Lithographic apparatus, device manufacturing method and substrate holder

A substrate holder has burls having a height not less than 100 μm and at least 10 vacuum ports arranged within a central region extending to a radius of two thirds the radius of the substrate. Thereby concave wafers can be reliably clamped by generating an initial vacuum in a central region which exerts a clamping force tending to flatten the wafer and allowing the initial vacuum to deepen until the wafer is fully clamped.

The present application claims priority to European Application No. 02258833.9, filed on Dec. 20, 2002, the entirety of which is hereby incorporated into the present application by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus, a method of manufacturing a device, and a substrate holder.

2. Brief Description of Related Art

To hold the substrate to the substrate table, a so-called burl plate may be used. A burl plate described in patent U.S. Pat. No. 6,232,615 (which document is incorporated herein by reference thereto) comprises a plate with a matrix arrangement of protrusions, or burls, on one face and a wall surrounding the matrix of burls. The burls all have a height of 150 μm. Holes in the burl plate lead to a vacuum system whereby the space below the wafer can be evacuated. The pressure differential between the normal atmospheric pressure above the substrate and the evacuated region below clamps the substrate firmly to the burl plate. The vacuum ports are relatively numerous, e.g. 20 or more, and are disposed in two concentric rings.

Other known designs of substrate holder have a relatively small number of vacuum ports, e.g. 3 or 4. For example, U.S. Pat. No. 5,923,408 discloses a substrate holder with three vacuum ports and protrusions that have total height of not less than 550 μm—made up of a narrow section of diameter 100 μm and height 50 μm on top of a wider section of diameter not less than 1 mm and a height not less than 500 μm. U.S. Pat. No. 5,324,012 discloses a pin chuck-type holder with a single vacuum port. The pin-type protrusions are said to have a height of from 10 μm to 500 μm but no specific examples are given. EP 1 077 393 A2 describes substrate holders that have one, four or eight vacuum ports and various arrangements of pin-like protrusions, but does not disclose the height of the pins. EP 0 803 904 A2 discloses a substrate holder that has pins of a height between 17.8–30.5 μm and four vacuum ports in a central region. GB 2 149 697 A describes a vacuum chuck with a plurality of pin-type protrusions of 50 μm in height and six vacuum ports.

The known designs of substrate holder suffer from the problem that if a concave (dished) substrate is placed on them it fails to be clamped because the wide gap between the raised edges of the substrate and the surrounding wall means that no vacuum develops underneath the substrate. Substrates can become concave due to processes carried out on them to build devices and may be discarded if they become too dished to be clamped onto the substrate table. The need to discard such substrates reduces yield and throughput.

SUMMARY

One aspect of the present invention is to provide a substrate holder that can more reliably clamp concave substrates.

According to an aspect of the invention, there is provided a lithographic apparatus comprising: an illumination system constructed to provide a beam of radiation; a support structure to support a patterning device, the patterning device constructed to impart a cross-section of the beam of radiation with a pattern to form a patterned beam of radiation; a substrate holder for holding a substrate having a radius, the substrate holder including a plurality of protrusions upstanding from a surface and having substantially coplanar extremities, a wall surrounding the plurality of protrusions, and a plurality of vacuum ports opening into a space bounded by the wall; and a projection system constructed to project the patterned beam of radiation onto a target portion of the substrate, each of the plurality of vacuum ports opening into a central region of the space bounded by the wall, the central region having a radius of not more than 70% of the radius of the substrate.

By reducing the height of the protrusions (which are also sometimes referred to as pimples or burls) and ensuring that a relatively large number of vacuum ports opens into a central region of the substrate holder, it is possible to ensure that a vacuum develops under the substrate even when the substrate is significantly concave. The pressure differential across the substrate tends to flatten the substrate enabling the initial vacuum to deepen, increasing the pressure differential and further flattening the substrate. It is therefore only necessary to develop an initial vacuum under the central region of the substrate to successfully clamp a substrate. The initial vacuum is sufficient to have a flattening effect on the substrate but need not be as deep as the vacuum developed when the substrate is clamped. The necessary depth of the initial vacuum will depend on the mechanical properties and curvature of the substrates to be clamped. Once the substrate has been clamped it is flattened against the tops of the protrusions and the clamping effect is the same as if the substrate had been flat in the first place.

