NOVELTY - A composite membrane (30) forming method involves loading a polyimide substrate (100) into a chamber and performing a cycle step in the chamber to form a single aluminum oxide layer (10). Another cycle step is performed in the chamber to form a single hafnium oxide layer (20). The cycle steps involves introducing aluminum element containing reactant (A), oxygen element containing reactant (B) and hafnium element containing third reactant into the chamber, and removing reactant (A), reactant (B) and third reactant from the chamber.    USE - Method for forming a composite membrane on a polyimide substrate by an atomic layer deposition process, which is utilized for a gas-barrier packaging membrane for LED display, solar cell, or thin-film transistor (all claimed).    ADVANTAGE - The method enables easy formation of the composite membrane on the substrate in short duration, with improved stability.    DETAILED DESCRIPTION - A composite membrane forming method involves loading a polyimide substrate into a chamber and performing a cycle step (I) in the chamber to form a single aluminum oxide layer. Another cycle step (II) is performed in the chamber to form a single hafnium oxide layer. Cycle step (I) involves introducing an aluminum element containing a reactant (A) into the chamber, removing the reactant (A) from the chamber, and introducing an oxygen element containing a reactant (B) into the chamber, and removing the reactant (B) from the chamber. Cycle step (II) involves introducing an oxygen element containing reactant (B) into the chamber, removing the reactant (B) from the chamber, and introducing hafnium element containing a third reactant into the chamber, and removing the third reactant from the chamber. An INDEPENDENT CLAIM is also included for a composite membrane formed on a substrate by an atomic layer deposition process.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a composite membrane.    Single aluminum oxide layer (10)    Single hafnium oxide layer (20)    Composite membrane (30)    Polyimide substrate (100)