NL2023691A - Apparatus comprising an electrostatic clamp and method - Google Patents

Abstract

An apparatus comprising an electrostatic clamp for clamping a component, and a mechanism for generating free charges adjacent to the electrostatic clamp. The electrostatic clamp comprises an electrode or a plurality of electrodes. The apparatus is configured to: operate in a first mode in which the or each electrode is set 5 at a potential such that clamping electric fields are generated between the electrostatic clamp and the component to clamp the component, operate in a second mode in which the or each potential of the or each electrode is set such that the component is unclamped, and operate in a third mode in which the or each potential of the or each electrode is set such that the flux of free charges generated by the mechanism to a surface of the component adjacent to the electrostatic clamp is increased in comparison With operating in 10 the first or second mode.

Description

FIELD [0001] The present invention relates to an apparatus comprising an electrostatic clamp, and a method of its operation. More particularly, but not exclusively, the apparatus may comprise a lithographic apparatus, the electrostatic clamp being configured to clamp a component such as a patterning device during lithographic patterning.

BACKGROUND [0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask or reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0003] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0004] A lithographic apparatus may typically use high-voltage electrostatic clamps in order to clamp patterning devices, for example, during patterning operations. Electrostatic clamps and patterning devices are often maintained in a low pressure hydrogen rich environment. This environment is nonconductive. It will be understood, therefore, that electric charge can build up on dielectric or ungrounded surfaces. For example, during operation, electric charge can accumulate on dielectric or ungrounded surfaces by touching parts (e.g. mask clamping) or by particle collisions during gas flow.

[0005] It will, also be understood that EUV radiation may cause the hydrogen rich environment to become conductive due to the creation of an EUV-induced hydrogen plasma. Free charges generated within the EUV-induced hydrogen plasma may be attracted to (or repelled by) electric fields generated by the electrostatic clamp. On the other hand, in the absence of the EUV-induced plasma, or in regions distant or well screened from any EUV-induced plasma, electric charges may accumulate on dielectric or ungrounded surfaces, and may remain after any electric field has been removed.

[0006] In addition to the accumulation of electric charges, very strong electrostatic fields (e.g. in the region of ~ 1-100 kV/cm) may be generated between pints of the electrostatic clamp and other system components. In particular, the high voltages applied to electrodes of the electrostatic clamps result in nearby conductors (e.g. conductive coatings which may be present on surfaces of the mask) being polarised. As such, strong electrostatic fields are developed, especially at sharp features (e.g. edges of conductive mask coatings).

SUMMARY [0007] According to a first aspect of the invention, there is provided an apparatus comprising an electrostatic clamp for clamping a component, and a mechanism for generating free charges adjacent to the electrostatic clamp: wherein the electrostatic clamp comprises an electrode or a plurality of electrodes, wherein the apparatus is configured to: operate in a first mode in which the or each electrode is set at a potential such that clamping electric fields are generated between the electrostatic clamp and the component to clamp the component, operate in a second mode in which the or each potential of the or each electrode is set such that the component is unclamped, and operate in a third mode in which the or each potential of the or each electrode is set such that the flux of free charges generated by the mechanism to a surface of the component adjacent to the electrostatic clamp is increased in comparison with operating in the first or second mode.

[0008] The component may be a patterning device such as a reticle or a mask. Alternatively, the component may be a substrate such as a wafer. The first aspect may enable acceleration of residual charge neutralization during unload of the component to be achieved. Thus, compensation of residual charge on the component by EUV-induced plasma during unload/load actions may be enhanced. This may avoid the occurrence of electrical breakdown leading to collateral component surface damage. In addition, throughput neutral grounding of the component may be achieved. Load and unload respectively refers to loading and unloading the component onto and from the electrostatic clamp.

[0009] The electrostatic clamp may comprise the plurality of electrodes, and wherein in the third mode the apparatus may be configured such that the potential of an edge electrode closest to an edge of the electrostatic clamp is set to be positive. This may provide additional negative bias to a second surface of the component (i.e. the surface adjacent to the clamp) which promotes positive ion flux towards the component.

[00010] In the third mode the apparatus may be configured such that the or each potential of the or each electrode is set such that the potential of the electrode or the average potential of the plurality of electrodes is negative. This may capacitively induce negative potential on the component which attracts positive ions towards the second surface of the component, e.g. reticle.

[00011] In the third mode the apparatus may be configured such that the potentials of the plurality of the electrodes are set such that the average potential of the plurality of electrodes is substantially 0V. This means that the capacitively induced potential on the component, e.g. reticle, (particularly on the second surface of the component) remains unchanged.

[00012] The electrostatic clamp may comprise the plurality of electrodes, and wherein in the third mode the apparatus may be configured such that the potential of an edge electrode closest to an edge of the electrostatic clamp is set to be negative and the potentials of the rest of the plurality of electrodes are set such that the average potential of the plurality of the electrodes is more negative than the potential of the edge electrode.

[00013] In the third mode the apparatus may be configured such that the or each potential of the or each electrode is set such that prior to the component moving from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp, the surface of the component adjacent to the electrostatic clamp has a positive potential. This means that the residual charge of the component may be neutralized at unload by elections (rather than by positive ions) therefore realizing neutralization much faster.

[00014] In the third mode the potential of the electrode or the average potential of the electrodes may be set to a predetermined negative value such that the surface of the component has a potential of substantially the same as the potential of the electrode or the average potential of the electrodes during the movement of the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp. This may prevent charging of the component, e.g. reticle, during exposure.

