Luc Van den Hove

Comparison of various phase-shift strategies and application to 0.35-μm ASIC designs

Phase shifting masks for real circuits have been investigated extensively only for DRAMs. In this paper, we report on the applicability of i-line phase shifting lithography to the production of application specific ICs (ASICs). The performance of several phase shift strategies is compared, using an i-line stepper with a numerical aperture of 0.48. Data preparation and mask technology considerations are taken into account. Emphasis is placed on the two most critical levels: poly gate and contact window. Results on poly topography are shown. For the poly level, the frequency doubling alternating shifter strategy in combination with a positive resist seems to be capable of printing features down to 0.35 micrometers CD, but the development of automatic phase shift level generation software is still in a preliminary phase. Edge contrast enhancement strategies in combination with a negative resist are considerably simpler, in particular the halftone PSM strategy. These strategies are also very useful in combination with a positive resist for the contact level, where a doubling of the process latitudes was obtained.

Characterization and correction of optical proximity effects in deep‐ultraviolet lithography using behavior modeling

We present the characterization of optical proximity effects and their correction in deep‐UV lithography using an empirically derived model for calculating feature sizes in resist. The model is based on convolution of the mask pattern with a set of kernels determined from measuring the printed test structures in resist. The fit of the model to the measurement data is reviewed. The model is then used for proximity correction using commercially available proximity correction software. Corrections based on this model is effective in restoring resist linearity and in reducing line‐end shortening. It is also more effective in reducing optical proximity effects than corrections based only on aerial image calculations.

Optical proximity effects and correction strategies for chemical-amplified DUV resists

Optical proximity effects (OPE) are narrowing the process window in the 0.25micrometers - 0.18micrometers CD range. Hence optical proximity correction (OPC) might be required. These proximity effects and correction strategies are studied in detail in this work. First, an evaluation methodology is derived for the three types of OPE (linewidth differences with pitch, end-of-line effects and corner rounding). Hence, the influence of various parameters on OPE is investigated for negative tone and positive tone resists, since clear differences exist in OPE for dark field and bright field masks. Linewidth differences with pitch are small for negative tone resists, end-of-line effects are less pronounced for positive tone materials. Obviously, optical parameters have an important influence on OPE. Also, loading effects during etch processes deserve attention. Aerial image based proximity correction is evaluated. With respect to CD variations with pitch, important improvements are obtained for some resists, but not for all materials. End-of-line effects and corner rounding are improved by the use of OPC in all our experiments. Superior proximity correction results are expected with the expansion of aerial image based OPC by implementation of resist models.

Liquid phase silylation for the DESIRE process

Liquid phase silylation is presented in this paper as a technique with improved silylation and dry development selectivity and hence improved process windows. Using this technique, the requirements imposed on silylation and dry development equipment are relaxed. The diffusion enhanced silylated resist process (DESIRE) has been presented as an attractive solution to overcome the inherent limitations of conventional wet develop lithography, such as the control of critical dimensions over highly reflective topography. Traditionally, the silylation process has been carried out using hexamethyl disilazane (HMDS), although lately, alternative gaseous agents such as tetramethyl disilazane (TMDS) have begun to exhibit certain advantages. The liquid phase silylation process simplifies the process and equipment requirements, and results in improved silylation selectivity. Liquid silylation has been found to be useful both for i-line and DUV exposures. The influences of resist and silylation solution composition have been investigated. Characterization of the silylation reaction and mechanism have been performed using thickness measurements, Fourier transform infrared spectrometry (FTIR), and Rutherford backscattering spectroscopy (RBS). Surface imaging based on liquid phase silylation has also been evaluated using phase shifting masks. The influence of partial coherence on the resolution and process latitudes using both conventional transmission masks and phase shifting masks have additionally been studied.

Feature biasing versus feature-assisted lithography: a comparison of proximity correction methods for 0.5*(lambda/NA) lithography

The effectiveness of two methods of optical proximity correction based on feature biasing and subresolution assisting features is compared by simulation and experiments. Parameters examined are overlapping focus- exposure windows for dense lines, semi-isolated and isolated lines, and line-end shortening. Binary and phase-shifting masks containing test and real IC design features are proximity corrected either by commercial software (in the case of feature biasing) or by manual correction using optimized size and placement of assisting features. The results indicate that, while both methods are effective in reducing optical proximity effects, the feature-assisted method is more advantageous in many cases.

