Synchrotron X-ray Lithography Research Articles

Stainless steel imitative crack using X-ray LIGA with suspension electroplating technique

In this research, the Synchrotron X-ray lithography (X-ray LIGA) was implemented to fabricate imitative Stress Corrosion Crack (SCC) for using as a reference specimen of electromagnetic NDT. A pattern of the imitative SCC was created by using X-ray LIGA with controllable shape and size in precise details. The pattern created had a high aspect ratio, which was formed on SU-8 polymer. As a preliminary experiment, the prepared pattern was inserted into a block of pure Nickel formed by an electroplating technique. To develop a reference comparable to stainless steel in terms of electromagnetic response, the technique called “Suspension Electroplating” was implemented to create a block of material from its powder form. A rough magnetic response was then measured by Eddy Current Testing (ECT). The results showed the possibility of creating the desired properties of stainless steel. The SCC pattern will be then inserted to become the reference specimen.

Development of oriented metallic patterns using LIGA process for focused anti-scatter grid application

X-ray projection imaging is troubled by beam scattering. Anti-scatter grids selectively remove such scattered X-rays and are classified as focused or unfocused, depending on the lamellar angle. Focused grids afford better transmittance than unfocused grids, but it is difficult to fabricate inclined microstructures with different angles. Here, we present a novel method for the fabrication of anti-scatter grids on bendable substrates. We utilized a synchrotron X-ray lithography process to obtain unique microstructures and subsequent copper-based electroforming process for metallization. Finally, lamellar micro-patterns were successfully realized with different angles according to each location of the focused anti-scatter grid application.

Fabrication of Polymer Microstructures of Various Angles via Synchrotron X-Ray Lithography Using Simple Dimensional Transformation.

In this paper, we developed a method of fabricating polymer microstructures at various angles on a single substrate via synchrotron X-ray lithography coupled with simple dimensional transformations. Earlier efforts to create various three-dimensional (3D) features on flat substrates focused on the exposure technology, material properties, and light sources. A few research groups have sought to create microstructures on curved substrates. We created tilted microstructures of various angles by simply deforming the substrate from 3D to two-dimensional (2D). The microstructural inclination angles changed depending on the angles of the support at particular positions. We used convex, concave, and S-shaped supports to fabricate microstructures with high aspect ratios (1:11) and high inclination angles (to 79°). The method is simple and can be extended to various 3D microstructural applications; for example, the fabrication of microarrays for optical components, and tilted micro/nanochannels for biological applications.

Implementation of high-pass subterahertz filters using high-aspect-ratio polimeric structures

This paper describes a technological approach to the implementation of quasioptical high-pass subterahertz filters with the use of high-aspect pseudometallic structures. The approach is based on micropatterning a solid polymer layer of polymethylmethacrylate by means of synchrotron X-ray lithography with subsequent metallization of the entire surface of the structure. A fabricated specimen and operational characteristics for the filter with a cutoff frequency of 0.275 THz, which has the thickness of 1 mm and is formed by hexagon-shaped through-holes arranged on a triangular lattice with 70-μm wide crosspieces, are demonstrated.

X-ray masks based on epoxygraphite

A simple fabrication technology of hard X-ray masks with thin (50–150μm thick) X-ray transparent bearing membranes made of a new material, i.e., polymerized epoxy resin with graphite filler (hereafter, epoxygraphite), is described. The first results of using such masks in pattern scanning synchrotron X-ray lithography in the exposing radiation wavelength range λ ≈ 0.5–3 A are presented. The prospects of applications of such masks in the softer spectral range λ ≈ 3–7 A are considered.

Gold-ruby glass in a new light: On the microstructuring of optical glasses with synchrotron radiation

We describe the microstructuring of gold-ruby glasses with synchrotron radiation. Plasmonic or luminescent microstructures with a lateral width of minimum 5 μm can be written directly into the glasses by implementing X-ray lithography. The technique involves two steps: First, gold containing glass samples are irradiated with synchrotron X-rays through a microstructured mask. And second, subsequent annealing at minimum 500°C induces the growth of gold nanoparticles. The patterned sites are ruby coloured due to the gold surface plasmon resonance of gold nanoparticles. Furthermore we investigated the photoluminescence of the microstructured glass. After synchrotron irradiation a red photoluminescence is observed under UV light excitation. Subsequent annealing for a few minutes at 300°C induces the quenching of the red luminescence. If the irradiated sample is annealed for 5 minutes at a higher temperature of 500°C a bright green light emission is detected. The green photoluminescence decreases after further annealing and finally vanishes. We assume that the origin of the luminescence are silicate hole centres. The technique of generating gold particles with synchrotron X-ray lithography has potential to produce micro-optical devices like optical storage units, photonic crystals, gratings or sensors.

