FP7 Unione Europea

Existing Technology

Non volatile semiconductor memories in space market have been faced with old antifuse technologies till now.

This means that media can be written (or burned) before the mission and only read during later on.

This kind of device is called OTP (One Time Programmable) memory and is typically used for star maps or boot software storage and is the only one actually chosen by space market.

 

The real breakthrough for the introduction of programmable and erasable non volatile memories is the introduction of floating gate (or floating trap) memory cells.

 

What are the problems?

Many problems arise on this technology well known since ’60 but never used for rad-environments:

  • the retention of the bit: a floating gate cell stores electrons in a sandwich of polysilicon (or silicon nitride for floating trap, also known as charge trap) surrounded by silicon dioxide thus leading to a variable threshold proportional to the amount of electrons trapped;

  • the exposure of the cell to an amount of radiations: this could move, during time, the threshold itself so changing the bit.

  • migration of trapped electrons: in terms of single events, an high energy particle can collide with a floating gate and if the energy is sufficiently high it is possible that trapped electrons can gain energy enough to migrate outside their traps.

 

It is worth mentioning that even if these aspects has been studied the technology and the methodology adopted were commercial: no efforts were profunded to design a non volatile device to be rad hard.