Replacement of Forming Gas Anneal by an Applied Voltage
Original Publication Date: 2001-Apr-14
Included in the Prior Art Database: 2003-Jun-18
The further scaling of MOSFETs is endangered by the early breakdown of smaller devices, when the gate oxide reaches a thickness of 1-2 nm. In the production process, gate oxides are exposed to a forming gas anneal (H 2 ,N 2 ), which saturates any dangling bonds near the interface. The hydrogen introduced however is responsible for the dielectric breakdown. Thus it would be desirable to saturate dangling bonds and limit the extent of the oxygen defi- cient suboxide near the Si/SiO 2 interface without introducing hydrogen. It is proposed to remove the dangling bonds by applying a voltage to the oxide after oxygen in a hydrogen- and oxygen-free ambient. The polarity of the voltage needs to be such that the substrate is positive Si/SiO 2 and the surface is at the near the anode. Dangling bonds can be attributed to the oxygen deficiency expressed as suboxide and to open bonds near the interface. Every dangling bond is considered as a defect. These defects are charged and their diffusive motion can be controlled by a magnetic field. In particular the defects are such that positive defects correspond to silicon excess defects and negative defects are oxygen excess defects. By applying a voltage across the structure silicon rich defects can be guided to the interface where they accumulate and form silicon, thus shifting the phase boundary. Thus the oxide will become thinner, but less oxygen-deficient. Thus the same effect is obtained as with a forming-gas anneal, that is dangling bonds are removed from the oxide. However, no extra hydrogen is introduced into the structure, that may cause early dielectric breakdown. A negative side effect of interface roughening due to electric stress is caused by the preferential deposition of silicon at silicon protrusions, which is due to the enhanced electric field near the protrusions, therefore magnifying the effect. This difficulty, if relevant, can be alievated by applying an electric field parallel to the oxide, which flattens these protrusions exploiting the "wind" of defects, which averages out the fluctuations of the silicon concentration parallel to the interface. This process avoids the detrimental effects of hydrogen without introducing any new material into the process. Rather a critical material namely hydrogen is removed from the process, or better it is not artificially introduced into the structure. The stability with regard to dielectric breakdown is increased, and with it the life span of MOSFETs.