Fabrication of quantum wires and dots by X-ray lithography and Ga + implantation enhanced intermixing

The mass fabrication of Ga/AsAl xGa 1-xAs quantum-well wires and dots is described, using a high resolution synchrotron x-ray lithography system to define Au/W implantation masks and employing a Ga + implantation enhanced intermixing technique on a GaAs/AlAs/AlGaAs double barrier quantum well. The large area line and dot arrays with sizes ranging from 40 – 200 nm show a very large photoluminescence blue-shift and the influence of the mask geometry is discussed. The defined fabrication of even smaller structures by x-ray lithography is possible, but secondary effects which stem from the substrate radiation inhibit a further size decrease and the potential importance of x-ray or VUV photo-fluorescence in the resist insolation process is pointed out.

Performance of a wide-field flux delivery system for synchrotron x-ray lithography

An x-ray flux delivery system consisting of a compact storage ring and dedicated beamlines is now in routine use in the IBM Advanced Lithography Facility in East Fishkill, New York. The beamlines have been designed for use with a stepper that scans the mask and wafer vertically through a stationary beam. The performance of the system from a lithographic standpoint is reported. The measured size and stability of the synchrotron x-ray source will be presented, and the design and performance of the beamlines will be discussed. The beamlines have been designed to provide high intensity, uniform illumination over a wide field. The overall performance of the system has been evaluated by measuring the flux at the mask plane in the stepper. A horizontal uniformity of better than ±2% over a field more than 50 mm wide has been achieved; the vertical uniformity is limited by the uniformity of the stepper scanning mechanism. The delivered mask-incident flux per milliampere of stored beam current has been found to be 1.5 mW/(mA cm), approximately four times greater than obtained with a simple planar mirror. A 100 mJ/cm2 resist with a mask transmission of 50% therefore requires a less than 2 s exposure (plus overscan overhead) for a 2.5 cm high field at a typical stored current of 200 mA, independent of the field width (up to the maximum of more than 50 mm). In addition, the source has proved itself to be highly reliable, with beam available for 99% of the scheduled time over the most recent seven month period.

Novolak Based Resists for X-ray and Electron Beam Lithography.

X-ray and elctron beam lithography is expected to be leading candidates for the future VLSI's having a minimum dimension below the resolution limit of optical lithography. Since the concept of chemical amplification has been introduced, tremendous progress has been achieved in designing resists for deep-UV, x-ray and electron beam lithography. Recently three component resist systems which contain polymer matrix, photoacid generator and dissolution inhibitor have shown the enhanced sensitivity and excellent resolution. Novel novolak based positive resist containing three components has been developed. The properties of novolak resins with solvent treatment was characterized by GPC, TGA and DSC. The results suggest that the solvent extraction influences the thermal stability and molecular weight of novolak resins. Preliminary synchrotron x-ray lithography shows that KXPR resist containing 0.3 wt% PAG is capable of ca. 0.4μm resolution and 60mJ/cm2 sensitivity . The KXPR resist is also sensitive to electon beam exposure. The resist containing 0.7 wt% PAG has a high sensitivity of ca. 3μC/cm2 with γ 5.1, and is capable of 0.3μm resolution. Etch resistance was evaluated and reported on silicon dioxide and polysilicon layers.

Effects of mirror surface roughness on exposure field uniformity in synchrotron x-ray lithography

In this paper we discuss the surface roughness tolerances for mirrors used in synchrotron x-ray lithography beamlines. These requirements are different for lithography beamlines than they are for traditional beamlines, which are imaging systems. For imaging optical systems it is desirable to minimize the surface roughness of the mirror in order to reduce the intensity of the scattered light. Proximity print x-ray lithography requires nonimaging optical systems designed to deliver a uniform power distribution at the exposure field. In such applications, the scattering of x rays is less important, provided that the total flux is not too severely degraded. From this point of view it is important to notice that scattering removes power from the central, coherent, beam and redistributes it over a wider angle. The loss in reflectivity is only apparent because energy is conserved in the process. Furthermore, the presence of scattered x rays can be used to improve the uniformity of the power distribution. The requirement on the surface quality of mirrors used for scanning x-ray illumination is thus relaxed with respect to that of typical imaging systems, providing considerable cost savings and simplifying manufacturing. The effect of the scattered light on the uniformity of illumination is dependent on intensity and angular distribution of the scattering. Experimental results will be compared to the results predicted by calculations.

