Aberrations, aberrations, here there everywhere but how do we collect useful data that can be incorporated into our simulators? Over the past year there have no less than 18 papers published in the literature discussing how to measure aberrations to answering the question if Zernikes are really enough. The ability to accurately measure a Zernike coefficient in a timely cost effective manner can be priceless to device manufacturers. Exposure tool and lens manufacturers are reluctant to provide this information for a host of reasons, however, device manufacturers can use this data to better utilize each tool depending on the level and the type of semiconductors they produce. Dirksen et al. first discussed the ring test as an effective method of determining lens aberrations in a step and repeat system, later in a scanning system. The method is based on two elements; the linear response to the ring test to aberrations and the use of multiple imaging conditions. The authors have been working to further enhance the capability on the test on the first small field 157 nm exposure system at International SEMATECH. This data was generated and analyzed through previously discussed methods for Z5 through Z25 and correlated back to PMI data. Since no 157nm interferemetric systems exist the lens system PMI data was collected at 248nm. Correlation studies have isolated the possible existence of birefringence in the lens systems via the 3-foil aberration which was not seen at 248nm. Imaging experiments have been conducted for various geometry's and structures for critical dimensions ranging from 0.13micrometers down to 0.10micrometers with binary and 0.07micrometers with alternating phase shift mask. The authors will review the results of these experiments and the correlation to imaging data and PMI data.
The design of 157nm photoresist is a daunting task since air, water, and most organic compounds are opaque at this wavelength. Spectroscopic studies led to the observation that fluorinated hydrocarbons improve the transparency of 157nm resist materials rather dramatically. These fluorinated resists have quickly become the prominent material platform for a variety of research activities. Regardless of wavelength, developing a practical photoresist material is always challenging; the added difficulties associated with 157nm radiation complicates the overall design problem and severely limits the choice of material classes to work with. This paper will discuss our 157nm simulation and parameter extraction efforts that have been completed over the past few months at International SEMATECH. During the past year we have developed the methodologies and practical test methods that are needed to study the lithographic behavior of 157nm resist systems. Our work is based on procedures in the open literature and augmented by internal research.
Fluorocarbon based polymers have been identified as promising resist candidates for 157nm material design because of their relatively high transparency at this wavelength. This paper reports our recent progress toward developing 157nm resist materials based on transparent dissolution inhibitors. These 2 component resist systems have been prepared and preliminary imaging studies at 157nm are described. Several new approaches to incorporating these transparent monomers into functional polymers have been investigated and are described. The lithographic performance of some of these polymers is discussed.
The design of 157 nm photoresists is a daunting task since air, water, and most organic compounds are opaque at this wavelength. Spectroscopic studies1 led to the observation that fluorinated hydrocarbons and siloxanes offer the best hope for the transparency that is necessary for the design of an effective 157nm photoresist, and these classes of materials have quickly become the prominent platforms for a variety of research activities in this field. There have been a number of authors that have suggested that negative resists have unique attributes for specific device applications. Numerous authors have discussed negative photoresists over the years. There are many uses for such materials at various levels in a semiconductor device. One such use is with complementary phase shift mask thus eliminating the need for a second exposure step. This paper reports our recent progress toward developing a negative 157nm resist materials based on fluoropolymers with crosslinkers that are transparent at 157nm. The authors will report on the synthesis of the polymers used in this work along with the crosslinkers and other additives used in the formulation of the photoresist. Imaging experiments at practical film thicknesses at 157nm with binary and strong phase shifting masks will be shown demonstrating imaging capabilities. Spectroscopic data demonstrating chemical mechanisms and material absorbance will be shown along with other process related information.
The development of sufficiently transparent resin systems is one of the key elements required for a successful and timely introduction for 157 nm lithography. This paper reports on the Simple Transmission Understanding and Prediction by Incremental Dilution (STUPID) model, a quick back-of-the-envelope increment scheme to estimate the absorption of polymers at 157 nm. A number of promising candidate resins based on norbornenes are discussed, and results with a first 157 nm resin system developed at the University of Austin are presented. The new system is based on copolymers of norbornene-5-methylenehexafluoroisopropanol (NMHFA) and t-butyl norbornene carboxylate (BNC), formulated with an acetal additive obtained by copolymerization of t-butyl norbornene-5-trifluoromethyl-5-carboxylate (BNTC) with carbon monoxide. Lithographic performance of this system extends to 110 nm dense features using standard illumination and a binary mask, or 80 nm semi-dense and 60 nm isolated features with a strong phase shift mask. The dry etch resistance of this resist is found to be slightly lower than APEX-E DUV resist for polysilicon but superior to it for oxide etches.
