Lithographic apparatus and device manufacturing method

A lithographic apparatus comprises an illumination system for supplying a beam of radiation, a patterning arrangement incorporating an array of individually controllable elements for imparting a pattern to the beam cross-section, a substrate table for supporting a substrate, and a projection system incorporating a microlens array for projecting the beam onto a target portion of the substrate. An error compensator is provided for supplying error correction values for compensating for the effect of positional errors in the microlens array, and a grey scale modulator is provided for supplying drive signals to controllable elements of the patterning arrangement in dependence on the error correction values in order to compensate for the effect of positional errors in the microlens array by varying the intensity of some parts of the pattern relative to other parts of the pattern.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic apparatus and a device manufacturing method.

2. Related Art

A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.

In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.

The sequence of exposures, or shots, forms a printed pattern. When considering maskless lithography, each shot results from an image of a spatial light modulator (SLM) array being projected onto a photosensitive surface, such as a layer of photoresist on a wafer substrate. This results in deposition of a dose, or a quantity of irradiation from a light source, within a certain exposure zone on this surface. Exposure zones are created when the substrate surface is illuminated by flashes of light from the light source.

In a pixel grid imaging lithographic system utilizing a micro lens array (MLA), the MLA is one of the most critical optical components in the system. The MLA focuses patterned beam projected by the projection lenses so as to form a large array of image spots in the image plane, which coincides with the photoresist layer on the substrate surface. For optimal resolution of the projected image the image spots should be very accurately located, for example within about 20 nm relative to the idea grid in the XY plane and within a few microns (e.g., +/−3 microns) in the focal direction. This small tolerance can lead to errors. These errors can be randomly distributed and can be magnification errors in the X direction or in the Y direction or in both the X and Y directions, or can be positional errors of the order of about 0.1 to 1 micron.

However, it is extremely difficult to manufacture MLA's that are capable of generating image spots having position errors of less than about 20 nm, and currently there are no techniques available to permit correction of position errors in the XY plane to the required level of accuracy.

Therefore, what is needed is a system and method that at least partially compensate for an effect of manufacturing errors in a MLA when used for focusing a patterned beam projected onto a target portion of a substrate.

SUMMARY

According to one embodiment of the present invention, there is provided a lithographic apparatus comprising an illumination system, a patterning arrangement, a projection system, an error compensator, and a gray scale modulator. The illumination system supplies a beam of radiation. The patterning arrangement patterns the beam. The projection system projects the beam onto a target portion of a substrate. The error compensator supplies error correction values for compensating for the effect of errors in the projection system. The grey scale modulator supplies drive signals to the patterning arrangement based on the error correction values in order to compensate for the effect of errors in the projection system by varying the intensity of some parts of the pattern.

In accordance with one embodiment of the present invention, there is provided a device manufacturing method comprising the following steps. Imparting a pattern to a beam using an adjustable patterning arrangement. Projecting the patterned beam of radiation onto a target portion of a substrate. Providing error correction values for compensating for the effect of errors in the projection system. Supplying drive signals to the patterning arrangement based on the error correction values in order to compensate for the effect of errors in the projection system by varying the intensity of some parts of the pattern.

In one example, the patterning arrangement can comprise an array of individually controllable elements. The grey scale modulator can be arranged to vary the intensity of radiation transmitted by at least some of the controllable elements of the array in order to compensate for the effect of errors in the projection system.

In one example, each of the individually controllable elements has a grey scale in intensity of, for example, 256 levels.

In one example, the projection system incorporates a micro lens array for producing image spots on the target portion of the substrate.

The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers can indicate identical or functionally similar elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Overview and Terminology

Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of integrated circuits (ICs), it should be understood that the lithographic apparatus described herein can have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, thin-film magnetic heads, micro and macro fluidic devices, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track (e.g., a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example, in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

The term “array of individually controllable elements” as here employed should be broadly interpreted as referring to any device that can be used to endow an incoming radiation beam with a patterned cross-section, so that a desired pattern can be created in a target portion of the substrate. The terms “light valve” and “Spatial Light Modulator” (SLM) can also be used in this context. Examples of such patterning devices are discussed below.

