Particle removal from wafer table and photomask

A method includes moving a sticky structure to a wafer table such that a first particle on the wafer table is adhered to the sticky structure, moving the sticky structure away from the wafer table after the first particle is adhered to the sticky structure, and performing a lithography process to a wafer held by the wafer table after moving the sticky structure away from the wafer table.

BACKGROUND

The fabrication of integrated circuits (“IC”) devices involves the performance of a range of processing steps. In particular, patterned layers of various materials are applied to a substrate to create the desired device. The patterns of the layers are accurately aligned to ensure proper operation of the resultant circuit. Misalignment of the layers will degrade the performance of the IC. As IC designs have become increasingly complex, the critical dimensions (“CDs”) thereof have been correspondingly reduced, resulting in a reduction in acceptable relative displacement of the various IC device layers.

DETAILED DESCRIPTION

As the scaling down process continues to advance, alignment and overlay issues in lithography process becomes more challenging due to the ever-decreasing device sizes. A small alignment or overlay error during fabrication may lead to the failure of a wafer. Various devices and techniques have been utilized to minimize misalignment during fabrication. For example, alignment marks may be used to ensure correct alignment between wafers as they are loaded into a semiconductor fabrication tool. As another example, a wafer leveling system may be used to ensure the wafer is flat during fabrication. However, particles generated by various fabrication processes may still cause alignment problems for semiconductor fabrication processes, particularly if these particles are located on a wafer table or a photomask (also referred to as reticle). Therefore, embodiments of the present disclosure provide a clean device to remove particles from the wafer table and/or the photomask.

FIG. 1schematically illustrates a lithographic apparatus according to some embodiments of the present disclosure. The apparatus includes an illumination system (illuminator)130configured to condition a radiation beam RB1(e.g., UV radiation or any other suitable radiation), a support structure (e.g., a photomask table)140constructed to support a patterning device (e.g., a photomask) MA and connected to a first positioning device (also referred to as a photomask positioning device)150configured to accurately position the patterning device MA in accordance with certain parameters. The apparatus also includes a substrate table (e.g., a wafer table)170or “substrate support” constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioning device (also referred to as a wafer positioning device)180configured to accurately position the substrate W in accordance with certain parameters. In some embodiments, the wafer table170can be referred to as a holding device because it can hold the wafer W. The apparatus further includes a projection system (e.g., a refractive projection lens system)160configured to project a pattern imparted to the radiation beam RB1by patterning device MA onto a target place of the substrate W.

In some embodiments, the illumination system130may 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 beam RB1.

The support structure140supports, i.e., bears the weight of, the patterning device MA. In some embodiments, the support structure140can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure140may be a frame or a table, for example, which may be fixed or movable. The support structure140may ensure that the patterning device MA is at a desired position, for example with respect to the projection system160.

In some embodiments, the patterning device MA is any device that can be used to impart a radiation beam with a pattern in its cross-section so as to create a pattern in the substrate W (i.e., wafer). It is noted that the pattern imparted to the radiation beam RB1may not exactly correspond to the desired pattern in the substrate W, for example if the pattern includes phase-shifting features. Generally, the pattern imparted to the radiation beam RB1will correspond to a particular functional layer in a device being created in the substrate W, such as an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples of patterning devices MA include photomasks (also referred to as reticles), programmable mirror arrays, and programmable LCD panels. Masks 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.

In some embodiments, the projection system160is 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.

As here depicted, the lithographic apparatus is of a transmissive type (e.g., employing a transmissive photomask). Alternatively, the lithographic apparatus may be of a reflective type (e.g., employing a programmable mirror array, or employing a reflective photomask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or “substrate supports” (and/or two or more photomask tables or “photomask supports”). In such “multiple stage” machines, the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system160and the substrate W. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the photomask MA and the projection system160. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that the substrate W must be submerged in liquid, but rather only means that a liquid is located between the projection system160and the substrate W during exposure.

Referring toFIG. 1, the illuminator130receives a radiation beam RB1from a radiation source110. The radiation source and the lithographic apparatus may be separate entities, for example when the radiation source is an excimer laser. In such cases, the radiation source110is not considered to form part of the lithographic apparatus and the radiation beam RB1is passed from the source110to the illuminator130with the aid of a beam delivery system120including, for example, suitable directing mirrors and/or a beam expander. In other cases, the radiation source110may be an integral part of the lithographic apparatus, for example when the radiation source110is a mercury lamp. The source110and the illuminator130, together with the optional beam delivery system120, may be referred to as a radiation system.

In some embodiments, the illuminator130may include an adjuster132configured to adjust the angular intensity distribution of the radiation beam RB1. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator130can be adjusted. In addition, the illuminator130may include various other components, such as an integrator134and a condenser136. The illuminator130may be used to condition the radiation beam RB1, to have a desired uniformity and intensity distribution.

