Abstract: A semiconductor processing apparatus is disclosed. The semiconductor processing apparatus comprises a process chamber configured to receive a plurality of substrates; at least one accumulator vessel in fluid communication with the process chamber, configured to store a precursor in gas form; and at least two solid state precursor storage vessels, each being configured to receive a precursor in gas form, to cause said received gas form precursor to be converted to a solid state inside the respective solid state precursor storage vessel, and to cause said solid state precursor to be converted to gas phase. Each of the at least two solid state precursor storage vessels are in fluid communication with the at least one accumulator vessel so as to allow provision of precursor in gas form to the accumulator vessel. The at least one accumulator vessel comprises an actuatable element configured to cause an internal volume of the at least one accumulator vessel to be changed upon actuation.