In this paper, we expand on our earlier work1,2 reporting the use of high sensitivity DUV transmission metrology as a
means for detection of progressive transmission loss on mask and pellicle surfaces. We also report a use case for
incoming reticle qualification based on DUV transmission uniformity.
Traditional inspection systems rely on algorithms to locate discrete defects greater than a threshold size (typically >
100nm), or printing a wafer and then looking for repeating defects using wafer inspection and SEM review. These
types of defect inspection do not have the ability to detect transmission degradation at the low levels where it begins
to impact yield. There are numerous mechanisms for transmission degradation, including haze in its early, thin film
form, electric-field induced field migration, and pellicle degradation.
During the early development of haze, it behaves as a surface film which reduces 193nm transmission and requires
compensation by scanner dose. The film forms in a non-uniform fashion, resulting from non-uniformity of exposure
on the pattern side due to varying dose passing through the attenuating layers. As this non-uniformity evolves, there
is a gradual loss of wafer critical dimension uniformity (CDU) due to a degradation of the exposure dose
homogeneity. Electric-field induced migration also appears to manifest as a non-uniform transmission loss,
typically presenting with a radial signature.
In this paper we present evidence that a DUV transmission measurement system, GalileTM, is capable of detecting
low levels of transmission loss, prior to CDU related yield loss or the appearance of printing defects. Galileo is an
advanced DUV transmission metrology system which utilizes a wide-band, incoherent light source and non-imaging
optics to achieve sensitivities to transmission changes of less than 0.1%. Due to its very high SNR, it has a fast
MAM time of less than 1 sec per point, measuring a full field mask in as little as 30 minutes. A flexible user
interface enables users to easily define measurement recipes, threshold sensitivities, and time-based tracking of
transmission degradation. The system measures through pellicle under better than class 1 clean air conditions.