Nitrogen implantations for rapid thermal oxinitride layers

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Abstract

Oxidation of nitrogen implanted substrates results in so called silicon-oxinitride layers (SixOyNz layers) which are dependent on implantation dose and energy always thinner than pure silicon-oxides (SiO2) produced under the same oxidation conditions. Elastic recoil detection profiles indicate that the implanted nitrogen diffuses out of the substrate into the silicon-oxide layer what improves the electrical quality of these insulators. The SixOyNz layers show lower Fowler–Nordheim tunnelling currents as well as lower interface state densities (Dit) than the corresponding SiO2 layers or N2O–silicon-oxinitride insulators. NH3–SixOyNz layers show the lowest Dit values because of H2-annealing effects but contain fixed charges.

Introduction

Ultra thin gate dielectrics of high quality are required when metal oxide semiconductor devices are scaled down for ultra large scale integration (ULSI) applications.

Although for many reasons oxidising silicon (SiO2) is preferred compared to nitration (Si3N4) or other fabrication methods in terms of gate or tunnel dielectrics, pure silicon-oxides often cannot provide the electrical qualities expected from sub 5 nm insulating layers.

Combining the advantages of conventional SiO2 and Si3N4 layers there are several basically different possibilities to produce so called silicon-oxinitrides (SixOyNz). First, rapid thermal oxidation (RTO) nitrogen-implanted substrates. Second, adding N2O to the process gas and third, a NH3 rapid thermal nitridation (RTN) of an RTO-layer. CVD methods are not discussed here.

Section snippets

Experimental procedure

All experiments were performed on p(B)-type 〈100〉 silicon substrates with a relative resistivity of 6 to 12 Ω cm.

For nitrogen implanted samples, nitrogen was implanted using doses (Φimp) from 1.6×1012 cm−2 to 2.3×1014 cm−2 at energies (Wimp) of 25, 100 and 180 keV through 2 to 3 nm thick scattering oxides which were removed after implantation.

Following a standard RCA clean dry and wet rapid thermal oxides were grown in a RTP system (Steag SHS 2000) at process temperatures (Tox) ranging from

Relative SixOyNz layer thickness

Oxidation processes mainly depend on process temperature and time (tox). Process times for pure RTO were adjusted to reach equal dox (5 nm) for different Tox. Therefore, insulating layers grown at fixed oxidation temperatures show different dox,imp dependent on Φimp and Wimp (Fig. 1).

Depth profiles of nitrogen and oxygen concentrations

ERD measurements (Fig. 2) prove, that less than 1% of nitrogen is incorporated in the SiO2 layers. Despite of that high implantation energies as well as high implantation doses result in increasing z-values for SixO

Conclusion

We have shown that SixOyNz layers grown with RTO processes on nitrogen implanted substrates (RTON processes) provide better electrical data (FNT, Dit) at lower insulating layer thickness than pure rapid thermal oxides and N2O oxinitrides. NH3–SixOyNz layers show insulator charges and are always hydrogen annealed and have therefore the best Dit values.

ERD showed lowest nitrogen concentrations for nitrogen implanted oxinitrides and only in this case no significant nitrogen enrichment at the SiO2

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