Sub-half-micron deep-UV lithography using wet and dry developable resist schemes

DUV lithography is an emerging technology which promises excellent resolution coupled with an improved depth of focus. Whilst the hardware connected with this technology is rapidly maturing, there remain question marks over the suitablilty of present DUV resists in the fabrication of actual circuits. Two widely differing approaches are typified by Plasmask (DESIRE) which involves surface imaging and dry development and by the wet developable Shipley Megaposit SNR 248-1.0, which utilises acid catalysed chemistry. We have studied both materials using a contact printing system and the ASM-L PAS 5000110 DUV stepper (both at 248 nm wavelength). In particular we have compared the lithographic performance of both resists on a variety of substrates and topographical features, commonly encountered during processing. Practical issues such as the intrinsic adhesion, photospeed, exposure latitude, focus latitude, linearity, thermal resistance and etch resistance are presented. For the Shipley material we have compared its performance under various development conditions, with respect to resolution, photospeed, profile and residues. On the Plasmask material we have investigated the degree of silicon incorporation for various silylation conditions and for various Plasmask formulations. Significant differences have been noted for exposures made with this wavelength (248 nm) and those commonly reported with g and i-line exposure. Finally, the feasibility of using TMDS (1,1,3,3 Tetramethyldisilazane) as a silylating agent is presented.

CD control comparison of step-and-repeat versus step-and-scan DUV lithography for sub-0.25-μm gate printing

In this paper, the intra-field critical dimension (CD) control of a KrF step&scan and step&repeat system are investigated and compared. The scanners are expected to replace the conventional steppers in the manufacturing of integrated circuit generation of 0.18 micrometer and beyond, because of the larger field size and the intrinsic improvement in intra- field CD and overlay control using comparable lens design, complexity and cost. The work has been focused on sub-0.25 micrometer critical dimensions. A reticle design for both top- down CD measurements and electrical linewidth probing has allowed massive data collection and investigation of the impact of the metrology technique in CD control studies. From this study, it can be concluded that the stepper and scanner exhibit similar CD control at best focus, but the scanner improves the CD control of the stepper if the considered focus range increases. The CD control is governed by the reticle CD non-uniformity. Focus budget calculations indicate that reticle CD ranges of 40 nm (4x) are needed to bring the CD control of 0.2 micrometer grouped lines within acceptable ranges for realistic gate levels. For isolated lines, dedicated deep-UV resists and resolution enhancement techniques will be needed on top of this to obtain similar CD control.

Lithographic strategies for 0.35-μm poly gates for random logic applications

DUV lithography using wet developable resists can be used for the poly gate definition of 0.35micrometers CMOS processes. Four years ago, we demonstrated a resolution of 0.3micrometers L/S obtained with Shipley XP 89131 resist. Nevertheless, in order to make this and other resist processes suitable for real device applications, several problems had to be overcome. First, reflective notching and linewidth variations over steps turned out to be an important limitation. Some strategy to reduce this sensitivity to reflections has to be applied. Furthermore, in order to obtain a stable and reproducible lithographic process, process latitudes should be wide. Furthermore, a comparison between positive tone and negative tone resists is made with respect to their suitability for poly gate patterning. It was observed that negatively sloped resist profiles, as a result of the use of negative tone resists, create a controllability problem during in-line SEM inspections, and such profiles result in positively sloped poly profiles after etching. Positive tone resists have positively sloped resist profiles, but they require the use of a bright field mask, and hence reflections are much more of a problem. Also, positive tone resist are more sensitive to the delay effects.

Dry Development of Surface Imaging Resists: a Major Parameter for Process Optimization

Most surface imaging resist processes are based on selective incorporation of silicon during silylation, followed by dry development of the resist. The dry development is an important parameter in the resist processing, because it will influence the resist profile and the sensitivity to residues, and, hence, the process latitude. For the DESIRE process, the silicon diffusion profile will be more steep by applying a so called two step dry development, which will result in a steeper resist profile and less residue. For several dry development schemes, the resist profile and the sensitivity to residue are investigated. The etch selectivities corresponding to the various development processes are measured, since they play a major role in the resulting lithographic performance. Further, the useful silylation window with respect to silicon incorporation and DUV induced crosslinking is determined for each type of development. Finally, the resolution and the process latitudes resulting from the different development processes are compared.

Molybdenum silicide based attenuated phase‐shift masks

The use of phase shifting masks (PSMs) causes revolutionary improvements of the performance of existing wafer steppers. Nowadays the attenuated PSM, also referred to as halftone, is found to be most attractive, as the technique is self‐aligned. Moreover, the number of additional process steps in mask fabrication is limited to a dry etching step. Typically, both focus and exposure latitudes for contact holes of 0.35 μm are improved by a factor of 1.5–2 over a conventional mask. As linewidths on mask shrink towards 1 μm (a critical dimension of 0.25 μm, at 4× magnification) and below, the required critical dimension control becomes much tighter. It becomes more and more clear that the accuracy cannot be met anymore with wet etching of chrome. Sputtered molybdenum silicide (MoSi) is easier to dry etch than chrome and is therefore an important candidate as alternative opaque material on masks. The use of MoSi for attenuated PSMs is discussed. This approach has the advantage over the use of an SOG/Cr combinati...