Robert Kiefer

DUV laser lithography for photomask fabrication

In the recent past significant work has been done to isolate and characterize suitable single layer Chemically Amplified Resist (CAR) systems for DUV printing applicable to photomask fabrication. This work is complicated by the inherent instability of most DUV CAR systems, particularly in air, showing unacceptable CD degradation over the normal photomask write time in today’s DUV mask pattern generators. The high reflectivity of most photomask substrates at DUV wavelengths, creating unacceptable standing waves in the photo resist profile, further compounds this problem. A single layer CAR system suitable for 90nm technology node mask fabrication with DUV printing has been characterized and optimized. Results of this optimization in terms of relevant mask making parameters will be detailed. Furthermore, comparison of the properties of this resist system to other commercially available systems, including FEP-171, will be shown. The pattern fidelity of DUV laser generated masks has been studied in considerable detail. A demonstration of the capabilities of the Etec Systems ALTA 4300 will be shown. The pattern fidelity achieved will be compared/contrasted to that achieved with today’s leading edge 50KeV vector scan e-beam systems. Advanced methods for modulating the DUV printed patterns’ fidelity will be detailed. Finally, the cost and cycle time implications of inserting the DUV laser pattern generator into the mask manufacturing flow will be discussed.

Next generation DUV ALTA mask patterning capabilities

The capability of the DUV ALTAÒ 4300 system has been extended by the development of two new optical subsystems: a 0.9 NA, 42X reduction lens and a high-bandwidth acousto-optic deflector based beam position and intensity correction servo. The PSM overlay performance has been improved by modifications to the software algorithms. Characterization data show improved resolution performance in line end shortening, through pitch CD bias and feature corner acuity. The AOD subsystem reduces stripe beam placement errors and random and systematic beam intensity errors. This has enabled local CD uniformity to be reduced to 4.3 nm (3σ) and global CD uniformity to be reduced to 5.8 nm (range/2). Second layer overlay performance is now 20 nm (max error). A split lot wafer evaluation has demonstrated the equivalence of unmodified ALTAÒ 4300 reticles to those printed on a 50 KeV electron beam system for a 130/110 nm device. Wafer lithography results show equivalent CD uniformity, depth of focus and pattern registration results.

Imaging properties of a leading-edge DUV laser generated photomask

In the recent past Deep Ultra Violet (DUV) Laser generated photomasks have gained widespread acceptance for critical and semi-critical applications in semi-conductor lithography. The advent of stable, highly capable, single-layer Chemically Amplified Resist (CAR) processes has made fabrication of this type of mask very robust in todays mask manufacturing environment. This platform affords mask makers benefits of the highly parallel architecture available in todays DUV Laser pattern generators - providing excellent cost and cycle time advantages when compared with alternative leading-edge processes using 50 KeV VSB e-beam systems. To date literature on this topic has focused mostly on characterization and optimization of DUV mask making processes. Meanwhile treatment of the resultant aerial image for critical litho applications has been largely ignored. In this paper details of the aerial image produced using DUV Laser generated photomasks will be detailed. Both 248nm and 193nm source printing with multiple types of illumination will be discussed. Details of a print test comparison performed on photomasks from two popular mask lithography platforms in use today; DUV, and 50 KeV VSB, will be documented. Finally, the most recent process improvements achieved in DUV Laser mask fabrication will be detailed. Special attention will be given to the impact of these enhancements on image quality.

Integration of an advanced laser writer into a manufacturing environment

The integration of an advanced high resolution laser lithography system for sub-0.50 micron technology, utilizing a 32 beam, 8 pass exposure writing strategy, in a merchant production mask-making environment, is explored. The ALTA-3000 tool represents the latest evolution of mask lithography platforms and, as such, has several advantages over the traditional e-beam or CORE exposure systems. For example, on a laser tool there is no requirement for a high vacuum system, and the complexities associated therein. Unexposed masks are not carried in individual cassettes, as in an e-beam system, and therefore system performance is not subjected to these error sources. Freedom from the potentially adverse affects of these sub- systems should appear as improvements on product performance in the areas of registration and defect control. The additional gray scales of the 32 beam architecture enable increased averaging of the individual beams, thus leading to improvements in print performance, relative to earlier versions of laser technology. The production capability of the ALTA-3000 is analyzed to determine what type of routine performance can be expected in terms of resolution, linewidth control, linearity, registration, defect control, and printing speed. The primary goal of this evaluation is to determine if the performance of the system is capable of meeting reticle requirements for 64 and 256 megabit chips and 5th and 6th generation microprocessors, which are expected to require 0.35 and 0.25 micron geometries on the wafer. The evaluation revealed the basic capability exists, however, additional work must be done in the area of registration control to improve the yields necessary to support high volume production for these technologies.

