Separation By Implanted Oxygen Wafers Research Articles

Influence of nitrogen dose on the charge density of nitrogen-implanted buried oxide in SOI wafers

To harden silicon-on-insulator (SOI) wafers fabricated using separation by implanted oxygen (SIMOX) to total-dose irradiation, the technique of nitrogen implantation into the buried oxide (BOX) layer of SIMOX wafers can be used. However, in this work, it has been found that all the nitrogen-implanted BOX layers reveal greater initial positive charge densities, which increased with increasing nitrogen implantation dose. Also, the results indicate that excessively large nitrogen implantation dose reduced the radiation tolerance of BOX for its high initial positive charge density. The bigger initial positive charge densities can be ascribed to the accumulation of implanted nitrogen near the Si-BOX interface after annealing. On the other hand, in our work, it has also been observed that, unlike nitrogen-implanted BOX, all the fluorine-implanted BOX layers show a negative charge density. To obtain the initial charge densities of the BOX layers, the tested samples were fabricated with a metal-BOX-silicon (MBS) structure based on SIMOX wafers for high-frequency capacitance–voltage (C–V) analysis.

Effects of Si implantation on the total dose hardness of fully-depleted SIMOX wafers

Total dose hardened fully-depleted SOI materials are fabricated on separation by implanted oxygen (SIMOX) materials by silicon ion implantation and annealing. The ID–VG characteristics of pseudo-MOS transistors pre- and post-irradiation are tested with 60Co gamma rays. The chemical bonds and the structure of Si in the buried oxide are also studied by X-ray photoelectron spectroscopy and cross-sectional high-resolution transmission electron microscopy, respectively. The results show that Si nanocrystals in the buried oxide produced by ion implantation are efficient deep electron traps, which can significantly compensate positive charge buildup during irradiation. Si implantation can enhance the total-dose radiation tolerance of the fully-depleted SOI materials.

Characterization of Distribution of Trap States in Silicon-on-Insulator Layers by Front-Gate Characteristics in n-Channel SOI MOSFETs

We characterized the distribution of trap states in silicon-on-insulator (SOI) layers in epitaxial layer transfer (ELTRAN) wafers and in low-dose separation by implanted oxygen (SIMOX) wafers. We measured the front- and back-gate characteristics of MOSFETs with SOI layers of different thicknesses. We used the current-Terman method to estimate the trap states at the gate oxide (GOX)/SOI interface and at the SOI/buried oxide (BOX) interface separately. As a result, we concluded that the high-density trap states in the SOI layers in SIMOX wafers cause a gate-voltage shift, which is attributed to the charged trap states only in the inversion layer. We also found that the trap states are distributed within about 30 nm from the SOI/BOX interface in the SOI layer in SIMOX wafers, which indicates that the distribution of trap states originates from the oxygen implantation that is peculiar to the SIMOX process.

Research on ion implantation effect on SIMOX material modification technique by X-ray photoelectron spectroscopy

Separation-by-implanted-oxygen (SIMOX) material was modified by silicon ions implantation with different techniques in this work, and the chemical bonds of Si in the buried oxide layer (BOX) of the SIMOX wafers were investigated with X-ray photoelectron spectroscopy (XPS) method. The results of Si 2p were presented to demonstrate the modification mechanism of the SIMOX materials, which could be resulted from the formation of different Si chemical structures in the silicon oxidation by the ions implantation with the post anneal process. The results of ID–VG characteristics on the wafers with Pseudo-MOS method showed that the processes of the buried oxide affected the character of the ID–VG curves of the Pseudo-MOS.

