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Title:
APPARATUS FOR AND METHOD OF IN SITU CLAMP SURFACE ROUGHENING
Document Type and Number:
WIPO Patent Application WO/2019/224040
Kind Code:
A1
Abstract:
Disclosed is a dedicated roughening substrate provided with abrasive element useful for in situ roughening of a surface of a clamp in a semiconductor photolithography apparatus. Also disclosed is a method of using the roughening substrate in which the roughening substrate is loaded, positioned opposite the clamp, and then pressed against the clamp and moved laterally.

Inventors:
ZORDAN ENRICO (US)
Application Number:
PCT/EP2019/062252
Publication Date:
November 28, 2019
Filing Date:
May 14, 2019
Export Citation:
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Assignee:
ASML HOLDING NV (NL)
International Classes:
G03F7/20; B24B27/00; H01L21/68; H01L21/683
Domestic Patent References:
WO2016081951A12016-05-26
Foreign References:
US20100279586A12010-11-04
JP2006013308A2006-01-12
US20120024318A12012-02-02
US9455172B22016-09-27
Attorney, Agent or Firm:
SLENDERS, Peter (NL)
Download PDF:
Claims:
CLAIMS

1. Apparatus comprising:

a substrate base;

a plurality of abrasive elements, each of the abrasive elements being mechanically coupled to the substrate base by a respective coupling element.

2. Apparatus as claimed in claim 1 wherein the substrate base comprises a reticle base.

3. Apparatus as claimed in claim 1 further wherein each of the respective abrasive elements comprises a piezoelectric element arranged to move the respective abrasive element laterally under the control of a control unit.

4. Apparatus as claimed in claim 1 wherein each of the respective coupling elements comprises a respective flexure.

5. Apparatus as claimed in claim 4 wherein each respective flexure is preloaded.

6. Apparatus as claimed in claim 4 wherein each respective flexure has a low spring constant in a direction substantially perpendicular to the substrate base.

7. Apparatus as claimed in claim 4 wherein each respective flexure has a spring constant in a direction substantially perpendicular to the substrate base in a range of about 50 N/m to about 5000 N/m.

8. Apparatus as claimed in claim 1 wherein the each of the plurality of abrasive elements has substantially the same shape and size.

9. Apparatus as claimed in claim 1 wherein the plurality of abrasive elements is arranged in an array.

10. Apparatus as claimed in claim 9 wherein the array is an ordered array.

11. Apparatus as claimed in claim 9 wherein the array substantially covers the substrate base. 12. Apparatus as claimed in claim 1 wherein each of the abrasive elements comprises a ceramic material.

13. Apparatus as claimed in claim 1 wherein for each of the abrasive elements a distance between the abrasive element and the substrate base is limited by a stop.

Description:
APPARATUS FOR AND METHOD OF IN SITU CLAMP SURFACE ROUGHENING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Provisional Patent Application Number 62/674,678, which was filed on May 22, 2018, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to clamps that can be used to hold a reticle or substrate in a device for semiconductor photolithography, and more particularly to the treatment of the surface of such a clamp that comes into contact with the reticle or substrate.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the“scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0005] The object such as the patterning device or substrate is attached to an object support, such as a mask table or a wafer table, respectively, through the use of clamps. An electrostatic clamp may be provided to electrostatically clamp the object to the object support. As part of holding the object, the clamp comes into contact with the object. It is necessary to roughen the surfaces of the clamp that come into contact with the object. Otherwise there is a tendency for the object to continue to adhere to the clamp even after the electrostatic clamping forces are removed because the extremely flat surfaces of the clamp and the object can stick via optical contact. This “stickiness” can complicate and extend the time necessary for removing the object from the clamp or even make nondestructive removal impossible.

[0006] Over the lifetime of the clamp the clamp surface must be roughened periodically or when, for example, it starts to exhibit stickiness. Clamp roughening is typically carried out with the scanner/stepper offline and the clamp removed from its operating environment. This can result in significant downtime.

[0007] There is therefore a need for a system for clamp roughening which reduces machine downtime.

