Detection of progressive transmission loss due to haze with Galileo mask DUV transmittance mapping based on non imaging optics

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 su rface 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 transmissi on measurement system, Galileo™, is capable of detecting low levels of transmission loss, prior to CDU related yield lo ss or the appearance of prin ting 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 throug h pellicle under better than class 1 clean air conditions. Keywords: Transmission, DUV, 193nm, Haze, Electric-field induced migration (EFM), Photomask, Non imaging optics, Dose, CDU INTRODUCTION A key feature of a photomask is the tr ansmission property of its many surfaces . Typical advanced 6" masks have 4 surfaces: back side quartz, front side pattern, inside pellicle and outside pellicle. In addition to the surfaces themselves the bulk of the transparent materials- fused silica which is the mate rial out of which the blank quartz is made and fluoropolymer out of which the pellicle is made, have specific optical transmission properties which contribute to the total Tr properties of the mask. Also surface coating materials like Cr, MoSi and anti-reflective