In this paper, an outline of materials-related activities in the national X-Ray Lithography Program is given. The program is directed towards the development of sub-quarter micron design-rule x-ray lithography together with the ancillary technologies required to fabricate defect-free x-ray masks. Work done at the Naval Research Laboratory is highlighted and used for examples. Most materials related work occurs in conjunction in the x-ray mask fabrication pro. cess. Topics discussed include electron beam-matter interaction in the mask patterning process, membane fabrication, stress control in thin membranes, reactive-ion etching of absorber-layers, mask inspection and repair.

In proximity X-ray lithography there is no imaging system in the traditional sense of the word. There are no mirrors, lenses or other means of manipulating the radiation to form an image from that of a pattern (mask). Rather, in proximity X-ray lithography, mask and imaging systems are one and the same. The radiation that illuminates the mask carries the pattern information in the region of the wavefronts that have been attenuated. The detector (photoresist) is placed so close to the mask itself that the image is formed in the region where diffraction has not yet been able to deteriorate the pattern itself. The quality of the image formation then is controlled directly by the interaction between the mask and the radiation field. In turn, this means that both the illumination field and the mask are critical. The properties of the materials used in making the mask thus play a central role in determining the quality of the image. For instance, edge roughness and slope can strongly influence the image by providing the equivalent of a blur in the diffraction process. This blur is beneficial in reducing the high frequency components in the aerial image but it needs to be controlled and be repeatable. The plating (or other physical deposition) process may create variation in density (and thickness) in the deposited film, that will show up as linewidth variation in the image because of local changes in the contrast; the same applies to variations in the carrier membrane. In the case of subtractive process, variations in edge profile across the mask must be minimized.

The variations in material composition, thickness and density may all affect the finale image quality; in the case of the resist, local variations in acid concentration may have strong effect in linewidth control (this effect is of course common to all lithographies).

Another place where materials will affect the final image quality is in the condensing system. Mirrors will exhibit some degree of surface roughness, leading to a scattered radiation away from the central (coherent) beam. For scanning systems, this is not harmful since no power is lost in the scattering process and a blur is actually created that reduces the degree of spatial coherence. Filters may also exhibit the same roughness; typically it will not affect the image formation. The presence of surface (changes of reflectivity) or bulk (impurities) defects may however strongly alter the uniformity of the transmitted beam. This is particularly true of rolled Be filters and windows, which may include contaminants of high-Z materials. Hence, the grain structure of the window plays a very important role in determining image uniformity.

Finally, a seemingly minor but important area is that of the gas used in the exposure area, typically helium. The gas fulfills several needs: heat exchange medium, to thermally clamp the mask to the wafer; low-loss X-ray transmission medium; protection from reactive oxygen radicals and ozone formation. Small amounts of impurities (air) may have a very strong effect on the transmission, and non-uniform distributions are particularly deleterious.

All these factors need to be controlled so that the final image is within the required tolerances. Unfortunately, some of these are difficult to characterize in the visible (e.g., reflectivity variations) and testing at X-ray wavelengths is necessary. Although these obstacles are by no means unsurmountable, foresight is necessary in order to deliver a functional X-ray lithography process.

This work was supported by various agencies, including ARPA/ONR/NRL and the National Science Foundation.

A systematic approach for predicting radiation stability of materials and devices is presented. The conceptual basis is a consideration of the radiolysis potentially occurring in x-ray-irradiated materials as a multi-stage process: a) primary radiation excitations (PRE); b) secondary generation of isolated electronic and structural defects resulting from PRE relaxation; and c) interaction, accumulation, and coagulation of defects resulting in change of device electronic properties and in degradation of optical material. This predictive technique involves specific relevant information in a relatively simple context for the purpose of modeling the radiolysis process. It should find general application in selecting materials for x-ray optical elements such as masks, filters, windows where resistance to radiolysis is required as well as for photoresists where radiolytic sensitivity is necessary.