Influence of Oxygen Partial Pressure on Thickness Change of Buried Oxide in Silicon-on-Insulator Structure during High-Temperature Oxidation Processes

High-temperature oxidation (HTO), which is used to create an internal thermal oxide (ITOX) layer at the superficial silicon/buried oxide (BOX) interface in the silicon-on-insulator structure, is an important process in the production of low-dose separation-by-implanted-oxygen (SIMOX) wafers. We studied the influence of oxygen partial pressure in the HTO atmosphere on ITOX growth. Low-dose SIMOX wafers were subjected to HTO processes in which the oxygen partial pressure was varied. ITOX growth at high oxygen partial pressures agreed well with the HTO model proposed by Schroer et al. At low oxygen partial pressures, however, ITOX growth was demonstrated for the first time to change to BOX shrinkage, which cannot be explained by previous HTO models. BOX shrinkage occurred simultaneously with top oxide growth, suggesting that the thermal oxidation at the top oxide/superficial silicon interface can proceed even on silicon crystals with an oxygen concentration below the solubility limit. We proposed a new HTO model that allows top oxide growth on silicon crystals below the solubility limit. Furthermore, a diffusion-limited BOX thickness change was assumed. The new HTO model showed excellent agreement with the experimental data and effectively explains the changing behavior from ITOX growth to BOX shrinkage.

Effects of the technology of implanting nitrogen into buried oxide layer on the characteristics of partially depleted SOI nMOSFET

The effects, caused by the process of the implantation of nitrogen in the buried oxide layer of SIMOX wafer, on the characteristics of partially depleted silicon_on_insulator nMOSFET have been studied. The experimental results show that the channel electron mobilities of the devices fabricated on the SIMON (separation by implanted oxygen and nitrogen) wafers are lower than those of the devices made on the SIMOX (separation by implanted oxygen) wafers. The devices corresponding to the lowest implantation dose have the lowest mobility within the range of the implantation dose given in this paper. The value of the channel electron mobility rises slightly and tends to a limit when the implantation dose becomes greater. This is explained in terms of the rough Si/SiO2 interface due to the process of implantation of nitrogen. The increasing negative shifts of the thre shold voltages for the devices fabricated on the SIMON wafers are also observed with the increase of implanting dose of nitrogen. However, for the devices fabri cated on the SIMON wafers with the lowest dose of implanted nitrogen in this pap er, their threshold voltages are slightly larger on the average than those prepa red on the SIMOX wafers. The shifts are considered to be due to the increment of the fixed oxide charge in SiO2 layer and the change of the density of the int erface-trapped charge with the value and distribution included. In particular, t he devices fabricated on the SIMON wafers show a weakened kink effect, compared to the ones made on the SIMOX wafers.

Comparison of ordered structure in buried oxide layers in high-dose, low-dose, and internal-thermal-oxidation separation-by-implanted-oxygen wafers

The ordered SiO 2 in the buried oxide (BOX) layer of high-dose, low-dose, and internal-thermal-oxidation (ITOX) separation-by-implanted-oxygen (SIMOX) wafers was investigated by X-ray diffraction. From the results, it was found that the SiO 2 molecules in the low-dose and ITOX SIMOX wafers are better ordered than those in the high-dose SIMOX wafer and that the ordered structure of the ITOX layer is different from that of the originally formed BOX layer, suggesting that the ITOX layer has a structure similar to that of the ordered SiO 2 in the thermal oxide layer.

Scanning kelvin-probe characterization of heavy metal contamination in patterned SIMOX wafers

The behaviour of nickel (Ni) contamination in silicon-on-insulator (SOI) wafers has been characterized by the Kelvin method combined with surface photovoltage (SPV) measurements. In order to characterize the SOI wafers contaminated with Ni atoms, we used the “patterned” separation by implanted oxygen (SIMOX) wafers with partially no buried oxide (BOX) region, which acts as an opening of the top-Si layer to the substrate. It is found that the smaller size of patterned SIMOX area help the gettering of Ni atoms in the top-Si layer through the openings to the substrate because of its shorter lateral migration distance for Ni atoms. In addition, it is found that the intensity of SPV for the SIMOX areas increases with increasing a line width of the openings because of its wider vertical diffusion path away from the top-Si layer toward the substrate. These results clearly suggest that the Ni atoms in the SOI layer even with the underneath BOX layer can diffuse laterally and escape to the bulk region through the BOX-less region.

