Akihiko Yasuoka

Application of nitrogen implantation to ULSI

Abstract Nitrogen is not a commonly used ion species in Si ULSI. It cannot be used as an n-type dopant because of its low solubility in Si. However, it shows interesting properties such as the suppression of boron diffusion when applied to source/drain doping and the nitridation of gate oxide when applied to gate doping. In this report, first, the effects of nitrogen preimplantation to the formation of boron-doped shallow p+n junctions are described. The technique is successfully applied to 0.25 μm PMOSFETs, forming shallow junctions and thus suppressing short channel effects. Next, the effects of nitrogen implantation into p+ poly-Si gates are studied. The implanted nitrogen diffuses to the gate oxide during annealing and nitrides the gate oxide. As a result, boron penetration through the gate oxide is suppressed and the reliability and hot carrier resistance are improved.

A built-in self-test circuit with timing margin test function in a 1 Gbit synchronous DRAM

This paper describes the implementation of a BIST circuit with timing margin test functions to a 200 MHz 1 Gbit synchronous DRAM. 220 ps-resolution timing signals with up to 80 ns cycle time are generated by a phase-locked loop (PLL) circuit and a delayed timing generator. These timing signals are used not only as actual control signals but also as reference signals in an AC timing comparator. The entire BIST circuit, which includes 20/spl times/4 bit LFSRs, occupies only 0.8% of the chip area. A cost evaluation of the BIST shows that the technology is effective for 64 Mbit high-speed DRAMs and beyond.

The effects on metal oxide semiconductor field effect transistor properties of nitrogen implantation into p+ polysilicon gate

We have studied in detail the effects of nitrogen implantation into a p+ polysilicon gate on gate oxide properties for the surface p-channel metal oxide semiconductor (PMOS) below 0.25 µm. The nitrided oxide film can be easily formed by the pile-up of nitrogen into the gate oxide film from the polysilicon gate. It was found that boron penetration through the gate oxide film can be effectively suppressed by nitrogen implantation into a p+ polysilicon gate because nitrogen in the polysilicon film can suppress boron diffusion, and the nitrided oxide film can also act as a barrier to boron diffusion. Moreover the hot-carrier hardness can be remarkably improved by the nitrided oxide film since interface state generation can be suppressed by the nitrided oxide film. Furthermore the number of electron traps in the gate oxide film can also be reduced by nitrogen implantation.

Focused ion beam technologies for lithographic applications

Abstract Focused ion beam (FIB) technologies have the advantages of high resolution and maskless fabrication, which play an important role in the lithographic process for the development of advanced semiconductor devices. FIB lithography has the added advantage of obtaining a T-shaped edge profile for the fabrication of a “mushroom gate” of GaAs FETs by the use of different ion species. A GaAs HEMT fabricated by using hybrid exposure of Be2+ and Si2+ shows a minimum noise figure of 0.83 dB with an associated gain of 7.7 dB at 18 GHz. The capability of maskless fabrication has been applied to defect repair of the lithographic photomask. An opaque defect is removed by ion beam sputtering, and a clear defect is repaired by chemical vapor deposition with the assistance of ion beam irradiation. A repair with high positional accuracy can be performed because the ion beam can be precisely positioned Extra ions penetrating the glass substrate reduce the light transmissivity, and change the characteristics of the pattern transfer. This problem is now eliminated by incorporating a post-dry etching process.

Effects of the oxidation process on the electrical characteristics of oxidized nitride films

The effects of the oxidation process on the electrical characteristics of metal–oxide–semiconductor (MOS) capacitors were studied. It was found that electrical and time dependent dielectric breakdown (TDDB) characteristics are improved for the MOS capacitor with a wet oxidized nitride film over the N2O oxidized nitride film. The depth profile of the oxidized nitride film is also studied by Auger electron spectroscopy. It is observed that from the top surface of an oxidized nitride film of about 2 nm (nitride film oxidized in the wet oxidation process or in N2O ambient by rapid thermal processing), the level of oxygen is the same but, compared to a N2O oxidized nitride film, the level of nitrogen is greater in a wet oxidized nitride film. The improvement of the electrical and TDDB characteristics of the wet oxidized nitride film over the N2O oxidized nitride can be thought due to the increased amount of nitrogen from the top surface to a depth of nearly 2 nm of the oxidize nitride films.

