Supply system for an extreme ultraviolet light source

A supply system for an extreme ultraviolet (EUV) light source includes an apparatus configured to be fluidly coupled to a reservoir configured to contain target material that produces EUV light in a plasma state, the apparatus including two or more target formation units, each one of the target formation units including: a nozzle structure configured to receive the target material from the reservoir, the nozzle structure including an orifice configured to emit the target material to a plasma formation location. The supply system further includes a control system configured to select a particular one of the target formation units for emitting the target material to the plasma formation location. An apparatus for a supply system of an extreme ultraviolet (EUV) light source includes a MEMS system fabricated in a semiconductor device fabrication technology, and the MEMS system including a nozzle structure configured to be fluidly coupled to a reservoir.

TECHNICAL FIELD

This disclosure relates to a supply system for an extreme ultraviolet light source.

BACKGROUND

Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer.

Methods to produce EUV light include, but are not necessarily limited to, converting a material that includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma (“LPP”), the required plasma may be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

In one general aspect, a supply system for an extreme ultraviolet (EUV) light source includes an apparatus configured to be fluidly coupled to a reservoir configured to contain target material that produces EUV light in a plasma state, the apparatus including two or more target formation units, each one of the target formation units including: a nozzle structure configured to receive the target material from the reservoir, the nozzle structure including an orifice configured to emit the target material to a plasma formation location. The supply system further includes a control system configured to select a particular one of the target formation units for emitting the target material to the plasma formation location.

Implementations may include one or more of the following features. The control system may include a temperature system configured to control an amount of heat in the apparatus, the control system being configured to select the particular target formation unit for activation or deactivation by controlling the temperature system. The temperature system may include two or more heaters, and each target formation unit may be associated with one or more of the heaters. The control system may be configured to select the particular target formation unit for activation by controlling the one or more particular ones of the heaters associated with the particular target formation unit. A thermally insulating material may be disposed between any two of the target formation units. In some implementations, the temperature system includes two or more active temperature control mechanisms, each target formation unit is associated with one or more of the active temperature control mechanisms, and the one or more active temperature control mechanisms associated with a target formation unit are configured to heat or cool that target formation unit.

Each target formation unit may further include a channel between the reservoir and the orifice, and one or more filters in the channel. Each target formation unit may further include an actuation chamber fluidly coupled to the channel and a modulator coupled to the actuation chamber, the modulator configured to modulate a pressure in the actuation chamber. The channel of each target formation unit may include more than one branch extending from a first end of the respective target formation unit facing the reservoir to the actuation chamber, and an outlet channel fluidly coupled to the actuation chamber, the outlet channel extending from the actuation chamber to the orifice.

The apparatus configured to be fluidly coupled to a reservoir may be a MEMS system fabricated in a semiconductor device fabrication technology. The apparatus configured to be fluidly coupled to a reservoir may be an integral, single piece. The single, integral piece may be a microelectromechanical (MEMS) system fabricated in a semiconductor device fabrication technology.

In some implementations, the supply system also includes a holder accommodating the apparatus. The apparatus and the holder may be configured to move relative to each other. The holder may be configured to move relative to the plasma formation location. Each target formation unit of the apparatus may include at least one of a plurality of capillary tubes, and the plurality of capillary tubes may extend away from the holder.

The control system may be configured to select the particular target formation unit based on one or more of: (a) an indication of an amount of the EUV light produced at the plasma formation location, (b) an indication of an absence of target material at the plasma formation location, and (c) an input from a human operator.

In some implementations, each target formation unit further includes an actuation chamber fluidly coupled to the orifice and a modulator coupled to the actuation chamber. The modulator may be configured to modulate a pressure in the actuation chamber. The control system may be further configured to drive the actuator of the particular target formation unit at two or more frequencies, at least one of the frequencies being based on a geometric configuration of the particular target formation unit.

In another general aspect, an apparatus for a supply system of an extreme ultraviolet (EUV) light source includes a MEMS system configured to be accommodated in a housing of the supply system. The supply system is configured to supply a target material to a plasma formation location. The MEMS system is fabricated in a semiconductor device fabrication technology. The MEMS system includes a nozzle structure configured to be fluidly coupled to a reservoir that is configured to contain the target material that produces EUV light in a plasma state, the nozzle structure including an orifice configured to emit the target material to the plasma formation location.

Implementations may include one or more of the following features. The MEMS system may further include: a channel between the reservoir and the orifice; and one or more filters in the channel. The MEMS system may further include: a channel between the reservoir and the orifice; a chamber fluidly coupled to the channel, the chamber configured to receive the target material from the channel; and a modulator coupled to the chamber, the modulator configured to modulate a pressure in the chamber. The channel may include one or more supply channels fluidly coupled to the chamber, and an outlet channel fluidly coupled to the chamber and the orifice. In operational use, the modulator may be under substantially the same pressure or partial pressure as that of the target material in the chamber.

In another general aspect, an EUV source includes an optical source configured to produce an optical beam; a vessel configured to receive the optical beam at a plasma formation location; and a supply system. The supply system includes: an apparatus configured to be fluidly coupled to a reservoir configured to contain target material that produces EUV light in a plasma state, the apparatus including two or more target formation units, each one of the target formation units including: a nozzle structure configured to receive the target material from the reservoir, the nozzle structure including an orifice configured to emit the target material to the plasma formation location; and a control system configured to select a particular one of the target formation units for emitting the target material to the plasma formation location. The optical beam produced by the optical source is configured to convert the emitted target material to plasma.

Implementations may include one or more of the following features. The control system may include a temperature system configured to control an amount of heat in the apparatus, the control system being configured to select the particular target formation unit for activation or deactivation by controlling the temperature system. The temperature system may include two or more heaters. Each target formation unit may be associated with one or more of the heaters, and the control system may be configured to select the particular target formation unit for activation by controlling the one or more particular ones of the heaters associated with the particular target formation unit.

The apparatus configured to be fluidly coupled to a reservoir may be a single, integral piece. The apparatus configured to be fluidly coupled to a reservoir may be fabricated in a semiconductor device fabrication technology.

Implementations of any of the techniques described above may include an EUV light source, a system, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

DETAILED DESCRIPTION

Referring toFIG. 1, a block diagram of a supply system110used in an EUV light source100is shown. The supply system110emits a stream of targets121such that a target121pis delivered to a plasma formation location123in a vacuum chamber109. In operational use, the supply system110is fluidly coupled to a reservoir112that contains target material under pressure P. The target material is any material that emits EUV light when in a plasma state. For example, the target material may include water, tin, lithium, and/or xenon. The target material may be in a molten state that is capable of flowing in, for example, a channel. The targets in the stream of targets may be considered to be droplets of target material or targets.

The supply system110includes a target formation apparatus116. In the example ofFIG. 1, the target formation apparatus116is accommodated in a housing114. The target formation apparatus116includes a nozzle structure118, which defines an orifice119. The orifice119is fluidly coupled to the reservoir112. For example, the orifice119may be fluidly coupled to the reservoir112via a channel (not shown inFIG. 1). The pressure in the vacuum chamber109is much lower than the pressure P applied to the reservoir, and the target material flows through the orifice119. Target material emitted from the orifice119forms the stream of targets121. The plasma formation location123receives a light beam106. The light beam106is generated by an optical source105and delivered to the vacuum chamber109via an optical path107. An interaction between the light beam106and the target material in the target121pproduces a plasma196that emits EUV light.

The target formation apparatus116may include a microelectromechanical (MEMS) system117that is fabricated in a semiconductor device fabrication technology. In these implementations, the nozzle structure118and the orifice119are formed as part of the MEMS system117, and the target formation apparatus116, which includes the MEMS system117, may be considered to be a MEMS-based target formation apparatus116. The MEMS-based target formation apparatus116may be used in the supply system110instead of a target formation apparatus fabricated using traditional machining techniques.

