Gregory P. Hughes

Phase standard based on profilometer metrology standard

A NIST traceable phase1 shift standard has been designed, fabricated, and tested on three phase shift measurement tools using different wavelengths. By using the fundamentals of NIST traceable step height, quartz index, and the understanding of the illumination optics of the Lasertec phase metrology tool, a phase standard has been created which can be used to calibrate Lasertec phase metrology tools. The pattern that is used is compatible with the recommended best practices for calibrating and measuring step heights and phase on the Lasertec tools. The mask is made with multiple depths. The three mask depths allow for the mask to be calibrated to three NIST traceable depth heights. This was done using the FEI SNP XT depth metrology tool. Since the mask format is mask based (6x250 Cr on quartz), it can be easily used on mask manufacturing metrology systems. The depths are targeted at the 180-degree phase shift for 157nm, 193nm, and 248nm lithography. The mask can be used to set targets and check the linearity of the phase metrology tools. The patterns are compatible with AFM and Profilometer depth metrology tools as well as multiple Lasertec spot sizes and shearing distances. The quartz depths are fabricated using a wet quartz etch process. The wet etch minimizes the quartz roughness and removes that error source from the metrology. The pattern is also arrayed so that multiple sites can be used to confirm the metrology and the prime measurement site could be changed if there was a suspicion of pattern damage or contamination.

Strategies for predictive control of chrome stress-induced registration errors

The focus of this paper is on the development and implementation of a correction strategy that enables mask manufacturers to maintain the yields at current levels while simultaneously reducing registration errors by several nanometers. An alternate consequence is that yields at current registration specifications are improved. Previous work has shown that one source of image placement error is the chrome stress relief caused by etching. This can cause over 25 nm of distortion from the resist pattern to the final etched chrome pattern. Theoretical and experimental data have shown that the distortion has a radial signature, which can be significantly reduced by traditional magnification correction. If the magnitude of this correction term can be predicted before patterning, the magnification can be implemented as a correction term in the writing process, minimizing registration errors. Studies have shown that the percent clear area of the mask, x-field size, y-field size, and chrome stress are the key parameters that will affect the correction term. Data based on finite element simulations were first fit to these parameters to obtain a predictive curve based upon theory. Experimental reticles were then written to test the theoretical prediction. The predictions were found to coincide well with the experimental data; registration improvements of over 20 nm were observed. The correlation was then applied to a set of production reticles. There was an observable improvement in registration after the correlation was implemented, although less than that seen in the experimental reticles.

Mighty high-T lithography for 65-nm generation contacts

Contact patterning for the 65nm device generation will be an exceedingly difficult task. The 2001 SIA roadmap lists the targeted contact size as 90nm with +/-10% CD control requirements of +/-9nm. Defectivity levels must also be below one failure per billion contacts for acceptable device yield. Difficulties in contact patterning are driven by the low depth of focus of isolated contacts and/or the high mask error (MEF) for dense contact arrays (in combination with expected reticle CD errors). Traditional contact lithography methods are not able to mitigate both these difficulties simultaneously. Inlaid metal trench patterning for the 65nm generation has similar lithographic difficulties though not to the extreme degree as seen with contacts. This study included the use of multiple, high transmission, 193nm attenuated phase shifting mask varieties to meet the difficult challenges of 65nm contact and trench lithography. Numerous illumination schemes, mask biasing, optical proximity correction (OPC), mask manufacturing techniques, and mask blank substrate materials were investigated. The analysis criteria included depth of focus, exposure latitude and MEF through pitch, reticle inspection, reticle manufacturability, and cost of ownership. The investigation determined that certain high transmission reticle schemes are strong contenders for 65nm generation contact and trench patterning. However, a number of strong interactions between illumination, OPC, and reticle manufacturing issues need to be considered.

Printability of quartz defects in a production Cr-less mask process

Chromeless PSM photomasks have been successfully applied to a production memory application. This 248-nm application has allowed an extremely aggressive, dense design to be successfully deployed without changing wavelength. This was achieved with an advanced resolution enhancement technique, a chromeless phase-shifting mask, to provide a more cost-effective total lithographic solution. The key to this technology is a mask that delivers high wafer-die yields, while delivering resolution at low k1. Therefore, the mask must have zero printing defects. In order to understand printing defects, many types of potential defects were analyzed and correlated back to the mask locations using both a 248-nm AIMs tool and SEM images. These defects were also correlated to a 257-nm KLA 576 tool using die-to-die inspection runs. This paper will examine chromeless mask phase-defect printing effects by using inspection capture at the key manufacturing steps (post-Cr etch, post-Qz Etch, and post-Cr removal). These defects will then be tracked through processes using SEM, AIMs, RAVE repair, and post-repair AIMs.

