Patent Application
20080220382
The present invention relates to a lithographic apparatus and method.
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, 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) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called 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.
When irradiating a target portion which is located in or around the center of a substrate, the radiation beam is only incident on those target portions. However, when irradiating target portions which are on the periphery of the substrate, a part of the radiation beam used to expose the target portion will not be incident on the substrate. The radiation which is not incident on the substrate may be incident on, for example, a substrate table which holds the substrate in position. Radiation not incident on the substrate is effectively wasted. The wasted radiation will also unnecessarily heat up the patterning device, lenses and anything else which it comes into contact with. Irradiation of the substrate table may cause it to heat up and expand. Expansion of the substrate table (or any other part of the lithographic apparatus) is, in general, undesirable since it makes it more difficult to accurately apply a pattern to the target portion of a substrate. This is because expansion of parts of the lithographic apparatus can affect the path of the radiation beam, or the positioning of the substrate which the radiation beam is projected onto.
It is desirable to provide, for example, a lithographic apparatus and method which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.
According to an aspect of the present invention, there is provided a lithographic apparatus that includes an illumination system constructed and arranged to condition a beam of radiation, and a support structure constructed and arranged to support a patterning device. The patterning device serves to impart the beam of radiation with a pattern in its cross-section. The apparatus also includes a substrate table constructed and arranged to hold a substrate, a projection system constructed and arranged to project the patterned beam of radiation onto a target portion of the substrate, and a shutter system constructed and arranged to mask out parts of the patterning device or the substrate from the beam of radiation. The shutter system comprising at least one moveable shutter. The apparatus further includes a control unit arranged to receive information regarding the extent to which the patterned beam of radiation would extend across a periphery of the substrate when being projected onto the target region of the substrate. The control unit is configured to move the shutter by a predetermined amount to affect the parts of the patterning device or the substrate masked out from the beam of radiation if projecting the patterned beam of radiation onto the target portion would involve the patterned beam of radiation extending across the periphery of the substrate.
According to a second aspect of the present invention, there is provided a lithographic method that includes patterning a beam of radiation with a patterning device to impart the beam of radiation with a pattern in its cross-section, and projecting the patterned beam of radiation onto a target portion of a substrate. The method also includes masking out parts of the patterning device or the substrate from the beam of radiation using a shutter of a shutter system and, if projecting the patterned beam of radiation onto the target portion of the substrate would involve the patterned beam of radiation extending across the periphery of the substrate by a predetermined amount, moving the shutter to affect the parts of the patterning device or the substrate masked out from the beam of radiation.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:
a and 2b depict known arrangements of a lithographic apparatus, and use of those arrangements,
a-3c depict an arrangement and operating principles according to an embodiment of the present invention;
a and 4b depict known arrangements of a lithographic apparatus, and uses of those arrangements; and
a-5f depict an arrangement and operating principles of another embodiment of the present invention.
Although specific reference may 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 may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), 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 may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may 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 may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 436, 405, 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A 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.
The support structure holds the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support can use mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required 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”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may 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 may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above).
The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.
The illuminator IL may comprise adjusting means AM for adjusting 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 radiation beam PB is incident on the patterning device (e.g. mask) MA, which is held on the support structure MT. Having traversed the patterning device 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 device 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 device PM and another position sensor (which is not explicitly depicted in
The depicted apparatus can be used in the following preferred modes:
1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam PB 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.
2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam PB is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT is determined by the (de-) magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the beam PB is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
a depicts an embodiment of a shutter system SS that may be used with the lithographic apparatus shown in
The shutters S are generally in the same plane. However, this is not essential. For example, one or more of the shutters S may be located at any appropriate position in the lithographic apparatus. For example, one or more of the shutters S may be located after the patterning device MA, or located in the illuminator IL of
Referring back to
When used in step mode, the shutters S are moved to a desired position to define a certain size aperture A through which the radiation beam PB may pass. The same sized aperture A is used to irradiate all of the target portions of the substrate W. After irradiation of a given target portion, the substrate W is moved so that a different target portion may be irradiated. Before the substrate is moved, another shutter (not shown) is closed to prevent the substrate W from being irradiated while it is moved.
b shows the substrate W held in position by the substrate table WT. Irradiation of target portions TP of the substrate W has already been undertaken. It can be seen that in target portions TP in or around the center of the substrate W, the radiation beam PB which has passed through the aperture A of
If the radiation beam PB is incident on the substrate table WT, he substrate table will heat it up and expand. Expansion of the substrate table WT may affect the position or positioning of the substrate W. This may make it difficult to accurately apply a pattern to target regions of a substrate W. Additionally, it can be seen from
a-3c illustrate an arrangement and method in accordance with an embodiment of the present invention.
a illustrates an embodiment of the shutter system SS of
For target portions on the periphery of the substrate W, the radiation beam PB will extend across and beyond the periphery of the substrate W, leading to possible irradiation of the substrate table WT. If is known (or can be determined) that the radiation beam PB will, when projected onto the substrate W, extend across and beyond its periphery, the shutters may be moved. Whether or not the shutters S are moved may depend on just how far across the periphery of the substrate W the radiation beam PB will extend. For example, if it is determined or known that the radiation beam PB will, if projected, extend over the periphery by a predetermined amount, the shutters S will be moved.