Contamination of optical elements during photoresist exposure is a serious issue in optical lithography. The outgassing of photoresist has been identified as a problem at 248nm and 193nm in production because the organic films that can be formed on an exposure lens can cause transmission loss and sever image distortion. At these exposure energies, the excitation of the photo acid generator, formation of acid, and cleavage of the protecting group are highly selective processes. At 157nm, the exposure energy is much higher (7.9 eV compared to 6.4 eV at 193nm) and it is known from laser ablation experiments that direct laser cleavage of sigma bonds occurs. The fragments formed during this irradiation can be considered as effective laser deposition precursors even in the mid ppb level. In this study, methods to quantify photoresist outgassing at 157 nm are discussed. Three criteria have been set up at International SEMATECH to protect lens contamination and to determine the severity of photoresist outgassing. First, we measured film thickness loss as a function of exposure dose for a variety of materials. In a second test we studied the molecular composition of the outgassing fragments with an exposure chamber coupled to a gas chromatograph and a mass spectrometer detector. Our third method was a deposition test of outgassing vapors on a CaF2 proof plate followed by analysis using VUV and X-ray photoelectron spectroscopies (XPS). With this technique we found deposits for many different resists. Our main focus is on F- and Si- containing resists. Both material classes form deposits especially if these atoms are bound to the polymer side chains. Whereas the F-containing films can be cleaned off under 157nm irradiation, cleaning of Si-containing films mainly produces SiO2. Our cleaning studies of plasma deposited F-containing organic films on SiO2 did not indicate damage of this surface by the possible formation of HF. Despite that we strongly recommend engineering measures to overcome contamination by resist, such as optimizing the purge flow between the final lens element and wafer surface or utilization of a lens pellicle.
157nm lithography is expected to be the lithography choice for the 100nm-technology node, which is scheduled to be in full-production in 2003. However, due to 157nm photons being strongly absorbed by commonly used polymeric organic materials, a completely new class of material (containing F and Si-O) will be needed for 157nm Single Layer Resist (SLR) system. It is expected that the 157nm SLR system development will take greater than 3 years, which the industry will barely have, until the projected 2003 production schedule. In an attempt to fill the gap and to provide working resist system, using thin (<100nm)films of existing resist materials along with inorganic thin hardmask/BARC films is an attractive approach. In this paper, we report the optical constants (n % k at 157nm as well as 193nm and 248nm) of various thin film hardmask/BARC candidate materials (SixNyHz, SixOyNz, SixCyCVD and TixNyPVD films) measured by VUV-VASE. The films' atomic compositions, determined by RBS/HFS, were varied by controlling feed gas flow rates in order to vary the optical behavior. However, we limited our study within the low process temperature PE-CVD and PVC films due to our intention of using these films along with LowK(2.7approximately equals 2.0) dielectric materials. In addition, we will also report the optical constants of two types of LowK materials (PE-CVD OSG film and Spin- On/Cure low-density organosilicate dielectrics by JSR.) The data is, then, used to optimize the physical properties (n & k) and utilized to determine suitable hardmask/BARC material for 157nm exposure using Prolith II simulation. The results containing property of these hardmask/BARC candidate films and our optimization analysis along with the first successful pattern transfer feasibility demonstration into realistic substrate material (poly-Si) using ultra thin resist (currently existing) at 157nm optical lithography are reported.
193 nm photoresists on the market today can be classified into three different chemical platforms. The first platform involves acrylate type polymers, the second one cycloolefin- maleic anhydride (COMA) type polymers, and the third one a mixture of both. In this paper, we present a complete review of the lithographic performances at the 130 nm node, for 10 different commercially photoresists, coming from the three different chemical platforms. The results include various criteria: linear resolution, depth of focus, dose latitude, proximity bias and edge roughness for 130 nm lines (various pitches from isolated to 1:1 dense), depth of focus for 100 nm isolated lines, depth of focus and dose latitude for 140 nm contact holes, PEB temperature sensitivity (CD variation vs PEB temperature), thermal stability (post development bake stability), exposure -- PEB delay stability. Also, pattern collapse tendency and etch selectivity to both polysilicon and SiO2 are presented. We then correlate some of these results to the thermal properties (glass transition and decomposition temperatures) of the materials. We finally conclude about the pros and cons of each chemical platform for achieving the 130 nm node requirements.
