Delta-doped CCDs for Ultravioletand Low-energy Particle Detection J. Shouleh Nikzad, Aimee Smith, Qiuming Yu, Todd J. Jones, Paula Grunthaner, and S.Tom Elliott Center for Space Microelectronics Technology .' Jet Propulsion Laboratory, California institute of Technology, Pasadena, CA 9 1 109 Abstract Processing of delta-doped CCDs was described previously.12 A 2.5 nm delta-doped Si layer is were Delta-doped CCDs developed at the grown onthe back surface of thinned, fully- Microdevices Laboratory at the Jet Propulsion processed CCDs at low-temperature. Delta-doped Laboratory to address quantum efficiency and CCDs have been extensively tested and have quantumefficiencyhysteresis in UV CCDs. shown 100% internal quantum efficiency in the Using molecular beam epitaxy, fully-processed ultraviolet and visible part of the spectrum thinned CCDs were modified by growing 2.5 nm indicating that the deleterious backside potential of Boron-doped silicon on their back surface. well responsible for the detector dead layer has Because of the sharply-spiked dopant profile in been effectively eliminated. Because the delta- the thin epitaxial layer, these devices are called dopedlayerisincorporateddirectlyinto the delta-doped CCDs and they exhibit stable and silicon lattice, themodified CCDs are robust uniform, 100% internal quantum efficiency in the enough to withstand direct deposition of anti- visible and ultraviolet regions of the spectrum. In reflection coatings for enhanced UV quantum addition,because delta-doped the layer is efficiency. incorporated in the lattice, it is possible to directly deposit antireflection coatings on these CCDs to UV characterization of Delta-doped CCDs further enhance the total quantum efficiency. UV and charged-particle detection with delta-doped The quantum efficiency(QE) and stability of CCDs, and data from field measurements will be delta-doped CCDs in the UV and visible regions presented. of the spectrum has been extensively measured. Figure 1 shows the typical quantum efficiency in Introduction the 250-700 nm region of the spectrum and the enhancement of the QE in the 300-400 nm region Because of their large format, high resolution,low by direct deposition of single layer HfOz.2 The noise, and maturityof their technology, CCDs are solid line in figure 1 is the silicon transmittance the detector of choice for many scientific which represents 100% internal quantum or the applications. However, frontside- standard maximum QE that be can obtained without illuminated CCDs do not respond in the UV addition of antireflection coatings. We have also because of short absorption of photons in this measured the QE of delta-doped CCDs in the wavelength range. Untreated back-illuminated 121.6-3 10 nm region of the spectrum. It was silicon CCDs have limited sensitivity to radiation shown in thosemeasurements that the delta-doped with short penetration depth (e.g., UV photons CCD shows 100% internal QE throughout the and low-energy particles), due to the surface entire 120-700 nm waveband. depletion caused by the inherent positive charge in the native oxide. Because of surface depletion, Applications in astronomy require stable device internally-generated electrons are trapped near the performance. Figure 2 shows quantum efficiency and irradiated surface therefore cannotbe data over a three-year period. No degradation of transported to the detection circuitry. This surface the device quantum efficiency was observed. The potential can be eliminated by low-temperature device with stability respect to historyof molecular beam epitaxial(MBE) growth of a illumination has also been examined. Increasing delta-doped layer onthe Si surface. This effect the exposure time by a factor of 100 and returning hasbeen demonstrated through achievement of tothe original exposure time yielded identical 1 0 0 % internal quantum efficiency for UV photons quantum efficiency for the delta-doped CCD, detected with delta-doped CCDs. thatquantum demonstrating no efficiency hysteresis existsin the device. Low-energy particle detection withdelta-doped CCDs 80 Imaging systems for low energy particles 60 generally involve the use of microchannel plate electron multipliers followed by position sensitive solid state detectors,or phosphors and CCDs. 40 These systems work well and can process up to Ban &doped CCD 