The radiation beam RB1is incident on the photomask MA, which is held on the photomask positioning device150, and is patterned by the photomask MA. Having traversed the photomask MA, the radiation beam RB1passes through the projection system160, which focuses the radiation beam RB1onto a target portion of the wafer W on the wafer table170. With the aid of the wafer positioning device180, the wafer table170can be moved accurately, e.g., so as to position different target portions of the wafer in the path of the radiation beam RB1. Similarly, the photomask positioning device150can be used to accurately position the photomask MA with respect to the path of the radiation beam RB1, e.g., after mechanical retrieval from a photomask library, or during a scan. In general, movement of the photomask table140may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the photomask positioning device150. Similarly, movement of the wafer table170may be realized using the wafer positioning device180. In the case of a stepper (as opposed to a scanner), the photomask table140may be connected to a short-stroke actuator only, or may be fixed. Photomask MA on the photomask table140and the wafer W on the wafer table170may be aligned using photomask alignment marks on the photomask MA and wafer alignment marks on the wafer W.

In some embodiments, the apparatus further includes a level sensor200that can determine a height map of a top surface of the wafer W. This height map of the wafer W may be used to correct the position of the wafer W during projection of a pattern on the wafer W. Moreover, the level sensor200can be also used to measure heights of various portions of the top surface of the wafer table170, so as to identify particles on the top surface of the wafer table170.FIGS. 2 and 3show exemplary particle identification using an exemplary level sensor200, whereinFIG. 2is a schematic side view, andFIG. 3is a schematic bottom view. The level sensor200can determine a height map on the wafer table170. Because the height variation on the wafer table170results from particles distributed on the wafer table170, the height map can be in turn used to identify particles on the wafer table170and thus be used to generate a particle map. In some embodiments, the level sensor200may be arranged in a stand-alone device, or integrated in a lithographic apparatus similar to the lithographic apparatus as shown inFIG. 1.

In some embodiments, the level sensor200includes a projection unit210, a detection unit220and a processing unit230. In some embodiments, the projection unit210comprises a light source212and a projection grating214. The light source212may be, for example, a broadband light source but a polarized or non-polarized laser beam can also be used. The light source212provides a measurement beam MB, which is directed to the projection grating214. The projection grating214comprises a pattern resulting in a patterned measurement beam MB.

In some embodiments, the wafer table170and/or the level sensor200may be moved with respect to each other to align different measurement areas WA on wafer table170with the measurement beam MB. For example, the wafer table170may be moved with respect to the level sensor200by using movement of the wafer positioning device180. In some embodiments, a single light source covers the whole measurement area WA. In alternative embodiments, two or more light sources may be provided to cover the measurement area WA.

The detection unit220is arranged to receive the measurement beam MB after reflection on the wafer table170. In some embodiments, the detection unit220includes a detection grating222and at least a one-dimensional array of detection elements224, for example a CMOS or CCD sensor. In some embodiments, the one-dimensional array of detection elements224may be a continuous array of detection elements made up by a single CMOS or CCD sensor. The array may for instance comprise a row of 1024 pixels, each pixel forming a detection element224. A continuous array may have the advantage that all detection elements224, for example pixels, are arranged adjacent to each other and fit together so that there is less light loss between the detection elements224, and as a result also less information loss with respect to the height of the surface area of the wafer table170within the measurement area WA.

As illustrated inFIG. 3, each detection element224is associated with a measurement subarea WA1of the measurement area WA, i.e., the detection element224receives a part of the measurement beam MB reflected by the respective measurement subarea WA1of the measurement area WA. Thus, the amount of light received by a detection element224relates to a height level of the associated measurement subarea WA1on the wafer table170.

The detection grating222is arranged to receive, at least partly, the measurement beam MB, and the light of the measurement beam MB transmitted by the detection grating222will be received by the detection elements224. The detection elements224are each configured to provide a measurement signal based on the amount of light received by the respective detection element224. The measurement signals generated from the detection elements224are fed to the processing unit230.

The processing unit230is configured to calculate a height level within the measurement area WA based on the measurement signals generated from the detection elements224. Each detection element224is associated with a measurement subarea WA1of the measurement area WA. Thus, each detection element224can be used to determine a height level of the respective measurement subarea WA1of the measurement area WA. When the processing unit230calculates a height level for each measurement subarea, a height map of the top surface of the wafer table170with high resolution can be obtained. Because the height variation of the wafer table results from particles on the wafer table170, the processing unit230can identify particles on the wafer table170based on the calculated height map and thus generate a particle map. In some embodiments, the processing unit230is a central processing unit (CPU) in a computer or the like.

FIG. 4illustrates an exemplary wafer positioning device180in accordance with some embodiments of the present disclosure. In some embodiments, the wafer positioning device180includes one or more first actuators182and one or more first guides184. The one or more first actuators182are movable along respective first guides184that extend in X-direction. Moreover, the wafer positioning device180further comprises a second actuator186and a second guide188. The second guide188is connected between the first actuators182and extends in Y-direction. The second actuator186is movable along the second guide188. The wafer table170is fixed to the second actuator186. In this way, the wafer positioning device180can move or transport the wafer table170either in X-direction or in Y-direction. The wafer positioning device180can thus referred to as a transportation device in some embodiments. In some embodiments, the actuators182and/or186may include any type of positioning actuator, such as a piezo-electric actuator, a pneumatic actuator, a linear motor, a Lorentz actuator, a cam disk or a spindle.

