H. C. Cheng

Cross‐sectional transmission electron microscope study of the growth kinetics of hexagonal MoSi2 on (001)Si

Cross‐sectional transmission electron microscopy (XTEM) has been applied to the study of the growth kinetics of hexagonal MoSi2 on (001)Si. Growth rates, interface structures, and microstructures in the growth layers were determined for samples annealed at 560–580 °C for various periods of time. The silicide thickness was found to be proportional to the square root of annealing time. The activation energy and the rate constant of the growth were measured to be 2.3 eV and 14 cm2/s, respectively. Growth mechanisms are discussed. The results of the previous studies on the MoSi2 growth are summarized. Advantages of the present approach in the study of the growth kinetics of silicides on silicon, both in sample preparation and in the employment of XTEM technique, are outlined.

Characterization of H2N2 plasma passivation process for poly-Si thin film transistors (TFTs)

Abstract The effects of nitrogen additives on the plasma hydrogenation of polycrystalline silicon thin film transistors (poly-Si TFTs) have been investigated with various radio-frequency (RF) power densities, substrate heating temperature, gas flow rates, as well as chamber pressures. The nitrogen-containing hydrogen ( H 2 N 2 ) plasma treatments show better passivation effects on the electrical characteristics of the poly-Si TFTs than the pure H2 hydrogenation. It is attributed to the passivation effect of the nitrogen radicals themselves and the promotion of the hydrogen plasma generation due to the radical collision. The passivation effects have been enhanced by properly chosen RF power density, gas flow rate and chamber pressure. Furthermore, the H 2 N 2 plasma were also utilized to passivate the poly-Si TFTs with different grain structures.

EFFECTS OF O2- AND N2O-PLASMA TREATMENTS ON PROPERTIES OF PLASMA-ENHANCED-CHEMICAL-VAPOR-DEPOSITION TETRAETHYLORTHOSILICATE OXIDE

The high quality silicon oxide films were prepared by plasma-enhanced chemical vapor deposition (PECVD) using tetraethylorthosilicate (TEOS)-oxygen based chemistry. The O2- or N2O-plasma treatments were performed on the as-deposited films as an attempt to improve the properties of the TEOS oxide films. TEOS oxide film deposited at lower pressure had lower Si–OH content, less carbon impurity, and flatter surface, and hence had better electrical properties. Both O2- and N2O-plasma would decrease the oxygen content of the oxide film, which led the composition of the film to deviate from the stoichiometric SiO2. The O2-plasma treatment did not show the encouraging effect on the chemical structure and electrical properties of the TEOS oxide films. In contrast, the N2O-plasma treatment could be a promising means to improve the breakdown field and leakage current density of the TEOS oxide films, which was accomplished by the N2O-plasma effect to facilitate the passivation of dangling bonds, linking reaction of Si–OH bonds, nitridation reaction and densification of the amorphous silicon oxide network.

Effects of rapid thermal annealing on the formation of shallow p+n junction by implanting BF2+ ions into thin metal films on Si substrate. I. Thin titanium films

High‐quality Ti‐silicided shallow p+n junctions have been fabricated by implanting BF+2 ions into thin Ti films on Si substrate and subsequent silicidation/drive‐in by rapid thermal annealing (RTA) or conventional furnace annealing (CFA) under proper implant and anneal conditions. For both the RTA and CFA techniques, annealing temperatures higher than 800 °C degrade the junction formation because of more severe dopant confinement within the silicides and more serious diffusion of knock‐on Ti into junction regions. The high‐dose implant greatly enhanced the dopant activation and thus improved the junctions. The high heating rate for RTA caused an immediate formation of Ti–B compounds at high temperatures, while CFA considerably promoted the drive‐in efficiency because of its low heating rates and long annealing times. Hence, CFA yielded better low‐bias rectifying characteristics than RTA due to larger dopant activation. However, CFA caused much worse high reverse‐bias characteristics. A rapid increase of r...

FORMATION OF SHALLOW P+N JUNCTIONS BY IMPLANTING BF2+ IONS INTO THIN COBALT FILMS ON SILICON SUBSTRATES

Silicided shallow p+n junctions have been formed by implanting BF2+ ions into a-Si/Co bilayer films on Si substrates and subsequent drive-in/silicidation. The effects of implantation dose and energy on the electrical characteristics were studied. The modulation ranges of implantation energy and dose employed to form good junctions are large for the present fabrication scheme. However, high-performance silicided shallow junctions must be optimized by using moderate implant energy and dosage. The junction depth, drive-in efficiency, dopant concentration and electrical characteristics must be compromised to achieve the goal of good shallow junctions. A silicided shallow junction formed by the implantation of 70 keV at a dose of 5 × 1015cm−2 and subsequently annealed at 700°C exhibited characteristics with a leakage current density lower than 0.5 nA cm−2, a forward ideality factor better than 1.01 and a junction depth of about 0.13 μm.

