The advance in microlithography has greatly helped the development of micro optical elements. Large array of microlenses can now be fabricated in the same fashion as manufacturing of integrated circuit at low cost and high yield [1-2].
Because microlens array requires well-defined and continuous surface relief profile, special methods are needed to supplement the normal microlithography to produce those spherical structures [3]. Various techniques have been developed, and the most widely used is multi-step photolithography with thermal resist reflow. However, the alternative grayscale photolithography technique appears to be the one as the most flexible and versatile method [4].
Indeed, this approach is a one-level lithography process enabling the development of 3D profiles in a photoresist masking layer. In addition, with the need to maintain or improve image quality at an ever-smaller pixel size, grayscale technic can offer one way to compensate the loss of the photosensitive area by achieving zero-gap microlens. One other advantage of grayscale is the possibility to have, from a single lithography, objects of different shapes, but also at the same time of different sizes (especially heights); which is possible with classical lithography only by doing multi-patterning.
There are several options for performing grayscale lithography, for example the HEBS mask (high energy beam sensitive) which darkens under exposure to electrons. The option that has been chosen is to use a grayscale reticle, with varying chromium features densities that locally modulate the intensity of transmitted UV light. Being non-uniformly exposed, this allows the creation of a relief structure in the resist layer after development. The resist height after development depends on the intensity of the incident light, the exposure time and the contrast of the resist. So contrary to conventional lithography where the goal is to achieve straight resist pattern profiles, grayscale lithography enables the realization of progressive profiles, which requires smooth resist contrast curve. The other specificity of these resists is that they must crosslink without flowing.
In this paper, we evaluate resists from different suppliers to generate microlenses smaller than 5μm via a grayscale mask. The study consists in establishing the contrast curves of these resists according to different process parameters, giving the designer great control of grayscale levels that can be achieved for a given resist. Then, pattern various microlenses shapes in these resists to evaluate the residual resist thickness according to the gray levels. With the final objective of establishing a relationship between these contrast curves and the profile variations at the microlens level to compute a suitable and accurate grayscale mask [5].
Microlens arrays are used on CMOS image sensors to focus incident light onto the appropriate photodiode and thus
improve the device quantum efficiency. As the pixel size shrinks, the fill factor of the sensor (i.e. ratio of the
photosensitive area to the total pixel area) decreases and one way to compensate this loss of sensibility is to improve the
microlens photon collection efficiency. This can be achieved by developing zero-gap microlens processes. One elegant
solution to pattern zero-gap microlenses is to use a grayscale reticle with varying optical densities which locally
modulate the UV light intensity, allowing the creation of continuous relief structure in the resist layer after development.
Contrary to conventional lithography for which high resist contrast is appreciated to achieve straight resist pattern
profiles, grayscale lithography requires smooth resist contrast curve. In this study we demonstrate the efficiency of
grayscale lithography to generate sub-2μm diameter microlens with a positive-tone photoresist. We also show that this
technique is resist and process (film thickness, development normality and exposure conditions) dependent. Under the
best conditions, spherical zero-gap microlenses as well as aspherical and off-axis microlenses, which are impossible to
obtain with the conventional reflow method, were obtained with satisfying process latitude.
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.
In this paper we investigate an in situ measurement of polymer and lithographic resist film mechanical properties on a silicon substrate, with a Dynamical Mechanical Analysis tool (DMA).
This technique allows the measure of the glass transition temperature (Tg) of the resist film (thickness range: several μm to few nm) and its elastic and viscous modulus variations with a high
precision and reproducibility. Indeed, DMA appears to be more sensitive than other thermal analysis methods like Differential Scanning Calorimetry (DSC), to monitor Tg variations induced by film
thickness reduction.
First we will discuss the performance of the tool and present the variations of the glass transition
temperature of a PMMA (PolyMethylMethAcrylate) layer as a function of its thickness: we observe a shift towards higher temperatures up to 30°C when the film thickness decreases from one
micrometer to 10nm. This behavior highlights the importance of surface properties versus bulk. We will also discuss the interest of the DMA technique applied to more complex chemistries, as it is the
case for lithographic resist formulations, i.e. a blend of polymer with grafted functionalities, photoactive compounds and various additives. We successfully applied this technique to
characterize different resist film thicknesses and we observed the effect of the thickness on the thermal events. Such kind of change is important to take into account in the optimization of material
performance for thin film applications. This material understanding allows to better define the process conditions and can be applied to different microelectronic topics such as: thermal flow
treatment of positive tone photo resists, hot-embossing nanoimprint, cross linking reactions with negative tone resists or so called "hardening" processes.
In this paper we develop a methodology in order to monitor the organic outgassing level of
BARC and resist materials, during the exposure or bake steps of the lithographic process. We
present two different approaches, both based on thermal desorption-gas chromatography/mass
spectrometry (TD-GC/MS) techniques. First we used an indirect method to monitor the byproducts
outgassed during the exposure step. Secondly we check with an in-situ measurement
the outgassing behaviour as a function of bake conditions. These two approaches are
illustrated using different resist and BARC formulations. Finally, TD-GC-MS technique is
integrated in a largest outgassing evaluation protocol, and results obtained by this technique
are correlated with other characterization methods such as TGA, FTIR and defectivity
monitoring.
