Sanjay Yedur

CD and profile metrology of EUV masks using scatterometry based optical digital profilometry

Research and development efforts on EUV technology for the 32 nm node and beyond are progressing rapidly. Although a big concern is defect control on EUV mask blanks, control of linewidth and profile will be an important factor in acceptance of EUV technology. In this paper, we discuss the issues and strategies surrounding CD and profile metrology of EUV masks. EUV mask blanks from Hoya and Asahi Glass Company were used in this study, and were measured on a Nanometrics Atlas-M measurement tool, generating CD and profile results using Timbre Technologies ODP analysis software. The Atlas-M tool has dual optics, enabling use of either normal incidence Reflectometry or oblique incidence Ellipsometry, either of which may be used for Scatterometry. The relative merits of each of these technologies are discussed. The complex EUV stack presents numerous challenges for metrology; the critical task is to accurately measure the optical constants of the numerous layers in the stack. The multilayer MoSi stack is effectively modeled as a single layer for optical constants determination. Photoresist FEP171 was used for the patterning, and the CD and profile of the resist were measured, after which the absorber layer was etched. Parameters characterized in this study include photoresist CD and height, etched Absorber CD, and capping layer over etch. Correlation to top-down CD-SEM, cross-sectional SEM, and AFM is reported. No charging or other deformation was observed on the EUV masks. The data show that ODP Scatterometry provides a non-destructive method for monitoring resist CD and profile, as well as etched structure CD and over/underetch on EUV masks.

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.

Enhanced optical CD metrology by hybridization and azimuthal scatterometry

Reducing parameter correlations to enhance scatterometry measurement accuracy, precision and tool matching is a crucial component of every modeling effort. Parameter sensitivity can largely depend on the orientation of the plane of incidence relative to the grating orientation. Conventional scatterometry is done with the plane if incidence normal to the grating orientation, whereas azimuthal scatterometry allows measurements at an arbitrary angle or set of angles. A second technique examined in this paper is hybrid metrology where inputs from source tools such as CD-SEM and CD-AFM are used to determine values of critical parameters. The first examples shows LER sensitivity gains by measuring narrow resist lines in an orientation parallel with the long axis of the grating. Hybridization of LER results in a CD and SWA FMP improvement of about 60%. We also showcase the benefits of azimuthal scatterometry measuring resist lines with CD larger than the wavelengths of the incident light. A CD and SWA FMP reduction of about 60% and 30% is obtained using azimuthal scatterometry at 0, 45 and 90 degrees azimuth angles. Hybridization of the ARC SWA after RIE results in CD and resist SWA FMP improvements by over 60% and 30%, respectively.

Scatterometry on pelliclized masks: an option for wafer fabs

Optical scatterometry-based metrology is now widely used in wafer fabs for lithography, etch, and CMP applications. This acceptance of a new metrology method occurred despite the abundance of wellestablished CD-SEM and AFM methods. It was driven by the desire to make measurements faster and with a lower cost of ownership. Over the last year, scatterometry has also been introduced in advanced mask shops for mask measurements. Binary and phase shift masks have been successfully measured at all desired points during photomask production before the pellicle is mounted. There is a significant benefit to measuring masks with the pellicle in place. From the wafer fabs perspective, through-pellicle metrology would verify mask effects on the same features that are characterized on wafer. On-site mask verification would enable quality control and trouble-shooting without returning the mask to a mask house. Another potential application is monitoring changes to mask films once the mask has been delivered to the fab (haze, oxide growth, etc.). Similar opportunities apply to the mask metrologist receiving line returns from a wafer fab. The ability to make line-return measurements without risking defect introduction is clearly attractive. This paper will evaluate the feasibility of collecting scatterometry data on pelliclized masks. We explore the effects of several different pellicle types on scatterometry measurements made with broadband light in the range of 320-780 nm. The complexity introduced by the pellicles optical behavior will be studied.

