Effects of Ge/Ge preamorphization combined with sub-keV boron implantation in pMOSFET fabrication

The reduction of transient enhanced diffusion (TED) and suppression of short-channel effect (SCE) are very critical for the formation of ultra shallow junctions required for deep sub-micron devices. This article reports the nanoscale gate length of p-type metal-oxide-semiconductor field-effect transistor (pMOSFET) technology using ⁷²Ge/⁷⁴Ge germanium preamorphization implantation (Ge PAI) upon the (1 0 0)-oriented silicon substrates. It is demonstrated that the channeling can be eliminated by the formation of a Ge-implantation induced thin amorphous layer near the surface prior to boron implantation. Optimizing the amorphous layer thickness by controlling a high ⁷²Ge/⁷⁴Ge ratio, the device performance of pMOSFETs can be enhanced. In addition, the optimum conditions of Ge PAI would help the confinement of boron ions to avoid the channeling phenomenon. It is also found that the thin Ge PAI amorphous layer formed by a low ⁷²Ge/⁷⁴Ge ratio would cause the degradation of threshold voltage (V_th) roll-off characteristics, I_on/I_off ratio and the fluctuation of 62.14% in gain factor, as compared to that formed by a high ⁷²Ge/⁷⁴Ge ratio. It is attributed to a thinner Ge amorphous layer that has a weak ability to suppress the channeling tail of boron, as compared to a thicker Ge amorphous layer at the same implanted doses and acceleration energies among various ⁷²Ge/⁷⁴Ge ratios.     

Optimization of the germanium preamorphization conditions forshallow-junction formation

Shallow p⁺-n and n⁺-p junctions were formed in germanium preamorphized Si substrates. Germanium implantation was carried out over the energy range of 50-125 keV and at doses from 3×10¹⁴ to 1×10¹⁵ cm^-2. p ⁺-n junctions were formed by 10-keV boron implantation at a dose of 1×10¹⁵ cm^-2. Arsenic was implanted at 50 keV at a dose of 5×10¹⁵ cm^-2 to form the n⁺-p junctions. Rapid thermal annealing was used for dopant activation and damage removal. Ge, B, and As distribution profiles were measured by secondary ion mass spectroscopy. Rutherford backscattering spectrometry was used to study the dependence of the amorphous layer formation on the energy and dose of germanium ion implantation. Cross-sectional transmission electron microscopy was used to study the residual defects formed due to preamorphization. Complete elimination of the residual end-of-range damage was achieved in samples preamorphized by 50-keV/1×10¹⁵ cm^-2 germanium implantation. Areal and peripheral leakage current densities of the junctions were studied as a function of germanium implantation parameters. The results show that high-quality p⁺-n and n⁺-p junctions can be formed in germanium preamorphized substrates if the preamorphization conditions are optimized     

Characterization of ultra-shallow p+-n junction diodesfabricated by 500-eV boron-ion implantation

Ultrashallow gated diodes have been fabricated using 500-eV boron-ion implantation into both Ge-preamorphized and crystalline silicon substrates. Junction depths following rapid thermal annealing (RTA) for 10 s at either 950°C or 1050°C were determined to be 60 and 80 nm, respectively. These are reportedly the shallowest junctions formed via ion implantation. Consideration of several parameters, e.g. reduced B⁺ channeling, increased activation, and reduced junction leakage current, lead to the selection of 15 keV as the optimal Ge preamorphization energy. Transmission electron microscope results indicated that an 850°C/10-s RTA was sufficient to remove the majority of bulk defects resulting from the Ge implant. Resulting reverse leakage currents were as low as 1 nA/cm² for the 60-nm junctions and diode ideality factors for crystalline and preamorphized substrates ranged from 1.02 to 1.12. Even at RTA temperatures as low as 850°C, the leakage current was only 11 nA/cm ². The final junction depths were found to be approximately the same for both preamorphized and nonpreamorphized samples after annealing at 950°C and 1050°C. However, the preamorphized sample exhibited significantly improved dopant activation     

The effects of doping impurities and substrate crystallin on the formation of nickel suicides on silicon at 200–280° C