The MEMS system117is fabricated in a semiconductor device fabrication technology instead of in a traditional machining technology. For example, the MEMS system117may be fabricated using wet etching and/or dry etching, electro discharge machining (EDM), and any other technology that is capable of manufacturing small devices. Traditional machining may use techniques that are different from those used to fabricate the MEMS system117. Traditional machining techniques include, for example, sawing, milling, drilling, laser machining, and/or turning. A target formation apparatus may be fabricated by applying traditional machining techniques applied to high-strength metal (such as molybdenum, tungsten, or titanium) to form features (such as orifices, channels, and/or chambers). However, applying traditional machining to the high-strength metal may introduce rough surfaces, particles, and/or chemical contamination. These effects may be mitigated through surface treatments such as etching, cleaning, and/or polishing, but particle and/or chemical contamination that is not mitigated may affect the performance of a target formation apparatus made with traditional machining techniques.

For example, traditional machining may produce particles with extents that are larger than the diameter of an orifice through which target material passes. If not removed by cleaning, these particles may block the flow of target material. When target material does not flow from the orifice, target material does not arrive at the plasma formation location123and no EUV light is produced. Moreover, the blockage may result in damage to the target formation apparatus. Traditional machining also may produce particles with extents that are smaller than the diameter of the orifice. These particles may become lodged in the nozzle or the orifice and may partially block the orifice. When the orifice119is partially blocked, the target material emitted by the orifice119may be launched on a trajectory deviating from an expected path and may not reach the plasma formation location123, leading to reduced or no EUV production. Chemically formed contamination also may block or partially obstruct the orifice.

However, because the target formation apparatus116includes the MEMS system117, the target formation apparatus116may have improved performance and reliability with less stringent cleaning as compared to a target formation apparatus formed solely with traditional machining techniques. For example, the MEMS system117is fabricated using a semiconductor device fabrication technology, thus, the MEMS system117may be manufactured and assembled under clean room conditions and at a single location. The MEMS system117may be fabricated in a semiconductor device fabrication technology that is based on lithographic patterning and different etching processes, such as wet etching, reactive ion etching, focused ion beam etching and the like, and chemical reactions such as chemical vapor deposition. The chemical vapor deposition techniques include, for example, atmospheric pressure chemical vapor deposition (APCVD), atomic layer chemical vapor deposition (ACVD or ALCVD), hot filament chemical vapor deposition (HFCVP), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), microwave plasma-assisted chemical vapor deposition (MPCVD), plasma enhanced chemical vapor deposition (PECVD), rapid thermal chemical vapor deposition (RTCVD), remote plasma enhanced chemical vapor deposition (RPECVD), ultra high vacuum chemical vapor deposition (UHVCVD). These various techniques are used to form thin layers of silicon oxide, silicon nitride, silicon carbide, and/or metals, such as, for example, gold, tungsten, chromium, and the like. As no abrasive manufacturing steps are involved, the MEMS system117results in the target formation apparatus116having less particle and/or chemical contamination than an apparatus fabricated using traditional machining techniques.

Furthermore, by using the MEMS technology portfolio to form the MEMS system117, the target formation apparatus116may be easier to reproduce in large numbers with tight tolerances than an apparatus made using traditional machining techniques. For example, some target formation apparatuses include a capillary tube that is formed manually from quartz or a similar material. The capillary tube includes a nozzle that defines a small orifice formed through a complex manual process that may be challenging to reproduce reliably and challenging to keep free of particle and chemical contamination. Further, the materials used for the capillary tube may be prone to cracking, which may present challenges to using a traditionally formed capillary tube at pressures of up to 8,000 pounds per square inch (psi). The target formation apparatus116that includes the MEMS system117may operate without a capillary tube. Moreover, the materials that may be used for the MEMS system117are suitable for use with the target material and at pressures up to and exceeding 8,000 psi. In some implementations, the MEMS system117is made out of silicon (Si) with nitride coatings. Other materials that the MEMS system117may be made out of include silicon carbide (SiC), silicon nitride (SiN), and/or silicon dioxide (SiO2). Moreover, any material used in the MEMS system117may be coated with nitride. For example, the MEMS system117may be made out of SiC coated with nitride, SiN coated with nitride, and/or SiO2coated with nitride.

Using the MEMS system117in the target formation apparatus116may provide additional advantages. For example, filters may be integrated into the MEMS system117. The filters may be used to reduce particle contamination introduced between the target formation apparatus116and the reservoir112. In some implementations, the filters may be placed at the beginning and end (considered with respect to the direction of flow from the reservoir112to the orifice119) of the MEMS system117such that the MEMS system117is substantially closed to the outside environment.

Furthermore, components made in MEMS technology may be much smaller than similar components made in traditional machining techniques such as those noted above. For example, components made in MEMS technology may be an order of magnitude smaller than a similar component made in a traditional machining technique. Therefore, the MEMS system117itself may be made much smaller than similar components made in traditional machining techniques, and a target formation apparatus116that includes the MEMS system117may thus be smaller than a target formation apparatus that does not include the MEMS system117. The smaller footprint of the target formation apparatus116may be advantageous as the overall amount of space available in the supply system110may be limited or restricted.

Moreover, including the MEMS system117in the target formation apparatus116may allow the target formation apparatus116to be fabricated as a monolithic structure that includes more than two individually controllable target formation apparatuses, each target formation apparatus including a respective MEMS system.FIGS. 5A and 5Bshow an example of this implementation. The target formation apparatuses within the monolithic structure are individually controllable such that any of the target formation apparatuses may be activated to produce the stream of targets121or deactivated to not produce the stream of targets121. Thus, if any of the target formation apparatuses fail, have degraded performance, are nearing the end of their expected life time, or are desired to be switched out of service for any other reason, another of the target formation apparatuses may be activated such that EUV light continues to be produced.

Although the MEMS system117is well suited for use in forming a monolithic structure that includes more than two target formation apparatuses, a group of target formation apparatuses, each fabricated using traditional machining techniques, also may be individually controlled and used together in a similar manner.FIG. 6shows an example of such an implementation.

FIGS. 2A and 2Bshow a MEMS system217fabricated in a semiconductor device fabrication technology. The MEMS system217is an example of an implementation of the MEMS system117(FIG. 1). The MEMS system217may be used in the target formation apparatus116and the supply system110(FIG. 1), and the MEMS system217is discussed with reference to the supply system110.FIG. 2Ais a block diagram of a side cross-sectional view of the MEMS system217.FIG. 2Bis a block diagram of a top view of the MEMS system217as seen from the line2B-2B. The dotted lines inFIG. 2Brepresent hidden elements that are below a first end225of the MEMS system217.

The MEMS system217includes the first end225and a second end226. In the MEMS system217, the first end225and the second end226are at opposing sides. First channels227extend from the first end225along the X axis. In operational use, the first end225is oriented to face the reservoir112, and target material from the reservoir112flows into the first channels227. Each first channel227is fluidly coupled to a chamber228via a respective intermediate channel229. The chamber228is, for example, a disk-shaped volume that is partially formed by a wall233. The chamber228may have a shape other than a disk. For example, the chamber228may be a square-shaped or a rectangular-shaped volume. The chamber228is fluidly coupled to a second channel230. The second channel230is fluidly coupled to a nozzle structure218, which defines an orifice219.

In the example of the MEMS system217, the first channels227have mirror symmetry in the X-Z plane. In other implementations, the first channels227do not have mirror symmetry in the X-Z plane. For example, the MEMS system217may be implemented with three first channels227spaced 120° from each other in the Y-Z plane. In the MEMS system217, the diameter of each of the first channels227in the Z-Y plane varies depending on the position along the X axis. Specifically, in the MEMS system217, the diameter of the first channels227is greater in a portion237at the first end225than in a portion238at the intermediate channel229. The change in diameter of the first channel227forms an acoustic filter that helps to filter acoustic disturbances arising from regions upstream of the first channel227(for example, in the reservoir112).

When the MEMS system217is fluidly coupled to the (pressurized) reservoir112, target material flows from the reservoir112into the first channels227and the intermediate channels229. The target material flows from the intermediate channels229into the chamber228, and from the chamber228into the second channel230. The target material is emitted from the orifice219as a jet of target material that breaks into targets. Collectively, the first channels227, the intermediate channels229, the chamber228, the second channel230, and the orifice219form a target material path from the first end225to the second end226.

