Gerhard Stengl

Ion Beam Lithography

Abstract Ion beam lithography (IBL) is still in the hands of researchers. The immediate aim is to investigate the production of micro-circuits with 0.1 μm line spacings in commercial prototypes. However such instruments can also be used for mask repair. The present paper, intended as an introduction to the more specialised contributions to IBL in these Proceedings, concentrates on basic physical principles. While ion sources of different types are covered, particular attention is paid to liquid metal ion sources, in view of their popularity and potential for a variety of applications in focused ion beam technology.Apparatus and method for projection ion beam lithography are described which allow formation of low distortion, large field, reduced images of a mask pattern at a wafer plane using an optical column of practical size. The column shown is comprised of an accelerating Einzel lens followed by a gap lens, with numerous cooperating features. By coordinated selection of the parameters of the optical column, lens distortion and chromatic blurring are simultaneously minimized. Real time measurement of the position of the image field with respect to the existing pattern on the wafer is employed before and during the time of exposure of the new field and means are provided to match the new field to the existing pattern even when the latter has been distorted by processing. A metrology system enables convenient calibration and adjustment of the apparatus.

Ion projection lithography: International development program

Ion projection lithography (IPL) has demonstrated not only the resolution required for next-generation lithography (50 nm resolution at >4:1 aspect ratio) [Bruenger et al., Microelectron Eng. 46, 477 (1999)] but also cost advantages with respect to other competing technologies [Gross et al., J. Vac. Sci. Technol. B 16, 3150 (1998)]. This article reports on the progress of a worldwide development program, with the target to manufacture a process development tool and create the necessary mask infrastructure to demonstrate that IPL is a viable industrial lithography technology for the future. An overview of papers, reporting on the progress in critical areas, is given and new, experimentally validated, simulations of complementary mask stitching are shown for the first time. Longitudinal and lateral offsets of up to 32 nm for 100 nm critical dimensions are possible with linewidth variations less than 11 nm. Our concept for beta tools, based on a powerful new stitcher strategy, is described. This will lead to...

Ion projection lithography machine IPLM‐01: A new tool for sub‐0.5‐micron modification of materials

An ion projection lithography machine uses ions as the information carrier in a demagnifying step‐and‐repeat exposure system. With an IPLM sub‐0.2‐micron resolution can be obtained combined with an extremely high depth of focus—more than 100 μm. The use of a reliable and stable duoplasmatron ion source with high brightness and high angular current density is significant for this machine. A preprojective lens octupole permits an electrostatic shift of the ion image in x and y directions, and a solenoid at this site enables a rotation of the ion image through the action of the axial magnetic field. Furthermore, the scale of the projected ion image can be adjusted electronically within ±3%. Thus fine adjustment for die‐by‐die alignment can be done without mechanical movements. An ion projection lithography machine IPLM‐01 with a total height of 2.5 m and a floor space of 2 m2 has been built. By inserting a mask consisting of a grid of pinholes an ion multibeam scanning tool (IMBS) is generated. Using demagni...

Ion projection lithography: Status of the MEDEA project and United States/European cooperation

Structure and targets of the European MEDEA project on ion projection lithography as well as related U.S./European cooperation are explained. By assuming 10 μm virtual source size and 1 eV (full width half maximum) energy spread calculations for a multielectrode electrostatic ion–optical system (1.25 m between ion source and stencil mask, ≈1.8 m between mask and wafer) we realize the possibility of 100 nm resolution (line and space) over an exposure field of 22×22 mm2 even when using the MONTEC model for calculating the stochastic blur and when running 3.3 μA He+ ion beam current through the ion–optical column, thus more than twice exceeding target specifications. Thus, for 100 nm resolution and 50% pattern density the raw throughput is ≈12 cm2/s corresponding to >75 WPH (pattern within 80% of 300 mm wafer area).

Ion projection lithography for sub-micron modification of materials

Ion projection lithography (IPL) uses demagnifying ion-optics for reduction printing of open stencil masks at 5 x or 10 x scale. A feasibility study with a research type Ion projection lithography machine (IPLM-01) demonstrates sub-0.1 Jim resolution combined with a high depth of focus and the possibility of an electronic alignment of the projected ion image in X, Y, rotation and scale and an electronic adjustment of intrafield distortion. The IPL technique combines lithography with direct submicron technique for ion beam modification of materials.

