Deh-Ming Shyu

Angular Scatterometry for Line-Width Roughness Measurement

We propose using angular scatterometry as a means to investigate LWR (line-width roughness) and CD (critical dimension). The grating target is illuminated by a single wavelength light source which has large angular aperture both in incidence angle θ and azimuth angle φ. A preliminary scatterometry model was first built by assuming perfect critical dimension printed without any line-width roughness. The difference between the model prediction and actual measurement is cased by line-width roughness contribution. We developed a calibration curve as a function of line-width roughness based on the statistical quantity of the incidence and azimuth angle dependence. The results demonstrate that scatterometry can indeed be used to extract line-width roughness and critical dimension information in production line with nano-scale resolution.

Angle-resolved scatterfield microscope for linewidth measurement

Angle-resolved scatterfield microscope (ARSM) is developed for several years. It combines the optical microscope and angle-resolved scatterometer with a relay lens and an aperture. In our research, the spatial light modulator (SLM) is used to instead of the relay lens and the aperture. In the SLM, the phase modulation is used to simulate the Fresnel lens, and then an incident plane wave is modulated and focused on the back focal plane of the objective lens. A plane wave with an angle which is according to the position of focused point on the back focal plane is emitted from the entrance pupil of the objective lens. By modulating the SLM, the angle of plane wave from the objective lens can be changed. In our system, an objective lens with NA 0.95 and the magnification of 50 is used for wide angle scan. A bare silicon wafer and a grating with the pitch of 417nm are measured with full-angle scan. By using the SLM, the advantage is full-optical modulation, that is, the mechanical motion is not needed in the ARSM. Thus, the system will have higher throughput and stabilization.

Accuracy of diffraction-based overlay metrology using a single array target

We focus on the capability and theoretical limits of a model-based scatterometry method to determine overlay using a single two-dimensional array target. We use our modeling capability to design an optimized test target for scatterometer-based overlay measurements in a range of semiconductor films. We propose a methodology to measure the overlay using a single two-dimensional array target designed with intentional offsets, Δx and Δy, between the top and bottom grid arrays along the X and Y directions. This method allows extraction of the two-dimensional overlays from first diffraction order measurements through bi-azimuth angle analysis (0 and 90 deg with respect to the incidence plane), and includes a simple linear response algorithm. Two critical issues are taken into account: correlation of Δx and Δy and lithography process errors. We have simulated the diffraction signatures of a two-dimensional target with a pitch of 400 nm and linewidth of 100 nm, and optimized the overlay target design to maximize the measurement sensitivity and minimize the correlation of two axial measurements. We also investigate the influence of parameter variations on overlay measurement error

In-line metrology of 3D interconnect processes

The continuous development of three-dimensional chip/wafer stacking technology has created the metrology requirements for in-line 3D manufacturing processes. This paper summarizes the developing metrology that has been used during via-middle & via-last TSV process development at ITRI (Industrial Technology Research Institute). An IR metrology tool including broadband infrared microscopic imaging module and a specific infrared laser confocal module is developed for the thinned wafers thickness measurement with spatial resolution of 0.5 μm. An existing spectral reflectometer is used and enhanced by implementing novel theoretical model and measurement algorithm for HDTSV inspection. It is capable of measuring via depth/bottom roughness/bottom profile in one shot measurement. A metrology module based on two sets of dual-channel capacitive sensors for metallization film thickness measurement is applied to make critical process control in the fab. We will share real metrology results and discuss possible solutions for 3D interconnect processing.

A novel method for overlay measurement by scatterometry

The potential of scatterometry has been developed for many years, but it is challenging to accurately and quickly obtain the overlay error from diffraction data. We presented a method to measure the overlay error by choosing an optimal measurement target design for scatterometry. All of the simulations in this study were calculated by rigorous coupled wave analysis. A set of two layer grating model were developed for evaluation of overlay measurement sensitivity at different incident angle, such as theta (0° to 90°) and phi (0° to 180°). We also compared the optical response of zero order and first order diffraction signature. We can use appropriate target design and measured condition to maximize the overlay measurement sensitivity and reduce the noise from lithography printing error. In addition, the diffractive signature imaging microscope (DSIM) is introduced to measure the diffraction signature. This instrument is a full-optical operation system without any mechanical movement, so it has good stabilization.

Optimal measurement method for scatterometer-based overlay metrology

Scatterometry takes advantage of the sensitivity exhibited by optical diffraction from periodic structures, and hence is an efficient technique for lithographic process monitoring. Even though the potential of this technique has been known for many years, it is challenging to accurately and quickly extract the multilayer grating overlay from diffraction data. We propose a method to measure the overlay by selecting an optimal measurement design based on the theoretical modeling of differential signal scatterometry overlays. A set of two grating overlay targets are designed with an intentional offset difference between the top and bottom gratings, to maximize the differential signal measurement sensitivity and to minimize the response to the process noise. We model the measurement sensitivity to overlays of two layer gratings, at a fixed wavelength and with a range of azimuth incidence angles from 0 to 180 deg, by means of rigorous diffraction theory. We compare the optical response of the zero- and first-order diffractive overlays. We show that with the appropriate target design and algorithms, scatterometry overlay achieves improved accuracy for future technology nodes.

