A deposition unit for a thin film deposition system, comprising: a machine base; one or more of deposition heads, each deposition head including: an output face having a plurality of gas openings; a mounting face opposite to and parallel with the output face, the mounting face including a plurality of deposition head gas ports; and gas passages connecting each of the gas openings on the output face to one of the deposition head gas ports; and a gas manifold rigidly attached to the machine base including: an attachment face having one or more interface regions, each interface region being configured to interface with the mounting face of a deposition head and including a plurality of manifold gas ports in positions corresponding to the deposition head gas ports; a plurality of gas connections; and gas passages connecting each of the manifold gas ports to one of the gas connections; wherein each of the deposition heads is rigidly fastened to the gas manifold in an interface region with sealing elements positioned between the manifold gas ports and the deposition head gas ports; wherein the mounting face of each deposition head and the attachment face of the gas manifold include alignment features for aligning each deposition head with the interface region of the gas manifold; wherein the gas openings are gas slots extending in a cross-track direction on the output faces of the deposition heads, and wherein a motion controller moves a substrate relative to the deposition heads in an in-track direction which is perpendicular to the cross-track direction; and wherein the interface regions corresponding to two of the deposition heads are adjacent to each other in the cross-track direction such that the adjacent deposition heads have abutting ends, wherein the abutting ends of the adjacent deposition heads include interlocking structures having an alternating sequence of protrusions and recesses such that the protrusions on the abutting end on one of the adjacent deposition heads fit into the recesses on the abutting end on the other of the adjacent deposition heads. a machine base; one or more of deposition heads, each deposition head including: an output face having a plurality of gas openings; a mounting face opposite to and parallel with the output face, the mounting face including a plurality of deposition head gas ports; and gas passages connecting each of the gas openings on the output face to one of the deposition head gas ports; and an output face having a plurality of gas openings; a mounting face opposite to and parallel with the output face, the mounting face including a plurality of deposition head gas ports; and gas passages connecting each of the gas openings on the output face to one of the deposition head gas ports; and a gas manifold rigidly attached to the machine base including: an attachment face having one or more interface regions, each interface region being configured to interface with the mounting face of a deposition head and including a plurality of manifold gas ports in positions corresponding to the deposition head gas ports; a plurality of gas connections; and gas passages connecting each of the manifold gas ports to one of the gas connections; an attachment face having one or more interface regions, each interface region being configured to interface with the mounting face of a deposition head and including a plurality of manifold gas ports in positions corresponding to the deposition head gas ports; a plurality of gas connections; and gas passages connecting each of the manifold gas ports to one of the gas connections; wherein each of the deposition heads is rigidly fastened to the gas manifold in an interface region with sealing elements positioned between the manifold gas ports and the deposition head gas ports; wherein the mounting face of each deposition head and the attachment face of the gas manifold include alignment features for aligning each deposition head with the interface region of the gas manifold; wherein the gas openings are gas slots extending in a cross-track direction on the output faces of the deposition heads, and wherein a motion controller moves a substrate relative to the deposition heads in an in-track direction which is perpendicular to the cross-track direction; and wherein the interface regions corresponding to two of the deposition heads are adjacent to each other in the cross-track direction such that the adjacent deposition heads have abutting ends, wherein the abutting ends of the adjacent deposition heads include interlocking structures having an alternating sequence of protrusions and recesses such that the protrusions on the abutting end on one of the adjacent deposition heads fit into the recesses on the abutting end on the other of the adjacent deposition heads.