X-ray Stepper Research Articles

Control of fixturing-induced distortion in x-ray masks

Mask distortion can be one of the major contributions to overlay error in x-ray lithography. Although some overlay errors due to mask patterning errors (limited by the accuracy of the electron-beam writing tool) will always be present, a significant amount of additional overlay error can be attributed to mask distortion caused in part by the way the mask is mechanically held in place. This fixturing-induced distortion can be present not only during the x-ray exposure step, but also during the mask writing itself. It is clearly important to reduce such distortion to a minimum if x-ray lithography is to fulfill its potential. We have developed a kinematic mask holder to effectively eliminate the distortion due to fixturing that has been observed with other mask holders. A description of the fixture is presented, along with experimental results showing the resulting distortion when used in an electron-beam writing tool, a Nikon 2I measuring tool, and an x-ray stepper. The stepper results are also compared to results using a nonkinematic fixture. In all cases, the kinematic fixture results in distortion of less than 0.02 μm (1σ), consistent with the noise of the measurements in all three tools.

Fabrication of 0.5 μm n- and p-type metal–oxide semiconductor test devices using x-ray lithography

Partially scaled 0.5 μm NMOS and PMOS test devices have been fabricated using synchrotron radiation x-ray lithography for all four levels. All exposures were done at the IMT lithography lab at the BESSY storage ring in Berlin, using the Suss MAX I x-ray stepper. The four levels of each device have been processed with the new Hoechst RAY/PF x-ray sensitive resist. A total overlay accuracy of 130 nm (1σ) in x and y direction for all aligned levels and an overall linewidth variation of 23 nm (1σ) across a 100 mm wafer have been achieved. Reproducible NMOS and PMOS processes have been carried out in the CMOS process line of the institute, using gateoxide thicknesses between 15 and 10 nm, appropriate channel implants, and a spacer technology. Electrical results of functional test transistors with effective gate lengths down to 0.25 μm will be presented.

X-ray Lithography: Novel Fabrication Process for SiC/W Steppermasks

This paper deals with the fabrication of an X-ray stepper mask based on a rigid SiC membrane and an almost stress free W absorber. As a result of extensive process optimization the poly-crystalline SiC membranes can be fabricated with a smooth surface (Rt<15 nm) and a Young's modulus as high as the bulk value (4.6×1011 N/m2). Membranes of 2.5 µm in thickness are being prepared routinely with excellent transparency for synchrotron and optical radiation. A proper stress adjustment of the SiC membrane can be either realized by an implantation of e.g. boron at a dose of 1015 cm-2 or by a thermal oxidation at 1100°C. The W absorber technology enables absorber stresses of less than 1.107 N/m2, resulting in mask distortions of <100 nm for 25×25 mm2 step fields. Precise sub-0.5-micron pattern have been generated by use of e-beam lithography and RIE techniques. High doses SOR experiments indicate an excellent long-term stability of these SiC/W masks.

Fabrication of surface acoustic wave devices by using x-ray lithography

Surface acoustic wave (SAW) devices with 0.6 ∼ 1.7 μm pattern size (fo=287.5∼812.5 MHz) designed as band-pass filters (BPF) were fabricated by using x-ray lithography. An x-ray stepper SX-5 with a conventional source (Pd terget), high sensitive x-ray resist chlorinated polymethylstyrene (CPMS) (negative) and EBR-9 HS (positive), and a low-distortion x-ray mask with W–Ti alloy absorber were used. The SAW BPF devices were fabricated by forming Al transducer patterns on an LiNbO3 substrate. The Al film was chemically etched with an acid etchant. The frequency characteristic of the fabricated SAW BPF devices was evaluated. As a result, it is confirmed that the SAW BPF devices satisfied the superior specification, with a center frequency of f0± 2 MHz, an insertion loss of &amp;lt; 15 dB and a band width of 30 MHz. It is thus demonstrated that the x-ray lithography has the potential for use in the fabrication of submicron SAW devices.

A vertical stepper for synchrotron x-ray lithography

A newly developed synchrotron x-ray stepper installed at a port of an electron storage ring of the NTT Synchrotron Facility1–3 is described. The stepper exposes wafers in a normal atmosphere to vertically scanned synchrotron x rays with continuous alignment control throughout the exposure. The key devices of the stepper are a vertical x–y stage and an optical-heterodyne alignment system. The x–y stage fully utilizes air lubricated ceramic components including air bearing lead screws and achieves 5 nm motion precision over a 200 mm stroke. The optical-heterodyne alignment system detects both lateral and gap displacements between the mask and wafer gratings with accuracies of 10 and 100 nm, respectively. After wafer step positioning using a laser interferometer, alignment between the mask and wafer is executed using the optical-heterodyne signal. In several exposure experiments, alignment capability is estimated to be better than ±0.14 μm as a 3σ value of machine repeatability. Also, several applications to device fabrication confirmed that the vertical stepper is feasible in quarter- to half-micron x-ray lithography.