Confirmation of electric properties of traps at silicon-on-insulator (SOI)/buried oxide (BOX) interface by three-dimensional device simulation

We succeeded in reproducing experimental phenomena characteristic of metal-oxide-semiconductor field-effect-transistors (MOSFETs) fabricated on separation by implanted oxygen (SIMOX) wafers by device simulation. Previously, we have found that SIMOX wafers have energetically localized traps at silicon-on-insulator (SOI)/buried oxide (BOX) interface, and we have also found that the traps cause current drops, suppression of kink and dependence of carrier lifetime on substrate bias in SIMOX MOSFETs. We developed a three-dimensional device simulator considering energetically localized traps at SOI/BOX interface, and we clarified that the novel phenomena in SIMOX MOSFETs come from the traps. We applied this simulator to three-dimensional MOSFET structures, and we estimated surface recombination velocity at SOI/BOX interface on SIMOX wafers.

Confirmation of electric properties of traps at silicon-on-insulator (SOI)/buried oxide (BOX) interface by three-dimensional device simulation

We succeeded in reproducing experimental phenomena characteristic of metal-oxide-semiconductor field-effect-transistors (MOSFETs) fabricated on separation by implanted oxygen (SIMOX) wafers by device simulation. Previously, we have found that SIMOX wafers have energetically localized traps at silicon-on-insulator (SOI)/buried oxide (BOX) interface, and we have also found that the traps cause current drops, suppression of kink and dependence of carrier lifetime on substrate bias in SIMOX MOSFETs. We developed a three-dimensional device simulator considering energetically localized traps at SOI/BOX interface, and we clarified that the novel phenomena in SIMOX MOSFETs come from the traps. We applied this simulator to three-dimensional MOSFET structures, and we estimated surface recombination velocity at SOI/BOX interface on SIMOX wafers.

Characterization of Trap States at Silicon-On-Insulator (SOI)/Buried Oxide (BOX) Interface by Back Gate Transconductance Characteristics in SOI MOSFETs

We characterized trap states at silicon-on-insulator (SOI)/buried oxide (BOX) interfaces in epitaxial layer transfer (ELTRAN) wafers and in low-dose separation by implanted oxygen (SIMOX) wafers. We formulated a simple analytical form to obtain the distribution of density of trap states from a comparison of the experimental transconductance and simulated transconductance of SOI metal oxide semiconductor field effect transistors (MOSFETs). We observed dips in back gate transconductance in MOSFETs on low-dose SIMOX wafers. Consequently, we concluded that “acceptor-like electron traps” with the density of 5 ×1012 cm-2·eV-1 near the conduction band edge Ec and “donor-like hole traps” with the density of 1012 cm-2·eV-1 near the valence band edge Ev are distributed in the forbidden band of Si at SOI/BOX interfaces in low-dose SIMOX wafers, while the trap density at SOI/BOX interfaces is less than 1011 cm-2·eV-1 in ELTRAN wafers. We also found that the kink effect is suppressed in MOSFETs on low-dose SIMOX wafers with a high density of trap states at the SOI/BOX interface.

Characterization of the Interface between the Top Si and Buried Oxide in Separation by Implanted Oxygen Wafers

The electrical characteristics of the top Si-buried oxide interface in low-dose separation by implanted oxygen (SIMOX) wafers have been investigated from high-frequency and quasi-static capacitance–voltage (C–V) measurements of the buried metal oxide semiconductor (MOS) diodes proposed here. The structure of the interface has been also analyzed using cross-sectional transmission electron microscopy (XTEM). SIMOX wafers with an internal thermal oxidation (ITOX) process have a fixed charge density of 2.7×1010 cm-2 and an interface trap density of 5×109 cm-2 eV-1. XTEM analysis revealed the undulation of the interface with ITOX is about 1 lattice. From the small fixed charge density and the smooth interface, we conclude that the strain field in the top Si-buried oxide interface is small, and comparable to that of thermal oxide-Si interface. The obtained results indicate that ITOX-SIMOX wafers are useful for the fabrication of fully depleted MOSFETs.

