X-ray Stepper Research Articles

X-ray stepper aiming at 0.2 μm synchrotron orbital radiation lithography

A vertical x-ray stepper has been developed for 0.2 μm synchrotron orbital radiation lithography. The key features of this prototype stepper are a new gap setting algorithm, an optical heterodyne alignment system, and a newly developed fine motion mask stage. Gap setting is executed so as to make the mask and wafer parallel to the travel plane of the wafer x–y stage so that only one gap setting per wafer is required. The gap setting accuracy between 20 and 50 μm gaps is better than ±1.5 μm (3σ) for each exposure position. The optical heterodyne alignment signal obtained by detecting the diffraction beams from two checkerboard gratings has a detectable resolution of better than 0.01 μm and has only a small dependence of ±0.02 μm on gap variation. The alignment signals are fed back to the mask stage which can align the mask and wafer with a resolution of 5 nm. In exposure experiments, 0.15 μm lines and spaces were printed on a negative resist (SAL 601) and a 0.05 μm overlay accuracy has been obtained.

Meeting the pattern definition challenge of 256MBit DRAM x-ray masks

Introduction The progress of ever increasing memory density continues to challenge the minimum linewidth and overlay accuracies of lithographic tools. Error budgets for the printed wafer continue to be driven downwards and in some manner must be apportioned between the stepper/wafer printing process and the manufacture of the master reticle image. In reduction steppers the former of these two processes usually takes the major share in order to give most latitude to the high volume production process leaving a reasonable tolerance for the magnified reticle manufacture. However, where X-ray stepper lithography is the selected printing technique its lx reticle technology requires a more even apportionment of the error budget if practical results are to be obtained. Table 1 shows the predicted requirement situation for a range of future memory products, both in the expected overall wafer level tolerances and in their apportionment where a lx reticle technology is used.

Design Optimization of Synchrotron Radiation Lithography Beamline for a Compact Storage Ring

We have developed a novel beamline for SR (synchrotron radiation) lithography using a compact storage ring. The beamline consists of two ellipsoidal focusing mirrors, collecting 34.9 mrad of the horizontal angle of the fan-shaped SR, which delivers X-rays of high intensity onto the wafer. Moreover, an X-ray stepper of Karl Süss, XRS-200, will be attached to the end of the beamline; it offers the exposure field of 25×25 mm2 in the atmospheric environment. An optical design was realized using a sophisticated method which combines a ray-tracing method with a nonlinear optimization method, the Davidon-Fletcher-Powel (DFP) method. This optimized optical design yielded excellent transmission of photon flux throughout the beamline. We will also discuss the spectral and the spatial uniformity of the illumination, and the quality of the printed image.

Recent Developments for Sub-Quarter Micrometer Fabrication

Resolution capability of synchrotron radiation based proximity lithography has been investigated in the sub-quarter micrometer range using an advanced X-ray stepper and high resolution SiC/W X-ray masks. High resolution tungsten gratings and single lines as small as 60 nm are now currently available. Such masks were tested by contact printing in the L2M lithography station implemented at Super-ACO synchrotron facility in Orsay-France. Also, the influence of diffraction on feature sizes was studied with 30-40 µm proximity gap using various positive and negative resists: PMMA, RAY-PF, SAL 601. It was found that 0.10 to 0.12 µm linewidths can be achieved for gratings and 0.15 to 0.20 µm for isolated lines. This resolution limit can be overpassed by a double exposure technique associated with a lateral mask-stage translation. Examples of feature size down to 80 nm were readily achieved at 40 µm gap.

First x-ray stepper in IBM advanced lithography facility

This article describes the first commercially available state-of-the-art x-ray stepper, the Karl Suss XRS200/3, installed at the IBM Advanced Lithography Facility (ALF) at the end of 1991. ALF was built for the development of x-ray lithography for future generations of electronics devices [G. Lesoine, K. Kukkanen, and J. Leavey, Proc. SPIE 1263, 131 (1990)]. This stepper is attached to the first lithography beamline in ALF [J. Oberschmidt, R. Rippstein, R. Ruckel, A. Chen, J. Grandlund, and A. Palumbo, Proc. SPIE 1671, 324 (1992)]. The architecture of the tool and its two main improvements, (a) kinematic mask handling and (b) alignment system, will be discussed. Then the qualification test methodology and resulting data on key lithographic properties and throughput will be presented.

Evaluation of heterodyne alignment technique for x-ray steppers

Registration accuracy, as well as resolution capability, is one of the critical factors to identify an appropriate lithography tool. For deep submicron devices represented by the 256 Mbit dynamic random access memory, an alignment accuracy of better than 30 nm is needed. In this article, the alignment accuracy of an x-ray stepper was evaluated using the double-exposure method with a posiresist. The alignment accuracy obtained was better than 38 nm (3σ). Moreover, making a quantitative analysis of alignment error factors, we identified the possibility of improvement toward better than 30 nm (3σ).

