H. Niimi

Bonding constraints and defect formation at interfaces between crystalline silicon and advanced single layer and composite gate dielectrics

An increasingly important issue in semiconductor device physics is understanding of how departures from ideal bonding at silicon–dielectric interfaces generate electrically active defects that limit performance and reliability. Building on previously established criteria for formation of low defect density glasses, constraint theory is extended to crystalline silicon–dielectric interfaces that go beyond Si–SiO2 through development of a model that quantifies average bonding coordination at these interfaces. This extension is validated by application to interfaces between Si and stacked silicon oxide/nitride dielectrics demonstrating that as in bulk glasses and thin films, an average coordination, Nav, greater than three yields increasing defective interfaces.

Controlled nitrogen incorporation at the gate oxide surface

Nitrogen has been incorporated selectively at the top surface of a conventional thermal gate oxide by nitridation with a remote He–N2 plasma at low temperatures, 23 and 300 °C. On‐line Auger electron spectroscopy (AES) has been used to characterize the process. A peak shift in the Si‐LVV feature establishes that the nitrogen is bonded to the silicon. The concentration of nitrogen can be varied by a combination of substrate temperature and duration of plasma exposure. Ex situ glancing‐angle x‐ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) confirm that the nitrogen is confined to the immediate vicinity of the surface. Rapid thermal annealing (RTA) of the nitrided oxide at 900 °C in N2 and N2O does not change the N content.

Integrated processing of silicon oxynitride films by combined plasma and rapid‐thermal processing

Silicon oxynitride (OXN) gate dielectric thin films have been prepared in a cluster tool using a low thermal‐budget process combining plasma and rapid‐thermal step. This N2‐based process has (i) increased process latitude for the formation of N‐rich alloys, and (ii) resulted in lower bonded‐H concentrations, in comparison to NH3‐based processes. On‐line Auger electron spectroscopy and off‐line infrared spectroscopy have been used to characterize chemical bonding, showing that the deposited films are pseudobinary alloys, (SiO2)x(Si3N4)1−x. The processing steps are (i) a 300 °C plasma‐assisted oxidation, (ii) a 300 °C plasma‐assisted chemical vapor deposition of oxynitride films, and (iii) a 30 s, 900 °C postdeposition rapid‐thermal anneal. Electrical characterization of O–OXN–O structures in metal–oxide–semiconductor capacitors was performed using capacitance–voltage techniques to evaluate the effect of alloy composition on midgap interface state density, Dit, and flat‐band voltage, Vfb.

STRUCTURE OF ULTRATHIN SIO2/SI(111) INTERFACES STUDIED BY PHOTOELECTRON SPECTROSCOPY

Device-grade ultrathin (9–22 A) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4+ (with respect to Si0) are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I1, I2, and I3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ∼2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N2 plasma. We find that annealing produces more abrup...

Band offsets for ultrathin SiO2 and Si3N4 films on Si(111) and Si(100) from photoemission spectroscopy

High resolution soft x-ray photoelectron spectroscopy with synchrotron radiation is used to study the interfaces of SiO2/Si(111), SiO2/Si(100), Si(111)/Si3N4, and SiO2/Si3N4 for device-quality ultrathin gate oxides and nitrides. The thin oxides and nitrides were grown by remote plasma deposition at a temperature of 300 °C. Aftergrowth samples were further processed by rapid thermal annealing for 30 s at various temperatures from 700 to 950 °C. The Si(111)/Si3N4 samples were air exposed and formed a thin ∼6 A SiO2 layer with a Si(2p) core-level shift of 3.9 eV, thus allowing us to study both the Si(111)/Si3N4 and SiO2/Si3N4 interfaces with a single type of sample. We obtain band offsets of 4.54±0.06 eV for SiO2/Si(111) and 4.35±0.06 eV for SiO2/Si(100) with film thicknesses in the range 8–12 A. The Si(111)/Si3N4 nitrides show 1.78±0.09 eV valence-band offset for 15–21 A films. This value agrees using the additivity relationship with our independent photoemission measurements of the nitride–oxide valence-ba...

