In one embodiment, a transfer robot for transferring a substrate includes a supporting means, a transfer robot arm including a first sub-robot arm and a second sub-robot arm arranged over the supporting means, an inner rail and an outer rail adjacent to the inner rail overlying the supporting means. The first sub-robot arm is adapted to move in a straight line motion along the inner rail and the second sub-robot arm is adapted to move in a straight line motion along the outer rail. The second sub-robot arm surrounds the first sub-robot arm.