In this work we demonstrate the application of a unique type of scatterometer to the measurement of advanced geometry semiconductor devices. Known as a dome scatterometer, the technology is capable of measuring multiple diffraction orders at multiple angles of incidence, thereby providing a means for gathering a large amount of scatterometry data in a short period of time. Dome scatterometers are also capable of measuring light scattered as a function of both theta (zenith) and phi (azimuth) incident angles, and for a variety of polarimetric configurations, all of which provide more information about the scattering structure and contribute to improved sensitivity. A dome scatterometer can also measure a grating structure regardless of its orientation, so that horizontal and vertical structures can be measured without the need for a wafer rotation.
Prior to initiating measurements with the dome scatterometer, we surveyed available laser sources and modeled their expected sensitivity theoretically to determine the best illumination wavelength for the applications we intended to study. Our findings demonstrated that a wavelength around 405nm is suitable for a wide variety of applications, but provides the best improved sensitivity for etch applications. We then modified our dome scatterometry optical system to accommodate a Using a 405nm laser, and performed measurements were performed on several types of grating structures. Examples of the excellent signal-to-noise ratio of dome scatterometry measurements across these applications are provided. Measurement data from applications including patterned photoresist, patterned poly lines and back-end trench interconnect structures will be presented. Comparisons to metrology tools such as AFM and CD-SEM will be made. Precision data will also be summarized, and the extendibility of dome scatterometry will be discussed.
Scatterometry is a novel metrology approach for process control that has recently been gaining more momentum in microlithography applications. The method can simultaneously measure Critical Dimension (CD), Side Wall Angle (SWA), and thickness of more than one layer. It analyzes the scattered and diffracted light from a periodic array of lines or holes that represent the surface structure of the measured sample. Scatterometry provides a non-destructive technique offering high precision and stability along with high tool-uptime performance. As such, it offers an excellent approach for real-time high volume production control with significant advantages as compared to traditional technologies such as CD-SEM and Profilometry. As the structure dimension shrinks considerably, producing high precision results becomes more critical. To date, reports on the deployment of scatterometry in real production environment have focused on Front End of Line (FEOL) applications such as STI and Gate. However, Back End of Line (BEOL) process control has not been widely reported. In this work, we will discuss the results of our study specifically for metal trench and contact layer on both patterned and etched wafers for 65nm technology node. We will also report the comparison between Scatterometry results to Critical Dimension Scanning Electron Microscope (CD-SEM) and Atomic Force Microscope (AFM). Finally we will provide a statistical analysis of our scatterometry results including precision, fleet precision, and TMU analysis. In contrast to the relatively simple stacks that comprise a FEOL structure, BEOL layers are typically complex structures with a large number of underlying layers. Generation of simulated scatterometry signatures that constitute a reference library for complex structures can require long computational times and result in large file sizes. To mitigate the computational overhead, it is necessary to intelligently decide which parameters to fix and which to vary. An additional complication is presented due to similarities in the optical properties of BEOL stack materials, which can introduce potential for parameter cross-correlation in the measurement. We will discuss methodologies for optimally selecting parameters to be fixed or varied to minimize these effects.
Scatterometry is a fast, non-destructive critical dimension (CD) optical metrology technique based on the analysis of light scattered from a periodic array of features. With technological advances in manufacturing, semiconductor devices are made in ever shrinking geometries. In recent years, the ability of scatterometry metrology tools to measure these devices at a gage-capable level for parameters such as CD, thickness or profile has become more challenging. The focus of this research is to analyze the acquired diffraction signature and determine an optimum diffraction signature "scan path." An optimized scan path can result in higher precision, reduced development time, smaller pre-generated library databases and faster real-time optimization speeds.
In this work, we will first review several methods for scan path selection and optimization. Our results indicate that the method choice can influence the scan path selection, and that some of the methods are complementary to one another. For example, one method, which we term orthogonal sensitivity, uses intelligent algorithms to select optimal scan path points based on enhancing single parameter sensitivity. While the method works well, it neglects parameter correlation effects. Thus, we will also review a method where correlation effects are considered. Finally, we will calculate and summarize the effectiveness of optimal scan path selection techniques using challenging lithography applications.
Scatterometry is receiving considerable attention as an emerging optical metrology in the silicon industry. One area of progress in deploying these powerful measurements in process control is performing measurements on real device structures, as opposed to limiting scatterometry measurements to periodic structures, such as line-space gratings, placed in the wafer scribe.