A programmable mirror array can comprise a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate spatial filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light to reach the substrate. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter can filter out the diffracted light, leaving the undiffracted light to reach the substrate. An array of diffractive optical micro electrical mechanical system (MEMS) devices can also be used in a corresponding manner. Each diffractive optical MEMS device can include a plurality of reflective ribbons that can be deformed relative to one another to form a grating that reflects incident light as diffracted light.

A further alternative embodiment can include a programmable mirror array employing a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation devices. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means.

In both of the situations described here above, the array of individually controllable elements can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference in their entireties.

A programmable LCD array can also be used. An example of such a construction is given in U.S. Pat. 5,229,872, which is incorporated herein by reference in its entirety.

It should be appreciated that where pre-biasing of features, optical proximity correction features, phase variation techniques and multiple exposure techniques are used, for example, the pattern “displayed” on the array of individually controllable elements can differ substantially from the pattern eventually transferred to a layer of or on the substrate. Similarly, the pattern eventually generated on the substrate can not correspond to the pattern formed at any one instant on the array of individually controllable elements. This can be the case in an arrangement in which the eventual pattern formed on each part of the substrate is built up over a given period of time or a given number of exposures during which the pattern on the array of individually controllable elements and/or the relative position of the substrate changes.

Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein can have other applications, such as, for example, the manufacture of DNA chips, MEMS, MOEMS, integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

The illumination system can also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the beam of radiation, and such components can also be referred to below, collectively or singularly, as a “lens.”

Further, the apparatus can be provided with a fluid processing cell to allow interactions between a fluid and irradiated parts of the substrate (e.g., to selectively attach chemicals to the substrate or to selectively modify the surface structure of the substrate).

Lithographic Projection Apparatus

FIG. 1schematically depicts a lithographic projection apparatus100according to an embodiment of the invention. Apparatus100includes at least a radiation system102, an array of individually controllable elements104, an object table106(e.g., a substrate table), and a projection system (“lens”)108.

Radiation system102can be used for supplying a beam110of radiation (e.g., UV radiation), which in this particular case also comprises a radiation source112.

An array of individually controllable elements104(e.g., a programmable mirror array) can be used for applying a pattern to beam110. In general, the position of the array of individually controllable elements104can be fixed relative to projection system108. However, in an alternative arrangement, an array of individually controllable elements104can be connected to a positioning device (not shown) for accurately positioning it with respect to projection system108. As here depicted, individually controllable elements104are of a reflective type (e.g., have a reflective array of individually controllable elements).

Object table106can be provided with a substrate holder (not specifically shown) for holding a substrate114(e.g., a resist coated silicon wafer or glass substrate) and object table106can be connected to a positioning device116for accurately positioning substrate114with respect to projection system108.

Projection system108(e.g., a quartz and/or CaF2lens system or a catadioptric system comprising lens elements made from such materials, or a mirror system) can be used for projecting the patterned beam received from a beam splitter118onto a target portion120(e.g., one or more dies) of substrate114. Projection system108can project an image of the array of individually controllable elements104onto substrate114. Alternatively, projection system108can project images of secondary sources for which the elements of the array of individually controllable elements104act as shutters. Projection system108can also comprise a micro lens array (MLA) to form the secondary sources and to project microspots onto substrate114.

Source112(e.g., an excimer laser) can produce a beam of radiation122. Beam122is fed into an illumination system (illuminator)124, either directly or after having traversed conditioning device126, such as a beam expander, for example. Illuminator124can comprise an adjusting device128for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in beam122. In addition, illuminator124will generally include various other components, such as an integrator130and a condenser132. In this way, beam110impinging on the array of individually controllable elements104has a desired uniformity and intensity distribution in its cross section.

