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An innovative platform for high-throughput high-accuracy lithography using a single wafer stage
Abstract
For 32 nm half-pitch node, double patterning is recognized as the most promising technology since some significant obstacles still remain in EUV in terms of technology and cost. This means much higher productivity and overlay performance will be required for lithography tools. This paper shows the technical features of Nikon's new immersion tool, NSR-S620 based on newly developed platform "StreamlignTM" designed for 2nm overlay, 200wph throughput and 2week setup time. The S620 is built basically upon Nikon's Tandem Stage and Local Fill Nozzle technology, but has several additional features. For excellent overlay, laser encoders with short optical path are applied for wafer stage measurement in addition to interferometers. By using this hybrid metrology, the non-linearity of the encoder scale can be easily calibrated, while eliminating the air fluctuation error of interferometer. For high throughput, a method with a new alignment microscope system and a new auto focus mapping, called Stream Alignment is introduced. It makes it possible to reduce the overhead time between the exposures remarkably. The target productivity is 4,000 wafer outs per day. Accuracy is also improved because many more alignment points and a continuous wafer height map without stitching are available. Higher acceleration and faster scan velocity of the stages are also achieved by optimal vibration dynamics design and new control system. The main body, including the projection lens, is isolated by Sky Hook Technology used already on the NSR-SF150 and SF155 steppers, and also the reticle stage is mechanically isolated from the main body. With this new platform, the imaging performance can be maximized.
... 149 In high-accuracy applications such as microlithography, turbulence can often be the leading source of uncertainty. 150 Turbulence also affects the measurement through direct interactions of the airflow with components within the metrology loop such as mirrors and mounts, resulting in vibrations that contribute to the observed optical path length fluctuations. ...
... 18,220 Modern lithography tools may use a combination of optical encoders (see Section 4.8) and conventional interferometers to overcome air turbulence. These two systems complement each other: The short air path typical of encoders minimizes the effects of air turbulence, thus realizing an improvement in short-term repeatability, 150 while the conventional interferometer provides superior linearity when compared to the nonlinearities encountered in the encoder gratings. 150,205 The limitations imposed by air turbulence disappear in vacuum environments, such as those encountered in reflective electron-beam lithography 153,221 and EUVL 152,222 systems. ...
... These two systems complement each other: The short air path typical of encoders minimizes the effects of air turbulence, thus realizing an improvement in short-term repeatability, 150 while the conventional interferometer provides superior linearity when compared to the nonlinearities encountered in the encoder gratings. 150,205 The limitations imposed by air turbulence disappear in vacuum environments, such as those encountered in reflective electron-beam lithography 153,221 and EUVL 152,222 systems. Microlithography for flat-panel displays poses its own special challenges. ...
The wavelength of light provides an exceedingly precise measure of distance and is the foundation for commercial interferometric measurement tools that monitor object positions with a resolution better than 1 nm for objects traveling at 2 m/s. A wide range of applications include machine tool stage positioning and distance monitoring over length scales from a few millimeters to hundreds of kilometers in space-based systems. Displacement measuring interferometry or DMI enjoys multiple advantages with respect to other methods of position monitoring. In addition to high resolution, wide measurement range, and fast response, the laser beam for a DMI is a virtual axis of measurement that can pass directly through the measurement point of interest (POI) to eliminate Abbe offset errors. The measurement is noncontact and directly traceable to the unit of length. Since the first practical demonstration of automated, submicron stage control using displacement interferometry in the 1950s,1 DMI has played a dominant role in high-precision positioning systems. This chapter is intended as an overview of the current state of the art in DMI as represented by the technical and patent literature.
... Linear motors or ball screws are normally used for the coarse stages, but the electric motors become larger and heavier due to the high acceleration and larger stage demands, thereby increasing the generation of heat (3) . Variations in temperature have severe effects on the measurement system and lens apparatus (4) (5) , and thus precise temperature control (5) is problematic. ...
... Linear motors or ball screws are normally used for the coarse stages, but the electric motors become larger and heavier due to the high acceleration and larger stage demands, thereby increasing the generation of heat (3) . Variations in temperature have severe effects on the measurement system and lens apparatus (4) (5) , and thus precise temperature control (5) is problematic. ...