Nitride thickness dependence of trap generation and negative stress-induced current in oxidized nitride films (<5 nm)

Abstract The trap generation and negative stress-induced current in oxidized nitride films for two film thickness values have been investigated using MIS capacitors and p-channel MISFET transistors. It was already observed that in thin nitride film the gate current is two orders of magnitude larger than that for the thick film. But for a constant top oxide on the nitride films, the gate current was almost the same. After subjecting both films to a constant current stress, the gate current measured for the oxidized thick nitride film was larger than that of the oxidized thin nitride film. The hole and electron currents were measured independently before and after stress application. The current increase in oxidized thin nitride film is caused by the stress-induced generation of trapped electrons, while the current increase in the oxidized thick nitride film is caused by the stress-induced generation of trapped holes in the top-oxide film and in the bulk nitride.

Improved reliability of wet oxidized nitride MOS capacitors in comparison to RTP N2O oxidized nitride films

Abstract A nitride film has been oxidized in a wet (O:H = 8:1) ambient or N 2 O atmosphere at different temperatures and times and the electrical characteristics have been studied. It was found that the reliability of MOS capacitors with a wet oxidized nitride film is better than that of N 2 O oxidized nitride film grown (1100°C, 30 s) samples. In addition it was found that the TDDB characteristics are improved with an increase of the N 2 O oxidation temperature and time. The improvement of the MOS capacitors with a wet oxidized nitride film can be attributed to the more efficient replacement of excess Si atoms and H-related species such as Siue5f8H bonds, which makes the ON layer more resistive and with less trap sites than the N 2 O oxidized nitride film.

Gate oxide breakdown by focused ion-beam irradiation

Dielectric breakdown of gate oxide by the focused iou beam (FIB) irradiation of the MOS structure has been studied. The sample device structure was that of an MOS transistor about to be ion-implanted in the source and drain (S/D) regions. The gate poly-silicon electrode was extended to the pad electrode on the thick field oxide. The thicknesses of the gate and field oxide layers were 25 iim and 500 mu, respectively. That of the poly-silicon layer was 250 nm. The typical areas of the gate and the pad electrodes were 31-100 jzm2 aiid 2-4x iO pm2, respectively. Gallium FIB, accelerated by 40 keV, was irradiated to sample devices. The total dose until the breakdowii depends on the irradiated location of the device. In the case that the edge of the polysilicon layer adjacent to the S/D regions is irradiated, the dose is the least. If the gate oxide layer has not been removed on the S/D regions, the dose exceeds that for the sample with the gate oxide layer etched there, although not greater than that in the case that any other part of the poly-silicon layer is irradiated. The electroluminescence measurement of some samples indicates that such edge irradiation iiiduces damages at a spot on the irradiated edge of the gate oxide. Such damages are probably due to the combiiiation of conductive microbridge formation and the electrical dielectric breakdown around the bridge. The microbridge is formed by the implantation of gallium ions, and/or adhesion of gallium and silicon atonis at the sidewall of the gate oxide. In the experiments where an area around the center of the polysilicon pad electrode was irradiated, two breakdown modes were observed: breakdown due to gate-culTent stress and instant breakdown. The gate potential during the FIB irradiation was obtained from the measured secondary electron current. The potential is approximately equal to that measured during the conventional constant-current-stress (005) lifetime experiment. This proves that gate oxide is stressed by current during irradiation. The time to breakdown becomes shorter with decrease in the gate area if the FIB current is the same. The smaller the devise becomes, the 005-like mode will be the more serious. Instant breakdown mode appears when the beam current is high. Detailed analysis reveals discrepancies between the phenomena and simple conjectures based on the 005 lifetime. By the irradiation, some samples are broken in far shorter time than expected from the 005 lifetime measurement, whereas the rest of the samples survive longer than the expectation. This suggests that there is a stronger stress at the onset of the irradiation. This breakdown mode is, at least at present, uncontrollable and most hazardous, to be overcome for improved