Meaning
Atomic defects at the interface of semiconductor and insulator layers consist of unsatisfied chemical valences that trap charge carriers. The presence of unpassivated dangling bonds creates localized energy states that interfere with the smooth flow of current in a transistor. These defects are often the result of incomplete chemical reactions during the deposition of the gate oxide.
Surface Chemistry
Silicon atoms at the edge of a crystal lattice lack a neighbor to complete their fourth covalent bond. Without a hydrogen atom to fill this gap, these unpassivated dangling bonds remain reactive and ready to capture passing electrons. The density of these sites is a direct indicator of the quality of the fabrication process.
Device Performance
Trapped charges at the interface cause the threshold voltage of the transistor to fluctuate over time. Unpassivated dangling bonds also increase the noise level in sensitive analog circuits and reduce the overall switching speed of digital logic. Over time, the accumulation of charge at these sites can lead to the permanent degradation of the device.
Mitigation Process
Hydrogen annealing at high temperatures is the standard method for neutralising these interface traps. During this step, hydrogen atoms migrate to the interface and bond with the unpassivated dangling bonds to create a stable and electrically neutral surface. This process is essential for ensuring the long term stability and reliability of modern integrated circuits.
Proper annealing reduces the trap density by several orders of magnitude. Engineers verify the success of this treatment by measuring the subthreshold swing of the transistors and looking for signs of hysteresis in the electrical curves.
