Ralph R. Dammel

Liquid immersion lithography: evaluation of resist issues

We address in this report a set of key questions tied to the implementation of liquid immersion lithography, from the perspective of the resist materials. We discuss the broad question of whether chemically amplified resists are capable of achieving the spatial resolution that ultimately will be required for the most advanced immersion scenario. Initial studies undertaken using model 193 nm resist materials provide some insight into how an aqueous liquid immersion process can affect the resist material.

Photoresist materials: a historical perspective

This paper provides a short history of the development of resist materials. We trace the development of resists from the very beginnings of photography in the early 1800s to todays efforts to develop 193 nm resist materials.

Deep-UV immersion interferometric lithography

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/NA), where λ is the optical wavelength and NA=nsin(θ) is the numerical aperture of the exposure device with n the refractive index of the exposure medium. Through the use of exposure media such as purified water (n 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.

Imaging capabilities of resist in deep ultraviolet liquid immersion interferometric lithography

Liquid immersion lithography (LIL) extends the resolution of optical lithography to meet industry demands into the next decade. Through the use of exposure media such as purified water (n of 1.44 at 193nm), it is possible to reduce minimum pitches compared with traditional air/vacuum exposures media 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 immersion lithography on a lithography processes. This article addresses 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 immersion. 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.

CD changes of 193-nm resists during SEM measurement

CD linewidth change during SEM inspection has been one of the issues encountered in the introduction of 193 nm resists. As a general tendency, the methacrylate resists exhibit faster line width reduction than the cycloolefin- maleic anhydride (COMA) systems; however, other resist components as well as CD SEM settings paly an important role. Based on the exposure time vs. CD loss, the line width slimming (LSW) is found to proceed in three steps, which are assigned as: 1) chemical change of outer resist layer, 2) evaporation of volatiles and 3) bulk chain scission or deprotection. Countermeasures for CD degradation are proposed form both the formulation and process sides. A calculation of e-beam penetration depth suggests that deprotection, chain scission and other reactions occur in the first 20-40nm, and these reaction rates combined with thermal effects determine LWS. The CD SEM measurement method has been improved to minimize e-beam exposure and to spread out the thermal load over a larger period of time. An optimized formulation exhibits less than 0.2% LWS per measurement with the improved CD measurement program.

Refractive index measurements of photoresist and antireflective coatings with Variable Angle Spectroscopic Ellipsometry

Lithography requires accurate knowledge of film thickness and refractive index (n and k) for photoresists (PR) and antireflective coatings. It is becoming increasingly necessary to track changes in refractive index over the process cycle. The refractive index can change by as much as 0.04 in both n and k simply by bleaching the film. These changes can be caused by changes in film chemistry by processing such as baking and bleaching by UV exposure. Thus, keeping track of changes in the refractive index is useful to both resist and antireflective coating manufacturers as well as the process engineer. This work uses Variable Angle Spectroscopic Ellipsometry (VASER) to determine the refractive index of photoresist and antireflective coatings over the spectral range 190 - 1700 nm. Theory, hardware, and applications of Spectroscopic Ellipsometry are discussed along with procedures used to simultaneously extract the refractive index and film thickness of photoresist and antireflective coatings. Examples of commonly used films are presented.

Photoresist characterization for lithography simulation: II. Exposure parameter measurements

Lithography simulation has become an increasingly important tool for the semiconductor industry as attempts are made to extend current lithographic technologies. The usefulness of this simulation capability has been somewhat hindered by the lack of availability of accurate modeling parameters for the various commercial resist systems. In particular, three sets of data are required to model a typical non-chemically amplified photoresist: the refractive index as a function of wavelength and exposure, the exposure or ABC parameters (the Dill parameters), and the development parameters. This work focuses on the accurate extraction of exposure parameters for non-chemically amplified resists. Previous exposure parameter extraction techniques such as those proposed by Dill involve a number of simplifying assumptions including the assumption that the refractive index of the resist does not change during exposure and that the index of the substrate is matched to the resist throughout the exposure process. However, the refractive index of the photoresist does vary during exposure as the chemical composition of the photoresist changes. A rigorous analysis technique for extracting exposure parameters which accounts for this refractive index change and other previously ignored factors has been developed. An apparatus has been constructed to perform bleaching experiments on non- chemically amplified resists and this new, rigorous analysis technique has been used to extract exposure parameters for a series of commercial resists. Some of the results of these studies are presented together with comparisons to previous parameter extraction techniques.

Application of photodecomposable base concept to 193 nm resists

This paper reports on the use of trimethyl sulfonium hydroxide as a base additive for 193 nm applications, which is found to stabilize the latent image as well a act as a photodecomposable base. Delay time stability (exposure to post-exposure bake) of formulations consisting of trimethylsulfonium hydroxide is compared to that of a non- photodecomposable base (diethanolamine) in both methacrylate- and cycloolefin-based 193 nm resists. Resist formulations made using the trimethylsulfonium base were stable for more than one hour, while the reference formulation with diethanolamine showed T-top formation within 10 minutes delay time under the same conditions. The trialkylsulfonium hydroxide base additives were found to be photodecomposable by measuring the acid produced upon exposure. Compared to a non- photodecomposable base containing resist, the photodecomposable base containing resist produced more acid in the exposed areas under identical PAG/BASE molar ratios.

Modeling of bottom antireflection layers: sensitivity to optical constants

Bottom anti-reflective layers provide a number of benefits including significant reduction in swing curve amplitude and reflective notching. The key to understanding how a bottom antireflective layer improves lithography lies in the interaction of the thin film system with the exposing radiation. Bottom antireflective layers function primarily via their absorption which is significantly larger than that of the overlying photoresist at the actinic wavelength. In the simplest physical picture, a bottom antireflective layer must be thick enough to effectively extinguish radiation that has passed through it twice, with the turning point being at the substrate/bottom antireflective layer interface. It might therefore seem that the larger the bottom antireflective layer absorption coefficient, the better the performance. More precise studies show that this simplistic view is incorrect. We have modeled the general photoresist/bottom antireflective layer/substrate film stack using the standard theory of thin film optics. It follows from the complete mathematical model that at very high absorption coefficients, bottom antireflective layers may act as mirror elements of their own. Reflection from the bottom antireflective layer/photoresist interface comes both from differences in the absorption coefficients of the two materials (the dominant effect), as well as from differences in the refractive indices. Theory therefore predicts an optimum set of optical constants for every desired film thickness range, a relationship which can be summarized in simple contour diagrams.

Modified polyhydroxystyrenes as matrix resins for dissolution inhibition type photoresists

It is generally accepted that the production of shrink versions of the 16 MB DRAM and the 64 MB DRAM generations will be patterned using deep UV radiation. This provides a new challenge to the photoresist suppliers, as the standard photoresist formulations are not suitable for this technology, mainly because the presently used novolak resins are highly opaque in the 200 - 300 nm region. This is especially true for the 248 nm wavelength of KrF eximer lasers. Poly 4- hydroxystyrene [PHS] has several advantages in transmission and thermal stability; however, its dissolution rate in commercial grade developers is unacceptably high. We report some recent results on modified, alkyl-substituted PHS derivatives. These polymers combine reduced alkaline solubiity with adequate optical and thermal properties, making them acceptable for future deep UV based production processes. Selected data of these new (co)polymers are discussed.