In this work we will discuss applications of 3D scatterometry to the measurement of advanced trench memory devices. This is a challenging and complex scatterometry application that requires exceptionally high-performance computational abilities. In order to represent the physical device, the relatively tall structures require a high number of slices in the rigorous coupled wave analysis (RCWA) theoretical model. This is complicated further by the presence of an amorphous silicon hard mask on the surface, which is highly sensitive to reflectance scattering and therefore needs to be modeled in detail. The overall structure is comprised of several layers, with the trenches presenting a complex bow-shape sidewall that must be measured. Finally, the double periodicity in the structures demands significantly greater computational capabilities.
Our results demonstrate that angular scatterometry is sensitive to the key parameters of interest. The influence of further model parameters and parameter cross correlations have to be carefully taken into account. Profile results obtained by non-library optimization methods compare favorably with cross-section SEM images. Generating a model library suitable for process control, which is preferred for precision, presents numerical throughput challenges. Details will be discussed regarding library generation approaches and strategies for reducing the numerical overhead. Scatterometry and SEM results will be compared, leading to conclusions about the feasibility of this advanced application.
The deployment of angular scatterometry as a powerful and effective process control methodology has recently included the measurement of etched metal features in a typical complex Aluminum stack. With the control of metal process steps taking a more critical role in silicon manufacturing, a fast, reproducible and accurate methodology for measuring CD and depth is necessary. With the half-pitch of the metal pattern being as low as the minimum device feature, etch rate measurements on above-micron test structures are hardly indicative of the pattern-dependent etch profiles and behavior. Angular scatterometry offers a non-destructive, fast and powerful approach for measuring the profiles of the yield-relevant array features in metal applications.
In this work we demonstrate the application of angular scatterometry to the qualification of metal etchers. Etch depth is difficult to control and must be inspected with slow techniques such as profilometry. In addition to the slow response time and sparse radial sampling, contact profilometry is susceptible to residual resist and polymer residue as well as to the variations in the TiN ARC layer affecting the measurement of the Aluminum etch rates. We show that the choice of a suitable profile model and accurate knowledge of the optical properties allow scatterometry to overcome all of these traditional challenges.
We demonstrate that angular scatterometry is sensitive to the parameters of interest for controlling metal etchers, specifically etch depth, CD and profile. Across an experimental design that introduced intentional variations in these parameters, angular scatterometry results were able to track the variations accurately. In addition, profile results determined through scatterometry compare favorably with cross-sectional SEM images and measurements. Measurement precision results will also be presented.
Lithography process control remains a significant challenge in modern semiconductor manufacturing. Metrology efforts must overcome the complexity of the lithography process, as well as the number of process elements that contribute to overall process yield. One specific area of concern is lithography tool focus control. It is vital to control photolithography tool focus during the photoresist development step with a high degree of precision and accuracy. Furthermore, dose variations can compound the difficulty in determining focus. The lenses used in photolithography tools have a very limited depth of focus, so utmost precision is necessary. Tools that are in focus will result in sharper and better controlled features, while tools that are out of focus will result in improperly developed photoresist features.
Angular scatterometry is a technology well-suited for lithography inspection and process control because it provides rapid measurement data and can be used for the measurement of resist line profiles. We report on model-based methods for focus control and their application towards photolithography control in a production setting. Topics of discussion include the effect of model parameter selection for focus metrics on focus curve quality and accuracy, as well as the effect of grating target design on focus sensitivity and accuracy. Measurement data using this focus technique in a production setting will be presented.
As DRAM (Dynamic Random Access Memory) device continuously decreases in chip size, an increased speed and more accurate metrology technique is needed to measure CD (critical dimension), film thickness and vertical profile. Scatterometry is an optical metrology technique based on the analysis of scattered (or diffracted) light from periodic line and space grating and uses 2θ angular method (ACCENT Optical Technologies CDS-200). When a light source is irradiated into the periodic pattern, the scattered intensity signal of zero-th order as a function of incident angle is measured. By analyzing these scattered signals, various parameters of the periodic pattern such as CD, vertical profile, mapping of substrate structure, film thickness and sidewall angle can be determined. Advantages of scatterometry are that drastic decreased measuring time and acquirement of CD, vertical profile, film thickness and sidewall angle by just one measurement. In this paper we will discuss various applications of scatterometry to sub-100nm DRAM structures of straight line and space and curved line and space patterns. Details of the correlation with CD-SEM (Scanning Electron Microscope) of standard metrology tool and repeatability of measured CD values will be discussed. As diverse applications, results of in-field, in-wafer and wafer-to-wafer CD monitoring, STI (Shallow Trench Isolation) depth monitoring and matching of vertical profile with V-SEM (Vertical SEM) will be also presented.
