US Patent 11489044 Nanosheet transistor bottom isolation

Semiconductor devices and methods of forming the same include forming slanted dielectric structures from a first dielectric material on a substrate, with gaps between adjacent slanted dielectric structures. A first semiconductor layer is grown from the substrate, using a first semiconductor material, including a lower portion that fills the gaps and an upper portion above the first dielectric material. The lower portion of the first semiconductor layer is replaced with additional dielectric material.