T. Wetteroth

Effects of irradiation temperature and dose on exfoliation of H+-implanted silicon carbide

H+ implantation of SiC is the basis for a thin-film transfer process, which when combined with oxidation and hydrophilic wafer bonding, can be exploited to produce silicon carbide-on-insulator material useful as a wide-band-gap semiconductor. This thin-film transfer process has been successfully applied to Si to produce a commercial silicon-on-insulator material. The efficacy of hydrogen to produce thin-film separation was studied by investigation of H+-induced exfoliation in implanted SiC. Results showed that the onset and degree of exfoliation of SiC depends initially upon the concentration of implanted H+. However, the dose dependence of exfoliation exhibits a rather marked retrograde behavior. The degree of exfoliation eventually starts to decrease with increasing ion dose until exfoliation is completely suppressed. This behavior is attributed to a competition between the positive effects of hydrogen on exfoliation and the negative effects of ion-induced damage. Experiments were done to isolate the ef...

Leakage current models of thin film silicon-on-insulator devices

Thin film silicon-on-insulator (SOI) devices have an advantage of excellent isolation due to the buried oxide layer leading to reduced capacitance coupling and no latchup in complementary metal-oxide-silicon circuits compared with bulk silicon devices. Reduced junction area should lead to lower leakage for a given device. However, because of the buried oxide, stress is built up in the Si island during isolation processes, especially near the island edges, inducing new kinds of leakage currents, which are not observed in bulk silicon devices. This letter proposes five leakage current models of the partially depleted SOI devices, identifies their origins, and suggests methods to prevent each type.

Thin-film metrology of silicon-on-insulator materials

Spectroscopic rotating-analyzer ellipsometry employing a compensator was used to measure the ellipsometric angles and depolarization from 0.73 to 5.4 eV of commercial separation by implantation of oxygen wafers. The data were analyzed to find the thicknesses of the native oxide cap, the top Si layer, and the buried oxide (BOX). From the depolarization in the spectral region of interference fringes, we determine layer thickness nonuniformities. Although a reasonable agreement between the data can be found by describing the BOX with the optical constants of thermal oxide, it can be improved by modeling the BOX as an effective medium consisting of thermal oxide and amorphous Si. The physical justification for this model is the presence of Si islands near the BOX/substrate interface. We compare our ellipsometry results with a destructive analysis using electron microscopy and secondary ion mass spectrometry.

IMPROVEMENT IN GATE OXIDE INTEGRITY ON THIN-FILM SILICON-ON-INSULATOR SUBSTRATES BY LATERAL GETTERING

Lateral gettering is implemented in thin-film silicon-on-insulator (TFSOI) substrates by introducing crystalline defects in the vicinity of metal-oxide-semiconductor device channel regions prior to gate oxidation. As a result of the gettering a significant improvement in gate oxide integrity is achieved, with increased oxide breakdown voltages and charge-to-breakdowns, as well as a reduction in localized oxide charge trapping. The same gettering effect on separation-by-implantation-of-oxygen and bonded silicon-on-insulator substrates suggests that the lack of effective gettering is mainly responsible for the oxide degradation regardless of the TFSOI type. This work also demonstrates the feasibility of achieving bulk-comparable gate oxides on TFSOI substrates.

The Effects of Damage on Hydrogen-Implant-Induced Thin-Film Separation from Bulk Silicon Carbide