Characterization of a process development tool for ion projection lithography

This article describes the performance of a process development tool for ion projection lithography (IPL), realized as part of the MEDEA program of the European Union. This system was designed for a 12.5×12.5 mm2 exposure field and 4:1 reduction ratio between the mask and the wafer. The design incorporates several novel concepts, including a negative electrostatic lens at the mask to reduce distortion and field-composable lenses to provide electronic fine alignment of the system. Continuous control of magnification, position offset, distortion, and telecentricity is provided by a real-time feedback system (pattern lock) that monitors the position of reference beamlets traveling in parallel with the integrated circuit image through the ion-optical system. After mechanically aligning the center and tilt of the lenses relative to the optical axis to within 10 μm and 50 μrad, respectively, we achieved 100 nm resolution over the full design field, with 75 nm resolution in local areas within the field. The redu...

Control of temperature gradients and distortion of ion projection lithography masks

In ion projection lithography a stencil mask is back ‘‘illuminated’’ by a broad beam of ions, and its demagnified image (∼3×) is projected onto a resist covered substrate. The stencil mask that will be used in the next generation systems consists of a 120‐mm‐diam 2.5‐μm‐thick single crystal silicon membrane etched in a 150‐mm‐diam wafer whose edge acts as part of a supporting frame. The main sources of distortion are thought to be stress relief due to the cutting of holes and the thermal expansion due to mask heating by the incident ions. In this paper we calculate the temperature distribution in the mask for the expected heat input by the beam, the resulting mask distortion, and the effect of optimized radiation cooling. The central 100‐mm‐diam area of an unpatterned membrane is assumed to be irradiated with an energy input of 3.3 mW/cm. The edge of the membrane at the frame is assumed to be at room temperature and held rigidly. Heat is lost by radiation and by conduction. An effective method of reducing...

Ion projection lithography in (in)organic resist layers

Abstract Ion Projection Lithography (IPL) was applied for structuring organic and inorganic resists. To prove the feasibility of Ion Projection Lithography for printing submicron features the pattern transfer characteristics was determined in PMMA and SiO 2 resist layers. Furthermore, Response Surface Methodology (RSM) was used to evaluate the dependence of the obtained results on various process parameters. By application of RSM the machine setup was optimized and process latitudes were established.

Analysis of stencil mask distortion in ion projection lithography

Abstract Accurate feature placement on the wafer in ion projection lithography requires that the distortion due to stress relief produced by the pattern of holes in the mask be controlled. We have used the finite element method provided by the ANSYS software package to calculate the two dimensional mask distortion due to non-symmetrical patterns of openings in stencil masks and to analyze a method for controlling distortion. The ion projection lithography mask is a circular silicon membrane 120 mm in diameter 2.5 μm thick rigidly held around the perimeter. The central 60 × 60 mm square area will contain the pattern to be printed, demagnified by 3x. We take the intrinsic stress of the membrane to be 10 MPa, the Youngs modulus of the membrane to be E = 1.5 × 1011 Pa and the Poisson ratio ν = 0.17. We have explored the x-y distortion produced by various simple geometries. For example, consider an asymmetric grid of square holes which is 22% open on one half and 10% open on the other half. High distortion in this case will occur along the diameter, between the two halves. The boundary between the two halves (along the diameter) is bowed by 182 nm. A method for reducing the distortion is to cut a pair perforation rings along the perimeter. The perforation rings will have the effect that the central part of the membrane will have the stress relieved and it will be held in effect by “springs” at a constant, low stress level. We have calculated that the bow of the diameter can be reduced to 18 nm by a suitable geometry of the perforations. The perforation ring is an effective means of reducing distortion to well below an acceptable level. The uniform shrinkage of the pattern due to stress relief is simply a small change in magnification which is automatically corrected by the beam lock in the ion optical column.

Distortion analysis of stencil masks with stress‐relief structures

We present an exact solution to an axially symmetric continuum model of distortion in stencil masks. A correction procedure is studied where the pattern displacement vectors are calculated from a linear approximation to the pattern distribution function. For practical mask patterns this can reduce distortion to near the levels needed for very large scale integration. Additional gains can be achieved by using a ring of perforations around the integrated circuit field to reduce stress. Corrected distortion figures below 20 nm on a 60 mm mask seem possible.