Contamination Sensing and Control for Multilayer Opticseuvlsymposium.lbl.gov/pdf/2005/pres/44 3-CC-14 Grant.pdfChemical Laboratory University of Cambridge Sensor Characterisation -

Chemical LaboratoryUniversity of Cambridge

Contamination Sensing and Control for Multilayer Optics4th International EUVL SymposiumSan Diego 7-9 Nov 2005

Anthony Keen, Rob Grant, George Kyriakou, David Davis and Richard Lambert

2


Outline

EUV Vacuum environment – the problem for multilayer mirrors

Need for contamination monitoring Hydrocarbon sensor development

• Fundamental characterisation• Impact on the sensing environment• Hydrocarbon selectivity

Comparison to Ruthenium capped multilayer mirrors Conclusions

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Vacuum Environment of the EUV Tool

An EUV tool is highly complex with pumping achieved via combination of various pump technologies

Both the illumination & projection optics are expensive& highly sensitive to contamination

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HYDROCARBONS

Mo / Si

Mask

Photo-

resist

Wafe

r

13.5 nm (92 eV) photons

e-e-

e-

The presence of contaminant molecules in the EUVL environment can seriously compromise the integrity of the process

Dangerous molecules include: unsaturated hydrocarbons, H2O, O2 Even hydrocarbon species present at pressures lower than 10-9 Torr, can

seriously degrade the reflectivity of the multilayer mirrors during their lifetime

Mirror Contamination – The Problem

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Contamination Monitoring in EUVL Systems

Contamination levels need to be strictly controlled and monitored in EUVL tool environments

• Defined specifications (ASML – SPIE 2003 vol. 5037 pp24) :• CxHy spec. 1x10-9 mbar, H2O spec. 1x10-5 mbar

Typical molecules likely to be present in EUV tool could include• Alkanes, alkenes, alcohols, aromatics, acids, esters, PFCs…

Evidence to suggest not all molecules have same contaminating effecti.e. different organic species lead to different levels of carbon growth under EUV stimulation

• K. Boller et al, Nucl. Inst Meth. 208 (1983) 273• R. Kurt et al, Proc. SPIE 4688 (2002) 702-709

Conventionally RGA is used to monitor CxHy in vacuum environments• Two approaches to quantification of CxHy level

• Measure sum of masses Σ(45…200) amu • Take an average level of 45…200 amu

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Limitations of RGA

Ultimate sensitivity of RGA is high, 10-14 Torr, but always a trade-off with mass resolution Even when only a few simple hydrocarbon species are present, RGA has limitations

•Cross-sensitivity to less active or inactive hydrocarbon species•Complex mass spectra, which can be very difficult to de-convolute given variations in molecular fragmentation and ionisation cross sections

Hydrocarbon mass spectrum

1

10

100

1000

10000

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97mass (amu)

Rel

ativ

e In

tens

ity (a

.u.)

cyclohexanehexanebutanetoluene

Hydrocarbon mass spectrum synthesised from standard cracking patterns.

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Urgent need for hydrocarbon sensors deployed at key points in the process

It is not desirable to use multilayer mirrors as the contamination monitor!

We need a hydrocarbon sensor to protect the optics Ideal Device features include:

• High Sensitivity (i.e. at least x10 that of a MLM surface)• Selective – Distinguish between harmful & benign species• Rapid response• Robust (Reliable)• Compact • Ultra high vacuum compatible

Highly functionalised molecules such as toluene are considered as dangerous, whereas, it is generally thought that light alkanes are benign

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Hydrocarbon Sensor

Oxygen ion conducting solid electrolyte (yttria-stabilized zirconia, YSZ) interfaced with platinum (or other) sensing and reference electrodes operating at 400 – 600 °C

Output voltage varies with hydrocarbon partial pressure as a result of a reaction between oxygen transported through the YSZ from the reference side and hydrocarbon molecules impinging on the (hot) sensing electrode on the vacuum side

However a solution to minimise the thermal impact on the EUV tool is required

Electrodes

Oxygen conducting materiale.g. Yttria doped Zirconia

Heater

Thermocouple

Reference oxygen

V

CxHy Sample

Vacuum flange

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Isolation of the Sensor from the EUV Chamber Environment

• Main chamber is isolated from any impact of the sensor e.g. thermal emissions, outgassing, charged particle emissions etc. through the high compression ratio of the turbopump• System response time is unaffected or improved• Sensor is buffered, by the high compression ratio of the turbopump, from back diffusion of species in the exhaust line

Mounting vacuum sensors mid-way along the blade stack of a turbomolecular pump offers significant system design advantages

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Sensor Characterisation & Optimisation