A pneumatic actuator has several advantages such as low heat generation, a high power-to-weight ratio, and low costs; however, it also has disadvantages such as time delays, nonlinearities, and position-dependent multiple pressure resonances. In this study, we propose a wave equation-based model, which can fit the position-dependent pressure resonances based on delay elements, taking into account the damping. Using this model, a wave cancellation filter is proposed for canceling all the resonances and anti-resonances. The effectiveness of both the model and the filter were verified through experiments.
... However, interferometers cannot satisfy the ever-tightening requirement of high accuracy because of their long beam length and sensitivity to atmospheric fluctuation, which are caused by temperature, pressure, humidity, and air turbulence, etc. [3,4]. New metrologies based on encoder theory with a short beam path are used to upgrade accuracy [5,6]. ...
We propose and demonstrate a new, to the best of our knowledge, optical encoder, which can measure in-plane and out-of-plane displacements simultaneously and independently. The symmetrical structure of the optical path can eliminate the impact from out-of-plane displacement on the measurement of in-plane displacement. The innovative new geometry also facilitates the multi-reflected diffracted beam to interfere with the same-order diffracted beam, so as to eliminate the impact from in-plane displacement on the measurement of out-of-plane displacement. An experimental setup is established to verify the two-dimensional independent measurement. The experiment result coincides with the one measured by two independent interferometers. The output of spectrum analysis shows that the two-dimensional independent encoder can be used for nanometric measurement.
... In a lithography machine with a decline of the CD, the correlation analysis shows that over 80% of the total errors are related to the influence of the air fluctuations on a laser interferometer (de Groot et al., 2016). A GI, as a substitute for a laser interferometer, has already been used in photolithography to monitor the critical platform motion during the wafer exposure step (Shibazaki et al., 2009). ...
A grating interferometer, called the “optical encoder,” is a commonly used tool for precise displacement measurements. In contrast to a laser interferometer, a grating interferometer is insensitive to the air refractive index and can be easily applied to multi-degree-of-freedom measurements, which has made it an extensively researched and widely used device. Classified based on the measuring principle and optical configuration, a grating interferometer experiences three distinct stages of development: homodyne, heterodyne, and spatially separated heterodyne. Compared with the former two, the spatially separated heterodyne grating interferometer could achieve a better resolution with a feature of eliminating periodic nonlinear errors. Meanwhile, numerous structures of grating interferometers with a high optical fold factor, a large measurement range, good usability, and multi-degree-of-freedom measurements have been investigated. The development of incremental displacement measuring grating interferometers achieved in recent years is summarized in detail, and studies on error analysis of a grating interferometer are briefly introduced.
... As CD values have decreased, the use of interferometers with long path lengths has reached a practical limit: fluctuations or turbulence in the air and environmental sensitivities related to variations in air index have proved to be the largest contributors to stage metrology error. For extreme ultraviolet (EUV) systems working in a vacuum, this error source is less of an issue; but the majority of semiconductor exposure processes still take place in air and the resulting air turbulence from airflow, stage motions and other disturbances contributes as much as 80% of the error budget for the stage metrology [4]. ...
A revolutionary development in optical displacement metrology has been the transition from free-space laser interferometers to extreme-precision optical encoders for positioning measurements in semiconductor photolithography systems. An increasingly demanding error budget for stage motion control has mandated a dramatic reduction in sensitivity to air turbulence and refractive index changes that is only achievable with multi-dimensional sensors with
intrinsically short air paths. Here we describe the design, implementation and performance of these new optical heterodyne encoder systems, which achieve sub-nm displacement measurements for stages moving at over 8 m/s.
... Recognizing that optical encoders minimize the air turbulence contribution by reducing the air paths, the photolithography industry now uses high-performance encoders to monitor the critical stage motions during wafer exposure [3,[7][8][9][10][11]. ...
The demands of modern precision engineering exemplified by semiconductor photolithography systems have propelled encoder technology to a performance level surpassing that of line-of-site interferometry. The key advantage in these systems is the nearly complete suppression of air turbulence. This advance has come at the price of increased demands on calibration and compensation systems, as well as on the encoder design itself. Here we have reviewed the basic concepts and geometries underlying the current and future generation of optical encoders. Driven by ever decreasing critical dimensions, there will continue to be demands on developers for new designs with even higher precision.
