N. P. Rao

Nanoparticle formation using a plasma expansion process

We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.

Focused nanoparticle-beam deposition of patterned microstructures

A method was developed for fabricating nanocrystalline microstructures. This method involves synthesizing nanoparticles in a thermal plasma expanded through a nozzle, and then focusing the nanoparticles to a collimated beam by means of aerodynamic lenses. High-aspect-ratio structures of silicon carbide and titanium were deposited on stationary substrates, and lines and two-dimensional patterns were deposited on translated substrates. Linewidths equalled approximately 50 μm. This approach allows the use of much larger nozzles than in previously developed micronozzle methods, and also allows size selection of the particles that are deposited.

Hypersonic impaction of ultrafine particles

The performance of a highly supersonic aerosol impactor as a size-discriminating instrument is explored experimentally with ultrafine particles having diameters as small as 51 A. A hypersonic jet is formed by expansion of a gas-particle mixture through an orifice of diameter dn, from a source region maintained at a pressure po into an evacuated region kept at a background pressure p1 several hundred times smaller than po. Facing the jet perpendicularly, at a variable distance, L, from the nozzle exit, is a bounce-free flat target plate which collects a fraction, E, of the incoming particles. The collection efficiency, E(dp, L), is determined on-line by using monodisperse charged particles of diameter dp and measuring the electrical current they transport to the conducting target plate, which is grounded through an electrometer. The impactor shows sharp separation efficiency curves E(dp, L) only when L is smaller than some critical distance L∗. For L > L∗, the curves E(L) become non-monotonic as a result of some gas dynamic reasons not fully understood yet. Provisionally, over the limited range of pressure ratios explored, it appears that L∗dn = 0.13 √(pop1). The size discrimination behaviour deteriorates also when Ldn becomes smaller than one, though this lower limit seems to be somewhat more flexible and geometry dependent than the upper one. Within the region 0.8 < Ldn < 0.13 √(pop1), E(dp, L) depends in a step-like manner on both these variables. The relatively sharp transition between E = 0 and E = 1 occurs roughly when the Stokes number in the impact region, S ∼ 32SoLdn, is approximately one, where So is a standard Stokes number based on the nozzle exit diameter, dn, and the gas sound speed, co, at source conditions. For particles sufficiently small to be in the free-molecule limit, So = 0.1983 ρpdpco2(dnpo), where ρp is their density. The fact that the fundamental variable S governing the impaction process is proportional to the product Ldp, makes these two factors conjugates of each other, and allows determining the diameter of an unknown monodisperse aerosol by measuring the value of L at which the capture efficiency E undergoes a step. This feature makes it possible to operate this impactor as an aerodynamic size spectrometer for ultrafine particles. With a pumping capacity of 500 l/min and an electrical noise level of 10−16 A, we have been able to ‘measure’ NaCl particles with a diameter as small as 51 A, and to discriminate between particle diameters only 3–5 A apart.

Nucleation and growth of aerosol in chemically reacting systems: A theoretical study of the near-collision-controlled regime

A theoretical treatment of the formation and growth of aerosols in systems where condensable molecules are generated at a constant rate is presented. Previous investigations of this type have focused either on collision-controlled (coagulation-limited) nucleation or on condensation /evaporation-controlled nucleation. In the latter case, classical nucleation theory has typically been used to determine particle formation rates, and coagulation of subcritical clusters is neglected. The present theory accounts for both coagulation and condensation / evaporation processes, and serves to bridge the gap between the two limiting cases. The aerosol population balance equations are cast in a nondimensional form and are solved numerically for the time-dependent size spectrum. A key aspect of this work is the identification of dimensionless parameters that have a significant influence on aerosol formation. The most important of these parameters is the evaporation parameter, E = , where N s is the saturation concentra...

Nanostructured materials production by hypersonic plasma particle deposition

Abstract We report on a new process for producing nanostructured materials, hypersonic plasma particle deposition (HPPD), wherein a thermal plasma seeded with vapor-phase precursors is supersonically expanded through a nozzle to nucleate ultrafine particles, which are then deposited by hypersonic impaction onto a temperature-controlled substrate. Results from preliminary experiments aimed at synthesizing nanostructured silicon are presented.

