Boron diffusion and activation in polysilicon multilayer films for P MOS structure: Characterization and modeling

This work deals with in situ boron diffusion and activation in multilayer films: polysilicon (Poly1)/amorphous silicon (Poly2). These films are deposited by LPCVD technique. However, several heat treatments were carried in order to determine the optimal annealing conditions to suppress boron penetration from the gate to the substrate through the gate oxide in MOS structure. The boron concentration is monitored by secondary ion mass spectrometry (SIMS). To investigate SIMS profiles we proposed a model of boron diffusion into these multilayer structures. It is important to note that the parameter values of the studied films such as the diffusion coefficient, the activation percentage of boron as well as the acceleration rate of boron diffusion are deduced from adjustment of simulated profiles with experimental profiles. From these results, we inferred that the boron is electrically active and its distribution does not reach the oxide layer and consequently, the Poly2 may reduce the boron diffusion in optimal annealing conditions.     

Verification of models for fabrication of Arsenic source-drains in VLSI MOSFET's

The understanding of the effects of both low- and high-temperature anneals of arsenic implanted into silicon is critical in the calculation of p-n junction profiles of sources and drains in short-channel MOSFET's. The work reported here uses a sample matrix of arsenic implanted into silicon over a wide range of fluences and annealed in both the low- and high-temperature regimes. This matrix of samples was measured by means of Rutherford Backscattering Spectrometry (RBS), spreading resistance (Rsp), and Secondary Ion Mass Spectrometry (SIMS). The measurement techniques are compared with each other, with the predictions of ion-implantation models, and with the annealing/diffusion models. Comparison of the RBS data from more than one experiment indicates that high-quality quantitative analysis requires more complex calibration data for the detector than is usually available. The Rsp data obtained on the low-temperature annealed samples did not yield reasonable arsenic profiles, both with respect to the peak location and profile shape. The measurement technique which was most consistent with theoretical models and most reproducible from one experimenter to another is the SIMS technique. Calculations of the annealed profiles were found to be in agreement with the SIMS data, at temperatures greater than 900°C, when the form used by Fair was employed. A large adjustment in the parameters of the charge vacancy reaction is necessary; a much smaller adjustment is required in the parameters of the extrinsic diffusion reaction. The accuracy obtained here is typical of much available data and is sufficiently accurate for 2-µm MOS device characterization, but not sufficient for submicrometer devices.     

Fluorine diffusion on a polysilicon grain boundary network inrelation to boron penetration from P+ gates

The combined effect of boron penetration and fluorine transport from P⁺ polycrystalline gates on flat-band voltage is studied. The SIMS concentration depth profiles elucidate the effect of annealing temperature on the fluorine transport, which in turn affects the boron penetration induced change in flat-band voltage. The fluorine diffusion in the poly gate is dominated by grain boundary diffusion. The identification of this mechanism is supported by SIMS profiles and a simulation based on a new methodology of network diffusion     

Nitrogen doped silicon films heavily boron implanted for MOS structures: Simulation and characterization

In this work we study the boron diffusion and its activation into recrystallized nitrogen doped silicon thin films (NIDOS) and we also discuss the influence of the chemical interaction between boron and nitrogen in NIDOS films. These films are deposited by low pressure chemical vapor (LPCVD) for the development of a P⁺ polysilicon gate for MOS structures. The reduction of boron diffusion with increasing nitrogen content is observed by SIMS profiles. SUPREM IV software is used in order to estimate the boron diffusion coefficients in NIDOS films. FTIR analyses show the appearance of a B–N complex whose density strongly depends on the annealing treatment in terms of temperature and duration. It is deduced through resistivity measurements and SEM observation that the formation of B–N complexes tends to degrade the electrical properties of polysilicon thin layers through the decrease of both electrically active boron and polycrystalline grains growth.     

