Spatial light modulator fourier transform

A filter to selectively block light from passing through the filter and to selectively permit light to pass through the filter. The filter includes an array with a plurality of individually addressable filter elements. Each filter element is selectively settable to have a variable transmittance to the light of between substantially zero percent and substantially one hundred percent. In this manner, the filter according to the present invention provides areas that pass only a portion of the light, and thus can block the light using patterns other than just an abrupt on/off filtering. By so doing, the filter is able to dramatically reduce, and in some embodiments eliminate, the light ringing that typically accompanies such on/off filters.

FIELD

This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to the imaging processes, such as inspection, that are performed during integrated circuit fabrication.

BACKGROUND

Integrated circuit fabrication makes use of many different optical processes. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, InP, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. Fourier filters are sometimes used in the systems that implement these optical processes, such as inspection systems, alignment systems, and exposure systems.

At least two previous methods exist for implementing a Fourier filter, which methods are a mechanical method and a liquid crystal method. The mechanical method utilizes physical rods that are placed so as to block the regular pattern bright areas present at the Fourier plane. The liquid crystal method uses a one-dimensional or two-dimensional, fully blocking or fully transmissive liquid crystal system with a digital output to block the regular pattern bright areas present at the Fourier plane.

The mechanical method has several disadvantages. First, this method induces ringing. Since the transition from full transmission to no transmission occurs instantaneously at an interface between the two extremes, the system has a step input response. The output response contains a large amount of ringing that reduces the signal to noise ratio, especially in the array region. Second, this method can only be implemented as a one-dimensional Fourier filter. Since the rods span the entire Fourier plane from one end to the other, blocking a single spot requires blocking the entire coordinate along the axis of the rod that defines the spot. Third, since the rods must have a relatively large diameter in order to be structurally sound, only a limited number of rods can be used, or else the entire Fourier plane would be blocked. Fourth, this method entails the typical issues associated with mechanical systems, such as reliability issues, large size, and low operating speed.

The liquid crystal method has many of the same problems as the mechanical method. The liquid crystal Fourier filter operates on the principle of light scatter, therefore providing areas of either full transmission or zero transmission. Because of this, ringing is present after the image is filtered. Although the two-dimensional implementation allows for the filtering of individual spots, the one-dimensional implementation presents the same issues as the mechanical method when trying to block individual spots. In addition, since this method utilizes light scatter, it cannot block light only partially, thus it can behave only as a digital output spatial light modulator. Light scatter also tends to introduce wave front phase aberrations.

What is needed, therefore, is a system whereby problems such as those described above can be overcome, at least in part.

SUMMARY

The above and other needs are met by a filter to selectively block light from passing through the filter and to selectively permit light to pass through the filter. The filter includes an array with a plurality of individually addressable filter elements. Each filter element is selectively settable to have a variable transmittance to the light of between substantially zero percent and substantially one hundred percent.

In this manner, the filter according to the present invention provides areas that pass only a portion of the light, and thus can block the light using patterns other than just an abrupt on/off filtering. By so doing, the filter is able to dramatically reduce, and in some embodiments eliminate, the light ringing that typically accompanies such on/off filters.

In various embodiments, the variable transmittance is a continuously variable transmittance. The array is preferably one of either a one dimensional array of bars or a two dimensional array of pixels. In use, preferably a first portion of the filter elements are set to be opaque, a second portion of the filter elements are set to be transparent, and a third portion of the filter elements are set to be partially translucent. Most preferably, the three portions of filter elements are disposed to selectively block the light in a sinusoidal pattern, with the first portion disposed in an interior position, the third portion disposed around outer edges of the first portion, and the second portion disposed around outer edges of the third portion. In some embodiments there are several portions, each with a different degree of opacity.

Each of the filter elements is preferably comprised of two mask levels of liquid crystal pixels, where the liquid crystal pixels are registered relative to each other such that extraordinary axes of the liquid crystal pixels are arranged at substantially ninety degrees to each other. Most preferably the extraordinary axes of the liquid crystal pixels are arranged at substantially forty-five degrees to a polarization of the light. Polarizers are preferably disposed on either side of the array, where the polarizers are oriented to a common axis of polarization. The filter is variously used in an optical inspection system, an optical alignment system, or an optical exposure system.

DETAILED DESCRIPTION

The preferred embodiments of the present invention use an analog, dual-mask spatial light modulator as a Fourier filter for a dark field imaging system. Such use of the spatial light modulator preferably reduces and most preferably eliminates ringing s problems that are associated with such imaging applications when a traditional Fourier filter is used. The spatial light modulator preferably allows for smooth transitions between full transmission areas and no transmission areas. This accounts for the preferred analog output. Such a configuration differs from a digital output spatial light modulator, which is only capable of full or no transmission or reflectivity, and no gradations in between these two extremes. Additionally, the spatial light modulator preferably does not introduce any wave front phase aberrations. In other words, the optical wave front is preferably preserved and the spatial light modulator preferably prevents image quality degradation thereby

The analog output, dual-mask spatial light modulator is preferably placed in the Fourier plane where it preferably acts as a sinusoidal Fourier filter. It is appreciated that only an analog output, dual-mask spatial light modulator can achieve a sinusoidal Fourier filter without introducing wave front phase aberrations. Since the spatial light modulator is capable of modulating the amplitude without inducing phase changes, a sinusoidal Fourier filter is implemented. Simulations show that a sinusoidal Fourier filter reduces ringing in the filtered image.FIG. 1shows the implementation of this filtering method.

