Electrostatic clamp and a method for manufacturing the same

An electrostatic clamp (300) and a method for manufacturing the same is disclosed. The electrostatic clamp includes a first layer (302) having a first ultra-low expansion (ULE) material, a second layer (304) coupled to the first layer, having a second ULE material, and a third layer (306), coupled to the second layer, having a third ULE material. The electrostatic clamp further includes a plurality of fluid channels (316) located between the first layer and the second layer and a composite layer (308) interposed between the second layer and the third layer. The method for manufacturing the electrostatic clamp includes forming the plurality of fluid channels, disposing the composite layer on the third layer, and coupling the third layer to the second layer. The plurality of fluid channels is configured to carry a thermally conditioned fluid for temperature regulation of a clamped object.

FIELD

The present disclosure relates to an electrostatic clamp for supporting an object, for example, a patterning device and/or a substrate in a lithographic apparatus, and a method for manufacturing the same.

BACKGROUND

Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
CD=k1λ/NA  (1)where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by decreasing the value of k1.

In order to shorten the exposure wavelength, and thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

The radiation generated by such sources will not, however, be only EUV radiation and the source may also emit at other wavelengths including infra-red (IR) radiation and deep ultra-violet (DUV) radiation. DUV radiation can be detrimental to the lithography system as it can result in a loss of contrast. Furthermore unwanted IR radiation can cause heat damage to components within the system. It is therefore known to use a spectral purity filter to increase the proportion of EUV in the transmitted radiation and to reduce or even eliminate unwanted non-EUV radiation such as DUV and IR radiation.

A lithographic apparatus using EUV radiation may require that the EUV radiation beam path, or at least substantial parts of it, must be kept in vacuum during a lithographic operation. In such vacuum regions of the lithographic apparatus, an electrostatic clamp may be used to clamp an object, such as a patterning device and/or a substrate to a structure of the lithographic apparatus, such as a patterning device table and/or a substrate table, respectively.

In addition, a lithographic apparatus using EUV radiation may require temperature regulation of, for example, the patterning device and/or the substrate. Heat produced by the EUV radiation or the unwanted non-EUV radiation may cause deformations in, for example, the patterning device and/or the substrate during a lithographic operation because of the heat absorbed by the patterning device and/or the substrate. To reduce the deformation, a coolant may be circulated through the electrostatic clamp. However, configuring an electrostatic clamp for circulating a coolant can create stress in the clamp structure. This stress may be transferred to the object (e.g., patterning device, substrate) clamped to the electrostatic clamp, resulting in deformations in the clamped object.

SUMMARY

Accordingly, there is a need for an electrostatic clamp that can be configured to securely hold an object and prevent heat-induced and stress-induced deformation in the clamped object.

According to an embodiment, an electrostatic clamp includes a first layer, a second layer, and a third layer. Each of the first, second, and third layer includes a first, second, and third ultra-low expansion (ULE) material, respectively. The first layer may be coupled to the second layer and the second layer may be coupled to the third layer. The electrostatic clamp further includes a plurality of fluid channels located between the first layer and the second layer and a composite layer interposed between the second layer and the third layer. The plurality of fluid channels may be configured to carry a thermally conditioned fluid.

In another embodiment, a device manufacturing method is provided. The method includes forming a plurality of fluid channels between a first layer and a second layer. The plurality of fluid channels may be configured to carry a thermally conditioned fluid. The method further includes disposing a composite layer on a third layer and coupling the third layer to the second layer. The composite layer may include alternating electrically conductive layers and electrically insulating layers.

Yet in another embodiment, a lithographic apparatus includes a chuck and an electrostatic clamp coupled to the chuck. The electrostatic clamp may be configured to releasably hold a patterning device. The electrostatic clamp includes a first layer having opposing first and second surfaces, a second layer having opposing third and fourth surfaces, and a third layer having opposing fifth and sixth surfaces. The fourth surface may be coupled to the first surface and the sixth surface may be coupled to the third surface. The electrostatic clamp further includes an array of channels located between the first surface and the fourth surface.

The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTION

Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

Example Reflective Lithographic System

The support structure MT holds the patterning device MA 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.

The term “patterning device” 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. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be reflective (as in lithographic apparatus100ofFIG. 1) or transmissive. 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.

The projection system, like 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, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

As here depicted, the lithographic apparatus is of a reflective type (e.g., employing a reflective mask).

