18 X-Ray and E-beam Lithography

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

Like the conventional optical lithography counterparts, Extreme Ultraviolet (EUV) lithography and x-ray lithography utilize photons to image and pattern devices. But unlike their conventional optical lithography counterparts, they utilize extremely short-wavelength photons in the soft (for EUV lithography) and hard x-ray (for x-ray lithography) regions of the electromagnetic spectrum. Of these two lithographies, x-ray lithography was the first to be successfully demonstrated to be useful in lithographic patterning.The resolution potential of both x-ray and EUV lithographies, given their short wavelengths, was the main motivation for their development. The two main printing modes of these lithographies are in proximity mode (for x-ray lithography) and projection mode (for soft x-ray lithography, also called EUV lithography). In the following sections, we discuss the essential attributes of these lithographies.

2. Proximity X-Ray Lithography

Lithography using x-ray photons with extremely short wavelengths in the range of 0.4 nm to 2 nm can inherently overcome the diffraction effects associated with imaging features with sizes comparable to the exposure wavelengths of UV lithographies. This was the main motivation underlying x-ray lithography. However, at x-ray wavelengths, there are no known materials for making image-forming lenses or mirrors. Consequently, proximity x-ray lithography (figure 1), where the mask is brought to within a few microns of the wafer and the x rays are passed directly through the mask and onto the wafer, was invented.

The year 1972 witnessed the invention of proximity x-ray lithography by Hank Smith at Massachusetts Institute of Technology. Many companies (IBM, Canon, Nikon), universities (MIT and the University of Wisconsin), and governments (United States and Japan) spent well over a billion dollars on its development. The first attempts at production of x-ray aligners were made in the mid-1970s by AT&T and MIT, and in the early 1980s by Micronix Partners, Hampshire Instruments, Nikon, and Karl Suss. X-ray step-and-repeat and step-and-scan systems were eventually made commercially available. The promise of x-ray lithography lies in the fact that it offers the shortest optical wavelength, and theoretically the highest resolution of all of the other optical lithographies. The minimum resolvable linewidth w in x-ray lithography is given by

Where is the x-ray exposure wavelength, z is the gap between the x-ray mask and the wafer,and α is a parameter that is a measure of the contributions from the resists. Typically, α ranges between 0.5 and 1.5.

Furthermore, x-ray lithography offers excellent Critical Dimension (CD) control, superior depth of focus, and the ability to project through surface contamination on wafers and masks. By using x rays, wavelength-related diffraction problems, which limit resolution of other optical lithographies, are eliminated. High-aspect-ratio pattern fabrication can be achieved with x-ray lithography, given the high transparency of resist films to x rays. In addition, there are no field size limitation issues in x-ray lithography. Unwanted scattering and reflection are negligible in x-ray lithography because the index of refraction is about the same (close to unity) for all materials in the x-ray spectral region. As a result, standing wave effects and other reflection based problems that limit resolution in other optical lithographies are eliminated in x-ray lithography. There are, however, some major limitations of x-ray lithography, and these include the difficulty of fabricating 1 x masks, the high cost of generating the x rays at sufficient power, penumbral and diffraction blurs that limit resolution (in the case of point sources), and overlay control.

2.1 Synchrotron sources

Synchrotron storage rings produce x rays (0.1–10-nm wavelength) that have been used in x-ray lithography. In the synchrotron, electrons are accelerated to up to 1 GeV and maintained at that energy, and circulated in an evacuated ring of pipe surrounded by a strong magnetic field. As the orbiting electrons in the storage ring change direction in the magnetic field, they emit shortwave dipole radiation. Under certain conditions, when the electron velocity approaches the velocity of light, the radiation pattern changes from dipole type to a forward lobe, resulting in a strong continuous output of an almost parallel (collimated) beam of radiation with a wide range of wavelengths. One synchrotron storage ring can produce up to 20 beam lines that can be used in as many x-ray exposure tools. X rays produced in the synchrotron are ideally suited for proximity lithography. Because they are naturally collimated, x rays produced in synchrotrons do not cause penumbra; but they produce deep resist profiles with vertical sidewalls and astonishing aspect ratios. In addition, because of the extremely short wavelength of x rays, diffraction effects caused by the mask-substrate gap in proximity printing are minimized. Synchrotrons are not without drawbacks. One of their major limitations is their extremely large size and the huge cost of ownership associated with them. Reliability is also a concern with these systems. For instance, a synchrotron can take a whole day or more to be brought up because they operate at 10-9 torr or higher vacuum, and beam lifetimes last only 24 hours.