Movement of the wafer table170using the transportation device180not only allows the level sensor200to obtain the particle map from an entire top surface of the wafer table170, but also allows a cleaning operation to remove particles from the wafer table170using a particle removal device300as shown inFIG. 1.

The particle removal device300includes a motion actuator310and a cleaner320fixed to the motion actuator310via a cantilever330. The cleaner320has a sticky structure322to which particles can be adhered. For example, the sticky structure322may be made of polyimide, kaoline or other sticky materials. As a result, when the transportation device180moves the wafer table170is to a position vertically below the sticky structure322, the cleaner320can be lowered to the wafer table170by the motion actuator310, so that particles on the wafer table170can be adhered to the sticky structure322.

In some embodiments, the motion actuator310refers generally to any type of machine or mechanism capable of actuating motion of the cantilever330. For example, the motion actuator310may be a liner motor, a tubular electromagnetic actuator, a hydraulic actuator, a ball screw drive or the like.

In some embodiments where the motion actuator310is a linear motor, the linear motor310has a stator312and a reaction plate314. The stator312is typically a three phase winding in a laminated iron core. When the stator312is energized from an AC power source, a traveling wave magnetic field is produced. The reaction plate314is the equivalent of a rotor in a rotary induction motor and comprises a sheet of conductive material often having a flat plate of backing material. The magnetic field produces a force that propels the reaction plate314linearly along the surface of the stator312. Reversing two phases of the power supply reverses the direction of the magnetic wave and, thus, the reaction plate314. In this way, the reaction plate314can perform a liner movement along the stator312, which in turn will lift or lower the cleaner320fixed to the reaction plate314. As a result, after the wafer table170is moved to a position vertically below the cleaner320by the transportation device180, the cleaner320can be lowered such that the sticky structure322can be in contact with the top surface of the wafer table170, so that particles on the top surface of the wafer table170can be adhered to the sticky structure322.

In some embodiments, the stator312extends vertically. More particularly, the stator312is vertically elongated and has a length that is determined by the desired travel distance of the cleaner320. The reaction plate314is attached to the stator312in such a manner that it is free to move relative thereto vertically along the stator312. The position and movement of the reaction plate314relative to the stator312is determined by a controller, which will be explained in detail later.

Also shown inFIG. 1are two frames: a reference frame420, which is also known as a so-called “metrology” frame, and a base frame410.

The reference frame420provides a reference surface with respect to which the wafer and/or the wafer table170is measured, and is mechanically isolated from the main apparatus structure. For example, the reference frame420is dynamically and thermally isolated from the base frame410. The reference frame420supports sensitive components such as level sensor200. Additionally, depending on the particular lithographic apparatus, the reference frame420may also support the projections system160. Moreover, in some embodiments, the reference frame420supports the stator312of the linear motor310.

In some embodiments, a bottom end of the stator312is fixed to a top surface of the reference frame420, and the projection unit210and the detection unit220of the level sensor200is fixed to a bottom surface of the reference frame420. In some embodiments, the reference frame420has a through hole422vertically below the cleaner320. The through hole422has a diameter greater than a diameter of the bar-shaped cleaner320, so that the bar-shaped cleaner320can pass through the through hole422. In some other embodiments, the stator312is fixed to a bottom surface of the reference frame420, and movement of the cleaner320is confined under the reference frame420. In such embodiments, the through hole422can be omitted from the reference frame420.

In some embodiments, the sticky structure322is a roller ball that is multi-directionally rotatable, so that the sticky structure322is free to roll when the sticky structure322is in contact with the wafer table170. In this manner, the wafer table170can be horizontally moved (e.g., moved in the X-Y plane) by the transportation device180when the sticky structure322is in contact with the wafer table170, so that the particles on different portions on the wafer table170can be adhered to the sticky roller ball322.

An example of the roller ball structure is illustrated in a cross-sectional view ofFIG. 5. In some embodiments, the cleaner320includes the sticky roller ball322, a housing324, a seal326and a plurality of bearing balls328. The sticky roller ball322protrudes from the housing324but is sealed to it by the seal326. A large number of small bearing balls328rotatably support the sticky roller ball322against a hemi-spherical surface of the housing324. In some embodiments, the bearing balls328may be made of an anti-adhesive material or coated with an anti-adhesive layer, so as to prevent the bearing balls328from being adhered to the sticky roller ball322, which in turn will facilitate free rolling of the sticky roller ball322.