Exposure instrumentation for the application of x-ray lithography using synchrotron radiation

This paper reviews the recent progress in the development of exposure equipment for x-ray lithography consisting of compact synchrotron radiation source, beamline, and x-ray stepper. This exposure facility is situated door by door to a CMOS-pilot line. The compact storage ring COSY, under assembly in Berlin, is described and calculations are presented, that show that relatively low accumulated current exposure times of some seconds can be expected using a highly sensitive x-ray resist. A simple, inexpensive, and highly reliable beamline has been realized to connect an x-ray stepper and a light source. Several x-ray steppers have been installed for field testing in the nearby lithography laboratory at BESSY, that are equipped with x-ray reflecting mirrors, or a scanning mask and wafer unit for full-field exposure.

Parameters of exposure equipment for SOR x-ray lithography (abstract)

The potential of x-ray lithography makes it a promising method to fulfill the demands of ultralarge-scale integration (ULSI) device fabrication. The main advantages compared to competing lithographic methods are simple shadow printing exposure arrangement, large field size, the excellent resolution down to approximately 150-nm pattern size, high depth of focus, process latitude, and low sensitivity against wafer topography and against dust particles. An increasing worldwide activity can be observed to solve the remaining problems: the small source for production environment, the 1:1 x-ray mask with high quality concerning positioning and linewidth, the x-ray stepper with large throughput and accurate alignment strategy, and a quick repair system for subhalf micron defects. The high sensitive x-ray resist will probably be available in the near future. At Fraunhofer-Institute für Mikrostrukturtechnik (IMT) in Berlin there is an exposure instrumentation under construction, consisting of the race-track compact storage ring COSY, the beamline, and the x-ray stepper XRS 200 from Suss Company. This contribution presents an investigation of choosing the parameter of the above mentioned setup to ensure high-throughput requirements. Experimental results based on experiments at BESSY and theoretical results obtained by means of the simulation program x-ray lithography modeling and simulation (XMAS) are presented concerning, e.g., resolution and transfer of defect particles.

Scanning x-ray stepper for synchrotron radiation

With the SUSS XRS 200 a synchrotron compatible stepper is available which combines high positional accuracy and high throughput using the scanning exposure method. With 1 s each for exposure, alignment, and stepping time, 20 6-in. wafers/h can be processed with the smallest stepfield size of 26×26 mm. The excellent linewidth control, irrespective of topography and surface characteristics, with a large gap and exposure dose window, show the capability of x-ray lithography to be used as the next generation production lithography method. The SUSS XRS-200 provides the user with a tool to take full advantage of this wide process latitude and make production of sub-half-micron features an economical operation.

Vertical x-ray stepper for synchrotron-based production lithography (abstract)

The second generation SUSS XRS-200 stepper will be described. The development of this stepper was based on three years of experience with the first generation SUSS MAX-I steppers operated at the BESSY synchrotron ring in Berlin. These MAX-I steppers have been in use since 1984 in mask, resist, and source research and development. Circuits with sub-0.5-micron feature sizes were produced using the MAX-I. In contrast to the first generation stepper, the XRS-200 is designed for production volumes with fully automatic mask and wafer handling. With the production oriented XRS-200, gap setting is performed optically using a trinocular microscope. After coarse prealignment of each field, the maskholder and wafer chuck are clamped together. Fine alignment of mask to wafer is then performed, and the complete clamped unit is scanned through the horizontal stationary x-ray beam. Because the mask and wafer are scanned, the beamline end window need only be a few millimeters in height. With this design, a thinner end window can be used, thereby maximizing x-ray flux to the mask and wafer. The first of the XRS-200 series has been installed at BESSY. Initial alignment and exposure results will be presented.

Application of X-ray steppers using optical alignment for synchrotron-based X-ray lithography

The results obtained with a first generation of synchrotron-compatible x-ray steppers are reported. Detection, alignment and overlay accuracy data are given. A number of devices have been manufactured with at least the critical mask levels made with x-ray lithography.