2.2 X-ray masks

Given that there are no materials that have excellent transparency to x rays, x-ray masks are comprised of very thin membranes (with thickness ,2 mm), typically made of low-atomic-number materials, on which the circuit patterns are placed in the form of high-atomic-number absorber materials. A high percentage of the x rays pass through the low-atomic-number material, but are absorbed or scattered by the high-atomic-number materials, thus generating a contrast for the pattern. Typical materials used as x-ray mask membranes include silicon, boron nitride, silicon carbide, diamond, and silicon nitride. Typical materials used as x-ray mask absorbers include gold, tungsten, Ta, TaN, TaSiN, and Ta4B, with the last four materials being the most preferable because they are compatible with various etch and cleaning processes. Membrane materials must have a high Young’s modulus (for silicon carbide, the value is 450 GPa, while it is 900 GPa for diamond), a characteristic that minimizes mechanical distortion as well as makes the membrane damage resistant to prolonged exposure to x rays. The use of thin membranes in x-ray masks presents many challenges. These challenges include film deformation because of stresses, and susceptibility to vibration when stepped or scanned in an exposure tool. Mask deformation is particularly problematic in x-ray lithography, given that the imaging is 1:1 printing, with no reduction. This necessitates very tight tolerances for x-ray masks relative to reduction printing systems. On the other hand, there are no lens distortions or feature-size-dependent pattern-placement errors associated with x-ray lithography since there are no lenses involved. This implies that a significant portion of the overlay budget can be allocated to the mask in x-ray lithography.

3. Extreme UltraViolet (EUV) Lithography

Projection soft x-ray lithography, also called EUV lithography, was invented independently in the 1980s by research teams at NTT in Japan and at Bell Laboratories in the United States, who were inspired by the availability of mirrors of reasonable efficiency at soft x-ray wavelengths ( 4–40 nm). EUV lithography uses photons with 13.5-nm wavelength to expose wafers. It has higher resolution (with suitable numerical aperture) than longer-wavelength photons used in conventional DUV lithography. This result indicates that the resolution potential of the EUV system is approximately five times better than that of the hyper-NA immersion 193-nm lithography system, provided that the resist process used to print the EUV image can support the same k1 factor as the 193-nm lithographic process. Designing and fabricating such a resist that can enable such enhanced resolution is one of the main difficulty in implementing EUV lithography in manufacturing. In addition, producing an imaging system that can exploit the advantages of the short EUV wavelength is a very complex undertaking in light of the fact that traditional materials used to fabricate lenses and mirrors do not have the required optical characteristics in the EUV range. The source chamber cannot be separated from the main chamber by a physical barrier because no known material has sufficient transparency to EUV photons. This places a significant challenge in being able to prevent debris in the form of particles, energetic ions, and neutrals generated by the hot plasma from getting into the main chamber where the illumination optics are housed. Vacuum clamping cannot be used in EUV exposure tools because of the risk of generating particles. EUV radiation cannot be transmitted through air, which makes it necessary to maintain the entire EUV exposure tool in a vacuum environment (1029 mbar). Outgassing of resists and components of the exposure tool inside the exposure chamber therefore poses a significant contamination risk to the EUV optics, which are completely based on mirrors. Furthermore, hydrocarbons and water vapor are cracked by EUV radiation, contaminating mirror surfaces via carbon deposition and oxidation of multilayer coatings. Both carbon deposition and oxidation of the surfaces of EUV optical elements reduce the reflectivity of the mirrors and introduce wavefront aberrations to the projection optics. The overall effect decreases not only the throughput of the exposure tool, but also the imaging performance. Thus, it is necessary to minimize hydrocarbons and water vapor content in the tool. This is yet another reason that imaging in EUV lithography is done in vacuum.