Scatterometry is a novel optical metrology that has received considerable attention in the silicon industry in the past few years. Based on the analysis of light scattered from a periodic sample, scatterometry technology can be thought of as consisting of two parts known as the forward problem and the inverse problem. In the forward problem, a scatterometer “signature” is measured. The signature is simply the measured optical response of the scattering features to some incident illumination, like laser light. In the inverse problem, the signature is analyzed in order to determine the parameters (such as linewidth, thickness, profile, etc) of the scattering features. Typically a rigorous electrodynamic model is used in the solution to the inverse problem, but due to the complexity of the model there is no direct analytic solution. Instead, a variety of numerical methods to solve the inverse problem have been proposed and utilized. The earliest widely used method of solution to the inverse problem involved the generation of a “library” of scatter signatures corresponding to discrete parameter combinations of the structure being measured. Once the library was generated, it was then searched in order to determine the best match to the measured signature. The parameters of the best match were then reported as the parameters of the measured signature. As the technology matured, other methods such as model optimization techniques also emerged. In fact, a variety of alternate techniques have been explored and reported, but a general study comparing the results (and hence the strengths and weaknesses) of the various techniques has yet to be performed. In this research, we shall report results from using several different solutions to the inverse problem on two applications (patterned resist and etched poly). The solution methods shall include the classic library search method as well as three common optimization methods. The results will show that each technique has strengths and weaknesses. For example, the library search methods are generally the most robust but also the most time consuming, and the optimization methods, while fast, are prone to reporting a local but not global minima.
The ability to accurately, quickly and automatically fingerprint the lenses of advanced lithography scanners has always been a dream for lithographers. This is truly necessary to understand error sources of ACLV, especially when the optical lithography is pushed into 130 nm regimes and beyond. This dream has become a reality at Texas Instruments with the help of scatterometry. This paper describes the development and characterization of the scatterometer based scanner lens testing technique (ScatterLith) and its application in 193 nm and 248 nm scanner lens fingerprinting. The entire procedure includes a full field exposure through focus in a micro stepping mode, scatterometer measurement of focus matrix, image field analysis and mapping of lens curvature, astigmatism, spherical aberration, line-through pitch analysis and ACLV analysis (i.e. across chip line width variation). ACLV has been directly correlated with image field deviation, lens aberration and illumination source errors. Examples are given to illustrate its applications in accurate focus monitoring with enhanced capability of dynamic image field and lens signature mapping for the latest ArF and KrF scanners used in manufacturing environment for 130nm node and beyond. Analysis of CD variation across a full scanner field is done through a step-by-step image field correction procedure. ACLV contribution of each image field error can be quantified separately. The final across slit CD signature is further analyzed against possible errors from illumination uniformity, illumination pupil fill, and higher order projection lens aberrations. High accuracy and short cycle time make this new technique a very effective tool for in-line real time monitoring and scanner qualification. Its fingerprinting capability also provides lithography engineers a comprehensive understanding of scanner performance for CD control and tool matching. Its extendibility to 90nm and beyond is particularly attractive for future development and manufacturing requirements.
Lens spherical error is an important lens aberration used to characterize lens quality and also has a significant contribution to across chip line width variation (ACLV). It also impacts tool-to-tool matching efforts especially when the optical lithography approaches sub-half wavelength geometry. Traditionally, spherical error is measured by using CD SEM with known drawbacks of poor accuracy and long cycle time. At Texas Instruments, an in-house scatterometer-based lens fingerprinting technique (ScatterLith) performs this tedious job accurately and quickly. This paper presents across slit spherical aberration signatures for ArF scanners collected using this method. The technique can successfully correlate these signatures with Litel lens aberration data and Nikon OCD data for spherical aberration errors as small as 10mλ. ACLV contributions from such small spherical errors can be quantified using this method. This provides the lithographer with an important tool to evaluate, qualify and match advanced scanners to improve across chip line width variation control.