FIG. 6is a flowchart of a wafer table cleaning method M1in accordance with some embodiments.FIGS. 7-10illustrate the wafer table cleaning method M1at various stages in accordance with some embodiments. The wafer table cleaning method M1may be implemented in a fab for fabricating a semiconductor device on a wafer using a photomask. It is understood that additional operations may be implemented before, during, and after the method M1, and some of the operations may be replaced, eliminated, or moved around for additional embodiments of the method M1.

The method M1begins at block S11where the wafer table is inspected to generate a particle map. With reference toFIG. 7, in some embodiments, inspecting the wafer table170can be performed by the level sensor200, as discussed previously with respect toFIG. 2. For example, the level sensor200can measure heights of various measurement areas on the wafer table170, so as to identify particles P1on the top surface of the wafer table170and thus generate a particle map PM1showing one or more identified particle locations PL1, as illustrated inFIG. 11. For example, if the measured height of a measurement area is higher than a predetermined value, the processing unit230(SeeFIG. 2) can determine there are one or more particles on this measurement area. In some embodiments, during inspecting the wafer table170, the wafer table170is moved either in X-direction (indicated by the double headed arrow D11inFIG. 7) or in Y-direction using the transportation device180as illustrated inFIG. 4. Various measurement areas on the wafer table170are aligned with the measurement beam MB generated from the level sensor200. In this way, the level sensor200can measure heights of various measurement areas on the wafer table170, so as to inspect the entire top surface of the wafer table170.

In some embodiments, the transportation device180is controlled by a controller C1, as illustrated in the block diagram ofFIG. 11. The controller C1is electrically connected to a transportation path library TPL that stores at least predetermined transportation paths P1and P2. The predetermined transportation path P1is a path that allows every measurement areas on the wafer table170to move to align with the measurement beam MB generated from the level sensor200. The predetermined transportation path P2is a path that allows the wafer table170to move from the position under the level sensor200to the position under the cleaner320. In block S11of the method M1, the controller C1is programmed to control the wafer transportation device180to move the wafer table170along the predetermined transportation path P1during the wafer table inspection.

In some embodiments, the controller C1is a programmable processor or microprocessor. In some embodiments, the transportation path library TPL is a non-transitory computer-readable media. Examples of a non-transitory computer readable recording medium include, but are not limited to, external/removable and/or internal/built-in storage or memory unit, e.g., one or more of an optical disk, such as a DVD, a magnetic disk, such as a hard disk, a semiconductor memory, such as a ROM, a RAM, a memory card, and the like.

Returning toFIG. 6, the method M1then proceeds to block S12where the wafer table is transported to a position under the cleaner. With reference toFIG. 8, in some embodiments, the wafer table170is moved in X-direction (indicated by the arrow D12) using the transportation device180to the position vertically below the cleaner320. In some embodiments, moving the wafer table170includes actuating the actuators182to move along the X-directional guides184, as illustrated inFIG. 4. In some embodiments, the controller C1is programmed to control the transportation device180to move the wafer table170along the predetermined transportation path P2after completing the wafer table inspection. In other words, the controller C2triggers transporting the wafer table170from the position under the level sensor200to the position under the cleaner320after completing the wafer table inspection.

Returning toFIG. 6, the method M1then proceeds to block S13where the cleaner is lowered to the wafer table. With reference toFIG. 9, in some embodiments, the cleaner320is lowered using the motion actuator310(as indicated by the arrow D13). In some embodiments, the motion actuator310is controlled by a controller C2, as illustrated in the block diagram ofFIG. 11. The controllers C1and C2can be programmed in such a way that the controller C2triggers the actuation of the motion actuator310after completing transportation of the wafer table170using the transportation device180controlled by the controller C1.

Referring toFIG. 6, the method M1then proceeds to block S14where the wafer table is moved when being in contact with the sticky structure. With reference toFIG. 10, in some embodiments, when the sticky structure322is in contact with the wafer table170, the wafer table170can be moved either in X-direction (indicated by the double headed arrow D14) or in Y-direction using the transportation device180. In some embodiments where the sticky structure322is a sticky roller ball as discussed previously with respect toFIG. 5, the sticky roller ball322in contact with the wafer table170will roll due to the movement of the wafer table170in X-Y plane. In this way, the particles P1on the wafer table170will be adhered to the sticky roller ball322and thus removed from the wafer table170.

In some embodiments, the controller C1is in electrical communication with the processing unit230, so that the controller C1can receive the particle map PM1generated from the processing unit230. In some embodiments, the controller C1is programmed to control the transportation device180to move the wafer table170along a path associated with identified particle locations PL1in the particle map PM1when the sticky structure322is in contact with the wafer table170.

After removing particles P from the wafer table170, the method M1then proceeds to block S15where the cleaner320can be lifted away from the wafer table170using the motion actuator310, resulting in separating the sticky structure322from the wafer table170. Afterwards, in block S16of the method M1, the wafer table170can be moved to a position under the projection system160using the transportation device180. Thereafter, in block S17of the method M1, a lithography process is performed to the wafer W held by the cleaned wafer table170, as illustrated inFIG. 1. In some embodiments, the lithography process can be performed using the radiation beam RB1passing through the photomask MA, as illustrated inFIG. 1.