It is preferred that the protrusions have a height of no less than 60 μm to ensure that the vacuum pressure under the substrate rapidly becomes uniform when the substrate is clamped. It is most preferred that the height of the protrusions is in the range of from 70 to 80 μm. With protrusions of such a height, the inventors have discovered that silicon substrates of standard dimensions with a curvature of up to 800 μm across a 300 mm wafer can be successfully clamped.

Preferably, the number of vacuum ports is in the range of from 20 to 40, all of which open into the space within the central region. In a particularly preferred embodiment of the invention, all of the vacuum ports open into an annular region having an outer radius not more than 70% of the radius of the substrate (about 100 mm for a 300 mm substrate) and an inner radius of not less than 40% of the radius of the substrate (about 60 mm for a 300 mm substrate).

According to a further aspect of the invention, there is provided a method of manufacturing a device, comprising: providing a substrate holder having a plurality of protrusions upstanding from a surface and having substantially coplanar extremities, a wall surrounding the plurality of protrusions and a plurality of vacuum ports opening into a space bounded by the wall, each of the vacuum ports being open into a central region of the space bounded by the wall, the central region having a radius of not more than 70% of the radius of a substrate configured to be held by the substrate holder; holding the substrate by evacuating the space between the substrate and the substrate holder; and projecting a patterned beam of radiation onto a target portion of the substrate.

According to yet a further aspect of the present invention, there is provided a substrate holder, the substrate holder comprising: a plurality of protrusions upstanding from a surface of the substrate holder and having substantially coplanar extremities; a wall surrounding the plurality of protrusions; and a plurality of vacuum ports opening into a space bounded by the wall, each of the vacuum ports opening into a central region of the space bounded by the wall, the central region having a radius of not more than 70% of the radius of the substrate.

Patterning device may be transmissive or reflective. Examples of patterning device include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. In each example of patterning device, the support structure may be a frame or table, for example, which may be fixed or movable and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

FIG. 1schematically depicts a lithographic apparatus according to an embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL for providing a projection beam PB of radiation (e.g. UV radiation or DUV radiation), which in this particular case also comprises a radiation source SO; a first support structure (e.g. a mask table) MT for supporting patterning device (e.g. a mask) MA and connected to first positioning structure PM for accurately positioning the patterning device with respect to a projection system (“lens”) PL; a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist-coated wafer) W and connected to second positioning structure PW for accurately positioning the substrate with respect to the projection system PL; and the projection system (e.g. a refractive projection lens) PL for imaging a pattern imparted to the projection beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

The illuminator IL may comprise an adjustor AM that adjusts the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation, PB, having a desired uniformity and intensity distribution in its cross-section.

The beam PB is incident on a patterning device, illustrated in the form of the mask MA, which is held on the mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning structure PW and position sensor IF (e.g. an interferometric device), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning structure PM and another position sensor (which is not explicitly depicted inFIG. 1) can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning structure PM and PW. However, in the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes.

In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam is projected onto a target portion C in one go (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

FIG. 2is a plan view of a substrate holder10which is positioned on the substrate table WT to hold a substrate thereon during exposures. The substrate holder10comprises a flat circular plate, the upper face of which is provided with an array of burls or projections12and is bounded by a wall11, as seen inFIG. 3. The burls12support the substrate W and have a total area of usually less than about 4% of the area of the substrate. Whilst for illustrative purposes the burls12are shown as arrayed in a regular rectagonal matrix, other arrangements are possible, e.g. concentric rings.