SUMMARY

[0008] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify key or critical elements of all embodiments nor set limits on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0009] According to one aspect of an embodiment there is disclosed a device and method in which clamp roughening can be carried out in situ by providing a roughening substrate which can be loaded in the same fashion as a reticle or substrate and then moved to and pressed against the surface(s) of the clamp requiring roughening. The roughening substrate includes abrasive elements connected to a substrate base by flexures so that the abrasive elements can conform to the surface(s) of the clamp and exert a substantially uniform normal force to the clamp surface(s). The abrasive elements are then moved back and forth laterally to roughen the clamp surface(s).

[0010] According to another aspect of an embodiment each abrasive element has a respective actuator such as a piezoelectric element that is controlled to move the abrasive element laterally.

[0011] According to another aspect of an embodiment there is disclosed an apparatus comprising a substrate base and a plurality of abrasive elements, each of the abrasive elements being mechanically coupled to the substrate base by a respective coupling element. The substrate base may comprise a reticle base. Each of the respective abrasive elements may comprise a piezoelectric element arranged to move the respective abrasive element laterally under the control of a control unit. Each of the respective coupling elements may comprise a respective flexure which may be preloaded and which may have a low spring constant in a direction substantially perpendicular to the substrate base. The spring constant in a direction substantially perpendicular to the substrate base in a range of about 50 N/m to about 5000 N/m.

[0012] Each of the plurality of abrasive elements may have substantially the same shape and size. The plurality of abrasive elements may be arranged in an array which may be an ordered array. The array may substantially cover the substrate base.

[0013] Each of the abrasive elements may comprise a ceramic material. For each of the abrasive elements a distance between the abrasive element and the substrate base may be limited by a stop.

[0014] According to another aspect of an embodiment there is disclosed a method of roughening the surface of a clamp in a semiconductor photolithography apparatus, the method comprising the steps loading a roughening substrate onto a stage, the roughening substrate having a face provided with a plurality of abrasive elements, aligning the roughening substrate with the clamp so that a surface of the clamp with burls is brought into opposition with the face of the roughening substrate having abrasive elements, pressing the abrasive elements against the surface of the clamp, roughening the burls by translating the abrasive elements substantially in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained, moving the roughening substrate away from the clamp; moving the roughening substrate to a position where the roughening reticle can be unloaded from the stage, and unloading the roughening substrate from the stage. The step of translating the roughening substrate in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained comprises determining when the burls have the desired roughness. Determining when the desired roughness has been obtained comprises determining when an amount of time has passed which is known to result in the desired roughness. Determining when the desired roughness has been obtained comprises sensing the roughness of the burls. Roughening the burls by translating the abrasive elements substantially in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained may comprise moving the substrate. A plurality of the abrasive elements may be mechanically coupled to a respective actuator and roughening the burls by translating the abrasive elements substantially in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained may comprise causing the actuators to move their respective abrasive elements substantially in a plane substantially parallel to the surface of the clamp. The actuators may comprise a piezoelectric element.

[0015] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0016] FIG. 1 depicts a lithographic apparatus in accordance with an aspect of an embodiment of the invention.

[0017] FIG. 2 depicts a partial cross-section of a conventional edge clamp and schematically illustrates the clamp pressure.

[0018] FIG. 3 is a plan elevation of a roughening substrate according to an aspect of embodiment of the invention.

[0019] FIG. 4A is a cross sectional view of a roughening substrate according to an aspect of embodiment of the invention.

[0020] FIG. 4B is a cross sectional view of a roughening substrate in engagement with a clamp according to an aspect of embodiment of the invention.

[0021] FIG. 5 is a flowchart illustrating a method of roughening a clamp surface according to an aspect of embodiment of the invention.

[0022] FIG. 6 is a plan elevation of a roughening substrate according to an aspect of another embodiment of the invention.

[0023] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein. DETAILED DESCRIPTION

[0024] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In the description that follows and in the claims the terms“up,”“down,”“top,” “bottom,”“vertical,”“horizontal,” and like terms may be employed. These terms are intended to show relative orientation only and not any orientation with respect to gravity unless otherwise indicated.

[0025] FIG. 1 schematically depicts a lithographic apparatus 100 according to an embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or EUV radiation); a support structure or support or pattern support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. including one or more dies) of the substrate W.

[0026] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0027] The support structure holds the patterning device in a manner that depends 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 structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure 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.”

[0028] The term“patterning device” as used herein should be broadly interpreted as referring to any 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, for example if the pattern includes phase- shifting features or so called assist features. 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.

