Phase masks are used to eliminate the Fourier-plane hotspot that otherwise degrades holographic data storage
performance. In order to eliminate the cost, bulk, and precision alignment difficulties of inserting a discrete phase mask
into an optical system we have designed phase masks integrated directly into the structure of a spatial light modulator
used as the storage system's write head. A micron-thick ferroelectric liquid crystal film is confined between the surface
of a VLSI integrated circuit and a window containing planarized relief structures on its inward-facing surface. This
arrangement avoids depth-of-field problems encountered by designs that place the phase mask on the outer surface of the
window. Any of a variety of phase mask designs can be implemented in this fashion. An alternative architecture in
which pixel surfaces of the CMOS VLSI backplane are etched to differing heights is also investigated.
We report design and test of a high brightness laboratory-breadboard LED/LCOS HMD system employing a 0.78-inchdiagonal
1280× 768 ferroelectric liquid-crystal-on-silicon microdisplay and a red-green-blue LED. With an 8× viewing
optic giving a 35°-diagonal field of view, the system yielded brightnesses greater 40,000 cd/m2 (12,000 fL) in colorsequential
mode and greater than 100,000 cd/m2 (30,000 fL) in monochrome mode, at LED power consumptions of
1.1 W and 3.3 W, respectively. The illumination optics employed a rectangular light pipe and tailored diffuser to efficiently
fill the microdisplay panel aperture and exit pupil. The high efficiency of such image generators facilitates
display readability in see-through HMDs operating in high-ambient-light environments, as well as enabling ultra-low
power HMDs (less than 100 mW total) for dismounted users of battery-powered systems.
The stressed liquid-crystal (SLC) electro-optic effect promises fast electro-optic response times even for design wavelengths
in the infrared (IR). Here we report characteristics of SLC devices appropriate for use as liquid-crystal-onsilicon
(LCOS) spatial light modulators (SLMs) in the near ( λ = 1.8-2.5 μm), mid (3-5.5 μm) and far (8-14 μm) IR
bands. For these three bands we fabricated SLC devices with 5, 10, and 20 μm thicknesses; at drive voltages of 25, 50,
and 125 V respectively these devices gave half-wave modulation with response speeds in the 1.3-1.6 ms range. Visiblelight
measurements on a 20-μm-thick SLC device between crossed polarizers gave a contrast ratio of 360:1 which
improved to nearly 18,000:1 with a Babinet-Soleil compensator offsetting residual SLC retardance. Widely available
high-voltage options in standard CMOS processes offer sufficient drive for near- and mid-IR SLCOS devices; with
modest increase of SLC material birefringence Δn and dielectric anisotropy Δε far-IR devices would be feasible, too.
Pixel drivers utilizing these options have pitches less than 24 μm, making 1000 ×1000 SLMs feasible.
Phase-modulation scatterometry is a metrology technique for determining the parameters of gratings using as a key device a phase modulator. For measurement purposes the phase modulator requires a complicated calibration procedure that is analyzed here in detail. The main source of error to be dealt with are the fluctuations of the phase modulation amplitude. The measurables are the direct term and the first two harmonics of the output. For the fitting of the experimental data we used the ratio of the harmonics to the direct term because it improves significantly the accuracy. A sensitivity analysis was performed for two samples, one real and one theoretical, to find the measurement configuration that insures optimum determination precision for the grating parameters. For the real sample, comparisons of the theoretical predictions for sensitivity with the actual values showed a good agreement. For both samples the sensitivity analysis indicated sub-nanometric precision for the critical dimension (grating linewidth).
In the work reported here, we discuss the measurement precision of two scatterometry techniques, the variable angle and the variable wavelength techniques. The issue of interest is the measurement precision of the sample parameters. This is determined by both the sensitivity of the diffraction measurable to changes in sample parameters and the precision with which the measurable can be determined. This approach includes taking into account the correlation effect between the contribution to the measurable of the various grating parameters to be determined, such as linewidth and height. The comparison of the theoretical predictions of precision for angle-resolved and wavelength-resolved scatterometer measurements shows no conclusive hierarchy. Practical considerations, however, indicate that angular-resolved scatterometry is a more advantageous technique. For both methods, decreasing the wavelength of the light source improves the determination precision of the sample parameters.
