gates to the offload transistors.

As mentioned, a key function of the electrodes is to act as transfer gates to control the charge transfer in CCDs. To delve a bit deeper in understanding how this process works, let's look at a "four-phase CCD," which has four electrodes per pixel. (Most CCDs are multi-phase devices and the number of the phases/electrodes varies by sensor model.)

The first phase of each pixel has the same voltage applied, as do the second, third, and forth phases. If an electrode receives a high voltage, a potential well is formed beneath the electrode in the silicon substrate, and if it receives a low voltage, a potential barrier is formed, which helps keep the captured electrons (the pixel data) in the potential well. Then by varying the voltages applied to adjacent electrodes in a properly timed sequence, the potential wells can actually be shuttled across the pixel and ultimately into the next pixel, enabling the bucket brigade effect as described above.

Simple but Complex
The four-phase operation is a simple process, though a bit complex to describe in words. We'll try here.

The process starts by first turning off phase one and phase two electrode (gate) voltages in the first clock period, while turning on phase three and phase four electrode voltages in that period. During the second clock period, phase one is turned on and phase three is turned off. Then phase two is turned on and phase four is turned off in the third clock period. Finally phase three is turned on and phase one of the next pixel is turned off during the fourth clock period. This process is repeated to move the charge along the sensor.

Four-phase CCD technology is a popular sensor architecture because it can be created using two layers of material. In addition, according to Philips which uses a four-phase design, it allows for at least 50 percent of the pixel well for storage and also offers the highest charge capacity among competitive designs. A three-phase CCD provides only 33 percent of the pixel well for storage.

Mintron Question and Answers

http://www.mintron.com/

1.What is Minimum Illumination? What is Sensitivity? What does 0.0001 lux stands for?

2.What is 10 bit DSP all about why other company used only 8~9bit DSP?

3.What is F2.0, f3.4mm stand for, how do I choose lenses from these number?

4.What is smear rejection ratio what's the physical meaning of it? How do you measure is?

5.What is CMOS camera? What is it different from CCD camera?

6.What is ROAD MAP of Mintron DSP? What is next exciting product?

7.What is peak detecting mode?

8.What is instantaneous response?

9.What is STAR LIGHT camera?

10.What is S filter, how can s filter be beneficial to image quality?