Large Size CaF₂ Single Crystal for Lithography Lens

Hitachi CaF₂ single crystals are developed for next generation lithography lens applications. The design rule of semiconductor has been continuously shrinking which requires stepper machines employing shorter wavelength. This makes CaF₂ single crystals, which excel synthetic quartz in transmittance, crucial. With the original know-how cultivated into the growth technology for GSO single crystal, we are capable of producing large size CaF₂ single crystal with excellent properties.

Features

High Laser Resistance	: No performance degradation under ArF laser irradiation
High refractive-index homogeneity and low birefringence	: Original low stress growth technology and high precision annealing technology to achieve the high refractive-index homogeneity and low birefringence
Good transmittance	: Very good transmittance at 193 nm achieved by original know-how in scavenger optimization
High growth yield and stable supply	: Two times more growth efficiency than that is attained conventionally
Selective orientations	: Both <111> and <100> with ease

Characteristics

Crystal size	≧Ф240 mm
Crystal orientation	<111> & <100>
Birefringence	≦0.2 nm/cm at 633 nm (mean)
Homogeneity	<20 ppb (RMS)
Transmittance	>99.9% @ 193 nm
Laser resistance	<0.1% @ 193 nm

Applications

• Illumination and projection lens for ArF & ArF immersion exposure tools
• Window material for the laser light sources of ArF and KrF exposure tools

CaF₂ single crystal for semiconductor lithography

The design rule for the mass production of advanced LSI and memory IC products shifts from the current 90 nm to 65 nm and 45 nm nodes and the development of corresponding system engineering is also being accelerated further. Lithography technology plays an important role for patterning the original circuit from the mask (reticle) on to the wafer through the photoresistive resin (photoresist) using exposure equipment, such as a stepper and a scanner. For the pattern size of 65 nm and beyond, ArF and ArF immersion equipment become in use, for which CaF₂ single crystal, which excels synthetic quartz in transmittance and laser resistance, is important.

The technical issues of manufacturing of CaF₂ single crystal

CaF2 Crystal structure
<111> single crystal
<100> single crystal	<110> single crystal

CaF₂ is very difficult to obtain in the form of single crystal because of its tendency to form multi-grains and its preferential growth direction different from the required <111> and <100>. Although the vacuum Bridgman method generally used for growing CaF₂ single crystal has simple equipment structure and enlargement of crystal size is easier, there is also a problem of easy formation of polycrystals.
Moreover, since the preferential growth direction of CaF₂ single crystal is close to the <110> direction, the controlled growth along <111> direction is difficult.
In addition, there are some technical issues also in obtaining the crystal without defects and stress which is very important when the crystal is to be used in the semiconductor lithography field.

Technology of CaF₂ single crystal growth at Hitachi Chemical

The heat flow in vacuum Bridgman furnace visualized by computer simulation	High quality CaF2 lenses suitable for lithography applications

In Hitachi Chemical, stable supply of high quality CaF₂ single crystal is ensured with the efficient direction controlled growth technology combined with advanced system engineering, such as understanding of growth environment based on crucible structure and simulation analysis of heat flow.
We succeeded in obtaining large size CaF₂ single crystal for the semiconductor lithography lenses which have a high level of optical quality with good reproducibility by further establishing low stress growth technology and high precision annealing technology.
It is expected that this CaF₂single crystal will contribute to the progress in the semiconductor lithography field from now on.

Example for use

Term description

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Crystal	: The periodic arrangement of atoms in a solid.
Single crystal	: The crystal in which crystal orientation is equal in all the places.
Polycrystal	: The crystal which consists of crystal grains from which crystal orientation is differed.