The intensity of radiation diffracted from periodic structures is extremely sensitive to slight variations in the geometry and composition of the diffracting structure. Rigorous diffraction theory provides a mechanism for accurate analysis of the scattered waves. Scatterometry is a metrology technique that combines the sensitivity of diffraction from periodic structures with a first principle solution of electromagnetic wave diffraction from these structures. The technique is self-calibrating, and sub-nm static and dynamic precision has been demonstrated for the measurement of sub-0.25 μm structures. We provide an overview of the development of this metrology technique, along with the theoretical foundation of rigorous diffraction analysis and its application to the analysis of the scattered data measured by the scatterometer.
The sensitivity analysis of fitting (SAF) is a formalism that determines the type of measurements that yields optimum determination precision. SAF is applied to ellipsometric- scatterometry of surface relief gratings and for the optimum measurement configuration predicts a significant improvement compared to the conventional scatterometry measurement configurations. The SF predictions for precision are compared to actual values obtained experimentally, and a qualitative agreement is obtained. The discrepancies between theory and experiment are likely due to inaccurate modeling of the grating.
Scatterometry, the analysis of light diffraction from periodic structures, is shown to be a versatile metrology technique applicable to a number of processes involved in the production of microelectronic devices, flat panel displays, and other technologies which involve precise dimensional control of micron and sub-micron features. This paper reviews metrology issues and requirements of these technologies and gives details on one application of scatterometry for illustration. Scatterometer results are compared to measurements of the same samples using other metrology techniques, including cross- section SEM, top-down SEM, AFM, and ellipsometry.
Conventional scatterometry measures the intensity of a diffraction order from a periodic structure as one or more measurement parameters is changed. We have previously demonstrated conventional techniques to characterize developed photoresist linewidths as small as approximately 0.15 micrometer, with scatterometer results agreeing well with measurements performed using other techniques. For developed photoresist, the measurement sensitivity of conventional scatterometer techniques diminishes considerably for sample linewidths that are sub-0.1 micrometer, using 633 nm laser illumination. We present a modified scatterometer configuration which combines aspects of conventional scatterometry and ellipsometry that provide increased sensitivity for characterizing sub-0.1 micrometer linewidth periodic photoresist structures. The complex reflection coefficients representing the grating sample are extracted, both in magnitude and in relative phase, through intensity measurements at selected polarizer/analyzer/compensator orientations. The cross-polarization terms of the reflection coefficient matrix are shown to be equal for a symmetric photoresist grating structure with line widths approximately 0.5 micrometer. Theoretical results for nominal 70 nm photoresist lines are presented that show phase measurement sensitivity to linewidth changes on the order of 2 - 4 deg/nm and reflectance sensitivity of at least 3%/nm. This results in linewidth measurement resolutions that are sub-nm.
Optical diffraction tomography (ODT) attempts to reconstruct the complex refractive index profile of an object by
inverting its backscattered and/or transmitted fields. Owing to its integral formulation of the diffracted plane, the inverse scattering
problem in ODT, i.e., reconstructing the object from its diffracted field, can be linearized via the Born approximation.
The validity range of the Born approximation is limited to weakly scattering objects, or objects whose refractive index distributions
are slowly varying and comparable in magnitude to their background. Such constraints are easily met in microlithography
when considering the area of latent image metrology. Indeed, latent images are generally characterized by their relatively
small and slowly varying refractive indices. An algorithm is presented for reconstructing the refractive index distribution of
latent images from their first (+1) and second (+2) reflected diffraction orders at the Bragg angle.
In previous applications scatterometry has shown promise as a metrology for several process measurements. The linewidths of both resist and etched features, and the thicknesses of several underlying film layers, have been accurately characterized using the technique1 . Up until recently these results have been obtained by assuming the features being measured possessed a nominally square profile. However, as metrology tolerances shrink in proportion to device dimensions, errors in the measurement technique due to non-square line profiles could become significant. To test the ability of the scatterometry technique to measure non-square profiles, two models have been developed. The first profile model assumes the top and bottom corners of a resist line can be approximated as a segment of some circle with a given radius. With the center of the circle fixed in space by the overall height of the resist and a nominal linewidth, the sidewall of the line is then modeled as the tangent line that connects the two circles. This particular model can accommodate both overhanging (<900) and trapezoidal sidewalls (<900) with just four parameters: the radius of the top and bottom corners, and the nominal top and bottom linewidths. Comparisons between cross-section SEM images and scatterometry profiles using this model will be presented. The second model, which we call the "stovepipe' model, is a modified version of a simple trapezoid model and has applications to etched features. In this model an etched line is parameterized by assuming the trapezoidal portion of the sidewall starts at some distance below the top of the line, with the top portion of the line remaining square. In this manner an etched profile can be modeled with four parameters: the overall height of the etched line, the nominal etched linewidth, and the overall height and sidewall angle of the trapezoid layer. Once again, scatterometry profile results in comparison to cross-section SEM images will be presented. The use of both of these models has reduced the difference between scatterometry and SEM CD measurements. For example, the average difference of twelve resist CD measurements, when compared to crosssection SEM measurements, improves from 19.3 to 10.1 urn when the full profile model is incorporated.
