A method for improving the thickness uniformity of the silicon layer of a silicon-on-insulator structure 108 comprising a handle wafer, the silicon layer, and a dielectric layer between the handle wafer and the silicon layer, the silicon layer having a cleaved surface 5 defining an outer surface of the structure, the method comprising: providing a CVD reactor 300 including a processing chamber 302, a rotatable susceptor 304 positioned within the processing chamber 302 and configured to support a silicon-on-insulator structure, a gas injection port 308 disposed at one end of the processing chamber 302, and a gas discharge port 310 disposed at an opposite end of the processing chamber 302; positioning the silicon-on-insulator structure 108 on the rotatable susceptor 304 within the processing chamber 302 of the CVD reactor 300 with the cleaved surface 5 facing upwards; determining a desired removal map profile for the silicon layer of the silicon-on-insulator structure; determining a set of process parameters for use in a gaseous etchant smoothing process based on the desired removal map profile, wherein determining a set of process parameters includes selecting, based on the desired removal map profile, a first set of process parameters for a first portion of a smoothing process and a second set of process parameters for a second portion of the smoothing process from a plurality of pre-determined process parameter sets, each pre-determined process parameter set associated with a pre-determined removal map profile; and selectively removing material from the silicon layer according to the removal map profile by contacting the cleaved surface 5 with a gaseous etchant in the CVD reactor 300 using the first set of process parameters for the first portion of the smoothing process and the second set of process parameters for the second portion of the smoothing process, thus performing the smoothing process on the cleaved surface 5 using the determined set of process parameters, wherein the determined set of process parameters includes a susceptor rotation speed and a position of the silicon-on-insulator structure 108 relative to a rotational axis of the susceptor 304, wherein positioning the silicon-on-insulator structure 108 on the rotatable susceptor 304 includes positioning the silicon-on-insulator structure 108 at the position, and wherein selectively removing material from the silicon layer includes controlling a rotation speed of the susceptor 304 according to the determined set of process parameters. providing a CVD reactor 300 including a processing chamber 302, a rotatable susceptor 304 positioned within the processing chamber 302 and configured to support a silicon-on-insulator structure, a gas injection port 308 disposed at one end of the processing chamber 302, and a gas discharge port 310 disposed at an opposite end of the processing chamber 302; positioning the silicon-on-insulator structure 108 on the rotatable susceptor 304 within the processing chamber 302 of the CVD reactor 300 with the cleaved surface 5 facing upwards; determining a desired removal map profile for the silicon layer of the silicon-on-insulator structure; determining a set of process parameters for use in a gaseous etchant smoothing process based on the desired removal map profile, wherein determining a set of process parameters includes selecting, based on the desired removal map profile, a first set of process parameters for a first portion of a smoothing process and a second set of process parameters for a second portion of the smoothing process from a plurality of pre-determined process parameter sets, each pre-determined process parameter set associated with a pre-determined removal map profile; and selectively removing material from the silicon layer according to the removal map profile by contacting the cleaved surface 5 with a gaseous etchant in the CVD reactor 300 using the first set of process parameters for the first portion of the smoothing process and the second set of process parameters for the second portion of the smoothing process, thus performing the smoothing process on the cleaved surface 5 using the determined set of process parameters, wherein the determined set of process parameters includes a susceptor rotation speed and a position of the silicon-on-insulator structure 108 relative to a rotational axis of the susceptor 304, wherein positioning the silicon-on-insulator structure 108 on the rotatable susceptor 304 includes positioning the silicon-on-insulator structure 108 at the position, and wherein selectively removing material from the silicon layer includes controlling a rotation speed of the susceptor 304 according to the determined set of process parameters.