Uwe Vogler

Advanced mask aligner lithography: Fabrication of periodic patterns using pinhole array mask and Talbot effect

The Talbot effect is utilized for micro-fabrication of periodic microstructures via proximity lithography in a mask aligner. A novel illumination system, referred to as MO Exposure Optics, allows to control the effective source shape and accordingly the angular spectrum of the illumination light. Pinhole array photomasks are employed to generate periodic high-resolution diffraction patterns by means of self-imaging. They create a demagnified image of the effective source geometry in their diffraction pattern which is printed to photoresist. The proposed method comprises high flexibility and sub-micron resolution at large proximity gaps. Various periodic structures have been generated and are presented.

Advanced mask aligner lithography: new illumination system

A new illumination system for mask aligner lithography is presented. The illumination system uses two subsequent microlens-based Köhler integrators. The second Köhler integrator is located in the Fourier plane of the first. The new illumination system uncouples the illumination light from the light source and provides excellent uniformity of the light irradiance and the angular spectrum. Spatial filtering allows to freely shape the angular spectrum to minimize diffraction effects in contact and proximity lithography. Telecentric illumination and ability to precisely control the illumination light allows to introduce resolution enhancement technologies (RET) like customized illumination, optical proximity correction (OPC) and source-mask optimization (SMO) in mask aligner lithography.

Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 µm (half-pitch) at a proximity distance of 30 µm is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 µm (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 µm proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 µm was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.

Advanced mask aligner lithography (AMALITH)

Mask aligner lithography is very attractive for less-critical lithography layers and is widely used for LED, display, CMOS image sensor, micro-fluidics and MEMS manufacturing. Mask aligner lithography is also a preferred choice the semiconductor back-end for 3D-IC, TSV interconnects, advanced packaging (AdP) and wafer-level-packaging (WLP). Mask aligner lithography is a mature technique based on shadow printing and has not much changed since the 1980s. In shadow printing lithography a geometric pattern is transferred by free-space propagation from a photomask to a photosensitive layer on a wafer. The inherent simplicity of the pattern transfer offers ease of operation, low maintenance, moderate capital expenditure, high wafers-per-hour (WPH) throughput, and attractive cost-of-ownership (COO). Advanced mask aligner lithography (AMALITH) comprises different measures to improve shadow printing lithography beyond current limits. The key enabling technology for AMALITH is a novel light integrator systems, referred to as MO Exposure Optics® (MOEO). MOEO allows to fully control and shape the properties of the illumination light in a mask aligner. Full control is the base for accurate simulation and optimization of the shadow printing process (computational lithography). Now photolithography enhancement techniques like customized illumination, optical proximity correction (OPC), phase masks (AAPSM), half-tone lithography and Talbot lithography could be used in mask aligner lithography. We summarize the recent progress in advanced mask aligner lithography (AMALITH) and discuss possible measures to further improve shadow printing lithography.

Customized illumination for process window optimization and yield improvement in mask aligner lithography systems

A novel illumination system for mask aligners will be described. The illumination system provides improved exposure light uniformity and customized illumination. It is based on two consecutive Kohler integrators and an exchangeable illumination filter plate. It allows a free choice of illumination settings, e.g., ring-illumination, quadrupole, multipole, Maltese cross, and free-forms in a standard mask aligner. The so-called micro optical exposure optics significantly increases the field of applications for mask aligners. The well defined illumination allows optical proximity correction (OPC) of the mask pattern to compensate for the remaining image errors due to diffraction or process effects. Customized illumination and OPC-like structures introduce well-known tools of projection lithography for mask aligners for the first time. Lithography process simulations as well as experimental results will be shown.

Intensity and phase fields behind Phase Shifting Masks studied with High Resolution Interference Microscopy

The proximity printing industry is in real need of high resolution results and it can be done using Phase Shift Mask (PSM) or by applying Optical Proximity Correction (OPC). In our research we are trying to find out details of how light fields behind the structures of photo masks develop in order to determine the best conditions and designs for proximity printing. We focus here on parameters that are used in real situation with gaps up to 50 μm and structure sizes down to 2 μm. The light field evolution behind the structures is studied and delivers insight in to precisions and tolerances that need to be respected. It is the first time that an experimental analysis of light propagation through mask is presented in detail, which includes information on intensity and phase. The instrument we use is known as High Resolution Interference Microscopy (HRIM). HRIM is a Mach-Zehnder interferometer which is capable of recording three dimensional distributions of intensity and phase with diffraction limited resolution. Our characterization technique allows plotting the evolution of the desired light field and therefore printable structure till the desired proximity gap. In this paper we discuss in detail the evolution of intensity and phase fields of elbow or corner structure at different position behind a phase mask and interpret the main parameters. Of particular interest are tolerances against proximity gap variation and the resolution in printed structures.

