Malahat Tavassoli

Novel solution for in-die phase control under scanner equivalent optical settings for 45nm node and below

As lithography mask process moves toward 45nm and 32nm node, phase control is becoming more important than ever. Both attenuated and alternating Phase Shifting Masks (PSM) need precise control of phase as a function of both pitch and target sizes. However conventional interferometer-based phase shift measurements are limited to large CD targets and require custom designed patterns in order to function properly, which limits phase measurement Zeiss is currently developing an optical phase measurement tool (PhameTM), providing the capability of extending process control from large CD test features to in-die phase shifting features with high spatial resolution. The necessity of designing this optical metrology tool according to the optical setup of a lithographic exposure tool (scanner) has been researched to be fundamental for the acquisition of phase information generated from features close to the size of the used wavelength. It was found by simulation that the image phase of a scanner depends on polarization and the angle of incidence of the illumination light due to rigorous effects. Additionally, for small features the phase value is strongly influenced by the imaging NA of the scanner due to the loss of phase information in the imaging pupil. Simulations show that the resulting scanner phase in the image plane only coincides with the etch-depth equivalent phase for large test features, exceeding the size of the in-die feature by an order of magnitude. In this paper we introduce the PhameTM phase metrology tool, which enables the industry to perform in-die phase control for Alternating PSM, Attenuated PSM and CPL masks. The PhameTM uses a 193nm light source with the optical capability of phase measurement at scanner NA up to the equivalent of a NA1.6 immersion scanner, under varying, scanner relevant angle of incidence for Attenuated PSMs and CPLs, and with the possibility of polarizing the illuminating light. New options for phase shifting mask process control on in-die features will be outlined with first phase measurement results for varying states of polarization.

CD metrology of binary and phase shift masks using scatterometry

In this paper, we report on a Scatterometry based metrology system that provides line width and thickness measurements on binary, APSM, EPSM masks both on FCCD (final check CD) and DCCD (develop check CD), fabricated on 193nm process. Measurements were made with scatterometer in DUV to visual wavelength range. Calculation of profile information was performed by a library-based analysis software. We characterized the CD uniformity, linearity, trench depth uniformity. Results show that linearity measured from fixed-pitch, varying line/space ratio targets show good correlation to top-down CD-SEM, meanwhile linearity from wide range of different pitch generally does not correlate well and therefore post-measurement calibration is needed. Depth measurements from APSM show that scatterometer makes good correlations to AFM. The effect of optical properties of the film layers on metrology performance is discussed. The data show that Scatterometry provides a nondestructive of monitoring basic etch profile combined with relatively little time loss from CD measurement step.

Laterally resolved off-axis phase measurements on 45-nm node production features using Phame

As lithography mask process moves toward 45nm and 32nm node, phase control is becoming more important than ever. To ensure an accurate printing both attenuated and alternating PSMs (Phase Shift Masks) need precise control of phase as a function of both pitch and target sizes. However critical target CDs fall much below conventional phase metrology tools capabilities. Interferometer-based phase shift measurements are limited to large CD targets and require custom designed features in order to function properly, which limits phase measurement. AFM (Atomic Force Microscopy) methods are able to capture small feature sizes but do not consider any diffraction effects which are caused by the topography of the features itself when getting close to the used wavelength. Imaging simulations, both, in a rigorous and a Kirchhoff regime, show the dependency of the phase in the image plane of a microlithography exposure tool on numerical aperture and pitch due to the loss of phase information in the imaging pupil. Additionally, for small features the phase is strongly impacted by polarization and 3D mask effects. For these feature sizes, the image phase does not coincide with the etch depth equivalent phase calculated from the nominal depth and optical constants of the shifter material. Deviations up to 20° have been observed leading to strong variations in the imaging quality and process window variations during scanner printing. Considerations of CD variation between 0 and pi features by simulation show lowest 0/pi CD variation and therefore largest process window if the scanner relevant phase is at 180°. The simulation results illustrate the importance to measure the scanner relevant phase, effective in the image plane of the scanner. Consequently Zeiss, in collaboration with Intel, has developed a laterally resolving Phase Metrology Tool - Phame® - for in-die phase measurements. The optical metrology tool is able to perform in-die phase measurement on alternating PSM, attenuated PSM and CPL masks down to 120nm half pitch at mask. On-axis measurement results have already been published. In this paper we elaborate on off-axis phase measurement theory and procedure. Furthermore we present first off-axis measurement results over varying features sizes using different illumination conditions.

Magnetron reactive sputtering of TaN and TaON films for EUV mask applications

We have developed and characterized a stack of TaN (absorber) and TaON (ARC) using reactive magnetron sputtering method. Two DOE (design of experiments) were performed with varying gas and power parameters and their effects on the various film parameters are discussed. We characterized the stress, uniformity, reflectivity (for defect inspection and EUV wavelengths), defect adders, and etch performance. Film property characterization was performed with AFM, Optical reflectance measurement tool, Particle inspection tool and profilometer. Optimized film stack met or exceeded ITRS guideline for EUV lithography mask with film stress less than 200MPa, inspection wavelength reflectivity at 9%, and thickness uniformity less than 5%. Defect adder number (< 0.5 / cm2) was a strong function of underlying film surface roughness and cleanliness of surface as well as deposition parameters.