光ヘテロダイン干渉式ギャップ検出法

A new gap detection technique is developed for quarter micron X-ray lithography, whitch technique depends on an optical-heterodyne interferometry with diffraction gratings. To obtain detection accuracy as good as a few tens of nanometers, a phase signal is utilized instead of a conventional intensity signal. The gap between mask and wafer is detected by measuring the phase difference between two heterodyne beat signals generated by projecting two laser beams from-1st-order and-3rd-order diffraction directions on the mask and wafer grating marks. The gap signal with a few tens of microns cycle makes it possible to set an arbitrary wide range gap. The gap can be detected in resolution of less than 20 nm by an experimental setup. Combining this gap detection method with the relative position detection technique using symmetric optics, a 4-channel alignment system is developed for synchrotron X-ray lithography. Six-axis alignment servo control is established by combining this system with highly accurate stages. The total overlay accuracies of ± 0.05 μm (3σ) are achieved.

Radiation damage and its effect on hot-carrier induced instability of 0.5 μm CMOS devices patterned using synchrotron x-ray lithography

The device characteristics and the radiation damgae ofn-channel andp-channel MOSFETs patterned using synchrotron x-ray lithography are examined. The effect of radiation damage caused by x-ray lithography on the device reliability during hot electron injection is investigated. In addition to neutral traps, large amounts of positive oxide charge and interface states, particularly acceptor-like interface states, which cause degradation of MOSFET characteristics are found to be created by x-ray irradiation during the lithography process. Although several annealing steps are performed throughout the entire fabrication process, the radiation damage, particularly neutral traps, is not completely annealed out. The hot-electron induced instability inp-channel MOSFETs is significantly increased due to the enhanced electron trapping in the oxide by residual traps. The effect of radiation damage on hot electron induced instability is found to be more severe inn +-poly buried-channelp-MOSFETs than inp +-poly surface-channel p-MOSFETs. However, the degradation inn-channel MOSFETs due to channel hot carriers is not significantly increased by x-ray lithography. These results suggest that the major degradation mechanism due to hot-carrier inp-channel MOSFETs is electron trapping and inn-channel MOSFETs is interface state generation. It also suggests thatp-channel MOSFETs, in addition ton-channel MOSFETs, needs to be carefully examined in terms of hot carrier induced instability in CMOS VLSI circuits patterned using x-ray lithography.

Application of synchrotron x-ray lithography to fabricate fully scaled 0.5 μm complementary metal–oxide semiconductor devices and circuits

High performance fully scaled 0.5 μm complementary metal–oxide semiconductors very large scale integrated (CMOS VLSI) circuits have been fabricated using synchrotron x-ray lithography technology. X-ray lithography is employed at all levels to attain a minimum feature size of 0.5 μm. The wafer exposures are done at the VUV storage ring from the National Synchrotron Light Source, Brookhaven National Laboratory. A stepper built at IBM Yorktown Heights is used at the beamline to perform the wafer exposures. All the lithography levels are aligned to the prefabricated 0.5 μm deep silicon trench zero level with an overlay less than 0.1 μm (1σ) between levels. Single level resists (both positive and negative) are used throughout the entire CMOS process. Linewidth control better than 0.01 μm and alignment tolerance less than 0.10 μm are accomplished. The patterning of this x-ray lithography mask is accomplished through a vector scan electron beam direct writing system. Masks made of boron doped silicon membranes with electroplated gold are used as the absorber. An average linewidth variation less than 0.2 μm can be attained for the (25 mm)2 field size masks. A retrograded N well 0.5 μm CMOS process is used to exercise the fabrication of x-ray lithography VLSIs. In the (25 mm)2 testsite, CMOS devices, ring oscillators and fully scaled 0.5 μm CMOS SRAM (static random access memory) are densely populated to exercise this technology. In this experiment CMOS devices and ring oscillators are successfully fabricated. The device characteristics are essentially the same as those fabricated using other lithography methods. A fully scaled 0.5 μm 61 stage inverter ring oscillator was measured at a delay of 95 ps/stage when operating at a 3.5 volts power supply. Radiation effects on the fabricated CMOS devices are studied. With the final 400 °C hydrogen ambient annealing, there is no apparent difference on the device characteristics in comparison to the same devices fabricated using optical lithography.