Nature of Surface and Bulk Defect Induced by Low Dose Oxygen Implantation in Separation by Implanted Oxygen Wafers

Two new metrologies have been proposed to characterize unknown surface and bulk defects in separation by implanted oxygen (SIMOX) wafers. They included the following: i) laser scattering particle counter plus coordinated atomic force microscope (AFM) and Cu-decoration for defect isolation and ii) cross-sectional transmission electron microscope (TEM) foil preparation using focused ion beam (FIB) and TEM investigation for defect morphology observation. Three types of surface defects were found and described as follows: i) a hole with a locally thinned top silicon layer (Park Defect A), ii) a hole formed locally without either a silicon or a buried oxide layer (Park Defect B), and iii) a hole formed with both a locally thinned top silicon and a buried oxide layer (Park Defect C). In addition, a bulk defect was found to occur as locally distributed square-shaped pits accompanied by the disappearance of the buried oxide layer (isolation defect). All surface and bulk defects in SIMOX wafers originate from oxide particle generation by electrostatic discharge and mechanical abrasion between the wafer and the pin of the wafer holder during oxygen ion implantation.

Ellipsometric analysis of ultrathin oxide layers on SIMOX wafers

Ellipsometric analysis of surface SiO 2 on Separation by IMplanted OXygen (SIMOX) wafers has been performed. Spectroscopic ellipsometry data in the range of 500–850 nm were fitted based on a structure model composed of surface SiO 2, surface Si, and buried SiO 2 layers as well as transition layers at the SiO 2/Si interfaces. From the fitting results, it is found that there exist transition layers with a thickness of 1.6 and 2.0 Å at the surface SiO 2/surface Si and buried SiO 2/Si interfaces, respectively. Angle-resolved He–Ne laser ellipsometry has been also applied for in-situ monitoring of oxidation processes of SIMOX wafers and it is shown that ultrathin oxide thickness can be determined with an accuracy of 3 Å.

Suppression of Substrate Crosstalk in Mixed-Signal Complementary MOS Circuits Using High-Resistivity SIMOX (Separation by IMplanted OXygen) Wafers

We investigated the effectiveness of high-resistivity SIMOX (Separation by IMplanted OXygen) wafers to suppress substrate crosstalk. Small- and large-signal crosstalk measurements yielded similar results, which indicated that the frequency dependence of both types of crosstalk could be understood by analyzing the small-signal data. With the measured data, we constructed a model of substrate crosstalk with lumped elements, and found that the model described substrate crosstalk accurately. We also measured the crosstalk between actual digital and analog circuits. The measured and calculated data showed that high-resistivity SIMOX wafers could reduce substrate crosstalk by 5–10 dB in mixed-signal complementary metal-oxide-semiconductor (CMOS) circuits. Hence, we concluded that using high-resistivity SIMOX wafers was an effective way to suppress substrate crosstalk in mixed-signal CMOS circuits.

X-ray diffraction moiré topography as a means to reconstruct relative displacement fields in weakly deformed bicrystals.

X-ray diffraction topographs of wafers produced by separation by implanted oxygen (SIMOX) show moiré fringes in both reflection and transmission geometry. These fringes reveal deformations of the order of 10(-6) to 10(-8) between the layer and the substrate of the SIMOX material. A new method for a quantitative analysis of moiré fringes is developed and allows reconstruction with a high sensitivity of the three components of the relative displacement field between layer and substrate directly from a set of topographs. This method is used for the interpretation of moiré topographs of entire 4 in SIMOX wafers and of regions around crystal defects. Finally, the capabilities of an analysis of moiré fringes are compared with those of the usual diffraction topo-graphy.

