Contact hole definition in the resist image is investigated as a function of reticle fidelity. It is found that for typical levels of corner rounding on reticle features, whether manufactured using a laser or shaped e-beam mask writer, the printed resist image at wafer level is largely unaffected. The loss of definition, which is also seen in supporting simulations using perfectly formed reticle features, is defined by the resolution limit of the resolution limit of the optical system of the stepper rather than the quality of the photomask.
KEYWORDS: Optical proximity correction, Lithography, Matrices, OLE for process control, Nanoimprint lithography, Optical lithography, Reticles, Semiconducting wafers, Semiconductors, Data processing
This work describes how rules for optical proximity correction, derived from lithography simulation, can be favorably changed to improve process latitudes through a metric called a dense-isolated focus/exposure matrix. Example calculations are given to demonstrate the derivation of amended rules.
The optimization of a dielectric anti-reflective coating (ARC) on a transparent substrate with significant topography is described. Supporting theory is provided and although it is not possible to obtain the ultimate performance of an ARC over planar film stacks and flat substrates, the critical dimension (CD) swing ratio is greatly reduced and a manufactureable solution achieved using response surface modeling (RSM) in combination with data generated form the lithography simulation tool, PROLITH/2.
Resist swing curves have been constructed practically, and by lithography modeling, for a conventional I-line resist both for dimensions at the substrate interface and at the top of the resist. Top SEM linewidth measurements were difficult to take repeatably in automatic mode so an enhanced measurement technique was developed. The effect of stepper focus on swing curve amplitude has been studied and, for substrate dimensions, amplitude increases as focus moves away from its optimum setting.
A development rate model for lithography simulation of extremely thick resist films is presented. Methods used in the extraction of lithographic modeling parameters for simulation packages such as PROLITH/2 are examined. The results account for hitherto little-considered aspects of the development process which, when implemented in the simulations, give good agreement with practical result. Effects examined include the variation in photoresist dissolution properties as a function of depth into the resist film and diffusion of the developer/resist in the spaces surrounding features. These refinements, which are particularly useful for MEMS and lithography, galvanoformung und abformung (LIGA) applications, can also be applied to conventional lithography in situations where the development models of commercially available software are unable to provide accurate results. The technique also provides a method for quantifying the effectiveness of mechanical, megasonic or ultrasonic during development.
Rule tables for describing optical proximity correction can be derived either from practical measurements or by means of lithography simulation. This paper shows that one set of rules may not adequately apply across the whole of the image field depending on CD uniformity across the lens field. Example measurements illustrate the problem and proposals are made for its solution.
This work describes assessment of `long range' and `short range' developer depletion using production and test reticles respectively. Long range developer depletion refers to the case where development of large unpatterned field areas remote from critical dimensions influences their size, whether the field areas are exposed or unexposed. Results show no significant effect at the 0.35 micron scale. Short range developer depletion refers to the case, on test reticles, where single lines are defined in large open field areas or in a small open box set within a dark field area. Differences in CD's can be readily measured on test reticles and attributed to the effects of developer depletion.
The variation of dimensional control between center and edge of a stepper lens field is measured both practically and by lithography simulation for both lines and slots at various pitches. Results for lines show that the sign of the center- edge offset is pitch dependent but for slots the dimension is always larger at center field irrespective of pitch.
A production application of optical proximity correction (OPC) aimed at reducing corner-rounding and line-end shortening is described. The methodology, using critical shape error analysis, to calculate the correct serif size is given and is extended to show the effect of OPC on the process window (i.e. depth-of-focus and exposure latitude). The initial calculations are made using the lithography simulation tools PROLITH/2 and SOLID-C, the results of which are transferred to the photo-cell for practical results.
A new metric for set-up of resist processes on transparent films is described using CMOS active area layer as a test vehicle for lithography simulations. Results show that the metric Es/Ec, where Es is exposure to size and Ec exposure to clear a high resolution feature, is more useful than the conventional ratio Es/Eo and where Eo is exposure to clear an open field area. High values of Es/Ec can protect against resist scumming if film thicknesses are chosen correctly, but if not accounted for at the sub-half-micron scale can be as low as 1.1. Where film thicknesses predict low Es/Ec, the ratio can be increased by use of bottom ARC.
Optical proximity correction in terms of linewidth correction at different pitches is used to demonstrate improvement in critical dimension control at the polysilicon layer of a sub- half-micron CMOS process. Further measurements across the image field show the effect on wafer linewidth distributions of different generations of the laser write tool used in reticle manufacturing.
