Influence of TiSi2 formation temperature on film thermal stability

This work is addressed to investigate thermal stability of a thin TiSi₂ film, that is its ability to resist degradation due to heat treatments at high temperatures. The study was carried out as a function of the formation RT treatment (675–750°C) at the end of a common process flow. Sheet resistance measurements were employed in order to evaluate this degradation. Electrical measures were performed on large and narrow poly-Si lines, on Van Der Pauw structures and on doped mono-Si substrates. An increase in sheet resistance value of an order of magnitude for silicide formed at temperatures below 700°C with respect to the one formed at temperatures above 700°C was found, particularly on poly-Si lines. The effect is detectable independently of the structure: it was observed also on 0.75-μm wide poly-Si lines, increasing when line width decreases. Different morphological analyses were carried out for investigating the influence of the formation temperature. We explain the increase of the final sheet resistance decreasing the formation temperature as a lower thermal stability of the TiSi₂ film, leading to a thermal grooving of the silicide grains.     

Small area versus narrow line width effects on the C49 to C54transformation of TiSi2

Polycrystalline silicon (poly) structures of equal area and varying line width were subjected to a standard self-aligned TiS₂ process. The poly lines had widths of 0.4, 0.5, 1.0, and 2.0 μm and the total contiguous silicided area ranged from 7 to over 2500 μm₂. On all large area structures (silicided area >100 μm₂) our findings were consistent with the well-known narrow line width effect; i.e., the kinetics of transformation were adversely affected as the line width decreased, but with a sufficiently high thermal budget complete transformation to the C54 phase was possible. However, on structures with silicided area less than 100 μm ₂, a small area effect was observed in addition to the narrow line width effect. A percentage of small area lines remained entirely in the C49 phase (because of an apparent absence of C₅₄ nucleation sites) regardless of the thermal budget. This percentage increased with decreasing silicided area but was independent of the line width. Only the small area lines that did transform to the C54 phase exhibited the narrow line width effect. Therefore, for the range of line widths studied, the narrow line width mechanism and the small area mechanism were distinct     

Comparison of transformation to low-resistivity phase andagglomeration of TiSi2 and CoSi2

The phase transformation and stability of TiSi₂ on n ⁺ diffusions are investigated. Narrower n⁺ diffusions require higher anneal temperatures, or longer anneal times, than wider diffusions for complete transitions from the high-resistivity C49 phase to the low-resistivity C54 phase. A model is presented which explains this in terms of the probability of forming C54 nuclei on narrow diffusions and the influence of diffusion width on C54 grain size. The results are that more C49 and C54 nucleation events are required to completely transform narrow lines. For thin TiSi₂ (40 nm), there is a narrow process window for achieving complete transformation without causing agglomeration of the TiSi₂. The process window decreases with decreasing silicide thickness. A significantly larger process window is achieved with short-time rapid annealing. Similar studies are performed for CoSi₂ on n⁺ and p⁺ diffusions. No linewidth dependence is observed for the transformation from CoSi_x to CoSi₂. There is a broad process window from 575°C to 850°C using furnace annealing, for which the low-resistivity phase is obtained without causing agglomeration     

Molecular dynamics characterization of crystalline and amorphous TiSi2 phases

Molecular dynamics simulations by a tight binding potential provide new interesting information on the ground state properties of the TiSi₂ phases. In particular, we have compared some structural, elastic, thermodynamic and electronic properties of the C49, the C54 and the amorphous phases. It turns out that the C49 structure is much softer than the C54, also displaying a melting temperature some 300–400 K below the one for the C54, in agreement to very recent experimental results. The amorphous phase is energetically and structurally more akin to the C49 than to the C54 phase. On the basis of these results we suggest the higher formation kinetics of the former to be related to an intrinsic advantage in the growth stage.     

