The　flatness　of　semiconductor　substrates　is　an　important　parameter　for　evaluating　the　surface　quality　of　semiconductor　substrates.　However,　existing　technology　cannot　simultaneously　achieve　high　measurement　efficiency,　large-range　thickness　measurement,　and　nanometer-level　measurement　accuracy　in　the　thickness　measurement　of　semiconductor　substrates.　To　solve　the　problems,　we　propose　to　apply　the　method　that　combines　spectral-domain　optical　coherence　tomography　(SD-OCT)　with　the　Hanning-windowed　energy　centrobaric　method　(HnWECM)　to　measure　the　thickness　of　semiconductor　substrates.　The　method　can　be　employed　in　the　full-chip　thickness　measurement　of　a　sapphire　substrate,　which　has　a　millimeter　measuring　range,　nanometer-level　precision,　and　a　sampling　rate　that　can　reach　up　to　80　kHz.　In　this　contribution,　we　measured　the　full-chip　thickness　map　of　a　sapphire　substrate　by　using　this　method　and　analyzed　the　machining　characteristics.　The　measurement　results　of　a　high-precision　mechanical　thickness　gauge,　which　is　widely　used　for　thickness　measurement　in　the　wafer　fabrication　process,　were　compared　with　the　proposed　method.　The　difference　between　these　two　methods　is　0.373%,　which　explains　the　accuracy　of　the　applied　method　to　some　extent.　The　results　of　10　sets　of　repeatability　experiments　on　250　measurement　points　show　that　the　maximum　relative　standard　deviation　(RSD)　at　this　point　is　0.0061%,　and　the　maximum　fluctuation　is　71.0　nm.　The　above　experimental　results　prove　that　this　method　can　achieve　the　high-precision　thickness　measurement　of　the　sapphire　substrate　and　is　of　great　significance　for　improving　the　surface　quality　detection　level　of　semiconductor　substrates.