Jyothi Singh

In situ laser diagnostic studies of plasma-generated particulate contamination

Laser light scattering measurements show that certain silicon etching plasmas produce a significant amount of in situ, particulate contamination. The particles are suspended at the sheath boundaries. Simultaneous measurement of plasma negative ions by the use of two‐photon laser‐induced fluorescence technique suggests that the particles are negatively charged and so are electrostatically trapped at the sheath boundaries. The parametric conditions for particle formation and growth in the plasma are identified. A mechanism for nucleation and growth is suggested involving formation of plasma negative ions from etch products, ion clustering with plasma species, and cluster growth into particles with electrostatic suspension and trapping. The particles drop onto the wafer when the rf is turned off. Implications for dry process technology are discussed.

Role of the chamber wall in low‐pressure high‐density etching plasmas

Ultraviolet‐adsorption spectroscopy has been used to examine how the chamber wall affects the concentration of gas‐phase reactants in high‐density etching plasmas. This technique was employed to detect CF2 in an inductively coupled discharge used for the selective etching of silicon dioxide relative to silicon nitride and polycrystalline silicon (polysilicon) films. In plasmas containing C2F6 and CF4, the concentration of CF2 depends strongly on the applied power and operating pressure as well as the amount of polymer on the walls of the chamber. Changes in the conditioning of the chamber during the etch process cause significant variations in the concentration of CF2 in the discharge. The selectivity of etching SiO2 relative to Si3N4 films closely follows the concentration of CF2 under a variety of plasma operating conditions. The ability to measure a fundamental plasma characteristic that reflects the level of conditioning of the chamber is an important step in the real‐time monitoring of a reactor para...

Ultraviolet absorption spectroscopy for the detection of CF2 in high‐density plasmas

Ultraviolet absorption spectroscopy has been employed to measure the density of CF2 in a high‐density discharge used for the selective etching of silicon dioxide relative to silicon films. In a plasma containing C2F4H2 and CF4, CF2 accounts for more than 10% of the gas in the reactor. The level of CF2 in the discharge is strongly dependent on the operating pressure and the applied power. A comparison of the intensity of optical emission from CF*2 with the ultraviolet absorption signal and microwave interferometry measurements shows that the optical emission signal is limited more by the electron density than by the availability of ground state CF2. The UV absorption signal for CF2 closely follows the selectivity of etching SiO2 to silicon. Both neutral fluorocarbon fragments and ions are believed to play a role in the deposition of fluorocarbon films which give rise to this selectivity. The ability to measure a fundamental plasma parameter which closely correlates with etch selectivity is an important ste...

Insitu infrared diagnostics of particle forming etch plasmas

In situ Fourier transform infrared absorption techniques are employed to characterize the gas‐phase plasma species and etch products present in halocarbon containing plasmas which produce particles. A correlation is demonstrated between the distribution of these species and the extent of particle formation as measured by laser light scattering. The effects of the presence of silicon and the addition of oxygen on both the plasma species distribution and the degree of light scattering are also characterized. Additionally, x‐ray photoelectron spectroscopy (XPS) and infrared (IR) microscopic techniques are employed to determine the chemical composition of the particulate material which is found on the silicon wafer after etching.

Plasma‐enhanced photoemission in argon discharges: Signal characterization and silicon doping effects

The recently reported technique of plasma‐enhanced photoemission (PEP) provides capability for in situ, noncontact and rapid monitoring of the surface of a wafer during plasma processing. In this work the technique is characterized as a function of laser power, rf power, laser synchronization with the rf cycle, and wafer doping. At higher laser power the signal is observed to be space‐charge limited. Synchronization of the laser pulse with the 13.6‐MHz rf plasma shows a strong dependence on the rf phase. The PEP signal is seen to be influenced by the type and density of the wafer dopant. Results are compared for low and high laser powers. These trends are interpreted in terms of the photoemissive properties of the semiconductor surface.

In-situ measurements of radicals and particles in a selective silicon oxide etching plasma

Selective silicon oxide etching plasmas typically use fluorocarbon feedgases under polymerizing conditions to provide etch selectivity and anisotropy. This work reports in—situ measurements of radical and particle distributions in the CF4-CHF3 plasma, one of the commonly used feedgas mixtures for selective oxide etching relative to silicon. Laser-induced—fluorescence (LIF) is used to monitor trends in the concentration of CF radicals in the ground electronic and vibrational state. Mie scattering observed in the perpendicular direction gives information about particles in the plasma. The effects of addition of Ar diluent on radical concentrations and etch rates are also reported. The dependence of plasma particle contamination on the proportion of CHF3 in the feedgas mix and its correlation with gas phase radical concentration is measured. It is also shown that an interrupted plasma discharge results in a significant reduction in the gas phase particle formation as compared to a continuous discharge.