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.
In this paper, we investigate the capabilities to form small contact holes with various 193nm
resists applying a thermal flow process. We first compare the material properties (glass
transition temperature Tg and thermal deprotection TD) of different 193nm resists to our
reference process for thermal reflow, namely the 248nm reference resist (RoR). The main
difficulty related to 193nm acrylate backbone is the high Tg value, which implies some flow
bake temperature closed to or superior to the deprotection temperature. Depending on the
resist chemistry, different behaviours have been observed such as acceleration of the flow
rate, formation of bubble defects linked to gaseous by-products or even contact hole diameter
increase. These results are strongly dependent on the chemical reactions occurring in the resist
film at the same time as the film softening. In order to better select the most promising 193nm
resist candidates for contact hole reflow technique, we also develop a polymer flow
measurement with Dynamic Mechanical Analysis (DMA). By measuring the creep
compliance of the resist film spin-coated onto a silicon wafer under various bake
temperatures, we are able to define the optimal temperature range for resist flow.
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.
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.
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.
ArF lithography is the current ramp-up technology for next generation devices. However, some manufacturing issues still remain when considering the resist design for the most advanced processes. Several polymer platforms have been proposed, among them, Methacrylate, CycloOlefin-alt Maleic Anhydride, and even pure Cyclo-Olefin. More recently, Vinyl-Ether Maleic Anhydride (VEMA) polymers have demonstrated potential in terms of both lithographic properties and etch capabilities. In this paper, the evaluation of some advanced samples of VEMA resists for 120nm and sub-120 nm gate applications will be discussed. The various criteria investigated for this study were; focus and exposure latitude for 120 and 100 nm lines (1/1.5 L/S to isolated lines), Iso-Dense bias, Line End Shortening (LES), Line Edge Roughness (LER), masking linearity, BARC compatibility, sensitivity to PEB temperature and electron beam, and finally etch resistance. Additionally some process optimizations were tested in order to minimize Iso-Dense Bias and the LER of the resists (See figure 1). In fact, this latter parameter has been a major focus of this work in improving the VEMA resist chemistry since its introduction and preparing it for device manufacture. The results obtained when varying parameters such as resist formulation, development conditions will be reported and so will demonstrate the current maturity of the most advanced VEMA samples.
The constant reduction in critical dimensions required for new device generations and the use of sub-wavelength lithography impact the quality of the aerial image transferred into the resist layer. As an example, the intrinsic bias between isolated and dense features in the aerial image is becoming more and more significant, requiring better performance from the resist process to cancel this effect. This work investigates the process mechanisms leading to Iso-Dense bias (I-D bias) reduction for two 193 nm methacrylate based resists at constant optical settings, as a function of PEB temperature. In both cases, it has been possible to find optimized process conditions leading to reduced I-D bias values, but it appears that the leading mechanisms involved during PEB are different and do not seem equivalent in terms of resist capabilities. Reaction controlled resists, which work with a Diffusion Well effect during PEB, that is a high diffusion contrast between exposed and unexposed areas, allow I-D bias compensation without degrading resolution performance. On the contrary, diffusion controlled resists, which usually require high Post Exposure Bake (PEB) temperature to thermally boost the deprotection reaction, do not keep a high diffusion contrast between exposed and unexposed areas during PEB. Consequently, for these resists, best process conditions for I-D bias reduction do not correspond to the optimized process conditions for other resist performance, such as resolution and DOF. In this paper, the two different mechanisms which drives the acid catalyzed deprotection during the Post Exposure Bake step have been studied using different characterization techniques (modulated Temperature DSC, Dielectric analysis, in-situ Ellipsometry) and process performance has been correlated with 193 nm resist component properties (Polymer matrix, protecting groups or PAG characteristics).
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.
Norbornene-alt-Maleic Anhydride polymers have been recently introduced to fulfill the transparency and plasma durability requirements demanded for 193 nm lithography single layer resist system. Very few information exist in the literature on these new materials. This paper investigates the properties of some representative polymers of this family, and tries to draw general rules. The investigation of the physico-chemical properties requires advanced characterization techniques such as modulated temperature DSC. The copolymers of N/MA and Methacrylate monomers appear to show interesting Tg switch effect. Additional information have been obtained with the implementation of Dielectric Analysis. The high rigidity of these polymers can explain their high resolution performance reported in the literature. More generally N/MA polymers exhibit unusual properties that raise new questions on the structure of the resist film and on the process mechanisms involved.
The implementation of new resist materials and advanced lithography processes requires new characterization techniques in order to understand the behavior of these systems and optimize their design. Examples are presented on two recent 193nm experimental formulations using three different characterization techniques, namely Modulated- Temperature DSC, in-situ ellipsometry and dielectric analysis. The results obtained provide new experimental evidence of the diffusion and reaction mechanisms involved in chemically amplified formulations.
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.
The large variety of protecting groups that can be employed for acrylate based resists increases the number of mechanisms encountered when implementing these new materials. Some of these phenomena, related to both their physical and chemical properties, are investigated.
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