Transmission electron microscopy and Auger electron spectroscopy have been applied to investigate the effects of doping impurities and substrate crystallinity on the formation of nickel suicides at 200–280° C in nickel thin films on silicon. The systems investigated included samples with as-implanted BF₂, B, F, As, and P and recrystallized (001) Si as well as P-doped low pressure chemical vapor deposited (LP-P) and B-doped plasma enhanced chemical vapor deposited (PE-B) amorphous silicon substrates. In samples annealed at 220–280° C, substantial amounts of epitaxial NiSi₂ were found to form on crystalline structure of BF₂, B and F implanted samples to various extents at different temperatures. High resolution lattice imagings of cross-sectional samples showed that the epitaxial NiSi₂/Si interfaces are coherent. No NiSi₂ was detected in all nickel thin films deposited on implantation-amorphous specimens. NiSi₂ epitaxy was found to be a sensitive function of annealing temperature. Good correlation was found between the atomic size factor and resulting stress and NiSi₂ epitaxy at low temperature. The formation of Ni₂Si and NiSi was observed to be influenced by the dopant species and crystallinity of the substrates. The vast difference in inducing the formation of nickel suicides in implantation-amorphous and recrystallized samples is likely due to variations in initial structure and/or dopant distribution. The finding that bothn-type andp-type dopants influenced the formation of Ni₂Si and NiSi suggested that they may be related to the electrical activity of the doping species in recrystallized samples. NiSi, possessing one of the lowest resistivity among all metal silicides, was found to be the only phase formed in all implantation-amorphous as well as LP-P and PE-B amorphous silicon samples annealed at 280° C. Nickel thin film appears to be an attractive candidate for the metallization of amorphous silicon devices.     

Material and electrical properties of ultra-shallow p+-njunctions formed by low-energy ion implantation and rapid thermalannealing

A study of low-energy ion implantation processes for the fabrication of ultrashallow p⁺-n junctions is presented. The resulting junctions are examined in terms of four key parameters: defect annihilation, junction depth, sheet resistance, and diode reverse leakage current. In the realm of very-low-energy ion implantation, Ge preamorphization is found to be largely ineffective at reducing junction depth, despite the fact that the as-implanted boron profiles are much shallower for preamorphized substrates than for crystalline substrates. Transmission electron microscopy (TEM) analysis of residual defects after rapid thermal annealing (RTA) reveals that the use of either a preamorphization implant or the implantation of BF₂ as a B source results in residual damage which requires higher RTA temperatures to be removed. A reasonable correlation is observed between residual defect density observed via TEM and junction leakage current. It is concluded that the key to an optimized low-energy implantation process for the formation of ultrashallow junctions appears to be the proper selection of preamorphization and annealing conditions relative to the dopant implant energy     

Epitaxially Grown Monoisotopic Si,Ge, and Si<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Ge<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> Alloy Layers: Production and Some Properties

The technology of the growth of Si, Ge, and Si_1–xGe_x layers by molecular-beam epitaxy with the use of a sublimation source of monoisotopic ³⁰Si or ²⁸Si and/or gas sources of monogermane ⁷⁴GeH₄ is demonstrated. All of the epitaxial layers are of high crystal quality. The secondary-ion mass spectroscopy data and Raman data suggest the high isotopic purity and structural perfection of the ³⁰Si, ²⁸Si, ⁷⁴Ge, and ³⁰Si_1–x⁷⁴Ge_x layers. The ³⁰Si layers doped with Er exhibit an efficient photoluminescence signal.     

Anomalous distribution of germanium implanted into a SOI dielectric layer after the annealing of radiation defects

The germanium-distribution profile is investigated in a Si/SiO₂/Si structure after the implantation of ⁷⁴Ge into SiO₂ dielectric layer, bonding with the Si device layer, and high-temperature annealing. The anomalously high transport and accumulation of ⁷⁴Ge atoms near the SiO₂/Si interface far from the bonded boundary is found. The observed ⁷⁴Ge distribution is beyond the framework of the existing model of diffusion of Ge in Si and SiO₂ after postimplantation annealing. A modified model of diffusion of Ge atoms near the Si/SiO₂ interface qualitatively explaining the observed features is proposed.     

