Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-08-31
2001-09-04
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000, C250S492200, C250S492100, C430S005000, C430S395000, C355S067000
Reexamination Certificate
active
06285488
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to printing of patterns with extremely high precision on photosensitive surfaces, such as photomasks for semiconductor devices and displays. It also relates to direct writing of semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. Furthermore, it can have applications to other types of precision printing such as security printing. The term printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is not limited to mean visible light, but a wide range of wavelengths from infrared (IR) to extreme UV. Of special importance is the ultraviolet range from 370 nm (UV) through deep ultraviolet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) down to a few nanometers wavelength. EUV is in this application defined as the range from 100 nm and down as far as the radiation is possible to treat as light. A typical wavelength for EUV is 13 nm. IR is defined as 780 nm up to about 20 &mgr;m.
In a different sense the invention relates to the art and science of spatial light modulators and projection displays and printers using such modulators. In particular it improves the grey-scale properties, the image stability through focus and image uniformity and the data processing for such modulators by application of analog modulation technique. The most important use of the analog modulation is to generate an image in a high-contrast material such as photoresist with an address grid, i.e. the increment by which the position of an edge in the pattern is specified, that is much finer than the grid created by the pixels of the spatial light modulator.
BACKGROUND OF THE INVENTION
It is known in the current art to build precision pattern generators using projection of micromirror spatial light modulators (SLMs) of the micromirror type (Nelson 1988, Kück 1990). To use an SLM in a pattern generator has a number of advantages compared to the more wide-spread method of using scanning laser spots: the SLM is a massively parallel device and the number of pixels that can be written per second is extremely high. The optical system is also simpler in the sense that the illumination of the SLM is non-critical, while in a laser scanner the entire beam path has to be built with high precision. Compared to some types of scanners, in particular electrooptic and acoustooptic ones, the micromirror SLM can be used at shorter wavelengths since it is a purely reflective device.
In both references cited above the spatial modulator uses only on-off modulation at each pixel. The input data is converted to a pixel map with one bit depth, i.e. with the values 0 and 1 in each pixel. The conversion can be done effectively using graphic processors or custom logic with area fill instructions.
In a previous application by the same inventor Sandström (Sandström et. al. 1990), the ability to use an intermediate exposure value at the boundary of a pattern element to fine-adjust the position of the element's edge in the image created by a laser scanner was described.
It is also known in the art to create a grey-scale image, preferably for projection display of video images and for printing, with an SLM by variation of the time a pixel is turned on or by printing the same pixel several times with the pixel turned on a varying number of times. The present invention devices a system for direct grey-scale generation with a spatial light modulator, with a special view to the generation of ultra-precision patterns. Important aspects in the preferred embodiments, are uniformity of the image from pixel to pixel and independence of exact placement of a feature relative to the pixels of the SLM and stability when focus is changed, either with intention or inadvertently.
Specifically, the there are problems with the prior art related to stitching errors. When using SLM a lot of pattern fields have to be stitched together to create the whole pattern, and with the previously known equipment the user can not avoid stitching boundaries being placed in sensitive parts of the pattern. Further, reproduction with partially coherent light, which is used in the invention, is a non-linear process. Therefore it is impossible even in theory to stitch together a critical pattern without artifacts at the boundary.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus for creating patterns, whereby the problem with stitching errors is reduced.
This object is achieved with an apparatus according to the appended claims.
The apparatus according to the invention comprises
a source of light pulses with energy content in the wavelength range from EUV to IR,
a spatial modulator having multitude of modulating elements, illuminated by said radiation pulses
a projection system creating for a radiation pulse an image of the modulator on the workpiece,
an electronic data delivery system accepting a digital description of the pattern to be written, extracting a sequence of partial patterns from the digital pattern description, converting said partial patterns to modulator voltages, and feeding said voltages to the modulator,
a precision mechanical system moving said workpiece relative to said projection system
an electronic control system coordinating the movement of the workpiece, the feeding of the voltages to the modulator and the pulses of radiation, so that a large pattern is stitched together from the partial images created by the sequence of radiation pulses
where at least two adjacent images being stitched together are overlapping at the common boundary, the overlapping images have essentially the same pattern in the overlap region and a reduced light intensity.
Hereby, the overlapping smoothes the edges between the fields, and distributes the errors over a larger area.
REFERENCES:
patent: 4430571 (1984-02-01), Smith et al.
patent: 4467211 (1984-08-01), Smith et al.
patent: 5296891 (1994-03-01), Vogt et al.
patent: 5847959 (1998-12-01), Veneklasen et al.
patent: 5849436 (1998-12-01), Yamada et al.
patent: 6060224 (2000-05-01), Sweatt et al.
patent: A1-9804950 (1998-02-01), None
Ben Loha
Birch & Stewart Kolasch & Birch, LLP
Micronic Laser Systems AB
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