As depicted inFIG. 1, the Fourier plane image10ais depicted at the top of the figure, the Fourier filter12ais depicted in the middle of the figure, and the resultant filtered image14ais depicted at the bottom of the figure. As depicted, the filter12ahas elements16that are completely opaque and thus block any light that would otherwise pass through them. In addition, elements18are partially transmissive and only block a selective portion of the light that would otherwise pass unheeded through them. In this manner, the light that would pass through portions20in the Fourier plane is effectively filtered without the ringing effect that would tend to occur when the partially transmissive elements18are not disposed adjacent both sides of the opaque elements16.

FIG. 2provides three representation of prior art designs, which provide a depiction of the ringing problems inherent with the prior art designs. As before, the Fourier plane image is given as10in the top row of the figure, the Fourier filter is given as12in the middle row of the figure, and the filtered image is given as14at the bottom of the figure. Columns b, c, and d represent different embodiments of the combined plane10, filter12, and image14.

Filter12bis standard mechanical filter having physical rods22that can be moved about as necessary within the boundary of the filter12b. However, such rods22produce the ringing effect24as depicted in14b.Filter12cuses addressable lines26which can be selectively darkened, such as may be implemented with a liquid crystal screen. However, the same ringing problems24are evident in the image14c.Filter12duses a two dimensional array of addressable elements28which can be selectively darkened, again such as with a liquid crystal screen. However, because the elements28are either completing blocking or completely transmissive, a similar ringing problem24is evident in image14d.

One of the most important characteristics of the spatial light modulator used in this application is its ability to control amplitude without introducing phase aberrations. Put differently, the spatial light modulator is preferably able to control phase and amplitude independent of each other. Such a device is described in U.S. Pat. No. 5,868,951, the entire disclosure of which is incorporated by reference herein in its entirety.

The spatial light modulator30as depicted inFIG. 3preferably consists of two masks32and34that are each made up of a multitude of liquid crystal pixels. The two masks32and34are preferably registered relative to each other such that pixel “i”36in the first mask32matches pixel “i”38in the second mask34. Furthermore, the extraordinary axes40of the liquid crystals in the two masks32and34are preferably arranged at substantially ninety degrees to each other and at substantially forty-five degrees to the polarization of the incoming light. The polarization of the light is preferably accomplished by one, and most preferably two, polarizers31.

When compared with the mechanical implementation12bof the Fourier filter, the spatial light modulator implementation12aand30offers smaller size, higher resolution, greater flexibility in choosing the pitch, higher reliability and faster response time. When compared with both the mechanical and the old liquid crystal implementation, the spatial light modulator Fourier filter30and12ahas the ability to filter the light using a sinusoidal amplitude filter without introducing wave front phase aberrations. This results in reduced ringing, and preferably eliminated ringing, and thus a higher signal to noise ratio in the filtered image14a.

FIGS. 4 and 5illustrate this concept. InFIG. 4, the square tooth filters44represent the prior art methods, whereas the sinusoidal elements46represent the new methods. InFIG. 5there is depicted the tightly attenuated image as outlined in area50, as compared to the ringing pattern as outlined in area48.

The one-dimensional spatial light modulator12aas depicted inFIG. 1can also be implemented in a two-dimensional filter12e,as depicted inFIG. 6. The mask12eofFIG. 6also depicts very clearly the preferred used of the mask12e,where some of the elements16are set to be opaque and block the light. These elements16are preferably surrounded with elements18that partially block the light. The other elements are preferably set to pass whatever light may reach them. In this manner, the sinusoidal filtering effect as depicted inFIG. 4is enabled, and the reduced ringing as depicted inFIG. 5is accomplished.

FIG. 7depicts a possible implementation of the filters described above in a system60, where the system60may be an optical inspection system, an optical alignment system, or an optical exposure system, where the light51coming from the substrate52passes through a first lens54, then through the spatial light modulator Fourier filter12aor12eas described above, and then through a second lens56which focuses the filtered light onto a detector58.

Thus, the various embodiments of the present invention have several novel features, including the use of a sinusoidal Fourier filter, the use of a spatial light modulator as a Fourier filter, and the use of a dual mask (phase/amplitude) spatial light modulator for wafer inspection. Therefore, the embodiments of the present invention provide a practical way to implement a sinusoidal Fourier filter. It is more reliable than a mechanical Fourier filter, and in addition it reduces the ringing that is present in the filtered image when using the old methods of filtering.