Illumination system and method for efficiently illuminating a pattern generator

An illumination method and system use a light source and illumination optics to illuminate a pattern generator. The illumination optics can include at least two devices. For example, if first and second diffractive and/or refractive devices are used, one can be a pupil defining element (PDE) and one can be a field defining element (FDE). In another example, a third diffractive or refractive element can be used to make light entering the illumination system uniform. When only two are used, the PDE forms one or more light beams having a defined profile. The FDE directs the one or more light beams having the defined profile, such that each directed beam substantially corresponds in size and shape to a desired illumination area(s) on the pattern generator. The directed beams are directed to impinge substantially only on the desired illumination area(s). Thus, using the PDE and the FDE increases optical efficiency of light impinging on the pattern generator and substantially reduces or eliminates stray light caused by light impinging on undesired areas of the pattern generator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to an illuminating system and method for illuminating a pattern generator, and more particularly for illuminating a pattern generator in a lithography system.

2. Background Art

Pattern generators are used in many different environments to pattern objects or project patterns using light, for example, in lithography systems, televisions, biomedical systems, biotechnology systems, etc. Typically, reticles (or masks), spatial light modulators (SLMs) or contrast devices (hereinafter, both are referred to as SLMs), such as digital mirror devices (DMDs), liquid crystal displays (LCDs), grating light valves (GLVs), or the like, or any other elements that include a transmissive and/or reflective pattern can be used as pattern generators.

SLMs can include an active area having an n×m (wherein n and m are integers greater than 1) array of active devices (or pixels) (e.g., an array of mirrors on the DMD, an array of gratings on a GLV, or an array of reflective/transmissive devices on the LCD). Each active device is individually controlled to move the active devices between ON and OFF through one or more discrete states. For example, if the active devices are mirrors on the DMD, each of the mirrors is individually controlled to rotate or tilt the mirror to either binary or multiple positions. As another example, if the active devices are strips in a GLV, sets of strips can be bent or straight to allow reflection or diffraction of incoming light beams.

It is to be appreciated that controlling the active devices in active areas so that they are partially or fully ON or OFF is well know in the art, and not fully described here for brevity. Typically, a predetermined and previously stored algorithm based on a desired exposure pattern is used to turn ON (or partially ON) and OFF the active devices, as is known in the relevant arts.

FIGS. 1,2, and3show conventional systems100,200, and300, respectively, for illuminating a pattern generator, so that patterned light is formed and directed from the pattern generator. As is known, the illumination optics, and optional pattern generator optics, can include one or more optical elements (e.g., lenses, mirrors, etc.). In one arrangement, the illumination optics can include the pattern generator optics. In another arrangement, the pattern generator optics can be a separate element. A projection system would normally focus patterned light from the pattern generator onto a substrate.

FIG. 4shows a convention illumination field400that can result from systems100,200, and/or300for pattern generator402having desired illumination areas404. Each illumination area can be either an active area of an SLM or a desired portion of a pattern on a reticle. As discussed above, each active area will include the active devices. As can be seen, illumination field400is so large that it not only impinges on desired illumination areas404, but is larger than pattern generator402. Thus, a substantial amount, maybe up to about 80–90%, of the light may be wasted (i.e., not used during operation of system100,200, and/or300) because that amount of light does not impinge on desired illumination areas404.

One use for the pattern generator, or an array thereof, is in maskless lithography. Lithography is a process used to create features on the surface of a substrate. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or flat panel display glass substrate. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.

During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image (e.g., a pattern) formed by the pattern generator, or array thereof. The image is projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, an excimer laser, x-ray, ion, electron, or photon lithography can each require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.

The projected image produces changes in the characteristics of a layer (e.g., photoresist) deposited on the surface of the wafer. These changes correspond to features in the image projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to the features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.

Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time with the image formed by the pattern generator, individual fields are scanned onto the wafer one at a time. This is accomplished by moving the wafer and controlling active devices on the pattern generator, such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of the pattern formed by the active devices on the pattern generator to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.

Desired illumination areas on a pattern of a reticle (or mask) or the active area of the SLM are usually substantially smaller than a size of a surface incorporating the desired illumination areas of the pattern or the active area. For example, in an SLM, an active area may only be 10–20% of the SLM surface, with the remaining surface area of the SLM being an inactive area, which can include packaging, circuitry, and the like. Thus, a substantial amount of the light directed to the pattern generator may not impinge on the desired illumination area of the pattern or the active area, but instead impinges on undesired areas of the pattern or the inactive areas, which can result in stray light and/or wasted light. Some of the stray light can reach the surface of the substrate. The stray light reaching the surface of the substrate can cause errors in devices being fabricated on the substrate.