106 electrons/sec., however the spatial resolution 20 Si transmittance - of these compound systems is considerably less than that of a directly imaged CCD. Also, these 0- systems have difficulties with gain stability and 300 400 500 600 they require high voltages. Wavelength (nm) Similar to UV photons,low-energyparticles Figure 1. Quantum efficiency of a bare delta-doped CCD depositasignificantfraction of theirenergy (circles) compared with solid line (Si transmittance) shows within a few nanometers of thesurface, therefore, 1 0 0 % internal QE. QE is enhanced by the addition of anti- 300.400 nm regions. reflection coatings optimized for the frontside-illuminated untreated or back- illuminated CCDs cannotdetectlow-energy particles. Quantum efficiency measurements in X-raymeasurements performed were on the UV indicate that electrons generated near the 1024x1024 pixel, 9 pm thick CCDs using Fe, Ti, surface of delta-doped are CCDs detected Ca, K, Si, and A1 targets. these From efficiently and delta-doped CCDs are promising measurements, it was seen that very low-energy x as imaging detectors of low-energy particles. We have extended the characterization of delta-doped rays,such as thealuminum K a line,canbe CCDs to detection of electrons in the 50-1500 eV In addition, detected. measurements were energy range using both a custom UHV chamber performedusingcarbonandfluorinexrays. and a scanning electron microscope.3J Because of their short absorption length, these x produce rays that very photo-electrons are To gain an understanding of different aspects of vulnerable to backsiderecombinationeffects. low-energyelectronresponse of delta-doped These tests showed that no significant surface CCDs, we performed measurements using various recombination was occurring. sources electron and device different configurations. One set of measurements was performed inan SEM to take advantage of its highly-focused beam.SEM electron The apparatus was a JEOL, model JSM 6 4 0 0 , and the measurements were made with beam energies ranging between 200 eV and 1 keV. While it was not possible for modifications to be made to the 60- SEM in order to accommodate the electronics necessary for collecting CCD images, performing mode photo-diode measurements was quite - straightforward and informative. Another set of m After8-doping measurements wasmade in a UHV system in 16 Months after &doping' mode. photo-diode For thisof mode 3 y n after8-doping measurement, each CCD in turn was mounted in plane with a Faraday cup and a phosphor screen !#O 3 h 3iO 4h 4h 5 h 5iO 6b a onto manipulator. Using the custom UHV Wavelength (nm) system afforded the use of two different electron sources, one of very low energy and one of Figure 2. QE measuredover a three-yearperiodonthe energies similar as used i n the SEM samedelta-doped 5 12-by-512-pixel Reticon CCD. The measurements. The low-energy electron gun is a CCD was stored unprotected in a laboratory environment. hot-filamentcathode that produceselectron The bars the error represent accuracy (f5%) of the energies of several 10 eV while generating a measurement systems used. stronglight background. Comparisonwasmade between the observed response ofthe CCD and implantation6 at electron energies greater than 1 the response of the CCD with the electron beam keV. magnetically deflected. Because of the strong CCD response background to the light, The delta-doped CCD responds efficiently and measurements with the hot filament electron gun reliably to low-energy electrons. Moreover, a beam are reported only qualitatively (50-200 eV). delta-doped CCD responds with higher gain to The UHV system further allowed for the later low-energy electrons than other backside treated attachment of the electronics necessary for devices (e.g., twice that of a flashgate CCD). The operating the CCD in imaging mode. This mode untreated CCD backside-thinned showed a of of operation allows for the observation electron dramatically lower quantum efficiency than the irradiation on operating parameters only apparent delta-doped CCD (5% vs. 160% at 900 eV). The mode as in imaging suchcharge transfer response of the untreated CCD to electrons was efficiency (CTE), individual pixel response, and unstable, decaying with a time constant on the surface charging. order of 20 minutes at an incident electron energy of 1 