The demands on CD metrology techniques in terms of both reproducibility and measurement uncertainty
increase with decreasing critical dimensions (CD) on lithography masks. Additionally a full 3D characterization
of the mask structures becomes more and more important to understand and control the printing behavior of state
of the art photomasks. Furthermore, an extension of metrology characterization including material properties can
provide the final puzzle pieces for a better correlation of mask metrology to wafer metrology. Here, optical
metrology systems, especially at-wavelength systems, are very well suited to characterize structure features of a
photomask regarding their printing behavior on a wafer. In particular scatterometry is able to provide a better
understanding of the investigated structure and allows for modeling of secondary structure parameters as well as
material composition.
AMTC has a commercial scatterometer from n&k Technology (n&k 5700-CDRT) in use. This system measures
the spectral transmission and reflection, the 0th diffraction order. Beside thin film characterization this system is
used for CD and edge profile characterization, also. The analysis of the data uses a look-up table approach in
combination with a database, which has been generated and can be expanded, respectively, using a RCWA
based software. At PTB we have realized a new DUV hybrid scatterometer which combines essential elements
of a radiometer, an ellipsometer, and a diffractometer.
These two systems are different both in terms of the measurement modes, the data evaluation method and the
Maxwell-solver used. Therefore we started to compare the performance of both systems to traditional metrology
system for CD metrology and phase measurement. For this purpose we performed first comparative
scatterometric measurements on a MoSi phase shifting mask.
Uniformity distribution of the corner rounding radius of curvature is investigated using reflectance- and transmittance-based
optical scatterometry. Arrays of square contact holes are measured at multiple locations on an ACI photomask
using a broadband spectrophotometer capable of collecting polarized reflectance (Rs and Rp) and polarized
transmittance (Ts and Tp) spectra in 190 - 1000 nm wavelength range in one-nanometer intervals. The measured spectra
are analyzed using two-dimensional Rigorous Coupled-Wave Analysis algorithm (2D RCWA) in conjunction with the
Forouhi-Bloomer dispersion relations for n and k. As a result of the analysis, the values of contact hole width and the
radius of curvature associated with the corner rounding are determined at every measurement location. The
measurements are presented as uniformity distribution maps and correlation plots, comparing the results with the values
obtained using a conventional CD-SEM.
The current abilities for active feedback loops to correct for various parameters challenge metrology groups to
provide exact input data for these correction cycles. One of the most important feedback loops is the one that
deals with the improvement of the CD (critical dimension) uniformity of structures. Here, several processes
rely on exact metrology data to tackle systematic effects that either have to be overcome by finding better
process conditions or compensated actively, for instance, by tuning the writer data.
Right now most of these processes tackle long range effects on the order of millimetres and do not vary a lot
on the micrometer scale. On the other hand, CD measurements are usually performed with instruments that
measure single points with dimensions of a couple of micrometers (such as the conventional CD-SEM). Thus
noise from the micrometer scale is introduced in the global mapping of the uniformity.
Recently, numerical methods, such as the exponentially weighted penalty approach called TPS (thin plate
splines) have been developed that separate between the true signatures on the millimetre scale from the noise
of the micrometer measurements. In this paper, we will take one step further by showing that the acquired
statistically stable CD signature of a CD-SEM measurement matches the CD data measured by a
scatterometer. Furthermore, we will show that the residual of the CD data of the scatterometer measurement
compared to the found TPS fit has a noise level of about 0.1 nm (3σ), which essentially equals the short-term
reproducibility of the tool. This is of high importance since both methods do essentially the same - they
average out micrometer noise with the only difference being that TPS does it theoretically and a scatterometer
does experimentally. Thus, we have the extremely fortunate situation in which theory and experiment give the
same results. Hence, two separate conclusions can be drawn: the scatterometer measures indeed stable
macroscopic CD signatures and TPS is indeed the right method to extract these signatures from any given CD
data.
For the first time, Rigorous Coupled-Wave Analysis (RCWA) is used for the analysis of both polarized broadband
reflectance and transmittance spectra with the purpose of measuring the degree of corner rounding in 2D contact holes.
The use of transmittance spectra proves to be advantageous for the characterization of the shape of the contact holes. In
contrast with the conventional reflectance-only techniques, transmittance measurements prove to be more sensitive to
the angstrom-level variations in the shape of the contact hole. Therefore, the new technique is capable of accurately
determining the degree of rounding of the contact hole corners and characterizing a variety of shapes - from perfectly
round to perfectly square. Additionally, the high intensity of the transmitted spectra improves the signal-to-noise ratio
and guarantees better repeatability of the results.
For the current study, 2D arrays of square contact holes with 800 nm pitch are measured on an After Clean Inspection
(ACI) phase-shift mask, using a spectrophotometer-based instrument capable of collecting four continuous spectra
during one measurement - two polarized reflectance spectra (Rs and Rp) and two polarized transmittance spectra (Ts and
Tp). The measured spectra are analyzed using the Forouhi-Bloomer dispersion equations, in conjunction with RCWA.
