The present invention relates to a method for fabricating integrated circuits and in particular to a method for fabrication of data storage type integrated circuits for protection against errors induced by the bombardment of an ionizing radiation.
In the trend towards smaller components in integrated circuits to achieve higher density, an error causing mechanism, not previously considered a factor in larger component circuits, in the form of the passage of alpha particles into the integrated circuit, has become a major problem particularly in those circuits intended for memory storage. The passage of the alpha particles into the integrated circuit generates sufficient hole-electron pairs, by the dissipation of energy, to disrupt the integrity of stored data, as an example, in small memory storage cells requiring only a small number of electrons to be retained for the storage of a binary digit.
It has been shown that alpha particles are responsible for charge induced errors. These alpha particles originate from radioactive decay of radioactive impurities present in the materials used to fabricate the integrated circuit. A few parts per million or less of radioactive impurities present in the materials emit a sufficiency of alpha particles, as nuclear decay products, to cause an unacceptably high probability of the electrons generated by an alpha particle decay to change the charge stored in the integrated circuit.
Stated another way, soft errors in storage memories, such as MOS dynamic RAM's, are caused by their exposure to alpha particle radiation. Alpha particles are the natural product of the radioactive decay of uranium and thorium present in most materials used in the fabrication of RAM's and thus radiation produces an unwanted charged particle in the silicon semiconductor material which upsets the wanted charge during the normal device operation.
Accordingly, it is a prime object of this invention to provide a method of reducing alpha particle induced soft error sensitivity in integrated circuits.