Systematic study of shallow junction formation on germanium substrates

Published results on Ge junctions are benchmarked systematically using R_S-X_J plots. The electrical activation level required to meet the ITRS targets is calculated. Additionally, new results are presented on shallow furnace-annealed B junctions and shallow laser-annealed As junctions. Co-implanting B junctions with F is shown to degrade junction properties.     

0.2-μm p+-n junction characteristics dependent onimplantation and annealing processes

Junction depth, sheet resistance, dopant activation, and diode leakage current characteristics were measured to find out the optimal processing conditions for the formation of 0.2-μm p⁺-n junctions. Among the 2×10¹⁵ cm^-2 BF₂ implanted crystalline, As or Ge preamorphized silicon, the crystalline and Ge preamorphized samples exhibit excellent characteristics. The thermal cycle of furnace anneal (FA) followed by rapid thermal anneal (RTA) shows better characteristics than furnace anneal, rapid thermal anneal, or rapid thermal anneal prior to furnace anneal     

New Ti-SALICIDE process using Sb and Ge preamorphization forsub-0.2 μm CMOS technology

A new process for thin titanium self-aligned silicide (Ti-SALICIDE) on narrow n⁺ poly-Si lines and n⁺ diffusion layers using preamorphization implantation (PAI) with heavy ions of antimony (Sb) and germanium (Ge) has been demonstrated for application to 0.2-μm CMOS devices and beyond. Preamorphization enhances the phase transformation from C₄₉Ti_xSi _x to C₅₄TiSi₂ and lowers the transformation temperature by 80°C so that it occurs before conglomeration in narrow lines. Preamorphization by Sb and Ge implantation yields better results than that by As. The sheet resistance of TiSi₂ on heavily As doped poly-Si lines are 3.7 Ω/□ and 3.8 Ω/□ for the samples preamorphized by Ge and Sb implantations even with line width down to 0.2 μm. There is less leakage in the Ti-SALICIDE diode with preamorphization than without it. The probable reasons and mechanisms are discussed     

Diffusion of ion-implanted boron impurities into pre-amorphized silicon

Boron ion implantation into pre-amorphized silicon is studied. Pre-amorphization is performed either by F⁺ or Si⁺ implantation prior to B⁺ implantation at 10 keV with 3×10¹⁵ ions/cm². Broadening of the boron profile can be suppressed markedly in the pre-amorphized layers. For instance, the as-implanted depth at a B concentration of 1×10¹⁸ atoms/cm³ decreases from 0.19 to 0.1 μm for implantation into a pre-amorphized layer compared to B implantation into crystalline silicon. After annealing at 950°C, B atoms diffuse much more rapidly in the pre-amorphized layers than in the crystalline silicon case. Nevertheless, shallower junctions are obtained with the use of pre-amorphization. For dual F⁺ and B⁺ implantation at F⁺ doses above 1×10¹⁵ F⁺/cm², fluorine is found to segregate to the peak of the boron profile during annealing. Fluorine is also trapped at the peak of the as-implanted fluorine profile peak and near the amorphous–crystalline interface. The effects of fluorine dose and anneal temperature on the F precipitation are described and compared to results for BF⁺₂ implants.     

Rapid thermal chemical vapor deposition of germanium on silicon and silicon dioxide and new applications of ge in ULSI technologies

In this study, low pressure chemical vapor deposition of pure germanium on silicon and silicon dioxide has been considered for new applications in future ultra large scale integration (ULSI) technologies. Germanium depositions were performed in a lamp heated cold-wall rapid thermal processor using thermal decomposition of GeH₄. It is shown that Ge deposition on Si can be characterized by two different regions: a) at temperatures below approximately 450° C, the deposition is controlled by the rate of surface reactions resulting in an activation energy of 41.7 kcal/mole. b) Above this temperature, mass transport effects become dominant. The deposition rate at the transition temperature is approximately 800 Å/min. It is shown that Ge deposition on SiO₂ does not occur, even at temperatures as high as 600° C, resulting in a highly selective deposition process. Selectivity, combined with low deposition temperature makes the process very attractive for a number of applications. In this work, it is shown for the first time that selective Ge deposition can be used to eliminate silicon consumption below the gate level during the silicidation of the shallow source and drain junctions of deep submicron MOSFETs. In addition, a new in situ technique has been developed which allows polycrystalline germanium (poly-Ge) deposition on SiO₂. In this work poly-Ge has been considered as a low temperature alternative to polycrystalline silicon (poly-Si) in the formation of gate electrodes in single-wafer manufacturing where low-thermal budget processes are most desirable.     