[00015] In the third mode the potential of the electrode or the average potential of the electrodes may be set to the predetermined negative value such that after exposure of the component, the component has substantially zero charge. This may mean that during unloading, there will not be a relatively significant increase in the potential difference between the second surface of the component, e.g. reticle, and the clamping surface of the clamp.

[00016] In the third mode the or each potential of the or each electrode may be set for at least one of: a period of time before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp, part or all of the time that it takes to move the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp, and part or all of the time that it takes to move the component from being spaced apart from the electrostatic clamp to being clamped by the electrostatic clamp.

[00017] In the third mode the potential of the electrode or the average potential of the electrodes may be set for at least pail or all of time that the mechanism is generating free charges.

[00018] The mechanism for generating free charges adjacent to the electrostatic clamp may comprise a source of gas, and a source of ionising radiation configured to ionise gas provided by the source of gas.

[00019] The source of ionising radiation may comprise at least one of an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[00020] In accordance with a second aspect of the present invention, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus according to any preceding clause, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises the component to be clamped, wherein the lithographic apparatus comprises the apparatus as described above.

[00021] In accordance with a third aspect of the present invention, there is provided a method of operating an apparatus, the apparatus comprising an electrostatic clamp, and a mechanism for generating free charges adjacent to the electrostatic clamp, the electrostatic clamp comprising an electrode or a plurality of electrodes, the method comprising: providing a component adjacent to the electrostatic clamp; controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp, operating the apparatus in a first mode in which the or each electrode is set at a potential such that clamping electric fields are generated between the electrostatic clamp and the component to clamp the component, operating the apparatus in a second mode in which the or each potential of the or each electrode is set such that the component is unclamped, and operating the apparatus in a third mode in which the or each potential of the or each electrode is set such that the flux of free charges to a surface of the component adjacent to the electrostatic clamp is increased in comparison with operating in the first or second mode.

[00022] The electrostatic may clamp comprise a plurality of electrodes, the method may further comprise: in the third mode setting the potential of an edge electrode closest to an edge of the electrostatic clamp to be positive.

[00023] The method may further comprise: in the third mode setting the or each potential of the or each electrode such that the potential of the electrode or the average potential of the plurality of the electrodes is negative.

[00024] The method may further comprise: in the third mode setting the potentials of the plurality of electrodes such that the average potential of the plurality of the electrodes is substantially 0V.

[00025] In accordance with a fourth aspect of the present invention, there is provided a computer program comprising computer readable instructions configured to cause a processor to carry out a method as described above. This has the advantage that additional hardware is not required.

[00026] In accordance with a fifth aspect of the present invention, there is provided a computer readable medium carrying a computer program as described above.

[00027] In accordance with a fifth aspect of the present invention, there is provided a computer apparatus for operating an apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS [00028] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

Figures 2a-2c depict an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention;

Figures 3a-3c depict an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention;

Figures 4a-4c depict an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention;

DETAILED DESCRIPTION [00029] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask or reticle), a projection system PS and a substrate table WT configured to support a substrate W.

[00030] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field minor device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil minor device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other minors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil minor device 11.

[00031] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[00032] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00033] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00034] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[00035] Figure 2a shows a cross section of the support structure MT in more detail. The cross section is in an x-plane, extending vertically in the z-direction and horizontally in the y-direction in the orientation shown. The y-direction may be taken as the direction of scanning of the lithographic apparatus and the x-direction may be taken as perpendicular to the scanning direction. The support structure MT comprises an electrostatic clamp 100 which is configured to clamp the patterning device MA during lithographic operations. The clamp 100 comprises a generally planar clamping surface 102, and clamp electrodes A-D disposed within the clamp body. The electrodes 104A-104D are separated from the clamping surface 102 of the clamp 100 by a dielectric coating. Burls (not shown) may protrude from the clamping surface 102, and act to separate the clamped patterning device MA from the clamping surface 102. The burls may, for example, have a height of around 10 pm, and may, collectively, cover around 1% of the surface of the clamp 100. It will be appreciated that many features of the clamp 100 (e.g, wiring, additional electrodes) are omitted for simplicity.

[00036] The clamp 100 may be considered to be part of the lithographic apparatus LA or may be considered to be part of an apparatus that forms part or is separate to the lithographic apparatus LA. The lithographic apparatus LA or the apparatus may comprise a mechanism for generating free charges. [00037] The patterning device MA is generally planar, and has first and second planar surfaces 122, 124 opposed to one another. In use (e.g. as shown in Figure 1) the first surface 122 is configured to reflect the radiation beam B, and to cause a pattern to be imparted to the beam B. In particular, a region of the first surface 122 may be patterned so as to cause the radiation beam B to become patterned. The patterning region of the first surface 122 is provided with a conductive coating. The first surface 122 may be called the patterning device MA frontside. That is, the reticle frontside is the surface of the patterning device MA facing away from the electrostatic clamp 100.

[00038] To enable the electrostatic clamp 100 to clamp the patterning device MA, the second surface 124 is provided with a conductive coating which typically covers a majority of the second surface 124. The second surface 124 may be called the patterning device MA backside. That is, the patterning device MA backside is the surface of patterning device MA facing towards the electrostatic clamp 100, In other words, the patterning device MA backside is the surface of the patterning device MA adjacent to the electrostatic clamp 100.

[00039] A baseplate 126 facing the first surface 122 of the patterning device MA is located on the opposite side from the electrostatic clamp 100. The baseplate 126 is part of an exchange device for transferring the reticle MA to and from the lithographic apparatus LA. The baseplate 126 is grounded and may be at a potential of substantially 0V.

