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Detectors. RIT Course Number 1051-465 Lecture CMOS Detectors. Aims for this lecture. To describe CMOS hybrid and monolithic detectors physical principles operation and performance of CMOS detectors Given modern examples of CMOS detectors. Lecture Outline. CMOS detector definition

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RIT Course Number 1051-465

Lecture CMOS Detectors

aims for this lecture
Aims for this lecture
  • To describe CMOS hybrid and monolithic detectors
    • physical principles
    • operation
    • and performance of CMOS detectors
  • Given modern examples of CMOS detectors
lecture outline
Lecture Outline
  • CMOS detector definition
  • CMOS detector principles of operation
  • Performance of modern CMOS detectors
  • Examples of CMOS detectors
  • Historical context of CMOS detectors
cmos detector definition
CMOS Detector Definition
  • CMOS detectors are made of complimentary MOSFET circuits connected to light-sensitive materials.
  • The voltage change due to integrated photogenerated charge is generally sensed directly through a source follower amplifier in each pixel, instead of via a charge transfer process, i.e. in CCDs.
  • Charge is sensed as a voltage directly in the pixel and is not reset every time it is sensed, unlike in a CCD.
  • The readout circuit is often called a “multiplexer” because it can sequentially direct signals from multiple pixels to an individual output amplifier.
  • Historically, they have been developed later than CCDs, and first for infrared astronomy detectors.
cmos vs ccd readout
CMOS vs. CCD Readout
  • CMOS has “direct readout (DRO)” random access architecture.
  • (Note that the CMOS device in the figure has readout circuitry that takes up some real estate – it is frontside illuminated, producing non-ideal fill factor.)
cmos detector types
CMOS Detector Types
  • Monolithic (“one piece”)
    • readout and photodiode integrated in same part, which means that light sensitive layer is made of silicon (only sensitive to optical photons)
    • frontside or backside illuminated
    • can be made with mostly standardized CMOS processes that are common to the commercial semiconductor industry
  • Hybrid (“two pieces”)
    • readout circuit is separate from photodiode
    • readout is made of silicon
    • photodiode is made of semiconductor with suitable cutoff wavelength
    • requires many custom steps
hybrid array benefits
Hybrid Array Benefits
  • Hybrid arrays are used when one wants to detect light of wavelengths that are not absorbed by silicon, i.e. wavelengths beyond ~1um.
  • Infrared arrays are “hybrids” – they use one material to detect light and silicon for the readout circuit.
periodic table
Periodic Table
  • Semiconductors occupy column IV of the Periodic Table
  • Outer shells have four empty valence states
  • An outer shell electron can leave the shell if it absorbs enough energy
periodic table continued
Periodic Table Continued
  • The column number gives the number of valence electrons per atom. Primary semiconductors have 4.
  • Compounds including elements from neighboring columns can be formed. These alloys have semiconductor properties as well (e.g. HgCdTe & InSb).
  • Mercury-cadmium-telluride (HgCdTe; used in NICMOS) and indium-antimonide (InSb; used in SIRTF) are the dominant detector technologies in the near-IR.
dark current
Dark Current
  • Dark current is the signal that is seen in the absence of any light.
  • The dominant components are diffusion across the pn junction, thermal generation-recombination (G-R) of charges within the bulk of the semiconductor, and leakage currents typically through surfaces.
  • Dark current adds an effective noise due to the shot noise of the dark charge.
  • Dark current can be reduced by cooling.
read noise
Read Noise
  • Read noise is the uncertainty in the signal measurement due to electrical fluctuations produced by the detector.
  • For CMOS devices, it is due to:
    • Johnson noise of FETs
    • random telegraph signal (a.k.a. popcorn noise) in the output FET
    • interface states at material surfaces
  • Typical read noise values for CMOS devices are around 10 electrons.
sampling schemes
Sampling Schemes
  • By being a bit clever about reading out the array, one can minimize or eliminate some of these noise modes.
  • During an exposure, typically each pixel is sampled several times.
  • The most common approaches are correlated double sampling (CDS), multiple non-destructive reads (aka “Fowler Sampling”), & fitting a line (aka “up the ramp”).
up the ramp
“Up the Ramp”
  • Fit best line to multiple non-destructive samples.
  • Sample spacing does not need to be uniform.
  • Not clear whether this or Fowler sampling is best.
  • This is what is done in NICMOS MULTIACCUM mode.
well depth and non linearity
Well Depth and Non-linearity
  • Well capacity is defined as the maximum charge that can be held in a pixel.
  • “Saturation” is the term that describes when a pixel has accumulated the maximum amount of charge that it can hold.
  • The “full well” capacity in a CCD is typically a few hundred thousand electrons per pixel for today’s technologies.
  • Persistence is the afterimage that a detector can produce if it traps charge from a previous exposure and releases it during the current exposure.
  • It is produced by charge traps.
  • Charge traps will decay with an exponential timescale.
pixel to pixel crosstalk
Pixel-to-pixel Crosstalk
  • Crosstalk is the generic term that describes signal contamination due to the presence of a signal in another pixel or electrical channel.
  • Charge diffusion from one pixel to a neighbor is an important crosstalk mechanism in IR arrays and CCDs.
  • Once charge carriers are created, their motion is governed by charge diffusion.
  • Interpixel capacitance (IPC) is a form of crosstalk.
  • In this case, charge in a pixel induces a voltage change in a neighbor, just like the behavior between parallel plates in a capacitor.
  • The effect is to blur the point spread function.
  • The induced voltage does not have noise.
  • In this example, IPC is very large for the H4RG SiPIN device (10 um pixel size).
teledyne h4rg si pin
Teledyne H4RG Si PIN
  • This is an image of the H4RG device in the Rochester Imagingn Detector Laboratory (RIDL).
history of infrared detection
History of Infrared Detection
  • Herschel’s detection of IR from Sun in 1800
  • Johnson’s IR photometry of stars (PbS) mid 60’s
  • Neugebauer & Leighton: 2mm Sky Survey (PbS), late 60’s
  • Development of bolometer (Low) late 60’s
  • Development of InSb (mainly military) early 70’s
  • IRAS 1983
  • CMOS Hybrid Arrays (InSb, HgCdTe, Si:As IBCs) mid-80’s
  • NICMOS, 2MASS, IRTF, UKIRT, KAO, common-user instruments, Gemini, etc.
  • JWST
science motivation
Science Motivation
  • Exploration & discovery
    • Neugebauer, Leighton, Low, Fazio, Townes
  • Technological opportunities
    • Bolometer (Low), PbS (Neugebauer), balloons (Fazio), IR lasers & interferometry (Townes)
  • A few, key problems
    • Bolometric luminosities (Herschel, Johnson)
    • The Galactic Center (Becklin)
    • Star formation