Off-axis illumination

Called less often inclined exposure or off-axis exposure - - oblique illumination (English off-axis illumination ) refers to semiconductor technology an advanced exposure techniques in photolithographic patterning. It offers the possibility of improving the resolution and increasing the depth of focus in certain structures without changing the numerical aperture (eg immersion lithography ) or the wavelength used. A similar technique has been known for a long time to improve the contrast in optical microscopes and in 1989 presented to the field of semiconductor technology and integrated in the early 1990s for the company Canon and Nikon in their photolithography systems. It is in addition to the use of phase masks (English phase -shifting mask ) are currently (2011), the preferred technique for the production of structures below 40 nm in the industrial mass production.

Operation

Conventional lighting

Wherein photolithographic structuring a local photochemical reaction ( photoresist ) (especially the solubility) of the photoresist is by using light in a photosensitive resist layer is triggered and thus the properties of locally changed. The local exposure is carried out by the shading of light using a photomask containing the desired pattern of structuring, so some places lit and others are not illuminated. In this way remains after removing the soluble areas of the photoresist masking layer on the substrate (usually a silicon wafer ) having the structure present on the photomask. However, this relatively simple description of the figure of the photo mask structures located on only applies to structures, which are larger than the wavelength of light used. If the structures to be imaged on the photomask in the range of the wavelength, as it has for some years is the case in semiconductor technology, the wave nature of light can not be neglected, especially diffraction effects at small periodic structures have a decisive influence on the image of the structures in the photoresist. It will therefore be simply described why in very dense structures the picture becomes progressively worse and how to improve the image can be achieved by the oblique illumination.

Typically, the illumination is perpendicular to the mask, that is, parallel to the axis of the optical system, often referred to as Kohler illumination ( after August Köhler ) respectively. Here, the 0th diffraction order propagates in the direction of the direction of incidence on the optical system, while the other orders are bent sideways. Since the deflection angle increases for higher orders with smaller feature sizes, only the 0th order arrives with very small structures through the mask to the objective lens and thus the photoresist layer. However, the 0 th diffraction order contains only information about the light source, resulting in that the structure of the photoresist mask is not shown in the photoresist layer. For a successful image, therefore, at least one other diffraction order are necessary and generally takes the picture quality with the number of diffraction orders, the impact on the quality of higher diffraction orders is quickly reduced. In order to obtain the highest possible resolving power and yet a sufficiently high image quality, it is sufficient that the 0, 1. and -1. Diffraction order to achieve the objective lens ( 3-beam illumination ).

Oblique illumination

An improvement is provided by the so-called monopole oblique illumination, in which the light is obliquely incident on the mask. In this way, also the lateral position of the diffraction orders is shifted, so that even with denser structure, not only the 0 -order diffraction is incident on the objective lens. An optimum image is obtained when the beams of the 0th and the 1st order ( 1 or -1) in the same angles and intersects the optical axis are detected by the objective lens ( 2 jet illumination). The result of this arrangement would be an improvement in the resolving power (highest possible value ) for periodic structures perpendicular to the planes of incidence of the light. Structures parallel to the plane of incidence, however, experiences no improvement, a clear disadvantage in the practical use. Also in this arrangement is the large energy loss by the non-detection of the other first diffraction order and the fact that the angle of incidence must be reset for each structure size, adversely.

The increased effort in the technical implementation ( complete redesign of existing facilities ) and the inflexible application possibilities of a " tilted optical system " have led to the implementation of an oblique illumination takes place on a different path. Starting point is an almost unchanged structure of the lithography equipment, which was supplemented only by a specially shaped aperture between the condenser and the photomask substantially. Is characteristic of the panel that it shades the region in the area of ​​the optical axis and thus prevents the perpendicular incidence of the structures. This amendment Light enters only from the region of the middle radius or the edge of the mask where it is obliquely incident circularly shaped by the used light source with a circular cone-shaped angular distribution on the mask. This has the effect, as in the above-described light oblique incident angle that all the diffraction orders are tilted. The system is now adjusted so that only the 0th order and one of the first orders of the incident radiation boundary rays are captured by the objective lens onto the photosensitive layer ( photoresist ) fall and thus produce a better contrast in photoresist.

The German term oblique illumination (and also the English term off-axis illumination ) is therefore somewhat misleading, especially as in the conventional partially coherent illumination components also are used in parallel and oblique to the axis. The term " oblique illumination " therefore refers exclusively to techniques after 1992 in which no components are used in parallel to the axis.

Types of lighting source distributions

Lighting source distributions are frequently used are:

In addition to these basic forms numerous combinations of the basic shapes were still (usually with small amounts of conventional pinhole ) (usually size) presented or forms with detail changes that are advantageous for specific structures. In addition, each system controller provides its own illumination distributions, which are often based on the quadrupole. This includes the quadrupole ring segment lighting (English quadrupole annularly segmented ring, QUASAR ) of ASML, the cquest ( Canon quadrupole effect for stepper technology) from Canon and SHRINC ( super high-resolution illumination control) from Nikon.

In recent years, research has for more complex shapes, such as Hexapolbeleuchtung or " freeform illumination" ( FlexRay) of ASML.

Pros and Cons

A major advantage of this resolution -enhancing technique is that it was relatively easy to integrate into existing imaging systems and it also hardly trouble to make all the exposure source distributions mentioned above as well as conventional lighting available to a plant. Therefore, a fast and cost- efficient implementation of the otherwise very expensive exposure systems (several million euros ) was possible. This is important because the oblique illumination allows no general improvement in the resolving power.

The improvement of the resolving power by oblique illumination is highly dependent on the location and size of the structures to be imaged and required for each structure a separate optimization. This is especially important, therefore, to say, since the structures are very diverse in terms of size, frequency and orientation on a photomask in general. Thus, the use of oblique illumination close to the dense spacing (English pitch) is the periodic structures bound, for which it was optimized. In this case, a greater distance even a worse figure than cause it would be feasible with a conventional lighting. A transfer of the exposure settings on different structure sizes is therefore not easily possible in general. This also applies to patterns with not being optimized axle located structures, such as an angle to the xy axis extending structures in Kreuzquadrupolbeleuchtung.

Generally experienced only periodic structures with a certain distance optimal imaging, no improvement in the resolving power, however, find isolated lines. The reason for this is that isolated lines do not produce discrete diffraction orders but only continuous diffraction pattern. The different effects of lighting on isolated and dense lines leads to a systematic deviation in the resolution of the two types that must be considered in the development. One way to reduce this deviation, is adding additional structures with sizes below the resolution limit near the isolated structure.

Also with respect to the usable energy from the light source has the oblique illumination disadvantages compared to the 3-beam illumination. Because by the deflection of a beam of the first diffraction order is a part of the exposure energy is lost and must be compensated by longer exposure times.