The MEMS system217also includes filters241a-241d. In the example ofFIGS. 2A and 2B, the MEMS system217includes four filters, with filters241a,241bin a first channel227at the first end225, the filter241cin the second channel230between the chamber228and the second channel230, and the filter241din the nozzle structure218near the orifice219. The filter241dis placed in the X direction relative to the orifice219. Thus, in operational use, the target material flows through the filters241a,241bat the first end225prior to flowing into the first channels227, through the filter241cto enter the second channel230, and through the filter241dbefore passing through the orifice219.

The filters241a-241dmay be, for example, any type of filter that is fabricated in a semiconductor device fabrication technology. The filters241a-241dmay include arrays of filter channels or openings that allow target material to pass and a filtration surface that captures particles that are larger than the diameter of the filter channels. For example, the filter channels may have diameters that are smaller than the diameter of particles that could block or partially block the orifice219. The particles are captured on the filtration surface, and the filters241a-241dfilter the target material by retaining particulate debris such that the debris is prevented from being transported toward the orifice219.

FIG. 2Cis a perspective view of a filter241eand a filter241f. Either of the filters241e,241fmay be used as any of the filters241a-241d. The filters241e,241finclude respective filtration surfaces248e,248fand supports249e,249f. In the filtration surfaces248e,248f, the dots represent filter channels or openings through which target material may flow. The channels or openings may pass entirely through the filter241e,241falong the X axis.

The filter channels may have a diameter of, for example, 1-20 μm. The filter channels may have a diameter that is smaller than the diameter of the orifice219. For example, the orifice219may have a diameter of 2-5 μm, and the filter channels may have a diameter of 1-2 μm or less. The filters241a-241dmay have the same characteristics, or the filters241a-241dmay be different from each other. For example, the filter241dmay have channels that have a smaller diameter than the channels of the filters241a,241b, or241c. In some implementations, the filters241aand241bmay have channels that have a diameter that is larger than the diameter of the orifice219to block larger particles, the filter241cmay have channels that have a diameter that is the same as the diameter of the orifice219, and the filter241dmay have channels that have a diameter that is smaller than the diameter of the orifice219.

The filters241a-241dmay be membrane filters that include a membrane that forms a filtration region held in place by supports. In some implementations, one or more of the filters241a-241dare high-porosity microsieve filters. The filters may be manufactured to have a burst pressure of greater than 100 bars (or greater than about 1450 psi). The filters241a-241dalso may be acoustically transparent.

Referring again toFIGS. 2A and 2B, the MEMS system217also includes an actuator231in a space234. The actuator231is coupled to the chamber228and is configured to modulate a pressure in the chamber228. In the MEMS system217, the actuator231is mechanically coupled to the chamber228through a membrane232. The membrane232is mechanically coupled to the actuator231such that the modulation of the actuator231is transferred to the chamber228. For example, the modulation of the position of the membrane232may produce a corresponding change in the pressure in the chamber228. The membrane232separates the actuator231from the chamber228. Thus, the membrane232also may help to protect the actuator231from target material in the chamber228. In the MEMS system217, part of the membrane232is formed from the wall233.

The space234is bounded by walls, two of which (walls240and243) are labeled inFIG. 2A. The walls240and243are separated along the X axis and are at opposite sides of the space234. In some implementations, the actuator231is clamped between the walls240and243. In these implementations, the actuator231may be configured to expand and contract along the X axis to cause the membrane232to move. In another implementation, the actuator231is rigidly attached to the wall240and the membrane232by, for example, gluing, soldering, or brazing. In these implementations, the actuator231may be configured to bend against the wall240to cause the membrane232to move.

The actuator231may be any suitable actuation mechanism that is able to modulate the pressure in the chamber228by modulating the position of the membrane232. For example, the actuator231may be a piezo actuator, which includes a piezo material that exhibits the inverse piezoelectric effect such that the piezo material elongates, bends, contracts, expands, and/or otherwise changes shape when an electric field is applied. In implementations in which the actuator231is a piezo actuator, the actuator231may include lead-zirconium-titanate (PZT) or another similar material. In these implementations, the actuator231may be a single piezo actuator (for example, a single piezo-platelet or a single layer of material that exhibits the inverse piezoelectric effect), two piezo actuators, or a multi-layer piezo assembly. In some implementations, the actuator231is formed from a single layer of piezo material deposited directly onto a wall or side240of the membrane232that faces the space234.

In implementations in which the actuator231is a piezo actuator that includes a piezo material, the piezo material may have any shape. For example, the shape of the piezo material may be, for example, a disk, a square, a rectangle, a cylinder, a tube, or an annulus. In implementations in which the actuator231is a piezo actuator, the configuration of the piezo material may be selected based on how the piezo actuator is mounted in the space234. As discussed above, the actuator231may be clamped between the walls240and243. In these implementations, a piezo actuator that is configured to change shape (for example, elongate) along the X axis in response to the application of an electric field may be used. In implementations in which the actuator231is attached to the wall240and not to the wall243, a piezo actuator that bends in response to the application of an electric field may be used.

Electrodes (not shown) may be placed near the piezo material to apply an electric field across the piezo material, and the modulation of the electric field causes the mechanical modulation of the piezo material. The electric field generated by the electrodes may be controlled to apply an electric field across the piezo material using signal control lines or a similar mechanism that reaches the electrodes through a conduit239(FIG. 2B). The conduit239passes through the MEMS system217along the Y axis. The conduit239provides access to the space234from an exterior of the MEMS system217.

The conduit239(or another conduit similar to the conduit239) may be used to control the pressure in the space234such that the pressure in the space234(at the actuator231) and the pressure in the chamber228are substantially the same. In this way, the only pressure difference experienced by the membrane232is the pressure difference that arises from the actuator231modulating the membrane232.

Applying the pressure P (FIG. 1) to the target material in the reservoir112causes the target material to exit the orifice219as a jet of target material. The jet of target material eventually breaks into targets. The natural break-up of a jet of liquid issuing from an orifice is known as the Rayleigh break-up. The Rayleigh frequency is the rate of production of individual droplets through Rayleigh break-up. The Rayleigh frequency is related to the average (or mean) velocity of the target material at the orifice219and the diameter of the orifice219. The Rayleigh break-up may occur without modulating the pressure in the chamber228. However, modulating the pressure in the chamber228allows more control over the break-up of the jet of target material. For example, modulating the pressure of the chamber228at a frequency that is much lower than the Rayleigh frequency also modulates the exit velocity of the target material from the orifice, thus allowing the break-up of the stream to occur in a more controlled manner.

As discussed above, the actuator231is used to modulate the pressure in the chamber228. In one implementation, a low-frequency modulated continuous jet technique may be applied to the actuator231to form the stream of targets121. In the low-frequency modulated continuous jet technique, the actuator231is controlled with an input signal that has at least two frequencies. The at least two frequencies include a first frequency and a second frequency. The first frequency may be in the megahertz (MHz) range. The first frequency may be close to the Rayleigh frequency. Modulating the pressure in the chamber228at the first frequency causes the jet of target material to break into relatively small targets of desired sizes and speeds.

The second frequency is lower than the first frequency. For example, the second frequency may be in the kilohertz (kHz) range. The second frequency is used to modulate the velocity of the targets in the stream and to determine the rate of target production. Modulating the pressure in the chamber228at a frequency that is much lower than the Rayleigh frequency causes groups of targets to form. In any given group of targets, the various targets travel at different velocities. The targets with higher velocities may coalesce with the targets with lower velocities to form larger coalesced targets that make up the stream of targets121. These larger targets are separated from each other by a larger distance than the non-coalesced droplets. The larger separation helps to mitigate the influence of the plasma formed from one target on the trajectory of the subsequent targets in the stream121.