Excellent thermal stability of cobaltaluminum alloy Schottky contacts on GaAs substrates

Abstract Schottky contact properties of films with codeposited CoAl mixture of tri-layered Co/Al/Co structure on GaAs substrate after different rapid thermal annealings (RTAs) have been investigated. Interfacial thermal stability between co-deposited CoAl mixture and GaAs was attributed to the formation of CoAl compound. However, cobalt at the Co/GaAs interface would strongly react with GaAs to produce Co 2 GaAs phase for tri-layered Co/Al/Co structure on GaAs. Subsequently, Co 2 GaAs compound would decompose into CoGa and CoAs phases. Capped with a 3500-A-thick SiO 2 layer, co-deposited CoAl films could withstand high-temperature annealing and exhibited a barrier height of 0.88 eV and an ideality factor of 1.08 even after 1050°C rapid thermal annealing for 10 s. As far as the authors are aware, this rapid thermal annealing temperature is the highest in the literature for such excellent Schottky characteristics. The contact was found to achieve a barrier of 0.90 eV for the co-deposited CoAl alloy with Co : Al = 1:1.05 after 850°C rapid thermal annealing for 50 s. Tri-layered Co/Al/Co film was also found to be thermally stable on GaAs after short time annealing (10 s), but the contact property rapidly degraded with annealing time.

Degradation of passivated and non-passivated N-channel low-temperature polycrystalline silicon TFTs prepared by excimer laser processing

Abstract The instability mechanisms of passivated and non-passivated low-temperature polycrystalline silicon thin film transistors (LTPS TFTs) under various bias stress conditions have been investigated. Irrespective of plasma treatment, the degradation was more severe under negative gate bias stress than that under positive gate bias stress. This could be due to Fowler–Nordheim tunneling electron induced impact ionization. For hot carrier stress, TFTs with NH 3 plasma treatment degraded more severely than those without plasma treatment. This might be attributed to collapsing of weak Si–H bonds in NH 3 -plasma passivated devices. For the high current stress, it showed the opposite results against hot carrier stress.

SUPPRESSION OF ANOMALOUS DIFFUSION OF ION-IMPLANTED BORON IN SILICON BY LASER PROCESSING

Anomalous diffusion of ion‐implanted boron in silicon has been suppressed by using the laser annealing (LA) technique. For the rapid thermal annealing process, the high‐dosage boron implant significantly enhanced the anomalous diffusion of boron in Si largely due to increased density of interstitial clusters. The mechanism that electrically active dopants contribute to diffusion is confirmed. The dopant activation is primarily determined from the heating process rather than the holding time interval. Hence, the optimum annealing regime, attaining high‐performance shallow p+‐n junctions, is to increase the dopant activation efficiency during the temperature‐ramping process as well as to shorten the holding interval. Having an ultrahigh heating rate, the LA technique serves as an excellent annealing scheme to significantly suppress the anomalous diffusion and considerably promote the dopant activation.

The effect of gate electrodes using tungsten silicides and/or poly-silicon on the dielectric characteristics of very thin oxides

Abstract The effects of gate electrodes made of tungsten silicides and/or polysilicon on the dielectric characteristics of very thin gate oxides have been studied. For a WSix/SiO2 (100 A)/Si gate structure, the phase transition from hexagonal to tetragonal WSi2 and the induced stress demonstrated the degradation of dielectric strength of gate oxide annealed at 700°C. Tungsten diffusion to the interface between SiO2 and Si substrate as well as the enhnaced local strain caused the deterioration of breakdown fields for annealing temperatures above 900°C. Drastic generation of hole traps at 1000°C annealing further diminished the field to breakdown of thin oxide. For WSix/poly-Si/SiO2 (100 A)/Si and poly-Si/SiO2 (100 A)/Si structures annealed at 1000°C, phosphorus diffusion to the SiO2/Si interface was also harmful to the dielectric characteristics of gate oxides besides the high-temperature hole trap generation and the thermally induced stress. Hence, MOS capacitors of WSix/poly-Si/SiO2 (100 A)/Si structures seemed to be superior to the other two structures when the sheet resistivity factor was also considered. The effect of thickness of gate oxide on the dielectric properties of three gate structures was also investigated.

Effects of ion-beam mixing on the epitaxial growth of MoSi2 on (111)Si

Both plan‐view and cross‐sectional transmission electron microscopy as well as sheet resistance measurement have been applied to study the effects of As+ ion‐beam mixing on the epitaxial growth of MoSi2 on (111)Si. Significant improvement in the epitaxial growth of MoSi2 on (111)Si and lower electrical resistivity of the MoSi2 overlayer were found as a result of ion‐beam mixing. Island formation was almost completely alleviated in samples implanted under suitable conditions and subsequently annealed up to 1100 °C. The dispersion of impurities near the Mo/Si interface and/or the generation of defects as a result of the ion‐beam mixing are thought to be particularly beneficial to the growth of MoSi2 epitaxy on silicon. The ineffectiveness of the ion‐beam mixing for the improvement of the silicon surface coverage in some instances is attributed to the more pronounced accumulation of As atoms at the MoSi2/Si interface to increase the interface energy so that island formation becomes energetically more favorable.