Lithography of choice for the 45-nm node: new medium, new wavelength, or new beam?

Abstract

In order to clarify the direction of the lithography for the 45 nm node, the feasibilities of various lithographic techniques for gate, metal, and contact layers are studied by using experimental data and aerial image simulations. The focus and exposure budget have been determined from the actual data and the realistic estimation such as the focus distributions across a wafer measured by the phase shift focus monitor (PSFM), the focus and exposure reproducibility of the latest exposure tools, and the anticipated 45 nm device topography, etc. 193 nm lithography with a numerical aperture (NA) of 0.93 achieves the half pitch of 70 nm (hp70) by using an attenuated phase shift mask (att-PSM) and annular illumination. 193 nm immersion lithography has the possibility to achieve the hp60 without an alternative PSM (alt-PSM). For a gate layer, 50-nm/130-nm line-and-space (L/S) patterns as well as 50 nm isolated lines can be fabricated by an alt-PSM. Although specific aberrations degrade the critical dimension (CD) variation of an alt-PSM, ±2.6 nm CD uniformity (CDU) is demonstrated by choosing the well-controlled projection lens and using a high flatness wafer. For a contact layers, printing 90 nm contacts is very critical by optical lithography even if the aggressive resolution enhancement technique (RET) is used. Especially for dense contact, the mask error factor (MEF) increases to around 10 and practical process margin is not available at all. On the other hand, low-energy electron-beam proximity-projection lithography (LEEPL) can fabricate 80 nm contact with large process margin. As a lithography tool for the contact layers of the 45 nm node devices, LEEPL is expected to replace 193 nm lithography.