Polycrystalline Si Layer Research Articles

Boron Vapour Phase Doping of Silicon for Bipolar Device Applications

Boron vapour-phase doping (VPD) has been investigated as a possible processing tool for defect-free applications in shallow, vertical and deep junctions in devices. From literature and experiments it has become clear that silicon boride (SiB3) layers can form during VPD. Thermodynamical calculations and analytical techniques were used to establish boride-free doping conditions. SiB3 formation and decomposition have been investigated as a function of temperature, time and the presence of O-containing compounds in the gas system. The hydrogen anneal after boron VPD not only decomposes boride layers on the Si surface, but it is also a means to control dose and depth of a boron junction. Boride-enhanced boron diffusion with buried SiB3 layers has been demonstrated. Effects of patterned Si windows in masking layers and polycrystalline Si layers on the VPD process are described. Application of the boron VPD technique to make a p-type base in bipolar npn transistors shows excellent device and high-frequency characteristics.

Dopant diffusion and segregation in semiconductor heterostructures: Part III, diffusion of Si into GaAs

We have mentioned previously that in the third part of the present series of papers, a variety of n-doping associated phenomena will be treated. Instead, we have decided that this paper, in which the subject treated is diffusion of Si into GaAs, shall be the third paper of the series. This choice is arrived at because this subject is a most relevent heterostructure problem, and also because of space and timing considerations. The main n-type dopant Si in GaAs is amphoteric which may be incorporated as shallow donor species SiGa+ and as shallow acceptor species SiAs-. The solubility of SiAs- is much lower than that of SiGa+ except at very high Si concentration levels. Hence, a severe electrical self-compensation occurs at very high Si concentrations. In this study we have modeled the Si distribution process in GaAs by assuming that the diffusing species is SiGa+ which will convert into SiAs- in accordance with their solubilities and that the point defect species governing the diffusion of SiGa+ are triply-negatively-charged Ga vacancies VGa3-. The outstanding features of the Si indiffusion profiles near the Si/GaAs interface have been quantitatively explained for the first time. Deposited on the GaAs crystal surface, the Si source material is a polycrystalline Si layer which may be undoped or n+-doped using As or P. Without the use of an As vapor phase in the ambient, the As- and P-doped source materials effectively render the GaAs crystals into an As-rich composition, which leads to a much more efficient Si indiffusion process than for the case of using undoped source materials which maintains the GaAs crystals in a relatively As-poor condition. The source material and the GaAs crystal together form a heterostructure with its junction influencing the electron distribution in the region, which, in turn, affects the Si indiffusion process prominently.

Epitaxial Realignment of In-Situ Doped Polycrystalline Silicon for Advanced BiCMOS Technologies

AbstractIndustrial feasibility of an in-situ-doped (ISD) polycrystalline Si process using chemical vapor deposition for advanced BiCMOS technologies is presented. ISD As-doped amorphous and polycrystalline Si layers have been deposited on Si substrates at 610°C and 660°C, respectively, with the deposition rate varying from 120 to 128Å /minute. Samples are compared on the basis of having been subjected to a substrate preclean prior to deposition using an HF solution and an in-situ H2 bake. TEM micrographs reveal the presence of a thin (10-15 Å) native oxide at the deposited layer/substrate interface for samples not precleaned. This is confirmed for both the amorphous and polycrystalline Si depositions. However, for the 610°C-deposited samples given the substrate preclean, a polycrystalline structure with partial epitaxial layer growth is observed. Twins and stacking faults are found at the poly Si/single crystal Si interface, causing interfacial roughness. Post-deposition annealing of the Si films typically generates grain growth, but RBS-channeling characterization of the annealed Si provides evidence of some recrystallization, the extent of which is affected by the original growth condition. Analysis shows that the amorphous deposition at 610°C results in a mixture of epitaxial and polycrystalline Si. Epitaxial realignment of the polycrystalline Si film by post deposition annealing can result in significantly improved device performance.

Surface, stress, and impurity effects on room-temperature migration of ion-beam-generated point defects

We have analyzed the perturbations produced by recombination at surface, trapping at impurities, and stress fields on the room-temperature migration properties of point defects in Si. A stack consisting of a Si oxide (or a Si nitride) and a polycrystalline Si layer, deposited on Si samples, was patterned to open 2-μm-wide, 10-μm-spaced stripes. A 40-keV Si implantation to fluences of 1×1012–5×1013/cm3, performed through this mask at room temperature, was used to inject point defects into the bulk of the wafer. After implants, defect-induced dopant deactivation, in the cross section orthogonal to the direction of the stripes, has been monitored using two-dimensional spreading resistance profilometry. It has been found that, in highly pure epitaxial Si samples, dopant deactivation extends in depth to several microns beyond the region (∼0.4 μm) directly modified by the ions. Furthermore, the two-dimensional deactivation profiles exhibit a strong recess at the surface and a significant anisotropy, being markedly elongated in the lateral direction. Analysis of the data shows that long-range migration of defects is interrupted by trapping at impurities (C and O) or recombination at the surface, characterized by a coefficient of ∼100 μm−1. Moreover, the lateral elongation of the profiles is tentatively explained assuming an anisotropy in the defect diffusivity tensor produced by the strain field under the mask.

