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.
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.
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.
Semiconductor device packaging technology is rapidly advancing, in response to the demand for thinner and smaller
electronic devices. Three-dimensional chip/wafer stacking that uses through-silicon vias (TSV) is a key technical focus
area, and the continuous development of this novel technology has created a need for non-contact characterization. Many
of these challenges are novel to the industry due to the relatively large variety of via sizes and density, and new processes
such as wafer thinning and stacked wafer bonding. This paper summarizes the developing metrology that has been used
during via-middle & via-last TSV process development at EOL/ITRI. While there is a variety of metrology and
inspection applications for 3D interconnect processing, the main topics covered here are via CD/depth measurement,
thinned wafer inspection and wafer warpage measurement.
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, x and y, 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 x and y 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
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.
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.
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.
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.
We develop a novel target, dual-overlay grating, used in the overlay measurement with an optical bright-field
imaging tool. The dual-overlay grating is the combination of two overlay gratings with different pitch. The two
overlay grating are approached each other and the separation gap between them is sub-micrometer. The image in the
proximity of the boundary of two overlay gratings is measured at in-focus position, and a method is built to analyze
the image. The gradient value of image and a merit value are calculated. A series of dual-overlay grating is measured
and analyzed with different overlay offset. We found the relation between the merit value and overlay offset is linear
in certain region, and the dual-overlay grating has the nano-scale resolution to the overlay offset. Thus, the
dual-overlay grating has potential application in overlay metrology for the process control in the future semi-conductor manufacturing.
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.
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.
As overlay tolerances of microlithographic technology become increasingly severe, conventional bright-field metrology systems are limited by image resolution and precision. Scatterometer (angular scatterometer or spectroscopic reflectometer, for example) has the advantages of good repeatability and reproducibility, and is proposed as an alternative solution for overlay metrology. Previous studies have applied a spectroscopic reflectometer, which is as function of incident wavelength, to overlay measurement. This work investigated overlay measurement by using an angular scatterometer, which is as function of incident angle. A focused laser spot was incident on linear grating, an overlay target. An angular signature, a 0th-order reflective light beam, scattered from linear grating was measured when the incident and reflective angles were changed simultaneously. The overlay target consists of two linear gratings located on two different layers of a stacked structure, and the overlay error is the misalignment between these two different layers. The measured results using angular scatterometer (also known as the diffraction-based method) are compared with using the bright-field microscope (also known as the image-based method), which use a bar-in-bar target as an overlay target. Statistical data sets demonstrate that angular sctterometer has nearly one order better of repeatability and tool induced shift than conventional bright-field microscope. Additionally, a series of different parameters of overlay targets, such as different pitches, line-to-space ratios, and stacked structures is designed and manufactured. The sensitivity of overlay measurement of various linear grating targets is also measured and discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.