Rik Jonckheere

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.

Repair of natural EUV reticle defects

Defects of the multi-layer (ML) mirror on a EUV reticle, so-called ML-defects, are a prime aspect why EUV mask defectivity is considered a challenge before EUV lithography can be used for the production of future node integrated circuits. The present paper addresses the possibility to mitigate the printability of these defects by repair. Repair of natural EUV mask defects is performed using the electron beam based Carl Zeiss MeRiT® repair technology and is evaluated by wafer printing on the ASML EUV Alpha Demo Tool (ADT) installed at IMEC. Both absorber defects and ML-defects are included. The success of absorber defect repair (both opaque and clear type) is illustrated. For compensation repair of ML-defects experimental proof of the technique is reported, with very encouraging results both for natural pits and bumps. In addition, simulation is used to investigate the limitations of such compensation repair, inspired by the residual printability found experimentally. As an example it was identified that alignment of the compensation repair shape with the ML-defect position requires sub-20nm accuracy. The integration of an Atomic Force Microscope (AFM) into the repair tool has been an important asset to cope with this.

EUV lithography program at IMEC

IMEC has started an EUV lithography research program based on ASMLs EUV full field scanner, the Alpha Demo Tool (ADT). Currently, the ADT is in the final phase of installation. The program focuses on three main projects: EUV resists, EUV reticles and assessment of the ADT performance. The intent of this program is to help improve and establish the necessary mask and resist infrastructure. In this paper, the status and the progress of the program is reviewed. In preparation for a resist process for the ADT, interference lithography has been used to track the progress of resist performance. Steady progress in resist development is seen, especially in terms of resolution, as some materials are now able to resolve 25nm HP. In its initial phase, the reticle project has concentrated on working with the mask and blank suppliers to assure timely availability of reticles for the ADT. An overview is given of the other reticle related activities, as well as first results of a defect printability study by simulation. In the ADT assessment project, simulation studies are reported aimed at the development of optical correction for flare and reticle shadowing effects. The impact of flare and shadowing effects are well understood and strategies for flare mitigation and shadowing effect correction are proposed.

Influence of gate patterning on line edge roughness

It is shown by simulation that the line edge roughness (LER) on the gates causes fluctuations on transistor performance [J. A. Croon et al., “Line edge roughness: Characterization, modeling, and impact on device behavior,” Proceedings of the IEDM, 2002; “Experimental investigation of the impact of line-edge roughness on MOSFET performance and yield” (to be published)]. Efforts are underway to investigate the influence on device performance experimentally. In this article, the transfer of the LER of the resist pattern into the poly silicon layer is investigated. For the experimental setup isolated gate lines ranging between 50–180 nm were patterned with an e-beam lithography. The resist line patterns are generated with some additional programmed LER. After the e-beam lithography step the processing was continued with etching the poly-Si, resist strip, and SiON removal. Using an offline software analysis tool for the edge detection based on SEM pictures, it was possible to determine the influence of process...

Natural EUV mask blank defects: evidence, timely detection, analysis and outlook

A combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) is used to find as many as possible printing defects on two different EUV reticles. These multiple inspections result in a total population of known printing defects on each reticle. The printability of these defects is first confirmed by wafer review on wafers exposed on the full field ASML Alpha Demo Tool (ADT) at IMEC. Subsequently reticle review is performed on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology allows to separate absorber related mask defects and multi layer (ML) related mask defects. In this investigation the focus is on ML defects, because this type of reticle defects is EUV specific, and not as evolutionary as absorber defects which can be mitigated in more conventional ways. This work gives evidence of critical printing ML defects of natural origin, both pits as shallow as 3nm and bumps just 3nm high at the surface. Wafer inspection was the first inspection technique to detect these ML-defects with marginal surface height distortion, because both state-of-the-art PMI and especially standard BI on the Lasertec M1350 had failed to detect these defects. Compared to standard BI, the more advanced Lasertec M7360 is found to have much better sensitivity for printing MLdefects and our work so far shows no evidence of printing ML defects missed by this tool. Unfortunately it was also observed that this required sensitivity was only achieved at the cost of an unacceptable nuisance rate, i.e., with a too high number of detections of non-printing defects. Optical blank inspection is facing major challenges : It needs not only to find ML defects with height distortions of 3nm and less (and in theory maybe even 0nm), but also it must be able to disposition between such likely-printing and non-printing defects.