Analysis of thin film polysilicon on graphite substrates deposited in a thermal CVD system

Thin films of polycrystalline silicon of 10–30 μm were grown on graphite substrates. The deposition experiments were conducted in a horizontal, atmospheric pressure RTCVD reactor from 800 to 1200°C employing the precursor trichlorosilane (TCS) and the dopant trichloroborine (TCB) diluted in a hydrogen carrier gas. The polycrystalline Si layers were analyzed by means of Nomarski microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and by high-resolution transmission electron microscopy (HRTEM). The electrical properties of the deposited films were evaluated by spreading resistance profiler (SRP), by Van der Pauw, and by microwave-deduced photoconductivity decay (MW–PCD) measurements. Deposition rates of the as-grown Si films were in the range of 1–3 μm min −1 resulting in grain sizes varying from 0.1 to 6 μm. The grain sizes of the deposited layer increases when the temperature or/and the input partial pressure of the reactant were raised. Additionally, preferred growth orientation conditions were found. Thus, crystallites grown between 900 and 1100°C showed a maximum diffraction peak for the (2 2 0)-orientation. A highly resistive region was also observed at the silicon–graphite interface due to the formation of SiC agglomerates.

Structural and electrical properties of silicon epitaxial layers grown by LPE and CVD on identical polycrystalline substrates

We compare structural and electrical properties of polycrystalline Si layers grown by chemical vapour deposition (CVD) and liquid-phase epitaxy (LPE) on multicrystalline, cast silicon substrates with similar grain boundary structures. Time-resolved microwave conductivity shows a higher minority carrier lifetime in LPE than in CVD layers; the calculated diffusion lengths are up to three times the layer thickness for LPE-grown layers. After etching the samples in Secco or Sirtl solution, we measured in the p-type Si epitaxial LPE and CVD layers practically at the same dislocation density as in the same areas of the substrate. Electron-beam-induced current measurements reveal a low recombination strength of grain boundaries and dislocations in the LPE-grown layers compared to those of the CVD layers. Transmission electron microscope investigations indicate that the lower recombination strength at the grain boundaries of the LPE layers is due to a lower density of grain boundary dislocations.

Study of lateral non-uniformity as a function of junction depth in ultra-shallow junctions and its effect on leakage behavior in as-deposited polycrystalline Si and amorphous Si diodes

Heavily implanted polycrystalline Si films are finding increasing applications as solid diffusion sources, for example, in the formation of ultra-shallow junctions in elevated source/drain metal-oxide-semiconductor field effect transistors and polycrystalline Si emitter bipolar junction transistors. For these applications the dopants are implanted into the polycrystalline Si layer and subsequently diffused into the underlying Si substrate. The diffusion behavior, is however, determined by the evolution of the polycrystalline Si grain microstructure, A large final grain size could lead to laterally more non-uniform and hence leakier junctions. These non-uniformities are lesser for high thermal budget anneals (1100–1150°C, 30 s) than for lower thermal budget anneals (950°C, 30 s) because any doping inhomogeneities in the Si substrate get smeared out as the junction becomes deeper. The junction depth, however, cannot be increased indefinitely from a device viewpoint and a trade-off between leakage current and junction depth has to be made. In this paper, we will study the effect of grain microstructure of ion-implanted as-deposited amorphous Si and as-deposited polycrystalline Si diffusion sources on the diffusion of As and B in the Si substrate, correlate it to the electrical properties of the ultra-shallow junctions formed using this technique.

Raman Spectroscopy Analysis of Nitrogen Ion Implantation in Silicon and Correlation with Transmission Electron Microscopy

ABSTRACTIn this work Si samples implanted with nitrogen (N+ or N2+) at a dose of 1017 cm−2 are characterized by Raman spectroscopy and cross section transmission electron microscopy (XTEM). The correlation between the Raman spectra obtained with different excitation wavelengths and XTEM observations allows to determine the structural features related to the layers contributing to the total spectra. The evolution of these features with the annealing treatments (up to 1150°C) is studied. The results obtained show, after the annealing treatment at the highest temperature, the presence of silicon nitride precipitates in the silicon subsurface region, and the formation of a nitrogen rich polycrystalline Si layer with Si3N4 grains. The Raman spectra from the subsurface region show a remaining shift of -0.15 cm−1 when compared to the spectra from unimplanted Si. This shift, together with the similar shape of both Raman lines, suggests the presence in this region of an average tensile stress of 37.5 MPa.