Process limitation and device design tradeoffs of self-aligned TiSi2 junction formation in submicrometer CMOS devices

Submicrometer CMOS transistors require shallow junctions to minimize punchthrough and short-channel effects. Salicide technology is a very attractive metallization scheme to solve many CMOS scaling problems. However, to achieve a shallow junction with a salicide structure requires careful optimization for device design tradeoffs. Several proposed techniques to form shallow titanium silicide junctions are critically examined. Boron, BF₂, arsenic, and phosphorus dopants were used to study the process parameters for low-leakage TiSi ₂ p⁺/n and n⁺/p junctions in submicrometer CMOS applications. It is concluded that the dopant drive-out (DDO) from the TiSi₂ layer to form a shallow junction scheme is not an efficient method for titanium salicide structure; poor device performance and unacceptably leaky junctions are obtained by this scheme. The conventional post junction salicide (PJS) scheme can produce shallow n⁺/p and p⁺/n junctions with junction depths of 0.12 to 0.20 μm below the TiSi₂. Deep submicrometer CMOS devices with channel length of 0.40 to 0.45 μm can be fabricated with such junctions     

Effects of thermal processes after silicidation on the performanceof TiSi2/polysilicon gate device

The effects of thermal processes after silicidation on the gate depletion, threshold voltage (V_th) shift, drive current, and sheet resistance of TiSi₂/polysilicon (Ti-polycide) gate devices are evaluated. The dopant depletion of the polysilicon film, which is known to increase the V_th and to degrade the drive-current, increases with increasing temperature of the post-thermal process. However, the V_th roll-off characteristic in nMOSFETs is enhanced with increasing temperature. Furthermore, the drive-current is significantly degraded by the gate reoxidation process. The sheet resistance of the Ti-polycide gate increases with gate reoxidation as well as with increased post-thermal processes     

Anomalous Ti SALICIDE gate to source/drain shorts induced by dry Sietch during TiSi2 local interconnect formation

A new failure mode was observed in a 0.5 μm version of the silicided amorphous-silicon contact and interconnect (SAC) technology. Massive PMOS gate to source/drain shorts were found. The cause is attributed to formation of Ti during the Si etch. The fluorinated Ti surface fails to form adequate TiN diffusion barrier during subsequent rapid thermal annealing (RTA) in N₂ or NH₃ ambient. Si diffuses from the polycrystalline Si gate and/or the p-type source/drain onto the spacer, reacts with Ti and forms resistive leakage paths. A blanket low-dose, low-energy As implant prior to Ti deposition corrects this problem without adversely changing device characteristics     

Investigation of C49–C54 TiSi2 transformation kinetics

The C49C54 TiSi₂ polymorphic transformation has been investigated trying to elucidate the relative role played by nucleation and growth in silicide formation. Samples having an amorphous layer and C54 seeds have been made by heat treatment of a Ti/Si bi-layer structure and subsequent suitable Ar ion implantation. In situ resistance measurements performed during heat treatments in controlled and purified atmosphere at constant heating rates and ex situ X-ray diffraction, MeV ⁴He⁺ backscattering spectrometry and cross-section transmission electron microscopy have been used to characterize the samples. The results show that in a sample with C49 the C54 nucleation occurs at 800°C; when C54 seeds are present the growth is appreciable already at temperatures as low as 400–500°C. It has also been shown that nucleation and growth of the C49 phase is a process competitive to the growth of the C54 phase.     

Formation of TiSi2 and shallow junction byAs+ ion-beam mixing and infrared rapid heat treatment

A technique for forming shallow junctions with low-resistance silicide contacts developed for the use in VLSI with scaled MOSFETs is discussed. The salicide (self-aligned silicide) MOSFET gate and source-drain features self-aligned refractory metal silicide and are isolated from one another even without any insulating spacer on the gate sides. A critical step in such a MOSFET fabrication process is the ion implantation through metal silicidation technique, which includes As⁺ ion-beam-induced titanium-silicon interface mixing and infrared rapid heat treatment to form simultaneously the n⁺-p junction and a high-quality TiN covered TiSi₂ contact layer     