In such cases, the laser is not considered to form part of the lithographic apparatus and the laser beam is passed from the laser to the source collector apparatus with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander.

In an alternative method, often termed discharge produced plasma (“DPP”) the EUV emitting plasma is produced by using an electrical discharge to vaporise a fuel. The fuel may be an element such as xenon, lithium or tin which has one or more emission lines in the EUV range. The electrical discharge may be generated by a power supply which may form part of the source collector apparatus or may be a separate entity that is connected via an electrical connection to the source collector apparatus.

In scan mode, the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device222and a facetted pupil mirror device224arranged to provide a desired angular distribution of the radiation beam221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation221at the patterning device MA, held by the support structure MT, a patterned beam226is formed and the patterned beam226is imaged by the projection system PS via reflective elements228,230onto a substrate W held by the wafer stage or substrate table WT.

More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter240may optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the Figs., for example there may be 1-6 additional reflective elements present in the projection system PS than shown inFIG. 2.

Collector optic CO, as illustrated inFIG. 2, is depicted as a nested collector with grazing incidence reflectors253,254and255, just as an example of a collector (or collector mirror). The grazing incidence reflectors253,254and255are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

Example Embodiments of Electrostatic Clamp

FIG. 3illustrates a schematic of a cross-sectional view of an electrostatic clamp300that can be implemented as a part of lithographic apparatus100, according to an embodiment. In an example of this embodiment, electrostatic clamp300may be used for holding substrate W on substrate table WT or patterning device MA on support structure MT in lithographic apparatus100.

According to an embodiment, electrostatic clamp300may comprise a multi-layered structure including a first layer302having opposing and parallel surfaces302aand302b, a second layer304having opposing and parallel surfaces304aand304b, and a third layer306having opposing and parallel surfaces306aand306b. First layer302, second layer304, and third layer306may have vertical dimensions in the range of about 1-4 mm, 1-4 mm, and 50-200 microns, respectively, according to an example of this embodiment. First layer302may be coupled to second layer304with surface302abeing in substantial contact with surface304band third layer306may be coupled to second layer304with surface304afacing surface306b. Surface306aof third layer306may define a clamping surface306aof electrostatic clamp300. Clamping surface306amay be configured to receive an object307(e.g., substrate W or patterning device MA) to be clamped to electrostatic clamp300. Object307may be clamped to be in substantial contact with clamping surface306a. Optionally, clamping surface306amay include burls305configured to be in contact with object307during clamping operation. Burls305may help to provide less contaminated contact between object307and clamping surface306aas contaminants are less likely to be on the smaller surface area of burls305than the larger surface area of clamping surface306a.

In an embodiment, first layer302, second layer304, and third layer306may comprise materials different from each other. In another embodiment, first layer302, second layer304, and third layer306may be manufactured out of one or more dielectric materials configured to support an electrostatic field during operation of electrostatic clamp300, as further explained in below. The dielectric materials may have ultra-low thermal expansion coefficients that may be equal to zero or substantially zero, such as, but not limited to, an ultra-low expansion silicon-based material (e.g., ULE® manufactured by Corning), a glass material, a ceramic material, a silicon-based glass ceramic material (e.g., ZERODUR® manufactured by SCHOTT), or a combination thereof. Any of these ultra-low expansion thermal materials may help to reduce thermal stress in the structure of electrostatic clamp300during its manufacture. Thermal stress in electrostatic clamp300if not reduced may result in one or more unwanted deformations in first layer302, second layer304, and/or third layer306, which may be transferred to object307during clamping operation.

In another embodiment, first layer302and/or second layer304may be manufactured out of one or more non-dielectric insulating materials having ultra-low thermal expansion coefficients.

Yet in another embodiment, first layer302, second layer304, and third layer306may be manufactured out of the same one or more ultra-low thermal expansion dielectric materials. Manufacturing all three layers of electrostatic clamp300from similar materials may help to further reduce thermal stress due to thermal expansion mismatch between dissimilar materials. In an example of this embodiment, first layer302, second layer304, and third layer306may be manufactured out of the ULE® material, which provides higher electrical stability than the ZERODUR® material.