Pattern fidelity performance from next-generation DUV laser lithography on 65nm masks and wafers

Currently, the ALTA 4300 generation Deep Ultra-Violet (DUV) Laser tool is capable of printing critical and semi-critical photomasks for the 130nm and 90nm IC technology nodes. With improved optical elements, an improved objective lens, and a higher bandwidth datapath the capability of the tool has been dramatically enhanced. Both the tools diffractive optic element (DOE) and acousto-optic modulator (AOM) have been refined. Additionally, the tools 33x, 0.8NA objective lens has been replaced with a 42x, 0.9NA objective lens. Finally, the tools datapath enhancement has allowed critical level write times to remain less than four hours. Quantitative results of these enhancements will be detailed through reporting of critical feature resolution limits, CD uniformity control, and pattern placement accuracy on mask. Performance will be shown from masks printed pre- and post- hardware upgrade. Experimental results will show actual improvements. In this paper details of the aerial image created when printing wafers with DUV Laser generated photomasks pre- and post-upgrade will be shown. Both 248nm and 193nm source printing with multiple illumination conditions will be discussed. Details of a print test comparison performed on photomasks from each tool configuration will be documented. The print test comparison will include process window characterization from each mask type. A study of the inspectability of the DUV Laser generated photomasks will also be highlighted.

Print characterization of photomasks from next-generation deep-ultra-violet laser pattern generator

Currently, the ALTA 4300 generation DUV Laser tool is capable of printing critical and semi-critical photomasks for the 130nm and 90nm IC technology nodes. With improved optical elements, an improved objective lens, and a higher bandwidth data-path the capability of the tool has been dramatically enhanced - culminating with the introduction of the ALTA 4700. Both the ALTA 4300 system’s diffractive optic element (DOE) and acousto-optic modulator (AOM) have been refined. Additionally, the ALTA 4300 system’s 33x, 0.8NA objective lens has been replaced with a 42x, 0.9NA objective lens. Finally, the tool’s data-path has been enhanced to maintain the ALTA system’s superior write time on critical mask layers. Quantitative results of these enhancements will be detailed through reporting of critical feature resolution limits, CD uniformity control, and pattern placement accuracy on mask. Performance will be shown from masks printed pre- and post- hardware upgrade. Experimental results will be compared with theoretical calculations that show expected and actual improvements. In this paper details of the aerial image created when printing wafers with DUV Laser generated photomasks pre- and post- upgrade will be shown. 193nm print results will be shown with multiple illumination conditions. Details of a print test comparison performed on photomasks from each tool configuration will be documented. The print test comparison will include process window characterization from each mask type.

Enhancement of the image fidelity and pattern accuracy of a DUV laser generated photomask through next-generation hardware

Currently, the ALTA® 4300 generation DUV Laser system is capable of printing critical and semi-critical photomasks for the 130nm and 90nm IC technology nodes. With improved optical elements, an improved objective lens, and a higher bandwidth datapath the capability of the tool has been dramatically enhanced. Both the tool’s diffractive optic element (DOE) and acousto-optic modulator (AOM) have been refined. Additionally, the tools 33x, 0.8NA objective lens has been replaced with a 42x, 0.9NA objective lens. Finally, the tools datapath has been enhanced to maintain the ALTA systems superior write times on the critical layers. Quantitative results of these enhancements will be detailed through reporting of critical feature resolution limits, CD uniformity control, and pattern placement accuracy. Performance will be shown from masks printed pre- and post- hardware upgrade. Experimental results will be compared with theoretical calculations that show the expected improvement for each relevant parameter.

Qualification of a low-cost high-quality reticle process for 90-nm contact layers

Reticle costs are increasing as users tighten specifications to accommodate the shrinking process windows in advanced semiconductor lithography. Tighter specs often drive the use of e-beam based mask processes, which produce better mask pattern acuity than laser-based tools but suffer lower throughput (and thus higher costs). In some cases, such as contacts, the pattern acuity of an e-beam tool does not seem to be required -- but the tight effective CD uniformity typically produced by an e-beam mask writer is still necessary to prevent wafer level defect problems. This presents problems for the maskshop (e.g., low yield and long cycle time) as well as for the fab (more expensive new product introduction, uncertainty in mask delivery). This paper describes the results of qualifying a low cost, high quality mask making process for 90nm wafer production. The process uses a DUV laser-based mask writer to achieve low cost. Wafer photolithography process results using two masks fabricated with different mask making processes are presented, along with comparative electrical performance.