The 193 nm photoresist generation will need several technological approaches in order for it to be successfully integrated into manufacturing. These approaches include bilayer, single layer and top surface imaging resists. Bilayer resists offer the advantages of thin film imaging (resolution, depth of focus) and potential advantages in plasma etch resistance due to the possibility of incorporating aromatic components into the undercoat. We have developed a prototype bilayer resist system based on a silicon containing methacrylate imageable layer and a crosslinked styrenic copolymer undercoat which has shown 0.13 micrometers resolution. In this paper we will discuss the effects of O2-RIE and polysilicon etch on resist and substrate profile, selectivity and iso-dense resist.
Extension of optical resolution using technologies such as alternating phase-shifting mask, chromeless PSM, or attenuated PSM combined with off-axis illumination is necessary for manufacturing 180 nm devices. Strong shifters, like alternating and the chromeless type PSM, hold the most promise for optical extension to k1's less than 0.5. However, it is difficult to produce error free masks of these types using today's technologies. Focus-exposure data for 180 nm and 250 nm grouped lines produced with an alternating PSM and a KrF stepper show an asymmetric response about the center of focus (CoF) and an exposure dependent shift in CoF. The CoF changes with changes in phase for varying aerial image critical dimensions (CD), and thus explains that deviations from 180 degree phase cause the observed changes in CoF. Phase error also induces change in image placement. Modeling predicts that the observed CoF is the result of phase greater than or less than 180 degrees, the shape of the entire focus-exposure CD response curve can elucidate which error is observed as can image placement deviation. Monitoring shifts in image placement lends itself to measuring phase error using aerial image analysis. In this paper modeling using the vector analysis package of PROLITH/2 (FINLE Technologies) suggested that the experimentally observed CoF could be explained by a 10 degree error to either side of 180 degrees. Aerial image shifts measured with a MSM-100 AIMS tool (Zeiss) indicated that the error was above 180 degrees. Then combining simulation and aerial image data, the effective phase of the mask was estimated to be 190 degrees.
The properties of a new anti-reflective coating for 248 nm lithography are described. It is formed by thermally cross-linking a spin-on organic coating, and has an absorbance greater than 12/micrometers. It is compatible with UVIIHS and APEX-E photoresists. Thin films (less than 600 angstrom over silicon substrates) are found to completely suppress standing waves, to reduce EO swing curves to less than 3%, and to offer good CD control over typical field oxide topography. The etch rate was found to be comparable to that of the APEX-E photoresist.
Chemical changes within a resist material (for example, resulting from the exposure and subsequent chemical reactions during post exposure bake) will in general, result in a change in diffusivity of components within that material. In the case of positive chemically amplified resists, the diffusivity of the photo-generated acid changes as a function of the extent of polymer deprotection. The deprotection reaction leads to the generation of small reaction product molecules, some of which are volatile. The liberation of these reaction products causes an increase in the free volume and changes in the chemical behavior in the exposed area. These changes, primarily the increase in free volume, results in an increase in the diffusivity of the acid. Low exposure areas have lower acid diffusivity, leading to a lower efficiency of reaction. This results in a contrast enhancement of the latent image due to the concentration dependent diffusivity of the acid. In this paper, a concentration dependent diffusivity expression is incorporated into a lithography simulator to explore these effects on lithographic performance. Using the assumption of free volume, suitable expressions for the diffusivity are examined and compared to experimentally measured values. The experimental work consists of XP-9402 positive acting, chemically amplified resist that was imaged using different thermal doses.
Improvements in modeling of chemically amplified resists are necessary to increase the capability of doing `What if' simulations and to help interpret experimental data. One method to minimize the difference between modeled and experimental results is to use an underlying database of experimentally determined bulk dissolution rates as the source of the input parameters for the imaging engine of the lithographic model. In this paper, a R(E,z) to R(m,z) converter is discussed. The converter takes into account the amplification factor, kinetic effects and acid loss. The underlying data consist of a positive acting chemically amplified resist, XP-9402, that was processed using various post exposure bake conditions. With conversion to R(m,z), the energy of activation and Arrhenius coefficient for both the deprotection reaction and acid loss, the rate of photoacid formation, C, the chemical amplification factor for a given thermal dose and the ratio of deprotection rate constant to acid loss rate constant can be determined. These parameters are then used in the lithographic simulator PROLITH/2 version 4.1a. Results are used to understand lithographic results for photoresist that had been processed at different temperatures.
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