The effect known as mask error factor is investigated using the optical lithography simulation tool PROLITH/2 with a well-tried and tuned set of simulation parameters. These investigations are extended to include the effect of pitch, linewidth, optical proximity correction, focus, lens aberrations, partial coherence, resist contrast, resist thickness and exposure. Through the use of focus-exposure matrices, process windows and manufacturing critical dimension budgets, the impact on reticle procurement specifications is also examined.
Critical Dimensions (CD) are measured on reticles which are written incorporating an optical proximity correction for pitch dependent linewidth bias. Reticles are manufactured on different generations of laser write tools and the result in terms of mean ann range of dimensions compared. Lithography simulation is used to test the response of reticle CD variation at wafer level.
An analysis technique for quantifying the effectiveness of optical proximity corrected (OPC) photomasks is described. The methodology is able to account for reticle manufacturing tolerances and has a number of applications including the optimization of OPC features and, in the examples described, the analysis of defect printability. The results presented here are generated using aerial image measurements from PROLITH/2, but the technique can be directly transferred to resist image measurements using 3D simulation tools such as PROLITH/3D where other factors such as swing curve effects caused by wafer topography could also be analyzed. With inspection tools such as scanning electron or atomic force microscopes and appropriate image processing and analysis software it should also be possible to apply this methodology to practical results.
Resist contrast is related to usable depth-of-focus (UDOF) for I-line and DUV stepper lenses through the resists process linearity function. Results, supported by simulations in the resist image, show that stepper field center-edge critical dimensional offsets, which decrease UDOF, increase as resist contrast decreases. Measurements on intra-field CDs show that their uniformity decreases as processes reach non-linear processing regimes.
The application of Optical Proximity Correction for improving uniformity of printed dimensions at sub-half-micron resolution in a 0.35 micron CMOS process is described. Results are presented in terms of measurements made on polysilicon gates, at different pitches, which are compared to the uncorrected case. The impact of photomask and stepper lens qualities on dimensional control are also considered. Results presented are at the demonstrator stage but strategy for implementation in production is discussed.
This paper investigates photoresist profiles on low reflectivity substrates with respect to numerical aperture and position in lens field. Results suggest a link between resist wall angle and substrate reflectivity which is influenced by lens aberrations.
Popular and recent development models used in optical lithography simulation are examined for the accuracy of their fit to practical data and also the ease with which supplied parameters may be fine-tuned to match the results of a particular photo-cell. It is found that some models are better able to describe the dissolution characteristics than others. No one model is found to be preferred in all cases and the final choice is resist dependent. For example, effects such as the 'notch' found in the R(m) curves of some modern high performance photoresists are best described by new models. In addition, a new method for constructing R(m,z) development rate data files is described and parameters extracted for this purpose show how the characteristics vary as a function of depth into the resist film.
Control of contact hole sizes in a sub-half-micron CMOS process using planarisation by resist etch back and chemical mechanical polishing is discussed. The limitations of using top anti-reflective coatings to overcome thin film effects on transparent substrates are calculated by simulation. Use of bottom anti-reflective coatings to improve uniformity in the resist etch back process are described through practical results which additionally show that comparable results are achieved in the chemical mechanical polishing process but in the absence of a bottom anti-reflective coating.
Methods used in the extraction of lithographic modeling parameters for simulation packages such as PROLITH/2 are examined. The results reveal hitherto unconsidered aspects of the development process which, when implemented in the simulations, give improved agreement with practical results with regard to characteristics such as resolution, depth-of- focus, linearity and dense/isolated bias. These refinements, which are particularly influential in the sub-half-micron regime, include the variation in photoresist dissolution properties as a function of depth into the resist film and also a small but powerful development 'notch' which is observed in the development rate versus PAC concentration curve as it approaches the minimum dissolution rate. This work therefore shows that current development models may not be adequate for some applications, and that great care must be taken in deriving and using the correct set of parameters for any one situation.
This paper investigates, by computer simulation, the effect of reticle bias on the isofocal behavior displayed by a range of feature types down to 0.4 micron resolution. All simulations are in the developed resist image and exploit resist and development parameters of a contemporary i-line positive resist. Results show that, with the exception of isolated lines, the application of reticle bias is ineffective in achieving a corresponding shift in the isofocal dimension.
