Silicon Wafer Cleaning for Silicon Wafer Cleaning for EUV Reflectance EUV Reflectance

Measurements by Cold, Measurements by Cold, High-Pressure COHigh-Pressure CO22 Jet Jet

William EvansWilliam Evans

Brigham Young UniversityBrigham Young University

Why EUV Optics?Why EUV Optics?

EUV LithographyEUV Lithography

Soft X-Ray Soft X-Ray MicroscopesMicroscopes

EUV AstronomyEUV Astronomy

Hydrocarbons and “Stuff”Hydrocarbons and “Stuff”

Thin films and silicon wafers naturally build Thin films and silicon wafers naturally build up a layer of contaminants. up a layer of contaminants.

The contaminants interfere with The contaminants interfere with measurements of the optical properties of measurements of the optical properties of the mirror. the mirror.

Hydrocarbon Buildups Lower Hydrocarbon Buildups Lower ReflectanceReflectance

Reduced Reflectance with Hydrocarbon Thickness. Reduced Reflectance with Hydrocarbon Thickness. Theoretical change in reflectance vs. grazing angle and Theoretical change in reflectance vs. grazing angle and organic thickness. (at λ=40.0 nm)organic thickness. (at λ=40.0 nm)

Cleaning MethodsCleaning Methods

Many different cleaning methods have been used Many different cleaning methods have been used to clean silicon wafers for thin film deposition. to clean silicon wafers for thin film deposition.

OpticleanOpticlean®® Problem: 20 A ResidueProblem: 20 A Residue

Oxygen Plasma EtchOxygen Plasma Etch Problem: Not Local and Can Oxidize Non-Protected Problem: Not Local and Can Oxidize Non-Protected

SurfacesSurfaces High Intensity UV Light in AirHigh Intensity UV Light in Air

Problem: Oxidizes Metals, even GoldProblem: Oxidizes Metals, even Gold

Cold, High-Pressure COCold, High-Pressure CO22 Jet Jet

Cold, Pressurized COCold, Pressurized CO22

We used a We used a COCO22 Snow Cleaning Snow Cleaning

Solenoid UnitSolenoid Unit pressurized CO pressurized CO22 gun gun

by Applied Surface Technologies. by Applied Surface Technologies. The unit uses freezing, pressurized The unit uses freezing, pressurized

COCO22 to blow particles and to blow particles and

contaminates off of a surface. contaminates off of a surface. We tested this unit on We tested this unit on

silicon wafers with a silicon wafers with a native oxide layer. native oxide layer.

Testing ProcedureTesting Procedure

We tested the COWe tested the CO22 gun cleaning gun cleaning

system by the following procedure. system by the following procedure. Measured the apparent oxide layer. Measured the apparent oxide layer. Applied OpticleanApplied Opticlean®®. . Again measured the apparent oxide. Again measured the apparent oxide. Cleaned the samples with COCleaned the samples with CO22. .

Again measured the apparent oxide Again measured the apparent oxide thickness. thickness.

ResultsResultsSample #Sample # Exposure Exposure

Time to CO2 Time to CO2 (s)(s)

Thickness Thickness Before Before

OpticleanOpticlean®® ( (ÅÅ))

Thickness Thickness w/ w/ OpticleanOpticlean®® Residue (Residue (ÅÅ))

Thickness Thickness After CO2 After CO2

Exposure (Exposure (ÅÅ))

11 00 25.9325.93 46.3246.32 46.6746.67

22 00 25.2725.27 47.9847.98 47.7247.72

33 55 26.8226.82 41.6941.69 40.6840.68

44 55 25.1925.19 42.7742.77 38.4838.48

55 1010 24.8224.82 42.3942.39 42.0042.00

66 1010 25.7525.75 46.9346.93 21.0521.05

Results (cont.)Results (cont.)

-10

-5

0

5

10

15

20

25

S 1 (0 s) S 2 (0 s) S 3 (5 s) S 4 (5 s) S 5 (10 s) S 6 (10 s)

Angstroms

Residue After CO2 Cleaning

Why the Discrepancy?Why the Discrepancy?

The two 10-s samples showed some obvious The two 10-s samples showed some obvious differences in the effectiveness of the COdifferences in the effectiveness of the CO22 gun. gun.

This is probably due to the COThis is probably due to the CO22 gun. gun. COCO22 gun cleans macroscopically as compared the gun cleans macroscopically as compared the

others, which are atomistic. others, which are atomistic. Also, the non-uniform removal of contaminates could Also, the non-uniform removal of contaminates could

have been influenced by cleaning geometry. The have been influenced by cleaning geometry. The sample and gun were hand-held, so the distance and sample and gun were hand-held, so the distance and angle varied from cleaning to cleaning. angle varied from cleaning to cleaning.

Results, Conclusions, and Results, Conclusions, and Directions for Further StudyDirections for Further Study

In conclusion, the COIn conclusion, the CO22 jet appears jet appears promising, because it was able to remove promising, because it was able to remove the residue in one case. the residue in one case. Specifically the cold, high pressure COSpecifically the cold, high pressure CO22 unit unit

was effective in removing Opticleanwas effective in removing Opticlean®® from from silicon wafers when used for a sufficient silicon wafers when used for a sufficient amount of time at a proper distance. amount of time at a proper distance.

Further tests are needed to specify how to Further tests are needed to specify how to use this cleaning procedure for a variety of use this cleaning procedure for a variety of substrates. substrates.

AcknowledgementsAcknowledgements

We would especially like to thankWe would especially like to thank Dr. David AllredDr. David Allred Dr. LindfordDr. Lindford Richard SandbergRichard Sandberg Andrew JaquierAndrew Jaquier Chris VerhaarenChris Verhaaren Amy BakerAmy Baker Kristin BestorKristin Bestor The BYU Thin Film Optics Research GroupThe BYU Thin Film Optics Research Group