Lithographic Imaging System

This invention relates generally to lithographic techniques for forming semiconductor wafers and particularly to improving the resolution of a projected image formed on a wafer by a reticle.

Photomasks are used to form an image to be projected onto a silicon wafer treated with photoresist. The photomask typically has a light blocking and transmitting pattern. The photomask may be formed by patterning a mask formed of a metal, such as chrome, that was deposited on a substrate, such as quartz. The patterned substrate, often called a "reticle," may be used to define an image which is projected onto the semiconductor wafer. The photoresist covering the wafer is thereby exposed in the desired pattern. After the pattern is developed, the developed photoresist may be used to define structure such that the original pattern is reproduced in that structure on the semiconductor wafer.

Typically in wafer lithographic systems, a light source is projected through the reticle onto the wafer. The wafer is positioned under control of a device called a stepper. The stepper aligns the reticle to the wafer.

There is a continuing need to improve photolithographic techniques to enable smaller integrated circuits to be fabricated. As smaller integrated circuits are possible, their performance, size and cost may be improved. Successive generations of lithographic techniques used for forming such images have provided increasingly improved resolution and enabled ever smaller devices to be made.

However, these lithographic systems are expensive. Thus, it would be desirable to improve the resolution achievable with a given system without requiring the substantial capital investment to replace and upgrade the equipment.

As light passes through the photomask, some of the light is diffracted at the interface between the masking and nonmasking areas of the pattern. The diffracted light decreases the image resolution of the projected image. As a result, the projected image and the structures formed on the semiconductor wafer may be of diminished quality. Ultimately, the size of the smallest image which can be transferred to the wafer is reduced as a result of this diffraction effect.

The diffraction effect may be overcome without requiring replacement of the lithographic system by positioning an appropriately designed layer over the reticle. The layer is specially designed to counteract the diffraction effect at the edges of the masking pattern on the photomask.

In accordance with one aspect of the present invention, a reticle includes a light transmitting pattern. The reticle has a light altering member adjacent the light transmitting pattern, the light altering member adapted to alter the phase of a portion of a light projected through the light transmitting pattern.

In accordance with another aspect of the present invention, a reticle includes a reflective member adapted to reflect a light pattern when exposed to a light source. The reticle has a light altering member adjacent the reflective member, the light altering member adapted to alter the phase of a portion of the light diffracted by the reflective member.

In accordance with but another aspect of the present invention, an imaging device includes a light transmitting pattern. The imaging device has a light transmitting member adjacent the light transmitting pattern. The light transmitting member has an exit surface remote from the light transmitting pattern. The distance between the pattern and the exit surface is such that light diffracted by the pattern and reflected from the exit surface to the pattern and back through the exit surface is out of phase with light passing directly through the exit surface.

In accordance with yet another aspect of the present invention, an imaging method includes placing a mask adjacent a first light transmitting member. The thickness of the first light transmitting member is selected such that a portion of light diffracted by the mask is out of phase with light not diffracted by the mask.

In accordance with but another aspect of the present invention, an imaging method includes placing a transmissive member adjacent a photomask, the transmissive member having an exit surface. A light is projected through the photomask and the exit surface. The phase of a portion of the light diffracted by the photomask and reflected by the exit surface is altered.

In accordance with still another aspect of the present invention, an imaging device includes a light source. A mask is adapted to transmit light from the light source and to diffract a portion of the transmitted light. A transmissive member, adjacent the mask opposite the light source, has an exit surface remote from the mask. The exit surface is adapted to reflect a portion of the diffracted light. The transmissive member is adapted to alter the phase of the reflected light.

In accordance with yet another aspect of the present invention, a method of imaging includes interposing a light blocking pattern between a reflective member and a transmissive member. The thickness of the light blocking pattern is such that a first portion of light diffracted by the light blocking pattern is out of phase with a second portion of light reflected by the light blocking pattern.

FIG. 1 is a partial, enlarged cross-sectional view of a reticle in a schematically depicted lithographic system; and

FIG. 2 is a partial, enlarged cross-sectional view of a reticle in another photolithographic system indicated schematically.

As shown in FIG. 1, a photolithographic system 10 may include a member 12 having a mask 14 defined thereon. The mask 14 and quartz member 12 together define a reticle. The mask 14 may be formed by any conventional technique including the patterning of a chrome layer. The member 12 may be made of a variety of light transmissive materials including quartz.