... Additional encoder heads (not shown) are used to measure displacement in Z and rotation around Y and rotation around X. This type of 2D encoder measurement has also been shown to have much lower noise and higher repeatability than the laser interferometer solution [196]. ...
Precision positioning of an object relative to a reference point is a common task in many activities of production engineering. Sensor technologies for single axis measurement, either linear or rotary, which form the fundamentals of measurement technologies for precision positioning, are reviewed. Multi-axis coordinate measurement methods such as triangulation and multilateration, as well as Cartesian and polar systems for specifying the position in a plane or three-dimensional (3D) space are then presented, followed by a discussion on traceability and standards. Some advanced applications of measurement technologies for precision positioning in manufacturing industries are also demonstrated.
... In literature, many mechanical innovations have been effectively implemented. By reducing the minimize mechanical structure, the resulting new layout or structural design [2,3] to much higher overall servo bandwidth. The resonant frequency of the driving actuator, which is limiting the bandwidth was greatly improved by stiffening and lightening the servo system and new driving system design, but this would induce the longer design period and higher product cost. ...
In order to overcome the vital resonant frequencies of high speed and high precision stage assembly, which is build-up system usually used in microelectronic manufacturing facilities with extravagant requirements. The paper studies the vibration compensation in wafer stage assembly through an additional accelerometer embedded on a sensitive position without affecting the dynamics properties of the system. An optimal and robust control is presented. The position and acceleration are the feedback signals for the μ-synthesis. Both the parameter uncertainties and external disturbances are included in the controller design, the control loops are designed together to share their control information and thus to obtain better performance. The experimental results for servo control as well as for the improvements had been illustrated finally.
Wafer focus-measuring system (WFMS) with high efficiency and robustness is crucial to ensure the exposure quality when advanced optical projection photolithography method is applied. In this work, we proposed a structure-illumination based wafer focus measuring system (SI-WFMS) which requires only single-step sampling process to implement the measurement of vertical defocus distance of exposure area, with less than 0.06 μm error. Both the surface profile and tilt angle of the wafer are also available to be rapidly measured through SI-WFMS, thereby saving the time of the measuring process and improving the manufacturing efficiency.
To satisfy the miniaturization requirements of semiconductor manufacturing and metrology, orthogonal Z-axis stage controllability of equipment using highly accurate position detectors are necessary in addition to fundamental XY-axes stage movement control. The Z-axis position detector must have high accuracy, high resolution, and high stability against environmental changes in temperature, humidity, and air pressure. In this paper, a two-dimensional encoder based on a grating interferometer detection method is presented. A prototype XZ-axes encoder was built and evaluated for long-term stability against variations in environmental conditions as well as short-range linear accuracy. The evaluation results show that it is feasible to integrate the encoder with a highly accurate two-dimensional (XZ or YZ) stage control with sub-nanometer resolution. The performance of the encoder as a Z-axis position detector satisfies the demands of industry. By applying the methods developed to the third axis, it can be possible to upgrade the position detection system to three dimensions.
This review gathers together 15 special topics in modern interferometric metrology representing a sampling of historical, current and future developments. The selected topics cover a wide range of applications, including distance and displacement measurement, the testing of optical components, interference microscopy for surface structure analysis, form and dimensional measurements of industrial parts, and recent applications in semiconductor manufacturing and consumer electronics. Techniques range from laser Fizeau systems to dynamic ellipsometry using polarized heterodyne interferometry.
Displacement sensors based on laser or optical fiber technology are recognized for their capacity for non-contact distance measurement based on reflection from an object, but with limited measurement distance and resolution. The other option could be light wave interferometer built with the mirrors on the object. However, long distance measurement requires large laser source with stabilized-wavelength. This report presents another solution: a non-contact grating interferometer displacement sensor that enables pico-meter resolution with a semiconductor laser source.
To reduce the acceleration time and improve the stability of the scanning speed during the exposure process of the wafer stage, a fifth-order S-curve is proposed. The third derivative of the acceleration is designed as a constant, and the position and speed formulas are calculated based on integration method. Optimization index with minimum time and jerk is proposed, and a modified fifth-order S-curve is designed. Simulation results show that the S-cure can ensure that the wafer stage speeds up to the scanning speed quickly and can stabilize in a short time. The modified S-curve can reduce the step time effectively, and the step movement curve is smooth.