[0029] The patterning device may be transmissive or reflective. Examples of patterning devices 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. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0030] The term“projection system” as used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid 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.”

[0031] The support structure and the substrate table may also be hereinafter referred to as an article support. An article includes but is not limited to a patterning device, such as a reticle, and a substrate, such as a wafer.

[0032] As herein depicted, the apparatus is of a reflective type (e.g. employing a reflective mask). Alternatively, the apparatus may be of a transmissive type (e.g. employing a transmissive mask).

[0033] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). 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.

[0034] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e. g. water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0035] Referring to FIG. 1, the illuminator IL receives a radiation beam 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 including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system if required, may be referred to as a radiation system.

[0036] The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator and a condenser. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0037] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected by the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure (e.g. mask table) MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the support structure (e.g. mask table) MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the mask alignment marks may be located between the dies.

[0038] FIG. 2 depicts a partial cross-section of an electrostatic clamp 110 as may be applied to the edge of an article such as a reticle or wafer. FIG. 2 also schematically illustrates by way of the arrows in the Figure the clamp pressure that may be generated by the clamp. The clamp 110 comprises a clamp lower portion 120 formed of an insulating material, and a clamp upper portion 130 formed of a dielectric material. The clamp upper portion 130 is formed with a plurality of burls 140. The tops of the burls 140 determine a plane 150 in which an article (not shown) is to be held. A first electrode 160 is provided between the clamp lower portion 120 and the clamp upper portion 130 and the first electrode 160 is adapted to be held at a voltage (typically 3 kV) to generate an electrostatic clamping force. A ground electrode 170 is held at ground and is spaced from the first electrode 160 by a void 180 which acts as a barrier between the first and ground electrodes. The void 108 may be filled with an insulating material, a dielectric material, or be left empty.

[0039] The downwardly pointing arrows in FIG. 2 schematically illustrate the clamping pressure that the clamp of FIG. 2 may generate. The length of the arrows is indicative of the clamping force and it will be seen that a uniform clamping pressure may be generated across with width of the first electrode the magnitude of which will depend on a number of parameters including the applied voltage, the dielectric constant of the clamp upper portion 130, and the dimensions of the various parts of the clamp 110. An article may be held in plane 150 by an electrostatic clamping force when a voltage is applied to the electrode 160. Additional details concerning the electrostatic clamps of this nature are disclosed, for example, in U.S. Patent No. 9,455,172, issued September 27, 2016, the entire contents of which are hereby incorporated by reference.

[0040] As mentioned, in operation the tops of the burls 140 come into contact with a generally flat surface of an article to be held. This can cause optical contact bonding of these surfaces which can interfere with the efficient removal of the article from the clamp. To combat this the tops of the burls can be deliberately roughened to prevent full optical contact. In general the surface roughness Ra, the arithmetic average of the absolute values of the profile height deviations from the mean line, is preferably in the range of about 3 nm to about 7 nm.

[0041] During use the burl top surfaces may become too smooth due to wear so that optical contact bonding is again possible. When this happens, it is necessary to re-roughen the tops of the burls. One way to accomplish this is to remove the clamp from the tool and roughen it manually. This, however, incurs a significant downtime penalty. Instead, it is advantageous to have a system for reconditioning the burl tops in situ and with less downtime.

[0042] The discussion that follows uses a reticle as an example, but it will be understood that the disclosed principles can be applied to other clamped articles such as substrates. FIG. 3 shows an example of a roughening reticle 200 according to one aspect of an embodiment. The roughening reticle 200 includes an array of abrasive elements 210 mounted on a reticle base 220 in a manner described below. The roughening reticle 200 can be loaded into the tool just as any other reticle, but after it is positioned facing the clamp, it is pressed against the clamp and caused to move laterally. This causes abrasive elements 210 arranged to substantially cover the surface of roughening reticle 200 to come into contact with and then roughen the tops of the burls on the clamp. The abrasive elements 210 (e.g., ceramic“stones”) on the arrangement of FIG. 2 are arranged in an ordered array as an example but other arrangements are possible as well, including arrays that are not ordered and arrays that contain abrasive elements of differing sizes and shapes. Also, the array in FIG. 2 is a 4X4 array, but it will be clear to one of ordinary skill in the art that other size arrays can be used. The overall size of the array and the abrasive elements 210 is selected so that the roughening reticle is close enough to being the same size as a standard reticle that devices designed to work with a standard reticle will also work with the roughening reticle. [0043] The abrasive elements 210 may be comprised of any material used for roughening a surface such as a ceramic material. Other examples of materials that can be used include alumina and zirconium oxide.

