The resolution limits of optical lithography are usually described by the well-known Raleigh criterion, <i>CD = κ<sub>1</sub> (λ/NA)</i>. One of the biggest challenges in optical lithography is to reliably print contact holes patterns with κ<sub>1</sub> ~ 0.35 using a hyper <i>NA</i> system (<i>NA</i> > 1) especially for relatively small (<i>m</i> × <i>n</i>) arrays. Polarization effects cause deviations from a simple (<i>λ/NA</i>) scaling large <i>NA</i> values. For an isolated hole, <i>n</i> = 1 and for large arrays, <i>n</i> ⪆ 15, the spectral content is mainly contained in the lowest diffracted orders that are captured within the <i>NA</i> of the imaging lens. The most difficult situation is for small arrays (<i>m</i>, <i>n</i> ≈ 2, 3, 4) where the spectral features are broader more of the important image information is contained in the higher diffraction orders. The patterning of contact holes also suffers from tight dose tolerances and high mask error enhancement factors (MEEF) as both the feature and array sizes decrease. A detailed PROLITH<sup>TM</sup> vector simulation study is reported for three different approaches to printing, isolated contact holes and small to large contact hole arrays with a κ<sub>1</sub> of 0.35 and <i>NA</i>s of 1.05 and 1.3: 1) imaging interferometric lithography (IIL, with a single mask and multiple exposures incorporating pupil plane filters), 2) two-exposure dipole illumination, and 3) alternating phase shift masks (alt-PSM). Only the IIL scheme is capable of printing smaller (<i>m</i>, <i>n</i> ≤ 10) at this low κ<sub>1</sub> factor. Single exposure alt-PSM does not allow for the necessary polarization control. Periodic assist features provide improved resolution, depth of focus and MEEF, at the expense of a more complex mask and additional nonprinting area surrounding the contact holes.
Extending the resolution capability of 193nm lithography through the implementation of immersion has created new challenges for ArF B.A.R.C.s. The biggest of which will be controlling reflectivity over a wider range of incident angles of the incoming imaging rays. An optimum B.A.R.C. thickness will depend on the angle of incidence of the light in the B.A.R.C. and will increase as the angle increases. At high angles different polarization have different optimum thicknesses. These confounding effects will make it increasingly difficult to control reflectivity over a range of angles through interference effects within a single homogenous B.A.R.C. Unlike single layer B.A.R.C.s, multilayer B.A.R.C.s are capable of suppressing reflectivity through a wide range of incident angles. In fact, remarkable improvements in antireflective properties can be achieved with respect to CD control and through angle performance with the simplest form of a multilayer B.A.R.C., a dual layer. Here we discuss the attributes of an all organic dual layer B.A.R.C. through simulations and preliminary experiments. One attribute of an organic over inorganic B.A.R.C. in high-NA lithography is its ability to planarize topography. ArF scanners designed to meet the needs of the 45nm node will have a very small depth-of-focus (DOF) which will require planar surfaces.
We will describe our barrier coat approach for use in immersion 193 nm lithography. These barrier coats may act as either simple barriers providing protection against loss of resist components into water or in the case of one type of these formulations which have a refractive index at 193 nm which is the geometric mean between that of the resist and water provide, also top antireflective properties. Either type of barrier coat can be applied with a simple spinning process compatible with PGMEA based resin employing standard solvents such as alcohols and be removed during the usual resist development process with aqueous 0.26 N TMAH. We will discuss both imaging results with these materials on acrylate type 193 nm resists and also show some fundamental studies we have done to understand the function of the barrier coat and the role of differing spinning solvents and resins. We will show LS (55 nm) and Contact Hole (80 nm) resolved with a 193 nm resist exposed with the interferometric tool at the University of New Mexico (213 nm) with and without the use of a barrier coat.
Liquid immersion lithography (LIL) can extend the resolution of optical lithography well beyond today’s capabilities. The half-pitch limit is given by the well-known formula P=λ/(4/<i>NA</i>), where λ is the optical wavelength and <i>NA</i>=<i>n</i>sin(θ) is the numerical aperture of the exposure device with <i>n</i> the refractive index of the exposure medium. Through the use of exposure media such as purified water (<i>n</i> of 1.44 at 193 nm), it is possible to reduce minimum pitches by a factor of as much as 44% - a full technology node. Beyond this simple observation, there is a good deal of work necessary to fully understand the impact of LIL on a lithography processes. This paper will address issues con-cerning resist chemistry and the impact of water immersion on the imaging capabilities of different resist formulations. All resists were evaluated by imaging dense line-space structures at a 65-nm half-pitch both in air and with water im-mersion. Studies of dense 65-nm lines made by immersion imaging in HPLC grade water with controlled variations in resist components were performed. Significant differences were observed and will be discussed.
The limit of optical lithography is the minimum pitch between features. This pitch limit is given by Λ=λ/<i>2nNA</i>), where λ is the optical wavelength, <i>n</i> is the refractive index of the final medium of the optical system which is typically air (<i>n </i>= 1), and <i>NA</i> is the numerical aperture of the exposure device. A great deal of work has been done to decrease exposure wavelengths and increase the <i>NA</i> of exposure tools, however, until recently very little effort has been applied towards an immersion medium with <i>n </i>> 1. This paper examines extending minimum pitches through the use of such media. Exposures are at a wavelength of 213 nm, close to the current state-of-the-art 193-nm lithography node. The possible limits of lithography are examined using 193-nm resists exposed in air and comparing these limits to those possible when implementing liquid immersion lithography (LIL) exposures. Two immersion liquids were examined: deionized water, and Krytox a Perfluoropolyether (PFPE) oil. These liquids were compatible with 193-nm resist. A resolution enhancement factor of 28% for Krytox and of 41% for DI water was demonstrated. Images of good dense lines with a half pitch of 54 nm are presented.
In order to achieve controlled degree of intermixing in selected areas (CISA), SiO<SUB>2</SUB> gratings are checked first to be able to influence the degree of intermixing during high-temperature rapid thermal annealing of InGaAs/GaAs quantum wells. Subsequently, SiO<SUB>2</SUB>/MgF<SUB>2</SUB> gratings with different periods are used to cover different parts of MWQ sample and found to be suitable for achieving CISA after only a single annealing procedure.