The effects of mask bias on the Mask Error Enhancement Factor (MEEF) of 180 nm contact holes is studied through lithographic simulation using commercial software and a DUV (248 nm) ESCAP photoresist model. Dense contacts show higher MEEF than isolated or semi-dense contacts. However, dense features exhibit a minimum in MEEF at a single negative mask bias (CD on reticle > 180 nm). Aerial image simulations indicated that low MEEF correlates approximately with high normalized aerial image log-slope (NILS). Hence, factors that affect NILS, such as numerical aperture, partial coherence, and wavelength, also influence MEEF, although without altering the optimum mask bias for minimum dense MEEF. Numerical aperture and wavelength of exposure have the greatest influence on MEEF. For 180 nm contact holes worst case MEEF values below 2 can be achieved by increasing NA to 0.8 at 248 nm or by decreasing (lambda) to 193 nm at 0.6 NA. Resist identity has little influence on the magnitude of MEEF but was the only factor affecting the mask bias setting for minimum dense MEEF.
Via fill performance of AR7 (KrF anti-reflectant) and a prototype 193nm anti-reflectant were measured for 600 and 1000 nm deep vias in thermal oxide. Simple fitting functions were found which gave good agreement with experimental data (Rsq over 0.84). The most important factors were AR thickness, via duty ratio and via depth. The importance of these factors was different for the different anti-reflectants.
Proc. SPIE. 4345, Advances in Resist Technology and Processing XVIII
KEYWORDS: Lithography, Data modeling, Diffusion, Inspection, Photoresist materials, Promethium, Photoresist processing, Process modeling, Standards development, Picture Archiving and Communication System
The use of experimental development rate information is used to demonstrate various deficiencies in the dissolution rate equations commonly employed in commercial lithography simulation programs. An improved version of the Notch dissolution rate equation, incorporating one new parameter, is proposed, which addresses the observed deficiencies. Simulation work comparing the new equation to the standard Notch model reveals significant differences in process window and exposure margin, yet negligible changes in feature profile and iso-dense bias at best focus and exposure.
Chemically amplified resist models for Shipley 193nm resists S6 and V2 were developed for use with commercial lithographic modeling software. S6 and V2 are based on methacrylate and vinyl ether/maleic anhydride polymer platforms, respectively, and contain an onium salt photoacid generator and proprietary base quencher. Fundamental parameters for these resists were determined experimentally and subsequently tuned to establish valid models. Current modeling algorithms appear sufficient to predict the lithographic behavior of typical features of interest. Experimental measurements that indicate that these 2 resists are similar with respect to acid photogeneration efficiency (0.04 cm2/mJ), polymer deprotection rate constant (0.05- 0.1 l/s), and developer selectivity. However, S6 exhibits greater transparency (0.35 1/micrometers vs. 0.5 1/micrometers for V2), lower acid diffusion, and greater surface inhibition. V2 exhibits considerably smoother dissolution.
This study examines through simulation the effects of mask bias and illumination settings on the MEEF and process window of 180nm contact holes. Previous work has shown that application of a global mask bias of -40 or -60 nm collectively minimizes MEEF for 180 nm contacts of varying pitches printed simultaneously with binary mask or 6 percent transmittance attenuated phase shifting mask respectively. Simulations in the present work show that in addition to reducing MEEF, negative mask bias lowers sizing energy and reduces sidelobe formation in patterns printed with 6 percent AttPSM. However, increased film loss from dense contacts and slightly reduced process window also result from the use of negative mask bias. These drawbacks can be partly mitigated by optimizing the illumination parameters. Higher (sigma) , higher NA, and shorter wavelength of exposure all reduce or eliminate top loss and increase overall exposure latitude, while higher (sigma) also increases focus latitude at low NA. At higher NA, a tradeoff exists between lower MEEF with negative mask bias and loss of focus latitude with 6 percent AttPSM.
The thickness and complex refractive indices of the thin films on a silicon wafer during lithographic imaging are critical factors affecting the processing of integrated circuits. The inorganic materials involved, such as the silicon substrate and inorganic anti-reflection coatings, are usually well characterized or present few difficulties. The optical properties of organic materials, such as photoresists and anti-reflection coatings have been more difficult to determine with confidence. In general, for each material on the substrate, three values need to be determined; they include the thickness and the real and imaginary parts of the refractive index at the exposure wavelength. Single measurements of reflectance or ellipsometric parameters do not provide sufficient degrees of freedom for determining these three unknowns. Typically, this problem is resolved by collecting reflectance or ellipsometric data over a range of wavelengths. However, because n and k functions of wavelength, two additional unknowns are present for each additional wavelengths. This problem is typically resolved by fitting n and kn to various spectral functions in order to reduce the number of unknowns. Unfortunately, the functional forms used are frequently inappropriate for the organic materials of interest. The essence of the new method is the use of data from coatings having different thicknesses in order to provide the degrees of freedom necessary for a solution. This is achieved at a single wavelength and thereby avoids the spectral model fits that are frequently fraught with problems. The new method is demonstrated by determining thicknesses and n and k values at the exposure wavelength for a photoresist and an anti-reflection coating designed to be used with 193 nm exposures. The optical properties of two 248 nm anti-reflection coatings are also determined over a spectral range.