Microstructural evolution of oxygen implanted silicon during annealing processes

Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) have been employed to characterize the microstructures, oxygen distributions, and chemical bonding states of SIMOX (Separation by IMplanted OXygen) wafers. Silicon substrates were implanted at 550°C with 2 × 10 17 and 4 × 10 17 cm −2 180 keV oxygen ions, followed by annealing at various temperatures. EELS spectra indicated that the chemical bonding state between Si and oxygen changed at 1000°C anneal, though there was no appreciable change in the microstructures observed by TEM below 1000°C anneal. Above 1100°C, oxygen precipitates were preferentially formed around the peak positions of the defect and oxygen concentrations in the as-implanted samples. These precipitates aggregated to reduce the surface energy, and their size increased with temperature. After 1350°C anneal, a continuous buried oxide layer was formed in the higher-dose specimen, while the upper- and lower-precipitates remained separately in the lower-dose one. The XPS profiles did not change at the later stage of annealing processes in the 2 × 10 17 cm −2 sample, though the precipitates became larger in size. This suggests that the oxygen diffusion mostly occurs along the lateral direction but does not along the vertical one in this sample

Low-dose SIMOX wafers for LSIs fabricated with internal-thermal-oxidation (ITOX) process: electrical characterization

Electrical performance of separation by implanted oxygen (SIMOX) wafers manufactured by internal-thermal-oxidation (ITOX) process was evaluated. Breakdown behaviour of the buried oxide (BOX) layer was confirmed quantitatively to be dominated by Si islands therein, which were found to be reduced in size or eliminated by the ITOX process. By optimizing the oxygen dose and ITOX amount, a BOX breakdown field of about 8 MV/cm, comparable to those of thermally-grown oxide, was attained. The gate oxide integrity on an ITOX-SIMOX wafer was found to be superior to that of bulk Si wafers, indicating the wafer surface was improved by the high temperature annealing.

Impurity gettering effects in separation-by-implanted-oxygen (SIMOX) wafers: what getters what, where and how

Two technologically attractive concepts are currently being evaluated for front-side gettering in the vicinity of the active device region (proximity gettering). The first one employs high-energy carbon implantation and annealing to form a carbon-rich layer. The second approach involves the implantation of helium followed by annealing to create a cavity-containing layer. In this study we are concerned with the gettering behaviour of two typical transition-metal impurities, namely Cu and Fe, in as-received SIMOX material and in SIMOX wafers incorporating such proximity gettering layers. The structures are characterised by secondary-ion-mass spectrometry, Rutherford backscattering/channelling spectrometry, transmission electron microscopy and elastic recoil detection. It is shown that the final impurity distributions are sensitive to the depth location of the proximity-gettering layers. Both C and He implants are found to capture efficiently Cu atoms whereas cavity microstructures act as gettering centres for Cu, but provide no detectable trapping of Fe. Appreciable depletion of Fe is only achieved in the presence of C-rich gettering layers. From this viewpoint, implanted C layers may be preferable to cavity-containing zones formed by He implantation.

Spectroscopic Ellipsometry of SIMOX (Separation by Implanted Oxygen): Thickness Distribution of Buried Oxide and Optical Properties of Top-Si Layer

Spectroscopic ellipsometry has been used to characterize SIMOX (separation by implanted oxygen) wafers. High-dose SIMOX wafers with 450 nm thick buried SiO2 were measured using a four-zone null spectroscopic ellipsometer in a 230–840 nm range. The measured spectra of Ψ and Δ were analyzed based on a multilayer structure of air/surface- SiO2/top-Si/buried- SiO2/Si in which the optical constants of the top-Si layer were analyzed using the optical constants of bulk-Si and also using model dielectric functions proposed for Si. In the fitting of Ψ and Δ spectra, a thickness fluctuation of buried- SiO2 was considered with an arbitrary thickness distribution function. Measured Ψ and Δ spectra could be well fitted under the assumption of thickness distribution in the buried- SiO2. To examine the thickness dependence of the optical properties of the top-Si layer, the thickness of this layer was systematically reduced by repeating oxidation and etching with HF. It was found that the optical constants of the top-Si layer were equal to those of Si for a thickness above 5 nm.