Oblique lighting
Oblique illumination ( English off-axis illumination ) - more rarely called oblique exposure or off-axis exposure - describes an advanced exposure process in photolithographic structuring in semiconductor technology . It offers the possibility of improving the resolving power and enlarging the depth of focus for certain structures without changing the numerical aperture (e.g. immersion lithography ) or the wavelength used . A comparable technology has long been known for improving the contrast in optical microscopes and was introduced in 1989 for the field of semiconductor technology and integrated into their photolithography systems by Canon and Nikon in the early 1990s .
The oblique illumination belongs to the group of resolution enhancement techniques (RET), which are used for structure sizes below the light wavelength (193 nm). These also included Optical proximity correction , phase masks ( English phase-shifting mask ) and multiple patterning ( English double patterning ).
functionality
Conventional lighting
In photolithographic structuring, a local photochemical reaction is triggered in a photosensitive lacquer layer ( photoresist ) with the help of light, thereby locally changing the properties (especially the solubility ) of the photoresist. The local exposure is done by shading light with the help of a photo mask, which contains the desired pattern of the structuring, so that some areas are illuminated and others are not. In this manner, a masking photoresist layer remains on the substrate after removing the soluble areas (usually a wafer from silicon ) to the located on the photomask structure. This relatively simple description of the imaging of structures located on the photomask, however, only applies to structures that are larger than the wavelength of the light used. If the structures to be imaged on the photomask are in the wavelength range, as has been the case for several years in semiconductor technology, the wave character of the light can no longer be neglected, especially diffraction effects on small periodic structures have a decisive influence on the imaging of the structures in the Photoresist. In the following, it is therefore described in a simplified manner why the image becomes increasingly poor in the case of very dense structures and how an improvement in the image can be achieved through oblique lighting.
The illumination is usually perpendicular to the mask, that is to say parallel to the axis of the optical system , often referred to as Koehler illumination (after August Koehler ). The 0th order of diffraction spreads further in the direction of the direction of incidence in the optical system, while the other orders are bent laterally. Since the deflection angle for higher orders increases as the structure sizes become smaller, only the 0th order reaches the objective lens and thus the photoresist layer in the case of very small structures. However, the 0th order of diffraction only contains information about the light source, which means that the structure of the photoresist mask is not imaged in the photoresist layer. For a successful imaging, at least one further diffraction order is therefore necessary and in general the image quality increases with the number of diffraction orders, the influence on the quality quickly decreasing with higher diffraction orders. In order to achieve the highest possible resolution and nevertheless a sufficiently high image quality, it is sufficient that the 0, +1. and −1. Order of diffraction reach the objective lens ( 3-beam illumination ).
Oblique lighting
The so-called monopole oblique lighting, in which the light strikes the mask at an angle, brings an improvement. In this way, the lateral position of the diffraction orders is also shifted, so that not only the 0th order of diffraction falls on the objective lens, even with denser structures. An optimal image is obtained when the rays of the 0th and one of the 1st order (+1 or −1) intersect the optical axis at the same angle and are captured by the objective lens ( 2-beam illumination ). The result of this arrangement would be an improvement in the resolving power (highest possible value) for periodic structures perpendicular to the plane of incidence of the light. Structures parallel to the plane of incidence, however, experience no improvement, a clear disadvantage in practical use. Furthermore, with this arrangement the large energy loss due to the non-detection of the other 1st order of diffraction and the fact that the angle of incidence has to be readjusted for each structure size are disadvantageous.
The increased effort involved in the technical implementation (complete redesign of the existing systems) and the inflexible application possibilities of a "tilted optical system" have led to the practical implementation of angled lighting in a different way. The starting point is an almost unchanged structure of the lithography systems, which was essentially only supplemented by a specially shaped aperture between the condenser system and the photo mask. A characteristic of this diaphragm is that it shades the region in the area of the optical axis and thus prevents perpendicular incidence on the structures. As a result of this change, light only reaches the mask from the area of the central radius or the edge, where it falls obliquely onto the mask with a circular cone-like angular distribution due to the circularly shaped light source used. As in the case of the above-described illumination with an oblique angle of incidence, this has the effect that all diffraction orders are tilted. The system is now tuned in such a way that only the 0th order and one of the two 1st orders of the incident radiation marginal rays are captured by the objective lens, fall on the photosensitive layer ( photoresist ) and thus create a better contrast in the photoresist.
The German term oblique illumination (and also the English term off-axis illumination ) is therefore somewhat misleading, especially since components parallel and oblique to the axis are also used in conventional, partially coherent illumination. The term "oblique lighting" therefore only refers to techniques after 1992 in which no components are used parallel to the axis.
