A new type of chrome-on-glass (COG) photomask defect was observed in 2006. Absorber material migrated into vias on
dark field masks, partially obscuring the incident 193nm light and thereby causing the imaged photoresist to be
underexposed. Through detailed characterization of new and defective photomasks and their histories it was determined
that the migration is not caused by any unusual line events or faulty mask handling procedures. Rather, it is an inevitable
result of mask use under specific conditions. Four essential elements have been identified: the presence of Cr, 193nm
light exposure, charge, and water vapor and their roles elucidated through modeling studies and existing literature. We
have reproduced Cr migration in the laboratory, demonstrating that these four elements are necessary and sufficient for
this type of defect to occur. The only way to avoid Cr migration is to avoid reactions with water vapor.
Previous work has shown that photomask blank flatness as well as photomask patterning and pelliclization all play an
important role in finished photomask flatness. Other work has shown that pellicle mounting techniques and pellicle
adhesives play a role as well. In this work, a comparison of the impact of various pellicle types, frame flatness, frame
shape and pellicle mounting tools on final photomask flatness will be shown. Pellicles with various adhesives, frame
shapes and flatness were mounted on blanks and completed photomasks using several mounting tools and the pellicle
induced flatness change was measured. These data will be discussed with the objective of demonstrating the effects of
pellicle type and mounting tool on photomask flatness.
Existing cleaning technology using sulfuric acid based chemistry has served the mask industry quite well over the years. However, the existence of residue on mask surfaces is becoming more and more of a problem at the high energy wavelengths used in lithography tool for wafer manufacturing. This is evident by the emergence of sub-pellicle defect growth and backside hazing issues. A large source of residual contamination on the surface of masks is from the mask manufacturing process, particularly the cleaning portion involving sulfuric acid. Cleaning strategies can be developed that eliminate the use of sulfuric acid in the cleaning process for advanced photomasks and alternative processes can be used for cleaning masks at various stages of the manufacturing process. Implementation of these new technologies into manufacturing will be discussed as will the resulting improvements, advantages, and disadvantages over pre-existing mask cleaning processes.
Photomask manufacturing automation has lagged semiconductor wafer process automation development. In a highly complex wafer fab, data automation and advanced process control techniques are required. Given limited demand, photomask equipment manufacturers have not unilaterally made the investment necessary to implement a standardized data flow protocol which would allow photomask process automation enhancements. Surveys indicate that significant photomask yield loss is attributed to manufacturing and administrative errors. By working with individual photomask equipment suppliers and through internal application development, IBM photomask manufacturing automation has eliminated nearly all manufacturing and administrative errors. Some examples of processes that were automated over the past several years are process routing selection, photomask yield prediction, linkage of photomask blank to mask build part number, automated recipe and setup download and dispositioning criteria, statistical analysis of process parameters and defect density, defect information management and automated data upload for photomask and wafer engineering use. Project highlights will be discussed and the case will be made for standardization of data flow protocol and for further photomask process automation improvements.
Photomask pellicles play an important role in determining final photomask flatness, which is important to photomask optical performance. This study explores the impact of the pellicle frame flatness and pellicle-to-mask adhesive on photomask flatness. In addition, the change in mask flatness as a function of time after pellicle mounting is studied. Implications of these results on photomask manufacture and photolithography are discussed.
Focus on Design for Manufacturing (DFM) in semiconductor device design has increased as semiconductor manufacturing technology has become more complex. Many of the techniques developed to improve wafer yield and manufacturability can also be applied to the photomask manufacturing process. For example, for the last several technology nodes, semiconductor manufacturers have known that pattern density and uniformity can have significant impact on wafer processes such as etching and chemical mechanical polishing. Photomask manufacturing can also be impacted by pattern density and its uniformity.
Some of these DFM practices can be beneficial if applied directly to photomask manufacturing while some of them can make photomask manufacturing significantly more difficult. Optical proximity correction (OPC), which involves convoluting the design shape to account for optical, physical and chemical processes, is increasingly required to support advanced lithography; some of the operational parameters of the OPC, such as the fragmentation run length, challenge mask resolution capability, image fidelity, defect inspection, mask repair, and dimensional metrology of photomasks. Sub-resolution assist features (SRAFs), which are utilized to create robust wafer lithography are often the most challenging mask features to create. The size and placement of SRAFs on photomasks are factors that impact photomask manufacturability in terms of image resolution, inspection, and dispositioning criteria. As OPC and other DFM processes become more widely deployed in an effort to make robust wafer manufacturing processes, the photomask maker needs to be involved to evaluate the implications to photomask manufacturing and assist in optimizing these DFM procedures to maximally benefit both the photomask and semiconductor manufacturing processes.