Keywords: metrology, diffraction, optical metrology, scatterometry, process control
The silylation step in the top surface imaging process has been difficult to monitor and characterize for lack of appropriate metrology tools. Utilizing scatterometry to measure silylated wafers, we report successful monitoring of processing effects. Wafers were manufactured under nominally identical processing conditions. Applying scatterometry, we are able to discern location dependent variations within wafers. In addition, wafer to wafer variations are also observe. Both these variations are detrimental to yield. Variations in processing conditions cause modifications and perturbations in the gratings. Different gratings diffract light in a dissimilar manner. Processing conditions and their effects on the wafers are deduced from these measurements using computational analysis. This information is used to detect unwanted variations in processing conditions so that corrective responses can be implemented. This technique is rapid, non-destructive and sensitive to changes introduced by the silylation process.
Scatterometry, the characterization of periodic structures via diffracted light analysis, has been shown to be a versatile technique for measuring critical dimensions in photoresist as small as 0.160 micrometer. Rapid, non-destructive and inexpensive, scatterometry has the potential to be applied to other microlithographic features as well. This paper discusses applications of scatterometry in the measurement of etched sub-um poly-Si line/space patterns. Since etched features represent the final dimensions of a finished product, the characterization of such features is important. Initial attempts at measuring the etched linewidth and height using scatterometry assumed the sidewalls were perfectly vertical. Although results from these two parameter predictions were good, our measurement algorithms suggested that the etch profiles were not square. Thus, sidewall angle was left as an unknown in our model and three parameter predictions were made. These improved results from measuring the linewidth, height and sidewall angle are presented, and comparisons to SEM measurements of the same samples are made. Finally, experiments to determine the repeatability of the scatterometer for measuring etched features were performed. Results show that the repeatability of the instrument, for both static and dynamic measurements of nominal 0.25 micrometer structures, is sub-nanometer for all parameters measured; the 3(sigma) repeatability for static CD measurements is 0.63 nm, and for dynamic measurements is 0.78 nm.
Scatterometry, defined as the angle resolved characterization of light scattered from a surface, is an attractive tool for the metrology of semiconductor devices. It is simple, rapid, non destructive, relatively inexpensive and can be used in-situ. This paper illustrates the use of scatterometry to characterize fine pitch gratings having linewidths less than or equal to 0.1 micrometer. These gratings diffract light only in the zeroth order as their pitch-to-wavelength ratio is much smaller than one, hence they are also known as 0-order gratings. Metrology of 0-order gratings brings forth new issues, chiefly (1) lack of diffraction sensitivity to variation in the grating parameters, and (2) non-uniqueness of the 'diffraction signatures.' We use the gratings in conical mounting to enhance the diffraction sensitivity and have circumvented the non-uniqueness issue in two ways: (1) limiting the parameter space of the search algorithm and (2) using different incident field polarizations. We employ constrained optimization techniques to efficiently scan the parameter space. Our results agree well with cross-sectional SEM measurements and demonstrate the feasibility of scatterometry for these structures. We are also using shorter wavelengths for the metrology of 0-order gratings, and preliminary results using (lambda) equals 442 nm demonstrate that the diffraction is more sensitive to the variation in grating linewidth and etch depth.
