Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.

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Gas-assisted focused electron beam and ion beam processing and fabrication

The study of nanostructures is creating a need for microscopes that can see beyond the limits of conventional visible light and ultraviolet microscopes. X-ray imaging is a promising option. A new microscope described this week achieves unprecedented resolution, and has the ability to see through containing material. It features a specially made two-component zone plate — a lens with concentric zones rather like the rings in the Fresnel lenses familiar in overhead projectors and elsewhere — that makes use of diffraction to project an image into a CCD camera sensitive to soft X-rays. Spatial resolution of better than 15 nm is possible. Analytical tools that have spatial resolution at the nanometre scale are indispensable for the life and physical sciences. It is desirable that these tools also permit elemental and chemical identification on a scale of 10 nm or less, with large penetration depths. A variety of techniques1,2,3,4,5,6,7 in X-ray imaging are currently being developed that may provide these combined capabilities. Here we report the achievement of sub-15-nm spatial resolution with a soft X-ray microscope—and a clear path to below 10 nm—using an overlay technique for zone plate fabrication. The microscope covers a spectral range from a photon energy of 250 eV (∼5 nm wavelength) to 1.8 keV (∼0.7 nm), so that primary K and L atomic resonances of elements such as C, N, O, Al, Ti, Fe, Co and Ni can be probed. This X-ray microscopy technique is therefore suitable for a wide range of studies: biological imaging in the water window8,9; studies of wet environmental samples10,11; studies of magnetic nanostructures with both elemental and spin-orbit sensitivity12,13,14; studies that require viewing through thin windows, coatings or substrates (such as buried electronic devices in a silicon chip15); and three-dimensional imaging of cryogenically fixed biological cells9,16.

Soft X-ray microscopy at a spatial resolution better than 15 nm

An extensive review is given of the results from literature on electron beam induced deposition. Electron beam induced deposition is a complex process, where many and often mutually dependent factors are involved. The process has been studied by many over many years in many different experimental setups, so it is not surprising that there is a great variety of experimental results. To come to a better understanding of the process, it is important to see to which extent the experimental results are consistent with each other and with the existing model. All results from literature were categorized by sorting the data according to the specific parameter that was varied (current density, acceleration voltage, scan patterns, etc.). Each of these parameters can have an effect on the final deposit properties, such as the physical dimensions, the composition, the morphology, or the conductivity. For each parameter-property combination, the available data are discussed and (as far as possible) interpreted. By combining models for electron scattering in a solid, two different growth regimes, and electron beam induced heating, the majority of the experimental results were explained qualitatively. This indicates that the physical processes are well understood, although quantitatively speaking the models can still be improved. The review makes clear that several major issues remain. One issue encountered when interpreting results from literature is the lack of data. Often, important parameters (such as the local precursor pressure) are not reported, which can complicate interpretation of the results. Another issue is the fact that the cross section for electron induced dissociation is unknown. In a number of cases, a correlation between the vertical growth rate and the secondary electron yield was found, which suggests that the secondary electrons dominate the dissociation rather than the primary electrons. Conclusive evidence for this hypothesis has not been found. Finally, there is a limited understanding of the mechanism of electron induced precursor dissociation. In many cases, the deposit composition is not directly dependent on the stoichiometric composition of the precursor and the electron induced decomposition paths can be very different from those expected from calculations or thermal decomposition. The dissociation mechanism is one of the key factors determining the purity of the deposits and a better understanding of this process will help develop electron beam induced deposition into a viable nanofabrication technique.

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A critical literature review of focused electron beam induced deposition

Fresnel zone plates consisting of alternating transmissive and opaque circular rings can be used to focus X-rays1. The spatial resolution that can be achieved with these devices is of the order of the width of the outermost zone and is therefore limited by the smallest structure (20–40 nm) that can be fabricated by lithography today2. Here we show that a large number of pinholes distributed appropriately over the Fresnel zones make it possible to focus soft X-rays to spot sizes smaller than the diameter of the smallest pinhole. In addition, higher orders of diffraction and secondary maxima can be suppressed by several orders of magnitude. In combination with the next generation of synchrotron light sources (free-electron lasers) these ‘photon sieves’ offer new opportunities for high-resolution X-ray microscopy and spectroscopy in physical and life sciences.

