B. Mortini

Study of dynamical formation and shape of microlenses formed by the reflow method

Microlenses arrays are commonly used in CMOS images sensors to focus the incident light onto the photosensitive area of the pixel. These microlenses are fabricated using a thermal reflow method. Currently, due to the fast evolution of CMOS Imager technology, the understanding of the mechanisms involved in microlens formation becomes essential to better control what occurs during the process. We have seen in a previous study that the complexity of the reflow method comes from the competition between two phenomena occurring during the melt bake step: on one hand the surface tension tends to push the resist patterns into a spherical shape, on the other hand the resist crosslinking reaction drastically increases the resist viscosity hindering the microlens formation. In this paper the influence of resist crosslinking, resist volume and resist/substrate interface on the final shape of the microlens has been investigated. It appears that the contact angle between microlens and substrate varies depending on substrate wettability but is the same whatever the resist volume for a given substrate/resist combination. The microlens shape depends also significantly on bake temperature and crosslinking kinetics. In fact the right tuning of process conditions seems to be the key parameter in the control of the final microlens shape because it enables to adjust the kinetics of each mechanism and thus favour the microlens formation with regards to resist crosslinking.

Study of bake mechanisms by real-time in-situ ellipsometry

Film formation and bake processes have been studied using in-situ ellipsometry. This new experimental set-up based on a HeNe laser mounted over a hot-plate is shown to be mainly sensitive to physical changes in the resist layer and provides real-time monitoring of the modifications induced during bake steps. Pure polymer films as well as DUV 248 nm and 193 nm CA resists are investigated.

Impact of HBr and Ar cure plasma treatments on 193nm photoresists

Since they have been introduced to substitute poly(hydroxystyrene) based 248nm photoresists (PR), 193nm photoresists based on acrylate chemistry have raised issues regarding their dry etch resistance. These resists undergo severe degradations during typical dry etch processes involved in gate patterning, resulting in strong film loss, resist chemical modifications, critical surface roughening and also linewidth roughness (LWR). Other studies have shown that applying plasma treatments to 193nm photoresist patterns prior to the other plasma etching processes is a way to minimize PR degradation. Among these plasma treatments, the HBr plasma cure is known to reinforce the 193nm photoresist etch resistance and to reduce the resist LWR. In this study, we propose to go further in the understanding of cure plasma treatments impact on a 193nm model resist polymer (from Rohm & Haas Electronic Materials) using real time in-situ ellipsometry experiments correlated to several characterization techniques such as in-situ X-Ray Photoelectrons Spectroscopy (XPS), Fourier Transformed Infrared Spectroscopy (FTIR) and Dynamic Mechanical Analysis (DMA). The impact of Ar and HBr cure plasma treatments on 193nm PR is investigated and compared. Both treatments lead to surface and also bulk modifications of the resist films. XPS analyses show that the 10 first nanometers of the resist film are graphitized after only 20s plasma treatment, resulting in a higher carbon content and therefore indicating a better etch resistance following the Ohnishi parameter. Besides this superficial modification, FTIR show that the resist film can be completely modified after HBr cure plasma treatment with the removal of lactone and leaving groups present in the polymer. The same kinds of modifications are observed with Ar cure plasma treatment but only the first 80nm of the resist film are chemically modified. A significant decrease of the glass transition temperature is also observed with both treatments and is related to lactone and leaving group units that remain in the film Finally, we show that the resist etch resistance is indeed improved if the resist is previously cured. However, in the case of Ar plasma treatment, the etch resistance is only improved while etching the first 80nm chemically modified resist.

Dynamical formation of microlenses by the reflow method: numerical simulation and experimental study of the process fabrication

In order to study the dynamics of microlens formation by the reflow method, we develop a model based on the Navier–Stokes equation that describes the formation of a microlens from its initial state (photolithographic dot) up to its final shape (rounded microlens). In CMOS imagers, photoresists used to form microlenses contain crosslinkers in order to obtain chemically and thermally stable microlenses at the end of the process. This model takes into account the effect of surface tension as well as the viscosity evolution with bake time, due to the crosslinking reaction. The results of the dynamic modelling show that the microlens passes through different intermediate steps before achieving its expected final spherical shape. It also shows that fast crosslinking kinetics leads to non-spherical microlenses. Indeed the fast increase of viscosity has the effect of blocking the formation of the microlens and freeze the microlens profile in an intermediate shape. Thus, with such type of photoresist materials, contrary to what is reported in the literature, the final shape of the microlens does not depend only on the initial volume or the aspect ratio of the resist pattern. This is the competition, during the melting bake, between the microlens formation under the effect of surface tension and the crosslinking reaction that determines, over all, the final shape that the microlens will adopt.

Evaluating resist degradation during reactive ion oxide etching using 193 nm model resist formulations

The weaker etch resistance of 193 nm resists1 is raising questions concerning their usability for the coming nodes as a single layer resist. We have found that 193 nm positive tone resists, that have been designed2 incorporating etch resistant groups like adamantyl or isobornyl3-7, exhibit chemical modifications concerning these grafted functions while undergoing an oxide etch step. Previously performed experiments have pointed out that the photoacid generator (PAG) that is still contained in the unexposed regions of the sacrificial layer might be a reason for the modifications in the chemical buildup of this resists. Therefore, this work has focused on evaluating the impact of reactive ion oxide etching8-10 on 193nm materials, for positive and negative tone chemically amplified resists. We used Thermo Gravimetric Analysis (TGA), Fourier Transformed Infra Red Spectroscopy (FTIR) and Atomic Force Microscopy (AFM) in order to check model formulations based on PHS, methacrylate or cyclic olefin polymers with various protecting groups having different activation energies and formulated with or without PAG and in order to understand the impact of the photoactive compound in the resist degradation behavior during plasma etch.