Therefore, what is needed is a system and method that increases optical efficiency by directing light such that it substantially impinges on desired illumination areas of a pattern generator and that reduces or substantially eliminates stray light caused from light impinging on undesired areas of the pattern generator.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the present invention provides a system that substantially increases optical efficiency and substantially reduces or eliminates the generation of stray light generated when illumination light impinges on undesired areas of a pattern generator. The system includes a light source, a pupil defining element (PDE), a field defining element (FDE), an optical system, and the pattern generator. The PDE and FDE can be diffractive or refractive devices. Light emitted from the light source is transmitted through the PDE and FDE to form shaped and directed beams that are directed onto one or more desired illumination areas of the pattern generator and are directed substantially away from undesired areas of the pattern generator.

Other embodiments of the present invention include a method that substantially increases optical efficiency and substantially reduces or eliminates the generation of stray light generated when illumination light interacts with a pattern generator (e.g., a reflective or transmissive reticle or SLM). The method includes transmitting light through a PDE and a FDE to form beams of light, directing the beams of light using an optical system towards the pattern generator, and substantially illuminating only the desired illumination areas of the pattern generator and substantially no undesired areas of the pattern generator, based on the transmitting and directing steps.

The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number may identify the drawing in which the reference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

Overview

While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications.

An embodiment of the present invention provides an illumination system having a light source and illumination optics. The illumination system is used to illuminate a pattern generator. The illumination optics include at least two devices. For example, if first and second diffractive and/or refractive devices are used, one can be a pupil defining element (PDE) and one can be a field defining element (FDE). In another example, a third diffractive or refractive element can be used to make light entering the illumination system uniform. When only two devices are used, the PDE forms one or more light beams having a defined profile. The FDE directs the one or more light beams having the defined profile, such that each directed beam substantially corresponds in size and shape to a desired illumination area(s) on the pattern generator. The directed beams are directed to impinge substantially only on the desired illumination area(s). Thus, using the PDE and the FDE increases optical efficiency of light impinging on the pattern generator and substantially reduces or eliminates stray light caused by light impinging on undesired areas of the pattern generator.

For the sake of brevity, a complete description of the operation and functionality of an SLM device (e.g., a DMD, a GLV, an LCD, etc.) being used as the pattern generator is not described herein. For example, it is to be appreciated that controlling the active devices in active areas so that they are partially or fully ON or OFF is well know in the art, and not fully described here for brevity. Typically, a predetermined and previously stored algorithm based on a desired exposure pattern is used to turn ON (or partially ON) and OFF the active devices, as is known in the relevant arts.

It is also to be appreciated that illumination optics and pattern generator optics can include one or more optical devices, for example lenses, mirrors, and the like, or other light manipulating devices, as would be known to a person of ordinary skill in the art upon reading this description.

Illumination Optics

FIG. 5shows an arrangement of elements in illumination optics500according to an embodiment of the present invention. The illumination optics can be placed in any of systems100,200, or300, or any other system used to illuminate a pattern generator. The illumination optics500can be used to form shaped and directed beams, such that substantially only desired illumination areas (e.g., pattern areas of a reticle or active areas of an SLM device) are illuminated. This can be used to increase optical efficiency and decrease stray light.

A first device502(e.g., a pupil defining element (PDE)) is positioned in a pupil plane of illumination optics500and receives a light beam504from a light source (not shown, but similar to those shown inFIGS. 1,2, or3).

PDE502can comprise a refractive (e.g., lens array, etc.) or diffractive (e.g., diffraction grating, etc.) optical element. PDE502can be used to shape light beam504as it is transmitted therethrough. For example, PDE502can be used to define a light beam profile for one or more light beams506. For example, PDE502can form conventional, dipole, quadrapole, etc. profiles for beam(s)506.

An optical device507(e.g., one or more optical elements) directs beam(s)506′ onto a second device508(e.g., a field defining element (FDE)). FDE508can be a refractive (e.g., a lens array, etc.) or diffractive (e.g., diffraction grating, etc.) optical element, which is positioned at a field plane of illumination optics500. FDE508can be used to form one or more light beams510that maintain a shape of beam(s)506′, but are directed to specific areas of a pattern generator (not shown) via optical device512.

Although PDE502and FDE508are shown in a particular order in illumination optics500, it is to be appreciated their order in the illumination path can be reversed, as is contemplated within the scope of the present invention.

It is to be appreciated that first and second diffractive devices502and508, respectively, can include more than one diffractive or refractive optical element. For example, in one embodiment using diffractive optical elements, first diffractive device502can also include a uniform beam profiling (UBP) diffractive optical element positioned before the PDE. In this embodiment, the UBP generates a uniform beam profile from light beam504.

Another exemplary optical system is shown inFIG. 2aof U.S. Ser. No. 10/270,556 (U.S. Patent Application Publication 2003/0076679 A1 (“the '679 PPA)), which is incorporated by reference herein in its entirety. The '679 PPA shows an optical system200that can be used to shape and direct light.