[00040] It will be understood that the electrostatic clamp 100 may use voltages in the order of several kV in order to clamp the patterning device MA. For example the clamp 100 may be a bipolar electrostatic clamp, in which a first subset 104A, 104C of the electrodes 104A-104D are connected to one or more voltage supplies (not shown) of around + 1... 10 kV (e.g. +2 kV), and a second subset 104B, 104D of the electrodes 104A-104D are connected to one or more voltage supplies of around -1...10kV (e.g. -2 kV). As such, a high electric field may be established between the clamp 100 and the patterning device MA, causing the patterning device MA to be attracted to the clamp 100. In particular, a charge is induced in the regions of the conductive coating of the second surface 124 adjacent to the electrodes 104A-104D having an opposite sign to the applied voltages, and an attractive force established between the opposite charges at various positions across the clamp 100 and patterning device MA. The region of the clamp 100 which is configured to support the patterning device MA may be referred to as a support region. Moreover, when the clamp 100 is operated to clamp the patterning device MA, the region of the clamp 100 which is configured to generate a clamping force may be referred to as a clamping region.

[00041] The electrostatic clamp 100 may operate in a first mode in which the electrodes 104A-104D are set to potentials such that clamping electric fields are generated between the electrostatic clamp 100 and the reticle MA to clamp the reticle MA. In this first mode, the electrodes 104A-104D may be balanced, i.e. the average potential of the electrodes 104A-104D may be approximately 0V. The electrostatic clamp 100 may operate in a second mode in which the electrodes 104A-104D are set to potentials such that there is no or relatively little clamping electric fields generated between the electrostatic clamp 100 and the reticle MA. For example, typically a minimum of 300V is needed to overcome gravity of the reticle MA. So below 300V there would be no clamping. This value can vary depending on clamping surfaces quality, so it can be 100V or 200 V, etc. Thus, in the second mode the potentials of the electrodes 104A-104D are set such that the reticle MA is unclamped. In the second mode, the electrodes 104A-104D may also be balanced, i.e. the average potential of the electrodes 104A-104D may be approximately 0V. The first and second mode of operating the clamp 100 may be considered to be normal operating.

[00042] The electrodes 104A-104D each have a rectangular shape, and are arranged so as to be generally parallel to one another. In such an arrangement the four electrodes illustrated each span the width of the clamped patterning device MA in the x-direction, and each cover around a quarter of the length of the patterning device MA in the y-direction. It will be appreciated that, in other embodiments, a different number of electrodes may be used, such as 1, 2, 3, 5,6, 7, 8 or more.

[00043] During normal operation of the clamp 100, an electric field will be established between surfaces of the clamp 100 and patterning device MA. Moreover, due to the close separation between various charged surfaces, (including other components within the lithographic apparatus, such as, for example masking blades) electrostatic discharge can occur. That is, electrostatic discharge can occur between any charged surfaces with the likelihood of discharge increasing as the electric field strength increases. Electrostatic discharges can damage components. Electrostatic discharges can generate particles from surfaces, and can also release particles which were previously attached to surfaces within the lithographic apparatus. It will be understood that such release of particles is undesirable in a lithographic apparatus as particles can land on critical regions of the apparatus, possibly leading to patterning defects in processed substrates.

[00044] The high electric fields generated by electrostatic clamps strongly attract any free charges. By free charges, it is meant charges (positive - e.g. ions, or negative - e.g. electrons) that are not bound to a physical substrate, but are free to move according to electric field lines. Moreover, abundant free charges are generated during EUV exposure. For example, electrons may be generated by photoemission and also from an EUV-induced plasma which is typically generated in the presence of hydrogen gas (which is often present in lithography tools). Positive ions may also be generated within an EUV-plasma.

[00045] The plasma generation process will now be discussed in more detail. It is understood that EUV photons within the beam B will ionise hydrogen molecules, generating H2+ ions, and free electrons. In an example using 13.5 nm EUV radiation, each photon may have an energy of around 92 eV, with the ionisation energy of molecular hydrogen being around 15 eV. Thus, the generated free electrons may have sufficient energy (e.g. >75 eV) and range to create a secondary plasma relatively far from the initial ionisation event. Further, an electron released in this way (i.e. having an energy of around 75 eV) may ionise one, two, or even three further hydrogen molecules. Thus, even if the primary plasma is created only where EUV photons are incident, a secondary plasma may be created in the vicinity of the clamp.

[00046] In embodiments, it will be understood that an EUV induced plasma is required to be generated, which provides a source of free charges to the vicinity of the patterning device MA.

[00047] In some embodiments, a secondary ionisation source may be provided thereby allowing a plasma to be created in the vicinity of the electrostatic clamp by means other than the EUV source SO. Such an arrangement may reduce the overall output load of the EUV source SO. It will be understood that the above described embodiments may put additional requirements on the EUV source SO, by requiring additional EUV output than that which is required for imaging. Further, in some embodiments, it may not be possible for an EUV source to generate power continuously. Similarly, it may not be possible and/or desirable for an EUV source to provide arbitrary EUV pulse energy in the range 0-100% of the nominal output power, while also ensuring clean collector operation and pulse energy stability.

[00048] As such, in some embodiments it may be preferred to provide an alternative mechanism for the creation of a region of increased gas conductivity than a primary EUV source.

