Ion-beam Projection Lithography Research Articles

Ion-beam induced fcc-bcc transition in ultrathin Fe films for ferromagnetic patterning

Ar + ion irradiation is used to induce a structural change from fcc to bcc in a 1.5nm thick Fe film epitaxially grown on a Cu(100) crystal. Scanning tunneling microscopy and low-energy electron diffraction show the nucleation of bcc nanocrystals, which grow with increasing ion dose. As a consequence of the structural change, the irradiated iron film becomes strongly ferromagnetic at room temperature. We present a model for the process of the transformation and demonstrate writing a magnetic pattern at the 100nm scale by ion-beam projection lithography.

Research trends

Sophisticated dry-etching techniques using inhibitor films as sidewall protection have been applied to realize holes in silicon in the micron size and depths of the order of 10–15 μm. These holes are fabricated in silicon membranes which were produced by wet-etching with KOH. These structures are proposed as stress-free stencil hole masks for demagnifying electron- and ion-beam projection lithography. They show the good thermal conductivity and high mechanical stability of bulk silicon. Besides this, these techniques and structures may find wide application such as masks for deposition methods and MBE, quantitative mechanical filters for chemistry, biology, medicine, and micromechanics.

Simulation of latent image formation for ion beam projection lithography

For proper exposure of ion sensitive polymeric resists of thickness 400–500 nm light ions (H +, He +, Li +, Be + and B +) with energies in the range 60–100 kV are necessary. Characteristic data concerning the penetration of these ions in polymer resists (transverse and lateral ranges, their dispersions, electronic and nuclear stopping powers etc.) calculated using the code TRIM are presented and discussed. Indications for the predominant role of the electronic losses under these conditions are used. Issues related to the use of chemically amplified resists (CAR’s) for ion submicron micro structuring of integrated circuits are discussed. Some requirements and peculiarities of the mask needed for a projection ion lithography system, defined from the point of view of defect generation during its production and exploitation are also shortly discussed.

In-Plane Gravity Loading of a Circular Membrane

This paper develops the displacement field for a circular membrane which is statically loaded by gravity acting in its plane. Coupled to the displacements are the stress and strain distributions. The solution is applicable to the modeling of next generation lithographic masks, ion-beam projection lithography masks in particular. [S0021-8936(00)00803-5]

Directly sputtered stress-compensated carbon protective layer for silicon stencil masks

Silicon stencil masks for ion beam projection lithography have a protective layer stopping the ions and thus preventing a change in the Si membrane stress. This is needed to maintain extremely tight pattern placement specifications even when they are irradiated with high exposure doses. The fabrication of carbon protective layers by indirect sputter coating which are suitable for helium ion beam exposure has already been reported. This article describes a method of forming very low stress carbon protective layers based on direct radio frequency sputter coating with nitrogen added to the argon sputter gas and in situ thermal treatment using commercially available equipment. The carbon layers thus produced are stable in conventional environments. The article deals also with the physical characterization of carbon layers and the protection performances of these coatings under helium ion beam exposure using accelerated lifetime testing.

Finite element simulation of ion-beam lithography mask fabrication

Fabrication of low distortion stencil masks is one of the key issues for ion-beam projection lithography (IPL). Identifying the sources of distortion and optimizing the processing techniques are essentical to meet the stringent error budgets. Since experiments are time consuming and expensive, it is desirable to develop accurate simulation procedures which can be used for analysis and design. This paper presents results to show that finite element analysis is a viable alternative.

Stress relief structures for ion-beam projection lithography masks

Next Generation Lithography (NGL) will require masks with high resolution and positioning accuracy to meet the requirements for sub-0.13 @mm technology. This paper focuses on the design and development of low distortion stencil masks for ion-beam projection lithography. With the stencil mask structure, one source of pattern placement errors is the in-plane distortion which occurs during the pattern transfer process. To address this issue, the use of stress relief structures to minimize distortion within the pattern area has been proposed. Results of the mechanical analysis, design, and optimization of these structures are presented.

Stencil mask distortion control using nonsymmetric perforation rings

For ion beam projection lithography to be successful in the 0.13 μm regime, it is essential to understand and control the sources of distortion in the stencil mask. This paper reports on the use of circumferential perforation rings to reduce the initial membrane prestress and subsequent pattern distortions. Using finite element methods, optimization studies have been performed to identify the necessary design parameters. The use of nonsymmetric perforations rings to correct for differences in membrane stiffness due to anisotropic pattern features is demonstrated. Pattern perforations are also shown to alter vibration mode shapes and frequencies, influencing membrane dynamic response.

Multicusp sources for ion beam projection lithography

Multicusp ion sources are capable of producing positive and negative ions with good beam quality and low energy spread. The ion energy spread of multicusp sources has been measured by three different techniques. The axial ion energy spread has been reduced by introducing a magnetic filter inside the multicusp source chamber which adjusts the plasma potential distribution. The axial energy spread is further reduced by optimizing the source configuration. Values as low as 0.8 eV have been achieved.

Stochastic Coulomb interactions in ion projection lithography systems with aberration-broadened crossover

The allowable current in ion beam projection lithography is limited by stochastic Coulomb interactions. Theories for the dependence of these interactions on the parameters of the system, i.e., length, beam energy, print size, crossover diameter, etc. are thus important to guide the design of future machines. All existing theories on blur from stochastic Coulomb interaction assume a homogeneous current distribution in the crossover, where a large part of the blur is produced. However, in actual lithography machines, the crossover may be substantially broadened by the spherical aberration of the ion lenses, thus giving a very inhomogeneous current distribution in the crossover. In most analytical theories, the stochastic blur is independent of the radius of the homogeneously field crossover rc for very small crossovers, and relates to rc as (1/rc)1/2 or (1/rc)1/3 for larger ones. If aberration broadening could do the same, the effect of broadening the crossover from rc=1.5 to 50 μm would decrease the blur by a factor as much as 6, without the negative effect of increasing the curvature-of-field blur. We find physical arguments that such a large factor is impossible. Monte Carlo simulations first of all show that the blur reduction is larger near the edges of the image field than in the center, near the optical axis. For the simulated model system, the reduction on axis can be increased to a factor 2 at very large aberration coefficients.

Fabrication of 15 μm thick Si-hole masks for demagnifying projection systems for ion- or electron-beams

Sophisticated dry-etching techniques using inhibitor films as sidewall protection have been applied to realize holes in silicon in the micron size and depths of the order of 10–15 μm. These holes are fabricated in silicon membranes which were produced by wet-etching with KOH. These structures are proposed as stress-free stencil hole masks for demagnifying electron- and ion-beam projection lithography. They show the good thermal conductivity and high mechanical stability of bulk silicon. Besides this, these techniques and structures may find wide application such as masks for deposition methods and MBE, quantitative mechanical filters for chemistry, biology, medicine, and micromechanics.