Curt Jackson

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.

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.

Conversion from 50 KeV to DUV mask writer for 90-nm technology critical layers

One of Cypress’ primary goals for 90-nm generation mask strategy is to control mask costs while not compromising on performance. One key objective is to replace the use of 50-ke V electron beam pattern generation with DUV laser mask lithography where possible. The higher productivity of the DUV laser systems compared to the 50Ke V e-beam platforms offers a unique opportunity for mask cost reduction. Compared to previous i-line generations of laser lithography systems, the DUV laser systems provide significantly improved resolution and pattern fidelity that more closely approaches that of ebeam lithography. We have previously published experimental results demonstrating that the difference in fidelity on the mask between the laser and EB platforms does not always translate to a measurable difference in wafer litho performance or even more importantly to a measurable difference in electrical performance. Through this work, Cypress was able to eliminate the use of 50Ke V ebeam writers for all of their 130nm technology node layers. In some cases the improved performance of the DUV tools was sufficient to replace i-line produced masks where wafer performance was marginal without having to resort to EB lithography. This study addresses the conversion of 50Ke V ebeam layers to DUV laser platform specifically for the critical layers of the Cypress’ 90nm Technology node. EB lithography was originally specified for these layers as a conservative approach in part due to the timing of 90-nm technology development relative to the maturation of the DUV laser mask lithography process. In this study, the electrical performance and wafer yield are evaluated for equivalency in order to take advantage of the lower cost and faster cycletime that use of a ALTA DUV system provides over the 50Ke V VSB systems. In addition, the wafer OPC is not changed between the two mask writing systems in order to allow interchangeable use of the two writing systems if the experimental results indicated no difference in wafer performance.

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.

DUV mask writer for BEOL 90-nm technology layers

Mask CD resolution and uniformity requirements for back end of line (BEOL) layers for the 90nm Technology Node push the capability of I-line mask writers; yet, do not require the capability offered by more expensive 50KeV ebeam mask writers. This suite of mask layers seems to be a perfect match for the capabilities of the DUV mask writing tools, which offer a lower cost option to the 50KeV platforms. This paper will evaluate both the mask and wafer results from all three platforms of mask writers (50KeV VSB,ETEC Alta 4300TM DUV laser and ETEC Alta 3500TM I-line laser) for a Cypress 90nm node Metal 1 layer, and demonstrate the benefits of the DUV platform with no change to OPC for this layer.