FIG. 12illustrates a cross-sectional view of another cleaner320asimilar to the cleaner320, except for a heating device321having a heater321hthermally coupled to the sticky structure322. In greater detail, the heating device321a thermally conductive structure321thaving one end in contact with the sticky structure322and another end in contact with the heater321h. In this way, the heating device321can heat the sticky structure322. If the particles are adhered to the wafer table170, heating the particles may result in reduced adhesion between the particles and the wafer table170, which in turn will benefit removing particles from the wafer table170using the sticky structure322.

FIG. 13illustrates a cross-sectional view of another cleaner320bsimilar to the cleaner320, except for a vibrator323in the housing324. The vibrator323is configured to vibrate the particles on the wafer table170. In some embodiments, vibration frequency of the vibrator323is tuned such that the vibration will induce resonance of particles on the wafer table170. If the particles are adhered to the wafer table170, the induced resonance of particles may result in reduced adhesion between the particles and the wafer table170, which in turn will benefit removing particles from the wafer table170using the sticky structure322.

FIG. 14illustrates a cross-sectional view of another cleaner320csimilar to the cleaner320, except for a cleaning tool325in the housing324and above the sticky structure322. In some embodiments, the cleaning tool325is a scrapper having a tool tip325tin contact with the sticky structure322. As a result, when the sticky structure322adhered with particles rolls due to the horizontal movement of the wafer table170, the tool tip325tcan scratch the particles away from the sticky structure322, which in turn will clean the sticky structure322. Stated differently, the cleaning tool325can act as a particle blocking structure that blocks the particles on the sticky structure322when the particles arrive at the particle blocking structure. In this way, the particles can be detached from the sticky structure322.

FIG. 15is a flowchart of a wafer table cleaning method M2in accordance with some embodiments.FIGS. 16-21illustrate the wafer table cleaning method M2at various stages in accordance with some embodiments. The wafer table cleaning method M2may be implemented in a fab for fabricating a semiconductor device on a wafer using a photomask. It is understood that additional operations may be implemented before, during, and after the method M2, and some of the operations may be replaced, eliminated, or moved around for additional embodiments of the method M2.

In block S21of the method M2, the wafer table170is inspected to generate a particle map PM1, as discussed previously with respect to block S11of the method M1. Thereafter, in block S22of the method M2, the wafer table170is moved such that an identified particle P1on the wafer table170associated with an identified particle location PL in the particle map PM1is moved in X-direction (indicated by the arrow D21) to a position vertically below the cleaner320, as illustrated inFIG. 16. Movement of the wafer table170can be performed using the transportation device180, as discussed previously with respect to block S12of the method M1.

Afterwards, in block S23of the method M2, the cleaner320is lowered until reaching the identified particle P1on the wafer table170, as illustrated inFIG. 17where the cleaner320is moved downwards (indicated by the arrow D22). Lowering the cleaner320can be performed using the motion actuator310, as discussed previously with respect to block S13of the method M1. Thereafter, in block S24of the method M2, the cleaner320is lifted so as to lift the particle P1away from the wafer table170, as illustrated inFIG. 18where the cleaner320is move upwards (indicated by the arrow D23).

The method M2then proceeds back to block S22where the wafer table170is moved in X-direction (indicated by the arrow D24inFIG. 19) and/or in Y-direction such that another identified particle P1on the wafer table170, associated with another identified particle location PL in the particle map PM1, is moved to the position vertically below the cleaner320, as illustrated inFIG. 19. Afterwards, the method M2proceeds to blocks S23where the cleaner320is lowered until reaching the another identified particle P1on the wafer table170, as illustrated inFIG. 20where the cleaner320is moved downwards (indicated by the arrow D25). Thereafter, in block S24of the method M2, the cleaner320is lifted so as to lift the another particle P1away from the wafer table170, as illustrated inFIG. 21where the cleaner320is moved upwards (indicated by the arrow D26).

Blocks S22, S23and S24of the method M2are in combination serve as a cyclic operation. After one or more repetitions of the cyclic operation to remove all identified particles from the wafer table170, the method M2proceeds to block S25where the wafer table170can be moved to a position under the projection system160using the transportation device180. Thereafter, in block S26of the method M2, a lithography process is performed to the wafer W (SeeFIG. 1) held by the cleaned wafer table170.

In some embodiments of method M2, the wafer table170remains stationary during the period that the sticky structure322is in contact with the wafer table170. As a result, the sticky structure322might not roll because of lack of horizontal movement of the wafer table170. Embodiments of the present disclosure thus provide another cleaner320dthat is capable of actively rolling the sticky structure322thereof, as illustrated inFIG. 22.

