Deterministic feedback blender

A method of continuously mixing chemical fluids in an inline blender to produce a mixture having a concentration of the chemical fluids within 0.22% w/w of a target concentration, the method comprising:

• a) providing a HF, a NH4F and a H2O input, wherein the NH4F comprises free NH3 or HF, and wherein the H2O input is at a known and fixed flow rate,
• b) forming a mixture comprising HF, NH4F and H2O from the inputs of step a), wherein steps a) and b) are performed continuously to deliver a mixture suitable for use as a buffered oxide etch solution,
• c) only monitoring the mixture formed by step b) for a feedback control of one or more rates of input in the providing step a), wherein the monitoring step c) comprises measuring both a temperature compensated pH and a temperature compensated conductivity of the mixture formed in step b),
• d) comparing the temperature compensated pH and the temperature compensated conductivity to a chemical operating curve, the chemical operating curve representing a relationship of a pH of the mixture of step b) to a molar concentration of HF in the mixture, as a function of a molarity of the NH4F of the mixture of step b) determined from the temperature compensated conductivity,
• e) adjusting one or more rates of input of step a) based on the comparison in step d) to thereby maintain a concentration of HF and NH4F in the mixture of step b) to within 0.22% w/w of the target concentration of HF and NH4F.