Applications
CAMERAS
800 Series
850 Series
900 Series
1100 Series
1110 Series

What Is A CCD?

A variety of CCDs are used in Spectral Instruments cameras.

Overview

Fundamentally, a charge coupled device (CCD) is an integrated circuit etched onto a silicon surface forming light sensitive elements called pixels. Photons incident on this surface generate charge that can be read by electronics and turned into a digital copy of the light patterns falling on the device. CCDs come in a wide variety of sizes and types and are used in many applications from cell phone cameras to high-end scientific applications. Shown above are various CCDs, the largest is mounted on a 6" wafer and is used in some of Spectral Instrument’s products.

The function of a CCD can be visualized as an array of buckets (pixels) collecting rainwater (photons). Each bucket in the array is exposed for the same amount of time to the rain. The buckets fill up with a varying amount of water, and the CCD is then read one bucket at a time. This process is initiated by pouring water into the adjacent empty column. The buckets in this column transfer their ‘water’ down to a final pixel where the electronics of the camera read-out this pixel (the computer measuring the bucket) and turn it into a number that can be understood and stored by a computer.

Of course, this is an oversimplification – in fact, this ‘model’ (shown above) is actually wrong in some ways, all the pixels in a CCD are actually shifted simultaneously, not one column at a time. We’ll start the explanation process by explaining how a simple pixel works.

Pixels

Simmple CCD DiagramPhotons striking a silicon surface create free electrons through the photoelectric effect. Nature abhors a vacuum and thus a concomitant positive charge or ‘hole’ is generated as well. If nothing else is done the hole and the electron will recombine and release energy in the form of heat. Small thermal fluctuations are very difficult to measure and it is thus preferable to gather electrons in the place they were generated and count them in some manner to create an image. This is accomplished by positively biasing discrete areas to attract electrons generated while the photons come onto the surface. Shown at left is a simple diagram of a CCD pixel.

The substrate of a CCD is made of silicon, but this is not where most of the action occurs. Photons coming from above the gate strike the epitaxial layer – essentially silicon with different elements doped into it – and generate photoelectrons. The gate is held at a positive charge in relation to the rest of the device, which attracts the electrons to it. Because of the insulating layer – essentially a layer of glass – the electrons can’t make it through to the gate, and are held in place by the positive charge above them.

The top black trace shows the ‘potential well’ for the electrons that are represented by the blue color and is low, or downhill, where the potential is high since opposites attract. As the voltage adjacent to the electron’s pixel is brought high, they begin to migrate in this direction until the voltage in the preceding gate is then brought to zero, or low, thus effectively transferring all the electrons into its neighboring pixel.

Now that the electrons are held in place, they need to be moved to where the light signal they represent can be quantified. Shown at left is how this is accomplished.

Electrons are shifted in two directions on a CCD, called the parallel or serial direction. One parallel shift occurs from the right to the left ( shown at left). The serial shift is performed from top to bottom and directs the electron packets to the measurement electronics.

 

 

 

 

 

 

 

 

 

Many CCDs are built with multiple amplifiers at each corner of the CCD and can thus be read out faster. The image is split up into 2 or 4 different sections and read-out as shown below.

 

A/D Electronics

The analog to digital (A/D) electronics measures the voltage created by the packet of electrons at the serial output and turns this into an electronic number that can then be digitally transmitted to and saved by a computer. The method of reading this voltage called ‘dual slope integration’ (DSI) is used when the absolute lowest noise possible is needed. Methods such as successive approximation used in higher speed devices are faster, but noisier, and are not used in most Spectral Instruments cameras. Generally speaking, the faster a pixel is read, the more noise is introduced into the measurement. The A/D electronics output units called analog to digital units or ADU. If the gain of the measurement is known – SI cameras come with a test report showing the gain at a given read speed – the ADU number for each pixel generated can be directly correlated to the number of electrons found in that pixel.

A/D electronics have limits on the largest number they can describe. For instance, an 8-bit A/D system, cannot represent a number larger than 28 = 256. 16-bit electronics can’t describe a number larger than 216 = 65536. Thus, a 16-bit camera can never show more than 65,535 ADU in any given pixel. Scientific grade CCDs can generally hold anywhere from 70,000 to 500,000 electrons in any given pixel. Since this is more than the number of ADUs that the A/D electronics can express, different gains must be used for the electronics to access the entire dynamic range of the CCD. At slow read speeds, (i.e. low noise) gains of 0.25e-/ADU are common, thus reading only a maximum of 0.25*65535 = ~16.4ke- which is much lower than the dynamic range of modern CCDs. At higher read speeds, gains of 5e-/ADU can be reached allowing full access to the CCDs dynamic range, but this sacrifices noise for extra dynamic range. All SI cameras can be read at multiple speeds to ensure access to the most important features of the measurement.

Full Well Capacity/Dynamic Range

Andromeda GalaxyAndromeda Galaxy Each pixel in a CCD can hold a maximum number of electrons, called ‘Full Well Capacity.’ This number can vary widely (10ke- to 500ke-) and depends mostly on the physical dimensions of the pixel (the bigger, the more electrons it can hold). When a pixel has too many electrons in it, the excess begin to ‘spill’ into the neighbors and create imaging artifacts known as blooming. Shown at left is a picture of the Andromeda galaxy, where faint details of the dust in the spiral arms are visible but the closer stars in our own galaxy are blooming.

