Microwave Imaging Reflectometry System for KSTAR

Summary form only given, as follows. A millimeter wave phased antenna array (PAA) has been proposed for use in plasma imaging reflectometric diagnostics and is currently under design and fabrication. To account for the rapid changes of the cutoff layer with plasma density variation, a true time delay (TTD) controlled phased antenna array (PAA) is being developed as an artificial lens with controllable focal length for launching the illumination beam of the microwave imaging reflectometry (MIR) system. The beam shaping concept illustrates that programming the PAA generates concave (focusing/beam narrow) or convex (defocusing/beam broaden) wavefronts by properly arranging the time delay (or phase shift), therefore, the PAA functions like a real optical lens. The focal properties can be easily changed by different time delay arrangement at high speed so that real time control is possible on time scales significantly faster than plasma changes. The operation frequencies for the first implementation of beam shaping antenna array are set as 45-55 GHz based on the NSTX MIR system requirements. The time delay unit utilizes a piezoelectric translator (PET)-driven perturber to vary the propagation velocity along low loss microstrip lines. Tapered slot antenna will be used in the system as array element. To overcome the system loss and scattering loss from the plasma, the beam shaping array system is designed with an output power of at least 50 mW, and with a desired variable 3-dB beam width extending from 15/spl deg/ to 25/spl deg/.

In this article we perform a numerical study of microwave imaging reflectometry (MIR) and compare it with conventional reflectometry system. As an approximation to the reflections by real plasma fluctuations, a corrugated wheel is used. As far as general performance is concerned, our simulations confirm the results by Munsat et al. [Plasma Phys. Controlled Fusion 45, 469 (2003)] that the MIR system reproduces shape of corrugation far from the wheel while conventional systems fail to do so. We addressed the effects of asymmetry and defocusing of the wheel-reflectometer system as well as spectral sensitivity of the imaging reflectometer. For a particular geometry we estimated the deterioration of the MIR performance due to misalignments and existence of broadband fluctuations.

Imaging diagnostics, such as Electron Cyclotron Emission Imaging (ECEI) and Microwave Imaging Reflectometry (MIR), exhibit unique characteristics that make them particularly well suited to the validation of theoretical models for plasma instabilities and turbulent fluctuations. A 2‐D picture of plasma phenomena is provided unambiguously, from localized, time‐resolved measurements. After more than a decade of development and successful demonstrations on RTP [1,2] and TEXTOR [3, 4, 5, 6], ECEI has come into maturity as an electron temperature diagnostic technique, and systems at ASDEX‐UG [7] and DIII‐D [8] are making regular contributions to plasma physics. The next generation ECEI diagnostic is currently being installed on KSTAR [9, 10]. MIR is a radar reflectometric density fluctuation diagnostic, and hence the perfect complement to ECEI when realized to simultaneously image the same plasma volume. Experiments with MIR at TEXTOR have guided a recent surge in analysis and laboratory experiments aimed at resolving remaining issues [11, 12]. Both techniques are discussed in this tutorial with brief examples of data which illustrate the capabilities of these techniques and motivate future development for application on ITER and burning plasma experiments to come (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microwave diagnostics will be one of the few diagnostic techniques that can be operated in future fusion devices. In the past, they have contributed significantly to the understanding of the plasma dynamics, in particular electron cyclotron emission (ECE) and reflectometry. While these provide 1D measurements of plasma electron temperature and density along a line of sight, the advancement of electron cyclotron emission imaging (ECEI) and microwave imaging reflectometry (MIR) allows to obtain 2D images with high temporal and spatial resolution. Recent technological improvements will not only reduce the overall dimensions of these systems, thereby fulfilling requirements of future fusion devices, but also increase their sensitivity, reduce their costs, and ease maintenance, which increases operational time of the devices they are installed on. This paper aims to present an overview of ECE diagnostics and reflectometry. It first discusses their 1D implementations, followed by a more detailed examination of ECEI and MIR, including recent developments, and a perspective on future directions.

Two-dimensional (2D) turbulence measurement by using microwave imaging reflectometry (MIR) has been performed in the reversed field pinch (RFP) plasma in TPE-RX. In this work, the maximum entropy method (MEM) is used to estimate the 2D k-spectrum. The turbulences between the pulsed poloidal current drive (PPCD) and the high Θ plasmas are studied. The m = 1 modes are dominant in PPCD plasma, while in high Θ plasma the k spectrum expands to the high k range and the turbulence propagates in the electron drift direction. The preliminary result shows that PPCD plasma switches off the turbulence, and the high Θ plasma has strong turbulence.

