Mass Transport in Atomic Layer Deposition Carrier Gas Reactors

A detailed mode l for the operation of a f low type atomic layer deposition (ALD) reactor is developed and tested with the growth of tanta lum pentoxide thin films. The mode l incorporates evaporation, diffusion, and convection phenomena in mass transport and a kinetic description of the surface reactions and film growth. The theory gives a quantitative estimate for the deposition rate and thickness uniformity of the film. The necessary conditions for an ALD operation are discussed. The experiments support the predictions provided a pure ALD mode is prevailing. The method for depositing thin-film material one layer at a time by applying two alternating precursor compounds was introduced in the mid-1970s. 1 The original name for the technique was atomic layer epitaxy (ALE) 2 but when an amorphous substrate is used, a more specific term atomic layer deposition (ALD) 3 is now preferred, although many other appellations have been employed. The ALD or ALE has been demonstrated in various reac-tor systems. 4 The only class of equipment put to use in industrial semiconductor device production 2' ~ is a chemical vapor deposition (CVD) type apparatus and this type of reactor is commercially available. 5 This CVD type ALD, sometimes called traveling wave reactor, ~is characterized by an inert carrier gas which transports the precursor molecules to the substrates, pre-vents their premature mixing, and purges the reaction products with excess reactants from the reaction chamber. The deposition of thin films involves complex mass-transport and kinetic processes. In conventional CVD reac-tors these have been thoroughly studied in literature s but the details related to the pulsing of reactants in ALD have not been addressed before. A complete understanding of the interrelationships between flow, diffusion, adsorption, desorption, and chemical reactions requires detailed x-perimental investigations of each individual deposition process. ~' ~ However, the general f ramework for the opera-tion of this type of ALD reactor can be developed in rela-tively simple terms. This reactor model ing and its experi-mental verification with a tanta lum pentoxide process are the subjects of this paper