System200inFIG. 2aof the '679 PPA includes a first diffractive array or field array210, a second diffractive array or pupil array212, and a condenser system220placed in an optical path along an optical axis209between first diffractive array210and second diffractive array212. First diffractive array210can be used to provide spatial and temporal coherence treatment for conditioned light103entering illumination system200. Second diffractive array212can act as a pupil and can change a magnitude of a light. Also,FIGS. 5a,5b,6a, and6bof the '679 PPA, and the description thereof, show exemplary diffraction devices that can be used for first and second diffractive arrays210and212.

Illumination optics500(or200in the '679 PPA) can be configured so that PDE502and/or FDE508(or PDE210and FDE212in the '670 PPA) are easily replaced (swapped) by other PDEs and FDEs. This can allow for illumination optics500(or system200in the '679 PPA) to be flexible enough to produce any desired number, position, and/or size of directed beams510, as is appropriate for a particular desired illumination area on the pattern generator. For example, the pattern generator can include a plurality of SLMs and only a predetermined set of the plurality of SLMs form the desired illumination areas. In this case, a specific PDE502and FDE508can be inserted into illumination optics500to produce the desired pattern. As another example, the pattern generator can include one or more SLMs each having varying sized active devices and/or active areas. In this case, a specific PDE502and FDE508can be inserted into illumination optics500to produce the desired pattern. In other examples, the interchangeability of PDEs and FDEs can allow a system including illumination optics500to switch between high and low resolution modes, to adjust throughput requirements, or the like.

Exemplary Light Path through the Illumination Optics

FIG. 6shows a portion of a light path through illumination optics500ofFIG. 5according to an embodiment of the present invention. It is to be appreciated a similar light path can be formed through system200of the '679 PPA.

Light beam506′ impinges FDE508to form beams510-n. It is to be appreciated that a number of beams510inFIG. 6is for illustrative purposes. If pattern generator602included more or less desired illumination areas606then the number of beams510would respectively reflect that number. If FDE508is a diffraction grating, beams510can be desired orders of diffracted beams.

Beams510impinge on optical device512at areas604-n. Directed beams510′-nexit optical device512and form an illumination spots608on respective desired illumination areas606of pattern generator602. Therefore, through the use of illumination optics500, light502from the light source is shaped and directed so that each illumination spot608covers only a small portion of a surface610and all of desired illumination areas606, which is best shown inFIG. 7.

FIG. 7shows illumination areas608. Using FDE508and optic512, illumination spots608impinge substantially only on desired illumination areas606, while only covering a small (desired) portion of surface610. In the example using an SLM for pattern generator602, using illumination optics500of the present invention can increase illumination efficiency up to around the 80–90% range.

Therefore, through use of illumination optics500(or system200in the '679 PPA) an increase in an amount of light intensity at each desired illumination area606can result without a need to increase the power of the light source. Illumination optics500(or system200in the '679 PPA) can further decrease or substantially eliminate stray light from surface610, which decreases or substantially eliminates errors on fabricated devices caused by the stray light.

It is to be appreciated that, although illumination spots608are shown as certain shapes, the illumination spots608can be any shape. The shape, as discussed above, can be based on FDE508.

Exemplary Application of the Illumination System

One exemplary application that can use illumination system500(or system200in the '679 PPA) and pattern generator602can be a maskless lithography system800, as described below. However, it is to be appreciated that other systems, such as those described above, can also employ illumination system500to illuminate pattern generator602.

FIG. 8shows an exemplary lithography system800(e.g., a reticle or maskless lithography system) that includes illumination optics500and projection optics850according to an embodiment of the present invention. Light504from a light source (not shown) is shaped using PDE502to form shaped beams506. Shaped beams506are focused onto FDE508using optical device507(e.g., a converging lens, or the like). Directed beams510are formed from FDE508and are focused onto desired illumination areas606of pattern generator602using optical device512(e.g., a converging lens, or the like). Reflected light beams852are directed using an optical device854as beams852′ through an aperture856. The light beams852′ are then focused using optical device858onto a substrate860(e.g., a wafer, flat panel display, or any object that can retain a received pattern) to pattern a photosensitive surface of substrate860.

It is to be appreciated that although a reflective pattern generator602is shown, a transmissive pattern generator can also be used, as would be apparent to one of ordinary skill in the art upon reading this description.

In one embodiments, uniformity of illumination within a pattern generator and between pattern generators in an array of pattern generators can be set using pattern generator calibration systems and methods.

In another embodiment, an FDE can be configured to balance relative dose from pattern generator to pattern generator in an array. For example, a series of filters (e.g., neutral density filters) down stream of the FDE can be used to balance relative dose from one pattern generator to another pattern generator.

In an embodiment performing stitching of pattern generator images, a rollof profile may be required. In one example, the rollof profile is generated with an FDE. In another example, the rollof profile is generated prior to the FDE and the FDE is used to reproduce the rollof profile.

CONCLUSION