Impact of Corrosion/Erosion on Ta-2.5W alloys for Liquid Tin Transport in EUV TechnologyYuhui An, Manlin Pan, Eric Prussack, and Nick RichterFaculty Advisors: Dr. John Blendell and Dr. Carol HandwerkerIndustrial Sponsors: Dr. Wei-Hsun Chen

Project Background

Liquid tin transport is critical to EUV lithography and refractory metals are the material of choice totransport it. Corrosion between Ta-2.5W alloys and liquid tin was studied to understand materiallifetime. Static corrosion tests were performed to understand the fundamental mechanisms ofcorrosion present. Dissolution of Ta was prevalent and future work needs to be done to quantify it.A dynamic corrosion test setup was designed and constructed to accurately study the corrosion ofthe Ta-2.5W tin transfer line. Future work includes running these dynamic tests.

Corrosion Testing

Recommendations

Discussion

MSE 430-440: Materials Processing and Design

This work is sponsored by ASMLSan Diego, CA

Extreme Ultraviolet Lithography

• Experimental Procedure• Used static corrosion test to investigate fundamental

mechanisms of corrosion present in liquid tin andTa-2.5W alloy system

• 1mm ID tubes cut and placed in liquid tin bath in anAl2O3 crucible at varying temperatures inatmospheric conditions to quantify corrosion

• Tube cut longitudinally to study corrosion productsforming within tube using EDS

• Before and after micrographs were taken usingoptical and SEM imaging

Results

Sources[1] Cymer. (May 2012). How an EUV Light source works. Retrieved from https://www.youtube.com/watch?v=8xJEs3a-1QU&t=1s

[2] P. F. Tortorelli and J. H. DeVan. Liquid metal corrosion consideration in alloy development. Metals and Ceramics Division, Oak Ridge National Laboratory.

• High power CO2 laserhits liquid Sn droplets [1] inUHV generating plasma andEUV light (λ=13.5nm) [1]

• Liquid tin is transportedthrough the system in Ta-2.5W alloy tubing (known asthe tin transfer line (TTL)) andcorrosion from and erosion byliquid tin could limit its lifetimeand contaminate the tin.

• Objective: To understandinteractions between liquid tinand refractory Ta-2.5W tintransfer line to investigatelifetime

Mechanisms of Corrosion [2]

Future Work

Dissolution/Selective Leaching Erosion CorrosionAcceleration in corrosion ratedue to the relative motion of fluid

Intergranular Corrosion Galvanic Corrosion

• Pitting and stress-corrosion cracking are not asprevalent in this system, but were still considered

Path Forward

Dynamic Testing Method

Environmental Constraints | Safety Constraints

1. O2 is removed using a turbomolecular vacuum pump and replaced with Ar - 2% H2 gas.

2. Tin is melted using the reservoir’s heat jacket on the reservoirs and heat tape on the TTL.

3. Ar - 2% H2 pressure is increased on reservoirs to flow liquid Sn through TTL, inducing corrosion/erosion.

4. Flow is reversed multiple times at high pressures to simulate/accelerate cycles the TTL will experience while in use.

• Partial pressure of O2must be below 1 x 10-8

torr• Partial pressure of H2O

must be below 1 x 10-6

torr• Temperature must be

above 2550 C (but may be elevated to accelerate corrosion)

• Exhaust and safety valves to relieve gas pressure

• Thermally isolated from the rest of lab

• Control box with master shut-off

Further corrosion testing and analysis needs to bedone to fully assess the lifetime of Ta-2.5W tubesexposed to liquid Sn. Static corrosion tests should berun at varying lengths of time to get a more completeunderstanding and quantifying of dissolution rate. Inaddition, EBSD needs to be done to analyze the phaseof the corrosion product formed between the samplesand the liquid tin. The dynamic corrosion test designedwill be implemented to simulate the conditionsexperienced by the TTL and give an accurateassessment of the fundamental corrosion mechanismsand the lifetimes of the material. Slight modifications willneed to be made to the experimental test setup toensure the tests are run with the proper environment.

Ar/H2 Gas

Vacuum Pump

Sn Reservoir

Tin Transfer Line

Ta-2.5W• 500 oC for 40 hours• 350 oC for 14 days

Ta-2.5W (Vendor A&B)/Mo• 250 oC for 1 month• 350 oC for 14 days• 500 oC for 14 days

Ta-2.5W (Vendor A)250 oC 350 oC

Molybdenum

Schematics illustrating EUV Lithography [1] Schematic of Experimental Design

Relief ValvesPressure Gauge

Al2O3 crucible containing Ta-2.5W tube submerged partially in Sn

SEM image of corrosion product from 500 oC test, with the compositions of points 1

and 2 shown in Table 1.

Point 1 Element Wt%

Tin 91.1

Tantalum 8.9

Point 2 Element Wt%

Tin 32.1

Tantalum 67.9

Table 1: Point analysis results

Preliminary results indicate that dissolution of Ta willoccur between Ta-2.5W and liquid Sn at typical operationtemperatures and further studies are necessary to quantifydissolution rate. The dynamic testing method is ready to beimplemented to more accurately evaluate the interactionsbetween Ta-2.5W and Sn in the TTL system.

SEM images of interface between liquid Sn and the Ta-2.5W A sample at (A) 250 oC and (B) 350 oC.

SEM images of interface between liquid Sn and the Ta-2.5W B sample at (A) 250 oC and (B) 350 oC.

SEM images of interface between liquid Sn and Mo tube at (A) 250 oC and (B) 350 oC.

• Presence of Ta in the solidified Sn on the EDS scansindicates dissolution has occurred for each sample ateach temperature and the corrosion rate could increasewith the influence of erosion

• Thin corrosion layers form on the Ta-2.5W tubes fromVendor A and B, but needs further analysis(EDS/EBSD) to identify composition and phase. Cracksform in the corrosion layer of the Mo tubes.

• Dark spots in the solidified Sn have been attributed topolishing compound contamination or porosity

• Quantification of dissolution is a crucial next step todetermining if this corrosion is detrimental to the TTL

Corrosion layer with cracks forming on the Mo tubes at 350 oC

Ta-2.5W (Vendor B)

500 oC

Dissolution Quantification• A dissolution rate for Ta into the liquid Sn must be

quantified to understand material lifetime• Possible methods:

• Dissolve the solidified Sn out of the tube using HCland measure the mass before and after todetermine Ta loss per time

• Measure change in weight of rod or tube in moltentin as a function of time and calculate amount ofTa dissolved and dissolution rate using exposedarea.