[0044] Parameters relevant to the roughening process include the amount of force applied on the abrasive element 210 while sliding on the clamp surface, the uniformity of the pressure between the abrasive element 210 and the burls 140 over the abrasive element area, and the roughness of the stone. One of the major challenges to in-situ roughening is securing uniform and consistent pressure over all the reticle clamp burls. This design goal is met by the system described below. As for stone roughness, the same material can come in a variety of different roughnesses. In general the range for the stone roughness for use in the systems disclosed herein is about 200 nm to about 1000 nm.

[0045] FIG. 4A shows an example of a roughening reticle 200 according to one aspect of an embodiment. The roughening reticle 4A includes a multitude of abrasive elements 210, together substantially covering the entire surface of the reticle base 220.

[0046] As mentioned, two parameters relevant to the roughening process are the amount of force applied on the abrasive element while sliding on the clamp surface and the uniformity of the pressure between abrasive element 210 and burls 140 over the abrasive element area. It is most advantageous if the pressure is uniform and consistent over all the reticle clamp burls. To control these parameters and as shown in FIG. 4A, each of the abrasive elements is provided with at least one flexure 230. The flexure 230 may be, for example, a spring. The flexure 230 is preloaded to secure a uniform minimum normal force while complying to the shape and flatness of the clamp 110 as shown in FIG. 4B. Stops 240 limit the vertical travel of the abrasive element 210 under preload from the flexure 230.

[0047] Thus each abrasive element 210 is attached to the reticle base 220 by a flexure 230.

The flexure 230 is preloaded in the z direction and designed to have a low spring constant (i.e., to be“soft”) in the z direction. The flexure 230 has a spring constant in a direction substantially perpendicular to the reticle base 220 in a range of about 50 N/m to about 5000 N/m. This ensures the desired consistency of the normal force between clamp and abrasive element, which will be approximately the same as the preload force.

[0048] Compliant preloaded flexures ensure compliance with the clamp surface. Each abrasive element of the abrasive element array is mechanically coupled to the reticle base with a flexure having a low spring constant (e.g., a spring) preloaded in the z direction (orthogonal to the array which corresponds to the xy plane) and having a low spring constant (being soft) in z, Rx (rotation about the x axis) and Ry (rotation about the y axis) and a higher spring constant (being stiffer) for all other degrees of freedom. The stops 240 constrain the movement of the abrasive element in the z direction so that the abrasive element 210 stays at most a predetermined distance from the reticle base. As the applied force is sufficient to overcome the preload for all abrasive elements 210 , the low spring force springs allow for substantially constant spring force within the displacement range in the z direction.

[0049] Thus as shown in FIG. 4B, the roughening reticle 200 is moved in the z direction to so that the force created by the displacement of the abrasive elements 210 as they press against the burls 140 on the clamp 110 overcomes the upward spring force applied by the flexures 230. In this regime the force applied by the flexures 230 is substantially constant thus applying a substantially constant normal force across the burls 140. Every abrasive element 210 contacts a certain number n of the burls 140. Every abrasive element 210 complies independently to the clamp surface.

[0050] FIG. 5 is a flow chart of an example of a clamp roughening process using the roughening reticle 200. In a step S500 the roughening reticle 200 is loaded onto a reticle stage as would be a normal reticle. In a step S510 the roughening reticle 200 is then brought into alignment with the clamp 110 so that the surface of the clamp 110 with the burls 140 is brought into opposition with the surface of the roughening reticle 200 with the abrasive elements 210. In a step S520 the reticle stage is controlled to press the roughening reticle 200 against the clamp 110 so that the abrasive elements 210 will make contact with the burls 140 to achieve a desired contact force and displacement. The flexures 230 will deform to maintain the individual forces around the preload. In a step S530 the reticle stage is then commanded to translate the roughening reticle 200 in the xy plane to perform a polishing until the desired roughness is obtained. The desired roughness will have an Ra in the range of about 3 nm to about 7 nm. Then in a step S540 the roughening reticle 200 is pulled away from the clamp. Then in a step S550 the roughening reticle 200 is moved away from the clamp to a position at which the roughening reticle 200 can be unloaded. Then in a step S560 the roughening reticle 200 is unloaded from the reticle stage.