There are no reasonably EUV-transmissive refractive materials that can be used in EUV optics. Instead, most naturally occurring materials reflect only a very small fraction of the incident EUV photons, and tend to absorb most of them within a fraction of a micron from their surface. Therefore, only precisely figured reflective optics (with reflectivity on the order of 70%) comprising mirrors and reticles can be used in the EUV tool. Even with reflective optics of moderate reflectivity, there are still significant photon losses through the exposure optics after reflection from multiple mirror surfaces, i.e., only about 6% of the photons incident on the reflective mirrors reach the wafer plane. Also, because of the high EUV absorptivity of solid materials, including thin polymer films, there are no materials currently demonstrated with sufficient transparency that can be used as EUV pellicles. As a result, there are no pellicles for EUV masks, which presents significant defect control challenges in EUV lithography. Furthermore, EUV photons are sufficiently energetic to interact directly with bound electrons of the individual atoms of any given material. In this spectral range, there are a large number of atomic resonances, which leads to strong absorption. Given the small scattering cross section in the 10–15-nm wavelength range, the refractive indices of available materials approach unity; as a result, materials tend to exhibit low reflectivity in the EUV region of the spectrum. The reflectivity R at normal incidence for an interface between vacuum and a given material is expressed as

                                                          1 − 2 = (1 +  )

where n is the refractive index of the material and 1 is the refractive index of vacuum. In general, R increases roughly as the fourth power of the wavelength in this regime. The influence of atomic resonances also increases significantly at extremely short wavelengths. For example, at 12.4 nm, silicon has an absorption edge associated with photoionization of its 2p electrons, and its index of refraction is greater than 1, while its reflectivity shows a significant decrease. It must be emphasized that the short attenuation length and low normal incidence reflectivity of materials in the EUV range present very difficult problems regarding fabricating single-surface mirrors or lenses. The main solution to this problem is the use of multilayer reflectors. Absent these reflectors, EUV lithography as currently practiced would not be possible.

3.1 EUV masks

Figure 2 shows the cross section of a typical EUV mask. It comprises an absorber layer (typically Ta, TaN, or CrN), which is deposited on top of the buffer layer (typically SiO2) that separates it from the Mo-Si multilayer reflector. Note that the absorber material in Fig. 2 is TiN. A capping layer (typically Ru or Si) is often deposited on top of the Mo-Si stack to protect it from reflectivity-degradative carbon deposition and oxidation reactions. Fabrication of EUV masks involves the deposition of the buffer layer on the mask blank, followed by the deposition of the absorber layer, and finally the deposition of the capping layer. By coating an appropriate resist over the capping layer and exposing (with electron-beam lithography) and developing away the exposed areas of the resists, a relief image comprising the open areas (without resist) and the resist-covered absorber areas is printed on the mask blank. Pattern transfer into underlying layers through the open areas over the capping layer, absorber, and buffer layer is effected by means of appropriate etching, in a subtractive manner. Here, the resist protects the capping layer and absorber–buffer layer stack under it.

3.2 The EUV exposure system

The EUV exposure system comprises the plasma source (for generating the plasma that in turn emits EUV photons), the source collection optics (comprising the condenser and illumination optics), and the projection optics. A schematic of an EUV exposure tool is shown in Fig. 3. Specially designed optics are used to collect EUV photons generated in the plasma source. The collector optics directs the EUV photons from the plasma source to the intermediate focus, which is the entrance to the illumination optical system. It is conventional to use the power level measured at the intermediate focus to specify the source power. The intermediate focus is also where a measure of how spread out the radiation is in area and angle matching between the collector and the illuminator is implemented in order to ensure that all of the collected EUV photons (from the collector) are accepted into the illuminator to avoid being wasted. The function of the illumination system is to provide uniform EUV irradiation of the reticle, as well as the required degree of partial coherence for imaging. The projection optics constitutes the last part of the exposure tool and is designed to produce a high-resolution image of the reticle. It should also have very low distortion and aberration levels.

4. Summary

Proximity X-ray lithography was discussed

A detailed description about Synchrotron sources and X-ray masks Extreme UV lithography and its exposure system