The method provides accurate and repeatable results for the degree of corner rounding of the square contact holes. In
addition, the method provides trench depth, critical dimensions, film thickness, and optical properties (n and k spectra
from 190 - 1000 nm) of phase-shift photomasks. The results of the measurements are represented as high-resolution
uniformity maps obtained for all the parameters mentioned above. The results show excellent correlation with
conventional CD metrology techniques.
For the first time, polarized broadband transmittance (T) plus reflectance (R) measurements, combined with
the Rigorous Coupled-Wave Analysis (RCWA) and the Forouhi-Bloomer dispersion equations for n and k,
were used to measure 2D trench dimensions. This is in contrast to traditional scatterometry, which is based
on reflectance-only measurements. T and R were measured from 190 to 1000 nm in one-nanometer intervals.
Inclusion of the transmittance measurements proved to be advantageous, because there is a greater sensitivity
of the T spectra to the sub-nanometer structural and/or material variations, which are difficult to detect with
R-only measurements. Furthermore, the intensity of T is much higher than the intensity of R, resulting in a
much improved signal-to-noise ratio, since intensity is proportional to number of photons reaching the
detector, which in turn is proportional to the signal. Thus, the higher the intensity, the higher the signal-to-noise,
and the better the repeatability and reproducibility of the results.
For the current study, 2D arrays of square and circular contact holes of various pitches were measured on an
After-Clean-Inspection (ACI) phase-shift mask, using a spectrophotometer-based instrument, capable of
collecting four continuous spectra during one measurement - two polarized reflectance spectra (Rs and Rp)
and two polarized transmittance spectra (Ts and Tp). The measured spectra were analyzed using the Forouhi-Bloomer dispersion equations, in conjunctions with RCWA algorithm, applied simultaneously to R and T polarized spectra. The method provided accurate and repeatable results for contact hole depths, critical dimensions film thicknesses and n and k spectra. High-resolution uniformity maps were obtained for all the parameters mentioned above.
A novel scatterometry method, based on broadband measurements of reflectance and transmittance spectra is presented. For the first time Rigorous Coupled Wave Analysis (RCWA) algorithm is applied to the analysis of the transmittance spectra for the determination of trench depths, critical dimensions, profiles, film thicknesses, and optical properties (n and k spectra from 190 - 1000 nm) of phase-shift photomasks.
It is shown that very small structural and/or material variations, which are difficult to detect with reflectance (R) only measurements, can be readily distinguished with transmittance (T) measurements.
For the current study, a spectrophotometer-based instrument (n&k R-T Scatterometer) was used, capable of collecting four continuous spectra during one measurement - two polarized reflectance spectra (Rs and Rp) and two polarized transmittance spectra (Ts and Tp). The light source of the spectrophotometer was equipped with a rotating polarizer, facilitating TE and TM polarizations of the measurement beam. The analysis was performed using Forouhi-Bloomer dispersion equations, in conjunctions with RCWA algorithm, applied simultaneously to reflectance and transmittance spectra. The method provided accurate and repeatable results of above stated parameters, for all materials present in the structure.
A linearity study based on this novel reflectance-transmittance scatterometry method, demonstrated excellent correlation with the target values and the conventional CD-SEM measurements and improved repeatability compared to the traditional reflectance-only measurements. The advantages of the method are high throughput, non-destructive nature of the measurements, and capability to measure a wider variety of structures pertinent to the photomask manufacturing process.
For the first time Rigorous Coupled Wave Analysis (RCWA) has been applied to the analysis of the transmittance spectra for the determination of critical dimension (CD) of phase-shift photomasks. The use of transmittance spectra proved to be instrumental in improving the sensitivity of the measurement to minor (sub-nanometer) changes in the width of the trench. We present a novel unique metrology solution based on the simultaneous measurement of broadband reflectance and transmittance, covering a wavelength range from 190 to 1000 nm, in one nanometer intervals. The analyses of both types of spectra are performed simultaneously, using Forouhi-Bloomer dispersion equations, in conjunctions with RCWA. The method provides accurate and repeatable results for critical dimensions, thickness, and optical properties (n and k spectra from 190 - 1000 nm) for all materials present in the structure. In the current study, the method described above was used to examine grating structures on ACI (After-Clean Inspection) phase-shift mask. The use of transmittance spectrum proved to be essential for the accurate measurement of the CD, since the transmittance spectrum is more sensitive to the change in line width, compared to the reflectance spectrum. The results were compared with the measurements taken on the same sample using conventional CD-SEM. The CD linearity study demonstrated excellent correlation with CD-SEM. The advantages of the optical reflectance and transmittance method are high throughput, non-destructive nature of the measurements and capability to measure a wider variety of structures pertinent to the photomask manufacturing process.