The predetermined amount maybe greater than 0% and less then 100% of the cross sectional area of the patterned radiation beam that would otherwise have been projected onto the substrate. The predetermined amount may be greater than about 5%, about 10%, about 25%, or about 50% of the cross sectional area of the patterned radiation beam that would otherwise have been projected onto the substrate. The exact value and nature of the predetermined amount may vary for different applications, and for different parts of the substrate. For example, if the target portion of the substrate is square or rectangular, and three of the four corners of the target portion lie on the substrate's surface, it may not be efficient or practical to divide the target portion into smaller target portions which require multiple exposures. In this case, the fact that three corners of the target portion are on the substrate may mean that the radiation beam does not extend across the periphery of the substrate by the predetermined amount, and that therefore the shutters may not be moved. Conversely, if two or less corners of the target portion lie on the substrate, the radiation beam may be taken to extend across the periphery of the substrate by the predetermined amount, and the shutters will be moved (i.e. the predetermined amount is when less than three corners of the target portion lie on the substrate's surface). In another situation, changing the configuration of the shutters S may take longer than it would to move the substrate W for exposure of a different target region. Therefore, in the interests of throughput, it might not be beneficial to change the configuration of the shutters, especially if any expansion caused by radiation not falling on the substrate is negligible. Whether or not the shutters S are moved may therefore be a trade-off between heating and therefore expansion of parts of the lithographic apparatus and throughput.
The control unit CU may detect the position of the substrate W, or be provided with (or refer to) information regarding the position of the substrate W and the target portions TP thereon (e.g. the number of corners of a square or rectangular target portion lying inside or outside the periphery of the substrate W), and/or the movements of the shutters S. Alternatively, the movements of the shutters maybe determined beforehand, either from modelling or from calibrations and/or tests, and stored in a data file. The control unit CU can then access this data file when controlling the shutter S during a set of exposures.
It can be seen in
c depicts a schematic representation of the substrate W when its target portions TP have been irradiated using the method as described in relation to
It will be appreciated that the movement of the shutters S by the control unit CU between successive exposures may be optimized for any particular application. For example, it may be more advantageous to have slightly larger exposure areas around the periphery of the substrate W, and therefore undertake a few exposures. However, it may be more advantageous to further reduce the wastage around the periphery of the substrate W by using small exposure areas (e.g. by defining small aperture sizes in the shutter system SS), and using more exposures.
As mentioned above, by using the control unit CU and method according to the present invention, significant savings can be achieved in the amount of the radiation beam PB which is wasted (and, conversely, the amount of heat that is generated). Referring back to
a-3c describe the use of the lithographic apparatus in step mode, where static exposures are undertaken. The invention is equally applicable to scanned exposures, where the substrate W is moved relative to the radiation beam PB.
b schematically depicts the situation when the radiation beam PB has been scanned across each of the target portions TP of the substrate W in succession. It can be seen that when target portions TP around the periphery of the substrate W are irradiated, a large amount of the radiation beam PB is not incident on the substrate W. Instead, large portions of the radiation beam PB are incident on a substrate table WT supporting the substrate W. This is undesirable, for the reasons given above in relation to
a-5f illustrate an arrangement and method in accordance with an embodiment of the present invention.
b illustrates a peripheral target portion TPP of the substrate W. As described in relation to
f shows the situation when all the target portions TP of the substrate W have been exposed to the radiation beam PB according to the methods described in relation to
The shutter S shown in
In summary, the present invention may be used to minimize the areas of parts of a lithographic apparatus unnecessarily exposed to the radiation beam PB. The benefits of doing this have been described above in relation to the patterning device MA, projection system PL and the substrate table WT. It will, however, be appreciated that, depending upon the location of the shutters S of the shutter system SS, the amount of unnecessary radiation passing through or onto other parts of the lithographic apparatus may also be reduced. This reduction may lead to a reduction in the expansion of these other parts, which may make it easier to accurately apply a pattern to a target portion of a substrate.
Use of the present invention may also reduce costs. For example, it is often desirable to ensure that various parts of a lithographic apparatus have a very low thermal expansion coefficient. These parts can be very expensive, or difficult to engineer or manufacture. If the amount of heat passing onto or through such parts can be reduced, the parts may not need to exhibit the very low thermal expansion coefficients otherwise required. This is because present invention provides a way of reducing the heating of these parts. Additionally, the lifetime of parts of the lithographic apparatus may be increased by reducing the ‘wear’ on them caused by their exposure to the radiation beam (and consequential heating).
In previous lithographic apparatuses which utilize blade or shutter systems to mask out parts of the patterning device and/or substrate from the radiation beam, corrections are sometimes made for the radiation beam reflecting off the blades/shutters. For example, the intensity or exposure time may be varied to ensure that each target portion on the substrate is exposed to the same dose. It will be appreciated that such corrections may need to be made when using the method and apparatus according to embodiments of the present invention. The corrections may be different for different parts of the substrate, due to the fact that the position of the blades/shutters may be different for different parts of the substrate.
The control unit described above may be a processor or any other appropriate hardware or software. The control unit may be part of a computer program already used to control the movement of the shutters in prior art apparatuses and arrangements.
Although embodiments of the present invention have been described with reference to transmissive patterning devices, it will be appreciated that other types of patterning device (e.g. reflective patterning devices) may equally be used.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention, the invention being defined by the claims that follow.