The targets in the stream of targets121may be approximately spherical, with a diameter of about 30 μm. The targets may be generated at frequencies of, for example, between 40 to 310 kHz and may travel toward the plasma formation location123(FIG. 1) at a velocity of, for example, between 40 and 120 meters per second (m/s) or up to 500 m/s. The spatial separation between two adjacent targets in the stream of targets121may be, for example, between 1 and 3 millimeters (mm). Between 100 and 300 initial droplets (also called Rayleigh droplets) may coalesce to form a single larger target.

As discussed above, the actuator231may be driven by at least two frequencies, one that is close to the Rayleigh frequency and another frequency (a second frequency) that encourages coalescence. The second frequency that encourages coalescence determines the frequency at which targets arrive in the plasma formation location123.

The mechanical structure of the MEMS system217may be used to determine the second frequency. For example, a resonant frequency of the MEMS system217may be used as the second frequency. The Helmholtz frequency or a frequency determined by the length of the second channel230along the X axis may be used as the second frequency, depending on the geometry of the MEMS system.

The Helmholtz frequency (fHelmholtz) is shown in Equation (1):

fHelmholtz=c2⁢π⁢1Vc⁢(AorificeLorifice+n⁢AthrottleLthrottle),Equation⁢⁢(1)
where c is the speed of sound in the (flowing) target material corrected for the compliance of the surroundings of the material, Vcis the volume of the chamber228, Aorificeis the surface area of the orifice219, Lorificeis the length of the orifice219, Athrottleis the surface area of one of the intermediate channels229, Lthrottleis the length of one of the intermediate channels, and n is the number of intermediate channels229. For implementations that include the channel230(such as the implementation ofFIG. 2A), the second frequency may change based on the design parameters or geometry of the channel230.

The quarter-wavelength frequency, which is the frequency at which a quarter of a wave with that frequency fills the second channel230and the nozzle structure218along the X axis, may be used as the second frequency. Excluding acoustic corrections, the quarter-wavelength frequency (fqw) is shown in Equation (2):

fqw=c4⁢L,Equation⁢⁢(2)
where c is the speed of sound in the (flowing) target material corrected for the compliance of the surroundings of the material, and L is the total length along the X axis of the second channel230and the nozzle structure218. For the MEMS system217, the quarter-wavelength frequency may be used as the second frequency. For example, assuming that c is 2000 m/s, to generate targets at a frequency of 320 kHz, L is 1.5 mm.

The MEMS system217may be designed without the second channel230such that the orifice219is directly coupled to the chamber228or with a second channel230that has a negligible extent between the orifice and the chamber228. In these implementations, the second frequency is defined by the Helmholtz frequency (Equation 1).FIGS. 3A and 3Bshow an example of such an implementation.

In implementations in which the second frequency is determined by the extent of the second channel230(such as, for example, from Equation 2), the thickness of the layer that contains the second channel230may be controlled to within tight tolerances to control the extent of the second channel230. In this way, such an implementation provides a tightly controlled value for the second frequency. For example, in some implementations, the MEMS system217is implemented as a planar (or substantially flat) structure with as few layers as possible.

Other techniques may be used to modulate the pressure in the chamber228. For example, in some implementations, the coalescence may be enhanced by adding harmonic frequencies between the first and second frequencies. Additionally or alternatively, a technique based on amplitude modulation may be used. Moreover, although the example above discusses using the quarter-wavelength frequency as the second frequency, other frequencies may be used. For example, a frequency greater than or equal to the frequency at which three quarters of a wave with that frequency fills the second channel230and the nozzle structure218along the X axis, may be used as the second frequency.

An electrically conductive coating242may be formed on the second end226. The conductive coating242may be any electrically conductive material. The nozzle structure218, orifice219, and other portions of the MEMS system217may be made from an insulating material such as silicon nitride (SiN). Electrical charge may accumulate on the second end226and other portions of the MEMS system217as a result of friction between the target material and the walls of the nozzle structure218, of the first channels227, of the second channel230and of other portions of the MEMS system217. The presence of this charge may impede formation of the coalesced targets and/or change the path of the targets due to a Coulomb repulsion between the targets. To mitigate this, the conductive coating may be grounded to remove the accumulated charge. The conductive coating242may be any electrically conductive material that is resistant to being corroded or otherwise degraded by exposure to the target material (for example, molten tin). For example, the conductive coating may be silicon dioxide, silicon monoxide, molybdenum, tungsten, tantalum, iridium, or chromium.

The MEMS system217is used in the assembled state.FIGS. 2A and 2Bshow the MEMS system217in the assembled state. Referring toFIG. 2D, a block diagram of the MEMS system217in an unassembled state is shown. The MEMS system217includes layers245a-245f. In the assembled state, the layers245a-245fare joined to each other along the X axis. In the assembled state, the MEMS system217may be considered to be a monolithic part formed from more than one layer. The monolithic part that forms the MEMS system217is a single, integral piece or part.

In the example ofFIG. 2D, the nozzle structure218, the orifice219, and the filter241dare formed as part of the layer245a. The layer245aalso may include the electrically conductive coating242. The nozzle structure218may have a coating of, for example SiN. The second channel230is formed as part of the layer245b. The chamber228and the intermediate channel229are formed in the layer245c. The membrane232and the portion238of the first channel227are formed in layer245d. The actuator231and associated components (for example, electrodes) are formed in layer245e. The portion237of the first channel227, and the filters241a,241bare formed in layer245f.

Other arrangements are possible. For example, the features of the layers245band245cmay be fabricated as a single layer, thus reducing the total number of layers in the MEMS system217to five. In another example, the features of layers245c,245d, and245e(which form, for example, the chamber228and the membrane232) may be fabricated as a single layer, thus reducing the total number of layers in the MEMS system217to four.

The layers245a-245fmay be permanently joined through, for example, a thermal, chemical, and/or mechanical bond such that, after being assembled, the layers245a-245fare not separable from each other without causing damage. In other implementations, the layers245a-245fare joined temporarily while the MEMS system217is used in the assembled state. In these implementations, the layers245a-245fmay be separated from each other after being assembled without causing damage. Techniques for temporarily joining the layers245a-245finclude, for example, applying a force to the layers245a-245falong the X axis (in the X and/or −X directions) with a clamp or mechanical mount. In some implementations, the layers245a-245fmay be permanently joined and held in a clamp.

The layers245a-245fmay be fabricated separately. Thus, a different semiconductor device fabrication technology may be used for each of the layers245a-245f. Additionally, the features of the MEMS system217that perform different functions may be fabricated in separate layers. For example, the actuator231and the nozzle structure218are fabricated as different layers. The orifice219may be formed on a surface or an edge of a silicon wafer by etching. In some implementations, more than one orifice219is etched onto a silicon wafer to form a group of target formation units. An example of this implementation is shown inFIG. 8.

Because each of the layers245a-245fis fabricated using a semiconductor device fabrication technology, the layers245a-245fare manufactured with a much greater degree of cleanliness than is possible using traditional machining technique. Additionally, in the example of the MEMS system217, the filters241aand241balso close the entrance to the MEMS system217in the assembled state. Closing the entrance may further enhance cleanliness of the MEMS system217, for example, contamination of the interior portion of the MEMS system217is limited by the filters241aand241bduring assembly and during operational use.

Any type of semiconductor device fabrication technology may be used to form the layers245a-245f. For example, each of the layers245a-245fmay be formed from a silicon wafer. After the layers245a-245fare formed, the layers may be joined by bonding each layer to an adjacent layer or layers. After the layers245a-245fare bonded, coatings are applied (for example, oxide and/or nitride coatings may be applied to each layer). In some implementations, each of the layers245a-245fare formed and a coating is applied to each of the layers245a-245fbefore the layers245a-245fare joined. As is known, bonding strengths depend on the surface roughness of the bonded surfaces.

Thus, the MEMS system217is expected to be operational even when placed under a relatively high pressure (for example, 8,000 psi). Moreover, in some implementations, the target formation apparatus116is operated and/or designed in an iso-static manner. In these implementations, the actuator231and/or other parts of the MEMS system217, or the entire MEMS system217is at the same pressure as the pressure of the target material in the chamber228or at a partial (or reduced) pressure compared to the pressure of the target material in the chamber228.