The fabrication of a production-worthy phase shift mask requires, among other things, excellent uniformity of critical dimensions (trench width and depth) and optical properties of the phase shift material (MoSi). Traditionally, CD-SEM has been the instrument of choice for the measurement of width; AFP (Atomic Force Profilometer) or conventional profilometer for the measurement of depth; and Interferometer for the measurement of phase shift and transmittance of the phase shift material. We present an innovative optical metrology solution based on broadband reflectometry, covering a wavelength range from 190 to 1000 nm, in one nanometer intervals. The analysis is performed using Forouhi-Bloomer dispersion equations, in conjunctions with Rigorous Coupled Wave Analysis (RCWA). The method provides accurate and repeatable results for critical dimensions, thickness, and optical properties (n and k spectra from 190 - 1000 nm) for all materials present in the structure. In the current study, the method described above was used to examine photomasks at two stages of mask manufacturing process: After Etch Inspection (AEI) and After Strip Inspection (ASI). The results were compared with the measurements taken on the same samples using conventional CD-SEM. Two comparison studies were conducted - global CD uniformity and CD linearity. The CD linearity study demonstrated excellent correlation between the values of grating line width obtained using this new optical reflectometry approach and a CD-SEM for the grating structures of two pitches (760 nm and 1120 nm). The global CD uniformity study revealed that this presented reflectometry method can be used to produce CD uniformity maps which demonstrate excellent correlation with the results obtained using a conventional CD-SEM. The advantages of the optical method are high throughput, non-destructive nature of the measurements and capability to measure a wider variety of structures pertinent to the photomask manufacturing process.
Critical dimension (CD) metrology is an essential part of the mask manufacturing process. We present a metrology solution based on broadband reflectometry, covering a wavelength range from 190 to 1000 nm, in one nanometer intervals. The analysis is performed using Forouhi-Bloomer dispersion equations, in conjunctions with Rigorous Coupled Wave Analysis (RCWA). The method provides accurate and repeatable results for critical dimensions, thickness, and optical properties (n and k spectra from 190 - 1000 nm) for all materials present in the structure. In terms of throughput (several seconds per point) and suitability for integration, the method has many advantages over conventional metrology techniques. Measurements were performed on two masks, at two different stages of the mask manufacturing process - After Etch Inspection (AEI) and After Strip Inspection (ASI). CD uniformity distribution maps at 121 points on the mask were obtained for 800 nm pitch grating arrays. The results were compared to conventional CD-SEM measurements collected at the same locations. A linearity study was conducted on 760 and 1120 nm pitch grating arrays with systematically increasing CD width. The results demonstrate excellent correlation with CD-SEM.
One of the difficult challenges faced by the semiconductor manufacturingindustry is the pressure to create ever more powerful, complex chips with smaller geometries. Currently, the demands for smaller feature sizes are being met by utilizing exposure wavelengths in the deep-UV range (248 nm and 193 nm). To further reduce feature size and squeeze the very last potential out of optical lithography, the technology has moved towards the incorporation of phase-shift materials. For such materials, rapid and accurate measurement is imperative to produce and maintain the correct phase-shift. The complete and accurate characterization of phase-shift materials requires that the measuring instrument provide phase-shift information plus thickness of the phase-shift material and values oin (the index of refraction and k (the extinction coefficient) at the specified wavelength. Furthermore, transmittance through both the phase-shift material and substrate must be measured at that specified wavelength. Specifically, the characterization data must indicate whether the ideal phase-shift of 180°, in addition to pre-specified transmittance in the 5-10% range, has been achieved. In this article we present a method of data collection and analysis that allows the phase-shift, film thickness and values of n and k to be determined simultaneously from the concurrent measurements of transmittance and reflectance, allowing the detection of non-uniformities in phase-shift in either patterned or un-pattemed films, with close correlation to direct-measured values. This technique offers the advantage of high throughput (entire masks can be characterized in minutes) and can be applied equally well to patterned or unpattemed masks.
The optical properties of materials comprising photolithographic masks are investigated at wavelengths covering the vacuum-ultra-violet (VUV) to the near-infra-red (NIR). Broadband reflectance (R) and transmittance (T) spectra from 130 to 1000-nm are obtained from a variety of single layer absorber and bi-layer absorber/anti-reflection coating (ARC) samples deposited on MgF2 and CaF2 substrates. These experimental data are analyzed using the Forouhi-Bloomer (F-B) dispersion equations, in conjunction with a least squares fitting algorithm, to infer the thickness and n and k spectra of the materials under investigation. Once determined, the optical properties of the component materials are used to calculate the optical density of the single layer absorbers at 157-nm. These preliminary calculations are performed to investigate the feasibility of extending the use of traditional mask materials to wavelengths below 193-nm. In addition, theoretical swing-curve and standing wave functions are predicted for a mask structure based on the CrOxNy/Cr material system.
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