Application of an x-ray stepper for subquarter micrometer fabrication

We report on a study of proximity x-ray lithography with a commercialized x-ray stepper (Karl Suss XRS200) and synchrotron orbital radiation at Super-ACO (Orsay, France). Special attention has been paid to the replication capabilities using SiC/W based high-resolution x-ray masks. The results show that proximity printing at a gap of 40 μm can result in subquarter micrometer replication. Since the resolution of the proximity printing is limited by the Fresnel diffraction, we use a double exposure technique to generate higher density and higher resolution grating structures. Fabrication of sub-100 nm linewidth gratings has been achieved using this technique with a wide exposure latitude.

Current status and problems of SR lithography

A compact ring named Super-ALIS employing low-energy one-pulse injection and superconducting magnets achieves stable operation. The beam current is 200 mA and the lifetime is 5.5 hours. Beamlines consisting of two-mirror optics are connected to the SR ring. X-ray steppers with alignment accuracy higher than 0.07 μm (3σ) are set up in a clean room. The steppers always operate for fine pattern replication using X-ray masks having the position accuracy higher than 0.09 μm (3σ).

An advanced x-ray stepper for [formula omitted]-μm SR lithography

For the establishment of an advanced synchrotron radiation lithography system, it is necessary to develop an x-ray stepper with highly accurate alignment capability. A new version of a vertical stepper called SS-1 has been developed using an air-lubricated xy-stage, an optical heterodyne detection system and a digital software servo alignment control system. It is shown in this paper that the stepper has the capability of achieving an alignment accuracy better than the 3-sigma value of 70 nm and delineating 0.2 μm line and space pattern clearly.

E-beam written optically transparent x-ray masks: Four levels for an industrial VLSI chip with megabit design rules

For the acceptance of X-ray lithography in an industrial environment not only the high resolution and accuracy capabilities have to be demonstrated, but also the compability of the X-ray related processes with up to date requirements on complexity and integration of VLSI devices. To achieve this we improved our running mask technology by developing a manufacturing process for e-beam written X-ray masks for direct use in an optically aligning X-ray stepper. This mask concept was applied in a joint project with an industrial partner in which the four final lithographic steps in a VLSI CMOS fabrication were performed by means of X-ray lithography.

Overlay performance of an advanced x-ray stepper (XRS 200)

Stepper related contributions to the overlay budget, such as alignment accuracy and displacements during pattern replication were investigated. By improving the alignment system, a value of 70 nm (mean + 3σ) was realized. Displacement errors induced by the stepper mechanism during pattern replication found to be in the range of 35 nm (3σ). This value was improved using a wafer chuck with high flatness. Summerizing the results, the total overlay contribution of the stepper (including alignment) is 75 nm (mean + 3σ).

A Vertical X-ray Stepper for SOR Lithography

A vertical x-ray stepper for SOR lithography has been developed. The stepper exposes wafers in a normal atmosphere to vertically-scanned SOR with continuous alignment control during exposure. The key devices of the stepper are a vertical X-Y stage and an opticalheterodyne alignment system. The X-Y stage uses airlubricated components, including air bearing lead screws, linear sliders and radial/thrust bearings. The optical-heterodyne alignment system detects the displacement between the mask and wafer gratings with precision of 8.3nm. Several exposure experiments proved that the alignment capability is better than ±0.1μm for a 3σ-value.

Special Issue on Mechatronics

Mechatronics is a term created to represent the total technology of mechanisms and electronics. Mechanical engineering dealing with mechanisms has a very long history. Its recent organic combination with electronics has certainly brought about a striking advance in the functions and performance of machines. This striking advance lay in the background of the creation of the new term “mechatronics”. The initiation of mechatronics was no doubt due to the advent of NC machine tools. NC machine tools were accomplished by fusing mechanisms with servo unit drives and computer techniques. The technique using them was somewhat innovative in that servo units were driven by digital computer signals. Mechatronics is considered as essential to develop peripheral machines for computers such as plotters, printers and magnetic memories, and as an application to wire bonding machines and X-ray exposing machines in semiconductor manufacturing processes. For such machines, increasingly higher speed and accuracy are likely to be required, and engineering developments are actively underway accordingly. This special issue was planned to present the current status and recent trends of mechatronic research arid development in Japan. The contents can be classified into three categories. First, bearings and actuators as basic mechatronic elements are featured. For bearings, trends of research and development on magnetic types which permit ultrahigh-speed rotation and operation in vacuum in particular were chosen. For actuators, recent examples of research and development on ultrasonic motors, linear motors and piezoelectric actuators were selected. Second, this issue presents examples of development in the area of X-ray steppers, memory medium handling systems, and polygonal scanners. These are frontier mechatronic systems and the descriptions will be of some help in recognizing future problems in development. Finally, some studies from the point of view of force-torque control were selected. While conventional mechatronic control studies are primarily concerned with position and speed control, force-torque control is expected to become an important trend. I hope that this special issue will be helpful in recognizing the current situation and future trends of mechatronics, and contribute to future developments in this area.