As illustrated inFIG. 3, electrostatic clamp300further comprises a composite layer308interposed between second layer304and third layer306, according to an embodiment. In an example of this embodiment, composite layer308may have a vertical dimension in the range of about 50-200 nm. Composite layer308includes electrically conductive layers310and insulating layers312that are arranged in an alternating configuration. One of electrically conductive layers310is electrically isolated from the other by one of insulating layers312. AlthoughFIG. 3illustrates only two electrically conductive layers, it is to be understood that in other modifications of the invention composite layer308may include a single or more than two electrically conductive layers. In an embodiment, electrically conductive layers310and insulating layers312are coplanar.

In various examples of this embodiment, any suitable electrically conductive material such as, but not limited to, aluminium, chrome, platinum, gold, or a combination thereof, may be used to form electrically conductive layers310and any suitable insulating material such as, silicon oxide may be used to form insulating layers312. In another example, electrically conductive layers310may comprise a single layer of metal, multiple layers of a same metal, or multiple layers of different metals.

Electrically conductive layers310may be configured as electrodes310to generate an electrostatic field within third layer306for clamping object307to clamping surface306a, according to an example embodiment. The electrostatic field may be generated by providing a clamping voltage to electrodes310. The clamping voltage may induce surface image charges on a conductive surface307aof object307to electrostatically attract and clamp object307to clamping surface306a.

Electrostatic clamp300may optionally comprise an intermediate layer314interposed between composite layer308and second layer304, as illustrated inFIG. 3, according to an embodiment. Intermediate layer314may comprise a silicon-based material, such as silicon oxide, and may be configured as a bonding medium for bonding composite layer314to second layer304. In an example of this embodiment, intermediate layer314may have a vertical dimension of about 10-200 nm. Alternatively, intermediate layer314may be interposed between composite layer308and third layer306, as discussed in further details below with reference toFIGS. 5J-K.

In a further embodiment, electrostatic clamp300comprises fluid channels316, as illustrated inFIG. 3. Fluid channels316may be configured to run parallel to surface302aand carry a thermally conditioned fluid (e.g., liquids or gas), such as, but not limited to water, air, alcohols, glycols, or phase change coolants (e.g., Freons, carbon dioxide). A fluid conditioning system318coupled to electrostatic clamp300may be configured to condition the thermally conditioned fluid to a desired temperature before entering fluid channels316and to circulate it through electrostatic clamp300. The circulating thermally conditioned fluid may help to regulate temperature of electrostatic clamp300to a desired temperature. Temperature regulation of electrostatic clamp300may include absorbing unwanted heat by the thermally conditioned fluid from electrostatic clamp300. This unwanted heat may be transferred through clamping surface306aand/or burls to electrostatic clamp300from object307in a clamped state.

In an example of this embodiment, object307may be a patterning device and the unwanted heat may be transferred to the patterning device from, for example, projection system PS, and/or other systems of lithographic apparatus100during their operation. Presence of unwanted heat in the patterning device may cause deformation of the patterning device that may lead to errors in the patterns transferred from the patterning device to the substrate. To prevent this deformation, the temperature of the patterning device may be maintained at substantially room temperature (approximately 22 degrees Celsius) or any other defined operating temperature, according to various embodiments. This temperature regulation of the patterning device may include transferring of heat from the patterning device (e.g., through clamping surface306a, burls316) to electrostatic clamp300, as discussed above, and thereby reducing or eliminating the heat-induced deformation of the patterning device.

FIG. 4illustrates a schematic of a cross-sectional view of an electrostatic clamp400coupled to a chuck420, according to an embodiment. Electrostatic clamp400and chuck420can be implemented as a part of lithographic apparatus100, according to an example of this embodiment. Chuck420may be configured to couple electrostatic clamp400to substrate table WT and/or support structure MA in an example embodiment. Electrostatic clamp400may be similar to electrostatic clamp300in structure and function except for the differences described below. In an example embodiment, fluid channels316of electrostatic clamp400may be coupled to fluid channels422of chuck420through vias424, as illustrated inFIG. 4. Fluid channels422may be configured to run parallel to surface420aand to carry a thermally conditioned fluid as fluid channels316. A fluid conditioning system418coupled to chuck420may be configured to condition the thermally conditioned fluid to a desired temperature before entering fluid channels422and to circulate it through electrostatic clamp400and chuck404. The circulating thermally conditioned fluid may help to regulate temperature of electrostatic clamp400and chuck420to a desired temperature. Temperature regulation of electrostatic clamp400and chuck420may include absorbing unwanted heat by the thermally conditioned fluid from electrostatic clamp400and chuck420. The unwanted heat in electrostatic clamp400may be transferred from object307, as described above and the unwanted heat in chuck420may be transferred from electrostatic clamp400and/or other parts of lithographic apparatus100coupled to chuck420.

