High Quantum Efficiency

High quantum efficiency is a fundamentally important property of a high-sensitivity charge-coupled device (CCD) imaging device.  Quantum efficiency is a measure of the percentage of photons that are detected at the output of the device compared to photons incident on the device. Other fundamentally important device properties in achieving high sensitivity are the noise of the device charge-to-voltage readout amplifier and the CCD charge-transfer efficiency (moving the charge from the pixel to the output port without loss or added spurious charge). The overall goal is to convert most or all of the photons that impinge on the device to photoelectrons and then to read out these photoelectrons without losing any and without adding read noise.

This section specifically assumes a CCD type of imaging device, but many of the considerations apply to any silicon imager.

Quantum Efficiency (QE) 

We define QE to be the external quantum efficiency, that is,

QE = (Number of photoelectrons created in a pixel and read out off chip)
                Number of photons incident on the pixel area

This is a systems-based definition that takes account of all loss factors, such as
  • Reflection loss at the air-silicon interface
  • Absorption loss of photons in any dead layer or absorbing layer on the silicon surface
  • Fill factor, in which part of the pixel area may be obscured by opaque material, such as metal, and therefore is not sensitive to the incident photons 
  • Any internal loss of photoelectrons from the instant they are created to the time they are read out

Measurement of the QE is straightforward, but requires care, since it is an absolute measurement. The QE is wavelength dependent, as discussed below.

Figure 1. Comparison of QE for selected imagersFigure 1. Comparison of QE for selected imagers. (Click on image to see larger version.)

Compared to other common detectors, the QE of currently evolved CCD scientific-quality devices can be almost perfect for much of the visible wavelength range (400 nm to 700 nm), as shown in Figure 1.

How did CCDs reach this advanced state of performance? Early CCD devices were built to receive the light on the circuit side of the device. The light needed to first pass through various layers on the device. Some of these layers were partially absorbing, and some reflected considerable light back away from the collecting surface, and part of the pixel surface may have been covered with opaque material, such as metal.

Back-Illuminated Imagers 

To avoid light loss due to films and structures on the front of the device, Lincoln Laboratory and other researchers have developed back-illuminated (BI) device technology. The concept is that the device is first bonded circuit side down to a support wafer, then thinned to a thickness that will allow the fields from the CCD wells to penetrate to the back (non-circuit side) of the device.

Figure 2. Cross section of back-illuminated deviceFigure 2. Cross section of back-illuminated device. (Click on image to see larger version.)

The exposed silicon back surface must then be passivated (the blue top surface in Figure 2 represents the passivation layer) to prevent surface states or other external fields from penetrating into the silicon and disrupting the internal device operation. It is important that this passivation layer be thin compared to the absorption depth of the wavelength of light to be detected since many of the photons absorbed in this layer may be lost. Such a "dead layer" is usually associated with a particular passivation layer technique. Obviously, the dead layer thickness should be much less than the wavelength-dependent skin depth of the incoming photons to guarantee high QE.

Back-illuminated devices present a planar surface to the incident radiation, unlike front-illuminated (FI) devices that have many features for light to pass through or between. Because of this planar surface, BI devices may also be effectively coated with an antireflection layer to minimize reflection losses at the air-silicon interface.

The process of fabricating a BI device is more difficult and expensive than for an FI one, so use of these devices is limited to applications that demand the highest performance. Commercial devices all tend to be FI, with lower performance, because of cost considerations, but the number of commercial BI devices is increasing.

Wavelength Dependence of QE

Figure 3 shows the dependence of the absorption length (distance in which the absorption is 1/e) of silicon with wavelength of the incident radiation.

Figure 3. Wavelength dependence of absorption length in siliconFigure 3. Wavelength dependence of absorption length in silicon. (Click on image to see larger version.)

The silicon substrate of an imager must absorb a photon in order to create a photoelectron, so when the absorption length increases, so must the thickness of silicon for high QE. If the device is thinner than the absorption length, some of the radiation will simply pass through the device undetected. Figure 3 shows that for the wavelength range approaching 1 µm, and also for X-ray radiation with energy greater than about 5 keV, the silicon thickness must be many 10s of µm. However, as the thickness of silicon is increased, its resistivity must be decreased in order for the fields of the CCD wells to penetrate to the back surface. If there is a field-free region near the back surface, photoelectrons will tend to move laterally before being collected, therefore producing a blurred, low-resolution image. We have discussed the challenges of building deep-depletion devices in another section.

Extreme Ultraviolet and Low-Energy Back-Illuminated Passivation

In the ultraviolet (UV) and extreme ultraviolet wavelengths, where the absorption length drops to much less than 0.1 micron, there is a different problem. Any moderate-thickness dead layer on the back surface of the device can result in very significant loss of photoelectrons before they can be collected in a CCD well. Therefore, the back-surface passivation process for these wavelengths must result in a very thin passivation layer and dead layer.

Using molecular-beam epitaxy (MBE), we have developed a novel process for producing very thin (~5 nm) passivation layers. This process results in excellent QE over the range from near UV to soft X-ray. This passivation layer also has proven to be very stable with respect to UV exposure and radiation. Lincoln Laboratory has a 200-mm MBE system in its fabrication facility (this system is highly unusual for a silicon facility).



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