As illustrated inFIG. 22, the cleaner320dincludes the sticky structure322, a first rotation actuator327and a second rotation actuator329. The first and second actuators327and329are in contact with different portions of the sticky structure322. The first rotation actuator327is rotatable about an X-directional axis A1, and the second rotation actuator329is rotatable about a Y-directional Axis A2. In this way, the first and second rotation actuators327and329can rotate the sticky structure322multi-directionally. In some embodiments, the first and second rotation actuators327and329each include a stepper motor fabricated using micro-electro-mechanical system (MEMS) techniques and controlled using an optical encoder.

In some embodiments of method M2, after the cleaner320dadhered with a particle P1is lifted from the wafer table170(block S24), the first rotation actuator327and/or the second rotation actuator329rotate the sticky structure322, so that the particle P1adhered to the sticky structure322can be rotated upwardly. As a result, when the cleaner320dis lowered again to another particle P1, it will be adhered to a clean area on the sticky structure322that is free from the pre-adhered particle P1.

Embodiments as described above relate to removing particles from the wafer table using the sticky structure. However, this concept can also be used in other applications. For example, following embodiments relate to removing particles from the photomask using the sticky structure.

FIG. 23is a flowchart of a photomask cleaning method M3in accordance with some embodiments.FIGS. 24-28illustrate the photomask cleaning method M3at various stages in accordance with some embodiments. The photomask cleaning method M3may be implemented in a fab for fabricating a semiconductor device on a wafer using a photomask. It is understood that additional operations may be implemented before, during, and after the method M3, and some of the operations may be replaced, eliminated, or moved around for additional embodiments of the method M3.

The method M3begins at block S31where the photomask is inspected to generate a particle map. Photomask can be inspected for particles using, for example, scattered light techniques. With a scattered light technique, a laser beam is focused on a photomask and a radiation beam that is scattered away from a specular reflection direction is detected. Particles on the photomask surface will randomly scatter the light. By observing the illuminated surface with a microscope, the particles will light up as bright spots. In this way, the particle locations on the photomask can be identified, which in turn will facilitate the particle removing process using the sticky structure as discussed previously.

In some embodiments, a scatterometer operating with visible or ultraviolet (UV) light allows faster photomask inspection than scanning imaging systems (e.g., confocal, EUV or electron beam microscope systems). Scatterometer uses a laser radiation beam and a coherent optical system with a Fourier filter in the pupil plane that blocks light diffracted from a pattern on the reticle. This type of scatterometer detects light scattered by defects over the level of background coming from a periodic pattern on the reticle.

One exemplary photomask inspection system is shown inFIG. 24. The exemplary photomask inspection system500includes a channel510including a microscope objective512, a pupil filter514, a projection optical system (e.g., projection lens)516, and a detector518. A radiation (e.g., laser) beam RB2generated from a light source520illuminates a photomask MA gripped by gripper arms542and544of a reticle gripper540, whereinFIG. 25illustrates a top view of the reticle gripper540. In some embodiments, the reticle gripper540can be referred to as a holding device because it can hold the photomask MA. Pupil filter514is used to block optical scattering due to the pattern of the photomask MA. A processing unit530can be used to control the filtering of pupil filter514based on the pattern of the photomask MA. Accordingly, filter514is provided as a spatial filter in a pupil plane relative to the photomask MA and is associated with the pattern of the photomask MA so as to filter out radiation from the scattered radiation. Detector518detects a fraction of radiation that is transmitted by projection optical system516for detection of particles P2. The processing unit530thus generates a particle map PM2based on detection result generated from the detector518, as exemplarily illustrated inFIG. 29. In some embodiments, the processing unit530is a central processing unit (CPU) in a computer or the like.

Returning toFIG. 23, the method M3then proceeds to block S32where the photomask is transported to a position under the cleaner. With reference toFIG. 26, in some embodiments, the photomask MA is moved in X-direction (indicated by the arrow D31) using the reticle gripper540to the position vertically below the cleaner620. In some embodiments, the reticle gripper540is controlled by a controller C3, as illustrated in the block diagram ofFIG. 29. The controller C3is programmed to control the reticle gripper540to move the photomask MA to the position under the cleaner620after completing the photomask inspection. In some embodiments, the controller C3is a programmable processor, microprocessor or the like.

Returning toFIG. 23, the method M3then proceeds to block S33where the cleaner is lowered to the photomask. With reference toFIG. 27, in some embodiments, the cleaner620fixed to the motion actuator610via the cantilever630is lowered (indicated by the arrow D32) using the motion actuator610, as discussed previously with respect to the particle removal device300. In some embodiments, the motion actuator610is controlled by a controller C4, as illustrated in the block diagram ofFIG. 29. The controllers C3and C4can be programmed in such a way that the controller C4triggers the actuation of the motion actuator610after completing transportation of the photomask MA using the reticle gripper540controlled by the controller C3.