An Example Method for Manufacturing an Electrostatic Clamp

FIGS. 5A-Nillustrate cross-sectional views of electrostatic clamp300(as shown inFIG. 3) at select stages of its manufacturing process, according to an embodiment.

FIGS. 5A-Billustrate cross-sectional views of a partially formed electrostatic clamp300during formation of fluid channels316(as described above with reference toFIGS. 3-4), according to an embodiment. The formation of fluid channels316may include formation of trenches530on surface302aof first layer302(as shown inFIG. 5A) and formation of a stacked structure532(as shown inFIG. 5B).

The formation of trenches530may include polishing, machining, and etching of surface302a, according to an embodiment. The polishing of surface302amay be performed using any suitable polishing process, such as, but not limited to, cerium oxide slurry polishing process to obtain a smooth surface having a root mean square (RMS) roughness of about 0.5 mm or lower. Following the polishing, surface302amay be machined using standard glass machining techniques and/or patterned and etched using standard photolithography and glass etching process to form trenches530(as shown inFIG. 5A). It should be noted that the rectangular cross-sectional shape of trenches530, as illustrated inFIG. 5A, is for illustrative purposes, and is not limiting. Trenches530can have other cross-sectional shapes (e.g., conical, trapezoidal), according to various embodiments, without departing from the spirit and scope of the present invention. Subsequent to the machining, an acid etch may be performed on the machined surface302ausing, for example, an acid mixture comprising hydrofluoric acid. The acid etch may remove few microns (e.g., about 5 microns) of layer302material from machined surface302a. This removal of material from machined surface302amay help to relieve stress in layer302that may be induced from the machining process. The stress may be due to small deformations on surface302athat developed from the physical force of machining.

According to an embodiment, the acid etch process may be followed by the coupling of layer302to second layer304to form stacked structure532, as illustrated inFIG. 5B. The coupling process may include polishing of surface304b, cleaning of surfaces302aand304bfollowed by direct bonding of first layer302to second layer304. Surface304bmay be polished to a root mean square (RMS) roughness of about 0.5 mm or lower using any suitable polishing process, such as, but not limited to, cerium oxide slurry polishing process. Subsequently, first layer302may be direct bonded to second layer304to form a stacked structure532by pressing surface302aagainst surface304bunder a pressure suitable for the layer materials used. Optionally, the stacked structure532may be annealed at a temperature in the range of about 350-800 degrees Celsius to strengthen the direct bonded interface between first layer302and second layer304.

Direct bonding herein may refer to an optical contact bonding that is a bonding between substantially defect free and highly polished surfaces (e.g., surfaces302aand304b) without the use of any bonding material, such as epoxy or any other adhesive material, according to an embodiment. Optical contact bonding may result from attractive intermolecular electrostatic interactions, such as Van der Waals forces between bonding surfaces (e.g., surfaces302aand304b). Annealing the optical contact bond (as described above) may transform, for example, the Van der Waals bonds between the bonding surfaces into stronger covalent bonds, and thereby strengthen the optical contact bonded structure.

The formation of fluid channels316may be followed by thinning down of layer304to about 2 mm, according to an embodiment. Surface304bmay be polished using any suitable polishing and/or grinding technique to thin down second layer304. Alternatively, the thinning down process of second layer304may be performed prior to formation of fluid channel316by polishing surface304aand/or surface304b.

FIGS. 5C-Dillustrate cross-sectional views of a partially formed electrostatic clamp300during formation of electrically conductive layers310, according to an embodiment. The formation of electrically conductive layers310may include deposition of, for example, one or more metal layers510on third layer306, as shown inFIG. 5C. This metal deposition may be followed by a patterning and an etching process to define electrically conductive layers310, as shown inFIG. 5D. The deposition of layer510may be performed using any conventional methods suitable for metals such as, but not limited to, sputtering, thermal evaporation, atomic layer deposition (ALD), or chemical vapor deposition (CVD). The patterning process may be performed by conventional photolithography process and the etching process may be performed by wet etch methods or dry etch methods such as, but not limited to, reactive ion etching (RIE).