Study of rigorous effects and polarization on phase shifting masks through simulations and in-die phase measurements

As lithography mask process moves toward 45nm and 32nm node, phase control is becoming more important than ever. Both attenuated and alternating PSMs (Phase Shift Masks) need precise control of phase as a function of both pitch and target sizes. However conventional interferometer-based phase shift measurements are limited to large CD targets and requires custom designed target in order to function properly, which limits phase measurement. Imaging simulations, both, in a rigorous and a Kirchhoff regime, show the dependency of the phase in the image plane of a microlithography exposure tool on numerical aperture, polarization, and on the so-called balancing of the mask for features close to the size of the used wavelength. For these feature sizes, the image phase does not coincide with the etch depth equivalent phase calculated from the nominal depth and optical constants of the shifter material. Additionally, for PSMs generating phase jumps deviating from 180°, the resulting phase in the image plane of a microlithography exposure tool depends on the transmitted diffraction orders through the aperture of the imaging system. Consequently Zeiss, in collaboration with Intel, has started the development of a laterally resolving Phase Metrology Tool (Phame) for in-die phase measurements. In this paper we present this optical metrology tool capable of phase measurement on individual line/spaces down to 120nm half pitch. Alternating PSM, Attenuated PSM, Cr-less masks were measured on various target sizes and simulations were performed to further demonstrate the capability and implication of this new method to measure the scanner relevant phase in-die, taking into account NA, polarization, and rigorous effects.

Measurement of EUV absorber and resist CD using spectroscopic ellipsometer

Evaluation of lithography process or stepper involves very large quantity of CD measurements and measurement time. In this paper, we report on an application of Scatterometry based metrology for evaluation of EUV photomask lithography. Measurements were made on mask level with Ellipsometric scatterometer for develop-check CD (DCCD) and final check CD (FCCD). Calculation of scatterometer profile information was performed with in-situ library-based rigorous coupled wave analysis (RCWA) method. We characterized the CD uniformity (CDU) and metal film thickness uniformity. OCD results show that high precision CD measurement EUV absorber and resist is possible with this method. A series of simulations were also performed to investigate the feasibility of Ellipsometric scatterometry for various pitches/line CD sizes, down to 11nm half-pitch at 1x magnification. The data showed that Scatterometry provides a nondestructive and faster mean of characterizing mask CD performance for various EUV process generations.

Implementation of multiple ROI with single FOV for advanced mask metrology

As technology nodes go down to 45nm and below, mask metrology becomes more important as the critical features decrease in size, while, at the same time, the number of measurements that need to be performed increases. OPC and RET put further burden on metrology as it is typical to measure more than one dimension on a single feature. In order to maximize the throughput of metrology tools and to keep up with the demand for more measurements, we have implemented the ability to measure multiple CD sites within a field of view without any stage movement in fully automated ways in a production environment. This in turn reduces total mask measurement time and helps to increase tool capacity

Phase metrology on 45-nm node phase-shift mask structures

As PSM (Phase Shift Mask) process moves toward 45nm and 32nm node, phase control is becoming more important than ever. Both attenuated and alternating PSM need precise control of phase as a function of both pitch and target sizes. However conventional interferometer-based phase shift measurements are limited to large CD targets and requires custom designed target in order to function properly, which limits clear understanding and control of small target PSM features. New type of Phase metrology tool created by Zeiss, in collaboration with Intel has been introduced and Intels 45nm node PSM targets have been measured. In this paper we present test results from AAPSM/EAPSM targets with space CDs down to 45nm a wafer-level. Smallest pitch was 300nm print pitch, 150nm CD at mask (75nm pitch at wafer). In addition to this, phase and transmission matching between conventional phase metrology tool and new tool has been investigated and shown.

Design based mask metrology hot spot classification and recipe making through random pattern recognition method

Design Based Metrology (DBM) requires an integrated process from design to metrology, and the very first and key step of this integration is to translate design CD lists to metrology measurement recipes. Design CD lists can come from different sources, such as design rule check, OPC validation, or yield analysis. These design CD lists can not be directly used to create metrology tool recipes, since tool recipe makers usually require specific information of each CD site, or a measurement matrix. The manual process to identify measurement matrix for each design CD site can be very difficult, especially when the list is in hundreds or more. This paper will address this issue and propose a method to automate Design CD Identification (DCDI), using a new CD Pattern Vector (CDPV) library.

Scatterometry based CD and Profile Metrology of Chrome-less masks using Optical Digital Profilometry

Control of line width and profile is gaining more importance in photomask processes as the industry moves toward 45nm node and beyond. In this paper we report scatterometer measurements of CD and profile data from chrome-less mask profile processed using Intels 65nm and 45nm node technology. As opposed to the highly charging nature of chrome-less plate during CD SEM measurements, scatterometry provides a non-charging optical alternative to measure critical CDs. As for trench depth measurement, scatterometry has big advantage over AFM with its much higher throughput (about 5 seconds vs. >2min). Since quartz plate is very transmittive to lights, we use eliipsometer-based scatterometry instead of conventional reflective-photometer based one. Parameters characterized in this study include line/space CD, contact CD, and trench depth. Correlation to top-down CD-SEM, cross-sectional SEM, and AFM is reported. Line CD uniformity reduction is more than 30% compared to that from CDSEM, due to averaging effect of scatterometry as well less lack of charging during measurements. Depth bias to AFM was around 3nm in both DCCD and FCCD height measurements we performed. The data show that Scatterometry provides a nondestructive way to monitor basic etch profile combined with relatively little time loss from measurement step.