[00049] For example, a source may be provided close to the electrostatic clamp 100 and clamped patterning device MA. A plurality of sources may be used. The source may, for example, be a soft xray source or a VUV light source which is capable of operating at pressures below one bar in a clean environment. The source may comprise a low power ioniser having a power of around 0.1 -1W. In some embodiments, the source may comprise a radioactive source or an electron-beam source.

[00050] In general terms, the EUV source SO, and the source (which may, for example, comprise a soft x-ray source, or a VUV ioniser), may each be considered to be examples of a source of ionising radiation. Further still, such sources, in combination with a source of hydrogen (or other) gas, may be considered to be a mechanism for generating free charges. That is, the hydrogen plasma, which contains both positive ions, and free electi ons, can be considered to be a cloud of free charges. Further still, such free charges comprise both positive and negative free charges.

[00051] A significant voltage may be established between the clamp 100 and the patterning device MA upon removal of the patterning device MA from the clamp 100.

[00052] It will be understood that capacitances exist between several of the components in the lithographic apparatus LA. In particular the capacitance between the clamping surface 102 and the second surface 124 of the patterning device MA may be considered to be a variable capacitance, which varies as a function of a gap between the clamping surface 102 and the second surface 124. Similarly, the capacitance between the baseplate 126 and the first surface 122 of the patterning device MA may be considered to be a variable capacitance, which varies as a function of a gap between the baseplate 126 and the first surface 122.

[00053] It will be understood that in a closed system, with no charge able to enter or leave the system, and for a given initial charge state, any variation in the separation between the clamp 100 and the patterning device MA, and the separation between the patterning device MA and the baseplate 126, will result in the respective variable capacitances changing. Moreover, this change in capacitance will also result in the voltages across the capacitances changing, possibly significantly, in accordance with the changes in separation.

[00054] In particular, the relationship Q = GV must be maintained at all times for each capacitance (assuming no charge is injected). Therefore, if a capacitance C is changed, and the amount of charge Q contained in that capacitance is maintained the same, the voltage V must change in inverse proportion to the changing capacitance C. This can result in significant voltage amplification. The most significant change in voltage occurs at the back side of the patterning device MA, i.e. between the clamping surface 102 and the second surface 124 of the patterning device MA.

[00055] The high resulting voltages will be understood to significantly increase the risk of discharge due to break down of the hydrogen gas (e.g. due to the voltage at the patterning device MA surface exceeding the lowest Paschen limit for hydrogen, which is around 250V) in the vicinity of the electrostatic clamp 100 and patterning device MA.

[00056] Thus, there is a chance of electrostatic discharge within the lithographic apparatus LA during unloading of patterning devices MA after clamping. Charge can become trapped at the dielectric surfaces of the clamp 100. Furthermore, residual charge can remain on a clamped patterning device MA once it has been released. As the unclamped patterning device MA is moved away from the clamp surface, the increasing separation between the clamp surface and the patterning device surface can lead to a decrease in capacitance, and an amplification of the voltage. That is, given the proportional ίο relationship between charge and voltage (i.e. Q = C. V) in a closed system, when the capacitance changes (in inverse proportion to the separation between parallel plates), any reduction in capacitance will result in a proportional increase in voltage. Thus, as the patterning device MA and clamp 100 are separated, it is possible that the voltage of the patterning device will rise sufficiently to cause electrical breakdown of the hydrogen gas to occur. Such discharge can result in damage to the patterning device MA, the electrostatic clamp 100 and/or particle generation, which can lead to subsequent defects.

[00057] The effects of the varying capacitances can be mitigated to some extent by the introduction of free charges during the unload process. For example, the separate ionisation source, or indeed the EUV source SO, can be used to generate a hydrogen plasma, which provides free charges (as described in detail above), and allows the fields established across the various dielectric components (and gaps) to be relaxed during the removal process.

[00058] The provision of free charges may result in a significant reduction of the voltages established between the various system components. That is, the established electric fields which result from the high voltages can be compensated by the introduction of additional free charges. These charge sources are effectively provided by the hydrogen plasma. The free charges within the plasma are driven by any electric fields as they begin to be established, and cause those fields to collapse.

[00059] In this way, the potential problems associated with significant voltages being established across the patterning device MA upon removal from the electrostatic clamp 100 can be mitigated or avoided entirely. As noted above, it will be understood that this effect is not binary, and that if insufficient charge is provided, some (reduced strength) fields may still be established. It will be understood, however, that even a reduction in voltage amplification (rather than a complete avoidance) may be beneficial, especially if the voltages are thus always maintained below the lowest Paschen limit for hydrogen (of about 250V).

[00060] Moreover, the free charges may be provided at various times during the separation of the clamp 100 and patterning device MA. Indeed, it will be understood that, when the patterning device MA is clamped, it may be difficult for the free charges to penetrate between the adjacent surfaces. Thus, there may be an effective minimum separation at which free charges are to be optimally provided.

[00061] Reticles (patterning deGees) may suffer from irreversible damage due to residual charge on reticle front and back surfaces accumulated during EUV radiation exposure and reticle handling (e.g. loading and unloading of the reticle). As mentioned, this residual charge may lead to electrical breakdown during reticle unload leading to a total loss of the reticles. Reticles may come out of the lithographic apparatus LA with reticle potential at -600V corresponding to a very large negative residual charge of about 50 nC.

[00062] The reticle acquires charge due to the EUV radiation creating fast electrons and charging floating reticle surfaces to a small potential of about -10V. Alternatively, residual charge of the reticle back side may be caused by tribo-charging - friction between reticle surface and clamp burls which are made of a different material.

