Meaning
Formation of electrically active complexes from interstitial oxygen atoms occurs when silicon wafers are heated within a specific temperature range, typically between 400 and 500 degrees celsius. This process of thermal donor activation changes the electrical properties of the material by introducing additional charge carriers into the crystal lattice. The effect is most pronounced in silicon grown by the czochralski method due to the naturally higher oxygen content.
Resulting clusters of oxygen and silicon atoms act as n-type dopants that alter the baseline resistivity of the wafer. This change is permanent unless the material is heated to a level where the complexes break apart.
Temperature Window
Annealing steps during the fabrication of a device can inadvertently trigger the creation of these clusters. The rate of thermal donor activation depends heavily on the duration of the heat treatment and the initial concentration of oxygen in the substrate. At temperatures above 600 degrees celsius, these complexes typically dissolve and lose their electrical activity.
Resistivity Shift
Unintended increases in the carrier concentration can lower the resistivity of a wafer, potentially moving it out of the specified range for the device. In high precision analog circuits, thermal donor activation leads to unpredictable gain or offset values in the finished transistors. Engineers monitor the cumulative thermal budget of the entire fabrication sequence to prevent these shifts.
Process Mitigation
Rapid thermal processing and the use of low oxygen substrates are common strategies used to minimize the impact of this phenomenon. Proper management of thermal donor activation ensures that the electrical behavior of the semiconductor remains consistent over its operating life. A brief high temperature soak can be used at the end of the manufacturing flow to annihilate any donors that formed during previous lower temperature steps.