Secondary defect profile related to low energy implanted boron measured up to 3.5 µm depth into Si-substrates

Low energy implantation is one of the most promising options for ultra shallow junction formation in the next generation of silicon BiCMOS technology. Among the dopants that have to be implanted, boron is the most problematic because of its low stopping power (large penetration depth) and its tendency to undergo transient enhanced diffusion and clustering during thermal activation. This paper reports an experimental study of secondary defect profiles of low energy B implants in crystalline silicon. Shallow p⁺n junctions were formed by low energy B implantation—10¹⁵ cm⁻² at 3 keV—into a reference n-type crystalline silicon or pre-amorphized n-Si with germanium −10¹⁵ cm⁻² at 30 keV, 60 keV, and 150 keV. Rapid Thermal Annealing (RTA) for 15 s at 950°C was then performed. Secondary defect profiles induced by this process are measured with isothermal transient capacitance in association with Deep Level Transient Spectroscopy (DLTS). Relatively high concentrations of electrically active defects have been obtained up to 3.5 μm into the crystalline silicon bulk. The relation of these defects with boron is discussed. The results of this study are in agreement with boron transient enhanced diffusion in Si-substrate as has been reported by Collart using Secondary Ion Mass Spectrometry (SIMS) measurements.     

Modeling of effect of interfacial charges on impurity diffusions in silicon-on-sapphire device processing

The existence of interfacial states and fixed charges between the silicon and sapphire films create an electric field that affects the diffusion of impurities near the interface. The effect of this field is modeled as a new term in the diffusion equation. A computer program was written to numerically integrate the diffusion equation for a one-dimensional model. The results were correlated with SIMS data for high-concentration phosphorus profiles. A predicted peak in the phosphorus concentration at the silicon-sapphire interface, due to the effect of negative charges at that interface, was confirmed. A pipe diffusion model was incorporated to account for the higher (than bulk silicon) diffusivities observed. New values of skewness were also needed to match implanted profiles with Pearson-IV type distributions.     

Reduction of boron diffusion in silicon-germanium by fluorine implantation

This letter investigates the effect of a 185 keV, 2.3 /spl times/ 10/sup 15/ cm/sup -2/ F/sup +/ implant on boron transient enhanced diffusion (TED) and boron thermal diffusion in SiGe by characterizing the diffusion of a boron marker layer in samples with and without a 288 keV, 6 /spl times/ 10/sup 13/ cm/sup -2/ P/sup +/ implant. In samples implanted with F/sup +/ only, the fluorine suppresses boron thermal diffusion by 58%. In samples given both P/sup +/ and F/sup +/ implants, the fluorine completely eliminates boron transient enhanced diffusion caused by the P/sup +/ implant and also significantly reduces boron thermal diffusion. SIMS profiles after anneal show a fluorine concentration in the SiGe layer that is approximately 8 /spl times/ higher than after implant, indicating that fluorine accumulates in the SiGe layer during anneal. A comparison with fluorine profiles in comparable silicon samples also shows that the fluorine concentration after anneal is dramatically higher in SiGe samples than in Si samples. This accumulation of fluorine in the SiGe layer during anneal will have major benefits for boron diffusion suppression in devices like SiGe HBTs, where boron must be kept within the SiGe layer.     

Enhanced diffusion by electrical deactivation of arsenic and itsimplications for bipolar devices

In this paper, we present experiments designed to show enhanced diffusion of dopants due to the electrical deactivation of implanted arsenic or arsenic in-diffused from polysilicon. Results show a clear enhancement of diffusion in a nearby boron layer as well as an enhancement for the arsenic itself at an annealing temperature of 750°C. At 500°C, more typical of backend processing, no enhancement is detected in accordance with the very slow deactivation process at this temperature. Implications for bipolar devices were also investigated. Large differences in device characteristics were measured due to the enhanced diffusion. Secondary ion mass spectrometry (SIMS) analysis and simulation confirmed that enhanced diffusion of both arsenic and boron is the cause for the change in device characteristics. Evidence is also presented demonstrating that the order of the anneals is crucial, thereby rejecting the hypothesis of a full coupled diffusion effect as seen for phosphorus     

瞬态退火过程中离子注入层杂质浓度分布的计算

本文在菲克定律的基础上导出了瞬态退火过程中注入杂质浓度分布与退火时间的关系,并对此理论结果进行了实验验证,两者符合良好。本文还讨论了瞬态退火过程中的增强扩散效应。     

A comparison of MOS gate structures formed by depositing polycrystalline silicon on thin oxides using conventional low pressure chemical vapor deposition and rapid thermal chemical vapor deposition