The dimensions of elements in the figures may be exaggerated or contracted for illustration purposes. In addition, conventional components such as condenser and projection lenses are omitted for simplicity.

A light source illuminates the member 12. The light source may be any type of light source conventionally used for lithographic purposes including an ultraviolet light source. Preferably, the light source is photon based and produces light "L" in the wavelength traditionally associated with visible or ultraviolet light, including deep ultraviolet. While an on-axis light source is illustrated, off-axis light sources may be used as well.

Incident light L formed by the light source passes through the reticle formed of the member 12 and mask 14. The light is blocked by the mask 14 and transmitted through openings 16 in the mask. As the light passes through the openings 16 in the mask, it is diffracted, as indicated at L', at the mask edges 17.

A layer 18 adjacent the mask 14 enhances the resolution possible with an existing reticle. The layer 18 may be substantially light transmissive, allowing a substantial percentage of the incident light L to pass through the layer 18.

The layer 18 may be a layer of quartz sputter deposited on the mask 14. Alternatively, the member 12 and the mask 14 may be deposited on a previously formed layer 18. The deposition of quartz layers may also be accomplished by chemical vapor deposition. The layer 18 may also be made of silicon nitride. In some cases, it may be advantageous to form the layer 18 of multiple layers. In other embodiments, the layer 18 may be a discrete plate of transmissive material placed adjacent the mask 14.

A portion of the diffracted light L' is reflected upwardly from the lower surface 20 of the layer 18, towards the mask 14 where it is then reflected from the mask 14 back through the exit surface 20 towards the wafer "w".

The distance "d.sub.t " between the mask 14 and the surface 20 is such that the diffracted and reflected light L" passing through the surface 20 of the layer 18 is out of phase with the transmitted light L.sub.t.

The distance d.sub.t is advantageously approximately equal to the wavelength of the light L divided by four times the refractive index of the layer 18. The light L" is then 180.degree. out of phase with the light L.sub.t.

Advantageously, a substantial portion of the diffracted light L' is reflected back from the surface 20 to create an out of phase component L". If the out of phase light L" is too insubstantial, the image enhancing effect may be insubstantial. On the other hand, if the amount of diffracted and reflected light L" is too high, other resolution problems may be created. Thus, it is most desirable that the ratio of the out of phase light L" to the in phase light L.sub.t be less than about 0.25 and greater than about 0.05. The transmitted light may be determined as a percentage of the intensity of the original light source light L.

As in any lithographic system, it is desirable that Sigma (.sigma.), the ratio of the image of the light source on the projection lens pupil over the numerical aperture of the pupil on the projection lens, be less than about 0.5.

With the embodiment described above, existing lithographic systems can be upgraded to improve their resolution with a given light source wavelength. This avoids or delays the expense of replacing prior generation lithographic systems with more recent generation systems. In addition, up to date systems may achieve even greater performance through inclusion of the image enhancing layer.

With an appropriately designed layer 18, the amplitude profile on the wafer is sharper and the printability of the pattern is improved.

As shown in FIG. 2, the same effect can be achieved in a system 10a relying on reflection of light rather than transmission of light. In this case, the incident light L is either reflected at the interface 24 or absorbed by the mask 26. Other combinations of optical effects could be utilized as well, including using a reflective mask 26 and an absorptive material 24.

The light source may be situated slightly off-axis. The wafer "w" then must be slightly skewed to align with the light L.sub.r reflected from the layer 22. Any conventionally reflective surface may be used as the layer 22, including a silver coating.

The thickness d.sub.r of the layer 26 is governed by the same relationship described above. Namely, d.sub.r is advantageously equal to about the wavelength of the incident light L divided by four times the index of refraction of the layer 18. This produces 180.degree. out of phase light L" which improves the printability of the resulting image. Similarly, the ratio of the light L" diffracted by the edge 17a and reflected by the exit surface 28 and ultimately reflected again off the mask 26, divided by the reflected light L.sub.r should be maintained in the range of less than about 0.25 and greater than about 0.05.

Since the layer 18 may be economically added to existing optical systems, the present invention offers an extremely cost effective solution which may also satisfy the need for ever increasing resolution in lithographic systems.

While the present invention has been described with respect to a limited number of preferred embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the present invention.