For years there has been a mismatch between the photomask inspection wavelength and the usage conditions. While the non-actinic inspection has been a source for concern, there has been essentially no evidence that a defect "escaped" the mask production process due to the inspection mismatch. This paper will describe the discovery of one such defect, as well as the diagnostic and inspection techniques used to identify the location, analyze the composition, and determine the source of the printed wafer defect.
Conventional mask inspection techniques revealed no defects, however an actinic Aerial Image Metrology System (AIMS) revealed a 1.5 mm region on the mask with up to 59% transmission reduction at 193 nm. Further diagnostics demonstrated a strong wavelength dependence which accounted for the near invisibility of the defect at I line (365 nm) or even DUV (248 nm) wavelengths, which had 0% and 5% respective transmission reductions. Using some creative imaging techniques via AIMS tool and modeling, the defect was deduced to have a three dimensional Gaussian absorption character, with total width approximately 1.5 mm. Several non-destructive diagnostic techniques were developed to determine the composition and location of the defect within the substrate. These results will be described in addition to identifying methods for ensuring product quality in the absence of actinic inspection.
With ever increasing linewidth challenges per technology changes, the mask manufacturing process becomes more and more difficult. The challenges can be separated into two categories: image size and defects. Mask inspection detects hard defects most likely caused somewhere in the mask manufacturing process. Defect partitioning highlights the hard defects sources. They range from pre-exposure mask blank handling to the cleanliness of the process tools. A test vehicle was designed to allow for mask manufacturing defect partitioning via a die-to-die inspection tool. The process changes implemented range from pre-write mask handling to tool modifications. The methodology used to determine the process induced defects and the yield gains by making the necessary process changes will be presented.
Improvements in mask making techniques and metrology strategies have been required to satisfy the requirements of the 90nm technology node. With decreasing k1 and increasing MEF, critical dimension uniformity and defect specifications have faced severely tightened requirements. Many of the mask making process enhancements inspired by the 90nm node can be retrofitted into the 130nm node which improves mask quality as well as wafer-level performance. Mask critical dimension uniformity improvements directly impact wafer across chip linewidth variation which results in significantly improved chip performance. Specific examples of 130nm chip performance improvement will be discussed. Mask critical dimension and defect density improvements also result in improved mask yield and reduced mask costs. Driving 90nm mask process learning back into 130nm mask production significantly improves 130nm performance. Close interaction with the wafer lithography team allows focus on critical process window improvements for both the mask maker and wafer lithographer and allows rapid implementation of high-end process learning into older technologies.
Device performance and functionality can be impacted by many factors, both physical and electrical. Close interaction between the lithographer and mask maker is useful in the deconvolution of the mask contributions to device speed and functionality. Across plate image size variation, linearity, orientation and proximity effects (both local and global) influence the Across Chip Linewidth Variation (ACLV). ACLV, in turn, has a strong correlation to overall device performance. Several situations in which integrated circuit functionality and performance were correlated to mask systematics will be presented along with resolution of the described issues. Methodologies for separating the mask components from the wafer level process components will also be discussed. Mask specifications are often derived by simply scaling the previous technology, rather than basing the specifications on technical requirements. A methodology will be derived which links technological device specifications and the anticipated mask exposure conditions to the required mask specifications.
KRS-XE is a chemically amplified resist developed to enable electron-beam lithography for mask making at the 100nm node. This material has been shown to provide an excellent process window for mask manufacturing at this node. Characterization of this material using both 50keV raster and 75keV vector scan e-beam exposure systems will be presented. A higher sensitivity version of this material has been developed specifically for a vector, shaped beam 50keV application. Initial mask manufacturing results for this higher sensitivity version of KRS-XE will be presented for 75keV. In addition, recent developments using KRS-XE formulations modified to achieve high sensitivity and improved etch resistance will be discussed.
The reduction of post-develop defects in photomask making is significantly more critical than in wafer processing. While wafers can afford to experience some level of defect density, photomasks are required to be defect free. Defect density learning in photomask making is expensive and time-consuming given the material and exposure time costs. In a wafer fab, it is much easier to run factorial experiments to get large amounts of data in a short amount of time. Some photomask making and wafer processing defect generation mechanisms are the same. Here a study of the formation of resist material residues during develop will be compared between photomask and wafer processing. Wafer processing experience will provide insight into photomask post-develop defect formation. Several options for the elimination of this defect type will be discussed. Differences in implementation strategies between photomask makers and wafer lithographers will also be discussed.