Study of 193-nm resist degradation under various etch chemistries

The effectivity of 193nm photoresists as dry etch masks is becoming more and more critical as the size of integrated devices shrinks. 193nm resists are known to be much less resistant to dry etching than 248nm resists based on a poly(hydroxystyrene) polymer backbone. The decrease in the resist film budget implies a better etch resistance to use single layer 193nm photoresists for the 65nm node and beyond. In spite of significant improvements made in the past decade regarding the etch resistance of photoresists, much of the fundamental chemistry and physics that could explain the behaviour of these materials has to be better understood. Such knowledge is necessary in order to propose materials and etch processes for the next technology nodes (45nm and below). In this paper, we report our studies on the etch behaviour of different 193nm resist materials as a function of etch chemistry. In a first step, we focus our attention on the interactions between photoresists and the reactive species of a plasma during a dry etch step. Etch experiments were carried out in a DPS (Decoupled Plasma Source) high density chamber. The gas chemistry in particular was changed to check the role of the plasma reactive species on the resist. O2, Cl2, CF4, HBr and Ar gas were used. Etch rates and chemical modifications of different materials were quantified by ellipsometry, Fourier Transformed Infrared Spectroscopy (FTIR), and X-Ray Photoelectrons Spectroscopy (XPS). We evaluated different materials including 248nm model polymer backbones (pure PHS or functionalized PHS), and 193nm model polymers (PMMA and acrylate polymers) or resist formulations. Besides the influence of resist chemistry, the impact of plasma parameters was addressed.

Thermal phenomena in acrylic 193-nm resists

A combination of methods has been applied to determine the glass transition and decomposition temperatures for several series of methacrylic copolymers used in commercial 193 nm resist as a function of the environment experienced by the protective group. The decomposition of the MAdMa and MLMA monomers which are the basis of the commercial AZ EXP AX- 1000P system is not appreciably catalyzed by the presence of MAA comonomers, leading to the conclusions that there is no autocatalytic effect in the deprotection of photoresists using these groups. AZ EXP AX1000P is found to have a high Tg of about 154 degrees C, which is corroborated by thermal flow measurements of developed resist features. Due to a decomposition process initiated by one of the other resist components, the formulation is presently not of the annealing type.

Extensive thermal characterization of advanced resists by modulated temperature DSC (MT-DSC): application to acrylate based 193 nm resists

The extensive thermal characterization of a typical acrylate based 193 nm resist is presented using the new MT-DSC technique. Unlike conventional DSC, the deconvolution of the reversing component with the nonreversing deprotection signal allows the determination of the Tg of these systems. The influence of the concentration in THP protecting group is investigated. The results obtained point out some of the problems encountered in the design of acrylate based resists such as Tg value and microphase separation.

Investigation of 193-nm resist and plasma interactions during an oxide etching process

193 nm chemically amplified resists currently meet the lithographic targets for the 130 nm and 90 nm nodes. However, the integration of such 193 nm materials is still an issue due to lack of etch resistance of 193 nm resist chemistries. Indeed, depending on the etch conditions (etch chemistry, power, temperature, etc.) 193 nm resist pattern degradations can be observed such as strong surface roughness, pinholes or even a loss of mechanical stability. In this work, the interactions between an oxide etch plasma and different 193 nm Methacrylate based contact hole resists have been investigated for the 130 nm node. All the resists belong to the Fujitsu platform, with various activation energies for their protecting groups. As a result, it has been observed that depending on the resist, a partial or complete loss of the carbonyl groups can take place during the oxide etch step, leading to a loss of etch resistance and pattern stability. In addition, it has been shown that an uncontrolled deprotection reaction of such 193 nm resist film can induce some transient changes in their physico-chemical properties, such as a decrease of the resist glass transition temperature and flow temperature. Uncontrolled 193 nm resist deprotection leads to mechanical stress in the polymer film, inducing adhesion issues and bubble formation, as well as resist flow temperature decrease. As a conclusion, a stable and reliable photo-etch step involving a 193 nm resist should take into account the limitations introduced by possible plasma and resist interactions. This can be achieved by some etch recipe adjustments, such as the precise control of the cathode temperature during the etch step, as well as some 193 nm resist formulation optimization in order to avoid strong resist deprotection during the etch step.

Properties and mutual interactions of various acrylate monomers used in 193 nm resists

Both the physical and chemical properties of various acrylate monomers, typically used in polymer matrices for 193 nm lithography, are investigated. The influence of each monomer on the glass transition temperature (Tg) of the final polymer is determined. The high Tg values measured for the different polymers lead to the conclusion that most acrylate based formulations tend to be non-annealing type resists. Mutual chemical interactions between monomers are also demonstrated. These phenomena underline the limits of the introduction of alicyclic ester groups in acrylic polymers and the complexity of the mechanisms involved in these resists.