The process window of a-Si/Ti bilayer metallization for anoxidation-resistant and self-aligned TiSi2 process

The dependences of both oxidation-resistant and self-aligned silicidation properties on the thicknesses of top amorphous-Si (a-Si) and Ti metal in an a-Si/Ti bilayer process are presented. It is shown that a thin silicide layer formed during the reaction between a-Si and Ti films becomes a stable oxidation and nitridation barrier for oxygen- and nitrogen-related impurities. Moreover, the formation sequence of the silicide phase depends not only on the annealing temperature but also on the thickness of the Ti film. In addition, the preferential orientation of the silicide phase after annealing at high temperature also shows a strong dependence on the thickness of Ti film, which is attributed to the difference of the grain size in the polycrystalline silicide film. The allowed process window for the a-Si thickness can be determined experimentally and a reproducible and homogeneous self-aligned TiSi₂ film can be easily obtained by using the a-Si/Ti bilayer process in salicide applications despite high-level contaminations of oxygen impurities in both the as-deposited Ti film and the annealing ambient     

Frequency stabilization of laser diodes using 1.51-1.55 μmabsorption lines of 12C2H2 and 13C2

Frequency stabilization of a 1.55 μm DFB laser diode is demonstrated using vibrational-rotational absorption of ¹²C ₂H₂ and ¹³C₂H₂ molecules (named VRAM). Frequency stability within 2 MHz peak to peak fluctuation can be achieved in the wavelength region of 1.51-1.55 μm. Frequency-stabilized DFB laser compact modules have been constructed. Frequency stabilities are evaluated by measuring the beat spectrum of the two lasers. In addition, the temperature and pressure dependences of the acetylene absorption lines are characterized     

Stimulated rotational Raman conversion in H2, D2, and HD

Stimulated rotational Raman conversion has been efficiently demonstrated in all three molecular hydrogen isotopes with a 532-nm Q-switched pump source. Optimum gas pressures and pump focusing have been determined. Single pass conversion efficiencies of 80%, 75%, and 65% have been achieved at the first Stokes wavelengths with H₂ , D₂, and HD, respectively. The maximum efficiency is generally limited by further Raman conversion of the initially generated Stokes light. The thresholds were found to increase in the order H₂ 22. The relative thresholds of H₂ and D₂ are reasonably consistent with the calculated steady-state gains based on the population statistics and the linewidth data. Intermediate frequency shifts such as these may be useful by themselves, or in combination with other nonlinear techniques, as methods to achieve specific wavelengths for system applications     

SiSiO2 interface state spectroscopy using MOS tunneling structures

This paper provides a critical review and classification of the studies of SiSiO₂ interface state parameters and energy distributions by means of MOS tunneling in structures with ultrathin SiO₂ layers (10–100 Å). Suggestions are made of experiments that will help to elucidate the importance of materials and processing conditions on these states, and to separate the various mechanisms involving charge exchange with the metal, the conduction band, and the valence band of the semiconductor.     

Measurement and control of a residual oxide layer on TiSi2 films employed in ohmic contact structures

An inadvertent oxide layer is formed on a titanium disilicide (TiSi₂) film following various wet and dry processes in a manufacturing environment. The use of H₂SO₄:H₂O₂:H₂O (1:1:5) as a wet etch for excess Ti metal, prior to the high temperature anneal used to form a subsequent TiSi₂ layer, is identified as the source of the undesired oxide via multiwavelength spectroscopic ellipsometry and Auger electron spectrometry studies. This inadvertent oxide layer on TiSi₂ is shown to form bad electrical contacts and is a contributing source to large standby currents in polysilicon gate shunts. Spectroscopic ellipsometry is shown herein as a unique analytical tool to determine both the thickness and structure of this poorly structured oxide during process development. A single wavelength ellipsometer monitoring scheme for both the appearance as well as the thickness of this inadvertent oxide layer is proposed for use in high-volume manufacturing     