Referring toFIG. 23, the method M3then proceeds to block S34where the photomask is moved during when being in contact with the sticky structure. With reference toFIG. 28, in some embodiments, when the sticky structure622of the cleaner620is in contact with the photomask MA, the photomask MA can be moved either in X-direction (indicated by the double headed arrow D33) or in Y-direction using the reticle gripper540. In some embodiments where the sticky structure622is a sticky roller ball as discussed previously with respect to the sticky structure322, the sticky roller ball622in contact with the photomask MA will roll due to the movement of the photomask MA in X-Y plane. In this way, the particles on the photomask MA will be adhered to the sticky roller ball622and thus removed from the photomask MA.

In some embodiments, the controller C3is in electrical communication with the processing unit530, so that the controller C3can receive the particle map PM2generated from the processing unit530. In some embodiments, the controller C3is programmed to control the reticle gripper540to move the photomask MA along a path associated with identified particle locations PL2in the particle map PM2when the sticky structure622is in contact with the photomask MA.

After removing particles from the photomask MA, the method M3proceeds to block S35where the cleaner620is lifted using the motion actuator610. Afterwards, in block S36of the method M3, the photomask MA can be moved to the photomask table in a lithographic apparatus (e.g., the photomask table150in the lithographic apparatus as shown inFIG. 1). Thereafter, the method proceeds to block S37where a lithography process is performed to a wafer (e.g., the wafer W as shown inFIG. 1) using the cleaned photomask MA. For example, the lithography process can be performed using the radiation beam RB1passing through the cleaned photomask MA, as illustrated inFIG. 1.

In the depicted embodiments inFIGS. 24-28, the method M3uses the cleaner620to remove particles from the photomask MA. In some other embodiments, the cleaner620used in the method M3can be replaced with the cleaner320aas shown inFIG. 12, the cleaner320bas shown inFIG. 13, and/or the cleaner320cas shown inFIG. 14. Stated differently, a heating device (e.g., the heating device321as shown inFIG. 12) and/or a vibrator (e.g., the vibrator323as shown inFIG. 13) can be integrated into the cleaner620to heat and/or vibrate the particles when the particles is on the photomask MA, which in turn will improve particle removal performance. In some embodiments, the cleaning tool325can also be integrated into the cleaner620. As a result, when the sticky structure622rolls due to horizontal movement of the photomask MA, particles adhered to the sticky structure622can be detached from the sticky structure622by the tool tip325tof the cleaning tool325.

FIG. 30is a flowchart of a photomask cleaning method M4in accordance with some embodiments.FIGS. 31-36illustrate the photomask cleaning method M4at various stages in accordance with some embodiments. The photomask cleaning method M4may be implemented in a fab for fabricating a semiconductor device on a wafer using a photomask. It is understood that additional operations may be implemented before, during, and after the method M4, and some of the operations may be replaced, eliminated, or moved around for additional embodiments of the method M4.

In block S41of the method M4, the photomask MA is inspected to generate a particle map PM2, as discussed previously with respect to block S31of the method M3. Thereafter, in block S42of the method M4, the photomask MA is moved such that an identified particle P2on the photomask MA associated with an identified particle location PL2in the particle map PM2(as shown inFIG. 29) is moved to a position vertically below the cleaner620, as illustrated inFIG. 31where the photomask MA is moved horizontally (indicated by the arrow D41). Movement of the photomask MA can be performed using the reticle gripper540, as discussed previously with respect to block S32of the method M3.

Afterwards, in block S43of the method M4, the cleaner620is lowered until reaching the identified particle P2on the photomask MA, as illustrated inFIG. 32where the cleaner620is moved downwards (indicated by the arrow D42). Lowering the cleaner620can be performed using the motion actuator610, as discussed previously with respect to block S33of the method M3. Thereafter, in block S44of the method M4, the cleaner620is lifted so as to lift the particle P2away from the photomask MA, as illustrated inFIG. 33where the cleaner620is moved upwards (indicated by the arrow D43).

The method M4then proceeds back to block S42where the photomask MA is moved such that another identified particle P2on the photomask MA, associated with another identified particle location PL2in the particle map PM2, is moved to the position vertically below the cleaner620, as illustrated inFIG. 34where the photomask MA is moved horizontally (indicated by the arrow D44). Afterwards, the method M4proceeds to blocks S43where the cleaner620is lowered until reaching the another identified particle P2on the photomask MA, as illustrated inFIG. 35where the cleaner620is moved downwards (indicated by the arrow D45). Thereafter, in block S44of the method M4, the cleaner620is lifted so as to lift the another particle P2away from the photomask MA, as illustrated inFIG. 36where the cleaner620is moved upwards (indicated by the arrow D46).

Blocks S42, S43and S44in the method M4are in combination serve as a cyclic operation. After one or more repetitions of the cyclic operation to remove all identified particles from the photomask MA, the method M4proceeds to block S45where the photomask MA can be moved to the photomask table (e.g., the photomask table150as shown inFIG. 1) in a lithographic apparatus. Afterwards, in block S46of method M4, a lithography process is performed to a wafer (e.g., the wafer W as shown inFIG. 1) using the photomask MA.

