an improved method and apparatus for coating semiconductor substrates with organic photoresist polymers by extruding a ribbon of photoresist in a spiral pattern which covers the entire top surface of the wafer . the invention provides a more uniform photoresist layer and is much more efficient than are current methods in the use of expensive photoresist solutions . a wafer is mounted on a chuck , aligned horizontally and oriented upward . an extrusion head is positioned adjacent to the outer edge of the wafer and above the top surface of the wafer with an extrusion slot aligned radially with respect to the wafer . the wafer is rotated and the extrusion head moved radially toward the center of the wafer while photoresist is extruded out the extrusion slot . the rotation rate of the wafer and the radial speed of the extrusion head are controlled so that the tangential velocity of the extrusion head with respect to the rotating wafer is a constant .