Radiation Tolerances: Effects on CID Imaging Devices

Tony Chapman, Thermo Fisher Scientific, CIDTEC Cameras & Imagers

Like other CTDs, the CID imager uses hundreds of thousands (up to 4 million) pixel elements to capture optical images, and converts the light into an electronic charge which may be displayed on a monitor, or captured and processed by a computer.

Unlike other CTDs, the CID imager architecture may be configured as a radiation hardened device that can reliably operate in a wide range of radiation environments well beyond the typical lifetime of CCD or CMOS-based cameras (in some cases orders of magnitude). Or the CID may be configured as a RACID (random access CID) device which is capable of randomly addressing individual pixels and interrogating pixel charge nondestructively for higher signal quality, and extended linear dynamic range for scientific imaging applications.

While it’s the CID’s superior resistance to radiation and the unique random pixel addressing capabilities that form the basis for an exciting new generation of imaging products from Thermo CIDTEC, the following information will primarily provide detail on the CID’s tolerance to radiation in the following areas:

Radiation affects on the following key parameters in imaging devices:

• Gate threshold voltage
• Field/channel stop threshold voltage
• Charge transfer efficiency
• Dark current
• Noise

Gate threshold voltage

Ionizing radiation causes positive charge to accumulate in the MOS transistor gate oxide which reduces the gate and field voltage thresholds in N-channel transistors. Also, the drain-source stand-off voltage decreases with increased radiation exposure. If the accumulated radiation is sufficiently high enough, the transistor will turn on permanently and cease to function. This type of device will basically short out. Conversely, when P-channel transistors are exposed to radiation, there is an increased stand-off voltage, and the gate and field thresholds require higher voltages to turn-on. They basically go to an open.

Typical commercially available CCD imagers are fabricated using N-channel technology, consequently, exposure to radiation causes the CCD register photogates to turn-on into one continuous channel and the devices cease to function (short out), Figure 1. This catastrophic failure may occur in as little as 10 to 20 krads total dose gamma (Co60) exposure.

Radiation hardened CIDs are fabricated using P-channel technology and will continue to function in radiation environments. Photogate and logic operation is extended well into the megarad range by sensing the voltage threshold shifts that occur on the device, and dynamically adjusting the drive voltages to compensate for this increase in gate threshold. Limits of operation are determined by the CID design, speed of operation and choice of process.

Figure 1. Images from a commercial CCD and the standard CID8825DX6 camera when exposed to a gamma source (Co60).

Channel stop threshold voltage

As mentioned earlier, ionizing radiation causes positive charge to accumulate in the field oxide regions that isolate MOS transistors. This causes a channel to form and couple unrelated N-channel transistors and photopixel structures causing them to short out in as little as 10 to 20 krads total dose gamma (Co60).

Conversely, the isolation that is present in the P-channel CID radiation hardened devices improves between unrelated transistors and photopixels and the CID will continue to function in significant radiation levels to a minimum of 3 Mrads or 3 x 10⁶ rads total dose gamma (Co60).

Charge transfer efficiency

Incomplete lattice bonds are formed at the surface of the silicon substrate due to the lack of silicon neighbor atoms. These bonds are usually completed using a hydrogen annealing process. Ionizing radiation easily disrupts these hydrogen-annealed bonds and these bond locations become charge traps for photon-generated charge. Additionally, these surface traps degrade temporal noise and thermally generated dark current of devices.

For the case of a large surface channel CCD that might require 1000 charge transfers through a charge-coupled shift register, a transfer efficiency of 99.5 percent would result in 0.7 percent of the final signal charge read out at the output preamplifier. For a similar buried channel CCD, a transfer efficiency of 99.9995 percent would result in 99.5 percent of the final signal charge read out at the output preamplifier. Because of this issue, any charge transfer problem of a CCD is circumvented through the use of a buried channel. However, exposure to ionizing radiation significantly increases the trap density in the buried channel CCD device which in turn lowers transfer efficiency causing the visible scintillation noise most commonly seen with CCDs.

While the CID is a surface channel device, a pixels charge is read out using a single charge transfer occurring within the individual pixel structure itself, so the pixel charge is not shared with entire rows or columns as with CCDs. Hence, charge transfer efficiency is typically not an issue with CID devices until significantly higher damaging radiation flux rates.

Noise

1/f noise, which is a function of trap density, increases dramatically with ionizing radiation and higher temperature. Lowering the imager operating temperature and high-quality oxide growth helps to minimize the generation of charge traps in the CIDs. Also, use of correlated double sampling reduces the effect of low-frequency flicker noise.

In CIDs, gamma and neutron impact-induced noise is minimized by reducing the thickness of the active layer, however, in high radiation flux rates (>100 krads/hr), some scintillation or random noise in the image may be observed on the MegaRAD series that may not be present in the passive pixel version radiation hardened CID imagers. While the MegaRAD series of cameras exhibit significantly higher light sensitivity and higher total dose capabilities due to their preamplifier per pixel (PPP) design, it is because of the significantly increased pixel sensitivity that the noise exhibited in these devices are the result of individual IEL radiation events striking and being detected within the associated pixel sites.

Dark current

Dark current, which is also a function of trap density, increases dramatically with radiation and higher temperature. Lowering the imager operating temperature and high-quality oxide growth minimize the generation of charge traps and the starting dark current.

Total dose tolerance

As with any solid-state device operating in radiation environments, the MegaRAD series of imagers are susceptible to damage by both ionizing energy loss (IEL) and non-ionizing energy loss (NIEL). The IEL ionize gate oxide which results in degraded performance of MOSFET transistors, is mainly observed in the CIDs as FET threshold (Vth) shift.

Thermo CIDTEC compensates for FET voltage threshold shift (Vth) by driving imager circuitry at higher biases as set by an algorithm that senses the threshold shift (Vth). NIEL can lead to bulk damage such as displacement of silicon atoms in the EPI layer. NIEL induced damage introduces permanent defects that are the primary cause of elevated dark current and poor charge transfer efficiency after exposure to radiation. In order to compensate for the increased dark current resulting from NIEL, Thermo CIDTEC employs a thermoelectric cooler to maintain the operating temperature of the MegaRAD series of imagers at 25 °C even at elevated ambient temperatures up to 50 °C.

The total dose radiation tolerance of the MegaRAD series of imagers is dependent on numerous factors including, but not limited to: duty cycle, type of radiation (gamma ray, x-ray, neutron, etc.), typical dose rate, ambient temperature, and by some account wafer-to-wafer process variations.

Duty cycle is among the most important factors. The extent and severity of radiation damage is much greater when the camera is under power than when it is powered down. Therefore, a MegaRAD series camera that is powered and operated only for brief intervals on a daily or even weekly basis will have extended lifetime in radiation environments than one operated continuously.

Based upon the typical MegaRAD imager threshold shifts (Vth) and the bias/drive voltage adjustment range, with 100 percent duty cycle (i.e., with the camera always powered and running), as tested with a Co60 (gamma) source, the MegaRAD series of cameras will continue to function to at least a total dose exposure of 3 x 10(6) rads (total dose gamma).

Meet the Author

Tony Chapman can be reached at CIDTEC Cameras & Imagers, Scientific Instruments, Thermo Fisher Scientific, 101 Commerce Blvd., Liverpool, NY 13008, or at tony.chapman@thermofisher.com.