CD-etch depth measurement from advanced phase-shift masks and wafers using optical scatterometry

In this paper, we report on a Scatterometry based metrology system that provides line width, line thickness, and trench depth measurements on APSM and EPSM photomasks. 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 with R2 of more than 0.99. EPSM FCCD data was obtained from both scatterometer and CDSEM. Results show that MEEF calculation based on scatterometer CD shows about 40% improvement in removing mask-induced CD non-uniformity, compared to calculation based on CD SEM data. This is thought to be due to averaging effect of scatterometer. Depth measurements from APSM show that scatterometer makes good correlations to AFM, generally within 3nm of each other. The data show that Scatterometry provides a nondestructive means of monitoring PSM profiles combined with relatively little time loss.

Scatterometry based CD and profile metrology of MoSi/quartz structures

As the on-wafer transistor sizes shrink, and gate nodes run well below 90 nm, it is becoming extremely important to accurately characterize and control the CDs on the Mask. Since Phase shift technology for masks is essential to achieve the geometries of the future, CD and profile metrology on the phase shifting materials becomes critical. Phase shift materials, such as MoSi, present unique challenges for metrology. In this paper, we discuss the effect of the optical properties of MoSi on CD and profile metrology and the challenges in obtaining the correct optical constants needed for accurate metrology. Optical Scatterometry based metrology was used successfully with both Spectroscopic Ellipsometry (SE; λ~ 210nm-1000nm) and Spectroscopic Polarized Reflectometry (Rp; λ: 320nm-780nm). Spectra were collected with Nanometrics Atlas-M reticle measurement system and were analyzed using ODP software from Timbre Technologies, Inc. Unlike chrome, the optical properties of the MoSi on the grating structure differ significantly from that on the rest of the blanket area of the mask. Unique modeling techniques are required to account for this difference. Etching of the chrome also causes changes in the MoSi top layer that need to be accounted. Data will be presented showing the sensitivities of the CD structures on the mask to variations of Quartz and MoSi optical constants. CD and profile sensitivities to roughness of the MoSi grating structure are also demonstrated.

Scatterometry performance improvement by parameter and spectrum feed-forward

Optical critical dimension (OCD) metrology using scatterometry has been widely adopted for fast and non-destructive in-line process control and yield improvement. Recently there has been increased interest in metrology performance enhancement through a holistic approach. We investigate the benefits of feed-forward of metrology information from prior process steps using samples from magnetic hard disk drive manufacturing. The scatterometry targets are composed of rather isolated gratings that are designed to have better correlation with device features. Two gratings, one with pitch ≈ 10CD, and the other with pitch ≈ 15CD, are measured at post develop and post reactive ion etch (RIE) steps. Two methods: parameter feed-forward (PFF) and spectrum feedforward (SFF) are studied in which the measurement results or spectrum collected on the blanket target at photo step are fed forward to the measurements on the grating structures at post develop or post RIE step. Compared with standard measurement without FF, for the more isolated grating at photo step, both PFF and SFF improve CD correlation from R2=0.96 to R2=0.975 using CD-SEM results measured on device as the reference. Dynamic precision and fleet measurement precision are improved by 20-60%. For post RIE step, PFF and SFF significantly improve CD correlation from R2=0.95, slope=1.09 to R2=0.975, slope=1.03 for the denser grating, and from R2=0.90, slope=0.79 to R2=0.96, slope=0.96 for the more isolated grating. Dynamic precision is generally improved by 20-40%. It is observed that both PFF and SFF are equally efficient in reducing parameter correlation for the application studied here.