Results are presented of dimensional measurements on 6 inch photomasks manufactured for the critical layers of a sub-half- micron CMOS process. Measurements show that dimensional control is excellent across individual plates but that capability indices are impaired by drift of mean-mean between individual photomasks. The measurement tool used was shown to be incapable of supporting the advanced manufacturing specification but its performance was improved by taking multi-point measurements.
Calculations using lithography modeling programs are described to measure the variation in linewidth in polysilicon gates which pass over typical active area topography. All linewidths are calculated in the resist image, with and without top and bottom anti-reflective coatings, and range from 0.4 micrometer at 365 nm to 0.225 micrometer at 193 nm exposure.
This paper investigates, by computer simulation using Prolith/2 and SOLID, the performance of a number of i-line photoresists at half-micron resolution in terms of focus/exposure (F-E) plots and relates their isofocal behavior to exposure and development characteristics as described by the Dill and Mack simulation parameters. Isofocal behavior is found to be independent of the exposure parameters but greatly affected by the development process and is shown to be a function of the Mack parameter, n, which is related to the resist contrast, (gamma) . In addition, this behavior is also shown to be resolution dependant. The results presented in this paper therefore assist in the choice of resist directly from resist characteristics as defined by the modeling parameters, for any particular application, while retaining the desired isofocal exposure conditions.
The forms of optical proximity effect (OPE) known as dense/isolated offset and line-end shortening are investigated by computer simulation using the programs Prolith/2 and SOLID. It is shown that as the partial coherence and numerical aperture are varied, the printing error due to these OPEs can change from positive, through zero to negative, suggesting that if an appropriate set of operating conditions is selected proximity effect can be reduced and even eliminated thus relaxing one of the many stringent reticle design specifications. This not only enables the user to tailor the operating conditions of the imaging system to his particular requirements, but also highlights a possible requirement for future wafer steppers to be made variable over as wide a range of operating conditions as is practicable.
The reduction of image intensity in a rim phase shift mask (PSM) for contact holes has been investigated by computer simulation and experimental lithography. The reduction occurs at contact hole size below 1.0(lambda) /NA. The smaller the size the severer the reduction. A new design of rim PSM called partial rim PSM, is proposed to overcome the problem. Computer simulation of aerial images has shown that the partial rim PSM can enhance image intensity. The enhancement is more significant for smaller size of contact holes while the degradation of image quality is much less than a biased rim PSM proposed previously. The partial rim PSM has been fabricated using the same self-aligned process as for conventional rim PSM fabrication. The contact holes with partial rims and with conventional rims have been printed on a wafer by a g-line stepper. The experimental result has confirmed the improvement in image intensity achieved by the partial rim PSM.
The effect of defocus on the printability of submicron 5X reticle defects at g-line, i-line and duv wavelengths is assessed. Imaging of a specifically designed test reticle incorporating defects whose size, and proximity to adjacent features, varies within submicron line/space arrays was studied using the simulation package SOLID. The results were assessed and initially presented by plotting minimum printed defect (MPD) vs. array linewidth (L/W) to compare with a previously reported practical study. Following this the results were replotted as curves of MPD vs. defocus at differing linewidth values. Finally, the resist modelling program has been used to create 3D images of the printed defects and thus demonstrate the effect caused by defocus on the resist profiles. Results on defect printability enable future reticle procurement specifications to be established.
Rim and attenuated phase shift masks can extend the range of i-line optical stepper lithography to 0.4 micrometers with enhanced depth of field. The repair of defects through subtractive and additive methods using focused ion beams is critical to the future success of the technology. Printability tests and simulation studies demonstrate the hierarchy of defects in which both rPSM and aPSM types are less tolerant of absorbing/attenuating defects, but more tolerant of clear defects than the corresponding conventional BIM. Repair trials reveal the existence of a new type of post repair defect - the phase trench - but also clearly demonstrate the feasibility of repair using FIB methods.
'Swing curves' derived from the modelling program SOLID are presented for a typical conventional i-line resist. At a constant sub-half-micron feature size, results illustrate the effect of varying stepper lens conditions on 'swing curve' amplitude and indicate that, whilst initially defined by reflectivities, resist thickness and resist absorption coefficient, it increases as aerial image peak intensity decreases. 'Swing curves' are drawn in terms of exposure-to-size vs resist thickness which has the advantage of always producing complete plots.
