Optical Etch‐Rate Monitoring Using Active Device Areas: Lateral Interference Effects

Abstract

The techn ique of optical (laser) etch-rate moni tor ing is prov ing useful for integrated circuit manufacture and pro-cess deve lopment. This techn ique detects the change in film th ickness dur ing etch ing by moni tor ing the ref lectance of the film on the integrated circuit wafer. When the ref lectance is mon i tored over a region of the wafer conta in ing both etched and unetched areas, the results are affected by lateral interference, wh ich is inter ference between the l ight ref lected f rom the di f ferent regions. This lateral in ter ference has only a minor effect when less than 30 % of the region being mon i to red is covered by an etch-res istant layer (typical for metal- type l i thography levels), but it can cause severe distort ions when a larger fract ion of the region is covered by the resist. The effects of lateral in ter ference have been ex-amined us ing numer ica l s imulat ions. The results are in good agreement with exper imenta l etch-rate mon i to r signals from the etch ing of pat terns wi th resist coverage rang ing f rom 10 to 80 % of the total area. As integrated circuit d imens ions become smal ler and as the var ious conduct ing or insulat ing films used in these circuits become th inner, the to lerances on the etch pro-cesses for these fi lms become more severe. The need for careful mon i tor ing of these etch processes can be met by