In the implementation shown inFIG. 2A, the MEMS system217is placed in a mechanical support or mount203. The mount203may include a device (such as, for example, a clamp) that applies additional force along the X direction and/or −X direction to aid in maintaining the structural integrity of the MEMS system217during operational use. In the example shown inFIG. 2A, the mount203surrounds the MEMS system217. The mount203also includes an opening204that coincides with the orifice219such that target material may be emitted from the orifice when the MEMS system217is in the mount203.

FIG. 3Ais a block diagram of a side cross-sectional view of a MEMS system317.FIG. 3Bis a block diagram of a top view of the MEMS system317as seen from the line3B-3B. The dotted lines inFIG. 3Brepresent hidden elements that are below a first end325of the MEMS system317.

The MEMS system317is another example of an implementation of the MEMS system117(FIG. 1). The MEMS system317may be used in the supply system110(FIG. 1), and the MEMS system317is discussed with reference to the supply system110. The MEMS system317is similar to the MEMS system217, except the MEMS system317does not include a second channel such as the second channel230.

The MEMS system317includes first channels327, which extend along the X axis from an end325. As shown inFIG. 3B, the MEMS system317includes six of the first channels327that are radially separated from each other by 60°. For simplicity, only one of the first channels327is labeled inFIG. 3B. Each of the first channels327is fluidly coupled to a chamber328via intermediate channels329. The chamber328is fluidly coupled to a nozzle structure318, which defines an orifice319. As shown inFIG. 3A, the MEMS system317also includes filters241a,241b, and241dand the actuator231. Although not shown inFIG. 3B, each of the first channels327includes a filter similar to the filter241aor the filter241bpositioned at the end325.

When the MEMS system317is fluidly coupled to the pressurized reservoir112, target material flows from the reservoir112through the filters at the first end325(for example, the filters241a,241bshown inFIG. 3A) and into the first channels327. The target material flows from the intermediate channels329into the chamber328, and from the chamber328into the filter241dand then into the nozzle structure318. The target material is emitted from the orifice319as a jet of target material that breaks into targets. Collectively, the first channels327, the intermediate channels329, the chamber328, and the orifice319form a target material path from the first end325to a second end326.

The actuator231is coupled to the chamber328in a manner similar to the actuator231in the MEMS system217. In the MEMS system317, the actuator231may be driven with two or more frequencies, including a frequency that is close the Rayleigh frequency. The second frequency may be the Helmholtz frequency, shown in Equation (1). As shown in Equation (1), the Helmholtz frequency depends on the number of intermediate channels329and on the surface area and length of the intermediate channels329. Thus, the second frequency (and thus the frequency of target production) may be tuned by increasing the number of intermediate channels329and/or by modifying the geometry of the intermediate channels329.

The MEMS system317is shown in the assembled state. The MEMS system317includes three layers345a,345b, and345c. The boundary of the layers are shown inFIG. 3Awith two dashed lines. The layers345a,345b, and345cmay be fabricated separately and are joined to form the assembled MEMS system317. The assembled MEMS system317may be accommodated in a mount such as the mount203ofFIG. 2A.

Other implementations of the MEMS system117are possible. For example, the MEMS systems217and317are configured such that the target material path is between two opposing sides of the MEMS system. However, other configurations may be used. For example, target material may enter the MEMS system along the Z axis and exit the MEMS system in the −X direction such that the target material path extends from ends of the MEMS system that are not at opposing sides. The MEMS system217includes six of the first channels227, though other implementations may include more or fewer first channels. For example, in some configurations, a single channel may extend from the first end225to the chamber228. The MEMS systems217and317may include more or fewer layers than the respective examples shown inFIGS. 2D and 3.

Furthermore, the first channels227include the portions237and238, and the first channels337include the portions337and338. The relative dimensions of the portion237to the portion238, and the dimensions of the portion337to the portion338may be different in different implementations. For example, in some implementations of the MEMS system317, the portion337may extend along the X axis for the entire length of the layer345csuch that the entire portion338is in the layer345b. Similarly, in some implementations of the MEMS system217, the portion237may extend along the X axis for the entire length of the layer245fsuch that the portion238is in a separate layer.

FIGS. 12A, 12B, 13A, and 13Bshow additional example implementations of the MEMS system117. Before discussing other example implementations of a MEMS system,FIGS. 4-8are discussed.FIGS. 4-8provide examples of a supply system that includes two or more controllable target formation units.

Referring toFIG. 4, a block diagram of a supply system400is shown. The supply system400may be used in an EUV light source. The supply system400includes a target formation apparatus416and a control system450. The target formation apparatus416includes n target formation units462(labeled as462_1to462_n), where n is any integer number greater than or equal to two. Each of the target formation units462includes a nozzle structure418, which defines an orifice419. InFIG. 4, the nozzle structures418are labeled418_1to418_n, and the respective orifice419for each nozzle is labeled419_1to419_n.

The target formation units462may be MEMS-based. For example, each of the target formation units462may be an instance of the MEMS system117(FIG. 1). An example of such an implementation is shown inFIG. 5A and 5B. In other implementations, the target formation units462are not MEMS-based and are instead made using traditional machining techniques. For example, each of the target formation units462may be formed from a high-strength metal that is machined using traditional machining techniques but with a nozzle that does not include a capillary tube. In some implementations, such as shown inFIG. 6, each of the target formation units462is made with traditional machining techniques and also includes a capillary tube.

The target formation apparatus416is configured to be fluidly coupled to a reservoir that contains target material, such as the reservoir112ofFIG. 1. In operational use, the orifices419are fluidly coupled to the reservoir such that pressurized target material is able to flow from the reservoir to any of the orifices419. Each of the target formation units462is capable of being activated or deactivation. The control system450controls which of the n target formation units462are able to produce targets at any given time by activating or deactivating certain one or more of the target formation units462. A particular target formation unit462is activated when it is able to emit target material from the respective orifice419. A particular target formation unit462is deactivated when target material is not able to be emitted from the respective orifice419.

The control system450controls the deactivation and/or activation of the target formation units462. In some implementations, including the implementation shown inFIG. 4, the control system450includes a temperature system453that is configured to control an amount of heat in the target formation units462. As discussed above, the target material may be in a molten state that is able to flow. For example, the target material may include molten tin. In these implementations, the temperature system453may add or remove heat from the target formation apparatus416and/or particular ones of the target formation units462. Adding heat may ensure that the target material remains in the molten state, whereas removing heat or cooling the target formation apparatus416and/or particular ones of the target formation units462causes the target material to solidify. When the target material is in the molten state, target material may be emitted from the orifice419and the target formation unit462is active. When the target material is solidified, target material is not emitted from the orifice and the target formation unit462is not active.

The temperature system453may include individual temperature systems463_1to463_n, with each of the target formation units462_1to462_nbeing associated with a respective one of the temperature systems463_1to463_n. Each of the temperature systems463_1to463_nmay include one or more cooling devices and/or one or more heating devices. The cooling device is any device capable of lowering the temperature of the associated target formation unit. For example, the cooling device may be an element that absorbs heat. The heating device is any device capable of increasing the temperature of the associated target formation unit. For example, the heating device may be a heater or a collection of heaters. The heating device and/or the cooling device may be implemented using, for example, a Peltier device. A Peltier device is a solid-state active heat pump that transfers heat from one side of the device to the other. A Peltier device also may be referred to as a Peltier heat pump, a Peltier cooler, a Peltier heater, a thermoelectric heat pump, a solid state refrigerator, or a thermoelectric cooler (TEC).

The control system450may control the temperature systems463_1to463_nwith activation and deactivation signals sent via the communications link452. The communications link452may be any type of communication link capable of sending data and electronic signals. The control system450exchanges data and/or information with the target formation apparatus416and/or any component of the target formation apparatus416via the communications link452. The control system450also includes an electronic processor454, an electronic storage456, and an input/output (I/O) interface458. The electronic processor454includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, an electronic processor receives instructions and data from a read-only memory, a random access memory, or both. The electronic processor454may be any type of electronic processor.