Tunneling currents through ultrathin oxide/nitride dual layer gate dielectrics for advanced microelectronic devices

Direct and Fowler–Nordheim tunneling currents through oxide and dual layer silicon oxide–silicon nitride dielectrics are investigated for substrate and gate injection. The calculations include depletion effects in the heavily doped (n+) polysilicon gate electrodes as well as quantization effects in the less heavily doped n-type substrates. The Wentzel–Kramers–Brillouin (WKB) effective mass approximation has been compared with exact calculations for the tunneling probability, and based on these comparisons it has been found that the WKB approximation is adequate for single layer dielectrics, but is not for the dual layer dielectrics that are the focus of this article. Using exact tunneling transmission calculations, current-voltage (I–V) characteristics for ultrathin single layer oxides with different thicknesses (1.4, 2.0, and 2.3 nm) have been shown to agree well with recently reported experiments. Extensions of this approach demonstrate that direct tunneling currents in oxide/nitride structures with oxi...

Monolayer-level controlled incorporation of nitrogen in ultrathin gate dielectrics using remote plasma processing: Formation of stacked “N–O–N” gate dielectrics

A low thermal budget approach to monolayer-level controlled incorporation of nitrogen in ultrathin gate dielectrics using 300 °C, remote plasma processing is discussed. Incorporation of approximately 1 ML of nitrogen at the Si–SiO2 interface in an “N–O” structure has been achieved by remote plasma-assisted oxidation of the Si surface followed by N2/He remote plasma nitridation, each at a process pressure of 0.3 Torr. The interface nitridation reduces direct and Fowler–Nordheim tunneling by at least one order of magnitude, independent of film thickness. Incorporation of nitrogen at the top surface of the oxide in a concentration equivalent to about 1–2 molecular layers of silicon nitride in an “O–N” structure has been accomplished by N2/He remote plasma nitridation at 300 °C, but at a reduced process pressure of 0.1 Torr. Top surface nitridation has been shown to prevent boron diffusion out of p+ poly-Si gate electrodes during high-temperature activation anneals, e.g., at 1000 °C. Combining interfacial and...

Monolayer-level controlled incorporation of nitrogen at Si–SiO2 interfaces using remote plasma processing

We demonstrate three different ways to incorporate nitrogen at Si–SiO2 interfaces: (i) an O2/He plasma oxidation of the Si surface followed by an N2/He plasma nitridation, (ii) an N2/He plasma nitridation of the Si surface, and (iii) a Si3N4 film deposition on to the Si surface. The two-step interface formation, the O2/He plasma oxidation followed by the N2/He plasma nitridation, is shown to yield significantly better interface device properties than the other two approaches. These differences in interface properties are explained by an application of constraint theory based on comparisons of the average bonding coordination of the dielectric layer at the interface with the Si substrate.

New approach for the fabrication of device-quality Ge/GeO2/SiO2 interfaces using low temperature remote plasma processing

It has been shown that low temperature (300 °C) remote plasma enhanced processing can separately and independently control interface formation and bulk oxide deposition on silicon substrates. Plasma processing is followed by a low thermal budget thermal anneal, e.g., 30 s at 900 °C. In this article, this process has been modified and applied to germanium substrates to determine if it can provide a successful pathway to device-quality Ge–dielectric interfaces. The new process also employs a three-step process: (i) an O2/He plasma-assisted, predeposition oxidation of the germanium surface to form a superficial germanium–oxide passivating film, (ii) deposition of a SiO2 bulk film by remote plasma-enhanced chemical vapor deposition from SiH4 and O2, and (iii) a postdeposition anneal for chemical and structural relaxation. The resulting interfaces are improved by the predeposition, plasma-assisted oxidation step, but are still far too defective for device applications.

DEPTH-DEPENDENT SPECTROSCOPIC DEFECT CHARACTERIZATION OF THE INTERFACE BETWEEN PLASMA-DEPOSITED SIO2 AND SILICON

We demonstrate the use of low-energy cathodoluminescence spectroscopy (CLS) to study optical transitions at defect bonding arrangements at Si–SiO2 interfaces prepared by low-temperature plasma deposition. Variable-depth excitation achieved by different electron injection energies provides a clear distinction between luminescence derived from (i) the near-interface region of the oxide film, (ii) the Si–SiO2 interface, and (iii) the underlying crystalline Si substrate. Cathodoluminescence bands at ∼0.8 and 1 eV are assigned to interfacial Si atom dangling bonds with different numbers of back-bonded Si and O atoms. CLS also reveals higher photon energy features: two bands at ∼1.9 and 2.7 eV assigned to suboxide bonding defects in the as-grown oxide films, as well as a substrate-related feature at ∼3.4 eV. The effects of hydrogenation at 400 °C and rapid thermal annealing at 900 °C, and especially the combination of both process steps is shown to dramatically reduce the intensities of the CLS features assigne...