Rim and attenuated phase shift masks (PSMs) are the most promising candidates for mass ASIC production. Computer simulations have been carried out to compare their merits and limitations. It has been shown that both rim and attenuated PSMs improve the exposure latitude and depth of focus compared with a conventional binary intensity mask. An attenuated PSM demonstrates a higher exposure level and better defocus performance than a rim PSM, especially for sub-half micron contact holes. The comparison is based on a combination of criteria, since it is found that different conclusions may arise from computer simulations based on the analysis of an aerial image, if different evaluation criteria, such as log-intensity slope, image contrast or exposure-defocus tree, are applied independently. Examples illustrate the importance of choice of evaluation criteria. The superior performance of the attenuated PSM over conventional masks is confirmed by i-line experimental lithography.
Conventional resist, without and with an underlying anti-reflective coating, and a dyed resist are used to calibrate the sub-micron dimensional control across a 6 inch diameter wafer coated with LPCVD polysilicon, as used in the manufacture of advanced CMOS devices by i-line technology. Results are referenced to the dimensional control measured for the same resist process on bare silicon test wafers. The effect of variable substrate reflectivity, with respect to the different resist processes, is thus assessed. Intra-field dimensional control over typical circuit topography is also measured for the same resist processes. Results are related to the amplitude of the linewidth vs resist thickness functions of the appropriate process that are derived from simulations using the SOLID modeling package.
Focused ion beam repair of opaque defects in 5X reticles produces post repair stains which result in ghost defects in wafer prints produced at G-line, I-line, and DUV wavelengths. These stains can be removed using a post repair plasma process which restores transmission to almost 100%. However, a new in situ process is preferred which reduces stains to acceptable levels. The key to the new process is an understanding of factors affecting sputter yield. The effectiveness of the in situ antistain procedure is demonstrated through wafer lithography at all three wavelengths showing an absence of ghost defects following repair.
The printability of sub-micron 5X reticle defects at G-Line, I-Line, and DUV (248 nm) wavelengths is assessed by both practical experiment and computer simulation. The photoresist exposure and development parameters were measured, where necessary, and used for the modeling with the same specifically designed test reticle incorporating defects whose size, and proximity to adjacent features, varies within sub-micron line/space arrays. Results are presented by plotting minimum printed defect vs array linewidth for both adjacent and isolated defect sites. The resist modeling program SOLID has been used extensively, not only to simulate the practical work at all 3 wavelengths, but additionally to create a 3D representation of the effects caused by the printed defects. Results on defect printability enable future reticle procurement specifications to be established.
This paper describes the calibration of an automatic inspection system to size 5X reticle defects down to half-micron resolution, the defect printability limit of the stepper lens with which the 5X reticles are to be used. An enhanced technique using image analysis for defect sizing is also described. Whilst enabling more accurate defect sizing, this method is resolution limited in automatic operation.
Focused ion beam repair of opaque defects on 5X reticles cause a post repair stain due mainly to gallium ion implantation in the surface of the quartz substrate. The effects of these stains have been investigated through printability tests using a g-line optical lithography process. In some cases the effect of the post repair stain is worse than that of the original defect, highlighting the need for effective antistain processes for use in conjunction with FIB repair.
Printability of sub-micron 5X reticle defects at i-line and duv. wavelengths is assessed by use of a specifically designed test reticle incorporating defects whose size, and proximity to adjacent features, varies within sub-micron line/space arrays. Results are presented by plotting minimum resolved defect vs. array linewidth for both adjacent and isolated defect sites. Results on defect printability enable future reticle procurement specifications to be established. Results at i-line have been successfully modelled using SOLID which has a powerful graphics package enabling results to be displayed in 3D.
Lithography for via holes in polyimide is conventionally restricted by the need for a thick masking resist due to poor plasma selectivity during pattern transfer. Two novel techniques for via hole definition are described. The first is a single layer masking process using a silicon containing resist, which presents high resistance to oxygen plasma, while the other is a `hard' masking process using spin-on-glass. Processing and characterization for each technique is described and compared with the standard process.
Sub-micron resist features obtained by surface imaging and dry development have been used as masks in the pattern transfer of multilevel metal stacks in a single-wafer multichamber etcher. The calibration of such a negative working submicron lithographic process, derived from a g-line wafer stepper having a specified resolution of 1 micron, is described. Surface imaging was initiated by silylation of a proprietary resist using HMDS, while dry development was performed in a single-stage oxygen process in a conventional reactive ion etcher. Metal etch processes are outlined in conjunction with their effect on resist integrity and removal and the need for post-development deep-UV hardening. While not being part of the two-layer demonstrator, submicron 'via' hole definition in polyimide interlayer dielectric is demonstrated in a single-stage dry development/pattern transfer process.
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