The electronic storage456may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage456includes non-volatile and volatile portions or components. The electronic storage456may store data and information that is used in the operation of the control system450and/or components of the control system450.

The electronic storage456also may store instructions, perhaps as a computer program, that, when executed, cause the electronic processor454to communicate with components in the control system450, the target formation apparatus416, and/or components in an EUV light source that includes the target formation apparatus416.

The I/O interface458is any kind of electronic interface that allows the control system450to receive and/or provide data and signals with a human operator, the target formation apparatus416, and/or an EUV light source that includes the target formation apparatus416, and/or an automated process running on another electronic device. For example, the I/O interface458may include one or more of a visual display, a keyboard, and a communications interface. In some implementations, the I/O interface458may be configured to connect to and communicate with a remote computer via a network such as the Internet.

The control system450may communicate with the target formation apparatus416to select a particular one of the target formation units462for activation or deactivation in response to information received at the I/O interface458. For example, the I/O interface458may receive an indication from an EUV light source that includes the target formation apparatus416that no EUV light is being produced. In another example, the control system450may receive an indication of an amount of EUV light that is being produced, and may compare the amount to an expected value stored in the electronic storage456. The control system450may activate a different target formation unit in response to the indication of no EUV light production or an amount that is below the expected value. In another example, a human operator may interact with the I/O interface458to select a particular one of the target formation units462_1to462_nfor activation or deactivation.

The control system450also may control other aspects of the target formation apparatus416. For example, the control system450may be configured to cause the target formation apparatus416to move by activating a mechanical positioning stage or similar device upon which the target formation apparatus416is mounted. In some implementations, the control system450may control an actuator in the target formation unit, such as the actuator231. For example, the control system450may be used to provide the first and second frequencies to electrodes that are near the actuator231.

Referring toFIGS. 5A and 5B, a target formation apparatus516is shown. The target formation apparatus516is accommodated in a mount570.FIG. 5Ais a block diagram of a side cross-sectional view of the target formation apparatus516and the mount570.FIG. 5Bis a block diagram of a top view of the target formation apparatus516and the mount570. The target formation apparatus516is an example of an implementation of the target formation apparatus416(FIG. 4), and the target formation apparatus516may be used with the control system450.

The target formation apparatus516is an example of a MEMS-based supply system with individually controllable MEMS-based target formation units517_1,517_2, and517_3. The target formation apparatus516includes three instances of the MEMS system217, which is discussed above with respect toFIGS. 2A-2D. The target formation apparatus516is a monolithic part formed from joined layers. The three instances are the target formation units517_1,517_2, and517_3. Each of the target formation units517_1,517_2, and517_3includes the features of the MEMS system217. When accommodated in the mount570, the first end225of each of the target formation units517_1,517_2, and517_3faces a reservoir that contains target material. In the example shown, the target formation units517_1,517_2, and517_3are arranged in a linear array that extends along the Z axis. The MEMS-based target formation apparatus516is relatively small. For example, the orifices of each of the target formation unit517_1,517_2, and517_3may be separated from each other by about 1 mm along the Z axis.

The target formation apparatus516also includes a thermal insulator565_1between the target formation units517_1and517_2, and a thermal insulator565_2between the target formation units517_2and517_3. The material from which the target formation units517_1,517_2, and517_3are formed may have relatively high thermal conductivity. For example, the target formation units517_1,517_2, and517_3may be made of silicon. The thermal insulators565_1and565_2provide a thermal barrier or reduce the amount of thermal transfer between adjacent target formation units. This allows the target formation units517_1,517_2, and517_3to be individually controllable (for example, able to be activated or deactivated one at a time) even when the target formation units517_1,517_2, and517_3are made out of a material that is a good thermal conductor.

The thermal insulators565_1and565_2may be made from any thermally insulating material that is suitable for use in a MEMS system. For example, each of the thermal insulators565_1and565_2may be cavities filled with air, stone wool, or folded polyimide foil. In some implementations, small holes can be etched at the positions of the thermal barriers565_1and565_2. A silicon dioxide or silicon layer may be placed in the etched holes. The thermal conductivity of silicon dioxide is about 5-10 W/m/K and of silicon 130 W/m/K, and a layer made of either or both of these materials provides thermal insulation. The thermal insulators565_1and565_2may be identical in shape and/or material. In some implementations, an active heating and/or cooling mechanism (such as a Peltier device) is associated with each of the target formation units517_1,517_2,517_3to actively heat or cool a particular target formation unit and to provide precise control of the temperature of each target formation unit.

The target formation apparatus516also includes temperature systems563_1,563_2, and563_3, which are respectively associated with the target formation units517_1,517_2, and517_3. In the target formation apparatus516, the temperature systems563_1,563_2, and563_3are part of the associated target formation unit and are also fabricated in a semiconductor device fabrication technology. For example, the target formation apparatus516may be a monolithic part that includes the target formation units517_1,517_2, and517_3, the temperature systems563_1,563_2, and563_3, and the thermal insulators565_1and565_2.

In operational use, the temperature systems563_1,563_2, and563_3receive control signals from the control system450. The control signals cause the activation or deactivation of the temperature systems563_1,563_2, and563_3thus allowing a particular one of the target formation units517_1,517_2, and517_3to be activated at any given time.

The target formation apparatus516may be held in place in the mount570with, for example, O-rings or any other sealing device. When accommodated in the mount570, a space571may be formed between the holder and the target formation apparatus516. The space571may be at the same partial pressure as the space234(which holds the actuator231) and the chamber228.

The example ofFIGS. 5A and 5Bshows a supply system that includes three target formation units, each of which is an instance of the MEMS system217. However, other configurations are possible. For example, more or fewer instances of the MEMS system217may be used and/or the instances may be arranged in a geometric configuration other than a linear array. MEMS systems other than the MEMS system217may be used in the target formation apparatus516.

Referring toFIG. 6, a perspective view of a target formation apparatus616is shown. The target formation apparatus616is another example of an implementation of the target formation apparatus416(FIG. 4), and the target formation apparatus616may be used with the control system450. The target formation apparatus616is used with an EUV light source.

The target formation apparatus616includes target formation units662that are individually controllable but are not MEMS-based. The target formation apparatus616includes more than two target formation units662. The target formation units662are fabricated using traditional machining techniques. Each target formation unit662includes at least one capillary tube671surrounded by a piezo actuator (not shown) and fitted into a filter673. Only one of the target formation units662is labeled inFIG. 6, but, as shown, the other units of the target formation apparatus616have similar features.

The target formation units662of the target formation apparatus616are mounted to a block670. The block670is mounted into the vacuum chamber of the EUV light source (for example, the vacuum chamber109ofFIG. 1). The block670is also mounted to the reservoir112such that target material in the pressurized reservoir may flow into the target formation units662and exit from an orifice of the capillary tube. The block670includes temperature systems (not shown) that are able to heat or cool the target material that flows toward the target formation units. Each of the target formation units662has an associated temperature system that is controllable with the control system450. The control system450controls the temperature system to maintain the molten state of the target material of a particular target formation unit to activate that target formation unit. To deactivate a particular target formation unit or to activate that target formation unit, the control system450controls the associated temperature system to cool the target material until it solidifies such that target material does not flow through that target formation unit, or to heat the target material so that it flows through that target formation unit.

In some implementations, the target formation units662may be configured to rotate in the block670about an axis677. In these implementations, the target formation units662are rotated relative to the block670to direct targets emitted from the activated target formation toward the plasma formation location123. In other implementations, the target formation units662are mounted in the block670such that all of the target formation units662are aimed at the plasma formation location123. In these implementations, the target formation units662do not rotate relative to the block670. In yet another implementation, the block670and the target formation units662move together relative to the plasma formation location123to direct targets emitted from the activated target formation unit toward the plasma formation location123. In this implementation, the block670and the target formation units662may rotate about the axis677along an arc A and/or may translate in any direction relative to the plasma formation location123. For example, the block670and the target formation units662may move together along a path P and/or a path P′. In implementations in which the block670and/or the target formation units672move, the control system450may be used to control the motion.

