Hm Wong

Proximity gettering with mega‐electron‐volt carbon and oxygen implantations

We have demonstrated that a buried gettering layer can be formed with a single MeV ion implantation without damaging the top device region. The strong gettering efficiency of carbon implant and its linear dependence on dose are confirmed. A surprising feature of the carbon implanted layers is that no extended defects are formed after annealing for implant doses up to 2×1016 cm−2 at 3 MeV, compared to a layer of small precipitates and dislocations in the case of oxygen implantation. It is suggested that the carbon‐related gettering centers are point defects or their clusters.

Gettering of gold and copper with implanted carbon in silicon

The gettering effects of implanted carbon for Au and Cu are studied and compared with the gettering effects of implanted oxygen, nitrogen, BF2, neon, and argon. It is demonstrated that implanted carbon forms strong gettering centers in silicon which are an order of magnitude more effective than implanted oxygen. The amount of gettered Au by implanted carbon is found to be approximately linear with dose in the range from 1015 to 1016 cm−2 and no thermal instability is observed with annealing up to 12 h at 1000 °C. It is found that the gettering effect of carbon is reduced by the addition of oxygen. This indicates that the strong gettering effect of carbon is not due to carbon‐enhanced oxygen precipitation but a phenomenon of its own.

Profile studies of MeV ions implanted into Si

Abstract Experimental profile data for MeV arsenic, phosphorus and boron ions implanted into silicon are presented. The energy ranges are 3–11 MeV, 1–7 MeV and 1–4 MeV for arsenic, phosphorus and boron respectively. Profiles are obtained from secondary ion mass spectroscopy (SIMS). To make the profile data easily accessible in integrated circuit processing design, moments parameters are extracted and fitted into profile models. The Rp, ΔRp, skewness and kurtosis of the measured data were fitted by polynomials to facilitate convenient calculations for process simulators. It is found that MeV arsenic, phosphorus and boron profiles can all be adequately modeled by Pearson-IV distribution functions, while Gaussians result in large errors in the tail portions of the profiles.

Buried dopant and defect layers for device structures with high-energy ion implantation

Abstract Recent developments in the use of megavolt ion implantation for fabricating microelectronics structures are presented. Deep buried dopant implanted layers offer major advantages in: (1) processing simplicity, (2) low thermal budget, and (3) process flow flexibility for both CMOS and bipolar integrated circuits. Dopant profile design issues such as vertical and lateral straggles of high-energy implants are shown to be important for future scaling of device dimensions. The use of deep-buried defect layers created by high-energy implants (e.g. O and C) has many useful VLSI applications such as proximity gettering of impurities and localized minotiry-carrier lifetime control. Improved VLSI frabrication yield and monolithic integration of signal-processing and power-circuits are expected with these buried defects.

Cross-section Transmission Electron Microscopy Study of Carbon Implanted Layers in Silicon

We have used cross‐section transmission electron microscopy (XTEM) to study microstructures of carbon‐implanted silicon layers after high‐temperature annealing. It was found that the threshold dose for extended defect formation was much higher for carbon implantation than for other ion species such as B, P, and O. In 2.4 MeV carbon‐implanted layers, no dislocations were formed for doses as high as 2×1016 cm−2 after annealing at 1000 °C for 1 h. The threshold was found to be lower for low‐energy implantation (100 keV): at a dose of 2×1016 cm−2, when an amorphous layer was formed, microtwins were formed near the projected range upon annealing. Microprecipitates around 50 A in size were observed in low‐energy carbon‐implanted samples and the precipitates appeared to be under strain.