Evaluation of an advanced submicron X-ray stepper (XRS 200): Pattern transfer and alignment accuracy

For the application of X-ray lithography for ULSI fabrication, a high performance X-ray stepper is necessary. The investigated X-ray stepper XRS 200 uses an optical alignment system for adjusting mask and wafer parallel, for gap setting and for alignment. As a precondition for good alignment, the system shows excellent recognition accuracies for all the different mark signals, which appear during the alignment of different levels of a CMOS process. Based on this, the alignment accuracy and the overlay are in the range of 100 and 150 nm respectively. This second generation stepper uses mask/wafer scanning in order to expose an entire stepfield. No influence of the scanning procedure on pattern transfer was observed.

Evaluation of a vertical x-ray stepper

The alignment capability of a vertical x-ray stepper equipped with an air bearing x-y stage and optical-heterodyne alignment system has been evaluated. Phase difference detection is useful to achieve precision alignment regardless of surface coating, although sufficient signal amplitude is required for guarantee to assure alignment accuracy. The dominant error factor is deviation in phase difference of the alignment beat signals. The alignment capability of the stepper is evaluated to be better than a 3-sigma value of 100 nm for various wafer surface conditions.

ＳＯＲ用たて形Ｘ線ステッパの開発

A vertical X-ray stepper for SOR lithography has been developed. The stepper exposes wafers in a normal atmosphere to vertically-scanned SOR with continuous alignment control during exposure. The key devices of the stepper are a vertical X-Y stage and an opticalheterodyne alignment system. The X-Y stage uses air-lubricated components, including air bearing lead screws, linear sliders and radial/thrust bearings. The optical-heterodyne alignment system detects the displacement between the mask and wafer gratings with precision of 8.3 nm. Several exposure experiments proved that the alignment capability is better than ± 0.1μm for a 3σ-value.

X-ray stepper exposure system performance and status

An integrated x-ray align and expose system has been installed at Brookhaven National Laboratory National Synchrotron Light Source VUV beam line U2. The system includes the first commercially available x-ray align and expose system, the Karl Suss XRS-200 and matched beamline. Subsystem and aligned exposure tests were performed to characterize the entire system. Initial results are presented for key lithographic properties as well as throughput. Improvements and future activities are also described.

High-performance synchrotron orbital radiation x-ray stepper

A newly designed synchrotron orbital radiation x-ray stepper system has been developed to establish practical quarter-micrometer x-ray lithography and installed in the Tsukuba Research Laboratory for SORTEC Corporation, Japan. This stepper features the newly devised high-precision real-time full-closed alignment system and the ultraprecision vertical wafer stage which can be operated at an atmospheric helium pressure. In order to realize precision positioning of the wafer stage, we developed a 10-axis stage by combining a coarse XY positioning stage guided by hydrostatic air bearings and a fine positioning stage driven by six piezoelectric actuators. We also developed holographic alignment error detection optics, which consists of a three-axis Fourier transform lens system. Alignment error between the mask and the wafer can be detected during exposure and is fed back to the servo control system. By this real-time alignment error correction, an alignment precision of 0.06 μm(3σ) was successfully achieved.

An alignment system for X-ray lithography

Abstract A new alignment technique for x-ray lithography, projection linear Fresnel zone plate (LFZP) method, has been developed and applied to an x-ray stepper for synchrotron radiation lithography. A wafer alignment mark is magnified and projected using an LFZP on a mask. The wafer mark image position is measured very precisely by a scanning optical system. The alignment error between a mask and a wafer is detected with less than 0.01 μm resolution. Up to date, 0.1 μm (3σ) overlay accuracy has been achieved.

The vertical traverse stage in vacuum condition.

A vertical traverse wafer positioning stage was developed for X-ray stepper application in synchrotron radiation (SR) as an X-ray source. To ensure high accuracy of movement in vacuum /low-pressure helium conditions, the stage construction was split up into a coarse stepping stage and a fine positioning stage. For the coarse stepping stage, a linear air hydrostatic bearing (LAB) with a noncontact sealing device was developed to avoid lubrication problems. For the fine positioning stage, a multiaxis leaf spring guided and piezoelectric actuators employed stage was developed to attain high rigidity. The air leakage rate from each noncontact sealing device was less than 5.7x10-4 Pa·m3/sec, so that the pressure of the vacuum chamber could reach 3 x 10-3 Pa. Experiments on the coarse stepping stage show rotational movement error such as pitching and yawing to be less than ±6μrad with a repeatability of less than ±1μrad. The multiaxis fine positioning stage is also proved to have a sufticient small movement error.