Referring toFIG. 7, a perspective view of a target formation apparatus716is shown. The target formation apparatus716is another example of an implementation of the target formation apparatus416(FIG. 4), and the target formation apparatus716may be used with the control system450. The target formation apparatus716is used with an EUV light source.

The target formation apparatus716includes target formation units762that are individually controllable. Only one of the target formation units762is labeled inFIG. 7, but, as shown, the other units have similar features. The target formation apparatus716includes more than two target formation units762. The target formation units762are MEMS-based and each target formation unit762includes a MEMS system717. The MEMS system717may be similar to the MEMS system117,217, or317, or the MEMS system717may have a different configuration. In each target formation unit762, the MEMS system717is mounted onto the filter673, which is mounted to the block675.

Referring toFIG. 8, a perspective view of a target formation apparatus816is shown. The target formation apparatus816is another example of an implementation of the target formation apparatus416(FIG. 4), and the target formation apparatus816may be used with the control system450. The target formation apparatus816is used with an EUV light source.

The target formation apparatus816includes target formation units862that are individually controllable. Only one of the target formation units862is labeled inFIG. 8, but, as shown, the other units have similar features. The target formation apparatus816includes more than two target formation units862. The target formation units862are MEMS-based and each target formation unit862includes a MEMS system817. The MEMS system817includes at least one filter and may be similar to the MEMS system217or317, or the MEMS system817may have a different configuration. The MEMS system817may be fabricated as multiple orifices that are etched in a silicon wafer. In each target formation unit862, the MEMS system817is mounted directly onto the block670.

The target formation apparatus716and816may be implemented to move relative to the block670and/or with the block670similar to the implementations of the target formation apparatus616. Additionally, the temperature systems of the block670and the control system450allow the target formation units762and862of the target formation apparatus716and816, respectively, to be individually controlled as discussed with respect to the target formation apparatus616. Moreover, more than one target formation unit may be activated at any given time such that two or more streams of targets are emitted toward the plasma formation location123.

FIG. 9schematically depicts a lithographic apparatus900including a source collector module SO according to one implementation. The target formation apparatuses116,416,516,616,716, and816are examples of target formation apparatuses or droplet generators that may be used in the source collector module SO. The lithographic apparatus900includes:an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV radiation).a support structure (for example, a mask table) MT constructed to support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;a substrate table (for example, a wafer table) WT constructed to hold a substrate (for example, a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; anda projection system (for example, a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, comprising one or more dies) of the substrate W.

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 projection system PS, like the illumination system IL, 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 apparatus is of a reflective type (for example, employing a reflective mask).

In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (for example, 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 PS2(for example, an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, for example, 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 PS1can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. Patterning device (for example mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2and substrate alignment marks P1, P2.

The depicted apparatus could be used in at least one of the following modes:1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.2. In scan mode, the support structure (for example, 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 (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.3. In another mode, the support structure (for example, mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

FIG. 10shows an embodiment of the lithographic apparatus900in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure1020of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma2may be formed by a laser produced LPP plasma source. The function of source collector module SO is to deliver EUV radiation beam20from the plasma2such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture1021in the enclosing structure1020. The virtual source point IF is an image of the radiation emitting plasma2.

From the aperture1021at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device22and a facetted pupil mirror device24. These devices form a so-called “fly's eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference1060). Upon reflection of the beam21at the patterning device MA, held by the support structure (mask table) MT, a patterned beam26is formed and the patterned beam26is imaged by the projection system PS via reflective elements28,30onto a substrate W held by the substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated while substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on patterning device MA through the slit of illumination.

Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to enclosing structure1020. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown inFIG. 10.

Considering source collector module SO in more detail, a laser energy source comprising laser1023is arranged to deposit laser energy1024into a fuel that includes a target material.

The target material may be any material that emits EUV radiation in a plasma state, such as xenon (Xe), tin (Sn), or lithium (Li). The plasma2is a highly ionized plasma with electron temperatures of several 10's of electron volts (eV). Higher energy EUV radiation may be generated with other fuel materials, for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector3and focused on the aperture1021. The plasma2and the aperture1021are located at first and second focal points of collector CO, respectively.

Although the collector3shown inFIG. 10is a single curved mirror, the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In an embodiment, the collector may be a grazing incidence collector which comprises a plurality of substantially cylindrical reflectors nested within one another.

To deliver the fuel, which, for example, is liquid tin, a droplet generator1026is arranged within the enclosure1020, arranged to fire a high frequency stream1028of droplets towards the desired location of plasma2. In operation, laser energy1024is delivered in a synchronism with the operation of droplet generator1026, to deliver impulses of radiation to turn each fuel droplet into a plasma2. The droplet generator1026may be or include any of the target formation apparatuses116,416,516,616,716, or816discussed above. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy1024is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy1024is delivered to the cloud at the desired location, to generate the plasma2. A trap1030is provided on the opposite side of the enclosing structure1020, to capture fuel that is not, for whatever reason, turned into plasma.

The droplet generator1026comprises a reservoir1001which contains the fuel liquid (for example, molten tin) and a filter1069and a nozzle1002. The nozzle1002is configured to eject droplets of the fuel liquid towards the plasma2formation location. The droplets of fuel liquid may be ejected from the nozzle1002by a combination of pressure within the reservoir1001and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams20,21,26. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module, the X axis coincides broadly with the direction of fuel stream1028, while the Y axis is orthogonal to that, pointing out of the page as indicated inFIG. 10. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagramFIG. 10, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

Numerous additional components used in the operation of the source collector module and the lithographic apparatus900as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector3and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus900.

Referring toFIG. 11, an implementation of an LPP EUV light source is shown.FIG. 11shows a LPP EUV light source1100. The light source1100may be used as the source collector module SO in the lithographic apparatus900. Moreover, any of the target formation apparatus116,416,516,616,716, and816may be used with the light source1100. For example, any of the target formation apparatus116,416,516,616,716, and816may be used with a supply system1125of the source1100. Furthermore, the light source105ofFIG. 1may be part of the drive laser1115, and the control system450may be part of the master controller1155, any of the components of the master controller1155, or may be implemented as a separate control system.

The LPP EUV light source1100is formed by irradiating a target mixture1114at a plasma formation location1105with an amplified light beam1110that travels along a beam path toward the target mixture1114. The target material discussed with respect toFIGS. 1-8may be or include the target mixture1114. The plasma formation location1105is within an interior1107of a vacuum chamber1130. When the amplified light beam1110strikes the target mixture1114, a target material within the target mixture1114is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture1114. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

The light source1100also includes the supply system1125that delivers, controls, and directs the target mixture1114in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture1114includes the target material such as, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SnH4; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture1114may also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture1114is made up of only the target material. The target mixture1114is delivered by the supply system1125into the interior1107of the chamber1130and to the plasma formation location1105.

The light source1100includes a drive laser system1115that produces the amplified light beam1110due to a population inversion within the gain medium or mediums of the laser system1115. The light source1100includes a beam delivery system between the laser system1115and the plasma formation location1105, the beam delivery system including a beam transport system1120and a focus assembly1122. The beam transport system1120receives the amplified light beam1110from the laser system1115, and steers and modifies the amplified light beam1110as needed and outputs the amplified light beam1110to the focus assembly1122. The focus assembly1122receives the amplified light beam1110and focuses the beam1110to the plasma formation location1105.

In some implementations, the laser system1115may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system1115produces an amplified light beam1110due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system1115may produce an amplified light beam1110that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system1115. The term “amplified light beam” encompasses one or more of: light from the laser system1115that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system1115that is amplified and is also a coherent laser oscillation.

The optical amplifiers in the laser system1115may include as a gain medium a filling gas that includes CO2and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system1115may include a pulsed laser device, for example, a pulsed, gas-discharge CO2laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, 10 kW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system1115may also include a cooling system such as water that may be used when operating the laser system1115at higher powers.

