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The present invention relates to etch chemistry and methods for the etching of silicon substrates. The method is particularly useful for deep trench etching of silicon substrates and produces a trench having a high aspect ratio. In this type of deep trench etching, control of the profile of the trench is addressed by the etch chemistry disclosed herein. The etchant described in the present invention comprises silicon and a halogen component, and may be gaseous, liquid or solid. The etchant disclosed is also substantially free of hydrogen and carbon.