Selective epitaxial growth with oxide-polycrystalline silicon-oxide masks by rapid thermal processing chemical vapor deposition

We have used rapid thermal processing chemical vapor deposition for Si selective epitaxial growth using a mask consisting of a sandwich structure of SiO2 on doped polycrystalline Si on SiO2. Lateral polycrystalline Si growth from the sidewalls of the polycrystalline Si layer was also observed and resulted in polycrystalline ‘‘bumps’’ along the mask sidewalls. Otherwise, the epitaxial Si layer was defect-free.

Characterization of Si/Si3N4 Layered Structures Grown By Physical Vapor Deposition/Ion Beam Assisted Deposition

ABSTRACTLayered silicon/silicon nitride structures were fabricated on (100) Si with 0.1 mTorr of N2 background pressure. Evaporated Si (PVD) layers were alternated with layers grown by simultaneous nitrogen ion bombardment and Si evaporation (IBAD). The ion to vapor flux ratios were chosen to yield stoichiometric Si3N4 (XN=0.57) at 150, 500, 1000 and 1500 eV and varied at a constant energy of 500 eV to yield sub- and superstoichiometric Si1-xNx. The microstructures were examined by cross-sectional and plan-view TEM; composition of the layers was deteiTnined by RBS. Films grown at low substrate temperatures were amorphous. Stoichiometric and superstoichiometric compositions showed a cellular (5 nm) structure in the amorphous IBAD layers. In addition, the IBAD Si1-xNx layers showed variations in contrast parallel to the substrate interface, indicative of compositional fluctuations on a scale of <20 nm. The IBAD layers were dense, but the PVD-Si layers contained voids that were aligned with the direction of the impingement of the evaporant on the substrate surface. A high substrate temperature (750°C) yielded polycrystalline Si layers and phase-ordering in the Si1-xNx layers.

High-Resolution Electron Microscopy of Process-Induced Defects in Silicon

AbstractSeveral examples of defect characterization which are of topical inter-est to Si device technology are presented. The results obtained by high-reso-lution electron microscopy (HREM) are discussed in context with the actual defect problem. - Reinvestigation of platelike defects in Si produced by reactive ion etching in hydrogen containing plasmas (CHF3) shows that some of the {111} platelets are of extrinsic nature. The defects contain probably both constituents from the plasma and Si-interstitials created by the impin-ging ions. - A high dose As-implantation forms an amorphous Si surface layer which has a sharply curved amorphous/crystalline (a/c)-interface below the implantation mask edge. Annealing at 900°C leads to formation of vacancy-type defects under the mask edge. This is due to the different regrowth rates on the various lattice planes of the curved a/c-interface. - Metal-silicide pre-cipitation at the SiO2/Si interface reduces the breakdown field strength of thin oxides. The main failure mechanism observed in model experiments is the thinning of the oxide layer thickness. - Additional x-ray peaks which are frequently observed in low-pressure chemical vapour deposited (625 7deg;C) po-lycrystalline Si layers arise from a diamond hexagonal Si phase. Small inclu-sions and bands of this phase were for the first time directly observed by HREM.

Characterization of short-range leakage currents in undoped polycrystalline Si by means of capacitance-voltage measurement

Metal-oxide-semiconductor capacitors with undoped polycrystalline Si (polysilicon) electrodes were studied and a model was developed to explain the voltage and frequency dependence of the capacitance and serial resistance. The model assumes random distribution of leaky paths in the polycrystalline Si layer which supply the charge to the polysilicon-oxide interface.

Interaction of TiSi2 layers with polycrystalline Si

Interactions of silicide films with undoped polycrystalline layers of Si grown by chemical vapor deposition at 630 °C were investigated by MeV He ion backscattering spectrometry, scanning electron microscopy, and transmission electron microscopy. For TiSi2, heat treatment in vacuum at temperatures above 850 °C results in erosion of the polycrystalline Si layer and growth of Si crystallites in the silicide film. The same phenomenon is observed for NiSi, Pd2Si, and CrSi2 at temperatures above one-half of melting point of the corresponding silicide.

Properties of Ion‐Implanted Polycrystalline Si Layers Subjected to Rapid Thermal Annealing

Polycrystalline silicon films have been deposited on thermally oxidized wafers. The films have been implanted with As, B, or and annealed with a Varian IA‐200 rapid thermal annealer. The system uses infrared radiation from a resistively heated sheet of graphite to heat the entire wafer to temperatures in excess of 1000°C for times on the order of a few seconds. The effects on sheet resistance, sheet carrier concentration, and mobility due to exposure time, heater temperature, dopant species, and resultant grain size are discussed. Higher temperatures and longer exposure times produce the lowest sheet resistances. These results are compared to CW laser and conventional furnace‐annealed films. A substantial loss of As occurs during the anneal unless the films are capped with . Results on B and implanted films indicate no boron loss has occurred during the anneal. The As and B uniformly distribute throughout the film when exposed to anneal conditions that produce very little diffusion in single‐crystal Si. This implies that As and B diffusion is much faster in polycrystalline Si than in single crystal under these anneal conditions. The anneal causes both components of a bimodal grain size distribution to grow significantly until grain growth is slowed by geometrical constraints. Increased grain size results in a substantial increase in mobility. The grain growth has been modeled by an interfacial energy‐driven mechanism.