Micro-optics and lithography simulation are key enabling technologies for shadow printing lithography in mask aligners

Abstract Mask aligners are lithographic tools used to transfer a pattern of microstructures by shadow printing lithography onto a planar wafer. Contact lithography allows us to print large mask fields with sub-micron resolution, but requires frequent mask cleaning. Thus, contact lithography is used for small series of wafer production. Proximity lithography, where the mask is located at a distance of typically 30–100 μm above the wafer, provides a resolution of approximately 3–5 μm, limited by diffraction effects. Proximity lithography in mask aligners is a very cost-efficient method widely used in semiconductor, packaging and MEMS manufacturing industry for high-volume production. Micro-optics plays a key role in improving the performance of shadow printing lithography in mask aligners. Refractive or diffractive micro-optics allows us to efficiently collect the light from the light source and to precisely shape the illumination light (customized illumination). Optical proximity correction and phase shift mask technology allow us to influence the diffraction effects in the aerial image and to enhance resolution and critical dimension. The paper describes the status and future trends of shadow printing lithography in mask aligners and the decisive role of micro-optics as key enabling technology.

Mask aligner process enhancement by spatial filtering

Mask Aligners are used in the Semiconductor Industry to transfer structures with moderate resolution requirements onto substrates. With the casting of the shadow a photochemical reactive resist is exposed. As diffraction appears at the mask structures the exposure wavelength and the proximity gap between mask and wafer influence the quality of the image in the resist. As both parameters are very often not changeable for processes there is a big need to find another way to improve the resist image. In this paper a new approach to enhance the exposure result will be presented. MO Exposure Optics, a novel illumination system for Mask Aligners, uses a combination of two microlens Köhler Integrators. MO Exposure Optics decouples the illumination system in a Mask Aligner from the lamp and ensures a uniform angular spectrum over the whole mask plane. Spatial filtering of the illumination light allows to reduce the diffraction effects at the mask structures and to improve the lithographic process in a Mask Aligner.

Numerical optimization of illumination and mask layout for the enlargement of process windows and for the control of photoresist profiles in proximity printing

Although proximity printing is the oldest and, in view of the basic optical setup, simplest photolithographic technique, it still remains in heavy use in the semiconductor manufacturing industry. The fact that this technique exists for a long time does not mean that there is no more room for improvements or new applications. Lending concepts developed for modern projection scanners and steppers and adapting them for our purposes, we demonstrate how numerical simulation and optimization can help to make the proximity printing process more stable against process variations and to increase the resolution for critical features. For this purpose, we numerically optimize the angular spectrum of the illumination and the mask layout. Furthermore, we couple the optimization of the optical degrees of freedom to the simulation of photoresist development to assess the effects of changes to the illumination and mask on the final photoresist profile.

Simulation tools for advanced mask aligner lithography

Contact- and proximity lithography in a Mask Aligner is a very cost effective technique for photolithography, as it provides a high throughput and very stable mature processes for critical dimensions of typically some microns. For shadow lithography, the printing quality depends much on the proximity gap and the properties of the illumination light. SUSS MicroOptics has recently introduced a novel illumination optics, referred as MO Exposure Optics, for all SUSS MicroTec Mask Aligners. MO Exposure Optics provides excellent uniformity of the illumination light, telecentric illumination and a full freedom to shape the angular spectrum of the mask illuminating light. This allows to simulate and optimize photolithography processes in a Mask Aligner from the light source to the final pattern in photoresist. The commercially available software LayoutLab (GenISys) allows to optimize Mask Aligner Lithography beyond its current limits, by both shaping the illumination light (Customized Illumination) and optimizing the photomask pattern (Optical Proximity Correction, OPC). Dr.LiTHO, a second simulation tool developed by Fraunhofer IISB fro Front-End Lithography, includes rigorous models and algorithms for the simulation, evaluation and optimization of lithographic processes. A new exposure module in the Dr.LiTHO software now allows a more flexible definition of illumination geometries coupled to the standard resist modules for proximity lithography in a Mask Aligner. Results from simulation and experiment will be presented.