MOS structures with 80Å-200Å thick gate oxides were fabricated using polycrystalline silicon gate electrodes deposited by rapid thermal chemical vapor deposition (RTCVD) and by conventional chemical vapor deposition (LPCVD). Polycrystalline silicon doping was achieved by BF₂ or As implantation followed by rapid thermal annealing (RTA). The Q-C method was employed to study the electrical properties of the capacitors through high and low frequencyC- V profiles. The electrical properties of the MOS structures show that the devices fabricated using RTCVD polycrystalline Si are comparable in quality to those with LPCVD polycrystalline silicon gate electrodes. However, the results also indicate that boron diffusion through the thin oxide is a problem regardless of the deposition technique used for polycrystalline silicon. Boron penetration is accompanied by a shift in threshold voltage, inversion layer capacitance and a high density of midgap interface traps. Dopant diffusion in polycrystalline silicon and through the thin gate oxides was also studied by secondary ion mass spectroscopy (SIMS). The findings of the SIMS analysis correlate well with the electrical measurements. The results indicate that significant boron diffusion can occur through an 80A oxide if an RTA temperature higher than 1000° C is used. On the other hand, both SIMS and electrical measurements suggest that As penetration into the substrate is negligible even at temperatures as high as 1100° C.     

Nonmelt laser annealing of 5-KeV and 1-KeV boron-implanted silicon

Nonmelt laser annealing has been investigated for the formation of ultrashallow, heavily doped regions. With the correct lasing and implant conditions, the process can be used to form ultrashallow, heavily doped junctions in boron-implanted silicon. Laser energy in the nonmelt regime has been supplied to the silicon surface at a ramp rate greater than 10 ¹⁰°C/s. This rapid ramp rate will help decrease dopant diffusion while supplying enough energy to the surface to produce dopant activation. High-dose, nonamorphizing boron implants at a dose of 10¹⁵ ions/cm² and energies of 5 KeV and 1 KeV are annealed with a 308-nm excimer laser. Subsequent rapid thermal anneals are used to study the effect of laser annealing as a pretreatment. SIMS, sheet resistance and mobility data have been measured for all annealing and implant conditions. For the 5-KeV implants, the 308-nm nonmelt laser preanneal results in increased diffusion. However, for the 1-KeV implant processed with ten laser pulses, the SIMS profile shows that no measurable diffusion has occurred, yet a sheet resistance of 420 Ω/sq was produced     

硅中离子注入掺杂原子的增强扩散系数的分布

本文利用离子注入掺杂原子在热退火过程中的浓度分布SIMS数据计算了增强扩散系数的深度分布.这种增强扩散系数的分布反映了离子注入产生的损伤和缺陷分布及热运动特性     

Dopant diffusion studies and free carrier lifetimes during rapid thermal processing of semiconductors

Rapid thermal processing of semiconductors involves significant photonic and subsequent thermal excitation. In the past, photonic excitation during rapid thermal annealing had been speculated to lead to significant enhancement of dopant diffusion or activation. In this work we present some experimental results indicating the absence of any such enhancement at high temperatures (1000–1050°C) which most often are employed during the metal-oxide–semiconductor device processing. The implanted dopant (boron, arsenic or phosphorus) movement in silicon during different rapid thermal annealing conditions was studied using secondary ion mass spectroscopy (SIMS) technique. To understand the effect of point defects in controlling the diffusion process, the concentrations of charged and neutral point defects were calculated as a function of carrier concentration using previously published defect-carrier relations. The dependence of free carrier concentration on lattice perturbation parameters such as impurities and temperature was formulated and used in calculating carrier lifetimes (τ) in silicon. We qualitatively analyze two competing reactions, (i) the phonon release at the defect sites and (ii) the Auger electron process due to many electron interactions, to explain the apparent absence of any enhanced dopant diffusion. In our analyses, we obtain a highest free carrier lifetime of about 442 ns in the case of low dose (1e13/cm²) implanted sample during the transient stage (700°C) of the dopant activation cycle. The corresponding smallest (17 fs) free carrier lifetime was obtained for the high dose implanted sample (dopants already activated) at 1000°C, the steady state part of an extended anneal cycle. Based on the detailed free carrier lifetime analyses, we suggest that any enhanced dopant activation or diffusion, at the best, may occur only at very low temperatures in the samples implanted with low doses of dopant atoms.     

