A horizontal substrate carrier is provided, for example a carrier for holding semiconductor substrates during horizontal thermal processing. The horizontal substrate carrier has asymmetrically placed support rails. One side of the horizontal substrate carrier has no upper rail while the other side of the horizontal substrate carrier has an upper rail placed at a relatively high location, for example at an angular location of 60° or more, more preferably of 70° or more, and most preferably at 90°. The side without an upper rail may be used for robotic loading of the horizontal substrate carrier. In a preferred embodiment, only three rails are provided: one upper rail on one side and two lower rails. The use and placement of these three rails can hold the substrate in precise uniform locations, minimize substrate movement, and minimize particle generation, all while allowing for easy robotic access.