Charge-coupled device for low background observations

Details Charge-coupled device for low background observations Charge-coupled device for low background observations

1999-07-08

2002-07-16

Pyo, Kevin (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C257S437000, C348S311000

Reexamination Certificate

active

06420691

ABSTRACT:

BACKGROUND
FIELD OF THE INVENTION
The field of the invention is charge-coupled devices.
Charge-coupled devices (CCDs) have proven to be exceptionally versatile and effective detectors from the near-ultraviolet through the near infrared. As demands for detector format or size and sensitivity increase, a number of problems have become the limiting factor for low-background observations. With larger detector formats, for example 4096×4096 photosensors or pixels, the problem of cooling to reduce dark current from bulk silicon layers of a typical CCD has become challenging because the exposed area of the device presents a large surface for absorption of thermal infrared photons from the environment.
CCD readout noise, which is introduced by the readout amplifier and the electronics outside the detector when reading a pixel without any photoelectrons, is a second limitation for low-background observations, especially when the devices can be cooled to their optimal operating temperature.
Charge-coupled devices (CCDs) have been used for astronomical observations since the 1970s. Their high quantum efficiency, low noise, relative stability and ease of use have brought about dramatic improvements in observational efficiency for ground based and space observations, culminating in the unprecedented discoveries made by missions such as the Hubble Space Telescope (HST). Space-based telescopes are more likely to be photon starved than their ground-based counterparts so that detector quantum efficiency is a greater factor in discovery potential. Typical requirements for space instruments such as the Space Telescope Imaging Spectrograph (STIS) on the HST are <25 electrons/hr/pixel dark current and <4 electrons/pixel readout noise.
Three sources of noise in a typical CCD detector are photon noise, dark current and readout noise.
Dark current is caused by thermally excited electrons, which are collected by the CCD pixel wells in the same way as photoelectrons are collected.
FIG. 3
shows the relationship between dark current and temperature. The dark current drops very rapidly with decreasing temperature. The emissivity of a typical silicon backside-illuminated, thinned CCD can be high (i.e. 0.66). Although silicon has no absorption in the thermal infrared, the silicon is attached to a glass substrate, and SiO
2
, the main constituent in glass, has a strong absorption band from 200-2000 cm
−1
that covers the thermal infrared band. Also, there may be a 1 um-thick layer of SiO
2
for electrical insulation, and this layer also absorbs some thermal radiation. A typical photon in the thermal infrared is absorbed within tens of micrometers of entering the glass substrate. For longer integration times (i.e. >1000 seconds), the dark current becomes significant.
The total noise in a CCD system can be modeled as:
n
t
=(
n
s
+(
Dt
)+
n
r
2
)
½
Where n
t
is the total noise for a pixel, n
s
is the number of photoelectrons, D is the dark current in electrons/pixel/second, t is the integration time in seconds, and n
r
is the readout noise in electrons. Additional noise may also be generated by the environment of the detector, such as cosmic rays.
Because of its simple operation, the CCD detectors are used for many observations between 800 and 1000 nm but ghost images in these wavelength ranges reduce quantum efficiency. In the near infrared, 800-1000 nm, a sizable fraction of the light passes through the CCD without being absorbed and converted into free electrons. The absorption length in silicon is greater than the thickness of a typical CCD silicon layer of 13 um so that for wavelengths longer than 800 nm, less than 1/e of the light is absorbed in the first pass. At &lgr;<600 nm, all of the light that enters the CCD is detected (some of the light is reflected off of the silicon-vacuum interface), but at 900 nm, the quantum efficiency is much reduced, typically about 30%. The result is that the light that is not absorbed during the first pass through the silicon forms a ghost image, which is enlarged and, if the focal plane is tilted, displaced from the pixel nearest its point of entry into the CCD detector.
It is desired to decrease the dark current and readout noise to increase overall quantum efficiency of a charge-coupled device system in low background or low photon environments, and in particular, to reduce ghost images caused by light not immediately absorbed by the silicon.
SUMMARY
The addition of a low emissivity reflecting layer to a typical charge-coupled device has improved overall quantum efficiency. The present invention is a charge-coupled device comprising a substrate, a sensing layer and a reflecting layer wherein the reflecting layer is positioned between the sensing layer and the substrate. The reflecting layer may be a metal such as aluminum for example. Typically the substrate will be a glass substrate. The sensing layer will be silicon. In addition, an insulating layer of silicon dioxide will typically be positioned between the sensing layer and the metal reflecting layer.
A particular embodiment of the invention would comprise a charge-coupled device having a glass substrate have a front side and a back side. On its back side would be located a metal carrier layer and on its front side, the metal reflecting layer, typically aluminum. The other side of the metal reflecting layer would be adjacent to the silicon insulating layer which is positioned between the sensing layer and the metal reflecting layer.
It is also an object of the invention to provide a system that combines the above embodiments of the charge-couple device with a correlated double sampler that controls a readout rate for the device so that the rate may be slowed down to reduce readout noise.


REFERENCES:
patent: 5456762 (1995-10-01), Kariya et al.
patent: 5693968 (1997-12-01), Cherry et al.
patent: 6075256 (2000-06-01), Kaifu et al.

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