A scatterometric sensor measures the intensity of light diffracted from a periodic structure. When applied in-situ to the post exposure bake (PEB) process for chemically amplified resists, a scatterometric sensor can monitor the formation of a latent image. Sturtevant, et al and Miller, et al have shown that this application of scatterometry is viable for chemically amplified resist linewidth, or critical dimension (CD) control. A CD control system requires the prediction of the final developed CD using data collected throughout part of the PEB process. Previous work has not addressed the issue of variations in underlying film thickness; these variations may not dramatically affect latent images when anti-reflection coatings are used, but they can greatly affect the signals from scatterometric sensors. This requires a sensor design coupled to a CD prediction algorithm that can accommodate underlying film thickness variations. We have designed, constructed and tested an experimental scatterometric sensor for the PEB process which collects sufficient data to provide a robust CD prediction. The instrument was installed on a PEB module at SEMATECH, and the signals from 0.35 micrometer linewidth gratings were measured on bare silicon (Si) wafers and wafers with varying poly-Si and oxide thicknesses. Using data from the first 45 seconds of a nominal 60 second PEB, final developed linewidth predictions were achieved with a standard error of prediction of 5.08 nm for bare Si wafers and 6.89 nm for poly-Si/oxide/Si wafers. In parallel with our sensor development effort, we have developed a physical model for diffraction from latent image gratings during the PEB process. The model links a lithography simulation tool to rigorous coupled wave diffraction theory. Diffraction from a latent image grating occurs via two mechanisms: variations of the index of refraction within the resist and a slight surface- relief grating which results from dose-dependent volume loss within the resist. Simulations indicate that the surface-relief grating dominates first-order diffraction signals.
Scatterometry, the characterization of periodic structures via diffracted light analysis, is shown to be a versatile metrology technique applicable to several processes involved in microlithography. Unlike contemporary inspection technologies, such as scanning force microscopy (SFM) and scanning electron microscopy (SEM), scatterometry is rapid, non- destructive, inexpensive and has the potential for use in-situ. Furthermore, the flexibility of the technique allows it to be used for a number of different process measurements. In the production of a sub-micron microelectronic device, a typical series of process steps could involve the deposition of a poly-Si layer on oxide, followed by the application of an anti- reflection coating (ARC) and resist layer. Thus in total there are four parameters which will ultimately affect the overall quality of subsequent processing: the linewidth of the resist, the resist height, and the thicknesses of the ARC and poly-Si. We have demonstrated that the scatterometer measurement technique is robust to changes in the thickness of underlying films. Indeed, there is sufficient information in one signature to determine four parameters at once, even when the linewidth dimensions are as small as 0.16 micrometer and the poly-Si thickness is on the order of 2500 angstrom. Results from determining these dimensions on several wafers show excellent agreement between the scatterometry measurements and measurements made with other metrology instruments (top down and cross-section SEM, and ellipsometer). For example, the average bias between nine scatterometry and cross-section SEM measurements on nominal 0.35 micrometer lines is minus 1.7 nm; for 0.25 micrometer lines, the average difference is minus 7.3 nm. In addition, results from measuring the sidewall angle (a fifth parameter) from these same scatter signatures indicate that the resist profiles at optimum focus and exposure are near-vertical. Finally, the dynamic repeatability of this technique is shown to be excellent for all of the parameters measured (linewidth, resist height, ARC thickness and poly thickness). For example, the 3(sigma) repeatability of measurements on a 207 nm linewidth is 0.75 nm and the 3 sigma repeatability for measurements on a 311 nm linewidth is 1.08 nm.
A precise and accurate technique for the characterization of periodic line/space gratings is presented. The technique, known as scatterometry, derives its sensitivity and robustness from the wealth of information present in diffracted optical radiation. Scatterometry is capable of determining width, height, and overall shape of sub-half micron lines as well as the thickness of underlying thin films. The characterization process consists of three elements: a diffraction measurement apparatus, a model built on calibration data, and a statistical analysis routine that uses the model to correlate empirical data to the unknown parameters of the structure. The measurement technique was evaluated on twenty five wafers fabricated with deliberate deviation in focus, exposure dose, and underlying thin film thickness. Each wafer consisted of developed photoresist lines on an antireflecting layer, placed on layers of polycrystalline silicon on gate oxide on a silicon substrate. Scatterometry was used to simultaneously determine the width and height of the nominal 0.25 micrometers and 0.35 micrometers photoresist lines, as well as the thickness of underlying layers. Comparison of results obtained using reference methods (ellipsometry and scanning electron microscopy) are included.