The light source1100includes a collector mirror1135having an aperture1140to allow the amplified light beam1110to pass through and reach the plasma formation location1105. The collector mirror1135may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation location1105and a secondary focus at an intermediate location1145(also called an intermediate focus) where the EUV light may be output from the light source1100and may be input to, for example, an integrated circuit lithography tool (not shown). The light source1100may also include an open-ended, hollow conical shroud1150(for example, a gas cone) that tapers toward the plasma formation location1105from the collector mirror1135to reduce the amount of plasma-generated debris that enters the focus assembly1122and/or the beam transport system1120while allowing the amplified light beam1110to reach the plasma formation location1105. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation location1105.

The light source1100may also include a master controller1155that is connected to a droplet position detection feedback system1156, a laser control system1157, and a beam control system1158. The light source1100may include one or more target or droplet imagers1160that provide an output indicative of the position of a droplet, for example, relative to the plasma formation location1105and provide this output to the droplet position detection feedback system1156, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system1156thus provides the droplet position error as an input to the master controller1155. The master controller1155may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system1157that may be used, for example, to control the laser timing circuit and/or to the beam control system1158to control an amplified light beam position and shaping of the beam transport system1120to change the location and/or focal power of the beam focal spot within the chamber1130.

The supply system1125includes a target material delivery control system1126that is operable, in response to a signal from the master controller1155, for example, to modify the release point of the droplets as released by a target material supply apparatus1127to correct for errors in the droplets arriving at the desired plasma formation location1105.

Additionally, the light source1100may include light source detectors1165and1170that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector1165generates a feedback signal for use by the master controller1155. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

The light source1100may also include a guide laser1175that may be used to align various sections of the light source1100or to assist in steering the amplified light beam1110to the plasma formation location1105. In connection with the guide laser1175, the light source1100includes a metrology system1124that is placed within the focus assembly1122to sample a portion of light from the guide laser1175and the amplified light beam1110. In other implementations, the metrology system1124is placed within the beam transport system1120. The metrology system1124may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam1110. A beam analysis system is formed from the metrology system1124and the master controller1155since the master controller1155analyzes the sampled light from the guide laser1175and uses this information to adjust components within the focus assembly1122through the beam control system1158.

Thus, in summary, the light source1100produces an amplified light beam1110that is directed along the beam path to irradiate the target mixture1114at the plasma formation location1105to convert the target material within the mixture1114into plasma that emits light in the EUV range. The amplified light beam1110operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system1115. Additionally, the amplified light beam1110may be a laser beam when the target material provides enough feedback back into the laser system1115to produce coherent laser light or if the drive laser system1115includes suitable optical feedback to form a laser cavity.

Other implementations are within the scope of the claims. For example, and as noted above, other implementations of the MEMS system117(FIG. 1) are possible. These other implementations of the MEMS system117may be used as the MEMS system117in the target formation apparatus116and the supply system110. Moreover, other implementations of the MEMS system117may be used in the target formation apparatus416(FIG. 4). For example, an instance of another implementations of the MEMS system117may be used as a target formation unit462.

FIGS. 12A and 12Bshow a MEMS system1217fabricated in a semiconductor device fabrication technology. The MEMS system1217is an implementation of the MEMS system117.FIG. 12Ais a block diagram of a side cross-sectional view of the MEMS system1217in the X-Z plane. InFIG. 12A, the MEMS system1217is accommodated in a mount1203.FIG. 12Bis a block diagram of a top view of the MEMS system1217in the Y-Z plane as seen from the line12B-12B.

The MEMS system1217includes a first end1225and a second end1226. In the MEMS system1217, the first end1225and the second end1226are at opposing sides. The MEMS system1217is accommodated in the mount1203. The mount1203is a clamp that holds the MEMS system1217at the first end1225and the second end1226. The mount1203applies force to the MEMS system1217along the X axis. The mount1203includes openings1204to allow target material to enter and exit the MEMS system1217. The mount1203may be sealed to the MEMS system1217. In the example ofFIG. 12A, a seal1245seals a nozzle1218of the MEMS system1217to the mount1203. The seal1245may be, for example, an O-ring seal or any other type of mechanism capable of sealing the nozzle1218to the mount1203.

A first channel1227extends from the first end1225along the X axis. In operational use, target material flows from a pressurized reservoir (such as the reservoir112ofFIG. 1) into the opening1204through a filter1241aand into the first channel1227. The first channel1227is fluidly coupled to a chamber1228via an intermediate channel1229. The chamber1228is partially formed by a wall1233. The chamber1228is fluidly coupled to a second channel1230, which extends away from the chamber1228along the Z axis. Thus, in the MEMS system1217, the first channel1227and the second channel1230are perpendicular to each other. The second channel1230is fluidly coupled to the nozzle1218, which extends along the X axis. The nozzle defines an orifice1219.

Together, the first channel1227, the intermediate channel1229, the chamber1228, the second channel1230, and the orifice form a target material path from the first end1225to the second end1226. The MEMS system1217includes just one first channel1227, and the target material that flows in the MEMS system1217may be more localized within the MEMS system1217as compared to an implementation that includes more than one first channel. Thus, the temperature of the MEMS system1217may be easier to control when, for example, the MEMS system1217is used as a target formation unit in a supply system that includes multiple target formation units that are individually controllable.

When the MEMS system1217is fluidly coupled to the pressurized reservoir112, target material flows from the reservoir112through the filter1241a, into the first channel1227and the intermediate channel1229, and then into the chamber1228. From the chamber1228, the target material flows into the second channel1230and the nozzle1218and passes through a filter1241b. The filters1241aand1241bmay be similar to any of the filters discussed above. After passing through the filter1241b, the target material is emitted from the orifice1219.

The MEMS system1217also includes the actuator231in a space1234. The actuator231is coupled to the chamber1228and is configured to modulate a pressure in the chamber1228. The actuator231is mechanically coupled to the chamber1228through a membrane1232. The actuator231contacts a wall1243, which is on an opposite side of the space1234from the membrane1232. The wall1243may be pressed against the actuator231in the −X direction to more firmly mechanically couple the actuator231to the membrane1232.

FIG. 12Ashows the MEMS system1217in an assembled state. The MEMS system1217is formed from two layers1245aand1245b. The boundary between the layers is shown inFIG. 12Awith a dashed line that extends in the Z axis. The layers1245aand1245bmay be fabricated separately and are joined to form the assembled MEMS system1217. The MEMS system1217includes just two layers and may be simpler to manufacture as compared to an implementation that includes more layers.

FIGS. 13A and 13Bshow a MEMS system1317, which is another example of an implementation of the MEMS system117.FIG. 13Ais a cross-sectional block diagram of the MEMS system1317in the Y-Z plane.FIG. 13Bis a cross-sectional block diagram of the MEMS system1317in the X-Z plane.FIG. 13Bshows layers1345a,1345b, and1345cof the MEMS system1317.

The MEMS system1317includes a first channel1327that extends along the Z axis from a first end1325. The first channel1327is fluidly coupled to an intermediate channel1329, a chamber1328, and a second channel1330, all of which extend along the Z axis. The second channel1330is fluidly coupled to a nozzle1318, which extends along the X axis (into the page inFIG. 13A). The nozzle1318defines an orifice1319. The MEMS system1317also includes an actuator231in a space1334. The actuator231is mechanically coupled to the chamber1328through a membrane1332. The actuator231is configured to modulate a pressure in the chamber1328.

In operational use, the first channel1327receives target material from a pressurized reservoir, such as the reservoir112ofFIG. 1. The target material flows from the first channel1327into the intermediate channel1329and the chamber1328. The target material flows from the chamber1328into the second channel1330. Together the first channel1327, the intermediate channel1329, the chamber1328, the second channel1330, and the orifice1319form a target material path through the MEMS system1317from the first end1325(FIG. 13A) to a second end1326(FIG. 13B). In the implementation ofFIGS. 13A and 13B, the first end1325and the second end1326are not at opposing sides of the MEMS system1317. Instead, the first end1325is at a side of the MEMS system1317that extends in in the X-Y plane, and the second end1326is at a side of the MEMS system1317that extends in the Y-Z plane.