FIGS. 5E-Fillustrate cross-sectional views of a partially formed electrostatic clamp300during formation of insulating layers312, according to an embodiment. The formation of insulating layers312may include deposition of, for example, one or more dielectric layers512on electrically conductive layers310and exposed areas of surface306bof third layer306, as shown inFIG. 5E. This dielectric deposition may be followed by a patterning and an etching process to define insulating layers312as shown inFIG. 5F. The deposition of layer512may be performed using any conventional methods suitable for dielectric materials such as, but not limited to, CVD process, magnetron sputtering, thermal evaporation, or e-beam evaporation. The patterning and etching process may be performed by methods as mentioned above.

FIGS. 5G-Hillustrate cross-sectional views of a partially formed electrostatic clamp300during bonding of composite layer308to stacked structure532(as described with reference toFIG. 5B), according to an embodiment. This bonding process may include depositing intermediate layer314on composite layer308, as shown inFIG. 5G. The intermediate layer314may help to provide a bonding surface314ato composite layer308compatible for direct bonding with surface304a. The intermediate layer314may be deposited using any suitable methods for depositing, for example, silicon oxide, such as CVD process. The bonding process may further include pressing the combined structure ofFIG. 5Gagainst stacked structure532to bond surface314ato surface304aas shown in FIG. H. To strengthen the bonded interface between surface314aand surface304a, the bonded structure may be annealed at a temperature in the range of about 350-800 degrees Celsius.

Optionally, the bonding process may be followed by a thinning down process of third layer306to a vertical dimension in the range of about 50-200 microns, according to an embodiment. Surface306amay be polished using any suitable polishing and/or grinding technique to thin down layer306. Alternatively, the thinning down process of third layer306may be performed prior to formation of composite layer308by polishing surface306aand/or surface306b.

FIG. 5I-Jillustrate cross-sectional views of an electrostatic clamp300during formation of burls305on clamping surface306a, according to an embodiment. Burls305may be formed by depositing, for example, a polymeric layer505as shown inFIG. 5I. This deposition may be followed by patterning and etching polymeric layer505to define burls305, as shown inFIG. 5J. The patterning and etching process may be performed by methods as mentioned above. It should be noted that the rectangular cross-sectional shape of burls305is for illustrative purposes, and is not limiting. Burls305can have other cross-sectional shapes (e.g., spherical, conical, trapezoidal), according to various embodiments, without departing from the spirit and scope of the present invention.

In an alternative approach, composite layer308and intermediate layer314may be formed on surface304aof stacked structure532as illustrated inFIG. 5K, according to an embodiment. Third layer306may be direct bonded to intermediate layer314and thinned down to a vertical dimension in the range of about 50-200 microns, as shown inFIG. 5L. The direct bonding and the thinning down may be performed by methods as mentioned above.

As illustrated inFIG. 5M, in another alternative approach, a first portion508aand a second portion508bof composite layer308may be formed on surface306band surface304a, respectively, according to an embodiment. First portion508aand second portion508bmay be thermally fused together to form composite layer308, as shown inFIG. 5N. This approach of forming composite layer308in an electrostatic clamp may help to eliminate the use of a bonding medium (e.g., intermediate layer314).

An Example Method for Coupling an Electrostatic Clamp to a Chuck

FIG. 6illustrates a cross-sectional view of electrostatic clamp300during coupling of electrostatic clamp300to a chuck620. Chuck620may be similar to chuck420in structure and function, as described above with reference toFIG. 4. In an embodiment, the coupling process may include polishing and cleaning of surfaces302band620afollowed by direct bonding of these surfaces. Surfaces302band620amay be polished to a root mean square (RMS) roughness of about 0.5 mm or lower using any suitable polishing process, such as, but not limited to, cerium oxide slurry polishing process. Subsequently, surfaces302band620amay be pressed together to form a direct bond between surfaces302band620a. As would be appreciated by those skilled in the relevant art(s), other types of bonding or coupling may be used for coupling of electrostatic clamp300to chuck620.

Example Steps for Manufacturing an Electrostatic Clamp

FIG. 7illustrates a flowchart for manufacturing electrostatic clamp300and coupling electrostatic clamp300to a chuck, according to an embodiment. Solely for illustrative purposes, the steps illustrated inFIG. 7will be described with reference to example fabrication process illustrated inFIGS. 5A-5N and 6. Steps can be performed in a different order or not performed depending on specific applications.

In step702, trenches are formed on a first layer. For example, trenches such as trenches530may be formed a first layer such as first layer302, as illustrated inFIG. 5A. Trenches530may be formed using standard glass machining techniques.