Displacement laser interferometers and interferometric encoders currently are the dominating solutions to the displacement measurement applications which require measurement uncertainties in the order of a few nanometers over hundreds of millimeters of measurement range. But, in comparison with interferometric encoders, to achieve nanometer order or even lower measurement uncertainties, displacement laser interferometers require much stricter environmental control if not vacuum, which will increase their Total Cost of Ownership (TCO). Therefore interferometric encoders are getting more and more preferable. Furthermore, for some applications, the measurement of the out-of-plane displacement is required as well. Therefore, in this work, a one-dimensional interferometric encoder was built and investigated, a novel two-dimensional (one is in-plane, the other one is out-of-plane) interferometric encoder was devised and its principle was proven experimentally. For the one-dimensional encoder, a periodic nonlinearity of +/-50 pm with HEIDENHAIN EIB 741 and a periodic nonlinearity of less than 10 pm with a home built phase meter and off-line Heydemann correction were identified through a comparison measurement with a differential heterodyne interferometer. In addition, this one-dimensional encoder was identified to have a better measurement stability compared to the differential heterodyne interferometer.
Double patterning (DP) is widely regarded as the lithography solution
for 32 nm half pitch semiconductor manufacturing, and DP will be the
most likely litho technology for the 22 nm node [1]. When using the DP
technique, overlay accuracy and CD control are of critical importance
[2]. We previously introduced the NSR-S620D immersion scanner, which
provides 2 nm overlay capabilities. In the case of the latest generation
NSR-S621D system, improvements have been developed for further overlay
accuracy enhancement. In this paper, we will show the overlay accuracy
and Mix-and-Match performance of the NSR-S621D. Further, the marked
improvement in product overlay and the overlay result in Spacer DP as a
result of enhanced alignment accuracy will also be shown.
To achieve the 2 nm overlay accuracy required for double patterning, we have introduced the NSR-S620D immersion scanner that employs an encoder metrology system. The key challenges for an encoder metrology system include its stability as well as the methods of calibration. The S620D has a hybrid metrology system consisting of encoders and interferometers, in XY and Z. The advantage of a hybrid metrology system is that we can continuously monitor the position of the stage using both encoders and interferometers for optimal positioning control, without any additional metrology requirements or throughput loss. To support this technology, the S620D has various encoder calibration functions that make and maintain the ideal grid, and control focus. In this paper we will introduce some of the encoder calibration functions based on the interferometer. We also provide the latest performance of the tool, with an emphasis on overlay and focus control, validating that the NSR-S620D delivers the necessary levels of accuracy and stability for the production phase of double patterning.
Double patterning requires extremely high accuracy in overlay and high uniformity in CD control. For the 32 nm half pitch, the CDU budget requires less than 2 nm overlay and less than 2 nm CD uniformity for the exposure tool. To meet these requirements, Nikon has developed the NSR-S620D. It includes a new encoder metrology system for precise stage position measurement. The encoder system provides better repeatability by using a short range optical path. For CD uniformity control, various factors such as focus control, stage control, and dose control affect the results. Focus uniformity is evaluated using the phase shift focus monitoring method. The function of "CDU Master" provides dose and focus correction across the exposure slit, along the scan direction, and across the wafer. Stage synchronization variability will also influence CD control. In this paper, we will show the actual results and analysis of the overall performance of S620D, including the exposed result of pitch splitting double patterning. S620D has sufficient performance for the 32 nm half pitch double patterning generation and shows potential for double patterning at the 22 nm half pitch node.
The problem of the alignment tree for double patterning (DP) is presented. When the 2nd DP exposure is aligned to the underlying zero layer, the space CD uniformity is shown to be well outside the budget for the 32 nm HP node. Aligning the 2nd DP layer to the zero layer gives better overlay results, but aligning the 2nd DP pattern to the 1st DP pattern gives results well within the overlay requirements for the 32 nm HP. Aligning the 2nd DP layer to the 1st DP layer is recommended to give the best CD uniformity and overlay results. Experimental results show, qualitatively, the CD uniformity is significantly worse when the 2nd pattern is aligned to the zero layer, but the overlay for both alignment trees could be corrected to roughly the same levels. The raw overlay data shows a significantly different signature for the two alignment trees, possibly caused by alignment mark signal differences between the marks on the zero and 1st layers, or distortion of the zero layer after the first etch. The requirements for a DP exposure tool were reviewed and can be summarized as improved dose control, improved overlay performance, and significantly higher throughput.
