Achievement of ultimate zero thermal expansion in super invar
based alloy for stable optical systems

Paper No.: 12750-54

Hiromichi Fujii, Naoki Sakaguchi, Haruyasu Ohno, Kotaro Ona
Shinhokoku Mateiral Corp., 5-13-1 Arajuku-machi, Kawagoe, JAPAN 350-1124

We developed a novel extremely low thermal expansion alloy with high temporal stability and high Young’s modulus, which was based on the super invar (Fe64Ni31Co5). The conventional super invar has
been often used as the structural supporting components for stable optical systems and its average coefficient of thermal expansion (CTE) is approximately 0.5 ppm/K around room temperature. We clarified
that there is a linear relationship between the average CTE and Fe content around room temperature in the super invar based alloys. According to the linear approximation between them, the average CTE
was found to become zero when the Fe content is 62.5%. Based on this knowledge, we achieved an extremely low thermal expansion of 0.03 ppm/K average CTE in the temperature range between 18ºC and28ºC (291 K and 301 K) by precise control of chemical composition. In case of a cast alloy, carbon content was reduced below 0.02% to suppress the carbon-dependent expansion with time that is known tooccur in high carbon invar based alloy. As a result, dimensional change over the years of our developed cast alloy becomes lower than 0.05 ppm/year. Low Young’s modulus due to coarse and columnar
grains microstructure in the cast alloys can be improved by 30% with grain refinement technique using cryogenic and heat treatments.

INTRODUCTION

 Enhancement of dimensional stability over years

RESULTS

The demand for ultra-high dimensional stability of the structural
supporting components is growing with the development of
ultra-precision manufacturing technologies represented bysemiconductor lithography equipment.

 Effect of Fe content on thermal expansion

Suppression ofexpansion

Comparison of technological benefits between Invar, Zerodur and CFRP
Metal(Invar)

Glass(Zerodur)

Composite(CFRP)

Coefficient of thermal
expansion (CTE) [/K]

10-6

10-8

10-6

Young’s modulus [GPa]92 (cast)
130 (forged)91> 200(fiber direction)

Machining

Easy

Difficult

DifficultEase of repair

EasyDifficult

DifficultDuctility

DuctileBrittleBrittleα = 0, CFe = 62.5%

Dilation curves over years
obtained from highcarbon, commercial anddeveloped super invars.
The measurements wereconducted by AIST.
Effect of Fe content in
Fe-Ni-Co alloys on
average coefficients ofthermal expansion
between 18ºC and 28ºC.

A linear relationship between CTE and Fe content was clarified.
CTE α could be determined by using the following equation;

ϒ expansion over years is known to be a phenomenon caused
by carbon atoms in super invar. We clarified that this can be
drastically suppressed by reducing carbon content.
 Improvement of Young’s modulus

α (18-28ºC) = −0.4789 CFe + 29.927
 Achievement of ultimate zero thermal expansion
Thermal expansion curvesof commercial invar 36 andsuper invar, zero-expansion
glass-ceramics (Zerodur)and commercial CFRP.

We develop an extremely low thermal expansion alloy by precise
control of chemical composition based on super invar. In
addition, dimensional stability over time and rigidity of thedeveloped alloy are improved by metallurgical approaches.

EXPERIMENTAL

Cryogenic treatmentYoung’s modulus of as-cast and cryogenic and heat treated super invars
α = ~ 10−6 /K

Process

α = ~ 10−8 /K

Thermal expansion
curves of conventional
invar 36 and superinvar, developed superinvar and zero
expansion glassceramics (Zerodur).

As-cast93 GPaCryogenic and heat treatments

130 GPa

Young’s modulus of cast super invar based alloy can be
improved by 30% with grain refinement technique using
proprietary cryogenic and heat treatments.

CONCLUSIONS

Temperaturedependence of thecoefficients of thermal
expansion estimatedfrom thermal
expansion curves.

Based on knowledge on a linear relationship between CTE andFe content in Fe-Ni-Co super invar based alloy, ultimate zerothermal expansion can be achieved by precise control of Fecontent.

© 2023 SHINHOKOKU MATERIAL CORPORATION All Rights Reserved.

Young’s modulus

α = ~ 10−8 /K

 Sample preparation✓ Materials: Fe64+xNi31+yCo5−(x+y)✓ Processes;
Induction melting, Casting, Forging,
Cryogenic and heat treatment
 Evaluation of materials properties✓ Precise dilatometer (Thermal expansion, Dimensional
stability over time)✓ Measurement of Young’s modulus by resonance method

Microstructure

新報国マテリアル株式会社
SHINHOKOKU MATERIAL CORPORATION

1. A linear relationship between the average CTE around room
temperature and Fe content is clarified in Fe-Ni-Co superinvar based alloy. According to this relationship, ultimate
zero expansion can be achieved.
2. Dimensional stability over years can be drastically improvedby reducing C content in cast super invar based alloy.
3. Young’s modulus of cast super invar can be improved withgrain refinement technique using cryogenic treatment.

These are important understandings for the future developmentof EUV lithography equipment by the application of super invar
based alloys with extremely high dimensional stability.

fujii@shst.co.jp
www.shst.co.jp

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