π [00063] As mentioned, during reticle unload capacitance of reticle-clamp system is decreasing which leads to increasing potential to about -600 V on reticle back side. This may, in some cases, lead to occurrence of electrical breakdown leading to collateral reticle surface damage.

[00064] Residual charge on reticle BS also leads to the occurrence of high field between reticle front side and baseplate leading to particle jump from baseplate to reticle FS during reticle handling in the scanner.

[00065] Reticle grounding without using additional hardware may be achieved by utilizing EUVinduced plasma as the source of charge capable of reducing reticle potential. However, providing e.g. 50 nC of charge carriers to reticle back-side surface is limited by the amount of plasma produced by the EUV radiation. Furthermore, the plasma density is reduced by the constraints of complex geometry of the environment around the reticle and the hardware associated with the reticle unload. Therefore, realization of reticle grounding using EUV-induced plasma (i.e. soft grounding) without additional aid can have a throughput penalty, i.e. an increase in the time it takes for a substrate W to pass through the lithographic apparatus LA, which is undesirable.

[00066] Figure 2a shows the polarity of the four electrodes 104A-104D and the general relative magnitude of the potentials for the electrostatic clamp 100 operating in a third mode. That is, in one embodiment, the electrodes 104A and 104D have positive potentials (+), and the electrodes 104B and 104C have negative potentials (-, -), with the magnitude of the electrode 104B being larger than the magnitude of the other electrodes. This means that the average potential of all of the electrodes 104A104D is negative.

[00067] The patterning device MA has an edge 128 (or end) which, in this embodiment, is closest to the EUV radiation beam B and thus the EUV induced plasma. Tire electrode 104A may be referred to as an edge electrode. In this embodiment, the edge electrode 104A has a positive potential as mentioned above.

[00068] The electrodes 104-104D are set to these potentials prior to the EUV radiation being turned on to produce the plasma. In the absence of plasma, the reticle MA will be floating and since the average potential of all of the electrodes 104A-104D is negative and the baseplate 126 is located on the other side of the reticle MA, then there will be a capacitively induced negative potential on the reticle MA. For example, the baseplate 126 may be at approximately 0V, the clamp 100 may be at approximately -1000V, and so the reticle MA may be at approximately -900V due to the capacitively induced current. Since the reticle MA second surface 124 is closer to the clamp 100, which has a negative potential, then the second surface 124 will be more negative than the reticle MA first surface 122. In other embodiments, the baseplate does not need to be in position and, in other embodiments, the baseplate may be exchanged for a different component. In other embodiments, the electrodes may be set to these potentials when the EUV radiation is already on, i.e. during the production of the plasma. [00069] This electrode arrangement provides increased positive ion flux to the second surface 124 of the patterning device MA while positive ion flux to the clamping surface 102 (and other clamp surfaces) of the clamp 100 is suppressed. Tire configuration of the electrodes 104A-104D provides positive nearfield at the reticle edge 128 and provides additional negative bias to reticle MA second surface 124 (i.e. the surface adjacent to the clamp 100) which promotes positive ion flux towards reticle MA. This is due to the electrode 104A located at the edge 128 being positive and so pushing positive ions away from the clamp 100 towards the reticle MA and also because the capacitively induced potential on the reticle MA from the overall average negative potential of the electrodes 104A-140D attracts positive ions towards the second surface 124 of the reticle MA. Since the second surface 124 is more negative than the first surface 122 then the positive ion flux will be attracted more to the second surface 124.

[00070] It will be appreciated that the electrostatic clamp 100 operating in the third mode means that the electrodes 104A-104D have potentials which increase the flux of positive ions to the reticle MA when compared to normal functioning of the electrostatic clamp 100, i.e. operating in the first or second mode. Previously, the clamp would not have been set to have an overall average negative or positive potential of the electrodes and so there wouldn’t be the substantial increase in the flux of free charges (electrons or ions) to the reticle when the EUV radiation was switched on. Furthermore, it will be appreciated that the third mode of operation of the electrostatic clamp 100 may include operating such that the electrostatic clamp 100 clamps the reticle MA and/or operating such that the electrostatic clamp does not clamp the reticle MA.

[00071] Figures 2b and 2c show the electrostatic clamp 100 operating in the third mode with similar electrode arrangements having a positive edge electrode 104A and an average potential of the electrodes 104A-104D being negative. However, in Figure 2b, the electrode 104C has positive potential and the electrodes 104B and 104D have negative potentials. The magnitude of the potential of the electrode 104B is still larger than the magnitude of the potential of the other electrodes. In Figure 2c, the electrode 104B has a positive potential and the electrodes 104C and 104D have negative potentials, with the magnitude of the of the potential of the electrode 104C being larger than the magnitude of the potential of the other electrodes.

[00072] These arrangements of electrodes 104A-104D, and more specifically, the particular arrangement of voltages applied to the electrodes 104A-104D enable acceleration of residual charge neutralization during reticle MA unload to be achieved. The acceleration is realized through the use of the electrodes 104A-104D as a source of additional E-field to provide higher plasma fluxes towards reticle MA surfaces. That is, the electrostatic clamp 100 electrodes 104A-104D voltages are set in such a way that net positive charge from plasma would be attracted to the reticle MA (primarily) back 124 and (secondary) front surfaces 122.

[00073] Tirus, compensation of residual charge on reticle MA by EUV-induced plasma at reticle MA unload/load actions may be enhanced.