Via fill and intervia coverage of AR5 and AR7 anti-reflectants were measured for 608nm deep vias in thermal oxide. Fitting functions were found which gave god agreement with experimental data. The most important factors were AR thickness, via duty ratio and via width. The importance of these factors was different for via fill and intervia coverage, and for AR5 and AR7. AR7 was found to fill a range of vias to a depth of 25 percent to 50 percent, suitable for a partial planarization approach to dual damascene fabrication. Planarization was shown to be relatively insensitive to several coat process variations, but sensitive to solution surface tension.
The ability of a commercial lithography simulator to accurately predict the pitch dependent print bias of a conventional i-line resist is investigated, under conventional and annular illumination schemes for two critical geometries. The influence of the simulator settings and resist modeling parameters on the observed bias are determined. The result reveal that the simulation predictions are qualitatively, but not quantitatively, reflective of experimental data and are remarkably insensitive to changes in either the simulator settings or the parameters used to describe the resist process.
This paper describes the development of a lithographic model for Shipley UV6 DUV photoresist from fundamental materials constants. Parameters describing optical absorbance, acid generation, acid generation, acid diffusion, resin deprotection, and resists dissolution rate were measured and input into PROLITH/2. Initial simulations showed significant deviations from observed lithographic performance. Simulated E0 swing curves showed a large bulk effect absent in experiment data. Simulated lines showed excessive top loss and tapering which were corrected by the artificial introduction of a surface base contaminant. Lithographic process windows for lines and contacts cold be successfully simulated after considerable adjustment of the acid diffusion coefficient and development parameters, but accurate simulation of both profile and process window with a single set of parameters was only possible for contacts. The manipulations required to match simulated and experimental data did lead to some insights into resists materials design. Simulations suggest that lowering acid diffusion should increase exposure latitude and reduce film thickness loss, while suppressing the resist dissolution rate at the onset of deprotection should improve both focus and exposure latitude.
A systematic approach was taken in order to improve the planarity of a DUV anti reflectant (AR) utilized for various lithographic steps, particularly those involving a patterned transparent layer. These layers can occur in both front and back end processing. Two approaches were pursued to accomplish this. The first approach was to minimize the molecular weight of the AR polymer. Polymers with weight average molecular weights from 45,000 daltons to as low as 2,300 daltons were evaluated. The planarity of the AR improved significantly for polymers with Mw's below 20,000 daltons. The second approach was to add plasticizers in order to reduce the glass transition temperature of the precrosslinked film. The addition of plasticizers to the AR was effective in increasing the planarity. One of the plasticizers contained a DUV chromophore used to maintain the required optical density of the AR. It was proven possible to make these changes while maintaining lithographic performance in both resist profiles and reflection control.
The post-exposure delay (PED) stability of several chemically amplified DUV resists in unfiltered environments is shown to be strongly dependent on the standing wave intensity. The use of a bottom antireflective layer diminishes the rate of CD change for UVIIHSTM, UVIIITM, APEX-E and UV5TM resists by a factor of three or greater. Increasing the post exposure bake to diffuse outstanding waves results in a three to six fold improvement with UVIIHS, UVIII, UV5 and UV6TM. These resists show the greatest stability when soft baked at high temperatures to reduce the diffusion rate of airborne contaminants, and post-exposure baked at high temperatures to diffuse out the standing wave pattern.
A thermally cross-linking bottom anti-reflectant, AR2, is evaluated. The material can be made in a range of absorptivities. An optimum optical density of about 9/(mu) ( 248 nm) which lowers photoresist swing curves to less than 2%, was chosen from optical modeling and etch rate measurements. The material offers spin bowl compatibility with common spin- coating solvents, and etch rates and conformality improvements over commercially available materials. Good profiles were obtained for several photoresists, and wider process windows than on planar silicon.
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.
Two approaches that control the overflow of silylated material that can occur subsequent to surface imaging of acid-hardened resists are introduced. Treatment of the resist surface with a cross-linking agent [bis(dimethylamino)dimethylsilane] prior to silylation can produce a surface layer with the physical integrity to constrain silylated material. Alternatively, the unexposed areas of the resist may be partially removed by development with a basic solution. The surface depressions thus produced allow volume expansion to occur during silylation without causing overflow.