Detection of aluminum particles during the chemical vapor deposition of aluminum films using tertiaryamine complexes of alane (AlH3)

Two methods of analyzing particles were interfaced to a low pressure chemical vapor deposition (CVD) reactor to evaluate whether or not particles were formed in the gas phase during the growth of aluminum films using tertiaryamine complexes of alane. A laser light scattering particle counter was used to detect large (>200 nm) particles in real time and established that the appearance of particles corresponded to the flow of precursor into the CVD reactor. A particle impaction system was used to collect particles (>20 nm) for analysis using analytical electron microscopy and electron diffraction. This established that the particles were crystalline aluminum and that the particle sizes ranged from 20–700 nm. The median size was 85 nm.

Synthesis of nanophase silicon, carbon, and silicon carbide powders using a plasma expansion process

Nanophase powders of Si, C, and SiC with narrow size distributions are synthesized by dissociating reactants in a dc are plasma and quenching the hot gases in a subsonic nozzle expansion. The plasma is characterized by calorimetric energy balances and the powders by on-line aerosol measurcment techniques and conventional materials analysis. The measured nozzle quench rate is about 5 × 10 6 K/s. The generated particles have number mean diameters of about 10 nm or less, with Si forming relatively dense, coalesced particles, while SiC forms highly aggregated particles. Our data suggest that SiC particle formation is initiated by the nucleation of small silicon particles.

PARTICLE BEAM MASS SPECTROMETER MEASUREMENTS OF PARTICLE FORMATION DURING LOW PRESSURE CHEMICAL VAPOR DEPOSITION OF POLYSILICON AND SIO2 FILMS

We have recently built a particle beam mass spectrometer (PBMS) for measuring ultrafine particle size distributions (0.005–0.25 μm) at low pressures (≳100 mTorr). The PBMS is being used to study nucleation and growth in low pressure chemical vapor deposition processes relevant to the production of semiconductor devices. In this article, the function and performance of the PBMS is summarized, and results of measurements made while depositing polysilicon and silicon dioxide films in tube furnaces are discussed. Measurements made during deposition of polysilicon films showed that there was a critical reactor pressure below which particles were not present; this critical pressure varied in proportion to the residence time in the reactor, and was insensitive to reactor temperature. Above the critical pressure, however, the concentration of particles produced was sensitive to reactor temperature. The average particle size was in the 0.003–0.03‐μm‐diam range, with concentrations of ∼104 cm−3. In contrast, partic...

Investigation of particle formation during the plasma enhanced chemical vapor deposition of amorphous silicon, oxide, and nitride films

There is considerable interest in understanding particle formation in microelectronic fabrication processes since process generated particles are a major source of yield loss in the industry. In this work, particle formation in a plasma enhanced chemical vapor deposition process has been studied using a newly developed instrument—the particle beam mass spectrometer (PBMS)—capable of measuring number densities and size distributions of submicron particles in vacuum environments with pressures >50 mTorr. Experiments have been conducted during the deposition of amorphous silicon, oxide, and nitride films, and particle formation correlated with process parameters such as plasma power and substrate temperature. For the measurements reported, the PBMS has been operated in a downstream monitoring mode, i.e., the PBMS sampled gases from the reactor exhaust during the deposition. Particle formation was observed during the amorphous silicon and oxide runs, but not during the nitride experiments. For the processes i...

Model for ion‐induced nucleation based on properties of small ionic clusters

A method is presented for calculating ion‐induced nucleation rates. Whereas classical ion‐induced nucleation theory makes the approximation that small ionic clusters are charged droplets whose properties equal their values for the bulk liquid, the method presented gives a nucleation rate in the form of a summation over discrete cluster properties. The summation converges rapidly around the critical cluster size, which is often as small as a few atoms. This approach allows the direct utilization of experimental and/or computational data for cluster properties. Sample calculations are presented for nucleation of silicon particles via condensation of neutral silicon vapor onto silicon anions for conditions representative of microelectronics processing plasmas. As the temperature increases the predicted nucleation rates show a transition from the collision‐limited regime to the condensation–evaporation regime, where nucleation rates drop sharply with temperature. The value of the temperature where this occurs...