Structural characterization of thick, high-quality epitaxial Ge on Si substrates grown by low-energy plasma-enhanced chemical vapor deposition

In this paper, we report on the growth of epitaxial Ge on a Si substrate by means of low-energy plasma-enhanced chemical vapor deposition (LEPECVD). A Si_1?xGe_x graded buffer layer is used between the silicon substrate and the epitaxial Ge layer to reduce the threading dislocation density resulting from the lattice mismatch between Si and Ge. An advantage of the LEPECVD technique is the high growth rate achievable (on the order of 40 Å/sec), allowing thick SiGe graded buffer layers to be grown faster than by other epitaxial techniques and thereby increasing throughput in order to make such structures more manufacturable. We have achieved relaxed Ge on a silicon substrate with a threading dislocation density of 1 × 10⁵ cm^?2, which is 4?10x lower than previously reported results.     

Photothermal Activation of Shallow Dopants Implanted in Silicon

Dopant impurities were implanted at high dose and low energy (10¹⁵ cm⁻², 0.5–2.2 keV) into double-side polished 200 mm diameter silicon wafers and electrically activated to form p–n junctions by 10 s anneals at temperatures of 1,025, 1,050, and 1,075°C by optical heating with tungsten incandescent lamps. Activation was studied for P, As, B, and BF₂ species implanted on one wafer side and for P and BF₂ implanted on both sides of the wafer. Measurements included electrical sheet resistance (Rs) and oxide film thickness. A heavily boron-doped wafer, which is optically opaque, was used as a hot shield to prevent direct exposure to lamp radiation on the adjacent side of the test wafer. Two wafers with opposing orientations with respect to the shield wafer were annealed for comparison of exposure to, or shielding from, direct lamp illumination. Differences in sheet resistance for the two wafer orientations ranged from 4% to 60%. n-Type dopants implanted in p-type wafers yielded higher Rs when the implanted surface was exposed to the lamps, as though the effective temperature had been reduced. p-Type dopants implanted in n-type wafers yielded lower Rs when the implanted surface was exposed to the lamps, as though the effective temperature had been increased. Effective temperature differences larger than 5°C, which were observed for the P, B, and BF₂ implants, exceeded experimental uncertainty in temperature control.     

Si~+/B~+双注入单晶硅的快速热退火

用四探针法、扩展电阻法、背散射沟道谱和二次离子质谱等测试分析手段研究了Ｓｉ＋ ／Ｂ＋ 双注入单晶硅的快速热退火行为。结果表明：Ｓｉ＋ 预非晶化注入能有效地抑制注入硼原子的沟道效应;快速热退火Ｓｉ＋ ／Ｂ＋ 注入样品,其注入损伤基本消除,残留二次缺陷少,硼原子电激活率高;优化与控制快速热退火条件和Ｓｉ＋ ／Ｂ＋ 注入参数,制备出了电学特性优良的浅ｐ＋ ｎ 结,其二极管反偏漏电流仅为１．９ ｎＡ·ｃｍ － ２（－ １．４Ｖ）。     

Thermal stability of Si/Gen/Si heterostructures by photoreflectance

The thermal stability of Si/Ge_n/Si(001) heterostructures includingn = 1, 6, 20, and 100 monolayers (ML’s) is studied in connection with their electronic structures through the measurement of photoreflectance (PR). The PR spectra are observed at 90 K over the energy range 0.85–4.0 eV. Comparing the PR signals of Si/Ge _n /Si(001) heterostructures before and after thermal annealing at 600° C, it is found that the samples with less than 6 ML Ge show no change whereas those with more than 20 ML Ge show large changes. The result suggests that Si/Ge _n /Si heterostructures with Ge layer thickness less than 6 ML’s are thermally stable. For the heterostructures with 20 and 100 ML Ge, the relaxation of strain in the Ge layer is found to occur from the PR spectra ofE ₀(Ge),E ₁(Ge) andE ₁ +Δ ₁(Ge), andE ₁(Si).     