X-ray lithography introduces device radiation damage from the high energy photons during the lithography process. We have studied this effect on deep submicron n- and p-channel MOSFETs with gate dielectric thickness at 7 to 13 nm. After the x-ray irradiation the device characteristics are strongly affected by the generation of oxide charges, interface states and electron traps. These introduced damages cause the reduction of device transconductance, shift of the threshold voltages and increased leakage current. However, this degradation of device and circuit is lessened from technology scaling by thinning the gate oxide and lowering the supply voltage. The x-ray radiation damage, induces interface states and oxide charges which can be annealed out with a low temperature (400°C) forming gas (FG, 90% N2, 10% H2) annealing process. The device properties are essential unchanged after the annealing process. However, the residue damage is shown to enhance hot-carrier instability of p-channel devices if the remaining neutral traps act as electron or hole traps in the SiO2. In this paper, we investigate the radiation effects on the n- and p-channel MOSFETS fabricated with deep submicron device processes with thinner gate oxides and compare the hot carrier reliability of these devices after the synchrotron x-ray irradiation and also after the post metal forming gas annealing. The results indicate the device hot carrier instability has no effect on the devices with thin gate oxide with thickness approaching the electron tunneling range.

The drive towards < 0.5 μm design rules in silicon device fabrication has led to the emergence of evaluating lithographic technologies based on the use of x-rays as the exposure source. X-rays (centered @ λ = 1.4 nm) emanating from a pulsed laser point source based system has been used for resist design purposes. Concurrent with the development of this exposure source has been the search for a resist which will provide high sensitivity ( ≤ 20 mJ/cm2) and resolution. The vast majority of resists in which the x-ray induced chemistry is amplified are viewed as the most promising class of materials to meet the high sensitivity requirement. Central to these so-called chemically amplified resists is the exposure induced generation of an acid catalyst. The acid during the subsequent post exposure bake (PEB) processing step removes a functionality in the resist resulting in film loss in the exposed regions and dissolution of those areas in the aqueous base developer. To date, the majority of such resists are multi-component which adds complexity to processing of these materials. Chemically amplified resists based on poly 4- t-butoxycarbonyloxystyrene and sulfur dioxide are shown to minimize this complexityThe novel insitu generation of the catalytic acidic species is presented, and the performance of these resists is improved through the use of model compounds based on arylmethyl sulfones.

The study of photoelectron effects in X-ray Lithography motivates the need for modeling codes to simulate these effects to have an estimate of the influence of x-ray generated photoelectrons in the exposure of resists. We have performed a series of Monte Carlo simulations to study the spatial distribution of photoelectrons in a resist, PMMA, and parametrized this distribution with a set of energy-dependent gaussians for monochromatic X-rays within an energy range of 0.5 KeV to 2.5 KeV. We discuss the effects of the the redistribution of the photoelectron kinetic energy as a function of the electrons generated by the x-ray absorption in various atomic species.

Investigations of submicron-channel-length n- and p-channel MOSFETs subjected to channel hot-carrier stressing were performed, comparing the reliability of devices with and without exposure to simulated x-ray lithography processing steps. No significant differences were observed between the sample groups.

In this report we describe a system of technologies we have developed for fabricating, patterning, and replicating x-ray masks with linewidths as fine as 50 nm (0.05 μm). This effort has evolved into a fairly routine service supporting a growing community of researchers at MIT interested in fabricating sub-micron and nanometer scale structures and devices. Strong, transparent, silicon-rich nitride (SiN) membranes bonded to optically-flat Pyrex rings and patterned with gold absorbers form the basic x-ray mask structure. We have recently completed the installation of a vertical thermal reactor (VTR) for the deposition of SiN coatings. Results indicate superb film uniformity, decreased defect counts, and increased strength over previously available nitrides. Patterning of the masks is performed at an off-site location by electron-beam lithography followed by development and gold electroplating at MIT. We describe a new fountain plating bath and mask chuck which allows us to obtain clean, uniform gold films of low stress. Once completed, the so-called “master” masks are replicated onto new x-ray mask substrates which are then processed into “daughter” masks. The resulting polarity-reversed patterns are required in order to achieve the desired pattern in positive resist when printed onto substrates. We will describe our in-house built x-ray aligner which features a high-power electron bombardment source and SiN vacuum isolation window to achieve exposures in a helium environment.