CCD Cooling

Noise vs Temperature

All CCDs benefit from working at lower temperatures. Thermal energy alone is enough to excite extraneous electrons into the image pixels and these cannot be distinguished from the actual image photoelectrons. This process generates noise and is called ‘dark current.’ For every 6-7°C of cooling, there is about a 2X reduction in the total dark current generation rate. This of course has its limits, most CCDs don’t function well below –120°C. Below is an example of how the CCD temperature affects dark current. Note that cooling to around –100°C nearly removes the noise generated from dark current.

This can be seen visually below as well. At –100°C, the image looks as a CCD should, with only random read noise present. Note that a bit of over scan has been included – one can tell the electronics to ‘read’ more from the CCD than there actually is to help get a sense of the electronic read-noise unrelated to the photoelectrons in the CCD pixels. As the temperature increases, more thermal electrons are generated. Remember that the electrons in any given pixel are moved across the CCD from left to right, so those electron packets accumulate more charge as they are swept in the parallel shifting of the CCD. This creates a gradient of signal increasing from left to right – a characteristic of a warm CCD. Also of note is the defect in this particular device. Towards the bottom is a column defect. The CCD manufacturing process can produce pixels that generate thermal electrons at a rate greater than their neighbors and inject charge into each electron packet swept past it.

Cooling a CCD to –100°C requires that the device is thermally isolated from its environment and thus must be in an evacuated environment. CCDs are commonly cooled with liquid nitrogen, Peltier junctions (thermoelectric coolers), or mechanical pumps (cryo-coolers). Spectral Instruments offers TEC or cryo-cooling since liquid nitrogen cooling doesn’t easily allow the camera to be oriented in any direction

 

 

Quantum Efficiency

All CCDs generate photoelectrons at different rates depending on the wavelength of light incident on the surface. Many factors contribute to the conversion of photons into electric signal called quantum efficiency (QE). Anti-reflection coatings have affects on QE, but nothing has the magnitude of effect that back thinning does. As shown below, a normal front illuminated device creates signal after the light has passed through the gate structures resulting in an attenuation of the incoming radiation. A back-thinned, or back-illuminated CCD has the excess silicon on the bottom of the device etched away allowing photoelectron generation to occur unimpeded.

Front and Back Illuminated

A back-illuminated device, which needs optical wavelength sensitivity, must have an additional coating for proper function. Every company has their own proprietary manner of performing the act of back-thinning and further coating, so variation between manufacturers can be significant. However, shown below is an example of some typical values from a CCD manufacturer (E2V) Spectral Instruments commonly uses.

Front Illuminated

Back Illuminated

CCD Versus the Competition

CCDs have the highest light sensitivity of any light detection technique available, but they aren’t the best sensor for every application. Below is a table demonstrating some of the advantages of the more common detectors available. CCDs are capable of holding a large signal in any single pixel, and also have a very low read noise and dark current, thus they have a very large dynamic range. CMOS parts have more read noise, and generally smaller pixels than CCDs and thus don’t quite compare to CCDs. A photomultiplier tube (PMT) can generate an electric output after a photon strikes the photocathode in just a few nanoseconds – not even high speed CMOS devices can match this. However, nothing can touch the light sensitivity of a back-thinned CCD at its peak absorption wavelength. A PMT will generate a substantial signal when a photon is detected, but not all of them incident on the detection surface will create an electron cascade. CCDs turn up to 95% of incident photons into usable signal, which can be read with low noise electronics. CMOS devices can read an entire array of silicon very quickly, but at the expense of noise in comparison to CCDs. Film, CCDs and CMOS can make pictures with one input event, while a PMT is a single source (one pixel) detector. Film must be processed in order to see the image that was detected which can take some time – but CMOS, CCDs and PMTs can be actively monitored and recorded by computers or other display devices.

The table below shows film beating out CCDs in broadband sensitivity as well, but this isn’t really fair since there is really no one piece of film that has usable sensitivity over such a broad range of the electromagnetic spectrum – CCDs can be modified to extend detection out to 125nm into the blue, but they’re not very good into the infrared past 1100nm. Film can be quite noisy even with no incident photons, but a cooled CCD can image for over 30 minutes before a single electron of dark current is generated in a pixel. Some CMOS devices generate less dark current than CCDs at a given temperature, but this is generally irrelevant because of the enhanced read noise from each CMOS pixel. As a consequence, CMOS is not usually cooled to the extent that CCDs will be.

  PMT Film CCD CMOS
Dynamic Range >1000 <100 >10,000 >5,000
Detection Speed fastest slow slow fast
Quantum Efficiency 5-20% 5-20% 25-95% 15-35%
Multi-channel no yes yes yes
Real-time yes no yes yes
Spectral Sensitivity 300-900nm 200-1300nm 300-1100 400-1100
Dark Signal good poor best best
Read Noise good good best best

The comparison table above is particularly geared for scientific imaging where exposure times are long and light levels are low. In a general sense, CCDs are much better where low noise is essential, but CMOS can be better in applications where speed is important and cooling is not needed (professional digital photography). CMOS is undergoing rapid changes in technology, and some of the parameters listed above are likely to change in a relatively short period of time.
What is a CCD?
1000S Camera