[00074] Further advantages may be that hardware changes are not required which saves on costs of goods and shortens development time. Implementation can be realized on any lithographic apparatus

LA. Furthermore, embodiments can be customized for particular and unusual reticle MA cases such as trimmed back side coating reticle MA use. In addition, a contactless grounding solution may increase clamp 100/reticle MA lifetime.

[00075] Figures 3a-3c show further embodiments of the electrostatic clamp 100 operating in the third mode with the polarities of the four electrodes 104A-104D and the general relative magnitude of the potentials identified.

[00076] In the embodiment of Figure 3a, each of the electrodes 104A-104D have negative potentials, with the magnitude of potential of the electrodes 104B-104D being larger than the magnitude of the potential of the electrode 104A (the edge electrode). This means that the average potential of all of the electrodes 104A-104D is still negative. The edge electrode 104A is negative but the average potential of all electrodes is even more negative than the edge electrode 104A. Such an electrode arrangement provides increased flux to reticle MA second surface 124 while the flux to the clamp surface 102 is suppressed.

[00077] Thus, a similar approach can be applied to achieve <0 (negative) capacitively induced potential on reticle MA by applying net extra negative bias to electrodes 104B-104D and their combinations. The configuration of the electrodes 104A-104D provides a nearfield at the reticle edge 128 which is less negative than the rest of the reticle MA second surface 124 and provides additional negative bias to the reticle MA second surface 124 (i.e. the surface adjacent to the clamp 100) which promotes positive ion flux towards reticle MA. This can also be achieved through imbalance of potential on positive electrodes.

[00078] Figures 3b and 3c show the electrostatic clamp 100 operating in the third mode with similar electrode arrangements having a negative edge electrode 104A and the average potential of the electrodes 104A-104D being even more negative. However, in Figure 3b, the electrode 104B and 104D have positive potentials and the potential of the electrode 104C is negative and has a much larger magnitude of the potential than the other electrodes. The embodiment of Figure 3c is the same as the embodiment of Figure 3b except that the potentials of the electrodes 104B and 104C have been exchanged.

[00079] The imbalanced electrode potential can be applied before the EUV radiation beam B is “on” when the reticle MA is unclamped as well as while the reticle MA is still clamped by the clamp 100. Thus, embodiments enable reticle MA grounding while reticle MA is still on the electrostatic clamp 100 which enables reticle MA grounding before unclamp action is started and thus minimizes risk of reticle MA damage by discharge. Reticle MA residual charge can be brought to zero while reticle MA is still close to clamp 100, or even still physically connected to clamp 100. Thus, significantly minimizing risk of reticle MA damage by a discharge when gap between reticle MA and clamp 100 becomes too big during unload (in presence of fixed charge, voltage increases when capacitance is reduced by increasing gap).

[00080] Figures 4a-4c show further embodiments of the electrostatic clamp 100 operating in the third mode with the polarities of the four electrodes 104A-104D and the general relative magnitude of the potentials identified.

[00081] In the embodiment of Figure 4a, the edge electrode 104A is positive as in Figures 2a-2c but the other electrodes 104B-104D have potentials and magnitudes of potentials such that the average potential of all of the electrodes 104A-104D is substantially 0V. More particularly, electrodes 104A and 104D have positive potentials (+) and electrodes 104B and 104C have negative potentials (-), with the magnitudes of the potentials of each of the electrodes 104A-104D being substantially the same.

[00082] Therefore, the capacitively induced potential on the reticle MA (particularly on second surface 124) remains unchanged. The edge electrode 104A (which has a positive potential in this case) is protecting the clamp 100 from attracting positive ions and therefore increasing positive ion flux to reticle MA second surface 124 as well as protecting clamp burls (not shown) from sputtering. This helps maintain a relatively long lifetime of clamp functionality. Even though the capacitively induced potential on the reticle MA is substantially 0V in this embodiment, the positive ion flux is increased to the reticle MA as the positive ions are directed away from the clamp 100 by the positive edge electrode 104 A.

[00083] The configuration of electrodes 104A-104D provide positive nearfield at the reticle edge 128 promoting positive ion flux towards reticle MA while maintaining zero additional bias to reticle MA second surface 124.

[00084] Figures 4b and 4c show the electrostatic clamp 100 operating in the third mode with similar electrode arrangements having a positive edge electrode 104A and the average potential of the electrodes 104A-104D being substantially 0V. However, in Figure 4b, the electrode 104B and 104D have negative potentials and the electrode 104C is negative. The magnitudes of the potentials of each of the electrodes 104A-104D are substantially the same as in Figure 4a. The embodiment of Figure 4c is the same as the embodiment of Figure 4b except that the potentials of the electrodes 104B and 104C have been exchanged.

[00085] The embodiments of Figures 4a-4c create conditions for plasma flux (positive ions) dominantly towards the reticle MA second surface 124 while the positive ion flux to clamp 100 is suppressed. It primarily serves the purpose of minimizing damage to the clamping surface 102 (and other clamp surfaces).

[00086] It will be appreciated that the exact configurations of the electrodes 104A-104D in Figures 2a-4c described above are exemplary only and in other embodiments they may have different polarities and magnitudes as long as they provide the advantages described. For example, in Figure 4c, the polarities of the electrodes 104B, I04C could be exchanged and the magnitudes of the potentials of these electrodes 104B, 104C could be both increased to be substantially more than the electrodes 104A, 104D, as long as the magnitudes were both substantially matched. In this case, there would still be an edge electrode 104A with a positive potential and an overall average potential of the electrodes of substantially 0V.

