As exposure wavelengths decrease from 248 nm to 193, 157, and even 13 nm (EUV), small process defects can cause collapse of the lithographic process window near the limits of resolution, particularly for the gate and contact structures in high- performance devices. Such sensitivity poses a challenge for lithography process module control. In this work, we show that yield loss can be caused by a combination of macro, micro, CD, and overlay defects. A defect is defined as any yield- affecting process variation. Each defect, regardless of cause, is assumed to have a specific 'kill potential.' The accuracy of the lithographic yield model can be improved by identifying those defects with the highest kill potential or, more importantly, those that pose the highest economic risk. Such economic considerations have led us to develop a simple heuristic model for understanding sampling strategies in defect metrology and for linking metrology capability to yield and profitability.
Data collection is implicit in experimentation on and control of semiconductor manufacturing processes, but is often ignored for its effect on the outcome of testing. Improper sampling can increase the cost of testing, either by testing too much, or by increasing the risk of reaching a wrong conclusion; the costs can be significantly more than the cost of measurement, in some cases approaching several million dollars. This paper reviews methods and patterns of sampling. Issues associated with the nested process characteristics of semiconductor manufacturing are discussed, with specific attention to typical distributions. Sample size effects and recommendations are reviewed. The cost of uncertainty associated with sampling is examined. The paper also includes definitions of conunonly-used statistics and tests.
Optical lithography, when extended by phase shift mask technology and modified illumination techniques, is a promising technology for sub-half-micron devices. Modified illumination can improve the resolution limit and depth of focus, but the imaging profile is changed, with pattern type, direction, and density having an effect on the result. The uniformity of the illumination system also differs according to aperture type. Because lens distortion may be affected by the aerial image and structure of illumination optics, we can expect that a modified illumination system may affect lens distortion and overlay accuracy in a real process. A comparison of changes in overlay and lens distortion was done for different illumination conditions. Focus was varied for each combination. As a result, we can observe the variation of overlay error in a modified illumination system relative to the conventional system. To use modified illumination in sub-half-micron processes distortion error must be reduced.