To Fill Small Features, Etch First
By Katherine Derbyshire
Smaller feature sizes mean higher aspect ratios. Since resistance increases as cross-section decreases, narrow metal lines must be thicker to maintain constant resistance. The difficulty in filling these features has sparked interest in conformal PVD and CVD deposition methods (See related article). Before a high aspect ratio feature can be filled, however, the etch system first has to create it.
These deeper features require longer etch times. Unfortunately, engineers can't simply increase the photoresist thickness to compensate. Most exposure tools have extremely narrow depth-of-focus at sub-250 nm feature sizes, so a thicker resist layer might fall outside the exposure window. Instead, Steve Rhoades, Lam Research Corp.'s (Fremont, CA) senior director of product management for dielectric etch, explained, etch processes need improved oxide resist selectivity to prevent photoresist erosion.
For similar reasons, uniform center-to-edge photoresist etching is even more important than oxide etch uniformity. High (50:1) selectivity between Si and SiO2 is routine, so overetching the bottom of a contact hole won't damage the underlying silicon. Non-uniform photoresist etching is a potentially serious problem, however. Combined with low selectivity, non-uniform etching can erode away the protective photoresist in some areas before the contact holes are cleared in others.
Lam addresses these issues in the company's new 9100PTX etch system (See related article). The new design doubles the spacing between the transformer-coupled plasma (TCP) coil and the electrode, Rhoades told Semiconductor Online, improving uniformity at high power. In turn, the higher power allows use of more selective polymerizing etch chemistries. At lower power, polymer buildup threatens to shut down the etch, but higher power eliminates the problem. Thus, the new design improves the tradeoff among etch rate, selectivity, and etch uniformity.
Consistent with the current austere industry climate, the 9100PTX design also adds removable chamber liners and other cost of ownership improvements. Lam estimates a one-third reduction in consumable costs.
For more information:
Lam Research Corporation
4650 Cushing Parkway
Fremont, CA 94538-6470
Tel: 510 659 0200