In this paper, we expand on our earlier work<sup>1,2</sup> 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, Galile<sup>TM</sup>, 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.
A key feature of a photomask is the transmission (Tr) property of its many surfaces. Typical advanced 6" masks have 4
surfaces: back side Quartz (Qz), 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 material out of which the blank Qz is made and
fluoropolymer out of which the pellicle is made, have specific optical Tr properties which contribute to the total Tr
properties of the mask. Also surface coating materials like Cr, MoSi and Anti Reflective (AR) coatings have their
specific Tr contributions. Figure 1 (see paper) shows a schematic drawing with all the different contributors to Tr loss in a
photomask exposure system. Overall the wafer printed pattern fidelity to the design depends both on the physical size of the etched lines and spaces
and on the Tr properties of the spaces and of the coating material in the lines.
Factors which may contribute to transmission deviations may be:
1. Virgin Qz raw material non homogeneity.
2. Contamination by haze growth on any of the surfaces (Qz, absorber, pellicle).
3. Contamination by metal and oxide ions absorbed in the Qz and adsorbed on the Qz surface during mask
4. Photochemical degradation of the pellicle and fused silica substrates.
5. Degradation of absorber thickness, particularly of MoSi, due to clean processes.
6. Other factors.
Accumulated contributions of all those factors can give rise to several percents of transmission variation. Every percent
of exposure dose change may result in 1-2 nm CD change on wafer depending on exposure and process conditions. All the above raise the need for an advanced transmission measurement system that will be able to measure transmission
at the exposure wavelength with sensitivities better than 0.1%, preferably better than 0.01% (100 ppm). Such systems are
not currently available.
It has been previously demonstrated that wafer CD uniformity can be improved via an ultrafast laser system. The
system provides local CD Control (CDC) by writing inside the bulk of photomasks.
Intra-field CD variation correction has been implemented effectively in mask-shops and fabs based on CD-SEM and Scatterometry (Optical CD or OCD) as the CD data source. Using wafer CD data allows correction of all wafer
field CD contributors at once, but does not allow correcting for mask CD signature alone. For mask shops attempting to
improve CDU of the mask regardless of the exposure tool, it is a better practice to use only mask CD data as the CD
In this study, we investigate the use of an aerial imaging system AIMS<sup>TM</sup>45-193i (AIMS45) as the mask CD data
source for the CDC process. In order to determine the predictive value of the AIMS45 as input to the CDC process,
we have created a programmed CD mask with both 45nm and 65nm node L/S and hole patterns. The programmed
CD mask has CD errors of up to 20nm in 2.5nm steps (4X). The programmed CD mask was measured by AIMS45,
defining the CDU map of the programmed CD mask. The CDU data was then used by Pixer CDC200<sup>TM</sup> to correct the
CDU and bring it back to a flat, almost ideal CDU.
In order to confirm that real CDU improvement on wafer had been achieved, the mask was printed before and after
CDC on an immersion scanner at IMEC and results of pre and post CD data were compared.
Advanced wafer fabs are currently fabricating devices with 90nm and 65nm design rules using 193nm lithography. To meet the challenges at these sub-wavelength technology nodes, mask designers are using a variety of resolution enhancement techniques (RETs) in lithography which require new methods of processing, inspecting and qualifying photomasks. As a result, reticle inspection tools need to be capable of detecting smaller defects on ever tighter critical dimensions and background patterns that are considerably more complicated than before. To meet the challenges of current and future technology nodes, a variety of new inspection modes have been developed on the KLA-Tencor Deep UV TeraScan reticle inspection tool. These new inspection modes include Reflected light (Die-to-Die and Die-to-Database) modes, a Transmitted light Tritone (Die-to-Database) mode for inspecting Embedded Attenuated Phase Shift Masks (EAPSMs) with chrome in the inspection area, as well as a STARlight2 (SL2) mode for contamination detection. The SL2 inspection mode is the natural successor to the STARlight contamination detection algorithm on the previous generation of KLA-Tencor reticle inspection tools. Each of the inspection modes comes with its own set of inspectability and sensitivity capabilities and therefore the selection and/or optimization of a mode can depend upon a number of factors. In this paper we will present the inspection modes that are available on the TeraScan platform and discuss the appropriate use cases for each of the modes, based on reticle type and the intended objectives of the inspection.
Alternating Phase Shift Mask (APSM) reticles is critical to achieve sub 0.1 um poly gate lithography. Intrinsic APSM image inbalance can be resolved with various methods such as isotropic etch and aperture sizing, where positional line-shift can be reduced to within 5nm of final CD target. Defect reduction of APSM fabrication is addressed with multiple-option strategy to achieve high manufacturing yield. After Develop Inspection (ADI) capability was demonstrated with partial and complete missing 180 deg apertures, detected at post-develop with correlation to Qz defect after dry etch. Feasibility of APSM inspection and repair was demonstrated with existing toolsets and critical gap versus APSM defect specification remained to be bridged.
Fabrication of 0.18 micrometers generation clearfield logic device photomask with plasma etch was compared with wet etch method in current 0.25 micrometers mask technology. Spatial consistency between the resist develop and plasma etch modules was critical to achieve < 25 nm CD rng manufacturable process. CD linearity for 0.6 to 3.0 micrometers lines and isolated-nested CD bias for 1.0 micrometers lines were both improved with the plasma etch process. Resist loading and proximity effect is critical for plasma etched clearfield mask and can account for up to 20 nm range of overall CD budget.