Model-based OPC for first-generation 193-nm lithography

The first 193 nm lithography processes using model-based OPC will soon be in production for 0.13 micrometer technology semiconductor manufacturing. However, the relative immaturity of 193 nm resist, etch and reticle processes places considerable strain upon the OPC software to compensate increased non-linearity, proximity bias, corner rounding and line-end pullback. We have evaluated three leading model-based OPC software packages with 193 nm lithography on random logic poly gate designs for the 0.13 micrometer generation. Our analysis has been performed for three different OPC reticle write processes, two leading 193 nm resists and multiple illumination conditions. The results indicate that the maturity of the model-OPC software tools for 193 nm lithography is generally good, although specific improvements are recommended.

Evidence of printing blank-related defects on EUV masks missed by blank inspection

In this follow-up paper for our contribution at BACUS 2010, first evidence is shown that also the more advanced Lasertec M7360 has missed a few printing reticle defects caused by an imperfection of its EUV mirror, a so-called multilayer defect (ML-defect). This work continued to use a combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) to find as many as possible printing defects on EUV reticles. The application of more advanced wafer inspection, combined with a separate repeater analysis for each of the multiple focus conditions used for exposure on the ASML Alpha Demo Tool (ADT) at IMEC, has allowed to increase the detectability of printing MLdefects. The latter uses the previous finding that ML-defects may have a through-focus printing behavior, i.e., they cause a different grade of CD impact on the pattern in their neighborhood, depending on the focus condition. Subsequent reticle review is used on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology has allowed achieving clear evidence of printing ML defects missed by this BI tool, despite of an unacceptable nuisance rate reported before. This is a next step in the investigation if it is possible to avoid actinic blank inspection (ABI) at all, the only presently known technique that is expected to be independent from the presence of a (residual) topography of the ML-defect at the top of the EUV mirror, in detecting those defects. This is considered an important asset of blank inspection, because the printability of a ML-defect on the EUV scanner and its detectability by ABI is determined by the distortion throughout the multilayer, not that at the surface.

Imaging performance of the EUV alpha semo tool at IMEC

Extreme Ultraviolet Lithography (EUVL) is the leading candidate beyond 32nm half-pitch device manufacturing. Having completed the installation of the ASML EUV full-field scanner, IMEC has a fully-integrated 300mm EUVL process line. Our current focus is on satisfying the specifications to produce real devices in our facilities. This paper reports on the imaging fingerprint of the EUV Alpha Demo Tool (ADT), detailing resolution, imaging, and overlay performance. Particular emphasis is given to small pitch contact holes, which are a critical layer for advanced manufacturing nodes and one of the most likely layers where EUVL may take over from 193nm lithography. Imaging of contact holes, pattern transfer and successful printing of the contact hole level on a 32nm SRAM device is demonstrated. The impact of flare and shadowing on EUV ADT performance is characterized experimentally, enabling the implementation of appropriate mitigation strategies.

Closing the gap for EUV mask repair

The EUV-photomask is used as mirror and no longer as transmissive device. In order to yield defect-free reticles, repair capability is required for defects in the absorber and for defects in the mirror. Defects can propagate between the EUV mask layers, which makes the detection and the repair complex or impossible if conventional methods are used. In this paper we give an overview of the different defect types. We discuss the EUV repair requirements including SEMinvisible multilayer defects, and demonstrate e-beam repair performance. The repairs are qualified by SEM, AFM and through-focus wafer prints. Furthermore a new repair strategy involving in-situ AFM is introduced. Successful repair is demonstrated on real defects.