Charge trapping nanocrystal dielectric for non-volatile memory transistor

[0001] The invention relates to transistor construction and, in particular, to manufacture of nanocrystal layers which may be used in non-volatile memory cells.

[0002] Non-volatile nanocrystal transistor memory cells are known. For example, U.S. Pat. No. 6,690,059 to B. Lojek describes a non-volatile memory transistor that uses a floating gate as a charge storage region, transferring charge through a tunneling barrier to nanocrystals. The device relies on a separate charge reservoir for charge supply, while the substrate is doped for conductivity between source and drain electrodes. By pulling charge from the charge reservoir to a separated nanocrystal layer, the electrostatic properties of the nanocrystal layer are modified, influencing a subsurface channel between source and drain in a MOS transistor. The nanocrystals are used to modify electrostatic properties of a separated region and then directly influence channel behavior in the usual way, characteristic of a MOS transistor. In the simplest mode of operation, a threshold may be established for charge transfer from the charge supply layer to the nanocrystal layer and this threshold is similar to the threshold of non-volatile memory transistors. However, further voltage changes will cause further electron transitions from the charge supply layer to the nanocrystal layer whereby the conductivity of the channel is changed in a stepwise manner, like modulation. Reverse voltages will cause depletion of the nanocrystal layer, driving electrons from the nanocrystal layer back to the charge supply layer. Conduction between source and drain amplifies the gate voltage in the amplifier mode or senses the pinch-off characteristic in the memory mode.

[0003] In U.S. Pat. No. 6,808,986 Rao et al describe a nanocrystal layer made using chemical vapor deposition. In U.S. Pat. No. 6,344,403 Madhukar et al describe a similar nanocrystal growth procedure.

[0004] An object of the invention is to provide a uniform, high density nanocrystal layer for more efficient discrete charge trapping in a memory transistor.

[0005] The above object has been achieved by growing silicon crystal nucleation sites, then exposing the sites in an etch. The exposed sites are then regrown by chemical vapor deposition to an enhanced size and protected with an oxide coating.

[0009] With reference to FIG. 1, a silicon substrate 11 is shown having a planar surface 12 which is polished and not patterned. Most commercial wafers already have adequate planarity from polishing and further polishing is not needed. Substrate 11, typically a silicon wafer, with doped regions has a desired conductivity for building transistors, preferably MOS or CMOS transistors.

[0010] On the surface 12 of the planar silicon substrate 11 an insulating silicon dioxide surface layer 13 is deposited by any of the usual methods as a first insulating layer. This oxide layer, typically a high quality thermal oxide layer, has a thickness in the range of 20 to 60 Angstroms. Such a thin oxide layer will serve as a tunnel oxide layer for a memory cell in which a conductive member resides above the oxide layer. In EEPROM memories, a typical floating gate is built above a thin oxide layer and for this purpose, the present invention contemplates a nanocrystal layer as the floating gate layer. Such nanocrystal layers are known in the prior art as charge storage layers, but they are made by different techniques.

[0011] In FIG. 2, a silicon oxide layer 15 is deposited on the silicon dioxide layer 13 as a second insulative layer. The silicon oxide layer may be virtually any insulative silicon oxide compound, Si_xO_y (compound) where a smaller “x” than “y” is preferable for giving rise to small nucleation seed sites, although x=y will work. The silicon oxide compound may be applied by any known vapor deposition process, but the objective is to achieve a high density array of nucleation sites, with each site in an array having a surrounding plurality of nucleation sites, each less than approximately 100 Angstroms away from a neighboring site.

[0012] In FIG. 3, the silicon oxide layer 15 is seen to have a plurality of seeds or nuclei forming nucleation layer 17 near the boundary 14 between the silicon oxide layer 15 and the silicon dioxide layer 13. The seeds are molecular silicon introduced during formation of the silicon oxide, i.e. excess silicon molecules that aggregate and form tiny silicon crystals embedded in the insulative silicon oxide layer. The desired concentration of silicon nanocrystal seeds as obtained, as seen in FIGS. 4 and 9, by thermally annealing the silicon oxide layer 15 in an inert ambient gas 18, such as argon, in a thermal reactor or oven 20 at a temperature of between 800 to 1100 C for approximately 60 to 120 minutes. Alternatively, laser annealing could be used.

[0013] With reference to FIG. 5, the silicon oxide layer 15 is etched by means of a wet acid etch, at least partially exposing the silicon seed layer 17. Upon the tiny exposed seeds, nanocrystals are grown.

[0014] In FIG. 6, the nanocrystal layer 17 is seen to reside in proximity to the boundary layer 14 between the silicon oxide layer 15 and the oxide layer 13. The substrate is placed in a vapor deposition reactor 22 and silane 24 is introduced at a temperature and pressure wherein the silane will react upon contact with the silicon seed layer 17. The type of vapor deposition reactor is not critical and any type of reactor may be used, such as CVD, LPCVD, PECVD, EPI, etc. The silane or disilane is decomposed, allowing silicon atoms to be deposited on the seed layer forming silicon nanocrystals 19, seen in FIG. 7. In other words, the seed layer 17 in FIG. 6 is transformed into a nanocrystal layer 19 in FIG. 7. In other words, silicon crystals are grown upon the silicon seeds. While a typical size of a silicon seed is less than 10 Å, a typical size of a silicon nanocrystal is 40–60 Å. Vapor deposition continues until the nanocrystals grow to a size short of mutual contact. In other words, the silicon seeds of the seed layer are not allowed to grow into agglomerating members. On the other hand, the silicon nanocrystals of the present invention are larger than directly deposited silicon nanocrystals of the prior art. Growth can be controlled by observing the size of the agglomerating crystals and stopping vapor deposition short of agglomeration, as seen in FIG. 10.

[0015] With reference to FIG. 8, a protective insulative layer 21 is deposited over the grown nanocrystal layer 19 in a third insulative layer 15. The silicon substrate 11 may now be patterned into a memory array having EEPROMs with nanocrystal floating gates, i.e. non-volatile memory transistors.