Influence of semiconductor manufacturing process variation on device parameter measurement for angular scatterometry

The influence of semiconductor manufacturing process variation on device parameter measurements for angular scatterometry was studied. Process variations, e.g, temperature and pressure variation of poly deposition, are considered to affect the optical properties of the deposition layer, and hence cause inaccurate model-based scatterometry measurements. This study investigates measurement error of device parameters if the optical properties change but the model stays for the same in angular scatterometry. A series of diffracted signatures was generated whose optical properties change slightly but keep the same structure. This work measured n (refractive index) and k (extinction index) of materials on wafer from the nominal process condition. Then, n and k are used to create a comparison library. The comparison library fixes all parameters other than varying CD (critical dimension) parameter. When poly layer n changes, the scattering signatures also change. The inaccuracy of CD measurement could be evaluated by comparing varying signatures due to optical properties change to the nominal process condition. An optimal structure design and feature region selection algorithm was developed to reduce errors introduced by these process variations to CD measurement. For angular scatterometry, the reflectance at some scan angles performs lower sensitivity to the optical parameters variation than the reflectance at other scan angles. By determining which scan angles contain less sensitivity and further optimize target design within the process variation range, the influence of process variation on device parameter measurement and the number of measurements used in the inversion process can be reduced. By using 65nm and 45nm as design rules, optimized grating structure with most sensitivity to CD measurement and the least influence on poly refractive index variation were obtained. The optimized grating structures are suitable for inline semiconductor process control of CD measurement for scatterometry.

Improved diffraction-based overlay metrology by use of two dimensional array target

We report results of theoretical modeling into a scatterometry-based method relevant to overlay measurement. A set of two array targets were designed with intentional offsets difference, d and d+20 nm, between the top and bottom grid arrays along the X and Y directions. The correlation of bi-azimuth measurements is the first critical issue been taken into account. The method linearizes the differential values of scatterometry signatures at the first diffraction order with respect to designed offsets, and hence permits determination of overlay using a classical linear method. By evaluating the process variations (eg. CD, roundness and thickness) on overlay measurement error, a set of two overlay target design were optimized to minimize the correlation of bi-azimuth measurements and maximize the measurement sensitivity.

Infrared differential interference contrast microscopy for overlay metrology on 3D-interconnect bonded wafers

Overlay metrology for stacked layers will be playing a key role in bringing 3D IC devices into manufacturing. However, such bonded wafer pairs present a metrology challenge for optical microscopy tools by the opaque nature of silicon. Using infrared microscopy, silicon wafers become transparent to the near-infrared (NIR) wavelengths of the electromagnetic spectrum, enabling metrology at the interface of bonded wafer pairs. Wafers can be bonded face to face (F2F) or face to back (F2B) which the stacking direction is dictated by how the stacks are carried in the process and functionality required. For example, Memory stacks tend to use F2B stacking enables a better managed design. Current commercial tools use single image technique for F2F bonding overlay measurement because depth of focus is sufficient to include both surfaces; and use multiple image techniques for F2B overlay measurement application for the depth of focus is no longer sufficient to include both stacked wafer surfaces. There is a need to specify the Z coordinate or stacking wafer number through the silicon when visiting measurement wafer sites. Two shown images are of the same (X, Y) but separate Z location acquired at focus position of each wafer surface containing overlay marks. Usually the top surface image is bright and clear; however, the bottom surface image is somewhat darker and noisier as an adhesive layer is used in between to bond the silicon wafers. Thus the top and bottom surface images are further processed to achieve similar brightness and noise level before merged for overlay measurement. This paper presents a special overlay measurement technique, using the infrared differential interference contrast (DIC) microscopy technique to measure the F2B wafer bonding overlay by a single shot image. A pair of thinned wafers at 50 and 150 μm thickness is bonded on top of a carrier wafer to evaluate the bonding overlay. It works on the principle of interferometry to gain information about the optical path length of the stacked wafers, to enhance the image contrast of overlay marks features even though they are locating in different Z plane. A two dimensional mirror-symmetric overlay marks for both top and bottom processing wafers is designed and printed in each die in order to know and realize the best achievable wafer to wafer bonding processing. A self-developed analysis algorithms is used to identify the overlay error between the stacking wafers and the interconnect structures. The experimental overlay results after wafer bonding including inter-die and intra-die analysis results will be report in the full paper. Correlation of overlay alignment offset data to electrical yield, provides an early indication of bonded wafer yield.

Measurement of through silicon via etch profile by dark-field optical microscope

Currently there are no in-line TSV (through silicon via) etch profile metrology tools suitable for use in high volume manufacturing. Cross-section SEM analysis can be utilized for process development, but it is a destructive technique. In our research, a dark-field optical microscope tool is developed to in-line non-destructively measure the via profile. It is capable of measuring images with high contrast between the measuring object and the surrounding background field. As the name implies, the background is dark and the measuring object is relatively bright. Thus, a tiny structure of object can be more clearly resolved compares to the conventional bright-field optical microscope method. Analysis algorithms are developed to analyze the bottom profile and the sidewall profile of the vias separately. In this paper, vias with CDs (critical dimensions) from 30 um to 200 um are measured, and the experimental results are verified by the cross-section SEM results.