It should be noted, with regard toFIG. 1, that source112can be within the housing of lithographic projection apparatus100(as is often the case when source112is a mercury lamp, for example). In alternative embodiments, source112can also be remote from lithographic projection apparatus100. In this case, radiation beam122would be directed into apparatus100(e.g., with the aid of suitable directing mirrors). This latter scenario is often the case when source112is an excimer laser. It is to be appreciated that both of these scenarios are contemplated within the scope of the present invention.

Beam110subsequently intercepts the array of individually controllable elements104after being directed using beam splitter118. Having been reflected by the array of individually controllable elements104, beam110passes through projection system108, which focuses beam110onto a target portion120of the substrate114.

With the aid of positioning device116(and optionally interferometric measuring device134on a base plate136that receives interferometric beams138via beam splitter140), substrate table6can be moved accurately, so as to position different target portions120in the path of beam110. Where used, the positioning device for the array of individually controllable elements104can be used to accurately correct the position of the array of individually controllable elements104with respect to the path of beam110, e.g., during a scan. In general, movement of object table106is realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted inFIG. 1. A similar system can also be used to position the array of individually controllable elements104. It will be appreciated that beam110can alternatively/additionally be moveable, while object table106and/or the array of individually controllable elements104can have a fixed position to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table106can be fixed, with substrate114being moveable over substrate table106. Where this is done, substrate table106is provided with a multitude of openings on a flat uppermost surface, gas being fed through the openings to provide a gas cushion which is capable of supporting substrate114. This is conventionally referred to as an air bearing arrangement. Substrate114is moved over substrate table106using one or more actuators (not shown), which are capable of accurately positioning substrate114with respect to the path of beam110. Alternatively, substrate114can be moved over substrate table106by selectively starting and stopping the passage of gas through the openings.

Although lithography apparatus100according to the invention is herein described as being for exposing a resist on a substrate, it will be appreciated that the invention is not limited to this use and apparatus100can be used to project a patterned beam110for use in resistless lithography.

The depicted apparatus100can be used in five modes:

1. Step mode: the entire pattern on the array of individually controllable elements104is projected in one go (i.e., a single “flash”) onto a target portion120. Substrate table106is then moved in the x and/or y directions to a different position for a different target portion120to be irradiated by patterned beam110.

2. Scan mode: essentially the same as step mode, except that a given target portion120is not exposed in a single “flash.” Instead, the array of individually controllable elements104is movable in a given direction (the so-called “scan direction”, e.g., the y direction) with a speed v, so that patterned beam110is caused to scan over the array of individually controllable elements104. Concurrently, substrate table106is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of projection system108. In this manner, a relatively large target portion120can be exposed, without having to compromise on resolution.

3. Pulse mode: the array of individually controllable elements104is kept essentially stationary and the entire pattern is projected onto a target portion120of substrate114using pulsed radiation system102. Substrate table106is moved with an essentially constant speed such that patterned beam110is caused to scan a line across substrate106. The pattern on the array of individually controllable elements104is updated as required between pulses of radiation system102and the pulses are timed such that successive target portions120are exposed at the required locations on substrate114. Consequently, patterned beam110can scan across substrate114to expose the complete pattern for a strip of substrate114. The process is repeated until complete substrate114has been exposed line by line.

4. Continuous scan mode: essentially the same as pulse mode except that a substantially constant radiation system102is used and the pattern on the array of individually controllable elements104is updated as patterned beam110scans across substrate114and exposes it.

5. Pixel Grid Imaging Mode: the pattern formed on substrate114is realized by subsequent exposure of spots formed by spot generator130that are directed onto array104. The exposed spots have substantially a same shape. One substrate114the spots are printed in substantially a grid. In one example, the spot size is larger than a pitch of a printed pixel grid, but much smaller than the exposure spot grid. By varying intensity of the spots printed, a pattern is realized. In between the exposure flashes the intensity distribution over the spots is varied.