keV. This decay was not reversible by a TheCCDs used experiments in these were thermal anneal at -200°C. thinned, back-illuminated EG&G Reticon CCDs. All measurements were repeated with both delta- In measurements conducted in our laboratory, we doped and untreated CCDs. In some of the reportthe use of CCDs to imageelectrons. measurements, direct comparisons of delta-doped Images of 500 eV electrons with the delta-doped CCDs with untreated CCDs were made on the CCD show excellent qualitative similarity to UV same device, using a delta-doped CCD which images at 250 nm, with similar contrast between included acontrolled(untreated) region. The controlledregion was provided on theback delta-doped andcontrol regions of the CCD. surface of the array by masking off a portion of the surface during the MBE growth. All devices were fully-characterizedprior to the electron measurements usingUV illumination. Figure 3 shows the electron quantum efficiency of adelta-dopedCCDplottedasafunction of energy. incident Quantum was efficiency :. calculated by dividing the measured current from the CCD configured in photodiode mode to the measured electron beam current (measured by a Faraday cup), which is equivalent to the number of electron-hole pairs detected divided by the 50 1 0 number of incident The electrons. measured quantumefficiency of thedelta-dopedCCD increases with increasing energy of the incident beam. The dependence of quantum efficiency on Beam Energy (eV) incident is to energydue the complicated Figure 3 Ratio of detected electrons to incident electrons as interaction of electrons with silicon which results a function of energy. The response of the CCD increases in the generation of multiple electron-hole pairs in with increasing energy as result of multiple electron-hole the cascade initiated by each incident electron. A pair generation. significantfraction of the incident energy is undetected,due to backscattering of incident electrons energy and other dissipation Field observationsand feedback fromscientific mechanisms (e.g., secondary and Auger electron community emission). Multiple electron-hole pair production, also known in the literature as quantum yield, is Delta-doped CCDs have beenusedrecentlyin also observed in the measured UV andx-ray collaborations with a scientists several in a response of delta-doped CCDs and other devices. number of field observations. In collaboration Quantum yield greater than unity has been with Caltech, a delta-doped CCD wasused to previouslyobserved in backside-illuminated image galaxies in the near UV at Caltech's CCDs modified using the flashgate5 and ion Palomar observatory. In a sounding rocket experiment in collaboration with the University of Colorado,adelta-dopedCCD wasusedasthe References detector in the spectrograph for ozone concentration measurements in the upper 1 . M.E. Hoenk,P.J.Grunthaner, F.J.Grunthaner, atmosphere.Use of delta-dopedCCDs in very M. Fattahi, H.-F. Tseng and R.W. Terhune, Appl. high precision photometry in collaboration with Phys. Lett., 61 (9) 1084 (1992). NASA Ames has been carried out showing that 2. S. Nikzad, M.E. Hoenk, P.J. Grunthaner, R.W. delta-doped CCDs have the dynamic range and Terhune, R. Wizenread, M. Fattahi, H-.F. Tseng, stability necessaryfor high precision photometry. and F.J. Grunthaner, Proc. of SPIE, 2217, Surveillance Technologies 111, April 4-8, Acknowledgments Orlando, Fl. (1994). 3. A. Smith, Q. Yu, S.T. Elliott, T.A. Tombrello, The authors gratefully acknowledge the and S. Nikzad, Proc. of the MRS, 448, Boston, invaluableassistance of Drs. L.D. Bell, M.E. Dec. 3, (1996). H o e d , S. Manion, T. Van Zandt, J. Trauger, M. 4. S. Nikzad, A. Smith, T. Elliott, T.A. T.J. Jones, Lesser, Professors J. McCarthy, Mr. and W. Tombrello, and Q. Yu, Proc. SPIE, 3019, Feb. 11, Proniewicz. The work presented in this paper was San.Jose, ( 1997). performed by the for Space Center 5. T. Daud, Janesick, J.R. K. Evans, and T. Microelectronics Technology, Jet Propulsion Elliott, Opt. Eng.,26 (8) 686 (1987). Laboratory, California Institute of Technology, 6. D.G. Steams and J.K. Wiedwald,Rev. Sci. and was jointly sponsored by the National Instnun. 60 (6)1095 (1989). Aeronautics and Space Administration, Office of Space Scienceand the Caltech President’s Fund. I ..
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