Three-dimensional (3D) microwave imaging reflectometry has been developed in the large helical device to visualize fluctuating reflection surface which is caused by the density fluctuations. The plasma is illuminated by the probe wave with four frequencies, which correspond to four radial positions. The imaging optics makes the image of cut-off surface onto the 2D (7 × 7 channels) horn antenna mixer arrays. Multi-channel receivers have been also developed using micro-strip-line technology to handle many channels at reasonable cost. This system is first applied to observe the edge harmonic oscillation (EHO), which is an MHD mode with many harmonics that appears in the edge plasma. A narrow structure along field lines is observed during EHO.

Millimeter wave imaging diagnostics such as electron cyclotron emission imaging (ECEI) and microwave imaging reflectometry (MIR) are advanced technology to investigate electron temperature/density profiles and their fluctuations in magnetically confined plasmas. We have installed an ECEI system in the large helical device (LHD) at National Institute for Fusion Science (NIFS). The optimization of a receiving antenna array is essential for the imaging diagnostics. We have designed and fabricated dual-dipole antenna array. In this paper, the design and fabrication process of dual-dipole antennas are described. The characterizations of the antennas are also reported by comparing simulation and experimental results.

To study the mechanism of anomalous transport in tokamaks requires the use of sophisticated diagnostic tools for the measurement of short-scale turbulent fluctuations. In this article, we describe an attempt at developing a technique capable of providing a comprehensive description of plasma fluctuations with k⊥ρi<1, such as those driven by the ion temperature gradient mode in tokamaks. The proposed method is based on microwave reflectometry, and stems from a series of numerical calculations showing that the spatial structure of fluctuations near the cutoff could be obtained from the phase of reflected waves when these are collected with a wide aperture optical system forming an image of the cutoff onto an array of phase sensitive detectors. Preliminary measurements with a prototype apparatus on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [U. Samm, Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (IEEE, Piscataway, NJ, 1995), p. 470] confirm the validity of these conclusions. Technical issues in the application of the proposed technique to tokamaks are discussed in this article, and the conceptual design of an imaging reflectometer for the visualization of turbulent fluctuations in the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] is described.

An optics system for microwave imaging reflectometry (MIR) in the Large Helical Device (LHD) was newly developed to optimize the performance of the two-dimensional microwave receiver array. Reflected microwaves from the plasma and the first local oscillator (LO) wave are transmitted to the receiver array via the optics from the front. Finite-difference time-domain (FDTD) calculation was used to design the ellipsoidal or hyperboloidal shapes of the quasi-optical mirrors. It is confirmed that the LO beam in the constructed system covers the receiver antenna aperture area as intended. The S/N ratios of the signals are improved with this optimized optics system from those in the previous system.

A data analysis technique for microwave imaging reflectometry (MIR) in the Large Helical Devices (LHD) and TPE-RX plasmas has been investigated. In LHD, the fast Fourier transform (FFT) is employed. The statistical properties of the fluctuation spectra on MIR signals are quantified by the time-frequency analysis by the ensemble average technique. Statistical analyses using cross-correlation and coherence spectra reveal the characteristics of MHD modes, such as wave numbers, mode numbers, and phase velocity. In TPE-RX, the wavelet analysis is more useful because the phenomena are transient in TPE-RX plasma.

Microwave diagnostics, such as reflectometry and electron cyclotron emission, are widely employed in tokamak fusion plasmas, and are also particularly well suited to the burning plasma environment expected in the upcoming ITER device. In existing fusion plasmas, reflectometry has been used to measure electron density profile, density fluctuations, turbulent flow and internal magnetic field strength. The measurement of electron density profile via reflectometry is a particularly high priority in ITER, and so an assessment of electromagnetic wave propagation is important in estimating measurement capability. In ITER, the ratio of plasma size to the required microwave signal wavelength is significantly larger than in current fusion experiments ensuring a more realistic analysis of reflectometry via ray-tracing techniques. The analytical and numerical studies presented in this paper highlight the fact that the group velocity (or power flow) is strongly dependent on the direction of wave propagation relative to the magnetic field. It is shown that this dependence strongly modifies power flow near the cut-off layer in a manner that embeds the local magnetic field direction in the ‘footprint’ of the returned power at the launch antenna. It will be shown that this can potentially be utilized to determine the magnetic field pitch angle at the cut-off location. The resultant beam drift and distortion due to the magnetic field also have consequences on the design of reflectometry systems for large, high-field fusion experiments such as ITER.

Performance of pupil-filtering stepper-lens system

Many research teams have begun pursuing optical micromachining technology in recent years due to its associated noncontact and fast speed characteristics. However, the focal spot sizes and the depth of focus (DOF) strongly influenced the design requirements of the micromachining system. The focal spot size determines the minimum features can be fabricated, which is inversely proportional to the DOF. That is, smaller focal spot size led to shorter DOF. However, the DOF of the emitted visible or near-infrared light beam is typically limited to tens of nanometers for traditional optic system. The disadvantages of using nanosecond laser for micromachining such as burrs formation and surface roughness were found to further influence the accuracy of machined surfaces. To alleviate all of the above-mentioned problems, sub-wavelength annular aperture (SAA) illuminated with 780 nm femtosecond laser were integrated to develop the new laser micromachining system presented in this paper. We first optimized the parameters for high transmittance associated with the SAA structure for the 780 nm femtosecond laser used by adopting the finite difference time domain simulations method. A lateral microscope was modified from a traditional microscope to facilitate the measurement of the emitted light beam optical energy distribution. To verify the newly developed system performance the femtosecond laser was used to illuminate the SAA fabricated on the metallic film to produce the Bessel light beam so as to perform micromachining and process on silicon, PCB board and glass. Experimental results were found to match the original system design goals reasonably well.