Scatterometry, the analysis of light scaattered by diffraction from periodic structures, is shown to be a versatile metrology technique applicable to a number of processes involved in microelectronic manufacturing. Contemporary inspection technologies such as scanning force microscopy (SFM) and scanning electron microscopy (SEM), apart from being slow and possibly destructive, in general cannot be used in-situ. Scatterometry, on the other hand, is rapid, nondestructive, inexpensive, and has the potential for use in-situ. In the production of a sub-micron microelectronic device, a typical series of process steps could involve the deposition of a poly-Si layer on oxide, followed by the application of an anti-reflection coating (ARC) and resist layer. After the resist is exposed and developed there are four dimensions which will affect further wafer processing: the linewidth of the resist, the resist thickness, and the ARC and poly-Si thicknesses. By varying the angle of incidence and continuously monitoring the diffracted power in any diffraction order, a scatter 'signature' may be obtained. We have demonstrated that there is sufficient information in one signature to determine all these dimensions at once, even when the linewidth dimensions are as small as 0.25 micrometers and the poly-Si thickness is on the order to 2500 angstrom. Results from determining these dimensions on a 25 wafer study show excellent agreement between the scatterometry measurements and measurements made with other metrology instruments (SEM and ellipsometer). For example, there is a 22.6 nm average difference between SEM and scatterometry measurements of 0.25 micrometers nominal linewidths. In addition, the repeatability (1(sigma) ) of this technique is shown to be sub-nanometer for all of the parameters measured (linewidth, resist height, ARC thickness, and poly thickness).
Scatterometry, the characterization of periodic structures via diffracted light analysis, is shown to be a viable and versatile metrology for critical dimensions as small as 0.24 micrometers . Scatterometry is rapid, nondestructive, inexpensive, and potentially useful for on- line control during several microlithographic processing steps. This paper discusses two recent studies in which scatterometry was applied to the measurement of developed photoresist patterns. First, scatterometric measurements of developed resist lines in the 0.38 micrometers to 0.70 micrometers range will be presented. Results from four sample wafers are shown to be consistent with SEM measurements. For one wafer, the average deviation of scatterometry linewidth measurements form top-down SEM measurements, over a broad exposure range, is 14.5 nm. Moreover, our scatterometer is shown to be highly linear with the SEM; linearity coefficients have typically been above 0.99. The goal of our second project has been to determine whether scatterometry measurements are affected by variations in the integrated circuit production process. A set of twenty-five wafers was fabricated with deliberate variations in the exposure dose and the underlying film thicknesses. We are presently investigating the effects of the film thicknesses on the measurements of critical dimensions (CDs) as small as 0.24 micrometers . Preliminary results indicate that CDs and multiple thin films can be simultaneously measured by applying multi-parameter prediction algorithms to the scattered light data. Results from four different prediction algorithms are given. Finally, the repeatability of the scatterometer is shown to be excellent: 0.5 nm for consecutive measurements and 0.8 nm for day-to-day measurements. The results of an extensive repeatability/precision experiment are presented.
Scatterometry is presented as an optical metrology technique potentially capable of determining the critical parameters of a phase etched diffraction grating test structure (sidewall profile, etch depth, and linewidth). The technique is noncontact, rapid and nondestructive. The test grating structure is illuminated by a laser beam and the intensities in the different diffracted orders are measured as the angle of incidence of the laser beam is varied over a certain range. A phase shift mask consisting of an array of chrome and chromeless phase etched gratings was fabricated at AT&T Bell Labs using e-beam techniques. The grating linewidths varied from nominal 0.5 micrometers to 5.0 micrometers , while the etch depths varied from a nominal 190 nm to 400 nm depths. Both the chrome and the quartz gratings were measured, although only data for the quartz gratings is presented here. The measurements of the diffracted orders were made using the two theta scatterometer located at the University of New Mexico. The shape of the diffraction curves obtained in this manner has been shown to be sensitive to the grating structure parameters (sidewall profile, etch depth, linewidth, etc.). An estimate of the quarts phase etched structure parameters was obtained through a combination of rigorous coupled wave theory (CWT) and minimum mean square error (MMSE) analysis. Additionally, each grating was measured using an AFM located at AT&T. Comparison of the scatterometer and AFM measurements are presented along with their absolute differences. Finally, the long term and short term repeatabilities of the scatterometer measurements are shown to be excellent.
Scatterometry, the analysis of light scattered by diffraction from periodic structures, is shown to be a versatile process control and metrology technique for use in microelectronics manufacturing. Contemporary inspection technologies, such as scanning force microscopy (SFM) and scanning electron microscopy (SEM), in general cannot be performed in-situ and are slow for real-time process control. Scatterometry, on the other hand, is rapid, nondestructive, inexpensive and might be used on-line. This paper will discuss applications of 2 - (Theta) scatterometry to developed photoresist focus/exposure matrices, often related to the manufacture of microelectronic devices. To test this technique we obtained and measured five identically processed wafers with nominal 0.5 micrometers line/0.5 micrometers space grating patterns. Each wafer is comprised of gratings created in Shipley 89131 negative photoresist and arranged in a matrix of incremental exposure doses and focus settings. The scatterometric CD measurements are consistent in comparison to cross- section and top-down SEM measurements of the same structures. The average deviation of 11 linewidth measurements from top down SEM measurements, over a broad exposure range, is 14.5 nm. In addition, the repeatability (1 - (sigma) ) of the 2 - (Theta) scatterometer is shown to be excellent: 0.5 nm for consecutive measurements and 0.8 nm for day to day measurements.
