Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Activity 1 : Introduction to CCDs.In this activity the basic principles of CCD Imaging is explained.Simon Tulloch [email protected] is a CCD ?Charge Coupled Devices (CCDs) were invented in the 1970s and originally found application asmemory devices. Their light sensitive properties were quickly exploited for imaging applicationsand they produced a major revolution in Astronomy. They improved the light gathering power of telescopes by almost two orders of magnitude. Nowadays an amateur astronomer with a CCD camera and a 15 cm telescope can collect as much light as an astronomer of the 1960s equipped with a photographic plate and a 1m telescope.CCDs work by converting light into a pattern of electronic charge in a silicon chip. This pattern of charge is converted into a video waveform, digitised and stored as an image file on a computer.Photoelectric Effect.The effect is fundamental to the operation of a CCD. Atoms in a silicon crystal have electrons arranged in discrete energy bands. The lower energy band is called the Valence Band, the upper band is the Conduction Band. Most of the electrons occupy the Valence band but can be excited into the conduction band by heatingor by the absorption of a photon. The energy required for this transition is 1.26 electron volts. Once in this conduction band the electron is free to move about in the lattice of the silicon crystal. It leaves behind a ‘hole’ in the valence band which acts like a positively charged carrier. In the absence of an external electric field the hole and electron will quickly re-combine and be lost. In a CCD an electric field is introduced to sweep these charge carriers apart and prevent recombination.Increasing energyValence BandConduction Band1.26eV Thermally generated electrons are indistinguishable from photo-generated electrons . They constitute anoise source known as ‘Dark Current’ and it is important that CCDs are kept cold to reduce their number.1.26eV corresponds to the energy of light with a wavelength of 1m. Beyond this wavelength silicon becomes transparent and CCDs constructed from silicon become insensitive.photonphotonHole ElectronCCD AnalogyA common analogy for the operation of a CCD is as follows:An number of buckets (Pixels) are distributed across a field (Focal Plane of a telescope)in a square array. The buckets are placed on top of a series of parallel conveyor belts and collect rain fall(Photons) across the field. The conveyor belts are initially stationary, while the rain slowly fills thebuckets (During the course of the exposure). Once the rain stops (The camera shutter closes) the conveyor belts start turning and transfer the buckets of rain , one by one , to a measuring cylinder (Electronic Amplifier) at the corner of the field (at the corner of the CCD)The animation in the following slides demonstrates how the conveyor belts work.RAIN (PHOTONS)BUCKETS (PIXELS)VERTICALCONVEYORBELTS(CCD COLUMNS)HORIZONTALCONVEYOR BELT(SERIAL REGISTER)MEASURING CYLINDER(OUTPUT AMPLIFIER)CCD AnalogyExposure finished, buckets now contain samples of rain.Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyoris tipped into buckets on the horizontal conveyor.Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn intothe measuring cylinder .`After each bucket has been measured, the measuring cylinderis emptied , ready for the next bucket load.A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.Eventually all the buckets have been measured, the CCD has been read out.Structure of a CCD 1.The image area of the CCD is positioned at the focal plane of the telescope. An image then builds up that consists of a pattern of electric charge. At the end of the exposure this pattern is then transferred, pixel at a time, by way of the serial register to the on-chip amplifier. Electrical connections are made to the outside world via a series of bond pads and thin gold wires positioned around the chip periphery. Connection pins Gold bond wires Bond pads Silicon chipMetal,ceramic or plastic packageImage areaSerial registerOn-chip amplifierStructure of a CCD 2.CCDs are are manufactured on silicon wafers using the same photo-lithographic techniques used to manufacture computer chips. Scientific CCDs are very big ,only a few can be fitted onto a wafer. This is one reason that they are so costly.The photo below shows a silicon wafer with three large CCDs and assorted smaller devices. A CCD has been produced by Philips that fills an entire 6 inch wafer! It is the worlds largest integrated circuit.Don Groom LBNLStructure of a CCD 3.One pixelChannel stops to define the columns of the imageTransparenthorizontal electrodesto define the pixels vertically. Also used to transfer the charge during readoutPlan ViewCross sectionThe diagram shows a small section (a few pixels) of the image area of a CCD. This pattern is reapeated.ElectrodeInsulating oxiden-type siliconp-type siliconEvery third electrode is connected together. Bus wires running down the edge of the chip make the connection. The channel stops are formed from high concentrations of Boron in the silicon.Structure of a CCD 4.On-chip amplifierat end of the serial registerCross section ofserial registerImage AreaSerial RegisterOnce again every third electrode is in the serial register connected together. Below the image area (the area containing the horizontal electrodes) is the ‘Serial register’ . This also consists of a group of small surface electrodes. There are three electrodes for every column of the image areaStructure of a CCD 5.The serial register is bent double to move the output amplifier away from the edgeof the chip. This useful if the CCD is to be used as part of a mosaic.The arrows indicate how charge is transferred through the device.Edge of Silicon160mImage AreaSerial RegisterRead Out AmplifierBus wiresPhotomicrograph of a corner of an EEV CCD.Structure of a CCD 6.ODOSRDRSWOutput NodeSubstrateOutput