The invention relates to a method making a compact chemical reactor with high aspect ratio cavities comprising the steps: forming at least one intermediate assembly, consisting of: at least one frame comprising: a process layer, a perimeter barrier and at least one low aspect ratio cavity; at least one frame comprising: a process layer, two perimeter barriers, and at least two low aspect ratio cavities disposed on a second process layer; at least one layer comprising a perimeter barrier formed on one side of a process layer with at least one low aspect ratio cavity; or combinations of (i) and (iii) joined together; joining at least two intermediate assemblies together to create a compact chemical reactor with at least one high aspect ratio cavity; and joining the compact chemical reactor with at least one high aspect ratio cavity to two reactant plenums to facilitate a transport process between the reactant plenums and the process layers.