[00087] Modelling of plasma fluxes to reticle MA may show positive ion flux to reticle MA second surface 124 enabling charge compensation in only fraction of a second (e.g. -0.1 s). This enables throughput neutral realization of reticle MA soft grounding.

[00088] Embodiments may result in ~10x higher neutralization of residual charge on reticle MA second surface 124 enabling throughout neutral realization of soft reticle grounding at unload (and at load) of reticle MA.

[00089] Another embodiment relates to setting the electrostatic clamp 100 to operate in the third mode such that the electrodes 104A-104D have an average negative potential before exposure of the patterning device (reticle) MA (i.e. when the radiation beam B is incident on the patterning device MA providing a reflected patterned EUV radiation beamB’ onto the substrate). For example, the electrodes 104A-104D may be set to have the potentials as in Figures 2a-2c or 3a-3c or another configuration where the average potential of the electrodes 104A-104D is negative.

[00090] This approach is aimed at inducing positive charge on reticle MA (in particular reticle MA (second surface 124) so that reticle MA residual charge neutralization at unload would be realized by electrons (rather than by positive ions as in previous embodiments). This makes use of the higher electron mobility compared to that of ions, therefore realizing neutralization much faster (few orders faster, i.e. instead of 10 s, neutralization may be achieved in only 0.1s). Such an approach may also enable fast neutralization of back side trimmed reticles MA. This is because back-side trimmed reticles are slower in neutralization as about 1 mm of metal coating on reticle backside at the edge is removed. Ion flux to such trimmed back side coating needs to go through a narrow slit - i.e. space between clamp and reticle back side. There is a relatively very low chance that ions can penetrate into such a slit, while this is not a problem for electrons. For example, coating retraction by 2 mm would result in an indefinite time of back side neutralization by ions but by electrons it would still be only few seconds.

[00091] For example, in order to induce positive charge on reticle MA during exposure, the clamp electrodes 104A-104D are set to provide negative reticle MA offset potential of about -1...-100 V before exposure. Once exposure is stopped, i.e. once the EUV radiation beam B is no longer incident on the patterning device MA, the reticle MA will have a positive charge (rather than a negative charge as described above). To realize this, one or more negative electrodes are set to a higher potential than the positive electrodes. For example, two positive electrodes are set to positive +1 kV and two negative electrodes are set to negative -1.1 kV. Due to this, electrons will be repelled from the reticle MA and positive ions will be attracted to the reticle MA, thus providing positive charge on the reticle MA after exposure.

[00092] Reticle MA residual charge can be brought to zero while reticle MA is still close to clamp 100, or even still physically connected to clamp 100. Thus, significantly minimizing risk of reticle MA damage by a discharge when gap between reticle MA and clamp 100 becomes too big during unload.

[00093] The conditions described above, i.e. the average potential of the electrodes 104A-104D being set to be negative, may be maintained for the full duration of the exposure or for only part of the time preceding the reticle MA unload action. In some embodiments, the electrodes could be set to a certain state (e.g. configuration and/or specific potentials) for only prut of time of the exposure. The electrodes do not have to be in the same configuration for the whole duration of exposure. It may be desired to set the electrodes to a balanced state (i.e. on average to zero) to ensure the reticle comes out neutral.

[00094] A further embodiment is aimed at preventing reticle MA charging during exposure. Again, this embodiment relates to setting the electrostatic clamp 100 in the third mode such that the electrodes 104A-104D have an average negative potential before exposure of the patterning device (reticle) MA (i.e. when the radiation beam B is incident on the patterning device MA providing a reflected patterned EUV radiation beam B’ onto the substrate). However, in this embodiment, the potential of the electrodes 104A-104D are set to provide a negative offset in potential of a specific value. This specific value may be calibrated to the specific reticle MA and exposure conditions such as EUV dose. This specific value may be measured from a previous exposures.· and then fed forward. For example, for one reticle the specific value may be set so that it provides a potential on the reticle of -2V and for another reticle MA it may be -10V. Having the reticle MA set to the calibrated value (i.e. -2V) may mean that over the exposure there will not be a substantial overall increase or decrease in the potential of the reticle MA as the transfer of charge caused by the electrons and ions will be balanced out. Thus, at the end of the exposure, the reticle MA will have the same or similar charge as before exposure (e.g. -2V). In other embodiments, the specific value may be set so that it provides a potential on the reticle in the range of 0V to -20V .

[00095] The specific value of the negative potential of the electrodes 104A-104D may be chosen such that after exposure, the reticle MA is substantially not charged, i.e. is at substantially zero charge). This means that during unloading (i.e. when the reticle MA is being moved away from the clamp 100), there will not be a significant increase in the potential difference between the second surface 124 of the reticle MA and the clamping surface 102 of the clamp 100 as seen when the reticle MA is left charged after exposure. The reticle MA residual charge being close to zero while the reticle MA is still close to clamp 100, or even still physically connected to clamp 100 significantly minimizes the risk of reticle MA damage by a discharge when gap between reticle MA and clamp 100 becomes too big during unload. In other embodiments, the specific value may be chosen such that the potential on the reticle MA matches the potential on the clamp 100 so there will not be a relative increase in the potential difference between the second surface 124 of the reticle MA and the clamping surface 102 of the clamp 100.

[00096] The conditions described above, i.e. the average potential of the electrodes 104A-104D being set to a specific negative value, may be maintained for the full duration of the exposure or for only part of the time preceding the reticle MA unload action.

[00097] It will be appreciated that embodiments may be implemented by changing software procedure without changing hardware. This means that there may be a relatively fast implementation on lithographic apparatus LA in the field and little to no impact on production. Furthermore, embodiments may be reversible and flexible. They can be used as a temporary mitigation strategy (i.e. switched on and off when needed or tuned).