We have initiated an effort to develop metrology tools that isolate the effect of each process step. Light scattered from diffracting structures is analyzed to determine characteristics of the structure. The technique is rapid, non-destructive, and extremely sensitive to variations in the samples that were examined. Through our technical collaboration with Texas Instruments Inc. we obtained wafers coated with surface imaging resists and exposed under varying focus and exposure conditions. We present results that utilize scatterometry to monitor the exposure step to determine defocus and exposure variations in the latent image. We also report using scatterometry to monitor the post-exposure bake (PEB) process for chemically amplified resists. Wafer-to-wafer variations in resist and underlying film thicknesses result in CD variations for constant exposure. The PEB time can be adjusted for each wafer to account for some of the parameter variations. We present experimental data supporting the concept of a scatterometer PEB monitor.
Scatterometry is shown to be a viable alternative to current methods of post-developed line shape metrology. Five wafers with focus-exposure matrices of line-space grating patterns in chemically amplified resist were generated. The gratings were illuminated with a He-Ne laser and, utilizing only the specular reflected order measured as a function of incident angle, we were able to predict linewidth and top and bottom rounded features. The scatterometry results were verified with those obtained from scanning electron microscopy (SEM). A set of wafers having a SRAM device pattern was analyzed. These wafers contain columns of devices, each having received an incremental exposure dose. We present exposure predictions based on data taken with the dome scatterometer, a novel device which measures all diffraction orders simultaneously by projecting them onto a diffuse hemispherical `dome.' A statistical calibration routine was used to train on the diffraction patterns from die locations with known exposure values.
Advances in memory IC technology for dynamic random access memory (DRAM) devices have been achieved by increasing the number of memory cells occupying a certain chip area, consequently increasing memory size. Current methods of implementation include vertical topography, which relies on reducing the cell's thickness while increasing its depth in order to maintain the same capacitance of stored electrical charges. As the memory size on DRAM devices rises, memory cells have to reach deeper levels, thus making the process of measuring depth even harder. A novel metrology technique, which utilizes both 2-D diffraction analysis and multivariate statistical methods to measure deep trench depth, is discussed in this work. This technique was applied to two DRAM product wafers, and successful prediction of trench depth was obtained for both wafers with an accuracy of +/- 0.04 micrometers , or +/- 0.56% variation.
The trend towards smaller design geometries for microelectronics devices places unprecedented demands on the measurement of these small structures. Two sample problems are considered. In the first case we predict the shape of developed photoresist gratings from diffraction data obtained from an angle scanning scatterometer in which a detector tracks a particular diffraction order as the angle of incidence is varied. The second problem we consider is the prediction of depths of cylindrical cells etched into Silicon. A dome scatterometer is used to collect the 2-D diffraction signal and in this case the experimental data is used to construct a data base of scatter signals.
In this paper we discuss an optical metrology technique for the determination of optimum lithography parameters through an interrogation of the latent image. This technique, called the Lithography Process Monitor (LPM), involves illuminating a latent image grating with a laser beam. The intensity of the orders diffracted from the grating has been shown to be directly related to the photoactive compound (PAC) concentration profile, and consequently, to the profile of the developed resist. We have developed a method of modeling the intensity in the diffracted orders by using lithography simulation software in conjunction with rigorous coupled wave diffraction analysis. Experiments have been conducted with both positive and negative resist. In addition, we have been able to determine the 'absolute' location of the top of the photoresist with respect to the stepper focal reference and determine film thickness variations on the wafer.
