An atomic layer deposition method is disclosed for preparing polypeptides. The method comprises providing a solid-phase support comprising a reactive amine monolayer in an atomic layer deposition (ALD) chamber. The solid-phase support is contacted with a first protected amino acid substituted with a protecting group by atomic layer deposition, wherein the protecting group is bonded to a non-side chain amino group of the protected amino acid. A carboxylic acid group of the first protected amino acid is reacted with the reactive amine monolayer, thereby coupling the first protected amino acid to the solid-phase support to produce a coupled-product.