Technische Hochschule Wildau Technische Hochschule Brandenburg Master’s Program Photonics (PM17) − Summer Semester 2018 − Bildgebende Verfahren Prof. Dr. Martin Regehly Optischer Messtechnik Prof. Dr.-Ing. Eckhard Endruschat Matteo Elviretti (50061279) Contents 1 Digital cameras 3 2 CCD Image sensor 2.1 Working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 4 3 Variants of CCD structure 5 4 Architecture 4.1 Intensified CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Electron-multiplying CCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7 7 5 Noise sources in CCD 7 2 1 Digital cameras Digital cameras convert analog information, represented by fluctuating waves, into digital information, represented by bits. An analog camera would focus the light onto a photosensitive film. A digital camera focus the light into a semiconductor device that record the light electronically. In practice a sensor converts the light into electrical charges. Solid-state image sensors have in general two parts: • light sensitive detectors, pixels; • readout circuit. They consist of one or more rows of closely spaced light-sensitive detectors, the pixels. The light-sensitive elements of solid-state sensors are either reversed biased photodiode or so-called MOS capacitors. A MOS capacitor is composed of three layers. On a photodiode substrate (p-n junction), a thin insulation SiO2-layer is applied. On top of that an electrode follows. Solid-state image sensors have a readout circuit, which relays and multiplexes the signals from each pixel to the output. There are different designs for readout electronics. Most common are MOSFET readout, CCD readout and CMOS readout. Figure 1: Scheme of a MOS capacitor a) prior to exposure and b) after exposure 2 CCD Image sensor 2.1 Working principle A CCD chip consists of a large number of MOS capacitors. The photoactive region of a CCD is, generally, an epitaxial layer of silicon. It is lightly p doped (usually with boron) and is grown upon a substrate material, often p++ . In buried-channel devices, the type of design utilized in most modern CCDs, certain areas of the surface of the silicon are ion implanted with phosphorus, giving them an n-doped designation and thus, realizing a p-n junction. This region defines the channel in which the photogenerated charge packets will travel. The readout procedure is • Integration\exposure time: photons hit the surface, electrons are generated and accumulated in the pixel; • Transfer to the shift register; • Line by line readout. The electrode is initially held at a positive potential of, for example, about 10 V. Incident light generates electron-hole pairs in the substrate. The positively charged holes flow to the ground, while the electrons migrate towards the positive electrode. However, due to the insulating layer the electrons do not reach the electrode. They accumulate underneath the electrode. The number of stored electrons in a MOS capacitor is a direct measure of the local exposure. In order to read out the stored charges in the various MOS capacitors, the charge-coupled device transport principle is applied. At the beginning, only capacitor number 2 is energized and charged. Subsequently, electrode 3 is also set to a positive potential. Then, the stored charges are redistributed between 3 Figure 2: Illustration of charge-coupled transport principle the two electrodes 2 and 3. Then, electrode 2 is grounded. Therefore, all charges are gathered under electrode 3. Hence, the goal is achieved: charges stored in a certain MOS capacitor have been pushed into the adjacent capacitor. The charge transfer from one cell to another is virtually lossless, it means that the charge transfer efficiency is about 100 %. The principle of charge-coupled device transport is repeated in all MOS capacitors again and again. The charge packets generated by the exposure and stored in the MOS capacitors travel in the direction of the output stage along the interface between silicon substrate and insulator. The output stage converts the individual charge packets into a voltage signal, i.e. the actual video signal. The functioning of a CCD is described by the Block-diagram Figure 3: Functional diagram of a CCD 2.2 Parameters Important parameters are the following: • fullwell capacity; • pixel size; • quantum efficiency; • spatial and temporal resolution; • fill factor. The fullwell capacity defines the amount of charge an individual pixel can hold before saturating. It is dependent upon the pixel size of the CCD and the operating voltages used on the CCD. It can therefore be modified by varying the gate voltage, making a different doping concentration and varying the oxide thickness. The quantum efficiency is the measure of the effectiveness of an imager to produce electronic 4 charge from incident photons. It is defined as η= number of generated electrons 61 number of incoming photons (1) Silicon-based photodiode arrays and CCD detectors are sensitive in the visible and near infrared spectral range from 0.4 to 1.1 µm. Figure 4: Quantum efficiency in respect to the wavelength of a back-illuminated CCD (plain curve) and a front-illuminated CCD (dashed curve) The low quantum efficiency in the UV region shown in Figure 4 is due to high absorption so that the UV photons has a low penetration depth and don’t reach the depletion region. In the NIR region the energy of the incoming photons is lower than the energy bandgap so that it can’t excite electrons. The spatial resolution of an image sensor depends on the distance between two neighboring pixels. A resolution of more than 100 line pairs per mm is achievable. For the temporal resolution we need to consider that the shiftout is sequential and, considering that the displacement of charges from one MOS capacitor to the adjacent one lasts a period of few 10 ns, it means that this is not a very fast device. For instance, considering the best case of 10 ns for each displacement and 1000 × 1000 pixels= 1 Mpixel: t · pixels = 10 × 10−9 s · 1 × 106 pixels = 10 × 10−3 s (2) such a device can record at 100fps. The fill factor of a pixel describes the ratio of light sensitive area versus total area of a pixel, since a part of the area of an image sensor pixel is always used for transistors, electrodes or registers, which belong to the structure of the pixel of the corresponding image sensor (CCD, CMOS, sCMOS). Only the light sensitive part might contribute to the light signal, which the pixel detects. 