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What Do You Need for High-Rel & Rad-Hard Applications? Part 2

In the first part of this blog, we started looking at the mechanism by which high energy particles (alpha and beta particles, ions) and photons (typically γ rays) could affect or damage semiconductor devices.

To understand what kind of damages these sources can do in the silicon structures, let's look at the different mechanisms the radiation can do inside the atom.

The mechanism of energy deposition for beta particles — fast-moving free electrons — is related to the electrical interaction of the electron that crosses the silicon material and the silicon electron cloud (see Figure 1):

Figure 1a

Figure 1b

Energy deposition process of an electron impacting a silicon semiconductor material having low (a) or high (b) energy. In case (a) the electron snatches one electron during the impact. In case (b) the electron interacts with the electrical field of the nucleus and emits a photon.

Energy deposition process of an electron impacting a silicon semiconductor material having low (a) or high (b) energy. In case (a) the electron snatches one electron during the impact. In case (b) the electron interacts with the electrical field of the nucleus and emits a photon.

The mechanism of energy deposition for protons and heavy ions is quite the same: The radiation source (a proton or a heavy ion) interacts electrically with the electrons of the silicon electron cloud (see Figure 2):

Figure 2

Energy deposition process of a proton or a heavy ion. The h+ represents the holes or spots where electrons are missing.

Energy deposition process of a proton or a heavy ion. The h+ represents the holes or spots where electrons are missing.

Each radiation source has a particular mechanism of energy deposition, but the final effect is the same: the creation of couples e /p+ . The mechanism inside the material is the same for all types of radiation sources. It is important to understand the correct testing procedure of the device after it has been exposed to the ionizing radiations. You need to understand how the device is impacted by the creation of these e /p+ couples.

To understand the effect on an IC, let's examine the effect of the radiation on a power MOSFET switch which can be considered an elementary component of an IC. Due to ionizing radiations, e /p+ pairs are created inside the gate oxide (SiO2 ) and the electrical field moves the more mobile charges to the gate terminal. Meanwhile, the holes (shown as h+ in Figure 3) remain inside the oxide. Some of the electrons that flow from the source to the drain will recombine with the holes inside the gate oxide, resulting in some holes disappearing from the oxide.

Figure 3

Recombination of holes trapped in the gate oxide and electrons flowing inside the source-drain channel.

Recombination of holes trapped in the gate oxide and electrons flowing inside the source-drain channel.

Here are the parameters to consider with regard to irradiation:

VGS = V-thbefore_ irradiation     (1) condition: irradiation performed, switch MOSFET state OFF

There is no flow of electrons from source to drain, so the only recombination is related to the electrons present at the source and the holes in the oxide. After that the irradiation of the device has been performed, resulting in:

VGS > Vthafter_ irradiation     (2) condition: irradiation performed, switch MOSFET state ON)

Now, the electrons flow from the source to the drain and some electrons will recombine with the holes that are still trapped into the gate oxide and survived to the recombination with regard to condition (1); again, refer to Figure 3. An electron coming from the source and having enough energy will reach the drain, provided it does not recombine with a hole. This results in a new threshold value of the gate-source voltage. To monitor the effect of the radiation on the switch, a threshold voltage test is recommended.

In the next (and last) part of this series, we'll look at the details of this gate threshold voltage change. We will also look at the performance of bipolar junction transistors with respect to irradiation.

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10 comments on “What Do You Need for High-Rel & Rad-Hard Applications? Part 2

  1. etnapowers
    December 23, 2013

    The variation of the threshold voltage of a power MOSFET due to the presence of ionizing radiations, could be measured to detect the presence of radiation in a controlled environment.

  2. samicksha
    December 24, 2013

    I agree you, what's your view on Shielding the package against radioactivity, to reduce exposure of the bare device.

  3. Victor Lorenzo
    December 24, 2013

    Some MOSFET transistors could be placed on specific points in the die to detect radiation excess and, possibly, modify some circuit setpoints, correct drifts, offsets, or even shut the circuit down to avoid an unrecoverable phisical damage.

    Would it be possible?

    Is shielding the only countermeasure against radiation?

  4. etnapowers
    December 24, 2013

    The package is very important to protect the device from radiation. The design of the device has to be performed taking into account the effect of the ionizing radiation, that impact the device despite of the screen of the package, on the silicon structure

  5. Vishal Prajapati
    December 25, 2013

    Does it require the same metal shielding as being used with RF circuit shilding with ground? Does it require any calculation for the type or shape of the shield?

     

    Is there any other type of protection apart from shilding to protect devices from radiation hazard?

  6. etnapowers
    December 25, 2013

    Yes Victor, it would be possible, the drifting depends on the dose rate and the energy of the impacting radiation, hence some radiations might not be revealed. Moreover the radiation can cause a failure of the device, I'm thinking to write a blog on this subject.

  7. Victor Lorenzo
    December 25, 2013

    Thanks, @Etnapowers.

    I was talking some time ago with a colleague, comparing similar products from two FPGA manufacturers just before engaging in one new product design and the radiation hardness was a topic he mentioned.

    It will be even more insteresting to me if you could include information about radiation effects with respect to IC technologies (e.g. transistor size) and specially the ones used by FPGA manufacturers.

     

  8. etnapowers
    December 25, 2013

    @ Victor: I will try to provide these details. Usually the radiation hardness depends on the structure , the dimension , the technology, the package , etc… I will do my best.

  9. eafpres
    December 27, 2013

    @Vishal–on the one hand you could consider the shielding effectiveness and try to calculate the effects of small openings in the shield.  However, gamma rays in space outside the Earth's atmosphere can have very high energies (i.e., high frequency) such that what matters is really the mass of material between the source and what you are shielding.  Consider that x-rays can get through an aluminum briefcase at the airport quite easily, so you choice of shielding will take into account the density of material as well.  This is the reason lead is used–for the density per unit area, nto for any special shielding property.

  10. RedDerek
    January 2, 2014

    So basically, the gate threshold for a MOSFET tends to decrease with radiation damage? Then one of the solutions is to just ensure that there is always sufficient drive voltage to account for the decreased Vth – negative Vgs.

    I do know that smaller feature size is more susceptible to radiation than the old large feature devices. Not sure what is in the Space Shuttle at the time of decomission, but I would guess it was 2-micron or larger.

    And anything going outside of earth orbit would be susceptible to higher radiation than within earth orbit.

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