脉冲CO_2激光作用下光气在BCl_3中的分解与生成

研究了在脉冲能量为0.8焦耳的TEA CO_2激光作用下BCl_3中光气的分解与生成。峰值功率密度约10~6瓦/厘米~2的脉冲CO_2激光,既不引起BCl_3中COCl_2的分解,也不引起BOl_3中的CO和Cl_2生成COCl_2。在焦点区峰值功率密度约10~8瓦/厘米~2的CO_2激光,则引起BOl_3中光气的分解和生成。     

Characteristics of self-induced lightly-doped-drain polycrystallinesilicon thin film transistors with liquid-phase deposition SiO2 as gate-insulator and passivation-layer

As the passivation layer on the top of undoped offset region for offset-gate structured poly-Si TFTs is exposed to hydrogen plasma, a lightly-doped-like drain region could be equivalently self-induced. The hydrogenated polycrystalline silicon thin-film transistor of this structure, named self-induced lightly-doped-drain (SI-LDD) poly-Si TFTs, was first developed with liquid-phase deposition oxide as both the gate insulator and the passivation layer. This paper describes the optimum hydrogenation condition, and the electrical characteristics for the novel SI-LDD poly-Si TFTs. The effects of DC electrical stress on SI-LDD poly-Si TFTs are also described. Finally a model is proposed to explain the degradation phenomena observed in our SI-LDD devices     

拉曼激光雷达测量大气二氧化碳不确定性分析

中科院安徽光机所研制了一台ARL-1二氧化碳拉曼激光雷达,本文结合实际大气条件,利用拉曼激光雷达的观测例子和分析仪观测结果,分析拉曼激光雷达测量大气二氧化碳的不确定性。分析结果表明,拉曼激光雷达在对流层低层具有良好的稳定性和较高的测量精度,在较好的天气条件下,1km高度范围内,ARL-1拉曼激光雷达的测量不确定性可控制在1.2ppm内,在2km高度范围内可控制在2.5ppm内。     

Reactive ion etching of III-V compounds using C2H6/H2

Reactive ion etching of InP, GaInAs and GaAs using a mixture of ethane and hydrogen, C₂H₆/H₂, is demonstrated for the first time. It has been found that by choosing optimum etching parameters one can obtain excellent vertical sidewalls as well as very smooth surfaces, keeping the etching rate at a convenient value of 20-60 nm/min     

High-density formation of Ge quantum dots on SiO2

We formed high-density Ge quantum dots (QDs) on an ultrathin SiO₂ layer by controlling the early stages of low-pressure chemical vapor deposition (LPCVD) with a germane gas (GeH₄) assisted by a remote plasma of pure H₂. We then characterized the electronic charged states of the QDs by an AFM/Kelvin probe technique. The formation of single crystalline Ge-QDs with an areal dot density of ∼2.0 × 10¹¹ cm⁻² was confirmed after examining the surface morphology and lattice by atomic force microscopy and transmission electron microscopy, respectively. It has been suggested that an increase in the flux of deposition precursors due to efficient decomposition of GeH₄ by a supply of hydrogen radicals and the dehydration reaction of surface OH bonds plays a role in nucleation of Ge-QDs on SiO₂. Surface passivation with hydrogen may also promote the surface migration of deposition precursors during LPCVD. The surface potential of the dots changed in a stepwise manner with respect to the tip bias due to multistep electron injection into and extraction from the Ge-QDs.     

Spike anneal: RTP processing at reduced thermal budget with applications to TiSi2 formation towards 0.1-μm linewidths

I report a way out of the narrowing of the temperature process window for formation of low resistivity C54–TiSi₂ below 0.25-μm linewidth. On one hand, higher temperatures would be required to overcome the increasing difficulty of the C54–TiSi₂ transformation. On the other hand, temperature should be lowered to avoid the increasing tendency to thermal agglomeration. A spike anneal, at higher temperature and shorter duration with high ramp-rate, is found to give full transformation on 0.25-μm lines without parasitic effects and with a tighter resistance distribution.