Quantitative methods are developed to use optical scatter to measure the critical dimensions of gratings etched into bulk Si and developed photoresist patterns on silicon substrates. Previous work either classified microstructures qualitatively or employed a 'chi-by-eye' method to find that structures were similar or dissimilar. A single detector scanning scatterometer is used to measure large 32 micrometers pitch structures while another instrument that varies the angle of incidence and tracks diffracted orders via the grating equation is used to measure 2 micrometers pitch structures. A rigorous coupled wave light scatter model is used to simulate diffraction from a set of test wafers. Partial least squares and neural network analysis techniques are then employed to use correlations between the simulated diffraction and the critical dimensions of the modeled structures to produce a capability to measure the critical dimensions from scattered light measurements. The marriage of rigorous coupled wave diffraction modeling and optical scatterometry directly addresses the needs of the industry for a rapid and nondestructive metrology tool.
We describe an experiment in which the etch depth of a diffraction grating is measured. A simulated experiment is used to develop and calibrate the measurement technique. A scatterometer was used to measure the diffraction patterns of a set of 5 wafers at 14 die locations. The estimator already developed is then used to find the etch depths at the 70 measured locations. Finally, a scanning force microscope is used as a reference method to validate the scatterometer measurements.
Laser scatterometry is a noncontact, rapid method of collecting and analyzing light scattered from a structure. We have applied optical scatter techniques to measure the surface roughness as well as the etch depth of phase shifting masks (PSMs). Experimental results and theoretical modeling are discussed.
We have applied optical scatter techniques to improve several aspects of microelectronic manufacturing. One technique involves characterizing light scattered from two dimensional device structures, such as those from VLSI circuitry etched on a wafer, using a frosted dome which is imaged by a CCD camera. Previously, limited dynamic range available from affordable digital imaging systems has prevented the study of two dimensional scatter patterns. We have demonstrated a simple technique to increase the dynamic range by combining multiple images taken at different intensities. After the images have been acquired, image processing techniques are used to find and catalog the diffraction orders. Techniques such as inverse least squares, principal component analysis, and neural networks are then used to evaluate the dependence of the light scatter on a particular wafer characteristic under examination. Characterization of surface planarization over a VLSI structure and measurement of line edge roughness of diffraction gratings are presented as examples.
The attempt to eliminate subsurface damage in polished materials is a major objective in optical and semiconductor fabrication. The level of subsurface damage in optical components is proportional to the surface scatter and related to the laser damage threshold of the optic. The float polishing process has been shown to produce surfaces with low subsurface damage on ferrite materials. We have ground samples of rough cut Corning 7940 fused silica using synthetic polycrystalline diamond. These samples were then float polished on a precision machine manufactured by Toyoda Machine Works Limited. Our surfaces were characterized using differential phase interference microscopy, total internal reflection microscopy, and scatterometry. We will describe the fabrication process and report the results of the surface and subsurface characterization.
The formation of resist lines having submicron critical dimensions (CDs) is a complex multistep process, requiring precise control of each processing step. Optimization of parameters for each processing step may be accomplished through theoretical modeling techniques and/or the use of send-ahead wafers followed by scanning electron microscope measurements. Once the optimum parameters for any process having been selected, (e.g., time duration and temperature for post-exposure bake process), no in-situ CD measurements are made. In this paper we describe the use of scatterometry to provide this essential metrology capability. It involves focusing a laser beam on a periodic grating and predicting the shape of the grating lines from a measurement of the scattered power in the diffraction orders. The inverse prediction of lineshape from a measurement of the scatter power is based on a vector diffraction analysis used in conjunction with photolithography simulation tools to provide an accurate scatter model for latent image gratings. This diffraction technique has previously been applied to looking at latent image grating formation, as exposure is taking place. We have broadened the scope of the application and consider the problem of determination of optimal focus.
We discuss the use of light scattered from a latent image to control photoresist exposure dose and focus conditions which results in improved control of the critical dimension (CD) of the developed photoresist. A laser at a nonexposing wavelength is used to illuminate a latent image grating. The light diffracted from the grating is directly related to the exposure dose and focus and thus to the resultant CD in the developed resist. Modeling has been done using rigorous coupled wave analysis to predict the diffraction from a latent image as a function of the substrate optical properties and the photoactive compound (PAC) concentration distribution inside the photoresist. It is possible to use the model to solve the inverse problem: given the diffraction, to predict the parameters of the latent image and hence the developed pattern. This latent image monitor can be implemented in a stepper to monitor exposure in situ, or prior to development to predict the developed CD of a wafer for early detection of bad devices. Experimentation has been conducted using various photoresists and substrates with excellent agreement between theoretical and experimental results. The technique has been used to characterize a test pattern with a focused spot as small as 36 micrometers in diameter. Using diffracted light from a simulated closed-loop control of exposure dose, CD control was improved by as much as four times for substrates with variations in underlying film thickness, compared to using fixed exposure time. The latent image monitor has also been applied to wafers with rough metal substrates and focus optimization.