[00098] As embodiments may not require additional hardware, there may be cost of goods savings when compared to other methods of implementing grounding of the reticle (e.g. during unload). Embodiments may be directly applied to all EUV lithographic apparatus LA and may result in improved reliability and availability of the lithographic apparatus LA. In addition, embodiments may be implemented with throughput neutral reticle grounding. Embodiments may result in higher yield.

[00099] 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. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[000100] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[000101] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[000102] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. earner waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other derices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[000103] 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 descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses.

1. An apparatus comprising an electrostatic clamp for clamping a component, and a mechanism for generating free charges adjacent to the electrostatic clamp:

wherein the electrostatic clamp comprises an electrode or a plurality of electrodes, wherein the apparatus is configured to:

operate in a first mode in which the or each electrode is set at a potential such that clamping electric fields are generated between the electrostatic chimp and the component to clamp the component, operate in a second mode in which the or each potential of the or each electrode is set such that the component is unclamped, and operate in a third mode in which the or each potential of the or each electrode is set such that the flux of free charges generated by the mechanism to a surface of the component adjacent to the electrostatic clamp is increased in comparison with operating in the first or second mode.

2. The apparatus of clause 1, wherein the electrostatic clamp comprises the plurality of electrodes, and wherein in the third mode the apparatus is configured such that the potential of an edge electrode closest to an edge of the electrostatic clamp is set to be positive.

3. The apparatus of either of clauses 1 or 2, wherein in the third mode the apparatus is configured such that the or each potential of the or each electrode is set such that the potential of the electrode or the average potential of the plurality of electrodes is negative.

4. The apparatus of clause 2, wherein in the third mode the apparatus is configured such that the potentials of the plurality of the electrodes are set such that the average potential of the plurality of electrodes is substantially 0V.

5. The apparatus of clause 1, wherein the electrostatic clamp comprises the plurality of electrodes, and wherein in the third mode the apparatus is configured such that the potential of an edge electrode closest to an edge of the electrostatic clamp is set to be negative and the potentials of the rest of the plurality of electrodes are set such that the average potential of the plurality of the electrodes is more negative than the potential of the edge electrode.

6. The apparatus of clause 3, wherein in the third mode the apparatus is configured such that the or each potential of the or each electrode is set such that prior to the component moving from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp, the surface of the component adjacent to the electrostatic clamp has a positive potential.

7. The apparatus of clause 3, wherein in the third mode the potential of the electrode or the average potential of the electrodes is set to a predetermined negative value such that the surface of the component has a potential of substantially the same as the potential of the electrode or the average potential of the electrodes during the movement of the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

8. The apparatus of clauses 3 or 7, wherein in the third mode the potential of the electrode or the average potential of the electrodes is set to the predetermined negative value such that after exposure of the component, the component has substantially zero charge.

9. Tire apparatus of any preceding clause, wherein in the third mode the or each potential of the or each electrode is set for at least one of: a period of time before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp, part or all of the time that it takes to move the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp, and pail or all of the time that it takes to move the component from being spaced apart from the electrostatic clamp to being clamped by the electrostatic clamp.

10. The apparatus of any preceding clause, wherein in the third mode the potential of the electrode or the average potential of the electrodes is set for at least part or all of time that the mechanism is generating free charges.

11. The apparatus of any preceding clause, wherein the mechanism for generating free charges adjacent to the electrostatic clamp comprises a source of gas, and a source of ionising radiation configured to ionise gas provided by the source of gas.

12. The apparatus of clause 11, wherein the source of ionising radiation comprises at least one of an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

13. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus according to any preceding clause, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises the component to be clamped, wherein the lithographic apparatus comprises the apparatus of any preceding clause.

14. A method of operating an apparatus, the apparatus comprising an electrostatic clamp, and a mechanism for generating free charges adjacent to the electrostatic clamp, the electrostatic clamp comprising an electrode or a plurality of electrodes, the method comprising:

providing a component adjacent to the electrostatic clamp;

controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp, operating the apparatus in a first mode in which the or each electrode is set at a potential such that clamping electric fields are generated between the electrostatic clamp and the component to clamp the component, operating the apparatus in a second mode in which the or each potential of the or each electrode is set such that the component is unclamped, and operating the apparatus in a third mode in which the or each potential of the or each electrode is set such that the flux of free charges to a surface of the component adjacent to the electrostatic clamp is increased in comparison with operating in the first or second mode.

15. The method of clause 14, wherein the electrostatic clamp comprises a plurality of electrodes, the method further comprising: in the third mode setting the potential of an edge electrode closest to an edge of the electrostatic clamp to be positive.

16. The method of clause 15, further comprising: in the third mode setting the or each potential of the or each electrode such that the potential of the electrode or the average potential of the plurality of the electrodes is negative.

17. The method of clause 15, further comprising: in the third mode setting the potentials of the plurality of electrodes such that the average potential of the plurality of the electrodes is substantially 0V.

18. A computer program comprising computer readable instructions configured to cause a processor to cany out a method according to any one of clauses 1 to 17.

19. A computer readable medium carrying a computer program according to clause 18.

20. A computer apparatus for operating an apparatus comprising:

a memory storing processor readable instructions;

and a processor arranged to read and execute instructions stored in said memory;

wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of clauses 1 to 17.

Claims (1)

CONCLUSION A device adapted to illuminate a substrate. 1/4

MT

PS WT