Selectivity depends on the adsorption characteristic of the organic molecule on the electrode surface

Sensitivity/Response depends on the equilibrium surface concentration of oxygen on the electrode surface

Surface science studies of the selectivity and sensitivity of model electrodes towards particular classes of hydrocarbons clearly demonstrates we can control and tune both of these parameters to the application

Experiments performed in a UHV chamber• Pressure 2 x 10 -10 Torr• X-ray Photoelectron spectroscopy (XPS)• Mass spectrometer (TPD)• Sample preparation: Ar+ sputtering• Model electrode: Pt polycrystalline foil• Au dosing: vacuum evaporation of the metal

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Pt as a sensing electrode At 597 °C all hydrocarbons tested

resulted in formation of the same chemical species on the Pt surface

•Binding energy of C 1s = 284.4 eV, characteristic for graphitic carbon

Formation of the same carbonaceous species, regardless of the identity of the incident hydrocarbon molecule, is an essential requirement for a quantitative hydrocarbon sensor

278 280 282 284 286 288 290 292

C 1stoluene 56 L

1-butene 50 L

n-butane 400 L

XPS

Binding energy (eV)

G. Kyriakou, D. J. Davis and R. M. Lambert, Sensors and Actuators B : Chemical, in press

Sensor Characterisation

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Sensor Characterisation - Selectivity

Pt electrodes• The sensor shows selectivity between saturated hydrocarbons and more functionalised molecules determined by sticking probability

0 20 40 60 80 100 120

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

1-butene toluene n-butane

C 1

s / P

t 4f i

nten

sity

ratio

Exposure L

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Sensor Selectivity Enhancement

It is generally accepted that introducing a noble metal e.g. gold to a catalytic surface dramatically changes the surface chemistry

1-butene toluene n-hexane0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

upt

ake

grad

ient

s

Pt θ

Αυ = 0.7 ML

θΑυ

= 0.8 ML

Pt/Au Alloy Electrode Surfaces• Varying the amount of Au

changes the selectivity to different organic molecules

0 20 40 60 80 100 120 140 160

0

2

4

6

8

10

12

14

C 1

s in

tegr

ated

inte

nsity

Exposure L

toluene 1-butene n-hexane

θ Au = 0.7 ML

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Comparison of the Sensor with MLM Surface

Need to demonstrate the comparison between the response of the sensor surface and the response of a Ru capped multi layer mirror surface under EUV exposure

We can achieve this using several methods:• 1. High Power Pulsed EUV – Ideal method• 2. Synchrotron BUT is the chemistry the same under continuous

exposure compared with high power pulsed stimulation?• 3. Low energy electrons of the correct energy distribution

• Assuming that it is the secondary electron emissions that are responsible for the surface chemistry

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Results of Electron Stimulation

Comparison of the profile of secondary electrons between 13.5 nm EUV photons and low energy electron beam

Electron gun stimulation at 0.2mW/mm2 of toluene at (true) 1x10-9 Torr partial pressure results in 1nm carbon growth in approximately 80 hrs

Sensor response to 1x10-9 Torrpartial pressure toluene is in a time scale of the order a few 10s of seconds.

0 50 100 150 200

0

2000

4000

6000

8000

10000

Inte

nsity

Kinetic energy (eV)

Synchrotron (Elettra)hv = 91.8 eV

Electron beam (Cambridge)Ep = 200 V

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Conclusions & Further Work

We have previously demonstrated sensor response to toluene partial pressures of 10-9 Torr

We can control sensitivity of the sensor as previously reported by control of surface oxygen concentration on the sensor electrode

This work demonstrates we can tune the selectivity of the sensor to different functionalised organic species thus being able to distinguish between harmful & benign organic material

The sensor demonstrates orders of magnitude more sensitivity to hydrocarbon contamination compared with a Ru surface

We have a method to ensure the sensor has zero impact to the tool thermal budget etc.

The next stage is to combine high power pulsed EUV stimulation with surface science analysis to test the validity of the electron beam simulation of EUV and determine the real effects of various organic molecules on MLM performance

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Acknowledgements

The authors would like to express their thanks to the following people: Corrado Crotti at Elettra Synchrotron, Trieste Phil Pakianathan at BOC Edwards Mintcho Tihov at University of Cambridge

David Davis wishes to thank EPSRC & BOCE for Ph.D. funding George Kyriakou wishes to thank BOC Edwards for post doctoral support

Documents

Contamination Sensing and Control for Multilayer Opticseuvlsymposium.lbl.gov/pdf/2005/pres/44 3-CC-14 Grant.pdfChemical Laboratory University of Cambridge Sensor Characterisation -