Sharper images by focusing soft X-rays with photon sieves

Focusing of 8 keV x rays to a spot size of 150 and 90 nm full width at half maximum have been demonstrated at the first- and third-order foci, respectively, of a phase zone plate (PZP). The PZP has a numerical aperture of 1.5 mrad and focusing efficiency of 13% for 8 keV x rays. A flux density gain of 121 000 was obtained at the first-order focus. In this article, the fabrication of the PZP and its experimental characterization are presented and some special applications are discussed.

Nanometer focusing of hard x rays by phase zone plates

High-resolution electron-beam-assisted deposition and etching is an enabling technology for current and future generation photomask repair. NaWoTec in collaboration with Carl Zeiss NTS (formerly LEO Electron Microscopy) has developed a mask repair tool capable of processing a wide variety of mask types, such as quartz binary masks, phase shift masks, extreme ultraviolet masks, and e-beam projection stencil masks. Specifications currently meet the 65nm device node requirements, and tool performance is extendible to 45nm and below. The tool combines LEO’s ultra-high-resolution Supra scanning electron microscope platform with NaWoTec’s proprietary e-beam deposition and etching technology, gas delivery system, and mask repair software. In this article, we focus on tool performance results; that is, the reproducibility and accuracy of repair of clear and opaque programmed defects on Cr binary and MoSi phase shift masks. These masks have in the past been difficult to repair due to beam position instability caus...

Electron-beam-based photomask repair

A vector scan pattern generator, optimized for smooth curvilinear as well as rectilinear primitive shapes, has been designed and constructed. The pattern generator uses high‐speed hardware to implement a set of second‐order, quadratic equations to drive digital to analog converters and high‐speed array processors to calculate the coefficients for these equations. The digital pattern generator package contains the high‐speed digital, analog, and high‐resolution analog electronics. The initial lithography results confirm the operation of the system and stepping rates of 40 MHz have been achieved.

Electron beam lithography digital pattern generator and electronics for generalized curvilinear structures

Earlier spatial‐phase‐locked e‐beam lithography (SPLEBL) was proposed as a means of eliminating the well‐known problem of feature placement precision in scanning electron‐beam lithography. In SPLEBL, a grid with long‐range spatial‐phase coherence is created on a substrate (or on top of its resist coating) and this grid is used to feedback information on beam location to the control system. In initial tests a standard deviation (σ) of 0.3 nm for phase‐locking precision in one dimension was demonstrated, which represents the finest field stitching ever obtained with any lithographic method. In two dimensions (2D), σx, σy=0.6, 0.4 nm was obtained. Moire spatial‐phase locking was also demonstrated in 2D. Two strategies for the global‐fiducial grid appear feasible: plating base modulation and a thin film of holographically exposed photoresist on thin‐film Al above the e‐beam resist. Either would permit spatial‐phase locking without exposure of resist.

Spatial‐phase‐locked electron‐beam lithography: Initial test results

In this paper, we present the test results obtained from the first commercial electron beam mask repair tool. Repaired defect sites on chrome-on-glass masks are characterized with 193nm AIMS to quantify the edge placement precision as well as optical transmission loss. The electron beam mask repair tool is essentially based on a scanning electron microscope (SEM), therefore, it can be used for in-situ CD and defect metrology. E-beam for EUV mask defect repair is also discussed. These early results are very encouraging and demonstrate the basic advantages of electron beam mask repair as well as highlight the key challenge of charge control.

Demonstration of damage-free mask repair using electron beam-induced processes

Metrology in an electron‐beam lithography system is typically carried out by a combination of beam scan and laser‐interferometer‐controlled sample motion. The high‐resolution technique presented in this paper avoids the stage motion by using a holographically produced grid, which is essentially a permanently recorded interference pattern. This grid can be at least as accurate as the interferometer, and no stage motion, with the potential for additional error sources, is required to map out the distortion in the deflection field. The quality of the grid is critical since it is the reference to which the distortion is compared. With careful control of the holographic exposure system, high‐quality low‐distortion orthogonal grids were fabricated. We have produced grids with a period of 200 nm and orthogonality of a few arc seconds using an UV laser holographic system. Once the grid is processed to produce a high‐contrast signal for either back‐scattered or transmitted electrons, both scanning and signal‐proce...

Volker Boegli

Papers

Electron-beam-based photomask repair

Electron beam lithography digital pattern generator and electronics for generalized curvilinear structures

Spatial‐phase‐locked electron‐beam lithography: Initial test results

Demonstration of damage-free mask repair using electron beam-induced processes

Metrology of electron-beam lithography systems using holographically produced reference samples