硅中注入硼的异常扩散

<正> 浅结制备是超大规模集成电路发展的关键技术之一。硅中硼、磷等杂质注入,在退火时发生异常扩散,使浅结的控制困难。异常扩散是一个瞬态快速扩散过程。对于硼,在退火开始时,杂质分布尾部推移极快,随之减慢,恢复正常扩散。这一过程用衰变时间表征。     

Influence of the initial boron doping level on the boron atom distribution arising as a result of heat treatment in silicon implanted with boron ions

The dependence of the boron distribution on the initial boron concentration in the range (1–9)×10¹⁹ cm⁻³ was investigated by secondary-ion mass-spectrometry (SIMS) after heat treatment of boron-ion implanted silicon at 900 °C. It was found that when the initial boron concentration exceeds the solubility limit at the annealing temperature used, two additional peaks arise in the boron concentration profiles at the boundaries of the ion-implantation disordered region. It is suggested that their appearance in these regions at high doping levels is due to clustering of excess interstitial impurity atoms not built into the lattice sites following displacement of boron atoms from the lattice sites by intrinsic interstitials that leave the disordered region. Fiz. Tekh. Poluprovodn. 32, 417–420 (April 1998)     

硅中离子注入硼的异常扩散

本工作用不同的Si~+预注入能量,改变注入损伤分布与离子注入硼杂质分布的相对位置,观察快速热退火中注入损伤对硼异常扩散的影响.结果表明,引起注入硼异常扩散的是点缺陷,而不是硼间隙原子的快扩散.而注入损伤中的点缺陷和簇团分解释放的点缺陷是驱动硼异常扩散的因素之一.如果注入损伤形成了扩展缺陷,那么扩展缺陷重构和分解将发射点缺陷,这是驱动硼异常扩散的另一个因素.     

Activation annealing of ultra-low-energy implanted boron in silicon: a study combining experiment and process modeling

We present our investigations on the clustering, diffusion and electrical activation of ultra-low-energy (<1 keV) implanted boron in crystalline silicon during annealing in the temperature range between 900 and 1200°C. We show that during the initial stage of the annealing, boron is bound to non-diffusing clusters. The formation and the dissolution in dependence of temperature are analyzed. We determine a dissolution time constant with an average activation energy of 2.3 eV. The depth-dependent electrical activation as measured by spreading resistance profiling was used to determine the fraction of electrically active boron. From the analysis of the time dependence of the electrically active fraction, a time constant was determined which is faster compared with the boron cluster dissolution time constant in the investigated temperature range. We have simulated the annealing process to demonstrate the influence of the boron clustering on the broadening of the boron profile.     

A model for boron short time annealing after ion implantation

A simulation model is proposed for boron diffusion in silicon. It is especially useful for analyzing the short time annealing process subsequent to ion implantation. This model takes into account nonequilibrium diffusion and reactions of point defects and defect-dopant pairs, considering their charge states, and the dopant inactivation by the introduction of a boron clustering reaction. It is assumed that the boron-interstitial-silicon pair (BI) is a dominant diffusion species that contributes to the total boron diffusion. A primary model parameter, the binding energy of BI, is determined and used to reproduce the equilibrium gaseous source diffusion data. Using a single set of reasonable parameter values, the model covers not only the equilibrium diffusion conditions, from intrinsic, but also the nonequilibrium postimplantation diffusion. Experimental boro distribution profiles can be accurately reproduced. It is shown that the time constant for the BI dissociation reaction rules the transient behavior of boron diffusion enhancement during postimplantation annealing     

Use of transient enhanced diffusion to tailor boron out-diffusion

We report experimental results demonstrating the use of transient enhanced diffusion (TED) caused by silicon implant for “tuning” boron out-diffusion. The effect was measured as a function of the silicon implant dose and anneal temperature, and a range of boron junction depth movement from almost none up to 81 nm was observed with increasing TED at 750°C. The diffused profiles could be approximated by using a modified solubility limit model to describe the enhanced boron diffusion and clustering. However, by using a more sophisticated continuum model based on atomistic calculations, excellent agreement with the measured profiles could he obtained. In addition, the fit to the measured data yields the fraction of boron present in BI₂ precursor clusters after silicon implant as a function of the silicon implant dose. Two possible applications of the TED “tuning” are discussed, with device simulations which show that the effect is sufficiently large to tune the base width of a bipolar device from being depleted to that suitable for a high performance device     

磷（P31^＋）注入Si的快速退火电特性

本文报告了P_(31)~+离子注入Si中快速退火的电特性研究结果。采用高精度四探针测量了P_(31)~+注入层在不同注入剂量下,薄层电阻与退火温度和退火时间的关系。采用自动电化学测量仪PN-4200,测量了P_(31)~+离子注入Si中的载流子剖面分布。