Double patterning (DP) has become the most likely candidate to extend immersion lithography to the 32 nm node and beyond. This paper focuses on experimental results of 32nm half pitch patterning using NSR-S620D, the latest Nikon ArF immersion scanner. A litho-freeze-litho (LFL) process was employed for this experiment. Experimental results of line CDU, space CDU, and overlay accuracy are presented. Finally, a budget for pitch splitting DP at the 22 nm half pitch is presented.
Currently, it is considered that one of the most favorable options for the 32 nm HP node is pitch-splitting double patterning, which requires the lithography tool to achieve high productivity and high overlay accuracy simultaneously. In the previous work [1], we described the concepts and the technical features of Nikon's immersion scanner based on our newly developed platform, Streamlign, designed for 2nm overlay, 200wph throughput, and short setup time. In this paper, we present the latest actual performance of S620D with the Streamlign platform. Owing to the high repeatability of our new encoder metrology system, Bird's Eye Control, and Stream Alignment, S620D achieves less than 2 nm overlay accuracy, less than 15nm focus accuracy, and successful 32 and 22 nm L/S pitchsplitting double patterning exposures. Furthermore, the results at high scanning speed up to 700 mm/s are fine and we have successfully demonstrated over 4,000 wpd throughput, which confirms the potential for high productivity. Nikon has developed this Streamlign as an optimized long life platform based on the upgradable Modular2 structure for upcoming generations. The performance of S620D indicates the possibility of immersion extension down through the 22 nm HP node and beyond.
Double patterning (DP), an extension of immersion, is the leading contender for the manufacturing of 32 nm half pitch node devices. For DP, substantial improvement in overlay accuracy is required to meet the CDU requirements for the 32 nm node, and substantial increase in throughput is required to meet the cost requirements. To meet these challenges, Nikon introduced the NSR-S620. The S620 is based on the Streamlign platform, which is characterized by three innovations: Bird's Eye Control, Stream Alignment, and Modular2 Structure. In addition, many of the current systems and techniques have been refined to meet the requirements for DP. This presentation will discuss these technological improvements and show the latest technical results.
Critical processing factors in the lithography process include overlaying the pattern properly to previous layers and properly exposing the pattern to achieve the desired line width. Proper overlay can only be attained in the lithography process while the desired line width accuracy is achieved by both lithography and etch processes. Since CD is substantially influenced by etch processing, therefore, it is possible to say that overlay is one of the most important processing elements in the lithography process. To achieve the desired overlay accuracy, it is desirable to expose critical layers with the same exposure tool that exposed the previous or target layer. This need to dedicate a particular exposure tool, however, complicates the lot dispatching schedule and, even worse, decreases exposure tool utilization. In order to allow any exposure tool available to print the arriving lot, M&M (Mix and Match) overlay control becomes necessary. By reducing overlay errors in M&M control, lot dispatching scheduling will become more flexible and exposure tool utilization will improve. Since each exposure tool has a unique registration signature, high order errors appear when overlaying multiple layers exposed with different tools. Even with the same exposure tool, if a different illumination is used, a similar error will be seen. A correction scheme to make the signature differences has to be implemented, however manually characterizing each tool's signature per illumination condition is extremely tedious, and is subject human errors. The challenge is to design a system to perform the corrections automatically. In the previous paper(1), we have outlined concepts of the system scheme. The system has subsequently been developed and tested using exposure tools. In this paper test results are shown using automated distortion correction. By analyzing the results, suggestions for further improvements and further developments are shown.
Double patterning is recognized as the best candidate for 32 nm half-pitch lithography. Currently pitch splitting processes are being considered for logic processes and spacer processes are being considered for memory. In pitch splitting, errors in overlay between the first and second exposure become CD errors on the final pattern. For this reason, overlay requirements are severe for pitch splitting double patterning. Revised CD and overlay budgets are presented, as well as technical requirements to satisfy these budgets. Spacer processes do not have similar restrictions on overlay, so they can be achieved using current immersion tools. Exposure tool requirements for double patterning are discussed and modifications to current platforms are described.