Electrical and structural properties of pulse laser-annealed polycrystalline silicon films

Doped epitaxial films of Si on single-crystal high-resistivity Si substrates have been prepared using ion implantation and Q-switched ruby laser annealing of LPCVD polycrystalline Si layers. Films, doped with B or As in the range 1017to 5 × 1020cm-3were studied by the measurement of their resistivities, Hall mobilities, and doping density profiles. The good film quality achieved permitted the fabrication of p-channel MOS transistors which, through measurements of threshold voltage and transconductance, yielded additional data on the surface mobility and the integrity of the Si-SiO 2 interface. The electrical properties of the films compared favorably with those of similarly doped single-crystal material, and transmission electron microscopy was used to confirm the good structural quality of the epitaxial growth.

Phosphorus distribution in TaSi2 films by diffusion from a polycrystalline silicon layer

Secondary ion mass spectroscopy was used to measure phosphorus concentration profiles in TaSi2 thin films deposited on P doped polycrystalline Si/SiO2/Si wafers by cosputtering. Fast redistribution of P in TaSi2 takes place by its up-diffusion from the polycrystalline Si layer. Radioactive assay of 32P obtained by neutron activation confirmed the fast-diffusion character of P in TaSi2. From preliminary results the temperature dependence of the diffusion coefficients over the temperature range of 664 °C-913 °C can be expressed by the following relations: Dc=6.44×10−10 exp[−0.61/(kT)] and Da=2.0×10−8 exp[−0.76/(kT)], where Dc and Da represent diffusion in the silicide film near and away from the surface, respectively.

Optical characterization of thin silicon films deposited by CVD and annealed by pulsed laser

Abstract Polycrystalline Si layers 100 to 800 nm thick have been deposited on Si single crystal substrates by CVD and annealed with a Q-switched ruby laser at energies up to 1.5 J/cm2. The optical characteristics of these layers have been measured by SEM and ellipsometry. The results can be attributed to a change in surface roughness with film thickness and laser energy.

Electrically-alterable memory using a dual electron injector structure

A novel type of electrically-alterable memory which uses the phenomenon of enhanced electron injection into SiO 2 from Si-rich SiO 2 to charge or discharge a floating polycrystalline Si storage layer in a metal-oxide-semiconductor field-effect-transistor is described. This non-volatile memory differs from others using floating polycrystalline Si in the charging or discharging process. This improvement is accomplished by using a chemically-vapor-deposited stack of Si-rich-SiO 2 -SiO 2 -Si-rich-SiO 2 between the floating polycrystalline Si layer and the control gate electrode. This device is capable of being written or erased in 5 msec at voltages of ≤ 16 V and in 2 µsec at voltages ≤ 23 V with excellent charge retention.

TEM, AES and XPS Studies of Si Layer on Buried SiO2 Layer Formed by High-Dose Oxygen Ion-Implantation

Depth distributions of crystallographic and chemical properties in oxygen ion-implanted Si layers have been studied by TEM, AES and XPS. Oxygen ions (16O+) were implanted into (100)-Si wafers at 150 keV to a dose of 1.2×1018/cm2. The dose rate was about 15 µA/cm2. The implanted oxygen forms a buried SiO2 layer after annealing. The Si layer on this buried SiO2 layer is not completely amorphized by implantation, and changes after annealing to a single crystal Si layer at the surface region and a deeper polycrystalline Si layer. A twin layer is formed at the interface region between these two layers. The polycrystalline Si layer consists of a mixture of Si and SiO2.

Deposition and Properties of Polycrystalline Si for Solar Cells

A tin coated graphite substrate was used for growing polycrystalline Si layers by CVD in the SiCl4–H2 system. A mean grain size of poly-Si layers on the tin coated substrate was 50–100 µm, compared with several microns on an uncoated substrate. Moreover the tin intermediate layer seems to play an important role to make a flat surface. The crystalline size could be further enlarged to 100–500 µm by introduction of melting process before normal CVD. A Schottky barrier solar cell using those poly-Si layers showed the open-circuit voltage of 0.25 V, the shortcircuit current density of 12.8 mA/cm2, fill factor of 59% and conversion efficiency of 2.0% under the solar intensity of 92.8 mW/cm2.