Types of lighting source distributions
Frequently used lighting source distributions are:
- Dipole lighting (x- or y-axis), suitable for patterns in which all lines are parallel to the dipole arrangement.
- Quadrupole illumination ( quadrupole illumination , quad), suitable for patterns in which all lines are oriented in the x or y direction, but not for oblique lines (e.g. 45 °)
- Kreuzquadrupolbeleuchtung (engl. Cross quadrupole illumination , c-quad)
- Ring illumination improves the resolution of structures regardless of the orientation. However, this improvement is less than with specialized distributions and is also at the expense of a smaller depth of field.
In addition to these basic shapes, numerous combinations of the basic shapes (mostly with small proportions of the conventional perforated diaphragm) or shapes with changes in detail (mostly size) were presented, which are advantageous for special structures. In addition, each system operator offers its own lighting distributions, which are often based on quadrupole lighting. This includes the quadrupole segmented annular ring (QUASAR) from ASML , the CQuest ( Canon quadrupole effect for stepper technology ) from Canon and SHRINC ( super high-resolution illumination control ) from Nikon.
In the last few years, research has also been carried out on more complex shapes, for example hexapole lighting or " free form illumination " (FlexRay) from ASML.
Advantages and disadvantages
A major advantage of this resolution-improving technology is that it can be integrated relatively easily into existing lighting systems and that there are hardly any problems in making all of the above-mentioned exposure source distributions and conventional lighting available on a system. Therefore, a quick and cost-efficient implementation of the otherwise very expensive exposure systems (several million euros) was possible. This is important because the oblique illumination does not allow a general improvement in the resolution.
The improvement of the resolution through the oblique illumination is very much dependent on the position and size of the structures to be imaged and requires separate optimization for each structure. This is especially important to mention because the structures on a photomask are generally very diverse in terms of their size, periodicity and alignment. The use of oblique lighting is closely tied to the close pitch of the periodic structures for which it was optimized. A greater distance can even produce a poorer image than would be possible with conventional lighting. A transfer of the exposure settings to other structure sizes is therefore usually not possible without problems. This also applies to patterns with structures that are not located in the optimized axis, for example structures running obliquely to the xy axis in the case of cross quadrupole illumination.
In general, only periodic structures with a certain distance are optimally reproduced, whereas isolated lines do not experience any improvement in resolution. The reason for this is that isolated lines do not produce discrete diffraction orders, but only continuous diffraction patterns. The different effect of the lighting on isolated and dense lines leads to a systematic deviation in the resolution of the two types, which must be taken into account during development. One way to reduce this deviation is to add additional structures with sizes below the resolution limit close to the isolated structure.
In terms of the usable energy of the light source, oblique lighting has disadvantages compared to 3-beam lighting. Because when one of the rays of the 1st order of diffraction is deflected, part of the exposure energy is lost and must be compensated for by longer exposure times.
literature
- Stanley Wolf: Silicon Processing for the VLSI Era - Volume 4 Deep-Submicron Process Technology . Lattice Press, 2002, ISBN 0-9616721-7-X , pp. 275-279 .
- Burn J. Lin: Optical Lithography: Here Is Why . SPIE Society of Photo-Optical Instrumentation Engineering, 2009, ISBN 978-0-8194-7560-2 , pp. 268-287 .
Web links
- Optimize source: Illumination Filter Plates . SUSS MicroOptics (schematic illustration of a lithography system and illustration of various exposure distribution apertures).
Individual evidence
- ^ CA Mack: Understanding focus effects in submicron optical lithography: A Review . In: Optical Engineering . tape 32 , no. 10 , 1993, p. 2350-2362 , doi : 10.1117 / 12.968408 .
- ^ Stanley Wolf: Silicon Processing for the VLSI Era Volume 4 Deep-Submicron Process Technology . Lattice Press, 2002, ISBN 0-9616721-7-X , pp. 275-279 .
- ↑ Dietrich Widmann, Hermann Mader, Hans Friedrich: Technology of highly integrated circuits . Springer, 1996, ISBN 978-3-540-59357-7 , pp. 127-138 .
- ↑ Miyoko Noguchi: Subhalf-micron lithography system with phase-shifting effect . In: Proceedings of SPIE . San Jose, CA, USA 1992, p. 92-104 , doi : 10.1117 / 12.130312 .
- ↑ ASML fulfills “holistic litho” plan with two tools, custom packages . In: Solid State Technology. 52, No. 9, 2009.