Scatterometry performance enhancement by holistic approach

Abstract. Scatterometry performance enhancement is demonstrated through a holistic approach by utilizing comprehensive information from various sources, including data from different process steps, different toolsets, multiple structures, and multiple optical channels using samples from magnetic hard disk drive manufacturing. Parameter and spectrum feed-forward are performed across multiple targets at the photo step and the photo results are fed forward to the post-reactive ion etch (RIE) step. For an isolated structure with critical dimensions (CD) much smaller than the incident light wavelengths, feed-forward methods improve CD correlation with a general improvement of 20 to 60% in precision and fleet measurement precision (FMP). A second technique examined is hybrid metrology, where inputs from source tools, such as CD-SEM and CD-AFM, are used to determine critical parameters. Hybridization of line edge roughness results in CD and sidewall angle (SWA) FMP improvement of ∼60%. We also demonstrate improved CD accuracy using azimuthal scatterometry at 0, 45, and 90 deg azimuth angles measuring resist lines with CD larger than the incident light wavelengths. FMP reductions of ∼60 and 30% are obtained for CD and SWA. SWA hybridization after RIE results in CD and SWA FMP improvements by >50 and 30%, respectively.

Scatterometry measurement of nested lines, dual space, and rectangular contact CD on phase-shift masks

Evaluation of lithography process or stepper involves very large quantity of CD measurements and measurement time. In this paper, we report on a application of Scatterometry based metrology for evaluation of binary photomask lithography. Measurements were made on mask level with ODP scatterometer then on wafer with CD-SEM. 4 to 1 scaling from mask to wafer means 60nm line on wafer translates to 240nm on mask, easily measurable on ODP. Calculation of scatterometer profile information was performed by a in-situ library-based analysis (5sec/site). We characterized the CD uniformity, linearity, and metal film thickness uniformity. Results show that linearity measured from fixed-pitch, varying line/space ratio targets show good correlation to top-down CD-SEM with R2 of more than 0.99. ODP-SEM correlation results for variable pitch shows that careful examination of scatterometer profile results in order to obtain better correlation to CD SEM, since both tools react differently to the target profile variation. ODP results show that global CD distribution is clearly measurable with less outliers compared to CD SEM data. This is thought to be due to averaging effect of scatterometer. The data show that Scatterometry provides a nondestructive and faster mean of characterizing lithography stepper performanceprofiles. APSM 1st level (before Cr removal) dual-space CDs and EPSM rectangular contacts were also measured with and results demonstrates that Scatterometer is capable of measuring these targets with reasonable correlation to SEM.

Contact hole CD and profile metrology of binary and phase-shift masks: effect of modeling strategies in application of scatterometery

Scatterometers are widely used for line/space or 2D structure measurements in both wafer and mask industries. This technology is now gaining more acceptance and is being applied 3D structures such as contacts and pads. Contact CDs and trench depth in photomasks are critical monitoring parameters in mask industry and are discussed here. We are reporting contact CDs and profile results measured from targets from Binary, PSM, and Crless plates. The strategies of model creation such as using simple trapezoid versus more advanced shapes affect how well SWA and footings can be measured and reported from these structures. We are reporting CD and profile information obtained with Scatterometer, and then comparing CD SEM, AFM, and cross section SEM. Multiple different modeling configurations were used with different levels of complexity, and we report on optimum modeling strategy to obtain profile information from 3D structures. The relationship between the modeling strategy versus cross correlation between different parameters is discussed. CD linearity, uniformity, and other correlation parameters to the reference CD SEM tool are reported. Target CDs ranged from 60nm up to 600nm. CD uniformity reported from Scatterometry is 20~30% less than that from CD SEMs. This CD uniformity improvement is due to the fact that scatterometer beam samples dozens to hundreds of samples and averages profile parameters, thus eliminating local effect such as line edge roughness. Contact depth are also measured and compared to AFM, in which the bias between the two tools are usually around 3nm or less. In terms of smallest target CD measurable, in this paper we report routine measurement of small contacts with middle CD down to 65nm (bottom CD close to 50nm) with both RP and SE mode. Application of scatterometry method to mask contacts and pads leads to accurate and fast measurement of 3D profiles, and opens up possibility of in-line monitoring of profile information due to the higher runrate compared to traditional metrology tools.