On the temperature variation of threshold voltage of GaAs MESFETs

The authors have investigated the temperature dependence of the threshold voltage of depletion-mode GaAs MESFETs with epitaxially grown n channels. An approach to threshold shift analysis that allows direct comparison with threshold measurement is taken. The contributions from various temperature-dependent effects to the threshold-voltage shift were studied, including the built-in voltage of the Schottky barrier, deep-level transients, capping layer effects, the substrate-channel built-in voltage, and the k factor which is related to channel mobility. A quasi-DC method for threshold voltage measurement, which enables threshold voltage to be measured as a function of temperature with minimum deep-level transient effect is reported. A method has also been developed to measure the temperature dependence of built-in voltage which is completely free from transient effects. The results show that the major contributors to the temperature variation of threshold voltage are the temperature dependence of the Schottky barrier built-in voltage and the effect of the capping layer. >

Electronic defects in silicon induced by MeV carbon and oxygen implantations

Abstract Electrical properties of silicon above buried defect layers formed by MeV carbon and oxygen implantation were investigated. Energies ranging from 2 to 4 MeV and doses from 5 × 10 14 to 10 16 cm −2 were used. By studying the reverse bias leakage currents of P + /N junction diodes after MeV implantation and subsequent annealing at 950 ° C for 1 h, it was found that the electrical defects are mostly localized near the peaks of the implant profiles. In cases of 4 MeV carbon and oxygen implantations at a dose of 10 15 cm −2 , average leakage currents less than 5 nA/cm 2 at 10 V reverse bias were obtained. The average minority carrier lifetimes in the top 2 μm were about 100 μs.

Observation of multiple precipitate layers in MeV Au++‐implanted silicon

Room‐temperature MeV Au++ implantation into silicon with energies above 1.8 MeV shows a splitting of the Au concentration profile in the Rutherford backscattering spectrometry (RBS) spectra. Cross‐section transmission electron microscopy micrographs show two distinct regions of Au precipitates corresponding to the peaks in the RBS spectra. The double peaks can be explained by the segregation of Au into the highly damaged region near the end of the implant range and Au segregation along a dislocation network. These dislocations arise from dynamic beam annealing during the implant and act as paths for rapid diffusion. Precipitation occurs when the Au concentration exceeds the solubility limit. Lower energy implants resulted in the expected Gaussian distributions.

Parasitic effects of surface states on GaAs MESFET characteristics at liquid-nitrogen temperature

An abrupt increase of drain current has been observed at low drain voltages for GaAs MESFETs operated at liquid-nitrogen temperature. The drain voltage (V/sub k/) at which the anomaly occurs shifts slightly toward smaller drain voltages with increasing gate voltage. Two types of I-V behavior exist for drain voltages less than V/sub k/. For type I behavior, the devices show transistor-like characteristics with smaller drain output current and transconductance than expected. I-V characteristics similar to breakdown in short-channel devices are observed for drain voltages near V/sub k/. For type II behavior, the devices show space-charge-limited current characteristics. In both cases, the drain current returns to the normal MESFET value when the drain voltage exceeds V/sub k/. The phenomena can be explained by parasitic effects of surface states in the separation regions between the gate and the heavily doped drain/source. >

Effects of high energy boron ions implanted in MOSFETs

Abstract In recent years, high energy ion implantation has been found to be very attractive for device fabrication because of its ability to place dopants more than 1 μm beneath the substrates surface. Many new applications and structures are made possible with this feature. Examples are the replacement of epilayer growth by high energy ion implant to form bipolar transistor subcollectors [1] and the construction of retrograded wells to reduce latchup and soft-error in CMOS transistors [2,3]. In this work, we will investigate the feasibility of customizing integrated circuits by threshold adjustment implant as a late processing step. Different doses of boron ions with energies ranging from 0.75 to 0.90 MeV are implanted into n-MOSFETs after second-level poly-Si deposition. Rapid thermal annealing is used for postimplant annealing and dopant activation. For low-dose implants, full activation can be realized at about 750°C for 15 s. For high-dose implants, however, no full activation is seen even at 1000°C. I-V measurements are used to investigate the high-energy implantation effects on the threshold voltage, the channel leakage current, and the subthreshold slope of the MOSFET transistors. It is found that the threshold voltage shifts are no longer linearly proportional to the implant doses. A new relationship is presented. After annealing, the leakage current for the implanted transistors can be reduced to the same level as that of unimplanted devices. However, the subthreshold slope is larger for an implanted transistor compared to an unimplanted transistor.