This paper discusses the properties of materials for x-ray masks used in synchrotron radiation lithography system. Through computer simulations and experimental analysis, we evaluate three kinds of membranes (Si, SiN, SiC) in terms of mechanical distortions, surface smoothness and optical transmittance. SiC exhibits the smallest distortion, but SiN has the best characteristics in the surface smoothness and the optical transmittance. By virtue of their high x-ray absorption coefficient and dry etching capabilities, Ta, W and Re are selected as potential absorber materials. We study their stress control characteristics, etching characteristics and the relation between their Young's modulus and distortions. We find that Ta is an excellent absorber because of its good stress controllability and good etching characteristics, such as a high etch rate and etch selectivity. By adopting a Ta/SiN structure, highly accurate masks can be fabricated by a process in which patterning the absorber is carried out before removing the bulk Si.

A silicon nitride membrane growth process has been characterized. The films were grown by LPCVD on 100 mm diameter silicon substrates using ammonia and dichlorosilane as reactant gases. The films were grown using a range of gas ratios at three different temperatures. The film composition was determined by elastic recoil detection. The silicon-nitrogen absorption bands were characterized by FTIR spectroscopy. The photon transmission of the films was measured in the visible region from 350 – 850 nm, and in the x-ray region for photon energies from 1000 to 3000 eV. The film stresses were determined using the wafer bow technique. An increase in the silicon content of the films was observed for increased dichlorosilane gas flow and for increasing growth temperatures. The increased silicon content of the films is correlated to a decrease in the tensile stress and a decrease in the optical transmission of the films.

Highly accurate X-ray masks are strongly required to establish SR lithography technology. X-ray masks must be produced as accurately as the LSI devices, because a one-toone projection aligner system is used. To minimize the in-plane mask distortion, it is desirable to estimate the value of the stress and the non-uniformity in the membrane fabrication (SiN) process. The values of the stress were estimated from the measurement of the warpage and the calculation. It is very difficult to obtain the stress distribution in the SiN/Si wafer. Thus, we measured the minority carrier lifetime distribution using the non-contact laser/microwave method. The carrier injection was done by a 774nm or 904nm semiconductor laser diode, and their beam was focused to about 500 μmφ. The surface lifetime, τ s, of the SiN/Si wafer with the stress over ˜ 108dyn/cm2 decreased to 60–70% of that of the bare Si wafer. Thus, the contactless laser/microwave system can be adaptable for the characterization in the X-ray mask process.

Microwave driven discharges are finding increasing use in semiconductor areas such as deposition, etching, cleaning and ashing. One application that is of particular interest is the use of microwave discharge grown thin film diamond as a x-ray mask. Because of diamonds extreme properties these masks offer potential advantages over current silicon based mask technology. A review of diamond mask technology will be presented and compared to competing technologies.

Deposition parameters acting on nucleation and growth local conditions have been optimized in a bell jar microwave plasma reactor to obtain polycrystalline diamond thin films compatible with X-ray membrane requests. The microstructure and the chemical quality of the films were estimated by SEM and Raman spectroscopy respectively, the roughness was evaluated by AFM experiments and the residual stress was deduced from the substrate deflection method. Membranes were obtained by removing the silicon substrate on 15 mm diameter circular windows. The optical transparency depending on deposition conditions was optimized up to 65% at 630 nm for 1 μm thickness. A high resolution additive mask process, based on well taut membranes and low stress electroplated gold absorber, was carried out. Micrometric and submicrometric mask patterns were generated in photoresists both by electron beam lithography with a nanopattern generator and by X-ray lithography using the synchrotron radiation facility implemented at LURE-Orsay. Despite the diamond films roughness of the order of 30 nm, well defined dots and lines as narrow as 100–200 nm could be obtained.

