Selective oxidation of silicon in diode, TFT, and monolithic three dimensional memory arrays
View Patent ↗The present invention relates to use of selective oxidation to oxidize silicon in the presence of tungsten and/or tungsten nitride in memory cells and memory arrays. This technique is especially useful in monolithic three dimensional memory arrays. In one aspect of the invention, the silicon of a diode-antifuse memory cell is selectively oxidized to repair etch damage and reduce leakage, while exposed tungsten of adjacent conductors and tungsten nitride of a barrier layer are not oxidized. In some embodiments, selective oxidation may be useful for gap fill. In another aspect of the invention, TFT arrays made up of charge storage memory cells comprising a polysilicon/tungsten nitride/tungsten gate can be subjected to selective oxidation to passivate the gate polysilicon and reduce leakage.
1. A method for forming a thin film transistor, the method comprising:
providing a first polysilicon film;
forming a gate of the thin film transistor over the polysilicon film, the gate comprising silicon and tungsten or tungsten nitride; and
oxidizing a portion of the silicon in the presence of a surface of the tungsten or tungsten nitride in conditions that substantially do not oxidize the tungsten or tungsten nitride, wherein the thin film transistor is a memory cell.
2. The method of claim 1 wherein the memory cell is a floating gate cell.
3. The method of claim 1 wherein the memory cell is a SONOS-type cell.
4. The method of claim 1 wherein the memory cell is a portion of a first memory level formed at a first height above a substrate.
5. The method of claim 4 wherein the first memory level is a portion of a monolithic three dimensional memory array, the array further comprising a second memory level, the second memory level formed at a second height above the substrate, wherein the second height is different from the first height.
6. The method of claim 1 wherein the oxidizing step comprises flowing H 2 and H 2 O, and wherein a ratio of partial pressure of H 2 to partial pressure of H 2 O is at least about 3:1.