Kinetics of the reaction of intrinsic and N-type silicon with atomic and molecular bromine and chlorine
Creators: name: Walker, Zane Harry
Publisher: name: University of British Columbia
Year: 2011

description: The etching of silicon by atomic and molecular chlorine and bromine was studied as a function of etchant pressure and reaction temperature. Various types of silicon were employed in the etching experiments including intrinsic and n-type polycrystalline silicon as well as the (100) face of intrinsic single crystal silicon. The pressures of Cl₂ and Br₂ varied from 0.1 to 30 Torr and the partial pressure of Cl and Br atoms was between 0.08 and 0.2 Torr. Temperatures of between 365 and 600°C were required for CI₂ and Br₂ etching, while lower temperatures of 25 to 470°C were sufficient for the more reactive Cl and Br atoms. The reaction between silicon and Br atoms was shown to be first order with respect to the partial pressure of atoms and a first order dependence was assumed for Cl atom etching. The rate constants were determined for the Cl and Br atom etching of intrinsic and n-type polycrystalline silicon, with a dopant concentration of 5x10¹⁸ atoms cm⁻³. The reactivity of Cl atoms with n-type silicon was approximately 90 times greater than with intrinsic silicon. This enhancement in reaction rate is primarily due to an increase in the preexponential factor in k₁, with the activation enthalpy for the process remaining unchanged at approximately 28 kJ mol⁻¹. For Br atom etching, the reaction rate for the n-type silicon was over 300 times greater than for intrinsic silicon and was characterized by activation enthalpies of 55 and 63 kJ mol⁻¹ respectively. The enhancement in reactivity can also be attributed principally to an increase in the preexponential factor. The preexponential factors for the rate constants are larger than those expected, based on the collision frequencies of Cl and Br atoms. This is interpreted as evidence for a preadsorption step in these reactions. The reactions of silicon with CI₂ and Br₂ were found to display a complex pressure dependence. The etch rates varied linearly with (etchant pressure)¹′² and the intercepts from a linear regression of the data were slightly negative. To account for the half order pressure dependencies observed in these etching reactions, a reversible dissociative adsorption mechanism is proposed whereby Br₂ (or CI₂) is dissociatively adsorbed, in a reversible reaction, onto the silicon surface yielding two atoms bound to the surface. This step is then followed by a first order reaction leading to the formation of a species which is either gaseous product or some precursor which forms that product in a subsequent non rate-determining step. From the slopes of etch rate versus (pressure)¹′² plots, composite half order rate constants were calculated and from the intercepts it was possible to evaluate the rate constant for dissociative adsorption of the halogen molecules. At high etchant pressures, where the reaction was half order with respect to Br₂ (or CI₂), a half order "composite" rate constant characterized the etching reaction. Values for the half order rate constant were determined for a number of wafers at various temperatures. From the temperature dependencies of these rate constants, activation enthalpies of 131±8 and 116±7 kJ mol⁻¹ were calculated for Br₂ and CI₂ etching of intrinsic polycrystalline silicon respectively. A value of 121±7 kJ mol⁻¹ was deterrnined for the Br₂ etching of silicon (100). Higher reaction rates were observed for the etching of n-type polycrystalline silicon, with greater enhancements observed for Br₂ relative to Cl₂ etching. The enhancements in etch rates were found to be principally due to a lower activation enthalpy for the half order rate constant. An activation enthalpy for the composite rate constant of 82±3 kJ mol⁻¹ was determined for Cl₂ etching of n-type silicon with a dopant atom concentration of 5x10¹⁸ atoms cm⁻³. Br₂ etching of the same wafer yielded an activation enthalpy of 86±3 kJ mol⁻¹. At low pressures, the reaction becomes first order and the temperature dependence of the corresponding first order rate constant yielded activation enthalpies of 109 and 83 kJ mol⁻¹ for intrinsic and n-type polycrystalline silicon.