Identification of dimensional parameters of an arbitrarily shaped grating using scatter characteristics is presented. A rigorous diffraction model is used to predict the scatter from a known grating structure, and utilizing this information we perform the inverse problem of predicting line shape from a measurement of the scatter.
As the microelectronics industry strives to achieve smaller device design geometries, control of linewidth, or critical dimension (CD), becomes increasingly important. Currently, CD uniformity is controlled by exposing large numbers of samples for a fixed exposure time which is determined in advance by calibration techniques. This type of control does not accommodate variations in optical properties of the wafers that may occur during manufacturing. In this work, a relationship is demonstrated between the intensity of light diffracted from a latent image consisting of a periodic pattern in the undeveloped photoresist and the amount of energy absorbed by the resist material (the exposure dose). This relationship is used to simulate exposure dose control of photoresist on surfaces which have different optical properties chosen to represent surfaces typical of those found in operating process lines. Samples include a variety of photoresist materials and substrates with a wide variety of optical properties. The optical properties of the substrates were deliberately varied to determine the effect of these properties on CD (in the presence and absence of an exposure monitor) during lithography. It was observed that linewidth uniformity of the developed photoresist can be greatly improved when the intensity of diffracted light from the latent image is used to control the exposure dose. Diffraction from the latent image grating structures was modeled using rigorous coupled wave analysis. The modeling is used to predict the diffraction from a latent image as a function of the substrate optical properties and the parameters of the latent image (i.e., linewidth, sidewall angle). Good agreement is obtained between theoretical and experimental observations. Conversely, the inverse problem is solved in which the parameters of the diffracting structure (the latent image) are determined from a measurement of the diffracted power. Therefore, the diffracted power can be monitored for the purpose of determining when the latent image will produce the proper CD upon development.
Photoacoustic spectrososcopy is used to characterize the surface absorption of polished fused silica substrates and thin films deposited on fused silica substrates. The extreme sensitivity of this technique allows measurement of surface absorptions of a few tenths of a part per million. Characterization of samples with surfaces finished using a variety of methods is reported. Key words: Photoacoustic ion beam absorption thin films. 1.
Ion beam figuring has been demonstrated to be a deterministic efficient flexible technique for removing material from optical surfaces. Recent interest in using this process to produce high quality optical components has driven the need to fully characterize the resulting surfaces. We have performed a polishing parameter matrix investigation to optimize fused silica (Corning 7957) surfaces for subsequent ion milling. Samples were characterized for surface scatter surface absorption surface roughness subsurface damage and laser damage as a function of mill depth. Small defects (pits) were evident on surfaces after milling a few microns with pit density dependent to some degree upon the surface preparation technique. The defects were often in lines apparently following a surface or subsurface scratch in the materiaL Surface scatter decreased significantly (up to lOX) and laser damage threshold increased in some cases by 400. Laser damage was not correlated with defects in the material. Key words: ion beam milling laser damage scatter fused silica absorption. 1.
A novel laser scatterometer linewidth measurement tool has been developed for the CD metrology of
photomasks. Calculation of the linewidth is based on a rigorous theoretical model, thus eliminating the need
for any calibrations. In addition, the effect of the glass slab on which the grating is placed, is explicitly taken
into account. The experimental arrangement consists of a chrome-on-glass diffraction grating illuminated with
a converging spherical wave from a He-Ne laser. A photodiode mounted in the Fourier plane of the scatterer
measures the scattered power in each diffracted order. A rigorous theoretical model is used to provide a
lookup table giving the 0-order transmitted power as a function of the linewidth for a fixed pitch of the grating.
This table is then used to associate a linewidth with the experimentally measured value of the power in
the 0 transmitted order.
A local company manufactured various photomask gratings having a 2 micron pitch and varying
linewidths. The 0-order transmitted power for each of these gratings was measured by the scatterometer, and
a prediction of the linewidth was made based on the theoretical model. The linewidth measured by the scatterometer
system represents an average of the linewidths over the total lines illuminated by the laser. All
present CD measurement systems however, measure the linewidth of a single line. If the variation of
linewidth is assumed to be small, comparable results should be obtained from the two procedures. The
predicted linewidth values were compared to those obtained using commercial optical linewidth measurement
systems and excellent agreement was obtained.