Electrical characterization of p+/n shallow junctions obtained by boron implantation into preamorphized silicon

Electrical characteristics of p⁺/n diodes obtained by boron implantation into amorphous silicon layers formed by a prior implantation of Si⁺ ions are presented. The absence of channeling phenomena (preamorphization), the low boron implantation energy (10–20 keV), and the post-implantation low temperature anneal (600–1000°C) or rapid anneal (electron beam) allow to obtain very shallow junctions (0.1–0.3 μm). Particular attention is given to analyse effects on the reverse diode current from dislocation loops which are formed at the amorphous-crystalline interface during annealing. If the dislocation loops are outside of the space charge region, the diodes show a low leakage current (∼ 1 nA/cm² at - 1 v), but the reverse current increases strongly when this residual damage falls into the depleted n-region. Experimental I–V characteristics are in excellent agreement with a numerical simulation, which takes into account a strong lifetime degration associated with the dislocation loops.     

Two-step rapid thermal diffusion of boron into silicon using a boron nitride solid diffusion source

A two-step rapid thermal diffusion process of boron into silicon using a boron nitride solid diffusion source is described. During the first step, HBO² glass is transferred onto the silicon wafer from the diffusion source by keeping the temperature of the silicon wafer at 750° C while the diffusion source is at about 900° C. Boron is, then, diffused into the silicon wafer from HBO² glass at 1000° C or 1100° C in N₂ during the second step. Extremely shallow junctions with junction depths of about 20 nanometers and sheet resistances of about 350 ohms/sq can be achieved with this method as well as relatively deep junctions with junction depths of about 175 nanometers and sheet re-sistances of about 55 ohms/sq. When the diffusion is performed at 1100° C, both the junction depth and electrically active boron concentration at the surface increase as the ambient gas is changed from N₂ to O₂ while the sheet resistance decreases. A boron rich layer which has high resistivity is not formed at the surface when the diffusion is performed at 1100° C in O₂ ambient. This work was supported by Ministry of Science and Technology, Korea     

A physically based phenomenological model using boltzmann-matano analysis for boron diffusion from polycrystalline Si into single crystal Si

The diffusion of boron in single crystal Si from a BF₂-implanted polycrystalline Si film deposited on single crystal Si has been accurately modeled. The effective diffusivities of boron in the single crystal Si substrate have been extracted using Boltzmann-Matano analysis and the new phenomenological model for B diffusivity has been implemented in the PEPPER simulation program. The model has been implemented for a range of furnace anneal conditions (800 to 950°C, from 30 min to 6h) and implant conditions (BF₂ doses varied from 5×10¹⁵ to 2×10¹⁶ cm⁻² at 70 keV).     

The behaviour of boron molecular ion implants into silicon

Implants of boron molecular ions into silicon have been studied using a variety of experimental techniques, but with emphasis on sheet resistance annealing characteristics and transmission electron microscopy. Boron halide compound molecules have been implanted and equivalent dose sequential implants of atomic species used as control conditions. The implants studied were B⁺, BCl₂⁺, BCl⁺, Cl⁺ + B⁺, BF₂⁺, BF⁺ and B⁺ + F⁺ at 25 keV/B atom and B⁺, BBr₂⁺ and Br₂⁺ + B⁺ at 12 keV/B atom.The implantation of molecular ions enables conditions of varying damage to be studied with constant dose, dose rate and energy of the dopant species. In addition to damage effects the halogen atoms produce species effects in the implanted zone. The escape of the halogen atoms has been measured as a function of the annealing temperature.The significant differences which exist between the behaviour of silicon implanted with these various conditions are considered with reference to the damage structures observed by transmission electron microscopy. The boron-fluorine molecular implants are shown to offer some advantages as a means of implanting boron.