Prospects for photolithography at 121 nm

Abstract

We have evaluated several subsystems that would be required for lithography at 121 nm: radiation sources, optics, ambient control, and resists. Dense plasma sources can generate significant power, >0.5 J of 121 nm radiation per pulse. An extensive survey of commercially available LiF, as well as specially grown LiF, shows that transmission losses are ∼20% per cm, a value which is at least 5× too high if LiF is to be used as lens material. At present it is not clear whether there is a fundamental limitation to LiF transparency at 121 nm, but the main source of absorption arises from lithium oxide or hydroxide in the crystal. Polymers, fluorinated or not, exhibit very high absorption at 121 nm, indicating that ultrathin resists or bilayer resists will have to be used at 121 nm. However, the photochemistry at 121 nm is probably not much different from that at longer wavelengths. The resists are sensitive, ∼1 mJ/cm2 required dose, and they have high contrast at 121 nm. Finally, we have also performed preliminary patterning experiments. Using contact printing with a phase shifting mask we have printed 45 nm features in an ultrathin photoresist.

Direct metal pattern writing by VUV photodissociation

“…However, LiF is hygroscopic and mechanically soft, 13 so that in our experiment MgF 2 with 1 in. The optical materials that transmit 121.6 nm wavelength are MgF 2 and LiF.…”

High power 121.6 nm radiation source

“…1 In previous articles, 2,3 we have reported a vacuum ultraviolet ͑VUV͒ light source based on a high-pressure cylindrical dielectric barrier discharge ͑DBD͒, which was capable of producing intense and spectrally clean hydrogen Lyman-␣ line at 121.6 nm. The lamp generated Lyman-␣ line radiation via resonant energy transfer from neon excimer molecules to molecular hydrogen, which leads to dissociation and excitation.…”

Optical design and system characterization of an imaging microscope at 121.6 nm

“…4,6 The vacuum ultraviolet (VUV) Lyman-α line (λ ¼ 121.6 nm) emission from atomic hydrogen is a promising short wavelength for microscopy, as it coincides with a transparent window in the air absorption spectrum. [7][8][9] The transmission window provides flexibility in imaging samples and potentially eases system operation, as the object to be imaged can be outside the vacuum chamber. In this paper, we present the optical design and construction of a numerical aperture ðNAÞ ¼ 0.3 imaging microscope at λ ¼ 121.6 nm.…”

“…The availability of high-quality transmissive optical material in the VUV spectral range is extremely limited, where MgF 2 and LiF are the two materials most often used. 7,10 However, the birefringence of MgF 2 is 4.44 × 10 −3 at λ ¼ 121.6 nm, which excludes MgF 2 for use in strongly focusing optical elements due to the excessive polarization aberration. 11 LiF is optically isotropic, but transmission of LiF in the VUV spectral range depends on the material purity, growth process, polishing, storing and handling of the crystals, and is hard to maintain.…”