Imaging study of positive and negative tone weak phase-shifted 65 nm node contacts

CPL and aerial image mapping type contact designs for both negative and positive tones were created, built and tested for 100 nm and sub-100 nm contacts. Experimental results illustrated the need for electromagnetic-field corrections in the simulations. Resolution down to 80nm dense contacts were seen with both negative and positive resists with acceptable process windows though some process optimization is still required as unacceptable CD variation and a reentrant profile was observed. High MEEF requires strict CD control on the mask. Data volume for the isolated contact designs can also challenge the mask build.

The study of phase angle effects to wafer process window using 193-nm EAPSM in a 300-mm wafer manufacturing environment

As the semiconductor-process technology advances towards the 90nm-node, more and more wafer-fabs start to use 193nm EAPSM (Embedded Attenuated Phase-Shift Mask) technology as the main lithography strategy for the most critical-layers. Because the 193nm EAPSM is a relative new technology in the semiconductor industry, it is important for us to understand the key-mask-specifications in a 193nm EAPSM and their impact to the wafer process windows. In this paper, we studied the effects of phase-angle and transmission to the wafer process window of a 193nm-EAPSM in a 300mm wafer-manufacturing environment. We first fabricated a special multi-phase EAPSM by a combination of extra Quartz-etch and Mosi-removal. We then used a high NA 193nm scanner (ASML-ALTA1100) and high contrast resist to perform the wafer-level printing study. To fully understand the impact of phase-angle and transmission to wafer process windows, we also used AIMS (Aerial-Image Measurement System) and Prolith simulation software to study the lithographic performances of various phase-angle and transmission combinations. By combining the wafer-level resist imaging printing results, AIMS studies and Prolith-2 lithography simulations, we proposed the practical phase-angle and transmission specifications for the 90nm-node wafer process.

Analysis of off-axis-illumination-based phase-edge/chromeless mask technologies

Production readiness of phase-edge/chromeless reticles employing off-axis illuminations for 65nm node lithography is assessed through evaluation of mask design conversion and critical layer lithography performance. Using ASML /1100ArF scanners, we achieved k1=0.33 for chromeless phase shift mask (crlPSM) with more than 0.6um DOF for dense features. Subresolution assist features allow for acceptable depth of focus through pitch. However, chromeless feature linearity fall-off continues to be a major issue hampering the acceptance of crlPSM for production. Several mask data conversion schemes such as chromeless gratings and chrome patches have been proposed as viable solutions to mitigate the chromeless linearity fall-off issue. We evaluated chromeless gratings, chromeless rims and chrome patches and report on their performance in resolving the chromeless linearity fall-off issues as well as mask process complexity associated with each solution.

Through pitch intensity balancing and phase error analysis of 193-nm alternating phase shift masks

An investigation of the predominant industry approaches to transmission balance and phase error through pitch of Alternating Aperture Phase-Shifting Mask manufacturing approaches has been conducted. Previous theoretical studies have shown both clear pattern bias and phase error changes through pitch. These variations are significant for the Low K1 applications. Several approaches have been proposed and discussed in previous papers, including undercut, asymmetric pattern biasing, mask phase-only, dual trench, SCAA, and others. Although much of the discussion has focused on lithographic process performance, some of the constraints in the mask making infrastructure may differentiate between processes of similar performance. Two manufacturable approaches, wet etch undercut and asymmetric pattern biasing, have been studied by electromagnetic field simulation to explore the across pitch performance at 193nm. This has been compared to experimental measurement of photomasks measured with a 193 Zeiss AIMS (Aerial Image Microscope System). Both mask fabrication approaches are compared to the simulations. The performance of both mask approaches to pattern bias and phase error was evaluated, and the feasibility of through pitch correction and its impact on design and manufacturability of the photomask is discussed.

Study of OPC for AAPSM reticles using various mask fabrication techniques

AAPSM masks require OPC correction through pitch in order to print a linear dark line response vs the design CDs. The masks also require correction for the clear intensity imbalance caused by the phased etched Qz wall edge. The clear intensity can be balanced by two approaches;(or a combination of the two) data biasing or wet undercut etching of the Qz etched opening. IC manufacturers would like to use one OPC model that will work for any mask fabrication approach. This paper shows that there is no OPC difference observed in either the aerial image or the printed image of several OPC learning patterns. The study includes CD through pitch for dense (1:1) L/S Patterns and Isolated Line CD vs line-space ratio. The images were analyzed for the dark line linearity, the clear CD balance though pitch, and the clear CD balance with focus (phase error effects -PES).