Proc. SPIE. 1925, Advances in Resist Technology and Processing X
KEYWORDS: Deep ultraviolet, Etching, Image processing, Silicon, Manufacturing, Scanning electron microscopy, Very large scale integration, Reactive ion etching, Photoresist processing, Semiconducting wafers
Many approaches to surface imaging rely upon selective incorporation of silicon into an already imaged resist layer. SAHRTM (silylated acid hardened resist) obtains its selectivity by a lower rate of silicon incorporation into exposed and crosslinked areas, providing a positive tone image after RIE development. Two difficulties with the practical implementation of this approach have been the overflow of silylated material onto crosslinked areas, and reduced silicon incorporation in small openings. We have found that surface treatment with a bifunctional silylation agent (the `two gas process') can prevent overflow, and that removing part of the resist layer with dilute tetramethyl ammonium hydroxide (TMAH) (the `presilylation develop process') minimizes overflow and improves silicon incorporation in small features. With a predevelop step, feature size linearity is obtained below k1 equals 0.7, with uniformity and repeatability consistent with VLSI manufacturing practices.
A variety of analytical and process control techniques have been employed during process development activities for a 0.5 micrometers deep UV positive tone surface imaging process. Examples of applications of these methods for identification of primary positive tone surface imaging issues and process optimization for enhancement of ultimate resolution are described. Advantages and limitations for each technique are discussed.
This paper reports the initial results of an improved chemically amplified, positive-tone photoresist for use in DUV applications. This photoresist is shown to have the following properties: low absorbance at 248 nm (0.22/micrometers ), high resolution (0.35 micrometers lines and spaces in 1.0 micrometers thick resist), and good environmental stability. The resist did not show evidence of `T-tops' nor did any linewidth change occur over a five hour period.
Techniques commonly used to determine silicon penetration depth during deep UV surface imaging lithography are compared to a method referred to as plasma etch 'staining.' This methodology is described in detail and the results compared and correlated to Rutherford Backscattering Spectroscopy (RBS) and ellipsometric (film swelling) measurements. Effects of the staining parameters on the resulting silicon depth are also discussed.
A combination of metrology techniques was employed to fine tune the wavelength setting of a 248 nm excimer laser stepper to optimize performance. Scanning electron microscopy was used to document local resolution, proximity effects, and astigmatism, while GCA SMARTSETR and electrical resistance techniques were used to examine full field effects. Using the combined metrology methodologies, the authors documented the decrease in proximity effect, improvement in resolution, and increase in absolute lens distortion with negative shifts in laser wavelength setting, with a slight differential in the setting required to minimize horizontal versus vertical proximity effect and astigmatism. A wavelength offset of -2.3 angstroms from the nominal stepper setup wavelength was determined to be the best operating wavelength for these applications.
G- and i-line diazonaphthoquinone/novolak photoresist films are surface imaged with g-line, i-line and deep-UV steppers. Following optical exposure, the resist film is treated with aqueous solutions which deposit a catalyst for electroless metal deposition. Wet development of the exposed and catalyzed photoresist results in selective removal of catalyst along with the exposed portion of the underlying photoresist. Upon immersion in an aqueous electroless plating solution, metal is selectively deposited on the unexposed photoresist which is still bearing catalyst to yield a positive-tone plasma etch mask. Oxygen magnetron-enhanced reactive ion etching (O2 MERIE) provides high polymer etch rates (approximately equals 1 micrometers /min) with excellent selectivity (> 300:1) to 70-170 angstrom Ni films. In addition, large ion fluxes produce highly anisotropic etch profiles for faithful pattern transfer. The process has achieved 0.30 micrometers resolution with a 6:1 aspect ratio at 248 nm (0.35 NA). Printing of 0.40 micrometers lines and spaces has been achieved at i-line (0.45 NA) over Al steps.
This paper reports our recent studies of crosslinking of phenolic resins with melamines through 1H and 13C NMR, GPC, and dissolution rate changes. For the NMR studies, we used model phenolic compounds such as 4-ethylphenol, and the hexafunctional crosslinker, hexamethoxymethylmelamine (HMMM). The NMR clearly reveals that the crosslinking reaction occurs quantitatively at the hydroxyl site of the phenol. This result raises the question of whether the dissolution inhibition observed in the ANR resists is due to -OH site consumption or to the rapid rise in molecular weight of the phenolic polymer. Comparison of tetrahydrofuran (THF) extraction vs. aqueous tetramethylammonium hydroxide (TMAH) development shows that the dose required to insolubilize the resist is much higher for THF. Gel permeation chromatography on the soluble fraction extracted into THF showed a fraction with molecular weights up to 400,000 Daltons. We believe that crosslinking and -OH site protection provide synergistic dissolution selectivity in TMAH, leading to high contrast and high resolution. Finally, we present results on the effect of (chi) , which is proportional to the ratio of phenolic hydroxyl groups to melamine methoxy groups, on the lithographic performance of ANR photoresists. At low (chi) , the DUV resists can be used as increased absorption resists over topography, and development times can be shortened significantly. We have also found that increasing the melamine loading can lessen the degree of bridging residue observed between lines.