An optical-heterodyne alignment technique for quarter-micron x-ray lithography

A new optical-heterodyne interferometry alignment technique with diffraction gratings is developed for quarter-micron x-ray lithography. To obtain detection accuracy as good as a few tens of nanometers, a phase signal is utilized instead of a conventional intensity signal. The relative lateral displacement between mask and wafer is detected by measuring the phase difference between heterodyne beat signals generated by projecting two laser beams from + first-order and − first-order diffraction directions on the mask and wafer grating marks. The displacement signal is only slightly influenced by gap variation using symmetric optics. A lateral displacement detection resolution better than 10 nm is obtained by the experimental alignment setup. A nonsymmetric beam from the − third-order diffraction direction is added to the symmetric beams to detect the gap. The phase difference between two beat signals emitted to the second-order diffraction direction from the same mask and wafer marks is used as the gap detection signal. The cyclic gap signal makes it possible to set an arbitrary gap. A gap detection resolution of <20 nm is realized. Using this optical-heterodyne interferometry alignment method, a four-channel alignment system is developed for synchrotron x-ray lithography. Six-axis alignment servo control is established by combining this system with highly accurate stages.

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.

Hot-electron-induced instability in 0.5- mu m p-channel MOSFETs patterned using synchrotron X-ray lithography

Radiation damage caused by X-ray includes positive oxide charge, neutral traps, and interface states. Although several annealing steps are performed throughout the entire fabrication process, the radiation damage, particularly neutral traps, is not completely annealed out. The hot-electron-induced instability in p-channel MOSFETs is significantly increased due to the enhanced electron trapping in the oxide by residual traps. However, the degradation in n-channel MOSFETs due to channel-hot carriers is not significantly increased by X-ray lithography since n-channel MOSFETs are susceptible to interface state generation by hot carriers but are relatively insensitive to the degradation due to electron trapping. The results suggest that p-channel MOSFETs in addition to n-channel MOSFETs need to be carefully examined for hot carrier-induced instability in CMOS VLSI circuits patterned using X-ray lithography and/or when the radiation damage is incurred in the back-end-of-the-line processing. >

IBM uses synchrotron X-ray lithography to make 0.5 micron scale chips

IBM uses synchrotron X-ray lithography to make 0.5 micron scale chips

Fully scaled 0.5 μm metal–oxide semiconductor circuits by synchrotron x-ray lithography: Mask fabrication and characterization

Several sets of x‐ray masks for a metal–oxide semiconductor (MOS) device program were fabricated using boron‐doped silicon as the membrane and gold as the absorber. The metrology of the mask sets was characterized. Mask‐to‐mask overlay error is <0.12 μm (3σ) including measurement error. Absorber induced distortion is not the dominating mask error. (Lack of better measurement systems and techniques preclude exact analysis of errors.) Quarter‐micron patterns have been resolved using a multilayer resist system. Linewidth variations are <0.017 μm (1σ) across the entire membrane. The mask sets have been successfully used to fabricate fully scaled n‐type MOS circuits.

Fabrication of fully scaled 0.5-μm n-type metal–oxide semiconductor test devices using synchrotron x-ray lithography: Overlay, resist processes, and device fabrication

Functional, fully scaled n-type metal–oxide semiconductor (NMOS) circuits with 0.5-μm ground rules have been fabricated using synchrotron radiation x-ray lithography for all device levels. The exposures were done at the vacuum ultraviolet storage ring of the National Synchrotron Light Source at Brookhaven National Laboratory using a mask/wafer aligner developed at IBM Research. The system is capable of step-and-repeat exposures covering 125-mm wafers. The mask and wafer are held in a near-vertical plane and a scanning dark-field alignment technique is used to register mask to wafer with a proximity gap of 40 μm. A scanning mirror in the beamline sweeps the x-ray beam up and down over the exposure area to achieve uniform exposure over the 25×25 mm2 exposure field used for this experiment. Exposures were done in 20 Torr of helium to provide cooling for the mask. Both positive and negative tone single-layer resists were used. The mask set consists of four light-field masks patterned on silicon substrates with electroplated gold absorbers; patterns were written by a vector scan electron beam lithography system. The performance of the aligner on both test wafers and device wafers is discussed, with emphasis on overlay and linewidth control. The repeatability of the alignment and the overall level-to-level overlay are described. Characteristics of the fabricated devices are also presented.

A MOSFET, manufactured with synchrotron X-ray lithography

Using “mix and match” technique of optical and X-ray lithograpy, the gate level of a MOSFET tetrode was fabricated. As an X-ray source the synchrotron radiation of the Berlin electron storage ring (BESSY) was used. The employed X-ray mask is based on a stress compensated Si membrane.