[0051] The step S530 may be terminated when the relative motion is carried out an amount of time that is known a priori to produce the desired surface roughness. Alternatively, part of carrying out the step S530 may involve making an affirmative determination, for example optically using sensor data, that the desired surface roughness has been obtained in which case the relative xy motion is carried out until this determination is positive.

[0052] In an alternative embodiment shown in FIG. 6, each individual abrasive element

210 is individually actuated with an individual actuator 250 (for example with a piezoelectric element). The actuators 250 are under the control of a control unit 260 to move their respective abrasive elements 210 laterally, that is, in the xy plane. This force can be applied in a coordinated manner such that the total net exerted lateral force would be substantially zero with the motion of abrasive elements 210 in the xy plane in varying directions to that they neutralize each other overall.

[0053] The embodiments may further be described using the following clauses:

1. Apparatus comprising:

a substrate base;

a plurality of abrasive elements, each of the abrasive elements being mechanically coupled to the substrate base by a respective coupling element.

2. Apparatus of clause 1 wherein the substrate base comprises a reticle base.

3. Apparatus of clause 1 further wherein each of the respective abrasive elements comprises a piezoelectric element arranged to move the respective abrasive element laterally under the control of a control unit.

4. Apparatus of clause 1 wherein each of the respective coupling elements comprises a respective flexure.

5. Apparatus of clause 4 wherein each respective flexure is preloaded.

6. Apparatus of clause 4 wherein each respective flexure has a low spring constant in a direction substantially perpendicular to the substrate base.

7. Apparatus of clause 4 wherein each respective flexure has a spring constant in a direction substantially perpendicular to the substrate base in a range of about 50 N/m to about 5000 N/m.

8. Apparatus of clause 1 wherein the each of the plurality of abrasive elements has substantially the same shape and size.

9. Apparatus of clause 1 wherein the plurality of abrasive elements is arranged in an array.

10. Apparatus of clause 9 wherein the array is an ordered array.

11. Apparatus of clause 9 wherein the array substantially covers the substrate base. 12. Apparatus of clause 1 wherein each of the abrasive elements comprises a ceramic material.

13. Apparatus of clause 1 wherein for each of the abrasive elements a distance between the abrasive element and the substrate base is limited by a stop.

14. A method of roughening the surface of a clamp in a semiconductor photolithography apparatus, the method comprising the steps:

loading a roughening substrate onto a stage, the roughening substrate having a face provided with a plurality of abrasive elements;

aligning the roughening substrate with the clamp so that a surface of the clamp with burls is brought into opposition with a face of the roughening substrate having abrasive elements; pressing the abrasive elements against the surface of the clamp;

roughening the burls by translating the abrasive elements substantially in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained;

moving the roughening substrate away from the clamp;

moving the roughening substrate to a position where the roughening reticle can be unloaded from the stage; and

unloading the roughening substrate from the stage.

15. A method of clause 14 wherein the step of translating the roughening substrate in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained comprises determining when the burls have the desired roughness.

16. A method of clause 15 wherein determining when the desired roughness has been obtained comprises determining when an amount of time has passed which is known to result in the desired roughness.

17. A method of clause 15 wherein determining when the desired roughness has been obtained comprises sensing the roughness of the burls.

18. A method of clause 14 wherein roughening the burls by translating the abrasive elements substantially in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained comprises moving the substrate.

19. A method of clause 14 wherein a plurality of the abrasive elements are mechanically coupled to a respective actuator and wherein roughening the burls by translating the abrasive elements substantially in a plane substantially parallel to the surface of the clamp until a desired roughness is obtained comprises causing the actuators to move their respective abrasive elements substantially in a plane substantially parallel to the surface of the clamp.

20. A method of clause 19 wherein the actuators comprise a piezoelectric element.

[0054] The present disclosure is made the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. For example, the metrology module functions can be divided among several systems or performed at least in part by an overall control system.

[0055] The above description includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term“includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term“comprising” as “comprising” is construed when employed as a transitional word in a claim. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.