Soft Errors from Alpha Particles

Alpha Particle-Induced Soft Errors, or simply soft errors, refer to transient errors in the operation of a dynamic random access memory (DRAM) device caused by alpha particles emitted by traces of radioactive elements (such as thorium and uranium) present in the packaging materials of the device.  These alpha particles manage to penetrate the die and generate a high density of holes and electrons in its substrate, which creates an imbalance in the device's electrical potential distribution that causes stored data to be corrupted.

The alpha particles emitted by the device package can have energies of up to 8 MeV. It takes about 3.6 eV to generate an electron-hole pair in the substrate, so an 8 MeV alpha particle can generate 2.5 million electron-hole pairs within 2-3 microns of the alpha particle track.

The potential well of a memory cell that contains a '0' is filled with electrons (inversion mode), while that of a memory cell that contains a '1' is devoid of electrons (depletion mode).  When an alpha particle hits the substrate and generates holes and electrons, the holes will be pulled toward the substrate supply while the electrons will be pulled towards the potential well.  An empty well can fill up with enough electrons (assuming that enough electron-hole pairs were generated by the alpha particle) to have its stored information reversed from  '1' to '0'. Cells that already have electron-filled wells in the first place are not affected by alpha particles.

The corruption of stored information due to alpha particles is what's known as an alpha-induced 'soft error'. Soft errors are random and non-recurring.  The soft error rate depends on circuit sensitivity and the alpha flux emitted by the package of the device. A single alpha particle that possesses enough energy can cause a soft error all by itself. 

The amount of charge needed to corrupt stored information and result in a 'soft' error is referred to as the critical charge, or Qcrit. Qcrit becomes smaller as devices are reduced in size and operating voltages, making soft errors a bigger problem for smaller devices.  Qcrit is also a function of the stored charge in the memory cell.

Soft error rates due to alpha particles may be minimized by: 1) reducing the number of alpha particles emitted by the package; 2) coating the chip surface with a film (such as polyimide resin) that blocks alpha particle irradiation; and 3) better design of the memory device to make it less sensitive to alpha-induced soft errors. Table 1 presents some typical alpha flux data from various package materials.

Table 1. Typical Alpha Flux from Package Materials