In some embodiments of method M4, the photomask MA remains stationary when the sticky structure622is in contact with the photomask MA. As a result, the sticky structure622might not roll because of lack of horizontal movement of the photomask MA. As a result, in some embodiments of the present disclosure, the cleaner620may be replaced with the cleaner320dthat is capable of actively rolling the sticky structure322thereof, as illustrated inFIG. 22. In greater detail, after lifting the cleaner320dadhered with a particle P2from the photomask MA (block S44), the first rotation actuator327and/or the second rotation actuator329rotates the sticky structure322, so that the particle P2adhered to the sticky structure322can be rotated upwardly. As a result, when the cleaner320dis lowered again to another particle P2, it will be adhered to a cleaning area on the sticky structure322that is free from the pre-adhered particle P2.

Based on the above discussions, it can be seen that the present disclosure offers advantages. It is understood, however, that other embodiments may offer additional advantages, and not all advantages are necessarily disclosed herein, and that no particular advantage is required for all embodiments. One advantage is that particles on the wafer table and/or photomask can be adhered to the sticky structure and thus removed from the wafer table and/or photomask when the sticky structure is lifted. Another advantage is that the particle removal performance can be improved by heating the sticky structure and/or by inducing resonance of particles. Yet another advantage is that cleaning of the sticky structure can be achieved using a cleaning tool (e.g., a scrapper having a tool tip in contact with the sticky structure). Yet another advantage is that the sticky structure can be precisely rotated to various orientations using rotation actuators rotatable along non-parallel axes, so that particles can be adhered to different portions of the sticky surface.

In some embodiments, a method includes moving a sticky structure to a wafer table such that a first particle on the wafer table is adhered to the sticky structure, moving the sticky structure away from the wafer table after the first particle is adhered to the sticky structure, and performing a lithography process to a wafer held by the wafer table after moving the sticky structure away from the wafer table.

In some embodiments, the method further includes inducing resonance of the first particle when the first particle is on the wafer table.

In some embodiments, moving the sticky structure to the wafer table is performed such that the sticky structure is in contact with the wafer table, and the method further includes heating the sticky structure when the sticky structure is in contact with the wafer table.

In some embodiments, moving the sticky structure to the wafer table includes moving a cantilever downwards along a vertically extending guide, wherein the sticky structure is fixed to the cantilever.

In some embodiments, moving the sticky structure away from the wafer table includes moving a cantilever upwards along a vertically extending guide, wherein the sticky structure is fixed to the cantilever.

In some embodiments, moving the sticky structure to the wafer table is performed such that the sticky structure is in contact with the wafer table, and the method further includes horizontally moving the wafer table when the sticky structure is in contact with the wafer table, wherein horizontally moving the wafer table is performed such that the sticky structure is rolled.

In some embodiments, rolling the sticky structure is performed such that the first particle is scratched away from the sticky structure.

In some embodiments, the method further includes after moving the sticky structure away from the wafer table, moving the sticky structure back to the wafer table such that a second particle on the wafer table is adhered to the sticky structure.

In some embodiments, the method further includes rotating the sticky structure after moving the sticky structure away from the wafer table, and adhering a second particle on the wafer table to the sticky structure after rotating the sticky structure.

In some embodiments, a method includes moving a photomask such that a first particle on the photomask is under a sticky structure, moving the sticky structure to the photomask until the first particle is adhered to the sticky structure, moving the sticky structure away from the photomask after the first particle is adhered to the sticky structure, and performing a lithography process using the photomask after lifting the sticky structure.

In some embodiments, the method further includes vibrating the first particle when the first particle is on the photomask.

In some embodiments, the method further includes heating the first particle when the first particle is on the photomask.

In some embodiments, the sticky structure is lowered such that the sticky structure is in contact with the photomask, and the method further includes horizontally moving the photomask when the sticky structure is in contact with the photomask, wherein horizontally moving the photomask rolls the sticky structure.

In some embodiments, rolling the sticky structure is performed such that the first particle is detached from the sticky structure.

In some embodiments, the method further includes moving the sticky structure to the photomask after moving the sticky structure away from the photomask until a second particle on the photomask is adhered to the sticky structure.

In some embodiments, the method further includes rotating the sticky structure after moving the sticky structure away from the photomask, and adhering a second particle on the photomask to the sticky structure after rotating the sticky structure.

In some embodiments, a lithographic apparatus includes a projection lens, a sticky structure and a holding movable between a position under the projection lens and a position under the sticky structure. The sticky structure is movable along a direction non-parallel with a top surface of the holding device.

In some embodiments, the lithographic apparatus further includes a heater thermally coupled to the sticky structure.

In some embodiments, the lithographic apparatus further includes a cleaning tool having a tip in contact with the sticky structure.

In some embodiments, the lithographic apparatus further includes a first rotation actuator in contact with the sticky structure and rotatable about a first axis, and a second rotation actuator in contact with the sticky structure and rotatable about a second axis non-parallel with the first axis.