Examples of equipment environments subjected to continuous or intermittent ionising radiation include avionics for aircraft that will operate in the stratosphere, all types of spacecraft, control and monitoring systems for nuclear power plants and battlefield military systems.

The Nature and Sources of Radiation
Radiation is classed as ‘ionising’ when it has an energy level sufficient to strip electrons away from the atoms in an affected material. It takes many forms including high-energy electromagnetic radiation (X-rays and gamma rays) and high-energy particles (electrons, protons, alpha particles, neutrons and ions).

Natural radiation originates from two main sources: the solar wind (a stream of charged particles emanating from the sun, which is heavily influenced by the earth’s magnetic field) and cosmic rays (a wide mix of particles originating from deep space). The overall strength and mix of radiation varies widely with distance from earth so that, for example, a low earth orbit global positioning satellite will see a different radiation environment from an interplanetary space probe. The natural background level of radiation present at sea level is generally too low to affect most electronic devices.

Man-made radiation includes radiation from nuclear power sources, nuclear weapons and instruments employing high-energy particles (e.g. medical imaging equipment and particle accelerators used in high energy physics).

Effects of Radiation
The potential effects of radiation on semiconductor die are twofold: instantaneous effects on the functioning of devices and long-term cumulative damage, which results in permanent changes in device parameters.

Instantaneous Effects
Instantaneous effects of radiation are collectively known as single event effects (SEE). These are caused by the ionisation of atoms within the crystal lattice of the bare die, which results in the generation of hole-electron pairs; these act as parasitic charge carriers within the die. The precise effect of these carriers depends on their density and location in the device. In some cases they will cause a temporary parametric change in a device characteristic, known as a single event upset (SEU). The effect of this will depend on the magnitude of the effect, its location in the bare die and the characteristics of the application circuit; a typical result would be an unexpected logic transition in a digital circuit or a noise pulse in an analogue circuit. These effects can be significant at the system level, particularly in digital circuits.

A more serious instantaneous effect occurs when parasitic charge carriers affect isolation junctions between individual transistors and the semiconductor substrate by generating sufficient charge carriers to forward bias a junction that is normally reverse biased. This can cause the creation of a low resistance four layer parasitic device resulting in an effect known as a single event latch-up (SEL), which will disable the device until the power supply is removed and reapplied. In many cases SEL will result in excessive current flows that destroy the die.

Long Term Effects
There are two principal radiation induced effects that result in long term parametric changes. Displacement damage occurs when high-energy particles cause crystal lattice damage with resulting defects. These defects create local carrier recombination sites that reduce minority carrier lifetime with a consequent gain reduction in bipolar transistors. MOS devices are less affected.

The second long term effect results from ionisation of atoms in oxide layers. This results in trapped charges in the oxide, which particularly affect MOS devices by causing progressive shifts of threshold voltage (gate turn-on voltage). Non-volatile memory die relying on stored charge such as flash memory can also be affected with potential data corruption.

In general, long-term radiation effects are proportional to the level of radiation integrated over time, known as ‘total dose radiation’. This means that a short period of high intensity radiation will produce the same level of die degradation as a longer period at lower intensity. However, in recent years it has been observed that for many bipolar die, lower levels of radiation can have a proportionately greater effect on device performance over time than higher levels. This is commonly known as the ‘Enhanced Low Dose Rate’ (ELDR) effect.

Radiation Testing and Qualification
Because of the limited market requirements for “rad hard die”, the vast majority of semiconductor die are not routinely tested for radiation hardness as part of the manufacturer’s qualification process. However, in principle, any die can be qualified as radiation hard by performing the necessary testing.
In most applications for rad hard die the concern is for long-term effects as opposed to SEE. Qualification for SEE is generally only required for those military systems that are required to withstand the high intensity radiation pulses arising from the discharge of nuclear weapons.

A number of industry standards exist that describe test methods for total dose radiation (TDR) testing with those published by the US Department of Defence and the European Space Agency being the most commonly used. The unit of TDR normally specified is the rad (‘radiation absorbed dose’) where 1 rad is the cumulative radiation dose that results in the absorption of 100ergs of energy per gram of material. The SI unit for TDR is the Gray (Gy); 1Gy=100rad=1J/kg absorbed energy; this unit is rarely used in the context of semiconductor testing.

Since radiation absorption is material dependent, the unit for device testing is normally expressed as Rad(Si) or, less commonly, Rad(SiO2) for silicon die. TDR qualification levels typically range from 3krad to 100krad or higher dependent on the specific application. Application dependent factors include the radiation environment and the degree of shielding provided by the die and system packaging.

In many cases the requirement is for a bare die to provide reliable operation over a long period of time in a relatively low-level radiation environment. In these cases an accelerated qualification test would normally be employed in which sample die are subjected to a much higher radiation level for a shorter period of time. Where this type of accelerated testing is used on bipolar devices care needs to be taken that the ELDR effect mentioned above does not invalidate the calculated acceleration factor employed.

Radiation Testing and Qualification – Capabilities

• Qualification to MIL-STDs
• Qualification to ESA SCC Specifications
• Qualification to Customer’s Own Specification
• Visual Inspection of Rad Hard die to Space Level
• Custom design and assembly of Rad-Hard packaged parts
• Working partnerships with original manufacturers for Rad Hard and Space level Known Good Die