An x-ray stepper for synchrotron radiation lithography

A prototype synchrotron radiation (SR) stepper for quarter-micron devices has been developed and installed at the Photon Factory in the National Laboratory for High Energy Physics, Japan. The stepper features are, (i) exposure in an atmospheric environment, (ii) large exposure area (25-mm sq), and (iii) alignment error detection at all times, including during exposure. The stepper consists of an SR extracting chamber, precision mechanical stages, and an alignment error detection system. An SR beam in UHV goes through a beryllium window into an atmospheric environment, and covers the 25-mm sq exposure area by using an oscillating mirror. Patterns on a mask are imprinted onto a wafer. Mask and wafer are, respectively, held in place with vacuum chucks. Their subsequent positioning movements are driven in 6 degrees of freedom by piezoelectric actuators for fine alignment and gap setting. The alignment system, based on the previous Fresnel lens optical system, newly employs a differential mode linear Fresnel zone plate alignment method. As the optical system for this method is located at the outside of an SR beam, it can detect an alignment error between a mask and a wafer at all times, including during exposure. Patterns measuring 0.2 μm were completely successfully imprinted on a wafer. Until now, 0.03-μm (3σ) positioning accuracy and 0.2-μm (3σ) overlay accuracy were achieved.

Experimental results with a scanning stepper for synchrotron-based x-ray lithography

A second generation of synchrotron-compatible x-ray steppers is described which uses the scanning exposure method to minimize overhead time between alignment and exposure. Available x-ray intensity is increased, since a thin exposure window, only a few mm high, can be used to separate the beam line vacuum from the exposure tool which operates in ambient air. First results indicate sufficient mechanical stability to allow the mask/wafer unit to be scanned in an aligned position.

Overlay Accuracy Evaluation in Step-and-Repeat X-Ray Lithography

Overlay accuracy in step-and-repeat X-ray lithography is evaluated through MOS device fabrication using an X-ray stepper applied to four lithography levels. Though relatively good overlay accuracies of 0.15 µm (σ) between two levels are obtained, these values are not satisfactory for sub-half-micrometer ULSI fabrication. To improve overlay accuracy, error factors are investigated. It is found that overlay errors are strongly affected by mask pattern placement errors. The standard deviation of mask errors is as high as 0.1 µm. These errors are introduced by e-beam writing errors and pattern displacement caused by stress relief of absorber and membrane layers in the etching process. The influence of these error factors on pattern placement accuracy is discussed.

Improved tungsten absorber technology for sub-half-micron x-ray lithography

This paper reports on a novel absorber technology for X-ray stepper masks based on a stabilized tungsten layer. Effective stress reduction (σ⩽10 7 N/m 2) and excellent long term stability (Δσ<5·10 6 N/m 2) are being obtained by sputtering the 0.8 μm thick tungsten layers in the presence of oxygen, and subsequently annealing them in an oxidizing atmosphere. Investigations with respect to microstructure and composition indicate a highly textures W 3O layer, having grain sizes in the order of 60 nm. For etching the W pattern a CF 4 based RIE process has been optimized. In this case a beam lithography in combination with a tri-level system (HPR/SiN/PM15) is being used. W pattern with sub-half micrometer dimensions have been etched with steep edges.

A synchrotron compatible production oriented x-ray stepper

A second generation X-ray stepper is described, based on work performed at the Institute for Microstructure Technology in Berlin. The equipment presents specifications suitable for the production of sub-half micron design rule ULSI circuits with high throughput.

X-ray lithography

X-ray lithography with wavelengths between 0.2 nm and 5 nm provides both high structural resolution as good as 0.1 μm and a wide scope of advantages for the application in circuit production. Examples for this better process performance compared to optical techniques are: lower particle and dust sensitivity, applicability of simple single-layer resist technique, high depth of focus without any influence of substrate material and chip topography and presumably the highest throughput of all lithography methods which are able to go into the submicron range. However, the introduction of X-ray lithography into the semiconductor production means a revolutionary change of production technology. This begins with a completely different mask technology which makes, for example, the classical separation of mask substrate fabrication from pattern generation by different manufacturers very problematical and ends with the necessity to introduce X-ray lithography in relatively large production capacity units consisting of a larger number of X-ray steppers. The latter is caused by the fact that a storage ring - even in the smallest version, e.g. COSY - has to supply up to 10 X-ray steppers with light in order to clearly beat the optical techniques with respect to throughput and lower cost level. But, on the other hand, X-ray lithography provides a relatively good chance to win significant advantages in the worlwide semiconductor competition, especially for those who venture to start early with this new technology. To mention only one example, using X-ray lithography can possibly slow down the crazy spiral of future clean room demands. To prove such statements in pilot production lines, the necessary tools and components for X-ray lithography are already or will be available for the first time on a commercial basis in the very near future. Especially steppers, sources and resists with satisfying specifications have been announced by a growing number of vendors. The most critical problem at present is the mask technology and the tools for defect elimination. However, with the existing technologies, the requirements for 0.5 μm design-rules will be met very soon on a pilot scale. The first commercial suppliers of X-ray masks are now preparing for their production technologies. It seems probable that X-ray lithography can be established as a standard production tool at the beginning of the nineties.