Exfoliation of Sic by hydrogen implantation and subsequent annealing forms the basis for a thin-film separation process which, when combined with hydrophilic wafer bonding, can be exploited to produce silicon-carbide-on-insulator, SiCOI. Sic thin films produced by this process exhibit unacceptably high resistivity because defects generated by the implant neutralize electrical carriers. Separation occurs because of chemical interaction of hydrogen with dangling bonds within microvoids created by the implant, and physical stresses due to gas-pressure effects during post-implant anneal. Experimental results show that exfoliation of Sic is dependent upon the concentration of implanted hydrogen, but the damage generated by the implant approaches a point when exfoliation is, in fact, retarded. This is attributed to excessive damage at the projected range of the implant which inhibits physical processes of implant-induced cleaving. Damage is controlled independently of hydrogen dosage by elevating the temperature of the SiC during implant in order to promote dynamic annealing. The resulting decrease in damage is thought to promote growth of micro-cracks which form a continuous cleave. Channeled H{sup +} implantation enhances the cleaving process while simultaneously minimizing residual damage within the separated film. It is shown that high-temperature irradiation and channeling each reduces the hydrogen fluence required to affect separation of a thin film and results in a lower concentration of defects. This increases the potential for producing SiC01 which is sufficiently free of defects and, thus, more easily electrically activated.

Abnormal transconductance and transient effects in partially depleted SOI MOSFETs

Abstract A transconductance dip, observed in floating body partially depleted SOI devices, is due to transient effects and is reduced with a positive back bias in SOI nMOSFETs. MEDICI simulations show that both hole and electron densities near the front interface fluctuate during the turn-on transient, causing a small decrease and then an increase of the drain current that leads to the transconductance dip. Transient effects also cause an initial current ramp in I DS – V GS characteristics at the start of the gate voltage sweep when the back gate is inverted. The transient effect diminishes as the channel length and channel width decrease and as the back bias increases.

Materials, device and gate oxide integrity evaluation of SIMOX and bonded SOI wafers

In this paper, we will review our recent material and electrical device results on SIMOX and BESOI wafers. The substrates were obtained from 2 SIMOX suppliers (IBIS and SOITEC) and one bonded supplier (HDOS). Substrates were routinely obtained over a period of more than two years and this has given us some insight into the various manufacturers quality and reproducibility as well as improvement efforts. The material parameters such as film uniformity, contamination, defects, and wafer warp and bow will be discussed. In addition, the integrity of gate oxides grown on these substrates will be compared to those grown on bulk wafers. Device results such as threshold voltage control (Vt) and subthreshold leakage for devices built on SIMOX and BESOI wafers will be compared. These results have been obtained from several lots processed in our line and thus represent variations in both the material and the process.

Thin film silicon on insulator substrates and their application to integrated circuits

Thin film silicon on insulator (TFSOI) devices have been studied for years. The advantages of TFSOI devices include: a reduction in junction capacitance, potentially lower junction leakage, a simpler process, and many other well documented advantages. However, other than some military/space applications, TFSOI circuits are not currently available in commodity products. One of the reasons TFSOI circuits are not wide spread is that there has not been a reliable source of TFSOI substrates. Recently, however, several suppliers of TFSOI substrates, both SIMOX and bonded and etch-backed wafers (BESOI), have made significant improvements in their material quality and are increasing capacity to meet expected demands. In this paper, we will discuss the major materials issues and how these issues impact either the TFSOI device performance or the process integration. In addition, we will present gate oxide integrity data as well as device results from these TFSOI substrates.

Effect of nitrogen and argon anneals on the leakage current of SIMOX TFSOI devices

High temperature annealing treatment is a critical step in SIMOX technology. Inert gases such as Ar or N/sub 2/ can be used during this anneal along with a small amount of oxygen. Characterization of TFSOI near-fully-depleted devices built on Ar and N/sub 2/ annealed SIMOX indicate that, in the N/sub 2/ annealed material, nitrogen atoms may become trapped at the SOI/BOX interface and cause excessive sub-threshold leakage in NMOS devices. This paper will discuss the effect of nitrogen on the device characteristics based on electrical and chemical measurements.

Transient behaviors in partially depleted thin film SOI devices

The floating-body configuration in SOI devices is desirable because of area efficiency and parasitics reduction. It has been predicted recently that there exists a dynamic floating-body effect in partially depleted SOI devices, which can lead to transient currents during device turn-on/off. This paper presents the observed current transients due to the dynamic floating body effects. The transient behaviors are analyzed and device simulation was done to confirm our analysis.