The burl plate is also provided with through-holes13, in this example there are 24 through-holes, arranged regularly around two concentric rings14,15. Through-holes13line up with vacuum ports on the substrate table WT and form vacuum ports for evacuation of the space below the substrate W and bounded by the wall11.

The substrate W is removed from the substrate holder10by turning off the vacuum and lifting it from below by pins which extend through further holes (not shown) in the substrate holder10. These further holes may be surrounded by walls that rise to meet the substrate so that there is no leakage of air into the space under the substrate via these holes.

FIG. 4is a graph of vacuum pressure |Pvac|, that is the magnitude of the difference between the pressure in a space below the wafer W and normal atmospheric pressure above. When a wafer W is clamped correctly on the substrate table WT, the pressure underneath the wafer W in the area within the wall11is at a uniform high vacuum level P1.

FIGS. 5 and 6illustrate what happens when a concave wafer W′ is presented to the substrate holder10. At its outer edge, the curvature of the wafer W′ means that there is a large gap between the wafer W′ and the substrate holder10so that the pressure in this area is the same as above the wafer and there is no clamping effect. However, because according to an embodiment of the invention the heights of the projections12are reduced and the vacuum ports are arranged in a central region of the substrate holder10, a vacuum does develop in the central region below the wafer W′, as indicated by the solid curved line inFIG. 6. There is therefore a pressure differential across the wafer W′ causing a clamping force, albeit initially small, that clamps the wafer W′ to the substrate holder10and also tends to flatten the substrate W′. The flattening of the substrate W′ reduces the gap between it and the substrate holder10allowing the vacuum in the central region to deepen. This in turn increases the flattening force on the substrate W′ and consequently the substrate W′ is rapidly flattened and fully clamped to the substrate holder10. The vacuum level underneath the substrate W′ then reaches the normal level, as indicated by the dashed line inFIG. 6.

The inventors have determined that certain conditions on the height d5of the projections12and the number and positioning of the vacuum ports13are satisfied in order to enable concave substrates to be clamped. The height of the projections12should be sufficiently small so that there is some resistance to airflow inwards under a curved substrate W′ placed on the substrate holder10to allow an initial vacuum to develop underneath the central portion. At the same time however, the projections12should not be so short that the area of initial vacuum is confined too close to the vacuum ports13and a uniform vacuum level underneath the wafer cannot be obtained. The inventors have determined that to ensure clamping of curved wafers W′ the projections12should have a height no more than 100 μm. The height being measured from the surface representing most of the area of the substrate holder. Thereby the space below a substrate resting on the protrusions has a maximum depth of 100 μm (excepting where the vacuum ports open). It may also be advantageous that the height is no less than 60 μm to ensure the vacuum pressure under the substrate quickly becomes uniform when the wafer is fully clamped. Clamping is particularly effective if the burl height is in the range of from 70 to 80 μm. A substrate holder10with projections having a height of 75 μm was found to reliably clamp substrates where the maximum curvature was up to 800 μm for a 300 mm wafer.

For the number and arrangement of vacuum ports13, it is necessary that they be sufficient in number and distributed sufficiently close to the center of the wafer to generate an initial vacuum. However, the vacuum ports should not be too far from the edge of the burl plate to ensure that the clamping process of concave wafers W′ is initiated. The inventors have determined that there should be at least 10 vacuum ports within a central region. The central region is preferably bounded by a circle of radius less than or equal to 70% of the radius d1of the substrate, e.g. 100 mm for a 300 mm (diameter) substrate. There should be no vacuum ports opening outside this central region. It is particularly preferred that the vacuum ports open into an annular region having an outer radius no more than 70% of the radius of the substrate and an inner radius no less than 40% of the radius of the substrate. In the described embodiment the vacuum ports are provided on rings14,15having radii d2, d3of 90 mm and 70 mm respectively.

While specific embodiments of the invention have been described above, it will be appreciated that the aspects of the invention may be practiced otherwise than as described. The description is not intended to limit the invention.