Combinations and/or variations on the above described modes of use or entirely different modes of use can also be employed.

FIG. 2represents a part of an apparatus100, according to one embodiment of the present invention. In particular, it shows a contrast device in the form of an array16of individually controllable elements, an array of focusing elements in the form of a micro lens array (MLA)10for directing radiation onto a substrate13, projection elements of a projection system11for directing radiation onto the MLA10by way of a field lens17, and a beam splitter12illuminated by oblique radiation14. In this example, the beam splitter is arranged within the set of projection elements, such that it diverts the projection beam of radiation onto the reflective array of individually controllable elements. The reflected radiation from the individually controllable elements then passes straight through the beam splitter12into the remainder of the projection system.

In an alternative arrangement, the array of individually controllable elements is directly illuminated in a transmissive configuration.

In the embodiment shown inFIGS. 6 and 7, the MLA10comprises a body of transparent material having a flat upper surface and a lower surface shaped to define a plurality of lenses, although other shapes of MLA can be provided in alternative embodiments of the invention. In practice, the MLA can comprise as many as one million microlenses or more.

Returning toFIG. 2, the controllable elements of the array16in this example are mirrors whose tilt angles can be set by application of an appropriate control signal from a control system. It will be appreciated, however, that other forms of controllable elements can be used in alternative embodiments. For example, in certain embodiments the controllable elements can be movable diffraction gratings. Some of the controllable elements can be set to a nominal “white” position, while other controllable elements can be set to a nominal “black” position, so that the array16of elements can impart a pattern to the projection beam. The patterned beam reflected from the array16of controllable elements is then projected onto a target plane by the projection system11, which comprises a beam expander arranged to receive a substantially parallel patterned beam from the controllable array and focus that beam such that it passes through an aperture18. The patterned beam is then expanded by a field lens17into a substantially parallel beam which is delivered to the MLA10. Each of the microlenses of the MLA10receives and focuses a respective portion of the patterned beam, forming a corresponding spot15of radiation on the target plane. In one example, the portion of the patterned beam incident on each microlens should correspond to one or more of the controllable elements of the array16.

Each of the lenses of the MLA10focuses the radiation directed onto it to an associated area on the substrate. The intensity of the radiation in each area is dependant on the sum of the intensities of the portions of the patterned beam from each of the individually controllable elements associated with that lens. Therefore, the radiation at the area on the substrate associated with the microlens is dependant on the intensity of the radiation propagating from each of the individually controllable elements. Each of the individually controllable elements can be set to a plurality of states. In one example, the elements can be set to either direct radiation onto the associated focusing element or no radiation. Therefore, it has two states, namely full intensity and zero intensity.

For example, where there are three individually controllable elements associated with each microlens, four different levels of radiation intensity can be produced at the area on the substrate to which the radiation from one microlens is directed. Specifically, all of the elements can be set to zero radiation, all the elements can be set to full intensity, only one of the individually controllable elements can be set to full intensity, or two of the individually controllable elements can be set to full intensity. Each of the microlenses of the MLA10focuses the radiation directed onto it to an associated imaging spot15on the substrate13, and each imaging spot can be set to a plurality of states depending on the states of the associated controllable elements.

In one example, in addition to the fully on and fully off states corresponding to high and low intensity transmission levels, the controllable elements can be set to two or more intermediate states in which radiation at an intensity between the high intensity level and the low intensity level is transmitted. Each individually controllable element can, for example, be capable of generating up to 256 different intensity levels.

In this example, the individually controllable elements associated with each lens of the MLA10can be configured to transmit a different proportion of the radiation incident on them to the lens in each of their corresponding states.

In one example, the apparatus can be configured such that the radiation that is incident on each of the individually controllable elements associated with one lens has a different intensity level.