In step704, the first layer is coupled to a second layer to form a stacked structure. For example, a second layer such as second layer304may be coupled to first layer302to form a stacked structure similar to stacked structure532, as illustrated inFIG. 5B. The coupling process may include direct bonding of surfaces302aand304b. Direct bonding may be performed by pressing surface302aagainst surface304bunder a pressure suitable for the layer materials used. The stacked structure532may be annealed at a temperature in the range of about 350-800 degrees Celsius.

In step706, a composite layer is formed on a third layer. For example, a composite layer similar to composite layer308may be formed on third layer306, as illustrated inFIGS. 5C-F. Composite layer308may be formed by deposition, patterning, and etching of a metal layer such as metal layer510on third layer306followed by deposition, patterning, and etching of a dielectric layer such as dielectric layer512. The deposition of metal layer510may be performed using, for example, sputtering, thermal evaporation, atomic layer deposition (ALD), or chemical vapor deposition (CVD). The deposition of dielectric layer512may be performed using, for example, CVD process, magnetron sputtering, thermal evaporation, or e-beam evaporation.

In step708, an intermediate layer is formed on the composite layer. For example, an intermediate layer similar to intermediate layer314may be formed on composite layer308, as illustrated inFIG. 5G. Intermediate layer314may be deposited using, for example, CVD process.

In step710, the third layer is coupled to the stacked structure to form an electrostatic clamp. For example, third structure306may be coupled to stacked structure532by direct bonding intermediate layer314to surface304aof stacked structure, as illustrated inFIG. 5H.

In optional step712, burls are formed on clamping surface of the clamp. For example, burls such as burls305may be formed on clamping surface such as clamping surface306aof third layer306, as illustrated inFIGS. 5I-J. Burls305may be formed by deposition, patterning, and etching of a polymeric layer505.

In optional step714, the electrostatic clamp is coupled to a chuck. For example, electrostatic clamp300may be coupled to a chuck similar to chuck620, as illustrated inFIG. 6. The coupling may be performed by direct bonding surface302bof clamp300to surface620aof chuck620.

Although specific reference may be made in this text to the use an electrostatic clamp in lithographic apparatus, it should be understood that the electrostatic clamp described herein may have other applications, such as for use in mask inspection apparatus, wafer inspection apparatus, aerial image metrology apparatus and more generally in any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device) either in vacuum or in ambient (non-vacuum) conditions, such as, for example in plasma etching apparatus or deposition apparatus.

The term “etch” or “etching” or “etch-back” as used herein generally describes a fabrication process of patterning a material, such that at least a portion of the material remains after the etch is completed. For example, generally the process of etching a material involves the steps of patterning a masking layer (e.g., photoresist or a hard mask) over the material, subsequently removing areas of the material that are no longer protected by the mask layer, and optionally removing remaining portions of the mask layer. Generally, the removing step is conducted using an “etchant” that has a “selectivity” that is higher to the material than to the mask layer. As such, the areas of material protected by the mask would remain after the etch process is complete. However, the above is provided for purposes of illustration, and is not limiting. In another example, etching may also refer to a process that does not use a mask, but still leaves behind at least a portion of the material after the etch process is complete.

The above description serves to distinguish the term “etching” from “removing.” In an embodiment, when etching a material, at least a portion of the material remains behind after the process is completed. In contrast, when removing a material, substantially all of the material is removed in the process. However, in other embodiments, ‘removing’ may incorporate etching.

The terms “deposit” or “dispose” as used herein describe the act of applying a layer of material to a substrate. Such terms are meant to describe any possible layer-forming technique including, but not limited to, thermal growth, sputtering, evaporation, chemical vapor deposition, epitaxial growth, atomic layer deposition, electroplating, etc.

The term “substrate” as used herein describes a material onto which subsequent material layers are added. In embodiments, the substrate itself may be patterned and materials added on top of it may also be patterned, or may remain without patterning.

The term “substantially” or “in substantial contact” as used herein generally describes elements or structures in physical substantial contact with each other with only a slight separation from each other which typically results from fabrication and/or misalignment tolerances. It should be understood that relative spatial descriptions between one or more particular features, structures, or characteristics (e.g., “vertically aligned,” “substantial contact,” etc.) used herein are for purposes of illustration only, and that practical implementations of the structures described herein may include fabrication and/or misalignment tolerances without departing from the spirit and scope of the present disclosure.