Scatterometry is a novel optical metrology based on the analysis of light diffracted from a periodic sample. In the past the technology has been applied successfully to a variety of different grating types found in the manufacture of microelectronic devices. The scope of these applications, however, has been limited to structures that are singly periodic (periodicity = 1) in nature, i.e., gratings that are simple line and space structures with one periodic dimension. Rigorous coupled wave theory (RCWT), the underlying theory behind scatterometry measurements, can be applied to structures with a higher dimension of periodicity (periodicity > 1), although the computation is much more complex. In this paper we will discuss the application of scatterometry to structures with higher dimensions of periodicity, such as arrays of contact holes and DRAM cells. Details of the model, such as computation time and considerations for choosing a proper shape for the diffracting structures, will be presented. Sensitivity of the various parameters, such as the multiple critical dimensions and sidewall angles, will be discussed. Finally, results of measurements on contact hole and typical DRAM storage node patterns will be summarized. When compared to SEM, we will show correlation results that are greater than 0.9 for most applications, indicating that the technology can be applied successfully to such complicated structures. System matching between tools for these applications will also be discussed.
Lithography process control is one of the greatest challenges of modern semiconductor manufacturing. Due to the complexity of the overall lithography process, the number of process elements that can contribute to the process yield is large. One area of chief concern ins the control of focus on the lithography tool. Determination of the center of focus for a fixed dose during the photoresist development step in wafer processing is critical. Furthermore, dose variations can compound the difficulty in determining this center of focus. The lenses that are used in steppers have a very limited depth of focus, so utmost precision is necessary. Lense that are in focus will yield sharper printed photoresist images, and lack of focus will result in improperly developed photoresist features. Being at the center of focus also improves process repeatability. Scatterometry is a technology well-suited for lithography inspection and process control because it provides rapid measurement data and can be used for the measurement of resist line profiles. In this research we will report on several different methods for lithography focus control, including approaches that use conventional scatterometry models as well as a model-less algorithm. Comparisons will be drawn between commanded focus offsets, stage tilts and the focus response from the scatterometer. Use of the focus control techniques to map illumination modes and field non- linearity will also be shown.
Scatterometry is a non-destructive optical metrology based on the analysis of light scattered form a periodic sample. In this research angular scatterometry measurements were performed on three wafers processed using shallow trench isolation (STI) technology. The periodic features that were measured on these wafers were composite etched gratings comprised of SiN on oxide on Si. The wafers were processed at three different etch times in order to generate different etch depths (shallow, nominal and deep). It was at this point in the process that the scatterometry measurements were performed. The scatterometry model was comprised of four parameters: Si thickness, SiN thickness, linewidth and sidewall angle. For comparison purposes measurements were also performed using a critical dimension scanning electron microscope (CD-SEM), a cross-section SEM and an atomic force microscope (AFM). The results show good agreement between the scatterometry measurements and the other technologies.
As modern circuit architecture features steadily decrease in size, more accurate tools are needed to meaningfully measure critical dimensions (CD). As a general rule, a metrology tool should be able to measure 1/10 of the product tolerance. As CD's continue to shrink, gauge control becomes more relevant. The trend is illustrated in Table 1. The standard in-line critical dimension measurement tool is the top-down scanning electron microscope (SEM). An emergine technology for high speed, high accuracy CD measurement is scatterometry. This paper will compare the two technologies for in-line CD measurement for three applications: A product etch step (assessing gauge capability as well as trending), a product resist step (trending), and lithographic cell monitors (trending).
Scatterometry is an optical measurement technology based on the analysis of light scattered, or diffracted, from a periodic array of features. It is not an optical imaging technique, but rather a model based metrology that determines measurement results by comparing measured light scatter against a model of theoretical scatter signatures. Angular scatterometers in particular function by scanning the features to be measured through a range of incident angles, and measuring the light scattered into the zeroth, or specular, diffraction order. Prior work in angular scatterometry used the technique for the measurement of line profiles in resist and etched materials. In this work applications of scatterometry for the measurement of asymmetric line profiles (unequal sidewall angles, for example) are presented. Beginning with simulated results form the theoretical model, the importance of measuring through complementary (positive and negative) angles of incidence will be demonstrated. Then, actual measurement data from three different sample sets will be presented. The results show that the method has good sensitivity for measuring line asymmetry, and can therefore be used for qualifying processes for which symmetric results might be desired, such as lithography and etch processing.