Process techniques have been developed to produce diamond membranes for application as x-ray lithography masks. Residual stress and grain size of the films are influenced by substrate temperature, chamber pressure, and methane concentration, and optimization of these parameters narrows the range of acceptable process conditions. A reactor geometry was chosen which has the capability to produce membranes with thickness uniformity of ± 7 percent across a 50 mm diameter. A cyclic renucleation technique was developed to maintain the grain size at low levels independent of film thickness. Membranes with RMS roughness of about 25 nm, as determined by atomic force microscopy, are fabricated reproducibly. The elastic modulus of small grained membranes deviates from that of larger grained membranes. This deviation must be taken into account in membrane distortion modeling calculations.

The applications of multilayer x-ray mirrors to x-ray lithography are reviewed. Topics included are: multilayer performance and characterization, testing and fabrication of large mirrors with figure errors in the Angstrom range, multilayer x-ray masks, and collimators.

Reflectivity of Mo/Si multilayer x-ray mirrors with molybdenum as the top layer decreased over time when stored in air due to oxidation of the top Mo layer to MoO3 and MoO2. High reflectivity can be maintained by having silicon as the top layer. In fact, reflectivity of Si-top multilayer coatings increase slightly during the first few days after deposition.

In many applications, x-ray mulhilayer mirrors are exposed to high peak fluxes of x-rays with subsequent damage to the mirror. Mirror damage is a particularly severe problem with the use of multilayers as cavity optics for short wavelength x-ray lasers. Intense optical and x-ray radiation, from the x-ray laser plasma amplifier, often damages the multilayer mirror on time scales of hundreds of picoseconds. The phenomenon of multilayer mirror damage by pulsed xray emission has been studied using short duration (500 psec) bursts of soft x-rays from a laser produced gold plasma. The results of the experiments will be compared with some simple models and the possibility of increasing the damage thresholds of short wavelength multilayer mirrors will be discussed.

In recent years, several X-ray lithography experiments involving laser plasma sources have been reported. The high X-ray conversion efficiency (η – 10%) in the keV range observed in various laboratories, using high laser intensities (I > 5 × 1012 W cm−2), has already made these sources an attractive alternative to the electron storage rings for X-ray lithography in proximity printing (XRL). In addition, X-rayproduction in the region around 130 Å has also been investigated at intensities of about 1011 W cm−2 for applications to X-ray Projection Lithography (XRPL). Conversion efficiency up to 1% into a 3 Å bandwidth has been demonstrated. In principle, a large variety of X-ray spectra can be obtained from a laser plasma source depending on the laser-target parameters. However, high conversion efficiencies, in a given spectral range, can only be achieved under specific plasma conditions (target atomic number, plasma temperature, plasma volume and spatio-temporal expansion...). This leads to some restrictions in possible target materials and irradiation conditions (laser wavelength, laser intensity, pulse duration). In this paper, we will discuss this physical aspect basing our analysis on both the theoretical and experimental studies of the X-ray spectra in different X-ray energy ranges and for various target atomic numbers. In addition, we will examine issues related to the laser plasma X-ray source design for both XRL and XRPL applications.

Capillary optics appear to permit the fabrication of practical collimator systems for laserdriven x-ray point sources. With such collimators, the illumination of wafers can meet the tight requirements for lithographic patterning of sub-micron device structures. However, the use of x-radiation with photon energies between 800–1200 eV makes capillaries of conventional silica-based glass capillaries marginal due to their low reflectivity. The reflectivity of various materials were examined with the goal of finding alternate glasses, elements for doping glasses or surface coatings that may enhance these collimators for lithographic applications. Coating capillary surfaces with more reflective materials may be the most viable method for improving the radiation transport properties for capillary optics to be used in x-ray lithography.

This paper presents data on the soft x-ray behavior of the photochromic dosimeter film UVSC manufactured by Sensor Physics. In particular, the x-ray sensitivity measured with synchrotron storage ring x-ray sources will be presented and compared with a presently used dosimeter film which is many times less sensitive. Relevant properties of the film for x-ray source evaluation also will be presented together with data obtained with other x-ray lithography sources.