Referring toFIG. 3, the exposure of the substrate13is effected by moving the substrate table at a constant velocity in a scan direction21as the substrate is illuminated by the array20of imaging spots so that each imaging spot is caused to scan a line across the substrate. The pattern applied to the MLA can be updated as required between pulses of the radiation system, and the pulses timed such that successive target portions on the substrate are exposed at the required locations. Consequently, the projection beam is caused to scan across the substrate to expose the complete pattern for a strip of the substrate.

The array20of imaging spots comprises rows which are inclined at an angle a to the scan direction21. This has the effect of causing the lines scanned by the imaging spots to be offset relative to one another in such a manner as to cause the whole of the substrate to be scanned by the imaging spots. A region of the substrate scanned by the imaging spots at a particular time during the exposure process is shown inFIG. 3. In this example, the imaging spots are shown superimposed on the individual lenses24of the MLA spaced at a pitch Lmla.

FIG. 4shows the MLA with one of the microlenses having a positional error, according to one embodiment of the present invention. This causes its position to be displaced relative to the ideal grid as shown at25

FIG. 5is a section through a MLA10showing the positional error in the corresponding imaging spot26as a result of the positional error of the microlens25, according to one embodiment of the present invention. Alternatively positional imaging spot errors can be caused as a result of a magnification error in the X direction or in the Y direction or in both X and Y direction.

FIG. 6shows the effect on the positioning of the imaging spots27of a magnification error in the MLA10, according to one embodiment of the present invention.

FIG. 7shows an MLA10with arrows28illustrating the vector errors associated with the imaging spots26, according to one embodiment of the present invention. In one example, almost every MLA imaging spot will have a position error.

In these patterning positional error conditions, the resulting positioning errors of the imaging spots can be compensated for by a patterning device calibration procedure. For example, a procedure in which the positioning errors of the imaging spots are measured by a sensor array (e.g., a photodiode array) and the nominal intensity of each spot is calibrated. In this example, the positional errors relative to the ideal grid positions are determined in a computer program using an MLA error matrix Emla (i,j), where i and j denote the positions of each microlens within the grid having pitches Lmlax and Lmlay in the X and Y directions. In this example, an intensity correction matrix Imla (m,n) can be determined as a function of these values, as well as a function of a velocity v of movement of the substrate holder relative to the MLA, a refresh rate f (Hz) of the patterning device, and the tilt angle a between the rows of the MLA and the scanning direction.

This function also depends on the actual pattern to be imaged.

In one example, the positioning error of an imaging spot can be corrected by adjusting the grey level of adjacent spots. For example, this can be done using a spot compensation procedure in which the nominal position of each spot is calculated, and the required grey level to be applied to each spot is determined on the basis of the spot error lookup table. In other words, the compensation for the positioning error is made by varying the intensity values of the relevant image spots relative to the values they would have in the absence of such error compensation.

FIGS. 8,9and10show the intensity of an array of eleven imaging spots plotted against the position of each imaging spot in the array, according to various embodiments of the present invention.

FIG. 8is a graph of such an array in which none of the imaging spots has a position error. Each of the spots has the same intensity as denoted by the intensity peaks40associated with the spots. Also shown in this graph is the curve41showing the overall intensity of the array, and it will be noted that this curve41has a flat top.

FIG. 9is a graph of such an array in which the middle imaging spot has a position error of about 0.1 μm to the right, so that each of the spots has the same intensity as before, but the intensity peak42associated with the middle imaging spot has a position error of about 0.1 μm to the right. This causes the curve43showing the overall intensity of the array to have an uneven top.

FIG. 10is a graph of such an array in which the spot position error of the middle imaging spot has been compensated for by applying suitable error correction values to the adjacent imaging spots. For example, the intensity of two of the adjacent spots has been increased to provide peaks44and46of increased intensity and the intensity of one of the adjacent spots has been decreased to provide a peak45of decreased intensity. These error correction values applied to the adjacent imaging spots cause the curve47showing the overall intensity of the array to have a substantially even top, thus preventing the image from being compromised. The correction value are then used during normal operation to provide compensation for errors.

CONCLUSION