Stacked semiconductor device and method of forming same

A method of forming a semiconductor device, the method comprising:

• forming a semiconductor structure protruding above a substrate and a gate structure extending across the semiconductor structure, wherein the semiconductor structure comprises a layer stack, wherein the layer stack comprises a lower channel layer formed of a first material comprising SiGex, an intermediate layer formed of a second material comprising SiGey and an upper channel layer formed of a third material comprising SiGez, wherein 0≤x<z<y;
• etching back end surfaces of the intermediate layer exposed at opposite sides of the gate structure to form recesses in the semiconductor structure at opposite sides of the gate structure;
• depositing first spacers in the recesses, wherein the first spacers are formed on the intermediate layer end surfaces;
• forming oxidized end portions defining second spacers on end surfaces of the upper channel layer, wherein forming the oxidized end portions comprises oxidizing end portions of the upper channel layer at opposite sides of the gate structure using an oxidization process adapted to cause a rate of oxidation of the third material which is greater than a rate of oxidation of the first material, wherein the first spacers cover the intermediate layer end surfaces during the oxidation process;
• epitaxially growing lower source/drain regions on end surfaces of the lower channel layer exposed at opposite sides of the gate structure, while the first spacers cover the intermediate layer end surfaces and the second spacers cover the end surfaces of the upper channel layer;
• removing the second spacers to expose the end surfaces of the upper channel layer;
• epitaxially growing upper source/drain regions on the end surfaces of the upper channel layer, subsequent to removing the second spacers and while the first spacers cover the intermediate layer end surfaces;
• forming lower source/drain contacts for the lower source/drain regions and/or forming upper source/drain contacts for the upper source/drain regions prior to epitaxially growing the upper source/drain regions;
• and
• forming an insulating capping layer on at least one of the lower source/drain contacts prior to forming the upper source/drain contacts.