Stability of SiC-masks for high resolution synchrotron X-ray lithography

Synchrotron X-ray lithography is a promising technique for high volume production of Ultra-LSI devices with latteral resolution down to 0.2 μm. Besides the powerful source, the X-ray stepper, and the high sensitive resist, mask technology is one of the main features in the development of X-ray lithography. For X-ray masks with high contrast a relatively thick absorbing pattern (0.8 μm) on a thin membrane (2.0 μm) is necessary. The achievable overlay accuracy depends mainly on the alignment accuracy of the X-ray stepper and of the stability of the masks. For a given stress in the absorping layer we will discuss the stability of X-ray masks which is mainly determined by the Young's modulus of the membrane material if we compare identical membrane geometries. SiC-membranes deposited by a high temperature CVD process can be fabricated with a Young's modulus as high as the bulk value (4.6×10 11 N/m 2) which is roughly a factor of 3 higher than for other relevant membrane materials. Membranes of 2 μm in thickness have been prepared with excellent transparency for synchrotron and optical radiation. A description of the preparation of the rigid SiC-membrane, as well as results of the transparency and the stability of these membranes against strong X-ray exposure will be given. For a high X-ray absorption a tungsten layer has been selected. Because of the thermal expansion coefficient which is comparable to SiC and the high Young's modulus tungsten is a promising material for sub-half micrometre pattern in a stress compensated absorber system. In view of X-ray masks with minimum distortion a description of the etch performance of the tungsten absorber system on SiC-masks will be given.

A practical submicron lithography system using a conventional source X-ray stepper

A practical submicron lithography system using a conventional source X-ray stepper

High resolution lithography using synchrotron radiation

This paper reviews the current state of X-ray lithography (XRL) with special emphasis on the exposure equipment, the resolution limitations and on the expected overlay accuracy. Recent developments in mask technology will also be considered consisely. In order to take full advantage of XRL (high throughput, ultimate resolution), one has to use the parallel high-intensity synchrotron radiation. The compact storage ring “COSY” being built at BESSY presently shows all the features concerning cost and physical dimensions required for a semiconductor fabrication environment. The same holds for the low-cost beam line equipped with an X-ray mirror for the illumination of large areas on the wafer. The wobbling of the electron beam within the storage ring has also been investigated for the enlargement of the exposure area. The first X-ray stepper suitable for synchrotron sources being operated in the lithography laboratory in Berlin shows an alignment accuracy which meets the requirements of a sub-half micron lithography. Experimental and theoretical results show that a structure resolution down to 0.2 μm is achievable and that an exposure field of several cm 2 can probably be used even at a feature size well below 0.5 μm.

X-ray lithography

X-ray lithography with wavelengths between 0.2 and 2 nm provides a structural resolution as good as 0.1 μm under optimized conditions even in the case of high proximity gaps of 50 μm which are needed in practical applications. But the high performance of X-ray lithography can only be realized by parallel and highly intensive radiation. The wavelength distribution has to be tuned properly to the absorption properties of a special set of mask substrate, absorber and window material. As there are no suitable optics for X-rays in order to fabricate a condensor for a homogeneous illumination of mask and wafer, the type of X-ray source used is the most decisive parameter with respect to both resolution and wafer throughput. Comparing various X-ray sources, such as X-ray tubes, storage rings, and plasma sources, the conclusion is that at present synchrotron radiation offers most advantages. However, the existence of low cost compact storage rings especially designed for the lithography purposes is a precondition for an industrial application.The second most important problem is a mask technology with high geometrical stability (10−6 is needed for the large step field sizes mentioned) and low defect densities. Silicon, siliconnitride and boronnitride for the membrane and gold or tungsten for the absorber are the most promising materials for X-ray masks. Besides a sufficiently high X-ray transparency, the mask membranes have to show a high transparency in the visible wavelength range too. Highly sophisticated optical pattern recognition seems to be the best way to solve the alignment problems with an alignment accuracy of at least 0.05 μm (3δ).Moreover, synchrotron radiation provides a possibility to use single layer resist techniques even in the case of highest resolutions. New resist materials now appearing seem to provide a sufficiently good compromise between radiation sensitivity and technological stability. It is estimated that on this basis the wafer throughput of an X-ray stepper with optical alignment will be 50–100 wafers (125 mm) per hour at a cost level comparable to the optical wafer stepper but at a significantly higher process performance.