The Σ Chip (Sigma Chip) is a chip from the Mega Man X series. In Rockman X Mega Mission 3 it is a creation of Enemice-HL, containing Sigma's data. It is used by Enemice to transform into a copy of Sigma. In the Rockman X2 manga a Sigma Chip is installed in Zero's forehead and it turns him into a Maverick.
The chip has an octogonal format with purple, rectangular edges on the sides. The chip has Sigma's symbol in it, making it easily recognizable. The chip contains a copy of Sigma's personality that takes over the Reploid who installs it. Its appearance is similar in the Rockman X2 manga, with smaller rectangular edges.
Rockman X Mega Mission 3: VS Hyper Limited
Enemice-HL reveals the Sigma Chip in his possession when he is cornered by the combined efforts of X, Zero, and Return X (who had transformed himself into an armor for the duo). He attempts to use it as his trump card, planning to obtain Sigma's destructive power; however, Enemice's mind is erased and taken over by Sigma's, creating Sigma Limited.
Rockman X2 manga
When Zero was revived by the X-Hunters, a Sigma Chip was installed in his forehead to turn him into a Maverick. As a Maverick, Zero's armor becomes black and his power increases, but he becomes out of control, destroying Serges before fighting against X. His forehead is damaged during the battle and Zero returns to normal for a brief moment, but the chip is still active and his armor becomes black again. When Zero destroys X's armor, he believes that he destroyed his friend and tries to kill himself, but X stops him and destroys the Sigma Chip by punching his forehead.