Fan-Out wafer level packaging has seen rapid adoption over the last few years due to its form factor, performance, and cost advantages compared to 3D packaging techniques. Redistribution layers (RDL) are used to route the very high density connections on the chip to the much lower density connections of the substrate. Multiple layers of RDL are required in order to match the line density of the chip. This in turn increases the cost of the total package. Decreasing the metal line width for RDL supports reducing the number of redistribution levels and decreasing the total packaging cost. Reducing the RDL width requires tightening of requirements for the lithography tool and the photoresist process. Also, electroplating and etching processes need to be enhanced. This study investigates creation of RDL structures using a through resist electroplating process. A test vehicle was designed that contains various test structures used to characterize the process. This includes metrology structures for in-line monitoring, CD-SEM measurements, and comb and serpentine electrical test structures. The process starts with the deposition of a Cu seed layer needed for electroplating followed by lithography, electroplating, resist strip and Cu seed removal. The minimum thickness of the seed layer is constrained since the uniformity of the electroplating process also depends on the resistance and thus thickness of this layer. This thickness in turn has a large impact on line fidelity after removal of the Cu seed at the end of the process. In order to fabricate RDL lines with sufficient low resistance the plating thickness needs to be maximized. This translates to maximizing the aspect ratio of the photoresist pattern. In order to minimize the CD loss by Cu seed etching the wet process has to be controlled carefully. The lithography process window must have a large depth of focus in order to accommodate local height variation due to wafer topography in Fan-Out packaging. The optical lithography tool used in this study has a variable numerical aperture (NA) that can be optimized to maximize depth of focus while maintaining resolution performance in photoresist. After completing the development of the 1 μm RDL process the electrical performance was validated with meandercomb electrical test structures. The results show that lithography needs to be performed at the isofocal setpoint in order to obtain the optimal depth of focus of 7.5μm. Seed etch will reduce the Cu linewidth by 300 to 500nm and an offset in lithography CD may be required to compensate this loss. With optimal conditions good electrical yield is obtained for 1.0μm L/S meandercomb test structures.

The feasibility of Hot Spot Fixing (HSF) system in DfM flow is studied and reported. Hot spot fixing using process simulation is indispensable under low-k1 lithography process for logic devices with advanced design rule (DR). Hot spot such as pinching, bridging, line-end shortening will occur, mainly depending on local pattern context. Proper calibration of DR, mask data preparation (MDP), resolution enhancement technique (RET) and optical proximity effect correction (OPC) will reduce potential hot spots. However, pattern layout variety is so enormous that, even with most careful calibration of every process, unexpected potential hot spots are occasionally left in the design layout 1-2. OPC optimization is useful for maximizing common process margin, but it cannot expand individual pattern's process margin without modification of design layout. So, at an early design stage, hot spot extraction using lithography compliance check (LCC) and manual modification of design at hot spots will be a simple and useful method. The problem is that, it is difficult to determine how to modify layout in order to be consistent with DR, MDP/OPC rule. For proper layout modification, intimate knowledge of the entire process would be necessary, and moreover, the modification work often tends to be iterative, and thus time-consuming. Therefore, using our automated HSF system in the cell design stage and also the chip design stage is helpful for fixing design layout while avoiding fatal hot spot occurrence, with enough process margin and also with short turnaround time (TAT) 3-4. The basic system flow in the developed system is as follows; LCC extracts potential hot spots, and the hot spots are categorized by lithography error mode, grade, and surrounding context. And then, hot spot modification instructor, taking the surrounding situation into consideration, generates modification guide for every hot spot. Design data is automatically modified according to the instruction at every hot spot, complying with the design rule. The design modification process is verified with design-rule checker (DRC) and process simulation to confirm hot spot elimination without side effect. In this work, HSF is implemented in the design flow for various logic devices of 65 nm node. We extend modification target layers to multiple critical layers, including active area, poly, local metal wire and intermediate metal wire. The feasibility of the provided HSF system has been studied by applying it to around one hundred data of various sizes with respect to pattern fixing rate and turn around time (TAT). Moreover, process margin expansion including depth of focus (DOF) and exposure latitude (EL), in small layout was verified using process simulation and also by experimental results, namely, scanning electron microscope (SEM) images of focus exposure matrix. The detailed results are shown in the paper.