3 Variants of CCD structure A traditional, front-illuminated digital camera is constructed in a fashion similar to the human eye, with a lens at the front and photodetectors at the back. This traditional orientation of the sensor places the active matrix of the digital camera image sensor on its front surface and simplifies manufacturing. The matrix and its wiring, however, reflect some of the light, and thus the photocathode layer can only receive the remainder of the incoming light; the reflection reduces the signal that is available to be captured. A back-illuminated sensor contains the same elements, but arranges the wiring behind the photocathode layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light can strike the photocathode layer without passing through the wiring layer. This change can improve the chance of an input photon being captured from about 60% to over 90%. Since the problem to detect UV radiation, as mentioned in 2, is due to the low penetration depth, in this structure also the UV sensitivity is increased as shown in 4. Another alternative structure is the Deep-depletion CCD. In order to enhance the 5 QE at NIR region, since these photons have a great absorption path, the photosensitive region in the back-illuminated structure must be thicker. Furthermore, the higher resistivity material allows the electric fields, created by applying voltages to the electrodes, to penetrate the entire depth of the now thicker photosensitive region and hence better collect and confine photoelectrons within the pixels. 6 Figure 5: Frontside (left) and backside (right) illuminated CCDs 4 Architecture By changing architecture, adding some components, alternative device can be built. 4.1 Intensified CCD An intensified charge-coupled device (ICCD) is a CCD that is optically connected to an image intensifier that is mounted in front of the CCD. An image intensifier includes three functional elements: a photocathode, a micro-channel plate (MCP) and a phosphor screen. he photons which are coming from the light source fall onto the photocathode, thereby generating photoelectrons. The photoelectrons are accelerated towards the MCP by an electrical control voltage, applied between photocathode and MCP. The electrons are multiplied inside of the MCP and thereafter accelerated towards the phosphor screen. The phosphor screen finally converts the multiplied electrons back to photons which are guided to the CCD by a fiber optic or a lens. 4.2 Electron-multiplying CCD An electron-multiplying CCD is a charge-coupled device in which a gain register is placed between the shift register and the output amplifier. In practice it multiplies the electrons present in the well so that one can measure weaker signals. That’s why such devices are used for single photon detection. 5 Noise sources in CCD The main contributions to CCD noise are: • shot noise; • readout noise; • dark current noise. Photon noise, also known as photonic or photon shot noise, is a fundamental property of the quantum nature of light. The total number of photons emitted by a steady source over any time interval varies according to a Poisson distribution: Pλ (s) = λs −λ ·e s! (3) The charge collected by a CCD exhibits the same Poisson distribution, so that the noise is equal to the square root of the signal. Photon noise is unavoidable and is always present in imaging systems; it is simply the uncertainty in the data. The uncertainty in the number of photons collected during a given period of time is simply: √ σshot = S (4) 7 where σshot is the shot noise and S is the signal, both expressed in electrons. The readout noise is a source of noise related to the output amplifier. The primary sources that contribute to this noise are the white noise and the Flicker noise. This noise strongly depends on the readout frequency. For example, for consumers’ CCDs the readout frequency is tipically of 5 MHz corresponding to a σread ≈ 20 − 100 e− . In this case, for a signal S = 100000 e− , σshot = 314 e− , while for a signal S = 100 e− , σshot = 10 e− . It means that readout noise only counts for weak signals, otherwise the prominent noise is given by the shot noise. Dark current is the result of imperfections or impurities in the depleted bulk silicon or at the silicon-silicon dioxide interface. These sites introduce electronic states in the forbidden gap which act as steps between the valence and conduction bands, providing a path for valence electrons to sneak into the conduction band using thermal energy, adding to the signal measured in the pixel. The deviation in this case is given by √ (5) σdark = D − 3 Eg e where D( pixel·s ) = A · JD · T 2 · e 2kB T . The total signal to noise ratio, SNR, is given by SNR = Signal electrons accumulated in the pixel S =p 2 . 2 2 Standard devations of all the noise sources σshot + σread + σdark (6) − photon e The signal can be described by S = P( pixel·seconds ) · η( photons ) · t(s) so that, inserting the deviations of the noise sources the total SNR can be written as follows: − photon e ) · η( photons ) · t(s) P( pixel·seconds p S= 2 P · η · t + σread +D·t (7) We can distinguish two limiting cases, the shot-noise limited case and the readout noise limited case. SNR is read-noise limited for short integration time and photon-noise limited fo longer integration time. The upper limit for the signal to noise ratio is given by Figure 6: Signal to noise variation with integration time SNRmax = full well capacity (e+ ) σread 8 (8)
